International Journal of Coal Geology 94 (2012) 225–237
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International Journal of Coal Geology
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Distribution of selected trace elements in density fractionated Waterbergcoals from South Africa
N.J. Wagner ⁎, M.T. TlotlengCoal and Carbon Research Group, School of Chemical and Metallurgical Engineering, University of the Witwatersrand Johannesburg, Private Bag 3, Wits 2050, South Africa
TheWaterberg Coalfield, located in the Limpopo Province (north-western area) of South Africa, contains a sig-nificant proportion of the South African coal resource. This Permian coalfield typically contains high vitrinite,high ash coals in the Kungurian Grootegeluk Formation, and high inertinite, low ash coals in the ArtinskianVryheid Formation. Four run-of-mine coals and density fractioned samples were analysed to determinetheir trace element content. The concentrations of most trace elements in the run-of-mine coals exceed theglobal averages and certain global ranges, and generally exceed values reported for other South African coal-fields. Specifically, Hg concentrations are high to very high in all the coals (up to 2.43 ppm in a sample from theVryheid Formation), Cd and Se concentrations are comparable to or lower than global averages, and As is verylow in the Vryheid Formation (1.57 ppm). A sample from the Grootegeluk Formation is enriched in siderophileelements, and a sample from the Vryheid Formation is depleted in chalcophile elements. Studies of the densityfractionated samples indicate that Cd exhibits an organic affinity in the Grootegeluk Formation samples, andpossibly a pyritic (or sulphide) affinity in the Vryheid Formation sample. Selenium has a greater affinity forthe middle, clay-rich density fractions in all samples. Arsenic reports to the mineral-rich sink fractions, specif-ically the fractions enriched in pyrite. Mercury reports to both the float and sink fractions, indicating an organ-ic and inorganic affinity in these coals, although there is more enrichment in the sink fractions in theGrootegeluk Formation samples, and a definite organic correlation in the Vryheid Formation sample.
Coal is expected to be the dominant source of fuel for South Africafor the foreseeable future, generating around 88% of the national elec-tricity demand (to 2030) (Dempers, 2006). Approximately one-thirdof the mined coal is exported as a low ash, beneficiated product,and the mineral-rich middlings fraction and poor quality run-of-mine (ROM) coal is used for power generation. As the coal reservesbegin to decline, lower-grade, mineral-matter rich coals are beingused in increasing amounts for power generation and in small scaleindustrial boilers in South Africa. Stringent environmental legislationand potentially heavy financial penalties associated with exceedingregulatory limits will force coal utilization facilities to reduce their en-vironmental footprint for the benefit of human health and the envi-ronment. Coal producers claim it is not feasible to mine selectively,and hence coal cleaning (beneficiation) plays a key role in improvingthe quality of the coal to meet the requirements of the utilization pro-cess and environmental compliance as effectively as possible.
The combustion of coal may lead to the discharge of volatiletrace elements into the atmospheric environment. Trace elements
er).
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originating from coal utilization that are believed to be of majorconcern from a human health perspective include: As, Cd, Hg,Pb, and Se (Clarke and Sloss, 1992; Gibb et al., 2003). The Euro-pean Pollutant Emission Register (EPER) requires the reporting ofAs, Cd, Cu, Cr, Hg, Ni, Pb, and Zn (Gibb et al., 2003), and the USAClean Air Act Amendments Bill of 1990 lists 11 elements of po-tential concern, namely: As, Cd, Cr, Hg, Ni, Pb, Sb, Be, Mn, Se,and Co. Following combustion, volatile trace elements (such asHg, and As) and halogens (such as Br, Cl, and F) typically endup in the flue gas, while the less volatile trace elements (As,Cd, Ga, Pb, Sb, Sn, Te, Zn) vaporize, oxidize and, on cooling, con-dense on to fly ash particles (Hower et al., 2005; Suarez-Ruiz andWard, 2008). Trace elements (such as Hg) associated with sul-phide minerals are susceptible to becoming airborne pollutants,due to the fact that sulphide minerals vaporize during combus-tion (Xu et al., 2003). As the finest sized fly ash particles providethe greatest surface area, the volatilized elements tend to con-centrate on these particles, or are released in the gas phase.These pollutants (typically PM10 and finer) are the most difficultto collect in pollution control devices, and may escape up thestack into the atmosphere (Buhre et al., 2006).
Certain volatile, hazardous trace elements tend to have a chemicalaffinity for sulphide minerals, in particular: pyrite with As and Hg(Babu, 1975; Dai et al., 2006b; Diehl et al., 2004; Goodarzi, 2002;
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Hower et al., 2008; Kolker, 2011a, 2011b; Laban and Atkin, 1999;Luttrell et al., 2000; Pires et al., 1997; Ruppert et al., 2005; Senior etal., 2000; Toole-O'Neil et al., 1999; Xu et al., 2003; Yudovich andKetris, 2005a, amongst others). The chemical affinity, or mode of oc-currence, of trace elements, as well as their volatility, are largely con-sidered to be a controlling factor leading to the mobilization of theseelements during processes such as pyrolysis, combustion, and gasifi-cation (Ratafia-Brown, 1994). The mode of occurrence refers to theaffinity of a trace element towards minerals present in coal, or the or-ganic component. Pyrite and other reactive iron-sulphide mineralshave been found to provide sinks for trace elements introduced tothe coal from anthropogenic and natural sources (Burton et al.,2006). According to Yudovich and Ketris (2005a), Hg is a very ‘coal-phile’ element, having a strong affinity for the organic and inorganicmatter in coal, and it is bound to the organic or sulphide (typically py-ritic) mineral matter. Xu et al. (2003) collated the results obtainedfrom several studies conducted on coals from USA, Australia, andGreat Britain, and found that As, Cd, Co, Cu, Fe, Hg, Mo, Ni, Pb, S, Sb,Se, Ti, W, and Zn occur in iron sulphides, with Ba, Ca, Fe, and S report-ing to the sulphates. Data published by Pires et al. (1997) andGoodarzi (2002) agree with that presented by Xu et al. (2003),where Hg and As were found to be associated with sulphides. Labanand Atkin (1999) show that certain trace elements, such as Cu, Mn,Ni, Pb, and Zn, are associated with silicates/carbonates as well as py-rite. Diehl et al. (2004) found a positive correlation between Hg andorganic sulphur, as did Toole-O'Neil et al. (1999). The latter authorsfound that, whilst a significant portion of Hg is associated with pyrite,the remainder of the Hg (~50%) was detected in the lightest specificgravity fraction; that is, bound within the organic matter.
