The waste baths, contg. residual nitric and hydrofluoric acids and metal nitrates and fluorides, are ignited to give combustion products contg. CO2, N2, H2O, HF and metal oxides, the metal oxides are separated, the HF is dissolved to recover hydrofluoric acid soln., and the remaining gas mixt. of CO2, N2 and H2O is vented to atmos. The method is used to treat spent pickling baths, esp. from pickling stainless steel. No lime is added to the baths so that stoppages for descaling of the plant are less frequent than in prior processes. The hydrofluoric acid is transformed to gaseous HF in the combustion process so that it is easily recovered and does not contaminate the metal oxide by-products.
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