Oxidative Pressure Selective Leaching Silver From Silver-rich Residue Of The Demanganized Silver-manganese Sulfide Concentrate In Ammonium Thiocyanate Solution

To overcome the disadvantages existing in traditional hydrometallurgy of extracting silver, such as instability of leaching reagent, poor selectivity and environmental pollution, a new process of recovery silver in the ammonium thiocyanate solution has been proposed in this paper. The new process includes oxygen pressure leaching of silver in the ammonium thiocyanate solution, cementing silver ions with iron powder in the filtrate solution, and the cemented solution recycling to oxygen pressure leaching after adjust to pH.By the thermodynamic calculation, Ag(Ⅰ) solubility in the ammonium thiocyanate solution has been determined, the concentration and initial pH of the ammonium thiocyanate solution was choose as 1.5mol/L and 1.8, respectively. The E-pH curves for the Me-MeS-O2-NH4SCN-H2O system have been drawn at normal temperature, and the results indicated that Fe3+ could be used as an oxidant in the oxygen pressure leaching process when the pH was less than 2.5. And silver ions could be cemented by the iron powders.Based on the theoretical analysis, technical condition studies on oxidative pressure leaching and filtrate cementation were carried out respectively. The optimum conditions for the oxidative pressure leaching were obtained as follows:leaching temperature 120℃, reaction time 2h, oxygen pressure 1.4MPa, Fe3+ 4.2g/L, ligin 0.6g/L and inital pH 1.8, respectively. The optimum conditions for the iron powder cementation were determined as follows:iron powder dosage was two times of theoretic consumption (3.16g/L), reaction temperature and time was 25℃and 60min, respectively. Under the optimum conditions, integrated cycle experiments were obtained good results. The average extraction percentage of silver was about 88%, the concentration of Ag+ in the cemented solution was below 5mg/L, and the cement percentage of silver was about 98%. Only 6.5% of ammonium thiocyanate was lost after four cycles. The XRD and XPS analysis of the leaching residue indicated that the pyrite was the mainly phase in the leaching residue and the toxic lead and arsenopyrite were existed in the leaching residue. Therefore, the new process has many advantages, such as good selectivity, low pollution, good stability of leaching reagent and high leaching precentage of silver.