Efficiency of Sulfuric Acid on Selective Scandium Leachability from Bauxite

metals

Article Efﬁciency of Sulfuric Acid on Selective Scandium Leachability from Bauxite Residue †

Maria Ochsenkuehn-Petropoulou *, Lamprini-Areti Tsakanika *, Theopisti Lymperopoulou, Klaus-Michael Ochsenkuehn, Konstantinos Hatzilyberis , Paraskevas Georgiou , Chrysanthos Stergiopoulos, Olga Seriﬁ and Fotis Tsopelas

School of Chemical Engineering (Chem. Eng.), National Technical University of Athens (NTUA), 9 Iroon Polytechniou St., 15780 Athens, Greece; [email protected] (T.L.); [email protected] (K.-M.O.); [email protected] (K.H.); [email protected] (P.G.); [email protected] (C.S.); olga.seriﬁ@gmail.com (O.S.); [email protected] (F.T.) * Correspondence: [email protected] (M.O.-P.); [email protected] (L.-A.T.); Tel.: +30-210-772-3094 or +30-697-203-5984 (M.O.-P.); +30-210-772-4022 or +30-695-523-9222 (L.-A.T.) † This paper is part of the oral presentation entitled “Mineral Acid Leaching of Scandium from Bauxite Residue”, presented by Ochsenkuehn-Petropoulou, M. Proceedings of the International Bauxite Residue Valorization and Best Practices Conference-BR2018, Athens, Greece, 7–10 May 2018. Available online: http://svs2015-mtm.icts.kuleuven.be/indico/event/39/contribution/30 (accessed on 20 September 2018). The whole proceedings are not available online and only a hard copy of them exists.

Received: 3 October 2018; Accepted: 5 November 2018; Published: 7 November 2018

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.

Keywords: bauxite residue; scandium; leaching; sulfuric acid

1. Introduction Bauxite residue (BR), also called red mud, is the highly alkaline (pH > 11) and very ﬁne-grained slurry by-product after “Bayer” process for alumina production. Its huge global annual production of ~120 million tons [1] has resulted in increasing BR accumulation, causing deposition problems and serious environmental impacts. The management and the safe disposal of this waste are major issues for the bauxite and alumina industries, affecting production cost. The valorization of BR as a secondary raw material and as a metal resource of low cost could be a route for its reduction, introducing the waste again in the economic cycle. BR is rich in minerals and metals of high economical interest. Its chemical composition depends directly on the origin of the bauxite ore and the conditions of the Bayer

Metals 2018, 8, 915; doi:10.3390/met8110915 www.mdpi.com/journal/metals Metals 2018, 8, 915 2 of 16 process. It contains oxides and salts of the main elements Fe, Al, Ca, Na, Si, Ti, numerous trace elements such as V, Cr, Zn, Ga, Nb, Zr, and Ta, and rare earths elements (REEs) (Sc, Y, and lanthanides) [2]. In Greece, Mytilineos S.A., formerly “Aluminium of Greece” (AoG), produces annually about 750,000 tons of BR, which is characterized as chemically stable. Besides other metals, Greek BR is rich in REEs, especially in scandium (Sc). It was found to contain ~1 kg REEs/t of dry BR with an Sc content of ~120 g/t of dry BR (0.02% Sc2O3) close to Sc main resources. Greek BR is enriched in REEs by a factor of two referring to the original bauxites, and the concentration of REEs is almost constant for a period of about 25 years [2–5]. It is calculated that a full exploitation of Greek BR from AoG can produce about 1 kt/yr of REEs oxides. REEs, especially Sc, are elements of high techno-economical interest because of their use in high-tech materials and modern applications. Scandium is exceptionally expensive especially in high purity, presenting highly promising demand but low availability due to its few economically exploitable minerals and its difficult recovery from deposits where co-exists with radioactive elements [6]. European Commission’s classification of Critical Raw Materials (CRMs) [7] ranks Sc as critically high on both supply risk and economic importance due to the decreasing available stockpiles combined with the globally limited production and the evolution of new applications [8–16]. The lightweight Al–Sc alloys with a content of 0.5–2% Sc find applications in the aerospace and defense technology and athletic equipment. Additionally, the use of Sc as an electrolyte in solid oxide fuel cells as a result of its improved oxygen ion conductivity creates the conditions for a sustainable Sc supply route. The economic value of REEs/t in Greek BR according to recent target prices shows that Sc corresponds to more than 90% of the total value, rendering its selective recovery attractive for industrial-scale exploitation [17–20]. Hydrometallurgical treatment is the most commonly used technique for REE (including Sc) recovery from BR [21,22]. Several conventional mineral [3,4,17–19,21–29] and organic acids [3,28], as well as alkaline solvents (Na2CO3)[30], have been studied. The use of organic acids shows very low recovery yields, while alkaline solvents hinder the following ion exchange and solvent extraction processes for Sc isolation and purification [14]. The addition of oxidative additives such as H2O2 with H2SO4 has also been investigated [31]. The presence of H2O2 has a positive effect on Sc recovery and selectivity mainly against Ti as well as in leachate handling, but the requirement of high temperatures (90 ◦C) increases operation cost at the expense of method viability. Other researchers [32–34] have tested ionic liquids in BR leaching and bioleaching, but results fall short due to low Sc recovery yield, the requirement for stronger conditions, and the high cost of ionic liquids. Under these drawbacks, the process viability on an industrial scale seems quite uncertain. Concerning bioleaching and the requirement of specific microorganisms for the degradation of BR, there is still much research work to be done. In cases where BR pretreatment is applied prior to leaching such as the pyro-metallurgical roasting or smelting, the need for strong conditions and high-energy consumption limits their economic performance [35,36]. The present work is a continuation of over 20 years of research in REE recovery from BR that resulted in the development of an innovative method for the selective separation and purification of REEs, especially Sc, including nitric acid leaching, ion exchange, selective extraction-backstripping, and HPLC (High Performance Liquid Chromatography) processes [3–5,17–19,23–25,37,38]. The whole procedure has been performed on a lab scale and partially scaled up to a pilot plant with comparable results [4,24,25]. In the present study, acidic leaching of BR using H2SO4 as a leaching agent was investigated for scandium selective recovery. The selection of sulfuric acid was based on environmental and economic reasons as being of critical importance in industrial-scale applications. Different parameters such as acid molarity, solid-to-liquid ratio (S/L), leaching time, final pH, leaching temperature, stirring mode, multistage leaching of the same BR with fresh acid, and recycling of the leachate solution to fresh BR were tested and evaluated, aiming at high Sc concentration and selective recovery, especially with respect to iron. The results reveal that the proposed process for Sc recovery is rather fast, with Metals 2018, 8, 915 3 of 16 scandium extraction being completed within less than 60 min, using low concentrations of sulfuric acid (<3 M). The effects of S/L, the adjustment of final pH, the multistage leaching, and especially the recycling were proven to be critical in achieving scandium recoveries up to 60% with low (<10%) iron impurities. Increasing the leaching temperature enhances iron dissolution resulting in reduced selectivity. The impact of the above parameters was evaluated with respect to Sc concentration in the leachate, since this is the decisive factor for the implementation of any subsequent purification process ([19], Section 3.1). The results were compared to those obtained in previous studies with other mineral acids on lab and pilot plant scales. Sulfuric acid leads to a competent Sc recovery and selectivity over iron, and it is more economically viable and environmentally friendly than the other investigated mineral acids. A flow diagram for the scale-up of the investigated leaching process on an industrial scale under ambient and environmental friendly conditions is developed taking into consideration the requirements of the leachate for the subsequent ion exchange process for scandium purification.

