Efficient recovery of rare earth elements from apatite concentrate via pre-leaching, pressure leaching, and precipitation: Process evaluation

The depletion of primary resources and the ever-increasing demand for advanced products containing rare earth elements (REEs) have recently attracted significant attention in the processing of low-grade REEs resources. This study proposes a pre-leach stage to extract non-REEs and enrich the REEs in the residue from apatite concentrate (AP) containing 1.28 % REEs. Over 98.9 % calcium and 98.1 % phosphorous were leached in a solution of 2.18 M HNO₃, at 45 °C and a solid/liquid ratio of 10 % (w/v) after 40 min. In the same experiment, the leaching efficiency of REEs was lower than 5 %. The proposed method increased the concentration of REEs in AP from less than 1.28 % to over 7.20 % (w/w). The pressure leaching results of the upgraded REE concentrate in HNO₃, revealed that the increase in HNO₃ concentration, leaching time, and liquid/solid (L/s) ratio have a positive effect on leaching efficiency. Meanwhile, as temperature increases, the leaching efficiency of REEs decreases. The XPS analysis revealed the presence of both Ce(III) and Ce(IV) phases in the leach residue, indicating the oxidation of Ce(III) during the leaching process. Additionally, the XPS spectrum demonstrated that other REEs (i.e., Nd and La) are present in the pre-leach residue in a trivalent form. Furthermore, thermodynamic calculations of the product resulting from the pre-leaching of AP indicated that Ca²⁺ and H₃PO₄ species are predominant in the solution. Moreover, the key factors determining the re-precipitation of REEs in the pre-leaching stage are two species: H₃PO₄ and Ca[H₂PO₄]⁺. Finally, the leached calcium and phosphorous from the purified leachate were precipitated as a pure hydroxyapatite by adding ammonia. Besides, more than 95 % of REEs were converted to the aqueous phase from pre-leach residue using pressure leaching with HNO₃.