Arsenic enrichment in coals may be attributed to epigenetic hy-drothermal fluids, burial metamorphism, or groundwater runofffrom overlying volcanic strata (Yudovich and Ketris, 2005b). Studiesconducted by Kolker and Finkelman (1998) and Kolker et al. (2000)show that As is predominantly associated with pyrite. This is in agree-ment with the results published by Bergh (2009), Goodarzi (2002),Xu et al. (2003), Dai et al. (2011), and Kolker (2011b). In addition,Luttrell et al. (2000) associated Cd with pyrite, whilst Cr, Co, Pb, andNi were characterized primarily with ash forming minerals (silicates).Senior et al. (2000) determined a pyrite-As relationship in three ofthe four USA coals tested, but report an organic association or arse-nate occurrence in the fourth USA coal; this particular coal had avery low pyrite association. However, Riley et al. (2011) indicatedthat As in some Australian coals is not principally associated with py-rite, which probably is a consequence of the low pyrite content of thecoals or indicative of oxidation of some of the coals. Arsenic in theMukah coal from Sarawak, Malaysia, is mainly organic associated(Sia and Abdullah, 2011).
In terms of the mode of occurrence of Se, a review compiled byYudovich and Ketris (2006) confirms that Se can have different asso-ciations/forms in coal. Selenium is predominantly associated withsulphide and organic species, but other forms are known to occur,such as clausthalite and selenio-galena as reported by Hower andRobertson (2003) and Dai et al. (2006a), elemental Se, as well as fer-roselite (FeSe2). Diehl et al. (2004) determined that Se has a positivecorrelation with Hg and As in pyrite, and Senior et al. (2000) deter-mined a similar relationship in some USA coals. Senior et al. (2000)put this correlation down to the fact that Se and As both substitutefor S in the pyrite structure, as Se is more like S in its geochemical be-haviour than any other element in the periodic table.
A recent study conducted by Bergh et al. (2011) on coal samplesfrom the Witbank No. 4 seam (South Africa) showed that most of thetrace elements determined had a chemical affinity for sulphideminerals,specifically pyrite. These trace elements include: As, Cd, Co, Cu, Hg, Mo,Ni, Pb, and Se. Bergh (2009) also reported that Cr and V were found tohave an affinity for clays; this correlates with the results presented byXu et al. (2003). However, there is a difference between the two sets
of results; Bergh (2009) reports that Mn shows an affinity for clayswhereas Xu et al. (2003) report a carbonate association.
Removing volatile trace elements prior to combustion is per-ceived as an attractive emission control method; it is likely tobe less expensive than post-combustion clean up, and may wellprove to be more effective where the trace elements are able topass through pollution control devices. As beneficiation leads toa decrease in the mineral matter content of the coal (typicallyclays and pyrite), by association, the removal of sulphide min-erals by pyrite removal could lead to the lowering of certain vol-atile trace element emissions into the atmosphere (World CoalInstitute, 2008). For this reason, it is important that the modeof occurrence of trace elements be understood, so as to ascertainwhether reduction of a specific trace element concentration ispossible by coal beneficiation, bearing in mind the associated car-bon (and energy) loss, and the need to manage the solid wastegenerated. Relatively coarse-grained pyrite can readily be re-moved from coal during beneficiation, thereby potentially reduc-ing the concentration of pyrite and associated elements enteringa combustion system. Fine-grained pyrite may be less amenableto coal beneficiation, and thus the associated trace elements,along with the sulphur compounds derived from the pyrite,would require post-combustion capture. Organically-bound traceelements are unlikely to be removed during coal beneficiation.
The question is posed: does the positive relationship between pyrite,and As and Hg as volatile trace elements, hold for the Waterberg coals,and, if so, would pre-combustion removal of pyrite also enable the re-duction of hazardous volatile trace elements in the coal? The currentpaper addresses selected hazardous volatile trace elements (Hg, As, Cd,Se) in some run-of-mine (ROM)Waterberg coals and density fractionat-ed products. When comparing the results obtained from other trace el-ement studies conducted on South African coals (Table 1), it isapparent that the Waterberg coals, and especially those of the VryheidFormation, have enhanced trace element values; these are also higherthan the global values for hard coals as published by Ketris andYudovich (2009). Occurrence and concentrations of Cd, Hg, and Sbhave not previously been reported for Waterberg coals.
2. Geological setting
The Waterberg Coalfield is situated in the Permian, Karoo-ageEllisras Basin, and covers a relatively small area compared to othercoalfields in South Africa (Fig. 1). However, it contains a significantportion of the nation's in situ resources of bituminous coal (50%according to Dreyer, 2006). The coalfield is bounded by the Eenzaam-heid and Zoetfontein Faults on the southern and northern margins re-spectively, and extends approximately 80 km east–west and 40 kmnorth–south (Cairncross, 1990). The post-Karoo Daarby Fault, with adisplacement of 200 to 400 m, divides the coalfield into a shallowerwestern area and a deeper north-eastern area. Smaller faults, mostlyunmapped, subdivide the coalfield into blocks (Jeffrey, 2005). Thegeneral lack of data pertaining to these faults impacts on the uncer-tainty around the coal resource estimates (Fourie and Henry, 2009),although the CSIR (Council for Scientific and Industrial Research,South Africa) is conducting a detailed geophysical survey to removesome of the uncertainties and enhance the structural understandingof the basin (Fourie and Henry, 2009).
