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Hydrothermal extraction of Scandium and Yttrium from red mud

Diamantopoulos Alexandros

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Summary

Red mud (RM) is the main solid by-product of the aluminum industry and in particular results from the treatment of bauxite by the Bayer method for the extraction of alumina, a precursor of aluminum. Due to the heterogeneity and the abundance of minerals and chemical elements that the rock carries, RM ends up having, also due to the reactions that take place during the refining of bauxite, a complex mineralogical and chemical composition, combined with high pH and fine-grained particles. Mainly in the main components of RM, metal oxides of iron, aluminum, sodium, calcium and silicon predominate, while there are also metals of very high technological and economic importance, which belong to the rare earths. The object of the present work is the evaluation of hydrothermal leaching of red mud, as an effective method for the recovery of metal contents, scandium (Sc) and yttrium (Y). The optimal conditions of temperature and processing time as well as concentration of inorganic acid and EDTA in the selective recovery of Sc and Y, were investigated. Hydrothermal tests of scandium and yttrium leaching, lasting 0.5, 1, 2 and 24 h under subcritical conditions (120,140 and 160 °C), were performed in 100 ml batch reactors. In each test, the solid / liquid ratio was kept constant and equal to 1 g RM per 20 ml of aqueous solution of HCl, H2SO4 and HNO3 of molar concentration equal to 0.05, 0.1 and 0.2 M. The effect of EDTA on leaching performance and selectivity was investigated at concentrations 0.002, 0.009, 0.017 and 0.034 M EDTA. The above tests showed that using 0.2 M H2SO4 for leaching RM for 1 hour at 140 °C, led to significant results of Sc (79%) and Y (100%) recovery rates. The best selectivity for rare earths was obtained for leaching with 0.2 M HNO3 at 140 °C, although the yields of Sc and Y were only 28% and 34%, respectively. Regarding kinetic experiments, a contact time of 1 h is sufficient for satisfactory recovery rates of Sc and Y, as a further increase in contact time causes a significant decrease in the degree of leaching of Sc, which is likely to form insoluble oxide. Attempting to enhance the selectivity and leaching efficiency of the rare earths in the above solution by adding EDTA, no significant improvement in recovery was found. Finally, in order to investigate the leaching mechanism that leads to selective recovery of rare metals against Fe and Ti, characterization of the material took place, performing the techniques of chemical analysis (ICP-MS), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM-EDS).

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