Scandium and Titanium Recovery from Bauxite Residue by Direct Leaching with a Brønsted Acidic Ionic Liquid

Article Scandium and Titanium Recovery from Bauxite Residue by Direct Leaching with a Brønsted Acidic Ionic Liquid

Chiara Bonomi 1,* , Alexandra Alexandri 1, Johannes Vind 1,2 , Angeliki Panagiotopoulou 3,4, Petros Tsakiridis 1 and Dimitrios Panias 1,*

1 School of Mining and Metallurgical Engineering, National Technical University of Athens, Iroon Polytechniou 9, Zografou Campus, 15780 Athens, Greece; aalexandri@metal.ntua.gr (A.A.); [email protected] (J.V.); [email protected] (P.T.) 2 Department of Continuous Improvement and System Management, Aluminium of Greece Plant, Metallurgy Business Unit, Mytilineos S.A., Agios Nikolaos, 32003 Viotia, Greece 3 Institute of Biosciences & Applications, National Centre for Scientiﬁc Research “Demokritos”, Patr. Gregoriou E & 27 Neapoleos Str, Agia Paraskevi, 15310 Athens, Greece; [email protected] 4 Institute of Biosciences & Applications, National Centre for Scientiﬁc Research “Demokritos”, Neapoleos 10, Agia Paraskevi, 15310 Athens, Greece * Correspondence: [email protected] (C.B.); [email protected] (D.P.); Tel.: +30-210-7724054 (C.B. & D.P.) Received: 20 September 2018; Accepted: 15 October 2018; Published: 17 October 2018

Abstract: In this study, bauxite residue was directly leached using the Brønsted acidic ionic liquid 1-ethyl-3-methylimidazolium hydrogensulfate. Stirring rate, retention time, temperature, and pulp density have been studied in detail as the parameters that affect the leaching process. Their optimized combination has shown high recovery yields of Sc, nearly 80%, and Ti (90%), almost total dissolution of Fe, while Al and Na were partially extracted in the range of 30–40%. Si and rare earth element (REEs) dissolutions were found to be negligible, whereas Ca was dissolved and reprecipitated as CaSO4. The solid residue after leaching was fully characterized, providing explanations for the destiny of REEs that remain undissolved during the leaching process. The solid residue produced after dissolution can be further treated to extract REEs, while the leachate can be subjected to metal recovery processes (i.e., liquid–liquid extraction) to extract metals and regenerate ionic liquid.

Keywords: bauxite residue; red mud; ionic liquids; scandium recovery; titanium recovery

1. Introduction Bauxite residue (BR), also known as red mud, is the major byproduct of the Bayer process for alumina production, produced by the alkali leaching of bauxite. On average, for each metric ton of alumina, 1–1.5 metric tons of BR are generated [1,2], which leads to a global production of over 150 million metric tons per year [1,3]. BR composition can differ depending on the type of bauxite ore from which alumina are produced and Bayer processing techniques [4,5]. During the Bayer process, valuable base and trace elements like iron (Fe), some aluminum (Al), titanium (Ti), and rare earth elements (REEs) remain in the bauxite residue. As a consequence, REEs are enriched with a factor of about 2 in BR comparing to the initial ore [6,7]. Particularly interesting is the case of scandium (Sc), as its concentration in BR (in Greek BR accounts to 130 ppm on average) is much higher than in the Earth’s crust (22 ppm on average [8]); that means a notable enrichment of Sc in BR. Due to the high market price (Sc2O3—4600 US$/kg, 99.99% purity, in 2017) [9], Sc may represent 95% of the economic value of rare earths in BR [10]. It has also

Metals 2018, 8, 834; doi:10.3390/met8100834 www.mdpi.com/journal/metals Metals 2018, 8, 834 2 of 17 been listed as a critical raw material by the European Commission due to its high economic importance and supply risk [11]. In fact, Sc is mainly produced as a byproduct during the processing of various ores, from titanium and REEs ores (China), uranium ore (Kazakhstan and Ukraine), and apatite ore (Russia). It can also be recovered from previously processed tailings or residues [9,12,13]. For these reasons, BR can be accounted as a secondary raw material source [14], and the recovery of Sc could represent a high economic interest. BR can also be considered a secondary source for Ti, which is a photocatalyst and it is applied in the white pigment industry [15]. Since the availabilities and qualities of Ti ores are decreasing [16], it is important to find methods for extracting Ti from secondary sources. Many studies, patents, and pilot scale implementations have been carried out for Sc and Ti recovery from BR, mainly by investigating hydrometallurgical or combined pyro-hydrometallurgical processes [5,12,16–23], but none of them has reached an industrial scale. Nowadays, the impact of the zero-waste valorization policy motivates the research community on finding innovative, greener, and economical viable routes for metal extraction from complex polymetallic matrices, such as the bauxite residue [24]. Ionometallurgical approach can be exploited as an alternative to conventional hydrometallurgical processing. The term ionometallurgy indicates the use of ionic liquids (ILs) as solvents in metals processing. ILs are liquid at room temperature and consist solely of ions; generally an organic cation and inorganic/organic anion. ILs have superior properties against conventional organic solvents, such as nonflammability, a wide electrochemical window, high thermal stability, negligible vapor pressure, and low volatility [25]. For these reasons and thanks to the vast number of combinations of the cation and the anion during synthesis, ILs have potential for many applications, such as solvent extraction [26,27], catalytic reactions [28,29], and electrodeposition of metals [30,31]. In the past few decades, ILs have been used also as lixiviants for metals dissolution [25,32–35]. Applying ionic liquid leaching on secondary raw material resources eventually improves efficiency yields, reduce waste effluent, and increases selectivity. The aim of this work is to investigate the direct leaching of bauxite residue by using a Brønsted acidic ionic liquid, achieving high Ti and Sc recovery yields. To optimize the process, several parameters were studied. Moreover, solid residue after leaching was fully characterized and explanations of the destiny of REEs were given.