The coal seams were deposited between 260 and 190 millionyears ago in an east–west orientated graben. They are variable inthickness (splits of a few mm to over 8 m over a stratigraphic thick-ness of >120 m), and intercalated with carbonaceous mudstonesand mudstones (Dreyer, 2006). There are two main coal-bearing for-mations, with the upper formation referred to as the Grootegeluk(previously the Volksrust Formation), which is 60 m thick, up to110 m thick in the south, and the lower, more carbonaceous andinterbedded sequence referred to as the Vryheid Formation (55 m
Table 1Concentrations of trace elements in coals from different South African coalfields, together with global averages and ranges.
a These trace elements were not determined.b Corrected value.
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thick) (Jeffrey, 2005). Faure et al. (1996a, 1996b) provide a detaileddiscussion on the palaoenvironment of the Grootegeluk Formation,and concluded that the coals are humic in origin. The base of the for-mation was interpreted to be distal, and the upper parts to have amore proximal setting.
The coal bearing sequence in the Waterberg Coalfield is divided into11 coal-bearing zones, locally referred to as Benches (equivalent to coalseams). Benches 1 to 5 belong to the upper zone (Grootegeluk Forma-tion), and Benches 6 to 11 to the poorer quality lower zone (Vryheid For-mation). The benches in the Grootegeluk Formation typically containbright coals at the base and grade upwards into shale. At the GrootegelukCoal Mine, the only operating coal mine in the Waterberg Coalfield,Benches 2, 3, and 4 yield a coking coal on beneficiation, alongwith a ther-mal (steam) and an export product. Bench 5 is a thermal grade coal (witha high phosphorous content), and Benches 6 to 11 are dull coals, highlyinterbedded with mudstone and carbonaceous mudstone. Due to thefinemudstone intercalations in theWaterberg Coalfield, the coals requireextensive beneficiation, and the yield of saleable coal is typically less than50% (Jeffrey, 2005). The coking coal yield is approximately 10% of theROM coal, with the ROM ash being between 55 and 60% (Jeffrey, 2005).
Fig. 1. Location of the Waterberg Coalfie
The vitrinite reflectance of the Gootegeluk Formation is 0.72% (mean ran-dom) (Dreyer, 2006), corresponding toMedium Rank C bituminous coal.
In South African coals, the total sulphur content is under 2% (dry,ash-free), except for two mines in the Highveld Coalfield and avalue of 2.32% (dry, ash-free) reported for the Grootegeluk Colliery(Pinheiro et al., 1999). Roberts (1988) identified a variety of pyriteforms in some Permian South African coals, and concluded that mar-casite was rare, and that pyrite was more dominant in vitrinite-richcoal, with organic sulphur being higher in inertinite. Pyrite is thedominant sulphide mineral in the Grootegluk coals, observed as glob-ular aggregates, isolated euhedral crystals, fossil leaf pseudomorphsalong bedding planes, and less commonly as epigenetic fracture infill-ings (Faure et al., 1996b).
3. Methodology
Four ROM composite belt cut samples, each weighing over 20 kg,were received from the Grootegeluk Colliery, Waterberg Coalfield,Limpopo Province, South Africa. The samples supplied by the minewere identified as B3, B4, B5, and B11, representative of different
ld, South Africa. (Faure et al., 1995).
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benches. Samples B3 to B5 are from the Grootegeluk Formation, andB11 is from the Vryheid Formation. The coal samples were crushedto particle size less than 1 mm using a Reutsch mill, and cone-and-quartered to obtain representative samples. A split from each bench(identified as P) was retained, and a second split was density fraction-ated using mixtures of bromoform (RD=2.90) and chloroform(RD=1.50). As a key point of the investigation was to determine a re-lationship between pyrite and As and Hg, separations were made atRD=1.5, 1.9 and 2.4; the latter was intended to provide samplesenriched in pyrite. Thus, four samples per parent coal were obtained,namely: float products at 1.5 g/cm3 (1.5f), 1.9 g/cm3 (1.9f), and 2.4 g/cm3 (2.4f), and a sink product at 2.4 g/cm3 (2.4 s). In total, 19 sampleswere analysed. One parent coal (B11) did not produce enough mate-rial for the final density fraction (2.4 s); this parent ROM coal had avery low inherent mineral content.
Several authors, including Huggins et al. (2000), Senior et al.(2000), Querol et al. (2001), have used density fractionation forsample preparation in the determination of trace element occur-rence and affinities in coals, although Querol et al. (2001) high-light some limitations of using this technique. These limitationsinclude: 1) samples with a low mineral content (thus limited iso-lation during density fractionation); 2) mineralogical complexityof the samples (thus mineral group affinity, not definite mineralspecies); 3) finely dispersed mineral matter (thus preventingclear separations); 4) low concentrations of minerals with affini-ties for trace elements (thus theoretical discussion of results).These limitations were not considered to be of major concernduring the course of this research as the Waterberg coals areknown to be mineral-rich, with finely dispersed minerals aswell as mineral cleats, bands, concretions etc. Pyrite is knownto occur in these coals (Faure et al., 1996b). However, sampleB11 did report a low ash value, and thus limited isolation duringdensity fractionation; this sample would possibly benefit fromleaching rather than density fractionation in future tests, butenough material was obtained for ICP-MS analysis in this study.
