A New Process of Extracting Titanium from Vanadium– Titanium Magnetite

crystals

Article A New Process of Extracting Titanium from Vanadium– Titanium Magnetite

Yandong Li 1,2 , Shuangyin Chen 2,* and Huamei Duan 3

1 Key Laboratory of Extraordinary Bond Engineering and Advanced Materials Technology (EBEAM), Yangtze Normal University, Chongqing 408100, China; [email protected] 2 School of Artiﬁcial Intelligence and Automation, Huazhong University of Science and Technology, Wuhan 430074, China 3 College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China; [email protected] * Correspondence: [email protected]

Abstract: A new process of extracting titanium from vanadium–titanium magnetite (VTM) in the Panxi area in Sichuan, China is introduced in this work. Various experiments, including reduction– magnetic separation, leaching and hydrolyzing experiments, are carried out. The results show that the optimum conditions for leaching experiments are an acid/slag ratio of 4:1, a leaching temperature of 60 ◦C, a leaching time of 80 min, and a liquid/solid ratio of 3.2:1. The leaching rate of titanium in Ti-bearing slag is 92.41%. The optimum conditions for hydrolyzing experiments are an H+ concentration of 0.75 g·L−1, hydrolyzing temperature of 100 ◦C, and hydrolyzing time of 180 min, and the hydrolyzing rate of titanium in acid leaching liquor is 96.80%. After the leaching and hydrolyzing experiments, the recovery rate of titanium from the Ti-bearing slag is 89.45%.

Keywords: vanadium–titanium magnetite; Ti-bearing slag; leaching; hydrolyzing

Citation: Li, Y.; Chen, S.; Duan, H. A New Process of Extracting Titanium from Vanadium–Titanium Magnetite. 1. Introduction Crystals 2021, 11, 327. https:// Vanadium–titanium magnetite (VTM) owes its extremely high and comprehensive doi.org/10.3390/cryst11040327 utilization value due to abundant associating and valuable elements such as iron, vanadium and titanium [1]. The reserve of the VTM resources attains over 10 billion tons in the Academic Editor: Ing. José García Panzhihua and Xichang regions, in which the reserve of the TiO2 reaches 873 million tons and accounts for 90.5% of total national reserves [2,3]. In the last decades, VTM has been Received: 14 March 2021 treated by BF (Blast Furnace) ironmaking and a BOF (Basic Oxygen Furnace) vanadium Accepted: 23 March 2021 extracting/semi-steel smelting process in China [4]. As a result, a mountain of titanium Published: 25 March 2021 in VTM enters the BF slag, which is piled along the Jinsha River and is difficult to utilize. Consequently, this leads to a serious issue of environmental pollution and a large supply Publisher’s Note: MDPI stays neutral of valuable titanium is lost. with regard to jurisdictional claims in In order to achieve the comprehensive utilization of titanium from ilmenite and VTM, published maps and institutional affil- a large number of processes for extracting titanium have been put forward nowadays [5–8]. iations. The sulphate process is the first commercialized technology to produce TiO2 using ilmenite. However, the lower quality products are obtained associating with a high cost and acid consumption to removing impurities in iron and vanadium. In addition, the by-product, ferrous sulphate, is less marketable [9–16]. Copyright: © 2021 by the authors. The chloride process of producing TiO2 employs more complicated technology. First, Licensee MDPI, Basel, Switzerland. the high grade of feedstocks containing more than 90% of TiO2, such as natural rutile, This article is an open access article synthetic rutile and high grade of titanium slag, are obtained from upgrading the ilmenite distributed under the terms and and VTM, then these feedstocks are purified by chlorination at a high temperature to obtain conditions of the Creative Commons pure TiCl4. After condensation and distillation, the TiCl4 is oxidized at high temperatures Attribution (CC BY) license (https:// to produce titanium dioxide. In this process, the cost and waste materials are both re- creativecommons.org/licenses/by/ duced [17–20]. However, strict requirements for equipment and raw materials hindered 4.0/).

Crystals 2021, 11, 327. https://doi.org/10.3390/cryst11040327 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 327 2 of 10

the chloride process development in China. Therefore, a new process should be developed to extract titanium from VTM. In this work, the process of extracting titanium from VTM through roasting and leaching Ti-bearing slag obtained from metalizing reduction and magnetic separation process [21] with H2SO4 was carried out. Then the hydrolyzing experiments of acid leaching liquor were investigated. In this process, the valuable elements of iron and vanadium have been recycled by a pyro-metallurgical process. Thus, the elimination of by-products can be reduced and simultaneously, the energy consumption can be decreased. Meanwhile, the waste problem of titanium in the BF process is solved.

2. Experimental Methods 2.1. Raw Materials The chemical compositions of VTM and industrial analysis of reduction coal examined in this work are presented in Table1, where A ad for ash content, Vad for volatile content, Mad for moisture content, FCad for ﬁxed carbon content and ad means air-dried basis. The particle sizes of VTM and reduction coal are ground to less than 0.074mm. As listed in Table1, the VTM from PanXi area of China contains 53.91% Fe and 13.03% TiO 2, and the main minerals in the VTM are magnetite, titano-magnetite, ilmenite and vanadium spinel [21].

