Gold Contaminant Affecting Leaching - Cyanidation

Perhaps the most powerful contaminant during leaching is the presence of copper ores such as secondary sulphides and oxides. Copper forms a range of cyanide complexes depending upon the solution chemistry. Both the cyanide concentration employed and the ionic strength and composition of the solution can influence the composition of the particular copper complex formed.

• Cu⁺ + CN^- = CuCN
• CuCN + CN^- = [Cu(CN)₂]^-
• [Cu(CN)₂]^- + CN^- = [Cu(CN)₃]^2-
• [Cu(CN)₃]^2- + CN^- = [Cu(CN)₄]^3-

The copper dissolved can go in part to the tailings pond and recycle to the operation again. Thus, if copper is not removed from the tailings water, on recycle, [Cu(CN)₂]^- will bind to more free cyanide in the leach liquor to form [Cu(CN)₃]^2- and [Cu(CN)₄]^3-. The formation of [Cu(CN)₂]^- is favored at pHs below 6 and at very low cyanide concentrations. Whereas, the [Cu(CN)₄]^3- species is preferred at high pH, high cyanide concentrations and in the presence of hypersaline solutions. The lower adsorption of [Cu(CN)₄]^3- on to activated carbon accounts for the use of high cyanide solution concentrations to improve the selectivity of gold over copper when this absorbent is employed for gold recovery. 

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.

An indication of the relative proportions of the different species present in solution can be obtained by measurement of the molecular ratio of sodium cyanide to copper in solution. Typically, this ratio varies between 2.5 and 3.5, although a number of mines are known to favour a ratio of NaCN:Cu >4.5:1.

Analytical procedures further complicate the detrimental effect of copper on gold dissolution. During titration with silver nitrate (the “standard” method for free cyanide analysis), a portion of the cyanide complexed with copper is released, which then complexes with the silver to establish a new equilibrium. Furthermore, the precipitation of CuCN can mask the end-point, resulting in either an over- or under-estimation. Also, the fourth ligand on the [Cu(CN)₄]^3- species is titrated along with the free cyanide. This can only be accurately accounted for if a full cyanide speciation calculation is conducted for the sample. WAD cyanide distillation methods, Raman spectroscopy, UV/membrane flow injection analysis, HPLC analyses to date have not proven to be totally reliable. 

The high consumption of cyanide by copper is therefore directly related to the requirement for sufficient cyanide to be present to complex with all of the copper and to also dissolve all of the gold. It is therefore evident that if the ore contains a significant concentration of cyanide-soluble copper the copper can rapidly deplete the solution of available cyanide. In addition, other constituents, such as sulphides can then more readily interact with the cyanide in the leach slurry to form thiocyanates.

Thus, as given, Cu(I) minerals make cyanide unavailable due to the formation of cuprocyanide complexes. And, for Cu(II) minerals, additional cyanide loss is generated by the oxidation of cyanide to cyanate.

Cyanidation is also adversely affected by the presence of free sulfur or sulfide minerals in the ore. Cyanide will preferentially leach sulfide minerals and will react with sulfur to produce thiocyanate. These reactions will also enhance the oxidation of reduced sulfur species, increasing the requirement for lime addition to control the pH at a sufficient level to avoid the volatilization of hydrogen cyanide (HCN).