Effect of oxidation behavior of cerium during the roasting process on the leaching of mixed rare earth concentrate

Herein, the phases of mixed rare earth concentrate, oxidative roasted ore, non-oxidative roasted ore, together with their leaching processes and mechanisms, were studied. XRD and SEM-EDS patterns indicate that the phases of the ore roasted in both air and argon atmospheres at 550 °C for 2 h are (Ce, La)OF and REPO4. Ce oxidized from trivalent to tetravalent during the roasting process, causing the lattice distance of the crystals to decrease, and the cracks on the surface of mineral to widen. Cerium extraction in the oxidative roasted ore is 3.2% at a low temperature and low acidity, which is less than 8.6% of the other rare earths. However, at 80 °C, Ce extraction can reach 52% at 2 min in a 3 mol/L hydrochloric acid solution. Analysis of fluorine extraction and SEM-EDS analysis of the leaching residue reveal that Ce(IV) complexes with F− rapidly, and this complex can promote the leaching of total rare earths. With time, Ce(IV) is reduced to its trivalent form, while Cl− is oxidized to Cl2. F− in the solution and RE(III) generate REF3 and go into the slag, which decreases Ce extraction from 52% to 48.1% and those of other rare earths from 51% to 41%. Although the leaching rates of Ce and other rare earths in a non-oxidative roasted ore cannot reach the same level as the ore oxidative roasted at a high temperature, they are relatively high. F− extraction is slightly lower than that of the oxidative roasted ore. The kinetics of the leaching process and SEM analysis of the leaching residue also confirm this.