Total recycling of all the components from spent auto-catalyst by NaOH roasting-assisted hydrometallurgical route

Total recycling of all valuable metals such as PGMs, Ce, Al and Mg from the spent automobile catalysts has been explored. The alkali roasting was performed under NaOH, 0.5-3.0 g; temperature, 300-800 °C; and time, 10-60 min. The phase transformation from cordierite to the soluble products (NaAlO₂, Na₂MgSiO₄) was influenced by the temperature, following the diffusion-controlled model (E_a(roasting), 6.4 kJ/mol). XRD analysis of the roasted mass revealed that the refractory phases of cordierite and γ-alumina could be eliminated at ≥600 °C at sample-to-NaOH mass ratio, 1:1. The leaching of the roasted mass followed an intermediate-controlled mechanism for aluminum leaching with (E_{a(Al-leaching)} value of 20.3 kJ/mol), while it was diffusion-controlled for magnesium leaching (E_{a(Mg-leaching)}, 8.9 kJ/mol). At the optimum leaching condition (1.0 M H₂SO₄, 90 °C, 60 min, yielding >95% aluminum and magnesium), a significant amount of PGMs was also leached. Thereafter, the cementation process was investigated with Al⁰-powder that could precipitate >99% PGMs within 15 min at 90 °C. The yielded concentrate of PGMs and CeO₂ was subsequently leached in 6.0 M HCl with 2.0 M NaClO₃ that dissolved >97% PGMs, leaving the residue as the CeO₂ concentrate. Individual metals can be recovered by following established separation and purification techniques.