Leaching of rare earth elements (REEs) and impurities from phosphogypsum: A preliminary insight for further recovery of critical raw materials

Phosphogypsum is a pollutant waste generated by the fertilizer industry. Managing this pollutant is challenging due to the large volumes generated worldwide. A promising route is the valorization of phosphogypsum to recover rare earth elements. However, optimized recovery schemes are needed to create a cost-effective and environmentally friendly process. This paper studies the extraction efficiency of rare earth elements from phosphogypsum and the release of impurities during leaching in a variety of solutions and different working conditions. The best leaching performance was obtained using a 3 M nitric acid (above 80%) solution that achieved a dissolution rate of 63% of the gypsum originally present. In contrast, using 0.5 M sulfuric acid extracted between 46% and 58% of the rare earth elements contained in phosphogypsum, dissolving less than 6% of the gypsum. This higher dissolution of gypsum led to a higher release of impurities by nitric acid. Increasing reaction times from 2 h to 8 h yielded an improvement of leaching efficiency of around 8% for both leaching solutions, while also promoting an increase of 6% in the release of impurities. Adding DTPA resulted in poor leaching performance (from 13% to 22%). Pretreating phosphogypsum with water can remove a significant fraction of the impurities without scavenging rare earth elements. Mineralogical and chemical evidence suggests unreacted phosphate and fluoride are the most probable minerals hosting rare earth element minerals in phosphogypsum. The results of this study could contribute to optimizing recovery methods to extract rare earth elements from phosphogypsum worldwide, thus helping achieve the goals of the circular economy.