Mineral Matter and Ash Analysis

Moisture and ash (Chapter 3) are not determined as a part of the data presented for ultimate analysis but must be determined so that the analytical values obtained can be converted to comparable bases other than that of the analysis sample. In other words, analytical values may need to be converted to an as-received basis, a dry basis, or a dry, ash-free basis. When suitable corrections are made for any carbon, hydrogen, and sulfur derived from the inorganic material, and for conversion of ash to mineral matter, the ultimate analysis represents the elemental composition of the organic material in coal in terms of carbon, hydrogen, nitrogen, sulfur, and oxygen. 

When the product coal shown in Figure 3 was subjected to low temperature ashing as described above and that ash product subjected to particle size analysis, a curve as is illustrated in Figure 4 results. Clearly the enveloped mineral matter in the product coal particles is considerably smaller in diameter than the coal particles from which they came and as such are not available for separation by the T-Process. The T-Process separation rejects all particles of pure mineral matter and agglomerates as product coal all particles that have any fraction of coal exposed to the liquid system. 

Each of these is reported in the proximate analysis. Coal analyses can be reported on several bases as-received, moisture-free (or dry), and mineral-matter-free (or ash-free). 

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. 

Two types of coal ash samples have been prepared routinely for analysis at the Illinois Geological Survey. Low-temperature ash samples (12), in which the bulk of the mineral matter remains unchanged, are prepared by reaction of the coal with activated oxygen in a radiofrequency field. The effective temperature produced by this device is approximately 150 °C. Such samples were unsatisfactory for emission spectroscopic analysis. It is postulated that the presence of largely unaltered mineral matter, such as carbonates, sulfides, and hemihydrated sulfates (12), caused the observed nonreproducibility of results. High-temperature ash samples, prepared in a muffle furnace, consisted mainly... 

The elemental analyses of the products from the extraction of Bruceton coal are shown in Table III. The mineral matter was separated from the extract quite efficiently as shown by the ash content of the extracts and the insoluble residue. The elemental composition of all fractions was quite similar to that of the original coal. Only the hydrogen content varied to some extent, increasing with increased solubility. The elemental analysis of the products from the extraction of Ireland Mine coal was incomplete. 

The presence of extraneous mineral matters is detected as follows 4-5 grams of the powdered substance are shaken in a test-tube with chloroform and then left to stand any mineral matter then settles at the bottom of the tube, wliilst the starch floats at the surface of the liquid. Analysis of the ash of the product by the ordinary methods indicates the nature of the inorganic substances. 

Analysis of ebonite is more difficult than that of ordinary vulcanised rubber, as it is less readily attacked by solvents the sample for analysis should be finely powdered. The determinations of moisture, ash, sulphur, etc., are made as in manufactured rubber the extraction with acetone should be prolonged, sometimes to 1-2 days, to be complete. The residue insoluble in acetone is extracted first with epichlorhydrin for 3 hours to remove resins insoluble, or almost so, in acetone (copal, mastic, amber) and then with pyridine as indicated for manufactured rubber next comes the treatment with alcoholic potash to dissolve any brown factis present. The residue from this last treatment comprises the pure rubber, the sulphur combined therewith and the mineral matter in one part of it the ash and the sulphur of the ash are determined, and in another the total sulphur, the sulphur united with the rubber being obtained by difference the pure rubber is then calculated by difference. 

The ash of true leather tanned with tannin consists essentially of calcium carbonate with traces of iron and of phosphates. Coloured leathers may contain metals from the mordants used (tin, copper, iron, chromium, aluminium) tin may also be introduced as stannous chloride used for bleaching. Small quantities of silicates (talc, kaolin) may be employed in the treatment of the leather. Finally, other mineral matters (barium, magnesium and lead salts and sodium chloride) may have been added as filling to increase the weight. Complete quantitative analysis of the ash is rarely necessary, but determination of its calcium content is sometimes required, this being made by the ordinary methods. 

Ash.—The dry residue from the determination of the water is incinerated and the ash weighed. If this exceeds 1%, adulteration with mineral matter is probable, this being confirmed by qualitative analysis of the ash. Such analysis is useful in any case to detect the presence of heavy metals (especially iron), which may be introduced during the manufacturing processes and are harmful in the dyeing. The ash of alizarin consists normally of sodium or calcium salts. 

Thus, to classify coal, the calorific value and a proximate analysis (moisture, ash, volatile matter, and fixed carbon by difference) are needed. For lower-rank coals, the equilibrium moisture must also be determined. To calculate these values to a mineral-matter-free basis, the Parr formulas are used (ASTM D-388). 

A wide range of trace elements occurs in coal, primarily as a part of the mineral matter. The release of certain trace elements into the environment as combustion products or in the disposal of ash is a concern for coal-burning facilities. Determination of certain trace elements in coal and coal ash is becoming an increasingly important part of coal analysis. 

It is therefore impossible to determine accurately the composition of the pure coal substance from the usual ultimate analysis simply by making allowance for the quantity of ash left behind as a residue when the coal is burned. Results obtained in this fashion are, as a consequence, quoted as being on a dry, ash-free basis, and no claim is therefore made that these results do in fact represent the composition of the pure coal substance. If, however, it were possible to calculate accurately the quantity of mineral matter originally present in the coal sample, then by making due allowance for this material, the composition of the pure coal material could be deduced with reasonable precision and certainly with a greater accuracy than could be obtained by adopting the analytical figures calculated to a dry, ash-free basis. 

The evaluation of coal mineral matter by the ashing technique can be taken further insofar as attempts can then be made to determine the individual metal constituents of the ash. On the occasion when the mineral matter has been separated from the coal successfully, it is then possible to apply any one of several techniques (such as x-ray diffraction, x-ray fluorescence, scanning electron microscopy and electron probe microanalysis) not only to investigate the major metallic elements in coal but also to investigate directly the nature (and amount) of the trace elements in the coal (Jenkins and Walker, 1978 Prather et al., 1979 Raymond and Gooley, 1979 Russell and Rimmer, 1979 Jones et al., 1992). Generally, no single method yields a complete analysis of the mineral matter in coal and it is often necessary to employ a combination of methods. 

ASTM method D3175 (H), and forms of sulfur by ASTM method D2492 (21). Elemental analysis of the ash was performed using ASTM method D3682 (22). Carbon aromaticity was determined using 13c NMR CP-MAS procedures described elsewhere (13). X-ray powder diffraction analysis of the mineral matter in the whole coal was performed using a Rigaku powder dif-... 

Mineral matter was a Deis ter table concentrate from Robena mine coal. It contained 68% pyrite and less than 4% organic material. The remainder was largely clay. In one case, a handpicked sample taken from a pyrite nodule found in a Pittsburgh seam coal was used. The microcrystals were crushed and sieved to 325 x 400 mesh. X-ray diffraction analysis indicated the only major component was pyrite, with a trace of marcasite also present. After heating in tetralin at 450°C for 15 min., the X-ray diffraction patterns of the recovered microcrystals indicated conversion was complete to pyrrhotite 1C. The coal was hvB, Homestead Mine, Kentucky, ground to pass 200 mesh. Ash and pyrite contents were 16.8% and 4.9%, respectively. The asphaltene was a homogenized mixture of samples isolated from liquid products derived from Pittsburgh seam, hvA coal. Its ash content was <0.1%( ). 

Analytical methods are described in (2). Standard methods were employed for total organic carbon (TOC) and Fischer Assay (FA) analyses. Kerogen was isolated by solvent extraction to remove bitumen and acid dissolution to remove mineral matter. Elemental analysis was performed only on kerogen with low ash content (<25% by weight). 

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