The Effect Of Polysulfate On The Leaching Of Gold By Thiosulfate And Its Regulation

Thiosulfate leaching of gold is considered as the most likely alternative which may replace the cyanide process.The scholars over the word do a lot of works to improve this technology for many years.These problems include reducing the regent consumption,increase gold dissolution,recovery of Au(S2O3)23-,etc.However,in the industry,the decomposition of thiosulfate is unavoidable because the complex composition of ores.The catalytic decomposition of thiosulfate by semiconductor minerals is more serious.The decomposition of thiosulfate will generate polythionate.The polythionate is bad for the leaching system.It not only reduces the Cu(?)concentration,burt also accounts for the gold passivation.Besides,the polythionate is bad for the adsorption of Au(S2O3)23-.So,it is important to studuy the production of polythionate with the presence of associated minerals and effect of polythionte on gold leaching and solve the bad influence.It is good for the industrial application of gold thiosulfate leaching that establish control method and enrich the gold leaching theory.First,the decomposition of thiosulfate by associated minerals and the generation of polythionate was discussed.The exist of pyrite?chalcopyrite or chalcocite will increase the thiosulfate decomposition.The consumption is also increased with the addition ammonia in the chalcopyrite?chalcocite or bornite system.Polythionate generates from the thiosulfate decomposition.And with the pH>9,the polythionate exists as the form of trithionate.In lighting conditions,photo induced catalytic effect of semiconductor minerals such as:pyrite or chalcopyrite,will increase the thiosulfate decomposition.The decomposition product is mianly tetrathionate.doped with cobalt or nickel will increase thiosulfate decomposition,but Pyrite doped with fluorine will reduce thiosulfate decomposition.In the pyrite system,thiosulfate decomposition will be decreased with the addition of Sodium citrate,as hole scavenger.The micro-mechanism shows that Fermi energy of pyrite doped with cobalt or nickel is increased,but its work function is decreased.This change increases the reductiblity of pyrite.Pyrite doped with cobalt or nickel helps oxygen accept electronics from itself.The growth of O-O will increase thiosulfate consumption.But pyrite dopped with fluorine has no effect on thiosulfate decomposition.Second,the effect of polythionate on pure gold foils leaching in thiosulfate system was investigated.The addition of trithionate will reduce the Cu(NH3)42+concentration and Redox potential of solution.The heavier passivation on gold surface was confirmed by SEM and the passivation layer contains Cu?S and O,which was confirmed by EDX.Surface enhanced Raman spectroscopy(SERS)results show that there are sulfide and polythionate adsorbed on gold surface.Also,there is more sulfur cover layer on gold surface,which is confirmed by electrochemical meaurement.The addition of tetrathionate will also reduce the Cu(NH3)42+concentration and Redox potential of solution.However,with the extend of leaching time,the gold concentration is higher than reference.There is still sulfur species on gold surface,which is mainly contains sulfur and polysulfide.The sulfur cover layer is also weaker.Then,the effect of polythionate on gold leaching and the control mechanisms were studied from the anodic process using electrochemical meaurement.Electrochemical impedance spectroscopy(EIS)show that polythionate will be adsorbed on gold surface when the gold electrode was held at 0.25V.Thiosulfate will be oxidized to form passivation layer on gold surface at 0.7V.Desorption of passivation layer will occur when the gold electrode was held at 1.1V.When the gold electrode was held at 0.25V,gold dissolution will be depressed by the addition of polythionate in thiosulfate solution.The passivation on gold surface was confirmed by SEM in this situation.SERS characterization results show that the sulfur species on gold surface is mainly contains sulfide in trithionate-added solution.For the tetrathioanate-added solution,the sulfur species is mainly contains polyslfide.The addition of ammonia?thiourea or potassium thiocyanate will reduce the bad effect caused by polythionate in gold anodic dissolution process.This small molecule will take part in the competitive adsorption on gold surface with thiosulfate and polythionate.This is confirmed by the change of electric double layer of gold surface.The alternate arrangement of these small molecule and thiosulfate will change the thiosulfate oxidize mechanism.The S-S adsorbed on gold surface is hard to form in this structure.So the passivation in this situation will be reduced,thus increased the gold dissolution.Finally,the effect of polythionate on cathodic process and control mechanisms were studied.The results show that the passivation was formed during the reduction process of Cu2+ in the thiosulfate?trithionate or tetrathionate-added copper-ammonia solution.The passivation layer contains O?S and Cu.There are ? s-s-s of S8,v(Cu-S)or v(Au-S)of Cu2S or Au2S,v(s-s)of Au(S2O3)23-or S4O62-,V sym(s-s)of S8 or Sn,v sym S2O32-of S2O32-or S4O62-in SERE results.There are enhanced v(Au-S)or v(Cu-S)in SERE results for the trithionate-added solution.And the amount of sulfur polymer is higher on gold surface for the tetrathionate-added solution.Also,the passivation of gold in trithionate media is heavier than tetrathionate media,which is similar to thiosulfate media.The addition of ethylenediamine or EDTA will reduce the bad effect caused by polythionate during the cathodic process.The existent form of Cu(NH3)42+ will be changed because the higher complex stability of ethylenediamine or EDTA with Cu2+.And the reduction potential of Cu(NH3)42+ to form Cu(NH3)4+ will be positive shift with the addition of ethylenediamine or EDTA.From the dissolution above and SEM results,the addition of ethylenediamine or EDTA will reduce the passivation layer formed during the reduction of Cu2+,which contains copper and sulfur.The effect of polythionate on gold leaching were discussed in this paper from the role of polythionate's generate,anoic process and control meachanisms,cathodic process and control meachanisms.The results show that ores with semiconductor properties(pyrite)can increase the decomposition of thiosulfate to form polythionate.Also,the addition of ammonia can reduce the surfur adsorption layer caused by polythionate in anoic process.The addition of ethylenediamine or EDTA will reduce the passivation layer formed during the reduction of Cu2+,which contains copper and sulfur.These conclution could serve as reference for the industry application of gold thiosulfate leaching.