Pretreatment of a refractory arsenopyritic gold ore using hydroxyl ion

•An arsenopyrite refractory ore was pretreated with NaOH to increase gold extraction.•After pretreatment, gold and silver were leached with copper ammonium thiosulfate solution.•Thiosulfate leaching of the as-received ore extracted only 21% and 29% of silver and gold, respectively.•Up to 81% of the gold and 50% of the silver were recovered after ore pretreatment with NaOH.•Gold, and to some extent silver, extractions were related to arsenic removal.

A sodium hydroxide pretreatment at ambient temperature was evaluated to increase gold extraction from a high-grade arsenopyrite mineral. A 2k factorial experimental design was established, with the following variables: NaOH concentration (0.1–3 M), solid concentration (40–200 g/L) and treatment time (6–48 h). After applying the pretreatment, standard leaching tests with thiosulfate solution were performed to quantify the effects of these three variables on the gold extraction. While only 23 and 29% gold extraction was achieved in the as received mineral with cyanide and thiosulfate leaching, respectively, up to 81% was obtained with the pretreatments. The lower solid concentration and the higher NaOH concentration gave the best results. This demonstrates that the use of the hydroxyl ion represents an effective treatment for minerals with gold encapsulated in arsenopyrite. Gold extraction was related to that of arsenic removal. Silver leaching was also improved for most pretreatments, but to a lesser extent.