2. Materials and Methods

2.1. Sample Collection and Characterization All experiments were conducted using a BR (red mud) sample produced in December 2016, provided by AoG. Mineralogical analysis of BR was performed by XRD (X-ray Diffraction) (BRUKER, Karlsruhe, Germany) (as shown in Figure S1). The sample had a pH value of 11.3, and a moisture content of about 26%.

2.2. Leaching Experiments

The sulfuric acid (H2SO4) used was of industrial grade, 96–98% w/w (Kalogeropoulos S.A., Athens, Greece). Leaching experiments were performed adding an appropriate amount of dry BR to 200–2000 mL of 1–3 M H2SO4 in order to result in 2–30% S/L values. For the leaching time effect study, a sample was collected at 5, 15, 30, 45, and 60 min. ◦ For temperature experiments, 2 M H2SO4 was preheated at 50 and 80 C in conical flasks immersed in water baths on a magnetic stirrer hot plate. An appropriate amount of dry BR was added to the hot H2SO4, resulting in a 10% S/L, and stirred for 60 min. High temperature experiments were performed in an autoclave, where 2 M H2SO4 was added to dry BR, resulting in a 10% S/L in the Teflon vessel. The vessel was introduced in the autoclave bomb and heated at 160 and 230 ◦C (hot plate temperature) for 60 min under magnetic stirring. In the above cases, the final pH of the leachate was measured, and the leachates were filtered under vacuum with 0.45 µm cellulose nitrate membranes supplied by Whatman plc (GE Healthcare Life Sciences, Little Chalfont, UK) and stored for analysis. Final pH adjustment of the leachate was performed by the addition of concentrated sulfuric acid to dry BR in order to achieve a final pH of the leachate about 0.1–0.2, and the mixture was stirred up to 60 min. pH and temperature were monitored by an electronic pH-meter (multi 350i, by WTW, Weilheim, Germany). The leachates were filtered (0.45 µm under vacuum) and stored for analysis. For the multistage leaching experimental procedure, an appropriate amount of dry BR was added to 1, 2, and 3 M H2SO4 in a conical flask, resulting in an S/L of 5 and 10%. The mixtures were stirred using a magnetic stirrer for 60 min at ambient conditions. After leaching and centrifugation procedures, the precipitate was collected and re-treated with fresh H2SO4, while the supernatant liquid was removed, filtered (0.45 µm under vacuum), and stored for analysis. The process was repeated thrice. The pH value of the leachate was measured after each repetition. The leachate recycling experiments were conducted in a conical flask or a sealed spherical flask by adding an appropriate amount of dry BR in 1 and 2 M H2SO4, resulting in an S/L of 10%. pH value was adjusted close to 0 with an appropriate amount of concentrated H2SO4. After 60 min of magnetic or mechanical agitation, the leachate was collected, filtered (0.45 µm under vacuum), and reused on Metals 2018, 8, 915 4 of 16

fresh BR sample. pH value was adjusted close to 0 with an appropriate amount of concentrated H2SO4 prior to each step. The process was repeated thrice, including the ﬁrst leaching. An agitation effect was evaluated using the options of magnetic and mechanical stirring, the latter being optionally accompanied by bottom air feed and mild bubble-agitation.

2.3. Analytical Methods—Instrumentation All leaching experiments were conducted thrice, and the resulting ﬁltered leachate was analyzed in three replicates after appropriate dilution, for Sc (361.383 nm) and main elements such as Fe (259.939 nm), Ti (334.940 nm), and Si (251.611 nm), using ICP-OES Optima 7000DV supplied by Perkin Elmer (Waltham, MA, USA). A single standard Sc solution TraceCERT 92279 of 1000 mg/L supplied by Fluka Chemie GmbH (Buchs, Switzerland) and a multielement standard solution IV for ICP CertiPUR 1.11355 of 1000 mg/L for each element supplied by Merck KGaA (Darmstadt, Germany) were used for the preparation of working standard solutions. ICP-OES measurements were performed in axial mode for all elements after appropriate dilution. Matrix effects were taken into account by preparing standards of similar matrix. An internal standard method (Rh) was also used as a performance test. For the determination of all these elements, the calibration curve method was applied. More details about the instrument conditions, analysis parameters, and reference material can be found in [2–4]. In particular, for the determination of initial Sc in the BR batches, the reference material used was BX-N from ANRT (Association Nationale de la Recherche Technique, Paris, France). XRD analysis of BR before and after leaching was conducted using D8 ADVANCED TWIN–TWIN with detector LYNEXEYE (1D Mode) supplied by BRUKER (Karlsruhe, Germany). pH measurements were carried out by the multi 350i pH-meter, with a SenTix® 41 precision electrode. A HeiTec magnetic stirring plate with heating and a mechanical stirrer Hei-Torque 100, both supplied by Heidolph (Schwabach, Germany), were used. High temperature experiments were performed in an autoclave bomb 4744 of a 45 mL capacity by Parr Instrument Company (Moline, IL, USA).

3. Results Table1 presents Sc and total REE concentrations in different Greek BR lots from the AoG production, for a period of more than 20 years, revealing for Sc a small variation of about 11%. The initial Sc concentration of the BR batches used in this study was estimated at 98.0 ± 3.0 µg of Sc/g of BR after three replicates of lithium tetraborate fusion [4]. The XRD diagrams before and after leaching with their interpretation are attached as supplementary material in Figure S1.

