Cyanide regeneration from Thiocyanate with the use of Anion exchange resins

It is known in many gold operations that less than 2% of the cyanide consumed accounts for the gold and silver dissolution. The majority of the cyanide is consumed by minerals contained in the gold ore to produce many different cyanide soluble complexes or used in converting cyanide to other related species such as thiocyanate and cyanate. The high costs associated with cyanide and thiocyanate detoxification and excessive cyanide utilisation encountered when treating ores with high cyanide consumption constitutes a significant proportion of the overall processing costs. This study examines the possibility of recovering free cyanide from thiocyanate using aprocess based on the Acidification-Volatilisation-Regeneration (AVR) circuit in conjunction with a pre-concentration stage using commercially available ionexchange resin. From thermodynamic modelling based on the STABCAL program it was found that it was thermodynamically possible to recover cyanide from thiocyanate if the oxidation of cyanide to cyanate can be stopped. Addition of copper to the system found that the majority of the thiocyanate exists as copper(I) thiocyanate (CuSCN) solid. Using ion-exchange resins can be an effective way to concentrate thiocyanate fromtailing solutions or slurries. Four different models were successfully used to model the equilibria between thiocyanate and chloride on commercial ion-exchange resins. By normalising the equilibria data when applying the Mass action law the equilibria becomes independent of ionic strength within the range of concentration considered. An advantage of this is that only one unique equilibrium constant is used to describe the ion-exchange process.The electrochemical and kinetic studies showed that the reaction between thiocyanate and hydrogen peroxide is catalysed by hydrogen ions. Secondly under acidic conditions the rate of cyanide recovery by the AVR circuit was faster than at higher pH conditions. The overall reaction of thiocyanate with respect to the concentration of thiocyanate and hydrogen peroxide is an overall third order reaction. The derived third order rate expression is first order with respect to thiocyanate concentration and second order with respect to hydrogen peroxide concentration. Previous studies showed that the production of cyanide inhibits the reactions between thiocyanate andhydrogen peroxide, but by removing cyanide from the reaction by air stripping, this was not observed. Addition of copper to the system did not show a catalytic effect on the reaction but it was found that copper (II) ions suppresses competing reactions that occurred without affecting the reaction between thiocyanate and hydrogen peroxide