Solvent extraction separation and recovery of palladium and platinum from chloride leach liquors of spent automobile catalyst

Spent catalysts from automobile industry contain environmentally critical and economically valuable metals such as Pt, Pd, Fe, Ni, Mn, and Cr. In this paper, we report the solvent extraction separation and recovery of palladium (Pd) and platinum (Pt) from hydrochloric acid leach liquors of spent automobile catalyst employing precipitation and solvent extraction methods. Composition of synthetic leach liquor (LL) used for the present study contains (mg/L): Pd-150, Pt-550, Mn-500, Ni-1000, Fe-1500, Cr-100 and 3 M HCl. Variation of tri-n-butyl phosphate (TBP) in the range from 0.0018 to 0.183 M in kerosene indicated selective and quantitative extraction of Pd even at the lowest extractant concentration. McCabe-Thiele plot for Pd extraction with 0.0054 M TBP predicted complete separation of metal in two stages at an aqueous to organic (A/O) phase ratio of 3.75. Counter-current batch extraction simulation (CCES) of Pd at an A/O of 3.75 resulted in a raffinate containing 0.13 mg/L of Pd, corresponding to Pd extraction efficiency of 99.9%. Stripping of Pd from loaded organic (LO) phase at O/A ratio of 5 with 0.5 M thiourea and 0.1 M HCl indicated 1.1 mg/L of Pd in spent organic (SO), with a stripping efficiency of 99.8%. The strip solution (SS) contains 2.8 g/L of Pd, with overall enrichment factor of 18.8 times. From the Pd-raffinate, complete removal of iron (Fe), 72% Cr and 5% Ni and 9.2% Mn was achieved at an aqueous pH of 3.2; whereas the loss of Pt was ∼9.1%. The observed loss of Pt was due to adsorption of metal on the hydrated iron oxide generated in situ. Variation of Aliquat 336 in the range from 0.002 to 0.22 M on the extraction of Pt from Pd-Fe free raffinate showed selective extraction of Pt and reached to 99.7% at 0.011 M concentration. McCabe-Thiele plot for Pt extraction with 0.011 M Aliquat 336 predicted >99% extraction of Pt in two stages at A/O phase ratio of 3 and two stages of stripping with 0.5 M each of thiourea and HCl at an O/A phase ratio 4. Extraction and stripping simulation tests confirmed Pt extraction and stripping efficiency of 99.83 and 99.9%, respectively with an enrichment factor of 12 times. Finally, complete flow sheet of the process for the separation and recovery of Pd and Pt as chlorides was demonstrated.