Sintering Optimisation and Recovery of Aluminum and Sodium from Greek Bauxite Residue

minerals

Article Sintering Optimisation and Recovery of Aluminum and Sodium from Greek Bauxite Residue

Pritii Wai Yin Tam 1,2,* , Dimitrios Panias 2 and Vicky Vassiliadou 1

1 Department of Continuous Improvement and Systems Management, Aluminum of Greece plant, Metallurgy Business Unit, Mytilineos S.A., Agios Nikolaos, 32003 Boeotia, Greece; [email protected] 2 School of Mining and Metallurgical Engineering, National Technical University of Athens, 9, Iroon Polytechniou, Zografou Campus, 15780 Athens, Greece; [email protected] * Correspondence: [email protected]; Tel.: +61-4-8109-0871

Received: 29 August 2019; Accepted: 19 September 2019; Published: 20 September 2019

Abstract: Bauxite residue is treated for the recovery of aluminum and sodium by sintering with the addition of soda, metallurgical coke and other reagents such as CaO, MgO and BaO. A thorough thermodynamic analysis using Factsage 7.0™ software was completed together with XRD mineralogy of sinters with different fluxes and reagents additions. Through both thermodynamic interpretation and mineralogical confirmations, it was observed that the type of desilication product in bauxite residue influences the total aluminum recovery through the sintering process and formation of sodium aluminum silicate exists in equilibrium with sodium aluminate, unless silica is consumed by additives (such as CaO, MgO, BaO etc.) forming other more thermodynamically favorable species and liberating alumina. Addition of barium oxide improves the aluminum and sodium recovery to 75% and 94% respectively. Complex sinter product formation that are triggered due to high calcium content in the Greek bauxite residue reduces aluminum recovery efficiency. Optimised and feasible recovery of aluminum and sodium for Greek bauxite residue was proved to be 70% and 85% respectively, when sintered with 50% excess stoichiometric soda. It was observed that stoichiometric carbon addition in inert atmosphere only assisted recovery up to 75% of aluminum and 83% of sodium, though there are benefits gained from pre-reducing iron from hematite for downstream recovery.

Keywords: bauxite residue; sintering; aluminum; sodium; recovery

1. Introduction The aluminum sector has seen an increase of demand and production within the recent years with current annual estimate in 2017 being 132,390 thousand metric tonnes of alumina [1]. The global market shows no signs of reduction with China’s competitive focus in alumina production estimated surpassing more than half of annual global production in 2017 [1]. Meanwhile, West Europe reports about 5,890,000 metric tonnes of alumina in 2017 with Aluminum of Greece contributing up to 820,000 metric tons of alumina in their yearly production [1,2]. However, as 1 tonne of alumina often generates about 1–1.5 tonnes of bauxite residue (BR) during its production in the Bayer process route, the uncertainty of effectively managing bauxite residue stockpiles in areas of increasingly tightening regulations has encouraged further research efforts into total valorization. Power, Klauber and Gräfe consolidated a series of papers that reviewed extensively towards methods of disposal, management and best practices as well as options in the interim that dealt with the reuse of bauxite residue [3,4]. They have also reported that from the period of 1964 to 2008, patents relating to bauxite residue filed in civil and building constructions consisted of about a third from the total amount, followed by around equal amounts of catalyst and adsorbent patents; ceramics, plastics, coating or pigments,

Minerals 2019, 9, 571; doi:10.3390/min9100571 www.mdpi.com/journal/minerals Minerals 2019, 9, 571 2 of 21 and wastewater treatment. Steel and slag patents and the recovery of major metals were around 7% and 9% respectively. With recent years sustainable economy focus, minor elements in enriched secondary resource such as bauxite residue had gained larger traction in developing rare earth recovery processes [5–8]. It is reported that iron often found in the range of 5–60% Fe2O3 depending on the mineral types influencing bauxite residue in Bayer processing, followed by 5–30% of Al2O3, 3–50% of SiO2 and 0.3–15% of TiO2. Calcium and sodium averages about 2–14% of CaO and 1–10% of Na2O in bauxite residue. Calcium’s addition also ranges from pre-desilication step, scales suppression up until mineral retardations and transformations [9,10]. Mishra and Gostu published a recent review into several different treatment options of the residue, with pyrometallurgical paths either involving smelting or sintering-type processing [11]. High temperature smelting of bauxite residue for complete iron reduction and removal and the subsequent conditioning of slag are commonly found in current researches [6,12–16]. The sintering or reductive-sintering of bauxite residue has also been explored in literature [7,11,14,17–19], but inefficient separation of varied magnetic iron phases often hinders iron recovery. However, sintering and reductive-sintering are able to recover aluminum and sodium without suffering sodium gaseous losses when smelted at high temperatures. As bauxites contain an amount of reactive silica that is soluble in caustic conditions of the Bayer process, desilication becomes necessary to reduce silica impurities and to avoid scale build-up within the evaporator and digester units. A desilication product that is often formed is a type of sodium aluminosilicate with a formula which may be represented as Na8(AlSiO4)6X2 nH2O where X 1 2 1 2 · could be 2 CO3 −, Cl−, OH−, and 2 SO4 − [20,21]. The sodium aluminosilicate product often exists as sodalite and cancrinite, expressed in similar stoichiometry, and sometimes substituting Na+ with K+, 2+ 2 2 Ca , OH−, SO4 −, Cl−, CO3 − and H2O in its framework [20]. If calcium is inherently abundant in bauxite or if lime is added to aid the silica removal, tricalcium aluminate (Ca3Al2(OH)12) may also be formed within the Bayer liquor, which transforms into hydrogrossular or hydrogarnet type species (Ca3Al2(SiO4)3 x(OH)4x)[10]. Smith further characterizes the reactions and phase transformations that − occur with lime’s presence particularly in high temperature Bayer digestion condition. He observed that lime addition encourages sodium recovery, suppressing the sodalite form in favor of hydrogrossular products [10]. Rivera et al. adds that when bauxite residue is subjected to CO2 neutralisation, cancrinitic and grossular phases is stabilized with carbonate ions, thus reducing alkalinity [22]. Meanwhile, the simple aluminum-bearing minerals are often originated from the unreacted boehmitic (γ-AlO(OH)), diasporic (α-AlO(OH)) and gibbsitic (γ-Al(OH)3) minerals in bauxite ores. Through the addition of soda and metal oxides, coupled with also a possible reductive stage of iron by adding carbon, the aluminum-bearing minerals is then transformed into leachable sodium aluminate phase, while suppressing reactive silica formation. This will also allow ease of the introduction of leachates back again into the Bayer circuit.

Previous Studies in Soda and Lime-Soda Sintering Processes Soda sintering or lime-soda sintering process, began from Louis Le Chatelier’s work in 1855 in obtaining aluminum hydroxide (Al(OH)3) from sintering bauxite with soda at around 1000 ◦C which forms a leachable sodium aluminate species [23]. It was further improved by introducing a calcination step to produce alumina (Al2O3) from Al(OH)3. Deville–Pechiney process also introduced lime and coke into the sintering processes [24]. Therefore, soda often targets the aluminum-bearing mineral species into sodium aluminate, as described in Reactions (1–3).

2AlOOH + Na CO 2NaAlO + CO + H O, (1) 2 3 → 2 2 2 2Al(OH) + Na CO 2NaAlO + CO + 3H O, (2) 3 2 3 → 2 2 2 Na Al Si O 2CaCO nH O + Na CO + Fe O + 10CaO = 6NaAlO + 6Ca SiO + 6 6 6 24· 3· 2 2 3 2 3 2 2 4 (3) 2NaFeO2 + 3CO2+ nH2O. Minerals 2019, 9, 571 3 of 21

Literature leaning towards Bayer process efficiency often targets leaching conditions in Bayer digestion and selectivity of aluminum precipitation species for activated alumina recovery, and from the by-product valorisation, the sintering of bauxite residues for the recovery of aluminum and sodium [25–28]. The scope of this work will be focused on the latter. Kaussen et al. explored a wide range of methods in recovering aluminum from bauxite residue in the Lünen landfill [14,29]. This expounded from secondary Bayer leaching at higher temperature and pressures in concentrated caustic solution [14], smelting to produce leachable calcium aluminate phases, sometimes coupled with concentrated caustic leaching again [13,14], and soda/lime-soda sintering [14,19]. Here, along with many other studies [30–33], it is noted that sodium ferrite species and sodium titanate species are potentially transformed after sintering and leaching, into hematite and titanium oxide species respectively. The converted salts of other present impurities in bauxite residue do not affect the sodium recovery, with exception to sodium silicate which increases silica impurity into the leachate [29]. Bruckard et al. reports that a solid-solution series of sodium aluminate–carnegeite species formed when bauxite residue is treated at high temperatures of 1400 ◦C with lime, and though it is a leachable species, up to 55% Al and 90% Na recovery were achieved. Previous attempts of the soda sintering process in Greek bauxite residue achieved the maximum Al recovery of 76% [34], may be suggestive that the recovery of aluminum is inhibited greatly through the bulk presence of hematite and aluminum-bearing mineral types existing in the system, which will be followed up in another study. Tathavadkar et al. investigated soda sintering for three different types of bauxite residues from Hungarian, Indian and UK origins [35]. By introducing soda to bauxite residues, accounting for total silica, aluminum and titanate formations during the sintering process at 550–850 ◦C, they reported a varied recovery of aluminum ranging at a low 55–60% for MAL (Magyar Alumínium Termel˝o és Kereskedelmi Zrt.), Hungarian refinery bauxite residue, followed by 83–95% in ALCAN (Aluminum Company of Canada, Ltd., Toronto, ON, Canada), UK refinery bauxite residue and 77–84% in INDAL (Indian Aluminium Company, Pathalam, India), Indian refinery bauxite residue [35]. A major difference noted in Hungarian bauxite residue is the higher calcium content (5.7%) compared to both ALCAN and INDAL bauxite residue (<1%), coinciding with the raised levels of silica. Whittington [36] notes that Ajka’s refinery introduces lime addition to the causticization step, where Ca(OH)2 interacts with desilication product (DSP) to form calcium aluminum hydrosilicate or possibly hydrogarnet and calcium aluminum hydrates intermediate which then transforms into calcite and recovered soda. AoG’s Greek bauxite residue behavior in the sintering process is more comparable with MAL’s Hungarian bauxite residue due to similarities in the existing calcium-bound aluminum bearing minerals i.e., hydrogarnet (Ca3AlFe(SiO4)(OH)8) and cancrinite (Na Ca Al Si (CO ) O 2H O) [37]. Iron-to-aluminum ratio in INDAL bauxite residue (1.5) is lesser 6 2 6 6 3 2· 24· 2 compared to MAL and ALCAN bauxite residues (2.3–2.4), with sodalite phase as desilication product in both INDAL and ALCAN bauxite residues. Meher and coworkers investigated the effects of divalent alkaline earth metal oxides, such as lime (CaO), barium oxide (BaO) and magnesium oxide (MgO), in soda sintering process using NALCO’s bauxite residue from Damanjodi, India [31,32]. The introduction of these alkaline earth metals was designed to capture the free and bound silica in bauxite residue, transforming them to its divalent metal silicate (M2SiO4 or the less thermodynamically stable MSiO3 phase). NALCO’s bauxite residue consisted of about 51–57% Fe2O3, 16–18% Al2O3 and 8–12% SiO2. In Meher et al.’s researches, the addition of metal oxides was performed by weight basis, with 10, 15 and 20 g of either MgO, CaO or BaO added to 100 g bauxite residue and fixed amount of Na2CO3 of 25 g. By adding minimal amounts of metal oxide in a soda sinter system, optimized levels of alumina extraction was achieved at 97.64%, 98.70% and 99.50% using CaO, MgO and BaO respectively [31,32]. Calcium levels in NALCO’s bauxite residue were reported below 2.3% with about 34% aluminum trapped as muscovite KAl2(Si3AlO10)(OH)2, sillimanite Al2SiO5, and kaolinite Al2Si2O5(OH)4, in bauxite residue, as reported Minerals 2019, 9, 571 4 of 21 by Mohapatra et al. [38]. The rest of aluminum was found in gibbsite, boehmite and alumo-goethite, indicative of the 14% alumina losses within NALCO’s bauxite extraction process [38].

2MO/MO + SiO M SiO /MSiO (where M represents metals such as Ba, Mg, Ca) (4) 2 → 2 4 3 Minerals 2019, 9, x FOR PEER REVIEW Fe O + 3C 2Fe + 3CO4 of (5)20 2 3 → The addition of lime, and subsequently explorationexploration of other metal oxides into the sinteringsintering process through Meher and coworkers’ eeffortsﬀorts [[31,3231,32,,39,40],39,40], focuses on capturingcapturing the free silicasilica during sintering (Reaction (Reaction (4)). (4)). Adding Adding metallurgical metallurgical coke coke into into the the system, system, such such as Reaction as Reaction (5), can (5), canencourage encourage the recovery the recovery of iron of downstream iron downstream by pre- byreducing pre-reducing hematite hematite and other and iron other species. iron species. It may Italso may suppress also suppress potential potential unwanted unwanted product product formatio formationn such as such complex as complex garnet-type garnet-type product product (e.g., (e.g., 3(Ca,Fe,Mg)O(Al,Fe) O 3SiO ) that may entrap both iron and aluminum into its matrix. 3(Ca,Fe,Mg)O(Al,Fe)2O3·3SiO2 32·) that2 may entrap both iron and aluminum into its matrix. Figures1 1 and and2 2reports reports the the Gibbs Gibbs free free energy energy of of formation formation of of di differentﬀerent silicate phases and titanate phases at a range ofof temperature. Silica is more likely to be bonded together with barium or calcium species andand predictedpredicted toto formform thethe moremore stablestable speciesspecies ofof BaBa33SiOSiO55 oror CaCa33SiO5,, whereas whereas titania is more likely to formform calcium-typecalcium-type bondsbonds suchsuch asas CaCa33Ti2O7..

