Regenerating spent ammonium thiosulphate fixing baths - by decomposing with sodium hydroxide and sulphuric acid and recombining to form ammonium sulphite soln.

摘要

Spent ammonium thiosulphate fixing baths from printing-, X-ray and photographic processing are decomposed into their constituent components which are subsequently used for the synthesis of fresh ammonium thiosulphate which is used to make fully quality new fixing bath. USE/ADVANTAGE - Process allows spent fixing batchs sent for disposal to be reprocessed and recycled as fully quality fresh fixing baths. The spent (NH4)2S2O3 soln. is first treated by metered addn. of NaOH to drive off NH3, followed by treatment with H2SO4 to drive off SO2. The NaOH and H2SO4 are produced electrolytically from Na2SO4 formed in the process. The equiv. amts. of the gases produced are reacted in a gas washer to form ammonium sulphite which is boiled with sulphur powder (recovered by filtration from the Na2SO4 sump) to form ammonium thiosulphate. Finally the prod. is diluted analytically to form a fresh fixing bath which is returned to the user.