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

Abstract

Cadmium sulphide thin layers in polycrystalline solar cells are produced by chemical bath deposition (CBD). This process produces wastes containing ammonia and cadmium. These two chemicals must be recovered, and if possible recycled, from an environmental and economic point of view. A process has been implemented in order to recover ammonia and to confine cadmium. A new closed reactor has been designed and fabricated in order to trap ammonia vaporized during the deposition step. Ammonia is absorbed from the exhaust gas from the CBD reactor. The wastes are treated in an ex situ process which enables 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 very small amount, is precipitated, mainly as cadmium sulphide, owing to decomposition of the thiourea introduced in excess. Microfiltration with a glass-fiber filter of about 0.7 μm cut-off produces a filtrate of very low cadmium concentration (< 10 μg/l). Optimization of both pH and residual ammonia concentration should make it possible to lower this residual cadmium concentration. This effluent is the only waste product of the process. The filtration cake containing cadmium compounds may be stored in specialized landfills after standard treatment or, better, treated to recover and recycle the metal without generation of any other harmful effluent. Copyright © 2001 John Wiley & Sons, Ltd.