Coal cleaning effects

NORTON ET AL. Coal Cleaning Effect on Trace Element Removal... 

Table 1. Effect of Coal Cleaning on Trace Elements ...

C. T. Fold and A. A. Price, "Evaluation of the Effect of Coal Cleaning on Eugitive Elements," Phase II, Part I, Bituminous Coa/Research, BCR Report L-1082,1980. 

Table V. Theoretical Effects of Coal Cleaning on Sulfur Reduction for Two Northern Appalachian Coals Mined in 1982...

Levels of various elements such as As, Cd, Cr, Cu, Pb, Mi, Se, and Zn were reduced by 50% or more in some coals cleaned by combinations of heavy media cyclones, froth flotation cells, and hydraulic classifiers (21-24). Cleaning was less effective for other coals. 

In our own work, the effectiveness was examined of trace element removal by two chemical coal cleaning methods being studied for possible large-scale application. One method involved treatment with aqueous Na,C03 solutions, and the other involved treatment with molten NaOH/KOH mixtures. In each case, acid washes were used as subsequent cleaning steps to decrease the ash content, followed by water washes in the final step. Although the primary objective of these treatments was to remove sulfur and ash-forming minerals, the extraction of trace elements from these coals was examined as an additional benefit of chemical coal cleaning. 

Ford, C.T. Care, R.R. Bosshart, R.E. "Preliminary Evaluation of the Effect of Coal Cleaning on Trace Element Removal" Report 3, Trace Element Program, Bituminous Coal Research, Inc., 1976. 

Growing concern over environmental effects of acid rain has resulted in increased Interest in development of pre-combustion removal of sulfur from coal. Physical coal cleaning processes are effective for pyritic sulfur removal but do little to reduce the organic sulfur content of coal This paper reports the removal of organic sulfur from coal, employing ethyl or methyl alcohols as the solvent/ reactant. The process is based on the observation that, under supercritical conditions, reactions occur that selectively remove organic sulfur from the coal matrix. 

The vast majority of the coal cleaned commercially is separated by processes that rely on the density differential of the various components. Technologies that make separations on this basis are particularly effective for treating the coarse to intermediate sizes of coal (i.e., particle sizes ranging from greater... 

Fig. 19.6. Different levels of coal cleaning and the effect on coal recovery and economics.

Coal cleaning, which involves the separation of some of the ash from the coal prior to liquefaction, can yield several benefits. Primarily, it reduces the load on the solid-liquid separator, thereby reducing the amount of organic material rejected with the ash. Other benefits include the reduction of wear on plant components and better utilization of reactor volume. However, removal of mineral matter prior to reaction may eliminate beneficial catalytic effects that coal minerals, particularly iron pyrite, provide. 

Mineral matter analysis can provide a more accurate and complete representation of the minerals present in bituminous coal and the effects of beneficiation processes upon them. Comparisons with conventional analytical methods are made which demonstrate some of the potential advantages of this technique in the monitoring or design of coal cleaning operations. 

Thus, coal preparation is, of necessity, an integral part of the production and use of coals. The effect on costs can be as important as the planning of mine layouts decisions concerning mining systems should be an essential element in aU mining feasibility studies, especially in view of new (and/or renewed) environmental regulations such as the Clean Air Act Amendments in the United States (Elliott, 1992 Tumati and DeVito, 1992 Rosendale et al., 1993 Paul et al., 1994). 

In the context of coal cleaning, oxygen is often considered unimportant because it is nonpolluting. But some consideration should be given to the effect of oxygen on the fate of its nitrogen and sulfur compatriots as well as its effect when water (a product of the combustion of hydrocarbons which also contain oxygen) condenses with other by-products on the cooler parts of combustion systems corrosive, aqueous acids can be the result. 

In coal cleaning by cyclone (Figure 6.21), raw coal and fluid enter tangentially close to the top of the cylindrical section, forming a powerful vertical flow (Couch, 1991). The separation of the impurity from the coal occurs due to a buoyancy effect which arises because of the difference between the mass of any particle and mass of an equivalent volume of displaced fluid. When the fluid is water, this factor is equivalent to the specific gravity. The densest particles therefore move to the... 

Rosendale, L.W., DeVito, M.S., Conrad, V.B., and Meenan, G.F. 1993. The effects of coal cleaning on trace element concentration. Air and Waste Management Association, Annual Meeting, Denver, CO, June. 

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