Arsenic behavior during the treatment of refractory gold ores via POX: Characterization of Fe-AsO4-SO4 precipitates

Arsenic is a common contaminant in refractory gold ores/concentrates and its accepted that total pressure oxidation (POX) is the most appropriate technology to treat these due to their refractoriness and ability to stabilize arsenic via ferric arsenate compounds (Fe-As). However, information gaps about the behavior and stability of the various Fe-As' s formed at high temperatures in downstream gold processing steps remain and may have significant practical implications. This paper focuses on the precipitation behavior of arsenic during autoclaving of various arsenopyrite containing ore concentrates from around the world. The first portion involved the precipitation of different synthetic precipitates at POX conditions found in the gold industry by varying Fe/As ratios in the feed solutions. Mineralogical characterization results showed that arsenate-containing basic ferric sulphate (As-BFS), basic ferric arsenate sulphate (BFAS), and ferric arsenate sub-hydrate (FAsH) formed. In the second portion, five pyrite/arsenopyrite concentrates received from gold mines around the world were submitted to batch POX and mineralogical analysis. We observed that the mechanism of precipitation for pyrite/arsenopyrite concentrates appears to be different vs. synthetic solutions. Upon processing of the gold concentrates under POX, the initial Fe/As ratio in the concentrates was retained to the final generated residues. The major Fe-As' s generated in the POX residues from the concentrates were As-BFS and BFAS, while non-As containing ferric phases included hematite and some small fraction of jarosite. Finally, we observed that as the Fe/As molar ratio in the concentrate feed increased, the amount of As-BFS decreased while that of BFAS increased.