-4000

Ba₃SiO₅

Thousands -3500 Ca₃SiO₅

-3000 BaSi₂O₅ CaSiTiO₅ Ba₂SiO₄ Ca₂SiO₄

Mg₂SiO₄ Na₂Si₂O₅ -2500 Na₄SiO₄ Fe₂SiO₄ NaAlSiO₄ -2000 BaSiO₃ CaSiO₃ Gibbs Energy (kJ/mol) Energy Gibbs MgSiO₃

-1500 Na₂SiO₃ FeSiO₃

500 700 900 1100 1300 1500 Temperature (ᵒC)

Figure 1.1. Comparison ofof GibbsGibbs free free energy energy of of phase phase formation formation of of several several silicate silicate species species throughout throughout the temperaturethe temperature range range by Factsage by Factsage 7.0™ database7.0™ database (Version (Version 7.3, Thermfact 7.3, Thermfact/CRCT/CRCT (Montreal, (Montreal, QC, Canada) QC, andCanada) GTT-Technologies and GTT-Technologies (Aachen, Germany)),(Aachen, Germany)), with silica with notably silica favoring notably bariumfavoring presence. barium presence.

-4900 Ca₃Ti₂O₇

-4400 Ca₃Ti₂O₆

Thousands Na₂Ti₆O₁₃

-3900

Ca₂Ti₂O₅ -3400 Na₂Ti₂O₅

-2900 Ba₂TiO₄ FeTi₂O₅ Mg₂Ti₂O₅ Na₄TiO₄ -2400 Mg₂TiO₄ FeTi₂O₄ Fe₂TiO₄ Gibbs Energy (kJ/mol) Energy Gibbs FeTi₂O₅ -1900 CaTiO₃ Na₂TiO₃ MgTiO₃ FeTiO₃ -1400

500 700 900 1100 1300 1500 Temperature (ᵒC) Figure 2. Comparison of Gibbs free energy of phase formation of several titanate species throughout the temperature range by Factsage 7.0™ database, with calcium titanate being most favourable.

Minerals 2019, 9, x FOR PEER REVIEW 4 of 20

The addition of lime, and subsequently exploration of other metal oxides into the sintering process through Meher and coworkers’ efforts [31,32,39,40], focuses on capturing the free silica during sintering (Reaction (4)). Adding metallurgical coke into the system, such as Reaction (5), can encourage the recovery of iron downstream by pre-reducing hematite and other iron species. It may also suppress potential unwanted product formation such as complex garnet-type product (e.g., 3(Ca,Fe,Mg)O(Al,Fe)2O3·3SiO2) that may entrap both iron and aluminum into its matrix. Figures 1 and 2 reports the Gibbs free energy of formation of different silicate phases and titanate phases at a range of temperature. Silica is more likely to be bonded together with barium or calcium species and predicted to form the more stable species of Ba3SiO5 or Ca3SiO5, whereas titania is more likely to form calcium-type bonds such as Ca3Ti2O7.

-4000

Ba₃SiO₅

Thousands -3500 Ca₃SiO₅

-3000 BaSi₂O₅ CaSiTiO₅ Ba₂SiO₄ Ca₂SiO₄

Mg₂SiO₄ Na₂Si₂O₅ -2500 Na₄SiO₄ Fe₂SiO₄ NaAlSiO₄ -2000 BaSiO₃ CaSiO₃ Gibbs Energy (kJ/mol) Energy Gibbs MgSiO₃

-1500 Na₂SiO₃ FeSiO₃

500 700 900 1100 1300 1500 Temperature (ᵒC) Figure 1. Comparison of Gibbs free energy of phase formation of several silicate species throughout Mineralsthe 2019temperature, 9, 571 range by Factsage 7.0™ database (Version 7.3, Thermfact/CRCT (Montreal, QC, 5 of 21 Canada) and GTT-Technologies (Aachen, Germany)), with silica notably favoring barium presence.

-4900 Ca₃Ti₂O₇

-4400 Ca₃Ti₂O₆

Thousands Na₂Ti₆O₁₃

-3900

Ca₂Ti₂O₅ -3400 Na₂Ti₂O₅

-2900 Ba₂TiO₄ FeTi₂O₅ Mg₂Ti₂O₅ Na₄TiO₄ -2400 Mg₂TiO₄ FeTi₂O₄ Fe₂TiO₄ Gibbs Energy (kJ/mol) Energy Gibbs FeTi₂O₅ -1900 CaTiO₃ Na₂TiO₃ MgTiO₃ FeTiO₃ -1400

500 700 900 1100 1300 1500 Temperature (ᵒC)

FigureFigure 2. 2. ComparisonComparison of of Gibbs Gibbs free free energy energy of of phase phase format formationion of of several several titanate titanate species species throughout throughout thethe temperature temperature range range by by Factsage Factsage 7.0™ 7.0™ databasedatabase,, with with calcium calcium titanate titanate being being most most favourable.

2. Materials and Methods

2.1. Characterisation of Bauxite Residue Bauxite residue (BR) from Aluminum of Greece’s (AoG) high temperature Bayer processing was sampled, homogenised and ground to below 150 µm. Chemical analysis of the bulk elements in BR is indicated in Table1. This was performed using the XRF glass disk fusion method with lithium tetraborate in AoG. 1 g of BR sample is fused as melt in platinum crucible with 1 g LiNO3 oxidizer, and mixture of flux (8 g 83.33% LiT and 12.82% LO-KBr). X-ray diffraction (XRD) and mineralogical analysis was performed using Brucker D6 Focus (XRD, Bruker, Billerica, MA, USA) equipment with integrated EVA software (Version 10.0) for identification of minerals. SEM images were obtained using Jeol6380LV Scanning Electron Microscope (SEM, JEOL, Tokyo, Japan) with INCA software for Energy Dispersive X-Ray Spectroscopy (EDS, EVO MA15 (ZEISS, Oberkochen, Germany) coupled with AZtec X-MAX 80 (Oxford Instruments, Abingdon, UK)) identification. Figure3 shows the XRD identification of mineralogical phases of AoG’s bauxite residue. AoG’s high temperature digestion process that uses quick lime to suppress silica levels, indicated similar phase transformations by the identification of hydrogrossular products in the bauxite residue as confirmed by literature. Diaspore, boehmite and gibbsite have been identified as the simple aluminum-bearing minerals, whereas cancrinite and hydrogrossular forms of desilication product are also detected. Chamosite mineral that exists originally from the bauxite ore, reacts slowly in Bayer cycle and remains in bauxite residue [41]. The composition of the Greek bauxite residue is representative of sampling of years 2010 to 2015 with an error margin of 1–10% for both chemical and mineral assays. Further variations of Greek bauxite residue is explained by Vind et al. that further expresses the trace elements alongside bulk elements in his characterization efforts [41]. Minerals 2019, 9, x FOR PEER REVIEW 5 of 20

2. Materials and Methods

2.1. Characterisation of Bauxite Residue Bauxite residue (BR) from Aluminum of Greece’s (AoG) high temperature Bayer processing was sampled, homogenised and ground to below 150 μm. Chemical analysis of the bulk elements in BR is indicated in Table 1. This was performed using the XRF glass disk fusion method with lithium tetraborate in AoG. 1 g of BR sample is fused as melt in platinum crucible with 1 g LiNO3 oxidizer, and mixture of flux (8 g 83.33% LiT and 12.82% LO-KBr). X-ray diffraction (XRD) and mineralogical analysis was performed using Brucker D6 Focus (XRD, Bruker, Billerica, MA, USA) equipment with integrated EVA software (Version 10.0) for identification of minerals. SEM images were obtained using Jeol6380LV Scanning Electron Microscope (SEM, JEOL, Tokyo, Japan) with INCA software for Energy Dispersive X-Ray Spectroscopy (EDS, EVO MA15 (ZEISS, Oberkochen, Germany) coupled with AZtec X-MAX 80 (Oxford Instruments, Abingdon, UK)) identification. Figure 3 shows the XRD identification of mineralogical phases of AoG’s bauxite residue. AoG’s high temperature digestion Minerals 2019, 9, 571 6 of 21 process that uses quick lime to suppress silica levels, indicated similar phase transformations by the identification of hydrogrossular products in the bauxite residue as confirmed by literature. Diaspore, boehmite and gibbsite haveTable been 1. Chemical identified analysis as the of AoGsimple bauxite aluminum-bearing residue. minerals, whereas cancrinite and hydrogrossular forms of desilication product are also detected. Chamosite mineral Major Metals Others that existsOxides originally from the bauxite ore, reacts slowly in Bayer cycle and remains in bauxiteLOI * residue [41]. The compositionFe2 Oof3 theAl Greek2O3 bauxiteSiO2 residueTiO2 is Narepresentative2O CaO of V sampling2O5 SO of3 years 2010 to 2015 with an errorwt % margin 43.51 of 1–10% 19.26 for both 6.50 chemical 5.49 and 2.81 mineral 9.59 assays. 0.17 Further 0.47 variations 9.40 of Greek bauxite residue is explained by Vind et *al. LOI: that Loss further on Ignition. expresses the trace elements alongside bulk elements in his characterization efforts [41].

LEGEND H H: Hematite - Fe2O3 140 Gi: Gibbsite - Al2O3·3H2O Go: Goethite - Fe2O3·H2O D: Diaspore - α-AlOOH P: Perovskite- CaTiO3 120 H B: Boehmite - γ-AlOOH Q: Quartz - SiO2 100 Gr: Grossular (Ca₃FeAl(SiO₄)((OH)₄) R: Rutile - TiO2 Ca: Cancrinite (Na₈(Al,Si)₁₂O₂₄(OH)₂.2H₂O A: Anatase - TiO2 80 Ch: Chamosite (Fe1.8Mg 0.2Al0.8(Si1.3Al0.7)O5(OH)4 Ca: Calc ite - CaCO3 H Gr H 60 H

Intensity Ca H H D H 40 Cn Q Cn D Cn Gi K Gr P Go K Redmud Gi G R DGr H H 20 Ch B P A Gi D P A o D 0 5 152535455565752-θ

Figure 3. XRD (X-ray diffraction) diﬀraction) mineralogical analysis of AoG bauxite residue.

The quantification of phasesTable 1. of Chemical AoG bauxite analysis residue of AoG isbauxite given residue. in Table 2, performed using XDB software designed by Sajó that uses aMajor full-profile Metals fit method that fits measured Others spectra to a simulated compositeOxides of mineral standards contained in the database [42]. This result was also verifiedLOI * in another Fe2O3 Al2O3 SiO2 TiO2 Na2O CaO V2O5 SO3 study [43], where where minerals were quantified in different stages of soda sintering and leaching wt % 43.51 19.26 6.50 5.49 2.81 9.59 0.17 0.47 9.40 process. The mineral phases shown in Table2 were utilized for the prediction of phases in sinter * LOI: Loss on Ignition. products by using the thermodynamic software Factsage 7.0™. However, the simplification of complex mineralsThe withquantification simpler ones of phases existing of inAoG the bauxite software re databasesidue is given was necessary. in Table 2, performed using XDB software designed by Sajó that uses a full-profile fit method that fits measured spectra to a simulated Table 2. XRD Mineralogical Quantification using XDB Software by Sajo [42,43]. composite of mineral standards contained in the database [42]. This result was also verified in another studyMiner [43], whereAnat whereBoeh mineralsGibb Hemawere quantifiedGoet Perovs in differentCancri stagesDias of Calsoda sinteringRut Gross and leachingChamo process.alogy The mineralase mite phasessite showntite in Tablehite 2 werekite utilizednite forpore the predictioncite ile of phasesular in sitesinter productsPhase by using the thermodynamic software Factsage 7.0™. However, the simplification of complexpercentage minerals1.0 with 2.1 simpler 1.0 ones 37.5 existing 5.2 in the 5.2 software 11.5 database 13.0 was 4.2 necessary. 0.5 14.6 2.1 %

2.2. Laboratorial Methods in Sintering Optimisation Figure4 indicates the procedure of sintering and leaching in this lab-scale study. Lab-grade reagents of Na2CO3 (99.8%, Chembiotin), CaO (>98%, Merck), BaO (99.5%, Alfa Aesar), MgO (>98%, Sigma Aldrich) and metallurgical coke (80.3% C) were used as reactive additives to the bauxite residue in this study. Dried homogenised bauxite residue (<150 µm) were mixed together with the additives and placed into alumina crucibles. For the lab-scaled experiments, the muﬄe furnace (Tmax: 1700 ◦C) shown in Figure5 is as below. Minerals 20192019,, 9,9, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 2020

Table 2. XRD Mineralogical Quantification usinging XDBXDB SoftwareSoftware byby SajoSajo [42,43].[42,43].

Minerr Anat Boeh Gibb Hem Goet Perov Cancrr Dias Cal Rut Gross Cham alogyalogy asease mite sitesite atiteatite hite skiteskite initeinite pore citecite ileile ular osite Phase percen 1.01.0 2.12.1 1.01.0 37.537.5 5.25.2 5.25.2 11.511.5 13.013.0 4.24.2 0.50.5 14.614.6 2.12.1 tagetage %%

2.2. Laboratorial Methods in Sintering Optimisation Figure 4 indicates the procedure of sintering and leaching in this lab-scale study. Lab-grade 2 3 reagentsreagents ofof NaNa2CO 3 (99.8%,(99.8%, Chembiotin),Chembiotin), CaOCaO (>98%,(>98%, Merck),Merck), BaOBaO (99.5%,(99.5%, AlfaAlfa Aesar),Aesar), MgOMgO (>98%,(>98%, Sigma Aldrich) and metallurgical coke (80.3% C) were used as reactive additives to the bauxite residueresidue inin thisthis study.study. DriedDried hohomogenised bauxite residue (<150 μm) were mixed together with the additives and placed into alumina crucibles. For the lab-scaled experiments, the muffle furnace (Tmax: additivesMinerals 2019 and, 9, 571placed into alumina crucibles. For the lab-scaled experiments, the muffle furnace (7T ofmax 21: 1700 °C) shown in Figure 5 is as below.

FigureFigure 4. 4. ProcedureProcedure for for sintering sintering of of AoG’s AoG’s Greek Greek bauxite bauxite residue.

10001000 900900 800800

C) 700 o C) 700 o 600600 500500 400400 300 Temperature ( Temperature 300 Temperature ( Temperature y = 2837.6e-0.395x 200 yy == 1758.4x1758.4x -- 1169.2 1169.2 y = 2837.6e-0.395x 200 R² = 0.9658 R² = 0.9978 R² = 0.9658 100100 00 024681012024681012 Time (Hours) Time (Hours) FigureFigure 5. 5. TemperatureTemperature heating heating profile proﬁle of muffle muﬄe furnace.

