Effect of lead nitrate on cyanidation of gold ores: progress on the study of the mechanisms

Abstract

This paper discusses some of the latest efforts to improve the understanding of the use of lead nitrate in cyanidation. The study is based on an electrochemical approach to establish the nature of the mechanisms related to gold, a surface analysis study, using X-ray photoelectron spectroscopy (XPS), to determine the modifications on gold and sulphide minerals (pyrite, pyrrhotite and chalcopyrite) and an investigation that focus on the improvements of cyanidation. In a cyanide solution, lead nitrate, lead sulphide and lead sulphite react with gold to form AuPb 2, AuPb 3 and metallic lead, which clearly accelerate the gold dissolution. The nature of the sulphide minerals affects the formation of lead or lead alloys on the gold surface. XPS did not find any lead on the surface of gold in the presence of pyrite or pyrrhotite but found a very thin layer (...50 A) in presence of chalcopyrite. Further investigation is required to study the effect of the presence of other sulphides. It is proposed that in presence of sulphide minerals, sometimes lead does not report on gold because of its high affinity for sulphide minerals (competing reactions). Pyrite, chalcopyrite and pyrrhotite showed different reaction mechanisms with lead nitrate. Lead nitrate forms a hydroxide layer on pyrite particles, which reduces the reaction rate with cyanide. The disolution of pyrite generates a sulphur layer on gold. This layer is less important in presence of lead. The effect of lead nitrate is subtler for chalcopyrite and pyrrhotite because it was less effective to retard the reaction of sulphides with cyanide and the reaction of iron with oxygen. For gold, the addition of lead nitrate has the same effect with pyrrhotite than with pyrite; inhibiting partially the formation of a sulphur layer. This was not observed for the gold in the chalcopyrite system. The results indicate that the strategy of lead nitrate addition, to be optimal, should be a function of the mineralogical composition of the ore. The formation of a passive layer on gold particles has a significant influence in the initial stages of leaching. The addition of lead nitrate notably decreased its inhibiting effect. In one case study, the lead nitrate treatment increased the overall gold extraction and decreased the cyanide consumption for an additional gross revenue of $ CND2.2 millions.