Zinc and silver recoveries from zinc–lead–iron complex sulphides by pressure oxidation

Abstract

Acid pressure oxidation followed by cyanide leaching of the residue is a promising process for the treatment of complex sulphides and the recovery of precious metals along with the base metals will improve the economy of the process. However, silver is incorporated into the jarosite specie during the pressure oxidation and cyanide leaching of the residue yields very low silver extraction. In this work, iodide was added to the pressure oxidation of zinc–lead–iron complex sulphides to prevent the deportment of silver ions into the jarosite phase. At low temperature range (110–130 °C), the silver ions were completely sequenced into the silver iodide phase because of the fast precipitation kinetics of silver iodide and its stability at low temperatures. The leaching of the residue in cyanide solution yielded high silver extraction (above 90%). Silver extraction from the residue decreased when the pressure oxidation was conducted at high temperatures (140–150 °C). At this temperature range, the enhanced stability and the precipitation kinetics of the jarosite specie posed a challenge by competing (with iodide) for silver ions. This competition was minimised by using moderately high initial acid for the pressure oxidation. High zinc extraction was achieved during the pressure oxidation. Also, there were appreciable iron precipitation and acid neutralisation of the slurry. The resulting pregnant solution is suitable for zinc recovery by electrowinning and the residue can be leached for silver and gold extraction.