The heating rate was kept to 30–35 C per min, and after sintering within set holding time, indicated The heating rate was kept to 30–35◦ °C per min, and after sintering within set holding time, in the plateau region of temperature profile in Figure5, was then left to cool overnight. The cooling indicatedindicated inin thethe plateauplateau regionregion ofof temperaturetemperature profileprofile inin FigureFigure 5,5, waswas thenthen leftleft toto coolcool overnight.overnight. TheThe kinetics were out of the scope of this study. Introduction of inert atmosphere for carbon-reduction coolingcooling kineticskinetics werewere outout ofof thethe scopescope ofof thisthis ststudy. Introduction of inertt atmosphereatmosphere forfor carbon-carbon- aided sintering process with 5 N L/min argon flow rate, where stoichiometric to 50% excess carbon reductionreduction aidedaided sinteringsintering processprocess withwith 55 NN L/minL/min argonargon flowflow rate,rate, wherewhere stoichiometricstoichiometric toto 50%50% excessexcess required to reduce iron were used. Current leaching conditions during sintering optimization were carboncarbon requiredrequired toto reducereduce ironiron werewere used.used. CurrentCurrent leachingleaching conditionsconditions duringduring sinteringsintering optimizationoptimization kept at 80 C for 4 h, at 240 rpm stirring rate, solid-to-liquid ratio of 1.5 g/100 mL (1:66) in 0.1 M NaOH were kept ◦at 80 °C for 4 h, at 240 rpm stirring rate, solid-to-liquid ratio of 1.5g/100 mL (1:66) in 0.1 M solution. Leachates were then filtered, diluted and sent to Atomic Absorption Spectroscopy (AAS, NaOH solution. Leachates were then filtered, dilutedted andand sentsent toto AtomicAtomic AbsorptionAbsorption SpectroscopySpectroscopy Waltham, MA, USA) for chemical analysis of aluminum and silica. Leached residue was collected and sent for XRF (Perform’X, Thermo Fisher Scientific™, Waltham, MA, USA) analysis, with mass balance of filtered solids accounted for sodium analysis to reduce experimental error and interference during AAS analysis when leached in 0.1 M NaOH. Two experimental series were performed. In the first the soda sintering process was studied in the presence of 50% excess of soda over stoichiometric amount required for sodium aluminate conversion and in the absence of any additives. In the second the effect of additives was studied. Particularly, AoG’s bauxite residue was subjected to 20 g of additives (divalent metal oxides) to 100 g of bauxite residue, with 25 g of Na2CO3 (~50% excess soda amounts required to form NaAlO2). Minerals 2019, 9, x FOR PEER REVIEW 7 of 20

(AAS, Waltham, MA, USA) for chemical analysis of aluminum and silica. Leached residue was collected and sent for XRF (Perform’X, Thermo Fisher Scientific™, Waltham, MA, USA) analysis, with mass balance of filtered solids accounted for sodium analysis to reduce experimental error and interference during AAS analysis when leached in 0.1 M NaOH. Two experimental series were performed. In the first the soda sintering process was studied in the presence of 50% excess of soda over stoichiometric amount required for sodium aluminate conversion and in the absence of any additives. In the second the effect of additives was studied. Particularly, AoG’s bauxite residue was Mineralssubjected2019 to, 9 ,20 571 g of additives (divalent metal oxides) to 100 g of bauxite residue, with 25 g of Na82CO of 213 (~50% excess soda amounts required to form NaAlO2).

3. Results and Discussion

3.1. Effect Eﬀect of Sintering Temperature andand RetentionRetention TimeTime onon MetalsMetals RecoveriesRecoveries Figure6 6 shows shows thethe thermodynamic thermodynamic predictionspredictions ofof solidsolid phase’sphase’s evolutionevolution duringduring sinteringsintering temperaturetemperature increase.increase. For For performing performing this this thermo thermodynamicdynamic analysis, the real desilicationdesilication phasesphases existing in AoG’s AoG’s Bauxite Bauxite residue residue (namely, (namely, cancrinite, cancrinite, so sodalitedalite and and hydrogrossular) hydrogrossular) were were replaced replaced by byones ones (such (such as Ferrocordierite, as Ferrocordierite, Grossularite Grossularite and andAlbite) Albite) which which was available was available in the in Factsage the Factsage 7.0TM TM 7.0software.software.

100% (FeO)₂(TiO₂) CaFe₂O₄ 100% 90% CaFe₄O₇ CaFe₂O₄ CaFe₄O₇ NaFe₂O₃ 90% 80% NaFeO₂ 80% Fe₂O₃ 70% Fe₂O₃ 70% CaTiO₃ Na₃Fe₅O₉ 60% CaTiO₃ 60% Ca₂Fe₂O₅ 50% Na₄TiO₄ NaFeO₂ 50% Na₄TiO₄ Ca₂Al₂SiO₇ CaAl₄O₇ 40% 40% Ca₂Al₂SiO₇ NaAlSiO₄ CaAl₄O₇ 30% NaAlSiO₄ 30% NaAlSiO₄ NaAl₉O₁₄ (Nepheline) NaAl₉O₁₄ (Nepheline) NaAlSiO₄

Phase Distribution Phase Distribution (%) (Carnegeite) 20% 20% Na₂Al₁₂O₁₉ (Carnegeite) CaCO₃ Na₂Al₁₂O₁₉ CaCO₃ 10% 10% NaCO₃ NaAlO₂ NaCO₃ NaAlO₂ 0% 0% 500 600 700 800 900 1000 1100 1200 1300 1400 1500 500 600 700 800 900 1000 1100 1200 1300 1400 1500 Temperature (ᵒC) Temperature (ᵒC) (a) (b)

100% 100% CaFe₂O₄ CaFe₄O₇ (FeO)₂(TiO₂)Na₂Ca₂SiO₄ Na₂Ca₂Si₂O₇ 90% 90% Ca₂SiO₄ NaFe₂O₃ 80% Fe₂O₃ Na₃Fe₅O₉ 80% Na₄TiO₄ NaFeO₂ 70% 70% Fe₂O₃ CaTiO₃ Fe (s1) Fe (s2) 60% 60% Fe (s1) 50% Ca₂Al₂SiO₇ Na₄TiO₄ 50% Ca₂Al₂SiO₇ 40% CaAl₄O₇ 40% NaAlSiO₄ NaAlSiO₄ NaAlSiO₄ 30% 30% C (Nepheline) (Carnegeite) NaAl₉O₁₄ (Nepheline) NaAlSiO₄ Phase Distribution Phase Distribution (%) NaAl₉O₁₄ 20% Na₂Al₁₂O₁₉ (Carnegeite) 20% CaCO₃ CaCO₃ 10% NaAlO₂ 10% NaAlO₂ NaCO₃ NaCO₃ 0% 0% 500 600 700 800 900 1000 1100 1200 1300 1400 1500 500 600 700 800 900 1000 1100 1200 1300 1400 1500 Temperature (ᵒC) Temperature (ᵒC) (c) (d)

Figure 6.6. Thermodynamic predictions of sinter phase formationsformations with didifferentﬀerent representativesrepresentatives ofof 2 4 5 18 3 2 3 12 desilication productproduct suchsuch as as ( a(a)) Ferrocordierite, Ferrocordierite, Fe Fe2AlAl4SiSi5OO18 ((bb)) Glossularite,Glossularite, Ca Ca3AlAl2SiSi3O12 (c)(c) Albite, Albite, 3 8 3 8 NaAlSi3O8 ((dd)) Albite, Albite, NaAlSi NaAlSi3OO8 withwith 50% 50% excess excess of of soda soda over over stoichiometric stoichiometric amount. amount.

As seen seen in in Figure Figure 6,6 ,at at 50% 50% stoichiometric stoichiometric excess excess of soda of soda in sintering in sintering system system achieves achieves only partial only partialconversion conversion into sodium into sodium aluminat aluminatee species, species, and sodium and sodium aluminosilicate aluminosilicate forms forms of nepheline of nepheline and andcarnegeite carnegeite were werestill present still present in the insystem. the system. NaAl9O NaAl14 and9 ONa142Aland12O Na19 represented2Al12O19 represented unreacted β unreacted-alumina β-alumina and β”-alumina phases respectively. Silica presence was predicted to be a constraint for the total transformation of aluminum bearing phases to leachable sodium aluminate which is in agreement with previous studies on soda sintering at high temperatures [12]. It is interesting to note that despite glossularite’s high calcium content when replaced as a desilication product, the result of the predictive transformation was still the same. The calcium showed an affinity towards iron, with calcium ferritic sinter products (CaFe2O4, CaFe4O7 and Ca2Fe2O5) formed instead of binding towards the available silica. This agrees with Hodge et al., noting that calcium has the tendency to preferentially react with iron phases instead of silica during bauxite residue soda sintering process [43]. In Figure6d, albite Minerals 2019, 9, x FOR PEER REVIEW 8 of 20 and β’’-alumina phases respectively. Silica presence was predicted to be a constraint for the total transformation of aluminum bearing phases to leachable sodium aluminate which is in agreement with previous studies on soda sintering at high temperatures [12]. It is interesting to note that despite glossularite’s high calcium content when replaced as a desilication product, the result of the predictive transformation was still the same. The calcium showed an affinity towards iron, with calcium ferritic sinter products (CaFe2O4, CaFe4O7 and Ca2Fe2O5) formed instead of binding towards the available silica. This agrees with Hodge et al., noting that calcium has the tendency to preferentiallyMinerals 2019, 9, 571react with iron phases instead of silica during bauxite residue soda sintering process9 of 21 [43]. In Figure 6d, albite was used as the extrapolative value of desilication product, and the addition of carbon as reducing agent promoted iron reduction and was predicted to suppress the calcium– ironwas usedinteraction, as the extrapolativethough it presents value of only desilication a slight product, increase and of the sodium addition aluminate of carbon phase as reducing over nepheline/carnegeiteagent promoted iron species. reduction and was predicted to suppress the calcium–iron interaction, though it presentsFigure only 7a ashows slight the increase metal ofrecoveries sodium aluminateas a function phase of sintering over nepheline temperature/carnegeite at 2 h species. retention time and 5%Figure stoichiometric7a shows the excess metal of recoveriessoda in bauxite as a functionresidue. ofAt sintering900 °C it reaches temperature at 55% at due 2 h to retention entrapment time ofand part 5% of stoichiometric aluminum in excess sodium of soda aluminosilicate in bauxite residue. and calcium At 900 ◦ Caluminosilicate it reaches at 55% species due to which entrapment are by of definitionpart of aluminum very difficult in sodium soluble aluminosilicate in diluted andNaOH calcium solutions. aluminosilicate This conclusion species which is supported are by definition by the behaviourvery difficult of solublesilica which in diluted was recovered NaOH solutions. at a valu Thise lower conclusion than 20% is supported showing by that the the behaviour dissolution of silica of silicatewhich wasphases recovered formed at during a value sintering lower than is limited 20% showing in diluted that NaOH the dissolution solutions of hindering silicate phases the alumina formed recovery.during sintering In Figure is limited7b, once in excess diluted soda NaOH levels solutions were investigated hindering the and alumina 50% stoichiometric recovery. In Figureexcess 7ofb, sodaonce in excess bauxite soda residue levels werewas used, investigated it was noted and 50% that stoichiometric increased retention excess oftime soda ofin sintering bauxite residuefrom 2 to was 4 hused, displayed it was notedvery that slight increased change retention towards time ofreco sinteringvery. Aluminum from 2 to 4 hrecovery displayed agrees very slight with change the thermodynamictowards recovery. predictions. Aluminum recovery agrees with the thermodynamic predictions.

100 100 90 %Al₂O₃-AAS %SiO₂-AAS 90 %Al₂O₃-AAS %SiO₂-AAS 80 80 70 70 60 60 50 50 40 40 Recovery (%) 30 Recovery (%) 30 20 20 10 10 0 0 800 900 1000 1100 1.5 2.5 3.5 4.5 Temperature (⁰C) Time (Hour of Sintering)

(a) (b)

Figure 7. Recovery of Al2O3 and SiO2 for (a) temperature-variable sintering experiments with 5% Figure 7. Recovery of Al2O3 and SiO2 for (a) temperature-variable sintering experiments with 5% 2 3 2 3 stoichiometricstoichiometric excess excess of of Na Na2COCO,3 and, and (b (b) )time-variable time-variable with with 50% 50% stoichiometric stoichiometric excess excess of of Na NaCO2CO.3 .

3.2.3.2. Effect Eﬀect of of Na Na2CO2CO3 3ExcessExcess on on Metals Metals Recoveries Recoveries

FigureFigure 88aa shows the thermodynamic predictionspredictions ofof sinter phasephase formationformation whenwhen sodasoda (Na(Na22COCO33) ) isis added added in in excess excess to to stoichiometric stoichiometric amounts. amounts. The The so solidlid sinter sinter phase phase distributi distributionon of of sodium sodium aluminate aluminate whenwhen soda soda is is introduced introduced in inexcess excess is noted is noted reach reach a plateau a plateau of equilibrium of equilibrium with nepheline with nepheline phase when phase thewhen soda the excess soda excess is equal is equal or greater or greater than than 200%. 200%. The The metastable metastable states states of NaNa22Ca3Al1616OO2828 andand NaNa22CaCa8Al8Al6O6O18 18werewere suppressed suppressed and and excluded excluded from from the the th thermodynamicermodynamic prediction prediction in in the the software. software. TheThe calcium contentcontent was was observed observed to partiallyto partially form fo calciumrm calcium aluminate aluminate and change and change into sodium into sodium calcium calciumsilicate insteadsilicate ofinstead beginning of beginning at 100% excess. at 100% Hematite excess. isHematite predicted is to predicted initially bindto initially preferentially bind preferentiallytowards calcium towards and thencalcium shift and to sodium then shift instead, to sodium forming instead, potential forming soluble potential sodium soluble ferrite sodium phases. ferriteActual phases. experimental Actual results experimental further conﬁrmedresults further the linear confirmed increase the until linear reaching increase plateau, until albeit reaching at an plateau,earlier range albeit with at an levels earlier of 67–70% range beingwith reachedlevels of when 67–70% 50% excessbeing ofreached stoichiometric when 50% soda excess required of stoichiometricwas added. soda required was added. Figure 9 shows the XRD mineral identification of sinters and leached residue at 50% excess soda addition. Sodium aluminum silicate (NaAlSiO4) observations in leached residue and sinters

Minerals 2019, 9, x FOR PEER REVIEW 9 of 20 confirmed that the nepheline-sodium aluminate equilibrium does exist. The consumption of soda reagent into iron products such as sodium iron titanium oxide (NaFeTiO4) and calcium products such as harmunite (CaFe2O4) which were the unwanted side reactions further fortifies the existence of nepheline-sodium aluminate equilibrium despite soda excess levels during sintering, though several overlapping peaks of both species lean to a larger confirmed harmunite presence with peaks from 40–48 degrees. Though silica is observed to be within the range of 10–20%, silica dissolution is still observedMinerals 2019 in, 9the, 571 leaching system. Sodium recovery at 80% to 90% range throughout the experimental10 of 21 data indicates that a variety of sodium-soluble products exists.

