Physical Coal Cleaning Research Articles

A physical coal cleaning approach for clean energy production from low grade Lakhra coal of Pakistan using diester table

In this work coal from Lakhra (coal mine in Pakistan) was subjected to a physical technique for mineral matter and sulfur reduction by using diester table. Effect of particle size and coal-to-water ratio of slurry were studied. A reduction of 85% in sulfur content and 88% in ash content for 500 µm coal sample was observed. While at 300 µm size sulfur and ash content were reduced about 77% and 80% respectively. At 150 µm size, the reduction in sulfur and ash was observed to be lesser (54% and 55% respectively) due to the decrease in specific gravity difference of coaland impurity (i.e. sulfur and ash). GCV increased from 17.2 MJ/kg of the raw coal to 23.3 MJ/kg of cleaner fraction obtained at 500 µm. Based on the results of this study, diester table can be proposed to be a simpler however an effective physical coal cleaning method to reduce sulfur and ash content. Besides other parameters, particle size is a crucial parameter to impact the cleaning efficiency and because of the greater difference in specific gravity of coal and impurity, higher particles size used in this study resulted in the cleanest product.

Mineralogy and Geochemistry of the M9 High-Sulfur Coal from the Renjiazhuang Mining District, China

According to coal lithotypes, the bottom, parting, roof, and 15 coal samples were collected by finely partitioning the M9 seam from the Renjiazhuang Mining District, Ningxia, China. Conventional chemical analysis, optical microscopy, scanning electron microscopy equipped with energy-dispersive X-ray spectrometry, X-ray diffractometry, inductively coupled plasma atomic emission spectrometry, inductively coupled plasma mass spectrometry, and atomic absorption spectrophotometry techniques were used on these samples to research the vertical variation between geochemistry and mineralogy in the high-sulfur coal. The weighted average content of total sulfur calculated from 15 coal samples is 3.07%, which belongs to the high-sulfur coal. However, the contents of morphological sulfur of 15 piles are significantly different: the contents of pyritic and organic sulfur are observed to range from 0.02 to 1.55% and from 1.88 to 3.91%. The results show that these differences are mainly controlled by marine conditions and the contents of organic matter and kaolinite. The mineralogy of the M9 coal is dominated by kaolinite, followed by dolomite, and it also contains minor amounts of illite, feldspar, pyrite, siderite, hematite, chalcopyrite, calcite, and marcasite. Moreover, pyrite is the main sulfide in coal, and agglomerated chalcopyrite and granular galena are partially visible. The forms of pyrite include fine-grained, spherical, irregular block-shaped, and clumps. Trace elements are mainly carried by pyrite and ash so that physical coal cleaning can be applied to partially remove them, while thalassophile elements Na, Ca, and Mg are closely related to organic sulfur, indicating that the coal blending can be used to decrease their contents.

Partitioning of hazardous trace elements during coal preparation

The samples of feed coal, cleaned coal, middlings, slime and gangue were collected from Xiqu, Antaibao and Heidaigou coal preparation plants, China. The analyses of minerals, ash, sulfur and 20 hazardous trace elements were performed on the samples by SEM/EDX, INAA, ICP-AES, CV-AAS, GAAS and also on the samples from experimental products (by heavy-media, electrostatic separation and forth-floatation). Partitioning of sulfur and these trace elements under the conditions of different coal ranks and coal preparation types were discussed. The results show that the partitioning behavior of a given element during the cleaning processes is mainly controlled by its modes of occurrence, the distribution forms of its carrier minerals and the cleaning technique types. Although hazardous trace elements have diverse degrees of removal due to different coal ranks and coal preparation types, the elements associated with fine minerals (e.g. Pb, U and Be) and organic constituents (e.g. Br and Cl ) will not be effectively reduced by physical coal cleaning and may still be enriched in the cleaned coal. As to the coarse sized feeds (particle size >0.5 mm), the elements associated with coarse/epigenetic minerals can be removed more easily by gravity separation methods and report to the gangue and slime. As to the finer-grained feeds (<0.5 mm), the elements associated with minerals with a high dielectric coefficient are easily removed by electrostatic separation; in comparison with the larger-sized fractions (74–150 µm), the smaller-sized fractions (<74 µm) show a high removability of elements by forth-floatation as a whole. The physical coal cleaning is not only effective in the removal of ash and sulfur, but also in reducing the concentration of most hazardous trace elements. However, as byproducts, the middlings, especially the slimes are preferentially enriched in many hazardous elements (e.g. Cr, Ni and Mo), so that these fractions have a potential of environmental risk and should not be directly used as fuel.

