Recovery of platinum group metals from spent automotive converters by leaching with organic and inorganic acids and extraction with quaternary phosphonium salts

The paper presents studies on the application of hydrometallurgical recovery of platinum group metals (PGM) from spent automotive converters (SAC), which is not only commercially viable but also important from an ecological perspective. The work proposes an integrated four-stage process, which includes: leaching with oxalic or lactic acids, then leaching with mineral acids from catalyst left after the first stage and next extraction of PGM with phosphonium or ammonium salts: Cyphos IL 101 (trihexyl(tetradecyl)phosphonium chloride), Cyphos IL 104 (trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate), and in the final stage, stripping with 0.1 M thiourea in 0.5 M HCl and 3 M HNO3 to recover PGM. The use of carboxylic acids in the first step of leaching reduces the amount of non-precious metals (Fe ions, Mg(II), Zn(II)) leached in the second stage. The highest concentration of the leached Al(III), Fe ions, Mg(II) and Zn(II) was 600, 25, 110 and 45 mg/dm3. To leach PGM from SAC, it was necessary to use stronger acidic solutions (e.g. a mixture of inorganic acids with an oxidant). The concentrations of Pt(IV), Pd(II), Rh(III) in the solutions after the second step of leaching were 34–40, about 3, 4.5–5 mg/dm3, respectively. After two stages of extraction, Rh(III) and Al(III) are left in the raffinate, therefore Pt(IV) and Pd(II) can be separated from Rh(III). Afterward, Pd(II) and Pt(IV) extracted to the organic phases can be separated in the first stage of stripping.

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