Pyrite surface characterization and control for advanced fine coal desulfurization technologies. Tenth quarterly technical progress report, January 1, 1993--March 31, 1993

The objective of the project is to conduct extensive fundamental studies on the surface chemistry of pyrite oxidation and flotation and to understand how the alteration of the coal-pyrite surface affects the efficiency of pyrite rejection in coal flotation. This report summarizes the studies in the following three aspects: (1) the effects of borate, used as pH buffer or electrolyte, on the pyrite surface oxidation and flotation; (2) the quantification of pyrite surface oxidation kinetics under different oxidation potentials; and (3) finding new coal-pyrite depressants. It has been demonstrated, for the first time, that borate, a pH buffer and electrolyte used by many previous investigators in studying pyrite oxidation, actively participates in the surface oxidation of pyrite. In high borate concentration solutions, the surface oxidation of pyrite is strongly enhanced. The anodic oxidation potential of pyrite is lowered by more than 0.4 volts. At low borate concentration, borate is chemisorbed on pyrite surfaces. In the intermediate concentration range, borate dissolves surface iron compounds. Consequently, the flotation of pyrite in borate solutions (using fuel oil as collector) displays depression-flotation-depression phenomena as the borate concentration is increased. The oxidation kinetics of pyrite surfaces has been determined by AC impedance spectroscopy. At low oxidation potentials, only capacitive behavior is observed. However, at high oxidation potentials, an inductive loop appears. The charge transfer resistance decreases with increasing potential, indicating that the oxidation rate increases with increasing potential. A chemical reagent has been found to be very effective in depressing the flotation of coal-pyrites from different sources, while it has little effects on the flotation of coal. The surface chemistry involved in the selective pyrite depression by this new reagent has been investigated by electrochemical studies and contact angle measurements