The Reducibility of the Greek Nickeliferous
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Transcript of The Reducibility of the Greek Nickeliferous
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 19
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
The reducibility of the Greek nickeliferouslaterites a review
E Zevgolis C Zografidis and I Halikia
This paper refers to a critical review of the Greek nickeliferous laterites roasting reduction studies
for better understanding of the process thermodynamic and kinetic mechanisms affecting
decisively the smelting step in pyrometallurgical extraction of nickel from these ores From this
work it is deduced that iron and nickel oxide reduction degree does not exceed 33 and 76
respectively the reductive reactions being stopped within the first 20ndash30 min Thus part of ferric
iron is transformed into ferrous iron (in the form of magnetite wustite or fayalite) instead of metallic
iron production Also no total conversion to metallic nickel takes place Variation of roasting
temperature (700ndash850uC) grain size of the ores and type of solid reductants affect the reduction
rates and the final reduction degrees obtained Diffusion or mixed control mechanisms have been
found to prevail during reduction Low reduction degrees obtained are attributed to kinetic
phenomena such as the formation of fayalite (2FeOSiO2) which probably covers oxide grains
and impedes reduction
Keywords Reducibility Laterites Review Ferronickel Pyrometallurgy
Introduction
Nickel is a lustrous silvery white metal with a highmelting point (1453uC) high resistance to corrosion and
oxidation good thermal and electrical conductivity
ferromagnetic properties catalytic behaviour ease of
electroplating and excellent strength and toughness at
elevated temperatures1 properties that are indicative of
the wide range of its applications Laterite (oxide) and
sulphide ores constitute the two basic natural resources
of nickel accounting for about 60 and 40 of the
worldrsquos primary nickel reserves respectively Nickel
production and demand has presented an eightfold rise
since 1950 and laterite source nickel though it formed
only a small fraction of the worldrsquos production (10 in
1950) its participation has risen ever since to 42 (2003)and according to laterite projected economic scenarios
and forecasts it is expected to rise further more to 51
by 20122
The three types of nickel laterite deposits with
economic interest for nickel are limonitic intermediate
and saprolitic (or garnieritic) The main mineralogical
phase of the saprolitic ores is garnierite
(NiMg)6Si4O10(OH)8 having a high magnesia and low
iron content with Ni grade 15 t o 35 Limonitic
laterites contain iron oxides as the main mineralogical
phase ndash goethite (a-FeOOH) hematite (Fe2O3) or
magnetite (Fe3O4)- thus having a high iron content
and a lower Ni grade (1ndash2) Laterite ores are classifiedinto three classes3
(i) class A garnieritic type of laterites (Fe 12and MgO 25)
(ii) class B limonitic type of laterites (high Fecontent 15ndash32 or 32 and MgO 10)
(iii) class C intermediate type of laterite ores that liebetween garnieritic and limonitic type of ores (Fe12ndash15 and MgO 25ndash35 or 10ndash25)
Greek nickeliferous laterites chemicalndashmineralogical characterisation andindustrial treatmentGreek nickeliferous laterite deposits are mainly locatedin three different regions Evia Island Lokrida inCentral Greece and Kastoria in Northern GreeceGreek reserves represent 90 of nickel laterite reservesin the European Union the remainder occurring in
Finland Evia Island and Lokrida laterites are classifiedin class B while Kastoria laterite deposit is approachinga typical example of class C laterite In Table 1 a typicalchemical analysis of the Greek nickeliferous laterites isgiven
Evia Kastoria and Lokrida laterite samples have beencharacterised by using techniques such as XRDdifferential thermal analysis and thermogravimetry X-
ray fluorescence ore microscopy and electron microp-robe analysis All laterite types examined are charac-
terised by a pissolitic texture Evia and Lokiridalateritesrsquo main mineralogical constituents were foundto be hematite (Fe2O3) and quartz (SiO2) while the mainnickel bearing mineral phases are the chlorite group[(MgNiFeAl)6(AlSi)4O10(OH)8]45 Some other nickel
Laboratory of Metallurgy School of Mining and Metallurgical EngineeringNational Technical University of Athens Heroon Polytechniou 9 15780Zografou Athens Greece
Corresponding author email zografidismetalntuagr
2010 Institute of Materials Minerals and Mining and The AusIMMPublished by Maney on behalf of the Institute and The AusIMMReceived 12 March 2009 accepted 16 August 2009DOI 101179174328509X431472
Mineral Processing and ExtractiveMetallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 9
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 29
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
bearing mineral phases that have been found in selective
Lokrida deposit parts are nepouite [(NiMg)3Si2O5(OH)4]
and takovite [(Ni5Al4O2(OH)86H2O]5 The main miner-
alogical constituent of the Kastoria ore was found to be
serpentine (MgFeNi)6Si4O12(OH)6 The high goethite (a-
FeOOH) content of Kastoria ore indicates a higher
content of the crystallic moisture compared to the other
types of Greek laterites Serpentine and nickeliferous
magnesian crostendite (Fez28 Fez3
4 )(Si4Fez34 )O20(OH)6-
where part of Fez2 can be replaced by Mg- represent the
main nickel carrier minerals in Kastoria ore6
Microcrystallity of the nickel bearing mineral phases
renders their liberation and upgrading by mineral proces-
sing techniques extremely difficult something which would
improve drastically the cost of pyrometallurgical or
hydrometallurgical laterite treatment
Greek nickeliferous laterites are processed by the
pyrometallurgical method lsquoLarco Processrsquo at the metal-
lurgical plant of LARCO GMM SA for the production
of a ferronickel alloy (FendashNi) with y20Ni and low C
S and P suitable for austenitic stainless steel productionlsquoLarco Processrsquo involves the following steps
(i) handling and mixing of raw materials (ie
laterite solid fuels and agglomerated rotary
kiln (RK) dust in form of pellets)
(ii) drying preheating and controlled prereduction
of the metallurgical mixture in RKs and
production of a calcine
(iii) smelting reduction of the calcine in open-bath
submerged-arc electric furnaces (EFs) and
production of an FendashNi alloy with 12ndash15Ni
(iv) refining and enrichment of FendashNi with 12ndash
15Ni to FendashNi with 18ndash23Ni in OBM
converters and granulation of the final market-able alloy product
Mechanisms of laterite solid statereductionThe main reactions that take place during coal based
reduction of iron nickel and cobalt oxides contained in
laterites can be summarised as follows
(i) three-step reduction of hematite
Step 1 hematite to magnetite
3Fe2
O3zCO2Fe
3O
4zCO
2 (1)
Step 2 magnetite to wustite
Fe3O4zCO3FeOzCO2 (2)
Step 3 wustite to iron
FeOzCOFeozCO2 (3)
One-step reduction of nickel and cobalt oxides as
follows
(a) NiOzCONiozCO2 (1a)
(b) CoOzCOCoozCO2 (1b)
(i) coal de-volatilisation
solid fuel (coal lignite coke)
carbon (C)zVolatile matter (4)
(ii) carbon gasification (Boudouard reaction)
Cz
CO2
2CO (5)
Hydrocarbons are the main gas constituents of the
volatile matter In high temperatures the high hydro-
carbons are cracked into low ones so that gaseous
reductants CO and H2 are evolved7
There is a general acceptance7ndash9 that solid-state
reduction in the Fe-C-O system (in the presence of solid
fuel) is mainly carried out via the gaseous intermediates
(mainly CO) which are produced by the solid fuelrsquos
gasification reaction (5) Regeneration of gaseous reduc-
tant through the Boudouard reaction provides the
reductive atmosphere needed for the transformation of iron nickel and cobalt oxides Direct contact of the ore
with carbon particles is probably responsible for the
production of CO during the very early stages of
reduction according to the following reaction
AxOyzCAxOy1zCO (6)
where A5Fe Ni or Co
But the overall reductive reaction mainly occurs
through the combination of reaction (7) below and
reaction (5) as follows
AxOyzCOAxOy1zCO2 (7)
AxOyzCAxOy1zCO
Physico-chemical parameters affecting reducibility of nickeliferous laterites
Greek lateritesReducibility of Greek laterites has been a subject of
extended research in order to find optimum values of
the physico-chemical parameters affecting their roasting
reduction process Given that energy requirementsconstitute one of the basic pillars for competitiveness
and development of the metallurgical process applied
the highest necessary reduction degree (37) of the
calcine is of decisive importance as it corresponds to
important energy saving during the smelting step of the
process and also because with this reduction degree
Table 1 Typical chemical analysis of Greek nickeliferouslaterites
Component Evia Lokrida Kastoria
SiO2 282 186 322Al2O3 70 109 29Fe2O3 475 450 248Fetot 332 314 173Cr2O3 31 27 14MnO 004 004 001
MgO 32 40 154Ni 103 115 145Co 005 006 006S 04 045 040CaO 30 66 145LOI 50 75 125Total 9888 9741 9903
(ii)
(iii)
Zev gol is et al The reducibility of the Greek nickeliferous laterites
10 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 39
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
smelting step is a quiet process since no reduction gasesare evolved through the slag in the EF
Table 2 presents a summary of the conditions
employed and the main conclusions drawn by the
published work on reducibility of Greek nickeliferous
laterites Temperature has proved to be one of the most
important operational parameters in RK roastingreduction of the Greek laterites According to the
equilibrium diagram of the Fe-C-O system as presentedin Fig 1 magnetite reduction (Fe3O4zCO53FeOzCO2) and wustite reduction (FeOzCO5FeozCO2) can take place above 680 and 700uC
respectively10 Chemical analysis of representative sam-
ples taken along the length of industrial RKs indicatedthat up to 700ndash800uC laterite reduction remained in low
levels (up to 5ndash7) but it was drastically increased for
higher temperatures11 (however always 300)
Calcine temperature is not increased above y850uC
due to agglomeration of the calcine Moreover all
experiments carried out in the laboratory by different
researchers regarding reducibility of Greek laterites withsolid fuels in the temperature range of 700ndash900uC
showed that an increase of temperature favours reduc-
tion rate of both iron and nickel oxides Nevertheless
influence of temperature on the reduction degree of iron
oxides is positive between 700 and 750uC1213 or 700 and
800uC according to nickeliferous RK dust and pellets
reducibility tests14 and then it diminishes for higher
temperatures Within the same temperature range
examined nickel oxide reduction degree is increased
for temperatures 700ndash800uC and then it remains
practically constant1213
Reducibility of laterite is considerably affected by the
ore grain size as shown during ore treatment in RKsand EFs of industrial scale for ferronickel produc-
tion1516 The conclusion drawn from this work is that
mean size of the ore and reduction degree are inversely
proportional Indeed even by macroscopic observation
of calcined ore grains topochemical reactions are
taking place that is reductive reactions take place
upon the ore grainrsquos surface within the temperature
range 700ndash850uC So by decreasing ore particle sizefree surface is increased and therefore total rate of
reduction is increased It was proved that gradual
decrease of feed size from 240 to 212 mm had very
satisfactory results in terms of the calcinersquos reduction
degree achieved Additionally a finer ore correspondsto a higher mean temperature of the calcine therefore
it corresponds to a more energy saving during smeltingof the calcine Nevertheless studying the effect of ore
grain size not only in the pre-reduction but also in the
smelting step of the Greek laterites it has been
concluded that a high portion of fine particles fed into
the RKs favoured high levels of dust production
(which means a higher nickel loss in dust operational
problems of the RKrsquos dedusting installation and
additional cost for dust handling and treatment in
order to be pelletised and recycled) and was responsible
for lower height of calcine-self-lining for prevention of EF walls chemical attack from slag Laboratory
studies using Greek laterite ore and solid fuels of
various grain sizes have also confirmed that iron and
nickel oxide reduction rate increases with decreasing
ore and carbonaceous material grain size This
phenomenon is more evident for grain size bigger than
3 mm as the difference in reduction obtained for finer
particles is less significant12
The type of solid fuel used as reductant constitutesanother critical parameter affecting reducibility of
Greek laterites Its role is much more complex than
being just a heat source as for example in cementproduction where fuel calorific value is the ultimate
critical parameter17 In case of laterite reduction solid
fuels are utilised both for heating and reduction thustheir reactivity is of great importance Also they can be
classified according to their reactivity as follows lignite
coal coke and graphite11 Lignite is characterised by its
high volatile matter inherent moisture and low percen-
tage of fixed carbon (Cfix) It evolves its thermal energy
in lower temperatures compared to coal or coke and
graphite which contain a higher percentage of fixed
carbon and a lower percentage of volatile matter than
lignite Laboratory studies of the Greek laterites withdifferent types of solid reductants maintaining the other
parameters fixed (temperature ore and fuel grain size
C
fix
Fe ratio etc) confirmed that the use of lignite
18
resulted in higher iron and nickel oxide reduction
degrees in comparison with the use of coal and pet
coke9 Moreover increasing the amount of carbonac-
eous material relative to iron and nickel oxide content
increased the rate of reduction Also industrial scale
experiments have shown that reduction degree of Greek
laterites decreases as we go from lignite to coke in the
following sequence ligniteRcoalRcoke11
The role of solid fuels used for reduction of Greek
laterites in RKs has also been studied1116 The conclu-
sions drawn after extensive evaluation of operational
data is that optimum results with respect to calcine
reduction are obtained in the appropriate combinationof solid fuels so that the necessary amount of volatiles
and Cfix respectively is contained Moreover increase of
the lignite amount has proved to result in an increased
reduction rate due to burning of the volatile matter in
the first zone of the RKs (drying preheating zone) andtherefore maintaining of the required reductive atmo-
sphere so that a higher reduction is obtained
Nevertheless very high content of volatile matter in
the metallurgical mixture corresponds to high tempera-
ture of RK flue gases which cause increased thermal
losses and risk for fire in the dedusting installation For
this reason decrease of lignite participation in the
metallurgical mixture is required and increased amountof less reactive coal with higher content of the main
reductive agent Cfix resulting in a lower specific
consumption of fuel per ton of natural laterite
There are various approaches published in interna-
tional literature concerning the effect of solid fuel
volatile matter in iron ore reduction All of them
however seem to agree that volatiles increase the rate of
reduction There is also an agreement among different
researchers that participation of volatiles in reductiondepends mainly on temperature Thus at temperatures
below 900ndash1000uC78 reduction by volatile matter is
regarded as quite significant whereas at higher tem-peratures the role of volatiles is negligible so that at
higher temperatures reduction mainly takes place
through solid carbon This is due to the fact that most
of volatiles are evolved at temperatures 1000uC11 Liu
et al and Strezov et al1920 also investigated the
fundamental mechanisms for iron ore fines reduction
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 11
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 49
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
T a b l e
2
L a b o r a t
o r y
a n d
i n d u s t r i a l r e s u l t s
o n
r e d u c i b
i l i t y
o f G r e e k
n i c k e l i f e r o u s
l a t e r i t e s
R e f e r e n c e
Z e v g o l i s 1 1
H a l i k i a - N e o u
e t
a l 1 2
H a l i k i a
a n d
S k
a r t a d o s
1 8
N e o u
e t
a l 9
N e o u
e t a l 1 3
Z e v g o l i s
e t
a l 1 4
O r i g i n o f
l a t e r i t e o r e
E v i a ndash L o k r i d a ndash P e l l e t s
( f r o m
r o t a r y k i l n d u s t )
7 0 ndash 3 0 ndash 5 w t -
E v i a
ndash L o k r i d a ndash K a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
E v i a ndash L o k r i d a ndash K
a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
E v i a ndash L o k r i d a ndash K a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
K a s t o r i a
G r e e k N i -
f e r r o u s l a t e r i t e
ndash r o t a r y k i l n d u s t
O r e g r a i n s i z e
O r e 2 1 5 m m
p e l l e t s 2 1 2 m m
2 2 0
8 z 3 8 m m
2 1 0
0 z 3 8 m m
2 5 3
z 3 8 m m
2 6 z 0 0 3 8 m m
2 2 0 8 z 3 7 m m
O r e 2 0 2 5 0 z 0 0 3 8 m m
p e l l e t s 2 1 0 z 5 6 m m
2 1 4 z 1 0 m m
D u s t 2 1 0 5 m m p e l l e t s 9 m m
R e d u c t a n t
C o a l l i g n i t e
C f i x
( o f d u s t )
L i g n
i t e
L i g n i t e c o a l
P e t c o k e
C o a l
C f i x
( o f d u
s t )
C f i x F e t o t
1 5 2 4
1 4
3 1
1 6 0 2 ndash 1 5 0 1
1 4 6 3
1 3 5 1
1 2 4 ndash 1 2 6
R e d u c t a n t
g r a i n s i z e
2 3 0 m m 2 5 m m
2 1 0 0 m m
2 2 0
8 z 1 4 7 m m
2 1 4
7 z 1 0 4 m m
2 1 0
4 z 7 4 m m
2 6 z 0 0 3 8 m m
2 2 0 8 z 3 7 m m
2 0 1 5 0 z 0 1 0 6 m m
hellip
S p e c i m e n f o r m
B u l k o r e a n d s o l i d f u e l s
G r o
u n d l a t e r i t e
a n d
l i g n i t e
B u l k o r e a n d
s o l i d f u e l s
G r o u n d l a t e r i t e
a n d p e t c o k e
P u l v e r i s e d o r e ndash
p e l l e t s b i n d i n g a g e n t
b e n t o n i t e 1
R o t a r y k i l n d u s t p e l l e t s
( o f s a m e
d u s t )
T e m p e r a t u r e
u C
9 2 0
7 0 0
ndash 8 5 0
8 6 0 ndash 9 0 0
7 5 0 ndash 9 0 0
7 0 0 ndash 9 0 0
7 0 0 ndash 8 5 0
M a x i m u m
r e d u c t i o
n
d e g r e e ndash
c o n d i t i o n s
2 5
3 2 7
3
2 9 5 5
2 3 1 6
3 1 7 4
2 8
9 2 0 u C
8 5 0
u C
9 0 0 u C
9 0 0 u C
9 0 0 u C
8 5 0 u C
2 1 5 m m
o r e
2 1 2 m m
p e l l e t s
2 3 0 m m
c o a l
2 5 m m
l i g n i t e
2 5 3
z 3 8 m m
o r e
2 1 0
4 z 7 4 m m
l i g n i t e
L i g n i t e
P u l v e r i s e d o r e
R o t a r y k i l n d u s t
A c t i v a t i o n
e n e r g y E k J m o l 2
1
hellip
4 0 1
7 ( i r o n o x i d e )
hellip
7 2 4 ( i r o n o x i d e )
hellip
4 2 E 8 4
( 7 0 0 ndash 7 5 0 u C )
8 7 4
5 ( n i c k e l o x i d e )
E 4 2 ( 8 0
0 ndash 8 5 0 u C )
R a t e c o n t r o l l i n g s t e p
hellip
D i f f u s i o n ( i r o n o x i d e )
M i x e d k i n e t i c m o d e l
c h e
m i c a l r e a c t i o n
ndash d i f f u s i o n ( n i c k e l o x i d e )
hellip
D i f f u s i o n ( i r o n o x i d e )
M i x e d k i n e t i c m o d e l
c h e m i c a l
r e a c t i o n ndash d i f f u s i o n
( n i c k e l o x i d e )
hellip
M i x e d k i n
e t i c m o d e l c h e m i c a l
r e a c t i o n ndash
d i f f u s i o n ( i r o n o x i d e )
Zev gol is et al The reducibility of the Greek nickeliferous laterites
12 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 59
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
in coalndashore mixtures and the role of the fuel volatile
matter They reported that volatile matter constituents
were methane (CH4) within the temperature range 450ndash
700uC and CO kai H2 for temperatures 800uC Gas
agents CO kai H2 are mainly responsible for hematite
(Fe2O3) reduction up to 800uC while additional CO
generated due to the Boudouard reaction at elevated
temperatures resulted in acceleration of the reduction
procedure Concerning laboratory work with Greek
laterite the fact that remaining carbon in the calcine is
in most cases much higher than the theoretically
expected indicates according to researchers that evolvedvolatiles participate in the oxide reduction912 but this
requires further investigation
Reducibility tests have been conducted by different
researchers on Greek laterites in the form of dust and
pellets as well Dust produced during industrial treat-
ment of laterite in the RKs has a higher Ni content than
the ore and a carbon content 8 which is higher than
the stoichiometrically required (y65) for FendashNi 13
production Thus it is evident that agglomeration and
recycling of dust which is y7 of the laterite feed
contribute to the economics of the metallurgical process
So the reductive behaviour of the industrially produced
cement bonded laterite pellets has been studied forcomparison with the behaviour of same origin laterite
dust14 Reducibility tests of pulverised Kastoria laterite
ore have been also conducted using coal as a reductant
in order to compare with the reductive behaviour of
agglomerated mixture (pellets) of Kastoria pulverised
ore and coal (the stoichiometrically required for FendashNi
13 production) with bentonite as the binding agent13
The temperature range of the experimental procedure
was 700ndash900uC It was deduced from both series of tests
that iron oxide reduction degree obtained for pellets was
lower than for dust (with same laterite) under the same
conditions though no serious difference is observed
concerning nickel oxide reduction This difference in thereductive behaviour between laterite dust and pellets can
be attributed to the fact that the binding agent covers
part of the oxide grains decreasing their reactive surface
area
The conclusion drawn from all reducibility studies of
Greek nickeliferous laterites either conducted in an
industrial or in a laboratory scale within the tempera-
ture range of 700ndash900uC is that independently of the
conditions employed ie ore grain size temperature and
type and amount of the solid reductant in no case
calcine reduction degree exceeds 33 though the
remaining carbon in the calcine is almost always higher
than the theoretically required for further progress of reductive reactions Given that the conversion of Fe2O3
1 Equilibrium curves of CO-CO2 versus temperature for Fe-C-O system10
2 Reduction degree versus time typical diagrams at various temperatures of a laterite ore mixture (Evia IslandndashLokridandash
Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lignite (20149z00940 mm)12 b electrostatic filter dust14 and c elec-
trostatic filter pellets14
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 13
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 69
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
