Grinding Behavior and Potential Beneficiation Options of Bauxite Ores

Article Grinding Behavior and Potential Beneﬁciation Options of Bauxite Ores

Evangelos Petrakis 1,* , Georgios Bartzas 2 and Konstantinos Komnitsas 1

1 School of Mineral Resources Engineering, Technical University of Crete, University Campus, Kounoupidiana, 731 00 Chania, Greece; [email protected] 2 School of Mining and Metallurgical Engineering, National Technical University of Athens, Zografos, 157 80 Athens, Greece; gbartzas@metal.ntua.gr * Correspondence: [email protected]; Tel.: +30-28210-37608

Received: 6 March 2020; Accepted: 30 March 2020; Published: 31 March 2020

Abstract: This laboratory study investigates selective grinding and beneficiation options for a Greek bauxite ore. First, a series of batch grinding tests were carried out in order to investigate the grinding behavior of the ore and the effect of the material filling volume (f c) on the distribution of aluminium- and iron-containing phases. Then, the ground ore was subjected to magnetic separation either as received or after reduction roasting in order to further explore potential beneficiation options. The results showed that grinding of the ore exhibits non-first order behavior, while the breakage rate varies with grinding time. Additionally, Al2O3 tends to concentrate in the coarser than 0.300 mm product fraction, while fc 10% and 2 min of grinding time are considered optimum conditions for good distribution of Al2O3 and Fe2O3. When different product fractions were subjected to magnetic separation, it was seen that the non-magnetic product obtained from the 0.300–1.18 mm fraction was more rich in Al2O3. In this fraction, the Al2O3 content increased from 58 wt% in the feed to 67.9 wt%, whereas the Fe2O3 content decreased from 22.4 wt% in the feed to 13.5 wt%. When the ore was subjected to a two-step treatment, involving reduction roasting followed by magnetic separation, the Fe2O3 grade decreased from 20.8 to 5.1 wt%, but in this case the recovery was very low.

Keywords: bauxite ores; selective grinding; grinding kinetics; magnetic separation; reduction roasting

1. Introduction Bauxite is an important rock which is used for the production of alumina of metallurgical or chemical grade through the Bayer process [1]. The global proven bauxite reserves are almost 30 billion tons and are located in Guinea (26.4%), Australia (19.2%), Brazil (12.1%) and Jamaica (7.1%), while other significant reserves exist in India, China, Greece and Suriname [2]. Bauxite deposits belong to two major types, namely the lateritic-type which account for 88% and the karst-type deposits which account for 12% of the world reserves. Lateritic-type are generally residual deposits derived from primary aluminosilicate rocks, while karst-type deposits are associated with carbonate rocks, where the bauxite body fills former karst cavities [3–5]. Karst-type deposits have different mineralogical composition compared to lateritic-type due to the presence of carbonates in the parent rock and the different weathering conditions [6]. In Greece, the major bauxite deposits were formed in the geotectonic zone of Parnassos-Ghiona, while economically important occurrences are found in Kallidromon, Iti, Nafpaktos, Smerna and Pylos [7,8]. The deposits of Parnassos-Ghiona are hosted within a carbonate sequence and belong to the Mediterranean karst bauxite belt. These allochthonous karst-type deposits are considered the most significant bauxite reserves in the European Union [9]; the estimated reserves are approximatelly 300 million metric tons—the 11th largest bauxite reserves globally [10]. In addition, Greece is the 12th

Minerals 2020, 10, 314; doi:10.3390/min10040314 www.mdpi.com/journal/minerals Minerals 2020, 10, 314 2 of 21 largest bauxite producer worldwide and a leading producer in the EU, with 1800 thousand metric tons production in 2017 [9,11]. Greek bauxite ores consist mainly of gibbsite [Al(OH)3], diaspore [α-AlO(OH)] and/or boehmite [γ-AlO(OH)], while impurities such as kaolinite [Al2Si2O5(OH)4], hematite [Fe2O3], goethite [FeO(OH)], titanium oxide [TiO2] and silicon oxide [SiO2] are also present. The Al2O3 content varies between 49% and 65%, the Fe2O3 content between 18% and 24%, the CaO content between 0 and 5%, the SiO2 content between 2% and 10%, the TiO2 content between 0.5% and 3%, while the Cr and Ni content may be as high as 2000 mg/kg. Smaller amounts of magnesium, manganese, vanadium and gallium as well as other elements may be also found [12,13]. Greek bauxites are high grade compared to ores treated in several other parts of the word [14–16]. In Greece, 1.8 million tons are treated annualy mainly for the production of alumina (820,000 tons of which 480,000 tons are exported) and aluminium (185,000 tons of which 125,000 tons are exported). Minor quantities are used in cement plants, as a flux for the production of cast iron as well as for the production of rockwool and abrasive materials. It has been mentioned that in recent years, in order to reduce the underground mining cost, some quantities of tropical bauxites (0.4 million tons out of 1.8 million tons) have also been imported to and treated in Greece. The production of aluminum metal is generally accomplished by the Bayer process in which finely ground bauxite ore is digested in strong sodium hydroxide solution at a temperature range 100–250 ◦C[17]. The dissolved alumina is separated by physical means, such as settling and filtration, from the insoluble residue, known as “red mud” or recently as “bauxite residue”. The amount of residue generated per ton of alumina produced, varies greatly depending on the type of bauxite used, from 0.3 tons for high grade bauxite to 2.5 tons for very low grade [18]. The bauxite residue mainly contains iron and silicon oxides, and other phases [19]. The Bayer process can economically produce alumina when Al2O3 to SiO2 mass ratio in bauxite is greater than 10, while sintering or a combination of sintering and Bayer process can be used for ores with lower ratios. However, sintering is an energy-intensive process and increases the production cost of alumina [20–23]. Beneficiation is often considered for the upgrade of bauxite ores in order to also render them suitable for the refractory/ceramics industry [13,16]. Studies focusing on the removal of iron-bearing minerals include magnetic/gravimetric separation [24–26], reductive roasting followed by magnetic separation [17,22,27], froth flotation [28–30], leaching of iron oxides [31], chlorination followed by leaching [32] and microwave-magnetising roasting [33]. Other studies have attempted to upgrade bauxite ores through selective grinding by exploiting the different grindability between the minerals of interest, e.g., diaspore (6.5–7 Mohs hardness) and gangue minerals, e.g., kaolinite (2–2.5 Mohs hardness) and some other aluminosilicates [20,34]. Prior to any ore processing, grinding is the principal operation attracting particular attention due to the associated CO2 emissions and the high energy consumption that affects the overall processing cost. Thus, in order to efficiently recover the economically valuable minerals an investigation of the ore behavior during grinding in relation to the operating parameters is required. Kinetic models which are based on two functions, namely the breakage rate and the breakage function are mainly used [35,36]. These models consider that grinding follows a first-order law as reported in previous studies [37,38]. However, deviations from first-order kinetics have been observed for several mineral phases indicating that the breakage rate varies with grinding time [39–41]. For non-first-order kinetics, the following Equation (1) proposed by Alyavdin can be used to describe the grinding process [42,43],

K tM R = R t e− · (1) i,t 0, · where Ri,t is the mass fraction of the mill hold-up that is of size i, R0,t is the mass fraction in the feed, K is the grinding rate constant, t is the grinding time and M is a constant depending on the material properties and grinding conditions. The present laboratory study aims to investigate selective grinding and various beneficiation options for a Greek bauxite ore. First, selective grinding was carried out, through a series of batch Minerals 2020, 10, 314 3 of 21 grinding tests, to elucidate the grinding behavior of the ore and assess the effect of the material filling volume in the mill. Second, the ore was subjected to magnetic separation to determine the distribution of Al- and Fe-containing phases. Third, reduction roasting followed by magnetic separation was investigated to enable the conversion of weak magnetic iron-bearing minerals to strong magnetic phases, i.e., magnetite or metallic iron, and further explore potential beneficiation options.

2. Materials and Methods The bauxite ore used in the present study was provided by the mining company Delphi-Distomon S.A., which is one of the three Greek mining companies exploiting bauxite from the Parnassos-Ghiona zone, in central Greece. The ore is a typical red-brown (Fe-rich) bauxite composed of diaspore and boehmite as aluminum bearing mineral phases [44]. The received ore, approximately 150 kg, with a particle size of 30 mm was homogenized by − the cone and quarter method, and a representative sample was crushed to 3.35 mm using a jaw − crusher (Fritsch pulverisette 1, Fritsch GmbH, Idar-Oberstein, Germany). To determine the particle size distribution of the crushed products, the ore was wet screened at 0.106 mm, while the +0.106 mm fraction was dried and screened using a series of screens with an aperture ratio 2 to obtain six size fractions. Chemical analyses were carried out by X-ray fluorescence (XRF) spectroscopy using a Bruker S2 Ranger Energy-dispersive ED-XRF (Bruker, Karlsruhe, Germany) spectrometer. The composition of major elements was also determined after complete dissolution of each fraction by acid digestion followed by atomic absorption spectroscopy (AAS). The differences between the two analytical approaches were marginal, so the results obtained by XRF analysis are presented in this study. The identification of the mineral phases was carried out by X-ray powder diffraction (XRD) using a Bruker-AXS D8 Advance type (Bruker) diffractometer. Quantitative phase analysis by the Rietveld method was also performed [45]. The microstructure of the ore and the products obtained after each treatment was examined by scanning electron microscopy (SEM) using a JEOL 6380LV microscope (JEOL Ltd., Tokyo, Japan) equipped with an Oxford INCA energy dispersive X-ray spectrometer (EDS). The thermal behavior of the ore was studied using a Perkin Elmer TGA-6/DTG differential thermogravimetric analyser (Perkin Elmer, Waltham, MA, USA). A mass of about 40 mg was heated to temperatures of up to 900 ◦C at a heating rate of 10 ◦C/min under pure N2 atmosphere with a flow rate of 35 mL/min. Three series of wet grinding tests were carried out in a laboratory ball mill (Sepor, Los Angeles, CA, USA) with an internal diameter of 204 mm using the crushed product 3.35 mm as feed material. − Different grinding times, namely 2, 4, 8 and 12 min, were considered, while the grinding media used 3 were stainless steel balls (ρb = 7.85 g/cm ) of three different sizes, i.e., 40, 25.4, and 12.7 mm. In this test series, the total ball mass was kept constant at about 10 kg corresponding to ball filling volume J = 40%, while the material filling volume f c varied between 5% and 15%. The interstitial filling U of the void spaces of the balls filled by material was calculated using Equation (2). The product obtained after each grinding period was wet sieved using a series of screens, namely 1.18, 0.300 and 0.075 mm. The mill specification data and test conditions are shown in Table1.

fc U = (2) 0.4 J · The magnetic separation tests involved the treatment of three product fractions, namely 0.300 mm, − 0.300–1.18 and 1.18–3.35, obtained after a specific grinding period. More specifically, the feed material in the mill, 3.35 mm, was ground using different material filling volumes f c (5%, 10% and 15%) in − order to obtain a product size of 50% passing 0.300 mm. Then, each grinding product was subjected to magnetic separation using three different types of magnetic separators, depending on the sample size fraction. The two coarse size fractions, namely 1.18–3.35 mm and 0.300–1.18 mm were treated with the use of a HS10 Perm Roll belt separator (Outokumpu Technology Inc., Jacksonville, FL, USA) Minerals 2020, 10, 314 4 of 21 with magnetic field strength of 0.5 T and a Carpco model MIH(13)111-5 high intensity induced roll magnetic separator (Outokumpu Technology Inc., Carpco Division), respectively, while for the fine fraction 0.300 mm a Carpco model WHIMS 3x4L wet high intensity magnetic separator (Outokumpu − Technology Inc., Carpco Division) was used. The magnetic separation process of the coarse fractions involved two stages, as seen in Figure1a. According to the procedure followed, each fraction was subjected to magnetic separation and two products were obtained, namely a magnetic product (MI) and a non-magnetic product (NMI). After the first stage, the non-magnetic product was further treated in the same magnetic separator using a lower roll speed and a second magnetic product (MII) and a final non-magnetic (NMII) product were obtained. The 1.18–3.35 mm fraction was treated using 180 and 140 rpm roll speed in the first and second stage, respectively. For the 0.300–1.18 mm fraction the roll speed was maintained at 120 and 100 rpm in the first and second stage, respectively, while the magnet coil current was kept at 3 amp. The magnetic separation of the 0.300 mm fraction involved a strong − magnetic field created within a cell containing iron spheroids which was placed between two poles. The process included three separation stages with decreasing magnet coil current ranging from 7 to 1.5 amp, corresponding to magnetic field strength between 0.8 and 0.3 T, as seen in Figure1b. The feed to the magnetic separator was a slurry with a pulp density of 10 wt%. At each stage, the slurry was fed through the cell, the magnetic product was retained on the sphere-poles and the non-magnetic product was flushed through. Then, the magnetic field strength was removed and the obtained magnetic product was further treated using a lower coil current. Finally, three non-magnetic products, namely NMI, NMII, NMIII and one magnetic MIII were collected. All the products were filtered, oven dried at 105 ◦C for 24 h, weighed and characterized by XRF, XRD and SEM.

