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Removal of Metallic Iron from Reduced Ilmenite by Aeration Leaching

Removal of Metallic Iron from Reduced Ilmenite by Aeration Leaching

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Article Removal of Metallic from Reduced by Aeration Leaching

Qiuyue Zhao 1,2,3 , Maoyuan Li 1, Lei Zhou 1, Mingzhao Zheng 1 and Ting’an Zhang 1,2,3,*

1 School of Metallurgy, Northeastern University, Shenyang 110004, China; [email protected] (Q.Z.); [email protected] (M.L.); [email protected] (L.Z.); [email protected] (M.Z.) 2 Engineering Research Center of Metallurgy of Non-Ferrous Metal Materials Process Technology of Ministry of Education, Shenyang 110004, China 3 Key Laboratory of Ecological Utilization of Multi-metal Intergrown of Ministry of Education, Shenyang 110004, China * Correspondence: [email protected]; Tel.: +86-24-83686283

 Received: 24 June 2020; Accepted: 27 July 2020; Published: 29 July 2020 

Abstract: Aeration leaching was used to obtain synthetic from a reduced ilmenite. The reduced ilmenite, obtained from the carbothermic reduction of ilmenite concentrate in a rotary kiln at about 1100 ◦C, contained 62.88% TiO2 and 28.93% Metallic iron. The particle size was about 200 µm and the size distribution was uniform. The effects of NH4Cl and HCl concentrations, stirring speed, and aeration leaching time on the extent of removal of metallic iron from the reduced ilmenite were studied at room temperature. The results revealed that aeration leaching is feasible at room temperature. When using the NH4Cl system, the metallic iron content was reduced to 1.98% in synthetic rutile, but the TiO2 content only reached 69.16%. Higher NH4Cl concentration did not improve the leaching. Using 2% NH4Cl with 3% HCl, we were able to upgrade the synthetic rutile to 75%, with a metallic iron content as low as 0.14% and a total iron content of about 4%. Synthetic rutile could be upgraded to about 90% using HCl solution alone. HCl and NH4Cl are both effective on the aeration leaching process. However, within the scope of this experiment, is more efficient in aeration leaching.

Keywords: reduced ilmenite; synthetic rutile; aeration leaching; Becher process

1. Introduction

Titanium dioxide (TiO2) is the most widely used product, being employed as , as filler in the , , and rubber industries, and as flux in glass manufacture. Synthetic rutile (SR) is one of the major sources of TiO2 [1–3]. Industrial processes usually involve the initial preparation of , followed by titanium metal production [4,5]. Several commercial or proposed processes are available to produce SR or high-grade titanium from ilmenite which is mainly composed of FeTiO3. These involve a combination of thermal oxidation and reduction by roasting, leaching, and physical separation steps. Iron is converted to soluble ferrous or elemental forms by reduction at a high temperature, followed by acid leaching to obtain a SR product. Ilmenite generally contains impurities such as iron, which to its low grade and cannot be directly used. Synthetic rutile is a kind of titanium rich raw material with the same composition and structural properties as natural rutile by separating most iron components from ilmenite. An industrial process for upgrading ilmenite to SR is typically represented by the Becher process [6–8]. Ilmenite contains 40–65% titanium as TiO2, with the rest being iron . The Becher process removes the , leaving a residue of SR that contains more than 90% TiO2. The Becher process comprises

Metals 2020, 10, 1020; doi:10.3390/met10081020 www.mdpi.com/journal/metals Metals 2020, 10, 1020 2 of 9 four major steps: oxidation, reduction, aeration, and acid leaching [9,10]. Oxidation involves heating the ilmenite in a rotary kiln with air to convert the contained iron to iron oxide:

4FeTiO +O 2Fe O TiO +TiO (1) 3(s) 2(g) → 2 3 · 2(s) 2(s) This allows for the use of a wide range of ilmenite materials with various Fe(II) and Fe(III) contents for the subsequent step. Reduction is performed in a rotary kiln with a mixture of pseudobrookite (Fe O TiO ) and at about 1200 C to reduce iron oxide to metallic iron: 2 3· 2 ◦ Fe O TiO + 3CO 2Fe + 2TiO + 3CO (2) 2 3 · 2(s) → (s) 2(s) 2(g) Metallic iron is then oxidized and precipitated from the solution as a slime in an aeration or ‘rusting’ step in large tanks using 1% chloride solution at 80 ◦C:

