THE LOG-NORMAL DISTRIBUTION OF PYRITIC INCLUSIONS IN COAL - CONSEQUENCES FOR DEPYRITIZATION

A computer model is described for the geometry of the coal-pyrite system based on the log normal size distribution for the pyritic inclusions and specific assumptions about what happens when coal is ground. It gives approximate predictions for the fraction of pyrite that sinks during float/sink tests, the fraction accessible to microbial attack, and the fraction of the exposed surface area that is pyrite, all as a function of coal particle size. Results suggest that the 'representative sample volume' recommended for pyrite analysis may not be adequate for some coals. While 50% pyrite reduction can be achieved by removing a few thousand inclusions per cm3 from a typical Ohio coal, 90% reduction requires the removal of several million small (approximately 10 mum) inclusions. Microbial degradation should be considered for these small inclusions because it requires less coal grinding than physical separations. The tendency of coal to fracture along planes containing pyritic inclusions is illustrated by comparing the model with X-ray fluorescence measurements of the exposed pyrite surface area.