The parent coals and products from the density fractionation weresubjected to proximate and ultimate analysis, determination of totalsulphur and sulphur forms, ash analysis for major elements,
Table 2Properties of ROM (parent) coals and density fractionated products (as received basis), withdlings component and/or loss during separation. GF=Grootegeluk Formation; VF=Vryhei
petrographic analysis, and trace element determinations, utilisinglaboratory facilities at the University of the Witwatersrand and a cer-tified analytical facility based in South Africa (UIS Analytical, Centuri-on). Proximate analysis was carried out using an in-house TGAtechnique on a Perkin Elmer TGA, and the results may differ slightlyto conventional proximate analytical data. Total sulphur and carbonwere determined using a Leco S-C analyser, and inductively coupledplasma optical emission spectrometry (ICP-OES) (Perkin Elmer Opti-ma 5300 DV) was used to determine the oxides of major elements foreach coal ash (ashed to 815 °C), including SiO2, Al2O3, CaO, K2O, Na2O,Fe2O3, MnO, MgO, TiO2, BaO, and P2O5. For ICP-OES analysis, the coalash samples were prepared using a lithium tetraborate fusion in aplatinum crucible and leached in diluted hydrochloric acid to getthe sample into solution. The solutions were analysed by ICP-OES todetermine the percentage of the total oxides in the ash.
Maceral group andmean random vitrinite reflectance analyses wereconducted on a Leitz Orthoplan polarising microscope (×500) fittedwith a Daytronic mainframe 9005 spectrophotometer at the Universityof Kentucky Centre for Applied Energy Research (CAER), following thestandard ISO technique (ISO 7405 part 3 and 5; Taylor et al., 1998). Ob-servations were made with regards to the mode of occurrence of pyrite,and petrographic imageswere taken using Leica imaging software in as-sociated with a Leica DMP4500 polarising microscope (×500) at theUniversity of theWitwatersrand. Scanning electronmicroscopy with el-emental detection (SEM-EDS) was undertaken, using a FEI FEI Quanta400 E-SEMwith an IncaOxford X-act EDS detector andPoint and ID soft-ware housed at the University of the Witwatersrand, to confirm thepresence of pyrite and other minerals.
For trace element determination, the coal samples were first sub-jected to microwave digestion using a Milestone Ethos MicrowaveHigh Pressure Labstation. The reagents used for digestion of each200-mg coal sample were: 10 ml 65% HNO3, 3 ml 40% HF, and 1 ml30% H2O2; and the Guaranteed-Reagent (GR Grade) HNO3 and HFfor sample digestion were further purified by sub-boiling distillation.Samples were digested until a maximum pressure of about 30 barswas reached, and the highest temperature was set at 220 °C for30 min. NIST traceable multi-element standards and SARM 18, 19,and 20 were used for calibration of trace element concentrations
yields for each density fraction. Difference between yield totals and 100% is due to mid-d Formation.
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and quality checks respectively, for both major oxides in ash and traceelement determination.
Inductively coupled plasma mass spectrometry (ICP-MS) (PerkinElmer Elan 6100 DRC, automatic pulse/analog counting mode) wasused to determine trace elements in the coal samples. Arsenic andSe were determined by ICP-MS using dynamic reaction cell technolo-gy (DRC) in order to avoid disturbance of polyatomic ions, also ap-plied by Dai et al. (2011). Arsenic was measured at mass 75 and Sewas measured at mass 78. Methane gas was used for the DRC cell tominimize the interferences of ArCl on As and Ar2 on Se. ICP-MS isthe analytical technique widely used for the determination of trace el-ement concentrations in coal due to the low detection limits achieved(Depoi et al., 2008; Guo et al., 2002; Hlatshwayo, 2008; Hu et al.,2006; Karayigit et al., 2000; Levandowski and Kalkreuth, 2009;Wagner and Hlatshwayo, 2005).
Fig. 2.Micrographs depicting forms of pyrite observed in the coals assessed during the projeframboidal pyrite (syngenetic); c) massive pyrite (most likely epigenetic); d) fine pyrite nopyrite flecks distributed through vitrinite particle (sub-micron in size, syngenetic; note scalsion lens ×50, excluding f. lens ×20.
4. Results and discussion
4.1. General characterisation ROM and density fractions
Data from the proximate and ultimate analysis, sulphur forms, ashanalysis, and density fractionation yields are given in Table 2. TheROM coal samples used in this study can be considered typical ofthe ROM products obtained from the Waterberg Coalfield, with highto very high ash percentages reported for the upper Grootegeluk For-mation (36.6–53.7%), and low ash in the Vryheid Formation (10.7%).The ash chemistry is dominated by SiO2 and Al2O3 (52–62.4%, and22.5–36.7% respectively), with Fe2O3 being around 5% and SO3 rang-ing from 1.07 to 2.61%.
Differing from that reported by Faure et al. (1996b) and Pinheiro etal. (1999), total S is well under 2% for all ROM coal samples, with pyritic
ct. a) pyrite infilling cleats and fissures (epigenetic; late stage course grained pyrite); b)dules (most likely syngenetic); e) pyrite infilling cell cavities (epigenetic); f) very finee difference in this micrograph).All micrographs taken under reflected light, oil immer-
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sulphur (FeS2) being under 0.9%. A significant proportion of the total Sconsists of organic S, approximated by the difference between total S,and FeS2 and SO4; SO4 is very low. Sulphur bound within carbon-richparticles (macerals) was frequently identified during SEM-EDS (spot)analysis. The deportment of pyrite as observed in petrographic examina-tion of these coals is depicted in Fig. 2. Multiple generations of pyrite arepresent, including syngenetic framboids, fine grained pyrite within thecoal matrix, massive pyrite, and cleat- and vein-infilling pyrite. Synge-netic and epigenetic processes affected pyrite formation and distribu-tion, with marine influence and deposition via hydrothermal fluidsboth evident. Thus, it would be expected that the different generationsof pyrite formed in different geochemical conditions, and, dependingon Fe availability, may have different proportions of Fe and S (Ryanand Ledda, 1997). Different Fe:S ratios were determined via SEM-EDS(Fig. 3), with Fe detected in the vein-infilling pyrite, but not in thebotanical-infilling pyrite (the Fe concentration was probably too lowfor detection by the SEM-EDS technique in this instance).