to Fe3O4 and to FeO corresponds to a reduction of 111and 333 respectively it comes that magnetite and
wustite should coexist in the calcine and no furtherconversion to metallic iron exists Additionally it is
concluded that reductive reactions occur within the first 20
to 30 min of the process and then they practically stop for
all cases examined This can be attributed to kineticphenomena such as the formation within the temperature
range examined of iron-silicate minerals such as fayalite
(2FeOSiO2) which probably covers oxide grains thusimpeding progress of the reduction In Fig 2 typical
reduction degree diagrams versus time within a tempera-
ture range 700ndash850uC are presented for reduction of
pulverised laterite ore mixture (Evia islandndashLokridandash Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lig-
nite (20149z00940 mm) and also the reduction of
electrostatic filter dust from RKs as well as the reduction
of laboratory made pellets from this dust From these
diagrams it comes that reduction degree does not exceed
33 The same is shown in Fig 3 with reduction in
industrial RKs (In Fig 3 reduction is defined as the ratio
Fe2z
Fetot
|100 so the real reduction degree ie
Oi Of eth THORN=Oifrac12 |100 where Oi is the initial oxygen and
Of is the final oxygen in iron oxides is 333)
Industrial operation with garnierite type of ores shows
that reduction in the RK goes up to 37 which meansthat we have metallic iron in the calcine21
World experienceReducibility tests concerning different types of nickelifer-
ous laterites according to international literature2223
indicate that chemical mineralogical composition of the
laterite ore is a critical parameter for the final degree of reduction obtained Reducibility of several types of
nickeliferous laterites from the Dominican Republic22
(grain size 20074 mm) has been studied within the
temperature range 400ndash1000uC using hydrogen as a
reductant The aim of the study was to compare the
reductive behaviour of garnieritic limonitic and inter-
mediate type of ores The experimental results clearly
indicated that reducibility of each oxide (iron nickel and
cobalt oxide) depends on the ore type Moreover
chemical and mineralogical analysis of the reduced
samples indicated that nickel and iron (oxide) degree of
metallisation was higher in limonitic than in garnieritic
and intermediate type of ores Specifically both iron andnickel oxide degree of metallisation obtained after 40 minprocessing of the garnieritic type of ore at 1000uC was
y10 Temperature variation between 400 and 1000uC
did not affect significantly the reduction process The
respective degrees of metallisation were 80 and 70 at
1000uC concerning limonitic type of ore and they were
dramatically increased 800uC Cobalt degree of metal-
lisation at 1000uC was slightly lower (y65) in limonitic
compared to garnieritic type of ore (70) and the effect of
temperature variation was negligible concerning the first
and significant 850uC concerning the second one This
is different from what happens in reduction by solid fuelsThat is in solid fuel reduction limonitic type of ores
cannot be reduced to metallic iron so reduction stops
when reduction degree of iron oxides is 333 Low
nickel oxide reducibility in the garnieritic type ores was
attributed to the formation of olivine a nickelndashironndash
magnesium orthosilicate and the tendency of nickel to
3 Temperature and reduction degree in RK
Zev gol is et al The reducibility of the Greek nickeliferous laterites
14 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 79
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
exchange places with magnesium in silicates which are
stable in high temperatures On the contrary the low
magnesia and silica content of the limonitic type of ores is
not adequate to result in hosting nickel in the olivine
phase thus the per cent of nickel oxide reduction degree
increases for high temperatures Cobalt oxide reducibility
was higher for the garnieritic laterite type and the effect of
temperature was more evident for lower temperatures
due to the cobalt tendency to replace iron in the limonitelattice Reductive reactions concerning all types of laterite
ores examined after 40 min practically stopped In the
Greek laterite reducibility tests as it has already been
mentioned above reductive reactions practically stopped
after 20 to 30 min
Reducibility tests of a garnieritic type of laterite ore in
the form of pellets in the range 700ndash1000uC using a CO
CO2 mixture as a reducing agent has also been
conducted23 Olivine (MgFe)2SiO4 formation was
proved to be critical for the reduction progress since it
is stated that reducibility (determined by percentage of
weight loss after first calcined in the same temperaturelaterite ore pellets) reaches the highest possible values for
temperatures above serpentine (Mg Fe Ni)6Si4O12(OH)6decomposition (y600uC) and below transformation
temperature of amorphous olivine to a stable miner-
alogical phase (y810uC) It was also deduced that a
strongly competitive relation exists among reduction
progress and olivine formation ie a slow reduction rate
(by employment of mild reducing atmosphere through
gas reducing agent) relative to olivine formation results to
a lower reduction degree On the contrary rapid
reduction rate (by employment of intensive reducing
atmosphere during the first minutes of the reductive
procedure) relative to the crystallisation of the olivinephase results in higher reduction degree values
Reduction kinetics of Greek nickeliferous lateritesKinetic analysis carried out for the Greek nickeliferous
laterite roasting reduction is based on the unreacted
shrinking core model Roasting reduction kinetics of
Greek laterite fine particles (EviandashLokridandashKastoria ore
mixture 60ndash25ndash15 wt- granulometry 20250z
0037 mm temperature range 700ndash900uC) with lignite
and pet coke as reductants respectively were con-ducted912 The methodology of work used for
approaching the rate controlling step of the process is
the application of the diagnostic equation
lnln 1aeth THORNfrac12 ~n ln tzln b (8)
where a is reduction degree () of iron or nickel oxides t
is time (s) b is constant and n is constant depending on
the rate controlling mechanism and the geometrical
characteristics of the ore and solid reductant particles
The obtained n values from application of the experi-
mental data a ndash t which represent the slopes of the linear
graphic representation of equation (8) are comparedwith the theoretical values of the widespread used kinetic
equations of Table 3 It is noted that equations (D1)ndash
(D5) correspond to the diffusion rate controlling step
equations (F1) (R1) and (R2) correspond to chemical
reaction mechanism and equations (A2) and (A3)
correspond to the nucleation rate controlling step
Linearity assessment of diagrams ln[2ln(12a)] versus
lnt and determination of the slope n were obtainedthrough the application of the least squares method It isnoted that no time greater than 15 min was used for the
kinetic analysis given that after the first 20 min thereactions tended to stop in all cases examined Theconclusion deduced from this work was that diffusion
kinetic equations (D1)ndash(D5) best fitted the experimental
data concerning iron oxide reduction Moreover it hasbeen reported that the ValensindashCarter equation was themathematical model that best represented the proposeddiffusion mechanism with Z value equal to 05 Another
value of coefficient Z would probably result in an evenmore representative mathematical model With respectto the nickel oxide reduction kinetics it was reported
that chemical reaction is the rate controlling step for thelower temperatures (up to 800uC) but no certain kineticmodel from Table 3 (equations (F1) (R1) and (R2)) canbe stated to be the best fitting due to the fact that all of
the aforementioned three equations fit very well
The rate controlling step though changes for highertemperatures (up to 900uC) but there has not been a
conclusion based on reported approaches about themechanism that prevails within the temperature range800ndash900uC
Reduction kinetics of RK dust and laboratory made
pellets of the same origin in the temperature range 700ndash 850uC were studied14 The experimental data obtainedconcerning iron oxide reduction were applied to thefollowing mathematical models
(i) CrankndashGinstlingndashBrounshtein (CGB) kineticmodel
1(2=3)a(1a)2=3~Kt (D4) (9)
rate controlling step diffusion through the
product layer
(ii)(1a)1=3
~Kt (R2) (10)
rate controlling step chemical reaction at the
interface between the unreacted core and the
product layer
(iii) 1(2=3)a(1a)2=3z1(1a)1=3
~Kt (D4zR2) (11)
Generalised equation that is a combination of
equations (9) and (10) based on the additivity of
reaction times
rate controlling step mixed controlled mechanism
Table 3 n values of kinetic equations for gasndashsolidreactions
Kinetic equation n
D1 a 25Kt 062D2 (12a )ln(12a )za 5Kt 057D3 [12(12a )13]25Kt 054D4 12(23)a 2(12a )235Kt 057D5 Kt 5Z z[1z(Z 21)a ]232(Z 21)(12a )23(Z 21) (ValensindashCarter equation)
F1 2ln(12a )5Kt 10R1 12(12a )125Kt 111R2 12(12a )135Kt 107A2 [2ln(12a )]125Kt 20A3 [2ln(12a )]135Kt 20
Z coefficient representing the product volume per volume ofthe reactants consumed Z is assumed to be 05
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 15
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 89
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
It was deduced from the kinetic analysis that the
mixed kinetic model equation (11) fits well the experi-mental data for all dust samples and pellets of a certainorigin (the washing tower) up to 750uC This kinetic
model also verified the experimental data for pellets of different origin (ie electrofilter and polycyclone dusts)up to 800uC At higher temperatures ie up to 850uC ithas been confirmed the predominance of diffusion
mechanism according to the CGB equation (9)The activation energy value of the rate-determining
step at various conditions has been evaluated concern-ing coal based laterite reduction by means of thewidespread rate expression (12) that follows
r~kC n (12)
where r is the reaction rate k the rate constant C thefluid reactant concentration (mol L21) and n the orderof the reaction Assuming that the fluid reactantconcentration is constant the Arrhenius law[lnk 5lnAo ndash E R(1T )] can be used for calculation of the
activation energy through slope determination of thefollowing linear equation
lnr~ln(AoC n)(E =R)(1=T ) (13)
where r is the mean initial reduction rate valuecalculated through the expression r5DaDt21 (a is thereduction rates obtained from the experimental data)
Ao is the frequency factor of the Arrhenius law C is thefluid reactant concentration (mol L21) n the order of the reaction E the activation energy T the temperatures
(K) examined for the kinetic analysis and R the universalgas constant Activation energy has also been calculatedby Zevgolis et al concerning RK dust and laboratory
made pellets of the same origin through application of the Arrhenius law to the slopes of the generalisedequation (13) lines mixed controlled mechanism at 700and 750uC ie by means of the relationship
ln k (T 2)
ln k (T 1)~
E
R
1
T 2
1
T 1
(14)
The activation energy values concerning iron oxidereduction912 (724 and 402 kJ mol21) indicate that theprevailing kinetic mechanism may be either the mixedcontrolled mechanism (diffusionndashchemical reaction) ordiffusion Activation energy value reported concerningnickel oxide reduction (874 kJ mol21) indicates thepredominance ndash at least for a certain temperature rangeof chemical reaction mechanism The mixed controlledmechanism for iron oxide reduction was proposed byZevgolis et al for low temperatures (700ndash750uC) and
diffusion through the product layer for higher tempera-tures (800ndash850uC)14
The conclusions drawn from the kinetic analysis of the Greek nickeliferous laterite solid state reductionare in agreement with the conclusions drawn by otherstudies concerning reducibility tests of different originlaterite samples either in pellet form23 or in form of cement bonded laterite briquettes with CO-CO2 gas
reducing mixture24
within the temperature range (700ndash 1000uC) Mixed control (chemical reaction and diffu-
sion of the reducing gas agent through the productlayer) is the mechanism that seems to prevail duringthe reductive procedure It is apparent from theaforementioned that diffusion constitutes a seriouskinetic factor affecting solid state reduction of laterites
either on its own or in combination with chemicalreaction in the interface product layer ndash gas reactantKinetic analysis of Greek nickeliferous lateritesenhances the conclusion drawn by all the reportedreducibility tests according to which iron silicatemineralogical phases formed in the temperature rangeexamined (700ndash900uC) like fayalite (2FeOSiO2)probably cover the iron oxide grains and thus impede
the progress of the reaction
ConclusionsThe main physicochemical parameters affecting reduci-bility of the Greek nickeliferous laterite ores have beenreviewed It is concluded that no matter if controllingthe values of physicochemical parameters such astemperature grain size of materials and type of solidreductant to be optimum iron and nickel oxide
reduction degrees obtained do not exceed 33 and 76respectively though the remaining carbon in the calcineis adequate for further progress of the reductive
reactions This means that practically no metallic ironis formed during roasting reduction of Greek lateritesand nickel oxide is partially transformed to metallicnickel Decrease of the ore and solid reductant grain size(mainly for granulometry z3 mm) and use of reactivesolid fuels (lignite) instead of less reactive (coal or coke)favour considerably iron and nickel oxide degree of
reduction Increase of temperature within the tempera-ture range 700ndash900uC examined results in increase of oxides reduction rate but its influence on iron and nickeloxide degree is evident up to 750ndash800uC and then itdiminishes Reduction process progresses initially with arelatively high rate for the first ten minutes then the rate
starts to decrease until it diminishes to zero after thefirst 20 min reaction The conclusion that reductivereactions practically stop after a time of 20ndash40 min isverified by published approaches concerning reducibility
studies of different origin laterite ores It can beattributed to the formation within the temperaturerange examined of iron silicate minerals such as fayalite(2FeOSiO2) or forsterite (Mg2SiO4) which probablycover oxide grains and impede further progress of thereduction Thus future reducibility studies of Greeknickeliferous laterites should be focused on mineralogi-cal analysis of the reduction products in relation withthe factors affecting the formation and reduction of complex nickelndashironndashmagnesium silicate phases so that
the highest possible reduction degree is obtainedKinetic studies of the reductive procedure showed thatmixed control and diffusion mechanisms have been
found to prevail during reduction of oxides
References1 wwwlibmurdocheduauadtpubfilesadt-MU20051004114504
02Wholepdf
2 A D Dalvi W Bacon and C Osborne lsquoThe past and the future of
nickel lateritesrsquo Proc PDAC 2004 Int Convent Toronto
Canada March 2004 PDAC 1ndash27
3 E N Zevgolis lsquoExtractive metallurgy of nickel part I
Pyrometallurgical methodsrsquo 2000 Athens National TechnicalUniverity of Athens
4 E Mposkos A Orfanoudaki and Th Perraki lsquoThe Ni distribution
in the mineral phases of Greek FendashNi laterite depositsrsquo Proc 3rd
Symp on lsquoMineral wealthrsquo Athens Greece November 2000
Technical Chamber of Greece 107ndash115
5 N Albadakis lsquoNi-minerals in the deposits of the sub-pelagonic
zonersquo Miner Wealth 1984 31 9ndash32
Zev gol is et al The reducibility of the Greek nickeliferous laterites
16 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 29
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
bearing mineral phases that have been found in selective
Lokrida deposit parts are nepouite [(NiMg)3Si2O5(OH)4]
and takovite [(Ni5Al4O2(OH)86H2O]5 The main miner-
alogical constituent of the Kastoria ore was found to be
serpentine (MgFeNi)6Si4O12(OH)6 The high goethite (a-
FeOOH) content of Kastoria ore indicates a higher
content of the crystallic moisture compared to the other
types of Greek laterites Serpentine and nickeliferous
magnesian crostendite (Fez28 Fez3
4 )(Si4Fez34 )O20(OH)6-
where part of Fez2 can be replaced by Mg- represent the
main nickel carrier minerals in Kastoria ore6
Microcrystallity of the nickel bearing mineral phases
renders their liberation and upgrading by mineral proces-
sing techniques extremely difficult something which would
improve drastically the cost of pyrometallurgical or
hydrometallurgical laterite treatment
Greek nickeliferous laterites are processed by the
pyrometallurgical method lsquoLarco Processrsquo at the metal-
lurgical plant of LARCO GMM SA for the production
of a ferronickel alloy (FendashNi) with y20Ni and low C
S and P suitable for austenitic stainless steel productionlsquoLarco Processrsquo involves the following steps
(i) handling and mixing of raw materials (ie
laterite solid fuels and agglomerated rotary
kiln (RK) dust in form of pellets)
(ii) drying preheating and controlled prereduction
of the metallurgical mixture in RKs and
production of a calcine
(iii) smelting reduction of the calcine in open-bath
submerged-arc electric furnaces (EFs) and
production of an FendashNi alloy with 12ndash15Ni
(iv) refining and enrichment of FendashNi with 12ndash
15Ni to FendashNi with 18ndash23Ni in OBM
converters and granulation of the final market-able alloy product
Mechanisms of laterite solid statereductionThe main reactions that take place during coal based
reduction of iron nickel and cobalt oxides contained in
laterites can be summarised as follows
(i) three-step reduction of hematite
Step 1 hematite to magnetite
3Fe2
O3zCO2Fe
3O
4zCO
2 (1)
Step 2 magnetite to wustite
Fe3O4zCO3FeOzCO2 (2)
Step 3 wustite to iron
FeOzCOFeozCO2 (3)
One-step reduction of nickel and cobalt oxides as
follows
(a) NiOzCONiozCO2 (1a)
(b) CoOzCOCoozCO2 (1b)
(i) coal de-volatilisation
solid fuel (coal lignite coke)
carbon (C)zVolatile matter (4)
(ii) carbon gasification (Boudouard reaction)
Cz
CO2
2CO (5)
Hydrocarbons are the main gas constituents of the
volatile matter In high temperatures the high hydro-
carbons are cracked into low ones so that gaseous
reductants CO and H2 are evolved7
There is a general acceptance7ndash9 that solid-state
reduction in the Fe-C-O system (in the presence of solid
fuel) is mainly carried out via the gaseous intermediates
(mainly CO) which are produced by the solid fuelrsquos
gasification reaction (5) Regeneration of gaseous reduc-
tant through the Boudouard reaction provides the
reductive atmosphere needed for the transformation of iron nickel and cobalt oxides Direct contact of the ore
with carbon particles is probably responsible for the
production of CO during the very early stages of
reduction according to the following reaction
AxOyzCAxOy1zCO (6)
where A5Fe Ni or Co
But the overall reductive reaction mainly occurs
through the combination of reaction (7) below and
reaction (5) as follows
AxOyzCOAxOy1zCO2 (7)
AxOyzCAxOy1zCO
Physico-chemical parameters affecting reducibility of nickeliferous laterites
Greek lateritesReducibility of Greek laterites has been a subject of
extended research in order to find optimum values of
the physico-chemical parameters affecting their roasting
reduction process Given that energy requirementsconstitute one of the basic pillars for competitiveness
and development of the metallurgical process applied
the highest necessary reduction degree (37) of the
calcine is of decisive importance as it corresponds to
important energy saving during the smelting step of the
process and also because with this reduction degree
Table 1 Typical chemical analysis of Greek nickeliferouslaterites
Component Evia Lokrida Kastoria
SiO2 282 186 322Al2O3 70 109 29Fe2O3 475 450 248Fetot 332 314 173Cr2O3 31 27 14MnO 004 004 001
MgO 32 40 154Ni 103 115 145Co 005 006 006S 04 045 040CaO 30 66 145LOI 50 75 125Total 9888 9741 9903
(ii)
(iii)
Zev gol is et al The reducibility of the Greek nickeliferous laterites
10 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 39
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
smelting step is a quiet process since no reduction gasesare evolved through the slag in the EF
Table 2 presents a summary of the conditions
employed and the main conclusions drawn by the
published work on reducibility of Greek nickeliferous
laterites Temperature has proved to be one of the most
important operational parameters in RK roastingreduction of the Greek laterites According to the
equilibrium diagram of the Fe-C-O system as presentedin Fig 1 magnetite reduction (Fe3O4zCO53FeOzCO2) and wustite reduction (FeOzCO5FeozCO2) can take place above 680 and 700uC
respectively10 Chemical analysis of representative sam-
ples taken along the length of industrial RKs indicatedthat up to 700ndash800uC laterite reduction remained in low
levels (up to 5ndash7) but it was drastically increased for
higher temperatures11 (however always 300)
Calcine temperature is not increased above y850uC
due to agglomeration of the calcine Moreover all
experiments carried out in the laboratory by different
researchers regarding reducibility of Greek laterites withsolid fuels in the temperature range of 700ndash900uC
showed that an increase of temperature favours reduc-
tion rate of both iron and nickel oxides Nevertheless
influence of temperature on the reduction degree of iron
oxides is positive between 700 and 750uC1213 or 700 and
800uC according to nickeliferous RK dust and pellets
reducibility tests14 and then it diminishes for higher
temperatures Within the same temperature range
examined nickel oxide reduction degree is increased
for temperatures 700ndash800uC and then it remains
practically constant1213
Reducibility of laterite is considerably affected by the
ore grain size as shown during ore treatment in RKsand EFs of industrial scale for ferronickel produc-
tion1516 The conclusion drawn from this work is that
mean size of the ore and reduction degree are inversely
proportional Indeed even by macroscopic observation
of calcined ore grains topochemical reactions are
taking place that is reductive reactions take place
upon the ore grainrsquos surface within the temperature
range 700ndash850uC So by decreasing ore particle sizefree surface is increased and therefore total rate of
reduction is increased It was proved that gradual
decrease of feed size from 240 to 212 mm had very
satisfactory results in terms of the calcinersquos reduction
