No Process And Mechanism Of Ammonia Thiosulfate Method Leaching Sulfide Silver

Argentite is a very important silver mineral, but the recovery is not high by traditional cyanidation. Firstly, solubility of silver sulfide is low. Secondly, the silver cyanide complex is not stable. Finally, sulfide ion released from the dissolved sulfide minerals combine with silver ion to generate silver sulfide, which eventually led to low silver extraction. In addition, cyanide is not friendly to the environment. In recent years, many researchers pay more attention to the ammoniacal thiosulfate solution, which is a low toxicity solution, could leach gold and silver rapidly and effectively. However, leaching with the ammoniacal thiosulfate solution consumes large quantities of reagents, and it is difficult to control reaction conditions. Through experiments, we found that ammonia (or ammonium) is not conducive for silver sulfide leaching. Thus, a non-ammonia thiosulfate leaching technology has been developed in this paper, which is environmentally benign, and a high silver recovery could be got without controlling pulp pH.The chemical reactions that may occur in the copper-thiosulfate solution were calculated, and potential-pH diagrams of copper-thiosulfate system and silver sulfide-thiosulfate system were drawn. Thermodynamic studies have shown that in the thiosulfate solution, copper(Ⅰ) ion and silver(Ⅰ) ion combine with thiosulfate ion to form stable complexes exist in aqueous solution.The composition of ore is very complicated, in order to avoid the effects of non-silver minerals, dissolution of pure silver sulfide in the copper-thiosulfate solution was studied. Researches have shown that the concentration of thiosulfate, the ratio of copper to thiosulfate, silver sulfide particle size, stirring speed, oxygen, temperature and reaction time will affect extraction of silver sulfide. Through the optimization of process conditions, in air atmosphere, [S2O32-]=0.12mol·L-1, [Cu2+]/[S2O32-]= 0.4, stirring speed of 250 r/min, 298K, after 4h, the largest silver sulfide extraction is 96.50%. Zinc can be used to recover 99.31% silver from the pregnant solution, when the amount of Zn:Ag=4:1, reaction time of 15min. The total silver recovery is 95.83%.The kinetics study shows that the dissolution of silver sulfate in the copper-thiosulfate solution is a two-step process. The first step occurs within 30 minutes, the reaction is mixed chemically/diffusion controlled. After 30min, the second step occurs, which is inner diffusion controlled.The electrochemical behavior of copper-thiosulfate solution on platinum electrode was investigated by cyclic voltammetry. The results of experiments indicate that thiosulfate solution is relatively stable in the range of-0.5-0.5V, there is no obvious redox reaction. Cupric ion changes the mechanism of thiosulfate oxidized, the cyclic voltammograms figure shows redox peaks at low potential, thiosulfate is oxidized to tetrathionate, while cupric ion is reduced to cuprous ions.The reaction mechanism of silver sulfide dissolved in copper-thiosulfate solution was investigated. The results of SEM and XRD confirm the conjecture of two-step process:the first step, copper thiosulfate complexes replace partly silver in silver sulfide and generate silver-copper sulfide (AgCuS), silver dissolves as silver thiosulfate complexes; the second step, copper thiosulfate complexes replace silver of silver-copper sulfide, ultimately generate cuprous sulfide (Cu2S). Cupric ions and sulfide ions combination carry out to promote the silver sulfide dissolution.A silver sulfide flotation concentrate leaching studies have shown that argentite mineral can be leached effectively using copper-thiosulfate solution. When [S2O32-]=0.3mol·L-1, [Cu2+]/[S2O32-]=0.4, the pulp concentration of 30%(the amount of Na2S2O3·5H2O 170kg/t, CuSO4·5H2O 68kg/t), leaching at room temperature for 48h, silver recovery is 90.20%. In addition, Pb, Zn, Fe and Sb minerals which associate with silver minerals are not dissolved in copper-thiosulfate solution. To deal with the silver flotation concentrate in a cyanide solution, the cyanide concentration of 6000ppm, pulp concentration of 30%(the amount of sodium cyanide 15kg/t), leaching for 96 hours, silver recovery is 87.40%. Compared with cyanidition, leaching argentite mineral with copper-thiosulfate is more quickly, and silver recovery is higher, too.Based on the study in this paper, the argentite mineral can be leached effectively using copper-thiosulfate solution under appropriate conditions. The problem of high reagent consumption needs to be further studied.