Table 1. Concentration [g/t] of Sc and rare earths elements (REEs) in Greek bauxite residue (BR) batches (1993–2016).

Element BR 1993 BR 2001 BR 2007 BR 2012 BR 2014 BR 2016 Mean Sc 127.9 107.0 130.0 110.0 104.7 98.0 112.9 ± 13.0 Total REEs 986.1 868.0 1010.5 1040.3 729.7 856.0 915.1 ± 118.2

3.1. Selection of Leaching Agent Different mineral acids have been previously investigated as leaching agents on lab and pilot plant scales, and the obtained results are presented in Figure1. For the lab-scale experiments, acid molarity is 0.5 M concerning HNO3, HCl, and H2SO4, while that for aqua regia (AQREG) is conc. HCl: conc. HNO3, 3:1. In all cases, S/L is 2% and reaction time is 24 h. The pilot plan experiment refers to 0.5 M HNO3, an S/L of 2%, and a total reaction time of 60 min. The final pH of the leachate is in all cases close to zero. The uncertainty of the method is estimated ~7% based on the reproducibility of the results. As shown, aqua regia, followed by nitric acid, hydrochloric acid, and sulfuric acid, gives the highest recovery for Sc and the other REEs. However, the selectivity of the process shows Metals 2018, 8, 915 5 of 16 the opposite trend. Co-extraction of iron and other main elements (Si, Ti, and Al) of BR hinder the subsequent processes of Sc isolation and purification. Sulfuric acid and nitric acid show the best Metals 2018, 8, x FOR PEER REVIEW 5 of 16 selectivity concerning iron recovery. Although the use of nitric acid results in higher Sc recovery, sulfuric acidprocess is preferred shows the insteadopposite duetrend. to Co-extraction its mild environmental of iron and other impact, main elements which (Si, isin Ti, accordance and Al) of with the EU environmentalBR hinder the regulations subsequent for processes landfill of disposals.Sc isolation Furthermore,and purification. theSulfuric market acid priceand nitric of sulfuricacid acid is show the best selectivity concerning iron recovery. Although the use of nitric acid results in higher almost halfSc thatrecovery, of nitric sulfuric acid, acid resulting is preferred in lowerinstead operatingdue to its mild costs environmental and enhancing impact, the which economic is in viability of the wholeaccordance method with [39 the–45 EU]. Additionally,environmental regulations the selected for landfill agent shoulddisposals. be Furthermore, at acceptable the market safety levels for industrialprice handling of sulfuric [19]. acid is almost half that of nitric acid, resulting in lower operating costs and The leachabilityenhancing the ofeconomic other elementsviability ofof the techno-economical whole method [39–45]. interest, Additionally, such the as Ga,selected Zr, andagent Nb, is also should be at acceptable safety levels for industrial handling [19]. more favoredThe by leachability sulfuric acid of other than elements by nitric of techno-econo acid [18]. mical interest, such as Ga, Zr, and Nb, is also Optimizationmore favored of by the sulfuric leaching acid than process by nitric is acid focused [18]. on Sc concentration rather than recovery %, by taking intoOptimization consideration of the the leaching requirements process is offocused the consecutiveon Sc concentration separation rather than and recovery purification %, by processes. Moreover,taking the leaching into consideration process the economics requirements show of the strong consecutive dependence separation between and purification the process processes. unit cost and Moreover, the leaching process economics show strong dependence between the process unit cost Sc concentrationand Sc concentration [19]. [19].

Figure 1. RecoveryFigure 1. Recovery (%) of (%) Sc of and Sc and other other REEs REEs byby leaching with with different different acids (ambient acids (ambient conditions conditions and and a solid-to-liquida solid-to-liquid ratio (S/L) ratio (S/L) of 2%). of 2%). *after * after two two successive leaching leaching steps. steps.

Scandium recoveries obtained by lab scale and pilot plant experiments carried out with HNO3 Scandiumare quite recoveries similar, although obtained a large-scale by lab process scale mi andght pilotrequire plant two leaching experiments steps due carried to the different out with HNO3 are quite similar,dispersion although conditions a[17,18,24]. large-scale process might require two leaching steps due to the different dispersion conditions [17,18,24]. 3.2. Effect of Leaching Time 3.2. Effect of LeachingLeaching Timeexperiments with sulfuric acid were conducted for a prolonged period of 24 h. The leaching reaction found to be fast and practically completed within a 60 min period. Sc concentration Leachingshows experimentsonly a slight increment with sulfuric above 30 acid min. were Almost conducted 90% of the for final a prolongedSc concentration period can ofbe 24 h. The leaching reactionachieved foundwithin to10 bemin fast under and higher practically acid molarities completed (3 M). within A leaching a 60 mintime period.of 60 min Sc concentrationwas shows onlyconsidered a slight for increment all experiments above as 30 an min. adequate Almost amou 90%nt of oftime the for final the completion Sc concentration of the reaction, can be achieved justified by a relevant kinetic investigation for a sufficiently wide range of leaching conditions within 10presented min under in [19]. higher The entire acid set molarities of the relevant (3 M).results A is leaching presented time in Figure of60 2. min was considered for all experiments as an adequate amount of time for the completion of the reaction, justified by a relevant kinetic investigation for a sufficiently wide range of leaching conditions presented in [19]. The entire set of the relevant results is presented in Figure2. Metals 2018, 8, 915 6 of 16 Metals 2018, 8, x FOR PEER REVIEW 6 of 16 Metals 2018, 8, x FOR PEER REVIEW 6 of 16