Fig. 3. SEM-EDS data from sample B5 2.4 sink. a) SEM backscattered image of a pyrite enricEDS spectrum for position 14 on (a) showing high S and no Fe; d) EDS spectrum for positio
As anticipated, the 1.5 float samples are very low in ash (under9%), and high in volatile matter (25–36%), whereas the 2.4 sink sam-ples are the converse (ash values 50–80%). The 1.9 float samples arelower in total S than the 1.5 float fraction for all bench samples, indi-cating that the 1.5 float samples have a greater proportion of organicsulphur, and the 1.9 float samples have a greater proportion of FeS2.Pyritic sulphur is concentrated in the sink samples (e.g. 25% in sampleB5 2.4 s). The yield for B5 2.4 sink is extremely low (4.2%), with mostof the bench sample reporting to the 2.4 float, which reports a verylow total S value; this would indicate that the pyrite in this sampleoccurs predominantly as epigenetic vein- or cleat- infilling, or as mas-sive particles (Fig. 2a,c,f). Samples B5 2.4 sink and B11 2.4 sink areenriched in Fe2O3 and depleted in SiO2 and Al2O3, relative to theother fractions of those bench samples.
The petrographic data reported in Table 3 indicate a range of mac-eral compositions for the ROM coals, with increasing vitrinite contentfrom the Vryheid to the overlying Grootegeluk Formation (12.7 to
hed coal particle; b) petrographic image of same particle (×20 oil immersion lens); c)n 15 on (a) showing S and Fe.
Table 3Petrographic characterisation of parent coals and density fractions: maceral group(vol.%), and reflectance (RoVmr%=percent mean random vitrinite reflectance).
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40.5% by vol. respectively). Vitrinite reflectance analysis places thecoals in the medium-rank bituminous C category (following theECE-UN in seam coal classification system), which is in line withother petrographic assessments on coals from this area (Faure et al.,1996b; Jeffrey, 2005). There is a slight upward decrease in reflectancefrom the Vryheid to the Grootegeluk Formation. The 1.5 float prod-ucts for coals B3, B4, and B5 all have very high vitrinite concentrations(71–81 vol.%, derived from parent coals with 16–40% vitrinite by vol-ume); the yields at this density are low (Table 2), but are in line withthe recoveries at the Grootegeluk washing plant (around 10% ROM forBenches 3 and 4). Sample B11 P has a comparable vitrinite content tosample B5 P, but very different inertinite and mineral matter con-tents. The B11 1.5 float product does not show a significant differencein vitrinite content compared to its parent sample, nor a significantlydifferent ash percentage (Table 2). The coals of the Vryheid Formationare low in mineral matter (in agreement with proximate analysisdata), and have very high inertinite contents. The coals of the overly-ing Grootegeluk Formation are higher in vitrinite (especially near thetop of the sequence) and are very high in mineral matter. This is es-sentially in agreement with the findings of Faure et al. (1996b). This
R² = 0.9931
0
20
40
60
80
0 20 40 60 80 100
pyr
ite
(pet
rog
) %
vo
l
pyritic sulphur corrected to vol. %
Correlation pyrite & pyritic sulphur
R² = 0.8546
Fig. 4. Relationship between petrographically determined pyrite and pyritic sulphur(as FeS2) corrected to equate weight % with volume %. If the top right point is removed,the R2=0.86 (as indicated in insert), which still indicates a good fit. Ta
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implies that the coals from the different formations may benefit fromdifferent beneficiation techniques, and that the Vryheid Formationcoals may not require beneficiation at all. The low ash, inertinite-rich (primarily semi-fusinite, with significant amount of inertodetri-nite with little or no associated mineral matter) coal of the VryheidFormation makes this a relatively unique coal from a South Africanperspective, potentially ideally suited, for example, to fixed-bed gasi-fication using the Sasol gasification technology (author's opinion).
In the density fractionated samples from theGrootegeluk Formation,the liptinite-rich particles contain a high proportion of finely dissemi-nated clays. In the detailed studies by Faure et al. (1996b), no mentionismade of the high liptinite content, nor of the unusual response to den-sity fractionation. Further consideration and significance of this findingis explored by Wagner (2010) andWagner et al. (in preparation).
According to Ryan and Ledda (1997), 1% by volume ofmicroscopically-visible pyrite equates to 3.5 wt.% FeS2. Fig. 4 showsa fit of R2=0.99 (or R2=0.86 if far left point is removed) betweenthe petrographically determined pyrite and pyritic sulphur. Thus itcan be assumed that the pyritic sulphur (expressed as FeS2) is agood representation of the pyrite in these coals, and as such is usedfor correlation when discussing the trace elements.
4.2. Trace elements in ROM coals
Table 4 provides data on a number of trace elements determinedduring the investigation; the ROM coals are discussed here, and thedifferent density fractions are discussed more fully below. For thepurposes of this paper Hg, As, Cd, and Se are considered in more de-tail; the other trace elements of concern will be addressed in a futurepaper.