Table 1. Chemical compositions of VTM and industrial coal (mass%).

VTM Coal

TFe FeO V2O5 TiO2 CaO SiO2 MgO Al2O3 Aad Vad Mad FCad 53.91 31.13 0.52 13.03 0.68 3.20 2.71 3.82 8.88 27.52 1.92 61.68

Other chemical agents mainly include concentrated sulfuric acid (98 vol.%) and deionized water in the leaching and hydrolyzing process.

2.2. Experimental Procedure The first step is the testing of valuable elements separation rate and the preparation of Ti-bearing slag. During the tests, the process parameters, such as reduction and magnetic separation, specifically, the magnetic intensity, reduction temperature, reduction time and carbon ratio, were changed within the default ranges. Then, the optimal parameters were determined, and the best Ti-bearing slag was obtained [3]. The second step involves acid leaching the Ti-bearing slag. The Ti-bearing slag, concentrated sulfuric acid, and deionized water were added into a three-necked, round-bottomed flask with a mechanical stirrer. Then, the leaching experiments were carried out at the preset temperature for the preset time. The third step involves hydrolyzing enrichment experiments of the acid leaching liquor. The acid leaching liquor was added into another three-necked, round-bottomed flask with a mechanical stirrer. The flask was placed in an electric heating jacket setting with the preset temperature and the preset time. The experimental flow accompanying the main apparatus is shown in Figure1. CrystalsCrystals 20212021, ,1111,, x 327 FOR PEER REVIEW 33 of of 10 10

Concentrated tailings

Fine grinding

H SO Ripening 2 4

H2SO4+H2O Leaching

Residuals Ti-enriched solution

H2O Hydrolysis

Deposition Discard solution

Roasting

TiO2

FigureFigure 1. 1. SchematicSchematic diagram diagram of of experimental experimental procedure procedure and and apparatus. apparatus. In this study, the leaching and hydrolysis mechanism were analyzed using X-ray 3. Results and Discussions diffraction (XRD, D8 Discover, Bruker, Karlsruhe, Germany) with Cu Ka radiation for 3.1.phase Preparation structures, of Ti scanning-Bearing electronSlag microscopy (SEM, Ultra Plus, ZEISS, Jena, Germany) for microstructuralDuring the previous analysis reduction and energy-dispersive and magnetic separat X-ray spectroscopyion experiments (EDS, of UltraVTM Plus,and coal,ZEISS, four Jena, key Germany) process factors for element, including content, carbon and ratio the leachingof 0.8–1.8, kinetics reduction was temperature determined of by 1250the unreacted–1355 °C , reduction core model. time of 10–60 min, and magnetic intensity of 25–125 mT, have been considered. The optimal process parameters for reduction and magnetic separation are3. Resultsa carbon and ratio Discussions of 1.0, a reduction temperature of 1350 °C , reduction time of 60 min and magnetic3.1. Preparation intensity of Ti-Bearing of 50 mT Slag [21]. Under the above conditions, the Ti-bearing slag was obtainedDuring and thethen previous analyzed reduction by chemical and methods, magnetic SEM separation and XRD. experiments of VTM and coal, four key process factors, including carbon ratio of 0.8–1.8, reduction temperature of Table1250–1355 2. Chemical◦C, reduction composition time of the of 10–60Ti-bearing min, slag and (mass%). magnetic intensity of 25–125 mT, have been considered. The optimal process parameters for reduction and magnetic separation TiO2 CaO SiO2 MgO Al2O3 TFe are a carbon ratio of 1.0, a reduction temperature of 1350 ◦C, reduction time of 60 min and 40.91 1.47 14.04 12.95 12.34 8.78 magnetic intensity of 50 mT [21]. Under the above conditions, the Ti-bearing slag was obtained and then analyzed by chemical methods, SEM and XRD. TheThe chemical chemical composition composition of ofthe the Ti- Ti-bearingbearing slag slag is given is given in Table in Table 2. As2 .listed As listedin Table in 2, the TiO2 content reaches 40.91%. As the XRD pattern and EDS analysis of the Ti-bearing Table2, the TiO 2 content reaches 40.91%. As the XRD pattern and EDS analysis of the slagTi-bearing are shown slag are in shown Figure in2, Figure the main2, the mineralogical main mineralogical phases phases of the of Ti the-bearing Ti-bearing slag slag are MgAl2O4, MgxTi3−xO5, FeTiO3, Mg2SiO4 and TiO2. are MgAl2O4, MgxTi3−xO5, FeTiO3, Mg2SiO4 and TiO2.

Table 2. Chemical composition of the Ti-bearing slag (mass%).

TiO2 CaO SiO2 MgO Al2O3 TFe 40.91 1.47 14.04 12.95 12.34 8.78

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FigureFigure 2. 2.XRD XRD pattern pattern and and EDS EDS analysis analysis of ofseparated separated tailings. tailings.

