Solvometallurgical Recovery of Platinum Group Metals from Spent Automotive Catalysts

Extraction and separation of platinum group metals (PGMs) from secondary raw materials are usually carried out via hydrometallurgical processes. These processes use strongly oxidizing acidic solutions, such as aqua regia or hydrochloric acid in the presence of chlorine gas, which may have negative environmental impacts and are dangerous. In this paper, a solvometallurgical approach was developed for the sustainable recovery of PGMs from spent automotive exhaust catalysts. The PGMs were leached using FeCl3 or CuCl2 as oxidizing agents in the organic solvents dimethyl sulfoxide (DMSO) or acetonitrile (CH3CN). Palladium was quantitatively dissolved in CuCl2/CH3CN, CuCl2/DMSO, and FeCl3/DMSO with only 10-20% codissolution of Pt and Rh. By adjusting the concentration of the oxidizing agent in CH3CN, Pd was selectively leached with 0.01 mol FeCl3 whereas Pt and Rh could be dissolved in a more concentrated FeCl3 solution (0.3 mol L-1). The solvoleaching of PGMs was investigated in depth by UV-vis spectra and electrochemical properties (i.e., cyclic voltammograms and formal reduction potentials of the Fe3+/Fe2+ and Cu2+/Cu+ couples in DMSO and CH3CN). After leaching, CH3CN was easily recovered by distillation. The Pd-containing residue was dissolved in water, from which Pd sponge was produced by reduction with formic acid. Meanwhile, the residue containing the solid chloride salts of Fe(III), Pt(IV), and Rh(III) was redissolved in ethylene glycol or DMSO for further purification by nonaqueous solvent extraction (NASX). The ionic liquid Aliquat 336 diluted in pcymene showed selective extraction of Fe(III) and Pt(IV) while leaving Rh(III) in the raffinate. The loaded ionic liquid was recycled after selective stripping of Fe(III) with water and Pt(IV) with a thiourea solution. A flow sheet comprising solvoleaching and NASX is proposed. The closed-loop solvoleaching of PGMs with less-hazardous chemicals (FeCl3/CH3CN) avoids the emission of toxic or flammable gases (Cl-2, H-2, and NOx) while reducing the consumption of acids and bases and limiting the generation of waste water. In addition, NASX with an ionic liquid may be a more sustainable alternative for the conventional separation of PGMs.

Automated summary

This paper presents a solvometallurgical approach for the sustainable recovery of platinum group metals (PGMs) from spent automotive exhaust catalysts. By investigating solvoleaching and nonaqueous solvent extraction (NASX), effective separation and purification of palladium, platinum, and rhodium were achieved. The closed-loop solvoleaching process using less-hazardous chemicals reduces environmental impacts and waste generation, while NASX with an ionic liquid offers a more sustainable alternative for PGMs separation.