Distribution of Precious Metals (Ag, Au, Pd, Pt, and Rh) Between Copper Matte and Iron Silicate Slag

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PRECIOUS metals, among others silver, gold, platinum, palladium, and rhodium, are both environmentally[1] and economically important by-products of copper production. There is also a growing trend in the copper smelters to use recycled copper[2] and complex, low-grade ore sources as raw materials. Hence, the concentrations of precious metals are increasing in smelting as well as the losses of precious metals deporting to slag. The key factors to improve the recovery of precious metals in flash smelting are to determine the amounts of truly chemically dissolved precious metals and mechanically entrained matte in the slag. Considering the distributions of precious metals in smelting, silver is the most extensively investigated metal in the literature. Although, experimental results show disagreements regarding the solubility of silver in the slags as well as with the mechanism of dissolution, the recent experiments have shown a good agreement copper matte (0 to 80 pct Cu) and an iron silicate slag, KATRI AVARMAA, Doctoral Candidate, and PEKKA TASKINEN, Professor, are with the School of Chemical Technology, Department of Materials Science and Engineering, Metallurgical Thermodynamics and Modelling Research Group, Aalto University, Espoo, Finland. HANNU JOHTO, Research and Development Engineer, is with Boliden Harjavalta Oy, 29200 Harjavalta, Finland. Contact e-mail: pekka.taskinen@aalto.fi Manuscript submitted May 24, 2014. Article published online November 13, 2015. 244—VOLUME 47B, FEBRUARY 2016

equilibrated in MgO crucibles. The experiments were carried out at temperatures 1523 K or 1573 K (1250 C or 1300 C), under different sulfur dioxide partial pressures (0.1, 0.5, and 1 atm). The experimental methods based on the equilibration of relatively large samples and quenching with argon gas, followed by a physical separation of matte and slag phases. The measurements of the bulk compositions were made using inductively coupled plasma spectrometry. The results for iron silicate slag are nearly unanimous and follow the same trend: the distribution coefficient of silver, Lm/s[Ag] = [pctAg]/ (pctAg), increases linearly up to the matte grade of approximately 65 pct Cu, then the coefficient turns to a radical decrease. The distribution coefficient of silver according to available studies[3–5] in the matte grade of 65 wt pct Cu was between 150 and 250. Louey et al.[6] also investigated the distribution of silver between copper matte (50 pct Cu) and an iron silicate slag. The experiments were carried out at 1523 K (1250 C) under inert N2 gas atmosphere and using MgO crucibles. The distribution coefficient of silver Lm/s[Ag] was found to be 120 ± 40. Kashima et al.[7] studied the distribution of silver between copper, white metal, and an iron silicate slag. The silver distribution coefficient between almost pure Cu2S matte (80 wt pct Cu, ‘white metal’) and the slag at 1473 K (1200 C) was approximately Lm/s[Ag]  20, although, there was a considerable scatter in the experimental data. These both studies agree well with the results of Ro