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THE FERRIC ION -GODS GIFT TOHYDROMETALLURGISTS
TO KEEP EM HUMBLE
By Chris Fleming
SGS Lakefield Research Ltd.
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TOPICS
The first SEx war
Theres nothing basic about basic ironsulphate
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THE FIRST SEx WAR
History
Great interest in solvent extraction in the 1960s, initially
for uranium and then for copper.
Low grade oxide copper ores were being processed in the
USA and Chile by acidic heap leaching (small scale).
Copper sulphate in solution was recovered by cementation
onto scrap iron, and then smelted and refined.
Most leach liquors contained ferric ions, which reacted
with the scrap iron wastefully, and the process was:(i) Expensive
(ii) Yielded an impure copper product
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THE FIRST SEx WAR
History
Early economic projections predicted the cost of SEx/EWwould be half the cost of cementation/smelting/refining.
The perfect reagent had to be able to extract copper (II)
from weak acid leach solution (pH 1-2), be strippable instrong acid (50 -100g/L H2SO4) to be compatible with EW,and be very selective for copper particularly versus theferric ion.
Copper SEx reagent development was spearheaded bytwo US companies, General Mills, who produced the LIX
reagents, and Ashland Chemical, who produced Kelex100.
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H
O
RN
HYDROXYQUINOLINE
KELEX
C = N
R
HO
LIX
HYDROXYOXIME
OH
COPPER SEx REAGENTS
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THE FIRST SEx WAR
History
The first copper SEx plant was built at the Blue BirdMine in Arizona in 1968 (6000 tpa Cu), soonfollowed by a much bigger plant at Nchanga minein Zambia (65,000 tpa Cu).
More plants followed, and 3% of world copperproduction was via SEx/EW by 1975. By 2007, thishad grown to 22% of annual Cu production, (3.5Mtons of cathode copper). This was being producedin 70 SEx/EW plants in 16 countries (60% in Chile).
But who was making the SEx reagents and whowas winning the reagent war?
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Copper extraction with LIX and KELEXreagents as a function of pH
-10
10
30
50
70
90
0 1 2 3 4 5 6
pH
Copper
Extraction
(%)
LIX 63 4.8
LIX 64 3.3
LIX 64N 2.9
LIX 70 2.6
KELEX 100 1.8
0
Initial Rate of Copper Extraction (g/L/min)
KELEX 100 0.98
LIX 64N 0.11
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KINETIC AND STABILITY CONSTANTSFOR THE REACTION OF Cu(II) AND Fe(III)
WITH HYDROXYQUINOLINES
Initial Rate ofExtraction with Kelex
100 (g/L/min)
III
Metal
Cu( )Fe( II)
0.980.067
Stability with8 HydroxyQuinoline
Log 2 = 23.0Log 3 = 36.9
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RATES OF EXTRACTION OF Cu(II) ANDFe(III) BY KELEX 100
0
25
50
75
100
0 50 100 150 200
Stirring time, min
Extraction with
10% KELEX 100
(%)
Cu
2+
from pH 1 solutionCu
2+from pH 2 solution
0
25
50
75
100
0 50 100 150 200
Stirring time, min
Extraction with
10% KELEX 100
(%)
Cu
2+
from pH 1 solutionCu
2+from pH 2 solution
Fe3+
from pH 1 solution
Fe3+
from pH 2 solution
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61575910005552508020
49483940604038200100
CuCuCu432 h24 h1 hmol %mol %
Metal Extracted (%)Salicyl-aldoxime
LIX65N
RATES OF EXTRACTION OF Cu(II) ANDFe(III) BY LIX65N AND ITS PRECUSOR,
SALICYLALDOXIME
Fe21
21
Fe21
22
Fe21
0.30.1
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THE MORAL OF THE STORY
If you want to win a SEx war, it is
better to be slow and selectivethan to be fast and flirtatious.
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THERES NOTHING BASIC ABOUT
BASIC IRON SULPHATE
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BACKGROUND
Basic iron sulphate (BFS) is a solid compound that isformed under certain conditions during the oxidation ofpyrite or other iron sulphide minerals with oxygen at hightemperatures in an autoclave.
Iron sulphide minerals are oxidized in an autoclave toproduce ferric sulphate and sulphuric acid in solution. Theferric sulphate then hydrolyzes slowly, precipitating backout of solution as hematite and/or BFS.