degree achieved Additionally a finer ore correspondsto a higher mean temperature of the calcine therefore
it corresponds to a more energy saving during smeltingof the calcine Nevertheless studying the effect of ore
grain size not only in the pre-reduction but also in the
smelting step of the Greek laterites it has been
concluded that a high portion of fine particles fed into
the RKs favoured high levels of dust production
(which means a higher nickel loss in dust operational
problems of the RKrsquos dedusting installation and
additional cost for dust handling and treatment in
order to be pelletised and recycled) and was responsible
for lower height of calcine-self-lining for prevention of EF walls chemical attack from slag Laboratory
studies using Greek laterite ore and solid fuels of
various grain sizes have also confirmed that iron and
nickel oxide reduction rate increases with decreasing
ore and carbonaceous material grain size This
phenomenon is more evident for grain size bigger than
3 mm as the difference in reduction obtained for finer
particles is less significant12
The type of solid fuel used as reductant constitutesanother critical parameter affecting reducibility of
Greek laterites Its role is much more complex than
being just a heat source as for example in cementproduction where fuel calorific value is the ultimate
critical parameter17 In case of laterite reduction solid
fuels are utilised both for heating and reduction thustheir reactivity is of great importance Also they can be
classified according to their reactivity as follows lignite
coal coke and graphite11 Lignite is characterised by its
high volatile matter inherent moisture and low percen-
tage of fixed carbon (Cfix) It evolves its thermal energy
in lower temperatures compared to coal or coke and
graphite which contain a higher percentage of fixed
carbon and a lower percentage of volatile matter than
lignite Laboratory studies of the Greek laterites withdifferent types of solid reductants maintaining the other
parameters fixed (temperature ore and fuel grain size
C
fix
Fe ratio etc) confirmed that the use of lignite
18
resulted in higher iron and nickel oxide reduction
degrees in comparison with the use of coal and pet
coke9 Moreover increasing the amount of carbonac-
eous material relative to iron and nickel oxide content
increased the rate of reduction Also industrial scale
experiments have shown that reduction degree of Greek
laterites decreases as we go from lignite to coke in the
following sequence ligniteRcoalRcoke11
The role of solid fuels used for reduction of Greek
laterites in RKs has also been studied1116 The conclu-
sions drawn after extensive evaluation of operational
data is that optimum results with respect to calcine
reduction are obtained in the appropriate combinationof solid fuels so that the necessary amount of volatiles
and Cfix respectively is contained Moreover increase of
the lignite amount has proved to result in an increased
reduction rate due to burning of the volatile matter in
the first zone of the RKs (drying preheating zone) andtherefore maintaining of the required reductive atmo-
sphere so that a higher reduction is obtained
Nevertheless very high content of volatile matter in
the metallurgical mixture corresponds to high tempera-
ture of RK flue gases which cause increased thermal
losses and risk for fire in the dedusting installation For
this reason decrease of lignite participation in the
metallurgical mixture is required and increased amountof less reactive coal with higher content of the main
reductive agent Cfix resulting in a lower specific
consumption of fuel per ton of natural laterite
There are various approaches published in interna-
tional literature concerning the effect of solid fuel
volatile matter in iron ore reduction All of them
however seem to agree that volatiles increase the rate of
reduction There is also an agreement among different
researchers that participation of volatiles in reductiondepends mainly on temperature Thus at temperatures
below 900ndash1000uC78 reduction by volatile matter is
regarded as quite significant whereas at higher tem-peratures the role of volatiles is negligible so that at
higher temperatures reduction mainly takes place
through solid carbon This is due to the fact that most
of volatiles are evolved at temperatures 1000uC11 Liu
et al and Strezov et al1920 also investigated the
fundamental mechanisms for iron ore fines reduction
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 11
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 49
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
T a b l e
2
L a b o r a t
o r y
a n d
i n d u s t r i a l r e s u l t s
o n
r e d u c i b
i l i t y
o f G r e e k
n i c k e l i f e r o u s
l a t e r i t e s
R e f e r e n c e
Z e v g o l i s 1 1
H a l i k i a - N e o u
e t
a l 1 2
H a l i k i a
a n d
S k
a r t a d o s
1 8
N e o u
e t
a l 9
N e o u
e t a l 1 3
Z e v g o l i s
e t
a l 1 4
O r i g i n o f
l a t e r i t e o r e
E v i a ndash L o k r i d a ndash P e l l e t s
( f r o m
r o t a r y k i l n d u s t )
7 0 ndash 3 0 ndash 5 w t -
E v i a
ndash L o k r i d a ndash K a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
E v i a ndash L o k r i d a ndash K
a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
E v i a ndash L o k r i d a ndash K a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
K a s t o r i a
G r e e k N i -
f e r r o u s l a t e r i t e
ndash r o t a r y k i l n d u s t
O r e g r a i n s i z e
O r e 2 1 5 m m
p e l l e t s 2 1 2 m m
2 2 0
8 z 3 8 m m
2 1 0
0 z 3 8 m m
2 5 3
z 3 8 m m
2 6 z 0 0 3 8 m m
2 2 0 8 z 3 7 m m
O r e 2 0 2 5 0 z 0 0 3 8 m m
p e l l e t s 2 1 0 z 5 6 m m
2 1 4 z 1 0 m m
D u s t 2 1 0 5 m m p e l l e t s 9 m m
R e d u c t a n t
C o a l l i g n i t e
C f i x
( o f d u s t )
L i g n
i t e
L i g n i t e c o a l
P e t c o k e
C o a l
C f i x
( o f d u
s t )
C f i x F e t o t
1 5 2 4
1 4
3 1
1 6 0 2 ndash 1 5 0 1
1 4 6 3
1 3 5 1
1 2 4 ndash 1 2 6
R e d u c t a n t
g r a i n s i z e
2 3 0 m m 2 5 m m
2 1 0 0 m m
2 2 0
8 z 1 4 7 m m
2 1 4
7 z 1 0 4 m m
2 1 0
4 z 7 4 m m
2 6 z 0 0 3 8 m m
2 2 0 8 z 3 7 m m
2 0 1 5 0 z 0 1 0 6 m m
hellip
S p e c i m e n f o r m
B u l k o r e a n d s o l i d f u e l s
G r o
u n d l a t e r i t e
a n d
l i g n i t e
B u l k o r e a n d
s o l i d f u e l s
G r o u n d l a t e r i t e
a n d p e t c o k e
P u l v e r i s e d o r e ndash
p e l l e t s b i n d i n g a g e n t
b e n t o n i t e 1
R o t a r y k i l n d u s t p e l l e t s
( o f s a m e
d u s t )
T e m p e r a t u r e
u C
9 2 0
7 0 0
ndash 8 5 0
8 6 0 ndash 9 0 0
7 5 0 ndash 9 0 0
7 0 0 ndash 9 0 0
7 0 0 ndash 8 5 0
M a x i m u m
r e d u c t i o
n
d e g r e e ndash
c o n d i t i o n s
2 5
3 2 7
3
2 9 5 5
2 3 1 6
3 1 7 4
2 8
9 2 0 u C
8 5 0
u C
9 0 0 u C
9 0 0 u C
9 0 0 u C
8 5 0 u C
2 1 5 m m
o r e
2 1 2 m m
p e l l e t s
2 3 0 m m
c o a l
2 5 m m
l i g n i t e
2 5 3
z 3 8 m m
o r e
2 1 0
4 z 7 4 m m
l i g n i t e
L i g n i t e
P u l v e r i s e d o r e
R o t a r y k i l n d u s t
A c t i v a t i o n
e n e r g y E k J m o l 2
1
hellip
4 0 1
7 ( i r o n o x i d e )
hellip
7 2 4 ( i r o n o x i d e )
hellip
4 2 E 8 4
( 7 0 0 ndash 7 5 0 u C )
8 7 4
5 ( n i c k e l o x i d e )
E 4 2 ( 8 0
0 ndash 8 5 0 u C )
R a t e c o n t r o l l i n g s t e p
hellip
D i f f u s i o n ( i r o n o x i d e )
M i x e d k i n e t i c m o d e l
c h e
m i c a l r e a c t i o n
ndash d i f f u s i o n ( n i c k e l o x i d e )
hellip
D i f f u s i o n ( i r o n o x i d e )
M i x e d k i n e t i c m o d e l
c h e m i c a l
r e a c t i o n ndash d i f f u s i o n
( n i c k e l o x i d e )
hellip
M i x e d k i n
e t i c m o d e l c h e m i c a l
r e a c t i o n ndash
d i f f u s i o n ( i r o n o x i d e )
Zev gol is et al The reducibility of the Greek nickeliferous laterites
12 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 59
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
in coalndashore mixtures and the role of the fuel volatile
matter They reported that volatile matter constituents
were methane (CH4) within the temperature range 450ndash
700uC and CO kai H2 for temperatures 800uC Gas
agents CO kai H2 are mainly responsible for hematite
(Fe2O3) reduction up to 800uC while additional CO
generated due to the Boudouard reaction at elevated
temperatures resulted in acceleration of the reduction
procedure Concerning laboratory work with Greek
laterite the fact that remaining carbon in the calcine is
in most cases much higher than the theoretically
expected indicates according to researchers that evolvedvolatiles participate in the oxide reduction912 but this
requires further investigation
Reducibility tests have been conducted by different
researchers on Greek laterites in the form of dust and
pellets as well Dust produced during industrial treat-
ment of laterite in the RKs has a higher Ni content than
the ore and a carbon content 8 which is higher than
the stoichiometrically required (y65) for FendashNi 13
production Thus it is evident that agglomeration and
recycling of dust which is y7 of the laterite feed
contribute to the economics of the metallurgical process
So the reductive behaviour of the industrially produced
cement bonded laterite pellets has been studied forcomparison with the behaviour of same origin laterite
dust14 Reducibility tests of pulverised Kastoria laterite
ore have been also conducted using coal as a reductant
in order to compare with the reductive behaviour of
agglomerated mixture (pellets) of Kastoria pulverised
ore and coal (the stoichiometrically required for FendashNi
13 production) with bentonite as the binding agent13
The temperature range of the experimental procedure
was 700ndash900uC It was deduced from both series of tests
that iron oxide reduction degree obtained for pellets was
lower than for dust (with same laterite) under the same
conditions though no serious difference is observed
concerning nickel oxide reduction This difference in thereductive behaviour between laterite dust and pellets can
be attributed to the fact that the binding agent covers
part of the oxide grains decreasing their reactive surface
area
The conclusion drawn from all reducibility studies of
Greek nickeliferous laterites either conducted in an
industrial or in a laboratory scale within the tempera-
ture range of 700ndash900uC is that independently of the
conditions employed ie ore grain size temperature and
type and amount of the solid reductant in no case
calcine reduction degree exceeds 33 though the
remaining carbon in the calcine is almost always higher
than the theoretically required for further progress of reductive reactions Given that the conversion of Fe2O3
1 Equilibrium curves of CO-CO2 versus temperature for Fe-C-O system10
2 Reduction degree versus time typical diagrams at various temperatures of a laterite ore mixture (Evia IslandndashLokridandash
Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lignite (20149z00940 mm)12 b electrostatic filter dust14 and c elec-
trostatic filter pellets14
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 13
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 69
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
to Fe3O4 and to FeO corresponds to a reduction of 111and 333 respectively it comes that magnetite and
wustite should coexist in the calcine and no furtherconversion to metallic iron exists Additionally it is
concluded that reductive reactions occur within the first 20
to 30 min of the process and then they practically stop for
all cases examined This can be attributed to kineticphenomena such as the formation within the temperature
range examined of iron-silicate minerals such as fayalite
(2FeOSiO2) which probably covers oxide grains thusimpeding progress of the reduction In Fig 2 typical
reduction degree diagrams versus time within a tempera-
ture range 700ndash850uC are presented for reduction of
pulverised laterite ore mixture (Evia islandndashLokridandash Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lig-
nite (20149z00940 mm) and also the reduction of
electrostatic filter dust from RKs as well as the reduction
of laboratory made pellets from this dust From these
diagrams it comes that reduction degree does not exceed
33 The same is shown in Fig 3 with reduction in
industrial RKs (In Fig 3 reduction is defined as the ratio
Fe2z
Fetot
|100 so the real reduction degree ie
Oi Of eth THORN=Oifrac12 |100 where Oi is the initial oxygen and
Of is the final oxygen in iron oxides is 333)
Industrial operation with garnierite type of ores shows
that reduction in the RK goes up to 37 which meansthat we have metallic iron in the calcine21
World experienceReducibility tests concerning different types of nickelifer-
ous laterites according to international literature2223
indicate that chemical mineralogical composition of the
laterite ore is a critical parameter for the final degree of reduction obtained Reducibility of several types of
nickeliferous laterites from the Dominican Republic22
(grain size 20074 mm) has been studied within the
temperature range 400ndash1000uC using hydrogen as a
reductant The aim of the study was to compare the
reductive behaviour of garnieritic limonitic and inter-
mediate type of ores The experimental results clearly
indicated that reducibility of each oxide (iron nickel and
cobalt oxide) depends on the ore type Moreover
chemical and mineralogical analysis of the reduced
samples indicated that nickel and iron (oxide) degree of
metallisation was higher in limonitic than in garnieritic
and intermediate type of ores Specifically both iron andnickel oxide degree of metallisation obtained after 40 minprocessing of the garnieritic type of ore at 1000uC was
y10 Temperature variation between 400 and 1000uC
did not affect significantly the reduction process The
respective degrees of metallisation were 80 and 70 at
1000uC concerning limonitic type of ore and they were
dramatically increased 800uC Cobalt degree of metal-
lisation at 1000uC was slightly lower (y65) in limonitic
compared to garnieritic type of ore (70) and the effect of
temperature variation was negligible concerning the first
and significant 850uC concerning the second one This
is different from what happens in reduction by solid fuelsThat is in solid fuel reduction limonitic type of ores
cannot be reduced to metallic iron so reduction stops
when reduction degree of iron oxides is 333 Low
nickel oxide reducibility in the garnieritic type ores was
attributed to the formation of olivine a nickelndashironndash
magnesium orthosilicate and the tendency of nickel to
3 Temperature and reduction degree in RK
Zev gol is et al The reducibility of the Greek nickeliferous laterites
14 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 79
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
exchange places with magnesium in silicates which are
stable in high temperatures On the contrary the low
magnesia and silica content of the limonitic type of ores is
not adequate to result in hosting nickel in the olivine
phase thus the per cent of nickel oxide reduction degree
increases for high temperatures Cobalt oxide reducibility
was higher for the garnieritic laterite type and the effect of
temperature was more evident for lower temperatures
due to the cobalt tendency to replace iron in the limonitelattice Reductive reactions concerning all types of laterite
ores examined after 40 min practically stopped In the
Greek laterite reducibility tests as it has already been
mentioned above reductive reactions practically stopped
after 20 to 30 min
Reducibility tests of a garnieritic type of laterite ore in
the form of pellets in the range 700ndash1000uC using a CO
CO2 mixture as a reducing agent has also been
conducted23 Olivine (MgFe)2SiO4 formation was
proved to be critical for the reduction progress since it
is stated that reducibility (determined by percentage of
weight loss after first calcined in the same temperaturelaterite ore pellets) reaches the highest possible values for
temperatures above serpentine (Mg Fe Ni)6Si4O12(OH)6decomposition (y600uC) and below transformation
temperature of amorphous olivine to a stable miner-
alogical phase (y810uC) It was also deduced that a
strongly competitive relation exists among reduction
progress and olivine formation ie a slow reduction rate
(by employment of mild reducing atmosphere through
gas reducing agent) relative to olivine formation results to
a lower reduction degree On the contrary rapid
reduction rate (by employment of intensive reducing
atmosphere during the first minutes of the reductive
procedure) relative to the crystallisation of the olivinephase results in higher reduction degree values
Reduction kinetics of Greek nickeliferous lateritesKinetic analysis carried out for the Greek nickeliferous
laterite roasting reduction is based on the unreacted
shrinking core model Roasting reduction kinetics of
Greek laterite fine particles (EviandashLokridandashKastoria ore
mixture 60ndash25ndash15 wt- granulometry 20250z
0037 mm temperature range 700ndash900uC) with lignite
and pet coke as reductants respectively were con-ducted912 The methodology of work used for
approaching the rate controlling step of the process is
the application of the diagnostic equation
lnln 1aeth THORNfrac12 ~n ln tzln b (8)
where a is reduction degree () of iron or nickel oxides t
is time (s) b is constant and n is constant depending on
the rate controlling mechanism and the geometrical
characteristics of the ore and solid reductant particles
The obtained n values from application of the experi-
mental data a ndash t which represent the slopes of the linear
graphic representation of equation (8) are comparedwith the theoretical values of the widespread used kinetic
equations of Table 3 It is noted that equations (D1)ndash
(D5) correspond to the diffusion rate controlling step
equations (F1) (R1) and (R2) correspond to chemical
reaction mechanism and equations (A2) and (A3)
correspond to the nucleation rate controlling step
Linearity assessment of diagrams ln[2ln(12a)] versus
lnt and determination of the slope n were obtainedthrough the application of the least squares method It isnoted that no time greater than 15 min was used for the
kinetic analysis given that after the first 20 min thereactions tended to stop in all cases examined Theconclusion deduced from this work was that diffusion
kinetic equations (D1)ndash(D5) best fitted the experimental
data concerning iron oxide reduction Moreover it hasbeen reported that the ValensindashCarter equation was themathematical model that best represented the proposeddiffusion mechanism with Z value equal to 05 Another
value of coefficient Z would probably result in an evenmore representative mathematical model With respectto the nickel oxide reduction kinetics it was reported
that chemical reaction is the rate controlling step for thelower temperatures (up to 800uC) but no certain kineticmodel from Table 3 (equations (F1) (R1) and (R2)) canbe stated to be the best fitting due to the fact that all of
the aforementioned three equations fit very well
The rate controlling step though changes for highertemperatures (up to 900uC) but there has not been a
conclusion based on reported approaches about themechanism that prevails within the temperature range800ndash900uC
Reduction kinetics of RK dust and laboratory made
pellets of the same origin in the temperature range 700ndash 850uC were studied14 The experimental data obtainedconcerning iron oxide reduction were applied to thefollowing mathematical models
(i) CrankndashGinstlingndashBrounshtein (CGB) kineticmodel
1(2=3)a(1a)2=3~Kt (D4) (9)
rate controlling step diffusion through the
product layer
(ii)(1a)1=3
~Kt (R2) (10)
rate controlling step chemical reaction at the
interface between the unreacted core and the
product layer
(iii) 1(2=3)a(1a)2=3z1(1a)1=3
~Kt (D4zR2) (11)
Generalised equation that is a combination of
equations (9) and (10) based on the additivity of
reaction times
rate controlling step mixed controlled mechanism
Table 3 n values of kinetic equations for gasndashsolidreactions
Kinetic equation n
D1 a 25Kt 062D2 (12a )ln(12a )za 5Kt 057D3 [12(12a )13]25Kt 054D4 12(23)a 2(12a )235Kt 057D5 Kt 5Z z[1z(Z 21)a ]232(Z 21)(12a )23(Z 21) (ValensindashCarter equation)
F1 2ln(12a )5Kt 10R1 12(12a )125Kt 111R2 12(12a )135Kt 107A2 [2ln(12a )]125Kt 20A3 [2ln(12a )]135Kt 20
Z coefficient representing the product volume per volume ofthe reactants consumed Z is assumed to be 05
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 15
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 89
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
It was deduced from the kinetic analysis that the
mixed kinetic model equation (11) fits well the experi-mental data for all dust samples and pellets of a certainorigin (the washing tower) up to 750uC This kinetic
model also verified the experimental data for pellets of different origin (ie electrofilter and polycyclone dusts)up to 800uC At higher temperatures ie up to 850uC ithas been confirmed the predominance of diffusion
mechanism according to the CGB equation (9)The activation energy value of the rate-determining
step at various conditions has been evaluated concern-ing coal based laterite reduction by means of thewidespread rate expression (12) that follows
r~kC n (12)
where r is the reaction rate k the rate constant C thefluid reactant concentration (mol L21) and n the orderof the reaction Assuming that the fluid reactantconcentration is constant the Arrhenius law[lnk 5lnAo ndash E R(1T )] can be used for calculation of the
activation energy through slope determination of thefollowing linear equation
lnr~ln(AoC n)(E =R)(1=T ) (13)
where r is the mean initial reduction rate valuecalculated through the expression r5DaDt21 (a is thereduction rates obtained from the experimental data)
Ao is the frequency factor of the Arrhenius law C is thefluid reactant concentration (mol L21) n the order of the reaction E the activation energy T the temperatures
(K) examined for the kinetic analysis and R the universalgas constant Activation energy has also been calculatedby Zevgolis et al concerning RK dust and laboratory
made pellets of the same origin through application of the Arrhenius law to the slopes of the generalisedequation (13) lines mixed controlled mechanism at 700and 750uC ie by means of the relationship
ln k (T 2)
ln k (T 1)~
E
R
1
T 2
1
T 1
(14)
The activation energy values concerning iron oxidereduction912 (724 and 402 kJ mol21) indicate that theprevailing kinetic mechanism may be either the mixedcontrolled mechanism (diffusionndashchemical reaction) ordiffusion Activation energy value reported concerningnickel oxide reduction (874 kJ mol21) indicates thepredominance ndash at least for a certain temperature rangeof chemical reaction mechanism The mixed controlledmechanism for iron oxide reduction was proposed byZevgolis et al for low temperatures (700ndash750uC) and
