Suppression of arsenopyrite oxidation by microencapsulation using ferric-catecholate complexes and phosphate.

Mineral processing, pyro- and hydrometallurgical processes of auriferous sulfide ores and porphyry copper deposits (PCDs) generate arsenopyrite-rich wastes. These wastes are disposed of into the tailings storage facilities (TSF) in which toxic arsenic (As) is leached out and acid mine drainage (AMD) is generated due to the oxidation of arsenopyrite (FeAsS). To suppress arsenopyrite oxidation, this study investigated the passivation of arsenopyrite by forming ferric phosphate (FePO4) coating on its surface using ferric-catecholate complexes and phosphate simultaneously. Ferric iron (Fe3+) and catechol form three types of complexes (mono-, bis-, and triscatecholate complexes) depending on the pH, but mono-catecholate complex (i.e.,[Fe(cat)]+) became unstable in the presence of phosphate because the chemical affinity of Fe3+-PO43- is most probably stronger than that of Fe3+-catechol in [Fe(cat)]+. When two or more catechol molecules were coordinated with Fe3+ (i.e., [Fe(cat)2]- and [Fe(cat)3]3-), however, these complexes were stable irrespective of the presence of phosphate. The treatment of arsenopyrite with [Fe(cat)2]- and phosphate could suppress its oxidation due to the formation of FePO4 coating, evidenced by SEM-EDX and XPS analyses. The mechanism of FePO4 coating formation by [Fe(cat)2]- and phosphate was confirmed by linear sweep voltammetry (LSV): (1) [Fe(cat)2]- was oxidatively decomposed and (2) the resultant product (i.e., [Fe(cat)]+) reacts with phosphate, resulting in the formation of FePO4.