3.2.3.2. Leaching Leaching of ofTi-bearing Ti-Bearing Slag Slag by byH2 HSO2SO4 4 3.2. Leaching of Ti-bearing Slag by H2SO4 3.2.1. Effects of Acid/Slag Ratio on Leaching Rate 3.2.1.3.2.1. Effects Effects of of Acid/Slag Acid/Slag ratio ratio on on Leaching Leaching Rate Rate InIn order order to to get get the the optimum optimum acid/slag acid/slag ratio, ratio, the the Ti-bearing Ti-bearing slag slag and and H H2SO2SO4 mixtures4 mixtures In order to get the optimum acid/slag ratio, the Ti-bearing slag and H2SO4 mixtures werewere prepared prepared with with the the acid/slag acid/slag ratios ratios of of 2:1, 2:1, 3:1, 3:1, 4:1 and 5:1. The otherother processprocess parameters parame- were prepared with the acid/slag ratios of 2:1, 3:1, 4:1 and 5:1.◦ The other process parame- terswere were kept kept constant constant as as follows: follows:roasting roasting temperaturetemperature of 200 °C,C, roasting roasting time time of of 90 90 min, min, ters were kept constant as follows:◦ roasting temperature of 200 °C, roasting time of 90 min, leachingleaching temperature temperature of of70 70°C, leachingC, leaching time time of 60 of min, 60 min, liquid/solid liquid/solid ratio of ratio 3.2:1. of The 3.2:1. var- The variationleaching temperature of leaching rate of 70 of °C, titanium leaching at differenttime of 60 acid/slag min, liquid/solid ratios is givenratio of in 3.2:1. Figure The3. var- iationiation of of leaching leaching rate rate of of titanium titanium at atdiffe differentrent acid/slag acid/slag ratios ratios is isgiven given in in Figure Figure 3. 3.

95 95 90.97 90.97 90 89.01 90 89.01 86.25 86.25 85 85

80 80 Leaching rate/% Leaching rate/%

75 75

71.56 71.56 70 70 1.52345 2.5 3.5 4.5 5.5 2345 1.5 2.5Acid/Slag 3.5 4.5 5.5 Acid/Slag Figure 3. Effect of acid/slag ratios on leaching rate of titanium. FigureFigure 3. 3.Effect Effect of ofacid/slag acid/slag ratios ratios on on leaching leaching rate rate of oftitanium. titanium. As shown in Figure3, the leaching rate of titanium increases from 71.56% to 90.97% as theAs acid/slagAs shown shown in ratio in Figure Figure increases 3, 3,the the from leaching leaching 2:1 to rate 4:1.rate of With of titanium titanium further increases increases from infrom acid/slag 71.56% 71.56% to ratioto 90.97% 90.97% from as4:1as the the to acid/slag 5:1,acid/slag the leaching ratio ratio increases increases rate of titaniumfrom from 2:1 2:1 presents to to 4:1. 4:1. With a With downward further further increases trend. increases The in deﬁnitein acid/slag acid/slag reason ratio ratio for fromthisfrom 4:1 phenomenon 4:1 to to 5:1, 5:1, the the isleaching thatleaching a rising rate rate acid/slagof of tita titaniumnium ratio presents presents can effectively a adownward downward restrain trend. trend. hydrolysis The The definite reactiondefinite reasonofreason the for acidfor this this leaching phenomenon phenomenon liquor is as isthat that well a risinga as rising improve acid/slag acid/slag the ratio kineticratio can can conditionseffectively effectively restrain of restrain the acidolysis hydrol- hydrol- ysisreactionysis reaction reaction between of of the the Hacid 2acidSO leaching4 andleaching Ti-bearing liquor liquor as slag. as well well However, as as improve improve a high the the acid/slagkinetic kinetic conditions conditions ratio may of lead of the the to acidolysis reaction between H2SO4 and Ti-bearing slag. However, a high acid/slag ratio twoacidolysis adverse reaction factors between of the leaching H2SO4 process.and Ti-bearing On the slag. one hand, However, the consumption a high acid/slag of H 2ratioSO4 mayincreases;may lead lead to to ontwo two the adverse adverse other hand,factors factors the of of emergencethe the leac leachinghing of process. gelatinous process. On On matter the the one one results hand, hand, in the lotsthe consump- ofconsump- soluble tion of H2SO4 increases; on the other hand, the emergence of gelatinous matter results in titaniumtion of H ions2SO4 absorbed,increases; whichon the can other decrease hand, the leachingemergence rate of of gelatinous titanium. matter Accordingly, results the in optimum acid/slag ratio of 4:1 is selected for the subsequent tests.

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Crystals 2021, 11, 327 5 of 10 lots of soluble titanium ions absorbed, which can decrease the leaching rate of titanium. Accordingly, the optimum acid/slag ratio of 4:1 is selected for the subsequent tests.

3.2.2.3.2.2. Effects Effects of of Temperature Temperature on on Leaching Leaching Rate Rate ToTo obtain obtain the the optimum optimum leaching leaching temperature, temperature, a a series series of of leaching leaching experiments experiments were were carriedcarried out out at at temperatures temperatures of of 40, 40, 50, 50, 60, 60, 70 70 and and 80 80 °C.◦C. The The other other process process parameters parameters were were setset as as follows: follows: roasting roasting temperature temperature of of 200 200 °C,◦C, roasting roasting time time of of 90 min, acid/slag ratio ratio of of 4:1,4:1, leaching leaching time of 6060 minmin andand liquid/solid liquid/solid ratioratio of of 3.2:1. 3.2:1. The The inﬂuence influence of of temperature temperature on onleaching leaching rate rate of of titanium titanium is shownis shown in in Figure Figure4. 4.

