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The behaviour of scandium in the Bayer process
Abstract
Scandium (Sc) is enriched in some bauxites of karst type compared to its crustal average concentration (13.6
mg/kg). The Bayer process uses bauxite as raw material for alumina production and discards bauxite residue
(BR) as a by‐product, in which insoluble compounds are roughly doubled compared to their original
concentrations. Sc is also enriched into BR and can be extracted from it. Sc has been estimated to comprise
over 95% of the commercial value of BR.
Our aim is to clarify the fate of Sc during the entire flowsheet of Bayer process, to understand its distribution
behaviour between products and by‐products and mineralogical nature before and after the process. The
present work will build upon and develop the understanding of Sc distribution previously explained by
Derevyankin et al.
... The lateritic and bauxitic weathering processes have the ability of relative Sc enrichment in the weathered rock up to a factor of ~10 compared to the original source rock . Furthermore, the processing of bauxites for production of alumina (Al 2 O 3 ) leads to a further twofold enrichment in the accumulating residue (Vind et al., 2017). This enrichment makes the so-called bauxite residue (BR) an interesting secondary raw material for Sc production. ...
This paper focuses on the scandium speciation in bauxite residues of different origin. Insights into mineral-chemical similarities and differences of these materials will be presented and links to their natural geological background discussed. The presented research should provide fundamental knowledge for the future development of efficient and viable technologies for Sc-recovery from bauxite residues derived from different bauxites and accumulating at different localities. In total, five bauxite residues were investigated which originated from Greece, Germany, Hungary and Russia (North Ural & North Timan) using a combination of different analytical tools. Those included: laser ablation inductively coupled plasma mass spectrometry, X-ray absorption near edge structure (XANES) spectroscopy, μ-Raman spectroscopy as well as scanning electron microscopy and electron microprobe analyses. X-ray fluorescence and inductively coupled plasma mass spectrometry were used to determine the overall chemical composition. The investigated samples were found to exhibit a relatively homogenous distribution of Sc between the larger mineral particles and the fine-grained matrix except for Al-phases like diaspore, boehmite and gibbsite. These phases were found to be particularly low in Sc. The only sample where Sc mass fractions in Al-phases exceeded 50 mg/kg was the Russian sample from North Ural. Fe-phases such as goethite, hematite and chamosite (for Russian samples) were more enriched in Sc than the Al-phases. In fact, in Greek samples goethite showed a higher capacity to incorporate or adsorb Sc than hematite. Accessory minerals like zircon, rutile/anatase and ilmenite were found to incorporate higher mass fractions of Sc (>150 mg/kg), however, those minerals are only present in small amounts and do not represent major host phases for Sc. In Russian samples from North Ural an additional Ca-Mg rich phase was found to contain significant mass fractions of Sc (>500 mg/kg). μ-XANES spectroscopy was able to show that Sc in bauxite residue occurs adsorbed onto mineral surfaces as well as incorporated into the crystal lattice of certain Fe-phases. According to our observations the bauxite type, i.e. karstic or lateritic, the atmospheric conditions during bauxitization, i.e. oxidizing or reducing, and consequently the dominant Sc-bearing species in the primary bauxite influence the occurrence of Sc in bauxite residues. In karstic bauxites, underlying carbonate rocks can work as a pH-barrier and stabilize Sc. This prevents the Sc from being mobilized and removed during bauxitization. Hence, karstic bauxites are more prone to show a Sc enrichment than lateritic bauxites. Reducing conditions during bauxitization support the incorporation of Sc into clay minerals such as chamosite, which can dissolve and reprecipitate during Bayer processing causing Sc to be redistributed and primarily adsorb onto mineral surfaces in the bauxite residue. Oxidizing conditions support the incorporation of Sc into the crystal lattice of Fe-oxides and hydroxides, which are not affected in the Bayer process. The genetic history of the bauxite is therefore the major influential factor for the Sc occurrence in bauxite residues.
