The role of nano-perovskite in the negligible thorium release in seawater from Greek bauxite residue (red mud)

Abstract

We present new data about the chemical and structural characteristics of bauxite residue (BR) from Greek Al industry, using a combination of microscopic, analytical, and spectroscopic techniques. SEM-EDS indicated a homogeneous dominant “Al-Fe-Ca-Ti-Si-Na-Cr matrix”, appearing at the microscale. The bulk chemical analyses showed considerable levels of Th (111 μg g−1), along with minor U (15 μg g−1), which are responsible for radioactivity (355 and 133 Bq kg−1 for 232Th and 238U, respectively) with a total dose rate of 295 nGy h−1. Leaching experiments, in conjunction with SF-ICP-MS, using Mediterranean seawater from Greece, indicated significant release of V, depending on S/L ratio, and negligible release of Th at least after 12 months leaching. STEM-EDS/EELS & HR-STEM-HAADF study of the leached BR at the nanoscale revealed that the significant immobility of Th4+ is due to its incorporation into an insoluble perovskite-type phase with major composition of Ca0.8Na0.2TiO3 and crystallites observed in nanoscale. The Th LIII-edge EXAFS spectra demonstrated that Th4+ ions, which are hosted in this novel nano-perovskite of BR, occupy Ca2+ sites, rather than Ti4+ sites. That is most likely the reason of no Th release in Mediterranean seawater.

Easily Leachable Rare Earth Element Phases in the Parnassus-Giona Bauxite Deposits, Greece

“…Although there have been few studies on the REE potential of Greek bauxites, a number have found significant concentrations in the red mud waste from their processing using the Bayer Process (e.g. Ochsenkühn-Petropulu et al, 1994;Deady et al, 2014;Borra et al, 2015;Gamaletsos et al, 2016).…”

Process Design Aspects for Scandium-Selective Leaching of Bauxite Residue with Sulfuric Acid

“…The appended graph in Figure 1 supports the hypothesis of 1st order kinetics by presenting three characteristic examples of Sc-selective leaching conditions with the adaptation of experimental data to be highly sufficient (large values of R 2 ), given that dry BR is a fine powder with significantly variable Sc concentration in the micro-structure ( [37] (pp.52-53), [38]). The 1st order kinetic approach for leaching is in accordance with a conditional version of the general diffusion mechanism (i.e., thin film model) as seen in [39], (Equation (2) of [31]), and Supplementary Materials, Text S2.…”

“…Evaluation of leaching yield data shows that the size of this residual core depends on the cationic element and leaching conditions. Moreover, studies of BR micro-structure [38] have shown that the residual content of trace elements is trapped in unyielding micro-cores inside the particle with bound main cationic elements, and mostly Fe ( [23], heavily extractable Sc). However, the validity of the overall model extension set could be tested in a future study.…”

“…The appended graph in Figure 1 supports the hypothesis of 1st order kinetics by presenting three characteristic examples of Sc-selective leaching conditions with the adaptation of experimental data to be highly sufficient (large values of R 2 ), given that dry BR is a fine powder with significantly variable Sc concentration in the micro-structure ( [37] (pp. 52-53), [38] Regarding the rest of the parameters, they did not show any significant Sc yield variation within the range of Sc-selective conditions, such as: (i) Temperature effect for the investigated range (ambient −80°C), but with taking into account that increasing temperature up to 80 °C resulted in enhanced Fe dissolution, rendering the process non selective; (ii) Particle size, as dry BR is a material with exclusively fine particles; (iii) Different agitation modes proved not to have a detectable effect. Relevant experimental studies [28,29,31,34], considering a variety of acids and conditions, show similar results.…”

Recovery of Rare Earths and Other Valuable Metals From Bauxite Residue (Red Mud): A Review

“…However, it is difficult to find the mineralogy of REEs in bauxite residue due to their concentration and fineness of the bauxite residue [41]. Nonetheless, Gamaletsos et al, and Sugita et al, proposed that cerium and other lanthanides are present in the perovskite (CaTiO 3 ) mineral in bauxite residue [49,54]. Borra et al, proposed that scandium could be in an iron oxide phase because chemical similarity was found between iron and scandium and the subsequent relation between iron and scandium dissolution during leaching [37].…”