Acid based recovery of PGMs from spent autocatalytic convertors using AlCl3 and HOCl

Saguru, Collins Tatenda
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The environmental importance of the platinum group metals (PGMs) for catalytic reduction and oxidation of toxic gases emitted by internal combustion engines has been responsible for the increasing demand for these metals. Consequentially, the majority of PGMs have been reporting to end-of-life autocatalytic convertor scrap, providing an opportunity for recycling, to recover the PGMs. A combination of pyrometallurgy and hydrometallurgy is employed in most of the current global industrial recycling centres. However, an exclusively hydrometallurgical process has some advantages over the current industrial establishments, chief amongst them being the lower energy consumption as a result of eliminating the smelting step. The aim of this study was to investigate the leaching of the PGMs via a purely hydrometallurgical process, using readily available and environmental friendly reagents. Platinum (Pt), palladium (Pd) and rhodium (Rh) were leached into a chloride solution under acidic, oxidizing conditions. AlCl3 and HOCl obtained after Ca(OCl)2 dissolution, were used as the complexing/acidifying and oxidising agent respectively. Maximum leaching efficiencies of 15%, 78% and 86% for Rh, Pt and Pd respectively were obtained from preliminary experiments designed using the central composite methodology. A second battery of experiments using the one factor at a time methodology and compounding on insights obtained from the preliminary set of experiments was conducted. Recoveries of Pt, Pd and Rh were improved to 99%, 99% and 68% under optimised conditions of 100 oC, 1.2 M AlCl3, 0.15 M Ca(OCl)2 and S/L ratio of 3g : 10ml. Solvent extraction of both Pt and Pd was conducted to illustrate the feasibility of co-extracting the 2 PGMs into a tri-iso octyl amine (Alamine 308) in Kerosene with n-Decanol organic phase. Recoveries from pregnant leach liquor of 99.6% and 87% for Pd and Pt were recorded, using a 4 stage batch simulation process for a continuous counter current solvent extraction process. There was negligible co-extraction of other metals except for Fe, which had an extraction into the organic phase of 98 %. Pt and Pd could then be selectively stripped using 0.5 M HCl with 2 different concentrations of thiourea. An exclusively hydrometallurgical process was therefore developed with overall recoveries for Pt, Pd and Rh of 85 %, 97 % and 67 % respectively. A preliminary cost benefit analysis based on operating costs alone was conducted, which revealed that the process could attain an operational profit margin of approximately 9.9 % / oz. (3E).
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg 2018
Saguru, Collins Tatenda (2018) Acid based recovery of PGMs from spent autocatalytic convetors using AICI3 and HOCl, University of the Witwatersrand, Johannesburg,