Effect of polar organic solvents on the separation of rare earths and transition metal chloride complexes: comparison of ion exchange, extraction chromatography and solvent extraction

Abstract

In the search for more efficient purification and separation methods for rare earths, remarkable results were obtained in the field of solvometallurgy. Replacing the aqueous phase partially or largely by polar molecular organic solvents can significantly improve extraction efficiency and selectivity in the separation of rare earths and transition metals. The effect of polar organic solvents on the sorption of rare-earth elements and transition metals was investigated for strong anion exchanger Amberlite IRA 402 (Cl^-), and for extraction chromatography resins, TEVA (Cl^-) and DGA. The sorption of metal ions was tested from ethanol, ethylene glycol, water and formamide. While Amberlite IRA 402 selectively recovered the iron, copper and cobalt, while leaving the rare-earth elements in solution, the DGA resin was selective for rare-earth elements and could be used for the separation of heavy rare earths from light rare earths. The efficiency of metal uptake by the resins increased with decreasing dielectric constant of the feed solvent. These results were compared to non-aqueous solvent extraction using Aliquat 336 and TODGA from the different polar organic solvents. Significant mutual miscibility observed during these solvent extraction tests demonstrated the advantage of using ion-exchange resins over solvent extraction. However, in the case of chromatographic resins, the functional molecules are only physically bonded by the resin, and loss of sorption capacity due to extractant loss was observed with both TEVA and DGA resin, especially in combination with ethanolic solutions.

Fig. 1. Effect of HCl concentration on the sorption of transition metals and REEs from feeds containing (A) 50, (B) 80 or (C) 95 vol% ethylene glycol by Amberlite IRA 402 (chloride form). Conditions: 25 mg of resin, 2.5 mL of feed solution, metal concentration 0.5 mmol L⁻¹ (each), t = 30 min, T = 21 ± 1 °C.

Fig. 2. Effect of HCl concentration on the sorption of transition metals and REEs from feeds containing (A) 50, (B) 80 or (C) 95 vol% ethanol by Amberlite IRA 402 (chloride form). Conditions: 25 mg of resin, 2.5 mL of feed solution, metal concentration 0.5 mmol L⁻¹ (each), t = 30 min, T = 21 ± 1 °C.

Fig. 3. Effect of HCl (0.5–6 mol L⁻¹) on the sorption of transition metals and REEs from aqueous feeds by (A) Amberlite IRA 402 (chloride form), (B) TEVA (chloride form) and (C) DGA resin. Conditions: 25 mg of resin, 2.5 mL of feed solution, metal concentration 0.5 mmol L⁻¹ (each), t = 30 min, T = 21 ± 1 °C.

Fig. 4. Effect of solvents on the particle size of (A) TEVA (chloride form) and (B) DGA resin at room temperature.

Fig. 5. Effect of HCl concentration on the sorption of transition metals and REEs from feeds containing (A) 50, (B) 80 or (C) 95 vol% ethylene glycol by DGA resin. Conditions: 25 mg of resin, 2.5 mL of feed solution, metal concentration 0.5 mmol L⁻¹ (each), t = 30 min, T = 21 ± 1 °C.

Fig. 6. Effect of HCl concentration on the sorption of transition metals and REEs from feeds containing (A) 50, (B) 80 or (C) 95 vol% ethanol by DGA resin. Conditions: 25 mg of resin, 2.5 mL of feed solution, metal concentration 0.5 mmol L⁻¹ (each), t = 30 min, T = 21 ± 1 °C.

Fig. 7. Extraction of transition metals and REEs from (A) 95 vol% ethylene glycol, (B) 50 vol% ethanol and (C) 95 vol% formamide by Aliquat 336 diluted in Shellsol A150. Conditions: [metal] = 2.0 mmol L⁻¹ each, 0.2 mol per L HCl, LP : MP = 1 : 1 (4 mL : 4 mL), 15 min of contact time at room temperature.

Fig. 8. Extraction of transition metals and REEs from (A) 95 vol% ethylene glycol, (B) 50 vol% ethanol and (C) 95 vol% formamide by TODGA diluted in Shellsol GS190. Conditions: [metal] = 2.0 mmol L⁻¹ each, 0.2 mol per L HCl, LP : MP = 1 : 1 (4 mL : 4 mL), 15 min of contact time at room temperature.