Article Mineralogical Characteristics and Isothermal Oxidation Kinetics of Ironsand Pellets

Yaozu Wang , Jianliang Zhang and Zhengjian Liu *

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30th Xueyuan Road, Haidian district, Beijing 100083, China; [email protected] (Y.W.); [email protected] (J.Z.) * Correspondence: [email protected]; Tel.: +86-010-6233-2550

 Received: 23 January 2019; Accepted: 20 February 2019; Published: 23 February 2019 

Abstract: An in-depth understanding of mineralogical characteristics and the oxidation behaviors of ironsand is of great significance to make the best of ironsand and develop Ti-containing pellets. This paper quantitatively characterized the mineralogical characteristics of the ironsand from East Java in Indonesia through X-ray diffraction (XRD-Rietveld) and scanning electron microscope (SEM-EDS). The results indicated that the mineral composition of the ironsand was (22.7%), titanomagnetite (40.9%), enstatite (17.1%), hematite–ilmenite solid solution (14.5%), and magnesium aluminum silicon oxide (5.8%). The microstructure characterization of pellets after oxidation showed that the porosity of the pellets decreased from 20.7% to 11.7% with temperatures ranging from 1073 to 1473 K. Moreover, the activation energies of ironsand pellets were calculated by using model-function method. The calculated data of different mechanism functions indicated that the chemical reaction mechanism for the early stage of the oxidation fit A2 (random nucleation and nuclei growth) well, the chemical reaction mechanism for the post-oxidation at 1073–1273 K fit F1 (chemical reaction) well, and the chemical reaction mechanism for the post-oxidation at 1373 and 1473 K fit D4 (diffusion) well. The reaction mechanism and the limited link was finally discussed based on the kinetic analysis and the mineralogical characteristics.

Keywords: isothermal oxidation; kinetics; ironsand pellets; mineralogy; Ti-containing pellets

1. Introduction Vanadium– magnetite is a type of important polymetallic mineral, which is mainly composed of Fe, V, Ti, and contains a small amount of Cr, Co, and Ni [1–3]. Vanadium–titanium magnetite (V-Ti) is widely distributed in Canada (Quebec), China (Panxi, Chengde, and Maoshan), South (Bushveld), Russia (Ural tusk), and [4–6]. For a long time, vanadium–titanium magnetite has been used as an essential raw material in and pelletizing processes for ironmaking. The use of vanadium–titanium magnetite not only helps alleviate the crisis of resources [7–9], but also contributes to prolonging the furnace life due to the deposition of Ti(Cx,N1−x) at the surface or crack of the firebricks on the hearth [10,11]. Therefore, the effective activation and utilization of vanadium–titanium magnetite in sintering and pelletizing processes are of interest. Domestic and foreign scholars [12–16] have undertaken a lot of research on the influences of vanadium–titanium magnetite concentrate proportion on mineralogy, performance, and productivity of sinter. They suggested that due to the low iron content, small mineral crystal size, complicated phase structure, and numerous mineral components, V–Ti magnetite was a polymetallic paragenic resource that may be harmful to the sinter quality. They indicated that the increase of TiO2 content in the sinter could decrease the drum index, yield, and vertical sintering speed of the sintering, which limits the use of V–Ti magnetite.

Metals 2019, 9, 265; doi:10.3390/met9020265 www.mdpi.com/journal/metals Metals 2019, 9, 265 2 of 13

In view of the drawbacks of Ti-containing sinter, the utilization of V–Ti magnetite in the pellets is another important research interest. Moreover, a deep understanding of the oxidation behavior of V–Ti magnetite pellets is important to determine operating parameters and improve the quality of the pellets. Han et al. [17] studied the isothermal oxidation kinetics of V–Ti magnetite pellets at the temperature of 1073–1323 K according to the shrinking unreacted-core model. The results indicated that the oxidation process of the Ti-bearing pellets was controlled by chemical reaction at temperatures of 1073–1173 K (activation energy, 68.64 kJ/mol), and was mixed-controlled by the chemistry reaction and the diffusion at a temperature range of 1173–1273 K (activation energy, 39.66 kJ/mol). In addition, the limited link of oxidation is the diffusion process when the temperature is higher than 1273 K with an activation energy 20.85 kJ/mol. Li et al. [18,19] conducted a series of experiments on the oxidation kinetics of Hongge vanadium titanium-bearing magnetite pellets. The results indicated that the phase transformations of the valuable elements could be described as follows: Fe3O4 → Fe2O3, Fe2VO4 → (Cr0.15V0.85)2O3, Fe2.75Ti0.25O4 → FeTiO3 → Fe9TiO15, FeCr2O4 → (Fe0.6Cr0.4)2O4, Fe0.7Cr1.3O3, (Cr0.15V0.85)2O3. The average value of activation energy was calculated to be 69.33 kJ/mol by the Flynn–Wall–Ozawa method. However, data are still scarce for the oxidation of ironsand pellets. The restriction factors are not clear at a high temperature. Identification of the kinetics mechanism for the ironsand pellets is deficient, especially oxidation at high temperatures. In this paper, mineralogical characteristics of ironsand were quantitatively characterized through X-ray diffraction (XRD-Rietveld) and the microstructures were observed by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). A series of thermal gravity (TG) experiments were conducted to investigate the isothermal kinetics of the ironsand pellets in the air. The mineralogical compositions, porosity evolution, and compressive strength of the pellets after oxidation were further characterized by SEM. Moreover, the activation energies of the ironsand pellets were calculated by using the model-function method. The reaction mechanism and the limited link was finally discussed based on the kinetics analysis and the mineralogical characteristics.

