Abstract
Concentrations < 0.5 mol m-3 (< 102 g m-3) of AuCl4- and AuCl2- ions in 1 kmol HCl m-3 aqueous electrolyte were reduced to elemental gold in a packed bed cathode of 2-3 mm graphite particles, in an electrochemical reactor incorporating a cation- permeable membrane and operated in batch recycle mode. Depletion to concentrations < 5×10-3 mol m-3 (< 1 g m-3) appeared to be mass transport controlled at an applied potential of 0.53 V (SHE) and occurred with < 4 F (mol Au)-1 and specific electrical energy consumptions of ca. 400 kW h (tonne Au)-1. However, atomic absorption and UV spectrophotometry established that, as the ([AuCl4-] + [AuCl2- ]) concentration decayed, the [AuCl2-] : [AuCl4-] molar ratio changed. A (multi-step) mechanism for reduction of AuCl4- ions is proposed, to explain this behaviour, in terms of changing overpotentials for AuCl4- and AuCl2- reduction as total dissolved gold concentrations decreased.
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