2. Materials and Methods Bauxite residue was provided by Aluminium of Greece (Mytilineos S.A.), dried at 100 ◦C overnight, homogenized, and split in order to take a representative sample that was next crushed and ground. The sample was then subjected to chemical, mineralogical and physical characterization. Chemical composition was analyzed after complete dissolution of the sample via fusion method: ◦ 0.1 g of BR was mixed with 1.5 g of Li2B4O7 and 0.1 g of KNO3 and then fused at 1000 C for 1 h, followed by dissolution in HNO3 10% v/v. The main elements were identified by a Perkin Elmer 2100 Atomic Absorption Spectrometer (AAS) (Waltham, MA, USA), while minor elements were analyzed by a Thermo Fisher ScientificTM X-series 2 Inductively Coupled Plasma Mass Spectrometer (ICP-MS) (Waltham, MA, USA) and a Perkin Elmer Optima 8000 Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-OES) (Waltham, MA, USA). The calcium oxide content was measured in the solid sample with a Spectro Xepos Energy Dispersive X-ray fluorescence spectroscopy (SPECTRO, Kleve, Germany) (ED-XRF). Mineralogical characterization was performed with a Bruker D8 focus X-ray powder diffractometer (XRD) (Bruker, Billerica, MA, USA) with nickel-filtered CuKa radiation, and quantitative evaluation was done via profile fitting by using XDB Powder Diffraction Phase Analytical System version 3.107 that targets specifically bauxite and bauxite residue [36,37]. Particle size analysis was carried out by a Malvern Mastersizer TM Laser particle size analyzer (Malvern Instruments, Malvern, UK). Metals 2018, 8, 834 3 of 17

The ionic liquid 1-ethyl-3-methylimidazolium hydrogensulfate ([Emim][HSO4]) was supplied by Iolitec (Iolitec Ionic Liquids Technologies, Heilbronn, Germany) with >98% purity and characterized. Infrared measurements were conducted with a Perkin Elmer FTIR spectrum 100 (Waltham, MA, USA). Viscosity analysis was performed with a Brookfield viscometer DV-I + LV supported by a Brookfield Thermosel accessory (Brookfield Ametek, Harlow, UK). Nuclear Magnetic Resonance (NMR) spectra ◦ were obtained in DMSO-d6 at 25 C on a Bruker Avance DRX 500 MHz (Bruker Biospin, Germany) (1H at 500.13 MHz and 13C at 125.77 MHz) equipped with a 5 mm multi nuclear broad band inverse detection probe. Batch leaching experiments were performed in a 50 mL Trallero and Schlee mini reactor (Trallero and Schlee, Barcelona, Spain) combined with a mechanical stirrer, a vapor condenser, and a temperature controller, by adding BR to the IL when the set temperature was reached. Vacuum filtration was executed by cooling the system at 120 ◦C and adding a nonviscous/volatile solvent (dimethyl sulfoxide, further denoted as DMSO) to the leachates, to decrease viscosity and ease the process. After filtration, pregnant leaching solutions (PLS) were digested through acidic treatment (HNO3 65% v/v and aqua regia) to oxidize and destroy the organics and then analyze with AAS, ICP-OES and ICP-MS. Solid residues were characterized via fusion method (already described above) and XRD. Microstructural characterization was carried out by a JEOL 6380 LV Scanning Electron Microscope (JEOL, Tokyo, Japan) coupled with Energy Dispersive System (SEM-EDS) and a JEOL 2100 HR (JEOL, Tokyo, Japan) 200 kV Transmission Electron Microscope (TEM) in order to detect and locate REEs.

3. Results and Discussion

3.1. Bauxite Residue Characterization

The main component of BR was found to be Fe2O3, accounting for 42.34 wt.%, followed by Al2O3 with 16.25 wt.%, while TiO2 was 4.27 wt.%, and total rare earth oxides (REO) assessed to 0.19 wt.%, as it is shown in Table1.

Table 1. Bauxite residue chemical analysis. Note: REO, rare earth oxides; LOI, loss of ignition.

Unit Fe2O3 Al2O3 SiO2 TiO2 CaO Na2O REO LOI Others Sum wt.% 42.34 16.25 6.97 4.27 11.64 3.83 0.19 12.66 1.85 100.00

In particular, cerium (Ce) was found to be the main rare earth element in concentration (402.2 mg/kg), followed by lanthanum (La) (145 mg/kg), scandium (Sc) (134 mg/kg), neodymium (Nd) (127.1 mg/kg), and yttrium (Y) (112 mg/kg). Identiﬁcation and quantiﬁcation of mineralogical phases (Table2) denoted hematite as the main mineral in BR with 30 wt.%, while Ti-containing phases were perovskite, anatase and rutile with 4.5, 0.5 and 0.5 wt.% respectively.

Table 2. Bauxite residue mineralogical phases and quantiﬁcation.

Mineralogical Phase Formula wt.%

Hematite Fe2O3 30 Calcium aluminum iron silicate hydroxide Ca3AlFe(SiO4)(OH)8 17 Cancrinite Na6Ca2(AlSiO4)6(CO3)2 15 Diaspore α-AlOOH 9 Goethite Fe2O3·H2O 9 Perovskite CaTiO3 4.5 2+ Chamosite (Fe ,Mg)5Al(AlSi3O10)(OH)8 4 Calcite CaCO3 4 Boehmite γ-AlOOH 3 Gibbsite Al(OH)3 2 Rutile TiO2 0.5 Anatase TiO2 0.5 Sum 98.5 Metals 2018, 8, x FOR PEER REVIEW 4 of 17 Metals 2018, 8, 834 4 of 17 MetalsFrom 2018, particle8, x FOR PEER size REVIEWdistribution analysis, it was found that 50% of the particles were below 41.87 of 17 μm, while 90% were smaller than 42.87 μm. From particle size distribution analysis, it was found that 50% of the particles were below 1.87 From particle size distribution analysis, it was found that 50% of the particles were below 1.87 µm, μm, while 90% were smaller than 42.87 μm. while3.2. Ionic 90% Liquid were Characterization smaller than 42.87 µm.