... Sc concentrations detected in bauxite residues worldwide range from 41 to 254 mg/kg (Borra et al., 2016;Zhang et al., 2016). Based on various publications that used different analytical techniques, the average concentration of Sc in AoG's bauxite residue is 121 ± 16 mg/ kg (n = 24) (Alkan et al., 2017;Borra et al., 2015;Davris et al., 2014Davris et al., , 2016Gamaletsos et al., 2016b;Laskou and Economou-Eliopoulos, 2013;Lymperopoulou et al., 2017;Ochsenkühn-Petropulu et al., 1994;Vind et al., 2017;Yagmurlu et al., 2017). It has been reported that over a 15-year period, the concentration of rare earth elements (REE) as well as Sc in the bauxite residue of AoG has fluctuated only about 8%, indicating to a stable and homogeneous occurrence of Sc in this material . ...
Bauxite and bauxite residue, a by-product of alumina production, were studied using a combination of micro-analytical 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 Sc 3+ probably substitutes Fe 3+. 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.
... The rate of formation of the titanium peroxo sulfate complex increases, thus the Ti leaching efficiency peaked at high leaching temperature. Since Sc mainly hosted by hematite and goethite minerals and minor in Ti minerals in BR, increase in the dissolution of those phases directly has a positive impact on the dissolution of Sc 36 . To further increase the Sc efficiency, all of these Sc hosting minerals have to be completely dissolved in the PLS. ...
The need of light weight alloys for future transportation industry puts Sc and Ti under a sudden demand.
While these metals can bring unique and desired properties to alloys, lack of reliable sources brought
forth a supply problem which can be solved by valorization of the secondary resources. Bauxite residue
(red mud), with considerable Ti and Sc content, is a promising resource for secure supply of these
metals. Due to drawbacks of the direct leaching route from bauxite residue, such as silica gel formation
and low selectivity towards these valuable metals, a novel leaching process based on oxidative leaching
conditions, aiming more efcient and selective leaching but also considering environmental aspects via
lower acid consumption, was investigated in this study. Combination of hydrogen peroxide (H2O2) and
sulfuric acid (H2SO4) was utilized as the leaching solution, where various acid concentrations, solid-toliquid
ratios, leaching temperatures and times were examined in a comparative manner. Leaching with
2.5M H2O2: 2.5M H2SO4 mixture at 90°C for 30min was observed to be the best leaching conditions
with suppressed silica gel formation and the highest reported leaching efciency with high S/L ratio for
Sc and Ti; 68% and 91%; respectively.
... Recent discoveries [6,25,26] indicate a close relation of Sc with Fe as Scandium associates mainly with iron mineral phases and to a less extent with the rest of the BR matrix. Additionally, taking into account the different leaching behavior of REE towards Sc and the dissolution mainly of Ca-Na-Si-Al mineral phases it can be suggested that REE are mainly associate with the desilication minerals generated in the Bayer process. ...
... The direct leaching of BR experiments using [Emim][HSO4] were carried out at 200 rpm, 200 °C, 12 hours retention time and 5 % w/v pulp density were conducted. Results shows ( Figure 6) high recovery yields of Sc (78 %), this is due to the fact that it occurs in the same mineralogical phases of Fe (hematite and goethite) and Ti (perovskite) [18]. ...
Bauxite residue (BR) is the solid waste generated from the Bayer process during alumina production from bauxite ore. It can be considered a secondary source of several base metals, but also critical metals such as rare earth elements (REEs). Hydrometallurgical methods have been mainly employed to extract REEs from BR. As an alternative to conventional mineral acid leaching, ionic liquids (ILs) could hold the key for developing an economical, sustainable and greener processing of low-grade REE resources. ILs are solvents consisting solely of ions and have been studied as leaching agents in metallurgical processes, hence the term ionometallurgy was coined. This paper aims to exploit two ionometallurgical approaches on treating BR, evaluating advantages and drawbacks of applying hydrophilic and hydrophobic ILs. The first leaching process involves a hydrophilic IL, [Emim][HSO4], which can dissolve base metals and REEs at elevated temperatures resulting to high recovery yields of scandium (up to 75 %), iron (100 %) and titanium (90 %), whereas the hydrophobic IL leaching process using [Hbet][Tf2N] is able to selectively dissolve REEs (up to 70 - 85 %) against iron (3 %). The findings of the studied processes provide great potential for exploiting ILs as lixiviants on low-grade REE-containing metallurgical by-products.
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