Suppression of Pyrite Oxidation by Formation of Iron Hydroxide and Fe(III)-silicate Complex under Highly Oxidizing Condition

Abstract

Acid drainage generated by pyrite oxidation has caused the acidification of soil and surface water, the heavy metal contamination and the corrosion of structures in abandoned mine and construction sites. The applicability of Na-acetate (Na-OAc) buffer and/or Na-silicate solution was tested for suppressing pyrite oxidation by reacting pyrite containing rock and treating solution and by analyzing solution chemistry after the reaction. A finely ground Mesozoic andesite containing 10.99% of pyrite and four types of reacting solutions were used in the applicability test: 1) H 2 O 2 , 2) H 2 O 2 and Na-silicate, 3) H 2 O 2 and 0.01M Na-OAc buffer at pH 6.0, and 4) H 2 O 2 , Na-silicate and 0.01M Na-OAc buffer at pH 6.0. The pH in the solution after the reaction with the andesite sample and the solutions was decreased with increasing the initial H 2 O 2 concentration but the concentrations of Fe and SO 4 2-were increased 10 -20 times. However, the pH of the solution after the reaction increased and the concentrations of Fe and SO 4 2-decreased in the presence of Na-acetate buffer and with increasing Na-silicate concentration at the same H 2 O 2 concentration. The solution chemistry indicates that Na-OAc buffer and Na-silicate suppress the oxidation of pyrite due to the formation of Fe-hydroxide and Fe-silicate complex and their coating on the pyrite surface. The effect of Na-OAc buffer and Na-silicate on reduction of pyrite oxidation was also confirmed with the surface examination of pyrite using scanning electron microscopy (SEM). The result of this study implies that the treatment of pyrite containing material with the Na-OAc buffer and Na-silicate solution reduces the generation of acid drainage.

Pyrite oxidation mechanism in aqueous medium

“…Huang and Evangelou found that PO 4 3− can inhibit the pyrite oxidation by forming ferric phosphate coatings in KH 2 PO 4 and H 2 O 2 solution. Furthermore, Kargbo et al showed that phospholipid can suppress the pyrite oxidation in the presence of silicate, where the Fe‐silicate complex is formed on the pyrite surface to impede the oxidation of pyrite . Afterward, Evangelou et al proved that ‐Fe(II)HCO 3 + or ‐Fe(II)SSCOOFe 2+ may be formed on the pyrite surface in a humidified CO 2 environment.…”