FigureFigure 2. Leaching 2. Leaching time time effect effect on on Sc Sc recovery recovery (%)(%) for diffe differentrent acid acid molarities molarities and and an anS/L S/Lof 5% of (1:20). 5% (1:20). Figure 2. Leaching time effect on Sc recovery (%) for different acid molarities and an S/L of 5% (1:20). 3.3. Effect3.3. Effect of Acid of Acid Molarity Molarity 3.3. Effect of Acid Molarity The effectThe effect of molarity of molarity depends depends on S/L. on S/L. Increasing Increasing molarity molarity raises raises Scconcentration Sc concentration inthe in leachate,the as presentedleachate,The inaseffect Figurepresented of 3molaritya. Standardin Figure depends 3a. deviation Standard on S/L. of de theIncreaviation methodsing of molaritythe has method been raises calculatedhas Scbeen concentration calculated at ~7%. Theat in ~7%. effectthe is signiﬁcantTheleachate, effect for as S/Lis presentedsignificant values higherin for Figure S/L than values 3a. 20%,Standard higher and thde theanviation same20%, ofand results the the method aresame also results has observed been are calculated also for observed iron at (Figure ~7%. for 3b). iron (Figure 3b). Acid molarities higher than 3 M were also tested and, despite the enhanced Sc AcidThe molarities effect is higher significant than for 3 MS/L were values also higher tested th and,an 20%, despite and the the same enhanced results Sc are concentration, also observed were for not concentration,iron (Figure 3b). were Acid not molarities selected higherdue to thaneven 3 hiMgher were iron also dissolution tested and, and despite reduction the enhanced of process Sc selected due to even higher iron dissolution and reduction of process selectivity. Lower molarities are selectivity.concentration, Lower were molarities not selected are moredue toeconomically even higher viable iron dissolutiondue to lower and acid reduction consumption of process and more economically viable due to lower acid consumption and environmentally friendly due to reduced environmentallyselectivity. Lower friendly molarities due areto reducedmore economically acidity, ensuring viable the due method’s to lower selectivity. acid consumption On the other and acidity,hand,environmentally ensuring silica gel the formation friendly method’s occursdue selectivity. to whenreduced low Onacidity, acid the molarities otherensuring hand, are the combined silicamethod’s gel with formationselectivity. high S/L occursOn values. the whenother This low acid molaritiesishand, an important silica aregel combined formationissue for hydrometallurgicaloccurs with highwhen S/L low values.acid processes molarities This under is are an importantcombinedacidic conditions, with issue high for especially S/L hydrometallurgical values. for Thisraw processesmaterialsis an important under such acidic as issue BR. conditions, Gelfor hydrometallurgicalformation especially is caused forprocesses due raw to the materials under presence acidic such of conditions,silica as BR. in the Gel especially solution formation as for silicic israw caused due toacidmaterials the (H presence4SiO such4) at as ofpH BR. silica <7. Gel The in formation the monomers solution is caused connect as silicic due to acid toform the (H polysilicicpresence4SiO4) at of acid, pH silica <7. and in The thecolloids monomerssolution result as insilicic connect gel to formformation,acid polysilicic (H4SiO as4 acid,) theyat pH andentrap <7. colloids The liquid monomers resultinside intheir connect gel structure. formation, to form The polysilicic as use they of acid entrap acid, molarities and liquid colloids >3 inside M confrontresult their in structure. thegel The useproblemformation, of acid even as molarities theyat high entrap S/L >3 liquidvalues M confront inside [31]. As their the shown problemstructure. in oureven The experiments, use at highof acid S/L a molarities silicon values content [>331 M]. Asaroundconfront shown 1000 the in our mg/L is considered to be a safe threshold such that rapid gel formation is avoided. However, the experiments,problem even a silicon at high content S/L values around [31]. 1000 As shown mg/L isin consideredour experiments, to be a a silicon safe threshold content around such that1000 rapid method’smg/L is considered selectivity toand be environmentala safe threshold benefits such that on rapidindustrial gel formationscale-up decline is avoided. by increasing However, acid the gel formation is avoided. However, the method’s selectivity and environmental beneﬁts on industrial molarity.method’s Anselectivity acid molarity and environmental of 2 M is selected benefits as the on bestindustrial option scale-upto compromise decline theseby increasing contradictory acid scale-up decline by increasing acid molarity. An acid molarity of 2 M is selected as the best option to effects.molarity. An acid molarity of 2 M is selected as the best option to compromise these contradictory compromiseeffects. these contradictory effects.

(a) (a) Figure 3. Cont. Metals 2018, 8, 915 7 of 16 Metals 2018, 8, x FOR PEER REVIEW 7 of 16

Metals 2018, 8, x FOR PEER REVIEW 7 of 16

(b)

FigureFigure 3. Acid 3. Acid molarity molarity effect effect on on( a(a)) Sc;Sc; ( b)) Fe Fe concentration concentration(b) for fordifferent different S/L values S/L values (2–30%) (2–30%) and 60 and 60 minmin leaching leaching period. period. Figure 3. Acid molarity effect on (a) Sc; (b) Fe concentration for different S/L values (2–30%) and 60 3.4. Effect3.4. Effect of S/Lmin of S/Lleaching period.

Sc recovery3.4.Sc Effectrecovery of is S/L decreased is decreased by by S/L S/L increment, increment, especiallyespecially with with low low acid acid molarities molarities and and high high S/L S/L values, as the amount of acid is not adequate to neutralize BR, and pH is not adjusted (Figure 4). The values, as theSc amountrecovery is of decreased acid is notby S/L adequate increment, to espe neutralizecially with BR, low and acid pH molarities is not and adjusted high S/L (Figure 4). The essentialessentialvalues, outcome outcome as the amount nevertheless nevertheless of acid isconcerns not concerns adequate the Sc to the concenneutralize Sc concentrationtration BR, andin the pH leachate. is in not the adjusted leachate.As presented (Figure As 4). in presented The Figure in Figure3a,3a, thereessential there is isan outcome an almost almost nevertheless linear linear correlation correlation concerns between the between Sc concen S/L and S/Ltration Sc and concentration, in the Sc concentration,leachate. for As allpresented acid for molarities all in acid Figure molarities and ratios up to 20%. For ratios higher than 20%, acid molarity of at least 3 M is required to obtain and ratios3a, upthere to is 20%. an almost For ratioslinear correlation higher than between 20%, S/L acid and molaritySc concentration, of at leastfor all 3acid M ismolarities required and to obtain sufficientlyratios up high to 20%. Sc recoveryFor ratios inhigher the leachatethan 20%, and acid avoid molarity gel offormation. at least 3 MThe is incapabilityrequired to obtain of lower sufficiently high Sc recovery in the leachate and avoid gel formation. The incapability of lower H SO H2SOsufficiently4 molarities high to Scneutralize recovery BRin thealkalinity leachate at and a high avoid S/L gel results formation. in high The pH incapability values, which of lower do not 2 4 molaritiesfavorH 2SOleaching to4 neutralizemolarities efficiency to BR neutralize alkalinity(see Section BR alkalinity at 3.5). a high The at a S/Lhigh higher results S/L values results in of highin S/L high pHinduce pH values, values, a certain which difficulty do do not not in favor leachinghandlingfavor efficiency leachingof the (see leachateefficiency Section due(see 3.5 ).Sectionto The silica higher 3.5). gel The formation. values higher of values S/LSince induceof the S/L keyinduce a certain point a certain difficultyis the difficulty leachate’s in handling in Sc of the leachateconcentrationhandling due of to rather the silica leachate than gel formation.the due Sc to recovery silica Since gel (%), theformation. keyhigher point SinceS/L is valuesthe the key leachate’s at point appropriate is Scthe concentrationleachate’s molarities Sc are rather concentration rather than the Sc recovery (%), higher S/L values at appropriate molarities are thanpreferred. the Sc recovery The use (%), of higher2 M H2 S/LSO4 combined values at appropriatewith S/L 20% molarities seems to be are a preferred.technically Theacceptable use of 2 M preferred. The use of 2 M H2SO4 combined with S/L 20% seems to be a technically acceptable compromise. H2SO4 combinedcompromise. with S/L 20% seems to be a technically acceptable compromise.