diffusion through the product layer for higher tempera-tures (800ndash850uC)14
The conclusions drawn from the kinetic analysis of the Greek nickeliferous laterite solid state reductionare in agreement with the conclusions drawn by otherstudies concerning reducibility tests of different originlaterite samples either in pellet form23 or in form of cement bonded laterite briquettes with CO-CO2 gas
reducing mixture24
within the temperature range (700ndash 1000uC) Mixed control (chemical reaction and diffu-
sion of the reducing gas agent through the productlayer) is the mechanism that seems to prevail duringthe reductive procedure It is apparent from theaforementioned that diffusion constitutes a seriouskinetic factor affecting solid state reduction of laterites
either on its own or in combination with chemicalreaction in the interface product layer ndash gas reactantKinetic analysis of Greek nickeliferous lateritesenhances the conclusion drawn by all the reportedreducibility tests according to which iron silicatemineralogical phases formed in the temperature rangeexamined (700ndash900uC) like fayalite (2FeOSiO2)probably cover the iron oxide grains and thus impede
the progress of the reaction
ConclusionsThe main physicochemical parameters affecting reduci-bility of the Greek nickeliferous laterite ores have beenreviewed It is concluded that no matter if controllingthe values of physicochemical parameters such astemperature grain size of materials and type of solidreductant to be optimum iron and nickel oxide
reduction degrees obtained do not exceed 33 and 76respectively though the remaining carbon in the calcineis adequate for further progress of the reductive
reactions This means that practically no metallic ironis formed during roasting reduction of Greek lateritesand nickel oxide is partially transformed to metallicnickel Decrease of the ore and solid reductant grain size(mainly for granulometry z3 mm) and use of reactivesolid fuels (lignite) instead of less reactive (coal or coke)favour considerably iron and nickel oxide degree of
reduction Increase of temperature within the tempera-ture range 700ndash900uC examined results in increase of oxides reduction rate but its influence on iron and nickeloxide degree is evident up to 750ndash800uC and then itdiminishes Reduction process progresses initially with arelatively high rate for the first ten minutes then the rate
starts to decrease until it diminishes to zero after thefirst 20 min reaction The conclusion that reductivereactions practically stop after a time of 20ndash40 min isverified by published approaches concerning reducibility
studies of different origin laterite ores It can beattributed to the formation within the temperaturerange examined of iron silicate minerals such as fayalite(2FeOSiO2) or forsterite (Mg2SiO4) which probablycover oxide grains and impede further progress of thereduction Thus future reducibility studies of Greeknickeliferous laterites should be focused on mineralogi-cal analysis of the reduction products in relation withthe factors affecting the formation and reduction of complex nickelndashironndashmagnesium silicate phases so that
the highest possible reduction degree is obtainedKinetic studies of the reductive procedure showed thatmixed control and diffusion mechanisms have been
found to prevail during reduction of oxides
References1 wwwlibmurdocheduauadtpubfilesadt-MU20051004114504
02Wholepdf
2 A D Dalvi W Bacon and C Osborne lsquoThe past and the future of
nickel lateritesrsquo Proc PDAC 2004 Int Convent Toronto
Canada March 2004 PDAC 1ndash27
3 E N Zevgolis lsquoExtractive metallurgy of nickel part I
Pyrometallurgical methodsrsquo 2000 Athens National TechnicalUniverity of Athens
4 E Mposkos A Orfanoudaki and Th Perraki lsquoThe Ni distribution
in the mineral phases of Greek FendashNi laterite depositsrsquo Proc 3rd
Symp on lsquoMineral wealthrsquo Athens Greece November 2000
Technical Chamber of Greece 107ndash115
5 N Albadakis lsquoNi-minerals in the deposits of the sub-pelagonic
zonersquo Miner Wealth 1984 31 9ndash32
Zev gol is et al The reducibility of the Greek nickeliferous laterites
16 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 39
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
smelting step is a quiet process since no reduction gasesare evolved through the slag in the EF
Table 2 presents a summary of the conditions
employed and the main conclusions drawn by the
published work on reducibility of Greek nickeliferous
laterites Temperature has proved to be one of the most
important operational parameters in RK roastingreduction of the Greek laterites According to the
equilibrium diagram of the Fe-C-O system as presentedin Fig 1 magnetite reduction (Fe3O4zCO53FeOzCO2) and wustite reduction (FeOzCO5FeozCO2) can take place above 680 and 700uC
respectively10 Chemical analysis of representative sam-
ples taken along the length of industrial RKs indicatedthat up to 700ndash800uC laterite reduction remained in low
levels (up to 5ndash7) but it was drastically increased for
higher temperatures11 (however always 300)
Calcine temperature is not increased above y850uC
due to agglomeration of the calcine Moreover all
experiments carried out in the laboratory by different
researchers regarding reducibility of Greek laterites withsolid fuels in the temperature range of 700ndash900uC
showed that an increase of temperature favours reduc-
tion rate of both iron and nickel oxides Nevertheless
influence of temperature on the reduction degree of iron
oxides is positive between 700 and 750uC1213 or 700 and
800uC according to nickeliferous RK dust and pellets
reducibility tests14 and then it diminishes for higher
temperatures Within the same temperature range
examined nickel oxide reduction degree is increased
for temperatures 700ndash800uC and then it remains
practically constant1213
Reducibility of laterite is considerably affected by the
ore grain size as shown during ore treatment in RKsand EFs of industrial scale for ferronickel produc-
tion1516 The conclusion drawn from this work is that
mean size of the ore and reduction degree are inversely
proportional Indeed even by macroscopic observation
of calcined ore grains topochemical reactions are
taking place that is reductive reactions take place
upon the ore grainrsquos surface within the temperature
range 700ndash850uC So by decreasing ore particle sizefree surface is increased and therefore total rate of
reduction is increased It was proved that gradual
decrease of feed size from 240 to 212 mm had very
satisfactory results in terms of the calcinersquos reduction
degree achieved Additionally a finer ore correspondsto a higher mean temperature of the calcine therefore
it corresponds to a more energy saving during smeltingof the calcine Nevertheless studying the effect of ore
grain size not only in the pre-reduction but also in the
smelting step of the Greek laterites it has been
concluded that a high portion of fine particles fed into
the RKs favoured high levels of dust production
(which means a higher nickel loss in dust operational
problems of the RKrsquos dedusting installation and
additional cost for dust handling and treatment in
order to be pelletised and recycled) and was responsible
for lower height of calcine-self-lining for prevention of EF walls chemical attack from slag Laboratory
studies using Greek laterite ore and solid fuels of
various grain sizes have also confirmed that iron and
nickel oxide reduction rate increases with decreasing
ore and carbonaceous material grain size This
phenomenon is more evident for grain size bigger than
3 mm as the difference in reduction obtained for finer
particles is less significant12
The type of solid fuel used as reductant constitutesanother critical parameter affecting reducibility of
Greek laterites Its role is much more complex than
being just a heat source as for example in cementproduction where fuel calorific value is the ultimate
critical parameter17 In case of laterite reduction solid
fuels are utilised both for heating and reduction thustheir reactivity is of great importance Also they can be
classified according to their reactivity as follows lignite
coal coke and graphite11 Lignite is characterised by its
high volatile matter inherent moisture and low percen-
tage of fixed carbon (Cfix) It evolves its thermal energy
in lower temperatures compared to coal or coke and
graphite which contain a higher percentage of fixed
carbon and a lower percentage of volatile matter than
lignite Laboratory studies of the Greek laterites withdifferent types of solid reductants maintaining the other
parameters fixed (temperature ore and fuel grain size
C
fix
Fe ratio etc) confirmed that the use of lignite
18
resulted in higher iron and nickel oxide reduction
degrees in comparison with the use of coal and pet
coke9 Moreover increasing the amount of carbonac-
eous material relative to iron and nickel oxide content
increased the rate of reduction Also industrial scale
experiments have shown that reduction degree of Greek
laterites decreases as we go from lignite to coke in the
following sequence ligniteRcoalRcoke11
The role of solid fuels used for reduction of Greek
laterites in RKs has also been studied1116 The conclu-
sions drawn after extensive evaluation of operational
data is that optimum results with respect to calcine
reduction are obtained in the appropriate combinationof solid fuels so that the necessary amount of volatiles
and Cfix respectively is contained Moreover increase of
the lignite amount has proved to result in an increased
reduction rate due to burning of the volatile matter in
the first zone of the RKs (drying preheating zone) andtherefore maintaining of the required reductive atmo-
sphere so that a higher reduction is obtained
Nevertheless very high content of volatile matter in
the metallurgical mixture corresponds to high tempera-
ture of RK flue gases which cause increased thermal
losses and risk for fire in the dedusting installation For
this reason decrease of lignite participation in the
metallurgical mixture is required and increased amountof less reactive coal with higher content of the main
reductive agent Cfix resulting in a lower specific
consumption of fuel per ton of natural laterite
There are various approaches published in interna-
tional literature concerning the effect of solid fuel
volatile matter in iron ore reduction All of them
however seem to agree that volatiles increase the rate of
reduction There is also an agreement among different
researchers that participation of volatiles in reductiondepends mainly on temperature Thus at temperatures
below 900ndash1000uC78 reduction by volatile matter is
regarded as quite significant whereas at higher tem-peratures the role of volatiles is negligible so that at
higher temperatures reduction mainly takes place
through solid carbon This is due to the fact that most
of volatiles are evolved at temperatures 1000uC11 Liu
et al and Strezov et al1920 also investigated the
fundamental mechanisms for iron ore fines reduction
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 11
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 49
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
T a b l e
2
L a b o r a t
o r y
a n d
i n d u s t r i a l r e s u l t s
o n
r e d u c i b
i l i t y
o f G r e e k
n i c k e l i f e r o u s
l a t e r i t e s
R e f e r e n c e
Z e v g o l i s 1 1
H a l i k i a - N e o u
e t
a l 1 2
H a l i k i a
a n d
S k
a r t a d o s
1 8
N e o u
e t
a l 9
N e o u
e t a l 1 3
Z e v g o l i s
e t
a l 1 4
O r i g i n o f
l a t e r i t e o r e
E v i a ndash L o k r i d a ndash P e l l e t s
( f r o m
r o t a r y k i l n d u s t )
7 0 ndash 3 0 ndash 5 w t -
E v i a
ndash L o k r i d a ndash K a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
E v i a ndash L o k r i d a ndash K
a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
E v i a ndash L o k r i d a ndash K a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
K a s t o r i a
G r e e k N i -
f e r r o u s l a t e r i t e
ndash r o t a r y k i l n d u s t
O r e g r a i n s i z e
O r e 2 1 5 m m
p e l l e t s 2 1 2 m m
2 2 0
8 z 3 8 m m
2 1 0
0 z 3 8 m m
2 5 3
z 3 8 m m
2 6 z 0 0 3 8 m m
2 2 0 8 z 3 7 m m
O r e 2 0 2 5 0 z 0 0 3 8 m m
p e l l e t s 2 1 0 z 5 6 m m
2 1 4 z 1 0 m m
D u s t 2 1 0 5 m m p e l l e t s 9 m m
R e d u c t a n t
C o a l l i g n i t e
C f i x
( o f d u s t )
L i g n
i t e
L i g n i t e c o a l
P e t c o k e
C o a l
C f i x
( o f d u
s t )
C f i x F e t o t
1 5 2 4
1 4
3 1
1 6 0 2 ndash 1 5 0 1
1 4 6 3
1 3 5 1
1 2 4 ndash 1 2 6
R e d u c t a n t
g r a i n s i z e
2 3 0 m m 2 5 m m
2 1 0 0 m m
2 2 0
8 z 1 4 7 m m
2 1 4
7 z 1 0 4 m m
2 1 0
4 z 7 4 m m
2 6 z 0 0 3 8 m m
2 2 0 8 z 3 7 m m
2 0 1 5 0 z 0 1 0 6 m m
hellip
S p e c i m e n f o r m
B u l k o r e a n d s o l i d f u e l s
G r o
u n d l a t e r i t e
a n d
l i g n i t e
B u l k o r e a n d
s o l i d f u e l s
G r o u n d l a t e r i t e
a n d p e t c o k e
P u l v e r i s e d o r e ndash
p e l l e t s b i n d i n g a g e n t
b e n t o n i t e 1
R o t a r y k i l n d u s t p e l l e t s
( o f s a m e
d u s t )
T e m p e r a t u r e
u C
9 2 0
7 0 0
ndash 8 5 0
8 6 0 ndash 9 0 0
7 5 0 ndash 9 0 0
7 0 0 ndash 9 0 0
7 0 0 ndash 8 5 0
M a x i m u m
r e d u c t i o
n
d e g r e e ndash
c o n d i t i o n s
2 5
3 2 7
3
2 9 5 5
2 3 1 6
3 1 7 4
2 8
9 2 0 u C
8 5 0
u C
9 0 0 u C
9 0 0 u C
9 0 0 u C
8 5 0 u C
2 1 5 m m
o r e
2 1 2 m m
p e l l e t s
2 3 0 m m
c o a l
2 5 m m
l i g n i t e
2 5 3
z 3 8 m m
o r e
2 1 0
4 z 7 4 m m
l i g n i t e
L i g n i t e
P u l v e r i s e d o r e
R o t a r y k i l n d u s t
A c t i v a t i o n
e n e r g y E k J m o l 2
1
hellip
4 0 1
7 ( i r o n o x i d e )
hellip
7 2 4 ( i r o n o x i d e )
hellip
4 2 E 8 4
( 7 0 0 ndash 7 5 0 u C )
8 7 4
5 ( n i c k e l o x i d e )
E 4 2 ( 8 0
0 ndash 8 5 0 u C )
R a t e c o n t r o l l i n g s t e p
hellip
D i f f u s i o n ( i r o n o x i d e )
M i x e d k i n e t i c m o d e l
c h e
m i c a l r e a c t i o n
ndash d i f f u s i o n ( n i c k e l o x i d e )
hellip
D i f f u s i o n ( i r o n o x i d e )
M i x e d k i n e t i c m o d e l
c h e m i c a l
r e a c t i o n ndash d i f f u s i o n
( n i c k e l o x i d e )
hellip
M i x e d k i n
e t i c m o d e l c h e m i c a l
r e a c t i o n ndash
d i f f u s i o n ( i r o n o x i d e )
Zev gol is et al The reducibility of the Greek nickeliferous laterites
12 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 59
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
in coalndashore mixtures and the role of the fuel volatile
matter They reported that volatile matter constituents
were methane (CH4) within the temperature range 450ndash
700uC and CO kai H2 for temperatures 800uC Gas
agents CO kai H2 are mainly responsible for hematite
(Fe2O3) reduction up to 800uC while additional CO
generated due to the Boudouard reaction at elevated
temperatures resulted in acceleration of the reduction
procedure Concerning laboratory work with Greek
laterite the fact that remaining carbon in the calcine is
in most cases much higher than the theoretically
expected indicates according to researchers that evolvedvolatiles participate in the oxide reduction912 but this
requires further investigation
Reducibility tests have been conducted by different
researchers on Greek laterites in the form of dust and
pellets as well Dust produced during industrial treat-
ment of laterite in the RKs has a higher Ni content than
the ore and a carbon content 8 which is higher than
the stoichiometrically required (y65) for FendashNi 13
production Thus it is evident that agglomeration and
recycling of dust which is y7 of the laterite feed
contribute to the economics of the metallurgical process
So the reductive behaviour of the industrially produced
cement bonded laterite pellets has been studied forcomparison with the behaviour of same origin laterite
dust14 Reducibility tests of pulverised Kastoria laterite
ore have been also conducted using coal as a reductant
in order to compare with the reductive behaviour of
agglomerated mixture (pellets) of Kastoria pulverised
ore and coal (the stoichiometrically required for FendashNi
13 production) with bentonite as the binding agent13
The temperature range of the experimental procedure
was 700ndash900uC It was deduced from both series of tests
that iron oxide reduction degree obtained for pellets was
lower than for dust (with same laterite) under the same
conditions though no serious difference is observed
concerning nickel oxide reduction This difference in thereductive behaviour between laterite dust and pellets can
be attributed to the fact that the binding agent covers
part of the oxide grains decreasing their reactive surface
area
The conclusion drawn from all reducibility studies of
Greek nickeliferous laterites either conducted in an
industrial or in a laboratory scale within the tempera-
ture range of 700ndash900uC is that independently of the
conditions employed ie ore grain size temperature and
type and amount of the solid reductant in no case
calcine reduction degree exceeds 33 though the
remaining carbon in the calcine is almost always higher
than the theoretically required for further progress of reductive reactions Given that the conversion of Fe2O3
1 Equilibrium curves of CO-CO2 versus temperature for Fe-C-O system10
2 Reduction degree versus time typical diagrams at various temperatures of a laterite ore mixture (Evia IslandndashLokridandash
Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lignite (20149z00940 mm)12 b electrostatic filter dust14 and c elec-
trostatic filter pellets14
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 13
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 69
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
to Fe3O4 and to FeO corresponds to a reduction of 111and 333 respectively it comes that magnetite and
wustite should coexist in the calcine and no furtherconversion to metallic iron exists Additionally it is
concluded that reductive reactions occur within the first 20
to 30 min of the process and then they practically stop for
all cases examined This can be attributed to kineticphenomena such as the formation within the temperature
range examined of iron-silicate minerals such as fayalite
(2FeOSiO2) which probably covers oxide grains thusimpeding progress of the reduction In Fig 2 typical
reduction degree diagrams versus time within a tempera-
ture range 700ndash850uC are presented for reduction of
pulverised laterite ore mixture (Evia islandndashLokridandash Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lig-
nite (20149z00940 mm) and also the reduction of
electrostatic filter dust from RKs as well as the reduction
of laboratory made pellets from this dust From these
diagrams it comes that reduction degree does not exceed
33 The same is shown in Fig 3 with reduction in
industrial RKs (In Fig 3 reduction is defined as the ratio
Fe2z
Fetot
|100 so the real reduction degree ie
Oi Of eth THORN=Oifrac12 |100 where Oi is the initial oxygen and
Of is the final oxygen in iron oxides is 333)
Industrial operation with garnierite type of ores shows
that reduction in the RK goes up to 37 which meansthat we have metallic iron in the calcine21
World experienceReducibility tests concerning different types of nickelifer-
ous laterites according to international literature2223
indicate that chemical mineralogical composition of the
laterite ore is a critical parameter for the final degree of reduction obtained Reducibility of several types of
nickeliferous laterites from the Dominican Republic22
(grain size 20074 mm) has been studied within the
temperature range 400ndash1000uC using hydrogen as a
reductant The aim of the study was to compare the
reductive behaviour of garnieritic limonitic and inter-
mediate type of ores The experimental results clearly
indicated that reducibility of each oxide (iron nickel and
cobalt oxide) depends on the ore type Moreover
chemical and mineralogical analysis of the reduced
samples indicated that nickel and iron (oxide) degree of
metallisation was higher in limonitic than in garnieritic
and intermediate type of ores Specifically both iron andnickel oxide degree of metallisation obtained after 40 minprocessing of the garnieritic type of ore at 1000uC was
y10 Temperature variation between 400 and 1000uC
did not affect significantly the reduction process The
respective degrees of metallisation were 80 and 70 at
1000uC concerning limonitic type of ore and they were
dramatically increased 800uC Cobalt degree of metal-
lisation at 1000uC was slightly lower (y65) in limonitic
compared to garnieritic type of ore (70) and the effect of
temperature variation was negligible concerning the first
and significant 850uC concerning the second one This
is different from what happens in reduction by solid fuelsThat is in solid fuel reduction limonitic type of ores
cannot be reduced to metallic iron so reduction stops
when reduction degree of iron oxides is 333 Low
nickel oxide reducibility in the garnieritic type ores was
attributed to the formation of olivine a nickelndashironndash
magnesium orthosilicate and the tendency of nickel to
3 Temperature and reduction degree in RK
Zev gol is et al The reducibility of the Greek nickeliferous laterites
14 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 79
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
exchange places with magnesium in silicates which are
stable in high temperatures On the contrary the low
magnesia and silica content of the limonitic type of ores is
not adequate to result in hosting nickel in the olivine
phase thus the per cent of nickel oxide reduction degree
increases for high temperatures Cobalt oxide reducibility
was higher for the garnieritic laterite type and the effect of
temperature was more evident for lower temperatures
due to the cobalt tendency to replace iron in the limonitelattice Reductive reactions concerning all types of laterite
ores examined after 40 min practically stopped In the
Greek laterite reducibility tests as it has already been
mentioned above reductive reactions practically stopped
after 20 to 30 min
Reducibility tests of a garnieritic type of laterite ore in
the form of pellets in the range 700ndash1000uC using a CO
CO2 mixture as a reducing agent has also been
conducted23 Olivine (MgFe)2SiO4 formation was
proved to be critical for the reduction progress since it
is stated that reducibility (determined by percentage of
weight loss after first calcined in the same temperaturelaterite ore pellets) reaches the highest possible values for
temperatures above serpentine (Mg Fe Ni)6Si4O12(OH)6decomposition (y600uC) and below transformation
temperature of amorphous olivine to a stable miner-
alogical phase (y810uC) It was also deduced that a
strongly competitive relation exists among reduction
progress and olivine formation ie a slow reduction rate