4Fe + 3O 2Fe O (3) (s) 2(g) → 2 3 The finer iron oxide is then separated from the larger SR particles. When most of the iron oxide is removed, the residual portion is leached using 0.5 M and then separated from the SR. In the aeration leaching step, the removal of metallic iron from the reduced ilmenite (RI) grains is essentially a reaction, which can be represented by the following half-cell reactions:

2+ 2Fe 2Fe + 4e− (anodic reaction) (4) → + O + 4H + 4e− 2H O (cathodic reaction) (5) 2 → 2 The oxidation of ferrous is then given by:

2+ 2Fe + 4OH− + 1/2O Fe O H O + H O (6) 2 → 2 3 · 2 2 In current industrial practice, the aeration step of the Becher process can take as long as 22 h to complete [11]. Some reports show that the rusting process can be accelerated by improving aeration [12] or by adding a component such as acetic, tartaric, or citric acid [13,14]; a ligand, such as ethylenediammonium dichloride; various phenolic and aldehyde compounds, such as pyrogallol, saccharin, starch, and ; sugars, such as glucose and sucrose; and -soluble redox catalysts, namely, methyl dichloride and diquat dibromide [11,15–18]. These additives differ in effectiveness and cost. Most prior research was carried out at relatively high temperature (70 ◦C). Other related hydrometallurgical processes include, for example, ultrasonic-assisted acid leaching for iron removal from quartz sand [19–21] and the process for iron removal from hydrochloric acid leaching solution of reduced [22]. In the present work, we report a study of aeration leaching of reduced ilmenite at room temperature. Aeration leaching experiments using the hydrochloric acid system with injection at room temperature are rarely studied. The effects of hydrochloric acid and chloride in improving the aeration efficiency were evaluated. The effects of leaching parameters, including stirring speed and NH4Cl and hydrochloric acid concentrations, were investigated. Through the above research, the method of strengthening the aeration process at room temperature is explored to provide a new way to obtain high-grade SR.

2. Materials and Methods

2.1. Materials A Chinese source of reduced ilmenite, produced by carbothermic reduction of ilmenite concentrate in a rotary kiln at about 1100 ◦C, was used. The chemical composition and particle size is reported in Table1 and Figure1, respectively. MFe stands for metal iron and TFe stands for all iron in Table1. Metals 2020, 10, 1020 3 of 9

The composition of reduced ilmenite and SR obtained by XRF analysis and MFe was determined by chemical titration. Figure1 shows that almost 80% of the particles were distributed between 90 and 400Metalsµ m,2020 with, 10, xa FOR mode PEER value REVIEW of about 200 µm and a uniform distribution. 3 of 9 Metals 2020, 10, x FOR PEER REVIEW 3 of 9

Table 1.1. Composition of reduced ilmenite (mass%). Table 1. Composition of reduced ilmenite (mass%).

ComponentComponent TiO2 TiOMFe2 MFe FeO FeO TFeTFe CaO CaO MgO MgO MnMn Al2O Al3 2OSiO3 2 SiO2 Component TiO2 MFe FeO TFe CaO MgO Mn Al2O3 SiO2 Content 62.88 28.93 3.69 31.90 0.15 0.23 1.89 1.55 1.84 ContentContent 62.88 62.88 28.9328.93 3.693.69 31.90 31.90 0.150.15 0.23 0.23 1.89 1.89 1.551.55 1.84 1.84

12 12 Reduced ilmenite Reduced ilmenite 10 10

8 8

6 6

Volume /% Volume 4 Volume /% Volume 4

2 2

0 0 1 10 100 1000 1 10 100 1000 Particle size /μm Particle size /μm Figure 1. Particle size distribution of reduced ilmenite. Figure 1. Particle size distribution of reduced ilmenite. 2.2. Aeration Conditions 2.2.2.2. AerationAeration ConditionsConditions The aeration leaching experiments were performed in a 1 L stirred reactor. Details of the The aeration leaching experiments were performed in a 1 L stirred reactor. Details of the experimentalThe aeration apparatus leaching are illustrated experiments in Figure were2 .perfor The innermed diameter in a 1 L of stirred the stirred reactor. reactor Details was 80 of mm the experimental apparatus are illustrated in Figure 2. The inner diameter of the stirred reactor was 80 andexperimental the agitator apparatus was a four-blade are illustrated propeller. in Figure The blade 2. The length inner as di 30ameter mm. of the stirred reactor was 80 mmmm andand thethe agitatoragitator waswas aa four-bladefour-blade propeller.propeller. TheThe bladeblade lengthlength asas 3030 mm.mm.

