Binderless briquetting of some selected South African...

eology 71 (2007) 303–312www.elsevier.com/locate/ijcoalgeo

International Journal of Coal G

Binderless briquetting of some selected South African prime coking,blend coking and weathered bituminous coals and the effect of coal

properties on binderless briquetting

S.J. Mangena a,⁎, V.M. du Cann b,1

a Division of Mining Technology, CSIR, PO Box 395, Pretoria, 0001, South Africab Coal and Mineral Technologies (Pty) Ltd, SABS, PO Box 73656, Lynnwood Ridge, Pretoria, 0040, South Africa

Received 18 April 2006; received in revised form 7 November 2006; accepted 8 November 2006Available online 14 December 2006

Abstract

The binderless briquetting of some selected South African prime coking and blend coking coals, as well as the effects ofweathering on the binderless briquetting of some inertinite-rich bituminous coals, were investigated in the laboratory. Selectedproperties of these coals were determined and the coals were briquetted at various moisture contents and pressures. Based on theresults obtained in this study, binderless briquetting was found to be most successful in the cases of the fresh, vitrinite-rich cokingand blend coking coals and satisfactory in the fresh inertinite-rich Witbank coals.

However, the bonding process seemed to be adversely affected by weathering. The negative impact on bonding could beameliorated to some extent by the presence of a significant kaolinite content when the percentage ash reports in the order of morethan 15% (air-dry basis). It should, however, be noted that kaolinite may reduce the water resistance of the briquettes.© 2006 Elsevier B.V. All rights reserved.

Keywords: Binderless coal briquetting; Weathering; Reactive macerals; Kaolinite

1. Introduction

Coal mining and processing operations inevitablyresult in large accumulations of fine coal (i.e. b1 mm),which constitute up to 12% of the annual Run of Mine(RoM) tonnage in South Africa. Although this materialcan be beneficiated efficiently using spirals, dense-

⁎ Corresponding author. Present address: Sasol Technology R&D;Coal and Syngas Technologies; P.O. Box 1; Sasolburg, South Africa;1947. Tel.: +27 16 960 2041; fax: +27 11 522 2356.

E-mail address: [email protected] (S.J. Mangena).1 Present address: PETROGRAPHICS SA, Suite 155, Private Bag X

025, Lynnwood Ridge, Pretoria, 0040.

0166-5162/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.coal.2006.11.001

medium cyclones and froth flotation, the products,particularly the ultra-fine coal (i.e. b100 μm), which isthe focus of this study, are very difficult to handle.

Until recently, in South Africa, ultra-fine coal wasdiscarded in slimes dams and old underground work-ings. This is now considered to be environmentally lessacceptable. With the world striving for sustainabledevelopment, the optimal and efficient utilisation ofenergy resources is an important issue. Ultra-fine coal isone such resource that can be converted into a valuablesource of energy. If not oxidised, the material can bebeneficiated efficiently using flotation methods.

However, beneficiation methods – flotation inparticular – leave the coal with a moisture content in

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http://dx.doi.org/10.1016/j.coal.2006.11.001

Table 1Some chemical and physical properties (air-dry basis) of the Sample A and Sample B coals tested

Colliery Product CV (MJ/kg) %Ash %H2O %Volatile matter %S % Fixed C FSI HGI

Sample A Flotation concentrate (coking) 32.02 10.9 0.9 25.6 0.80 62.6 9½ 91Sample B Spiral (coking) 29.12 10.1 2.8 35.9 0.90 51.2 3½ 56

HGI = Hardgrove grindability index.FSI = free swelling index.

304 S.J. Mangena, V.M. du Cann / International Journal of Coal Geology 71 (2007) 303–312

the region of ∼70%. The equipment most commonlyused for dewatering this material is a filter press, whichreduces the surface moisture of the coal to about 25%.However, the reduction in the moisture content of thismaterial by the filter press is still not sufficient as, byvirtue of the high surface area of the particles, theproduct remains difficult to handle and transport (i.e. itis sticky and therefore blocks the chutes, transfer points,silos and rail trucks).