Partitioning of elements and macerals during preparation of Antaibao coal

Abstract Analyses of the macerals, ash, sulfur and 43 major and trace elements were performed on samples of feed coal, cleaned coal, middlings and slime collected from the Antaibao coal preparation plant, China, and also on samples from coal preparation experiments. This study is focused on the partitioning of elements and macerals during coal preparation and potential environment aspects of the elements. The conclusions are as follows: (1) in comparison with the feed coal, the cleaned coal has a higher vitrinite content and relatively lower inertinite and exinite contents, whereas the middlings and slime have lower vitrinite and exinite contents, and relatively higher inertinite contents. The vitrinite contents in the size-segregated cleaned coals were observed to show a slightly increasing tendency with increasing particle size, while the inertinite contents decreased. (2) Physical coal cleaning is not only effective for removal of ash and sulfur, but also in reducing the concentration of most elements. As, Cd, Co, Cs, Hg, Fe, K, Mg, Nb and Ni are observed to show a high degree of removal, while Br, Be, Cu, U, Mn, Zn and organic sulfur are enriched in the cleaned coal and show a lower degree of removal. The large-sized cleaned coal is cleaner than the smaller sized fractions. (3) The middlings, especially the slime, are enriched in S, Hg, Cr, V, Zn, etc., so that these fractions should not be directly used as fuel. In addition, the concentration of Pb and V in the process water exceeds the limit of relevant environmental water quality standard. Consequently, it is necessary to develop new processes to remove ash, sulfur and hazardous trace elements to the maximum extent. Further studies on deep processing of the middlings and slime and cleaning of the process water should also be performed.

Element geochemistry and cleaning potential of the No. 11 coal seam from Antaibao mining district

Based on the analyses of sulfur and 41 other elements in 8 channel samples of the No. 11 coal seam from Antaibao surface mine, Shanxi, China and 4 samples from the coal preparation plant of this mine, the distribution of the elements in the seam profile, their geochemical partitioning behavior during the coal cleaning and the genetic relationships between the both are studied. The conclusions are drawn as follows. The coal-forming environment was probably invaded by sea water during the post-stage of peatification, which results in the fact that the contents of As, Fe, S, etc. associated closely with sea water tend to increase toward the top of the seam, and that the kaolinite changes into illite and montmorillonite in the coal-sublayer near the roof. These elements studied are dominantly associated with kaolinite, pyrite, illite, montmorillonite, etc., of which the As, Pb, Mn, Cs, Co, Ni, etc. are mainly associated with sulfides, the Mo, V, Nb, Hf, REEs, Ta etc. mainly with kaolintie, the Mg, Al etc. mainly with epigenetic montmorillonite, and the Rb, Cr, Ba, Cu, K, Hg, etc. mainly with epigenetic illite. The physical coal cleaning is not only effective in the removal of ash and sulfur, but also in reducing the concentration of most major and trace elements. The elements Be, U, Sb, W, Br, Se, P, etc. are largely or partly organically bound showing a relatively low removability, while the removability of the other elements studied is more than 20%, of which the Mg, Mn, Hg, Fe, As, K, Al, Cs, and Cr associated mostly with the coarser or epigenetic minerals show a higher removability than that of ash. The distribution of the elements in the seam profile controls their partitioning behavior to a great degree during the coal cleaning processes.

Arsenic-bearing pyrite and marcasite in the Fire Clay coal bed, Middle Pennsylvanian Breathitt Formation, eastern Kentucky