90 90.97 91.10

85 87.55

75 Leaching rate/% 72.95 70

65 67.60

60 3540 45 50 55 60 65 70 75 80 85 Leaching temperature/℃ FigureFigure 4. 4. EffectEffect of of temperature temperature on on leaching leaching rate rate of of titanium. titanium.

AsAs shown shown in in Figure Figure 4,4 ,after after the the leaching leaching tests, tests, the the valuable valuable element element of titanium of titanium in the in Ti-bearingthe Ti-bearing slag separates slag separates from fromthe residue the residue and enters and enters the acid the leaching acid leaching liquor. liquor.The leach- The ingleaching rate of rate titanium of titanium in the inacid the leaching acid leaching liquor liquorincreases increases from 72.95% from 72.95% to 91.1% to 91.1%as the astem- the ◦ ◦ peraturetemperature increases increases from from40 °C 40to 60C °C. to 60 HoweC. However,ver, the leaching the leaching rate sharply rate sharply decreases decreases when ◦ thewhen leaching the leaching temperature temperature is over is 70 over °C. 70ThisC. could This couldbe caused be caused by a variety by a variety of complex of complex reasons.reasons. On Onthe theone one hand, hand, when when the the leaching leaching temperature temperature increases increases within within the the reasonable reasonable range, the reactivity of the interfaces between H SO and Ti-bearing slag and the flowability range, the reactivity of the interfaces between 2H2SO4 4 and Ti-bearing slag and the flowa- bilityof the of acid the leachingacid leaching liquor liquor are improvedare improved effectively. effectively. Therefore, Therefore, the the leaching leaching reactions reac- tionsproceed proceed under under the optimumthe optimum dynamic dynamic conditions. conditions. On On the the other other hand, hand, when when the the leaching leach- ingtemperature temperature increases increases further, further, an an insoluble insoluble film film forming forming on on thethe Ti-bearing slag surface surface hinder the proceeding of leaching reaction. Accordingly, the optimum leaching temperature hinder the proceeding of leaching reaction. Accordingly, the optimum leaching tempera- of 60 ◦C is selected for the subsequent tests. ture of 60 °C is selected for the subsequent tests. 3.2.3. Effects of Reaction Time on Leaching Rate 3.2.3. Effects of Reaction Time on Leaching Rate To optimize the leaching time, a series of leaching experiments were conducted at variousTo optimize times of 20,the 40,leaching 60, 80, time, 100 and a series 120 min. of leaching The other experiments process parameters were conducted were kept at variousconstant times as follows: of 20, 40, roasting 60, 80, temperature 100 and 120 of mi 200n. ◦TheC, roasting other process time of parameters 90 min, acid/slag were kept ratio constantof 4:1, leaching as follows: temperature roasting temperature of 60 ◦C, liquid/solid of 200 °C, roasting ratio of 3.2:1.time of The 90 influencemin, acid/slag of time ratio on ofleaching 4:1, leaching rate of temperature titanium is shownof 60 °C, in Figureliquid/solid5. ratio of 3.2:1. The influence of time on leachingAs shownrate of titanium in Figure is5, shown the leaching in Figure rate 5. of titanium increases from 64.85% to 92.41% as the leaching time increases from 20 min to 80 min. When the leaching time is longer than 80min, the leaching rate of titanium begins to decline. This phenomenon can be explained by the two factors on which the leaching rate of titanium from Ti-bearing slags mainly depends: leaching reaction rate and hydrolyzing reaction rate. In the initial stage, the content of TiO2+ in the leaching system is low, the leaching reaction plays the leading role, and it makes the titanium leached from Ti-bearing slag to the acid leaching liquor. However, with the leaching reaction rising, the content of TiO2+ in the leaching system becomes higher and higher, the hydrolyzing reaction begins to play the leading role, and the hydrous titanium dioxide is generated rapidly. Therefore, the optimum leaching time of 80 min is selected for the subsequent tests.

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90 92.41 90.96 88.75

80 80.94

75 Leaching rate/% 70

CrystalsCrystals 20212021, 11, 11, x, 327FOR PEER REVIEW 65 6 6of of 10 10 64.85 65.37 60 1020 30 40 50 60 70 80 90 100 110 120 130 Leaching time/min 95 Figure 5. Effect of time on leaching rate of titanium. 90 92.41 90.96 88.75

