Direct recovery of cadmium and nickel from Ni-Cd spent batteries by electroassisted leaching and electrodeposition in a single-cell process

The investigation was aimed at developing a new electrochemical process for recovery of nickel, cobalt and cadmium mainly present in the form of hydroxides in Ni-Cd batteries, in view to obtain, a solid residue composed of metal nickel and carbon, a pure cadmium-free solution of metal salts and metal cadmium. This process relies upon coupling leaching of the metal hydroxides by electrogenerated protons to electrodeposition in a single divided cell. The feasibility of the technique has been first demonstrated in a lab cell operated in batch mode: cadmium-free solution of nickel and cobalt cations could effectively be produced at the end of the 5-h run, with nearly complete leaching of the hydroxides and a current efficiency of the cathode near 73%: occurrence of side hydrogen evolution at the cathode reduced the efficiency during the last hour of the treatment because of insufficient amounts of Cd2 + cations. In a second test, the waste (called black mass) was added at regular intervals, in view to allowing more regular transfer rates of Cd2 + species to the cathode chamber. The more even profiles of concentrations allowed avoiding depletion of this cation, which resulted in higher current efficiency in cadmium deposition. The efficiency of fed-batch mode is finally discussed in relation to the compromise to be found between purity of the cadmium-free solution and current efficiency of cadmium.