In fact, a relatively high moisture content is a majorproblem associated with fine and ultra-fine coal. InSouth Africa, most of the export contracts for steam coalspecify the heating value of coal on a gross as received(GAR) or net as received (NAR) basis, which impliesthat the moisture content of the coal becomes a verycritical factor in the quality of the coal delivered to theport. It is mainly for this reason that the ultra-fine coalhas in the past been discarded at most collieries.

The maximum total moisture content required forultra-fine coal to meet the quality specifications forexport steam coal is ∼10%. For fine and ultra-fine coal,thermal drying is the only method that can be used toreach this moisture content of 10%. However, whentransported in uncovered trucks and stockpiled outdoorsduring rainy seasons, the coal may be re-wetted, thusleading to the aforementioned problems.

In addition, if the material is too dry, it may bedispersed by wind, thus leading to a loss of product andenvironmental problems. Also, some coal fines areliable to spontaneous combustion subsequent to oxida-tion when stockpiled or stored in bunkers or silos. It istherefore necessary that this product be economicallyagglomerated into regular-sized lumps so as to providebulk, ease of handling and mechanical strength.

The economic agglomeration of coal has been underinvestigation, as part of Coaltech 2020 (a South African

Table 2Petrographic properties of Samples A and B coal

Colliery % Vitrinite % Liptinite % Reactive inertinite % Inert in

Sample A 91 1 1 7Sample B 86 6 3 5

Note: Maceral analysis — percentage by volume, mineral matter-free basis

coal mining collaborative research programme), since1999. Various methods of agglomeration, including butnot limited to binder briquetting, pelletising andbinderless briquetting, have been investigated (England,unpublished). Of all the agglomeration methods inves-tigated, binderless briquetting was found to be the mosteconomically feasible method and was hence investi-gated on a laboratory scale.

The initial study was aimed at investigating theamenability of some Witbank ultra-fine coals to binder-less briquetting (Mangena et al., 2004). The coals werefound to be amenable to binderless briquetting and thebonding in the binderless briquettes was found to bestrongly dependent on the presence of clay minerals inthe coals, particularly kaolinite, the effect of whichseemed to come into play when the ash content was in theregion of ∼15% (air-dry basis).

The reaction of coal with atmospheric oxygen, underparticular environmental conditions involving changesin temperature and moisture, results in the alteration ofthe chemical and physical properties of the coal (Nelson,1989). It is a well-known and documented fact that thesechanges in the physical and chemical properties of coaldue to weathering adversely affect the technologicalprocesses associated with the beneficiation and utilisa-tion of coal (Laskowski, 2001; van Krevelen, 1993).

This study was aimed at investigating the behaviourof some South African prime coking and blend cokingcoals during binderless briquetting and the effect ofweathering on the binderless briquetting process. Toachieve this aim, the following experimental strategywas applied:

(i) determine selected chemical, mineralogical andphysical properties, as well as some petrographic(i.e., microscopic) characteristics, of the coals

ertinite % Total inertinite % Reactives R̄r% Rank

8 93 1.26 Medium rank B8 95 0.72 Medium rank C

(mmf ).

Table 3Mineralogical properties of Samples A and B coal

Colliery % Pyrite % Quartz % Calcite % Dolomite % Kaolinite % K Feldspar % Siderite % Plagioclase

Sample A 0.8 2.7 0.3 2.6 3.1 0.4 1.0Sample B 6.0 0.4 2.9 0.8

Table 5Physical properties of the Sample A coal binderless briquettes

Pressure(Mpa)

% H2O Averagecompressivestrength (kPa);90% CL

σ forn=20

Waterresistance /Wet strength(kPa);90% CL

σ forn=10

305S.J. Mangena, V.M. du Cann / International Journal of Coal Geology 71 (2007) 303–312

(ii) briquette the coals at various feed moisturecontents and pressures

(iii) determine some physical properties (i.e. compres-sive strength and water resistance, also called ‘wetcompressive strength’) of the briquettes, andrelate them to the properties of the coals and tothe briquetting conditions.

2. Methodology

2.1. Samples studied

The following three samples were obtained fromthree different coalfields.

Sample A was a flotation concentrate of a primecoking coal from the Soutpansberg coalfield.

Sample B was a spiral product of a blend cokingcoal from the Waterberg coalfield.Sample C was a rawweathered coal, obtained from an old mine pillar, fromthe Witbank coalfield.