85As shown in Figure 5, the leaching rate of titanium increases from 64.85% to 92.41% as the leaching time increases from 20 min to 80 min. When the leaching time is longer 80 than 80min, the leaching80.94 rate of titanium begins to decline. This phenomenon can be explained by the two factors on which the leaching rate of titanium from Ti-bearing slags 75 mainly depends: leaching reaction rate and hydrolyzing reaction rate. In the initial stage, Leaching rate/% the 70content of TiO2+ in the leaching system is low, the leaching reaction plays the leading role, and it makes the titanium leached from Ti-bearing slag to the acid leaching liquor. 65 2+ However,64.85 with the leaching reaction rising, the content65.37 of TiO in the leaching system becomes60 higher and higher, the hydrolyzing reaction begins to play the leading role, and 1020 30 40 50 60 70 80 90 100 110 120 130 the hydrous titanium dioxideLeaching time/min is generated rapidly. Therefore, the optimum leaching time of 80 min is selected for the subsequent tests. FigureFigure 5. 5. EffectEffect of of time time on on leaching leaching rate rate of of titanium. titanium. 3.2.4. Effects of Liquid/Solid Ratio on Leaching Rate 3.2.4.As Effects shown of in Liquid/Solid Figure 5, the Ratioleaching on Leaching rate of titanium Rate increases from 64.85% to 92.41% as theTo Toleaching optimize optimize time the increases liquid/solidliquid/solid from ratio, 20 minaa seriesseries to 80 of of leachingmin. leaching When experiments experiments the leaching are are carriedtime carried is outlonger without withthandifferent 80min,different liquid/solid the liquid/solid leaching ratios rate ratios of 1.6,titaniumof 1.6, 2.4, 2.4, 3.2, begins 3.2, 4.0 4.0 and to and decline. 4.8. 4.8. The The This other other phenomenon process process parameters parameters can be ex- are ◦ areplainedkept kept constant by constant the astwo follows: as factors follows: roasting on roastingwhich temperature the temper leachingature of 200 rate ofC, of200 roastingtitanium °C, roasting time from of Ti-bearing 90time min, of acid/slag90 slagsmin, ◦ acid/slagmainlyratio of depends: 4:1,ratio leaching of 4:1,leaching leaching temperature reaction temperature rate of 60 andC, of hydrolyzing leaching60 °C, leaching time reaction of time 80 min. ofrate. 80 Themin.In the inﬂuence The initial influence stage, of the oftheacid/slag the content acid/slag ratioof TiO ratio on2+ leachingin on the leaching leaching rate ofrate system titanium of titanium is islow, shown is the shown leaching in Figure in Figure reaction6. 6. plays the leading role, and it makes the titanium leached from Ti-bearing slag to the acid leaching liquor. 94 However, with the leaching reaction rising, the content of TiO2+ in the leaching system 92.43 becomes92 higher and higher, the hydrolyzing reaction begins to play the leading role, and the hydrous titanium dioxide is generated rapidly. Therefore, the optimum leaching time of 8090 min is selected89.16 for the subsequent tests.

88 3.2.4. Effects of Liquid/Solid Ratio on Leaching87.99 Rate87.98 86To optimize the liquid/solid ratio, a series of leaching experiments are carried out Leaching rate/% 85.42 with84 different liquid/solid ratios of 1.6, 2.4, 3.2, 4.0 and 4.8. The other process parameters are kept constant as follows: roasting temperature of 200 °C, roasting time of 90 min, acid/slag82 ratio of 4:1, leaching temperature of 60 °C, leaching time of 80 min. The influence

of the80 acid/slag ratio on leaching rate of titanium is shown in Figure 6. 1.21.6 2.2 2.4 3.2 4 4.2 4.8 5.2 Liquid/Solid 94

FigureFigure 6. 6. EffectEffect of of liquid/solid liquid/solid ratio ratio 92.43on on leaching leaching rate rate of oftitanium. titanium. 92 AsAs shown shown in in Figure Figure 6,6, the the leaching leaching rate rate of of titanium obviously increases increases when when the the 90 89.16 liquid/solidliquid/solid ratio ratio increased increased from 1.61.6 to to 3.2. 3.2. However, However, the the leaching leaching rate rate of titanium of titanium decreases de- creasesfrom88 92.41% from 92.41% to around to around 88% as 88% the liquid/solidas the liquid/solid ratio increases ratio increases further. further. The reason The reason causing 87.99 87.98 causingthis above this above phenomenon phenomenon is because is because titanium titanium cannot cannot be leached be leached completely completely when when the 86 theliquid/solid liquid/solid ratio ratio is is relatively relatively low. low. However, However, when when the the liquid/solid liquid/solid ratio ratio is toois too high, high, the Leaching rate/% 85.42 hydrolyzing84 reaction is promoted and the hydrous titanium dioxide is generated. Therefore, the optimum liquid/solid ratio of 3.2:1 is selected for the subsequent tests. 82

3.3. Hydrolyzing of Acid Leaching Liquor 80 1.2After1.6 leaching 2.2 2.4 tests of the 3.2 Ti-bearing 4 slag, 4.2 the 4.8 optimum 5.2 process conditions for leaching Liquid/Solid are acid/slag ratio of 4:1, leaching temperature of 60 ◦C, leaching time of 80 min and Figureliquid/solid 6. Effect ratioof liquid/solid of 3.2:1. ratio Based on on leaching the above rate of experimental titanium. parameters, the acid leaching liquor was obtained and analyzed by the chemical method. The results showed that the contentAs shown of total in titanium Figure was6, the 56.5 leaching g·L−1. rate of titanium obviously increases when the liquid/solid ratio increased from 1.6 to 3.2. However, the leaching rate of titanium decreases from 92.41% to around 88% as the liquid/solid ratio increases further. The reason causing this above phenomenon is because titanium cannot be leached completely when the liquid/solid ratio is relatively low. However, when the liquid/solid ratio is too high,

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the hydrolyzing reaction is promoted and the hydrous titanium dioxide is generated. Therefore, the optimum liquid/solid ratio of 3.2:1 is selected for the subsequent tests.

