Characterization and hydrometallurgical processing of Korean rare earth oxides (REOs) for extraction of rare earth metals

Abstract

Monazite is the second most significant and copious source of rare earths minerals found in nature. These are mainly rare earth (RE) phosphate containing thorium and uranium. Various phosphides are formed during the reaction between the phosphorus and RE present in the monazite which causes pulverization of the RE-alloys. So, to decrease the amount of phosphorous entering into the alloys of RE removal of phosphorus is essential. The behavior of phosphate removal from monazite mineral by sodium carbonate roasting and alkaline leaching followed by water leaching was investigated. Various parameters viz., the effect of temperature, time, Na2CO3 concentration on roasting, NaOH concentration on leaching, etc was studied. The results indicate that 10 g Na2CO3 with 10 g monazite at 900°C roasting temperature and 120 min roasting time was required for the complete conversion of REPO4 into RE2O3. On the other hand, leaching with 50% NaOH (w/v) at a pulp density of 100 g/L and 170 °C, the recovery of 99.9% phosphate in 3 hours was noticed, leaving all REMs in the residue. Leaching kinetics of phosphate in alkaline solution fitted well to the “film diffusion control dense shrinking spheres model” i.e. 1-(1-X)2/3 = kct. Further, the roasted samples as well as the alkaline leached residue were washed with distilled water which leaves behind the rare earth oxide for further analysis. The rare earth oxides (REOs) left after these dephosphorization processes could be easily leached out using acids. Using 6 M HCl for 2 hours at 90°C keeping the pulp density to be 60 g/L was considered to be the optimum condition for RE dissolution. From the above experiments, it was noticed that the recovery of RE elements from the alkaline leached REOs was almost 70% which was greater as compared to the REOs obtained from Na2CO3 roasting. Further, precipitation/ solvent extraction methods can be used to separate the elements from the leach liquor obtained.