Triboelectric separation of granular materials

Triboelectric separation of granular materials is based on either the difference in electrical resistivity of the materials or on the difference in their effective work functions. We discuss here triboelectric separation based on different work functions of the materials. In this case, two granular materials to be separated from each other must acquire electrostatic charges of opposite polarity when tribocharged against a surface of a third material. While triboelectric separation of materials has been well documented in literature, the fundamental mechanisms of electrostatic charging and separation remain poorly understood primarily because of our lack of understanding of the electronic surface structures of particulate materials. The molecular orbital structures of the surface atoms of insulating and semiconducting powders exposed to moisture and contaminants are often unpredictable; as a result, experimental studies are needed to determine charging characteristics of the materials under different ambient conditions. We present here results of our studies on separating coal particles from mineral impurities for coal powders charged in a copper static charger. A parallel plate-separator was used for beneficiation of coal and experimental data are presented on the sulfur content of the powder samples taken from the "clean plate" and the "refuse plate". The effects of moisture content and exposure to oxygen on the charging level are discussed with experimental data.