Maximizing the exploitation of phosphogypsum wastes using soaking technique with citric acid, recovering rare-earth and residual phosphate contents

Phosphogypsum (PG), the main by-product of phosphoric acid production industries, is considered one of the most important secondary sources of rare earth elements (REEs). The current study focuses on the recovery of REEs content and the residual phosphate content existing in the PG with preserving on the CaSO4 skeleton to be used in other various applications. These attainments are carried out using citric acid leaching process via soaking technique. Several dissolution parameters for REEs using citric acid were studied, including soaking time, soaking temperature, citric acid concentration, solid-to-liquid ratio, and recycling of the citrate leaching solutions in the further REEs dissolution experiments. The best-operating conditions were 14 d of soaking time, 7.5% citric acid concentration, and the solid-to-liquid ratio of 1/5 at ambient temperature. About 79.57% dissolution efficiency of REEs was achieved using the optimal conditions. Applying four soaking stages by mixing different fresh PG samples with the same citrate solution sequentially, cumulative dissolution efficiency for REEs was found to be 64.7% under optimal soaking conditions. REEs were recovered using Dowex 50X8 resin from citrate solutions with 96% extraction efficiency. Dissolution kinetics proved the pseudo-first-order nature, reversible reactions, and two activation energies for all REEs.

Automated summary

Phosphogypsum (PG), a main by-product of phosphoric acid production industries, is considered as one of the most important secondary sources of rare earth elements (REEs). This study successfully recovers REEs from PG using citric acid leaching process, while preserving the phosphate content. The optimal conditions include 14 days of soaking time, 7.5% citric acid concentration, and a solid-to-liquid ratio of 1/5 at ambient temperature. The dissolution efficiency of REEs reaches 79.57% under the optimal conditions.