Figure 2. Aeration leaching reactor. FigureFigure 2.2. AerationAeration leachingleaching reactor.reactor.

TheThe initialinitial reactionreaction mixturemixture comprisedcomprised 640640 mLmL solutionsolution andand 320320 gg reducedreduced ilmenite,ilmenite, whichwhich werewere addedadded toto thethe stirredstirred reactor.reactor. TheThe solutionsolution containedcontainecontainedd didifferentdifferentfferent concentrationsconcentrationsconcentrations ofof ammonium ammoniumammonium chloridechloride andand/orand/or/or hydrochlorichydrochlorichydrochloric acid.acid.acid. The The pulp pulp was was stirred stirred byby aa four-bladefour-bladefour-blade agitator.agitator.agitator. Aeration Aeration gas gas waswas thenthen introducedintroduced andand and passedpassed passed throughthrough through thethe the pulppulp pulp forfor for thethe the entireentire entire durationduration duration ofof of thethe the experiment.experiment. experiment. AfterAfter 44 h,h, fine finefine ironiron oxidesoxides werewere separatedseparated fromfrom thethe SRSR bybyby wetwetwet screening.screening.screening. ParticlesParticles ofof ironiron oxidesoxides andand SRSR werewere washedwashed andand drieddried forfor analysis.analysis. We WeWe usedused thethe contentcontent ofof metallicmetallic ironiron (MFe)(MFe) remainingremaining inin thethe SRSR toto measuremeasure thethe efficiencyefficiency ofof aerationaeration leaching:leaching: itit waswas foundfound thatthat thethe lowerlower thethe residualresidual ironiron content,content, thethe betterbetter thethe effect.effect. AerationAeration leachingleaching isis aa processprocess ofof oxygenoxygen absorpabsorptiontion corrosioncorrosion ofof metametall iron.iron. ThreeThree kindskinds ofof corrosioncorrosion systemssystems areare generallygenerally selected:selected: ammoniumammonium chloride,chloride, ammoniumammonium chloridechloride plusplus hydrochlorichydrochloric acid,acid, andand hydrhydrochloricochloric acid.acid. ItIt isis recognizedrecognized thatthat thethe anionanion providedprovided byby hydrochlorichydrochloric acidacid cancan destroydestroy thethe passivationpassivation filmfilm onon thethe surfacesurface ofof metallicmetallic ironiron inin thethe aerationaeration processprocess [23].[23]. TheThe