Table 1. Mill speciﬁcation data and test conditions.

Item Description 1st Series 2nd Series 3rd Series Bulk density (g/cm3) 1.82 1.82 1.82 Material filling volume, f (%) 5 10 15 Material c Interstitial filling, U (%) 31 63 94 Pulp density, % (by weight) 60 60 60 Item Description In all series Diameter, D (cm) 20.4 Length, L (cm) 16.6 Mill Volume, V (cm3) 5423 Operational speed, N (rpm) 66 Critical speed, Nc (rpm) 93.7 Diameter, d (mm) 40, 25.4, 12.7 Number 13, 51, 407 3403.9, 3463.6, Balls Weight (g) 3407 Density (g/cm3) 7.85 Porosity (%) 40 Ball filling volume, J (%) 40

The 0.300 mm grinding fraction obtained using material filling volume f c = 10% in the mill − was also subjected to reduction roasting followed by magnetic separation. Reduction roasting, which is an option used also for the treatment of bauxite residues, was carried out to facilitate reduction of iron minerals and production of magnetic phases, as indicated in earlier studies [46–48]. The reduction roasting was carried out in a Linn High Therm model HK 30 furnace (Linn High Therm GmbH, Eschenfelden, Germany) at 800 ◦C. A commercial activated-charcoal/carbon type Norit GAC 1240 purchased from Sigma–Aldrich (Sigma–Aldrich GmbH, Taufkirchen, Germany) was used as a reductant and Na2CO3 as the flux. The ore was mixed with carbon using carbon to bauxite (C/B) mass ratio 0.15, while the mass ratio of Na2CO3 to bauxite ore was maintained at 0.20. The parameters Minerals 2020, 10, 314 5 of 21 Minerals 2020, 10, x FOR PEER REVIEW 5 of 21 parametersused in this used study, in i.e., this reductive study, i.e., roasting reductive temperature roasting andtemperature duration, and C/B duration, ratios and C/B flux ratio additions and wereflux additionbased on were previous based tests on previous carried out tests in carried the laboratory. out in the The laboratory. mixtures The were mixtures placed were in the placed furnace in andthe furnaceheated to and the heated required to temperature the required at temperature a heating rate at of a 17 heating◦C/min rate under of 17 pure °C/min N2 atmosphere under pure with N2 atmospherea flow rate ofwith 200 a mLflow/min. rate Theof 200 duration mL/min. of The the reductionduration of process the reduction was 1 h.process Then, was the samples1 h. Then, were the samplesremoved, were cooled-down removed, incooled a desiccator-down in and a storeddesiccator in sealed and stored plastic in containers sealed plastic until characterization.containers until characterization.The roasted samples The were roasted then samples subjected were to the then wet subjected magnetic toseparation, the wet as magnetic previously separation, mentioned, as previouslyby applying mentione 3.5 amp coild, by current applying (0.6 T 3.5 magnetic amp coil field current strength) (0.6 and T magnetic two products, field i.e.,strength) a magnetic and two and products,a non-magnetic i.e., a magnetic one, were and obtained. a non-magnetic one, were obtained.

(a) (b)

FigureFigure 1. 1. SchematicSchematic diagram diagram of the of theprocess process for the for magnetic the magnetic separation separation of (a) ofcoarse (a) coarsefractions fractions (1.18– 3.35(1.18–3.35 mm and mm 0.300 and– 0.300–1.181.18 mm) and mm) (b and) fine (b fraction) fine fraction (−0.300 ( mm)0.300 mm). − 3.3. Characterization Characterization of of Bauxite Bauxite Ore Ore TableTable2 2 shows shows the the main main chemical chemical composition, composition, in in the the form form of of oxides, oxides, of of each each size size fraction fraction of theof the as received ore. The respective mass ratios of Al O to SiO and Al O to Fe O are also presented. It is as received ore. The respective mass ratios of Al2 23O3 to SiO2 2 and Al2 2O33 to Fe22O33 are also presented. It is observedobserved that that the the main main oxides oxides present present are Al are2O3 and Al2O Fe3 2andO3 assaying Fe2O3 assaying 57.95 and 57.95 22.39 wt%,and 22.39respectively. wt%, respectivelyThe Al2O3 content. The Al2 inO3 general content decreasesin general withdecreases decreasing with decreasing size, while size, increased while increased content of content Fe2O3 ofis Feobserved2O3 is observed in the finer in the fractions. finer fractions. As a result, As a result, the mass the ratiomass Alratio2O 3Al/Fe2Ο2O3/Fe3 decreases2Ο3 decreases from from 2.75 2.75 in the in 1.70–3.35 mm fraction to 1.76 in the 0.106 mm fraction. The overall Al O /SiO ratio is much greater the 1.70–3.35 mm fraction to 1.76 in− the −0.106 mm fraction. The overall2 3 Al2Ο2 3/SiΟ2 ratio is much greater(36.55) than(36.55) 10 than and thus,10 and the thus, bauxite the is bauxite considered is considered to be high-grade to be high and-grade can be and processed can be directlyprocessed by directlythe Bayer by process the Bayer [49 ].process [49]. The XRD patterns of three representative different size fractions, namely 1.70 3.35 mm, The XRD patterns of three representative different size fractions, namely 1.70−3.35 mm,− 0.212– 0.212–0.425 mm and 0.106 mm are shown in Figure2. It is observed that the main aluminum 0.425 mm and −0.106 mm− are shown in Figure 2. It is observed that the main aluminum containing phasescontaining present phases in the present ore are in the diaspore ore are [α diaspore-AlO(OH)] [α-AlO(OH)] and boeh andmite boehmite[γ-AlO(OH)], [γ-AlO(OH)], while the while gangue the minerals include hematite [Fe2O3], anatase [TiO2] and kaolinite [Al2Si2O5(OH)4]. It is revealed from these patterns that in general the crystalline phases identified differ marginally between the different

Minerals 2020, 10, 314 6 of 21

Minerals 2020, 10, x FOR PEER REVIEW 6 of 21 gangue minerals include hematite [Fe2O3], anatase [TiO2] and kaolinite [Al2Si2O5(OH)4]. It is revealed fromsize fractions. these patterns However, that in the general characteristic the crystalline peaks ofphases diaspore identified have di higherffer marginally intensity in between the coarse the difractionfferent (1.70 size– fractions.3.35 mm), However, while the the intermediate characteristic (0.212 peaks–0.425 of diaspore mm) and have fine higher (−0.106 intensity mm) fractions in the coarse fraction (1.70–3.35 mm), while the intermediate (0.212–0.425 mm) and fine ( 0.106 mm) fractions contain more boehmite and hematite. In addition, kaolinite is more abundant− in the fine fraction. containThese results more boehmiteare consistent and with hematite. the quantitative In addition, phase kaolinite analysis is more based abundant on Rietveld in the method, fine fraction. which Theseindicated results that are the consistent diaspore content with the was quantitative 63 and 42phase wt% in analysis the coarse based and on fine Rietveld fraction, method, respectively, which indicatedwhile the thathematite the diaspore content was content 10 an wasd 14 63 wt%, and respectively. 42 wt% in the coarse and fine fraction, respectively, while the hematite content was 10 and 14 wt%, respectively. Table 2. Chemical composition of each bauxite fraction obtained after sieving. Table 2. Chemical composition of each bauxite fraction obtained after sieving. Size Mass Al2O3 Fe2O3 SiO2 TiO2 LOI a aAl2O3/SiO2 Al2O3/Fe2O3 Size (mm) Mass(%wtAl) 2O(%wt)3 Fe(%wt)2O3 (%wt)SiO2 (%wt)TiO2 (%wt)LOI Al2O3/SiO2 Al2O3/Fe2O3 (mm)1.70–3.35(%wt) 38.52(%wt) 58.76 (%wt)21.34 (%wt)1.68 3.90(%wt) 5.1(%wt) 34.98 2.75 1.70–3.350.850–1.70 38.52 20.93 58.7659.45 21.3421.38 1.681.43 3.81 3.90 2.7 5.141.51 34.98 2.78 2.75 0.850–1.700.425–0.850 20.93 14.69 59.4558.33 21.3822.32 1.431.44 3.90 3.81 1.9 2.740.63 41.51 2.61 2.78 0.425–0.8500.212–0.42514.69 9.95 58.3358.37 22.3222.38 1.441.41 3.94 3.90 1.3 1.941.35 40.63 2.61 2.61 0.212–0.4250.106–0.2129.95 6.66 58.3757.65 22.3823.00 1.411.64 3.99 3.94 0.9 1.335.23 41.35 2.51 2.61 0.106–0.212−0.106 6.669.25 57.6550.38 23.0028.68 1.642.62 4.43 3.99 1.2 0.919.25 35.23 1.76 2.51 0.106 9.25 50.38 28.68 2.62 4.43 1.2 19.25 1.76 − Total 100.0 57.95 22.39 1.65 3.94 13.0 36.55 2.62 Total 100.0 57.95 22.39 1.65 3.94 13.0 36.55 2.62 a Loss on Ignition at 1050 °C for 3 h. a Loss on Ignition at 1050 ◦C for 3 h.

Figure 2. XRD patterns of three representative bauxite fractions obtained after sieving, namely (a) 1.70–3.35Figure 2.mm XRD fraction, patterns (b )of 0.212–0.425 three representative mm fraction, bauxite and ( cfractions) 0.106 mmobtained fraction. after sieving, namely (a) 1.70–3.35 mm fraction, (b) 0.212–0.425 mm fraction, and (c)− −0.106 mm fraction. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) of the bauxiteThermogravimetric ore are presented a innalysis Figure (TGA)3. The andTGA d thermogramserivative thermogravimetric show a weight lossanalysis of about (DTG) 2% upof the to 78bauxite◦C, which ore are is presented attributed in to Figure the loss 3. ofThe the TGA inherent thermograms moisture show of the a ore.weight After loss this of about temperature 2% up to a three-stage78 °C, which weight is attributed loss was to observed. the loss Theof the weight inhere lossnt moisture (~3%) detected of the inore. the After 78–486 this◦C temperature range can be a attributedthree-stage to weight the dehydroxylation loss was observed. of boehmite, The weight while loss a ( sharp~3%) detected weight loss in the of 9%78– was486 °C observed range can at the be 486–580attributed◦C to temperature the dehydroxylation range. This of boehmite bigger weight, while loss a sharp is associated weight loss with of the9% peakwas observed on DTG curveat the at486 532–580◦C, °C which temperature indicates range. the removalThis bigger of theweight chemically loss is associated bound water with present the peak in on diaspore DTG curve [50,51 at]. Dehydration532 °C, which and indicates removal the of theremoval structural of the water chemical of boehmitely bound and water kaolinite present may in alsodiaspore occur [ at50, this51]. temperatureDehydration range. and removal However, of duethe structural to the lower water content of boehmite of boehmite and and kaolinit kaolinitee may in thealso ore occur compared at this totemperature diaspore, these range. eﬀects However, overlap with due the to dehydrationthe lower content eﬀect of of diaspore boehmite [52 ]. and The kaolinite third stage in of the weight ore losscompared (~1.4%) to is diaspore, recorded these in the effects range 580–900overlap ◦withC and the may dehydration be assigned effect to the of release diaspore of CO[522].due The to third the decompositionstage of weight ofloss calcite. (~1.4% However,) is recorded calcite in the was range not detected 580–900 by°C XRDand may and be this assigned may be to due the to release fact that of itsCO content2 due to is the below decomposition the detection of limitcalcite (<. 2However, wt%) of the calcite instrument was not [ 53detected]. by XRD and this may be due to fact that its content is below the detection limit (<2 wt%) of the instrument [53].

Minerals 2020, 10, 314 7 of 21 MineralsMinerals 20 202020, ,10 10, ,x x FOR FOR PEERPEER REVIEWREVIEW 7 7of of 21 21

Figure 3. TGA and DTG curves of the bauxite ore. Figure 3. TGA and DTG curvescurves ofof thethe bauxitebauxite ore. ore. SEM images of the bauxite ore (crushed product 3.35 mm) along with EDS composition data are SEMSEM imagesimages ofof thethe bauxitebauxite ore (crushed product− −−3.353.35 mm)mm) alongalong withwith EDS EDS composition composition data data shown in Figure4a–d. areare shown shown inin FigureFigure 4a4a––d.d.

