Silver precipitation using sodium dithionite in cyanide media

Isauro Rivera, Guillermo Juárez, Iván A. Reyes, Antonio Roca and María I. Reyes
J. Mex. Chem. Soc., 56:2, 156-162., 2012.


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 (<5 mg L-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 &#8776;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--S2O42- is controlled by chemical reaction, characterized by high activation energy, and is independent of the hydrodynamic variables of the system. Key words: Silver precipitation, sodium dithionite, sodium cyanide, chemical kinetics, reaction order.