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the efficiency of aeration leaching: it was found that the lower the residual iron content, the better the effect. Aeration leaching is a process of oxygen absorption of metal iron. Three kinds of corrosion systems are generally selected: ammonium chloride, ammonium chloride plus hydrochloric acid, and hydrochloric acid. It is recognized that the anion provided by hydrochloric acid can destroy Metals 2020, 10, x FOR PEER REVIEW 4+ of 9 the passivation film on the surface of metallic iron in the aeration process [23]. The role of NH4 is to combine with ferrous ions in the particles to form a complex that cannot be separated, so as to role of NH4+ is to combine with ferrous ions in the ore particles to form a complex that cannot be separated,prevent oxidationso as to prevent and oxidation in and the hydrolysis ore particles. in the The ore complex particles. immediately The complex decomposes immediately when decomposesencountering when water, encountering and so acts water, as a carrier.and so acts The as eff aect carrier. of ammonium The effect chlorideof ammonium was examined chloride was using concentrations of 2%, 4%, 6%, and 8% (m/v) NH4Cl. The stirring speed was 800 rpm. An ambient examined using concentrations of 2%, 4%, 6%, and 8% (m/v) NH4Cl. The stirring speed was 800 rpm. Antemperature ambient temperature was employed. was Wetemployed. separation Wet of separation the fine iron of oxidethe fine from iron the oxide coarse from titanium the coarse titaniumparticles mineral was done particles by using was done hydrocyclones by using hydrocyclones and spiral classifiers. and spiral We classifiers. measured We the measured MFe and the TiO 2 contents in the SR after aeration leaching for 4 h. MFe and TiO2 contents in the SR after aeration leaching for 4 h. TheThe particle particle size size of thethe samples samples was was analyzed analyzed by aby laser a dilaserffraction diffraction particle particle size analyzer size analyzer (Bettersize (BettersizeV8.0, Dandong V8.0, Dandong Baite Instrument Baite Instrument Co., Ltd., DandongCo., Ltd., city, Dandong China). city, The structureChina). The and structure morphology and of morphologythe reduced of ilmenite the reduced samples ilmenite and the productsamples after and aeration the product leaching after were aeration characterized leaching by anwere X-ray α characterizeddiffractometer by (BRUKERan X-ray Inc.,diffractometer Karlsruhe, (BRUKER Germany), Inc., applying Karlsruhe, Cu K Germany),radiation atapplying 40 kV and Cu 40 K mA,α θ radiationwith 2 atrecording 40 kV and from 40 10mA,◦ to with 80◦ 2withθ recording a step size from of 10° 0.02 to◦ and 80° awith counting a step timesize ofof 0.10.02° s perandstep. a countingThe metallic time of iron 0.1 contents per step. of theThe solids metallic was iron determined content of by the solids was dichromate determined titration by potassium in FeCl 3 solution. Other elements were determined by a ZSX PrimusIV X-ray fluorescence spectrum (Japan dichromate titration in FeCl3 solution. Other elements were determined by a ZSX PrimusIV X-ray fluorescenceNeo Confucianism spectrum Co., (Japan Ltd., Neo Tokyo, Confucianism Japan). Co., Ltd., Tokyo, Japan ) 3. Results and Discussion 3. Results and Discussion 3.1. Effect of Solution Composition 3.1. Effect of Solution Composition The results are shown in Figure3. With increasing NH 4Cl concentration from 2% to 8%, the metallic ironThe content results in are the shown SR increased in Figure from 3. 1.85% With toincreasing 6.75%, the NH total4Cl iron concentration content decreased from 2% from to 8%, 19.63% the to metallic iron content in the SR increased from 1.85% to 6.75%, the total iron content decreased from about 16%, and the TiO2 content increased from 64.97% to about 70%. It can be deduced that an increase 19.63% to about 16%, and the TiO2 content increased from 64.97% to about 70%. It can be deduced in NH4Cl concentration is not conducive to the aeration leaching process. When the concentration of that2% an ammonium increase in chloride NH4Cl concentration was the same is as not that conducive in Reference to th [21e ]aeration and the leaching reaction timeprocess. was When reduced the by concentrationone hour, the of removal 2% ammonium rate of MFe chloride in this was paper the was same as as high that as in 98.15%, Reference while [21] the and removal the reaction rate of time iron in wasReference reduced [ 21by] one was hour, less than the 50%.removal The rate TFe of content MFe in was this close paper to 20%was andas high this as is not98.15%, suffi cientwhile for the the removalaeration rate leaching of iron products. in Reference [21] was less than 50%. The TFe content was close to 20% and this is not sufficient for the aeration leaching products.

35 100 Conditions: MFe/% Temperature: room temperature FeO/% TiO2/% 90 30 t=4h TFe/% Oxygen:excess 80 25 Solutions: ammonium chloride 70

20 60

50 15 content / % 2 40 MFe Content/ % Content/ MFe 10 TiO 30 5 20

0 10 23456789 NH Cl Concentration / % (w/v) 4 Figure 3. Effect of ammonium chloride concentration on composition of synthetic rutile. Figure 3. Effect of ammonium chloride concentration on composition of synthetic rutile.

Aeration leaching tests using 2% NH4Cl with 0% to 3% hydrochloric acid were then carried out. The results are shown in Figure 4. For the same reaction time and other conditions, the contents of TFe and MFe in the SR monotonically decreased with an increase in hydrochloric acid concentration from 1% to 3%, while the TiO2 content increased. The addition of hydrochloric acid helped to improve the aeration leaching, but the SR was only upgraded to 75%, which indicated that the reaction needed more time to improve the purity. Similar to Figure 3, the reaction rate of MFe was high but there was over 10% content of TFe in the aeration leaching products.