3.3. Hydrolyzing of Acid Leaching Liquor After leaching tests of the Ti-bearing slag, the optimum process conditions for leaching are acid/slag ratio of 4:1, leaching temperature of 60 °C, leaching time of 80 min and liquid/solid ratio of 3.2:1. Based on the above experimental parameters, the acid leaching Crystals 2021, 11, 327 7 of 10 liquor was obtained and analyzed by the chemical method. The results showed that the content of total titanium was 56.5 g·L−1.

3.3.1.3.3.1. Effects Effects of of H H+ Concentration+ Concentration on on Hydrolyzing Hydrolyzing Rate Rate ToTo determine determine the the optimum optimum H+ Hconcentration,+ concentration, various various hydrolyzing hydrolyzing tests testswith withH+ con- H+ centrationsconcentrations of 0.5, of 0.75, 0.5, 1.00, 0.75, and 1.00, 1.25 and g·L 1.25−1 were g·L− carried1 were out. carried The out.other The hydrolyzing other hydrolyzing process parametersprocess parameters were kept wereconstant kept as constant follows: hydrolyzing as follows: hydrolyzingtemperature temperatureof 90 °C, hydrolyzing of 90 ◦C, timehydrolyzing of 180 min. time The of 180influence min. Theof H inﬂuence+ concentration of H+ concentration on hydrolyzing on rate hydrolyzing of titanium rate is of showntitanium in Figure is shown 7. in Figure7.

100

93.96 91.74 90

88.17 Hydrolyzing rate/% 85 86.42

80 0.4 0.50 0.75 1.00 1.25 H+ concentration/g·L-1 FigureFigure 7. 7. EffectEffect of of H H+ concentration+ concentration on on hydrol hydrolyzingyzing rate rate of of titanium. titanium.

AsAs shown shown in FigureFigure7 ,7, the the hydrolyzing hydrolyzing rate rate of titanium of titanium increases increases from 88.17%from 88.17% to 93.96% to − − 93.96%as the Has+ theconcentration H+ concentration increases increases from 0.5 from g·L 0.51 tog·L 0.75−1 to g 0.75·L 1g·L. However,−1. However, the hydrolyzingthe hydro- − lyzingrate of rate titanium of titanium begins begins to decrease to decrease when thewhen H+ theconcentration H+ concentration is over is 0.75 over g· L0.751. Thisg·L−1 is. Thisbecause is because the hydrolyzing the hydrolyzing reaction reaction of TiOSO of4 inTiOSO acid4 leaching in acid liquorleaching can liquor be promoted can be whenpro- + motedthe H whenconcentration the H+ concentration varies in a certainvaries in range. a certain But range. the stability But the of stability acid leaching of acid liquor leach- is + ingdestroyed liquor isas destroyed the H concentration as the H+ concentration increases further, increases and furthe the hydrolyzingr, and the hydrolyzing rate of titanium rate ofdecreases titanium asdecreases a result. as a result.

3.3.2.3.3.2. Effects Effects of of Temperature Temperature on on Hydrolyzing Hydrolyzing Rate Rate ◦ AA series series of of tests tests at atthe the temperatures temperatures of 70, of 70,80, 80,90, 90,and and 100 100°C wereC were carried carried out to out de- to determine the optimum hydrolyzing temperature under the conditions of H+ concentration Crystals 2021, 11, x FOR PEER REVIEWtermine the optimum hydrolyzing temperature under the conditions of H+ concentration8 of 10 0.75 g·L−1 and hydrolyzing time 180 min. The inﬂuence of temperature on hydrolyzing 0.75 g·L−1 and hydrolyzing time 180 min. The influence of temperature on hydrolyzing rate of titanium is shown in Figure8. rate of titanium is shown in Figure 8.

100

96.80 93.96 90

80 81.18 Hydrolyzing rate/% 70 70.08

60 6570 70 75 80 85 90 95 100 105 Hydrolyzing temperature/℃ FigureFigure 8. 8. EffectEffect of of temperature temperature on on hydrolyzing hydrolyzing rate rate of of titanium. titanium.

AsAs shown shown in in Figure Figure 8,8, the hydrolyzing rate of of titanium increases increases from from 70.08% to ◦ ◦ 96.80%96.80% as as the the temperature temperature increases increases from from 70 70 °CC to to 100 100 °C.C. The The increase increase in in hydrolyzing hydrolyzing temperaturetemperature is is beneficial beneﬁcial to to the the hydrolyzing hydrolyzing re reactionaction of of titanium titanium in in acid acid leaching leaching liquor. liquor. This is because, on the one hand, the hydrolyzing reaction of acid leaching liquor is one endothermic reaction, so the high temperature is propitious to the hydrolyzing reaction; on the other hand, Ti3+ in acid leaching liquor can be oxidized to Ti4+ at a higher temperature. The influence of hydrolyzing temperature on the product morphology is shown in Figure 9. As shown in Figure 9a,b, the size of product increases with the hydrolyzing temperature increasing from 80 °C to 100 °C. At the temperature of 80 °C, the product is pre- cipitated with a high nucleation rate and relatively low growth rate; thus, a large number of grains are generated, but the size of the grain is relatively small. At the temperature of 100 °C, two main reasons lead to the growth of grains. On the one hand, the saturation and viscosity of solution are decreased at high temperature and the mass transfer coeffi- cient is increased; thus, the growth rate of grains greatly increases. On the other hand, the energy of molecular motion increases with the temperature increases and the recrystallization of grains is enhanced.