2. Materials and Methods

2.1. Materials and Oxidation Experiments The ironsand used in the paper came from East Java in Indonesia. Table1 gives the chemical composition of ironsand determined by chemical analysis and fluorescence analysis (XRF-1800, Shimadzu, Kyoto, Japan). The results indicated that the ironsand had a high content of TiO2 and FeO. The mass fractions of Fe3+ and Fe2+ were 26.03% and 29.60%, respectively. The oxidation kinetics of the ironsand pellets were studied by conducting a series of oxidation experiments performed in an electric tube furnace in Figure1. Ironsand with sizes less than 74 µm was obtained through fine grinding and screening. Then the ironsand (50 g) and deionized water (4 mL) were mixed thoroughly. A total of 15 g of the blended mixture was pressed into a cylindrical block with a height of 10 mm and a diameter of 20 mm under the pressure of 10 MPa. After that, the pellets were dried in an oven at a constant temperature of 105 ◦C for 3 h to remove the free water. During a typical oxidation experiment, a pellet was placed in an iron––aluminum alloy wire basket and preheated under a protective atmosphere of Ar (3 L/min). The sample was then preheated to the target temperature. Subsequently, the gas was switched to compressed air with a flow rate of 5 L/min for 30 min. After oxidation, the samples were cooled to room temperature under an atmosphere of Ar (3 L/min). Each test was carried out three times and the mean was taken to remove any discrepancies.

Table 1. Chemical analysis results of typical magnetite (mass, %).

Item Total Fe FeO SiO2 Al2O3 CaO MgO TiO2 V2O5 PK2O Na2O Ironsand 55.63 29.60 4.13 3.38 0.60 3.74 11.41 0.76 0.03 0.02 0.02 Metals 2019, 9, 265 3 of 13 Metals 2019, 9, x FOR PEER REVIEW 3 of 14

Figure 1. Schematic of electric tube furnace. Figure 1. Schematic of electric tube furnace. 2.2. Characterization 2.2. Characterization This study quantitatively characterized the mineralogical characteristics of ironsand through XRD-RietveldThis study withquantitatively Cu Kα radiation characterized (M21XVHF22, the mineralogical MAC Science characteristics Co., Ltd., of Yokohama, ironsand through Japan). XRDThe- microstructuresRietveld with Cu of Kα ironsand radiation were (M21XVHF22, characterized MAC by Science an energy Co., dispersive Ltd., Yokohama X-ray, spectrometer Japan). The microstructures(EDS) in combination of ironsand with a scanningwere characterized electron microscopy by an energy (SEM, dispersive JSM-6460, X-ray JEOL spectrometer Ltd., Mitaka, (EDS) Japan) in combinationwith backscattering with a scanning modes. Sample electron particles microscopy were (SEM, fixed JSM in epoxy-6460, resin JEOL and Ltd., vacuum Mitaka, impregnated. Japan) with backscatteringBy using water modes as a lubricant,. Sample silicon particles carbide were paper fixed upin epoxy to 1000 resin grit, and and vacuum diamond impregnated. paste, these By sections using waterwere polishedas a lubrica carefullynt, silicon [20]. carbide The average paper porosity up to 1000 of the grit, pellets and wasdiamond obtained paste, through these area sections method were by polishedusing five carefully SEM patterns. [20]. The The average compressive porosity strengths of the ofpellets the pellets was obtained after oxidation through were area measured method by by a usingcompressive five SEM machine patterns. (QZYC-10C, The compressive Hebi metallurgical strengths of machinery the pellets equipmentafter oxidation co. LTD, were Hebi, measured China). by a compressive machine (QZYC-10C, Hebi metallurgical machinery equipment co. LTD, Hebi, China). 3. Results and Discussion 3. Results and Discussion 3.1. Mineralogical Characteristics of Ironsand 3.1. Mineralogical Characteristics of Ironsand Figure2 illustrates the XRD-Rietveld analysis of the ironsand. In the process of multiphase refinement,Figure 2 the illustrate Chebyshevs the polynomial XRD-Rietveld was analysis used to fitof thethe background.ironsand. In Thethe refinementprocess of significantlymultiphase refinement,reduced the the difference Chebyshev between polynomial the calculated was used patternto fit the and background. the experimental The refinement pattern. significantly There was reducedno residual the difference diffraction between peak, which the calculated indicated thatpattern the and crystal the structureexperimental model pattern. was reasonable There was [ 21no]. residTheual Rwp diffraction of the refinement peak, which reached indicated 5.05% that withthe crystal an accuracy structure of model 1%, which was reasonable indicated [21] that. The the Rwpequation of the analog refinement effect reached was good. 5.05% The with average an accuracy composition of 1%, which of mineral indicated phases that were the equation obtained analogthrough effect multiple was good. tests The as average shown incomposition Table2. The of mi resultsneral phases indicated were that obtained the main through minerals multiple of teststhe ironsand as shown were in Table titanomagnetite 2. The results (Fe2.814 indicateTi0.186dO 4that), magnetite the main (Fe minerals3O4), hematite–ilmenite of the ironsand were solid solution (Fe Ti O ), enstatite (Mg Fe Si O ), and magnesium iron aluminum silicon titanomagnetite1.696 (Fe0.2282.814Ti30.186O4), magnetite1.56 (Fe3O0.444), hematite2 6 –ilmenite solid solution (Fe1.696Ti0.228O3), oxide (Al Fe Mg O Si ). Titanium mainly exists in the form of Fe Ti O and enstatite (Mg0.2051.56Fe0.2810.44Si2O0.7196), and3 magnesium0.795 iron aluminum silicon oxide (Al0.205Fe0.2812.814Mg0.7190.186O3Si4 0.795). Fe1.696Ti0.228O3 at concentrations of 40.9% and 14.5%. Si and Mg combine with a small amount Titanium mainly exists in the form of Fe2.814Ti0.186O4 and Fe1.696Ti0.228O3 at concentrations of 40.9% and of iron to form Mg1.56Fe0.44Si2O6 and Al0.205Fe0.281Mg0.719O3Si0.795. 14.5%. Si and Mg combine with a small amount of iron to form Mg1.56Fe0.44Si2O6 and Al0.205Fe0.281Mg0.719O3Si0.795.

