Sphalerite in coals from the Illinois Basin

Abstract

Chemical analyses of 81 samples of coal from the Illinois Basin suggested the occurrences, in many instances, of a separate zinc sulfide phase. Field investigations proved this, and a number of sphalerite samples were collected for study from coals in northeastern, southeastern, and northwestern Illinois. The highest concentration of sphalerite in Illinois coals is in northwestern Illinois where sphalerite is present in all three of the coals mined. The sphalerite is generally cadmium rich; ratios of zinc to cadmium in the sphalerite samples extracted from coals range from 48/1 to 358/1. The Zn/Cd for sphalerite samples from Illinois and Kentucky that were collected from strata other than coal range from 29/1 to 1,050/1.Sphalerite occurs in coal principally as fillings in cleats (vertical fractures). Pyrite, quartz, kaolinite, and calcite are associated with the sphalerite. The paragenetic sequence began with a sulfide stage, continued with a silicate stage, and concluded with a carbonate stage. The sphalerite is epigenetic and was deposited slowly under conditions near equilibrium. The cleats provided channels in the coal along which the mineralizing solutions could move, and the coal in turn provided a suitable substrate for the bacterial reduction of sulfate to sulfide and/or a source of reduced sulfur.The sphalerite is readily removed from the coal by washing (separation by specific gravity techniques) and can be recovered. A zinc content as high as 5,350 ppm and cadmium content as high as 65 ppm, both on a whole coal basis, were observed. The economic recovery of these elements should be considered, especially when coal conversion processes are being planned.