FigureFigure 4. S/L 4. effect S/L effect on Sc on recovery Sc recovery (%) (%) at atdifferent different molarities molarities and and 60 min 60 minleaching leaching period. period. Figure 4. S/L effect on Sc recovery (%) at different molarities and 60 min leaching period. 3.5. Effect of pH Adjustment

The final pH value of the leachate is a definite indicator of leaching process performance. A pH value close to 0.1 is related to a significant improvement in Sc recovery, enhanced up to 25% in Metals 2018, 8, x FOR PEER REVIEW 8 of 16

3.5. Effect of pH Adjustment The final pH value of the leachate is a definite indicator of leaching process performance. A pH value close to 0.1 is related to a significant improvement in Sc recovery, enhanced up to 25% in Metals 2018 8 comparison, , 915to that at higher pH, especially in the case of high S/L values. The set of the pertinent8 of 16 results is presented in Table 2. The optimum pH range has been found to be from 0.0 to 0.2. Low pH valuescomparison are considered to that at mandatory higher pH, for especially the increment in the caseof Sc of concentration high S/L values. in the The leachate. set of Furthermore, the pertinent pHresults adjustment is presented with inthe Table addition2. The of optimum concentrated pH range sulfuric has acid been during found the to beleaching from 0.0 reaction to 0.2. improves Low pH Scvalues recovery are considered by up to 6%, mandatory due to the for temperature the increment synergy of Sc concentrationeffect. in the leachate. Furthermore, pH adjustment with the addition of concentrated sulfuric acid during the leaching reaction improves Sc recoveryTable by up2. pH to values 6%, due of tothe the leachate temperature for differe synergynt S/L values effect. and acid molarities (t = 60 min). S/L M Final pH Recovery % 30%Table 2. pH values of the1 leachate for different S/L values2.00 and acid molarities (t 21.2= 60 min). 30% S/L2 M 1.50 Final pH Recovery 28.0 % 30% 3 0.32 40.0 30% 1 2.00 21.2 20% 30%1 2 0.65 1.50 28.0 35.9 20% 30%2 3 0.28 0.32 40.0 42.5 20% 20%3 1 0.20 0.65 35.9 43.2 20% 20% pH adjusted * 2 0.082 0.28 42.5 49.2 20% 3 0.20 43.2 10% 20%1 pH adjusted * 0.32 0.082 49.2 41.8 10% 10%2 1 0.16 0.32 41.8 44.7 10% 10%3 2 0.07 0.16 44.7 46.6 10% 10% pH adjusted * 3 0.11 0.07 46.6 52.0 10% pH adjusted * 0.11 52.0 5% 1 0.09 44.7 5% 1 0.09 44.7 5% 5%2 2 0.01 0.01 49.0 49.0 5% 5%3 3 0.01 0.01 46.7 46.7

* pH pH adjustment adjustment at low low values values about about 0.1 0.1 by by the the addition addition of of concentrated concentrated 96–98% w/ww/w H H22SOSO4 duringduring leaching reaction. reaction.

3.6. Temperature Effect ◦ ◦ Increased reaction temperature from 25 to 80 °CC at ambient conditions, 160 and 230 °CC (hotplate temperature) underunder autoclave autoclave conditions conditions (see Figure(see Figu5a,b),re improves5a,b), improves only marginally only marginally the Sc leachability. the Sc leachability.On the other On hand, the iron other dissolution hand, iron is dissolution noticeably enhanced is noticeably reaching enhanced a recovery reaching of even a recovery 22% and of a even ﬁnal 22%concentration and a final of concentration almost 7 g/L of that almost diminishes 7 g/L that method diminishes selectivity. method selectivity.

(a) (b)

Figure 5. EffectEffect of of temperature temperature on on Sc Sc and and Fe Fe recovery recovery ( aa)) under under normal normal pressure pressure and and ( b) under autoclave conditions (S/L 10%, 10%, 2 2 M M H H22SOSO4,4 t, t= =60 60 min). min).

Additional leaching experiments under extreme conditions were also performed in order to assess the combinatorial effect of high molarities (>3 M) and high temperature. As expected, the results showed an essential increment both for Sc recovery (up to 70%) and concentration (about 20 mg/L), Metals 2018 8 with avoidance, , 915 of gel formation, but there was also an almost complete dissolution of iron at9 ofthe 16 expense of selectivity. Such high Fe concentration in the leachate impedes any following ion exchangeor solvent or extraction solvent extraction processes processes for Sc separation for Sc se andparation purification. and purification. The reason The lies reason in the lies chemical in the chemicalsimilarity similarity between scandium between andscandium iron due and to theiron close due associationto the close of association scandium with of scandium the microstructure with the microstructureof nano-perovskite of nano-perovskite in the iron oxide in phases. the iron Therefore, oxide phases. it is not Therefore, possible toit recoveris not possible 100% of to scandium recover 100%under of ambient/mild scandium under conditions ambient/mild [28]. Furthermore, conditions both[28]. elementsFurthermore, present both similar elements hydrolysis present behavior, similar hydrolysisas they are behavior, first row transition as they are metals first row [46]. transition metals [46]. All above-mentioned parameters (S/L, acid acid molarity, molarity, temperature, temperature, and and time) were also tested with respectrespect toto thethe concentrationconcentration of of other other main main elements. elements. Al Al and and Fe Fe concentrations concentrations were were increased increased by bytemperature, temperature, S/L, S/L, and and molarity, molarity, taking taking values values from from 3000 mg/L3000 mg/L to 13,000 to 13,000 mg/L mg/L and from and 1040from mg/L 1040 mg/Lto 32,500 to 32,500 mg/L, mg/L, respectively; respectively; Ti was Ti was increased increased with with S/L, S/L, rising rising from from 1430 1430 mg/L mg/L (for (for 20% 20% S/L) S/L) to 6550 mg/L (for (for 40% 40% S/L). S/L). Aluminum Aluminum was was not not regarded regarded as as impure impure since since Sc–Al Sc–Al alloys alloys are materials of technological interest.interest. InIn contrast,contrast, the the silicon silicon concentration concentration was was suppressed suppressed under under almost almost 100 100 mg/L mg/L at athigher higher molarities molarities (>3 (>3 M), M), solving solving the the silica silica gel gel formation formation problem. problem. Ambient conditions are generally more preferable, considering both the Sc selectivity and the energy consumption cost, ensuring an economically viable process design. Fu Furthermore,rthermore, Fe Fe and Ti are of major concern in this study due to the specificspecific requirements of the consecutiveconsecutive isolation and purificationpurification processes. The The selectivity selectivity criteria criteria focus focus on on Fe Fe content content since since iron iron oxide oxide constitutes constitutes 50% 50% of BR.of BR.

