An Efficient Leaching of Palladium from Spent Catalysts through Oxidation with Fe(III)

Reclamation of spent catalysts for the efficient recovery of palladium (Pd) is gaining growing attention due to its scarcity and high supply risk. Currently Pd extraction from spent catalysts through an efficient, economical, and green method has remained a challenge. In this study, Fe3+ is utilized for leaching through oxidation of Pd in a mild condition. Before leaching, distillation was proposed to remove and recover the organics from spent catalysts. The effects of HCl concentration, Fe3+ concentration, NaCl concentration, leaching time, and temperature on the leaching efficiency of Pd were investigated to determine the optimum leaching conditions. The results show that Pd extraction and dissolution of Al2O3 increase with higher HCl concentration. The effect of NaCl on Pd leaching efficiency is significant at low acid concentration (2.0 mol/L HCl). The leaching efficiency was 99.5% for Pd under the following conditions: 2.0 mol/L HCl, 4.0 mol/L NaCl, and 0.67 mol/L Fe3+ at 80 °C for 90 min. The leaching kinetics fits best to the shrinking-core model of surface chemical reaction. The activation energy for the leaching of Pd was 47.6 kJ/mol. PdCl42− was selectively adsorbed by anion exchange resin. The filtrate containing adequate H+, Cl-, and Fe3+ was reused as leaching agent. Pd leaching efficiency was over 96% after five cycle times. This study provides an efficient process for recovery of Pd from spent catalysts.

. The reduction potentials for PdCl4 2-in chloride media at the concentration of 0.001, 0.01 and 0.1 mol/L. Figure S2. The organics recovered from spent catalysts by distillation. Figure S3. XRD patterns of the spent catalysts before and after distillation.
The FTIR spectroscopy was used to identify the functional groups of recovered products by distillation. The results are shown in Figure S4. The yellow solid distillage is probably to be anthraquinones, which contain many kinds of functional groups, such as -OH, -CH2OH, -CH3, -COOH, -C=O. The characteristic peak in Figure S4(a) at 3450.2 cm -1 is caused by the absorbance of −OH stretch. The peak at 2943.2 cm -1 is assigned to -CH3 and -CH2OH vibrations. The bands at 1666.4 and 1593.1 cm -1 are assigned to -C＝C and -C＝O vibrations. The bands in the range of 1550 -700 cm -1 include the -C-C stretching and -C-O bending vibrations. The characteristic peaks are consistent with the functional groups of anthraquinones. The characteristic peaks (3477.4, 1645.2 and 715.5 cm -1 ) in Figure S4(b) are assigned to -OH, benzene and -C2H5 vibrations, respectively. The result shows that the liquid of distillage is aromatic solvent oil. Figure S4. The infrared spectra of recovered products by distillation, (a) solid product and (b) liquid product.  The lines at 40, 50, 80 ℃ in Figure S5 are non-linear as the correlation coefficients (R 2 =0.8943, 0.8601 and 0.7085, respectively) are below 0.9, indicating the linearity is poor. This result shows that Pd leaching is not controlled by mass transfer.  The lines at 50, 70, 80 ℃ in Figure S6 is non-linear as the correlation coefficients (R 2 =0.8601, 0.9162 and 0.9005, respectively) are about 0.9, indicating the linearity is poor. This result shows that Pd leaching is not controlled by ash layer diffusion. Moreover, the apparent activation energy calculated by the ash layer diffusion model was 59.52 kJ/mol, which was not within the reasonable range (10-20 kJ/mol) (Zhang et al. 2004).

