pickling acid for stainless steel and process for the processing of the consumed pickling acid

摘要

A hydrofluoric acid/nitric acid mixture with 5 to 12%, preferably 6 to 10%, of hydrofluoric acid and 1 to 5%, preferably 1 to 4%, of nitric acid is suitable for pickling austenitic steels. Spent pickling acid with an iron content controlled at about 35 to 60 g of iron per litre and an n(fluoride)/n(iron) mass ratio of 4 to 6, especially 4.4 to 5.5, is adjusted with alkali to pH 4 to 6, especially 4.5 to 5.5, for reprocessing, a crystalline precipitate (/= 98% fluoride) being formed. The suspension obtained can be evaporated as such to dryness, but preferably the precipitate is separated from the liquid and disposed of as special waste. The liquid can be concentrated in conventional evaporators without risk of corrosion and dried for harmless disposal as dry material (special waste). Especially caustic soda solution is used for adjusting the pH. The pickling acid is particularly suitable for pickling off radio-actively contaminated steels.