Trace Element Removal During Physical and Chemical Coal Cleaning

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Abstract
Trace elements in coal are detrimental from both an environmental and a technological perspective. Since most trace elements in coal are associated primarily with the mineral phases, they are often amenable to removal by conventional physical coal beneficiation methods. However, these methods generally are not adequate for removing those trace elements which are associated with the organic matrix or which are very finely disseminated. Thus, chemical treatments may be necessary for removal of these trace components. Data from a variety of studies indicate that cleaning by concentrating tables, jigs, heavy-media cyclones, hydrocyclones, froth flotation, and oil agglomeration could effectively remove many trace elements from some coals. However, trace element removal efficiencies varied widely between coals. Because cleaning efficiencies are strongly coal-dependent, conclusions regarding the relative effectiveness of various cleaning techniques are difficult to draw since different coals were used among studies. Although trace element data on chemically cleaned coals are relatively sparse, the available information shows that cleaning with chemical agents such as aqueous Na2Co3, molten NaOH/KOH, acidic ferric sulfate solutions, chlorine gas, and aqueous NaOH/Ca(OH)2 effectively remove many trace elements from some coals. However, the nature of the chemicals and the relatively severe reaction conditions used in some cases necessitate that care be taken to avoid trace element contamination from the corrosion of reaction vessels. Although definitive conclusions are hard to draw in view of the different coals used in the various studies, chemical cleaning appeared to be more effective at removing trace elements from coal than conventional physical cleaning techniques.