The values previously published by Faure et al. (1996b) for theVryheid Formation coals are significantly higher than those deter-mined for sample B11 P in this study; in this study, the Vryheid For-mation coal has a lower trace element footprint relative to those ofthe Grootegeluk Formation (Fig. 5, Table 4). The chemistry and mac-eral composition of the Vryheid Formation coal is more comparableto those of the Witbank and Highveld Coalfields (with an added ben-efit of being lower in ash than the Grootegeluk Formation coals), andthe trace element concentrations reported for sample B11 P reflect agreater similarity to the Witbank and Highveld coals than the datafrom samples B3, B4, and B5; this excludes Hg, which is far higher
Global Average [a]
3 B
As 9 9.36
Cd 0.2 0.17
Hg 0.1 0.9
Se 1.6 1.1
0
4
8
12
16
pp
m
Fig. 5. Graphical representation of selected trace elements determined for the Waterberg Rhard coals).
(2.43 ppm) than typical Witbank and Highveld values. The As andHg concentrations in the coals of the Grootegeluk Formation arehigher than those of coals from the Witbank and Highveld Coalfields;Cd values are lower, with comparable Se values (Table 1). Sample B5has a high concentration of trace elements, indicative of a differentgeochemical environment compared to the other Grootegeluk Forma-tion samples; the high ash, total S, and P2O5 content (Table 2) are inagreement with this. Spears et al. (1988) identified a possible ton-stein horizon between the Grootegeluk Formation and the VryheidFormation, indicative of volcanic ash, which may have resulted inhigher concentrations of trace and rare earth elements in the coals.Faure et al. (1996b) did not identify this tonstein horizon, rather con-cluding that the minerals in this horizon may be derived from clasticvolcanic ash. Sample 5B, immediately above the “tonstein” layer, doeshave a high trace element fingerprint, different in many aspects fromsample 11B, which is lower in rare earth elements (Nd and Sc) andmost other trace elements.
As depicted in Fig. 5, the ROM samples have lower or comparableSe concentrations, variable As and Cd concentrations, and a higher Hgconcentration compared to the global average as reported by Ketrisand Yudovich (2009). Many other trace elements reported inTable 4 have higher to significantly higher concentrations than globalaverages for hard coals (Ketris and Yudovich, 2009), although thereare differences between the benches. Sample B11 P has an enrich-ment factor of 20 for Hg compared to the Coal Clarke values publishedby Ketris and Yudovich (2009), and As values are approximately 6times lower than the Coal Clarke values. The Hg concentrations forall theWaterberg samples are above the maximum value of 1 ppm in-dicated by Swaine (1994), although Zheng et al. (2007) report Hgconcentrations of more than 1 ppm for some Chinese coals, Dai etal. (2011, in press) report Hg values of 3.165 ppm (low rank Chinesecoals), and Kolker (2011a) reports values of 1.68 ppm for Donbasmines in Ukraine. Other trace elements in theWaterberg coals exceedthe maximum global values reported by Swaine (1994) (Table 1). Forexample, the Mn concentration for sample B3 P is 375 ppm, andSwaine (1994) reports a maximum value of 300 ppm. The Cr concen-tration for sample B4 is at the maximum of 60 ppm indicated bySwaine (1994), and the Ni concentrations for B3, B5, and B11 exceedthe maximum value of 50 ppm. Such high trace element contents inthe Waterberg ROM coal samples could be of environmental concern,especially the potentially volatile Hg content.
4 B 5 B 11 B
15 8.45 1.57
0.26 0.3 0.08
1.03 1.39 2.43
1.6 0.9 1
OM parent coals (ppm) compared to global averages. ([a] Ketris and Yudovich, 2009 –
233N.J. Wagner, M.T. Tlotleng / International Journal of Coal Geology 94 (2012) 225–237
Following the categorisation by Goldschmidt (1954), and appliedby Babu (1975) and Wagner and Hlatshwayo (2005), the trace ele-ments have been grouped into chalcophile, siderophile, and lithiphileelements (Table 5); chacophile elements have an affinity for S, sidero-phile elements have an affinity for Fe, and lithophile elements have anaffinity for Si. According to Goldschmidt (1954), siderophile elementsare most likely in their native state, and chalcophile elements com-bine with sulphur as sulphides, whereas lithophile elements occurprimarily as rock forming minerals. Wagner and Hlatshwayo (2005)found that Highveld coals are depleted or comparable in chalcophileelements relative to the global averages reported by Ketris andYudovich (2009), comparable in siderophile elements, and generallyenriched in lithophile elements. The Vryheid Formation coal (sampleB11 P) is comparable to the Highveld coals, with B11 essentially beingdepleted in all chalcophile elements, apart from Hg. Sample B5 isenriched to very enriched in all siderophile and lithophile elements,and variable in most of the chalcophile elements considered. SampleB3 is more enriched in siderophile and lithophile elements than chal-cophile elements. Sample B4 is comparable to the global averages interms of siderophile elements, enriched in lithophile elements, andcomparable to or enriched in terms of chalcophile elements.
4.3. Trace elements in density fractionated samples
Fig. 6 displays washability curves for each data set for Cd, Hg, Se,and pyritic sulphur (displayed as FeS2); As is displayed in Fig. 7 dueto different scaling. Whilst the graphs depicted are not typical wash-ability curves as used in the coal preparation industry, these curveshave been constructed ( following Gluskoter et al., 1977) to showthe percentage recovery for each element (left-hand graphs) andthe concentration of the selected trace elements at the different den-sity splits (right-hand graphs). A negative slope of the curve wouldindicate a concentration in the clean coal fractions; a uniform distri-bution in would indicate that washing the coal does not affect thetrace element concentration (that is, the element is uniformly distrib-uted throughout all fractions); a positive slope would indicate thatthe element is concentrated in the inorganic portion of the coal; thesteeper the slope, the stronger the association with the mineral mat-ter (Gluskoter et al., 1977).