FigureFigure 4. 4. (a(a––c)c )Cross Cross section sectionalal SEM SEM-backscattered- backscattered electron (BSE) images of polished surfaces of bauxitebauxite ore Figure(crushedore (crushed 4. product (a–c) Cross product −3.35 section mm)3.35. alEDS mm).SEM spectra- backscattered EDS obtained spectra in obtainedelectron several (BSE)spot in several locations images spot of (b polished– locationsd) show surfacesthe (b– presenced) show of bauxite of the oxide ore − (crushedphases,presence the product ofmineral oxide −3.35 formation phases, mm).the EDS of mineral the spectra bauxite formation obtained clasts ofandin theseveral mixed/intergrown bauxite spot clasts locations and oxides mixed(b–d) inshow/intergrown the diasporicthe presence oxides matrix of in oxide (D: phases,Diasporethe diasporic the; B: mineral Boehmite matrix formation; (D: H: Diaspore;Hematite of the; B:A:bauxite Boehmite; Anatase clasts; K: H: andKaolinite Hematite; mixed/intergrown; F A:-T: Anatase; Fe-Ti oxide) K:oxides Kaolinite; in the F-T: diasporic Fe-Ti oxide). matrix (D: Diaspore; B: Boehmite; H: Hematite; A: Anatase; K: Kaolinite; F-T: Fe-Ti oxide) MicroscopicMicroscopic characterizationcharacterization ofof polishedpolished bauxite surfaces proved proved that that the the main main Al Al-bearing-bearing and and Fe-richFe-richMicroscopic phasesphases presentpresent characterization inin thethe oreore areofare polished diasporediaspore bauxite [[αα-AlO(OH)],-AlO(OH)], surfaces boehmite proved that [γ [γ--AlO(OH)] AlO(OH)]the main Al and and-bearing hematite hematite and [FeFe[Fe-2richO2O33],], phases respectivelyrespectively present (Figure(Figure in the4 a).4a). ore In In are accordance accordance diaspore with [αwith-AlO(OH)], the the XRD XRD results, results,boehmite remarkable remarkable [γ-AlO(OH)] minor minor and quantities quantities hematite of anatase[Feof 2 anataseO3], [TiOres pectively2 [TiO] and2] kaolinite and (Figure kaolinite [Al 4a).2Si In2 [AlO accordance52(OH)Si2O5(OH)4] (mixed4 ]with (mixed with the Al-oxides)XRD with results, Al-oxides) were remarkable also were found also minor in found the quantities diasporic in the ofdiasporic anatase clasts [TiO 2] of and the kaoliniteore (Figure [Al 4b).2Si2 O More5(OH) specifically4] (mixed , withFigure Al -4coxides) (zoom were of rectangular also found area in ofthe diasporic clasts of the ore (Figure 4b). More specifically, Figure 4c (zoom of rectangular area of

Minerals 2020, 10, 314 8 of 21 Minerals 2020, 10, x FOR PEER REVIEW 8 of 21 clastsFigure of 4b) the shows ore (Figure the presence4b). More of speciﬁcally, anatase that Figure occurs4c (zoomin the ofform rectangular of individual area of microscale Figure4b) particles shows the(<2 presenceμm) widely of anatasedispersed that within occurs the in diasporic the form ofcore individual (center of microscale the clast) particlesas well as (< in2 µthem) form widely of dispersedintergrown within particles the diasporicwithin the core internal (center rings of the of clast) hematite as well (inner-outer as in the form dashed of intergrown line area) particles in the withindiasporic the clast internal [11]. rings In this of context, hematite EDS (inner-outer point anal dashedyses confirmed line area) the in thepresence diasporic of Ti clast in the [11 brightest]. In this context,particles, EDS i.e., pointintergrown analyses with conﬁrmed hematite the (Fe-Ti presence oxide) of as Ti well in the as brightestindividually particles, (particles) i.e., intergrown within the withdiaspore hematite core, (Fe-Ti while oxide) Si was as detected well as individually in the gray (particles)region of withinthe diasporic the diaspore matrix core, due while to kaolinite Si was detectedcontribution in the (Figure gray region 4b–d). of the diasporic matrix due to kaolinite contribution (Figure4b–d).

4.4. Results and Discussion

4.1.4.1. Kinetic Behavior Behavior of of Grinding Grinding Process Process TheThe grindinggrinding behaviorbehavior ofof thethe bauxitebauxite oreore waswas evaluatedevaluated byby identifyingidentifying thethe relationshiprelationship betweenbetween thethe remainingremaining mass mass (%) (%) fraction fraction of eachof each particle particle size vs.size grinding vs. grinding time. Figuretime. 5Figurea shows, 5a as shows, an example, as an theexample, experimental the experimental data obtained data for obtained four selected for four sizes, selected namely sizes, 0.600, namely 0.300, 0.600, 0.150 0.300, and 0.075 0.150 mm and when 0.075 themm material when the filling material volume filling (f c) was volume 5%. It ( isfc) observedwas 5%. thatIt is the observed experimental that the data experimental are expressed data well are by Equationexpressed (1) well and by thus, Equation grinding (1) ofan thed thus, ore exhibitsgrinding non-first-order of the ore exhibi behaviorts non-first-order and the grinding behavior rate and of eachthe grinding size fraction rate isof time each dependent. size fraction High is time values dependent. of correlation High coevaluesfficient of Rcorrelation2, ranging coefficient from 0.996 R to2, 1.0,ranging were from obtained 0.996 for to the 1.0, di werefferent obtainedf c values for used. the Indifferent general, fc the values reduction used.rate In general, of larger the particle reduction sizes israte higher of larger than thatparticle of smaller sizes is particles higher than with increasingthat of smaller grinding particles time, with indicating increasing that largergrinding particles time, areindicating ground that more larger efficiently. particles Figure are ground5b shows more the effi fittingciently. curves Figure of the5b shows particle the size fitting 0.300 curves mm when of the Equationparticle size (1) was0.300 applied mm when to the Equation experimental (1) was data, applied for di fftoerent the fexperimentalc values (5%, data, 10% andfor different 15%). It isfc apparentvalues (5%, that 10% the and reduction 15%). rateIt is forapparent a specific that particle the reduction size depents rate for on a the specific material particle filling size volume depentsf c and on thusthe material the grinding filling rate volume and consequently fc and thusthe the grinding grinding e ffirateciency and increases consequently with decreasingthe grindingf c. Asefficiency can be seenincreases in Table with3, decreasing which shows fc. As the can calculated be seen parametersin Table 3, whichK and showsM by fittingthe calculated the Alyavdin parameters equation K and to experimentalM by fitting data,the Alyavdin the grinding equation rate constant to experimentalK decreases data, from the 0.206 grinding to 0.043 rate when constantf c increases K decreases from 5%from to 0.206 15%. Ittois 0.043 also seenwhen that fc increasesM values from range 5% between to 15%. 0.780 It isand also 1.084, seen confirmingthat M values the non-first-orderrange between grinding0.780 and behavior 1.084, confirming of the ore the [37 ,non-first-order41,54]. grinding behavior of the ore [37,41,54].

(a) (b)

FigureFigure 5.5. RemainingRemaining mass mass fraction fraction (wt%) (wt%) vs. vs. grinding grinding time time for for (a) four(a) four selected selected sizes sizes when whenf c = 5% fc = and 5% (andb) particle (b) particle size 0.300size 0.300 mm whenmm when diferrent diferrentf c values fc values (5%, (5%, 10% 10% and 15%)and 15%) were were used. used.

Table 3. Parameters of Alyavdin formula (Equation (1)) for particle size 0.300 mm at diﬀerent f c values. Table 3. Parameters of Alyavdin formula (Equation (1)) for particle size 0.300 mm at different fc

values. Parameter f c = 5% f c = 10% f c = 15%

KParameter 0.206fc = 5% fc 0.059= 10% fc = 15% 0.043 M K 0.7800.206 0.059 1.084 0.043 0.987 2 R (adj.) M 0.9990.780 1.084 0.999 0.987 0.996 R2 (adj.) 0.999 0.999 0.996

4.2. Al2O3 and Fe2O3 Content of the Selective Grinding Products

Minerals 2020, 10, 314 9 of 21 Minerals 2020, 10, x FOR PEER REVIEW 9 of 21 Minerals 2020, 10, x FOR PEER REVIEW 9 of 21 4.2. Al2O3 and Fe2O3 Content of the Selective Grinding Products Figure 6 shows the Al2O3 and Fe2O3 content as well as the mass recovery (yield) in the product Figure 6 shows the Al2O3 and Fe2O3 content as well as the mass recovery (yield) in the product fractionsFigure (as6 cumulativeshows the Al oversize)2O3 and after Fe2O 23 mincontent of grinding as well as when the mass the material recovery filling (yield) volume in the product(fc) was fractions (as cumulative oversize) after 2 min of grinding when the material filling volume (fc) was 10%.fractions It is (asseen cumulative that as size oversize) decreases after the 2yield min in of the grinding product when coarser the materialthan this ﬁllingsize increases. volume (Inf c) light was 10%. It is seen that as size decreases the yield in the product coarser than this size increases. In light 10%.of this, It mass is seen recovery that as sizeincreases decreases from the 43.2 yield to 87.6 in the wt% product when coarserthe product than size this decreases size increases. from In1.18 light to of this, mass recovery increases from 43.2 to 87.6 wt% when the product size decreases from 1.18 to 0.106of this, mm. mass With recovery decreased increases product from size 43.2 the to Fe 87.62O3 grade wt% when of the the coarser product particles size decreases increased fromby 5.6 1.18 wt%, to 0.106 mm. With decreased product size the Fe2O3 grade of the coarser particles increased by 5.6 wt%, while0.106 mm.the Al With2O3 grade decreased decreased product slightly size the by Fe 1.72O wt%,3 grade from of the65 to coarser 63.3 wt%. particles The increasedresults show by 5.6that wt%, the while the Al2O3 grade decreased slightly by 1.7 wt%, from 65 to 63.3 wt%. The results show that the whilecontent the of Al Al2O2O33,grade which decreased is mainly slightly present by in 1.7 diaspore, wt%, from increases 65 to 63.3 slightly wt%. in The coarser results fractions, show that while the content of Al2O3, which is mainly present in diaspore, increases slightly in coarser fractions, while Fecontent2O3 grade of Al becomes2O3, which higher is mainly in finer present fractions in indicating diaspore, increasesgood selectivity slightly during in coarser grinding. fractions, However, while Fe2O3 grade becomes higher in finer fractions indicating good selectivity during grinding. However, theFe2 Omass3 grade recovery becomes in coarser higher fractions in ﬁner fractions and consequently indicating the good overall selectivity Al2O3 duringrecovery grinding. is very low. However, the mass recovery in coarser fractions and consequently the overall Al2O3 recovery is very low. the mass recovery in coarser fractions and consequently the overall Al2O3 recovery is very low.

Figure 6. Al2OO33 grade,grade, Fe Fe22OO33 gradegrade and and mass mass recovery recovery (yield) (yield) in in the the cumulative cumulative oversize after 2 min Figure 6. Al2O3 grade, Fe2O3 grade and mass recovery (yield) in the cumulative oversize after 2 min of grinding and when f c == 10%.10%. of grinding and when fc = 10%.

The eeffectﬀect of grindinggrinding timetime onon AlAl22O3 grade and recovery for the productproduct fractions coarser than The effect of grinding time on Al2O3 grade and recovery for the product fractions coarser than 1.18 mm and 0.300 mm is presentedpresented in FigureFigure7 7a,ba,b respectively. respectively. ItIt is is seen seen from from Figure Figure7 a7a that that Al Al22OO33 recovery1.18 mm inand the 0.300 product mm is fraction presented coarser in Figure than 1.18 7a,b mm respectively. decreases It from is seen 45.2 from to 21.8 Figure wt% 7a as that grinding Al2O3 recovery in the product fraction coarser than 1.18 mm decreases from 45.2 to 21.8 wt% as grinding time increasesincreases fromfrom 2 2 to to 12 12 min, min, respectively, respectively, while while the Althe2 OAl3 2gradeO3 grade remains remains almost almost constant, constant, ~65 wt%, ~65 time increases from 2 to 12 min, respectively, while the Al2O3 grade remains almost constant, ~65 wt%,during during grinding. grin Higherding. Higher Al2O3 recoveryAl2O3 recovery can be achievedcan be achieved for the product for the fractionproduct coarser fraction than coarser 0.300 than mm, wt%, during grinding. Higher Al2O3 recovery can be achieved for the product fraction coarser than 0.300as seen mm, in Figureas seen7 b.in InFigure this context,7b. In this Al context,2O3 recovery Al2O3 reachesrecovery almost reaches 73 almost wt% after 73 wt% 2 min after of grinding2 min of 0.300 mm, as seen in Figure 7b. In this context, Al2O3 recovery reaches almost 73 wt% after 2 min of withgrinding a corresponding with a corresponding Al2O3 grade Al 642O wt%.3 grade At 64 this wt%. grinding At this period, grinding the Fe period,2O3 grade the wasFe2O 18.33 grade wt% was and grinding with a corresponding Al2O3 grade 64 wt%. At this grinding period, the Fe2O3 grade was 18.3the corresponding wt% and the corresponding recovery 65.4 wt%. recovery Thus, 65.4 better wt%. results Thus, can better be obtained results can in termsbe obtained of Al2O in3 recoveryterms of 18.3 wt% and the corresponding recovery 65.4 wt%. Thus, better results can be obtained in terms of Alfor2O the3 recovery product for fraction the product coarser fraction than 0.300 coarser mm andthan after 0.300 2 mm min and of grinding. after 2 min of grinding. Al2O3 recovery for the product fraction coarser than 0.300 mm and after 2 min of grinding.

