Recovery of gold and silver in secondary slag from gold refining process by co-precipitation with metal hydroxides

Abstract

Purification of gold using chlorine gas as an extracting agent is one of the oldest and most widely used process in large scale gold refining, so called "Miller Chlorination Process" Slag derived from this process is usually called "Primary slag" generally containing 5-10 wt% gold. Secondary slag produced after smelting of the primary slag generally contains approximately 200-300 ppm gold and 1000 ppm silver, which indicates the high potentiality for further recovery of precious metals (Au and Ag). The aim of this study is to develop an effective recovery system for gold and silver, from secondary slag during gold refining process. The recovery process consists of both physical process (comminution and shaking table) and chemical separation (extraction with nitric acid and aqua regia respectively). The extracted gold and silver were recovered by co-precipitation with metal (e.g. Fe, Cu) hydroxides at pH 6, and then, the co-precipitation including goldwas dissolved with 1 M hydrochloric acid or 1 M nitric acid. The result of physical separation process showed alloy of Au-Ag-Cu was liberated from the slag under the size of 70 Mesh. After passing the shaking table, the concentration of slag was upgraded from 0.015% to 0.126% of gold and from 0.174% to 1.56% of silver. Then, the two-stage leaching with nitric acid and aqua regia was applied to recover silver as AgCl precipitate, and gold as [AuCl[subscript 2]] with AgNo[subscript 3] impurities. Then, the solution or leachate was further concentrated by co-precipitation with metal (Fe, Cu) hydroxides at pH 6. The chlorohydroxy Au (III) complexes co-precipitated on metal hydroxides are reduced to elemental gold (Au[superscript 0]) without adding any reducing reagents. The gold and silver was further purified by (comparatively) dissolved using 1M HNO[subscript 3] and 1M HCI. The results showed that gold and silver could be effectively purified by treatment of 1M HNO[subscript 3] rather than that of 1M HCI.