3.2.3.2. Ionic 1-ethyl-3-methylimidazoliumIonic Liquid Liquid Characterization Characterization hydrogensulfate ([Emim][HSO4]) is a Brønsted acidic ionic liquid whose molecular weight is 208.24 g/mol and density (ρ) at room temperature is 1367.9 kg/m3. The 1-ethyl-3-methylimidazolium hydrogensulfate ([Emim][HSO4]) is a Brønsted acidic ionic liquid 1-ethyl-3-methylimidazolium hydrogensulfate ([Emim][HSO4]) is a Brønsted acidic ionic liquid molecular structure of the IL is shown in Figure 1. 3 whosewhose molecular molecular weight weight is is 208.24 208.24 g/mol g/mol and and density density ( ρ()ρ at) at room room temperature temperature is is 1367.9 1367.9 kg/m kg/m3.. The The molecularmolecular structure structure of of the the IL IL is is shown shown in in Figure Figure1. 1.

Figure 1. [Emim][HSO4] molecular structure. Figure 1. [Emim][HSO4] molecular structure. Figure 1. [Emim][HSO4] molecular structure. Viscosity measurements (Figure 2) have revealed that even though [Emim][HSO4] is very Viscosity measurements (Figure2) have revealed that even though [Emim][HSO 4] is very viscous atviscous room temperatureat room temperature (1642 mPa (1642·s), by mPa·s), increasing by increasing temperature temperature its viscosity its dramatically viscosity dramatically decreases, Viscosity measurements (Figure 2) have revealed that even though [Emim][HSO4] is very decreases, reaching 221 mPa·s◦ at 60 °C and 33 mPa·s◦ at 120 °C. reachingviscous 221at room mPa· stemperature at 60 C and (1642 33 mPa mPa·s),·s at 120 byC. increasing temperature its viscosity dramatically Midinfrared spectrum have shown bands (cm−−11) at 3452 (OH), 3151 (aromatic/imidazole CH), decreases,Midinfrared reaching spectrum 221 mPa·s have at shown 60 °C and bands 33 (cmmPa·s )at at 120 3452 °C. (OH), 3151 (aromatic/imidazole CH), 3106 (imidazole ring), 2985 (CH), 2944 (CH), 2881 ((CH2)n–CH3), 2583–2497 (HOSO•••HOSO), 1636 3106 (imidazoleMidinfrared ring), spectrum 2985 (CH), have 2944 shown (CH), bands 2881 (cm ((CH−1)2 )atn–CH 34523 ),(OH), 2583–2497 3151 (aromatic/imidazole (HOSO•••HOSO),1636 CH), (OH),(OH), 15721572 (C=C,(C=C, C=N,C=N, C–N),C–N), 14541454 (CH(CH3),), 14311431 (S=O(S=O2),), 13891389 (CH(CH3),), 12111211 (S=O(S=O2), 1160 (S–O attached toto 3106 (imidazole ring), 2985 (CH), 29443 (CH), 2881 2((CH2)n–CH3), 2583–2497 2(HOSO•••HOSO), 1636 CC2H5),), 1089 (HSO4−−),), 1023 1023 (C–N–C), (C–N–C), 960 960 (O–S–O), (O–S–O), 832 832 (imidazole (imidazole ring), ring), 757 757 (CH (CH of imidazole ring)ring) (OH),2 5 1572 (C=C,4 C=N, C–N), 1454 (CH3), 1431 (S=O2), 1389 (CH3), 1211 (S=O2), 1160 (S–O attached to andand 701701 (C–H–C).(C–H–C). 11H NMR (500 MHz,MHz, DMSO)DMSO) δδ (ppm):(ppm): 1.361.36 (t,(t, 3H,3H, CHCH3), 3.853.85 (s,(s, 3H,3H, CHCH3), 4.19 (q, C2H5), 1089 (HSO4−), 1023 (C–N–C), 960 (O–S–O), 832 (imidazole ring),3 757 (CH of imidazole3 ring) 2 2H,2H, CHCH ), 7.71 (s, 1H,1 CH=CH),CH=CH), 7.78 (s, 1H, CH=CH), 9.19 9.19 (s, (s, 1H, 1H, N–CH–N). N–CH–N). and 7012 (C–H–C). H NMR (500 MHz, DMSO) δ (ppm): 1.36 (t, 3H, CH3), 3.85 (s, 3H, CH3), 4.19 (q, 2H, CH2), 7.71 (s, 1H, CH=CH), 7.78 (s, 1H, CH=CH), 9.19 (s, 1H, N–CH–N). 1800

16001800 14001600 12001400 10001200 1000800 600800 Viscosity (mPa·s) Viscosity 400600 Viscosity (mPa·s) Viscosity 200400 2000 25 45 65 85 105 0 25 45 65T (°C) 85 105 FigureFigure 2.2. ViscosityViscosity measurementsmeasurements ofof [Emim][HSO[Emim][HSOT (°C) 44]] versusversus temperature.temperature.

