Abstract
This paper examines a three-stage countercurrent hydrochloric acid leaching of phosphogypsum to enhance the recovery of rare earth elements while simultaneously producing a technologically valuable gypsum product. The phosphogypsum under study has a ΣREE content of 340.53 g/t and consists primarily of CaSO4·2H2O dihydrate with minor silicate and phosphate impurities. The countercurrent leaching process is shown to ensure efficient reagent distribution and profound destruction of the gypsum matrix. At a solid: liquid ratio of 1:10, the overall recovery of rare earth elements reaches 88%, while at a solid: liquid ratio of 1:4, it is 82.4%, accompanied by a threefold increase in the ΣREE concentration in the productive solutions. X-ray phase analysis of the residue confirmed that complete removal of calcium sulfate from the solid phase is achieved at a solid-liquid ratio of 1:10, while at a solid-liquid ratio of 1:4, some sulfate phases are retained. The precipitate that forms during cooling of the solution is represented by CaSO4·2H2O and is successfully converted by heat treatment (165°C, 4 h) into the hemihydrate form CaSO4·0.5H2O. The strength of the obtained material (5.1 MPa) corresponds to that of G5 gypsum binder. The obtained results demonstrate the effectiveness of three-stage countercurrent leaching and confirm the possibility of comprehensive processing of phosphogypsum, which ensures the production of productive solutions suitable for further sorption or extraction enrichment of rare earth elements, as well as a gypsum residue that meets the requirements of the construction industry.