Raman study on the speciation of copper cyanide complexes in highly saline solutions

Abstract

Previous studies have established that the following three copper(I)-cyanide complexes form in aqueous solution: [Cu(CN) 2] −, [Cu(CN) 3] 2− and [Cu(CN) 4] 3−. The distribution of these complexes in solution at equilibrium is highly dependent upon the CN/Cu molar ratio. The speciation of copper cyanide complexes in highly saline solutions is of interest to Australian mining companies because of the unusually highly saline process water used on the goldfields of Western Australia (ca. 200 g/l total dissolved solids). This study has used the vibrational technique of Raman spectroscopy to determine the effect of highly saline water on the equilibrium distribution of copper cyanide complexes in solution for various CN/Cu molar ratios. For the first time it has been shown that in highly saline solutions the equilibrium distribution of copper cyanide complexes changes significantly. It has been established that [Cu(CN) 3] 2− predominantly forms in highly saline solutions for CN/Cu molar ratios of 2.2 to 2.5, where previously it has been shown that both [Cu(CN) 2] − and [Cu(CN) 3] 2− exist in non-saline solutions. Furthermore, in saline solutions containing an excess of cyanide (ca. 200 mg/l) only [Cu(CN) 4] 3− exists in solution and the formation of [Cu(CN) 3] 2− has not been observed. This phenomenon has been used to explain the increased selectivity of ion exchange resins for gold cyanide in highly saline solutions.