Reaction MechanismFigure 2. Viscosity measurements of [Emim][HSO4] versus temperature. 3.2.1. Reaction Mechanism To investigate the mechanism of the reaction that takes place, two monometallic solutions of 3.2.1.To Reaction investigate Mechanism the mechanism of the reaction that takes place, two monometallic solutions of 11 11 g/L of Sc and 11 g/L of Al were prepared, by dissolving Sc2O3 and Al2O3 in [Emim][HSO4]. g/L of Sc and 11 g/L of Al were prepared, by dissolving Sc2O3 and Al2O3 in [Emim][HSO4]. TheTo investigate two monometallic the mechanism solutions of werethe reaction then analyzed that takes with place,1H two and monometallic13C NMR. Assignment solutions of of 11 1 13 1 1 1 13 Hg/L and of ScC and chemical 11 g/L of shifts Al were was prepared, based on theby dissolving combined Sc analyses2O3 and ofAl a2O series3 in [Emim][HSO of H– H and4]. H– C correlation experiments recorded using standard pulse sequences from the Bruker library. Metals 2018, 8, x FOR PEER REVIEW 5 of 17

MetalsThe2018 ,two8, 834 monometallic solutions were then analyzed with 1H and 13C NMR. Assignment 5of of 1 17H and 13C chemical shifts was based on the combined analyses of a series of 1H–1H and 1H–13C correlationFrom theexperiments results (Appendix recorded Ausing, Figures standard A1 andpulse A2 sequences), it could from be concludedthe Bruker library. that there is no signiﬁcantFrom rearrangementthe results (Appendix in the carbon A, Figures chain afterA1 and the dissolutionA2), it could procedure be concluded in all three that metalthere cases.is no significant1H and rearrangement13C NMR spectra in the did carbon not indicate chain anyafter notable the dissolution differences procedure in the chemical in all three shifts metal depending cases. on the1H leached and 13 metal.C NMR The spectra similar did chemical not indicate shifts for any the protonsnotable anddifferences the carbons in localizedthe chemical in between shifts thedepending two nitrogen on the atoms leached indicate metal. metalThe similar interaction chemical through shifts the for anion the protons of the IL.and the carbons localized in betweenThere isthe not two any nitrogen steric effectatoms of indicate electron metal clouds interaction changing through of electrostatic the anion interactions of the IL. between ionicThere charges. is not any steric effect of electron clouds changing of electrostatic interactions between ionicThe charges. results obtained from NMR analysis of two monometallic leachates have led to the following proposedThe results reaction: obtained from NMR analysis of two monometallic leachates have led to the following proposed reaction: − + Me2On + n[Emim][HSO4] = Me2(SO4)n + nOH + n[Emim] (1) Me2On + n[Emim][HSO4] = Me2(SO4)n + nOH− + n[Emim]+ (1) where MeMe isis thethe metalmetal andand nn isis thethe oxidationoxidation statestate ofof thethe metal.metal.

3.3. Leaching Process Optimization: Parameters Affecting the System In orderorder toto optimize optimize the the process, process, stirring stirring rate, rate retention, retention time, time, temperature, temperature, and pulpand pulp density density were investigated.were investigated. Each Each parameter parameter was studiedwas studied separately, separately, keeping keeping the others the others constant, constant, and and choosing choosing the combinationthe combination that gavethat gave the best the results.best results. In each In case, each Ca case, and Ca Si inand leachates Si in leachates were below were the below detection the limitdetection and Ce,limit Nd, and Y Ce, and Nd, La recoveryY and La was recovery lower was than lower 1%. than 1%.

3.3.1. Stirring Rate Initially, experimentsexperiments were were carried carried out out by by examining examining four four different different stirring stirring rates, rates, 100, 100, 200, 200, 400 and400 ◦ 600and revolutions600 revolutions per minuteper minute (rpm), (rpm), while while keeping keeping constant constant all the all other the other parameters parameters at 150 at 150C, 5% °C,w 5%/v pulpw/v pulp density density and and 24 h. 24 Results h. Results are givenare given in Figure in Figure3. 3.

100

60 Fe 50 Al

40 Ti Recovery (%) Recovery Na 30 Sc 20

0 100 200 400 600 Stirring rate (rpm)

Figure 3.3. InvestigationInvestigation of of the the stirring stirring rate rate effect effect on on metals dissolution by leaching BR withwith ◦ [Emim][HSO44] at 150 °C,C, 5% 5% w//vv pulppulp density density for for 24 24 h. h.

At these conditions, it is possible to observe an increase of Fe, Ti, andand ScSc extractionextraction when the stirring raterate increasesincreases from from 100 100 to to 200 200 rpm rpm (from (from 55% 55% of of Sc, Sc, 57% 57% of Fe,of Fe, and and 65% 65% of Ti of at Ti 100 at rpm100 rpm to 67% to of67% Sc, of 75% Sc, of75% Fe of and Fe 72%and 72% of Ti of at Ti 200 at rpm).200 rpm). On theOn otherthe other hand, hand, as the as stirringthe stirring rate rate increases increases from from 200 to200 600 to rpm,600 rpm, Fe, Ti,Fe, and Ti, and Sc extraction Sc extraction is observed is observed to linearly to linearly decrease decrease (from (from 67% 67% of Sc, of 75% Sc, 75% of Fe, of and Fe, 72%and 72% of Ti of at Ti 200 at rpm 200 rpm to 37% to of37% Sc, of 46% Sc, of46% Ti, of and Ti, onlyand 9%only for 9% Fe for at Fe 600 at rpm). 600 rpm). Na and Na Al and recovery Al recovery were Metals 2018, 8, 834 6 of 17 Metals 2018, 8, x FOR PEER REVIEW 6 of 17 slightlywere slightly affected affected by the by stirring the stirring rate as rate they as remained they remained almost stablealmost in stable a range in ofa range 17–32% of of17–32% recovery. of Thisrecovery. effect This of stirring effect rateof stirring on metal rate recovery on metal is typicalrecovery in hydrometallurgy.is typical in hydrometallurgy. Under low stirring Under rates, low astirring thick boundaryrates, a thick layer boundary was developed layer was on developed the surface on of the the surface solid particles, of the solid making particles, the diffusionmaking the of chemicaldiffusion speciesof chemical from species and to from the solid and particlesto the solid surface particles inefﬁcient. surface Therefore,inefficient. at Therefore, stirring rates at stirring lower thanrates 200lower rpm, than the 200 leaching rpm, the process leaching is process slowed is down slowed and down metal and recovery metal recovery decreases, decreases, as it is seen as it inis Figureseen in3 .Figure At stirring 3. At rates stirring higher rates than higher 200 rpm, than the 200 thickness rpm, the of thickness the boundary of the layer boundary is substantially layer is decreased,substantially but decreased, the high convective but the high mass convective transfer ofma reactantsss transfer from of thereactants surface from of the the particles, surface makesof the theparticles, surface makes reactions the surface again inefﬁcientreactions again and thus ineffici theent recovery and thus yields the recovery are diminishing, yields are as diminishing, it is seen in Figureas it is 3seen. Therefore, in Figure a compromise3. Therefore, isa alwayscompromise found is under always intermediate found under stirring intermediate rates which, stirring for rates this system,which, for is aroundthis system, 200 rpm.is around At this 200 stirring rpm. At rate, this Fe,stirring Ti, and rate, Sc Fe, have Ti,the and highest Sc have recovery the highest yields recovery (75%, 72%yields and (75%, 67% 72% respectively). and 67% respectively).