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Aeration leaching tests using 2% NH4Cl with 0% to 3% hydrochloric acid were then carried out. The results are shown in Figure4. For the same reaction time and other conditions, the contents of TFe and MFe in the SR monotonically decreased with an increase in hydrochloric acid concentration from 1% to 3%, while the TiO2 content increased. The addition of hydrochloric acid helped to improve the aeration leaching, but the SR was only upgraded to 75%, which indicated that the reaction needed moreMetals 2020 time, 10 to, x improve FOR PEER the REVIEW purity. Similar to Figure3, the reaction rate of MFe was high but there was5 of 9 Metals 2020, 10, x FOR PEER REVIEW 5 of 9 over 10% content of TFe in the aeration leaching products.

Figure 4. EffectEffect of 2% NH4ClCl with with hydrochloric acid on composition of synthetic rutile rutile.. Figure 4. Effect of 2% NH4Cl with hydrochloric acid on composition of synthetic rutile. The aeration leaching was better with the additionaddition of hydrochlorichydrochloric acid thanthan withwith thethe NHNH44Cl alone,The so aeration pure hydrochloric leaching was acid better was with considered the addi fortion comparativecomparative of hydrochloric analysis. acid The than hydrochloric with the NH acid4Cl alone,concentration so pure was washydrochloric selectedselected asas acid 1.5% 1.5% was (m (m/v)./ v).considered The The resulting resulting for comparative MFe MFe and and TiO TiO analysis.2 contents2 contents The in in the hydrochloric the SR SR are are shown shown acid in concentrationFigurein Figure5. 5. was selected as 1.5% (m/v). The resulting MFe and TiO2 contents in the SR are shown in FigureIn thethe 5. hydrochlorichydrochloric acid acid system, system, the the TFe TFe content conten int thein the SR SR was was about about 4%, 4%, compared compared with with over over 10%, and10%, evenIn and the upeven hydrochloric to 18%,up to in 18%, theacid NHin system, the4Cl NH system. the4Cl TFe system. This conten proved Thist in proved thatthe SR hydrochloric wasthat abouthydrochloric acid4%, compared is better acid is for withbetter aeration over for 10%,leachingaeration and thanleachingeven ammonium up thanto 18%, ammonium chloride. in the NH However,chloride.4Cl system. Howeve it is moreThisr, proved diitffi iscult more that to store difficulthydrochloric and to transport store acid and hydrochloricis bettertransport for aerationacid,hydrochloric and leaching the acid, accumulation thanand theammonium accumulation of chloride chloride. ionof chloride is Howeve not conducive ionr, is it not is conducivemore to recycling difficult to ofrecycling to the store corrosion of and the transportcorrosion solution. hydrochloricTherefore,solution. Therefore, comprehensive acid, and comprehensive the accumulation consideration consideration of is neededchloride to ionis select neededis not the conducive bestto select aeration tothe recycling leachingbest aeration of solution. the corrosion leaching solution.solution. Therefore, comprehensive consideration is needed to select the best aeration leaching solution. 35 100 35 100 30 30 80 25 Conditions: 80 25 Conditions:Temperature: room temperature 20 60 Temperature:t=4h room temperature 20 MFe t=4hOxygen:excess 60 15 FeO Oxygen:excessSolutions: 1.5% hydrochloric acid MFe content /%

TiO 2 15 FeO2 Solutions: 1.5% hydrochloric acid 40 TFe content /% 10 TiO2 40 2 TiO MFe content /% 10 TFe TiO MFe content /% 5 20 5 20 0 0 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 0 0 20 40 60 80 100time 120 /min 140 160 180 200 220 240 260 time /min Figure 5. EffectEffect of hydrochloric acid on composition of synthetic rutile.rutile. Figure 5. Effect of hydrochloric acid on composition of synthetic rutile. 3.2. Effect of Stirring Speed 3.2. Effect of Stirring Speed Stirring is one of the most important factors in mixing processes in the and metallurgy.Stirring Theis one purpose of the mostis to importantmix evenly, factors accelera in mixingte the processesdissolution, in theor acceleratechemical industry the reaction and metallurgy.process. Generally, The purpose too slow is ato stir mixring evenly, speed willaccelera teto theuneven dissolution, mixing and or acceleratetoo fast a stirring the reaction speed process.can damage Generally, the product. too slow High-sp a stirringeed speed mixing will consumes lead to uneven more electricmixing andenergy, too fastwhich a stirring results speed in an canincrease damage in production the product. cost. High-sp Selectioneed of mixing an appropriate consumes mixing more speedelectric is, energy,therefore, which essential results [23]. in The an increaseeffect of instirring production speed cost.on the Selection removal of of an metallic appropriate iron from mixing the speedreduced is, ilmenitetherefore, was essential investigated [23]. The by effect of stirring speed on the removal of metallic iron from the reduced ilmenite was investigated by