Metals 2019, 9, x FOR PEER REVIEW 4 of 14 Metals 2019, 9, 265 4 of 13

8000 Bos Calc

Fe3O4 6000 Fe1.7 Ti0.3 O3 Fe2.8 Ti0.2 O4

Fe0.4Mg1.6Si2O6 4000 Al0.205Fe0.281Mg0.719O3Si0.795

2000

Intensity (arb. units) (arb. Intensity 0

-20002000 diff 0 -2000 30 40 50 60 70 80 90 2 Theta(°)

Figure 2. Rietveld refinement of the obtained XRD pattern for the ironsand. Figure 2. Rietveld refinement of the obtained XRD pattern for the ironsand. Table 2. XRD-Rietveld quantitative analysis of the mineral phase in the ironsand (mass ratio, %). Table 2. XRD-Rietveld quantitative analysis of the mineral phase in the ironsand (mass ratio, %). Phase Chemical Formula Quantitative Analysis

MagnetitePhase Chemical Fe3 OFormula4 Quantitative21.7 Analysis TitanomagnetiteMagnetite Fe2.814FeTi3O0.1864 O4 21.40.97 Enstatite Mg1.56Fe0.44Si2O6 17.1 Titanomagnetite Fe2.814Ti0.186O4 40.9 Hematite–ilmenite solid solution Fe1.696Ti0.228O3 14.5 Magnesium ironEnstatite aluminum silicon oxide Al0.205MgFe0.2811.56FeMg0.440.719Si2O36 Si0.795 17.5.81 Hematite–ilmenite solid solution Fe1.696 Ti0.228O3 14.5 MagnesiumFigure3 gives iron the aluminum microstructures silicon o ofxide the ironsand.Al0.205Fe0.281 AsMg can0.719 beO3 seenSi0.795 from Figure35.8a, the shapes of the ironsandFigure 3 gives particles the microstructures were mostly regular of the and ironsand. had smooth As can surfaces. be seen from The particleFigure 3a diameter, the shapes varied of thefrom ironsand 100 to 300 particlesµm. SEM were patterns mostly in regular Figure 3and suggested had smooth that iron surfaces. oxides The in ironsand particle haddiameter two different varied fromkinds 100 of to microstructures. 300 μm. SEM patterns One was in particles Figure 3 with suggested the layered that iron phases oxides deposited in ironsand on the had substrate two different at an kindsangle, of as microstructures. shown in Figure One3b,c. was The particles EDS results with in the Figure layered3 (P1, phases P2) indicated deposited that on the substrate at was an angle,titanomagnetite as shown in (TTM) Figure and 3b the,c. The layered EDS phase results was in titanohematiteFigure 3 (P1, P2) (TTH). indicated Another that was the asubstrate homogeneous was titanomagnetiteparticle with smooth (TTM surfaces) and and the relatively layered averagephase was element titanohematite distribution, as(TTH expressed). Another in Figure was3 e,f.a homogeneousThe EDS results particle in Figure with3 (P4, smooth P6) confirmed surfaces the and main relatively phase in theaverage homogeneous element particle distribution, was TTM. as expressedThe EDS patternsin Figure in 3e Figure,f. The3 (P3, EDS P5) results confirmed in Figure the existence 3 (P4, P6) of enstatite,confirmed which the main was consistent phase in with the homogeneousthe results of XRD.particle More was remarkable, TTM. The EDS the EDS patterns results in showedFigure 3 that (P3, a P5) small confirmed amount the of Mg, existence Al, and of V enstatite,existed in which iron oxides was consistent to form the with complex the results solid of solution. XRD. More remarkable, the EDS results showed that a small amount of Mg, Al, and V existed in iron oxides to form the complex solid solution.

Metals 2019, 9, x FOR PEER REVIEW 5 of 14 Metals 2019, 9, 265 5 of 13

FigureFigure 3. 3.SEM SEM and and energy energydispersive dispersive X-rayX-ray spectroscopy spectroscopy (EDS (EDS)) images images ofof the the morphology morphology of ofthe the ironsand.ironsand. (a (–af))– SEM(f) SEM images images of ironsand of ironsand particles. particles. (P1 –(P1P6))– indicated(P6) indicated the EDS the resultsEDS results in the in SEM thepatterns. SEM patterns. 3.2. Effect of Oxidation Temperature on the Microstructure of the Pellets 3.2. Effect of Oxidation Temperature on the Microstructure of the Pellets Figure4 gives the microstructures of the pellets with the oxidation time of 30 min at a temperature range ofFigure 1073–1373 4 gives K. the The microstructures results indicated of that the thepellets oxidation with thetemperature oxidation significantly time of 30 min affected at a the mineralogicaltemperature compositionsrange of 1073– and1373 microstructure.K. The results indicated As can that be seen the oxidation from Figure temperature4a,b, clear significantly boundaries existedaffect betweened the mineralogical the iron ore compositions particles due toand the microstructure small amount. ofAs liquid can be phase seen from in the Figure pellets. 4a The,b, clear pellets boundaries existed between the iron ore particles due to the small amount of liquid phase in the were mainly composed of hematite, silicate, magnetite, and ilmenite. The Fe2O3 crystallization started atpellets. 1273 K, The and pellets some were crystalline mainly phasescomposed were of formedhematite, in silicate, Figure 4magnetite,c. As shown and inilmenite. Figure 4The, hematite Fe 2O3 grainscrystallization grew gradually, started andat 1273 the K, morphology and some crystalline transformed phases from were graininess formed in toFigure crystal 4c. stockAs shown with in the increaseFigure 4, of hematite oxidation grains temperature. grew gradually Moreover,, and the the morphology generated transformed liquid phase from under graininess high temperature to crystal stock with the increase of oxidation temperature. Moreover, the generated liquid phase under high bonded the fine grains together to form a continuous cohesive zone. More liquid phase can also facilitate ion exchange during the oxidation, which is good for crystal growth. The silicate, hematite, Metals 2019, 9, x FOR PEER REVIEW 6 of 14 temperature bonded the fine grains together to form a continuous cohesive zone. More liquid phase can also facilitate ion exchange during the oxidation, which is good for crystal growth. The silicate, hematite, magnetite, pseudobrookite, and perovskite in pellets formed the melting corrosion structure under high temperature. The porosity in pellets reduced due to the existence of the liquid phase. Five SEM patterns were randomly selected to calculate the average porosity of the pellets to further confirm the microstructures variation, as shown in Figure 4f. The results indicated that the porosity of the pellets decreased from 20.7% to 11.7% with temperatures ranging from 1073 K to 1473 K. Accordingly, the compressive strength showed the reverse change trend and increased from 663 NMetals to 11572019, 9N., 265 6 of 13 Figure 4g–i further indicated that the liquid phase in pellets was composed of Fe, O, Si, Ca, Ti, and Al. With the rising of oxidation temperature, the unoxidized magnetite and titanomagnetite magnetite, pseudobrookite, and perovskite in pellets formed the melting corrosion structure under reacted with the silicates to form the low melting point silicate liquid phase. The liquid phase high temperature. The porosity in pellets reduced due to the existence of the liquid phase. Five SEM composition was similar at different temperatures. In summary, the increase of oxidation patterns were randomly selected to calculate the average porosity of the pellets to further confirm the temperature was propitious to the growth of grain. For the oxidation of ironsand, the formation of microstructures variation, as shown in Figure4f. The results indicated that the porosity of the pellets Fe2O3 crystallization was marked when the oxidation temperature was over 1273 K. The decrease of decreased from 20.7% to 11.7% with temperatures ranging from 1073 K to 1473 K. Accordingly, the porosity in pellets could enhance the consolidation performance of the pellets, prevent the air compressive strength showed the reverse change trend and increased from 663 N to 1157 N. diffusion inside the pellets, and slow down the oxidation rate.

