Removal of fluoride and chloride from waste acid by Bi₂O₃: comparison between the synchronous and two-step methods

Removal of chloride (Cl(− I)) and fluoride (F(− I)) ions is a major issue in metallurgical waste acid recycling. This study explored the synchronous and two-step precipitation approaches using bismuth oxide (Bi₂O₃) to remove Cl(− I) and F(− I). Under the mole ratio of Bi₂O₃ and halide ions at respectively 0.3 and 0.7 for the first and second steps, residual concentrations of Cl(− I) and F(− I) below 100 and 50 mg/L, respectively, were obtained, where the removal proportions of Cl(− I) and F(− I) in the first and second steps were respectively 1:1.1 and 1:3.3. During the treatment, Bi₂O₃ was dissolved to produce Bi³⁺, which reacted with Cl(− I) first and then F(− I), leading to the precipitation of bismuth oxychloride and bismuth oxyfluoride. After the removal of Cl(− I) and F(− I), residual Bi³⁺ was removed to a final concentration of 1.3 mg/L through introduction of hydrogen sulfide (H₂S) and recovered as bismuth sulfide (Bi₂S₃). In this method, approximately 97.6% of Bi₂O₃ was regenerated, additionally, Cl(− I) and F(− I) were transferred to a NaOH/NaCl/NaF concentrate with minimal volume. This two-step approach for the removal of Cl(− I) and F(− I) using Bi₂O₃ exhibited excellent performance in terms of high efficiency, high reagent recovery rate and low secondary pollution risk.