100% 100 Minerals 2019, 9, x FORCaFe₂O₄ PEER REVIEW 9 of 20 90% 90 80% Na₃Fe₅O₉ 80 NaFeO₂ confirmed70% that the nepheline-sodium aluminate equilibrium70 does exist. The consumption of soda Fe₂O₃ 60 4 reagent60% into iron products such as sodium iron titanium oxide (NaFeTiO ) and calcium products%Al₂O₃-AAS such as harmunite50% (CaFe2O4) whichNa₄TiO₄ were the unwanted side50 reactions further fortifies the%SiO existence₂-AAS of CaAl₄O₇ nepheline-sodium40% aluminate equilibriumNa₂CaSiO₄ despite soda 40excess levels during sintering, though%Na₂O-XRF several 30% Recovery (%) 30 overlapping peaksNaAlSiO₄ of both species lean to a larger confirmed20 harmunite presence with peaks from Phase Distribution (%) Distribution Phase 20% 40–48 degrees.(Nepheline) Though silica is observed to be within 10the range of 10–20%, silica dissolution is still 10% observed in the leaching system. SodiumNaAlO₂ recovery at 80%0 to 90% range throughout the experimental 0% 0 50 100 150 200 0 5 data indicates that10 15 a 25 variety50 of sodium-soluble products exists. 100 200 300 400 500 600 700 800 900 Excess Stoichiometric Addition of Na₂CO₃ (%) 1000 Excess Na₂CO₃ (%) 100% 100 CaFe₂O₄ (a) (b) 90% 90 80% Na₃Fe₅O₉ 80 FigureFigure 8. 8. ((aa)) Thermodynamic Thermodynamic predictions predictionsNaFeO₂ of of sinter sinter phase phase formation formation and and ( (bb)) results results of of experimental experimental 70% 70 recoveryrecoveryFe₂O₃ of of Al Al2OO3, SiO, SiO2, and, and Na Na2O atO the at the range range of excesses of excesses of stoichiometric of stoichiometric Na2 NaCO3CO additionsadditions at 900 at 60% 2 3 2 2 60 2 3 °C900 forC 2 forh. 2 h. %Al₂O₃-AAS 50% ◦ Na₄TiO₄ 50 %SiO₂-AAS CaAl₄O₇ 40% Na₂CaSiO₄ 40 %Na₂O-XRF 0: Quartz (SiO₂) 8: Grossular (Ca₃FeAl(SiO₄)((OH)₄) Figure9 shows the XRD1: Hematite mineral (Fe₂O₃) identiﬁcationRecovery (%) 30 of9: Anatase sinters (TiO₂) and leached residue at 50% excess 30% 2: Goethite (α-FeOOH) NaAlSiO₄ 2010: Rutile (TiO₂) soda (%) Distribution Phase 20% addition. Sodium aluminum3: Cancrinite (Na₈(Al,Si) silicate₁₂O₂₄(OH)₂.2H (NaAlSiO₂O 411:) observationsPerovskite (CaTiO₃) in leached residue and sinters (Nepheline) 4: Chamosite 1012: Calcite (CaCO₃) conﬁrmed10% that the nepheline-sodium(Fe1.8Mg 0.2Al0.8(Si1.3Al0.7)O aluminate5(OH)4) equilibrium13: Sodium aluminate does (NaAlO exist.₂) The consumption of soda 5: Boehmite (γ-AlOOH)NaAlO₂ 14: Sodium aluminium silicate (NaAlSiO₄)

16,15 0 reagent0% into iron products6: suchGibbsite (Al(OH) as sodium₃) iron titanium15: oxide Sodium ferrotitanate (NaFeTiO (NaFeTiO)₄) and calcium products such 7: Diaspore (α-AlOOH) 16: 0Harmunite (CaFe₂O 50₄) 4 100 150 200 0 5 1,15 10 15 25 50 1,11 as harmunite (CaFe O )100 which200 300 400 were500 600 the700 unwanted800 900 side reactionsExcess Stoichiometric further fortiﬁes Addition of theNa₂CO existence₃ (%) of 1000 2 4 15, 16 11 16,15 1 1 16 14 Excess Na 11

₂CO₃ (%) 1 14 16 16 1 nepheline-sodium aluminate equilibrium despite14 soda excess levels during sintering, though several

Leached 16 overlapping peaks of both(a) species lean13 to a larger conﬁrmed harmunite(b presence) with peaks from

Residue 1,11 13 1 16,15

40–48 degrees. Though silica is observed to13,14 be within the range of 10–20%, silica dissolution is still 1 11 1 11 1 16 1 14 15,16 16 14 Figure 8. (a) Thermodynamic predictions of sinter phase formation16 and (b) results of experimental observed in the leaching system. Sodium recovery at 80% to 90% range throughout the experimental recovery of Al2O3, SiO2, and Na2O at the range of excesses of stoichiometric Na2CO3 additions at 900

Sinter 1,8

data indicates that a variety of sodium-soluble1,11 products exists. °C for 2 h. 7 1 7 12 5 8 3 1 7 7 3 6 1 1 1 0 3 10 8 7 8,10 11,12 5 4 8 8 8 2 8 8 9 0: Quartz (SiO₂) 8: Grossular (Ca₃FeAl(SiO₄)((OH)₄) 1:Bauxite Hematite Residue (Fe₂O₃) 9: Anatase (TiO₂) 2: Goethite (α-FeOOH) 10: Rutile (TiO₂) 103: Cancrinite (Na 20₈(Al,Si)₁₂O₂₄(OH)₂ 30.2H₂O 4011: Perovskite 50(CaTiO₃) 60 2-θ 4: Chamosite 12: Calcite (CaCO₃) degrees (Fe1.8Mg 0.2Al0.8(Si1.3Al0.7)O5(OH)4) 13: Sodium aluminate (NaAlO₂) 5: Boehmite (γ-AlOOH) 14: Sodium aluminium silicate (NaAlSiO₄) Figure 9. XRD of mineralogical6: Gibbsite (Al(OH) phases₃) for 50%16,15 stoichiometric15: Sodium ferrotitanate excess (NaFeTiONa2CO₄) 3 at 900 °C and for 2 h. 7: Diaspore (α-AlOOH) 16: Harmunite (CaFe₂O₄) 1,15 Red font: Existing minerals in bauxite residue;1,11 black font: Transformed sinter and leached minerals. 15, 16 11 16,15 1 1 16 14 11 1 14 16 16 1 14 3.3. Effect of Carbon Additions on Metals Recoveries Leached 16 13

Residue 1,11 13 1 16,15 13,14 1 11 After fixing to 50% excess of soda which reduces the amount of soda1 required per aluminum 11 1 16 1 14 15,16 16 14 recovery, the study next focused on the suppression of side16 reactions through the reduction of iron

Sinter 1,8 with metallurgical coke additions as a carbon1,11 source in inert conditions. The thermodynamic 7 1 7 12 5 8 3 1 7 7 3 6 1 1 1 0 3 prediction of sinter phases in Figure 10a indicate10 s that the equilibrium that exists between leachable 8 7 8,10 11,12 5 4 8 8 8 2 8 8 9 sodium aluminate and nephelineBauxite Residue form still exists despite the conversion of hematite or goethite into metallic iron. 10 20 30 40 50 60 2-θ Figure 11 shows the XRD confirmations of mineral phases presentdeg reesin the sinter product and leachedFigure residue. 9. XRD Part of mineralogicalof the iron formed phases forsodium 50% stoichiometric iron oxide while excess hematite Na2CO3 atwas 900 partially °C and for reduced 2 h. to Figure 9. XRD of mineralogical phases for 50% stoichiometric excess Na2CO3 at 900 ◦C and for 2 h. Red font: Existing minerals in bauxite residue; black font: Transformed sinter and leached minerals. Red font: Existing minerals in bauxite residue; black font: Transformed sinter and leached minerals. 3.3.3.3. Effect Eﬀect of of Carbon Carbon Additions Additions on on Metals Metals Recoveries Recoveries AfterAfter fixing ﬁxing to to 50% 50% excess excess of of soda soda which which redu reducesces the the amount amount of of soda soda required required per per aluminum aluminum recovery,recovery, the the study study next next focused focused on onthe the suppression suppression of side of sidereactions reactions throug throughh the reduction the reduction of iron of withiron withmetallurgical metallurgical coke cokeadditions additions as a as carbon a carbon source source in ininert inert conditions. conditions. The The thermodynamic thermodynamic predictionprediction of of sinter sinter phases phases in in Figure Figure 10a 10a indicate indicatess that that the the equilibrium equilibrium that that exists exists between between leachable leachable sodium aluminate and nepheline form still exists despite the conversion of hematite or goethite into metallic iron. Figure 11 shows the XRD confirmations of mineral phases present in the sinter product and leached residue. Part of the iron formed sodium iron oxide while hematite was partially reduced to

Minerals 2019, 9, x FOR PEER REVIEW 10 of 20 magnetiteMinerals 2019 and, 9, wustite.x FOR PEER Though REVIEW the calcium iron oxide species were now suppressed, as it can be10 seen of 20 from Figure 10a thermodynamic predictions, there was a secondary issue of gehlenite (Ca2Al2SiO7) formationMineralsmagnetite2019 on and, 9 ,top 571 wustite. of nepheline Though throughout the calcium theiron range oxide ofspecies stoichiometric were now suppressed,carbon additions, as it can causing be11 seen of 21 additionalfrom Figure aluminum 10a thermodynamic losses downstream. predictions, there was a secondary issue of gehlenite (Ca2Al2SiO7) formation on top of nepheline throughout the range of stoichiometric carbon additions, causing sodiumadditional aluminate aluminum and losses nepheline downstream. form still exists despite the conversion of hematite or goethite into 100% metallic iron.CaFe₂O₄ Na₂CaSiO₄ 100 90% Na₄TiO₄ 90 80% 100% 80 Na₃Fe₅O₉CaFe₂O₄ Na₂CaSiO₄ 100 70% 90% NaFe₂O₃ Fe 7090 60% Na₄TiO₄ 80% 60 %Al₂O₃-AAS Na₃Fe₅O₉ 80 50% 70% Ca₂Al₂SiO₇ 50 Fe 70 %SiO₂-AAS 40% CaAl₄O₇NaFe₂O₃ 60% 4060 %Al₂O₃-AAS

30% NaAlSiO₄ Recovery (%) %Na₂O-XRF 50% 30

Phase Distribution (%) Distribution Phase 50 (Nepheline)Ca₂Al₂SiO₇ %SiO₂-AAS 20% CaAl₄O₇ 40% 2040 10%

Recovery (%) %Na O-XRF 30% NaAlO₂NaAlSiO₄ 1030 ₂

Phase Distribution (%) Distribution Phase 0% (Nepheline) 20% 0 1 2 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 20 10% 0 1020304050 Stoichiometric carbon-to-ironNaAlO₂ (C:Fe2O3) ratio addition 10 Excess Carbon (%) 0% Fe2O3 + 3C → 2Fe + 3CO 0 1 2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 0 1020304050 Stoichiometric carbon-to-iron(a) (C:Fe2O3) ratio addition (b) Excess Carbon (%) Fe2O3 + 3C → 2Fe + 3CO Figure 10. (a) Thermodynamic predictions of sinter phase formation and (b) results of experimental (a) (b) recovery of Al2O3, SiO2, and Na2O at the range of excesses of stoichiometric carbon additions (based onFigure Carbon:Fe 10.10. ((a2O)) ThermodynamicThermodynamic3 ratio from bauxite predictions residue) with of sinter 50% phasephaseexcess formation soda, 900 and°C for ((b )2) resultsresultsh. ofof experimentalexperimental

recovery ofof AlAl22O3,, SiO SiO2,, and and Na Na22OO at at the the range range of of excesses of stoi stoichiometricchiometric carbon additions (based Figure 1 shows2 3 the mineralogy of sinter and leached residue, confirming the theoretical on Carbon:Fe2O3 ratioratio from from bauxite bauxite residue) residue) with with 50% excess soda, 900 °C◦C for 2 h. formations of sodium aluminum silicates and gehlenite, though latter phases were minor. The suppressionFigure of111 solubleshows thethesodium-iron XRDmineralogy conﬁrmations products of sinter occurs of mineraland pastleached phases 0.7 stoichiometricresidue, present confirming in the ratio sinter indicatedthe product theoretical that and carbonleachedformations addition residue. of sodiumis Part favorable of aluminum the iron at formedstoichiometric silicates sodium and ratio irongehl enite, oxideonwards. whilethough This hematite latter was phases wasalso partiallyconfirmed were minor. reduced in theThe to experimentalmagnetitesuppression and resultsof wustite. soluble in ThoughFigure sodium-iron 10b, the calciumand products an ironincr ease oxideoccurs from species past 68% 0.7 were tostoichiometric now75% suppressed,is observed ratio as inindicated italuminum can be seenthat recovery,fromcarbon Figure additionthough 10a thermodynamicsodiumis favorable recovery at predictions,stoichiometric reduces to there 3% ratio waslesser onwards. a secondaryamount Thisthat issue wasthe of previous gehlenitealso confirmed soda (Ca2 Alexcess in2SiO the7 ) additionsformationexperimental in onatmospheric results top of nephelinein environment. Figure throughout10b, andSilica an amount theincr rangeease remains from of stoichiometric relatively68% to 75% unchanged carbonis observed additions, within in thealuminum causing range ofadditionalrecovery, 18% recovery. though aluminum sodium losses recovery downstream. reduces to 3% lesser amount that the previous soda excess additions in atmospheric environment. Silica amount remains relatively unchanged within the range 0: Quartz (SiO₂) 11: Perovskite (CaTiO₃) of 18% recovery. 1: Hematite (Fe₂O₃) 12: Calcite (CaCO₃) 2: Goethite (α-FeOOH) 13: Sodium aluminate (NaAlO₂) 3: Cancrinite (Na₈(Al,Si)₁₂O₂₄(OH)₂.2H₂O 14: Jadeite (NaAlSi₂O₆) 4: Chamosite0: Quartz (SiO (Fe1.8₂) Mg 0.2Al0.8(Si1.3Al0.7)O5(OH)4) 15: Gehlenite (Ca₂Al₂SiO₇) 5: Boehmite (γ-AlOOH) 11: Perovskite (CaTiO₃) 1: Hematite (Fe₂O₃) 16:12: Wustite Calcite (FeO) (CaCO₃) 6: Gibbsite2: Goethite (Al(OH) (α-FeOOH)₃) 17: Magnetite (Fe₃O₄)/Maghemite(γ-Fe₂O₃) 7: Diaspore (α-AlOOH) 13: Sodium aluminate (NaAlO₂) 3: Cancrinite (Na₈(Al,Si)₁₂O₂₄(OH)₂.2H₂O 18:14: Sodium Jadeite iron (NaAlSi oxide ₂O₆(Na)₅FeO₄)