Figure 4. SEM images of pellets under different oxidation temperature. (a) 1073 K; (b) 1173 K; (c) 1273 K; Figure 4. SEM images of pellets under different oxidation temperature. (a) 1073 K; (b) 1173 K; (c) 1273 (d) 1373 K; (e) 1473 K; (f) Porosity and compressive strength of pellets after oxidation; (g) EDS pattern K; (d) 1373 K; (e) 1473 K; (f) Porosity and compressive strength of pellets after oxidation; (g) EDS of P1; (h) EDS pattern of P2; (i) EDS pattern of P3. pattern of P1; (h) EDS pattern of P2; (i) EDS pattern of P3. Figure4g–i further indicated that the liquid phase in pellets was composed of Fe, O, Si, Ca, Ti, 3.3.and Thermogravimetric Al. With the rising Analysis of oxidation and Reaction temperature, Mechanism the unoxidized magnetite and titanomagnetite reacted with the silicates to form the low melting point silicate liquid phase. The liquid phase 3.3.1. Thermogravimetric Analysis composition was similar at different temperatures. In summary, the increase of oxidation temperature wasFig propitiousure 5 gives to the the TG growth curves of and grain. the oxidation For the oxidationdegrees during of ironsand, the oxidation the formation of ironsand of Fein 2theO3 aircrystallization at the temperatures was marked ranging when from the oxidation 1023 to 1423 temperature K. Due to was the over oxidization 1273 K. Theof Fe decrease2+ in theof ironsand, porosity thein pellets weight could of each enhance sample the increased consolidation as the performance reaction proceed of theed pellets,. The results prevent also the airindicated diffusion that inside the increasethe pellets, in reaction and slow temperature down the oxidation could accelerate rate. the oxidation rate. For all the pellets, the reaction rate reached the maximum at the beginning of the reaction and gradually decreased with time. This was3.3. Thermogravimetricmostly because the Analysis growth and and Reaction recrystallization Mechanism of hematite in the pellets, as well as the

3.3.1. Thermogravimetric Analysis Figure5 gives the TG curves and the oxidation degrees during the oxidation of ironsand in the air at the temperatures ranging from 1023 to 1423 K. Due to the oxidization of Fe2+ in the ironsand, the weight of each sample increased as the reaction proceeded. The results also indicated that the increase in reaction temperature could accelerate the oxidation rate. For all the pellets, the reaction rate reached the maximum at the beginning of the reaction and gradually decreased with time. This was mostly because the growth and recrystallization of hematite in the pellets, as well as the formation of silicate liquid phase, promoted the bonding of the small particles. As analyzed in Section 3.2., the shrinkage Metals 2019,, 9,, 265x FOR PEER REVIEW 7 of 1314

formation of silicate liquid phase, promoted the bonding of the small particles. As analyzed in Section and consolidation of the pellets could reduce the porosity and prevent the diffusion of the air, which 3.2., the shrinkage and consolidation of the pellets could reduce the porosity and prevent the slows down the oxidation rate. As described in Equation (1), the oxidization degree of the pellets diffusion of the air, which slows down the oxidation rate. As described in Equation (1), the defined as α. ∆m and m respectively represent the mass increase of the pellets and the theoretical oxidization degree of theT pellets defined as 훼. ∆푚 and 푚 respectively represent the mass increase maximum increase of the sample mass. As shown in Figure푇5, the change rule of the oxidation degrees of the pellets and the theoretical maximum increase of the sample mass. As shown in Figure 5, the at different temperatures tends to follow lines similar to the TG curves in Figure5. The maximum change rule of the oxidation degrees at different temperatures tends to follow lines similar to the TG oxidation degrees of the pellets at different temperatures can be obtained in Figure5. The results curves in Figure 5. The maximum oxidation degrees of the pellets at different temperatures can be further confirmed that the increase of the oxidation temperature could significantly promote the obtained in Figure 5. The results further confirmed that the increase of the oxidation temperature oxidation degree of the pellets from 0.3426 to 0.8833. could significantly promote the oxidation degree of the pellets from 0.3426 to 0.8833. ∆∆m푚 α = (1) 훼 =m (1) 푚T푇

0.5 1.0

1073 K 0.4 1173 K 0.8 1273 K 1373 K 0.3 1473 K 0.6

0.2 0.4

TG (g) TG

Oxidation degree Oxidation 0.1 0.2

0.0 0.0

0 5 10 15 20 25 30 Time (min) Figure 5. Thermal Gravity (TG) curves and oxidation degrees variation of ironsand at different temperatures. Figure 5. Thermal Gravity (TG) curves and oxidation degrees variation of ironsand at different 3.3.2.temperatures. Reaction Mechanism

