Association of mineral matter

Advances in Quantitative Assessment of the Association of Mineral Matter with Coal... 

Ash is the product of combustion of mineral matters associated with coal. It consists principally of silica, alumina, and ferric oxide with variable amounts of other oxides such as CaO, MgO, and NazO. A high ash content is in general not desirable. A coal with high ash... 

Table I shows that most of the South African bituminous coal contain high quantities of mineral matter which is often intimately associated with the organic matter of the coal. About half of the resources yield between 30 and 35 per cent ash.

Particulates include solid-phase materials entrained in the raw product gas as it exits the gasifier. They include the inorganic ash that is associated with mineral matter in the biomass feedstock, unconverted biomass in the form of char, or material from the gasifier bed. 

The first method chosen to express the coal-mineral association results is in terms of the weight fraction of mineral matter in the individual particles, as determined from their cross section. The resulting distribution is comparable to the so-called "grade distributions" used in the mineral industry [8,9]. Such a distribution is included in Table II for the Upper Freeport coal. The data in the table indicate that pyrite is preferentially liberated as compared to quartz or kaolinite. About 78% of the pyrite is in particles containing more than 80% mineral matter, which should be easily removed by density-based separations. 

Figures 6 and 7 show the coal-mineral association for the same Upper Freeport coal sample expressed in terms of the amount of mineral matter on the surface of the particles. There is considerable difference between these two figures and Figures 4 and 5. While Figure 4 shows that about 80% of the mineral matter is present in particles containing more than 40% mineral matter (i.e., less than 60% coal). Figure 6 indicates that only 16% of the mineral matter is associated with particles for which more than 40% of the surface is covered by mineral matter. Indeed, about 70% of the mineral matter is found in particles with more than 80% of the surface covered by coal. Also, the preferential liberation of pyrite noted in terms of bulk composition is absent when association is measured in terms of particle surface.

The above conclusions based on SEM-AIA measurements of association in terms of particle surfaces are somewhat more tentative than the conclusions drawn from bulk association distributions. There are more analytical difficulties when characterizing the particle surfaces than when characterizing the bulk sample. In addition, the factors determining the overall surface nature of a particle are more complex than just the relative amount of the phases present on the particle surface. However, the SEM-AIA results can still provide a useful and heretofore unavailable insight into the nature of mineral matter in coal. 

Analysis of coal-mineral association in samples of Upper Freeport and Indiana No. 3 coals showed significant differences in the association of minerals with the coal matrix. In terms of bulk properties, the mineral matter was generally more liberated in the Upper Freeport coal than in the Indiana No. 3 coal. Also, based on bulk properties, pyrite was found to be preferentially liberated in the Upper Freeport coal, while no such preferential liberation was observed for minerals in the Indiana No. 3 coal. For both coals, the... 

Coal contains detrital minerals that were deposited along with the plant material, and authigenic minerals that were formed during coalification. The abundance of mineral matter in coal varies considerably with its source, and is reported to range between 9.05 and 32.26 wt% (Valkovic 1983). Minerals found in coal include (Table 2) aluminosilicates, mainly clay minerals carbonates, such as, calcite, ankerite, siderite, and dolomite sulphides, mainly pyrite (FeS2) chlorides and silicates, principally quartz. Trace elements in coal are commonly associated with one or more of these minerals (see Table 2). 

Physical cleaning of various coals by oil agglomeration reduced levels of As, Cr, Pb, Mn, Mo, Ni, and V by 50-80%, while levels of some other trace elements were reduced by lesser amounts (20). Oil agglomeration appeared to be more effective at removing trace elements than the wet concentrating table or float/sink density separations. This may be related to an increase in the liberation of mineral matter associated with grinding to produce the relatively fine particle sizes required in the oil agglomeration technique. 

Dormaar, J.F., 1968. Infrared absorption spectra of mineral matter in saxicolous lichens and associated mosses. Can. J. Earth Sci., 5 223—230. 

With regard to the designation of humin as a separate fraction, it is possible that this material consists of portions of other fractions so intimately associated with mineral matter that they cannot be solubilized by extraction with alkali. Also, it is not known whether hymatomelanic acid is a distinct chemical entity. This material may be an artifact produced from humic acid during fractionation. The simple process of redissolving the alcohol-insoluble material in alkali followed by reprecipitation with acid results in a further increase in alcohol-soluble material. 

Thirty-four minor and trace elements are of potential environmental concern (n ). Sulfur is the element of major concern due to its abundance in flue gases from some coal-burning plants and its subsequent contribution to "acid rain." Sulfur as acidic ions of sulfate can also contribute to pollution of surface water and groundwater. Other elements of greatest concern are As, B, Cd, Pb, Hg, Mo, and Se. With the exception of B and Se, these elements are strongly associated with mineral matter in the coal and are concentrated in waste piles from coal preparation plants. If the waste disposal site is not constructed as a closed system, pollution of nearby groundwater is possible. Boron and Se may contribute to the pollution risk as they are associated with both mineral and organic components. On the other hand, certain coal-mine wastes have potential for recovery of valuable metals such as zinc and cadmium (18). 

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