It has been estimated that 70% of the world’s Sc resources might be found in bauxite minerals and bauxite residue (Petrakova et al., 2016). Scandium is a very rarely concentrated element, making commercially usable deposits of this critical material very rare. As a result it usually extracted as by- product from the metallurgical residues from primary extraction of TiO2 from ilmenite and other ores, Nickel-Cobalt from laterites, U3O8 from various ores, Rare Earth oxides and others (Wang et al., 2011). For this reason scandium concentrations of 50-60 mg/kg are considered as exploitable (Wang et al., 2011). The main application of scandium is in the production of high strength aluminum alloys suitable for aerospace applications (Ahmad, 2003). Due to the potentially large Al alloy market, the production of scandium from bauxite residue is easily connected with the primary production of aluminum to produce Al-Sc alloy.
There have been many studies on the extraction of REEs, and particularly scandium, from bauxite residue. Several authors have reported different methods based mainly on hydro- or combined pyro- and hydro-metallurgical methods and these are reviewed in the paragraphs below.

Direct leaching with inorganic acids

REE are a small fraction of the BR. Direct leaching of BR with mineral acids, requires significant acid consumption, as BR is by nature highly alkaline, and results in leach solutions with low REE concentrations (<1000mg/l), while the major BR metals such as Fe, Al, Ti, Ca, Na and Si dissolve extensively and are thus found in concentrations of several g/l. This complicates the REE extraction and refining from such solutions, as established methods (solvent extraction – ion exchange) start with solutions of several g/l of REEs. Hence, most researchers focus on the selective solubilization of REE against major elements like Fe, looking to produce leach solutions that can be further processed economically.
Initially, selective solubilization of REE with weak acids was performed in BR from Jamaica (Fulford et al., 1991a). Sulfurous acid H2SO3, produced by SO2gas pumped through bauxite residue pulp, reduces gradually the pH of the pulp, allowing the selective leaching of REE against iron. The leaching takes place in 2-3 stages and the recoveries of REE reach 85% (expressed by the recovery of Y) while the recovery of Sc is not reported. Τhe same team also separated the REE directly from the pregnant solution using DEHPA organic solvent (Fulford et al., 1991b). Selectivity achieved for pH values in the range of 1,5-3. In this range, Fe remains insoluble, while other metals such as Ca, Na are completely dissolved.
The use of inorganic acids (Η2SO4, HCl, HNO3), results in selective leaching of REEs from Greek Bauxite Residue, but the recovery levels are quite low (Borra et al., 2016a). For instance, Sc selective leaching ranges always between 30-50% even at 150°C (Sugita et al., 2016). Ιt has been found that HCl addition in the Greek Bauxite Residue leads to a maximum Sc leaching of 50% against Fe (5%), while the increased recovery levels correspond to high Fe solubilization, indicating indirect correlation between Sc and mineral Fe phases (Borra et al., 2015). The same team reports that during solubilization the complete Ca, Na dissolution takes place, while Al, Ti, Si are recovered in the range of 30-40%. REE recoveries varied between 30-60% after 24 hours treatment in a 2% w/v pulp density with 0.5-1,5 mol/L HCl. Increasing the HCl concentration to 3-6 mol/L, increases REE and Fe recovery above 70%. At the same time 30-50% of Al, Ti and Si is dissolved. Increasing the pulp density during leaching (at the same acid concentration) reduces the recovery of REE. Finally, the increase of temperature during the HCl leaching does not affect REE recoveries (Borra et al., 2015). The absence of significant effect of factors such as temperature and pressure during the leaching of Greek BR are also reported in previous studies (Ochsenkühn-Petropulu et al., 1996). On the other hand, there are studies that report opposite results (Sugita et al., 2016). This happens because the leaching stage takes place under different conditions and there is heterogeneity between bauxite residues from different plants (processing different bauxite ores). Heterogeneity also occurs even from the same bauxite residue supplier in samples taken at different times due to possible changes in Bayer process conditions or more importantly the bauxite ore (mixture) used in the plant. This fact does not always allow the direct comparison of results between different BR acids leaching.
Greek BR leached with nitric acid (HNO3) results in selective recovery of Rare Earths against Fe (Ochsenkühn-Petropulu et al., 1996). The selectivity of Rare Earths from Bauxite Residue based on the consumed acid increases with the following order H2SO4<HCl< HNO3. During the HNO3 leaching, Υ recovery was about 90% , heavy lanthanides (Dy, Er, Yb) recovery was up to 70%, for the middle (Nd, Sm, Eu, Gd) up to 50% and for the light lanthanides (La, Ce, Pr) up to 30% (Ochsenkühn-Petropulu et al.,1996). Leaching with diluted HNO3 was applied and optimized on a pilot scale (Ochsenkühn-Petropoulou et al., 2002), where scandium recovery reaches 70% after a 3 steps of consecutive HNO3 leaching. Furthermore, pH must be kept between 0 and 0.2 for the highest Sc/Fe ratio in the pregnant solution. The final solution undergoes purification stages to recover REE using ion exchange resin Dowex 50 and then solvent extraction with DEHPA (Ochsenkühn-Petropoulou et al., 2002; Ochsenkühn-Petropulu et al., 1995). Finally, Sc extraction from the organic solvent is performed with NaOH, while REE are separated in a subsequent step with liquid chromatography (Tsakanika et al., 2004). The main drawback of this method is the large volume of liquid wastes containing nitrate anions when produced by the solid residue washing after leaching (Petrakova, 2014).