Arsenic concentrations determined on 11 lithotype samples from the Middle Pennsylvanian Breathitt Group Fire Clay coal bed, Leslie County, KY, range from 1 to 418 ppm (whole coal basis). The 11 lithotype samples, which vary in thickness from 4 to 18 cm, were sampled from a continuous 1.38 m channel sample, and were selected based on megascopic appearance (vitrain-rich versus attrital-rich). A lithotype that contains 418 ppm As is located near the top of the coal bed and is composed of 10.5 cm of bright clarain bands containing fusain that, within short distances, grade laterally into Fe sulfide bands. To determine the mode of occurrence of As in this lithotype, the coal was examined with scanning electron microscopy and analyzed by energy dispersive X-ray fluorescence. Massive, framboidal, cell filling, cell-wall replacement, and radiating forms of Fe sulfide were observed in the high As lithotype; many of the radiating Fe sulfide forms, and one of the cell-wall replacements contained As. Examination of the grains with optical light microscopy shows that the majority of radiating morphologies are pyrite, the remainder are marcasite. Selected Fe sulfide grains were also analyzed by electron microprobe microscopy. Arsenic concentrations within individual grains range from 0.0 wt.% to approximately 3.5 wt.%. On the basis of morphology, these Fe sulfides are presumed to be of syngenetic origin and would probably be removed from the coal during physical coal cleaning, thus eliminating a potential source of As from the coal combustion process. However, because the grains are radiating and have high surface area, dissolution and release of As could occur if the pyrite is oxidized in refuse ponds.

Modes of Occurrence and Cleaning Potential of Trace Elements in Coals from the Northern Ordos Basin and Shanxi Province, China

Abstract Based on the analyses of 43 elements in 16 samples of the raw coal and feed coal collected from the northern Ordos basin and Shanxi Province, the modes of occurrence of these elements were studied using the method of cluster analysis and factor analysis, and the cleaning potential of the hazardous elements relatively enriched in the coals was discussed by analyzing six samples of the cleaned coal from the coal‐washing plants and coal cleaning simulation experiments. The results shows that the elements Br and Ba show a strong affinity to the organic matter, Cs, Cd, Pb, Zn and Hg partly to the organic matter, and the other trace elements are mainly associated with the mineral matter. Cs, Mo, P, Pb, Zn and S have positive correlations with the two principal factors, reflecting the complexity of their modes of occurrence. Some elements that were thought to show a faint relationship (Be with S and Sb with carbonates) in other rocks are found to have a strong interrelation in the coals. Clay minerals (mainly kaolinite) dominate in the coals, and Ta, Th, Ti, Sc, REE, Hf, U, Se, W, V, Nb, Mo, Al, P, Cr, Pb and Zn are distributed mostly in kaolinite, while K, Rb, Cs, and Na have much to do with illite. Conventional cleaning can reduce the concentrations of most hazardous elements in various degrees. The hazardous elements S, As, Sb, Se, Mo, Pb, Cd and Hg relatively enriched in some coals from the area studied have a relatively high potential of environmental risks. However, by physical coal cleaning processes, more than 60% of As and Hg were removed, showing a high degree of removal, more than 30% of Sb, as well as S, Pb and Cd partly associated with the inorganic matter were removed. Se and Mo showing a relatively low degree of removal could be further removed by deep crushing of the coal during physical cleaning processes, and the concentrations of S, Pb, Cd and Hg with a partial association with the organic matter could be decreased in such ways as the coal blending. Cluster analysis together with factor analysis is a rapid and effective way to deduce the mode of occurrence of an element from bulk samples, and the removability data of most hazardous elements are basically consistent with their modes of occurrence suggested, which indicates that the statistical analysis could predict the cleaning potential of hazardous elements during the physical coal cleaning.

04/00611 Biodesulphurization as a complement to the physical cleaning of coal: Martínez, O. et al. Fuel, 2003, 82, (9), 1085–1090

04/00611 Biodesulphurization as a complement to the physical cleaning of coal: Martínez, O. et al. Fuel, 2003, 82, (9), 1085–1090

Trace element abundances in major minerals of Late Permian coals from southwestern Guizhou province, China