3.3.2.According Reaction M toechanism Avrami–Erofeev, the relation between oxidation rate and constant reaction rate can be expressed as Equation (2) [22], where α is the oxidation rate; kr is the constant reaction rate; t is the timeAccording (min); andto Avramin is the–Erofeev, Avrami the exponent. relation Equationbetween oxidation (2) can also rate be and expressed constant as reaction Equation rate (3) aftercan be taking expressed the logarithm. as Equation Figure (2) [22]6 gives, where the α relation is the oxidation between rate;ln( −푘푟ln is(1 the− α constant)) and lnt reaction at different rate; t is the time (min); and n is the Avrami exponent. Equation (2) can also be expressed as Equation (3) temperatures. The slopes and intercepts of the fitting lines are n and ln kr, respectively. The calculated parametersafter taking arethe summarizedlogarithm. Figure in Table 6 gives3. The the calculated relation standardbetween deviationln(− ln(1 − in훼 Table)) and3 waslnt at close different to 0, whichtemperatures. indicated The that slopes the obtainedand intercepts model of is the credible. fitting li Thenes are results n and suggested ln푘푟, respectively. that the The oxidation calculated of theparameters pellet had are two summarized stages (the in early Table stage 3. The of calculated the oxidation standard and thedeviation post-oxidation) in Table 3 as was the close reaction to 0, proceeded.which indicated The oxidation that the obtained temperatures model greatly is credible. influenced The results the turning suggest pointed that between the oxidation the two stages. of the Itpelle cant behad concluded two stages that (the the early oxidation stage temperatures of the oxidation and the and oxidation the post- timeoxidation) have a as great the influence reaction onproceed the oxidationed. The oxidation rate. The temperatures effect of temperature greatly influence on the oxidationd the turning rate of point the pellets between became the two stronger stages. withIt can the be increaseconcluded of that temperature. the oxidation While temperatures the effect of reactionand the oxidat time onion the time oxidation have a great rate of influence the pellets on decreasedthe oxidation with rate. the The extension effect ofof time.temperature It can be on also the concluded oxidation thatrate theof the reaction pellets is bec mainlyame controlled stronger with by thethe secondincrease stage of temperature. at 1073, 1173, While and 1273 the K.effect The of kinetics reaction analysis time on can the be oxidation divided into rate three of the parts: pellets the earlydecrease staged with of the the oxidation, extension the of post-oxidation time. It can be atalso 1073–1273 concluded K, andthat post-oxidation the reaction is atmainly 1373 and controlled 1473 K. by the second stage at 1073, 1173, and 1273 K. The kinetics analysis can be divided into three parts: n the early stage of the oxidation, the post-−oxidationln(1 − α )at= 1073krt –1273 K, and post-oxidation at 1373 and(2) 1473 K. (− ( − )) = + ln ln 1 α nlnt푛 ln kr (3) − ln(1 − 훼) = 푘푟푡 (2)

ln(− ln(1 − 훼)) = nln 푡 + ln푘푟 (3)

Metals 2019, 9, x FOR PEER REVIEW 9 of 14 Metals 2019, 9, 265 8 of 13

Figure 6. The relation between ln(− ln(1 − α)) and lnt at different temperatures. (a) 1073 K; (b) 1173 K; Figure 6. The relation between ln(− ln(1 − 훼)) and lnt at different temperatures. (a) 1073 K; (b) 1173 (c) 1273 K; (d) 1373 K; (e) 1473 K. K; (c) 1273 K; (d) 1373 K; (e) 1473 K. Table 3. Important data-fitting parameters at different temperatures. 3.4. Oxidation Reaction Kinetics Analysis Period Temp (K) 1073 1173 1273 1373 1473 As shown in Equation (4), the isothermal conversion rate is a function of time and temperature, Slope (n) 1.397 ± 0.020 1.383 ± 0.018 1.341 ± 0.037 1.180 ± 0.010 1.256 ± 0.013 where the Intercept conversion (lnk ) rate− 4.159 is dα/dt,± 0.030 s −1; −the3.821 reaction± 0.027 rate−3.428 k(T±)0.054 is a function−2.898 ± 0.020of temperature;−2.923 ± 0.030 the First r α 0–0.209 0–0.333 0–0.467 0–0.774 0–0.844 mechanismstage function is f(α); and the time is t, s. According to the Arrhenius formula, k(T) can be R-Square 0.997 0.997 0.982 0.998 0.996 −1 expressed as StandardEquation (5), where the pre-exponential factor is A, s ; the activation energy is E, 0.047 0.043 0.085 0.041 0.060 kJmol−1; and deviationR is the standard molar gas constant, kJmol−1K−1. Equation (6) can be obtained by combining EquationSlope (n) (5) and0.303 Equation± 0.006 (1). 0.103 Equation± 0.010 (7) can 0.254 also± 0.016 be expressed 0.289 ± 0.013as Equation 0.188 ±(6)0.021 after Intercept (lnk ) −1.889 ± 0.020 −0.971 ± 0.030 −0.949 ± 0.045 −0.382 ± 0.041 −0.104 ± 0.065 Second r taking the logarithm.α 0.280–0.340 0.333–0.410 0.468–0.591 0.774–0.835 0.844–0.875 stage R-Square 0.981푑훼 0.925 0.955 0.960 0.813 Standard = 푘(푇)푓(훼) (4) 0.015 0.021 0.036 0.001 0.011 deviation 푑푡