8: Grossular (Ca₃FeAl(SiO₄)((OH)₄) 11,17,1 4: Chamosite (Fe1.8Mg 0.2Al0.8(Si1.3Al0.7)O5(OH)14 4) 19:15: Lorenzenite Gehlenite (Na (Ca₂₂Al₂SiO₇)Ti₂SiO₉) 9: Anatase5: Boehmite (TiO (γ-AlOOH)₂) 11 14 16: Wustite (FeO) 10:6: Rutile Gibbsite (TiO ₂(Al(OH)) ₃) 17 17 17: Magnetite (Fe₃O₄)/Maghemite(γ-Fe11 ₂O₃) 17,1 17 19 14 16 7: Diaspore (α-AlOOH) 14,1 15 16 18 18: Sodium iron oxide (Na₅FeO19 ₄)

8: Grossular (Ca₃FeAl(SiO₄)((OH)₄) 11,17,1

14 19: Lorenzenite (Na₂Ti₂SiO₉) Leached9: Anatase (TiO₂) 11 14 17

10: Rutile (TiO₂) 13

Residue 17 11 11,1 13,14 17,1 17 19 14 16 14,1 15 16 18 19 11 17,1 11 17 17 16 15 16 18 17 14,1 14 15 Leached 19

Residue 13 11,1 Sinter 13,14 1,8 11 17,1 1,11 11 17 17 16 15 16 18 17 14,1 14 15 19 7 1 7 12 5 8 1 3 7 7 3 6 1 1 1 0 3 10 8 7 8,10 11,12 5 8 8 8 2 8 9 Sinter 8 4 1,8 Bauxite Residue 1,11 7 1 7 12 5 8 1 3 7 7 3 6 1 1 1 0 3 10 8 7 8,10

11,12 2-θ 5 8 8 8 2 8 9 8

104 20 30 40 50 60 degrees Bauxite Residue FigureFigure 11. 11. XRDXRD of ofmineralogical mineralogical10 phases 20phases for for stoichiometric 30 stoichiometric 40 carbon carbon addition 50 addition needed 60needed2-θ to to produce produce Fe Fe from from degrees ReactionReaction (5), (5), with with 50% 50% stoichiometric stoichiometric excess excess Na Na2CO2CO3 at3 at inert inert conditions conditions at at 900 900 °C◦ Cand and for for 2 h. 2h. Red Red font: font: ExistingExistingFigure minerals11. minerals XRD of in inmineralogical bauxite bauxite residue; residue; phases black black for font: font:stoichiometric Transformed Transformed carbon sinter sinter addition and and leached leached needed minerals. minerals. to produce Fe from Reaction (5), with 50% stoichiometric excess Na2CO3 at inert conditions at 900 °C and for 2 h. Red font: 3.4. EffectFigureExisting of CaO1 showsminerals Additions the in mineralogybauxite on Phase residue; Transformations of sinter black andfont: leached Transformed during residue, Sintering sinter conﬁrming Process and leached the minerals. theoretical formations of sodium aluminum silicates and gehlenite, though latter phases were minor. The suppression of Following soda and carbon additions, the addition of lime into the sintering process of 20 g CaO soluble3.4. Effect sodium-iron of CaO Additions products on Phase occurs Transformations past 0.7 stoichiometric during Sintering ratio indicatedProcess that carbon addition is to 100 g bauxite residue (80% excess to stoichiometric requirements to form Ca2SiO4) is necessary to favorable at stoichiometric ratio onwards. This was also conﬁrmed in the experimental results in Following soda and carbon additions, the addition of lime into the sintering process of 20 g CaO Figure 10b, and an increase from 68% to 75% is observed in aluminum recovery, though sodium recovery to 100 g bauxite residue (80% excess to stoichiometric requirements to form Ca2SiO4) is necessary to

Minerals 2019, 9, x FOR PEER REVIEW 11 of 20 investigate the consumption of free and bound silica in desilication products, relying on higher thermodynamic affinity with calcium. Figure 12 shows the thermodynamic predictions of sinter phases at excess lime additions. The thermodynamic predictions made by Factsage 7.0™, however, shows that calcium indeed reacts with iron to form CaFe2O4 and Ca2Fe2O5, when iron is available in atmospheric conditions at 900 °C. Minerals 2019, 9, 571 12 of 21 Compared to Figure 8a, thermodynamics of soda only addition into the sinter of bauxite residue notes that sodium preferably reacts to form nepheline species and sodium titanate species at lower thanreduces 50% to soda 3% lesser excess amount levels, thatwhereas, the previous calciumsoda is bound excess to additions either aluminum in atmospheric or iron environment. initially. At betweenSilica amount 50% and remains 200% relatively soda excess unchanged levels, sodium within the ferrite range species of 18% of recovery. intermediate Na3Fe5O9 forms, transforming into NaFeO2 at even higher soda concentrations. Sodium ferrite then dissolves back as sodium,3.4. Eﬀect hence of CaO phases Additions do not on Phaseinhibit Transformations system. Also at during higher Sintering soda levels Process (Figure 8a), soda and calcium is predictedFollowing to preferentially soda and carbon react, additions, still existing the addition in equilibrium of lime intowith the nepheline sintering species. process In of Figure 20 g CaO 12, theto 100 excess g bauxite CaO predicted residue (80% to be excess added to in stoichiometric the sintering system requirements is divided to form into Catwo2SiO amounts,4) is necessary Figure 12a,to investigate one that encourages the consumption monocalcium of free silicate and bound formation silica and in desilication the other, Figure products, 12b, relyingdicalcium on silicate higher instead.thermodynamic Increased aﬃ additionsnity with calcium.of lime only Figure encourage 12 shows calcium the thermodynamic to complex into predictions Ca2Fe2O of5 stable sinter species phases instead,at excess with lime additions.nepheline Thephase thermodynamic still existing predictionsin equilibrium made amounts by Factsage to sodium 7.0™, however, aluminate shows species. that Grossularcalcium indeed species reacts in the with form iron of to Ca form2Al2 CaFeSiO7 2andO4 and sodium Ca2Fe calcium2O5, when silicate iron species is available (Na2 inCaSiO atmospheric4) exists whenconditions CaO atis added 900 ◦C. in excess.

100% 100% 90% Ca₂Fe₂O₅ 90% Ca₂Fe₂O₅ 80% 80% CaFe₂O₄ 70% 70% 60% 60% 50% CaFe₂O₄ 50% NaFeO2 NaFeO2 40% Na4TiO4 40% Na4TiO4 Ca₂Al₂SiO₇ 30% 30% Phase Distribution % Distribution Phase NaAlSiO₄ NaAlSiO₄ Na₂CaSiO₄ 20% 20% 10% NaAlO₂ 10% NaAlO₂ 0% 0% 0% 5% 10% 15% 25% 50% 100% 200% 0% 5% 10% 15% 25% 50% 100% Excess CaO Reagent % Excess CaO Reagent % CaO + SiO₂ → CaSiO₃ 2CaO + SiO₂ → Ca₂SiO₄ (a) (b)

Figure 12. Thermodynamic predictions of sinter phase formation ( a) when monocalcium silicate is expected or ( b) when dicalcium silicate is expected, at excess of lime additions at 50% excess soda, 900 °C for 2 h. 900 ◦C for 2 h.

FigureCompared 13 shows to Figure the 8XRDa, thermodynamics identification of of the soda sinter only phase addition and intoleached the sinterresidue, of bauxitewhereas residue Figure 14notes shows that the sodium SEM preferablyconfirmations reacts of to minerals form nepheline identified species in the and sinters sodium and titanateleached species residue. at The lower sinters than confirmed50% soda excess the presence levels, whereas, of sodium calcium aluminate, is bound calciu tom either ferrite aluminum and titanate or iron peaks initially. as well At betweenas dicalcium 50% silicateand 200% in both soda XRD excess and levels, SEM. sodiumIn Figure ferrite 13, both species presence of intermediate of harmunite Na (CaFe3Fe5O2O9 4forms,) and srebrodolskite transforming (Cainto2Fe NaFeO2O5) were2 at even detected, higher where soda angular concentrations. rods exist Sodium in both ferrite sinters then and dissolves leached residue back as images sodium, in hence SEM phases(Figure do14, not (2) inhibitand (6), system. nano-rods, Also atSEM higher empirical soda levels formula (Figure of 8CaFea), soda6.9O10 and). Iron calcium was isfound predicted to be entrainedto preferentially at varying react, levels still throughout existing in equilibriumSEM-EDS element with nepheline identification. species. In Figure 12, the excess CaOCalcium predicted tosodium be added aluminum in the sintering silicate system is(grossular, divided into SEM two amounts,empirical Figure formula 12a, one thatof Caencourages2.23Na1.11AlSi monocalcium1.15FeO9.6) trace silicate remainder formation in both and XRD the and other, SEM Figure images 12b, (Figure dicalcium 14, (5)) silicate confirms instead. the thermodynamicIncreased additions inhibition of lime onlyof aluminum encourage phase calcium that to complexconverts intofrom Ca nepheline/sodium2Fe2O5 stable species aluminum instead, withsilicate nepheline phases phasetowards still grossular existing in products equilibrium instead amounts of tosodium sodium aluminate aluminate phases, species. indicating Grossular incompletespecies in the recovery form of of Ca aluminum.2Al2SiO7 and Meher sodium et al. calcium [31,44] silicate also detected species (Na similar2CaSiO formations4) exists when of minerals CaO is inadded lime/calcite in excess. added soda sinter system, with calcium aluminum silicate detected instead of sodium aluminumFigure silicate 13 shows type the products, XRD identification though alumina of the sinter extraction phase was and more leached favourable residue, whereasat 10 g CaO Figure ratio 14 atshows 900 °C the [44]. SEM Kirschsteinite confirmations (CaFeSiO of minerals4) was identified also detected in thesinters in their and paper leached [44] residue.and it may The exist sinters in minimalconfirmed amounts the presence that remained of sodium undetected aluminate, in calcium our sinter ferrite system. and titanate peaks as well as dicalcium silicate in both XRD and SEM. In Figure 13, both presence of harmunite (CaFe2O4) and srebrodolskite (Ca 2Fe2O5) were detected, where angular rods exist in both sinters and leached residue images in SEM (Figure 14, (2) and (6), nano-rods, SEM empirical formula of CaFe6.9O10). Iron was found to be entrained at varying levels throughout SEM-EDS element identification. Minerals 2019, 9, 571 13 of 21 MineralsMinerals 2019 2019, 9,, 9,x FORx FOR PEER PEER REVIEW REVIEW 1212 of of20 20

0: Quartz (SiO₂) 10: Rutile (TiO₂) 1: Hematite0: Quartz (Fe (SiO₂O₂₃)) 11: 10:Perovskite Rutile (TiO (CaTiO₂) ₃) 2: Goethite1: Hematite (α-FeOOH) (Fe₂O₃) 12: 11:Calcite Perovskite (CaCO ₃(CaTiO) ₃) 3: Cancrinite2: Goethite (Na (α-FeOOH)₈(Al,Si)₁₂O₂₄(OH)₂.2H₂O 13: 12:Sodium Calcite aluminate (CaCO₃ )(NaAlO₂) 3: Cancrinite (Na₈(Al,Si)₁₂O₂₄(OH)₂.2H₂O 13: Sodium aluminate (NaAlO₂) 4: Chamosite (Fe1.8Mg0.2Al0.8(Si1.3Al0.7)O5(OH)4) 14: Sodium aluminium silicate (NaAlSiO₄) 5: Boehmite4: Chamosite (γ-AlOOH) (Fe1.8Mg0.2Al0.8(Si1.3Al0.7)O5(OH)4) 15: 14:Sodium Sodium ferrotitanate aluminium (NaFeTiO silicate (NaAlSiO₄) ₄) 6: Gibbsite5: Boehmite (Al(OH) (γ-AlOOH)₃) 16: 15:Harmunite Sodium (CaFeferrotitanate₂O₄) (NaFeTiO₄)

6: Gibbsite (Al(OH)₃) 1,11 16: Harmunite (CaFe₂O₄) 7: Diaspore (α-AlOOH) 17:Srebrodolskite (Ca₂Fe₂O₅) 1,11

7: Diaspore (α-AlOOH) 16 8: Grossular (Ca₃FeA l(SiO₄)((OH)₄) 18: 17:SrebrodolskiteDicalcium silicate (Ca₂FeSiO₂O₄)₅) 9: Anatase8: Grossular (TiO₂ (Ca) ₃FeA l(SiO₄)((OH)₄) 16 18: Dicalcium silicate (Ca₂SiO₄) 12 1,16

9: Anatase (TiO₂) 11 8 12 1,16 8 11 11 8 16 8 8 8 8 8 11 8 16 12 8 16 8 16 14 8 1 8 8 1 18 8 1 12 16 17 8 16 14 8 1 1 18 8 1 17 Leached 16

Leached 1,11 16

Residue 1,11

Residue 1,16 8 13 1,16 18 18,8 8 11 13 1 8 13 18 18,8 11 11 8 1 1,16 1 13,14 17 8 13 8,0 17 16 16 1 11 18 8 8 1,16 1 18 1 16 17 13,14 17 8,0 17 16 16 1 18 8 18 1 16 17 Sinter 1,8 Sinter 1,4 1,8 1,4 7 11 7 5 12 8 1 3 7 7 7 3 11 6 1 7 1 1 5 12 8 0 1 3 7 7 3 3 8 6 7 8,10 1 11,12 1 5 1 10 8 8 8 4 2 8 9 8 0 3 8 7 8,10 11,12 5 10 8 8 8 4 2 8 9 8 Bauxite Residue Bauxite Residue 2-θ 10 20 30 40 50 60 2-θ 10 20 30 40 50de 60grees degrees FigureFigure 13. 13. XRD XRDXRD of of ofmineralogical mineralogical mineralogical phases phases for for lime lime lime addition addition additionss (20s (20 g CaO/100g g CaO/100 CaO/100 g bauxiteg bauxite bauxite residue, residue, residue, equivalent equivalent to 80% excess CaO addition for Ca2SiO4 formation), and 50% excess soda at 900 °C and for 2 h. Red to 80%80% excessexcess CaOCaO additionaddition forfor CaCa22SiO4 formation),formation), and and 50% 50% excess soda at 900 ◦°CC and for 2 h.h. Red font:font: Existing Existing minerals minerals in in baux baux bauxiteiteite residue; residue; black black font: font: Transformed Transformed sinter sinter and and leached leachedleached minerals. minerals.minerals.