Fourteen samples of minerals were separated by handpicking from Late Permian coals in southwestern Guizhou province, China. These 14 minerals were nodular pyrite, massive recrystallized pyrite, pyrite deposited from low-temperature hydrothermal fluid and from ground water; clay minerals; and calcite deposited from low-temperature hydrothermal fluid and from ground water. The mineralogy, elemental composition, and distribution of 33 elements in these samples were studied by optical microscopy, scanning electron microscope equipped with energy-dispersive X-ray spectrometer (SEM-EDX), X-ray diffraction (XRD), cold-vapor atomic absorption spectrometry (CV-AAS), atomic fluorescence spectrometry (AFS), inductively coupled-plasma mass spectrometry (ICP-MS), and ion-selective electrode (ISE). The results show that various minerals in coal contain variable amounts of trace elements. Clay minerals have high concentrations of Ba, Be, Cs, F, Ga, Nb, Rb, Th, U, and Zr. Quartz has little contribution to the concentration of trace elements in bulk coal. Arsenic, Mn, and Sr are in high concentrations in calcite. Pyrite has high concentrations of As, Cd, Hg, Mo, Sb, Se, Tl, and Zn. Different genetic types of calcite in coal can accumulate different trace elements; for example Ba, Co, Cr, Hg, Ni, Rb, Sn, Sr, and Zn are in higher concentrations in calcite deposited from low-temperature hydrothermal fluid than in that deposited from ground water. Furthermore, the concentrations of some trace elements are quite variable in pyrite; different genetic types of pyrites (Py-A, B, C, D) have different concentrations of trace elements, and the concentrations of trace elements are also different in pyrite of low-temperature hydrothermal origin collected from different locations. The study shows that elemental concentration is rather uniform in a pyrite vein. There are many micron and submicron mosaic pyrites in a pyrite vein, which is enriched in some trace elements, such as As and Mo. The content of trace element in pyrite vein depends upon the content of mosaic pyrite and of trace elements in it. Many environmentally sensitive trace elements are mainly contained in the minerals in coal, and hence the physical coal cleaning techniques can remove minerals from coal and decrease the emissions of potentially hazardous trace elements.

Occurrence, emissions and deposition of mercury during coal combustion in the province Guizhou, China

Forty-eight coal samples from the major coal mines in Guizhou were collected by means of subterranean channel sampling. The average mercury content in coal from this Chinese province is 0.53 mg kg-1, which is substantially higher than coal produced in any other province of China. A strong link is proposed to exist between atmospheric mercury emissions and the combustion of coal. During the last two decades, the amount of mercury emitted from coal combustion in Guizhou has increased invariably to 8.3 tons in 1998. Being concentrated to the pyrite fraction in the coal, the implementation of physical coal cleaning is likely to be an effective way to reduce the mercury content. Total and fractionated mercury in air and precipitation was measured in the provincial capital. During a measurement campaign in October 1999, the reactive gaseous mercury (RGM) concentration averaged at 450 pg m-3, which is much higher than that in pristine area (less than 20 pg m-3). The average total mercury in rainwater of 1996 was estimated to be 33 ng L-1, and the total annual wet deposition of mercury in the area was 39 g km-2 in 1996.

Mercury concentration in coal—unraveling the puzzle

Based on data from the US Geological Survey's COALQUAL database, the mean concentration of mercury in coal is approximately 0.2 μg g −1. Assuming the database reflects in-ground US coal resources, values for conterminous US coal areas range from 0.08 μg g −1 for coal in the San Juan and Uinta regions to 0.22 μg g −1 for the Gulf Coast lignites. Recalculating the COALQUAL data to an equal energy basis unadjusted for moisture differences, the Gulf Coast lignites have the highest values (36.4 lb of Hg/10 12 Btu) and the Hams Fork region coal has the lowest value (4.8 lb of Hg/10 12 Btu). Strong indirect geochemical evidence indicates that a substantial proportion of the mercury in coal is associated with pyrite occurrence. This association of mercury and pyrite probably accounts for the removal of mercury with the pyrite by physical coal cleaning procedures. Data from the literature indicate that conventional coal cleaning removes approximately 37% of the mercury on an equal energy basis, with a range of 0% to 78%. When the average mercury reduction value is applied to in-ground mercury values from the COALQUAL database, the resulting `cleaned' mercury values are very close to mercury in `as-shipped' coal from the same coal bed in the same county. Applying the reduction factor for coal cleaning to eastern US bituminous coal, reduces the mercury input load compared to lower-rank non-cleaned western US coal. In the absence of analytical data on as-shipped coal, the mercury data in the COALQUAL database, adjusted for cleanability where appropriate, may be used as an estimator of mercury contents of as-shipped coal.

97/04139 An evaluation of physical coal cleaning plus flue gas desulfurization coal fired utility applications

97/04139 An evaluation of physical coal cleaning plus flue gas desulfurization coal fired utility applications

