Phosphogypsum−a byproduct of phosphoric acid production−piled near eco-sensitive regions, poses both environmental and health risks. By reacting phosphogypsum with NaOH, Ca(OH) 2 is produced, which can be utilized for CO 2 sequestration. The cost of NaOH, however, and the low market value of the byproduct, a concentrated Na 2 SO 4 stream, hinder the economic viability of this approach. Here, we propose to increase the cost-efficiency of phosphogypsum processing by converting the Na 2 SO 4 to NaOH and H 2 SO 4 using bipolar membrane electrodialysis (BMED). The result is a cleaner and more circular process, as NaOH can be recycled for dissolving phosphogypsum, while H 2 SO 4 can be reintroduced into the industrial phosphoric acid production or used to leach valuable metals out of the phosphogypsum. To assess the feasibility of BMED integration, we used a bench-scale system to probe the trade-off between obtaining high base-product concentration and obtaining high specific energy consumption. This was done for synthetic Na 2 SO 4 solutions at various concentrations, applied voltages, and types of anion-exchange membranes. Economic analysis based on the results showed that a two-step batch BMED process is able to reduce the chemical cost by 50%, as compared to purchasing NaOH. Integrating BMED with existing approaches can, therefore, improve the sustainability and cost-efficiency of phosphogypsum valorization.

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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 highrecovery (>90%) reverse osmosis step, generating high-quality water. A final reaction between the reverse osmosis concentrate and natural phosphate rock enabled P recovery and neutralization of remaining acidity. The electric-power requirement of the process was estimated to be 4.4 kWh per m 3 of wastewater, from which 0.78 m 3 of clean water, ~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.

No 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. A final reaction between the reverse osmosis concentrate and natural phosphate rock enabled P recovery and neutralization of remaining acidity. 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, ~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