1 1 2 2 6 6

4 4 5 5

3 3

(a()a ) (b()b )

Figure 14. SEM (scanning electron microscope) mineral identifications in CaO, Na2CO3 and bauxite Figure 14. SEM (scanning electronelectron microscope)microscope) mineralmineral identiﬁcationsidentifications inin CaO,CaO, NaNa22COCO33 and bauxite residueresidue (a ()a Sinter) SinterSinter product; product;product; (1) (1)(1) Sodium SodiumSodium aluminate, aluminate,aluminate, (2) (2)(2) Calcium Calcium ferrite, ferrite, Calcium Calcium titanate, titanate, (3) (3) Calcium Calcium silicate,silicate, sodium sodiumsodium calcium calcium calcium silicate;silicate; silicate; andand and ( b()b Leached()b Leached) Leached residue; residue; residue; (4) Calcium(4) (4) Calcium Calcium ferrite, ferrite, ferrite, calcium calcium calcium silicate, silicate, (5)silicate, Calcium (5) (5) Calciumferrite,Calcium ferrite, grossular ferrite, grossular grossular product, product, (6)product, Calcium (6) (6) Calcium ferriteCalcium (moreferrite ferrite details (more (more indetails details Supplementary in in supplementary supplementary Materials). materials). materials).

3.5.3.5. Effect EffectCalcium of ofMgO MgO Additi sodium Additionsons on on aluminum Phase Phase Transformation Transformation silicate during during (grossular, Sintering Sintering Process SEM Process empirical formula of Ca Na AlSi FeO ) trace remainder in both XRD and SEM images (Figure 14, (5)) 2.23FigureFigure1.11 15 15 shows shows1.15 the the9.6 thermodynamic thermodynamic prediction prediction of of the the effects effects of of MgO MgO added added in in excess excess of of conﬁrms the thermodynamic inhibition of aluminum phase that converts from nepheline/sodium stoichiometricstoichiometric amounts amounts to to form form magnesium magnesium silicate silicate product. product. aluminum silicate phases towards grossular products instead of sodium aluminate phases, indicating

incomplete100% recovery of aluminum. Meher et al. [31,44] also detected similar formations of minerals in 100% 100% CaFe₂O₄CaFe₂O₄ 100% CaFe₂O₄ lime/calcite90% added soda sinter system, with calcium90% aluminum silicateCaFe₂O₄ detected instead of sodium 90% 90% Na₃Fe₅O₉Na₃Fe₅O₉ Na₃Fe₅O₉ 80% 80% Na₃Fe₅O₉ aluminum80% silicate type products, though alumina extraction80% was more favourable at 10 g CaO ratio at Fe₂O₃ 70% Fe₂O₃ 70% Fe₂O₃ 900 ◦C[4470%]. Kirschsteinite (CaFeSiO4) was also detected70% in their paperFe₂O₃ [44] and it may exist in minimal 60% 60% amounts that60% remainedNa₄TiO₄ undetected in our sinter system.60% Na₄TiO₄ 50% Na₄TiO₄ 50% Na₄TiO₄ 50% 50% 40% NaAlSiO₄ 40% NaAlSiO₄ 3.5. E ect40% of MgO AdditionsNaAlSiO₄ on Phase Transformation during40% SinteringNaAlSiO₄ Process ﬀ 30% (Nepheline) 30% (Nepheline) 30% (Nepheline) 30% (Nepheline) Phase Distribution Phase Distribution (%)

20% Phase Distribution (%) 20% Figure20% 15 shows the thermodynamic prediction20% of the eﬀects of MgO added in excess of NaAlO₂ 10% NaAlO₂ 10% NaAlO₂ 10% NaAlO₂ MgO 10% MgO MgO stoichiometric amounts to form magnesiumMgO silicate product.0% 0% 0%0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0%0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Excess MgO reagent % Excess MgO reagent % Excess MgO reagent % Excess MgO reagent % 2MgO + SiO → Mg₂SiO₃ MgO + SiO → MgSiO₃ 2MgO + SiO → Mg SiO MgO + SiO → MgSiO₃ ₂ ₃ (a()a ) (b()b )

FigureFigure 15. 15. Thermodynamic Thermodynamic predictions predictions of of sinter sinter phase phase formation formation (a ()a when) when monomagnesium monomagnesium silicate silicate is isexpected expected or or (b ()b when) when dimagnesium dimagnesium silicate silicate is isexpected expected at atexcess excess of of MgO MgO additions additions at at 50% 50% excess excess soda,soda, 900 900 °C °C for for 2 h.2 h.

Minerals 2019, 9, x FOR PEER REVIEW 12 of 20

0: Quartz (SiO₂) 10: Rutile (TiO₂) 1: Hematite (Fe₂O₃) 11: Perovskite (CaTiO₃) 2: Goethite (α-FeOOH) 12: Calcite (CaCO₃) 3: Cancrinite (Na₈(Al,Si)₁₂O₂₄(OH)₂.2H₂O 13: Sodium aluminate (NaAlO₂) 4: Chamosite (Fe1.8Mg0.2Al0.8(Si1.3Al0.7)O5(OH)4) 14: Sodium aluminium silicate (NaAlSiO₄) 5: Boehmite (γ-AlOOH) 15: Sodium ferrotitanate (NaFeTiO₄) 6: Gibbsite (Al(OH)₃) 16: Harmunite (CaFe₂O₄) 7: Diaspore (α-AlOOH) 1,11 17:Srebrodolskite (Ca₂Fe₂O₅) 8: Grossular (Ca₃FeA l(SiO₄)((OH)₄) 16 18: Dicalcium silicate (Ca₂SiO₄) 9: Anatase (TiO₂) 12 1,16 11 8 8 11 16 8 8 8 8 12 16 8 16 14 8 1 1 18 8 1 17 Leached 16 Residue 1,11 1,16 8 13 18 18,8 11 1 8 13 11 8 1,16 1 13,14 17 8,0 17 16 16 1 18 8 18 1 16 17

Sinter 1,8 1,4 7 11 7 5 12 8 1 3 7 7 3 6 1 1 1 0 3 8 7 8,10 11,12 5 10 8 8 8 4 2 9 8 8

Bauxite Residue 10 20 30 40 50 60 2-θ degrees Figure 13. XRD of mineralogical phases for lime additions (20 g CaO/100 g bauxite residue, equivalent

to 80% excess CaO addition for Ca2SiO4 formation), and 50% excess soda at 900 °C and for 2 h. Red font: Existing minerals in bauxite residue; black font: Transformed sinter and leached minerals.

1 2 6

4 5

(a) (b)

Figure 14. SEM (scanning electron microscope) mineral identifications in CaO, Na2CO3 and bauxite residue (a) Sinter product; (1) Sodium aluminate, (2) Calcium ferrite, Calcium titanate, (3) Calcium silicate, sodium calcium silicate; and (b) Leached residue; (4) Calcium ferrite, calcium silicate, (5) Calcium ferrite, grossular product, (6) Calcium ferrite (more details in supplementary materials).

3.5. Effect of MgO Additions on Phase Transformation during Sintering Process

MineralsFigure2019, 9 ,15 571 shows the thermodynamic prediction of the effects of MgO added in excess14 of 21of stoichiometric amounts to form magnesium silicate product.

100% 100% CaFe₂O₄ CaFe₂O₄ 90% 90% Na₃Fe₅O₉ Na₃Fe₅O₉ 80% 80% 70% Fe₂O₃ 70% Fe₂O₃ 60% 60% Na₄TiO₄ Na₄TiO₄ 50% 50% 40% NaAlSiO₄ 40% NaAlSiO₄ 30% (Nepheline) 30% (Nepheline)

Phase Distribution Phase Distribution (%) 20% 20% NaAlO₂ 10% NaAlO₂ 10% MgO MgO 0% 0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Excess MgO reagent % Excess MgO reagent % 2MgO + SiO → Mg SiO MgO + SiO → MgSiO₃ ₂ ₃ (a) (b)

FigureFigure 15.15. ThermodynamicThermodynamic predictionspredictions of of sinter sinter phase phase formation formation (a ()a when) when monomagnesium monomagnesium silicate silicate is expectedis expected or (orb) ( whenb) when dimagnesium dimagnesium silicate silicate is expected is expected at excess at excess of MgO of MgO additions additions at 50% at excess 50% excess soda, Minerals900soda, 2019◦C, 9,900 for x FOR°C 2 h. for PEER 2 h. REVIEW 13 of 20

FiguresFigures 16 16 and and 17 17 shows shows the the XRD XRD identification identification and SEMSEM confirmationsconfirmations ofof minerals minerals identified identified in inthe the sinters sinters and and leached leached residue residue respectively. respectively. The The eff ecteffect of theof the addition addition of magnesiumof magnesium oxide oxide in the in sodathe sodasintering sintering system system of 20 gof MgO 20 tog 100MgO g bauxiteto 100 residueg bauxite (130% residue excess (130% to stoichiometric excess to requirementsstoichiometric to form Mg SiO ) when thermodynamically interpreted, is limited and not able to push the nepheline requirements2 to4 form Mg2SiO4) when thermodynamically interpreted, is limited and not able to push theproduct nepheline reaction product towards reaction sodium towards aluminate. sodium alumin Any calciumate. Any existing calcium in existing the system in the is system said to is change said into CaFe O product in Figure 15, and sodium reacts with iron and titanium to form Na Fe O and to change 2into4 CaFe2O4 product in Figure 15, and sodium reacts with iron and titanium3 to5 form9 Na TiO preferentially. Na3Fe4 5O94 and Na4TiO4 preferentially.

7: Diaspore (α-AlOOH) 1,14 17: Brow nmillerite 8: Grossular (Ca₃FeAl(SiO₄)((OH)₄) (Ca₂(Mg,Si,Al,Fe)₂O₅) 9: Anatase (TiO₂) 1,11, 16 1 14,15 11 14 17 14 16 14 1 14 16 11

Leached 1 Residue 15 1 14 13,17 1,11,16 13 14 11 14 16 16 14 1 14 11 Sinter 1,8 1,4 7 11 7 5 12 8 3 1 7 7 3 6 1 1 1 0 8 3 7 8,10 11,12 11,12 10 5 8 8 8 4 2 9 8 8

Bauxite Residue 10 20 30 40 50 60 2-θ degrees FigureFigure 16. 16. XRDXRD of ofmineralogical mineralogical phases phases for for MgO MgO additions additions (20 (20 g gMgO/100 MgO/100 g gbauxite bauxite residue) residue) at at

atmosphericatmospheric conditions conditions at at 900 ◦°CC and forfor 22h. h. Red Red font: font: Existing Existing minerals minerals in in bauxite bauxite residue; residue; black black font: font:Transformed Transformed sinter sinter and and leached leached minerals. minerals.

1 2 6

3 5 7

(a) (b)

Figure 17. SEM mineral identifications in MgO, Na2CO3 and bauxite residue (a) sinter products; (1) Calcium iron aluminum sodium oxide, (2) Magnesium oxide, (3) Calcium magnesium iron aluminum sodium oxide (4) Magnesium iron oxide, Calcium titanate, Calcium magnesium silicate, and (b) Leached residue; (5) Magnesium oxide coating, (6) Calcium magnesium iron oxide (7) Calcium magnesium iron aluminum sodium oxide (more details in supplementary materials).

This contrasts with the XRD of experimental results of 20 g MgO additions in 100 g bauxite residue in Figure 16, where complex sinter mineral formations such as magnesioferrite (Mg(Fe3+)2O4), bredigite (Ca2Mg2SiO4)8) and brownmillerite (Ca2(Mg,Al,Fe)2O5) were detected in addition to unreacted magnesia. Leached residue indicates that excess MgO as well as these complex phases remains as part of the leftover residue and that aluminum remains trapped still from the conversion towards these complexes. SEM mineral identifications in Figure 17 confirms a series of complex compound containing calcium iron aluminum sodium oxide and calcium magnesium iron aluminum sodium oxide (SEM

Minerals 2019, 9, x FOR PEER REVIEW 13 of 20

Figures 16 and 17 shows the XRD identification and SEM confirmations of minerals identified in the sinters and leached residue respectively. The effect of the addition of magnesium oxide in the soda sintering system of 20 g MgO to 100 g bauxite residue (130% excess to stoichiometric requirements to form Mg2SiO4) when thermodynamically interpreted, is limited and not able to push the nepheline product reaction towards sodium aluminate. Any calcium existing in the system is said to change into CaFe2O4 product in Figure 15, and sodium reacts with iron and titanium to form Na3Fe5O9 and Na4TiO4 preferentially.

7: Diaspore (α-AlOOH) 1,14 17: Brow nmillerite 8: Grossular (Ca₃FeAl(SiO₄)((OH)₄) (Ca₂(Mg,Si,Al,Fe)₂O₅) 9: Anatase (TiO₂) 1,11, 16 1 14,15 11 14 17 14 16 14 1 14 16 11

Leached 1 Residue 15 1 14 13,17 1,11,16 13 14 11 14 16 16 14 14 1 11 Sinter 1,8 1,4 7 11 7 5 12 8 3 1 7 7 3 6 1 1 1 0 8 3 7 8,10 11,12 11,12 10 5 8 8 8 4 2 9 8 8

Bauxite Residue 10 20 30 40 50 60 2-θ degrees Figure 16. XRD of mineralogical phases for MgO additions (20 g MgO/100 g bauxite residue) at Mineralsatmospheric2019, 9, 571 conditions at 900 °C and for 2 h. Red font: Existing minerals in bauxite residue; black15 of 21 font: Transformed sinter and leached minerals.

