Chenna Rao Borra

Recovery of scandium from leachates of Greek bauxite residue by adsorption on functionalized chitosan–silica hybrid materials

Bauxite residue (red mud) is a waste residue that results from the production of alumina by the Bayer process. Since it has no large-scale industrial application, it is stockpiled in large reservoirs. Nevertheless, it should be considered as a valuable secondary resource as it contains relatively large concentrations of critical metals like the rare earths, scandium being the most important one. In this work, we investigated the recovery of scandium from real leachates of Greek bauxite residue. In the separation of scandium from the other elements, the biggest challenge arose from the chemical similarities between scandium(III) and iron(III). This hampers high selectivity for scandium, especially because iron, as one of the major elements in bauxite residue, is present in much higher concentrations than scandium. In order to achieve selectivity for scandium, chitosan–silica particles were functionalized with the chelating ligands diethylenetriamine pentaacetic acid (DTPA) and ethyleneglycol tetraacetic acid (EGTA). Both organic ligands were chosen because of the high stability constants between scandium(III) and the corresponding functional groups. The adsorption kinetics and the influence of pH on hydrolysis and adsorption were investigated batchwise from single-element solutions of scandium(III) and iron(III). In binary solutions of scandium(III) and iron(III), it was observed that only EGTA-functionalized chitosan–silica appeared to be highly selective for scandium(III) over iron(III). EGTA–chitosan–silica shows a much higher selectivity over state-of-the-art adsorbents for the separation of scandium(III) from iron(III). The latter material was therefore used as a resin material for column chromatography in order to effectively separate scandium from bauxite residue. Full separation was achieved by eluting the column with HNO3 solution at pH 0.50; at this pH all other elements had already eluted.

Hydrometallurgical recycling of NdFeB magnets: Complete leaching, iron removal and electrolysis

Abstract NdFeB magnets currently dominate the magnet market. Supply risks of certain rare earth metals (REM), e.g. Nd and Dy, impose efficient recycling schemes that are applicable to different types and compositions of these magnets with minimum use of chemicals and waste generation. In this study, a hydrometallurgical method was studied that could be adjusted to recover not only REM, but also other valuable metals (e.g. Co, Ni and Cu) that co–existed in the magnet. The magnet powders were completely dissolved in a dilute sulfuric acid solution giving more than 98% of dissolved iron in the ferrous state. Chemical oxidation of Fe 2+ into Fe 3+ by the addition of MnO 2 required only 1 h at ambient temperature. It was then possible to precipitate more than 99% of this ferric iron by adjusting the pH of the solution above 3 with either Ca(OH) 2 or MnO additions. However, the addition of Ca(OH) 2 resulted in the formation of gypsum and up to ca. 23% REM losses, possibly via co-precipitation into the gypsum. MnO elevated the Mn 2+ concentration in the solution. However, it was found to be problematic that subsequent direct electrolysis removed Mn and Co. Low anodic current efficiencies (ACE) resulted in high energy consumption (EC), while incomplete Mn and Co removals and undesired REM losses were reported. Pre-electrolysis removals of REM and/or Co by oxalate and/or sulfide precipitation were proven to be successful and selective, but this enlarged the flowsheet considerably with only minor improvement of the Mn removal, ACE and EC.