Environmental impact of minerals in UK coals

Abstract Both detrital and diagenetic minerals are quantitatively important in UK coals. The detrital minerals include quartz, and the clay minerals illite, mixed-layer illite-smectite, kaolinite and chlorite. The diagenetic minerals are dominated by pyrite/marcasite, carbonates, mainly ankerite and calcite, and kaolinite. Pyrite makes an important contribution to the S contents of the coals and is largely responsible for the variation in total S. The S content of coals delivered to power stations could be reduced by selective extraction during mining and improved physical coal-cleaning. Pyrite is a major location for trace elements of environmental concern and the elimination of pyrite would reduce both SO 2 and trace element emissions. Pyrite is also important in the weathering environment due to its instability. Its breakdown in coal stocks and colliery discard heaps may lead to spontaneous combustion. In discard heaps, pyrite breakdown is also responsible for acid porewaters and toxic elements in solution which inhibit colonization by vegetation and thus hinder reclamation of derelict land. If carbonates are sufficiently abundant, acid conditions may be eliminated. Acid porewaters are one facet of the general problem of acid mine drainage, which has a major impact on water quality, particularly in streams and rivers. In the UK acid mine drainage is a problem of growing concern due to reduction of deep-mine pumping following closure of collieries. Although pyrite breakdown in colliery spoil has an important influence on porewater compositions, it does not greatly influence the engineering stability because the major component, the mudrocks, contain very little pyrite. Upon exposure, the mudrocks break down relatively rapidly due to sedimentary structures to form an aggregate. Loss of interparticle bonding is a much slower process but there are a few mudrocks associated with low rank bituminous coals with both a high mixed-layer clay content and porosity which disintegrate rapidly during cycles of wetting and drying. There are also regional variations in the relative proportions of the clay minerals which influence the engineering properties. Kaolinite is dominant in the northern coalfields. This is also detected in the composition of fly ash produced by coal-fired power stations. Fly ash is enriched in those trace elements which have an environmental impact. The volatile elements, many of which were sulphide-associated in the coal, are enriched on the surface of the ash particles where they are accessible to leaching in the weathering environment, resulting in the contamination of natural waters.

Precombustion control options for air toxics

Coal cleaning can reduce the concentration of most, of the elements named as hazardous air pollutants in the 1990 Amendments to the Clean Air Act. As an air toxics control measure, coal cleaning offers several advantages to utilities. Because physical coal cleaning is a relatively inexpensive technology, it may prove to be the lowest-cost control option in many cases. Also, coal cleaning is currently the only commercially available control technology for the highly volatile trace element mercury. Finally, removing trace elements before combustion reduces tile concentration of these elements in utility solid wastes, reducing possible long-term environmental liability.

95/01296 Changes in organic sulfur compounds in coal macerals during liquefaction

Environmentally sound use of coal for energy production involves effective sulfur removal from the feed coal and/or coal-derived products. Physical cleaning of coal is effective in removing substantial quantities of inorganic sulfur compounds such a pyrite. However, removal of organic sulfur by physical means has not been extremely successful. It is likely that only chemical methods will be useful in substantial removal of organic sulfur. A thorough knowledge of the chemistry of organic sulfur in coal will be valuable in attempts to remove organic sulfur from coal or its liquefaction or pyrolysis products by chemical methods. Since different coal macerals have different liquefaction reactivities, the analysis of sulfur functionalities on separated macerals is deemed to be more meaningful than studying the whole coal. Liquefaction behavior and organic sulfur speciation of the macerals separated from Lewiston-Stockton coal (Argonne Premium Coal Sample bank, APCS-7) by XPS, XANES, and HRMS has been previously described. This paper describes the preliminary speciation of sulfur compounds in the asphaltene fraction of the liquefaction products from these macerals.

Fate of Trace Metals During Agglomeration of Blue Creek Coals with Used Motor Oil

Selective agglomeration of fine coal with oil is a physical coal cleaning process. It is not practiced on a large commercial scale due to the high cost of virgin oil. Used motor oil, a waste product, is a possible alternative, but it contains many toxic metals of environmental concern. Agglomeration with used motor oil solves a potential disposal problem while simultaneously enhancing the coal's thermal value. The fate of lead and cadmium contained in used motor oil was studied during agglomeration of Blue Creek coal at different oil treatment levels. The threshold oil dosage for successful agglomeration was determined. At the threshold oil dosage, 83% of the lead and 88% of the cadmium in the thickener underflow from the coal cleaning was rejected. Agglomerate lead and cadmium concentrations were observed to increase with increased oil dosage. It was concluded that lead and cadmium can be removed by agglomeration with used motor oil and the amount of oil used is an important factor affecting the trace metal content in the agglomerates.

