Minerals in coal

Concern over the release of hazardous trace elements from the burning of coal has been highlighted by the 1990 Clean Air Act Amendments. Most toxic elements are associated with ash-forming minerals in coal (5). As shown in Table 1, levels of many of these toxic metals can be significantly reduced by physical coal cleaning (6). 

Despite considerable effort to understand the chemical nature of the organic and inorganic portions of coal, and substantial advances in instrumentation and methodologies, much of coal s chemical nature remains intractable. This paper reviews methods and their limitations for the determination of specific minerals in coal, and presents results of efforts at Consolidation Coal Company to develop FTIR methods for routine coal mineralogy. 

In the next section, the potential for factor analysis and related chemometric techniques for providing improvements in the determination of minerals in coal by FTIR are explored. 

Several dozen minerals are reported in coals, although most of these occur only sporadically or in trace amounts. The overwhelming majority of the minerals in coal are in one of four groups aluminosilicates, carbonates, sulfides, and silica (quartz) (Figure 1). 

Sulfides and Sulfates. Pyrite is the dominant sulfide mineral in coal. Marcasite has also been reported from many different coals. Pyrite and marcasite are dimorphs, minerals that are identical in chemical composition (FeS2) but differ in crystalline form pyrite is cubic while marcasite is orthorhombic. Other sulfide minerals that have been found in coals, and sometimes in significant amounts, are sphalerite (ZnS) and galena (PbS). 

Carbonates. The carbonate minerals, in general, vary widely in composition because of the extensive solid solution of calcium, magnesium, iron, manganese, etc. that is possible within them. There is also a wide range of mineral compositions for the carbonate minerals in coals. The relatively pure end members, calcite (CaC03) and siderite (FeC03),... 

Clay Minerals. The clay minerals in coal all contain water bound within their lattices. Kaolinite contains 13.96%, illite 4.5%, and mont-morillonite 5% bound water. In addition, the montmorillonite in the mixed-layer clays also contains interlayer or adsorbed water. All of the water is lost during the high-temperature ashing. 

Identification of Minerals in Coal. Once the low-temperature mineral matter residue has been obtained by radiofrequency ashing, the minerals can be identified, and their concentrations can be determined by a variety of instrumental techniques. The best developed, most inclusive, and probably most reliable method used thus far in distinguishing minerals in coal is x-ray diffraction analysis. It has been used extensively by Gluskoter (15), Wolfe (17), O Gorman and Walker (2), and Rao and Gluskoter (1) and has been somewhat successful in quantifying mineral analyses. 

The modern investigations of trace elements in coals were pioneered by Goldschmidt, who developed the technique of quantitative chemical analysis by optical emission spectroscopy and applied it to coal ash. In these earliest works, Goldschmidt (31) was concerned with the chemical combinations of the trace elements in coals. In addition to identifying trace elements in inorganic combinations with the minerals in coal, he postulated the presence of metal organic complexes and attributed the observed concentrations of vanadium, molybdenum, and nickel to the presence of such complexes in coal. 

The carbonate minerals in coals occur primarily as epigenetic fracture fillings (cleat filling). Magnesium, iron, and manganese are often associated with the sedimentary carbonate minerals and would reasonably be expected to be associated with the cleat fillings in coal. 

The literature concerned with trace elements and minerals in coals has increased in quantity as the interest in these materials, caused by concern about environmental quality and energy availability, has increased. A number of excellent review articles, particularly that by Watt (43), have been published and provide comprehensive summaries of the existing literature. 

This excellent review of minerals in coal and the chemical composition of coals does not, as the title may suggest, limit itself to British coals. The literature of the rest of Europe and of North America is generously cited. A large section devoted to the various methods of determining the amount of mineral matter is included. There is also a section concerned with the methods of identifying mineral species in coals. 

R. L. Bond In studying high temperature systems the problems associated with minerals in coal should also be investigated, and 1 would make a plea that as much work should be done on the minerals at high temperatures as on the carbon/hydrogen-containing particles. 

Environmental issues with arsenic-bearing sulfide minerals in coal and ore deposits... 