(by employment of mild reducing atmosphere through
gas reducing agent) relative to olivine formation results to
a lower reduction degree On the contrary rapid
reduction rate (by employment of intensive reducing
atmosphere during the first minutes of the reductive
procedure) relative to the crystallisation of the olivinephase results in higher reduction degree values
Reduction kinetics of Greek nickeliferous lateritesKinetic analysis carried out for the Greek nickeliferous
laterite roasting reduction is based on the unreacted
shrinking core model Roasting reduction kinetics of
Greek laterite fine particles (EviandashLokridandashKastoria ore
mixture 60ndash25ndash15 wt- granulometry 20250z
0037 mm temperature range 700ndash900uC) with lignite
and pet coke as reductants respectively were con-ducted912 The methodology of work used for
approaching the rate controlling step of the process is
the application of the diagnostic equation
lnln 1aeth THORNfrac12 ~n ln tzln b (8)
where a is reduction degree () of iron or nickel oxides t
is time (s) b is constant and n is constant depending on
the rate controlling mechanism and the geometrical
characteristics of the ore and solid reductant particles
The obtained n values from application of the experi-
mental data a ndash t which represent the slopes of the linear
graphic representation of equation (8) are comparedwith the theoretical values of the widespread used kinetic
equations of Table 3 It is noted that equations (D1)ndash
(D5) correspond to the diffusion rate controlling step
equations (F1) (R1) and (R2) correspond to chemical
reaction mechanism and equations (A2) and (A3)
correspond to the nucleation rate controlling step
Linearity assessment of diagrams ln[2ln(12a)] versus
lnt and determination of the slope n were obtainedthrough the application of the least squares method It isnoted that no time greater than 15 min was used for the
kinetic analysis given that after the first 20 min thereactions tended to stop in all cases examined Theconclusion deduced from this work was that diffusion
kinetic equations (D1)ndash(D5) best fitted the experimental
data concerning iron oxide reduction Moreover it hasbeen reported that the ValensindashCarter equation was themathematical model that best represented the proposeddiffusion mechanism with Z value equal to 05 Another
value of coefficient Z would probably result in an evenmore representative mathematical model With respectto the nickel oxide reduction kinetics it was reported
that chemical reaction is the rate controlling step for thelower temperatures (up to 800uC) but no certain kineticmodel from Table 3 (equations (F1) (R1) and (R2)) canbe stated to be the best fitting due to the fact that all of
the aforementioned three equations fit very well
The rate controlling step though changes for highertemperatures (up to 900uC) but there has not been a
conclusion based on reported approaches about themechanism that prevails within the temperature range800ndash900uC
Reduction kinetics of RK dust and laboratory made
pellets of the same origin in the temperature range 700ndash 850uC were studied14 The experimental data obtainedconcerning iron oxide reduction were applied to thefollowing mathematical models
(i) CrankndashGinstlingndashBrounshtein (CGB) kineticmodel
1(2=3)a(1a)2=3~Kt (D4) (9)
rate controlling step diffusion through the
product layer
(ii)(1a)1=3
~Kt (R2) (10)
rate controlling step chemical reaction at the
interface between the unreacted core and the
product layer
(iii) 1(2=3)a(1a)2=3z1(1a)1=3
~Kt (D4zR2) (11)
Generalised equation that is a combination of
equations (9) and (10) based on the additivity of
reaction times
rate controlling step mixed controlled mechanism
Table 3 n values of kinetic equations for gasndashsolidreactions
Kinetic equation n
D1 a 25Kt 062D2 (12a )ln(12a )za 5Kt 057D3 [12(12a )13]25Kt 054D4 12(23)a 2(12a )235Kt 057D5 Kt 5Z z[1z(Z 21)a ]232(Z 21)(12a )23(Z 21) (ValensindashCarter equation)
F1 2ln(12a )5Kt 10R1 12(12a )125Kt 111R2 12(12a )135Kt 107A2 [2ln(12a )]125Kt 20A3 [2ln(12a )]135Kt 20
Z coefficient representing the product volume per volume ofthe reactants consumed Z is assumed to be 05
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 15
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 89
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
It was deduced from the kinetic analysis that the
mixed kinetic model equation (11) fits well the experi-mental data for all dust samples and pellets of a certainorigin (the washing tower) up to 750uC This kinetic
model also verified the experimental data for pellets of different origin (ie electrofilter and polycyclone dusts)up to 800uC At higher temperatures ie up to 850uC ithas been confirmed the predominance of diffusion
mechanism according to the CGB equation (9)The activation energy value of the rate-determining
step at various conditions has been evaluated concern-ing coal based laterite reduction by means of thewidespread rate expression (12) that follows
r~kC n (12)
where r is the reaction rate k the rate constant C thefluid reactant concentration (mol L21) and n the orderof the reaction Assuming that the fluid reactantconcentration is constant the Arrhenius law[lnk 5lnAo ndash E R(1T )] can be used for calculation of the
activation energy through slope determination of thefollowing linear equation
lnr~ln(AoC n)(E =R)(1=T ) (13)
where r is the mean initial reduction rate valuecalculated through the expression r5DaDt21 (a is thereduction rates obtained from the experimental data)
Ao is the frequency factor of the Arrhenius law C is thefluid reactant concentration (mol L21) n the order of the reaction E the activation energy T the temperatures
(K) examined for the kinetic analysis and R the universalgas constant Activation energy has also been calculatedby Zevgolis et al concerning RK dust and laboratory
made pellets of the same origin through application of the Arrhenius law to the slopes of the generalisedequation (13) lines mixed controlled mechanism at 700and 750uC ie by means of the relationship
ln k (T 2)
ln k (T 1)~
E
R
1
T 2
1
T 1
(14)
The activation energy values concerning iron oxidereduction912 (724 and 402 kJ mol21) indicate that theprevailing kinetic mechanism may be either the mixedcontrolled mechanism (diffusionndashchemical reaction) ordiffusion Activation energy value reported concerningnickel oxide reduction (874 kJ mol21) indicates thepredominance ndash at least for a certain temperature rangeof chemical reaction mechanism The mixed controlledmechanism for iron oxide reduction was proposed byZevgolis et al for low temperatures (700ndash750uC) and
diffusion through the product layer for higher tempera-tures (800ndash850uC)14
The conclusions drawn from the kinetic analysis of the Greek nickeliferous laterite solid state reductionare in agreement with the conclusions drawn by otherstudies concerning reducibility tests of different originlaterite samples either in pellet form23 or in form of cement bonded laterite briquettes with CO-CO2 gas
reducing mixture24
within the temperature range (700ndash 1000uC) Mixed control (chemical reaction and diffu-
sion of the reducing gas agent through the productlayer) is the mechanism that seems to prevail duringthe reductive procedure It is apparent from theaforementioned that diffusion constitutes a seriouskinetic factor affecting solid state reduction of laterites
either on its own or in combination with chemicalreaction in the interface product layer ndash gas reactantKinetic analysis of Greek nickeliferous lateritesenhances the conclusion drawn by all the reportedreducibility tests according to which iron silicatemineralogical phases formed in the temperature rangeexamined (700ndash900uC) like fayalite (2FeOSiO2)probably cover the iron oxide grains and thus impede
the progress of the reaction
ConclusionsThe main physicochemical parameters affecting reduci-bility of the Greek nickeliferous laterite ores have beenreviewed It is concluded that no matter if controllingthe values of physicochemical parameters such astemperature grain size of materials and type of solidreductant to be optimum iron and nickel oxide
reduction degrees obtained do not exceed 33 and 76respectively though the remaining carbon in the calcineis adequate for further progress of the reductive
reactions This means that practically no metallic ironis formed during roasting reduction of Greek lateritesand nickel oxide is partially transformed to metallicnickel Decrease of the ore and solid reductant grain size(mainly for granulometry z3 mm) and use of reactivesolid fuels (lignite) instead of less reactive (coal or coke)favour considerably iron and nickel oxide degree of
reduction Increase of temperature within the tempera-ture range 700ndash900uC examined results in increase of oxides reduction rate but its influence on iron and nickeloxide degree is evident up to 750ndash800uC and then itdiminishes Reduction process progresses initially with arelatively high rate for the first ten minutes then the rate
starts to decrease until it diminishes to zero after thefirst 20 min reaction The conclusion that reductivereactions practically stop after a time of 20ndash40 min isverified by published approaches concerning reducibility
studies of different origin laterite ores It can beattributed to the formation within the temperaturerange examined of iron silicate minerals such as fayalite(2FeOSiO2) or forsterite (Mg2SiO4) which probablycover oxide grains and impede further progress of thereduction Thus future reducibility studies of Greeknickeliferous laterites should be focused on mineralogi-cal analysis of the reduction products in relation withthe factors affecting the formation and reduction of complex nickelndashironndashmagnesium silicate phases so that
the highest possible reduction degree is obtainedKinetic studies of the reductive procedure showed thatmixed control and diffusion mechanisms have been
found to prevail during reduction of oxides
References1 wwwlibmurdocheduauadtpubfilesadt-MU20051004114504
02Wholepdf
2 A D Dalvi W Bacon and C Osborne lsquoThe past and the future of
nickel lateritesrsquo Proc PDAC 2004 Int Convent Toronto
Canada March 2004 PDAC 1ndash27
3 E N Zevgolis lsquoExtractive metallurgy of nickel part I
Pyrometallurgical methodsrsquo 2000 Athens National TechnicalUniverity of Athens
4 E Mposkos A Orfanoudaki and Th Perraki lsquoThe Ni distribution
in the mineral phases of Greek FendashNi laterite depositsrsquo Proc 3rd
Symp on lsquoMineral wealthrsquo Athens Greece November 2000
Technical Chamber of Greece 107ndash115
5 N Albadakis lsquoNi-minerals in the deposits of the sub-pelagonic
zonersquo Miner Wealth 1984 31 9ndash32
Zev gol is et al The reducibility of the Greek nickeliferous laterites
16 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 49
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
T a b l e
2
L a b o r a t
o r y
a n d
i n d u s t r i a l r e s u l t s
o n
r e d u c i b
i l i t y
o f G r e e k
n i c k e l i f e r o u s
l a t e r i t e s
R e f e r e n c e
Z e v g o l i s 1 1
H a l i k i a - N e o u
e t
a l 1 2
H a l i k i a
a n d
S k
a r t a d o s
1 8
N e o u
e t
a l 9
N e o u
e t a l 1 3
Z e v g o l i s
e t
a l 1 4
O r i g i n o f
l a t e r i t e o r e
E v i a ndash L o k r i d a ndash P e l l e t s
( f r o m
r o t a r y k i l n d u s t )
7 0 ndash 3 0 ndash 5 w t -
E v i a
ndash L o k r i d a ndash K a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
E v i a ndash L o k r i d a ndash K
a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
E v i a ndash L o k r i d a ndash K a s t o r i a
6 0 ndash 2 5 ndash 1 5 w t -
K a s t o r i a
G r e e k N i -
f e r r o u s l a t e r i t e
ndash r o t a r y k i l n d u s t
O r e g r a i n s i z e
O r e 2 1 5 m m
p e l l e t s 2 1 2 m m
2 2 0
8 z 3 8 m m
2 1 0
0 z 3 8 m m
2 5 3
z 3 8 m m
2 6 z 0 0 3 8 m m
2 2 0 8 z 3 7 m m
O r e 2 0 2 5 0 z 0 0 3 8 m m
p e l l e t s 2 1 0 z 5 6 m m
2 1 4 z 1 0 m m
D u s t 2 1 0 5 m m p e l l e t s 9 m m
R e d u c t a n t
C o a l l i g n i t e
C f i x
( o f d u s t )
L i g n
i t e
L i g n i t e c o a l
P e t c o k e
C o a l
C f i x
( o f d u
s t )
C f i x F e t o t
1 5 2 4
1 4
3 1
1 6 0 2 ndash 1 5 0 1
1 4 6 3
1 3 5 1
1 2 4 ndash 1 2 6
R e d u c t a n t
g r a i n s i z e
2 3 0 m m 2 5 m m
2 1 0 0 m m
2 2 0
8 z 1 4 7 m m
2 1 4
7 z 1 0 4 m m
2 1 0
4 z 7 4 m m
2 6 z 0 0 3 8 m m
2 2 0 8 z 3 7 m m
2 0 1 5 0 z 0 1 0 6 m m
hellip
S p e c i m e n f o r m
B u l k o r e a n d s o l i d f u e l s
G r o
u n d l a t e r i t e
a n d
l i g n i t e
B u l k o r e a n d
s o l i d f u e l s
G r o u n d l a t e r i t e
a n d p e t c o k e
P u l v e r i s e d o r e ndash
p e l l e t s b i n d i n g a g e n t
b e n t o n i t e 1
R o t a r y k i l n d u s t p e l l e t s
( o f s a m e
d u s t )
T e m p e r a t u r e
u C
9 2 0
7 0 0
ndash 8 5 0
8 6 0 ndash 9 0 0
7 5 0 ndash 9 0 0
7 0 0 ndash 9 0 0
7 0 0 ndash 8 5 0
M a x i m u m
r e d u c t i o
n
d e g r e e ndash
c o n d i t i o n s
2 5
3 2 7
3
2 9 5 5
2 3 1 6
3 1 7 4
2 8
9 2 0 u C
8 5 0
u C
9 0 0 u C
9 0 0 u C
9 0 0 u C
8 5 0 u C
2 1 5 m m
o r e
2 1 2 m m
p e l l e t s
2 3 0 m m
c o a l
2 5 m m
l i g n i t e
2 5 3
z 3 8 m m
o r e
2 1 0
4 z 7 4 m m
l i g n i t e
L i g n i t e
P u l v e r i s e d o r e
R o t a r y k i l n d u s t
A c t i v a t i o n
e n e r g y E k J m o l 2
1
hellip
4 0 1
7 ( i r o n o x i d e )
hellip
7 2 4 ( i r o n o x i d e )
hellip
4 2 E 8 4
( 7 0 0 ndash 7 5 0 u C )
8 7 4
5 ( n i c k e l o x i d e )
E 4 2 ( 8 0
0 ndash 8 5 0 u C )
R a t e c o n t r o l l i n g s t e p
hellip
D i f f u s i o n ( i r o n o x i d e )
M i x e d k i n e t i c m o d e l
c h e
m i c a l r e a c t i o n
ndash d i f f u s i o n ( n i c k e l o x i d e )
hellip
D i f f u s i o n ( i r o n o x i d e )
M i x e d k i n e t i c m o d e l
c h e m i c a l
r e a c t i o n ndash d i f f u s i o n
( n i c k e l o x i d e )
hellip
M i x e d k i n
e t i c m o d e l c h e m i c a l
r e a c t i o n ndash
d i f f u s i o n ( i r o n o x i d e )
Zev gol is et al The reducibility of the Greek nickeliferous laterites
12 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 59
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
in coalndashore mixtures and the role of the fuel volatile
matter They reported that volatile matter constituents
were methane (CH4) within the temperature range 450ndash
700uC and CO kai H2 for temperatures 800uC Gas
agents CO kai H2 are mainly responsible for hematite
(Fe2O3) reduction up to 800uC while additional CO
generated due to the Boudouard reaction at elevated
temperatures resulted in acceleration of the reduction
procedure Concerning laboratory work with Greek
laterite the fact that remaining carbon in the calcine is
in most cases much higher than the theoretically
expected indicates according to researchers that evolvedvolatiles participate in the oxide reduction912 but this
requires further investigation
Reducibility tests have been conducted by different
researchers on Greek laterites in the form of dust and
pellets as well Dust produced during industrial treat-
ment of laterite in the RKs has a higher Ni content than
the ore and a carbon content 8 which is higher than
the stoichiometrically required (y65) for FendashNi 13
production Thus it is evident that agglomeration and
recycling of dust which is y7 of the laterite feed
contribute to the economics of the metallurgical process
So the reductive behaviour of the industrially produced
cement bonded laterite pellets has been studied forcomparison with the behaviour of same origin laterite
dust14 Reducibility tests of pulverised Kastoria laterite
ore have been also conducted using coal as a reductant
in order to compare with the reductive behaviour of
agglomerated mixture (pellets) of Kastoria pulverised
ore and coal (the stoichiometrically required for FendashNi
13 production) with bentonite as the binding agent13
The temperature range of the experimental procedure
was 700ndash900uC It was deduced from both series of tests
that iron oxide reduction degree obtained for pellets was
lower than for dust (with same laterite) under the same
conditions though no serious difference is observed
concerning nickel oxide reduction This difference in thereductive behaviour between laterite dust and pellets can
be attributed to the fact that the binding agent covers
part of the oxide grains decreasing their reactive surface
area
The conclusion drawn from all reducibility studies of
Greek nickeliferous laterites either conducted in an
industrial or in a laboratory scale within the tempera-
ture range of 700ndash900uC is that independently of the
conditions employed ie ore grain size temperature and
type and amount of the solid reductant in no case
calcine reduction degree exceeds 33 though the
remaining carbon in the calcine is almost always higher
than the theoretically required for further progress of reductive reactions Given that the conversion of Fe2O3
1 Equilibrium curves of CO-CO2 versus temperature for Fe-C-O system10
2 Reduction degree versus time typical diagrams at various temperatures of a laterite ore mixture (Evia IslandndashLokridandash
Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lignite (20149z00940 mm)12 b electrostatic filter dust14 and c elec-
trostatic filter pellets14
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 13
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 69
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
to Fe3O4 and to FeO corresponds to a reduction of 111and 333 respectively it comes that magnetite and
wustite should coexist in the calcine and no furtherconversion to metallic iron exists Additionally it is
concluded that reductive reactions occur within the first 20
to 30 min of the process and then they practically stop for
all cases examined This can be attributed to kineticphenomena such as the formation within the temperature
range examined of iron-silicate minerals such as fayalite
(2FeOSiO2) which probably covers oxide grains thusimpeding progress of the reduction In Fig 2 typical
reduction degree diagrams versus time within a tempera-
ture range 700ndash850uC are presented for reduction of
pulverised laterite ore mixture (Evia islandndashLokridandash Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lig-
nite (20149z00940 mm) and also the reduction of
electrostatic filter dust from RKs as well as the reduction
of laboratory made pellets from this dust From these
diagrams it comes that reduction degree does not exceed
33 The same is shown in Fig 3 with reduction in
industrial RKs (In Fig 3 reduction is defined as the ratio
Fe2z
Fetot
|100 so the real reduction degree ie
Oi Of eth THORN=Oifrac12 |100 where Oi is the initial oxygen and
Of is the final oxygen in iron oxides is 333)
Industrial operation with garnierite type of ores shows
that reduction in the RK goes up to 37 which meansthat we have metallic iron in the calcine21
World experienceReducibility tests concerning different types of nickelifer-
ous laterites according to international literature2223
indicate that chemical mineralogical composition of the
laterite ore is a critical parameter for the final degree of reduction obtained Reducibility of several types of
nickeliferous laterites from the Dominican Republic22
(grain size 20074 mm) has been studied within the
temperature range 400ndash1000uC using hydrogen as a
reductant The aim of the study was to compare the
reductive behaviour of garnieritic limonitic and inter-
mediate type of ores The experimental results clearly
indicated that reducibility of each oxide (iron nickel and
cobalt oxide) depends on the ore type Moreover
chemical and mineralogical analysis of the reduced
samples indicated that nickel and iron (oxide) degree of
metallisation was higher in limonitic than in garnieritic
and intermediate type of ores Specifically both iron andnickel oxide degree of metallisation obtained after 40 minprocessing of the garnieritic type of ore at 1000uC was
y10 Temperature variation between 400 and 1000uC
did not affect significantly the reduction process The
respective degrees of metallisation were 80 and 70 at
1000uC concerning limonitic type of ore and they were
dramatically increased 800uC Cobalt degree of metal-
lisation at 1000uC was slightly lower (y65) in limonitic
compared to garnieritic type of ore (70) and the effect of
temperature variation was negligible concerning the first
and significant 850uC concerning the second one This
is different from what happens in reduction by solid fuelsThat is in solid fuel reduction limonitic type of ores
cannot be reduced to metallic iron so reduction stops
when reduction degree of iron oxides is 333 Low
nickel oxide reducibility in the garnieritic type ores was
attributed to the formation of olivine a nickelndashironndash
magnesium orthosilicate and the tendency of nickel to
3 Temperature and reduction degree in RK
Zev gol is et al The reducibility of the Greek nickeliferous laterites
14 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 79
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
exchange places with magnesium in silicates which are
stable in high temperatures On the contrary the low
magnesia and silica content of the limonitic type of ores is
not adequate to result in hosting nickel in the olivine
phase thus the per cent of nickel oxide reduction degree
increases for high temperatures Cobalt oxide reducibility
was higher for the garnieritic laterite type and the effect of
temperature was more evident for lower temperatures
due to the cobalt tendency to replace iron in the limonitelattice Reductive reactions concerning all types of laterite
ores examined after 40 min practically stopped In the
Greek laterite reducibility tests as it has already been
mentioned above reductive reactions practically stopped
after 20 to 30 min
Reducibility tests of a garnieritic type of laterite ore in
the form of pellets in the range 700ndash1000uC using a CO
CO2 mixture as a reducing agent has also been
conducted23 Olivine (MgFe)2SiO4 formation was
proved to be critical for the reduction progress since it
is stated that reducibility (determined by percentage of
weight loss after first calcined in the same temperaturelaterite ore pellets) reaches the highest possible values for
temperatures above serpentine (Mg Fe Ni)6Si4O12(OH)6decomposition (y600uC) and below transformation
temperature of amorphous olivine to a stable miner-
alogical phase (y810uC) It was also deduced that a
strongly competitive relation exists among reduction
progress and olivine formation ie a slow reduction rate
(by employment of mild reducing atmosphere through
gas reducing agent) relative to olivine formation results to
a lower reduction degree On the contrary rapid
reduction rate (by employment of intensive reducing
atmosphere during the first minutes of the reductive
