Purification of industrial wet phosphoric acid solution by sulfide precipitation in batch and continuous modes: Performance analysis, kinetic modeling, and precipitate characterization

Abstract

The purification of wet-process phosphoric acid has always been a challenge for both academic and industrial researchers. A treatment unit was developed to mitigate metallic impurities, such as cadmium, from industrial dilute wet phosphoric acid solution (25% P₂O₅) by NaHS precipitation in batch and continuous modes. The effects of operating parameters on the purification performance were evaluated: namely Sulfur/Cd²⁺ molar ratio, the presence of other metallic ions besides Cd²⁺, temperature, and hydraulic residence time. In the batch mode, equilibrium was attained in less than 20 min; augmenting Sulfur/Cd²⁺ molar ratio increased the CdS precipitation effectiveness up to 86%, but temperature increment had an adverse effect. In the continuous mode, a hydraulic residence time of 40 min was necessary to reach the same yield as in the batch mode at 25 °C. Sulfidation was found to eliminate simultaneously other metallic impurities (especially Cu and As) originally present in the wet-produced phosphoric acid. Finally, NaHS precipitation was able to reduce the concentration of cadmium from 65.95 mg Cd/kg P₂O₅ to 11.15 mg Cd/kg P₂O₅ from untreated industrial WPA solution (25% P₂O₅) at 25 °C, which outperforms most of the published data based on alternative treatment processes.