Recovery of silver from X-ray film processing effluents by hydrogen peroxide treatment

Abstract

In this study, recovery of silver from X-ray film processing effluents by precipitation was studied. Hydrogen peroxide was used as the precipitating agent. A full factorial design (42) approach was adopted for the study. The results have shown that precipitation process is highly exothermic in nature with the evolution of copious amount of heat apparently owing to the concomitant oxidation of thiosulphate. The precipitation of silver by hydrogen peroxide is a fast reaction, which is almost complete within minutes. It is also an acid consuming reaction with the tendency of pH to increase towards neutral/alkaline conditions. The concentration of hydrogen peroxide was proved to be statistically the most significant factor affecting the precipitation process. High silver recoveries (≥95%) from the waste solution (1.1g/L Ag, 113g/L S2O32−) were obtained only at high levels of H2O2 (≥37.6g/L). Over the reaction period, a substantial increase in the concentration of sulphate was noted indicating the consumption of H2O2 mainly by the oxidation of thiosulphate. Increasing pH was found to have a beneficial effect on the recovery of silver noticeably at low H2O2 concentrations. The addition of ethylene glycol (0.5–10mL) enhanced the recovery of Ag (by 1.3–18.7%) presumably due to its stabilising effect on H2O2. SEM–EDS and XRD analyses of the precipitates have revealed that silver is present mainly as fine silver sulphide. These findings demonstrate that the waste photoprocessing solutions can be suitably treated by hydrogen peroxide to recover silver and remove thiosulphate.