Cyanidation of a copper-gold ore

Abstract

A free milling gold ore, which contains 0.4% copper as chalcopyrite, was treated with cyanide. In the pre-leaching, the kinetics of oxidation of soluble sulphides are not an accurate indication for the length of the treatment. Efficient gold leaching is achieved only under specific conditions using oxygen, lead nitrate and a high concentration of free cyanide. The addition of lead nitrate results in a higher gold extraction, although cyanide consumption cannot be reduced below 1.85 kg/t without a drop in the extraction. Without lead nitrate, the gold recovery is lower than 90% but with lead nitrate, it reaches 98%. The addition of oxygen increases the gold recovery by 1.5%. The extraction of gold is not sensitive to lead nitrate additions when higher than 300 g/t. The redox potential values could thus be used as control parameters for lead nitrate addition, at the different stages of cyanidation, to indicate the state of the system. The increase of lead nitrate concentration inhibits the dissolution of chalcopyrite but the approach used was not efficient enough to decrease the cyanide consumption. The high concentration of copper in solution requires a concentration of NaCN in the range of 700 mg/L. When the average NaCN concentration is lower than 640 mg/L, gold recovery drops significantly. It was also found that lead nitrate can be added directly at the start of cyanidation to achieve a performance equivalent to the situation in which it is added during pre-leaching.