Study of Rare Earths Leaching After Hydrothermal Conversion of Phosphogypsum

Phosphogypsum (PG) is classified as the main hazardous waste present in large quantities in Tunisia. It is a by-product of the fertilizer industry produced by a sulfuric acid attack on calcium phosphate. Storage and management of this toxic waste are environmental problems that concern many countries producing phosphate fertilizer, especially since waste is, in principle, prohibited by international law. Therefore, finding out a solution for the valorization or the removal of phosphogypsum has become a universal preoccupation and a topical research field. In this work, a fresh produced PG sample was firstly characterized by various analytical methods to determine its chemical composition as well as its rare earth elements (REEs) amount. Then, a hydrothermal conversion of the PG allowed obtaining an insoluble residue of calcium carbonate richer in REEs than the starting sample such that the enrichment rate was 66%. The obtained residue, considered after that as an interesting source of rare earths, was leached with a hydrochloric solution (5–6%) in the presence of ascorbic acid as reducing agent. The leaching method favored the migration of 89% of REEs to the leach liquor.