2.2. Apparatus

Pillow-shaped briquettes were produced from aKomarek B-100A roll-type briquetting machine, witha roll diameter of 130 mm and a maximum allowablepressure of 17 Mpa. The machine consisted of two rollsmounted on the ends of shafts cantilevered betweenbearing blocks. The main load carrying bearings weremounted close to the inside face of the rolls, and asecond bearing was mounted near the back of each shaft.The lower roll, shaft and bearing block assembly were

Table 4Particle size distribution of the Sample A coal

Particle size (μm) Fractional (% W/W)

+2 mm 3.4−2+1 mm 9.5−1 mm+500 18.1−500+300 15.2−300+212 8.7−212+106 20.1−106+75 7.8−75 17.2

held in a fixed position and could rotate about a pivotpoint located at the rear-bearing block. The hydrauliccylinders controlled the movement of the upper bearingblock and in turn pressed the two front bearing blocksagainst fixed stops located behind the tie bars. The stopsprevented rolls from making contact with each other.

Rolls were driven by a pinion and set of companiongears mounted in the rear of the bearing blocks. Thepaddle feeder agitated the material into the horizontalscrew at the feed inlet that fed it into the rolls. Thehorizontal screw was driven by a variable speed driveunit and the paddle feeder by a constant speed gearmotor.

The Chatillon model UTSE-2 tensile strength testinginstrument, operated in a compression mode, was usedfor the compressive strength tests.

2.3. Test methods and procedure

The chemical and petrographic characteristics (or-ganic composition, i.e. maceral analysis and reflectanceof vitrinite) and some physical properties of the coalsamples were determined according to the relevantstandard methods at the laboratories of Coal andMineralTechnologies (CMT).

7 0.9 (AD) 651±33 85.7 362±20 33.510 0.9 (AD) 794±37 98.2 466±27 a71.012 0.9 (AD) 831±50 133.7 548±38 62.814 0.9 (AD) 861±43 114.6 565±28 a73.817 0.9 (AD) 1112±90 238.3 694±31 a82.217 5.5 570±18 29.0 298±28 45.717 10.0 419±34 55.1 206±25 41.717 13.1 250±30 48.5 189±17 28.017 17.4 213±31 51.1 138±14 22.8

AD = air dry.σ = standard deviation.CL = confidence limit.a n=20.

Fig. 2. Compressive strength of briquettes against superficial moisturecontent of the Sample A.


Qualitative and quantitative determinations of theminerals in the coal samples were carried out at thelaboratory of the Council for Geoscience (CGS),Pretoria South Africa. The samples were subjected towhole rock powder X-ray diffraction (XRD) analysisusing a Siemens D500 diffractometer equipped with aCu tube and a graphite monochromator. Randompowder preparations, obtained by pressing the powderin a plastic frame with a round cavity against a roughfilter paper, were scanned from 2 to 65°2θ, with a0.02°2θ step size and a counting time of 1 s/step at atube voltage and current of 40 kVand 30 mA. The XRDpatterns were evaluated using Siemens “DIFFRACplus” software. The latter allows matching with thePDF-2 Database, of which sets 1–42 were at hand(ICDD, 1992). The semi-quantitative analysis was basedon peak height percentages (Brime, 1985). For mostminerals, their strongest reflection was used. However,in the case of quartz and albite, for which the mostintense peaks often overlap, other reflections werechosen.

Sample A, the prime coking coal, was air-dried,homogenised and split into four portions of ∼5 kg each.The four 5-kg portions were then wetted with cold tapwater to various feed moisture contents and thereafterbriquetted at 17 Mpa using the Komarek B100Abriquetting press. Various feed moisture contents wereachieved by physically adding cold tap water to the coal,and homogenising the mixture for about 15 min using aLödige automatic mixer. Prior to briquetting, thesuperficial moisture of the feed coal was measured bydrying a representative portion of the wetted sample at40 °C overnight and expressing the mass loss as apercentage. Five more batches of briquettes wereproduced at 7, 10, 12, 14, and 17 Mpa, respectively,from the remaining air-dried sample.

Fig. 1. Correlation between briquetting pressure and dry compressivestrength of briquettes produced from the air-dried Sample A.