(a) (b) Figure 9. SEM images of hydrolyzing product at different temperatures: (a) 80 °C, 180 min and (b) 100 °C, 180 min.

3.3.3. Effects of Time on Hydrolyzing Rate To determine the optimum hydrolyzing time, a series of tests at the times of 30, 60, 120, 180 and 240 min are carried out under the conditions of H+ concentration 0.75 g·L−1

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100

96.80 93.96 90

80 81.18 Hydrolyzing rate/% 70 70.08

60 6570 70 75 80 85 90 95 100 105 Hydrolyzing temperature/℃ Figure 8. Effect of temperature on hydrolyzing rate of titanium. Crystals 2021, 11, 327 8 of 10 As shown in Figure 8, the hydrolyzing rate of titanium increases from 70.08% to 96.80% as the temperature increases from 70 °C to 100 °C. The increase in hydrolyzing temperature is beneficial to the hydrolyzing reaction of titanium in acid leaching liquor. ThisThis is is because, because, on on the the one one hand, hand, the the hydrolyz hydrolyzinging reaction reaction of ofacid acid leaching leaching liquor liquor is one is one endothermicendothermic reaction, reaction, so so the the high temperature is propitious to the hydrolyzing reaction; on 3+ 4+ onthe the other other hand, hand, Ti Tiin3+ in acid acid leaching leaching liquor liquor can can be be oxidized oxidized to to Ti Ti4+at ata ahigher higher temperature.temperature. The inﬂuence of hydrolyzing temperature on the product morphology is shown in FigureThe9. influence As shown of inhydrolyzing Figure9a,b, temperature the size of on product the product increases morphology with the is hydrolyzing shown in ◦ ◦ ◦ Figuretemperature 9. As shown increasing in Figure from 9a,b, 80 theC to size 100 of productC. At the increases temperature with the of 80 hydrolyzingC, the product tem- is peratureprecipitated increasing with a highfrom nucleation80 °C to 100 rate °C. and At relativelythe temperature low growth of 80 rate;°C, the thus, product a large is number pre- cipitatedof grains with are generated,a high nucleation but the rate size and of the rela graintively is low relatively growth small.rate; thus, At the a large temperature number of of100 grains◦C, two are maingenerated, reasons but lead the tosize the of growth the grain of grains.is relatively On the small. one At hand, the thetemperature saturation of and 100viscosity °C, two of solutionmain reasons are decreased lead to the at growth high temperature of grains. On and the the one mass hand, transfer the saturation coefﬁcient is andincreased; viscosity thus, of solution the growth are ratedecreased of grains at hi greatlygh temperature increases. and On the mass other transfer hand, the coeffi- energy cientof molecular is increased; motion thus, increases the growth with rate the of temperature grains greatly increases increases. and On the the recrystallization other hand, the of energygrains isof enhanced.molecular motion increases with the temperature increases and the recrystallization of grains is enhanced.

(a) (b)

FigureFigure 9. 9. SEMSEM images images of of hydrolyzing hydrolyzing produc productt at atdifferent different temperatures: temperatures: (a) (80a) °C, 80 ◦180C, 180minmin and and(b) (b) 100100 °C,◦C, 180 180 min. min.

3.3.3.3.3.3. Effects Effects of of Time Time on on Hydrolyzing Hydrolyzing Rate Rate Crystals 2021, 11, x FOR PEER REVIEW 9 of 10 ToTo determine determine the the optimum optimum hydrolyzing hydrolyzing time, time, a series a series of oftests tests at the at the times times of 30, of 30,60, 60, 120,120, 180 180 and and 240 240 min min are are carried carried out out under under the the conditions conditions of of H H+ +concentrationconcentration 0.75 0.75 g·L g·−L1 −1 and hydrolyzing temperature 100 ◦C. The inﬂuence of time on hydrolyzing rate of titanium and ishydrolyzing presented in temperature Figure 10. 100 °C. The influence of time on hydrolyzing rate of titanium is presented in Figure 10. 100 96.80 97.08 90

86.87 80

76.93

70 Hydrolyzing rate/%

57.42

50 030 50 60 90 100 120 150 180 200 210 240 250 Hydrolyzing time/min FigureFigure 10. Effect 10. Effect of temperature of temperature on hydrolyzing on hydrolyzing rate rateof titanium. of titanium.

