Colin R. Ward

Analysis and significance of mineral matter in coal seams

Abstract The material described as “mineral matter” in coal encompasses dissolved salts in the pore water and inorganic elements associated with the organic compounds, as well as discrete crystalline and non-crystalline mineral particles. A range of technologies, including but not restricted to low-temperature oxygen-plasma ashing, may be used to evaluate the total proportions of minerals and other inorganic constituents in a coal sample. The relative proportions of the individual minerals in the coal may be further determined by several different techniques, including Rietveld-based X-ray powder diffractometry, computer-controlled scanning electron microscopy (CCSEM), and normative interpretation of chemical analysis data. The mode of occurrence of particular minerals may be evaluated by optical or electron microscopy techniques. The minerals in coal may represent transformed accumulations of biogenic constituents such as phytoliths and skeletal fragments, or they may be of detrital origin, introduced as epiclastic or pyroclastic particles into the peat bed. Other minerals are produced by authigenic precipitation, either syngenetically with peat accumulation or at a later stage in cleats and other pore spaces by epigenetic processes. They may represent solution and reprecipitation products of biogenic and detrital material, or they may be derived from solutions or decaying organic matter within the peat bed. Non-mineral inorganics may be derived from a range of subsurface waters, and possibly redistributed within low-rank seams by post-depositional ion migration effects. They may also be expelled in different ways from the organic matter with rank advance. Quantitative analysis of minerals and other inorganics contributes significantly to defining coal quality. It may also be useful as an aid to stratigraphic correlation, either between seams in a coal-bearing sequence or between sub-sections within an individual coal bed. Mineralogical analysis may help in identifying the mode of occurrence and mobility of particular trace elements, including potentially toxic components such as arsenic and mercury. Knowledge of the mineral matter can also be used to evaluate the behaviour of particular coals in different utilization processes, including the processes that control the characteristics of fly ashes, slags and other combustion by-products.

Mineral matter and trace elements in coals of the Gunnedah Basin, New South Wales, Australia

Abstract The concentrations of major and trace inorganic elements in a succession of Permian coals from the Gunnedah Basin, New South Wales, have been determined by X-ray fluorescence techniques applied to both whole-coal and high-temperature ash samples. The results have been evaluated in the light of quantitative data on the minerals in the same coals, determined from X-ray diffraction study of whole-coal samples using a Rietveld-based interpretation program ( Siroquant ™), to determine relationships of the trace elements in the coals to the mineral species present. Comparison of the chemical composition of the coal ash interpreted from the quantitative mineralogical study to the actual ash composition determined by XRF analysis shows a high degree of consistency, confirming the validity of the XRD interpretations for the Gunnedah Basin materials. Quartz, illite and other minerals of detrital origin dominate the coals in the upper part of the sequence, whereas authigenic kaolinite is abundant in coals from the lower part of the Permian succession. These minerals are all reduced in abundance, however, and pyrite is a dominant constituent, in coals formed under marine influence at several stratigraphic levels. Calcite and dolomite occur as cleat and fracture infillings, mostly in seams near the top and bottom of the sequence. The potassium-bearing minerals in the detrital fraction are associated with significant concentrations of rubidium, and the authigenic kaolinite with relatively high proportions of titanium. Zirconium is also abundant, with associated P and Hf, in the Gunnedah Basin coal seams. Relationships exhibited by Ti, Zr, Nd and Y are consistent with derivation of the original sediment admixed with the seams from an acid volcanic source. Pyrite in the coals is associated with high concentrations of arsenic and minor proportions of thallium; no other element commonly associated with sulphides in coals, however, appears to occur in significant proportions with the pyrite in the sample suite. Small concentrations of Cl present in the coal are inversely related to the pyrite content, and appear to represent ion-exchange components associated with the organic matter. Strontium and barium are strongly associated with the cleat-filling carbonate minerals. Ge and Ga appear to be related to each other and to the coals organic matter. Cr and V are also related to each other, as are Ce, La, Nd and Pr, but none of these show any relationship to the organic matter or a particular mineral component.

Quantification of mineral matter in the Argonne Premium Coals using interactive Rietveld-based X-ray diffraction

Abstract The mineral matter in the eight reference North American coal samples of the Argonne Premium Coal series has been investigated on a quantitative basis using X-ray diffraction (XRD) techniques. X-ray diffraction data obtained from electronic low-temperature (oxygen–plasma) ash (LTA) residues, from ashes produced by heating the coals in air at 370°C, and also from the raw coals themselves, were evaluated using an interactive data processing system ( siroquant ™) based on Rietveld interpretation methods. The results from the three types of material (LTA, 370°C ash and raw coal) were compared for each sample. This allowed the components present in the raw coals in crystalline form to be recognised separately from mineral artifacts produced, particularly in the low-rank coals, from interaction of organically associated elements (Ca, S, etc.) during the two ashing processes. After the allowance for the production of any artifacts, the quantitative mineral assemblages identified from XRD of the raw coals were found to be consistent, even for coals having a relatively low ash percentage (around 5%), with the results obtained from the respective mineral concentrates prepared by the ashing methods. The effects of heating the coal to 370°C could also be distinguished, relative to the raw coal or the LTA, through changes in components such as pyrite and the clay minerals. Although some areas of uncertainty exist, particularly with magnesium in the low-rank coals, the calculated chemical compositions of the coal ash derived from the mineral mixtures identified for each coal were also found to be consistent with the results of direct chemical analysis of the respective coal ash materials.

