Selective precipitation of cadmium from nickel cadmium sulphate solutions using sodium decanoate

Each year in France, about 1300 tons of spent Ni–Cd batteries are collected. Their treatment consists of crushing, physical separation, acidic leaching of electrode materials, separation and recovery of metals leached with different processes. In this work, the selective precipitation of cadmium in synthetic Ni–Cd mixtures was investigated using sodium decanoate as precipitant. A factorial design of experiments was used to improve the separation using variables such as cadmium concentration, pH, molar ratio between decanoate and cadmium, time of addition of precipitant and rest time of the solution. In the best conditions defined by a 25−2 fractional factorial design, starting with concentrations of 0.1 mol L−1 of nickel (5.9 g L−1) and 0.05–0.15 mol L−1 of cadmium (5.6–16.8 g L−1) it is possible to recover in only one stage about 99% of cadmium without precipitating more than 5% of nickel. The results of the separation are on one side a pure solution of nickel at 2.39 × 10−2 mol L−1 (1.4 g L−1) containing 3.44 × 10−4 mol L−1 of cadmium (about 40 mg L−1) and on the other side a precipitate of cadmium decanoate with a cadmium content equal to 23.6 wt% and nickel content lower than 0.8 wt%. These results demonstrated the viability of this separation.