Circular Process for Phosphoric Acid Plant Wastewater Facilitated by Selective Electrodialysis

Abstract

Phosphoric acid production generates large volumes of industrial wastewater that cannot be treated efficiently by existing processes because of its low pH and high precipitation potential. At present, the wastewater is generally stored in evaporation ponds that are prone to breaches, leakage, and flooding. We developed an alternative three-step process for the treatment of phosphoric acid wastewater including selective electrodialysis, reverse osmosis, and neutralization. Testing the process with synthetic wastewater yielded promising results. An exceptional Na/Ca selectivity (up to 18.3) was observed in low-pH electrodialysis, enabling the separation of concentrated H2SO4 without gypsum scaling. Sulfate removal from the electrodialysis diluate prevented scaling in the subsequent high-recovery (>90%) reverse osmosis step, generating high-quality water. Finally, the acidity remaining in the reverse osmosis concentrate was neutralized using phosphate rock which also enabled P recovery. The electric power requirement of the process was estimated to be 4.4 kWh per m3 of wastewater, from which 0.78 m3 of clean water (conductivity 1.4–4.4 μS cm–1), ∼3 kg H2SO4, and ∼2.5 kg P were recovered. Overall, lab-scale results indicate that this process would be a sustainable and techno-economically viable solution for the treatment of hazardous wastewater byproducts of the phosphoric acid industry.