Suppressive effects of ferric-catecholate complexes on pyrite oxidation

Abstract

Pyrite, a common gangue mineral in complex sulfide ores and coals, is rapidly oxidized in water by ferric ions and dissolved oxygen to form a very acidic and heavy metal-laden leachate called acid mine drainage (AMD). Carrier-microencapsulation (CME) using Ti⁴⁺, Si⁴⁺, and Al³⁺ was reported as a promising new approach to prevent pyrite oxidation by forming a passivating barrier on the pyrite surface. In CME, the presence of Fe³⁺-catecholate complexes is unavoidable but their effects on pyrite oxidation remain unclear. In this study, the effects of Fe³⁺-catecholate complexes on pyrite oxidation were investigated. Formations of mono-, bis-, and tris-catecholate complexes of Fe³⁺ were verified by UV-Vis spectrophotometry and their speciation with pH was consistent with thermodynamic considerations. Linear sweep voltammetry was conducted to evaluate the redox properties of Fe³⁺-catecholate complexes, and the results indicate that ligands in the three complexes were sequentially oxidized until Fe³⁺ is released. Coating formation on pyrite was confirmed after treatment with mono- and bis-catecholate complexes. Results of SEM-EDX and ATR-FTIR indicate that the coating is composed primarily of iron oxyhydroxide phases. The results of leaching experiments showed that pyrite oxidation was suppressed by Fe³⁺-catecholate complexes via two mechanisms: (1) electron donating effects of the complexes, and (2) formation of a protective coating on pyrite. The results provide not only a better understanding of the effects of Fe³⁺-catecholate complexes on pyrite oxidation but also some possible applications of Fe³⁺-based CME such as the suppression of pyrite oxidation to prevent AMD formation and depression of pyrite floatability in mineral processing.

Keywords: Acid mine drainage; Carrier-microencapsulation; Ferric-catecholate complexes; Pyrite oxidation; Redox properties.