The potential of QEMSCAN in predicting coal washability of Mpumalanga coals

Abstract

Traditional float and sink analyses are undertaken to determine the washability of coal. Float and sink analyses are costly, require toxic heavy organic liquids and the procedure to wash and dry the float and discard fractions is lengthy. QEMSCAN has the ability to characterise particle density based on the mineralogical composition of the particle. The objective of this research is to determine if QEMSCAN is a viable alternative to float and sink analysis. Float and sink analysis typically requires coarse size fractions while QEMSCAN analysis requires samples to be crushed down to 1mm. Any crushing will liberate minerals, which will alter the particle density distribution. Crushing a large particle generates 'puzzle pieces' of the original particle. The smaller 'puzzle pieces' have densities frequently different to the original particle. A mineralogical based particle density prediction model confirms that the float and sink analysis data used in this study is valid. The measured ash contents for the different float and discard fractions were within the expected limits. It is observed that there are a set of controls over the liberation of particles when crushed. Particles in the low float fractions (<1.6g/cm3) predominantly comprise vitrinite rich coal with fine lamellae of kaolinite. The higher float fractions (>1.6-2.0g/cm3) comprise bright and dull coal incorporated into an 'inertodetrinite' texture. Cleats and kaolinite laminae serve as preferential cleavage planes in the lower density fractions, while bright and dull coal serve as preferential breakage planes in the higher float fractions. As a result, these phases are liberated and there is evidence to support that liberation of minerals have controls that can be identified and corrected for. Thus, washability can be determined using QEMSCAN since the significant effect of liberation can be calculated and corrected, for a specific coal type.