procedure) relative to the crystallisation of the olivinephase results in higher reduction degree values
Reduction kinetics of Greek nickeliferous lateritesKinetic analysis carried out for the Greek nickeliferous
laterite roasting reduction is based on the unreacted
shrinking core model Roasting reduction kinetics of
Greek laterite fine particles (EviandashLokridandashKastoria ore
mixture 60ndash25ndash15 wt- granulometry 20250z
0037 mm temperature range 700ndash900uC) with lignite
and pet coke as reductants respectively were con-ducted912 The methodology of work used for
approaching the rate controlling step of the process is
the application of the diagnostic equation
lnln 1aeth THORNfrac12 ~n ln tzln b (8)
where a is reduction degree () of iron or nickel oxides t
is time (s) b is constant and n is constant depending on
the rate controlling mechanism and the geometrical
characteristics of the ore and solid reductant particles
The obtained n values from application of the experi-
mental data a ndash t which represent the slopes of the linear
graphic representation of equation (8) are comparedwith the theoretical values of the widespread used kinetic
equations of Table 3 It is noted that equations (D1)ndash
(D5) correspond to the diffusion rate controlling step
equations (F1) (R1) and (R2) correspond to chemical
reaction mechanism and equations (A2) and (A3)
correspond to the nucleation rate controlling step
Linearity assessment of diagrams ln[2ln(12a)] versus
lnt and determination of the slope n were obtainedthrough the application of the least squares method It isnoted that no time greater than 15 min was used for the
kinetic analysis given that after the first 20 min thereactions tended to stop in all cases examined Theconclusion deduced from this work was that diffusion
kinetic equations (D1)ndash(D5) best fitted the experimental
data concerning iron oxide reduction Moreover it hasbeen reported that the ValensindashCarter equation was themathematical model that best represented the proposeddiffusion mechanism with Z value equal to 05 Another
value of coefficient Z would probably result in an evenmore representative mathematical model With respectto the nickel oxide reduction kinetics it was reported
that chemical reaction is the rate controlling step for thelower temperatures (up to 800uC) but no certain kineticmodel from Table 3 (equations (F1) (R1) and (R2)) canbe stated to be the best fitting due to the fact that all of
the aforementioned three equations fit very well
The rate controlling step though changes for highertemperatures (up to 900uC) but there has not been a
conclusion based on reported approaches about themechanism that prevails within the temperature range800ndash900uC
Reduction kinetics of RK dust and laboratory made
pellets of the same origin in the temperature range 700ndash 850uC were studied14 The experimental data obtainedconcerning iron oxide reduction were applied to thefollowing mathematical models
(i) CrankndashGinstlingndashBrounshtein (CGB) kineticmodel
1(2=3)a(1a)2=3~Kt (D4) (9)
rate controlling step diffusion through the
product layer
(ii)(1a)1=3
~Kt (R2) (10)
rate controlling step chemical reaction at the
interface between the unreacted core and the
product layer
(iii) 1(2=3)a(1a)2=3z1(1a)1=3
~Kt (D4zR2) (11)
Generalised equation that is a combination of
equations (9) and (10) based on the additivity of
reaction times
rate controlling step mixed controlled mechanism
Table 3 n values of kinetic equations for gasndashsolidreactions
Kinetic equation n
D1 a 25Kt 062D2 (12a )ln(12a )za 5Kt 057D3 [12(12a )13]25Kt 054D4 12(23)a 2(12a )235Kt 057D5 Kt 5Z z[1z(Z 21)a ]232(Z 21)(12a )23(Z 21) (ValensindashCarter equation)
F1 2ln(12a )5Kt 10R1 12(12a )125Kt 111R2 12(12a )135Kt 107A2 [2ln(12a )]125Kt 20A3 [2ln(12a )]135Kt 20
Z coefficient representing the product volume per volume ofthe reactants consumed Z is assumed to be 05
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 15
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 89
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
It was deduced from the kinetic analysis that the
mixed kinetic model equation (11) fits well the experi-mental data for all dust samples and pellets of a certainorigin (the washing tower) up to 750uC This kinetic
model also verified the experimental data for pellets of different origin (ie electrofilter and polycyclone dusts)up to 800uC At higher temperatures ie up to 850uC ithas been confirmed the predominance of diffusion
mechanism according to the CGB equation (9)The activation energy value of the rate-determining
step at various conditions has been evaluated concern-ing coal based laterite reduction by means of thewidespread rate expression (12) that follows
r~kC n (12)
where r is the reaction rate k the rate constant C thefluid reactant concentration (mol L21) and n the orderof the reaction Assuming that the fluid reactantconcentration is constant the Arrhenius law[lnk 5lnAo ndash E R(1T )] can be used for calculation of the
activation energy through slope determination of thefollowing linear equation
lnr~ln(AoC n)(E =R)(1=T ) (13)
where r is the mean initial reduction rate valuecalculated through the expression r5DaDt21 (a is thereduction rates obtained from the experimental data)
Ao is the frequency factor of the Arrhenius law C is thefluid reactant concentration (mol L21) n the order of the reaction E the activation energy T the temperatures
(K) examined for the kinetic analysis and R the universalgas constant Activation energy has also been calculatedby Zevgolis et al concerning RK dust and laboratory
made pellets of the same origin through application of the Arrhenius law to the slopes of the generalisedequation (13) lines mixed controlled mechanism at 700and 750uC ie by means of the relationship
ln k (T 2)
ln k (T 1)~
E
R
1
T 2
1
T 1
(14)
The activation energy values concerning iron oxidereduction912 (724 and 402 kJ mol21) indicate that theprevailing kinetic mechanism may be either the mixedcontrolled mechanism (diffusionndashchemical reaction) ordiffusion Activation energy value reported concerningnickel oxide reduction (874 kJ mol21) indicates thepredominance ndash at least for a certain temperature rangeof chemical reaction mechanism The mixed controlledmechanism for iron oxide reduction was proposed byZevgolis et al for low temperatures (700ndash750uC) and
diffusion through the product layer for higher tempera-tures (800ndash850uC)14
The conclusions drawn from the kinetic analysis of the Greek nickeliferous laterite solid state reductionare in agreement with the conclusions drawn by otherstudies concerning reducibility tests of different originlaterite samples either in pellet form23 or in form of cement bonded laterite briquettes with CO-CO2 gas
reducing mixture24
within the temperature range (700ndash 1000uC) Mixed control (chemical reaction and diffu-
sion of the reducing gas agent through the productlayer) is the mechanism that seems to prevail duringthe reductive procedure It is apparent from theaforementioned that diffusion constitutes a seriouskinetic factor affecting solid state reduction of laterites
either on its own or in combination with chemicalreaction in the interface product layer ndash gas reactantKinetic analysis of Greek nickeliferous lateritesenhances the conclusion drawn by all the reportedreducibility tests according to which iron silicatemineralogical phases formed in the temperature rangeexamined (700ndash900uC) like fayalite (2FeOSiO2)probably cover the iron oxide grains and thus impede
the progress of the reaction
ConclusionsThe main physicochemical parameters affecting reduci-bility of the Greek nickeliferous laterite ores have beenreviewed It is concluded that no matter if controllingthe values of physicochemical parameters such astemperature grain size of materials and type of solidreductant to be optimum iron and nickel oxide
reduction degrees obtained do not exceed 33 and 76respectively though the remaining carbon in the calcineis adequate for further progress of the reductive
reactions This means that practically no metallic ironis formed during roasting reduction of Greek lateritesand nickel oxide is partially transformed to metallicnickel Decrease of the ore and solid reductant grain size(mainly for granulometry z3 mm) and use of reactivesolid fuels (lignite) instead of less reactive (coal or coke)favour considerably iron and nickel oxide degree of
reduction Increase of temperature within the tempera-ture range 700ndash900uC examined results in increase of oxides reduction rate but its influence on iron and nickeloxide degree is evident up to 750ndash800uC and then itdiminishes Reduction process progresses initially with arelatively high rate for the first ten minutes then the rate
starts to decrease until it diminishes to zero after thefirst 20 min reaction The conclusion that reductivereactions practically stop after a time of 20ndash40 min isverified by published approaches concerning reducibility
studies of different origin laterite ores It can beattributed to the formation within the temperaturerange examined of iron silicate minerals such as fayalite(2FeOSiO2) or forsterite (Mg2SiO4) which probablycover oxide grains and impede further progress of thereduction Thus future reducibility studies of Greeknickeliferous laterites should be focused on mineralogi-cal analysis of the reduction products in relation withthe factors affecting the formation and reduction of complex nickelndashironndashmagnesium silicate phases so that
the highest possible reduction degree is obtainedKinetic studies of the reductive procedure showed thatmixed control and diffusion mechanisms have been
found to prevail during reduction of oxides
References1 wwwlibmurdocheduauadtpubfilesadt-MU20051004114504
02Wholepdf
2 A D Dalvi W Bacon and C Osborne lsquoThe past and the future of
nickel lateritesrsquo Proc PDAC 2004 Int Convent Toronto
Canada March 2004 PDAC 1ndash27
3 E N Zevgolis lsquoExtractive metallurgy of nickel part I
Pyrometallurgical methodsrsquo 2000 Athens National TechnicalUniverity of Athens
4 E Mposkos A Orfanoudaki and Th Perraki lsquoThe Ni distribution
in the mineral phases of Greek FendashNi laterite depositsrsquo Proc 3rd
Symp on lsquoMineral wealthrsquo Athens Greece November 2000
Technical Chamber of Greece 107ndash115
5 N Albadakis lsquoNi-minerals in the deposits of the sub-pelagonic
zonersquo Miner Wealth 1984 31 9ndash32
Zev gol is et al The reducibility of the Greek nickeliferous laterites
16 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 59
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
in coalndashore mixtures and the role of the fuel volatile
matter They reported that volatile matter constituents
were methane (CH4) within the temperature range 450ndash
700uC and CO kai H2 for temperatures 800uC Gas
agents CO kai H2 are mainly responsible for hematite
(Fe2O3) reduction up to 800uC while additional CO
generated due to the Boudouard reaction at elevated
temperatures resulted in acceleration of the reduction
procedure Concerning laboratory work with Greek
laterite the fact that remaining carbon in the calcine is
in most cases much higher than the theoretically
expected indicates according to researchers that evolvedvolatiles participate in the oxide reduction912 but this
requires further investigation
Reducibility tests have been conducted by different
researchers on Greek laterites in the form of dust and
pellets as well Dust produced during industrial treat-
ment of laterite in the RKs has a higher Ni content than
the ore and a carbon content 8 which is higher than
the stoichiometrically required (y65) for FendashNi 13
production Thus it is evident that agglomeration and
recycling of dust which is y7 of the laterite feed
contribute to the economics of the metallurgical process
So the reductive behaviour of the industrially produced
cement bonded laterite pellets has been studied forcomparison with the behaviour of same origin laterite
dust14 Reducibility tests of pulverised Kastoria laterite
ore have been also conducted using coal as a reductant
in order to compare with the reductive behaviour of
agglomerated mixture (pellets) of Kastoria pulverised
ore and coal (the stoichiometrically required for FendashNi
13 production) with bentonite as the binding agent13
The temperature range of the experimental procedure
was 700ndash900uC It was deduced from both series of tests
that iron oxide reduction degree obtained for pellets was
lower than for dust (with same laterite) under the same
conditions though no serious difference is observed
concerning nickel oxide reduction This difference in thereductive behaviour between laterite dust and pellets can
be attributed to the fact that the binding agent covers
part of the oxide grains decreasing their reactive surface
area
The conclusion drawn from all reducibility studies of
Greek nickeliferous laterites either conducted in an
industrial or in a laboratory scale within the tempera-
ture range of 700ndash900uC is that independently of the
conditions employed ie ore grain size temperature and
type and amount of the solid reductant in no case
calcine reduction degree exceeds 33 though the
remaining carbon in the calcine is almost always higher
than the theoretically required for further progress of reductive reactions Given that the conversion of Fe2O3
1 Equilibrium curves of CO-CO2 versus temperature for Fe-C-O system10
2 Reduction degree versus time typical diagrams at various temperatures of a laterite ore mixture (Evia IslandndashLokridandash
Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lignite (20149z00940 mm)12 b electrostatic filter dust14 and c elec-
trostatic filter pellets14
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 13
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 69
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
to Fe3O4 and to FeO corresponds to a reduction of 111and 333 respectively it comes that magnetite and
wustite should coexist in the calcine and no furtherconversion to metallic iron exists Additionally it is
concluded that reductive reactions occur within the first 20
to 30 min of the process and then they practically stop for
all cases examined This can be attributed to kineticphenomena such as the formation within the temperature
range examined of iron-silicate minerals such as fayalite
(2FeOSiO2) which probably covers oxide grains thusimpeding progress of the reduction In Fig 2 typical
reduction degree diagrams versus time within a tempera-
ture range 700ndash850uC are presented for reduction of
pulverised laterite ore mixture (Evia islandndashLokridandash Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lig-
nite (20149z00940 mm) and also the reduction of
electrostatic filter dust from RKs as well as the reduction
of laboratory made pellets from this dust From these
diagrams it comes that reduction degree does not exceed
33 The same is shown in Fig 3 with reduction in
industrial RKs (In Fig 3 reduction is defined as the ratio
Fe2z
Fetot
|100 so the real reduction degree ie
Oi Of eth THORN=Oifrac12 |100 where Oi is the initial oxygen and
Of is the final oxygen in iron oxides is 333)
Industrial operation with garnierite type of ores shows
that reduction in the RK goes up to 37 which meansthat we have metallic iron in the calcine21
World experienceReducibility tests concerning different types of nickelifer-
ous laterites according to international literature2223
indicate that chemical mineralogical composition of the
laterite ore is a critical parameter for the final degree of reduction obtained Reducibility of several types of
nickeliferous laterites from the Dominican Republic22
(grain size 20074 mm) has been studied within the
temperature range 400ndash1000uC using hydrogen as a
reductant The aim of the study was to compare the
reductive behaviour of garnieritic limonitic and inter-
mediate type of ores The experimental results clearly
indicated that reducibility of each oxide (iron nickel and
cobalt oxide) depends on the ore type Moreover
chemical and mineralogical analysis of the reduced
samples indicated that nickel and iron (oxide) degree of
metallisation was higher in limonitic than in garnieritic
and intermediate type of ores Specifically both iron andnickel oxide degree of metallisation obtained after 40 minprocessing of the garnieritic type of ore at 1000uC was
y10 Temperature variation between 400 and 1000uC
did not affect significantly the reduction process The
respective degrees of metallisation were 80 and 70 at
1000uC concerning limonitic type of ore and they were
dramatically increased 800uC Cobalt degree of metal-
lisation at 1000uC was slightly lower (y65) in limonitic
compared to garnieritic type of ore (70) and the effect of
temperature variation was negligible concerning the first
and significant 850uC concerning the second one This
is different from what happens in reduction by solid fuelsThat is in solid fuel reduction limonitic type of ores
cannot be reduced to metallic iron so reduction stops
when reduction degree of iron oxides is 333 Low
nickel oxide reducibility in the garnieritic type ores was
attributed to the formation of olivine a nickelndashironndash
magnesium orthosilicate and the tendency of nickel to
3 Temperature and reduction degree in RK
Zev gol is et al The reducibility of the Greek nickeliferous laterites
14 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 79
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
exchange places with magnesium in silicates which are
stable in high temperatures On the contrary the low
magnesia and silica content of the limonitic type of ores is
not adequate to result in hosting nickel in the olivine
phase thus the per cent of nickel oxide reduction degree
increases for high temperatures Cobalt oxide reducibility
was higher for the garnieritic laterite type and the effect of
temperature was more evident for lower temperatures
due to the cobalt tendency to replace iron in the limonitelattice Reductive reactions concerning all types of laterite
ores examined after 40 min practically stopped In the
Greek laterite reducibility tests as it has already been
mentioned above reductive reactions practically stopped
after 20 to 30 min
Reducibility tests of a garnieritic type of laterite ore in
the form of pellets in the range 700ndash1000uC using a CO
CO2 mixture as a reducing agent has also been
conducted23 Olivine (MgFe)2SiO4 formation was
proved to be critical for the reduction progress since it
is stated that reducibility (determined by percentage of
weight loss after first calcined in the same temperaturelaterite ore pellets) reaches the highest possible values for
temperatures above serpentine (Mg Fe Ni)6Si4O12(OH)6decomposition (y600uC) and below transformation
temperature of amorphous olivine to a stable miner-
alogical phase (y810uC) It was also deduced that a
strongly competitive relation exists among reduction
progress and olivine formation ie a slow reduction rate
(by employment of mild reducing atmosphere through
gas reducing agent) relative to olivine formation results to
a lower reduction degree On the contrary rapid
reduction rate (by employment of intensive reducing
atmosphere during the first minutes of the reductive
procedure) relative to the crystallisation of the olivinephase results in higher reduction degree values
Reduction kinetics of Greek nickeliferous lateritesKinetic analysis carried out for the Greek nickeliferous
laterite roasting reduction is based on the unreacted
shrinking core model Roasting reduction kinetics of
Greek laterite fine particles (EviandashLokridandashKastoria ore
mixture 60ndash25ndash15 wt- granulometry 20250z
0037 mm temperature range 700ndash900uC) with lignite
and pet coke as reductants respectively were con-ducted912 The methodology of work used for
approaching the rate controlling step of the process is
the application of the diagnostic equation
lnln 1aeth THORNfrac12 ~n ln tzln b (8)
where a is reduction degree () of iron or nickel oxides t
is time (s) b is constant and n is constant depending on
the rate controlling mechanism and the geometrical
characteristics of the ore and solid reductant particles
The obtained n values from application of the experi-
mental data a ndash t which represent the slopes of the linear
graphic representation of equation (8) are comparedwith the theoretical values of the widespread used kinetic
equations of Table 3 It is noted that equations (D1)ndash
(D5) correspond to the diffusion rate controlling step
equations (F1) (R1) and (R2) correspond to chemical
reaction mechanism and equations (A2) and (A3)
correspond to the nucleation rate controlling step
Linearity assessment of diagrams ln[2ln(12a)] versus
lnt and determination of the slope n were obtainedthrough the application of the least squares method It isnoted that no time greater than 15 min was used for the
kinetic analysis given that after the first 20 min thereactions tended to stop in all cases examined Theconclusion deduced from this work was that diffusion
kinetic equations (D1)ndash(D5) best fitted the experimental
data concerning iron oxide reduction Moreover it hasbeen reported that the ValensindashCarter equation was themathematical model that best represented the proposeddiffusion mechanism with Z value equal to 05 Another
value of coefficient Z would probably result in an evenmore representative mathematical model With respectto the nickel oxide reduction kinetics it was reported
that chemical reaction is the rate controlling step for thelower temperatures (up to 800uC) but no certain kineticmodel from Table 3 (equations (F1) (R1) and (R2)) canbe stated to be the best fitting due to the fact that all of
the aforementioned three equations fit very well
The rate controlling step though changes for highertemperatures (up to 900uC) but there has not been a
conclusion based on reported approaches about themechanism that prevails within the temperature range800ndash900uC
Reduction kinetics of RK dust and laboratory made
pellets of the same origin in the temperature range 700ndash 850uC were studied14 The experimental data obtainedconcerning iron oxide reduction were applied to thefollowing mathematical models
(i) CrankndashGinstlingndashBrounshtein (CGB) kineticmodel
1(2=3)a(1a)2=3~Kt (D4) (9)
rate controlling step diffusion through the
product layer
(ii)(1a)1=3
~Kt (R2) (10)
rate controlling step chemical reaction at the
interface between the unreacted core and the
product layer
(iii) 1(2=3)a(1a)2=3z1(1a)1=3
~Kt (D4zR2) (11)
Generalised equation that is a combination of
equations (9) and (10) based on the additivity of
reaction times
rate controlling step mixed controlled mechanism
Table 3 n values of kinetic equations for gasndashsolidreactions
Kinetic equation n
D1 a 25Kt 062D2 (12a )ln(12a )za 5Kt 057D3 [12(12a )13]25Kt 054D4 12(23)a 2(12a )235Kt 057D5 Kt 5Z z[1z(Z 21)a ]232(Z 21)(12a )23(Z 21) (ValensindashCarter equation)