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OXIDATION
2FeS2 + 702 + 2H2O 2FeSO4 + 2H2SO4
4FeSO4 + 2H2SO4 + O2
2Fe2(SO4)3 + 2H2O
Overall:
4FeS2 + 1502 + 2H2O 2Fe2 (SO4)3 + 2H2SO4
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HYDROLYSIS
Ferric sulphate hydrolyzes to hematite at higher temperaturesand lower acidity
Fe2(SO4)3 + 3H2O Fe2O3 + 3H2SO4
and it hydrolyzes to BFS at lower temperatures and higheracidity
Fe2(SO4)3 + 2H2O 2Fe(OH)SO4 + H2SO4
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OXIDATION AND HYDROLYSIS
Overall reaction for the oxidation of pyrite to ferricsulphate followed by hydrolysis to hematite
4FeS2 + 15O2 + 8H2O 2Fe2O3 + 8H2SO4
Overall reaction for the oxidation of pyrite to ferricsulphate followed by hydrolysis to BFS
4FeS2 + 15O2 + 6H2O 4Fe(OH)SO4 + 4H2SO4
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Stability domains of compounds of the ferric ion inwater as a function of temperature and pH
20
60
100
140
180
220
260
0 2 4 6 8 10 12
pH
Temp
(C)
Fe3+
Fe (OH)3
Goethite
FeO.OH
Hematite
Fe2O3
Basic Iron Sulphate
Fe(OH)SO4
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WHY IS BFS BAD NEWS IN ACYANIDATION CIRCUIT?
BFS is not basic, it is actually acidic..and it must be
neutralized before cyanidation
The rate of release of acid by BFS is extremely slow inweakly acidic solution (pH 7 Fe(OH)3 + CaSO4
pH3.5
very slowFe(OH)SO4 + CaCO3 + H2O Fe(OH)3 + CaSO4 + CO2
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WHY IS BASIC IRON SULPHATE BADNEWS IN CYANIDATION CIRCUITS
For health and safety reasons related to HCN formation,the BSF must be fully neutralized prior to cyanidation.This will take 12-24 hours and add significantly to plantcapital cost.
As a result, most of the sulphate generated in theautoclave has to be neutralized with hydrated lime, ratherthan limestone. Lime can be at least 10 times the price oflimestone.
If not dealt with appropriately, the increased capex and
opex associated with BFS formation could eliminate POXfrom consideration for many refractory gold projects
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WHAT IS THE BEST SOLUTION?
THE HOT CURE PROCESS
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The basis of the hot cure process is the fact that thehydrolysis reaction that produces BFS in the autoclave isreversible at lower temperatures:
BFS Formation
BFS Decomposition
THE HOT CURE PROCESS
Fe2(SO4)3 + 2H2O Fe(OH)SO4 + H2SO4T>150C
Fe(OH)SO4 + H2SO4 Fe2(SO4)3 + 2H2O
90-140C
fastRT
very slow
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2Fe(OH)SO4 + H2SO4 Fe2(SO4)3 + 2H2O
THE HOT CURE PROCESS
20
60
100
140
180
220
260
0 2 4 6 8 10 12
pH
Temp
(C)
Fe3+
Fe (OH)3
Goethite
FeO.OH
Hematite
Fe2O3
BFS
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THE HOT CURE PROCESS
Once the basic iron sulphate has decomposed to ferric
sulphate, it can be separated from the solids by CCD orfiltration, and neutralized with limestone
Fe2(SO4)3 + 3CaCO3 + 3H2O 2Fe(OH)3 + 3CaSO4 +3CO2
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NEUTRALIZATION OF THE ACIDAND SULPHATE WITH LIMESTONE
(1) Fe2(SO4)3 + 3CaCO3 +H2O 2FeO.OH + 3CaSO4 + 3CO2
(2) Fe2(SO4)3 + 3CaCO3 +3H2O 2Fe(OH)3 + 3CaSO4 + 3CO2
20
60
100
140
180
220
260
0 2 4 6 8 10 12 14
pH
Temp
(C)
Fe3+
Hematite
Fe2O3
BFS
Goethite
FeO.OH
Fe(OH)3
(1)
(2)
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A REFRACTORY GOLD POX FLOWSHEETINCORPORATING HOT CURING
Oxygen
Concentrate
Pressure
Oxidation
Hot Cure
Solid/LiquidSeparation
SolidLiquid
Solid/Liquid
Separation
Neutralisation
Solid
CaSO4Fe(OH)3
Cyanide
Leach
Gold
Recovery
Cyanide
DestructionTailings
Steam90 100 C4 to 12 hours
CO2
NaCN
Ca(OH)2
Liquid
Base MetalRecovery ?
CaCO3
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QUIMSACOCHA PROJECT, ECUADOR(IAMGOLD CORPORATION)
Fe (mass/mass) = 18.1%
SO4 (mass/mass) = 32.0%Fe/SO4 = 0.57
0
10
20
30
40
-3 0 3 6Time at 90C (hr)
Fe
SO4
ACD
Conc. InAutoclaveDischarge
Solids(%)
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QUIMSACOCHA PROJECT
0
10
20
30
40
50
60
70
-3 0 3 6
Time (hr)
Autoclave
Discharge Solution(g/L)
Fe
H2SO4
ACD
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QUIMSACOCHA PROJECT
Concentrate head grade: 24 g/t Au, 104 g/t Ag
PRODUCT RECOVERY ALKALI CONSUMED APPROX. COST
Au % Ag % CaCO3 kg/t Ca(OH)2 kg/t $/tAutoclaveDischarge
99.6 94.8 370 260 43
Hot CureDischarge
99.4 91.9 704 15 9
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