Metals 2019, 9, 265 9 of 13

3.4. Oxidation Reaction Kinetics Analysis As shown in Equation (4), the isothermal conversion rate is a function of time and temperature, where the conversion rate is dα/dt, s−1; the reaction rate k(T) is a function of temperature; the mechanism function is f (α); and the time is t, s. According to the Arrhenius formula, k(T) can be expressed as Equation (5), where the pre-exponential factor is A, s−1; the activation energy is E, kJ·mol−1; and R is the standard molar gas constant, kJ·mol−1·K−1. Equation (6) can be obtained by combining Equation (5) and Equation (1). Equation (7) can also be expressed as Equation (6) after taking the logarithm. dα = k(T) f (α) (4) dt  E  k(T) = A exp − (5) RT dα  E  = A exp − f (α) (6) dt RT dα E ln = ln A − + ln f (α) (7) dt RT For the study of reaction kinetics, researchers have developed a lot of models. As summarized in Table4, these models can be categorized into nucleation, geometric contraction, diffusion, and reaction-order models [23]. All the models in Table4 have been tested for the reaction. The calculated data of different mechanism functions indicated that the chemical reaction mechanism for the early stage of the oxidation fit A2 well with the R-square value of 0.90, the chemical reaction mechanism for the post-oxidation at 1073–1273 K fit F1 well with the R-square value of 0.91, and the chemical reaction mechanism for the post-oxidation at 1373 and 1473 K fit D4 well with the R-square value of 0.97. The comparisons between the theoretical curves and the experimental curves for the early stage of the oxidation, the post-oxidation at 1073–1273 K, and the post-oxidation at 1373 and 1473 K are drawn in Figures7–9, respectively. The activation energies of ironsand pellets were calculated by using the model-function method. For the early stage of the oxidation, the reaction mechanism was random nucleation and nuclei growth (A2) due to the formation of new phases which was consistent with the results of the study [24,25]. For the post-oxidation, the reaction mechanism was chemical reaction (F2) at the temperatures of 1073, 1173, and 1273 K. As indicated in Figure6, the increase of oxidation temperature could significantly increase the oxidation degree. This means that for the post-oxidation at the relative low temperature, the oxidation of ironsand pellets was mainly controlled by chemical reaction. However, with the increase of reaction temperature, the reaction mechanism was the three-dimension diffusion control (D4) at the temperature of 1373 and 1473 K. Due to the growth and recrystallization of hematite, as well as the formation of silicate liquid phase, the porosity of the pellets sharply decreased, which prevented the air diffusion inside the pellets and slowed down the oxidation rate. Table5 summarizes the apparent activation energy obtained through Equation (7). The calculation results were as follows: for the early stage of the oxidation E = 15.39 kJ/mol, and A = 0.77; for the post-oxidation at 1073, 1173, and 1273 K, E = 34.29 kJ/mol and A = 4.97; for the post-oxidation at 1373 and 1473 K, E = 13.88 kJ/mol and A = 0.23. Metals 2019, 9, 265 10 of 13

Table 4. The common mechanism functions (data from [23]).

R-square Mechanism Code Differential Form Integral Form Stage 1 Stage 2 Stage 3 n = 1 F1 (1 − α) −ln(1 − α) −0.57 0.90 0.75 Chemical reaction n = 2 F2 (1 − α)2 (1 − α)−1 − 1 −1.85 0.51 0.91 n = 3 F3 (1 − α)3 [(1 − α)−1 − 1]/2 −2.60 0.06 0.89 The two- dimension Diffusion control D2 [−ln(1 − α)]−1 (1 − α)ln(1 − α)+α −4.11 −0.11 0.96 Diffusion The two- dimension Diffusion control D3 1.5(1 − α)2/3[1 − (1 − α)1/3]−1 [1 − (1 − α)1/3]2 −4.21 −0.42 0.88 The three- dimension Diffusion control D4 1.5[(1 − α)1/3 − 1]−1 (1 − 2α/3) − (1 − α)2/3 −4.21 0.02 0.97 Random nucleation Two dimension A2 2(1 − α)[−ln(1 − α)]1/2 [−ln(1 − α)]1/2 0.91 0.56 0.20 and nuclei growth Three dimension A3 3(1 − α)[−ln(1 − α)]2/3 [−ln(1 − α)]1/3 0.88 0.33 0.02 Power series law, n = 1/2 P2 2α1/2 α1/2 −0.72 −0.60 −0.40 Exponential Power series law, n = 1/3 P3 3α2/3 α1/3 −1.05 −0.76 −0.48 nucleation Power series law, n = 1/4 P4 3α3/4 α1/4 −1.20 −0.83 −0.52 Phase boundary Cylindrical symmetry R2 2(1 − α)1/2 1 − (1 − α)1/2 0.27 0.83 0.48 reaction Spherical symmetry R3 3(1 − α)2/3 1 − (1 − α)1/3 0.02 0.89 0.59 Metals 2019, 9, x FOR PEER REVIEW 10 of 14

퐸 k(푇) = Aexp(− ) (5) 푅푇 푑훼 퐸 = Aexp(− )푓(훼) (6) 푑푡 푅푇 푑훼 퐸 ln = ln 퐴 − + ln 푓(훼) (7) 푑푡 푅푇 For the study of reaction kinetics, researchers have developed a lot of models. As summarized in Table 4, these models can be categorized into nucleation, geometric contraction, diffusion, and reaction-order models [23]. All the models in Table 4 have been tested for the reaction. The calculated data of different mechanism functions indicated that the chemical reaction mechanism for the early stage of the oxidation fit A2 well with the R-square value of 0.90, the chemical reaction mechanism for the post-oxidation at 1073–1273 K fit F1 well with the R-square value of 0.91, and the chemical reaction mechanism for the post-oxidation at 1373 and 1473 K fit D4 well with the R-square value of 0.97. The comparisons between the theoretical curves and the experimental curves for the early stage of the oxidation, the post-oxidation at 1073–1273 K, and the post-oxidation at 1373 and 1473 K are drawn in Figure 7, Figure 8 and Figure 9, respectively. The activation energies of ironsand pellets were calculated by using the model-function method. For the early stage of the oxidation, the reaction mechanism was random nucleation and nuclei growth (A2) due to the formation of new phases which was consistent with the results of the study [24,25]. For the post-oxidation, the reaction mechanism was chemical reaction (F2) at the temperatures of 1073, 1173, and 1273 K. As indicated in Figure 6, the increase of oxidation temperature could significantly increase the oxidation degree. This means that for the post-oxidation at the relative low temperature, the oxidation of ironsand pellets was mainly controlled by chemical reaction. However, with the increase of reaction temperature, the reaction mechanism was the three-dimension diffusion control (D4) at the temperature of 1373 and 1473 K. Due to the growth and recrystallization of hematite, as well as the formation of silicate liquid phase, the porosity of the pellets sharply decreased, which prevented the air diffusion inside the pellets and slowed down the oxidation rate. Table 5 summarizes the apparent activation energy obtained through Equation (7). The calculation results were as follows: for the early stage of the Metals 2019, 9, 265 11 of 13 oxidation E = 15.39 kJ/mol, and A = 0.77; for the post-oxidation at 1073, 1173, and 1273 K, E = 34.29 kJ/mol and A = 4.97; for the post-oxidation at 1373 and 1473 K, E = 13.88 kJ/mol and A = 0.23.