(a) (b) (a) (b) Figure 7. EffectEﬀect of grinding time on Al 2OO33 gradegrade and and recovery recovery for for the the product product fraction fraction coarser coarser than than ( (aa)) Figure 7. Effect of grinding time on Al2O3 grade and recovery for the product fraction coarser than (a) 1.18 mm and (b) 0.3000.300 mm,mm, whenwhen ffc == 10%.10%. 1.18 mm and (b) 0.300 mm, when fc = 10%.

Figure 8a presents the effect of material filling volume fc (5%, 10% and 15%) on Al2O3 grade and recoveryFigure for 8a the presents product the fraction effect of coarser material than filling 0.300 volume mm after fc (5%, 2 min10% andgrinding. 15%) onIt isAl observed2O3 grade thatand recovery for the product fraction coarser than 0.300 mm after 2 min grinding. It is observed that

Minerals 2020, 10, 314 10 of 21

Figure8a presents the e ffect of material filling volume f c (5%, 10% and 15%) on Al2O3 grade and recovery for the product fraction coarser than 0.300 mm after 2 min grinding. It is observed that Al2O3 grade increases marginally from 63.5 to 64 wt% with increasing f c from 5% to 10% and then drops to 62.6 wt% when f c was 15%. However, Al2O3 recovery increases significantly from 57.7% to 73% when f cMineralsincreasesMinerals 2020 2020, to10,, 10x 10% ,FOR x FOR PEER and PEER REVIEW reaches REVIEW 73.4 wt% at f c = 15%. Similar results were obtained10 forof10 21 of the 21 Fe2O3 grade and recovery in the product fraction coarser than 0.300 mm, as seen in Figure8b. Fe O grade Al2O3 grade increases marginally from 63.5 to 64 wt% with increasing fc from 5% to 10% and then2 3 Al2O3 grade increases marginally from 63.5 to 64 wt% with increasing fc from 5% to 10% and then increased slightlydrops to 62.6 from wt% 17.6 when to 18.7fc was wt% 15%. with However, increasing Al2O3 recoveryf c to 15%, increases while significantly its recovery from reached 57.7% to 66.2 wt%. drops to 62.6 wt% when fc was 15%. However, Al2O3 recovery increases significantly from 57.7% to Based on these73% when results, fc increasesf = 10%to 10% can andbe reaches considered 73.4 wt% asat f thec = 15%. optimum Similar results material were obtained volume for in the the mill for 73% when fc increasesc to 10% and reaches 73.4 wt% at fc = 15%. Similar results were obtained for the Fe2O3 grade and recovery in the product fraction coarser than 0.300 mm, as seen in Figure 8b. Fe2O3 upgradingFe2O the3 grade bauxite and recovery ore during in thegrinding. product fraction Under coarser these than conditions, 0.300 mm, theas seen Al 2inO Figure3 grade 8b. increased Fe2O3 by grade increased slightly from 17.6 to 18.7 wt% with increasing fc to 15%, while its recovery reached 9.5 wt%grade compared increased with slightly the from raw 17.6 ore, to with 18.7 correspondingwt% with increasing recovery fc to 15%, 73 while wt%, its while recovery the reached Fe O grade 66.2 wt%. Based on these results, fc = 10% can be considered as the optimum material volume in the2 3 66.2 wt%. Based on these results, fc = 10% can be considered as the optimum material volume in the decreasedmill by 18.3for upgrading wt% and the its bauxite recovery ore wasduring 65.4 grinding. wt%. Under It is worth these conditions, mentioning the thatAl2O3 despite grade the fact mill for upgrading the bauxite ore during grinding. Under these conditions, the Al2O3 grade that a higherincreased grinding by 9.5 rate wt% of compared the material with the is raw achieved ore, with when correspondingf c = 5% recovery (Table3 73), thewt%, results while the in terms of increasedFe2O3 grade by 9.5 decreased wt% compared by 18.3 withwt% theand rawits recoveryore, with was corresponding 65.4 wt%. It recoveryis worth mentioning73 wt%, while that the Al2O3 grade and recovery are not satisfactory. Hence, the results of the present study show that the Fe2Odespite3 grade the decreased fact that a byhigher 18.3 grindingwt% and rate its of recovery the material was is65.4 achieved wt%. whenIt is worth fc = 5% mentioning (Table 3), the that usual industrial practice of using higher milling rate does not necessarily result in efficient separation despiteresults the in factterms that of aAl higher2O3 grade grinding and recovery rate of are the not material satisfactory. is achieved Hence, whenthe results fc = 5% of the(Table present 3), the betweenresults thestudy di in ffshow termserent that of minerals theAl2 usualO3 grade industrial of interestand recovery practice present ofare using not in higher oressatisfactory. or milling concentrates. Hence, rate does the not results Thus, necessarily of detailed the result present grinding kineticsstudy analysisin efficientshow is that required separation the usual in between orderindustrial tothedetermine practicedifferent of minerals using the optimum higher of interest milling grinding present rate doesin conditions ores not or necessarily concentrates. that maximize result the breakagein rateefficientThus, of detailed the separation minerals grinding between ofkinetics interest, th analysise different as foris requ minerals exampleired in oforder diaspore interest to determine present in this thein case, oresoptimum whileor concentrates. grinding minimizing the conditions that maximize the breakage rate of the minerals of interest, as for example diaspore in this respectiveThus, rate detailed of the grinding impurities kinetics [55]. analysis is required in order to determine the optimum grinding conditionscase, while that minimizing maximize thethe respective breakage rate ofof thethe impurities minerals of [55]. interest, as for example diaspore in this case, while minimizing the respective rate of the impurities [55].

(a) (b)

Figure 8. Effect of material filling volume fc on (a ) Al2O3 grade and recovery and (b) Fe2O3 grade and Figure 8. Effect of material filling volume f c on (a) Al2O3 grade and recovery and (b) Fe2O3 grade and recovery for the product(a) fraction coarser than 0.300 mm after 2 min of grinding.(b) recovery for the product fraction coarser than 0.300 mm after 2 min of grinding. Figure 8. Effect of material filling volume fc on (a) Al2O3 grade and recovery and (b) Fe2O3 grade and Figure 9 presents the Al2O3 enrichment ratio (Al2O3 grade in the product: Al2O3 grade in the recovery for the product fraction coarser than 0.300 mm after 2 min of grinding. Figurefeed)9 presents for the thecumulative Al 2O 3oversizeenrichment product ratio after (Al 2 min2O3 ofgrade grinding in the when product: different Al material2O3 grade filling in the feed) volumes fc were used. The results show that when the material filling volume fc was 10%, in all cases for the cumulativeFigure 9 oversize presents productthe Al2O3 afterenrichment 2 min ratio of grinding (Al2O3 grade when in ditheff erentproduct: material Al2O3 grade filling in volumes the f c a higher Al2O3 enrichment ratio was obtained, while the lowest enrichment ratio was achieved at fc = were used.feed) The for the results cumulative show thatoversize when product the material after 2 min filling of grinding volume whenf c was different 10%, material in all cases filling a higher 15%. The maximum ratio of 1.12 was achieved for the coarser than 1.18 mm product fraction with 65 Al O enrichmentvolumes fc were ratio used. was The obtained, results show while that thewhen lowest the material enrichment filling volume ratio f wasc was achieved10%, in all cases at f c = 15%. 2 3 wt% Al2O3 grade and corresponding recovery 45.2 wt%. The maximuma higher ratioAl2O3 enrichment of 1.12 was ratio achieved was obtained, for the while coarser the lowest than 1.18enrichment mm product ratio was fraction achievedwith at fc = 65 wt% 15%. The maximum ratio of 1.12 was achieved for the coarser than 1.18 mm product fraction with 65 Al2O3 grade and corresponding recovery 45.2 wt%. wt% Al2O3 grade and corresponding recovery 45.2 wt%.

Figure 9. Al2O3 enrichment ratio in the cumulative oversize after 2 min of grinding when different

material filling volumes fc were used.

Figure 9. Al2O3 enrichment ratio in the cumulative oversize after 2 min of grinding when different Figure 9. Al2O3 enrichment ratio in the cumulative oversize after 2 min of grinding when diﬀerent material filling volumes fc were used. material ﬁlling volumes f c were used.

Minerals 2020, 10, x FOR PEER REVIEW 11 of 21 Minerals 2020, 10, 314 11 of 21

4.3. Magnetic Separation Efficiency on Bauxite 4.3. Magnetic Separation Eﬃciency on Bauxite Figure 10a shows the Al2O3 grade in the magnetic separation products, namely MI, MII and

NMIIFigure for the 10 a1.18 shows–3.35 theand Al 0.3002O3 –grade1.18 mm in the fractions, magnetic when separation the material products, filling namely volume MI, fc was MII 10%. and NMIIThe Al for2O3 the grade 1.18–3.35 in the feed and 0.300–1.18is also presented mm fractions, for comparison. when the As materialexpected, filling the non-magnetic volume f c was product 10%. The(NMII) Al2O has3 grade higher in theAl2O feed3 grade is also compared presented tofor the comparison. magnetic one, As i.e., expected, MI and the MII. non-magnetic The Al2O3 productgrade in (NMII)this product, has higher although Al2O higher3 grade than compared the one to in the the magnetic feed, does one, not i.e., exceed MI and 66.3 MII. wt%. The The Al 2feedO3 grade fraction in thishas product,a great althougheffect on higherthe magnetic than the separation one in the feed,efficiency does notgiven exceed that 66.3the wt%.non-magnetic The feed fractionproduct hasobtained a great from effect the on the0.300 magnetic–1.18 mm separation fraction eisffi ciencymore rich given in that Al2 theO3 non-magneticdespite the fact product that finer obtained feed fromfractions the 0.300–1.18 contain less mm Al fraction2O3. The is more non-magnetic rich in Al2 Oproduct3 despite (NMII) the fact contained that finer feed14.6– fractions15.7 wt% contain Fe2O3 lessdepending Al2O3. Theon the non-magnetic feed fraction product tested, (NMII)as seen containedin Figure 10b. 14.6–15.7 Fe2O3 wt% grade Fe in2O this3 depending product corresponds on the feed fractionto a 9.6 tested,and 21.8 as seenwt% in reduction Figure 10 whenb. Fe2 Othe3 grade feed infraction this product used was corresponds 1.18–3.35 to and a 9.6 0.300 and– 21.81.18 wt%mm, reductionrespectively. when the feed fraction used was 1.18–3.35 and 0.300–1.18 mm, respectively.

(a) (b)

FigureFigure 10.10.( (aa)) Al Al22OO33 gradegrade andand ((bb)) FeFe22O33 grade in the magnetic separationseparation productsproducts forfor thethe 1.18–3.351.18–3.35 andand 0.300–1.180.300–1.18 mmmm fractions,fractions, when when the the material material ﬁlling filling volume volumef cfc waswas 10%.10%.

TheThe eeffectffect ofof thethe material material filling filling volume volumef cfcon onAl Al22OO33 gradegrade andand recoveryrecovery inin thethe non-magneticnon-magnetic productproduct NMIINMII whenwhen the 0.300–1.180.300–1.18 mm fraction fraction was was subjected subjected to to magnetic magnetic separation separation is ispresented presented in inFigure Figure 11a. 11 a.It Itis isobserved observed that that the the maximum maximum Al Al2O2O3 3gradegrade and and recovery, recovery, i.e., i.e., 67.85 and 47.8 wt%,wt%, respectivelyrespectively is is achieved achieved when whenf cfc= = 5%5% whilewhile atat higherhigher materialmaterial filling filling volume volume ( f(cfc= = 10%) thethe respectiverespective qualityquality slightlyslightly drops from 67.85 to to 66.2 66.2 wt% wt% and and from from 47.8 47.8 to to 44.7 44.7 wt%. wt%. In Inaddition, addition, Al2 AlO32 gradeO3 grade and andrecovery recovery improve improve slightly slightly when when fc increasesf c increases from from10% to 10% 15%. to 15%.Under Under the optimum the optimum material material filling fillingvolume volume (fc = 5%) (f c the= 5%) Al2 theO3 grade Al2O3 isgrade increased is increased by 9.2 bywt% 9.2 by wt% taking by takinginto account into account the grade the gradein the in0.300 the– 0.300–1.181.18 mm feed mm fraction. feed fraction. Under Under these conditio these conditions,ns, the NMII the NMIIproduct product contains contains 13.51 13.51wt% Fe wt%2O3 Fecorresponding2O3 corresponding to a decrease to a decrease of 30.1 of wt% 30.1 wt%compared compared to the to feed the feedfraction, fraction, as seen as seen in Figure in Figure 11b. 11 Asb. Asmaterial material filling filling volume volume increases increases from from 5 to 5 10 to wt% 10 wt% the theFe2O Fe32 gradeO3 grade also also increases increases from from 13.51 13.51 to 14.6 to 14.6wt%, wt%, while while the therespective respective grade grade in inthe the NMII NMII product product remains remains unchanged unchanged when when ffcc increasesincreases to 15%.

TheThe Fe MineralsFe2O2O33recovery 2020recovery, 10, x FOR ranges ranges PEER between REVIEW between 30.5 30.5 and and 32.7 32.7 wt% wt depending% depending on the on material the material ﬁlling volumefilling12 of volume21 tested. tested.