Modes of occurrence of potentially hazardous elements in coal: levels of confidence

The modes of occurrence of the potentially hazardous elements in coal will be of significance in any attempt to reduce their mobilization due to coal combustion. Antimony and selenium may be present in solid solution in pyrite, as minute accessory sulfides dispersed throughout the organic matrix, or in organic association. Because of these modes of occurrence it is anticipated that less than 50% of these elements will be routinely removed by conventional coal cleaning procedures. Arsenic and mercury occur primarily in late-stage coarse-grained pyrite therefore physical coal cleaning procedures should be successful in removing substantial proportions of these elements. Cadmium occurs in sphalerite and lead in galena. Both of these minerals exhibit a wide range of particle sizes and textural relations. Depending on the particle size and textural relations, physical coal cleaning may remove as little as 25% of these elements or as much as 75%. Manganese in bituminous coal occurs in carbonates, especially siderite. Physical coal cleaning should remove a substantial proportion of this element. More information is needed to elucidate the modes of occurrence of beryllium, chromium, cobalt, and nickel.

Influence of coal properties and pretreatment on ash aerosol yield

The formation of ash from a broad range of coals has been studied in a laboratory-scale flow reactor. The objectives of the work were to: (1) examine the factors governing the residual fly ash size distribution for a wide range of minimally processed coals; and (2) identify the influence of coal pretreatment (e.g. cleaning and sizing) on yields of ash aerosol relative to that from the minimally processed parent coals. The raw coals received only the pretreatment that would be appropriate for utility firing, i.e. pulverizing to a broad size distribution which corresponds to 70–80% −200 mesh. The results for the residual fly ash size distribution were correlated through the breakup model. In general, the number of ash particles produced from each coal particle increased for lower rank, western coals, with some notable exceptions. The size distribution predicted by the breakup model, after the breakup number was optimized, generally matched the measured ash size distributions well. Physical coal cleaning, via froth floatation, was applied to three of the coals. For two bituminous coals, the cleaned fractions yielded substantially more aerosol than the parent coals. The fraction of the parent coal that suppressed the formation of the aerosol was associated with the minerals removed in the first step of the cleaning process, i.e. the most easily removed excluded minerals. The various cleaned cuts of the western lignite showed essentially identical behaviour. Aerodynamic sizing yielded coals that produced significantly more aerosol than the parent coal. These results suggest that excluded minerals play a role in modifying the vaporization mechanism responsible for aerosol production.

Effect of Grinding Conditions on the Performance of a Selective Agglomeration Process for Physical Coal Cleaning

Abstract An extensive experimental study was conducted to investigate the effect of grinding conditions on the performance of a selective agglomeration process using n-pentane as agglomerant. Upper Freeport, Pittsburgh # 8, and Illinois # 6 coal samples were dry and wet ground to 28 mesh × 0 and 200 mesh × 0 particle sizes using a pulverizer, a ceramic jar mill, and a specially designed stirred-ball mill. The ground ;, samples were subjected to a standardized agglomeration test procedure using a Waring blender as the agglomeration unit. The effects of various operating variables, such as n-pentane to coal ratio, slurry concentration, mixing speed, and agglomeration time on energy recovery and ash rejection were statistically evaluated. The agglomeration test data showed dramatic differences between wet and dry grinding methods and between 28 mesh × 0 and 200 mesh × 0 particle sizes. Grinding to 200 mesh × 0 resulted in higher energy recoveries. Moreover, 200 mesh × 0 particle size obtained with wet grindi...

Determination of pyrite particle size distribution in coal by merging analyses of size distributions at different microscopic magnificiations: Optimum allocation of number of observations to different size groups

In the characterization of particle size distributions, it is often useful to measure the size of smaller particles using optical magnifications that are higher than the magnifications used to measure relatively large particles. This results in an examination of a smaller area in the sample specimen. This paper considers the question of how many fields should be microscopically analysed at different k magnifications to arrive at an efficient estimate of the mean particle diameter (by count). The resolution of this question depends upon the following three factors: 1, the relative sizes of areas scanned under different magnifications; 2, the expected number of particles of a given size range per optical field under a given magnification; and 3, the standard deviation within each size range scanned under the same magnification. A formula is provided for the optimum allocation of field counts at each magnification when measuring particle sizes in various size ranges. Numerical examples are also provided. The discussion in this paper is illustrated by reference to the measurement of pyrite particle sizes in a given coal. It is well known that the size distribution of pyrites in coal influences the potential amount of sulphur reduction that can be achieved by physical coal cleaning methods.