Moderately efficient leaching of rare earth elements from phosphogypsum via crystal regulation with EDTA-2Na during gypsum phase transformation and recovery by precipitation

Abstract

Phosphogypsum (PG) is a bulk solid waste generated during the production of wet-process phosphoric acid, which is considered an important secondary resource for rare earth elements (REEs) due to a large amount of REEs locked in them. However, a large part of REEs is encapsulated in the gypsum crystal lattices, and it is difficult to recover this fraction of REEs. In this study, REEs were effectively leached from PG through synergistic control of the phase transformation of PG into α-hemihydrate gypsum (α-HH) and the crystal morphology and particle size of α-HH in sulfuric acid solutions under atmospheric conditions. The dissolution process of the phase transition could effectively release REEs trapped in the gypsum lattice. Meanwhile, during the recrystallization process, thick and short columnar α-HH crystals with a low specific surface area were formed through the regulation of EDTA-2Na, which weakened the adsorption of the released REEs on recrystallized crystal surfaces and further improved the REE leaching efficiency. When EDTA-2Na was added at 0.5%, coarse columnar α-HH crystals with a specific surface area of only 0.57 m2/g could be prepared, and the REE leaching efficiency could reach 59.3% after 2 h of leaching. Notably, the coarse columnar α-HH is a high-strength gypsum product with high value-added. Additionally, the REEs in the leachate were enriched using the sequential precipitation method. Therefore, this study initially achieved the combination of the efficient leaching of REEs and the preparation of high-value-added gypsum products from PG in sulfuric acid solutions and provided a new idea for the comprehensive utilization of PG.