(a) (b)

Figure 11. Effect of material filling volume fc on (a) Al2O3 grade and recovery and (b) Fe2O3 grade and Figure 11. Eﬀect of material ﬁlling volume f c on (a) Al2O3 grade and recovery and (b) Fe2O3 grade and recoveryrecovery in the in NMII the NMII product product when when the 0.300–1.18 the 0.300 mm–1.18 fraction mm fraction was subjected was subjected to magnetic to magnetic separation. separation.

Figure 12a shows the Al2O3 grade and recovery in the cumulative non-magnetic product for the −0.300 mm fraction when the material filling volume fc was 10%. It is seen that the Al2O3 recovery is continuously increasing from 44.2 to 86.3 wt% in the cumulative non-magnetic product, but the Al2O3 grade slightly decreased from 60.7 to 58.6 wt%. A similar trend was observed for Fe2O3 recovery which increased from 33.9 to 77.2 wt% by taking into account the same products (Figure 12b), indicating that the separation is not satisfactory, especially in the NMIII product, and a large amount of iron is distributed in the non-magnetic products. Thus, considering the Al2O3 and Fe2O3 recoveries in the products a two stage magnetic separation (cumulative NMII product) is considered adequate for better product quality. In addition, no significant differences in Al2O3, Fe2O3 grades and recoveries at different material filling volumes were observed. In light of this, Al2O3 grade and recovery in the cumulative NMII product varied between 59.4–60.3 wt% and 65–70.4 wt%, respectively, when fc varied between 5% and 15%. This product contained about 21 wt% Fe2O3 with a recovery of about 56 wt%, which corresponds to a 10.9–14.1 wt% reduction depending on the grade of the feed fraction at each material filling volume.

(a) (b)

Figure 12. (a) Al2O3 grade and recovery and (b) Fe2O3 grade and recovery in the cumulative

non-magnetic product for the −0.300 mm fraction when the material filling volume fc was 10%.

Figure 13 presents the Al2O3 enrichment ratio for the non-magnetic products obtained after magnetic separation of different feed fractions for various material filling volumes fc. The feed size indicated in this figure is the upper size of the tested size class. It is observed, for all material filling volumes used, that no major changes of the Al2O3 enrichment ratio occur. More specifically, the Al2O3 enrichment ratio increased slightly when the feed fraction increased from −0.300 to 0.300–1.18 mm and then dropped slightly for the coarse fraction 1.18–3.35 mm. The maximum ratio of 1.16 was achieved for the product fraction 0.300–1.18 mm when the material filling volume was 5%.

Minerals 2020, 10, x FOR PEER REVIEW 12 of 21

(a) (b)

Figure 11. Effect of material filling volume fc on (a) Al2O3 grade and recovery and (b) Fe2O3 grade and recovery in the NMII product when the 0.300–1.18 mm fraction was subjected to magnetic Minerals 2020, 10, 314 12 of 21 separation.

FigureFigure 1212aa showsshows thethe AlAl22OO33 grade and recovery in the cumulative non-magneticnon-magnetic productproduct forfor thethe −0.300 mm fractionfraction whenwhen thethe materialmaterial fillingfilling volumevolumef fcc waswas 10%.10%. ItIt isis seenseen thatthat thetheAl Al2OO3 recoveryrecovery isis − 2 3 continuouslycontinuously increasingincreasing from from 44.2 44.2 to 86.3to 86.3 wt% wt% in the in cumulativethe cumulative non-magnetic non-magnetic product, product, but the but Al2 Othe3 gradeAl2O3 slightlygrade slightly decreased decreased from 60.7 from to 58.6 60.7 wt%. toA 58.6 similar wt%. trend A similar was observed trend forwas Fe observed2O3 recovery for whichFe2O3 increasedrecovery which from 33.9 increased to 77.2 from wt% 33.9 by taking to 77.2 into wt% account by taking the into same account products the (Figure same products12b), indicating (Figure that12b), the indicating separation that is the not separati satisfactory,on is especiallynot satisfactory, in the especially NMIII product, in the andNMIII a large product, amount and ofa large iron isamount distributed of iron in is the distributed non-magnetic in the products. non-magnetic Thus, products. considering Thus, the considering Al2O3 and Fethe2 OAl3 2recoveriesO3 and Fe2O in3 therecoveries products in athe two products stage magnetic a two stage separation magnetic (cumulative separation NMII (cumulative product) NMII is considered product) is adequate considered for betteradequate product for better quality. product In addition, quality. no In significantaddition, no di significantfferences in differences Al2O3, Fe2 inO 3Algrades2O3, Fe and2O3 grades recoveries and atrecoveries different materialat different filling material volumes filling were vo observed.lumes were In lightobserved. of this, In Al light2O3 grade of this, and Al recovery2O3 grade in and the cumulativerecovery in NMII the productcumulative varied NMII between product 59.4–60.3 varied wt% between and 65–70.4 59.4 wt%,–60.3 respectively, wt% and when65–70.4f c varied wt%, betweenrespectively, 5% and when 15%. fc varied This product between contained 5% and 15%. about This 21 product wt% Fe2 containedO3 with a recoveryabout 21 wt% of about Fe2O 563 with wt%, a whichrecovery corresponds of about 56 to wt%, a 10.9–14.1 which wt%corresponds reduction to dependinga 10.9–14.1 onwt% the reduction grade of thedepending feed fraction on the at grade each materialof the feed filling fraction volume. at each material filling volume.

(a) (b)

FigureFigure 12.12. (a) AlAl2O33 gradegrade and recovery and (b)) FeFe22O3 grade and recovery inin thethe cumulativecumulative non-magneticnon-magnetic productproduct forfor thethe −0.3000.300 mmmm fractionfraction whenwhen thethe materialmaterial ﬁllingfilling volumevolume ffcc was 10%. −

FigureFigure 1313 presentspresents thethe AlAl22OO33 enrichmentenrichment ratioratio forfor thethe non-magneticnon-magnetic productsproducts obtainedobtained afterafter magneticmagnetic separationseparation ofof didifferentfferent feedfeed fractionsfractions forfor variousvariousmaterial material fillingfilling volumes volumesf fcc.. TheThe feedfeed sizesize indicatedindicated inin thisthis figurefigure isis thethe upperupper sizesize ofof thethe testedtested sizesize class.class. ItIt isis observed,observed, forfor allall materialmaterial fillingfilling volumesvolumes used,used, that that no no major major changes changes of theof Althe2 OAl3 enrichment2O3 enrichment ratio ratio occur. occur. More More specifically, specifically, the Al2 Othe3 enrichment ratio increased slightly when the feed fraction increased from 0.300 to 0.300–1.18 mm and Al2O3 enrichment ratio increased slightly when the feed fraction increased− from −0.300 to 0.300–1.18 thenmm droppedand then slightlydropped for slightly the coarse for the fraction coarse 1.18–3.35 fraction mm.1.18– The3.35 maximummm. The maximum ratio of 1.16 ratio was of achieved 1.16 was Minerals 2020, 10, x FOR PEER REVIEW 13 of 21 forachieved the product for the fraction product 0.300–1.18 fraction 0.300 mm– when1.18 mm the materialwhen the filling material volume filling was volume 5%. was 5%.

Figure 13. Al2O3 enrichment ratio in the non-magnetic products when diﬀerent feed fractions were Figure 13. Al2O3 enrichment ratio in the non-magnetic products when different feed fractions were subjected to magnetic separation for various material ﬁlling volumes f c. subjected to magnetic separation for various material filling volumes fc.

The magnetic separation efficiency was also investigated using the Fuerstenau-II upgrading curves which relate recoveries of components in products [56,57]. Figure 14a shows the Fuerstenau-II upgrading plots of different feed fractions when the material filling volume fc was 10%. It is shown that better separation of Al2O3 from Fe2O3 in the concentrate (non-magnetic product) is achieved when the 0.300–1.18 mm feed fraction was used, while the efficiency of magnetic separation is lower when the coarse fraction 1.18–3.35 mm was used. As shown in Figure 14b, which presents the effect of the material filling volume on the separation efficiency for the 0.300–1.18 mm fraction, slightly better results were obtained when the material filling volume was 15%. Overall, it can be concluded that in terms of Al2O3 and Fe2O3 recoveries in the non-magnetic products, better separation efficiency was achieved when the 0.300–1.18 mm fraction was subjected to magnetic separation using 15% material filling volume.

(a) (b)

Figure 14. The Fuerstenau-II upgrading curves for different (a) feed fractions when fc was 10% and (b) material filling volumes fc when the 0.300–1.18 mm feed fraction was used.

4.4. Magnetic Separation Efficiency after Reduction Roasting of Bauxite A reduction roasting–magnetic separation process can modify the distribution of aluminium and iron phases. In the reduction roasting process, weak magnetic iron-containing minerals such as hematite can be converted into strong ones, e.g., magnetite, or even metallic iron, which can be easily separated after magnetic separation [58–60]. Higher reductant dosage may result in the formation of magnetite. However, other studies indicated that the excess of coal affects negatively the magnetic separation efficiency, due to the further reduction of magnetite to wüstite (FeO) [17]. The following reactions can take place during reduction roasting, (3) 3FeO +C→2FeO +CO (4) FeO + 3C → 2Fe + 3CO

Minerals 2020, 10, x FOR PEER REVIEW 13 of 21

Figure 13. Al2O3 enrichment ratio in the non-magnetic products when different feed fractions were Minerals 2020, 10, 314 13 of 21 subjected to magnetic separation for various material filling volumes fc.

TheThe magneticmagnetic separationseparation eefficiencyfficiency waswas alsoalso investigatedinvestigated usingusing thethe Fuerstenau-IIFuerstenau-II upgradingupgrading curvescurves whichwhich relate relate recoveries recoveries of components of components in products in products [56,57].Figure [56,57]. 14 aFigure shows the14a Fuerstenau-II shows the upgradingFuerstenau-II plots upgrading of different plots feed of fractions different when feed thefractions material when filling the volume materialf c fillingwas 10%. volume It is shownfc was that10%. better It is separationshown that of better Al2O 3separationfrom Fe2O 3ofin Al the2O3 concentrate from Fe2O (non-magnetic3 in the concentrate product) (non-magnetic is achieved whenproduct) the 0.300–1.18is achieved mm when feed the fraction 0.300 was–1.18 used, mm while feed thefraction efficiency was ofused, magnetic while separation the efficiency is lower of whenmagnetic the coarseseparation fraction is lower 1.18–3.35 when mm the was coarse used. fraction As shown 1.18 in–3.35 Figure mm 14 wasb, which used. presentsAs shown the in e ffFigureect of the14b, material which presents filling volume the effect on the of separationthe material effi ciencyfilling forvolume the 0.300–1.18 on the separation mm fraction, efficiency slightly for better the results0.300–1.18 were mm obtained fraction, when slightly the material better results filling were volume obtained was 15%. when Overall, the material it can be filling concluded volume that was in terms15%. Overall, of Al2O 3itand can Febe2 Oconcluded3 recoveries that in in the terms non-magnetic of Al2O3 and products, Fe2O3 betterrecoveries separation in the enon-magneticfficiency was achievedproducts, when better the separation 0.300–1.18 efficiency mm fraction was achieved was subjected when to the magnetic 0.300–1.18 separation mm fraction using was 15% subjected material fillingto magnetic volume. separation using 15% material filling volume.

(a) (b)

FigureFigure 14.14. TheThe Fuerstenau-IIFuerstenau-II upgrading upgrading curves curves for for di differentﬀerent (a )(a feed) feed fractions fractions when whenf c was fc was 10% 10% and and (b) material(b) material ﬁlling filling volumes volumesf c when fc when the the 0.300–1.18 0.300–1.18 mm mm feed feed fraction fraction was was used. used.

4.4.4.4. Magnetic Separation Separation Effi Efficiencyciency after after Reduction Reduction Roasting Roasting of Bauxite of Bauxite AA reductionreduction roasting–magneticroasting–magnetic separationseparation process cancan modifymodify thethe distributiondistribution ofof aluminiumaluminium andand ironiron phases.phases. In the reductionreduction roastingroasting process,process, weakweak magneticmagnetic iron-containingiron-containing mineralsminerals suchsuch asas hematitehematite cancan bebe convertedconverted intointo strongstrong ones,ones, e.g.,e.g., magnetite,magnetite, oror eveneven metallic iron,iron, whichwhich cancan be easily separatedseparated afterafter magneticmagnetic separationseparation [[5858––6060].]. Higher reductant dosage maymay result inin thethe formationformation ofof magnetite.magnetite. However, other studies indicated that thethe excess of coalcoal aaffectsffects negativelynegatively thethe magneticmagnetic separationseparation eefficiency,fficiency, duedue toto thethe furtherfurther reductionreduction ofof magnetitemagnetite toto wüstitewüstite (FeO)(FeO) [[17].17]. TheThe followingfollowing reactionsreactions cancan taketake placeplace duringduring reductionreduction roasting,roasting, (3) 3Fe3FeOO+C→2Fe+ C 2FeO O+CO+ CO (3) 2 3 → 3 4 (4) FeFeOO+ 3C+ 3C → 2Fe2Fe + 3CO+ 3CO (4) 2 3 → Additionally, in the presence of soda (Na2CO3), the main constituents of bauxite ore, e.g., Al2O3 and Fe2O3 may react according to Reaction (5), where M is a trivalent metal such as Al and Fe [61].

