Modes of occurrences of scandium in Greek bauxite and bauxite residue
Abstract
Bauxite and bauxite residue, a by-product of alumina production, were studied using a combination of microanalytical techniques — electron microprobe wavelength dispersive spectrometry, laser ablation inductively coupled plasma mass spectrometry and μ-Raman spectroscopy. The aim of the work was to reveal the modes of occurrence of scandium (Sc). The motivation behind this effort was to provide mineralogical insight for the support of ongoing development of Sc extraction technologies from bauxite residue. In the analyzed bauxites and residue, Sc is mainly hosted in hematite, where Sc3+ probably substitutes Fe3+. The average concentration of Sc in the hematite matrix of bauxite is about 200 mg/kg, while in the bulk sample it ranges from 42 to 53 mg/kg Sc. In bauxite residue, the average concentration of Sc in hematite matrix is about 170 mg/kg, and in the bulk sample it is 98 mg/kg. In bauxite residue, goethite was also identified to host Sc with a concentration of about two times more than in hematite — 330 mg/kg. In bauxite residue, hematite, goethite and zircon host respectively 55 ± 20%, 25 ± 20% and 10 ± 5% of the total Sc. The effect of the Bayer process to the modes of occurrences of Sc is minor. The secondary bauxite residue minerals formed during bauxite processing do not capture any or capture very low amounts of Sc. New evidences of Sc leaching behavior from bauxite residue show that Sc is first released from goethite, then from hematite and the unrecovered proportion of Sc is likely associated with zircon.