Coals are sedimentary and sometimes metamorphic rocks that contain at least 50 wt % and 70 vol % combustible organic materials, which are often valuable sources of energy (Craig, Vaughan and Skinner, 2001), 503. Unlike many other sedimentary and metamorphic rocks, coals often contain substantial pyrite and other sulfide minerals. The organic matter and sulfide minerals in coals frequently have strong affinities for arsenic. 

There are two types of minerals in coal (1) extraneous mineral matter and (2) inherent mineral matter. Extraneous mineral matter consists of materials such as calcium, magnesium, and ferrous carbonates pyrite marcasite clay shale sand and gypsum. Inherent mineral matter represents the inorganic elements combined with organic components of coal that originated from the plant materials from which the coal was formed. 

A large number of distinct mineral phases have been reported in various coals (Table 5.1), although lists of minerals in coal may contain as many as 50 to 60 minerals, most fall into one of five groups (1) aluminosilicate minerals (clay minerals), (2) sulfide and sulfate minerals, (3) carbonate minerals, (4) silicate minerals (principally quartz), and (5) other minerals that include minerals that may occur in trace amounts or may be specific to a particular coal having originated because of the localized deposition and maturation conditions (Speight, 1994, and references cited therein). 

The dimorphs pyrite (FeS2) and marcasite (FeS2) are the dominant sulfide minerals in coal pyrite is the more abundant. Pyrite and marcasite have different crystal forms pyrite is isometric and marcasite is orthorhombic. These two minerals are readily observed and, to some degree, easily removed as well as being especially interesting because they contribute significantly to the total sulfur content that causes boiler tube fouling, corrosion, and pollution by emission of sulfur dioxide when coal is burned (Beer et al., 1992). 

The major cations found in the carbonate minerals in coals are calcium, magnesium, and iron. The rather pure end member calcite (CaC03) is dominant in some coals, whereas siderite (FeCO ) is dominant in others. Calcite and ankerite (a mixed crystal composed of Ca, Mg, and Fe carbonates) are abundant in some coals. 

Infrared absorption bands from 650 to 200 cm 1 have been used extensively to analyze, both qualitatively and quantitatively, minerals in coal ash. One such test method (ASTM D-5016) is a procedure using a high-temperature tube furnace... 

Scanning electron microscopy with an energy-dispersive x-ray system accessory has been used to identify the composition and nature of minerals in coals and to determine the associations of minerals with each other. Examinations can be made on samples resulting from ashing techniques or whole coal. With this technique it is possible to identify the elemental components and deduce the mineral types present in coal samples. Computerized systems to evaluate scanning electron microscopy images have been developed and are useful in characterizing the minerals in coal mine dusts and in coal. 

Recent studies on iron sulfide minerals in coals, minerals in coals, and in situ investigation of minerals in coal all used the scanning electron microscope (SEM) as the primary analytical tool. The ion microprobe mass analyzer (IMMA) is more sensitive than either the energy-dispersive x-ray spectrometer or the wavelength-dispersive x-ray spectrometer, both of which are used as accessories to an electron microscope. 

Optical microscopy is another method that has been used to determine the distribution of minerals in coal. This method is based on the detailed microscopic examination of polished or thin sections of coal in transmitted and/or reflected light. In principle, observing several of its optical properties, such as morphology, reflectance, refractive index, and anisotropy, makes identification of a mineral type possible. 

But we must speak of candles as they are in commerce. Here are a couple of candles commonly called dips. They are made of lengths of cotton cut off, hung up by a loop, dipped into melted tallow, taken out again and cooled, then re-dipped until there is an accumulation of tallow round the cotton. In order that you may have an idea of the various characters of these candles, you see these which I hold in my hand-they are very small, and very curious. They are, or were, the candles used by the miners in coal mines. In olden times the miner had to find his own candles and it was supposed that a small candle would not so soon set fire to the fire-damp in the coal mines as a large one and for that reason, as well as for economy s sake, he had candles made of this sort-20, 30, 40, or 60 to the pound. They have been replaced since then by the steel-mill, and then by the... 

XRF data on coals treated with molten caustic at fitnes Laboratory showed that levels of Fe, which is predominantly associated with pyrite in coal, were reduced by about 90% for each of the two runs. Reductions in concentrations of other elements which form abundant minerals in coal, including Al, K, and Si, were also substantial. Because roughly 90% of the ash was removed, substantial concentration reductions in the major mineral-forming elements are expected. 

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