Silver Precipitation Using Sodium Dithionite in Cyanide Media

Abstract

The nature of the reaction and the main parameters affecting Ag precipitation rate in the system Ag⁺-CN^--S₂O₄^2- are studied. From the results obtained, a process of chemical precipitation for Ag recovery is proposed, using an environmentally-friendly reducing reagent (sodium dithionite, Na₂S₂O₄), leaving a residual content of Ag into the permitted environmental levels (-1 Ag). The levels of CN^- in liquids wastes are of the order of 0.816 mol L^-1, and they can be treated by ozonization processes for inerting cyanide. The results obtained indicate that Ag precipitation is achieved in less than 1 hour, with recoveries close to 99%, at room temperature. Silver precipitation is characterized by an induction period followed by a precipitation period, leaving a metal amount in solution less than 1 mg L^-1. The main kinetic parameters studied lead to obtain the following apparent reaction orders: with respect to the initial [S₂O₄^2-] was n = 1.675, with respect to [CN^-] was -1.24, with respect to [OH^-] was ≈0 (between 10^-4 and 10^-2 mol L^-1) and with respect to the initial [Ag⁺] was 0.524. An activation energy of 77 kJ mol^-1 was obtained. Under these conditions, Ag precipitation in the system Ag⁺-CN^--S₂O₄^2- is controlled by chemical reaction, characterized by high activation energy, and is independent of the hydrodynamic variables of the system.