3.7. Effect of Stirring Mode Three kinds of stirringstirring werewere tested:tested: magnetic stirring, mechanical agitation, and mechanical agitation with air bubbles, with stirring speeds of about 550–650550–650 rpmrpm (see(see FigureFigure6 ).6). AA standardstandard deviation ofof ~7% ~7% was was estimated. estimated. As seen,As seen, the stirringthe stir modering hasmode no has effect no on effect Sc concentration on Sc concentration regardless regardlessof the S/L valuesof the andS/L acid values molarities and acid tested. molariti Thesees ﬁndings tested. applyThese to findings the lab-scale apply experiments to the lab-scale of this experimentsstudy, but in largerof this scalestudy, setups but in more larger vigorous scale stirringsetups more conditions vigorous might stirring be required conditions due tomight diverse be requireddispersion due mechanisms. to diverse dispersion mechanisms.

Figure 6. Effect of stirring mode (S/L 10%, 10%, 11 M M H H2SO2SO4, 4t, =t =60 60 min). min). 1. 1.Magnetic Magnetic stirring stirring at 650 at 650 rpm; rpm; 2. mechanical2. mechanical agitation agitation at at550 550 rpm; rpm; 3. 3.mechanical mechanical agit agitationation at at 650 650 rpm; rpm; 4, 4, mechanical mechanical agitation agitation with with air at 550 rpm. 3.8. Effect of Multistage Leaching Process 3.8. Effect of Multistage Leaching Process The multistage process refers to successive leaching of the same BR sample with corresponding The multistage process refers to successive leaching of the same BR sample with corresponding multiple fresh sulfuric acid batches. The entire process performed in triplicates under ambient multiple fresh sulfuric acid batches. The entire process performed in triplicates under ambient conditions for 60 min. Different acid molarities and S/L values were studied, and the results are presented in Figure7a,b. Multistage leaching has a positive effect on Sc recovery (Figure7b) but causes a great reduction on Sc concentration (Figure7a), as expected due to the obvious increment of the ﬁnal leachate volume after the multiple adding of fresh sulfuric acid batches. The iron extraction remains Metals 2018, 8, x FOR PEER REVIEW 10 of 16 conditions for 60 min. Different acid molarities and S/L values were studied, and the results are presentedMetals 2018, 8in, 915 Figure 7a,b. Multistage leaching has a positive effect on Sc recovery (Figure 7b)10 ofbut 16 causes a great reduction on Sc concentration (Figure 7a), as expected due to the obvious increment of the final leachate volume after the multiple adding of fresh sulfuric acid batches. The iron extraction remainsrelatively relatively low even low after even the third after stage the third reaching stage a totalreaching recovery a total up torecovery 10%. Despite up to the10%. remarkable Despite the Sc remarkablerecovery increase, Sc recovery the multistage increase, process the multistage is not advantageous process is for not large-scale advantageous applications, for large-scale due to Sc applications,concentration due reduction to Sc concentration and the required reductio largen volumesand the required of acid feed. large volumes of acid feed.

(a) (b)

Figure 7. Multistage effecteffect on:on: ( a(a)) Sc Sc concentration; concentration; (b ()b Sc) Sc recovery recovery (%) (%) for for different different acid acid molarities molarities and andS/L S/L values values (t = ( 60t = min,60 min, ambient ambient conditions). conditions).

3.9. Effect of Leachate Recycling 3.9. Effect of Leachate Recycling The recycling process refers to multiple reusing reusing (reflux) (reflux) of of the the leachate leachate on on fresh fresh BR BR samples. samples. The target is threefold: (a) the leachate enrichment in Sc with the lower possiblepossible impurities in the main elements (mainly Fe Fe and and Ti), Ti), (b) the enhancem enhancementent of of the the leaching leaching proce process’ss’ economic economic viability, viability, especially on an industrial scale, and (c) the simplification simplification of Sc further purification purification after leaching leaching by by ion exchange/extraction processes processes [15]. [15]. The The entire entire process process was was performed performed at at three sequential cycles using 1 MM andand 22 M M acid acid molarities molarities with with additional additional pH pH adjustment adjustment close close to zero,to zero, before before each each cycle. cycle. The µ Theobtained obtained results results for Scfor (in Sc (ing/L), μg/L), Ti, Ti, and and Fe Fe (in (in mg/L) mg/L) for for both both molarities molarities are are givengiven inin FigureFigure8 8.. AsAs clearly illustrated, reusing of leachate solution thri thricece with combined pH adjustment results in almost 2 tripled Sc concentration (~12,000 µg/L). There is a linear correlation, with R = 1.0000 for 1 M H2SO4 tripled Sc concentration (~12,000 μg/L). There is a linear correlation, with R2 = 1.0000 for 1 M H2SO4 2 and 0.9997 for 2 M H2SO4. Fe and Ti also present a linear correlation, with R = 0.8712 (1 M) − 0.9999 and 0.9997 for 2 M H2SO4. Fe and Ti also present a linear correlation, with R2 = 0.8712 (1 M) − 0.9999 − (2 M) and 0.9892 (1 M) − 0.99870.9987 (2 (2 M), M), respectively. respectively. However, However, the the final final concentration remains at low acceptable levels for Fe and Ti. The maximum measured Fe concentration (3 cycles of 2 M H2SO4) is acceptable levels for Fe and Ti. The maximum measured Fe concentration (3 cycles of 2 M H2SO4) is 3000 mg/L, corresponding corresponding to to ~10% ~10% recovery. recovery. Recycling Recycling of of 1.5 1.5 times, times, which which is possible in continuous operation on an industrial scale satisfies satisfies the requiredrequired leachate specificationsspecifications for the subsequent ion exchange process, soso ScSc >> 66 mg/L,mg/L, Fe << 30,00030000 mg/L, mg/L, and and Ti Ti < < 1700 1700 mg/L mg/L [15]. [15]. In In addition, addition, the the leachate leachate recycling achieves a lower acid and water consumption, promoting the viability of a large-scale application while rendering the entire process more environmentallyenvironmentally friendly.friendly.

