Abstract: A process for treating an acid waste liquid containing Fe ions which has been used for acid washing of metallic materials or articles, comprising the first stage where an organic solvent (A) is added to the acid waste liquid to extract Fe ions, the second stage where an acid is added to the resulting acid waste liquid to convert chemical species of remaining metal salts and increase the hydrogen ion concentration followed by addition of an organic solvent (B) to recover the mineral acid of concern by extraction, the third stage where an organic solvent (C) is added to the resulting acid waste liquid to extract and recover other required mineral acids, and regenerating each of the organic solvents (A), (B) and (C) in respective stages.