Across all samples, Cd appears to have a negative slope, indicativeof an organic affinity; the Cd concentration is very low across all sam-ples. Selenium appears to concentrate in the 1.9 float fraction acrossall samples and is depleted in the sink samples, although the curvesfor B3 do show a different behaviour in the sink fraction; this behav-iour is related to the high yield in the 2.4 float fraction. Thus, Se mayhave a greater affinity for silicates in the middle fractions, rather thanthe cleaner coal fraction, or the sink fraction. Mercury appears to havea great affinity for the mineral-rich sink fraction in the GrootegelukFormation samples, and appears to have a slightly negative curve inthe Vryheid sample. The histograms show the highest Hg values
Table 5The enrichment or depletion of chalcophile, siderophile, and lithophile elements in the Wa2009). Categories are based on the geochemical classification of elements (Table VI) by Gol
Chalcophile eleme
As Cd Cu
Global averages (ppm) (Ketris and Yudovich, 2009) 9 0.2 16Highveld coals (ppm) (data from Wagner and Hlatshwayo, 2005) vd E dB3 P C C CB4 P E C EB5 P C C VEB11 P vd vd C
occur in the sink fractions. The pyritic sulphur (represented as FeS2)shows a very sharp slope in the washability curves for all samples, al-most vertical towards 100% recovery, clearly showing an associationwith the mineral-rich fraction. Arsenic has an almost vertical curvetowards the 100% recovery, indicating a great affinity for themineral-rich fraction, and presumably the pyrite.
By considering the curves, it appears that Hg does not have a closeaffinity with pyritic sulphur for all samples, with sample B11 clearlyshowing that Hg has an organic affinity. It is possible that Hg has aslight organic affinity in sample B5, may have a slight organic affinityin sample B4, and definitely has an organic and inorganic affinity insample B3. Senior et al. (2000) determined that Hg in the Wyodakcoals (USA) has an organic association, whilst coals from Pittsburgh,Illinois No.6, and Elkhorn/Hazard coals probably have higher pyrite-Hg associations. Toole-O'Neil et al. (1999) also determined an organicand inorganic affinity for Hg. Finely disseminated pyrite was observedin certain float products, specifically from samples B3 and B4; it ispossible that Hg is associated with pyrite in this form as well as, orrather than, the organic matter. Scanning electron microscopy-EDXwas undertaken on these samples, but no trace elements were identi-fied in association with the observed pyrite; it may be that the con-centrations were not high enough to be detected by the instrument.Dai et al. (2011), on the other hand, were able to determine highlevels of As in pyrite using SEM-EDS techniques.
Correlation coefficients (Table 6) show some interesting relation-ships, with good to very good correlations between As and Hg(R2=0.898), and between pyritic sulphur and Hg and As(R2=0.705 and 0.749 respectively). There is a very poor correlationbetween pyritic sulphur and Se (R2=0.037), and a poor correlationbetween Cd and pyritic sulphur (R2=0.304). Mercury and Se havean extremely poor correlation (R2 0.026), whilst Hg and Cd have amoderate to poor correlation (R2=0.474). These findings concurwith those in literature, apart from the finding for Se. In the presentstudy Se has an extremely poor correlation with pyritic sulphur, andis thought to be associated with the silicates.
If the correlation coefficients are compared for the full sequence(all samples) and the upper and lower formations separately, the R2
values show some interesting trends (Figs. 8 and 9). The R2 valuesfor the Grootegeluk Formation samples are based on sixteen samples,and thus can be considered reliable; the R2 values for the Vryheid For-mation are only based on four samples, and thus can only be consid-ered indicative at best. The Hg:As, Hg:pyritic sulphur, and As:pyriticsulphur R2 values all improve when only the Grootegeluk samplesare considered (all have R2≥0.9) (Figs. 8b and 9b). In the VryheidFormation samples the Hg:As and Hg:pyritic sulphur (Fig. 8c) correla-tion coefficients are around R2=0.1, clearly a poor relationship, butthe As:pyritic sulphur relationship is R2=0.99, clearly an excellentrelationship (Fig. 9c). This again indicates an organic affinity for Hgin the Vryheid coals, and that As is clearly associated with pyrite inall the Waterberg coals. The correlation coefficient between pyrite
terberg coals compared to global average values for hard coals (Ketris and Yudovich,dschmidt (1954).
nts (S) Siderophileelements(Fe)
Lithopile elements(Si)
Pb Sb Se Zn Hg Co Ni V Cr Mn
9 1.0 1.6 28 0.1 6 17 28 17 71d d C vd E C C C VE VEE C C VE VE E VE E E VVEVE C C C VVE C C VE VE VEVVE C d VE VVE E VVE VE VE VEC vd C vd VVE E VE C E C
; C=comparable.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 20 40 60 80 100
Tra
ce e
lem
ent
pp
m
Percent recovery
B3Hg
Cd
Se
FeS2
0
1
1
2
2
3
3
P 1.5 1.9 2.4 2.5
Tra
ce e
lem
ent
pp
m
Specific gravity fraction
B3
Hg Cd Se
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100
Tra
ce e
lem
ent
pp
m
Percent recovery
B4HgCdSeFeS2
0
1
2
3
4
5
P 1.5 1.9 2.4 2.5
Tra
ce e
lem
ent
pp
m
Specific gravity fraction
B4
Hg Cd Se
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80 100
Tra
ce e
lem
ent
pp
m
Percent recovery
B5HgCdSeFeS2
0
2
4
6
8
10
P 1.5 1.9 2.4 2.5
trac
e el
emen
t p
pm
Specific gravity fraction
B5Hg Cd Se
0
0.4
0.8
1.2
1.6
2
2.4
0 20 40 60 80 100
Tra
ce e
lem
ent
pp
m
Percent recovery
B11HgCdSeFeS2
0
1
2
3
P 1.5 1.9 2.0
trac
e el
emen
t p
pm
Specific gravity fraction
B11Hg Cd Se
234 N.J. Wagner, M.T. Tlotleng / International Journal of Coal Geology 94 (2012) 225–237
b
a
0
1
2
3
4
5
6
7
8
9
10
0 20 40 60 80 100
Ars
enic
pp
m
Percentage recovery
Arsenic B3
B4
B5
B11
0
20
40
60
80
100
120
p 1.5 1.9 2.4 2.5
Ars
eni c
pp
m
Specific gravity fraction
Arsenic
B3 B4 B5 B11
Fig. 7. a) Washability curves for arsenic, and b) distribution of element in the samplesets.
