Kinetics of gold(III) chloride complex reduction using sulfur(IV)
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I. INTRODUCTION
REACTIONS of reduction of noble metal complexes are often used in applied methods of these metals recovery from aqueous solutions. The choice of the reducing agent depends on the pH of the solution containing a noble metal under consideration. In turn, the type of the environment results directly from the leaching solution applied. So far the most common leaching medium, used in hydrometallurgy of gold containing ores is cyanides. Due to high efficiency of the leaching processes, they are still used by a number of metallurgical companies.[1,2] Gold cyanide complexes are very stable in aqueous solutions, which enables their recovery with different methods.[1,3] However, due to environmental as well as safety requirements, other leaching agents are investigated, e.g., thiourea solution, thiosulfate, and chlorine water.[4,5,6] As an example, the process used in the Glogów Copper Plant in Poland can be given where leaching medium is a mixture of HCl and Cl2 gas. In this process, gold is obtained by the reduction of gold complexes from chloride solutions. It is a part of a bigger technological cycle in which other metals such as Cu, Ag, Pb, and Se are produced. The starting material is a sludge left after silver electrorefining, which contains not only Au, Pt, and Pd but also elements such as Ag, Cu, Bi, and small amounts of Pb, Sb, As, Se, and Te. Purified sludge is leached with a mixture of HCl and gaseous Cl2. During this process, noble metals react, forming complexes. Next, solutions with complexes of noble metals are selectively reduced. One of the important steps is the reduction of gold(III) complexes with acid sodium sulfate(IV). Literature sources suggest[1,7] that pH of the reducing environment and resulting changes of the complexes as well as the type of reducing agent, its concentration, and temperature can influence the rate and morphology of the obtained product. In order to control all these changes, a deep knowledge of chemical processes is required. It was found the number of KRZYSZTOF PACLAWSKI, Assistant Professor, and KRZYSZTOF FITZNER, Full Professor and Head, are with the Physical Chemistry and Electrochemistry Laboratory, Faculty of Non-Ferrous Metals, AGH University of Science and Technology, 30-059, Krakow, Poland. Contact e-mail: [email protected] Manuscript submitted September 29, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B
studies on gold(III) complexes reduction with NaHSO3 in chloride solutions is small. Moreover, there is a disagreement as far as the mechanism of the process is concerned.[8,9] It was decided to investigate the mechanism and kinetics of the reduction process of gold(III) complexes under the conditions similar to industrial process and to learn about its mechanism. II. EXPERIMENTAL A. Materials Preparation HAuCl4 solution was prepared by dissolving pure Au (99.99 pct purity, obtained from Mennica Pan´ stwowa, Warsaw, Poland) in aqua regia. After several evaporations of this solution and dilution of the obtained solid in distilled water, the starti
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