Identification of nanominerals and nanoparticles in burning coal waste piles from Portugal.

A range of carbon nanoparticles, agglomerates and mineral phases have been identified in burning coal waste pile materials from the Douro Coalfield of Portugal, as a basis for identifying their potential environmental and human health impacts. The fragile nature and fine particle size of these materials required novel characterization methods, including energy-dispersive X-ray spectrometry (EDS), field-emission scanning electron microscope (FE-SEM), and high-resolution transmission electron microscopy (HR-TEM) techniques. The chemical composition and possible correlations with morphology of the nanominerals and associated ultra-fine particles have been evaluated in the context of human health exposure, as well as in relation to management of such components in coal-fire environments.

Chemical composition and minerals in pyrite ash of an abandoned sulphuric acid production plant.

The extraction of sulphur produces a hematite-rich waste, known as roasted pyrite ash, which contains significant amounts of environmentally sensitive elements in variable concentrations and modes of occurrence. Whilst the mineralogy of roasted pyrite ash associated with iron or copper mining has been studied, as this is the main source of sulphur worldwide, the mineralogy, and more importantly, the characterization of submicron, ultrafine and nanoparticles, in coal-derived roasted pyrite ash remain to be resolved. In this work we provide essential data on the chemical composition and nanomineralogical assemblage of roasted pyrite ash. XRD, HR-TEM and FE-SEM were used to identify a large variety of minerals of anthropogenic origin. These phases result from highly complex chemical reactions occurring during the processing of coal pyrite of southern Brazil for sulphur extraction and further manufacture of sulphuric acid. Iron-rich submicron, ultrafine and nanoparticles within the ash may contain high proportions of toxic elements such as As, Se, U, among others. A number of elements, such as As, Cr, Cu, Co, La, Mn, Ni, Pb, Sb, Se, Sr, Ti, Zn, and Zr, were found to be present in individual nanoparticles and submicron, ultrafine and nanominerals (e.g. oxides, sulphates, clays) in concentrations of up to 5%. The study of nanominerals in roasted pyrite ash from coal rejects is important to develop an understanding on the nature of this by-product, and to assess the interaction between emitted nanominerals, ultra-fine particles, and atmospheric gases, rain or body fluids, and thus to evaluate the environmental and health impacts of pyrite ash materials.

Quantitative X-ray powder diffraction analysis of clay minerals in Australian coals using Rietveld methods

The mineralogy of clay-rich mineral matter isolated from a range of Australian bituminous coals has been evaluated in quantitative terms from X-ray powder diffraction (XRD) patterns using a Rietveld-based data processing technique. The chemical composition of coal ash derived from this mineral matter has been calculated and compared to the directly determined composition of the ash prepared from the same coal samples. Although there are some minor differences due in part to uncertainty regarding the actual composition of several minerals, the compositions indicated by the two methods show a relatively high correlation, suggesting that the Rietveld technique provides mineralogical analyses that are consistent with independently determined chemical data. Comparison of the normalised clay mineral percentages from the Rietveld analysis to quantitative interpretations based on a peak intensities in glycolated and heat-treated oriented aggregates of the respective clay fractions also shows a high correlation, confirming mutual consistency of the two different mineralogical analysis methods. Such quantitative mineralogical data are significant to a range of coal exploration, mining and utilization activities, including seam correlation, material handling and ash and slag formation in combustion processes.

Minerals in bituminous coals of the Sydney basin (Australia) and the Illinois basin (U.S.A.)

Although other forms of inorganic components may be significant in lower-rank coals, the mineral matter in most bituminous coals is dominated by silicates, carbonates, sulphides, phosphates and other crystalline mineral groups. These may be present in various forms, including thin bands and laminae intimately associated with the organic matter as well as lenticles, nodules, crystal aggregates and cell infillings, all of which indicate approximately contemporaneous formation with the peat bed. Other minerals, however, occur as veins or cleat and fracture fillings, indicating precipitation after most of the compaction and, presumably, after most of the rank advance. ome of the penecontemporaneous mineral matter is of detrital origin, representing epiclastic or pyroclastic particles washed or blown into the peat swamp. Other mineral matter, however, including much of the pyrite, quartz and siderite, as well as abundant well-crystallized kaolinite, appears to have been formed by precipitation processes either within the swamp waters or in the pores of the peat deposit. Pyrite, largely formed by bacterial reduction of dissolved sulphate ions, seems to be related in abundance to marine transgressions, which allowed permeation of sea water into the swamp and through the peat bed beneath. Siderite appears to form in the absence of sulphate-rich sea water, probably from CO2 released by organic decay. The clay fraction of the noncoal rocks in many coal-bearing sequences is typically dominated by illite or interstratified clay minerals, with kaolinite, chlorite and possibly montmorillonite also present to a variable, though typically minor extent. Although perhaps modified by chemical reactions or ion-exchange mechanisms in the peat swamp, these clay minerals may also be present in the coal as well, especially near the base of top of the seam. The kaolinite in the noncoal rocks is usually poorly crystallized and typically a minor constituent, but well-crystallized kaolinite is characteristically present, and often the dominant component in the mineral fraction of bituminous coal beds, as well as in some of the lutites (including underclays) intimately associated with the seams. A progressive upward gradation to a particularly kaolinite-dominated mineral assemblage occurs in some seams of the Sydney basin, Australia. This gradation is thought to be due to either increased blockage of detrital contaminants as the swamp developed, or to in-situ chemical or biological leaching of the mineral fraction and reconstitution of the residues in the upper parts of the peat bed. Superimposition of such cycles is also noted within the seam in some instances, suggesting repetition of several phases of this type during major peat accumulations.