The air-dried Sample B (blend coking) coal wasmilled to b500 μm, split into homogeneous portions,which were wetted with cold tap water to various feedmoisture contents, and briquetted at 17 Mpa, as in thecase of the Sample A coal.

Sample C, a weathered, inertinite-rich Witbank coal,was split into two portions of about 25 kg each. Oneportion of 20 kg was milled to b300 μm and briquettedat various feed moisture contents and a pressure of17 Mpa. The other portion of the sample was upgradedto less than 15% ash (air-dry basis) using dense medium(made up using organic solvents) separation. The up-graded product was then milled and briquetted as above.To measure the degree of weathering, the Sample C coalwas investigated petrographically and microphoto-graphs of selected components were taken.

All the briquettes produced in this study were pillowshaped (40 mm (Length)×19 mm (Breadth)×13 mm(thickness)). They were tested for dry compressivestrength and water resistance (also called ‘wet compres-sive strength’) after being air-dried indoors for morethan two days.

To test for dry compressive strength, the crushingload (in N) was measured on 20 briquettes from eachbatch and the average value converted to compressivestrength (in kPa) with the aid of Eq. (1) (Richards,1990). The minimum target value for this technique was350 kPa (Clark et al., 1998).

P ¼ Crushing load ðNÞ � 1000

Cross� sectional area of plane of fracture ð760 mm2Þð1Þ

To test for water resistance, 10 briquettes from eachbatch were immersed in water for 2 h and their crushingloads measured immediately after removal from thewater. Eq. (1) was again employed to convert the average

Fig. 3. Photomicrograph of briquette produced at 17 Mpa from the air-dried Sample A coal, showing deformation and subsequent consolidation ofvitrinite particles.


maximum crushing load to wet compressive strength,which can also be expressed as water resistance.

3. Results and discussion

3.1. Coking and blend coking coals

Some chemical, physical, petrographic and mineral-ogical properties of the coking and blend coking coalstested are shown in Tables 1–3. Sample A is known tobe one of South Africa's prime coking coals, withunique characteristics. It is a vitrinite-rich, medium-rankB coal with a free swelling index (FSI) of 91/2 and aHardgrove grindability index (HGI) of 91, which ren-ders the coal highly friable.

Although vitrinite-rich, Sample B differs fromSample A in the fact that it is medium rank C (spe-cifically R̄r% of 0.72), with a lower FSI of 31/2 and HGIof 56. This coal is used mainly as a blend coking coal bythe metallurgical industry.

3.1.1. Sample A — prime coking coalThe particle size distribution of Sample A coal and

the data for the physical properties of the briquettesproduced from this coal are shown in Tables 4 and 5.

Table 6Particle size distribution of Sample B coal

Particle size (μm) Fractional (% w/w)

+500 1.3−500+300 20.1−300+212 29.2−212+106 29.9−106+75 11.6−75 7.9

The effect of pressure on the compressive strength of thebriquettes is shown in Fig. 1. The R2 value of 0.93 inFig. 1 implies a direct correlation between the twovariables; this is in line with the literature (Clark et al.,1998).

The briquettes produced from the air-dried Sample Acoal were very strong and also exceeded the minimumrequirements for both the dry compressive strength andwater resistance (i.e. 350 kPa). However, the addition ofwater to the feed coal led to a drop in the compressivestrength of the briquettes (Fig. 2). Briquettes producedfrom samples with ≥13% moisture did not meet theminimum requirements for dry compressive strength.

Briquettes produced from samples with feed mois-ture contents in the range 5–10% met the minimumrequirements for dry compressive strength. It appearsthat for the coal studied, and in terms of dry compressivestrength, binderless briquetting is favoured at a feedmoisture of b10%. This is in line with the literature andthe test work done at the CSIR, South Africa (Clarket al., 1998 and Coaltech 2020 project 4.4.1, unpub-lished work).

The water resistance of the briquettes produced fromsamples with feed moisture contents in the range 5–10%was lower than the required minimum value (Fig. 2).

Table 7Physical properties of briquettes produced from Sample B coal

% H2O Compressivestrength (kPa);90% CL

σ forn=20

H2O Resistance/Wet strength (kPa);90% CL

σ forn=10

2.8 (air dry) 545±36 62.2 206±10 16.94.4 866±85 231.3 408±20 34.811.9 390±18 31.0 151±16 27.116.2 302±21 35.8 148±16 27.8

Fig. 4. Plot of the mean dry and wet compressive strengths ofbriquettes against feed moisture content of Sample B.

