Kinetics of Pyrite Oxidation in Simulated Mine Tailings Pond Conditions

Abstract

Pyrite oxidation is the predominant source of acid mine drainage (AMD), which is one of the severe environmental impacts of mining. AMD destroys flora and fauna in the nearby ecosystem, and it causes grave health problems to the people. To further the understanding and help in the mitigation of AMD, this electrochemical study investigates the effect of the type and concentration of oxidant, and the presence of impurity (chalcopyrite) to the rate of pyrite oxidation, while simulating the conditions in tailings ponds. Open Cell Potential (OCP) and Electrochemical Impedance Spectroscopy (EIS) analysis were done using a three-electrode cell set-up, which is composed of a pyrite rotating disc electrode (RDE), mercury/mercurous sulfate reference electrode in saturated potassium sulfate, and a platinum counter electrode, with a sulfuric acid working electrolyte. Polarization resistance obtained after fitting an electrical circuit to the EIS data was related to the rate of pyrite oxidation, while OCP was used to describe its oxidation mechanism. It was found out that the oxidation of pyrite by ferric ions preferentially occurred rather than by dissolved oxygen. The parameter setting with the lowest relative oxidation rate were: electrolyte with no ferric addition, and with nitrogen purging; and RDE with 10 % chalcopyrite addition. Reaction orders calculated for the effect of ferric ions and dissolved oxygen were equal to 1; but ferric has a higher slope value in the plot. Presence of chalcopyrite in the RDE caused the preferential oxidation of chalcopyrite instead of the pyrite due to their galvanic interaction. To minimize pyrite oxidation, ferric and dissolved oxygen concentration should be minimized, and chalcopyrite impurities should be increased. Keywords: Electrochemical Impedance Spectroscopy, Acid Mine Drainage, Kinetics, Rotating-Disc Electrode, Pyrite