Metals 2018, 8, x FOR PEER REVIEW 11 of 16

Metals 2018, 8, 915 11 of 16 Metals 2018, 8, x FOR PEER REVIEW 11 of 16

Figure 8. Leachate recycling with pH adjustment (S/L 10%, 1 and 2 M H2SO4,2 pH 4≈ 0, t = 60 min, Figure 8.8. Leachate recycling withwith pHpH adjustmentadjustment (S/L(S/L 10%, 1 andand 22 MM HH2SOSO4, pH ≈ 0,0, tt == 60 60 min, ambientambient conditions). conditions).

3.10. Statistical Evaluation 3.10. Statistical Evaluation Method repeatability was evaluated by comparing the results from various leaching Method repeatability was evaluated by comparing the results from various leaching experiments, Methodexperiments, repeatability keeping the was same evaluated research team by and comparing analytical technique,the results as well from as the various same raw leaching experiments,keepingmaterial the same keeping and research leaching the teamconditions.same and research analytical The variation team technique, ofand leaching analytical as efficiency well astechnique, theis presented same rawas inwell materialFigure as 9,the where and same leaching raw materialconditions.it andis shown The leaching variationthat the conditions. average of leaching is 42.3% The variationefﬁciencywith a standard of is leaching presented deviation efficiency ±2.8%. in Figure is9 presented, where it isin shownFigure 9, that where the itaverage is shown is 42.3% that the with average a standard is 42.3% deviation with a ±standard2.8%. deviation ±2.8%.

Figure 9. Repeatability of leaching process (S/L 5%, 1 M H2SO4, t = 60 min).

One-way ANOVA statistical analysis was applied to the experimental data in order to evaluate the contribution of each investigated parameter on Sc recovery and concentration in the leachate solution. The three independent parameters considered were time, S/L, and molarity, while the two FigureFigure 9.9. RepeatabilityRepeatability ofof leachingleaching processprocess(S/L (S/L 5%, 1 M H2SO4,, tt == 60 60 min). min). dependent variables were recovery % and concentration. S/L has been confirmed to contribute mostly to Sc concentration, while leaching time is more critical for Sc recovery [18]. One-way ANOVA statistical analysis was applied to the experimental data in order to evaluate the contribution of each investigated parameter on Sc recovery and concentration in the leachateleachate solution. The three independent parameters considered were time, S/L, and molarity, while the two solution. The three independent parameters considered were time, S/L, and molarity, while the two dependent variablesvariables were were recovery recovery % and% and concentration. concentration. S/L hasS/L beenhas been conﬁrmed confirmed to contribute to contribute mostly mostlyto Sc concentration, to Sc concentration, while leaching while leaching time is moretime is critical more forcritical Sc recovery for Sc recovery [18]. [18]. Figure 10 presents the effect of two parameters (S/L and time) on Sc concentration in the leachate solution, showing that maximum Sc concentration is achieved with a high S/L and a long leaching duration.

Stefania Marin ([email protected]), Mădălina Georgiana Albu Kaya ([email protected]), Mihaela Violeta Ghica ([email protected]), Cristina Dinu-Pîrvu ([email protected]), Lăcrămioara Popa ([email protected]), Denisa Ioana Udeanu ([email protected]), Geanina Mihai ([email protected]), Marius Enachescu ([email protected]) Metals 2018, 8, 915 12 of 16

Figure 10. Two-factor (S/L and leaching time) interaction; 2D projection on the extracted Sc concentration in the leachate.

4. Assessment and Conclusions The core objective of this study is to determine the optimum leaching conditions for the scale-up of the proposed process to a demonstration plant to be established in the premises of Mytilineos S.A. (the main producer of BR in Greece), taking into consideration the feed requirements of the leachate to the subsequent ion exchange process for scandium purification. This investigation expands the existing research further by introducing and studying additional parameters, such as multistage leaching, leachate recycling, pH adjustment of the leachate, and different stirring modes. Based on the results obtained, the development of a leaching process flow diagram (see Figure 11) was possible in order to set up a detailed blueprint for applying this method in industry. BR leaching using H2SO4 at ambient conditions for Sc recovery is a rapid process, practically integrated within 30 min. Final pH of the solution is critical, requiring a pH adjustment in the range of 0.0–0.2. The proposed acid molarity of 2 M is a result of a technical compromise between the higher values needed in order to avoid Si-caused gel formation (Si < 1000 mg/L) and the lower values that ensure Sc selectivity, mainly with respect to Fe. The1 S/L and leachate solution reuse are crucial for Sc concentration. A minimum of 10% slurry concentration (S/L) and a maximum of two cycles of leachate reusing are suggested, resulting under ambient conditions in an Sc concentration of about 10 mg/L and Fe 1700 mg/L, corresponding to a recovery of nearly 50% and 3%, respectively. Elevated temperature is not recommended due to high Fe dissolution at the expense of Sc selectivity. A multistage process reduces Sc concentration despite the rise of its recovery. The criteria for selecting the parameters investigated rely on environmental and process economics aspects, meaning residues of low acidity and low operating and energy cost, as well as compliance with the requirements of the consecutive Sc separation and purification processes [19]. Any further improvement depends mostly on the leachate specifications range concerning the respective separation processes after leaching (i.e., ion exchange and solvent extraction), rather than the yields of the leaching process itself. Figure 11 presents the flow diagram of the industrial scale-up of the developed leaching process followed by the purification process, based on the conceptual design for continuous operation. The following notation is used in the above diagram: Metals 2018, 8, 915 13 of 16