Table 8Some chemical properties (air-dry basis) of Sample C coal

Raw coal Upgraded coal

CV (MJ/kg) 25.73 26.99H2O% 4.1 3.7Ash % 17.7 13.4Volatile matter % 21.9 22.0Fixed carbon % 56.3 60.9


However, when crushed during the water resistancetests, the briquettes broke into about 90% of pieces that,although not tested, appeared to be N2 mm in size. Thissuggested that the briquettes would still be handleablewhen moist.

A microphotograph of a Sample A binderlessbriquette produced from air-dried coal at 17 Mpa isshown in Fig. 3. According to the literature, vitrinite,due to its plastic behaviour, becomes deformed duringcompaction, thereby assisting with the bonding duringbinderless briquetting (Gregory, 1960).

Given the very high vitrinite content of 91% (mineralmatter-free basis), the level of maturity (1.26% Rr), theHardgrove grindability index (HGI) of 91, the freeswelling index (FSI) of 91/2 and the ash content of10.9% (air-dry basis), it is strongly suggested that the

Fig. 5. Photomicrograph of Sample B binderless coal briquette showing t

bonding in the Sample A briquettes was very stronglydependent on the very high vitrinite content. This isconfirmed by the deformation and subsequent consol-idation into cohesive masses of small vitrinite particles,after the compaction process, as illustrated in Fig. 3.

Compared with all the South African coals tested atthe CSIR South Africa to date, the Sample A coalappeared to be the best-case scenario (Coaltech 2020project 4.4.1, unpublished work). Due to its plasticbehaviour, the coal could even briquette relatively wellat a pressure of 7 Mpa (see Table 5) as compared withthe Witbank coals which formed strong briquettes whenbriquetted at a much greater pressure of N17 Mpa.

3.1.2. Sample B — blend coking coalThe particle size distribution of the Sample B coal is

shown in Table 6. The results for the physical propertiesof the briquettes produced from the Sample B coal withvarying feed moisture contents are shown in Table 7 andillustrated in Fig. 4. The briquettes produced from theair-dried sample met the minimum requirements for drycompressive strength. The optimum feed moisturecontent for this coal sample was attained at ±5%. Atthis feed moisture content, the briquettes not only metthe stated minimum requirements for dry compressive

he deformation and consolidation of vitrinite and liptinite macerals.

Table 9Petrographic properties of the raw Sample C coal

% Vitrinite % Liptinite % Reactive inertinite % Inert Inertinite % Total Inertinite % Heat-affected % Total reactives R̄r%

18 3 26 51 77 2 47 0.82

Note: Maceral analysis — percentage by volume, mineral matter-free basis (mmf).

Table 10Mineralogical composition of the raw Sample C coal

Kaolinite(%) Quartz(%) Pyrite(%) Anatase (%)

9.6 7.3 0.5 0.4


strength but also surpassed the minimum criteria forwater resistance. Further addition of water to the feedcoal reduced the quality of the briquettes.

When crushed during the water resistance test,briquettes that met the minimum requirements for drycompressive strength failed to meet the minimumcriteria for water resistance. Although not tested, theybroke into ca. 90% of pieces with a particle size ofN2 mm. This suggested that they would still behandleable when moist.

Like Sample A, the bonding in Sample B binderlessbriquettes seemed to have resulted from the high re-active macerals content (i.e. vitrinite, liptinite plus reac-tive inertinite), particularly the vitrinite content of 86%(mmf). This can be confirmed by the microphotograph(Fig. 5) of the binderless briquette produced at optimumconditions. The picture shows the deformation, subse-quent consolidation and integration into cohesive

Table 11Particle size distribution of Sample C coal

Size (μm) Fractional (% w/w)

Raw coal Upgraded coal

−212+106 18.0 3.6−106+45 39.0 44.9−45+25 13.0 7.8−25 30.0 43.7

Table 12Condition petrographic analysis of the raw Sample C coal

Fresh coal particles % 64Pyrite% (normal) 2Pyrite% (oxidised) 0Particles with extensive cracks or fissures% 21Severely weathered particles% 10Particles with desiccation cracks% 2Severely heat-altered char or anthracite% 1Total abnormal particles% 34

masses of the reactive macerals after the compactionprocess. The fact that the briquettes did not disintegrate,but instead maintained their strength to beyond theminimum value required during water immersion, mayalso serve to support the above statement since thereactive macerals are known to be hydrophobic.

