Thiosulfate Mechanism Of Gold Dissolution

Cyanidation, which has been applied to gold extraction for a long time, is till the most widely used technique in this field for the main reasons of its simpleness, good adaptability, and high coefficient of recovery, at the present time. However, certain disadvantages of this technique do exist. For instance, the speed of gold leaching is slow and the process can be easily interfered by impurities like copper, iron, lead, zinc, stibium, tellurium, etc; gold ores which contain sulfur, arsenic and carbonaceous and thus is difficult to treat can not be effectively leached; the cyanide is virulent and therefore does great harm to the environment.The thiosulfate technique, as a major non-cyaniding approach, has received increasing attention and is becomed as the most potential one to replace cyanidation. Thiosulfate possesses a number of advantages including innocuity, low price, convenience, quickness and high coefficient of recovery. Moreover, gold leaching process of this technique undergoes in the alkaline medium, which does not erode the facilities. Last but not least, the thiosulfate technique owns considerable advantages when deals with gold ores which contain sulfur, arsenic and carbonaceous and thus is difficult to treat. But,the liquor chemistry of thiosulfate gold leaching system is extremely complex, and the research about gold leaching mechanism is still insufficiently, and the literature report findings are different.In this paper, the spectrophotometry of gold is established using TMK as reagent. It has been found that a complex compound is formed by Au and this reagent in the buffer solution which pH is 3.6HAc-NaAc. The maximum absorption wavelength of this complex compound is 550nm, apparent molar absorptivity 1.34×10⁵, and Lambert-beer's Law is obeyed when the content of Au is between 0-15μg /25ml. TMK is used to measure the traces of gold in the thiosulfate gold leaching liquid and the result turns out to be satisfactory with high sensitivity, good selectivity, convenience and quickness.This paper has successfully verified the feasibility of thiosulfate technique from the perspective of thermodynamics and electrochemistry. The gold leaching mechanism is investigated through theoretical analysis and actual experiments. It has been found that [Cu(NH₃)₄]²⁺ plays a significant role in the process as the transfer carrier of oxygen and the catalyst of redox reaction.In the thiosulfate gold leaching solution, copper (Ⅱ) mainly exists in the form of [Cu(NH₃)₄]²⁺ with small amount of [Cu(NH₃)₃]²⁺ and copper (Ⅰ) mainly exists in the form of [Cu(S₂O₃)₃]^5- with small amount of [Cu(S₂O₃)₂]^3- and [Cu(NH₃)₂]⁺. NH₃ maybe act as the catalyst of the gold leaching process, the stabilizing agent of the thiosulfate and copper (Ⅱ).Using pure gold, it has been found through condition exploration tests that under room temperature and normal pressure, the amount of the leached gold and the consumption of the thiosulfate are affected by mixing speed, the duration of leaching, the acidity of the solution, the content of O₂,the addition of SO₄^2- and SO₃^2-, as well as the concentration of S₂O₃^2-,Cu²⁺ and NH₃·H₂OThe tests results show that NH₄HCO₃, (NH₄)₂CO₃, CH₃COONH₄, (NH₄)₃PO₄, NH₄Cl and (NH₄)₂SO₄ can replace NH₃ partly to reduce the environmental pollution. Meanwhile the complex formed by ethylenediamine and copper quite stable (the stability constant of [Cu(en)₂]²⁺ is 1.0×10²⁰), thus there is no detached H₂NCH₂CH₂NH₂ or less environmental pollution.The gold ore contianing As has been tested by thiosulfate leaching system. Grinding fineness, leaching duration, and the liquid-solid ratio are tested. The best technical condition is fixed : CuSO₄0.03mol/L, NH₃·H₂Olmol/L, Na₂S₂O₃ 0.3 mol/L, (NH₄)₂SO₄ 0.1 mol/L, the liquid-solid ratio being 3:1, pH=10, mixing speed being 200rpm and leaching duration being 4h. The final result shows that the leaching recovery stands 84.8%, and it is higher than the recovery by cyanidation.