3.3.2. Kinetic Studies Several sets of kinetic have been performed at 200 rpm stirringstirring rate,rate, 5% ww//vv pulppulp density, density, ◦ analyzing the behavior of the system at threethree differentdifferent temperatures:temperatures: 150,150, 175,175, andand 200200 °C.C. ◦ In Figure4 4 it it isis observedobserved thatthat atat lowlow temperaturetemperature (150(150 °C),C), all metals show the same trend in the firstfirst twelvetwelve hours;hours; ananinitial initial metal metal dissolution dissolution occurred occurred in in the the first first six six hours, hours, whilst whilst in thein the following following six hours,six hours, metals metals dissolution dissolution is decreased, is decreased, reaching reaching their their lowest lowest concentration concentration at 12 at h retention12 h retention time. time. This unusual behavior can be attributed to the precipitation of Ca as CaSO that massively occurs within the This unusual behavior can be attributed to the precipitation of Ca4 as CaSO4 that massively occurs firstwithin 6 h the (Appendix first 6 h A(Appendix, Figure A3 A,), whileFigure Fe A3), dissolution while Fe isdissolution low. In the is latter low. 6In h, the adsorption latter 6 h, phenomena adsorption werephenomena more important were more and important faster than and dissolutionfaster than and,dissolution being inand, contact being with in contact anhydrite, with metalsanhydrite, are removedmetals are from removed the leachates, from the attaining leachates, the attaining minimum the at minimum 12 h. Then, at 12 metals h. Then, continue metals their continue dissolution their anddissolution as the anhydrite and as the precipitation anhydrite hasprecipitation been completed has been they completed are gradually they desorbed, are gradually increasing desorbed, their concentrationincreasing their in concentration solution and reachingin solution the and equilibrium reaching atthe 24 equilibrium h retention at time, 24 h with retention the exception time, with of ironthe exception that continues of iron to that be dissolved continues but to be at adissolved substantially but at lower a substantially rate. lower rate.

100

60 Fe 50 Al

40 Ti Recovery (%) Recovery Na 30 Sc 20

0 0 6 12 18 24 30 36 42 48 Time (hours)

Figure 4. Kinetic curves for metals dissolution by leaching BR with [Emim][HSO[Emim][HSO44] at 1, 3, 6, 12, 18, 24 and 48 h, 200 rpm, 150 ◦°CC and 5% w/vv pulppulp density. density.

Kinetic studies have been carried out at 175 ◦°CC (Figure5 5),), inin thisthis casecase thethe unusualunusual dissolutiondissolution phenomenon observed at 150 °C◦C was not seen and the plateau has has been been reached reached faster, faster, after 12 h, achieving 90% of Fe, 70% of Ti and Sc dissolution and again moderate Al and Na recovery (30%). (30%). After 1 h, more than 35% of Sc has been dissolved, this,this, as mentioned, is due to the fact that goethite is totally dissolved and hematite starts to be leached as well. The equilibrium has been reached at 70% of Sc and 90% of Fe recovery, which is in agreement with Vind et al. studies, as the main Metals 2018, 8, 834 7 of 17 is totally dissolved and hematite starts to be leached as well. The equilibrium has been reached Metals 2018, 8, x FOR PEER REVIEW 7 of 17 Metalsat 70% 2018 of, 8 Sc, x FOR and PEER 90% REVIEW of Fe recovery, which is in agreement with Vind et al. studies, as the 7main of 17 mineralogical Sc containing phases in bauxite residue are hematite and goethite (55% and 25% on mineralogical Sc containing phases in bauxite residue are hematite and goethite (55% and 25% on average, respectively) [38]. average, respectively) [38].

100

70 60 60 Fe 50 Al 40 Ti

Recovery (%) Recovery 40 Recovery (%) Recovery Na 30 Sc 20

0 0 2 4 6 8 1012141618 Time (hours)

Figure 5. Kinetic curves for metals dissolution by leaching BR with [Emim][HSO4] at 1, 3, 6, 12, and Figure 5. Kinetic curves for metals dissolution by leaching BR with [Emim][HSO4]] at at 1, 1, 3, 6, 12, and ◦ 18 h, 200 rpm, 175 °CC and 5% w/vv pulppulp density. density.

◦ At 200200 C°C (Figure (Figure6), 6), Fe, TiFe, and Ti Scand are Sc considerably are considerably leached leached even after even 1 h (60–74%).after 1 h The(60–74%). maximum The maximumextraction ofextraction these metals of hasthese been metals reached has after been 12 h,reached where Feafter was 12 almost h, where totally Fe dissolved, was almost Ti recovery totally wasdissolved, over 90% Ti recovery and Sc reached was over nearly 90% 80%.and Sc Al reached and Na dissolutionnearly 80%. remainedAl and Na stable dissolution along the remained kinetic stablecurve inalong a range the kinetic of 30–40%. curve in a range of 30–40%.

