Electrochemical Studies Of Gold Leaching In Alkaline Thiourea Solutions

Many researchers have been investigating the use of non-cyanide substances in gold leaching,to replace the use of cyanide,which is toxic.With its high efficiency and low toxicity,in recent years thiourea has been considered as a potential substitute for cyanide.Under acid conditions,Au minerals are non-selectively leached,leading to a considerable consumption of thiourea meanwhile the corrosion of leaching devices and equipment is serious.To solve this problem,attempts have been made to leach Au in thiourea under alkaline conditions.But the theoretical study in the system of gold leaching in alkaline thiourea is still not perfect.In this paper,the cyclic voltammetry and steady state polarization were used to systematically study the electrochemical behavior of the dissolution of anodes occurring with Au or its associated minerals(chalcocite,stibnite,pyrite,and arsenopyrite),as well as the effects of different thiourea concentration,stabilizer and oxidant on the dissolution.The corrosion current and corrosion potential of gold in alkaline thiourea solutions thus calculating the kinetic parameters was studied with the Tafel test at different temperatures.The transient constant current step method was used to study the gold passivation and polarization in alkaline thiourea solution.The cyclic voltammetry and steady state curves showed that without stabilizer,the oxidation peak of gold dissolution in alkaline thiourea solutions is invisible.With stabilizer added,the peak current and potential increased and the peak is obvious,the Au anode dissolution peak current increased with the stabilizer concentration.At 0.38 and 0.42 V and an Na2SiO3 concentration lower than 0.09 M,the current density increased with the Na2SiO3 concentration;however,its growth rate decreased for Na2SiO3 concentrations higher than 0.09 M.Higher stabilizer concentrations were required as the pH increased.Under different pH values,the polarization of gold dissolution in alkaline thiourea solution is different:when pH=10,only electrochemical polarization was existed,when pH=ll,both electrochemical polarization and concentration polarization were existed,and the passive film was formed,when pH=12,the solution is unstable due to the high-alkaline pH.The Tafel curves showed that with the temperature increasing,the corrosion current of gold in alkaline thiourea solutions increased no matter whether Na2SiO3 is added.The apparent activation energy was calculated from the Arrhenius equation,without Na2SiO3,the apparent activation energy was 183.76 kJ/mol,whereas 98.07 kJ/mol with 0.09mol/L Na2SiO3.The results showed that Na2SiO3 could reduce the the apparent activation energy,thus the the gold dissolution rate could be accelerated.In the absence of a stabilizer,ores consisting mainly of chalcocite and arsenopyrite,the selective dissolution of Au could not be achieved.The addition of Na2SiO3 favored the selective dissolution of Au when it was associated with chalcocite or stibnite,but did not work when Au was associated with arsenopyrite and pyrite.