Abstract
Cyanidation is the main process for gold and silver recovery from its ores. In this study, a process is proposed to recover copper and cyanide from barren solutions from the Merrill-Crowe cementation process with zinc dust. This technology is based on inducing nucleated precipitation of copper and silver in a serpentine reactor, using sodium sulfide as the precipitator, and sulfuric acid for pH control. Results show that pH value has a significant effect on copper cyanide removal efficiency, and it was determined the optimal pH range to be 2.5 - 3. At this pH value, the copper cyanide removal efficiency achieved was up to 97 and 99%, when copper concentration in the influent was 636 and 900 ppm. respectively. In this process (sulphidization-acidification-thickening-HCN recycling), the cyanide associated with copper cyanide complexes, is released as HCN gas under weakly acidic conditions, allowing it to be recycled back to the cyanidation process as free cyanide. Cyanide recovery was 90%. Finally, this procedure was successfully run at Minera William in México.
Highlights
The most common process for gold and silver recovery from ores is cyanidation, due to the selectivity of free cyanide for both metals, and the stability of the cyanide complex ( Au(CN), k = × 1038) [1].Chemical recovery of gold from Merrill-Crowe process cyanide solution, involves two different operations: 1) gold dissolution, where it is oxidized and dissolved to form Au (I)ion and cyanide complex, and 2)
Results show that pH value has a significant effect on copper cyanide removal efficiency, and it was determined the optimal pH range to be 2.5 - 3
At this pH value, the copper cyanide removal efficiency achieved was up to 97% and 99%, when copper concentration in the influent was 636 and 900 ppm. respectively. In this process, the cyanide associated with copper cyanide complexes, is released as HCN gas under weakly acidic conditions, allowing it to be recycled back to the cyanidation process as free cyanide
Summary
The most common process for gold and silver recovery from ores is cyanidation, due to the selectivity of free cyanide for both metals, and the stability of the cyanide complex At room temperature and standard atmospheric pressure, approximately 8.2 mg of oxygen are present in one liter of water This corresponds to 0.27 × 10–3 mol/L. The corresponding sodium cyanide concentration for a complete reaction (molecular weight of NaCN = 49) should be equal to 4 × 0.27 × 10–3 × 49 = 0.05 g/L or approximately 0.01% This was confirmed in practice at room temperature by a very dilute solution of NaCN of 0.01% - 0.5% for ores, and 0.5% 5% for gold and silver concentrates [2]. Gold dissolution is an electrochemical reaction in which oxygen takes up electrons at one section of the metallic surface [cathodic zone], while the metal gives them up in another section [anodic zone]. Many common copper minerals are soluble in the dilute cyanide solution, typical of leach conditions found in the gold cyanidation process.
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