Study of silver precipitation in thiosulfate solutions using sodium dithionite. Application to an industrial effluent

This paper discusses a study of silver precipitation in elemental form in the S2O32−–S2O42− system. The nature and kinetics of this process were determined. Using the method described here, silver precipitation is characterized by an induction period, followed by a conversion (precipitation) period, until almost all the silver is precipitated, leaving in solution a metal amount less than 1 ppm. Aggregates of individual crystals with a size less than 0.5 μm were obtained. In thiosulfate medium 1 mol dithionite precipitated 2 mol silver, whereas without thiosulfate in the reaction system, 1 mol dithionite precipitated 6 mol silver. In the kinetic study and in the induction period, the inverse of this parameter was proportional to [S2O42−]0.4 and an apparent activation energy of 108 kJ mol− 1 was obtained. During the conversion period, the reaction order with respect to [S2O32−] was n = − 3, and with respect to the initial [S2O42−] it was a 0.9. An activation energy of 52 kJ mol− 1 was obtained. On the basis of the behavior described, an empirical mathematical model, including the induction and conversion periods, was established, which fits well with the experimental results. The precipitation process was applied to an industrial effluent (a fixing bath); metallic silver was obtained with a recovery of more than 99%.