(548f) Methods for Tracking the Evolution of Refractory REE Mineral Decomposition in Strong Acid Media

Rare earth elements (REEs) are considered critical materials for the development of advanced technologies in applications such as clean energy production. In many ore deposits, the REE-bearing minerals are very complex and refractory in nature, requiring the use harsh processes such as acid baking for their treatment. Acid baking is a hydrometallurgical process that employs elevated temperature and high-strength acid to break down and transform minerals in an ore concentrate in order to recover the REEs into water-soluble sulfates. Aggressive conditions used such as concentrated (98 wt%) sulfuric acid and temperatures in the range of 155-230^oC ^[1] render characterization of the resulting products very difficult, and as such the reaction mechanisms and associated kinetic limitations are not well understood. To this end, the focus of this research was on the development of innovative approaches for the characterization of the solid residue and precipitates throughout the reactive process, especially in the presence of residual sulfuric acid. This may provide an opportunity to optimize the REE recovery, and lead to cost reductions in the final extraction flowsheet.

Laboratory-scale experiments were performed using a time series at baking temperatures ranging from 80^oC to 250^oC, 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 ~8^oC 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 (~10^oC), to reduce the contribution of exothermic precipitation of alkali-bearing rare earth double sulphate salts^[2]. The <0.45 µm 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).

^[1] Sadri, F., et al., “A review on the cracking, baking and leaching processes of rare earth element concentrates,” J. Rare Earths, 35 (8), pp. 739-752 (2017). ^[2] Lim, H., et al., “Leaching of rare earths from fine-grained zirconosilicate ore,” J. Rare Earths, 34 (9), pp. 908-916 (2016).