Mineral matter in low-rank coals and associated strata of the Mae Moh basin, northern Thailand

Abstract The mineral and other ash-forming constituents in lignites of the Mae Moh basin in northern Thailand have been identified and evaluated in relation to the minerals present in the associated interseam non-coal strata and the intra-seam non-coal bands. Treatment of the coal with water, ammonium acetate and hydrochloric acid shows that a considerable part of the ash-forming inorganic matter occurs in water-soluble, acid-soluble or ion-exchangeable form. Although the distribution of elements may be modified by the effects of pyrite oxidation, the mobile fraction of the fresh coal is dominated by water-soluble, ion-exchangeable and acid-soluble calcium, water-soluble and exchangeable magnesium, water-soluble sodium and sulphur, and acid-soluble iron and aluminium. Calcium is most abundant, especially in coals where a low proportion of silicate minerals (quartz, clay, etc.) is present; it combines readily with the different sulphur forms on combustion to produce an ash with a distinctively high CaO and SO 3 content. The main minerals in the coals are quartz, pyrite, and a clay fraction consisting of kaolinite, illite and an expandable, irregular mixed-layer mineral. The mixed-layer clay appears to be more abundant in the coal, and illite less so, than in the mudrocks of the associated non-coal strata. The non-coal partings within the main seams contain quartz, pyrite and a similar clay mineral assemblage, but they also contain an abundance of calcite and/or aragonite, and in some cases consist entirely of carbonate minerals. Shell fragments and possibly plant replacement structures are present in some of these materials. Gypsum, bloedite, hexahydrite and other sulphate minerals, and calcite, are deposited as fracture fillings and surface coatings on some exposed open-cut mine faces.

Occurrence of phosphorus minerals in Australian coal seams

Abstract Although the average proportion of phosphorus in Australian coals is similar to that of P in world coal deposits generally (around 0.05% or 500 ppm), a number of individual seams have phosphorus contents above this average. Ply-by-ply studies of selected seams indicate that P is typically abundant in only a few sub-sections or plies; the remaining parts of the seam, even in high-phosphorus coals, commonly have significantly lower phosphorus levels. The bulk of the phosphorus in Australian coals is present as crystalline mineral particles, although some of these may be only a few micrometres in diameter and intimately associated with the organic components. X-ray diffraction of low-temperature oxygen-plasma ash residues, combined with EDAX investigations of polished sections of coal under the scanning electron microscope, show that the phosphorus usually occurs either as apatite or as a solid-solution of Sr, Ba and Ca aluminophosphates that represent minerals of the goyazite-gorceixite-crandallite group. Both apatite and a range of aluminophosphate minerals can occur in the one coal sample, although one phosphate mineral variety is usually the dominant component. The apatite and the aluminophosphates most commonly occur as infillings in the pore spaces and cell cavities of inertinite macerals. They sometimes occur alone, but in many cases are intimately associated with kaolinite and possibly quartz. Later-formed veins filled with either apatite or aluminophosphate minerals also occur in some coal seams, particularly in the more vitrinite-rich sub-sections. These typically cross-cut the pore-filling accumulations. The phosphorus that formed these minerals was probably derived mainly from phospho-proteins in the organic matter of the original peat deposits, although volcanic debris, shells or faecal matter may also have acted as phosphorus sources. The P was apparently released in a more soluble form during plant decay, and then reprecipitated with other available ions in the pores of appropriate parts of the peat bed. Factors such as pH and metal availability were probably critical in determining whether apatite or aluminophosphates were formed. Where they occur, phosphatebearing veins probably represent material remobilised from earlier-formed accumulations and precipitated from groundwaters after the coals had essentially reached their present rank.

SEDNORM-a program to calculate a normative mineralogy for sedimentary rocks based on chemical analyses

A method for calculating a normative mineral distribution, based on the major oxide percentages, has been developed specifically for clastic sedimentary rocks and coal ash. Unlike previous methods, SEDNORM allows some variation in the method of normative calculation depending on the sample type and the range of chemical data available. Options available include changing the clay chemistry, including or excluding certain minerals and, in the absence of certain component analyses, allowing sufficient CO2 and H2O to be added to the system to form selected phases such as calcite until the associated components are exhausted. The method has proved successful for a range of sediment types including sandstones, shales, and carbonate rocks.