3.2. Cyanide preparation
The chemicals, copper(I) cyanide (CuCN, 99.99%) and sodium cyanide (NaCN, 99.9%), were obtained from Aldrich. NaCN was used to study the effect of free cyanide on flotation with a cyanide concentration of 10 ppm (equals to 0.3846 mM) in flotation pulp and 100 ppm (equals to 3.846 mM) in the electrolyte solution for electrochemical studies. A stock solution of 0.1 M NaCN was prepared with 18 MΩ cm deionized water. The pH of the stock solution was stabilized at 11.5 using standardised 1 M sodium hydroxide (NaOH) to avoid any HCN loss by volatilisation. Cuprous cyanide solution was prepared by dissolving CuCN powders in excess of NaCN solution to give a final solution with CN/Cu = 3. The NaCN solution was purged with nitrogen gas of high purity to remove O2 before and during the addition of CuCN powders because relatively unstable Cu(CN)2− may form when the cyanide to copper ratio locally turns to be less than 3 during the addition of CuCN powders. Free cyanide or cuprous cyanide solution was added in flotation pulp with the same cyanide concentration.4 ppm Zn