Metals 2019, 9, x FOR PEER REVIEW 11 of 14 Metals 2019, 9, x FOR PEER REVIEW 11 of 14 FigureFigure 7. Comparison 7. Comparison of theoretical–experimentalof theoretical–experimental curves for for the the early early stage stage of the of theoxidation. oxidation.

Figure 8. Comparison of theoretical–experimental curves for the post-oxidation at 1073, 1173, and Figure 8. Comparison of theoretical–experimental curves for the post-oxidation at 1073, 1173, 1273Figure K. 8. Comparison of theoretical–experimental curves for the post-oxidation at 1073, 1173, and and 12731273 K.

Figure 9. Comparison of theoretical–experimental curves for the post-oxidation at 1373 and 1473 K. Figure 9. Comparison of theoretical–experimental curves for the post-oxidation at 1373 and 1473 K. Figure 9. Comparison of theoretical–experimental curves for the post-oxidation at 1373 and 1473 K. Table 4. The common mechanism functions (data from [23]). Table 4. The common mechanism functions (data from [23]). R-square Differential Integral R-square Mechanism Code Differential Integral Stage Stage Stage Mechanism Code form form Stage Stage Stage form form 1 2 3 n = 1 F1 (1 − α) −ln(1 − α) −0.571 0.902 0.753 n = 21 F2F1 (1(1 −− αα))2 (1−ln(1 − α) -−1 −α )1 −1.850.57 0.510.90 0.910.75 Chemical reaction n = 2 F2 (1 − α)2 (1[(1 − − α α)-)1 −−1 −1 −1.85 0.51 0.91 Chemical reaction 3 n = 3 F3 (1 − α) [(1 − α)−1 − −2.60 0.06 0.89 n = 3 F3 (1 − α)3 1]/2 −2.60 0.06 0.89 The two- 1]/2 (1 − α)ln(1 The two- −1 Diffusion dimension D2 [−ln(1 − α)] (1 − α)ln(1 −4.11 −0.11 0.96 Diffusion dimension D2 [−ln(1 − α)]−1 − α)+α −4.11 −0.11 0.96 Diffusion control − α)+α Diffusion control

Metals 2019, 9, 265 12 of 13

Table 5. Activation energy of the isothermal oxidation of ironsand pellets at different temperatures.

Stage Early Stage of the Oxidation Post-Oxidation Post-Oxidation Fitting Model A2 F2 D4 Temp (K) 1073–1473 1073–1273 1373–1473 E (KJ/mol) 15.39 34.29 13.88 A 0.77 4.97 0.23 R-square 0.90 0.91 0.97

4. Conclusions The mineralogical characteristics and isothermal oxidation kinetics of ironsand pellets in air were investigated through a series of experiments. (1) The quantitative analysis of mineral composition through XRD-Rietveld indicated that the mineral composition of the ironsand was magnetite (22.7%), titanomagnetite (40.9%), enstatite (17.1%), hematite-ilmenite solid solution (14.5%), and magnesium iron aluminum silicon oxide (5.8%). SEM-EDS patterns of ironsand suggested that iron oxides in ironsand were the heterogeneous particle with the layered phases deposited on the substrate at an angle and the homogeneous particle with smooth surface and relatively average element distribution. (2) The mineralogical compositions and microstructure of pellets after oxidation were characterized by SEM. The results showed that the porosity of the pellets decreased from 20.7% to 11.7% with temperatures ranging from 1073 to 1473 K. Accordingly, the compressive strength showed the reverse change trend and increased from 663 to 1157 N. (3) The oxidation of the ironsand pellets had two stages as the reaction proceeded. The calculated data of different mechanism functions indicated that the chemical reaction mechanism for the early stage of the oxidation fit A2 well, the chemical reaction mechanism for the post-oxidation at 1073–1273 K fit F1 well, and the chemical reaction mechanism for the post-oxidation at 1373 and 1473 K fit D4 well. Kinetic studies showed that for the early stage of the oxidation E = 15.39 kJ/mol, and A = 0.77; for the post-oxidation at 1073, 1173, and 1273 K, E = 34.29 kJ/mol and A = 4.97; for the post-oxidation at 1373 and 1473 K, E = 13.88 kJ/mol and A = 0.23.

Author Contributions: Conceptualization, Y.W. and Z.L.; methodology, Y.W. and Z.L.; software, Y.W.; validation, Y.W., Z.L. and J.Z.; formal analysis, Y.W. and Z.L.; investigation, Y.W.; resources, J.Z. and Z.L.; writing (original draft preparation), Y.W.; writing (review and editing), visualization, Y.W. and Z.L.; supervision, Z.L. and J.Z. Funding: The authors acknowledge the financial support of the National Natural Science Foundation of China (51874025), and the National Key R&D Program of China (2017YFB0304300 & 2017YFB0304302). Yaozu Wang gratefully acknowledges financial support from China Scholarship Council for one year study at the Montanuniversitaet Leoben, Austria. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Liang, X.; Zhong, Y.; Zhu, S.; Zhu, J.; Yuan, P.; He, H.; Zhang, J. The decolorization of Acid Orange II in non-homogeneous Fenton reaction catalyzed by natural vanadium–titanium magnetite. J. Hazard. Mater. 2010, 181, 112–120. [CrossRef][PubMed] 2. Liu, J.; Cheng, G.; Liu, Z.; Chu, M.; Xue, X. Reduction process of pellet containing high chromic vanadium–titanium magnetite in cohesive zone. Res. Int. 2015, 86, 808–816. [CrossRef] 3. Liu, S.; Guo, Y.; Qiu, G.; Jiang, T. Solid-state reduction kinetics and mechanism of pre-oxidized vanadium–titanium magnetite concentrate. Trans. Nonferrous Met. Soc. China 2014, 24, 3372–3377. [CrossRef] 4. Wang, Y.-Z.; Zhang, J.-L.; Liu, Z.-J.; Du, C.-B. Carbothermic reduction reactions at the –slag interface in Ti-bearing slag from a . JOM 2017, 69, 2397–2403. [CrossRef] 5. Zhang, J.; Chen, B. Occurrence and recycling of main elements in Panxi vanadium-titanium magnetite. Conserv. Util. Miner. Resour. 2008, 5, 12. Metals 2019, 9, 265 13 of 13

