Kinetics of reduction of gold(III) complexes using H 2 O 2
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UCTION
REDUCTION reactions are often used in precipitation of noble metals from aqueous solutions. As an example, a technology applied in the KGHM ‘‘G1ogo´w’’ (Poland) copper plant can be given. In the process of gold recovery, chloride gold(III) complexes are reduced with sodium hydrogensulfate(IV), yielding solid, metallic gold powder. The results of the kinetic study of this particular reaction were given in our previous article.[1] Because the complexes of platinum metals are also present in the same solution it is advisable to carry out the reduction process selectively reducing gold ions only. It would be beneficial also to keep platinum metal complexes unchanged, i.e., in the form of chloride complexes, because further separation depends on the composition of the solution. These facts stimulated further search for a reductant, instead of NaHSO3, which would be able to reduce gold(III) complexes leaving chloride Pt(IV) and Pd(II) complexes untouched. One of the possible choices seems to be H2O2. Taking into account its ‘‘reducing power,’’ it is a good candidate for such a selective reductant.[2] Writing the overall scheme of the reaction of reduction,[2] 2HAuCl4 13H2 O2 18NaOH 5 2AuY18NaCl13O2 18H2 O [1] one may suspect that the mechanism of this reaction is complex and consists of a number of elementary steps. Calculations of the equilibrium composition of the solutions enabled us to predict the forms of Au(III), Au(I) complexes as well as those of the reductant, which may appear under experimental conditions. Results of these calculations are shown in our previous article.[1] They showed that, in alkaline solutions (0.1 M NaOH), ions of [Au(OH)4] and [HO2] are predominant. Under the assumption that, during an elementary step, only one electron is transferred, one can suggest that intermediate products of this reaction are gold(II) and gold(I) ions. Moreover, it is known that, in the redox reactions with H2O2, the free radicals appear as products.[3,4] Taking this into account, the KRZYSZTOF PACŁAWSKI, Assistant Professor, and KRZYSZTOF FITZNER, Full Professor, are with the Physical Chemistry and Electrochemistry Laboratory, Faculty of Non-Ferrous Metals, AGH University of Science and Technology, Krakow, Poland. Contact e-mail: fitzner@ uci.agh.edu.pl Manuscript submitted February 22, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
paths of the reduction of complex gold(III) ions to metallic gold in alkaline solution (pH 5 13) can be postulated in the following form: Stage 1: ½AuðOHÞ4 1½HO2 ! ½AuðOHÞ3 1 HO2 1 OH [2] k1
Stage 2: ½AuðOHÞ3 1HO2 ! ½AuðOHÞ2 1O2 1OH 1H 1 [3] k2
Stage 3: ½AuðOHÞ2 1 ½HO2 ! Au 1 HO2 1 2OH [4] k3
The sum of Reactions [2] through [4] yields ½AuðOHÞ4 1 2½HO2 ¼ Au 1 HO2 1 O2 1 4OH 1 H 1 [5] If the sequence of elementary Reactions [2] through [4] is repeated, the model leads to the reaction 2½AuðOHÞ4 1 4½HO2 ¼
2Au 1 2HO2 1 2O2 1 8OH 1 2H 1
[6]
The free radicals (present on the right side of Reaction [6]) should recombine:[4] k4
HO2 1 HO
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