Copper cyanide removal by precipitation with quaternary ammonium salts

Cyanide is widely used in the mining industry to extract gold from ores. Some of the minerals processed for precious metals extraction contain copper species which may react with cyanide to form cuprocyanide complexes. The presence of these copper species affects adversely the process and causes high cyanide consumption. In order to overcome these limitations this laboratory work explores the feasibility of removing the copper–cyanide complexes by precipitation with quaternary ammonium salts, allowing the remaining solution, free of copper and containing free cyanide, to be recycled to the cyanidation process. The first part of the experimental work was performed with synthetic copper–cyanide solution simulating a high copper–cyanide solution (2700 mg/L cyanide, 730 mg/L copper and pH adjusted to 12 with CaO) and three quaternary ammonium salts: hexadecyl trimethyl ammonium chloride (HTA), octadecyl trimethyl ammonium chloride (OTA) and dioctadecyl dimethyl ammonium chloride (DDA). The results showed that it is possible to remove up to 90% of the copper in the precipitate when adding 12.32 g OTA/g copper at pH 12. The free cyanide remains unreacted in the solution and could be recycled to the process. Results of tests performed at different pH values suggest that regardless the initial species distribution in the solution, the solid formed will contain mainly copper tricyanide and some of copper tetracyanide. The molar ratio CN/Cu in the solid is around 3 while the molar ratio OTA/Cu is around 2. This implies that some amount of copper tetracyanide is transformed into copper tricyanide while reacting with the amine and forming the precipitate. When zinc is also present in the cyanide solution, the ammonium salt will react first with the zinc–cyanide complexes before precipitating the copper cyanides. Tests performed with an industrial solution corroborated the results obtained with synthetic solutions: quaternary ammonium salts (e.g. OTA) react with copper and zinc cyanides (but not with free cyanide) to form a precipitated that can be separated from the solution by filtration. An analytic technique for measuring quaternary ammonium salts HTA and OTA in solution was developed.