1 2 6

3 5 7

(a) (b)

Figure 17. SEMSEM mineral mineral identifications identiﬁcations in in MgO, MgO, Na Na2CO2CO3 and3 and bauxite bauxite residue residue (a) (sintera) sinter products; products; (1) (1)Calcium Calcium iron iron aluminum aluminum sodium sodium oxide, oxide, (2) (2) Magnes Magnesiumium oxide, oxide, (3) (3) Calcium Calcium magnesium magnesium iron iron aluminum aluminum sodium oxideoxide (4) (4) Magnesium Magnesium iron iron oxide, oxide, Calcium Calcium titanate, titanate, Calcium Calcium magnesium magnesium silicate, silicate, and (b) Leachedand (b) residue;Leached (5)residue; Magnesium (5) Magnesium oxide coating, oxide (6) Calciumcoating, magnesium(6) Calcium iron magnesium oxide (7) Calciumiron oxide magnesium (7) Calcium iron aluminummagnesium sodium iron aluminum oxide (more sodium details oxide in Supplementary (more details in Materials). supplementary materials).

This contrastscontrasts with with the the XRD XRD of experimentalof experimental results results of 20 gof MgO 20 g additions MgO additions in 100 g bauxitein 100 residueg bauxite in 3+ Figureresidue 16 in, whereFigure complex16, where sinter complex mineral sinter formations mineral formations such as magnesioferrite such as magnesioferrite (Mg(Fe )2 (Mg(FeO4), bredigite3+)2O4), (Cabredigite2Mg2SiO (Ca4)28Mg) and2SiO brownmillerite4)8) and brownmillerite (Ca2(Mg,Al,Fe) (Ca2(Mg,Al,Fe)2O5) were2O detected5) were indetected addition in toaddition unreacted to magnesia.unreacted magnesia. Leached residue Leached indicates residue thatindicates excess that MgO excess as well MgO as theseas well complex as these phases complex remains phases as partremains of the as leftoverpart of the residue leftover and residue that aluminum and that aluminum remains trapped remains still trapped from still the conversionfrom the conversion towards thesetowards complexes. these complexes. SEM mineral identificationsidentifications in Figure 17 confirmsconfirms a seriesseries ofof complexcomplex compoundcompound containingcontaining calcium iron iron aluminum aluminum sodium sodium oxide oxide and and calciu calciumm magnesium magnesium iron ironaluminum aluminum sodium sodium oxide oxide(SEM (SEM empirical formulas of CaFe AlNa O and CaFe Mg Al Na O ). Complex sinter area 2.4 1.6 9 4.5 7 1.2 1.6 13 that contains calcium magnesium iron aluminum and silicon (Total SEM empirical formula of Ca1.8Fe11.6SiTi1.48Mg6.7Al2.6O44.7) indicates that the sinter product becomes a heterogenous mix of elements that are harder to distinguish from topographical SEM-EDS method. In Figure 16, they appear as bredigite and brownmillerite species instead. A combination of calcium magnesium iron oxide (SEM empirical formula of CaMg8.9Fe11O16) contains minute traces of distributed calcium, and empirical formula indicates that particles such as Figure 17 (6) suggests that magnesioferrite species are present in the system, with partial calcium substitution in the species. Overall, adding MgO does not aid the sinter reactions as they still contain unfavourable complex garnet-type products. Meher and Padhi reports cancrinite and bayerite mineral identifications in their XRDs of MgO and Na2CO3 sintered bauxite residue system, though magnesium was found to be bound with ferrite, titanate and silica forms [40].

3.6. Eﬀect of BaO Additions on Phase Transformations during Sintering Process By introducing barium oxide into the soda sinter system, the thermodynamic prediction of minerals changes to improve the recovery of aluminum via sodium aluminate phases. Figure 18 shows that in the thermodynamic predictions, barium favourably reacts towards the available silica and preferentially binds them into either Ba2Ca2Si4O16 or Ba2SiO4 forms, pushing also the equilibrium of reaction from sodium aluminum silicate product towards sodium aluminate product, especially when added in excess of 100% stoichiometric requirements for Ba2SiO4 formation. Minerals 2019, 9, x FOR PEER REVIEW 14 of 20

empirical formulas of CaFe2.4AlNa1.6O9 and CaFe4.5Mg7Al1.2Na1.6O13). Complex sinter area that contains calcium magnesium iron aluminum and silicon (Total SEM empirical formula of Ca1.8Fe11.6SiTi1.48Mg6.7Al2.6O44.7) indicates that the sinter product becomes a heterogenous mix of elements that are harder to distinguish from topographical SEM-EDS method. In Figure 16, they appear as bredigite and brownmillerite species instead. A combination of calcium magnesium iron oxide (SEM empirical formula of CaMg8.9Fe11O16) contains minute traces of distributed calcium, and empirical formula indicates that particles such as Figure 17 (6) suggests that magnesioferrite species are present in the system, with partial calcium substitution in the species. Overall, adding MgO does not aid the sinter reactions as they still contain unfavourable complex garnet-type products. Meher and Padhi reports cancrinite and bayerite mineral identifications in their XRDs of MgO and Na2CO3 sintered bauxite residue system, though magnesium was found to be bound with ferrite, titanate and silica forms [40].

3.6. Effect of BaO Additions on Phase Transformations during Sintering Process By introducing barium oxide into the soda sinter system, the thermodynamic prediction of minerals changes to improve the recovery of aluminum via sodium aluminate phases. Figure 18 shows that in the thermodynamic predictions, barium favourably reacts towards the available silica and preferentially binds them into either Ba2Ca2Si4O16 or Ba2SiO4 forms, pushing also the equilibrium of reaction from sodium aluminum silicate product towards sodium aluminate product, especially Minerals 2019, 9, 571 16 of 21 when added in excess of 100% stoichiometric requirements for Ba2SiO4 formation.

100% 100% Ba₂Ca₆Si₄O₁₆ Ba₂Ca₆Si₄O₁₆ 90% 90% 80% Ba₂SiO₄ 80% Ba₂SiO₄ 70% Na₃Fe₅O₉ 70% 60% 60% Na₃Fe₅O₉ Fe₂O₃ 50% 50% Fe₂O₃ 40% Na₄TiO₄ 40% 30% 30% Na₄TiO₄ NaAlSiO₄ NaAlSiO₄ Phase Distribution (%) Distribution Phase 20% (Nepheline) 20% (Nepheline) 10% 10% NaAlO₂ NaAlO₂ 0% 0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Excess BaO reagent % Excess BaO reagent % BaO + SiO → BaSiO₃ 2BaO + SiO → Ba₂SiO₃ (a) (b)

FigureFigure 18. 18.Thermodynamic Thermodynamic predictionspredictions of sinter phase formation formation (a (a) )when when monobarium monobarium silicate silicate is is expected or (b) when dibarium silicate is expected at excess of BaO additions at 50% excess soda, 900 expected or (b) when dibarium silicate is expected at excess of BaO additions at 50% excess soda, 900 ◦C for°C 2 h.for 2 h.

UnlikeUnlike calcium-added calcium-added sinter sinter system,system, wherewhere the ad additiondition of of excess excess causes causes potential potential unwanted unwanted formationformation of of grossular-type grossular-type products products that that inhibit inhibit aluminum aluminum recovery, recovery, barium barium preferentially preferentially reacts reacts with calciumwith calcium during during the sintering, the sintering, leaving leaving aluminum aluminum component component to bind to bind with with sodium sodium instead. instead. Meher Meher et al. reducedet al. reduced the amount the ofamount barium-to-silica of barium-to-silica mol ratio mo requirementsl ratio requirements to 1, and encouragingto 1, and encouraging the formation the of sodiumformation barium of sodium silicate barium to allow silicate for sodium to allow and for aluminum sodium and extraction aluminum (99% extraction recovery) (99% with recovery) decreased BaOwith reagent decreased usage BaO [39 reagent]. usage [39]. They also detected varying degrees of barium ferrite, Ba2Fe2O5 and BaFe12O19, and barium silicate They also detected varying degrees of barium ferrite, Ba2Fe2O5 and BaFe12O19, and barium products, as calcium is low in NALCO’s bauxite residue system. In the AoG bauxite residue system, silicate products, as calcium is low in NALCO’s bauxite residue system. In the AoG bauxite residue the addition of 20 g BaO to 100 g bauxite residue (25% excess to stoichiometric requirements to form system, the addition of 20 g BaO to 100 g bauxite residue (25% excess to stoichiometric requirements BaSiO3), is investigated. XRD mineral identifications in Figure 19 indicates that batiferrite to form BaSiO3), is investigated. XRD mineral identiﬁcations in Figure 19 indicates that batiferrite (Ba(Fe10Ti2)O19), ankangite (BaTi8O16) and trasicite (Ba9Fe2Ti2Si12O3(OH)) were the favorable phases (Ba(Fe Ti )O ), ankangite (BaTi O ) and trasicite (Ba Fe Ti Si O (OH)) were the favorable phases that10 were2 formed19 experimentally,8 16 indicating that experimentally9 2 2 12 3 barium prefers to react with thatMinerals were 2019 formed, 9, x FOR experimentally, PEER REVIEW indicating that experimentally barium prefers to react with15 titaniumof 20 instead. This indicates a selective titanium recovery into the sinters and residue, though iron’s presence titanium instead. This indicates a selective titanium recovery into the sinters and residue, though can result into the complex Ba–Fe–Ti product instead. It is recommended to combine barium with iron’s presence can result into the complex Ba–Fe–Ti product instead. It is recommended to combine carbon reduction sintering to further improve the selectivity of BaTi O , as titanium could potentially barium with carbon reduction sintering to further improve the selectivity8 16 of BaTi8O16, as titanium becould a valuable potentially commercial be a valuable product commercial downstream. product downstream.

0: Quartz (SiO₂) 9: Anatase (TiO₂) 1: Hematite (Fe₂O₃) 10: Rutile (TiO₂) 2: Goethite (α-FeOOH) 11: Perovskite (CaTiO₃) 3: Cancrinite (Na₈(Al,Si)₁₂O₂₄(OH)₂.2H₂O 12: Calcite (CaCO₃) 4: Chamosite (Fe1.8Mg 0.2Al0.8(Si1.3Al0.7)O5(OH)4) 13: Sodium aluminate (NaAlO₂) 5: Boehmite (γ-AlOOH) 14: Sodium aluminium silicate (NaAlSiO₄) 6: Gibbsite (Al(OH)₃) 15: Batiferrite (Ba(Fe₁₀Ti₂)O₁₉) 7: Diaspore (α-AlOOH) 16: Ankangite (BaTi₈O₁₆) 8: Grossular (Ca FeA l(SiO )((OH) ) ₃ ₄ ₄ 1,11 17: Trasicite (Ba₉Fe₂Ti₂Si₁₂O₃₆(OH)) 14 15,17 7 15, 17 15 11 15,14 15 14 15,14 15 1 14 16 16 17

Leached 14 Residue 1,11 13 15 15,16 15 16 15 14 11 15 13 16 15,14 14 15,14 1

Sinter 1,8 1,4 7 11 7 5 12 8 1 3 7 7 3 6 1 1 1 0 3 8 7 8,10 11,12 11,12 5 10 8 8 8 4 2 9 8 8

Bauxite Residue 10 20 30 40 50 60 2-θ degrees

FigureFigure 19. 19. XRDXRD of mineralogical phases phases for for BaO BaO additions additions (20 (20 g gBaO/100 BaO/100 g bauxite g bauxite residue) residue) at at atmospheric conditions at 900 °C and for 2 h. Red font: Existing minerals in bauxite residue; black atmospheric conditions at 900 ◦C and for 2 h. Red font: Existing minerals in bauxite residue; black font: Transformedfont: Transformed sinter sinter and leached and leached minerals. minerals

In Figure 20, the SEM identification of barium titanate is visibly large tetragonal-type particles (1) with SEM empirical formula of Ba3.2TiO10, confirming barium affinity to titanium species. More complex agglomerates such as Figure 20 (2) that SEM empirical formula of the area is distributed as BaTi1.2Fe5.9Ca1.5AlSi1.5O24.3 and (3) as Ba3.7Fe3Ca1.2AlSi1.4O20.9 shows that there are partial losses of aluminum within the sinter product, though XRD only picks up trace amounts of existing sodium aluminum silicate as opposed to the complex product types.

1 2

(a) (b)

Figure 20. SEM mineral identifications in BaO, Na2CO3 and bauxite residue (a) sinter products; (1) Barium titanate, and (b) Leached residue; (2) Barium titanium iron calcium aluminum silicon oxide (3) Barium iron calcium aluminum silicon oxide (more details in supplementary materials).

3.7. Experimental Results of Leaching Tests from 50% Excess Soda Sinters with CaO, MgO and BaO Additions

The results of the leaching tests of Na2CO3 only system, CaO + Na2CO3 system, MgO + Na2CO3 system and BaO + Na2CO3 system as shown in Figure 21, further confirms the outcomes of the thermodynamic and experimental analysis. Aluminum recovery of the systems was in the range of 64% to 75% in total, with the highest to lowest recovery coming from BaO > Na2CO3 > MgO > CaO added systems. The co-recovery of silica was higher in both Na2CO3 only and MgO added system, at

Minerals 2019, 9, x FOR PEER REVIEW 15 of 20

titanium instead. This indicates a selective titanium recovery into the sinters and residue, though iron’s presence can result into the complex Ba–Fe–Ti product instead. It is recommended to combine barium with carbon reduction sintering to further improve the selectivity of BaTi8O16, as titanium could potentially be a valuable commercial product downstream.

