St. Petersburg Mining University (St. Petersburg, Russia):

Cheremisina O. V., Head of Chair, Doctor of Engineering Sciences, Professor, ovcheremisina@yandex.ru
Sergeev V. V., Associate Professor, Candidate of Engineering Sciences
Fedorov A. T., Assistant, Candidate of Engineering Sciences
Mikheeva V. Yu., Student

Phosphogypsum is a by-product obtained in the processing of apatite concentrates in the phosphate fertilizers industry. Mining enterprises at the Khibiny deposits generate over 8 Mtpa of phosphogypsum. The stale phosphogypsum reserves in the dumps here reach 200 Mt, with an average content of rare earth elements (REE) of 0.4 % wt. In phosphogypsum (CaSO₄·2H₂O), rare earth metals are present in both water-soluble and hard-to-recover forms. Chemical recovery methods based on the use of various mineral acids as leaching agents are most common. Considering that rare-earth metals may form soluble complexes with citrate ions used as an eluent, the effects of sodium citrate solutions were studied. Preliminary thermal activation of samples at up to 300 °C leads to lower lanthanum, cerium, and neodymium recoveries caused by the compaction of the phosphogypsum crystal lattice due to the removal of its crystallization water. Exposure to ultrasound has no significant effect on the recovery. A significant REM recovery improvement is ensured by phosphogypsum exposure to ionizing radiation. The resulting defects in the crystal structure of the material allow the eluent solution to easily penetrate into the crystal lattice. Here, however, waste storage time is the key factor for the REM recovery efficiency. Water treatment of any freshly mined material after exposure to ionizing radiation eliminates the use of mineral acids as leaching agents and improves REM recovery.
The work was carried out with the financial support of the Russian Science Foundation (project No. 19-19-00377).

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