F1 2ln(12a )5Kt 10R1 12(12a )125Kt 111R2 12(12a )135Kt 107A2 [2ln(12a )]125Kt 20A3 [2ln(12a )]135Kt 20
Z coefficient representing the product volume per volume ofthe reactants consumed Z is assumed to be 05
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 15
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 89
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
It was deduced from the kinetic analysis that the
mixed kinetic model equation (11) fits well the experi-mental data for all dust samples and pellets of a certainorigin (the washing tower) up to 750uC This kinetic
model also verified the experimental data for pellets of different origin (ie electrofilter and polycyclone dusts)up to 800uC At higher temperatures ie up to 850uC ithas been confirmed the predominance of diffusion
mechanism according to the CGB equation (9)The activation energy value of the rate-determining
step at various conditions has been evaluated concern-ing coal based laterite reduction by means of thewidespread rate expression (12) that follows
r~kC n (12)
where r is the reaction rate k the rate constant C thefluid reactant concentration (mol L21) and n the orderof the reaction Assuming that the fluid reactantconcentration is constant the Arrhenius law[lnk 5lnAo ndash E R(1T )] can be used for calculation of the
activation energy through slope determination of thefollowing linear equation
lnr~ln(AoC n)(E =R)(1=T ) (13)
where r is the mean initial reduction rate valuecalculated through the expression r5DaDt21 (a is thereduction rates obtained from the experimental data)
Ao is the frequency factor of the Arrhenius law C is thefluid reactant concentration (mol L21) n the order of the reaction E the activation energy T the temperatures
(K) examined for the kinetic analysis and R the universalgas constant Activation energy has also been calculatedby Zevgolis et al concerning RK dust and laboratory
made pellets of the same origin through application of the Arrhenius law to the slopes of the generalisedequation (13) lines mixed controlled mechanism at 700and 750uC ie by means of the relationship
ln k (T 2)
ln k (T 1)~
E
R
1
T 2
1
T 1
(14)
The activation energy values concerning iron oxidereduction912 (724 and 402 kJ mol21) indicate that theprevailing kinetic mechanism may be either the mixedcontrolled mechanism (diffusionndashchemical reaction) ordiffusion Activation energy value reported concerningnickel oxide reduction (874 kJ mol21) indicates thepredominance ndash at least for a certain temperature rangeof chemical reaction mechanism The mixed controlledmechanism for iron oxide reduction was proposed byZevgolis et al for low temperatures (700ndash750uC) and
diffusion through the product layer for higher tempera-tures (800ndash850uC)14
The conclusions drawn from the kinetic analysis of the Greek nickeliferous laterite solid state reductionare in agreement with the conclusions drawn by otherstudies concerning reducibility tests of different originlaterite samples either in pellet form23 or in form of cement bonded laterite briquettes with CO-CO2 gas
reducing mixture24
within the temperature range (700ndash 1000uC) Mixed control (chemical reaction and diffu-
sion of the reducing gas agent through the productlayer) is the mechanism that seems to prevail duringthe reductive procedure It is apparent from theaforementioned that diffusion constitutes a seriouskinetic factor affecting solid state reduction of laterites
either on its own or in combination with chemicalreaction in the interface product layer ndash gas reactantKinetic analysis of Greek nickeliferous lateritesenhances the conclusion drawn by all the reportedreducibility tests according to which iron silicatemineralogical phases formed in the temperature rangeexamined (700ndash900uC) like fayalite (2FeOSiO2)probably cover the iron oxide grains and thus impede
the progress of the reaction
ConclusionsThe main physicochemical parameters affecting reduci-bility of the Greek nickeliferous laterite ores have beenreviewed It is concluded that no matter if controllingthe values of physicochemical parameters such astemperature grain size of materials and type of solidreductant to be optimum iron and nickel oxide
reduction degrees obtained do not exceed 33 and 76respectively though the remaining carbon in the calcineis adequate for further progress of the reductive
reactions This means that practically no metallic ironis formed during roasting reduction of Greek lateritesand nickel oxide is partially transformed to metallicnickel Decrease of the ore and solid reductant grain size(mainly for granulometry z3 mm) and use of reactivesolid fuels (lignite) instead of less reactive (coal or coke)favour considerably iron and nickel oxide degree of
reduction Increase of temperature within the tempera-ture range 700ndash900uC examined results in increase of oxides reduction rate but its influence on iron and nickeloxide degree is evident up to 750ndash800uC and then itdiminishes Reduction process progresses initially with arelatively high rate for the first ten minutes then the rate
starts to decrease until it diminishes to zero after thefirst 20 min reaction The conclusion that reductivereactions practically stop after a time of 20ndash40 min isverified by published approaches concerning reducibility
studies of different origin laterite ores It can beattributed to the formation within the temperaturerange examined of iron silicate minerals such as fayalite(2FeOSiO2) or forsterite (Mg2SiO4) which probablycover oxide grains and impede further progress of thereduction Thus future reducibility studies of Greeknickeliferous laterites should be focused on mineralogi-cal analysis of the reduction products in relation withthe factors affecting the formation and reduction of complex nickelndashironndashmagnesium silicate phases so that
the highest possible reduction degree is obtainedKinetic studies of the reductive procedure showed thatmixed control and diffusion mechanisms have been
found to prevail during reduction of oxides
References1 wwwlibmurdocheduauadtpubfilesadt-MU20051004114504
02Wholepdf
2 A D Dalvi W Bacon and C Osborne lsquoThe past and the future of
nickel lateritesrsquo Proc PDAC 2004 Int Convent Toronto
Canada March 2004 PDAC 1ndash27
3 E N Zevgolis lsquoExtractive metallurgy of nickel part I
Pyrometallurgical methodsrsquo 2000 Athens National TechnicalUniverity of Athens
4 E Mposkos A Orfanoudaki and Th Perraki lsquoThe Ni distribution
in the mineral phases of Greek FendashNi laterite depositsrsquo Proc 3rd
Symp on lsquoMineral wealthrsquo Athens Greece November 2000
Technical Chamber of Greece 107ndash115
5 N Albadakis lsquoNi-minerals in the deposits of the sub-pelagonic
zonersquo Miner Wealth 1984 31 9ndash32
Zev gol is et al The reducibility of the Greek nickeliferous laterites
16 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 69
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
to Fe3O4 and to FeO corresponds to a reduction of 111and 333 respectively it comes that magnetite and
wustite should coexist in the calcine and no furtherconversion to metallic iron exists Additionally it is
concluded that reductive reactions occur within the first 20
to 30 min of the process and then they practically stop for
all cases examined This can be attributed to kineticphenomena such as the formation within the temperature
range examined of iron-silicate minerals such as fayalite
(2FeOSiO2) which probably covers oxide grains thusimpeding progress of the reduction In Fig 2 typical
reduction degree diagrams versus time within a tempera-
ture range 700ndash850uC are presented for reduction of
pulverised laterite ore mixture (Evia islandndashLokridandash Kastoria 60ndash25ndash15 wt- 20053z0037 mm) with lig-
nite (20149z00940 mm) and also the reduction of
electrostatic filter dust from RKs as well as the reduction
of laboratory made pellets from this dust From these
diagrams it comes that reduction degree does not exceed
33 The same is shown in Fig 3 with reduction in
industrial RKs (In Fig 3 reduction is defined as the ratio
Fe2z
Fetot
|100 so the real reduction degree ie
Oi Of eth THORN=Oifrac12 |100 where Oi is the initial oxygen and
Of is the final oxygen in iron oxides is 333)
Industrial operation with garnierite type of ores shows
that reduction in the RK goes up to 37 which meansthat we have metallic iron in the calcine21
World experienceReducibility tests concerning different types of nickelifer-
ous laterites according to international literature2223
indicate that chemical mineralogical composition of the
laterite ore is a critical parameter for the final degree of reduction obtained Reducibility of several types of
nickeliferous laterites from the Dominican Republic22
(grain size 20074 mm) has been studied within the
temperature range 400ndash1000uC using hydrogen as a
reductant The aim of the study was to compare the
reductive behaviour of garnieritic limonitic and inter-
mediate type of ores The experimental results clearly
indicated that reducibility of each oxide (iron nickel and
cobalt oxide) depends on the ore type Moreover
chemical and mineralogical analysis of the reduced
samples indicated that nickel and iron (oxide) degree of
metallisation was higher in limonitic than in garnieritic
and intermediate type of ores Specifically both iron andnickel oxide degree of metallisation obtained after 40 minprocessing of the garnieritic type of ore at 1000uC was
y10 Temperature variation between 400 and 1000uC
did not affect significantly the reduction process The
respective degrees of metallisation were 80 and 70 at
1000uC concerning limonitic type of ore and they were
dramatically increased 800uC Cobalt degree of metal-
lisation at 1000uC was slightly lower (y65) in limonitic
compared to garnieritic type of ore (70) and the effect of
temperature variation was negligible concerning the first
and significant 850uC concerning the second one This
is different from what happens in reduction by solid fuelsThat is in solid fuel reduction limonitic type of ores
cannot be reduced to metallic iron so reduction stops
when reduction degree of iron oxides is 333 Low
nickel oxide reducibility in the garnieritic type ores was
attributed to the formation of olivine a nickelndashironndash
magnesium orthosilicate and the tendency of nickel to
3 Temperature and reduction degree in RK
Zev gol is et al The reducibility of the Greek nickeliferous laterites
14 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 79
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
exchange places with magnesium in silicates which are
stable in high temperatures On the contrary the low
magnesia and silica content of the limonitic type of ores is
not adequate to result in hosting nickel in the olivine
phase thus the per cent of nickel oxide reduction degree
increases for high temperatures Cobalt oxide reducibility
was higher for the garnieritic laterite type and the effect of
temperature was more evident for lower temperatures
due to the cobalt tendency to replace iron in the limonitelattice Reductive reactions concerning all types of laterite
ores examined after 40 min practically stopped In the
Greek laterite reducibility tests as it has already been
mentioned above reductive reactions practically stopped
after 20 to 30 min
Reducibility tests of a garnieritic type of laterite ore in
the form of pellets in the range 700ndash1000uC using a CO
CO2 mixture as a reducing agent has also been
conducted23 Olivine (MgFe)2SiO4 formation was
proved to be critical for the reduction progress since it
is stated that reducibility (determined by percentage of
weight loss after first calcined in the same temperaturelaterite ore pellets) reaches the highest possible values for
temperatures above serpentine (Mg Fe Ni)6Si4O12(OH)6decomposition (y600uC) and below transformation
temperature of amorphous olivine to a stable miner-
alogical phase (y810uC) It was also deduced that a
strongly competitive relation exists among reduction
progress and olivine formation ie a slow reduction rate
(by employment of mild reducing atmosphere through
gas reducing agent) relative to olivine formation results to
a lower reduction degree On the contrary rapid
reduction rate (by employment of intensive reducing
atmosphere during the first minutes of the reductive
procedure) relative to the crystallisation of the olivinephase results in higher reduction degree values
Reduction kinetics of Greek nickeliferous lateritesKinetic analysis carried out for the Greek nickeliferous
laterite roasting reduction is based on the unreacted
shrinking core model Roasting reduction kinetics of
Greek laterite fine particles (EviandashLokridandashKastoria ore
mixture 60ndash25ndash15 wt- granulometry 20250z
0037 mm temperature range 700ndash900uC) with lignite
and pet coke as reductants respectively were con-ducted912 The methodology of work used for
approaching the rate controlling step of the process is
the application of the diagnostic equation
lnln 1aeth THORNfrac12 ~n ln tzln b (8)
where a is reduction degree () of iron or nickel oxides t
is time (s) b is constant and n is constant depending on
the rate controlling mechanism and the geometrical
characteristics of the ore and solid reductant particles
The obtained n values from application of the experi-
mental data a ndash t which represent the slopes of the linear
graphic representation of equation (8) are comparedwith the theoretical values of the widespread used kinetic
equations of Table 3 It is noted that equations (D1)ndash
(D5) correspond to the diffusion rate controlling step
equations (F1) (R1) and (R2) correspond to chemical
reaction mechanism and equations (A2) and (A3)
correspond to the nucleation rate controlling step
Linearity assessment of diagrams ln[2ln(12a)] versus
lnt and determination of the slope n were obtainedthrough the application of the least squares method It isnoted that no time greater than 15 min was used for the
kinetic analysis given that after the first 20 min thereactions tended to stop in all cases examined Theconclusion deduced from this work was that diffusion
kinetic equations (D1)ndash(D5) best fitted the experimental
data concerning iron oxide reduction Moreover it hasbeen reported that the ValensindashCarter equation was themathematical model that best represented the proposeddiffusion mechanism with Z value equal to 05 Another
value of coefficient Z would probably result in an evenmore representative mathematical model With respectto the nickel oxide reduction kinetics it was reported
that chemical reaction is the rate controlling step for thelower temperatures (up to 800uC) but no certain kineticmodel from Table 3 (equations (F1) (R1) and (R2)) canbe stated to be the best fitting due to the fact that all of
the aforementioned three equations fit very well
The rate controlling step though changes for highertemperatures (up to 900uC) but there has not been a
conclusion based on reported approaches about themechanism that prevails within the temperature range800ndash900uC
Reduction kinetics of RK dust and laboratory made
pellets of the same origin in the temperature range 700ndash 850uC were studied14 The experimental data obtainedconcerning iron oxide reduction were applied to thefollowing mathematical models
(i) CrankndashGinstlingndashBrounshtein (CGB) kineticmodel
1(2=3)a(1a)2=3~Kt (D4) (9)
rate controlling step diffusion through the
product layer
(ii)(1a)1=3
~Kt (R2) (10)
rate controlling step chemical reaction at the
interface between the unreacted core and the
product layer
(iii) 1(2=3)a(1a)2=3z1(1a)1=3
~Kt (D4zR2) (11)
Generalised equation that is a combination of
equations (9) and (10) based on the additivity of
reaction times
rate controlling step mixed controlled mechanism
Table 3 n values of kinetic equations for gasndashsolidreactions
Kinetic equation n
D1 a 25Kt 062D2 (12a )ln(12a )za 5Kt 057D3 [12(12a )13]25Kt 054D4 12(23)a 2(12a )235Kt 057D5 Kt 5Z z[1z(Z 21)a ]232(Z 21)(12a )23(Z 21) (ValensindashCarter equation)
F1 2ln(12a )5Kt 10R1 12(12a )125Kt 111R2 12(12a )135Kt 107A2 [2ln(12a )]125Kt 20A3 [2ln(12a )]135Kt 20
Z coefficient representing the product volume per volume ofthe reactants consumed Z is assumed to be 05
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 15
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 89
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
It was deduced from the kinetic analysis that the
mixed kinetic model equation (11) fits well the experi-mental data for all dust samples and pellets of a certainorigin (the washing tower) up to 750uC This kinetic
model also verified the experimental data for pellets of different origin (ie electrofilter and polycyclone dusts)up to 800uC At higher temperatures ie up to 850uC ithas been confirmed the predominance of diffusion
mechanism according to the CGB equation (9)The activation energy value of the rate-determining
step at various conditions has been evaluated concern-ing coal based laterite reduction by means of thewidespread rate expression (12) that follows
r~kC n (12)
where r is the reaction rate k the rate constant C thefluid reactant concentration (mol L21) and n the orderof the reaction Assuming that the fluid reactantconcentration is constant the Arrhenius law[lnk 5lnAo ndash E R(1T )] can be used for calculation of the
activation energy through slope determination of thefollowing linear equation
lnr~ln(AoC n)(E =R)(1=T ) (13)
where r is the mean initial reduction rate valuecalculated through the expression r5DaDt21 (a is thereduction rates obtained from the experimental data)
Ao is the frequency factor of the Arrhenius law C is thefluid reactant concentration (mol L21) n the order of the reaction E the activation energy T the temperatures
(K) examined for the kinetic analysis and R the universalgas constant Activation energy has also been calculatedby Zevgolis et al concerning RK dust and laboratory
made pellets of the same origin through application of the Arrhenius law to the slopes of the generalisedequation (13) lines mixed controlled mechanism at 700and 750uC ie by means of the relationship
ln k (T 2)
ln k (T 1)~
E
R
1
T 2
1
T 1
(14)
The activation energy values concerning iron oxidereduction912 (724 and 402 kJ mol21) indicate that theprevailing kinetic mechanism may be either the mixedcontrolled mechanism (diffusionndashchemical reaction) ordiffusion Activation energy value reported concerningnickel oxide reduction (874 kJ mol21) indicates thepredominance ndash at least for a certain temperature rangeof chemical reaction mechanism The mixed controlledmechanism for iron oxide reduction was proposed byZevgolis et al for low temperatures (700ndash750uC) and
diffusion through the product layer for higher tempera-tures (800ndash850uC)14
The conclusions drawn from the kinetic analysis of the Greek nickeliferous laterite solid state reductionare in agreement with the conclusions drawn by otherstudies concerning reducibility tests of different originlaterite samples either in pellet form23 or in form of cement bonded laterite briquettes with CO-CO2 gas
reducing mixture24
within the temperature range (700ndash 1000uC) Mixed control (chemical reaction and diffu-
sion of the reducing gas agent through the productlayer) is the mechanism that seems to prevail duringthe reductive procedure It is apparent from theaforementioned that diffusion constitutes a seriouskinetic factor affecting solid state reduction of laterites
either on its own or in combination with chemicalreaction in the interface product layer ndash gas reactantKinetic analysis of Greek nickeliferous lateritesenhances the conclusion drawn by all the reportedreducibility tests according to which iron silicatemineralogical phases formed in the temperature rangeexamined (700ndash900uC) like fayalite (2FeOSiO2)probably cover the iron oxide grains and thus impede
the progress of the reaction
ConclusionsThe main physicochemical parameters affecting reduci-bility of the Greek nickeliferous laterite ores have beenreviewed It is concluded that no matter if controllingthe values of physicochemical parameters such astemperature grain size of materials and type of solidreductant to be optimum iron and nickel oxide
reduction degrees obtained do not exceed 33 and 76respectively though the remaining carbon in the calcineis adequate for further progress of the reductive
reactions This means that practically no metallic ironis formed during roasting reduction of Greek lateritesand nickel oxide is partially transformed to metallicnickel Decrease of the ore and solid reductant grain size(mainly for granulometry z3 mm) and use of reactivesolid fuels (lignite) instead of less reactive (coal or coke)favour considerably iron and nickel oxide degree of
reduction Increase of temperature within the tempera-ture range 700ndash900uC examined results in increase of oxides reduction rate but its influence on iron and nickeloxide degree is evident up to 750ndash800uC and then itdiminishes Reduction process progresses initially with arelatively high rate for the first ten minutes then the rate
starts to decrease until it diminishes to zero after thefirst 20 min reaction The conclusion that reductivereactions practically stop after a time of 20ndash40 min isverified by published approaches concerning reducibility
studies of different origin laterite ores It can beattributed to the formation within the temperaturerange examined of iron silicate minerals such as fayalite(2FeOSiO2) or forsterite (Mg2SiO4) which probablycover oxide grains and impede further progress of thereduction Thus future reducibility studies of Greeknickeliferous laterites should be focused on mineralogi-cal analysis of the reduction products in relation withthe factors affecting the formation and reduction of complex nickelndashironndashmagnesium silicate phases so that
the highest possible reduction degree is obtainedKinetic studies of the reductive procedure showed thatmixed control and diffusion mechanisms have been
found to prevail during reduction of oxides
References1 wwwlibmurdocheduauadtpubfilesadt-MU20051004114504
02Wholepdf
2 A D Dalvi W Bacon and C Osborne lsquoThe past and the future of
nickel lateritesrsquo Proc PDAC 2004 Int Convent Toronto
Canada March 2004 PDAC 1ndash27
3 E N Zevgolis lsquoExtractive metallurgy of nickel part I
Pyrometallurgical methodsrsquo 2000 Athens National TechnicalUniverity of Athens
4 E Mposkos A Orfanoudaki and Th Perraki lsquoThe Ni distribution
in the mineral phases of Greek FendashNi laterite depositsrsquo Proc 3rd
Symp on lsquoMineral wealthrsquo Athens Greece November 2000
Technical Chamber of Greece 107ndash115
5 N Albadakis lsquoNi-minerals in the deposits of the sub-pelagonic
zonersquo Miner Wealth 1984 31 9ndash32
Zev gol is et al The reducibility of the Greek nickeliferous laterites
