Basics of ammonia-cyanide leaching of copper-gold ores. Review

One of the biggest challenges for the gold industry in the 21st century is the presence of copper in gold containing ore bodies. This is due to copper consumes large quantities of cyanide. Additionally, copper cyanide compounds are more stable than free cyanide, and hence make it difficult to destruct the tails. The ammonia-cyanide leaching system was first patented over 100 years ago and stands out as a unique method of selectively leaching up to 90% Au and <1% Cu from oxidized copper-gold. This paper reviews some of the recent applied and fundamental studies on the leaching of copper-gold ores with the ammonia-cyanide solutions and provides insights into the mechanism to give a better appreciation of the key parameters required for the optimum leaching of gold with minimum copper dissolution. The ammonia-cyanide system is known as an alternative system for the treatment of complex copper-gold ores. Thermodynamic research on the Au – Cu (Cu2O, Cu2S) – CN – NH4+ – O2 – OH– – H2O systems allowed to establish the possibility of coexistence of Au(CN)2– – CuCN compounds in a standard solution. Copper cyanidation is faster than gold cyanidation which leads to a decrease in the content of the active reagent in the solution and negatively affects the extraction of gold. The addition of ammonia to the solution causes the formation of a strong CuNH3 compound capable of dissolving gold in the absence of free cyanide ions. In this case, copper precipitation occurs as a parallel process, leaving significant amounts of Cu (I) cyanide involved in the dissolution of gold. The critical value of the concentrations of oxygen and added ammonia in the solution, which determine the interface between the thermodynamic stability zones CuCN and CuO/Cu(OH)2, is estimated. The presence of copper sulfides in ores contributes to the dissolution of gold, while copper oxides contribute to precipitation. When cyanide solutions did not contain copper, the addition of ammonia reduced the rate of gold leaching. When copper was added to the solution, the gold recovery rate also decreased due to the formation of copper cyanide complexes. However, in the absence of free cyanide in the solution, gold was easily leached due to the formation of the Cu(CN)32– complex, and the addition of ammonia did not affect the dissolution process. At the same time, the dominance of the Cu(CN)2– complex in the solution causes an increase in the rate of gold leaching, however, in the absence of ammonia, gold recovery is very low. In the absence of free cyanide, but with sufficient ammonia content, the addition of copper (II) increases the rate of dissolution of gold compared to cyanide complexes of copper (I). The recommended leaching solution compositions, Eh and pH values make it possible to adapt them to different types of ores. The issues related to the selective recovery of gold from ammonia-cyanide solutions by cementation, coal and ion exchange adsorption are also considered.