• L = Leaching reactor. Indicative type: CF-CSTR (Continuous Flow-Continuous Stirred Tank Reactor), with a heating/cooling mantle to be an option. • P = Unit for Purification of leachate from solids. Indicative configuration: Intermediate Tank, Filter Press or Flocculator/Filters, and leachate Tank. • M = Mixing unit. Indicative configuration: Continuous flow mixing device, with a heat dissipation system. • BR = Bauxite Residue streams with moisture content. Subscripts: i = input (alkaline solids stream), o = output (acidic solids stream). • A = Acid streams (sulfuric acid solutions). Subscripts: f = leaching feed, p = product (leachate), n = new (fresh solution), r = recycling leachate, d= dense acid (93–98% w/w). (Reflux ratio) = (Number of cycles in batch processing) − 1. •MetalsW 2018 =, Water 8, x FOR stream; PEER REVIEW mixture of raw water with used water from Filter Press cleaning. 13 of 16

Figure 11. Conceptual design for a continuouscontinuous BR leaching process.process. The leaching unit solid wastes (partially neutralized pH ≥ 2) could be used in a pyrometallurgical The following notation is used in the above diagram: unit for Fe recovery. Moreover the acidic Sc leachate (pH ≈ 0) could be continuously forward to an ion exchanged• L = Leaching process forreactor. further Indicative purification type: and theCF-CSTR neutralized (Continuous eluent couldFlow-Continuous be finally recycled Stirred toTank the leachingReactor unit), (L,with Figure a heating/cooling 11) to cover its mantle raw water to be demand.an option. • P = Unit for Purification of leachate from solids. Indicative configuration: Intermediate Tank, SupplementaryFilter Press Materials: or Flocculator/Filters,The following are and available leachate online Tank. at http://www.mdpi.com/2075-4701/8/11/915/s1 , Figure S1: Mineralogical analysis of bauxite residue before and after leaching process with sulfuric acid. • M = Mixing unit. Indicative configuration: Continuous flow mixing device, with a heat Author Contributions: This work was performed by the members of Laboratory of Inorganic and Analytical Chemistrydissipation of Department system. of Chemical Sciences (M.O.-P.; L.-A.T., T.L., K.-M.O., C.S., O.S., and F.T.) in collaboration with• theBR Department= Bauxite Residue of Process stream Analysiss with and moisture Plant Design content. (K.H. and P.G.) of the NTUA School of Chemical Engineering.Subscripts: L.-A.T., i = input T.L., (alkaline C.S., and solids O.S. performed stream), o the = output experiments; (acidic T.L. solids and stream). L.-A.T. integrated ICP-OES measurements;• L.-A.T. and T.L. evaluated the data and wrote the draft paper; M.O.-P supervised the whole work and correctedA = Acid the streams draft paper; (sulfuric K.S. designedacid solutions). the proposed continuous process for the future industrial scale-up; K.-M.O.,Subscripts: P.G., and F.T.f = reviewedleaching thefeed, paper. p = product (leachate), n = new (fresh solution), r = recycling Funding:leachate,This research d= dense was acid funded (93–98% by the w/w). European (Reflux Community’s ratio) = (Number Horizon 2020of cycles Program in batch (H2020/2014-2020), processing) − under1. Grant Agreement number 730105. • Acknowledgments: W= Water stream;The research mixture leading of raw to water these resultswith used wasperformed water from within Filter the Press SCALE cleaning. project, and funding was received from the European Community’s Horizon 2020 Program (H2020/2014-2020), under Grant Agreement No. 730105.The leaching The companies unit II-VIsolid Inc. wastes (Saxonburg, (partially PA, USA)neutralized and MYTILINEOS pH ≥ 2) S.A., could formerly be Aluminumused in ofa Greecepyrometallurgical S.A. (AoG), are unit also gratefullyfor Fe recovery. acknowledged Moreover for the fruitfulthe acidic collaboration Sc leachate and cooperation (pH ≈ 0) in thecould context be ofcontinuously SCALE and inforward previous to joint an projects. ion exchanged process for further purification and the neutralized Conflictseluent could of Interest: be finallyThe recycled authors declareto the leaching no conflict unit of interest.(L, Figure The 11) results to cover of this its work raw water were derived demand. in the context of the SCALE project (Grant Agreement No. 730105), and the publication is in accordance with its terms.Supplementary The funding Materials: sponsors (EU/EASME)The following had are no roleavailable in the designonline ofat the www.mdpi.com/xxx/s1, study, in the collection, analyses,Figure S1: or interpretation of data, or in the writing of the manuscript. Mineralogical analysis of bauxite residue before and after leaching process with sulfuric acid. ReferencesAuthor Contributions: This work was performed by the members of Laboratory of Inorganic and Analytical Chemistry of Department of Chemical Sciences (M.O.-P.; L.-A.T., T.L., K.-M.O., C.S., O.S., and F.T.) in 1.collaborationPower,G.; with Grafe, the Department M.; Klauber, of C.Process Bauxite Analysis residue and issues: Plant I.Design current (K.H. management, and P.G.) of disposal the NTUA and School storage of Chemicalpractices. Engineering.Hydrometallurgy L.-A.T.,2011 T.L.,, 108 C.S.,, 33–45. and O.S. [CrossRef performed] the experiments; T.L. and L.-A.T. integrated 2.ICP-OESOchsenkühn-Petropoulou, measurements; L.-A.T. M.;and Lyberopulu, T.L. evaluated T.; Parissakis,the data and G. wrote Direct the determination draft paper; ofM.O.-P lanthanides supervised yttrium the wholeand work scandium and corrected in bauxites the anddraft red paper; mud K.S. from design aluminaed production.the proposedAnal. continuous Chim.Acta process1994 ,for296 the, 305–313. future industrial[CrossRef scale-up;] K.-M.O., P.G., and F.T. reviewed the paper.

Funding: This research was funded by the European Community's Horizon 2020 Program (H2020/2014-2020), under Grant Agreement number 730105.

Acknowledgments: The research leading to these results was performed within the SCALE project, and funding was received from the European Community's Horizon 2020 Program (H2020/2014-2020), under Grant Agreement no. 730105. The companies II-VI Inc. (Saxonburg, PA, USA) and MYTILINEOS S.A., formerly Aluminum of Greece S.A. (AoG), are also gratefully acknowledged for the fruitful collaboration and cooperation in the context of SCALE and in previous joint projects.

Conflicts of Interest: The authors declare no conflict of interest. The results of this work were derived in the context of the SCALE project (Grant Agreement No. 730105), and the publication is in accordance with its terms. The funding sponsors (EU/EASME) had no role in the design of the study, in the collection, analyses, or interpretation of data, or in the writing of the manuscript.

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