M O + Na CO 2NaMO + CO (5) 2 3 2 3 → 2 2

Additionally, in the presence of soda (Na2CO3), the main constituents of bauxite ore, e.g., Al2O3 and Fe2O3 may react according to Reaction (5), where M is a trivalent metal such as Al and Fe [61]. (5) MO +NaCO →2NaMO +CO

Figure 15a shows the Al2O3 grade and recovery in the magnetic and non-magnetic products obtained after reduction roasting followed by magnetic separation. The non-magnetic product has higher Al2O3 grade compared to the magnetic one and reached 67.3 wt%, corresponding to an increase of 11.2 wt% compared to the feed fraction. However, its recovery is quite low and does not exceed the value of 35 wt%. On the other hand, the magnetic product contained 57.4 wt% Al2O3 with Minerals 2020, 10, 314 14 of 21 corresponding recovery 64.9 wt% which indicates the strong association of alumina containing phases with iron-bearing minerals. As seen in Figure 15b, which shows the Fe2O3 grade and recovery magneticin the magnetic separation separation products, products, Fe2O3 grade Fe2O3decreased grade decreased to 5.1 wt% to 5.1 in wt% the non-magnetic in the non-magnetic product product from a feedfrom fraction a feed gradefraction of 20.8grade wt% of (75.5%20.8 wt% decrease) (75.5% with decrease) corresponding with corresponding recovery equals recovery to 8 wt%. equals On theto 8 otherwt%. hand,On the the other magnetic hand, the product magnetic contained product 28 contained wt% Fe2O 283 with wt% itsFe2 recoveryO3 with its presenting recovery verypresenting high value,very i.e.,high 92.2 value, wt%. i.e., Overall, 92.2 wt%. the results Overall, indicate the thatresults reduction indicate roasting–magnetic that reduction roasting separation–magnetic is very eseparationﬃcient in reducingis very efficient the iron in content reducing of bauxites. the iron Incontent cases of where bauxites. a lower In gradecases where bauxite a waslower treated, grade unlikebauxite the was one treated, used inunlike this study,the one this used integrated in this stud approachy, this integrated could be beneﬁcialapproach incould reducing be beneficial the iron in contentreducing (e.g., the below iron 2–2.5content wt%), (e.g., thus below rendering 2–2.5 bauxite wt%), suitablethus rendering for other applicationsbauxite suitable such asfor for other the productionapplications of such refractories as for the and production ceramics. of refractories and ceramics.

(a) (b)

FigureFigure 15.15. ((aa)) AlAl22OO33 gradegrade andand recoveryrecovery andand ((bb)) FeFe22OO33 gradegrade andand recoveryrecovery inin thethe magneticmagnetic andand non-magneticnon-magnetic products products obtained obtained after after reduction reduction roasting roasting followed followed by by magnetic magnetic separation. separation.

4.5.4.5. CharacterizationCharacterization of Magneticof Magnetic Separation Separation Products Pproducts FigureFigure 16 16 shows shows the the XRD XRD patterns patterns of of the the products, products, namely namely the the non-magnetic non-magnetic product product NMI NMI and and the magnetic product MIII obtained after magnetic separation of the 0.300 mm fraction, following the the magnetic product MIII obtained after magnetic separation of the− −0.300 mm fraction, following procedurethe procedure seenin seen Figure in 1Figureb. The results1b. The indicate results that indicate the aluminum that the hydroxide aluminum phases, hydroxide i.e., diaspore phases, and i.e., boehmitediaspore presentand boehmite in this fractionpresent havein this higher fraction intensities have higher in the intensities non-magnetic in the product, non-magnetic while hematite product, iswhile more hematite abundant is in more the magnetic abundant one. in Thethe characteristicmagnetic one. peaks The ofcharacteristic anatase in the peaks non-magnetic of anatase product in the werenon-magnetic found to be product more intense were found than those to be inmore the magneticintense than product, those while in the no magnetic diﬀerences product, were observed while no fordifferences kaolinite inwere the twoobserved products for examined.kaolinite in However, the two based products on the examined. results of theHowever, quantitative based Rietveld on the analysis,results of it the is revealed quantitative that kaoliniteRietveld isanalysis, slightly it more is revealed abundant that in kaolinite the magnetic is slightly product more indicating abundant the in poorMineralsthe magnetic liberation 2020, 10, xproduct ofFOR this PEER mineralindicating REVIEW phase the whichpoor liberation is associated of this with mineral the iron-bearing phase which phases is associated prevailing15 ofwith in21 thethe ore. iron-bearing phases prevailing in the ore.

Figure 16. XRD patterns of (a) non-magnetic product NMI and (b) magnetic product MIII obtained afterFigure magnetic 16. XRD separation patterns of of ( thea) non-magnetic0.300 mm fraction. product NMI and (b) magnetic product MIII obtained after magnetic separation of the −0.300 mm fraction.

The XRD patterns of the non-magnetic and magnetic products obtained after reduction roasting followed by magnetic separation when carbon to bauxite (C/B) ratio was 0.15 are seen in Figure 17.

Figure 17. XRD patterns of (a) non-magnetic product and (b) magnetic product obtained after reduction roasting followed by magnetic separation when carbon to bauxite (C/B) ratio was 0.15.

Ιt is seen from Figure 17 that the mineral phases present in the raw ore, i.e., diaspore, boehmite, anatase and kaolinite were not detected in the magnetic separation products, whereas less intense peaks of hematite are shown after reduction. Phase transformation took place during reduction roasting and the main minerals identified by XRD were corundum [α-Al2Ο3], hematite [Fe2O3], metallic iron [Fe], maghemite [γ-Fe2O3], magnetite [Fe3O4], pseudo-mullite [Al5SiO9.5], perovskite [CaTiO3], sodium aluminate [NaAlO2] and sodium magnesium aluminum oxide [Na2OMgO(Al2O3)5]. As a result, all alumina containing phases in the raw ore were transformed into corundum, sodium aluminate, sodium magnesium aluminum oxide and pseudo-mullite, while hematite was partially converted to magnetite, maghemite and metallic iron. According to previous studies, at ~780 °C diaspore and kaolinite transform into corundum and meta-kaolinite, respectively, whereas at temperature higher than 980 °C the formation of γ-Al2O3, amorphous SiO2 and mullite is promoted [22]. In the temperature range of 200–250 °C, hematite transforms to magnetite, while the formation of metallic iron starts at temperatures higher than 900 °C [62]. In

Minerals 2020, 10, x FOR PEER REVIEW 15 of 21

Figure 16. XRD patterns of (a) non-magnetic product NMI and (b) magnetic product MIII obtained Minerals 2020, 10, 314 15 of 21 after magnetic separation of the −0.300 mm fraction.

The XRD patterns of the non-magnetic and magnetic products obtained after reduction roasting followed by magnetic separation when carbon toto bauxitebauxite (C(C/B)/B) ratioratio waswas 0.150.15 areare seenseen inin FigureFigure 1717..

Figure 17. XRD patterns of (a) non-magnetic product and (b) magnetic product obtained after reduction Figure 17. XRD patterns of (a) non-magnetic product and (b) magnetic product obtained after roasting followed by magnetic separation when carbon to bauxite (C/B) ratio was 0.15. reduction roasting followed by magnetic separation when carbon to bauxite (C/B) ratio was 0.15. It is seen from Figure 17 that the mineral phases present in the raw ore, i.e., diaspore, boehmite, anataseΙt is and seen kaolinite from Figure were not17 that detected the mineral in the magnetic phases present separation in the products, raw ore, whereas i.e., diaspore, less intense boehmite, peaks ofanatase hematite and are kaolinite shown were after reduction.not detected Phase in the transformation magnetic separation took place products, during reduction whereas roastingless intense and thepeaks main of mineralshematite identifiedare shown by after XRD reduction. were corundum Phase [αtransformation-Al2O3], hematite took [Fe place2O3], during metallic reduction iron [Fe], 2 3 2 3 maghemiteroasting and [γ the-Fe 2mainO3], magnetite minerals [Feidentified3O4], pseudo-mullite by XRD were [Al corundum5SiO9.5], perovskite[α-Al Ο ], [CaTiOhematite3], sodium[Fe O ], 2 3 3 4 5 9.5 aluminatemetallic iron [NaAlO [Fe], 2maghemite] and sodium [γ-Fe magnesiumO ], magnetite aluminum [Fe O oxide], pseudo-mullite [Na2OMgO(Al [Al2OSiO3)5].], Asperovskite a result, all[CaTiO alumina3], containingsodium aluminate phases in the[NaAlO raw ore2] wereand transformed sodium intomagnesium corundum, aluminum sodium aluminate, oxide sodium[Na2OMgO(Al magnesium2O3)5]. aluminumAs a result, oxideall alumina and pseudo-mullite, containing phases while in the hematite raw ore was were partially transformed converted into tocorundum, magnetite, sodium maghemite aluminate, and metallicsodium iron.magnesiu Accordingm aluminum to previous oxide studies, and pseudo-mullite, at ~780 ◦C diaspore while andhematite kaolinite was partially transform converted into corundum to magnetite, and meta-kaolinite, maghemite and respectively, metallic iron. whereas According at temperature to previous studies, at ~780 °C diaspore and kaolinite transform into corundum and meta-kaolinite, higher than 980 ◦C the formation of γ-Al2O3, amorphous SiO2 and mullite is promoted [22]. In the respectively, whereas at temperature higher than 980 °C the formation of γ-Al2O3, amorphous SiO2 temperature range of 200–250 ◦C, hematite transforms to magnetite, while the formation of metallic and mullite is promoted [22]. In the temperature range of 200–250 °C, hematite transforms to iron starts at temperatures higher than 900 ◦C[62]. In addition, despite the overlapping peaks of magnetite, while the formation of metallic iron starts at temperatures higher than 900 °C [62]. In hematite, maghemite and magnetite at around 2-Theta = 35.6◦, the remaining strongest peak of hematite at 2-Theta = 33.15◦ in the magnetic separation products reveals that this phase was not completely converted. The presence of perovskite in the roasted samples indicates that there was some amount of calcium in the raw ore, despite the fact that no Ca-bearing phases were identified by XRD. Comparing the magnetic separation products obtained after reduction roasting (Figure 17), it can be seen that the characteristic peaks of iron-bearing phases in the magnetic product have in general higher intensities than in the non-magnetic one, but no great difference was observed between the corundum peaks. Sodium aluminate, sodium magnesium aluminum oxide and pseudo-mullite although more abundant in the non-magnetic product are still present in the magnetic product indicating that the formed magnetic phases after reduction are associated with the non-magnetic ones. Thus, the complex mineralogy of the samples in combination with the incomplete hematite reduction results in a large amount of non-magnetic phases being trapped in the magnetic product. SEM-BSE images of the products along with EDS analyses obtained after magnetic separation of the 0.300 mm fraction, namely the non-magnetic product NMI and the magnetic product MIII are − shown in Figure 18a–d. As shown in Figure 18a, the bright particles corresponding to hematite almost disappeared in the non-magnetic product NMI and were limited to those Fe-rich particles (mostly Fe-Ti oxides) embedded in minor quantities in the diasporic matrix, thus their complete removal was difficult. Minerals 2020, 10, x FOR PEER REVIEW 16 of 21

addition, despite the overlapping peaks of hematite, maghemite and magnetite at around 2-Theta = 35.6°, the remaining strongest peak of hematite at 2-Theta = 33.15° in the magnetic separation products reveals that this phase was not completely converted. The presence of perovskite in the roasted samples indicates that there was some amount of calcium in the raw ore, despite the fact that no Ca-bearing phases were identified by XRD. Comparing the magnetic separation products obtained after reduction roasting (Figure 17), it can be seen that the characteristic peaks of iron-bearing phases in the magnetic product have in general higher intensities than in the non-magnetic one, but no great difference was observed between the corundum peaks. Sodium aluminate, sodium magnesium aluminum oxide and pseudo-mullite although more abundant in the non-magnetic product are still present in the magnetic product indicating that the formed magnetic phases after reduction are associated with the non-magnetic ones. Thus, the complex mineralogy of the samples in combination with the incomplete hematite reduction results in a large amount of non-magnetic phases being trapped in the magnetic product. SEM-BSE images of the products along with EDS analyses obtained after magnetic separation of the −0.300 mm fraction, namely the non-magnetic product NMI and the magnetic product MIII are shown in Figure 18a–d. As shown in Figure 18a, the bright particles corresponding to hematite almost disappeared in the non-magnetic product NMI and were limited to those Fe-rich particles Minerals(mostly2020 Fe-Ti, 10, 314 oxides) embedded in minor quantities in the diasporic matrix, thus their complete16 of 21 removal was difficult.

