Controlled dissolution experiments in the rare earth orthophosphate system: Investigating fractionation pathways during caustic alkali cracking

Abstract

The orthophosphate system includes two important minerals that represent economic sources of rare earth elements (REE). Monazite, with a monoclinic structure, can accommodate the large trivalent light REE (LREE). Xenotime is tetragonal and preferentially incorporates the smaller trivalent heavy REE (HREE). Through coupled substitutions, significant concentration of Th4+ can also be present, especially within monazite. Due to their refractory nature, caustic cracking using NaOH solution has been used to decompose the orthophosphate minerals. However, this aggressive process is considered as a bulk non-selective approach where all the REE and actinides concentrate in a solid hydroxide cake. In this context, controlled alkali dissolution experiments on synthetic and natural monazite and xenotime crystals were performed in order to evaluate if, through characterization of the evolution of the decomposition reaction, more selective pathways may be identified. Our results indicate that due to a systematic temperature-dependent increase in the relative solubility of the REE hydroxides with decreasing trivalent ionic radii, a more selective expression of the orthophosphate alkali leaching reaction can be defined at temperatures exceeding ≈ 165 °C. Of particular interest, in addition to a solid LREE-enriched hydroxide phase that can be dissolved in a weak HCl solution and an insoluble Th4+(±Ce4+) residue, the HREE are concentrated in an actinide-depleted dissolved fraction that, when preserved, can be recovered separately through precipitation upon cooling of the NaOH solution. Because the observed selectivity is not intrinsic to the orthophosphate decomposition but involves the partial dissolution of the hydroxide, up-front access to such an actinide-free HREE fraction could also be possible during alkali cracking of peralkaline-type refractory ores.