Cadmium recovery and recycling from chemical bath deposition of CdS thin layers
Résumé
Cadmium sulfide thin layers for polycrystalline solar cells are produced by chemical bath deposition (CBD). This process generates wastes containing mainly ammonia and cadmium. We have implemented and described in a previous paper a process to recover 90% of ammonia and to confine cadmium as a cake which is a mixture of cadmium sulfide and cadmium cyanamide. The present paper concerns cadmium recycling to the CBD step. The cake is first dissolved in a moderate sulfuric acid solution 0.2-0.5 mol/l mixed with hydrogen peroxide (about I mol/l). This last component must be used in large excess (molar ratio H2O2/Cd greater than or equal to 5) and must be completely removed from the solution before recycling or cadmium electrowinning. Hydrogen peroxide decomposition is electrochemically catalyzed by platinized platinum immersed into the solution. The resulting solution contains cadmium sulfate 0.2 mol/l, sodium and ammonium sulfate. One can either recycle it as a chemical bath, or recover the cadmium content by electrowinning; in this case the residual concentration of hydrogen peroxide is electrochemically reduced at the beginning of cadmium electrolysis. The raffinate solution is recycled into the stripping reactor. The pure cadmium metal recovered may be dissolved in a dilute sulfuric acid solution by means of internal electrolysis with two electrodes: a platinized platinum grid as cathode and the cadmium metal as anode. According to the mass of cadmium dissolved it is possible to obtain a concentrated solution of pure cadmium sulfate. The global process recovers at least 99.999% of cadmium and generates only solid sulfur and a liquid effluent containing traces of cadmium (< 10 mug/l).