Eco-threat Minimization in HCl Leaching of PGMs from Spent Automobile Catalysts by Formic Acid Prereduction

Reclamation of spent automobile catalysts via aqueous processing for the efficient recovery of Pt, Pd, and Rh (PGMs) has remained a challenge. In this research, the effect of prereduction by HCOOH on PGMs leaching using mild acid was investigated and compared to the typical leaching procedure that uses concentrated HCl. Prereduced samples with particle sizes of <0.59 mm demonstrated a remarkable enhancement in the leaching efficiency of PGMs at a lower lixiviant concentration. Using higher acid concentration (>4.0 M HCl), the prereduction effect on PGMs extraction was negligible compared to leaching without HCOOH prereduction. A detailed study of the influential parameters revealed the optimal prereduction conditions to be HCOOH concentration, 15 vol %; pulp density, 10%; temperature, 60 °C; and time, 1 h. Postreduction leaching in 2.0 M HCl at 90 °C for 2 h yielded > 80% Pt, > 85% Pd, and >62% Rh in leach liquor. A subsequent study on the addition of oxidant during the leaching step further enhanced the extraction efficacy up to ∼95% PGMs by introducing ≥1.5 M NaClO3. The results revealed that a HCOOH prereduction step can significantly minimize the environmental impact and cost of reagents with the maximum yield of PGMs in a less acidic solution.