Mineral matter associated 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... 

The mineral matter associated with coal undergoes various transformations during the coal gasification process, with the temperature of transformation of pyrite depending on either the reducing or oxidizing conditions present in the gasifier. The temperatures may vary due to possible trace elements present in the pyrite structure [16,17]. 

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. 

It is generally appreciated that the mineral matter associated with some coals may act as a catalyst for liquefaction. A common observation is that among bituminous coals from the eastern United States, those with a relatively high mineral matter content also provide relatively high liquefaction yields. Also, addition of coal-derived mineral matter increases the liquefaction yields from those coals with low mineral matter content. The involvement of pyrite in these effects has been fairly well established. The importance of clay and/or other minerals is less well defined. 

The mineral matter associated with the coal burnt in a combustor may be broadly classified ... 

Reactions of Goal Ash. Mineral matter impurities have an important effect on the utili2ation of a coal. One of the constituents of greatest concern is pyrite because of the potential for sulfur oxide generation on combustion. The highest concentrations of pyrite are associated with coal deposition under marine environments, as typified by the Illinois Basin, including parts of Illinois, Indiana, and Kentucky. Additionally, the mineral matter... 

Fluorine. Tables X, XI, and XII show the results obtained for fluorine in the coal-washing study with coals from Pennsylvania, Iowa, and West Virginia. In all three coals, the fluorine appears to be associated with the mineral matter in the coal. Therefore, removal of the higher specific gravity fractions would lower the fluoride content of the remaining coal. 

Because of the importance of coal as a major source of energy and the environmental hazards involved in its use, considerable research has become necessary in order to understand fully the different compounds appearing in the coal and their transformation during processing and to know how those compounds contribute to the pollution of the environment, i.e., acidity of water streams near the coal mines and pollution by power plants. Some positive properties can be associated with the mineral matter in coal. For example, recently several researchers have shown that the mineral matter in the coal may play an important role in the liquefaction process (i). Of all the minerals in the coal, the iron-bearing minerals seem... 

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). 

The fifth session looked at the possibility that there may be some desirable aspects associated with the mineral matter in the coal. The mineral matter may be a catalyst for some of the current or future uses of coal. Specifically, the mineral matter could have some effect on combustion and also on future synthetic fuels efforts that could provide either gaseous or liquid fuels. 

Impurities. The three categories of potential pollutants in coal are sulfur, nitrogen, and ash. Sulfur and ash are associated with both the mineral and organic portions of coal, whereas nitrogen is mainly associated with the organic matter (4). 

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.

A notable feature of the Western Canadian coals is their low sulphur content (usually <0.5%) which tends, however, to be partly offset by higher mineral matter contents than are associated with the Eastern coals. As well, bituminous coals in the mountain belts are typically deficient in vitrinite, which often represents less than 50% of the coal "substance" and only occasionally reaches 70-75%, but this is compensated by the fact that their micrinites and semifusinites tend to be "reactive" constituents when the coals are carbonized. Notwithstanding their low fluidity (rarely >1000 dd/min), Western mvb coals therefore make excellent metallurgical cokes when carbonized in suitably proportioned blends. 

The size distribution of the kaolinite is quite variable and again the Kingston sample differed in that most of the kaolinite has sodium associated with it and could therefore be considered a smectite rather than kaolinite. Marcasite is commonly present as a replacement of original coal particles. The gypsum is present mainly in the -20 ym size range. Overall the mineral matter content of the lignites is of the order of 5%-10% (i.e. a medium distribution density). 

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

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.

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