Gold leaching in thiosulfate solutions and its environmental effects compared with cyanide

Nowadays, keeping mining and the environment sustainable is a major concern all over the world. Using toxic chemicals such as cyanide and mercury in the gold leaching process is one of the main factors that need to be considered in terms of the environment. Finding alternative lixiviants is one solution that would decrease the uses of these toxic chemicals. Thiosulfate is one of the most promising alternative substitutes to cyanide kinetically and environmentally.Gold leaching in thiosulfate solutions was evaluated environmentally and kinetically in different ways: (1) applying a closed system to leach gold (2) evaluating the environmental and metallurgical optimum thiosulfate leaching characteristics for pure gold and pure silver and gold silver alloys (4, 8, 20 and 50 wt % silver), (3) studying the electrochemical behavior of gold, silver, and gold/silver alloys (4) optimizing the leaching conditions of gold ore samples supplied from Centamin Egypt Limited Company, (5) finally, studying the ability of thiosulfate solutions to dissolve mercury and evaluating gold, copper, and mercury recovery from ion-exchange resins.The study showed that the leaching rate of gold and silver in the closed vessel was greater than that obtained in the open vessel by 30% and 45% respectively. To avoid the losses of ammonia a closed leaching system is recommended. Gold and silver leaching in thiosulfate solution is preferable kinetically and environmentally if conducted in a closed vessel system.The effect silver alloyed with gold was evaluated and the result showed that the dissolution rate of pure gold is higher than that form 4 and 8 wt% Ag alloys. And the dissolution rate of gold from 20 and 50% silver alloys is more than that obtained for pure gold. The silver dissolution rate is sensitive to copper(II) concentration being 1.67 x 10-5 mol.m-2.s-1 at an initial copper(II) concentration of 1.25 mM and 6.6x10-5 mol.m-2.s-1 at an initial copper(II) concentration of 10 mM. Silver dissolution is more sensitive to ammonia than gold.From the electrochemical study on gold/silver alloys, it was found that an increase in silver in the alloy results in an increase of the current density from both gold and silver oxidation in thiosulfate solutions in the range of potentials 0.242- 0.542 V. Also, it was found that in thiosulfate solutions containing thiourea, the oxidation current of gold/silver alloys decreased and the oxidation current from pure gold after the standard potential for gold oxidation (150 mV) is higher than the total oxidation current from gold-silver alloys.Leaching of a selected gold ore using thiosulfate solutions for 24 hours suggested the optimum conditions to leach the ore to be 0.2 – 0.4 M thiosulfate, 400 mM NH3, 10 -20 mM Cu(II), 30 ºC, 300 rpm, no air supplied, solid/liquid ratio 20%, and mean particle size of < -106 μm.Finally, the study showed that mercury dissolves in thiosulfate solutions as it dissolves in cyanide. Mercury dissolves in thiosulfate solutions in the order: HgS2 <Hg0<Hg2Cl2