Abstract

Cyanidation, a primary gold and silver recovery method since the 1880s, dissolves these metals in alkaline cyanide solutions. Subsequent Merrill-Crowe processing recovers the metals. However, large-scale operations often yield low-quality precipitates due to co-precipitation of copper and lead. This increases zinc dust consumption, smelting flux requirements, crucible lifespan, and lowers Dore quality. In this study, two processes are used to clean the high concentrations of zinc, copper, lead and iron cyanide ions from the barren solutions of the Merrill-Crowe process, to produce in the next step a Dore of high quality (99%). The first step in this process is to induce nucleated precipitation of copper, zinc, lead, and silver in a conical hermetic reactor using sodium sulfide as a reagent precipitator and sulfuric acid for pH control, while also regenerating the NaCN with sodium hydroxide. The results reveal that pH has a considerable effect on zinc and copper cyanide removal efficiency, with the ideal pH range being 8 to 9. At this pH, lead, copper, and zinc cyanide removal efficiency was up to 70%, whereas cyanide linked with copper, zinc, and iron cyanide complexes recovered at 80%. The second step is used for the total elimination of these cyanide complexes and lead ions from the barren solution with an attractive electrocoagulation process. In this hermetic reactor, all the remaining complex metal ions and cyanide recoveries were 99%. Then, the clean solutions passed through the cementation step to give a first-order reaction which was faster when compared to the barren with cyanide complex of copper/zinc. Finally, this treatment procedure was successfully practiced at Minera Labri / William, La Sorpresa and Bacis Maning Company in México.

Keywords:Sulphide precipitation; Electrocoagulation; Merrill-crowe; Dore and gold/silver