100

70 60 60 Al 50 Fe 40 Ti

Recovery (%) Recovery 40 Recovery (%) Recovery Na 30 Sc 20

0 0369121518 Time (hours)

Figure 6. Kinetic curves for metals dissolution by leaching BR with [Emim][HSO4] at 1, 3, 6, 12, and Figure 6. Kinetic curves for metals dissolution by leaching BR with [Emim][HSO4]] at at 1, 1, 3, 6, 12, and 18 h, 200 rpm, 200 °C, and 5% w/v pulp density. 18 h, 200 rpm, 200 ◦°C,C, and 5% w/vv pulppulp density. density.

Extraction of Sc at these high recovery yields (nearly 80%), when Fe was also almost totally dissolved, again confirms that Sc was found to be hosted mainly in hematite and goethite mineralogical phases in bauxite residue [38], as already mentioned. It was hypothesized before that in the same experimental setup as given here, the unrecovered proportion of Sc (about 20%) may be associated mainly with the chemically durable zirconium orthosilicate (ZrSiO4), that contains around associated mainly with the chemically durable zirconium orthosilicate (ZrSiO4), that contains around Metals 2018, 8, 834 8 of 17

Extraction of Sc at these high recovery yields (nearly 80%), when Fe was also almost totally dissolved, again conﬁrms that Sc was found to be hosted mainly in hematite and goethite mineralogical phases in bauxite residue [38], as already mentioned. It was hypothesized before that in the same

Metalsexperimental 2018, 8, x FOR setup PEER as REVIEW given here, the unrecovered proportion of Sc (about 20%) may be associated 8 of 17 mainly with the chemically durable zirconium orthosilicate (ZrSiO4), that contains around 10% of the 10%total of Sc the in bauxite total Sc residue,in bauxite but residue, also with but other also undissolvedwith other undissolved (or partially (or dissolved) partially phases dissolved) as boehmite, phases asdiaspore, boehmite, and diaspore, titanium-containing and titanium-containing phases, which phas havees, been which determined have been to determined be carriers ofto Scbe in carriers Greek ofBR Sc [38 in]. Greek BR [38].

3.3.3. Pulp Density Four experiments have been conducted to investig investigateate the the effect effect of pulp density on on the system, at 2.5%, 5%, 5%, 10%, 10%, and and 14.3% 14.3% ww/v/ pulpv pulp density, density, under under constant constant temper temperature,ature, time time and andstirring stirring rate (200 rate ◦ (200°C, 12C, h, 12and h, 200 and rpm). 200 rpm). Results Results are shown are shown in Figure in Figure 7. 7.

100

60 Fe 50 Al

40 Ti Recovery (%) Recovery Na 30 Sc 20

0 2.5 5 10 14.3 Pulp density (%) Figure 7.7. StudyStudy on on the the system system behavior behavior for metalsfor metals dissolution dissolution by changing by changing pulp density pulp whendensity leaching when ◦ leachingBR with [Emim][HSOBR with [Emim][HSO4] at 12 h,4] 200 at 12 rpm, h, 200 200 rpm,C. 200 °C.

Fe, Ti, and ScSc presentpresent constantconstant recoveryrecovery inin thethe areaarea ofof 2.5–5%2.5–5% ww//v pulp density (almost total dissolution for for Fe, Fe, 88% 88% for for Ti, Ti, and and 78% 78% for for Sc). Sc). This This behavior behavior can canbe explained be explained by the by extremely the extremely high ionichigh ionicliquid liquid excess excess and the and relatively the relatively low lowviscosity viscosity of the of thesystem system due due to the to thelow low concentration concentration of dissolvedof dissolved metals. metals. By Byincreasing increasing pulp pulp density, density, the the ionic ionic liquid liquid excess excess decreases decreases and and viscosity substantially rises,rises, due due to to the the increase increase of the of numberthe number of suspended of suspended BR particles BR particles as well asas the well dissolved as the dissolvedmetal concentrations metal concentrations affecting the affecting ions mobility the io phenomenans mobility andphenomena the thickness and ofthe the thickness boundary of layer. the Thisboundary results layer. in a sharp This andresults linear in a decreasing sharp and recovery, linear decreasing reaching the recovery, minimum reaching at 14.3% thew/ minimumv (60% of Fe,at 14.3%70% Ti, w and/v (60% 14% of Sc).Fe, 70% Experiments Ti, and 14% at pulp of densitySc). Experiments higher than at14.3% pulp wdensity/v were higher not carried than 14.3% out due w to/v werethe high not viscosity, carried whichout due prevented to the ﬁltrationhigh viscosity, and caused which serious prevented problems filtration during and the leachingcaused process.serious problems during the leaching process. 3.4. Characterization of the Solid Residue after Leaching 3.4. CharacterizationThe solid residue of the collected Solid Residue after leaching after Leaching bauxite residue at optimum conditions (200 rpm, 200 ◦C, 12 h andThe 5%solidw /residuev pulp density)collected wasafter characterized leaching bauxite via fusion residue method, at optimum XRD, conditions SEM, and TEM(200 rpm, analyses. 200 The°C, 12 resulting h and residue5% w/v was pulp found density) to be was 48% characterized of the weight ofvia the fusion initial method, BR mass. XRD, SEM, and TEM analyses.As it The can resulting be seen from residue chemical was found analysis to be shown 48% inof Tablethe weight3, solid of residue the initial after BR leaching mass. is high in aluminum,As it can calcium, be seen and from silicon, chemical while analysis it is depleted shown in in Table iron and3, solid titanium. residue REEs after remain leaching in is the high solid in aluminum,residue (with calcium, the exception and silicon, of scandium) while it andis depleted can be leachedin iron and afterwards. titanium. REEs remain in the solid residue (with the exception of scandium) and can be leached afterwards.