16 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 79
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
exchange places with magnesium in silicates which are
stable in high temperatures On the contrary the low
magnesia and silica content of the limonitic type of ores is
not adequate to result in hosting nickel in the olivine
phase thus the per cent of nickel oxide reduction degree
increases for high temperatures Cobalt oxide reducibility
was higher for the garnieritic laterite type and the effect of
temperature was more evident for lower temperatures
due to the cobalt tendency to replace iron in the limonitelattice Reductive reactions concerning all types of laterite
ores examined after 40 min practically stopped In the
Greek laterite reducibility tests as it has already been
mentioned above reductive reactions practically stopped
after 20 to 30 min
Reducibility tests of a garnieritic type of laterite ore in
the form of pellets in the range 700ndash1000uC using a CO
CO2 mixture as a reducing agent has also been
conducted23 Olivine (MgFe)2SiO4 formation was
proved to be critical for the reduction progress since it
is stated that reducibility (determined by percentage of
weight loss after first calcined in the same temperaturelaterite ore pellets) reaches the highest possible values for
temperatures above serpentine (Mg Fe Ni)6Si4O12(OH)6decomposition (y600uC) and below transformation
temperature of amorphous olivine to a stable miner-
alogical phase (y810uC) It was also deduced that a
strongly competitive relation exists among reduction
progress and olivine formation ie a slow reduction rate
(by employment of mild reducing atmosphere through
gas reducing agent) relative to olivine formation results to
a lower reduction degree On the contrary rapid
reduction rate (by employment of intensive reducing
atmosphere during the first minutes of the reductive
procedure) relative to the crystallisation of the olivinephase results in higher reduction degree values
Reduction kinetics of Greek nickeliferous lateritesKinetic analysis carried out for the Greek nickeliferous
laterite roasting reduction is based on the unreacted
shrinking core model Roasting reduction kinetics of
Greek laterite fine particles (EviandashLokridandashKastoria ore
mixture 60ndash25ndash15 wt- granulometry 20250z
0037 mm temperature range 700ndash900uC) with lignite
and pet coke as reductants respectively were con-ducted912 The methodology of work used for
approaching the rate controlling step of the process is
the application of the diagnostic equation
lnln 1aeth THORNfrac12 ~n ln tzln b (8)
where a is reduction degree () of iron or nickel oxides t
is time (s) b is constant and n is constant depending on
the rate controlling mechanism and the geometrical
characteristics of the ore and solid reductant particles
The obtained n values from application of the experi-
mental data a ndash t which represent the slopes of the linear
graphic representation of equation (8) are comparedwith the theoretical values of the widespread used kinetic
equations of Table 3 It is noted that equations (D1)ndash
(D5) correspond to the diffusion rate controlling step
equations (F1) (R1) and (R2) correspond to chemical
reaction mechanism and equations (A2) and (A3)
correspond to the nucleation rate controlling step
Linearity assessment of diagrams ln[2ln(12a)] versus
lnt and determination of the slope n were obtainedthrough the application of the least squares method It isnoted that no time greater than 15 min was used for the
kinetic analysis given that after the first 20 min thereactions tended to stop in all cases examined Theconclusion deduced from this work was that diffusion
kinetic equations (D1)ndash(D5) best fitted the experimental
data concerning iron oxide reduction Moreover it hasbeen reported that the ValensindashCarter equation was themathematical model that best represented the proposeddiffusion mechanism with Z value equal to 05 Another
value of coefficient Z would probably result in an evenmore representative mathematical model With respectto the nickel oxide reduction kinetics it was reported
that chemical reaction is the rate controlling step for thelower temperatures (up to 800uC) but no certain kineticmodel from Table 3 (equations (F1) (R1) and (R2)) canbe stated to be the best fitting due to the fact that all of
the aforementioned three equations fit very well
The rate controlling step though changes for highertemperatures (up to 900uC) but there has not been a
conclusion based on reported approaches about themechanism that prevails within the temperature range800ndash900uC
Reduction kinetics of RK dust and laboratory made
pellets of the same origin in the temperature range 700ndash 850uC were studied14 The experimental data obtainedconcerning iron oxide reduction were applied to thefollowing mathematical models
(i) CrankndashGinstlingndashBrounshtein (CGB) kineticmodel
1(2=3)a(1a)2=3~Kt (D4) (9)
rate controlling step diffusion through the
product layer
(ii)(1a)1=3
~Kt (R2) (10)
rate controlling step chemical reaction at the
interface between the unreacted core and the
product layer
(iii) 1(2=3)a(1a)2=3z1(1a)1=3
~Kt (D4zR2) (11)
Generalised equation that is a combination of
equations (9) and (10) based on the additivity of
reaction times
rate controlling step mixed controlled mechanism
Table 3 n values of kinetic equations for gasndashsolidreactions
Kinetic equation n
D1 a 25Kt 062D2 (12a )ln(12a )za 5Kt 057D3 [12(12a )13]25Kt 054D4 12(23)a 2(12a )235Kt 057D5 Kt 5Z z[1z(Z 21)a ]232(Z 21)(12a )23(Z 21) (ValensindashCarter equation)
F1 2ln(12a )5Kt 10R1 12(12a )125Kt 111R2 12(12a )135Kt 107A2 [2ln(12a )]125Kt 20A3 [2ln(12a )]135Kt 20
Z coefficient representing the product volume per volume ofthe reactants consumed Z is assumed to be 05
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 15
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 89
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
It was deduced from the kinetic analysis that the
mixed kinetic model equation (11) fits well the experi-mental data for all dust samples and pellets of a certainorigin (the washing tower) up to 750uC This kinetic
model also verified the experimental data for pellets of different origin (ie electrofilter and polycyclone dusts)up to 800uC At higher temperatures ie up to 850uC ithas been confirmed the predominance of diffusion
mechanism according to the CGB equation (9)The activation energy value of the rate-determining
step at various conditions has been evaluated concern-ing coal based laterite reduction by means of thewidespread rate expression (12) that follows
r~kC n (12)
where r is the reaction rate k the rate constant C thefluid reactant concentration (mol L21) and n the orderof the reaction Assuming that the fluid reactantconcentration is constant the Arrhenius law[lnk 5lnAo ndash E R(1T )] can be used for calculation of the
activation energy through slope determination of thefollowing linear equation
lnr~ln(AoC n)(E =R)(1=T ) (13)
where r is the mean initial reduction rate valuecalculated through the expression r5DaDt21 (a is thereduction rates obtained from the experimental data)
Ao is the frequency factor of the Arrhenius law C is thefluid reactant concentration (mol L21) n the order of the reaction E the activation energy T the temperatures
(K) examined for the kinetic analysis and R the universalgas constant Activation energy has also been calculatedby Zevgolis et al concerning RK dust and laboratory
made pellets of the same origin through application of the Arrhenius law to the slopes of the generalisedequation (13) lines mixed controlled mechanism at 700and 750uC ie by means of the relationship
ln k (T 2)
ln k (T 1)~
E
R
1
T 2
1
T 1
(14)
The activation energy values concerning iron oxidereduction912 (724 and 402 kJ mol21) indicate that theprevailing kinetic mechanism may be either the mixedcontrolled mechanism (diffusionndashchemical reaction) ordiffusion Activation energy value reported concerningnickel oxide reduction (874 kJ mol21) indicates thepredominance ndash at least for a certain temperature rangeof chemical reaction mechanism The mixed controlledmechanism for iron oxide reduction was proposed byZevgolis et al for low temperatures (700ndash750uC) and
diffusion through the product layer for higher tempera-tures (800ndash850uC)14
The conclusions drawn from the kinetic analysis of the Greek nickeliferous laterite solid state reductionare in agreement with the conclusions drawn by otherstudies concerning reducibility tests of different originlaterite samples either in pellet form23 or in form of cement bonded laterite briquettes with CO-CO2 gas
reducing mixture24
within the temperature range (700ndash 1000uC) Mixed control (chemical reaction and diffu-
sion of the reducing gas agent through the productlayer) is the mechanism that seems to prevail duringthe reductive procedure It is apparent from theaforementioned that diffusion constitutes a seriouskinetic factor affecting solid state reduction of laterites
either on its own or in combination with chemicalreaction in the interface product layer ndash gas reactantKinetic analysis of Greek nickeliferous lateritesenhances the conclusion drawn by all the reportedreducibility tests according to which iron silicatemineralogical phases formed in the temperature rangeexamined (700ndash900uC) like fayalite (2FeOSiO2)probably cover the iron oxide grains and thus impede
the progress of the reaction
ConclusionsThe main physicochemical parameters affecting reduci-bility of the Greek nickeliferous laterite ores have beenreviewed It is concluded that no matter if controllingthe values of physicochemical parameters such astemperature grain size of materials and type of solidreductant to be optimum iron and nickel oxide
reduction degrees obtained do not exceed 33 and 76respectively though the remaining carbon in the calcineis adequate for further progress of the reductive
reactions This means that practically no metallic ironis formed during roasting reduction of Greek lateritesand nickel oxide is partially transformed to metallicnickel Decrease of the ore and solid reductant grain size(mainly for granulometry z3 mm) and use of reactivesolid fuels (lignite) instead of less reactive (coal or coke)favour considerably iron and nickel oxide degree of
reduction Increase of temperature within the tempera-ture range 700ndash900uC examined results in increase of oxides reduction rate but its influence on iron and nickeloxide degree is evident up to 750ndash800uC and then itdiminishes Reduction process progresses initially with arelatively high rate for the first ten minutes then the rate
starts to decrease until it diminishes to zero after thefirst 20 min reaction The conclusion that reductivereactions practically stop after a time of 20ndash40 min isverified by published approaches concerning reducibility
studies of different origin laterite ores It can beattributed to the formation within the temperaturerange examined of iron silicate minerals such as fayalite(2FeOSiO2) or forsterite (Mg2SiO4) which probablycover oxide grains and impede further progress of thereduction Thus future reducibility studies of Greeknickeliferous laterites should be focused on mineralogi-cal analysis of the reduction products in relation withthe factors affecting the formation and reduction of complex nickelndashironndashmagnesium silicate phases so that
the highest possible reduction degree is obtainedKinetic studies of the reductive procedure showed thatmixed control and diffusion mechanisms have been
found to prevail during reduction of oxides
References1 wwwlibmurdocheduauadtpubfilesadt-MU20051004114504
02Wholepdf
2 A D Dalvi W Bacon and C Osborne lsquoThe past and the future of
nickel lateritesrsquo Proc PDAC 2004 Int Convent Toronto
Canada March 2004 PDAC 1ndash27
3 E N Zevgolis lsquoExtractive metallurgy of nickel part I
Pyrometallurgical methodsrsquo 2000 Athens National TechnicalUniverity of Athens
4 E Mposkos A Orfanoudaki and Th Perraki lsquoThe Ni distribution
in the mineral phases of Greek FendashNi laterite depositsrsquo Proc 3rd
Symp on lsquoMineral wealthrsquo Athens Greece November 2000
Technical Chamber of Greece 107ndash115
5 N Albadakis lsquoNi-minerals in the deposits of the sub-pelagonic
zonersquo Miner Wealth 1984 31 9ndash32
Zev gol is et al The reducibility of the Greek nickeliferous laterites
16 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 89
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
It was deduced from the kinetic analysis that the
mixed kinetic model equation (11) fits well the experi-mental data for all dust samples and pellets of a certainorigin (the washing tower) up to 750uC This kinetic
model also verified the experimental data for pellets of different origin (ie electrofilter and polycyclone dusts)up to 800uC At higher temperatures ie up to 850uC ithas been confirmed the predominance of diffusion
mechanism according to the CGB equation (9)The activation energy value of the rate-determining
step at various conditions has been evaluated concern-ing coal based laterite reduction by means of thewidespread rate expression (12) that follows
r~kC n (12)
where r is the reaction rate k the rate constant C thefluid reactant concentration (mol L21) and n the orderof the reaction Assuming that the fluid reactantconcentration is constant the Arrhenius law[lnk 5lnAo ndash E R(1T )] can be used for calculation of the
activation energy through slope determination of thefollowing linear equation
lnr~ln(AoC n)(E =R)(1=T ) (13)
where r is the mean initial reduction rate valuecalculated through the expression r5DaDt21 (a is thereduction rates obtained from the experimental data)
Ao is the frequency factor of the Arrhenius law C is thefluid reactant concentration (mol L21) n the order of the reaction E the activation energy T the temperatures
(K) examined for the kinetic analysis and R the universalgas constant Activation energy has also been calculatedby Zevgolis et al concerning RK dust and laboratory
made pellets of the same origin through application of the Arrhenius law to the slopes of the generalisedequation (13) lines mixed controlled mechanism at 700and 750uC ie by means of the relationship
ln k (T 2)
ln k (T 1)~
E
R
1
T 2
1
T 1
(14)
The activation energy values concerning iron oxidereduction912 (724 and 402 kJ mol21) indicate that theprevailing kinetic mechanism may be either the mixedcontrolled mechanism (diffusionndashchemical reaction) ordiffusion Activation energy value reported concerningnickel oxide reduction (874 kJ mol21) indicates thepredominance ndash at least for a certain temperature rangeof chemical reaction mechanism The mixed controlledmechanism for iron oxide reduction was proposed byZevgolis et al for low temperatures (700ndash750uC) and
diffusion through the product layer for higher tempera-tures (800ndash850uC)14
The conclusions drawn from the kinetic analysis of the Greek nickeliferous laterite solid state reductionare in agreement with the conclusions drawn by otherstudies concerning reducibility tests of different originlaterite samples either in pellet form23 or in form of cement bonded laterite briquettes with CO-CO2 gas
reducing mixture24
within the temperature range (700ndash 1000uC) Mixed control (chemical reaction and diffu-
sion of the reducing gas agent through the productlayer) is the mechanism that seems to prevail duringthe reductive procedure It is apparent from theaforementioned that diffusion constitutes a seriouskinetic factor affecting solid state reduction of laterites
either on its own or in combination with chemicalreaction in the interface product layer ndash gas reactantKinetic analysis of Greek nickeliferous lateritesenhances the conclusion drawn by all the reportedreducibility tests according to which iron silicatemineralogical phases formed in the temperature rangeexamined (700ndash900uC) like fayalite (2FeOSiO2)probably cover the iron oxide grains and thus impede
the progress of the reaction
ConclusionsThe main physicochemical parameters affecting reduci-bility of the Greek nickeliferous laterite ores have beenreviewed It is concluded that no matter if controllingthe values of physicochemical parameters such astemperature grain size of materials and type of solidreductant to be optimum iron and nickel oxide
reduction degrees obtained do not exceed 33 and 76respectively though the remaining carbon in the calcineis adequate for further progress of the reductive
reactions This means that practically no metallic ironis formed during roasting reduction of Greek lateritesand nickel oxide is partially transformed to metallicnickel Decrease of the ore and solid reductant grain size(mainly for granulometry z3 mm) and use of reactivesolid fuels (lignite) instead of less reactive (coal or coke)favour considerably iron and nickel oxide degree of
reduction Increase of temperature within the tempera-ture range 700ndash900uC examined results in increase of oxides reduction rate but its influence on iron and nickeloxide degree is evident up to 750ndash800uC and then itdiminishes Reduction process progresses initially with arelatively high rate for the first ten minutes then the rate
starts to decrease until it diminishes to zero after thefirst 20 min reaction The conclusion that reductivereactions practically stop after a time of 20ndash40 min isverified by published approaches concerning reducibility
studies of different origin laterite ores It can beattributed to the formation within the temperaturerange examined of iron silicate minerals such as fayalite(2FeOSiO2) or forsterite (Mg2SiO4) which probablycover oxide grains and impede further progress of thereduction Thus future reducibility studies of Greeknickeliferous laterites should be focused on mineralogi-cal analysis of the reduction products in relation withthe factors affecting the formation and reduction of complex nickelndashironndashmagnesium silicate phases so that
the highest possible reduction degree is obtainedKinetic studies of the reductive procedure showed thatmixed control and diffusion mechanisms have been
found to prevail during reduction of oxides
References1 wwwlibmurdocheduauadtpubfilesadt-MU20051004114504
02Wholepdf
2 A D Dalvi W Bacon and C Osborne lsquoThe past and the future of
nickel lateritesrsquo Proc PDAC 2004 Int Convent Toronto
Canada March 2004 PDAC 1ndash27
3 E N Zevgolis lsquoExtractive metallurgy of nickel part I
Pyrometallurgical methodsrsquo 2000 Athens National TechnicalUniverity of Athens
4 E Mposkos A Orfanoudaki and Th Perraki lsquoThe Ni distribution
in the mineral phases of Greek FendashNi laterite depositsrsquo Proc 3rd
Symp on lsquoMineral wealthrsquo Athens Greece November 2000
Technical Chamber of Greece 107ndash115
5 N Albadakis lsquoNi-minerals in the deposits of the sub-pelagonic
zonersquo Miner Wealth 1984 31 9ndash32
Zev gol is et al The reducibility of the Greek nickeliferous laterites
16 Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17
8122019 The Reducibility of the Greek Nickeliferous
httpslidepdfcomreaderfullthe-reducibility-of-the-greek-nickeliferous 99
P u b l i s h e d b y M a n e y P u b l i s h i n g ( c ) I O M C
o m m u n i c a t i o n s L t d a n d t h e A u s t r a l a s i a n I n s t i t u t e o f M i n i n g a n d M e t a l l u r g y
6 S Agatzini lsquoA new approach to the metallurgical treatment
of nickeliferous lateritesrsquo Report within the framework of the
CEC BRITE-EURAM Programme ECU 368000 (In cooperation
with University of Hertfordshire University of Minho) 1993 1ndash
12
7 Q Wang Z Yang J Tian W Li and J Sun lsquoMechanisms of
reduction in iron orendashcoal composite pelletrsquo Ironmaking
Steelmaking 1997 24 (6) 457ndash460
8 E Donskoi D L S McElwain and L J Wibberley lsquoEstimation
and modeling of parameters for direct reduction in iron orecoal
composites part II Kinetic parametersrsquo Metall Mater Trans B 2003 34B 255ndash266
9 P Neou-Syngouna I Halikia and K Skartados lsquoPrereduction
of laterites with petroleum coke influence of the granulometric
size on its progress and kineticsrsquo Min Metall Ann 1997 1 25ndash
49
10 E N Zevgolis lsquoIron-cast iron metallurgy ndash theory and technologyrsquo
2004 Athens National Technical Univerity of Athens
11 E N Zevgolis lsquoA contribution to the study of problems of rotary
kilns in roasting reduction of greek nickeliferous lateritesrsquo Thesis
for lectureship National Technical Univerity of Athens Athens
Greece July 1982
12 I Halikia P Neou-Syngouna and M Katapotis lsquoReductive
roasting of iron-nickel ore using greek lignite thermodynamic and
kinetic approachrsquo In honor of Professor Emeritus of NTUA A Z
Fragiskos 1998 Athens National Technical Univerity of Athens
13 P Neou-Syngouna I Halikia C Skartados L Papadopoulou
and G Portokaloglou lsquoComparative study of laterite roasts in the
form of powder and pelletsrsquo Min Metall Ann 1999 1ndash2 85ndash
118
14 E N Zevgolis I Halikia and I-P Kostika lsquoReductive behavior of
the recycled dust during nickeliferous laterite treatmentrsquo Erzmetall
ndash the World of Metallurgy 2006 59 (6) 350ndash359
15 E N Zevgolis lsquoThe importance of iron ore grain size in rotary kiln
operationrsquo Miner Wealth 1986 45 103ndash110
16 E N Zevgolis lsquoThe ore grain size effect in ferroalloys production
by the rotary kiln ndash electric furnace methodrsquo Miner Wealth 1988
54 39ndash46
17 E N Zevgolis and A Tzamtzis lsquoThe role of solid fuels used for
reduction in rotary kilnsrsquo Techn Chron C 1987 7C (2) 5ndash19
18 I Halikia and K Skartados lsquoEffect of solid reductant on themetallurgical behaviour of laterite calcinersquo In Memory of NTUA
Professor Emeritus J Papageorgarakis 294ndash303 2001 Athens
National Technical Univerity of Athens
19 G-S Liu V Strezov L A Lucas and L J Wibberley lsquoTermal
investigations of direct iron ore reduction with coalrsquo Thermochim
Acta 2004 410 133ndash140
20 V Strezov G-S Liu J A Lucas and L J Wibberley
lsquoCalorimetric study of the iron ore reduction reactions in mixtures
with coalrsquo Ind Eng Chem Res 2005 44 621ndash626
21 E N Zevgolis Report from visit to FENIMAK Ferronickel Plant
Kavadarci Fyrom 24 February 2000
22 M Kawahara J M Toguri and R A Bergman lsquoReducibility of
laterite oresrsquo Metall Trans B 1988 19B 181ndash185
23 S Li and K S Coley lsquoKinetics and mechanism of reduction of
laterite ore high in serpentinersquo Proc J M Toguri Symp onlsquoFundamentals of metallurgical processingrsquo (ed G Kaiura et al)
179ndash192 2000 Ottawa CIM
24 H Purwanto T Shimada R Takahashi and J Yagi lsquoReduction
rate of cement bonded laterite briquette with CO-CO2 gasrsquo ISIJ
Int 2001 41 S31ndashS35
Zev gol is et al The reducibility of the Greek nickeliferous laterites
Mineral Processing and Extractive Metallurgy (Trans Inst Min Metall C) 2010 VOL 1 19 NO 1 17