Efficient enrichment of platinum group metals from spent automotive catalysts by alkaline hydrothermal treatment and the subsequent acid leaching method

Spent automotive catalysts (SAC) is the most important secondary resource for the recovery of platinum group metals (PGMs). This paper demonstrates that the acid-resistant and alkali-resistant cordierite carrier can be transformed into hydroxysodalite by a alkaline hydrothermal treatment (AHT), which can then be dissolved by subsequent acid leaching (AL) to enrich PGMs. Experiments were conducted to assess the influencing factors and the optimal conditions were determined as follows: holding temperature of 150 °C; NaOH to raw material mass ratio 10:10; holding time of 60 min; liquid to solid ratio of 3:1, and stirring speed of 400 rpm. The residue ratio in the AHT process was 132.57% under optimized conditions. The total residue ratio was 18.68% under fixed acid leaching experimental conditions of holding temperature of 60 °C, liquid to solid ratio of 15:1, holding time of 60 min, stirring speed of 400 rpm, and hydrochloric acid concentration of 2.5 mol·L-1. The contents of Pt, Pd, and Rh in the enriched residue reached 650.50, 2131.54, and 405.28 g·t−1, respectively, and the corresponding enrichment times were 7.13, 7.25, and 7.07. The additions of sodium formate before the AHT and aluminum powder during the AL were found to effectively inhibit PGMs leaching. The losses of Pt, Pd, and Rh throughout the entire process were only 2.77%, 1.89%, and 4.28%, respectively.