Gold electrowinning from thiourea solutions

The use of thiourea as an alternative non-polluting reagent for gold leaching is very well known. However, the processes of gold recovering from thiourea solutions have received little attention, particularly the electrowinning process. Gold electrowinning from thiourea solutions was studied by linear sweep voltammetry and chronopotentiometry. For potentials more cathodic than 0 mV vs. SCE gold reduction is mass transport controlled. The estimated diffusion coefficient for the complex gold (I)-thiourea, at 25 degrees C, was 1.0 x 10(-5) cm(2) s(-1). The influence of gold, thiourea and formamidine disulfide concentrations on the cathodic behavior of gold is described. A passivation due to thiourea adsorption on the electrode surface was observed at -300mV vs. SCE. Experiments shown that gold reduction is favored by low thiourea and formamidine disulfide concentrations. Chronopotentiometric measurements indicated the presence of an adsorption step before the reduction of the complex gold (I)-thiourea, although the reduction of the species in solution is also feasible. The influence of current density on the cathodic current efficiency and gold structure is presented. (C) 2000 Published by Elsevier Science Ltd. All rights reserved