Table 6Coefficient correlations (R2) between Hg, As, Cd, Se and pyrite (from pyritic sulphur)for all samples (Waterberg coals) and for coals from the Grootegeluk and VryheidFormations.
Fig. 8. Correlation coefficients: graphs depicting Hg:FeS2. a) full sequence; b) Grootege-luk Formation only; c) Vryheid Formation only.
235N.J. Wagner, M.T. Tlotleng / International Journal of Coal Geology 94 (2012) 225–237
and Cd is R2=0.905 for the Vryheid coals, compared to 0.304 overall,indicating that Cd is likely to be associated with pyrite in this bench,with a poor correlation in the upper formation.
5. Summary and conclusions
During the course of this project, ROM and density fractionatedsamples from the Waterberg Coalfield, South Africa, were assessedfor their trace element content. Selected trace elements (Hg, As, Se,and Cd) were considered in detail to determine their mode of occur-rence/affinity for organic and inorganic components, specifically withregards to pyrite. The coals of the Grootegeluk Formation have
Fig. 6.Washability curves (left) and distribution of elements in size fractions (right) foeach case.
different chemical and petrographic characteristics to the coal fromthe Vryheid Formation; the Vryheid Formation coal has very lowash and is high in inertinite. Sample B5 (Grootegeluk Formation)has the highest ash yield (over 50%). The total S content was below1.7% for all ROM samples, which is lower than previously reportedvalues. The presence of pyritic sulphur in some float products proba-bly represents finely distributed pyrite bound within the organic mat-ter. The petrographically determined pyrite content was over 60% forsample B5 2.4 sink, and pyrite was observed in syngenetic and epige-netic forms in all samples.
Generally, the trace element concentrations for the Waterbergcoals tested are high compared to global averages, with the excep-tion of Se and Cd, which are lower; As is depleted in the Vryheidsample. The trace element concentrations in the Grootegeluk For-mation samples were generally higher than for coals from theWitbank and Highveld Coalfields. Sample B11 (Vryheid Formation)has a different trace element fingerprint compared to the Groote-geluk Formation samples, and sample B5 (Grootegeluk Formation)formed under a different geochemical environment compared tothe upper benches. Sample B11 is 20 times enriched in Hg com-pared to the Coal Clarke values, which potentially raises causefor concern with respect to environmental impact, whether on awaste pile (via leaching), or released following coal conversion(air pollutant), or on an ash dump (again through leaching, or
r each sample set. Pyritic sulphur as % FeS2. P represents the ROM (parent) coal in
R² = 0.7495
0
40
80
120
160
0.0 5.0 10.0 15.0 20.0 25.0 30.0
As
pp
m
FeS2
a
R² = 0.9385
0
40
80
120
160
0.0 5.0 10.0 15.0 20.0 25.0 30.0
As
pp
m
FeS2
b
R² = 0.9999
0
40
0.0 5.0 10.0 15.0 20.0
As
pp
m
FeS2
c
Fig. 9. Correlation coefficients: graphs depicting As:FeS2. a) full sequence; b) Grootege-luk Formation only; c) Vryheid Formation only.
236 N.J. Wagner, M.T. Tlotleng / International Journal of Coal Geology 94 (2012) 225–237
via dust). The sink product for sample B5 also has an extremelyhigh Hg concentration, but a very low yield.
The density fractionated samples show variations in trace elementcontent. Cadmium primarily exhibits an organic affinity, reporting tothe carbon-rich float fractions, but may have an affinity with pyrite inthe lowest bench (11B). Selenium reports to the middle fractions, anda silicate affinity is proposed. Arsenic reports predominantly to themineral-rich sink fractions, especially the fraction enriched in pyrite.Mercury reports to both the float and sink fractions (although there ismore enrichment in the sink fractions), indicating both organic and in-organic affinities in the Waterberg coals; the Vryheid Formation sampleappears to have an organic affinity. Some Hg appears to be associatedwith pyrite, and most As is also associated with pyrite. Thus, pyritereduction by beneficiation could remove As and some Hg from thesecoals. However, this conclusion is based on laboratory based float-sinkseparations on−1 mm particles; actual beneficiation plant samples arerequired to test this hypothesis. Pyrite reduction in Bench 11 is unlikelyto reduce the Hg content, as it appears that this Hg is associated withthe organic matter rather than the mineral fraction.
Acknowledgements
This project was funded by the South African National Energy Re-search Institute (SANERI), with additional supporting funds from the
Sasol Hub and Spoke Initiative, and the National Research Foundation(NRF) Thutuka Grant. Any opinion, findings and conclusions or rec-ommendations expressed in this material are those of the author(s)and therefore the NRF do not accept any liability in regard thereto.Samples were provided by Exxaro. Japie Oberholzer at UIS under-took the ICP-OES and ICP-MS analyses, and Alex Ziegler at the Univer-sity of the Witwatersrand assisted with the SEM-EDX analysis. Theauthors are very grateful to the reviewer, editor, and guest editorfor their constructive and helpful criticism.
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