Study of precious metals coprecipitation with manganese hydroxide and recovery of gold nanoparticles from computer microporcessor scrap

Abstract

Recovery of gold (Au), platinum (Pt) and palladium (Pd) by coprecipitation with Mn(OH)₂ and their selective separation were investigated to develop an alternative method for recovery of precious metals from electronic wastes. It was found from the study that at pH 9, room temperature, nearly 99% of either Au or Pd was coprecipitated with Mn(OH)₂, while only 40% of Pt was coprecipitated after 24 hours. However, the coprecipitation of Pt was enhanced up to 90% within 24 hours when Pt(IV) coexisted with Au(III) and Mn(II) in the initial solution having Au:Pt:Mn mole ratio of 6:1:10. Au in coprecipitates was reduced to metallic Au (Au(0)) nanoparticles, which is considered to be caused by electron transfer through Au-O-Mn bonding, whereas Pt(IV) and Pd(II) were not reduced to metallic state. It is considered that Pt(IV) and Pd(II) were entrapped or incorporated into MnO2 structure instead of adsorption on Mn(OH)₂ surface like Au. The existence of Au nanoparticles was confirmed by its solubility in dilute HCl solution and TEM analysis, and coprecipitated Au nanoparticles exhibited catalytic ability for CO oxidation at the half conversion temperature (T½) of 80℃. It was also found that the coprecipitated Au and Pd can be selectively separated from coprecipitated Pt by 0.1 mol dm⁻³ HCl dissolution. Results of the study were then applied to recover Au from CPU microprocessor scrap composed of various metals by coprecipitation with Mn(OH)₂. It was found that Au can selectively separated from the sample solution by coprecipitation with Fe(OH)₃ at pH 4 as Au(III) complexes. The Au-Fe coprecipitate was dissolved in 0.01 mol dm⁻³ HCl, and then MnCl₂ solution was added to the Au-Fe solution at Au:Mn mole ratio of 6:10 to achieve up to 81% Au recovered as metallic Au in Fe(OH)₃ and Mn(OH)₂ coprecipitates, or metallic Au supported on manganese and iron oxides surfaces.