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3.2. Effect of Stirring Speed Stirring is one of the most important factors in mixing processes in the chemical industry and metallurgy. The purpose is to mix evenly, accelerate the dissolution, or accelerate the reaction process. Generally, too slow a stirring speed will lead to uneven mixing and too fast a stirring speed can damage the product. High-speed mixing consumes more electric energy, which results in an increase in production cost. Selection of an appropriate mixing speed is, therefore, essential [23]. The effect of Metalsstirring 2020 speed, 10, x FOR on PEER the removal REVIEW of metallic iron from the reduced ilmenite was investigated by varying6 of 9 the impeller speed in the range of 400 to 1000 rpm. The concentration of hydrochloric acid was 1.5% varyingm/v, the the reaction impeller time speed was 4in h, the and range the aeration of 400 to gas 1000 was rpm. present The inconcentration excess. of hydrochloric acid was 1.5%As shownm/v, the in reaction Figure6, time the metallic was 4 h, iron and remaining the aeration in gas the SRwas decreased present in from excess. 6.95% at 400 rpm to 0.49%As atshown 800 rpm, in Figure corresponding 6, the metallic to a reduction iron remain ining metallic in the iron SR contentdecreased of 6.46%from 6.95% points. at The400 metallicrpm to 0.49%iron contentat 800 rpm, increased corresponding to 2.85% atto 1000a reduction rpm and in themetallic iron content iron content increased of 6.46% by 2.36% points. compared The metallic with ironthe valuecontent of increased 2.85% at 800 to 2.85% rpm. at 1000 rpm and the iron content increased by 2.36% compared with the value of 2.85% at 800 rpm.

7 Conditions: 6 Temperature: room temperature t=4h 5 Oxygen:excess Solutions: 1.5% hydrochloric acid

4

3

MFecontent /% 2

1

0

400 600 800 1000 Stirring speed /rpm Figure 6. Effect of stirring speed on metallic iron content of synthetic rutile. Figure 6. Effect of stirring speed on metallic iron content of synthetic rutile. The presence of agitation can break up bubbles, increase the specific surface area of bubbles, The presence of agitation can break up bubbles, increase the specific surface area of bubbles, and and accelerate mass transfer from the gas phase to the liquid phase. Agitation can also promote uniform accelerate mass transfer from the gas phase to the liquid phase. Agitation can also promote uniform suspension of reduced ilmenite particles, increase the liquid–solid contact area, accelerate the internal suspension of reduced ilmenite particles, increase the liquid–solid contact area, accelerate the internal diffusion process, and prevent corroded iron ions from reducing ilmenite particles in an in situ reaction. diffusion process, and prevent corroded iron ions from reducing ilmenite particles in an in situ The vortex will be formed at high speed, which will lead to uneven mixing of gas, liquid, and solid. reaction. The vortex will be formed at high speed, which will lead to uneven mixing of gas, liquid, and3.3. solid. Phase Analysis

3.3. PhaseFigure Analysis7 presents XRD spectra of reduced ilmenite before and after iron removal by aeration under the conditions: room temperature, t = 4 h, excess oxygen, 1.5% hydrochloric acid. The major phases in Figure 7 presents XRD spectra of reduced ilmenite before and after iron removal by aeration the reduced ilmenite before iron removal were Fe, TiO2, and FeTi2O5. The diffraction peaks of FeTi2O5 under the conditions: room temperature, t = 4 h, excess oxygen, 1.5% hydrochloric acid. The major and TiO2 were strong. Peaks for the metallic iron phase were not to be found in the sample after aeration phases in the reduced ilmenite before iron removal were Fe, TiO2, and FeTi2O5. The diffraction peaks leaching, but diffraction peaks of FeO(OH) were detected. The diffraction peaks of FeO(OH) and TiO2 ofhad FeTi the2O same5 and intensity,TiO2 were which strong. indicated Peaks for that the metallic metallic iron iron transformed phase were intonot to FeO(OH). be found There in the were sample just aftertwo aeration main phases leaching, in the but sample diffraction after thepeaks aeration of FeO(OH) leaching were process. detected. These The results diffraction indicate peaks that theof FeO(OH)transformation and TiO of2 reduced had the ilmenite same intensity, into rutile which was achieved indicated under that these metallic experimental iron transformed conditions. into FeO(OH). There were just two main phases in the sample after the aeration leaching process. These results indicate that the transformation of reduced ilmenite into rutile was achieved under these experimental conditions.