Table 3. Chemical analysis of the residue after leaching BR at optimum conditions.

Metal Oxide Fe2O3 Al2O3 SiO2 TiO2 CaO Na2O REO SO3 LOI Others wt.% 3.71 27.44 14.51 1.46 24.73 2.75 0.19 18.69 6.00 0.52 Metals 2018, 8, 834 9 of 17

Table 3. Chemical analysis of the residue after leaching BR at optimum conditions.

Metal Oxide Fe2O3 Al2O3 SiO2 TiO2 CaO Na2O REO SO3 LOI Others wt.% 3.71 27.44 14.51 1.46 24.73 2.75 0.19 18.69 6.00 0.52 Metals 2018, 8, x FOR PEER REVIEW 9 of 17

FromFrom the the comparison comparison of of the the XRD XRD spectraspectra of baux bauxiteite residue residue and and residue residue generated generated after after leaching leaching (Appendix(AppendixA, A, Figure Figure A4 A4),), it it is is possible possible to to observe observe thatthat peakspeaks attributed to to hematite, hematite, goethite, goethite, calcium calcium aluminumaluminum iron iron silicate silicate hydroxide, hydroxide, gibbsite,gibbsite, and perovskite, perovskite, which which are are present present in inbauxite bauxite residue, residue, disappeardisappear after after leaching. leaching. Aluminum Aluminum phasesphases like diaspore and and boehmite boehmite remain remain relatively relatively intact intact after after leaching,leaching, as as well well as as cancrinite, cancrinite, chamosite, chamosite, andand calcite.calcite. On On the the other other hand, hand, a anew new mineralogical mineralogical phase phase calciumcalcium sulfate sulfate anhydrite anhydrite (causing (causing the the consumption consumption of of about about 2 wt.%2 wt.% of of the the IL), IL), which which was was formed formed due todue the interactionto the interaction between between calcium calcium and theand anion the anion of the of ionicthe ionic liquid, liquid, is created is created during during leaching. leaching. The aboveThe observationsabove observations explain explain well the well behavior the behavior of Al and of NaAl duringand Na leaching during leaching as their mainas their minerals main in BRminerals such as diaspore,in BR such boehmite as diaspore, and cancrinite boehmite remain and cancrinite insoluble, remain leading insoluble, to low to leading moderate to recoveries.low to Onmoderate the other recoveries. hand, Fe andOn the Ti bearingother hand, minerals Fe and were Ti be depletedaring minerals in leaching were depleted residue thusin leaching confirming residue their thus confirming their observed high recoveries. Regarding Ca leaching, phases like calcium observed high recoveries. Regarding Ca leaching, phases like calcium aluminum iron silicate hydroxide aluminum iron silicate hydroxide are substantially soluble, while phases such as cancrinite and are substantially soluble, while phases such as cancrinite and chamosite resist dissolution. Calcite chamosite resist dissolution. Calcite is partially dissolved in IL solution and in the presence of HSO4− is partially dissolved in IL solution and in the presence of HSO − anions, undertakes a transition to anions, undertakes a transition to anhydrite, which is a secondary4 precipitated phase during the anhydrite, which is a secondary precipitated phase during the leaching process. leaching process. SEM-EDSSEM-EDS analysis analysis of of the the solid solid residue residue after after leaching leaching confirmed confirmed the the findings findings of chemicalof chemical and and XRD analysesXRD analyses (Figure (Figure8). The 8). matrix, The matrix, which which is mainly is mainly composed composed of of Al, Al, Ca, Ca, andand Na silicates, silicates, surrounds surrounds phasesphases transitioning transitioning from from CaCO CaCO3 3to to CaSOCaSO44.

FigureFigure 8. 8.Scanning Scanning electron electronmicroscope microscope (SEM image of of the the matrix matrix of of the the residue residue after after leaching). leaching).

3.5.3.5. REEs REEs in in the the Solid Solid Residue Residue

SmallSmall REEs-containing REEs-containing particles particles (about(about 10 µμm) were detected detected in in SEM-EDS, SEM-EDS, in in particular particular YPO YPO4 4 particlesparticles including including heavy heavy rarerare earthsearths likelike gadoliniumgadolinium and dysprosium (Figure (Figure 9 left). This This is is consistentconsistent with with Vind Vind etet al. studies studies of ofraw raw bauxite bauxite residue residue [39] where [39] where the presence the presence of heavy of rare heavy earth rare earthphosphates phosphates with with the themajor major constituent constituent being being yttrium yttrium and and containing containing other other heavy heavy REEs REEs like like gadolinium, dysprosium, and erbium is reported. This is an indication that these grains endure the [Emim][HSO4] leaching process without being subjected to any dissolution and thus explaining negligible heavy REEs recoveries. Small mixed calcium–cerium phosphate particles were also identified; in this case, grains included light rare earths like neodymium, lanthanum, and samarium (Figure 9 right). Vind et al. Metals 2018, 8, x FOR PEER REVIEW 10 of 17 Metals 2018, 8, 834 10 of 17 reported the presence of light rare earths as calcium-containing phosphate phases in bauxite residue [39]. In the case of the solid residue after leaching, grains containing light REEs (LREEs) phosphates gadolinium, dysprosium, and erbium is reported. This is an indication that these grains endure are also present. This may indicate a partial dissolution of calcium from the mixed Ca-LREEs phases, the [Emim][HSO ] leaching process without being subjected to any dissolution and thus explaining leaving behind4 smaller phosphate particles which are beneficiated in LREEs. This was also implied negligible heavy REEs recoveries. by TEM analysis, detecting very fine (<500 nm) particles of Al-containing CePO4 (Figure 10).

