Gold recovery by leaching processes with cyanide is employed in most mining operations around the world and its presence in tailings is an aspect to be studied during the process selection and even during the operation. When gold is leached by cyanide solution, the reaction produces a gold-cyanide compound by oxidizing and other cyanide complex compounds, which are stable and based on theoretical considerations, the amount of cyanide needed is only slightly in excess. However, the consumption of cyanide in real operations is influenced by the presence of other metals.
The appropriate determination of cyanides in solution from tailings has some analytical problems. Depending on the pH, cyanide can be found as molecular acid, as cyanide ions and metal complexes of widely. Due to the high toxicity of HCN and CN-, the analytic procedure to be used must be accurate and sensitive. According to their stability, complexed metal cyanides have different levels of toxicity.
Cyanide can be found in three different forms, free cyanide, weak acid dissociable cyanide and total. The first one is characterized by the presence of only hydrogen cyanide and the cyanide ion in solution, which can be considered as free cyanide. The presence of HCN and CN- in solution is influenced by the equilibrium reaction. The latter one depends on the solution pH. The analytic procedures to determine this type of cyanide must not consider any weaker cyanide complex. In other words, the method must avoid any interference.
The second type include is also known as WAD cyanide and comprises all cyanide compounds produced at pH of 4.5. The main cyanide compounds are formed of copper, cadmium, nickel, zinc and silver. The analytic procedure must be able to detect the presence of more stable forms of cyanide compounds. The last one comprises free cyanide and all WAD cyanide complexes and even all the ferro-cyanide and ferri-cyanide compounds. Oxidized compounds such as cyanate (CNO-) thiocyanate (SCN-) are not included in this category. Analytic procedures have to determine the presence of stable complexes free of any cyanate or thiocyanate compound.