Galvanic microencapsulation (GME) using zero-valent aluminum and zero-valent iron to suppress pyrite oxidation

Abstract

Pyrite is a common gangue mineral in mine wastes, and its oxidation is the primary cause of acid mine drainage (AMD) formation, which is a very serious environmental problem encountered worldwide. To address this problem, we developed a new technique to suppress pyrite oxidation called galvanic microencapsulation (GME). Galvanic interaction occurs when two conductive or semi-conductive materials with different rest potentials interact with one another. The material with a lower rest potential becomes the anode and is oxidized while the other one with the higher rest potential becomes the cathode and is galvanically protected. In this study, the effects on pyrite oxidation of zero-valent aluminum (ZVAl) or zero-valent iron (ZVI) dosages, leaching time, and pH were elucidated. In addition, the suppression mechanisms involved during GME were investigated by electrochemical measurements and surface-sensitive characterization techniques. The results showed that pyrite oxidation was suppressed in the presence of ZVAl or ZVI. With time, galvanic interaction between pyrite and ZVAl in the first 3 days was negligible, which could be attributed to the Al-oxyhydroxide coating on ZVAl. After 7 days, however, ZVAl exhibited substantial suppressive effects on pyrite oxidation. In comparison, the suppressive effects of ZVI on pyrite oxidation were observed after just 1 day. Cyclic voltammetry and chronoamperometry measurements showed that the suppressive effects of ZVAl and ZVI were predominantly due to galvanic interactions. Although ZVAl and ZVI could limit pyrite oxidation, their suppressive effects were only temporary because the surface of pyrite was not passivated by an unreactive coating. To induce coating formation and prolong the suppression of pyrite oxidation, phosphate was added together with ZVI. Only ZVI was selected for these experiments because of the potential formation of iron phosphate, a very stable material even under acidic conditions. In the presence of phosphate, suppression of pyrite oxidation by ZVI was dramatically improved because of the combined effects of galvanic interactions and coating formation.