3.2. Sample C — weathered inertinite-rich Witbankcoal

Some chemical and petrographic properties, and themineralogical composition of Sample C tested in thisstudy are shown in Tables 8–10. The particle sizedistribution of the raw and upgraded samples is shownin Table 11.

Table 12 shows the degree of weathering of the rawSample C coal tested. The sample had 64% of freshparticles and 34% of abnormal particles. The reflectanceof the vitrinite particles averaged 0.82%, which isslightly higher than normal (i.e. the average vitrinitereflectance for the Witbank coals is ∼0.7% whichrenders them to be of medium rank C). Based on thevitrinite reflectance and the petrographic conditionanalyses (see Fig. 6 and Table 12), it appears that thecoal was not severely weathered. Figs. 7–9 illustratefeatures of this coal.

The physical properties of the binderless briquettesproduced from the Sample C tested are shown inTable 13 and depicted in Fig. 10. The raw coal formedstrong briquettes, which met the minimum criterion fordry compressive strength (i.e. 350 kPa). However, whentested for water resistance, the briquettes disintegrated to

Fig. 6. Vitrinite random reflectance histogram of Sample C coal.

Fig. 7. Fresh coal macerals of Sample C coal. Fig. 9. Coal char form displaying devolatilisation pores and increasedlevels of reflectance (indications of abnormal thermal effects) inSample C coal.


form slurry in water. The upgraded coal only formedvery weak briquettes at superficial moisture contents of∼5 and 7% respectively. The briquettes formed at ∼5%superficial moisture were, however, not water-resistantas they disintegrated to form a slurry in water. Thebinderless briquettes produced at ∼7% superficialmoisture were abraded along the edges and somebroke into smaller pieces, although a few retainedtheir shape subsequent to 2 h of water immersion.

The weathering of coal is known to reduce theplasticity of the coal macerals (Kruszewska and DuCann, 1996). The plasticity of the reactive macerals,particularly vitrinite, under pressure plays a crucial rolein binderless briquetting. As shown in Fig. 10, incontrast with the raw Sample C coal, the upgradedSample C coal did not form briquettes that met theminimum requirements for dry compressive strength(i.e. 350 kPa). However, the presence of at least somesufficiently fresh vitrinites in the upgraded Sample Ccoal, in combination with the kaolinite content (albeitreduced), appears to have been responsible for the weakbonding of the briquettes in this case.

Fig. 8. Extensive cracks and fissures (a result of weathering) in SampleC coal.

The raw Sample C coal, on the other hand, formedbinderless briquettes that met the minimum require-ments for dry compressive strength, although they werenot water resistant. It has been shown in previous studiesthat clay minerals, particularly kaolinite, play an impor-tant role in binderless briquetting, more especially if theash content is in the region of ∼15% (air-dry basis) orgreater (Mangena et al., 2004). This is mainly due to theplastic nature of kaolinite when mixed with water (Deeret al., 1997). Table 10 shows the predominance of kao-linite in the raw Sample C coal. It is therefore stronglysuggested that the bonding in the binderless briquettesproduced from the raw coal resulted mainly from theinfluence of the kaolinite content.

Some inertinite-rich coals from the Witbank coal-field have been tested at the CSIR South Africa, as partof the Coaltech 2020 programme, on both laboratoryand pilot scales (Coaltech 2020 project 4.4.1, unpub-lished work). The results obtained to date showed thatthe coals are amenable to binderless briquetting,

Table 13Physical properties of the briquettes produced from the Sample C coal

%Superficialmoisture

Compressivestrength(kPa);90% CL

σ forn=20

H2OResistance/Wet strength(kPa);90% CL

σ forn=10

0 371±28 75.1 0Raw coal 5.0 441±30 82.2 0

7.5 399±20 54.2 0Upgradedcoal

0 0 0 0

5.0 97±5 14.1 07.5 214±17 47.6 89±5 12.4

Fig. 10. Compressive strength of the Sample C coal binderlessbriquettes.