Selective Adsorption of Pd by R410 Ion Exchange Resins.
R410 anion exchange resin was used for Pd absorption. Column experiments were conducted in a 5.0 cm diameter × 80 cm height Plexiglas tube in which R410 anion exchange resin was wet-packed. The leachate containing PdCl4 2-was poured into the tube. The flow rate of column effluent was controlled at 5.0 ml/min. After the leachate had passed through the column, it was flushed with deionized water until the column effluent was colorless. And then the resin was eluted with 40 g/L NH4Cl + 8% NH3 solution at a flow rate of 5 mL/min to desorb [PdCl4] 2-. The resin was regenerated by being flushed with 20% NaOH solution, deionized waste, and 6.0 mol/L HCl solution in turn.
The Pd-containing solutions were from leaching processes under the condition of (1) Fe 3+ 0-1.0 mol/L, HCl 2.0 mol/L, NaCl 4.0 mol/L, S/L of 1:5 at 80 °C for 2.0 h; (2) HCl 2.0-6.0 mol/L, NaCl 4.0 mol/L, S/L of 1:5 at 80 °C for 2.0 h. Table S3 shows the effect of Fe 3+ on the absorption efficiency of Pd. It can be seen that R410 ion exchange resins exhibited a high absorption affinity to Pd. Pd absorption efficiency was over 97.5% when the concentration of Fe 3+ was below 0.67 mol/L. As the Fe 3+ concentration increased to 1.0 mol/L, the absorption efficiency decreased slightly to 93.37%. Table S4 shows the effect of H + on the absorption efficiency of Pd. Pd absorption efficiency varied from 97.98% to 99.41% as the concentrations of H + were between 2.0-6.0 mol/L, indicating that it has little influence on the sorption of Pd by R410 ion exchange resins.

Fe 3+ (mol/L) c1 (mg/L) v1 (mL) c2 (mg/L) v2 (mL) A (%)
where c1 and c2 are the concentrations of Pd in the leachate and tail liquid; v1 and v2 are the volumes of leachate and tail liquid. A is the adsorption efficiency of Pd.

Elution of Palladium from Loaded Ion Exchange Resins.
The Pd-loaded resins were stripped by using four different elution reagents: (i) 8.0% NH3, (ii) 40 g/L NH4Cl +8.0% NH3, (iii) 1.0 mol/L thiourea and (iv) 2.0 mol/L NaOH. The Pd in the resins were selectively eluted at the rate of 5.0 mL/min. Following the 2.0h elution, the resins were washed with deionized water. The result was shown in Table A5. The eluents 1.0 mol/L thiourea and 2.0 mol/L NaOH were relatively ineffective in eluting Pd from the loaded R410 ion exchange resins. The elution efficiency was less than 5% with the eluent solutions. The 8.0% NH3 and 40 g/L NH4Cl + 8.0% NH3 reagents desorbed 64.27% and 94.73% of Pd, respectively. The result showed that 40 g/L NH4Cl + 8.0% NH3 has successfully stripped Pd from the loaded resins.
The elution of PGMs from selective ion exchange resins is rather difficult because of strong chemical bonds of adsorbed metal ions with functional groups on the resins. Thus, it is necessary to choose appropriate eluent reagents that can form more stable complexes with the PGM ions than the existing complexes in the resins. The mixture solution of 40 g/L NH4Cl and 8.0% NH3 eluent was found to be effective.
where c0, c1 and c2 are the concentrations of leaching solution, after adsorption and after elution, respectively; v0, v1 and v2 are the volumes of leaching solution, after adsorption and after elution, respectively.

Leaching Efficiency of Pd by Reusing Leaching Agent
After the leachate had passed through the column of anion exchange resins, The tail liquid containing large amounts of Fe 3+ , H + , Cl -, and Fe 2+ was reused as leaching agent. It is assumed that the amounts of Fe ions and Clwere equal to the leaching agent before leaching since their consumptions were less than 1%. 1.0 ml H2O2 was added to oxide Fe 2+ into Fe 3+ . The composition of leaching agent was adjusted by HCl(aq) (37%), FeCl3·6H2O and NaCl. Table S7 shows the leaching efficiency of Pd after 5 cycle times of reusing leaching agent. v1 is the volume of the solution after filtration, which is large than 250 ml since deionized water has been added during filtration.