A critical review of the thermodynamics of hydrogen cyanide and copper(I)–cyanide complexes in aqueous solution

• Review of formation and equilibrium constants for aqueous copper–cyanide complexes
• Includes measured dissociation constants for hydrogen cyanide and water
• Also reviews equilibrium constants measured under non-standard conditions
• The corresponding enthalpies and entropies of reaction are also reviewed.
• Gaps in available information determined and recommendations for further research

Despite the importance of cyanide and of metal–cyanide complexes in gold hydrometallurgy, and the need for reliable thermodynamic data for modelling gold solution chemistry, no comprehensive critical overview of the thermodynamics of hydrogen/cyanide and metal/cyanide complex formation has appeared in the literature since that of Beck in 1987. In particular there has been little consideration of the values of the equilibrium constants (and related thermodynamic parameters) at the higher ionic strengths and non-standard temperatures more typical of hydrometallurgical processing. The copper(I)/cyanide system is of particular importance in gold hydrometallurgy as gold is often associated with copper sulfide minerals such as chalcopyrite, chalcocite, covellite and bornite, all of which except chalcopyrite are reasonably soluble in cyanide solutions due to the formation of copper(I)/cyanide complexes. This paper reviews the available thermodynamic data for the hydrogen/cyanide and copper(I)/cyanide systems in aqueous solution with special emphasis on measurements made at elevated ionic strengths and as a function of temperature. It has been found that, while reliable data are available at 25 °C and very low ionic strengths, the data for higher ionic strengths and temperatures are limited. An attempt has been made to rationalize the available data, and to point out areas where further careful measurements are desirable.