Improved thiosulfate leaching of a refractory gold concentrate calcine with additives

• Humic acid can increase gold extraction and decrease thiosulfate consumption.
• Humic acid weakens the interaction between Cu2 + and S2O32 −.
• Humic acid eliminates catalytic effect of iron minerals on thiosulfate decomposition.
• Humic acid prevents passivation species from coating calcine surface.
• Humic acid allows the extraction of some encapsulated gold.

One of the major problems of thiosulfate leaching was the high consumption of thiosulfate. In this work, additive was used to reduce thiosulfate consumption and simultaneously increase gold leaching. Thiosulfate leaching without additive was not effective to treat a refractory gold concentrate calcine with high iron, sulfur and arsenic due to its low gold extraction (72.7%) and high thiosulfate consumption (42.4%). Thus, several additives including sodium chloride, sodium sulfite, CMC (sodium carboxymethyl cellulose), CMS (sodium carboxymethyl starch) and HA (sodium humic acid) were used to improve the leaching. HA as a new kind of additive had the best performance, and it could improve gold extraction to 81.4%, which was obviously higher than the extraction by cyanidation (78.2%), and reduce thiosulfate consumption to 13.2%. As the XPS and SEM/EDS results indicated, there were less passivation species including copper sulfides, sulfur, iron hydroxide, etc. on the surface of calcine leached with the addition of HA. The role of the HA in the thiosulfate leaching was also preliminarily analyzed in this paper, and was proposed as follows: HA weakened the interaction between cupric ions and thiosulfate, and also eliminated the catalytic effects of iron minerals on thiosulfate decomposition; on the other hand, HA prevented passivation species from coating the surfaces of gold and gold-bearing minerals, and allowed the extraction of some encapsulated gold.