It is important to detect the presence of some cyanide consumers. These minerals take cyanide and there is only residual part for leaching gold minerals. When they are present in high levels, the cyanidation process will not have high recoveries. Obviously, there is a critical point that must be detected by studying metal content and mineralogical composition. These undesirable minerals form cyanides and their formation generates operational and environmental problems such as tailings disposal or water balance. This problem increases operative cost and reduces gold recovery due is required higher cyanide addition to compensate the lack of cyanide to leach gold.
Without any doubt, the most serious problem is the presence of oxidized copper minerals and some secondary sulphides. Basically, there are several types of copper cyanide compounds which are formed under different chemical conditions during contact with cyanide. Some of them are [Cu(CN)2]-, [Cu(CN)3]2- and [Cu(CN)4]3-. The Copper cyanide complexes have limited ability to dissolve gold, thus the free cyanide concentration in solution must be maintained at a level which ensures that the maximum amount of gold is dissolved. It a common practice to analyze the pregnant solution by different compounds and especially cyanide and copper in order to determine the presence of copper compounds.
Other problem is the presence of free sulfur or sulphides. Cyanide reacts with sulfur to produce thiocyanate and this situation favors the oxidation of reduced sulfur compounds. And consequently, the operation needs to increase the pH in order to don’t release hydrogen cyanide.
Malachite and chrysocolla are oxidized copper minerals