Metals 2020, 10, 1020 7 of 9 MetalsMetals 2020 2020, ,10 10, ,x x FORFOR PEERPEER REVIEW 7 of 79 of 9

A Product BA A - TiO Product B A - TiO2 B A 2 B B - FeO(OH) B A B - FeO(OH) B C - Fe B C - Fe A A B A B G - FeTi O A A 2 5 B A G - FeTi O B B 2 5 A B B B A B G G A Raw material A Raw material Intensity(a.u.) Intensity(a.u.) A A G A A G A A G C G A A A G C G A

10 20 30 40 50 60 70 80 10 20 302Theta 40 50 60 70 80 2Theta FigureFigure 7. 7.X-ray X-ray didiffractionffraction patterns of of reduced reduced ilmenite ilmenite before before and and after after iron iron removal removal.. Figure 7. X-ray diffraction patterns of reduced ilmenite before and after iron removal. PhilipsPhilips ssx-550 ssx-550 scanning scanning electron electron microscope microscope (SEM) (SEM) images images of theof the sample sample before before and and after after aeration leachingaerationPhilips are leaching shown ssx-550 inare Figure scanningshown8 .in There Figureelectron are 8. obviousTheremicroscope are di obviousff erences(SEM) differences images between ofbetween the the raw sample the material raw before material and theand and product after ofaeration thethe aerationproduct leaching of leaching the are aeration shown process: leaching in Figure the sampleprocess: 8. There beforethe ar samplee obvious aeration before differences leaching aeration wasbetween leaching compact the was raw andcompact material we could and and not findthewe holes product could in not the of find surface;the aerationholes after in theleaching aeration surface; process: leaching, after aeration the the sample interior leaching, before was the aeration full interior of holes, leachingwas givingfull ofwas holes, rise compact to giving a network and structure,werise could to a which networknot find maintained structure,holes in the which the surf sample maintainedace; after integrity. aeration the sample The leaching, holesintegrity. the are Theinterior attributed holes was are to full attributed the of transformationholes, to givingthe ofrise metallictransformation to a network iron into of structure,metallic iron oxide iron which into by theironmaintainedaeration oxide by the leachingthe sampleaeration reaction integrity. leaching and reactionThe its holes removal and are its attributedremoval from the from to interior the the interior of sample. oftransformation sample. of metallic iron into iron oxide by the aeration leaching reaction and its removal from the interior of sample.

(a)

(a)

(b)

(b)

Figure 8. Cont.

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(c)

Figure 8. Scanning electron micrographs of sample (a) before and ((b)) afterafter aerationaeration leachingleaching processprocess ((cc)) afterafter aerationaeration leachingleaching processprocess (enlarged).(enlarged).

4. Conclusions The following conclusions werewere drawndrawn onon thethe basisbasis ofof thethe resultsresults obtainedobtained inin thisthis work.work. Aeration leachingleaching is is feasible feasible at at room room temperature. temperature. For aFor reaction a reaction time oftime 4 h of and 4 adequateh and adequate stirring speedstirring of speed 800 rpm, of 800 the rpm, effectiveness the effectiveness of metallic of meta iron removalllic iron removal differed fordiffered different for different solution systems.solution Whensystems. using When NH using4Cl, theNH MFe4Cl, the content MFe content could be could reduced be redu to 1.98%ced to in1.98% SR, in but SR, the but TiO the2 contentTiO2 content only reachedonly reached 69.16%. 69.16%. A higher A higher NH4Cl NH concentation4Cl concentation did not did improve not improve aertion aertion leaching. leaching. Using 2%Using NH 42%Cl withNH4Cl hydrochloric with hydrochloric acid, the acid, presence the presence of the of acid the helped acid helped to improve to improve the leaching, the leaching, but the but SR the was SR onlywas only upgraded upgraded to 75%, to 75%, which which indicated indicated that that the the reaction reaction needed needed more more time time toto improveimprove the purity. purity. In the hydrochloric acid system, the MFe content was as low as 0.14% and TFe content was about 4%, indicatingindicating thatthat thethe SRSR couldcould bebe upgradedupgraded toto aboutabout 90%.90%.

