Integrated acid leaching and biological sulfate reduction of phosphogypsum for REE recovery

Sulfuric acid leaching of phosphogypsum (PG) waste and leachate treatment in sulfate-reducing bioreactor was studied, in order to recover rare earth elements (REE) and remove sulfate from the residual solution. The challenge was to balance efficient REE extraction from PG while maintaining suitable leach solution acidity and sulfate concentration for the microorganisms in the bioreactor. Stirred tank reactor experiments were conducted for PG leaching to form the scope for suitable sulfuric acid concentration and REE leaching efficiency. According to these experiments, moderate REE leaching started already at 0.01 M H2SO4 concentration (yield 6-15%), and increased steadily when increasing H2SO4 concentration to 0.05 M (yield 34-62%). Different leachates were treated in sulfate-reducing bioreactor to understand the limits of acidity and sulfate concentration for the biological system. It was shown that bioreactor requires rather a mild leachate (< 0.02 M H2SO4) to perform reliably and also precipitate REE from the influent. Finally, the precipitate from the bioreactor was collected and analyzed for both chemical and mineralogical composition. These results showed that REE were enriched in the precipitate and formed distinctive mineralogical phases in the MLA images. The method described in this research could be used to treat moderately acidic PG waste leachates with potential for REE recovery.