Metals 2018, 8, x FOR PEER REVIEW 10 of 17

reported the presence of light rare earths as calcium-containing phosphate phases in bauxite residue [39]. In the case of the solid residue after leaching, grains containing light REEs (LREEs) phosphates are also present. This may indicate a partial dissolution of calcium from the mixed Ca-LREEs phases, leaving behind smaller phosphate particles which are beneficiated in LREEs. This was also implied by TEM analysis, detecting very fine (<500 nm) particles of Al-containing CePO4 (Figure 10).

FigureFigure 9. SEM 9. SEM image image of of a YPOa YPO4 4particle particle ( (leftleft) )and and a aCePO CePO4 particle4 particle (right (right). ).

Small mixed calcium–cerium phosphate particles were also identified; in this case, grains included light rare earths like neodymium, lanthanum, and samarium (Figure9 right). Vind et al. reported the presence of light rare earths as calcium-containing phosphate phases in bauxite residue [39]. In the case of the solid residue after leaching, grains containing light REEs (LREEs) phosphates are also present. This may indicate a partial dissolution of calcium from the mixed Ca-LREEs phases, leaving behind smaller phosphate particles which are beneficiated in LREEs. This was also implied by TEM analysis, detecting very fine (<500 nm) particles of Al-containing CePO (Figure 10). Figure 9. SEM image of a YPO4 particle (left) and a CePO4 particle4 (right).

Figure 10. TEM image of a CePO4, Al containing, particle.

FigureFigure 10. 10.TEM TEM image imageof of aa CePOCePO44, ,Al Al containing, containing, particle. particle.

4. Conclusions In this study, Brønsted acidic ionic liquid 1-ethyl-3-methylimidazolium hydrogensulfate was used to directly leach bauxite residue. Experiments were carried out in a closed mini reactor, equipped with a condenser and a temperature controller. Stirring rate, time, temperature and pulp density were thoroughly examined to ﬁnd the optimum conditions for Sc (nearly 80%) and Fe (almost totally dissolved) high recovery yields. This outcome conﬁrms that Sc is mainly hosted in hematite and goethite mineralogical phases (55% and 25%, respectively) in bauxite residue, in accordance to the work of Vind et al. [38]. The undissolved Sc content might be attributed to ZrSiO4, containing around 10% of the total Sc in bauxite residue, but also to other phases, such as boehmite, diaspore, and titanium-containing phases that host Sc in Greek bauxite residue [38]. At the optimum conditions, 90% of Ti was dissolved, while Al and Na were partially extracted (in a range of 30–40%). Si and REEs dissolutions were found to be negligible, whereas Ca was partially dissolved and precipitated as CaSO4 consuming about 2 wt.% of the ionic liquid. Solid residue after leaching was fully characterized and found to be rich in Al, Ca, and Si, with the main minerals present being anhydrite, diaspore, and cancrinite; it could be further treated to extract REEs. SEM and TEM analyses of the solid residues provided explanations for the destiny of REEs, which remain undissolved enduring the leaching process. It can be concluded that [Emim][HSO4] ionic liquid is a good leaching agent for dissolving metals from bauxite residue and, since it is not selective against iron, high recovery yields of Sc can be achieved, reaching up to 80% of extraction.

Author Contributions: C.B. and D.P. conceived and designed the experiments; C.B. performed the experiments; A.P. performed NMR analysis and analyzed the spectra; A.A. performed chemical analyses; J.V. and C.B. performed the SEM-EDS analysis and analyzed the data; P.T. performed TEM analysis; C.B. analyzed the data and wrote the paper; D.P. contributed to the analysis of the data and the writing of the paper. Acknowledgments: The research leading to these results has received funding from the European Community’s Horizon 2020 Program (H2020/2014—2019) under Grant Agreement no. 636876 (MSCA–ETN REDMUD). Conﬂicts of Interest: The authors declare no conﬂict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results. Metals 2018, 8, 834 12 of 17 Metals 2018, 8, x FOR PEER REVIEW 12 of 17

Appendix A Appendix A

1 1 FigureFigure A1. A1.H NuclearH Nuclear Magnetic Magnetic Resonance Resonance (NMR) (NMR) comparison comparison between between [Emim][HSO[Emim][HSO4]] (blue), (blue), [Emim][HSO [Emim][HSO4]4 after] after leaching leaching Al Al2O23 O(green),3 (green), [Emim][HSO [Emim][HSO4] after4] after leachingleaching Sc2 OSc32O(red).3 (red).

Metals 2018, 8, 834 13 of 17 Metals 2018, 8, x FOR PEER REVIEW 13 of 17

13 13 FigureFigure A2. A2.C NMRC NMR comparison comparison between between [Emim][HSO [Emim][HSO4]4] (blue), (blue), [Emim][HSO [Emim][HSO44] after after leaching leaching Al Al2O2O3 (green),3 (green), [Emim][HSO [Emim][HSO4] after4] after leaching leaching Sc2O Sc3 (red).2O3 (red). Metals 2018, 8, 834 14 of 17 Metals 2018, 8, x FOR PEER REVIEW 14 of 17

FigureFigure A3. A3.Comparison Comparison between between X-ray X-ray powder powder diffractometer diffractometer (XRD) (XRD) of of bauxite bauxite residueresidue and solid residue after after leaching leaching bauxite bauxite residue residue at at15 1500 °C,◦ C,200 200 rpm, rpm, 5% 5%w/vw /v pulppulp density density for 1,for 6, 1, 12, 6, 12, and and 24 h.24 h. Metals 2018, 8, 834 15 of 17

Metals 2018, 8, x FOR PEER REVIEW 15 of 17

FigureFigure A4. A4.Comparison Comparison of of the the XRD XRD spectra spectra ofof bauxitebauxite residue an andd solid solid residue residue after after leaching leaching at atoptimum optimum conditions. conditions. Metals 2018, 8, 834 16 of 17

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