(548f) Methods for Tracking the Evolution of Refractory REE Mineral Decomposition in Strong Acid Media
Laboratory-scale experiments were performed using a time series at baking temperatures ranging from 80oC to 250oC, with a constant acid: ore ratio of 1:1.4 (g/g), with the goal of identifying progressive stages of the sulfation process. To effectively stop the reaction and prevent hydration of any newly-produced phases and the extremely hygroscopic residual sulfuric acid, rapid cooling down to ~8oC was achieved under desiccant. In order to obtain information on the phase transformations, a portion of the resulting paste-like material was retained to prepare high-quality polished cross-sections amenable to detailed characterization. To maintain the pristine nature of this hygroscopic and reactive acid-bearing product, vacuum drying techniques that minimize sample modification were explored. Another portion was subjected to water-rinsing, to identify the soluble phases that had formed up to that stage of the reaction. This was done using cold water (~10oC), to reduce the contribution of exothermic precipitation of alkali-bearing rare earth double sulphate salts. The <0.45 Âµm filtered rinsate was analyzed by inductively coupled plasma spectroscopy and polished sections of the stable dried residue could be easily prepared using conventional preparation techniques. Textural, structural and chemical properties of the stabilized paste and the rinsed sample were characterized by X-ray diffraction, field-emission scanning electron microscopy, wavelength-dispersive X-ray microanalysis, and the vibrational spectroscopy (Raman, Fourier-Transform Infrared).
 Sadri, F., et al., âA review on the cracking, baking and leaching processes of rare earth element concentrates,â J. Rare Earths, 35 (8), pp. 739-752 (2017).  Lim, H., et al., âLeaching of rare earths from fine-grained zirconosilicate ore,â J. Rare Earths, 34 (9), pp. 908-916 (2016).