Ammonia recycling and cadmium confinement in chemical bath deposition of CdS thin layers

Cadmium sulfide thin layers in polycrystalline solar cells are produced by chemical bath deposition (CBD). This process produces wastes containing amm onia and cadmium. These two chemicals must be recovered, and if possible re cycled, from an environmental and economic point of view. A process has bee n implemented in order to recover ammonia and to confine cadmium. A new clo sed reactor has been designed and fabricated in order to trap ammonia vapor ized during the deposition step. Ammonia is absorbed from the exhaust gas f rom the CBD reactor. The wastes are treated in an ex situ process which ena bles recovery of the major part of the large amounts of ammonia used in the process. Concentrated solutions of pure ammonia are obtained (more than 10 mol/l). In connection with this operation, all of the cadmium, except a ve ry small amount, is precipitated, mainly as cadmium sulfide, owing to decom position of the thiourea introduced in excess. Microfiltration with a glass -fiber filter of about 0-7 mum cut-off produces a filtrate of very low cadm ium concentration (< 10 pg/l). Optimization of both pH and residual ammonia concentration should make it possible to lower this residual cadmium conce ntration. This effluent is the only waste product of the process. The filtr ation cake containing cadmium compounds may be stored in specialized landfi lls after standard treatment or, better, treated to recover and recycle the metal without generation of any other harmful effluent. Copyright (C) 2001 John Wiley & Sons, Ltd.