Recovery of Gold from Thiourea,Thiocyanate, or Thiosulfate Solutions by Reduction-Precipitation with a Stabilized Form of Sodium Borohydride

Abstract

Sodium borohydride can be used as an efficient agent for reducing gold and silver in acidic solutions of thiourea, thiocyanate, thiosulfate, acid chloride, and acid nitrate. The Au(I) ion is reduced to metallic gold in the form of very fine crystals, as indicated by XRD and SEM analysis. The reduction reaction is carried out at ambient temperature for a very short period of time (30 s). The reduction of Au(I) in such solutions can be performed for a wide range of gold concentrations (2 mg/L-2 g/L). Different parameters controlling the rate of the reaction and the efficiency of borohydride were studied, namely: acidity, stoichiometry, thiourea concentration, the presence of foreign ions, and temperature. Gold reduction by sodium borohydride is more favorable in the presence of Ag⁺ and/or Zn²⁺ ions, while the presence of Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺ ions decreases the efficiency of borohydride. The reduction reaction is selective for gold and silver over base metals in a solution mixture since the reaction is carried out in acidic medium. High recovery is obtained (>99%) and the product is pure. Thiourea does not undergo any degradation reaction during the course of reduction, and it can be recycled. The process appears promising for the gold industry due to its simplicity, speed, and low capital investment requirements.