Effect of flotation and potassium hydroxide pretreatment of an antimonial refractory ore on the extraction of silver by cyanidation

Abstract

In this study, potassium hydroxide leaching as pretreatment before cyanide leaching of a flotation concentrate of an antimonial refractory gold-silver ore was investigated. The ore assaying 1157 g/t Ag and 3.96 g/t Au was found to be composed of predominantly quartz, barite, kaolinite, clay, mica/chlorite, and dolomite/ankerite. Electrum, pyrargyrite, acanthite, and tetrahedrite were identified to be the most important silver-bearing phases in the ore. A flotation concentrate with a high silver grade (0.6% Ag) at 94.8% silver recovery, as well as high recoveries of antimony (86.8%), zinc (98.6%), and lead (84.7%), was produced in flotation tests. Gold recovery to the concentrate was limited to be 74.4%. The cyanide leaching of the concentrate has yielded consistently low extractions of silver (≤13.8%) confirming the refractory nature of the ore. Alkaline pretreatment of the concentrate was performed in which effects of potassium hydroxide concentration (0.05–3 mol/L) and temperature (20–80 °C) on the removal of antimony and subsequent cyanide leaching of silver were investigated. Extraction of antimony was substantially enhanced from 11.5 to 93.8% with increasing the concentration of KOH and temperature. The activation energy (Ea) for leaching of antimony was calculated to be ≈17 kJ/mol with the implication of a diffusion-controlled leaching reaction. Silver extraction in the cyanidation improved from 13.8% to 95.7% as a consequence of the removal of antimony (i.e. 93.8%) by the alkaline pretreatment. This improvement in the silver extraction can be attributed to the decomposition of silver-bearing antimonial minerals i.e. pyrargyrite, and tetrahedrite in the concentrate. These findings confirm that potassium hydroxide leaching is a suitable approach for the pretreatment of antimonial refractory silver ores.