Clay–coal charge transfer and beneficiation by dry mineral removal

Most coal requires upgrading (beneficiation) prior to burning to reduce the amount of ash left after combustion. Normal washing techniques may produce large amounts of clayey sludge requiring expensive storage behind dykes and dams.Dry coal powder, pulverized to 150 μm for burning, is suitable for dry beneficiation using electrostatic techniques. Such a process would eliminate clay sludge. Agitation of the powdered coal and its mineral wastes results in a charge transfer on contact, the coal becoming positive and the mineral matter negative. Separation in an electrostatic tower produces high quality coal and dry clay mineral waste.Clay minerals form the main component of the ash in most Canadian coals; kaolinite in eastern coals and montmorillonite in western coals. The cumulative amount of charge transfer controls the efficiency of separation. The test results show that the cumulative amount of charge transfer increases with increasing number of contacts, montmorillonite generating four times more charge than kaolinite at a given dry density. Typical values of cumulative charge after 100 contacts on montmorillonite and kaolinite pellets of 1.9 g/cm3 dry density are 0.3 × 10−10 and 0.07 × 10−10 C, respectively.