FigureFigure 18. 18.Cross-sectional Cross-sectional SEM-BSE SEM- BSE images images of of polished polished surfaces surfaces of (ofa) ( non-magnetica) non-magnetic product product NMI NMI and (band) magnetic (b) magnetic product product MIII obtained MIII obtained after magnetic after magnetic separation separation of the of0.300 the − mm0.300 fraction. mm fraction. EDS spectra EDS − obtainedspectra obtained in regions in(marked regions (marked by white by rectangle) white rectangle) of (c) non-magnetic of (c) non-magnetic product NMIproduct and NMI (d) magnetic and (d) productmagnetic MIII product conﬁrm MIII the confirm metal contentthe metal (Fe content and Ti) (F increasee and Ti) in increase the MIII in sample the MIII and sample the subsequent and the decreasesubsequent of Al decrease content. of Al content.

OnOn the the other other hand, hand, microscopic microscopic characterization characterization of the of magneticthe magnetic product product MIII showsMIII shows much greatermuch brightgreater area bright compared area compared to the non-magnetic to the non-magnetic product NMIproduct due NMI to higher due to content higher in content hematite, in hematite, displayed bothdisplayed in the formboth ofin mixedthe form and of individual mixed and particles individual (Figure particles 18b). By(Figure comparing 18b). By regional comparing EDS spectraregional of non-magneticEDS spectra of product non-magnetic NMI and product magnetic NMI product and ma MIIIgnetic shown product in Figure MIII 18 shownc,d, respectively, in Figure 18c,d, it was foundrespectively, that the it metal was contentfound that (Fe andthe Ti)metal of the cont MIIIent sample(Fe and increased Ti) of the significantly; MIII sample the Feincreased content increased from 9.02 to 24.78 wt%, the Ti content increased from 1.35 to 4.75 wt% while the Al content decreased from 41.61 to 28.68 wt%. This result is in accordance with XRD results shown in Figure 16 and suggests that the methodological procedure seen in Figure1b e ffectively succeeded in separating the magnetic particles of the 0.300 mm fraction. − Figure 19 shows SEM-BSE images of the non-magnetic and magnetic products obtained after reduction roasting followed by magnetic separation when carbon to bauxite (C/B) ratio was 0.15. On the one hand, and in line with XRD results shown in Figure 17, SEM analysis along with EDS spectra data confirmed the depletion of the major Al-bearing phases present in the bauxite ore (feed material), namely diaspore and boehmite as well as gangue mineral phases such as anatase and kaolinite in both magnetic separation products; hematite was only detected in some places. On the other hand, bauxite particles have undergone excessive transformation during reduction roasting; this resulted in a micro-structure filled either with reduced particles entrapped in the matrix (non-magnetic product) or liberated/aggregated (magnetic product) as shown in Figure 19a,b–d, respectively. Minerals 2020, 10, x FOR PEER REVIEW 17 of 21 significantly; the Fe content increased from 9.02 to 24.78 wt%, the Ti content increased from 1.35 to 4.75 wt% while the Al content decreased from 41.61 to 28.68 wt%. This result is in accordance with XRD results shown in Figure 16 and suggests that the methodological procedure seen in Figure 1b effectively succeeded in separating the magnetic particles of the −0.300 mm fraction. Figure 19 shows SEM-BSE images of the non-magnetic and magnetic products obtained after reduction roasting followed by magnetic separation when carbon to bauxite (C/B) ratio was 0.15. On the one hand, and in line with XRD results shown in Figure 17, SEM analysis along with EDS spectra data confirmed the depletion of the major Al-bearing phases present in the bauxite ore (feed material), namely diaspore and boehmite as well as gangue mineral phases such as anatase and kaolinite in both magnetic separation products; hematite was only detected in some places. On the other hand, bauxite particles have undergone excessive transformation during reduction roasting; this resulted in a micro-structure filled either with reduced particles entrapped in the matrix (non-magneticMinerals 2020, 10product), 314 or liberated/aggregated (magnetic product) as shown in Figure 19a,b–d,17 of 21 respectively.

FigureFigure 19. Cross 19. Cross sectional sectional SEM- SEM-BSE BSE images images of polished of polished surfaces surfaces of (a) of non-magnetic (a) non-magnetic product product NMI NMI andand (b–d (b) magnetic–d) magnetic product product MIII MIIIobtained obtained after afterreduct reductionion roasting roasting followed followed by magnetic by magnetic separation separation whenwhen carbon carbon to bauxite to bauxite (C/B) (C ratio/B) ratio was was0.15. 0.15.EDS EDSspectra spectra obtained obtained in several in several spot locations spot locations show showthe the presencepresence of newly of newly formed formed metallic metallic oxides oxides (H: Hema (H: Hematite;tite; Mt: Magnetite; Mt: Magnetite; Mh: Maghemite; Mh: Maghemite; Fe: Metallic Fe: Metallic iron;iron; Pv: Perovskite). Pv: Perovskite).

In Inthis this context, context, and and duedue toto the the addition addition of Na of2CO Na3 2duringCO3 during the roasting the roasting process, theprocess, transformation the transformationof Al-bearing of phases Al-bearing into corundum, phases into sodium corundum, aluminate, sodium sodium aluminate, magnesium sodium aluminum magnesium oxide and aluminumpseudo-mullite oxide and is detected pseudo-mullite based on is EDS detected point analyses based on conducted EDS point in bothanalyses magnetic conducted separation in both products. magneticReduction separation of hematite products. present Redu inction the bauxite of hematite ore and present transformation in the bauxite into ore magnetite, and transformation maghemite and intometallic magnetite, iron ismaghemite more clearly and visible metallic in the iron magnetic is more product clearly as expected.visible in Morethe magnetic specifically, product large particlesas expected.of magnetite More andspecifically, maghemite large (up particle to 100s µofm) magnetite can be detected and maghemite in the magnetic (up to product 100 μm) while can metallicbe iron is also formed in smaller particles (up to 5 µm). Figure 19c (zoom of rectangular area of Figure 19b) shows in detail the presence of metallic iron developed in the rim region/shell of hematite particles as well as aggregated and scattered in the matrix [63]. As can be seen in Figure 19d, perovskite [CaTiO3] is the only Ti-rich oxide observed as needle-like infillings present at the internal space of the magnetic product thus indirectly confirming the calcium content in the bauxite ore (feed material). Finally, the average chemical composition (in oxides) of the non-magnetic product NMI and the magnetic product MIII obtained after reduction roasting followed by magnetic separation based on EDS analyses conducted on respresentative regions of their polished surfaces are shown in Table4. Results are given in comparison with the corresponding values obtained for the feed material (bauxite) and shown in Table2. It is seen that the metal oxide content (Fe and Ti) of the non-magnetic product NMI decreased significantly; the Fe2O3 content decreased from 22.39 to 16.58 wt%, the TiO2 content decreased from 3.94 to 1.14 wt% while the Al2O3 content increased from 57.25 to 68.23 wt% This result Minerals 2020, 10, 314 18 of 21 suggests that the approach involving reduction roasting and magnetic separation has a synergistic and noticeable effect on bauxite beneficiation.

Table 4. Average chemical composition of the non-magnetic product NMI and the magnetic product MIII obtained after reduction roasting followed by magnetic separation in comparison with the feed material.

a Sample Al2O3 (wt%) Fe2O3 (wt%) SiO2 (wt%) TiO2 (wt%) LOI (wt%) NMI 68.23 16.58 2.32 1.14 9.73 MIII 58.05 27.25 0.89 5.14 7.67 Feed material (bauxite) 57.95 22.39 1.65 3.94 13.0 a Loss on Ignition.

5. Conclusions The present experimental study aimed to optimize selective grinding of a Greek bauxite ore and explore potential beneficiation options that affect the distribution of Al- and Fe-containing phases. Grinding kinetics showed that the breakage rate varies with grinding time, and therefore grinding of bauxite exhibits non-first-order behavior. The reduction rate depents on the particle size range as well as the material filling volume used in the mill. Coarse particles were ground more efficiently compared to the fine ones while the lower the material filling volume the higher was the reduction rate. Selective grinding showed that the content of Al2O3 increased slightly in the coarser fractions, while the Fe2O3 grade increased in the finer ones. However, the mass recovery in the coarser fractions and consequently the overall Al2O3 recovery was very low. Grinding time and material filling volume affected the grade and recovery of Fe2O3 and Al2O3 in the products. Best results were obtained for the coarser than 0.300 mm product fraction, after 2 min of grinding and when the material filling volume used was 10%. Under these conditions, the Al2O3 grade increased by almost 10% to 64 wt% with corresponding recovery 73 wt%, while the Fe2O3 grade decreased by 18.3% and its recovery was 65.4 wt%. The magnetic separation of the products obtained after selective grinding showed that the feed fraction had significant effect on process efficiency. Thus, the non-magnetic product obtained from the 0.300–1.18 mm fraction was more rich in Al2O3 compared to 1.18–3.35 mm fraction, despite the fact that finer feed fractions contained less Al2O3. The maximum Al2O3 grade and recovery, i.e., 67.85 and 47.8 wt%, respectively, were obtained for a material filling volume of 5%. Under these conditions, the non-magnetic product contained 13.5 wt% Fe2O3, which corresponds to a decrease of 30.1%. The results of magnetic separation implemented after reduction roasting showed that a non-magnetic product containing 67.3 wt% Al2O3 but with quite low recovery (~35%) was obtained. In this product, Fe2O3 grade decreased by 75.5%, from 20.8 to 5.1 wt%, but in this case the corresponding recovery (8 wt%) was very low. The results of this study indicate that reduction roasting followed by magnetic separation can be an effective approach for reducing the iron content of bauxites, but due to the complex mineralogy of the ores significant amounts of non-magnetic phases may be trapped in the magnetic fraction. To optimize grinding and roasting and maximize the efficiency and the cost effectiveness of the proposed integrated process, complete mineralogical and modelling studies are required to fully characterize the bauxite ore, select the proper mill and its best operating parameters and carry out reduction roasting at the correct temperature. In this case, improved beneficiation results are anticipated, especially for lower grade bauxite ores.

Author Contributions: Conceptualization, E.P. and K.K.; methodology, E.P., K.K. and G.B.; writing—original draft preparation, E.P. and G.B.; data analysis E.P., K.K. and G.B.; writing—review and editing, K.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Minerals 2020, 10, 314 19 of 21

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Cardenia, C.; Balomenos, E.; Panias, D. Iron Recovery from Bauxite Residue Through Reductive Roasting and Wet Magnetic Separation. J. Sustain. Metall. 2019, 5, 9–19. [CrossRef] 2. Gibson, B.; Wonyen, D.G.; Chelgani, S.C. A Review of Pretreatment of Diasporic Bauxite Ores By Flotation Separation. Miner. Eng. 2017, 114, 64–73. [CrossRef] 3. Bogatyrev, B.A.; Zhukov, V.V.; Tsekhovsky, Y.G. Formation conditions and regularities of the distribution of large and superlarge bauxite deposits. Lithol. Miner. Resour. 2009, 44, 135–151. [CrossRef] 4. Ahmadnejad, F.; Zamanian, H.; Taghipour, B.; Zarasvandi, A.; Buccione, R.; Ellahi, S.S. Mineralogical and geochemical evolution of the Bidgol bauxite deposit, Zagros Mountain Belt, Iran: Implications for ore genesis, rare earth elements fractionation and parental affinity. Ore Geol. Rev. 2017, 86, 755–783. [CrossRef] 5. Vind, J.; Malfliet, A.; Blanpain, B.; Tsakiridis, P.E.; Tkaczyk, A.H.; Vassiliadou, V.; Panias, D. Rare Earth Element Phases in Bauxite Residue. Minerals 2018, 8, 77. [CrossRef] 6. Mucsi, G.; Cs˝oke,B.; Solymár, K. Grindability characteristics of lateritic and karst bauxites. Int. J. Miner. Process. 2011, 100, 96–103. [CrossRef] 7. Laskou, M.; Andreou, G. Rare earth element distribution and REE-minerals from the Parnassos-Ghiona bauxite deposits, Greece. In Proceedings of the 7th Biennial SGA Meeting, Athens, Greece, 24–28 August 2003; Society for Geology Applied to Mineral Deposit: Genéve, Switzerland, 2003; pp. 89–92. 8. Eliopoulos, D.; Economou, G.; Tzifas, I.; Papatrechas, C. The potential of rare earth elements in Greece. In Proceedings of the ERES2014: First European Rare Earth Resources Conference, Milos, Greece, 4–7 September 2014. 9. U.S. Geological Survey. Bauxite and Alumina. Available online: http://minerals.usgs.gov/minerals/pubs/ commodity/bauxite (accessed on 30 March 2020). 10. Laskou, M.; Economou-Eliopoulos, M.; Mitsis, I. Bauxite ore as an energy source for bacteria driving iron-leaching and bio-mineralization. Hell. J. Geosci. 2010, 45, 163–174. 11. Gamaletsos, P.N.; Godelitsas, A.; Kasama, T.; Church, N.S.; Douvalis, A.P.; Göttlicher, J.; Steininger, R.; Boubnov, A.; Pontikes, Y.; Tzamos, E.; et al. Nano-mineralogy and -geochemistry of high-grade diasporic karst-type bauxite from Parnassos-Ghiona mines, Greece. Ore Geol. Rev. 2017, 84, 228–244. [CrossRef] 12. Mutakyahwa, M.K.D.; Ikingura, J.R.; Mruma, A.H. Geology and geochemistry of bauxite deposits in Lushoto District, Usambara Mountains, Tanzania. J. Afr. Earth Sci. 2003, 36, 357–369. [CrossRef] 13. de Aquino, T.F.; Riella, H.G.; Bernardin, A.M. Mineralogical and physical-chemical characterization of a bauxite ore from Lages, Santa Catarina, Brazil, for refractory production. Miner. Process. Extr. Metall. Rev. 2011, 32, 137–149. [CrossRef] 14. Kumar, M.; Senapati, B.; Kumar, C.S. Beneficiation of high silica bauxite ores of India an innovative approach. In Light Metals 2013; The Minerals, Metals & Materials Series; Sadler, B.A., Ed.; Springer: Cham, Switzerland, 2016; pp. 187–190. 15. Rao, D.S.; Das, B. Characterization and beneficiation studies of a low grade bauxite ore. J. Inst. Eng. (India): Ser. D 2014, 95, 81–93. [CrossRef] 16. Sadler, L.Y.; Venkataraman, C. A process for enhanced removal of iron from bauxite ores. Int. J. Miner. Process. 1991, 31, 233–246. [CrossRef] 17. Pickles, C.A.; Lu, T.; Chambers, B.; Forster, J. A study of reduction and magnetic separation of iron from high iron bauxite ore. Can. Metall. Q. 2012, 51, 424–433. [CrossRef] 18. Komnitsas, K.; Bartzas, G.; Paspaliaris, I. Efficiency of limestone and red mud barriers: Laboratory column studies. Miner. Eng. 2004, 17, 183–194. [CrossRef] 19. Rivera, R.M.; Xakalashe, B.; Ounoughene, G.; Binnemans, K.; Friedrich, B.; Gerven, T.V. Selective rare earth element extraction using high-pressure acid leaching of slags arising from the smelting of bauxite residue. Hydrometallurgy 2019, 184, 162–174. [CrossRef] 20. Ou, L.M.; Feng, Q.M.; Zhang, G.F.; Chen, Y. Comminution property of bauxite and selective separation of Al and Si in bauxite. Miner. Process. Extr. Metall. 2008, 117, 179–184. [CrossRef] 21. Pehlivan, A.; Aydin, A.O.; Alp, A. Alumina extraction from low-grade diasporic bauxite by pyro- hydro metallurgical process. SAÜ Fen Bilimleri Enstitüsü Dergisi 2012, 16, 92–98. [CrossRef] Minerals 2020, 10, 314 20 of 21