As shownAs shown in Figure in Figure 10, the10, thehydrolyzing hydrolyzing rate rateof titanium of titanium increases increases from from 57.42% 57.42% to to 96.80%96.80% as the as hydrolyzing the hydrolyzing time time increases increases from from 30 min 30 minto 180 to 180min, min, and andthe thehydrolyzing hydrolyzing rate holds the line even if the hydrolyzing time keeps increasing. On the one hand, extend hydrolyzing time is beneficial to the growth of grains and the generation of hydrous titanium dioxide. On the other hand, the hydrous titanium dioxide would be re-dissolved after reaches equilibrium as the hydrolyzing time increases further. Therefore, an optimum hydrolyzing time of 180 min is selected for the subsequent tests. The influence of hydrolyzing time on the product morphology is shown in Figure 11. As shown in Figures 11a–b and 9b, the size of the product increases with the hydrolyzing time increasing from 30 min to 180 min. When the growth rate is larger than the generation rate of hydrous titanium dioxides, the product size starts to grow up. In addition, as the extension of hydrolyzing time, the crystal nucleus of the grains begins to aggregate and form the agglomeration. This may lead to a negative effect on product growth.

(a) (b) Figure 11. SEM images of hydrolyzing product at different times: (a) 100 °C, 30 min and (b) 100 °C, 120 min.

4. Conclusions According to the comprehensive comparisons at a series of experimental conditions, the optimum conditions for leaching experiments are obtained as follows: the acid/slag ratio of 4:1, the leaching temperature of 60 °C, the leaching time of 80 min and the liquid/solid ratio of 3.2:1. Under the above conditions, the leaching rate of titanium in Ti-

Crystals 2021, 11, x FOR PEER REVIEW 9 of 10

and hydrolyzing temperature 100 °C. The influence of time on hydrolyzing rate of titanium is presented in Figure 10. 100

96.80 97.08 90

86.87 80

76.93

70 Hydrolyzing rate/%

57.42

50 030 50 60 90 100 120 150 180 200 210 240 250 Hydrolyzing time/min Crystals 2021, 11, 327 9 of 10 Figure 10. Effect of temperature on hydrolyzing rate of titanium.

As shown in Figure 10, the hydrolyzing rate of titanium increases from 57.42% to 96.80%rate holds as the the hydrolyzing line even if thetime hydrolyzing increases from time 30 keeps min increasing.to 180 min, On and the the one hydrolyzing hand, extend ratehydrolyzing holds the line time even is beneﬁcial if the hydrolyzing to the growth time keeps of grains increasing. and theOn generationthe one hand, of extend hydrous hydrolyzingtitanium dioxide. time is On beneficial the other to hand,the growth the hydrous of grains titanium and the dioxide generation would of hydrous be re-dissolved tita- niumafter reachesdioxide. equilibrium On the other as hand, the hydrolyzing the hydrou times titanium increases dioxide further. would Therefore, be re-dissolved an optimum afterhydrolyzing reaches equilibrium time of 180 minas th ise selectedhydrolyzing for the time subsequent increases tests.further. Therefore, an optimum Thehydrolyzing inﬂuence time of hydrolyzing of 180 min is time selected on the for product the subsequent morphology tests. is shown in Figure 11. As shownThe influence in Figure of hydrolyzing11a,b and Figure time on9b, the the pr sizeoduct of morphology the product is increases shown in withFigure the 11. hy- Asdrolyzing shown in time Figures increasing 11a–b fromand 9b, 30 the min size to 180of the min. product When increases the growth with rate the is hydrolyzing larger thanthe timegeneration increasing rate from of hydrous 30 min titanium to 180 min. dioxides, When the the growth product rate size is starts larger to than grow the up. generation In addition, rateas the of extensionhydrous titanium of hydrolyzing dioxides, time, the theproduct crystal size nucleus starts to of grow the grains up. In begins addition, to aggregate as the extensionand form of the hydrolyzing agglomeration. time, This the crystal may lead nucleus to a negative of the grains effect begins on product to aggregate growth. and form the agglomeration. This may lead to a negative effect on product growth.

(a) (b)

FigureFigure 11. 11. SEMSEM images images of of hydrolyzing hydrolyzing pr productoduct at atdifferent different times: times: (a) ( a100) 100 °C,◦ C,30 30min min and and (b) (100b) 100 ◦C, °C,120 120 min. min.

4.4. Conclusions Conclusions AccordingAccording to to the the comprehensive comprehensive comparisons comparisons at ata series a series of ofexperimental experimental conditions, conditions, thethe optimum optimum conditions conditions for leaching experiments areare obtainedobtained asas follows:follows: thethe acid/slagacid/slag ra- ratiotio of of 4:1, 4:1, the the leaching leaching temperature temperature of 60of ◦60C, °C, the leachingthe leaching time time of 80 of min 80 andmin the and liquid/solid the liquid/solidratio of 3.2:1. ratio Underof 3.2:1. the Under above the conditions, above conditions, the leaching the rateleaching of titanium rate of intitanium Ti-bearing in Ti- slag gets 92.41%. The optimum conditions for hydrolyzing experiments are the H+ concentration of 0.75 g·L−1, the hydrolyzing temperature of 100 ◦C and the hydrolyzing time of 180 min. Under the above conditions, the hydrolyzing rate of titanium in acid leaching liquor

is 96.80%. After the leaching and hydrolyzing experiments, the recovery rate of titanium from the Ti-bearing slag reaches 89.45%, which is feasible at the technological level.

Author Contributions: Validation, H.D.; Writing—original draft, S.C.; Writing—review & editing, Y.L. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This work was supported by Research Project from Chongqing Committee of Education (KJQN201801408). Conﬂicts of Interest: The authors declare no conﬂict of interest. Crystals 2021, 11, 327 10 of 10

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