Pretreatment strategies for enhancing rare earth elements leaching efficiency from phosphogypsum – A review with a focus on hydrometallurgical processes

Abstract

The extraction of rare earth elements (REEs) from phosphogypsum (PG, CaSO 4 ·2H 2 O) using acid leaching has emerged as a promising approach to exploit the large quantities of PG waste generated globally during the production of phosphoric acid. However, the conventional leaching methods employed using mineral acids (i.e., H 2 SO 4 , HNO 3 , HCl, etc.) to extract REEs from PG suffer from low leaching efficiencies, which hinders large-scale application. Studies indicate that the low efficiency emanates from the insoluble sulfated PG matrix that immobilizes REEs, making it difficult for leaching agents to extract larger fractions of the REEs. Moreover, the major proportion of REEs in PG occurs within the gypsum crystal lattice, or as residual and organic REEs, which are less prone to acid dissolution. To overcome these challenges, novel chemical and physical pretreatment strategies are being explored to modify PG's chemical and physical properties and enhance the leaching efficiency. These strategies include methods such as phase transformation (CaSO 4 ·2H 2 O → CaCO 3 , and CaF 2 ), double lixiviation, subcritical water extraction, microwave irradiation, ultrasonication, combined mechanical grinding, ultrasonication, and resin-in-pulp. Based on the reported leaching efficiencies, two-step pretreatment, and microwave irradiation were the most efficient chemical and physical strategies, respectively. These strategies promise to unlock the potential for more efficient extraction routes for REEs and reduce reliance on unsatisfactory conventional leaching methods. This review paper aims to critically examine the chemical and structural modifications that occur during the pretreatment of PG, resulting in improved leaching efficiency of REEs. Various strategies used to recover REEs from PG leachates are also examined.

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