22. Gu, F.; Li, G.; Peng, Z.; Luo, J.; Deng, B.; Rao, M.; Zhang, Y.; Jiang, T. Upgrading Diasporic Bauxite Ores for Iron and Alumina Enrichment Based on Reductive Roasting. J. Miner. Met. Mater. Soc. (TMS) 2018, 70, 1893–1901. [CrossRef] 23. Marino, S.; Wang, X.; Lin, C.; Miller, J. The flotation of a gibbsite bauxite ore. In Proceedings of the 9th International Alumina Quality Workshop, Perth, Australia, 18–22 March 2012; pp. 345–350. 24. Bolsaitis, P.; Chang, V.; Schorin, H.; Aranguren, R. Beneficiation of ferruginous bauxite by high-gradient magnetic separation. Int. J. Miner. Process. 1981, 8, 249–263. [CrossRef] 25. White, J.C. Domestic saprolites as potential substitutes for refractory grade bauxite. Ind. Eng. Chem. Res. 1987, 26, 7–11. [CrossRef] 26. Kahn, H.; Tassinari, M.M.L.; Ratti, G. Characterization of Bauxite Fines Aiming to Minimize Their Iron Content. Miner. Eng. 2003, 16, 1313–1315. [CrossRef] 27. Rao, R.B.; Besra, L.; Reddy, B.R.; Banerjee, G.N. The effect of pretreatment on magnetic separation of ferruginous minerals in bauxite. Magn. Electr. Sep. 1997, 8, 115–123. [CrossRef] 28. Bittencourt, L.R.M.; Lin, C.L.; Miller, J.D. Flotation recovery of high-purity gibbsite concentrates from a Brazilian bauxite ore. In Advanced Materials-Application of Mineral and Metallurgical Processing Principles; Society of Mining Engineers of AIME: Salt Lake City, UT, USA, 1990; pp. 77–85. 29. Barbosa, F.D.M.; Bergerman, M.G.; Horta, D.G. Removal of iron-bearing minerals from gibbsitic bauxite by direct froth flotation. Tecnol. Metal. Mater. Min. 2016, 13, 106–112. [CrossRef] 30. Marino, S.L. The flotation of marginal Gibbstitic bauxite ores from Paragominas-Brazil. Master’s Thesis, University of Utah, Salt Lake City, UT, USA, 2012. 31. Patermarakis, G.; Paspaliaris, Y. The leaching of iron oxides in boehmitic bauxite by hydrochloric acid. Hydrometallurgy 1989, 23, 77–90. [CrossRef] 32. Szabo, I.; Ujhidy, A.; Jelinko, R.; Vassanyi, R.I. Decrease of iron content of bauxite through high-temperature chlorination. Hung. J. Ind. Chem. 1989, 17, 465–475. 33. Ofori-Sarpong, G.; Abbey, C.E.; Asamoah, R.K.; Amankwah, R.K. Bauxite enrichment by microwave-magnetising roasting using sawdust as reducing agent. Am. J. Chem. Eng. 2014, 2, 59–64. [CrossRef] 34. Zhu, Y.; Han, Y.; Tian, Y.; Hong, W. Medium Characteristics on Selective Grinding of Low grade Bauxite. Adv. Mater. Res. 2010, 158, 159–166. [CrossRef] 35. Herbst, J.A.; Fuerstenau, D.W. Scale-up procedure for continuous grinding mill design using population balance models. Int. J. Miner. Process. 1980, 7, 1–31. [CrossRef] 36. Austin, L.G.; Klimpel, R.R.; Luckie, P.T. Process Engineering of Size Reduction: Ball Milling; SME–AIME: New York, NY, USA, 1984. 37. Petrakis, E.; Stamboliadis, E.; Komnitsas, K. Identification of optimal mill operating parameters during grinding of quartz with the use of population balance modelling. KONA Powder Part. J. 2017, 34, 213–223. [CrossRef] 38. Katubilwa, F.M.; Moys, M.H. Effect of ball size distribution on milling rate. Miner. Eng. 2009, 22, 1283–1288. [CrossRef] 39. Rajamani, R.K.; Guo, D. Acceleration and deceleration of breakage rates in wet ball mills. Int. J. Miner. Process. 1992, 34, 103–118. [CrossRef] 40. Bilgili, E.; Scarlett, B. Population balance modeling of non-linear effects in milling processes. Powder Technol. 2005, 153, 59–71. [CrossRef] 41. Petrakis, E.; Karmali, V.; Bartzas, G.; Komnitsas, K. Grinding kinetics of slag and effect of final particle size on the compressive strength of alkali activated materials. Minerals 2019, 9, 714. [CrossRef] 42. Harris, C.C. The Alyavdin-Weibull Plot of Grinding Data and the Order of Kinetics. Powder Technol. 1973, 7, 123–127. [CrossRef] 43. Beke, B. The Process of Fine Grinding; Dr. W. Junk Publishers: The Hague, The Netherlands, 1981. 44. Gamaletsos, P.; Godelitsas, A.; Kasama, T.; Kuzmin, A.; Lagos, M.; Mertzimekis, T.J.; Göttlicher, J.; Steininger, R.; Xanthos, S.; Pontikes, Y.; et al. The role of nano-perovskite in the negligible thorium release in seawater from Greek bauxite residue (red mud). Sci. Rep. 2016, 6, 21737. [CrossRef] 45. Young, R.A. The Rietveld Method; Oxford University Press: Oxford, UK, 1993. 46. Smith, P. The processing of high silica bauxites—Review of existing and potential processes. Hydrometallurgy 2009, 98, 162–176. [CrossRef] Minerals 2020, 10, 314 21 of 21

47. Rai, S.; Nimje, M.T.; Chaddha, M.J.; Modak, S.; Rao, K.R.; Agnihotri, A. Recovery of iron from bauxite residue using advanced separation techniques. Miner. Eng. 2019, 134, 222–231. [CrossRef] 48. Deng, B.; Si, P.; Bauman, L.; Luo, J.; Rao, M.; Peng, Z.; Jiang, T.; Li, G.; Zhao, B. Photocatalytic activity of

CaTiO3 derived from roasting process of bauxite residue. J. Clean. Prod. 2020, 244, 118598. [CrossRef] 49. Yılmaz, K.; Birol, B.; Sarıdede, M.N.; Yi˘git,E. Pre-Beneficiation of Low Grade Diasporic Bauxite Ore by Reduction Roasting. World Academy of Science, Engineering and Technology. Int. J. Chem. Mol. Nucl. Mater. Metall. Eng. 2015, 9, 1075–1078. 50. Kloprogge, J.T.; Ruan, H.D.; Frost, R.L. Thermal decomposition of bauxite minerals: Infrared emission spectroscopy of gibbsite, boehmite and diaspore. J. Mater. Sci. 2002, 37, 1121–1129. [CrossRef] 51. Laskou, M.; Margomenou-Leonidopoulou, G.; Balek, V.Thermal characterization of Bauxite samples. J. Therm. Anal. Calorim. 2006, 84, 141–145. [CrossRef] 52. Ostojić, G.; Lazić, D.; Škundrić, B.; Škundrić, J.P.; Sladojević, S.; Kešelj, D.; Blagojević, D. Chemical-mineralogical characterization of bauxites from different deposits. Contemp. Mater. 2014, 1, 84–94. 53. Castaldi, P.; Silvetti, M.; Enzo, S.; Deiana, S. X-ray diffraction and thermal analysis of bauxite ore-processing waste (red mud) exchanged with arsenate and phosphate. Clays Clay Miner. 2011, 59, 189–199. [CrossRef] 54. Fuerstenau, D.W.; Phatak, P.B.; Kapur, P.C.; Abouzeid, A.-Z.M. Simulation of the grinding of coarse/fine (heterogeneous) systems in a ball mill. Int. J. Miner. Process. 2011, 99, 32–38. [CrossRef] 55. Petrakis, E.; Karmali, V.; Komnitsas, K. Factors affecting nickel upgrade during selective grinding of low-grade limonitic laterites. Miner. Process. Extr. Metall. Trans. Inst. Min. Metall. 2018.[CrossRef] 56. Fuerstenau, D.W.; Sastry, K.V.S.; Hanson, J.S.; Narayanan, K.S.; Urbina, R.H.; Diao, J.; Chen, W. Coal Surface Control for Advanced Fine Coal Flotation; Project No. DE-AC22-88PC88878, Final Report; University of California: Berkeley, CA, USA; University of Utah: Salt Lake City, UT, USA; Columbia University: New York, NY, USA; Praxis Engineers: Milpitas, CA, USA, 1992. 57. Drzymala, J.; Ahmed, H.A.M. Mathematical equations for approximation of separation results using the Fuerstenau upgrading curves. Int. J. Miner. Process. 2005, 76, 55–65. [CrossRef] 58. Li, X.; Xiao, W.; Liu, W.; Liu, G.; Peng, Z.; Zhou, Q.; Qi, T. Recovery of alumina and ferric oxide from Bayer red mud rich in iron by reduction sintering. Trans. Nonferrous Met. Soc. China 2009, 19, 1342–1347. [CrossRef] 59. Ravisankar, V.; Venugopal, R.; Bhat, H. Investigation on beneficiation of goethite-rich iron ores using reduction roasting followed by magnetic separation. Miner. Process. Extr. Metall. 2017.[CrossRef] 60. Chun, T.J.; Zhu, D.Q.; Pan, J. Simultaneously roasting and magnetic separation to treat low grade siderite and hematite ores. Miner. Process. Extr. Metall. Rev. 2015, 4, 223. [CrossRef] 61. Kaußen, F.K.; Friedrich, B. Methods for Alkaline Recovery of Aluminum from Bauxite Residue. J. Sustain. Metall. 2016.[CrossRef] 62. Jang, K.-O.; Nunna, V.R.M.; Hapugoda, S.; Nguyen, A.V.; Bruckard, W.J. Chemical and mineral transformation of a low grade goethite ore by dehydroxylation, reduction roasting and magnetic separation. Miner. Eng. 2014, 60, 14–22. [CrossRef] 63. Samouhos, M.; Taxiarchou, M.; Tsakiridis, P.E.; Potiriadis, K. Greek “red mud” residue: A study of microwave reductive roasting followed by magnetic separation for a metallic iron recovery process. J. Hazard. Mater. 2013, 254, 193–205. [CrossRef][PubMed]

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