H2SO4 was selected as the most suitable extracting agent for the Australian BR leaching. Sc recovery is 47% against Fe 7%, while Υ recovery is 5% at 10% w/v pulp, 1 mol/L H2SO4 at 50°C for 2 h (Wang et al., 2013). Recent leaching studies in Greek BR using H2SO4 didn’t show significant improvement in REEs recovery. Nevertheless, efforts have been made to reduce acid consumption by neutralizing BR with CO2(Rivera et al., 2017) as well as attempting to avoid the formation of silica gel by the method of digestion with concentrated acids and final water dilution (Rivera et al., 2018). Scandium recoveries are between 30-40% with high Fe dissolution due to the use of concentrated HCl and H2SO4. Furthermore, the avoidance of silica gel and Sc (68%), Ti (91%) recovery can, also, be achieved with 2,5Μ H2O2 solution in 2,5Μ H2SO4 but leaching is not selective (Alkan et al., 2018). Τhe main scandium leaching mechanism from BR selectively using H2SO4 is described by the shrinking core model- thin film diffusion (Hatzilyberis et al., 2018), while the reaction rate is first class with the scandium recovery up to 50% selectively at 2% w/v pulp density, 1-3 mol/L for 1 h.
Correspondingly, in Canadian BRs, three different acids were examined. H2SO4 selected as the most appropriate, based on the leaching efficiency, cost and practical part (Reid et al., 2017). In the kinetic model, the process of BR’s REEs leaching is described by the expansion of acid through the boundary layer at the boundary of grain. Pretreatment of BR in direct microwave exposure before leaching, nano-sized pores were grown on the surface of grains, allowing for an increase in REE leaching efficiency, which means 40,0-64,2% Sc and 54,3-78,7% Nd, no results on recoveries for other main metals are given (Reid et al., 2017).
The relatively moderate recoveries of REE and especially the Sc recovery, during the direct selective leaching, led many teams to use more concentrated acids in higher temperatures to achieve higher scandium recovery. The processing of BR with concentrated acids provides the extraction of critical and other metals with no selectivity. BR leaching with HCl in 6mol/L at 50°C for 1 h and 20% w/v pulp density can leach up to 80% Sc with 21,2 mol HCl consumption per kg of BR (Wang et al., 2010). In a bauxite residue digestion process with concentrated H2SO4 at high temperatures followed by leaching with water and addition of H2SO4, the optimum conditions were determined as an average particle size of 65-80 μm, 90°C, 3h and solid to liquid ratio 3:1 recovering 85% of Sc (Xue et al., 2010). Ιn a similar like process (Zhang et al., 2005), 6mol/L HCl, solid to liquid ratio 4:1 are leached at 50 ℃ for 1 h recovering 80% of Sc and then the residue is treated with H2SO4 at 92%wt H2SO4, with solid to liquid ratio 3:1, at 200℃, for 1.5 h to leach 97% of Ti. In these studies, the recoveries of other metals from BR as well as the subsequent recovery of Sc from the solutions produced are not reported. Nonselective leaching of Sc with concentered sulfuric acid (3-4 mol/L) at 95 C at 10-20% pulp densities has also been reported for the Greek BR, leading to 90% Sc and 95% Fe extraction. The produced PLS (Pregnant Liquid Solution) was reported having 7g/l Al, 5 g/L Ti, 42 g/L Fe and 15 mg/l of Sc (Davris et al. 2018c).
For the non-selective recovery of metals from BR, Orbite Technology Inc. (Boudreault et al., 2017) developed a hydrometallurgical process in which the BR is treated with concentrated HCl (18-33% wt.) solution in an autoclave at 140-170 °C. During the leaching, all metals (except Ti) are dissolved. Aluminum, iron and magnesium is recovered by treating the solution with HCl gas and subsequently removing steps (Figure 1). REE are separated from the pregnant solution using organic solvents. The use of corrosive HCl, which requires glass-lined reactors as well as valves and tubes of high chemical resistance polymers, results in high cost and maintenance demand. In addition, the handling and storage of the produced insoluble acidic solid is of high importance, rendering the process as less effective.