A Solvent Extraction Design for the Selective Separation of Cadmium, Nickel, and Cobalt from Spent Rechargeable Ni-Cd Batteries

The hydrochloric acid leaching of cadmium, nickel, and cobalt from the dismantled powder of spent rechargeable nickel-cadmium batteries was optimized by the response surface methodology. The optimized parameters included HCl concentration, powder mass, temperature, and solid/solution contact time. The optimal values were 100 mL of 2.6 M HCl, 4.4 g of powder, temperature 48.6 °C, and time 3.19 h. The concentrations of Cd, Ni, and Co in the leach solution were 13500, 12150, and 900 mg/L, respectively. The designed process involved a two-step solvent extraction/back-extraction procedure using 0.5 mol/L trioctylamine (TOA) in dichloromethane as the organic phase. The first step consisted of four successive extraction experiments of the metals from the aqueous phase into the organic phase. The results of this step showed that 92.7% of cadmium and 26.7% of cobalt are extracted into the organic phase. In contrast, nickel ions remain quantitatively in the source phase. The different stability of the anionic chloride complexes of cadmium and cobalt leads to different extractability of these metals. By employing an appropriate back-extraction procedure a selective separation of these ions was achieved. The extracted Cd(II) and Co(II) ion pairs (i.e. [TOAH+][MCl¯3], M is Cd or Co) were back-extracted into the stripping phase by using ammonia solutions as a suitable complexing reagent for these metal ions. Selective back-extraction of cadmium (86.3%) from the organic phase by using 0.5 M NH3 solution has resulted. The second step included five consecutive extraction experiments on the raffinate of the preceding step. The total extraction of cobalt into the accumulated organic phase reached 83.0%. Efficient recovery of the extracted cobalt (79.3%) was obtained by a single back-extraction experiment using 7 M NH3. The investigated design allows for improving Ni/Co ratio from 13.9 in the starting aqueous solution to 106 in the ultimate raffinate.