The reduction of Miller chlorides with sodium silicate and carbon

Abstract

In the refining of molten gold, the silver and the copper impurities are chlorinated and form a molten chloride mixture on the surface of the gold. This silver chloride-copper chloride salt is known as Miller chlorides. In the present work, the reduction of this chloride with sodium silicate and carbon was studied. A silver-copper alloy, a silica-based slag and a sodium chloride salt were produced. The overall reactions were as follows: 2AgCl + Na 2O.SiO 2 + C = 2 Ag + 2NaCl + SiO 2 + CO and 2CuCl + Na 2O.SiO 2 + C = 2Cu + 2NaCl + SiO 2 + CO The effects of charcoal, sodium silicate, borax additions and crucible type on the metal recovery and composition were determined. Experiments were performed in resistance, gas-fired and induction furnaces. The results show that even at relatively high temperatures, such as 1400°C sodium silicate additions considerably in excess of the stoichiometric are required to achieve silver recoveries approaching 100 percent. For a given sodium silicate addition, borax additions improved the silver recovery. Mechanical mixing of the solid reagents followed by smelting in a clay graphite crucible in an induction furnace resulted in the highest metal recoveries and rapid material throughput.