Kinetic studies of gold(III) chloride complex reduction and solid phase precipitation in acidic aqueous system using dimethylamine borane as reducing agent Marek Wojnicki ⁎, Ewa Rudnik, Magdalena Luty-Błocho, Krzysztof Pacławski, Krzysztof Fitzner AGH University of Science and Technology, Faculty of Non-Ferrous Metals, Department of Physical Chemistry and Metallurgy of Non-Ferrous Metals, al. Mickiewicza 30, 30‐059 Cracow, Poland abstract article info Article history: Received 8 December 2011 Received in revised form 28 April 2012 Accepted 25 June 2012 Available online 6 July 2012 Keywords: Gold recovery Kinetics Reduction Metal precipitation DMAB Kinetics of gold(III) chloride complex ions reduction with dimethylamine borane and precipitation of gold nanoparticles was investigated. The influence of concentrations of reductant, gold(III) chloride complex ions, neutral salt and chloride ions as well as temperature on the process was reported. Experimental results were used to determine parameters in the Arrhenius equation. A set of reactions occurring in the system was proposed. Rate equations were presented and verified. It was proposed that state of the catalytic surface of gold particles play a crucial role in the mechanism of the reaction and competition between AuCl 4 − adsorption and AuCl 4 − reduction on the metal particles exists in the system. Microscopic observations as well as analysis of the particles were also performed. © 2012 Elsevier B.V. All rights reserved. 1. Introduction KGHM Polska Miedź S.A. is one of the biggest producers of copper, silver and gold in Europe. In 2008, 924 kg of gold and more than 1200 tons of silver were produced in the plant as side products (KGHM, 2011). Noble metals (Au, Pt, Pd) are recovered in several steps from a slime after silver electrorefining. Initially, the slime is leached in hydrochloric acid to remove copper and bismuth contaminations. Pre- cious metals remained in the solid phase are then dissolved in 10% HCl in the presence of gaseous chlorine. In next stages, gold chloride complexes are reduced to metal using disodium disulphite (pH 0.2, 323 K), whereas palladium, platinum and remaining gold ions are pre- cipitated from the solution using sodium formate (pH 2.5, 353 K). Puri- fied solution is finally transferred to the waste-water treatment plant. Technology used currently in the Polish plant seems to be costly, since losses of precious metals (0.005 g/L in the waste solution), consumption of energy and reductants are quite significant. This inspired us to devel- op an alternative method for selective recovery of noble metals present in an acid chloride solution. From a number of various compounds, dimethylamine borane (DMAB) can be proposed as an alternative for HCOONa reductant. It shows good reactivity towards noble metals at ambient temperature. This reductant has been widely used in electro- less deposition of gold (Sadik et al., 2005; Sargent and Sadik, 2001; Sargent et al., 2001) or palladium (Lelental, 1973), although application for synthesis of platinum or gold nanoparticles was also reported (Hüttl et al., 2009; Wojnicki and Fitzner, 2011). There is a lack of information on the kinetics of reaction between noble metal ions and DMAB. Our previous study (Wojnicki et al., 2011) was focused on the kinetics of reduction of palladium(II) chlo- ride complex using DMAB. The aim of the present paper was to deter- mine the rate equation as well as mechanism of the reaction between gold(III) chloride complex and DMAB. The effect of gold(III) ions, Cl − ions and DMAB concentrations, ionic strength of the solution and temperature on the reaction rate was studied. The knowledge on the kinetics can help to select the best conditions for the selective re- duction of palladium and gold complexes in acidic chloride solutions. 2. Experimental Solution of gold(III) chloride complex was prepared from metallic gold (99.99%, provided by Mennica Polska S.A.). Metal was dissolved in aqua regia, and then mixture was evaporated several times to remove nitric acid and excess of HCl to obtain pure solid tetrachloroauric acid. Next, the acid was dissolved in deionized water. Required concentra- tions of gold(III) chloride complexes (2.5×10 −5 to 2.5×10 −4 M) in the investigated solutions were achieved by an appropriate dilution of the initial solution with 0.1 M HCl (from Chempur, p.a.). Total Cl − con- centration in the solutions was maintained in the range of 0.05–1.05 M by addition of NaCl (from POCh, >99.9%). DMAB (from Fluka, >97%) was dissolved in deionized water. Concentrations of the reductant were changed in the range of 1.25–25 mM. Neutral salt NaClO 4 (from Koch-Light Laboratories Ltd.) in the concentration range of 0.125–1M was used to salt effect studies. pH of solutions was 1.7. Experiments were carried out at temperatures from the range of 288–318 K. Hydrometallurgy 127–128 (2012) 43–53 ⁎ Corresponding author. Tel./fax: +48 126174126. E-mail address: marekw@agh.edu.pl (M. Wojnicki). 0304-386X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2012.06.015 Contents lists available at SciVerse ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet