Fundamental Studies on a Recycling System for Precious and Rare Metals Using a Propylene Carbonate Solvent Containing CuBr₂ and KBr

Abstract

Previously we studied a novel process for recycling gold from secondary sources: the leaching of gold using dimethyl sulfoxide solutions containing copper bromide and precipitation with water, which could offer a number of advantages, including eco-friendliness, ease of operation and low cost. In this study, we have further investigated a more environmentally benign solvent, Propylene Carbonate (PC), with CuBr₂ and KBr for the leaching and recovery of precious and rare metals. The mechanism of dissolution was investigated using electrochemical measurements. Metal wires were dissolved in a PC solution with 0.2 M of CuBr₂ and 0.2 M of KBr at 343 K. Next, 10 ml of dilute sulfuric acid aqueous solution at pH 1 was added to the solution at ambient temperature and shaken to biphasically separate the dissolved metals. The contents of each element in the sulfuric acid and PC phases were measured by ICP-OES. The results of the electrochemical measurements indicated that the anodic dissolution of sample metals in the PC containing CuBr₂ occurred at relatively negative potentials and was paired with the cathodic reduction of Cu²⁺ to Cu⁺. It was found that Au, Pd, Cu, Sn, Co, Ni and Zn could be dissolved at relatively fast rate, while Ag, Ta, Ti and W could not be dissolved. In addition, 98% of Au and 94% of Pd remained in the PC phase, while most other dissolved metals migrated to the sulfuric acid phase. This indicated that the dissolved Au and Pd could be effectively separated from other metals via biphasic separation with sulfuric acid. Next, the gold in the PC phase was recovered by the reduction of ascorbic acid or calcination. The cost analysis for recovering gold by this system resulted in 0.34 USD/g-Au.