metals Article Removal of Metallic Iron from Reduced Ilmenite 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 Ores 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 rutile 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, hydrochloric acid 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 titanium product, being employed as pigment, as filler in the paper, plastic, 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 titanium dioxide, followed by titanium metal production [4,5]. Several commercial or proposed processes are available to produce SR or high-grade titanium slag 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 leads 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 oxide. The Becher process removes the iron oxide, 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 coal 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% ammonium 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 sulfuric acid 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 redox 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 ions 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 formaldehyde; sugars, such as glucose and sucrose; and water-soluble redox catalysts, namely, methyl viologen 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 goethite process for iron removal from hydrochloric acid leaching solution of reduced laterite [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 oxygen injection at room temperature are rarely studied. The effects of hydrochloric acid and ammonia 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 CaOCaO 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 62.8862.88 28.9328.93 3.693.69 31.9031.90 0.150.15 0.230.23 1.891.89 1.551.55 1.841.84 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.