Effect of the composition of some sulphide minerals on cyanidation and useof lead nitrate and oxygen to alleviate their impact

An investigation was carried out on synthetic ores containing high purity p yrite, pyrrhotite and chalcopyrite and on two gold ores currently processed to evaluate the impact of cyanicides on cyanidation and to improve the lea ching performance by using a pre-leaching, injecting oxygen and adding lead nitrate. With regard to the synthetic ores, it was found that pyrrhotite d id not generate a high cyanide consumption while pyrite and chalcopyrite we re detrimental. Pre-leaching was deleterious for the ore containing chalcop yrite while pre-leaching with lead nitrate was very efficient to decrease t he reactivity of the ore containing pyrite. The two gold ores studied had v ery different compositions. The low sulphide ore had a low sulphide content (1.36% S), present as pyrrhotite while the second had a very high sulphide content (20.2% S), in the form of pyrite, pyrrhotite and chalcopyrite, The efficiency of the process conditions was peculiar to the ores. The high su lphide ore required a stronger, longer pre-leaching period (12 h) with grea ter amounts of Lime (7.0 kg/t) and lead nitrate (600 g/t) than the low-sulp hide ore. The ore with a low sulphide content required a pre-leaching of on ly 1 h with a small quantity of Pb(NO3)(2) (50 g/t) and leaching can be per formed at 360 ppm NaCN to allow a recovery of 96.4% Au and a low cyanide co nsumption at 0.18 kg/t. As for the high sulphide ore, cyanidation had to be conducted at 560 ppm NaCN to recover 88.4% Au with a cyanide consumption o f 0.80 kg/t, An increase in the amount of lime enhanced oxidation of solubl e sulphides. Lead nitrate stabilized copper and iron dissolution by forming a passivation layer at the surface of sulphide minerals. Lead nitrate also prevented the formation of a passive layer at the surface of gold. (C) 199 8 Published by Elsevier Science B.V. All rights reserved.