6. Liang, X.; Zhu, S.; Zhong, Y.; Zhu, J.; Yuan, P.; He, H.; Zhang, J. The remarkable effect of vanadium doping on the adsorption and catalytic activity of magnetite in the decolorization of methylene blue. Appl. Catal. B Environ. 2010, 97, 151–159. [CrossRef]

7. Qin, J.; Xue, X.; Deng, J. Study on CO2 emission in the process of dealing with vanadium-titanium magnetite. In Proceedings of the 2011 International Conference on Materials and Products Manufacturing Technology, Chengdu, China, 28–30 October 2011; Trans Tech Publications: Zurich, Switzerland, 2011; pp. 125–132. 8. Li, H.; Wang, H.; Qing, S.; Hu, J.; Li, Y.; Hou, Y.; Hu, L.; Li, L. Energy conservation and emissions reduction in iron and steel industry. In Proceedings of the Third International Conference on Measuring Technology and Mechatronics Automation, Shangshai, China, 6–7 January 2011; pp. 41–44. 9. Yu, C.; Li, H.; Jia, X.; Li, Q. Improving resource utilization efficiency in China’s mineral resource-based cities: A case study of Chengde, Hebei province. Resour. Conserv. Recycl. 2015, 94, 1–10. [CrossRef] 10. Shi, L.; Liu, Y.; Huang, J.; Zhang, S.; Zhao, X. Growth kinetics of cubic carbide free layers in graded cemented carbides. Int. J. Miner. Metall. Mater. 2012, 19, 64–71. [CrossRef] 11. Smolik, J.; Zdunek, K.; Larisch, B. Investigation of adhesion between component layers of a multi-layer

coating TiC/Ti (Cx,N1−x)/TiN by the scratch-test method. Vacuum 1999, 55, 45–50. [CrossRef] 12. Wang, Y.; Zhang, J.; Liu, Z.; Zhang, Y.P.; Liu, D.H.; Du, C.B. Effects of w(TiO2) on mineral structure and softening-melting property of sinter. Iron Steel 2017, 52, 20–28. 13. Dehghan-Manshadi, A.; Manuel, J.; Hapugoda, S.; Ware, N. Sintering characteristics of titanium containing iron ores. ISIJ Int. 2014, 54, 2189–2195. [CrossRef] 14. Zhou, M.; Yang, S.; Jiang, T.; Xue, X. Influence of basicity on high-chromium vanadium-titanium magnetite sinter properties, productivity, and mineralogy. JOM 2015, 67, 1203–1213. [CrossRef]

15. Paananen, T.; Kinnunen, K. Effect of TiO2-content on reduction of iron ore agglomerates. Steel Res. Int. 2009, 80, 408–414. 16. Liu, Z.; Chu, M.; Wang, H.; Zhao, W.; Xue, X. Effect of MgO content in sinter on the softening–melting behavior of mixed burden made from chromium-bearing vanadium–titanium magnetite. Int. J. Miner. Metall. Mater. 2016, 23, 25–32. [CrossRef] 17. Han, G.; Jiang, T.; Zhang, Y.; Huang, Y.; Li, G. High-temperature oxidation behavior of vanadium, titanium-bearing magnetite pellet. J. Iron Steel Res. Int. 2011, 18, 14–19. [CrossRef] 18. Li, W.; Fu, G.-Q.; Chu, M.-S.; Zhu, M.-Y. Oxidation induration process and kinetics of Hongge vanadium titanium-bearing magnetite pellets. Ironmak. Steelmak. 2017, 44, 294–303. [CrossRef] 19. Tang, J.; Zhang, Y.; Chu, M.; Xue, X. Preparation of oxidized pellets with high Chromium Vanadium-Titanium Magnetite. J. Northeast. Univ. Nat. Sci. 2013, 4, 22. 20. Wang, Y.; Liu, Z.; Zhang, J.; Du, C.; Niu, L. Central band structures: New insights into the coupling effects between the pores and minerals of sinter. Metall. Mater. Trans. B 2019.[CrossRef] 21. Bartels, W.J.; Hornstra, J.; Lobeek, D.J.W. X-ray diffraction of multilayers and superlattices. Acta Crystallogr. Sect. A 1986, 42, 539–545. [CrossRef] 22. Tan, S.; Su, A.; Li, W.; Zhou, E. New insight into melting and crystallization behavior in semicrystalline poly (ethylene terephthalate). J. Polym. Sci. B Polym. Phys. 2000, 38, 53–60. [CrossRef] 23. Lv, W.; Lv, X.; Zhang, Y.; Li, S.; Tang, K.; Song, B. Isothermal oxidation kinetics of ilmenite concentrate powder from Panzhihua in air. Powder Technol. 2017, 320, 239–248. [CrossRef] 24. Rekoske, J.E.; Barteau, M.A. Isothermal reduction kinetics of -based materials. J. Phys. Chem. B 1997, 101, 1113–1124. [CrossRef] 25. Zhang, Y.; Lü, W.; Lü, X.; Li, S.; Bai, C.; Song, B.; Han, K. Isothermal reduction kinetics of Panzhihua ilmenite concentrate under 30vol% CO–70vol% N 2 atmosphere. Int. J. Miner. Metall. Mater. 2017, 24, 240–248. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).