Leached 14 Residue 1,11 13 15 15,16 15 16 15 11 14 15 13 16 15,14 14 15,14 1

Sinter 1,8 1,4 7 11 7 5 12 8 1 3 7 7 3 6 1 1 1 0 3 8 7 8,10 11,12 11,12 5 10 8 8 8 4 2 9 8 8

Bauxite Residue 10 20 30 40 50 60 2-θ degrees

Minerals 2019Figure, 9, 571 19. XRD of mineralogical phases for BaO additions (20 g BaO/100 g bauxite residue) at 17 of 21 atmospheric conditions at 900 °C and for 2 h. Red font: Existing minerals in bauxite residue; black font: Transformed sinter and leached minerals In Figure 20, the SEM identification of barium titanate is visibly large tetragonal-type particles In Figure 20, the SEM identification of barium titanate is visibly large tetragonal-type particles (1) with SEM empirical formula of Ba3.2TiO10, confirming barium affinity to titanium species. (1) with SEM empirical formula of Ba3.2TiO10, confirming barium affinity to titanium species. More More complex agglomerates such as Figure 20 (2) that SEM empirical formula of the area is distributed complex agglomerates such as Figure 20 (2) that SEM empirical formula of the area is distributed as as BaTiBaTi1.21.2FeFe5.95.9CaCa1.5AlSiAlSi1.51.5O24.3O24.3 andand (3) (3)as asBa Ba3.7Fe3.73CaFe1.23CaAlSi1.21.4AlSiO20.9 1.4showsO20.9 thatshows there that are there partial are losses partial of losses of aluminumaluminum within within the the sintersinter product,product, though though XRD XRD only only picks picks up up trace trace amounts amounts of existing of existing sodium sodium aluminumaluminum silicate silicate as opposedas opposed to to the the complex complex productproduct types.

1 2

(a) (b)

Figure 20. SEM mineral identifications in BaO, Na2CO3 and bauxite residue (a) sinter products; (1) Figure 20. SEM mineral identiﬁcations in BaO, Na2CO3 and bauxite residue (a) sinter products; (1) BariumBarium titanate, titanate, andand ((bb)) LeachedLeached residue; residue; (2) (2) Barium Barium titanium titanium iron iron calcium calcium aluminum aluminum silicon silicon oxide oxide (3) Barium iron calcium aluminum silicon oxide (more details in supplementary materials). Minerals(3) Barium 2019, iron9, x FOR calcium PEER aluminumREVIEW silicon oxide (more details in Supplementary Materials). 16 of 20 3.7. Experimental Results of Leaching Tests from 50% Excess Soda Sinters with CaO, MgO and BaO 3.7.about Experimental 20%, indicating Results of that Leaching existence Tests of from partially 50% Excess leachable Soda silica Sinters phases with was CaO, formed MgO and during BaO sintering. Additions Additions These phases were most like Na2SiO3 phase, as part of the soda added reacted preferentially with the The results of the leaching tests of Na2CO3 only system, CaO + Na2CO3 system, MgO + Na2CO3 availableThe silicaresults instead. of the leaching In the CaO tests and of Na BaO2CO systems,3 only system, the thermodynamically CaO + Na2CO3 system, favourable MgO + Na formations2CO3 system and BaO + Na2CO3 system as shown in Figure 21, further conﬁrms the outcomes of the ofsystem Ca2SiO and4 and BaO Ba +2 SiONa24CO, and,3 system the formation as shown ofin aFigure myriad 21, offu rthercomponents confirms such the outcomesas sodium of calciumthe thermodynamic and experimental analysis. Aluminum recovery of the systems was in the range of silicate,thermodynamic grossular and and experimental garnet complex analysis. phases, Aluminum locked recovery silica intoof th ethe systems matrix was and in thehindered range of silica 64% to 75% in total, with the highest to lowest recovery coming from BaO > Na CO > MgO > CaO leaching64% to 75% from in sinters. total, with the highest to lowest recovery coming from BaO > Na2CO2 3 > 3MgO > CaO added systems. The co-recovery of silica was higher in both Na2 3CO only and MgO added system, addedThe systems. sodium The recovery co-recovery increased of silica to 94% was inhigher systems in both that Na managedCO2 only3 to and stop MgO silicon added from system, turning at into at about 20%, indicating that existence of partially leachable silica phases was formed during sintering. leachable products, whereas Na2CO3 only system exhibited the least amount of soda recovery. TheseBarium phases oxide were addition most like in the Na 2sodaSiO 3sinteringphase, as system part of assisted the soda theadded best recovery reacted for preferentially both aluminum with and the availablesodium silica while instead. suppressing In the silica CaO in and system, BaO systems,however,the the thermodynamicallyprice of barium oxide favourable (US $300–$500/metric formations of Catonne)2SiO 4whenand Bacompared2SiO4, and, to lime the (US formation $50–200/metric of a myriad tonne), of magnesium components oxide such (US as sodium$80–$200/metric calcium silicate,tonne) grossular and soda and garnet(US $210–$250/metric complex phases, tonne), locked be silicacomes into considerably the matrix and more hindered expensive, silica leachingfor the fromaluminum sinters. and sodium recovered in bauxite residue.

100 94.9 94.0 SiO₂ Al₂O₃ Na₂O 88.7 90 84.6 80 75.2 70.5 69.6 70 64.2 60

30 Percent of Recovery % of Recovery Percent 20.1 21.0 20

10 1.9 5.0 0 Na₂CO₃ CaO + Na₂CO₃ MgO + Na₂CO₃ BaO + Na₂CO₃ Reactants added to Bauxite Residue Weight Ratio of BR:Na₂CO₃:Metal Oxide is 1:0.25:0.2

FigureFigure 21. Results21. Results of experimentalof experimental recovery recovery of of Al Al2O23O,3 SiO, SiO2,2, and and Na Na22OO withwith CaO, MgO and and BaO BaO added added withwith 50% 50% excess excess soda, soda, compared compared to onlyto only 50% 50% excess excess soda, soda, Na Na2CO2CO3,3, atat 900900 ◦°CC for 2 h.

A simplified mass balance of the laboratory scale experiment of optimised recovery at 50% excess soda (soda only) and stoichiometric carbon with 50% excess soda (carbon + soda) is presented in the flowchart (Figure 22) and mass balance (Table 3). The recovery of an optimised sintering process will be be 70% and 75% for aluminum (0.2 g Al/L) and 85% and 83% for sodium (~1 g Na/L) for soda only system and carbon with soda system respectively. The recovery of silica is kept below 20% (~0.02 g Si/L) when washed with solid-to-liquid ratio of 1:66. The leaching optimisation of the system is recommended to be investigated following this research. The proportion of leached residue to sinter product is 68.2% and 58% for soda only system and carbon with soda system respectively. The latter will experience higher gaseous carbon monoxide/dioxide losses and reduce Fe2O3 in residue into FeO or Fe3O4, and therefore decreasing load for downstream processing.

Figure 22. Total proposed flowsheet for the recovery of aluminum and sodium.

Minerals 2019, 9, x FOR PEER REVIEW 16 of 20 about 20%, indicating that existence of partially leachable silica phases was formed during sintering. These phases were most like Na2SiO3 phase, as part of the soda added reacted preferentially with the available silica instead. In the CaO and BaO systems, the thermodynamically favourable formations of Ca2SiO4 and Ba2SiO4, and, the formation of a myriad of components such as sodium calcium silicate, grossular and garnet complex phases, locked silica into the matrix and hindered silica leaching from sinters. The sodium recovery increased to 94% in systems that managed to stop silicon from turning into leachable products, whereas Na2CO3 only system exhibited the least amount of soda recovery. Barium oxide addition in the soda sintering system assisted the best recovery for both aluminum and sodium while suppressing silica in system, however, the price of barium oxide (US $300–$500/metric tonne) when compared to lime (US $50–200/metric tonne), magnesium oxide (US $80–$200/metric tonne) and soda (US $210–$250/metric tonne), becomes considerably more expensive, for the aluminum and sodium recovered in bauxite residue.

100 94.9 94.0 SiO₂ Al₂O₃ Na₂O 88.7 90 84.6 80 75.2 70.5 69.6 70 64.2 60

Minerals 2019, 9, 571 30 18 of 21 Percent of Recovery % of Recovery Percent 20.1 21.0 20

10 5.0 The sodium recovery increased to 94%1.9 in systems that managed to stop silicon from turning into 0 leachable products, whereas NaNa2₂CO₃3 onlyCaO system + Na₂CO exhibited₃ MgO + Na the₂CO least₃ BaO amount + Na₂CO of₃ soda recovery. Barium oxide addition in the soda sintering systemReactants assisted added to the Bauxite best Residue recovery for both aluminum and sodium while suppressing silica in system,Weight however, Ratio of BR:Na the₂ priceCO₃:Metal of bariumOxide is 1:0.25:0.2 oxide (US $300–$500 /metric tonne) whenFigure compared 21. Results to lime of experimental (US $50–200 recovery/metric of tonne), Al2O3, SiO magnesium2, and Na2O oxidewith CaO, (US MgO $80–$200 and BaO/metric added tonne) and sodawith 50% (US excess $210–$250 soda,/ metriccompared tonne), to only becomes 50% excess considerably soda, Na2CO more3, at expensive,900 °C for 2 forh. the aluminum and sodium recovered in bauxite residue. A simpliﬁedsimplified massmass balance balance of of the the laboratory laboratory scale sc experimentale experiment of optimised of optimised recovery recovery at 50% at excess 50% excesssoda (soda soda only)(soda andonly) stoichiometric and stoichiometric carbon carbon with wi 50%th 50% excess excess soda soda (carbon (carbon+ soda) + soda) is presentedis presented in inthe the ﬂowchart flowchart (Figure (Figure 22) and22) and mass mass balance balance (Table (Table3). The 3). recovery The recovery of an optimised of an optimised sintering sintering process willprocess be bewill 70% be be and 70% 75% and for 75% aluminum for aluminum (0.2 g (0.2 Al/L) g Al/L) and 85% and and85% 83%and 83% for sodium for sodium (~1 g(~1 Na g /Na/L)L) for forsoda soda only only system system and and carbon carbon with with soda soda system system respectively. respectively. The The recovery recovery of silica of silica is kept is kept below below 20% 20%(~0.02 (~0.02 g Si/ L)g Si/L) when when washed washed with solid-to-liquidwith solid-to-liquid ratio of ratio 1:66. of The 1:66. leaching The leaching optimisation optimisation of the system of the systemis recommended is recommended to be investigated to be investigated following following this research. this research. The proportion The proportion of leached of residueleached toresidue sinter toproduct sinter isproduct 68.2% andis 68.2% 58% forand soda 58% onlyfor soda system only and system carbon and with carbon soda with system soda respectively. system respectively. The latter willThe experiencelatter will higherexperience gaseous higher carbon gaseous monoxide carbon/dioxide monoxide/dioxide losses and reduce losses Fe2 Oand3 in reduce residue Fe into2O FeO3 in residueor Fe3O 4into, and FeO therefore or Fe3O decreasing4, and therefore load fordecreasing downstream load processing.for downstream processing.

Figure 22. Total proposed ﬂowsheet for the recovery of aluminum and sodium. Figure 22. Total proposed flowsheet for the recovery of aluminum and sodium. Table 3. Mass balance of input and output from sintering system (Soda only vs Carbon + Soda).

Mass Balance Bauxite Leached Metallurgical Na CO Sinter CO /CO NaOH (kg) Residue 2 3 2 Residue Coke Soda only 1 0.3 1.08 0.22 0.288 0.737 - Carbon + 1 0.3 0.905 0.318 0.288 0.6325 0.122 Soda

4. Conclusions The recovery of aluminum from Greek bauxite residue was found to reach a plateau of 70–75% when excess soda, retention time of sintering and temperature of sintering were investigated. This was conﬁrmed in both thermodynamic and experimental results, whereas sodium recovery levels were optimized at 85–90%. It is recommended that the minimum amount of soda reagent required to convert sodium and aluminum into leachable products, through this optimized study, should be used at 50% excess soda levels, aiding sodium-aluminum recoveries at feasible amounts and thus, processing and conditioning residue for use downstream. Thermodynamic study of the bauxite residue notes that the type of complex desilication products that aluminum remains trapped in bauxite residue, such as sodium-based cancrinite or sodalite, or calcium-based grossular and garnet type product or cancrinite with calcium inclusions, transforms into equilibrium amounts of sodium aluminate and sodium aluminum silicates converted during sintering. Additional additives such as CaO, MgO and BaO are also tested to consume silica from adhering to sodium and aluminum, forming other thermodynamically favourable and stable silica-bound species and liberating the main elements for recovery. Minerals 2019, 9, 571 19 of 21

The addition of barium oxide aids the recovery to achieve leachate containing 75% of recovered aluminum and 94% recovered sodium respectively while keeping silica levels suppressed. The presence of calcium existing in the Greek bauxite residue prevents the recovery of aluminum due to NaAlSiO4/NaAlO2 sintered equilibrium formations. Increased lime additions will preferentially react with iron in this system. Addition of carbon within inert conditions assisted aluminum recovery to 75% and sodium recovery to 83%. The beneﬁt of addition of carbon and in inert system, however, is to assist pre-reduction of iron and its recovery downstream. Optimal recovery of aluminum and sodium for Greek bauxite residue is recommended to be 70% and 85% respectively, when sintered with 50% excess stoichiometric Na2CO3. Overall, soda sintering with other additional reagents may be more suitably adapted towards pure sodaline/cancrinite products as opposed to a calcium-rich desilication product. Depending on the diﬀerent types of bauxite residues and desilication minerals present in it after Bayer processing, a mineral characterization and understanding of thermodynamic equilibrium system is important for the improved recovery of aluminum and sodium.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/9/10/571/s1, In supplementary details titled “Additional Information_SEM Details”; EDS details for Figure 14: SEM mineral identifications in CaO, Na2CO3 and bauxite, Figure 17: SEM mineral identifications in MgO, Na2CO3 and bauxite, Figure 20: SEM mineral identifications in BaO, Na2CO3 and bauxite residue. Author Contributions: P.W.Y.T., D.P. and V.V. conceptualised and designed the experiments; P.W.Y.T. performed the experiments, writing and investigation; P.W.Y.T. performed XRD, Factsage 7.0™, and SEM-EDS and analyzed the data. D.P. contributed to data validation, software training and further conceptualization of project. V.V. provided resources and project administration. D.P. provided supervision, funding acquisition, writing and project administration. Funding: 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). Acknowledgments: Special thanks to NTUA researchers Ioanna Giannoupoulou and Alexandra Alexandri; and AoG’s team of helpers, Athina Dritsa and Athina Fillipou for experimental and analytical support and efforts and to the rest of the REDMUD team for the countless support. Constructive remarks by peer reviewers are greatly appreciated, as well to Koen Binnemans, Ken Evans, and György (George) Bánvölgyi for the support and thorough input for improving our work.This publication reflects only the author’s view, exempting the Community from any liability. Project website: http://www.etn.redmud.org. 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.

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