Efficiency of Sulfuric Acid on Selective Scandium Leachability from Bauxite Residue

Abstract

Bauxite residue (BR) is a well promising resource for critical metals, especially scandium (Sc), a rare and expensive metal with increasing applications in advanced technology. Greek BR seems to significantly favor a commercially viable recovery of Sc combining optimized leaching and advanced separation techniques. Leaching with mineral acids emerges as the dominant selection compared to other techniques. This study investigates an optimized leaching condition set for Sc recovery, using the most advantageous option of sulfuric acid. The main target is to develop a leaching scale-up process to be established in the premises of Mytilineos S.A. (formerly Aluminium of Greece, the largest Greek alumina and aluminum producer), taking into account the feed requirements of a subsequent advanced ion exchanged procedure. Several parameters were studied individually or combined in order to achieve high Sc concentration in the leachate and to ensure selectivity, especially concerning iron. The most significant parameters prove to be the solid-to-liquid ratio (S/L), the final pH value, and the leachate’s recycling. The proposed process, with low molarities of sulfuric acid and ambient conditions, integrates rapidly, leading to high and selective Sc recovery. Finally, a leaching process flow diagram under continuous operation on an industrial scale is developed.

Optimizing Conditions for Scandium Extraction from Bauxite Residue Using Taguchi Methodology

“…As mentioned elsewhere [63], the chemical composition of BR is Fe 2 O 3 (43.5%), Al 2 O 3 (19%), SiO 2 (7.3%), TiO 2 (5.6%), Na 2 O (3%), and CaO (9.4%). Mineralogical analysis of BR revealed phases of iron (hematite, goethite), alumina (gibbsite, diaspore, aluminium oxide) titania (rutile), silica (quartz), calcium (calcite), and aluminosilicates (kaolinite, anorthite, cancrinite, montmorilonite).…”

“…The experimental procedure as well as instrumentation and reagents used are reported in details elsewhere [63]. All experiments investigated in this study were conducted using sulfuric acid at three different acid concentrations (2, 4, and 6 M), as designed by Taguchi's experimental methodology ( Table 1).…”

“…The solution was magnetically stirred at 600 rpm in all cases on a magnetic stirring plate. Agitation rate was not involved in the parameters tested, since it is of lesser importance [63].…”

“…The "one-factor-at-a-time" approach was used in order to evaluate the effect of several parameters, such as solid-to-liquid ratio (S/L), acid concentration (M), time, temperature, final pH, and type of agitation [63].…”

“…The key point of the present work is to estimate the synergy effect of all leaching variables, which has not been previously presented. Acid molarity, leaching time, solid-to-liquid ratio, and reaction temperature were indicated as control parameters for Sc concentration in the leachate solution and its selective recovery in respect to iron [63]. Signal-to-noise (S/N) ratio and analysis of variance (ANOVA) statistical approach was employed for the calculation of percentage contribution of each parameter and optimization of leaching conditions.…”