Author Contributions: This is a joint work of the five authors; each author was in charge of their expertise Author Contributions: This is a joint work of the five authors; each author was in charge of their expertise and and capability: Q.Z. for writing, formal analysis and original draft preparation, M.L. for data curation, L.Z. for experimentalcapability: Q.Z. assistance, for writing, M.Z. formal for validation, analysis and T.Z. origin for methodology.al draft preparation, All authors M.L. have for readdata andcuration, agreed L.Z. to thefor publishedexperimental version assistance, of the manuscript.M.Z. for validation, T.Z. for methodology. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the National Natural Science of China (NSFC) (GrantFunding: No. This 51204040); research Fundamental was supported Research by the FundsNational for Natura the Centrall Science Universities Foundation (Grant of China No. N180725023). (NSFC) (Grant No. Acknowledgments:51204040); FundamentalThe Research authors gratefully Funds for acknowledge the Central Universities the Laboratory (Grant Center No. ofN180725023) Northeastern University for chemical analysis. Guangxi Yueqiao New Material Technology Co., Ltd. for providing the reduced ilmenite samples. Acknowledgments: The authors gratefully acknowledge the Laboratory Center of Northeastern University for Conflictschemical ofanalysis. Interest: GuangxiOn behalf Yueqiao of all authors,New Material the corresponding Technology authorCo., Ltd. states for that providing there is the no conflictreduced of ilmenite interest. samples. References Conflicts of Interest: On behalf of all authors, the corresponding author states that there is no conflict of interest. 1. Zhang, W.S.; Zhu, Z.W.; Cheng, C.Y. A literature review of titanium metallurgical processes. Hydrometallurgy References2011, 108 , 177–188. [CrossRef] 2. Lakshmanan, V.I.; Bhowmick, A.; Halim, M.A. Titanium dioxide: Production, properties, and applications. 1. Zhang, W.S.; Zhu, Z.W.; Cheng, C.Y. A literature review of titanium metallurgical processes. Chem. Phys. Res. J. 2014, 7, 37–42. Hydrometallurgy 2011, 108, 177–188. 3. Que, Y.; Weng, J.; Hu, L. Applications of titanium dioxide in solar cells. Prog. Chem. 2016, 28, 2. Lakshmanan, V.I.; Bhowmick, A.; Halim, M.A. Titanium dioxide: Production, properties, and applications. 40–50. Chem. Phys. Res. J. 2014, 7, 37–42. 4. Guo, Y.F.; Liu, S.S.; Jiang, T.; Qiu, G.Z.; Chen, F. A process for producing synthetic rutile from Panzhihua 3. Que, Y.; Weng, J.; Hu, L. Applications of titanium dioxide in perovskite solar cells. Prog. Chem. 2016, 28, titanium slag. Hydrometallurgy 2014, 134, 147–148. [CrossRef] 40–50. 5. Yaraghi, A.; Sapri, M.H.A.; Baharun, N.; Rezan, S.A. Aeration leaching of iron from nitrided Malaysian 4. Guo, Y.F.; Liu, S.S.; Jiang, T.; Qiu, G.Z.; Chen, F. A process for producing synthetic rutile from Panzhihua ilmenite reduced by polystyrene-Coal reductant. Procedia Chem. 2016, 19, 715. [CrossRef] titanium slag. Hydrometallurgy 2014, 134, 147–148. 6. Becher, R.G. Improved process for the beneficiation of ores containing contaminating iron. Aust. Patent 1963, 5. Yaraghi, A.; Sapri, M.H.A.; Baharun, N.; Rezan, S.A. Aeration leaching of iron from nitrided Malaysian 247, 110. ilmenite reduced by polystyrene-Coal reductant. Procedia Chem. 2016, 19, 715. 7. Farrow, J.B.; Ritchie, I.M.; Mangono, P. The reaction between reduced ilmenite and oxygen in ammonium 6. Becher, R.G. Improved process for the beneficiation of ores containing contaminating iron. Aust. Patent chloride solution. Hydrometallurgy 1987, 18, 21–38. [CrossRef] 1963, 247, 110. 7. Farrow, J.B.; Ritchie, I.M.; Mangono, P. The reaction between reduced ilmenite and oxygen in ammonium chloride solution. Hydrometallurgy 1987, 18, 21–38.

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