Table 14Selected properties of the KKC and KFNC flotation concentrates thatare important in binderless briquetting

KKC KFNC

% Ash (AD) 15.1 13.8%H2O (AD) 3.2 2.8% Volatile matter (AD) 22.8 23.6% Fixed carbon (AD) 58.9 59.8CV (MJ/kg) (AD) 27.40 27.56% Vitrinite (mmf) 30 28% Total Inertinite (mmf ) 67 68% Total reactive macerals (mmf) 55 53Mean R̄r% 0.75 0.73% Kaolinite 9.1 8.0

AD = air-dry basis.mmf = mineral matter-free basis.


irrespective of the grade as determined by the ashcontent. Examples of these coals are the flotationconcentrates of KFNC and KKC coals (see Fig. 11),which were from the Witbank coalfield. Some proper-ties of these coals, which are important in binderlessbriquetting, are shown in Tables 14 and 15.

Although it is acknowledged that the KFNC coal wasparticularly fine (see Table 15) as compared with theSample C coal, one of the major differences in terms ofthe microscopically observed characteristics of theorganic constituents between the KFNC, KKC andSample C coals was the significant proportion of imme-diately recognisable weathering features in the SampleC macerals. The fresh KFNC and KKC coals formedbriquettes that met the minimum criterion for dry com-pressive strength. The water resistance of the briquetteswas also satisfactory as during the test the briquettesbroke into large pieces (i.e. ∼90% of N2 mm particles)that were handleable.

Fig. 11. Compressive strength of the KFNC and KKC binderlessbriquettes.

It may therefore be concluded that the weathering ofthe Sample C coal had adverse effects on its potential forbinderless briquetting.

With the declining reserves of “virgin” good-qualitycoals, some collieries in the Witbank coalfield arefocusing on the extraction of old underground coalpillars. The fact that coal has been exposed to the at-mosphere for many years may render it weathered tovarious extents. In addition, the fact that weatheringrenders the coal more friable, while adversely affectingits amenability to froth flotation and oil agglomeration,poses a challenge to the coal preparation engineers asmore fines and more burning coal will have to behandled in the coal preparation plant (De Korte, 2004).

Hence, other methods of beneficiating weathered fineand ultra-fine coal (e.g. gravity-based processes) have tobe investigated. To make the beneficiated weatheredfine coal handleable, other methods of agglomeration,such as pelletising and extrusion, have to be consideredsince binderless briquetting, although relatively lesscostly, seems not to be the solution. However, the rawultra-fines can be briquetted without the use of bindersand be supplied to the local utility generators, providedthe economics are justifiable.

Table 15Particle size distribution of the KKC and KFNC flotation concentrates

KKC KFNC

−1 mm+500 μm% 6.9 0.0−500+212 μm% 28.2 0.0−212+106 μm% 16.4 0.0−106+75 μm% 3.8 0.0−75+45 μm% 6.1 0.4−45+25 μm% 3.0 14.7−25 μm% 35.6 84.9


4. Conclusions

Although the findings apply only to the coals studied,they may be at least qualitatively applicable to manyother South African bituminous coals. Based on theresults obtained in this study, the following conclusionsmay be drawn:

➢ Binderless briquetting was found to be mostsuccessful in the cases of the fresh, vitrinite-richcoking and blend coking coals. Bonding in boththe coking coal binderless briquettes was found tobe due mainly to the deformation and subsequentconsolidation of the very high proportion of freshvitrinite macerals in these two coals.

➢ As in other technological processes associatedwith the beneficiation and utilisation of coal,weathering was found to have an adverse effect onthe amenability of coal to binderless briquetting.This negative influence could be ameliorated tosome extent by the presence of a significantkaolinite content when the percentage ash reportsin the order of ∼15% (air-dry basis).

It is hoped that this study will assist the coal industryin further understanding the behaviour of various typesand grades of coal during binderless briquetting.

Acknowledgements

This work was part of Task 4.4.1 of the Coaltech2020 research program (a South African collaborativemining research initiative — http://coaltech.csir.co.za)which is highly acknowledged for permission to publishthe work.

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