Microstructure and Chemical Transformation of Natural Ilmenite during Isothermal Roasting Process in Air Atmosphere

: Ilmenite is a vital raw material for the production of metal titanium and titanium-contain-ing materials. In this paper, microstructure and chemical transformation of natural ilmenite in air atmosphere were investigated by the analysis of XRF, X-ray diffractometer, and SEM-EDS. Results showed that the untreated ilmenite had three layers after oxidation at 800 °C for 60 min, which were Fe 2 O 3 , TiO 2 and the inside mixture layer of Fe 2 O 3 and TiO 2 in turn. Subsequently, it was roasted at 900 °C, and Fe 2 Ti 3 O 9 was firstly developed between Fe 2 O 3 and TiO 2 layers. With the increase in the roasting time, the Fe 2 Ti 3 O 9 layer was decomposed into Fe 2 TiO 5 and TiO 2 , and Fe 2 Ti 3 O 9 continued to be formed along the diameter direction toward the center of the particle until Fe 2 TiO 5 and TiO 2 were formed as the final products in the center of particles. Pseudorutile in natural ilmenite was directly decomposed into TiO 2 and Fe 2 O 3 in the roasting process.


Introduction
Ilmenite as one of titanium-bearing ores is abundant and economical [1,2], being a vital material for the production of metal titanium and titanium-containing materials like ferrotitanium alloy [2][3][4][5][6], synthetic rutile [7][8][9], titanium dioxide nanomaterial [10][11][12] and anode material (Li4Ti5O12) [11,13]. The utilizing value of ilmenite is limited to this. Pseudobrookite and pseudorutile synthesized by pure reagents can be used as a photocatalyst material [14,15] and anode material [16]. Moreover, they are the key components of the oxidation product of natural ilmenite. If the pseudobrookite and pseudorutile is produced by ilmenite, the cost of which will be lower, which will promote the development of anode materials and photocatalytic materials. Therefore, the application of ilmenite in anode materials and photocatalytic materials has great potential. Moreover, it is meaningful for ilmenite to reveal its chemical transformation mechanisms in the oxidation process in order to produce pseudorutile and pseudobrookite by controlling reaction processes.
Ilmenite mainly consists of one or several phases of FeTiO3, Fe2O3, Fe3O4, TiO2, and pseudorutile (Fe2Ti3O9) [1,7,[17][18][19][20][21][22][23][24]. There are two different crystal types of pseudorutile. One is the phase of unprocessed weathering ilmenite [20,22,23], and the other is obtained in oxidation process of ilmenite above 500 °C. In this paper, the latter is written as H239 in accordance with the previous paper [19]. Researchers did some investigations about the chemical transformation of ilmenite in an oxidizing atmosphere. The consensus was that the final oxidation product consisted of Fe2O3 and TiO2 or/and H239 below 800 °C (Reactions (1) to (4)) and it comprised Fe2TiO5 and TiO2 above 900 °C (Reactions (5) to (7)) [19,[24][25][26]. Besides that, there still were some differences owing to the various roasting temperatures, varying distributions of particle size and different initial phase compositions. Fu et al. indicated that H239 and rutile were formed by parallel reactions during the oxidation process of ilmenite and small particle size could enhance the formation of H239. Gupta et al. revealed pseudorutile in natural ilmenite could be decomposed to Fe2Ti2O7 and TiO2 (Reaction (9)) [23]. With prolonged roasting time, Fe2Ti2O7 would decompose to Fe2O3 and TiO2 below 800 °C (Reaction (10)) and decompose to Fe2TiO5 and TiO2 above 800 °C (Reaction (11)). Besides that, Zhang and Ostrovski also elucidated that ilmenite was able to be oxidized to Fe2Ti2O7 between 600 and 800 °C (Reaction (8)), that Fe2Ti2O7 was able to decompose to Fe2O3 and TiO2 between 600 and 1000 °C, and that Fe2TiO5 was formed between 1000 and 1200 °C as Reactions (4) and (11) show [22].
Fe2Ti2O7 (s) → Fe2O3 (s) + 2TiO2 (s) Fe2Ti2O7 (s) → Fe2TiO5 (s) + TiO2 (s) (11) With regard to the microstructure of ilmenite in oxidation process, Zhang et al. revealed ilmenite was oxidized to form hematite and rutile, with a morphology of needlelike rutile grains intermingled by hematite grains bellow 800 °C [18]. When temperature increased to 800 °C, hematite and rutile were constantly consumed to form the morphology with irregular rutile grains and isolated hematite grains dispersed in the pseudobrookite matrix.
From the studies of previous researchers, there are some uncertainties for the chemical transformation mechanisms of ilmenite like the formation processes of H239 above 800 °C, the actual formation paths of Fe2TiO5, and the chemical transformation processes of Fe2Ti2O7 in air atmosphere. In addition, development processes of microstructure for ilmenite are also unclear in oxidation processes. Thus, this paper investigated the microstructure and chemical transformation of natural ilmenite without pseudorutile phase above 800 °C. After that, chemical transformation processes of pseudorutile were studied alone in order to eliminate the interference of existing compounds and the oxidation products of ilmenite.

Materials
Ilmenite, in this study, was from Liaoning, China, whose chemical composition is shown in Table 1. Ilmenite A0 and A1 were obtained by a sample crusher for 0 s and 240 s; the average grain diameters are 64.5 and 19.8 μm, respectively. Pseudorutile powder was synthesized by a hydrothermal method via tetra-n-butyl titanate (Ti(OBu)4) and ferric nitrate (Fe(NO3)3·9H2O) [27]. The synthetic product consists of pseudorutile and a small amount of titanium dioxide, as indicated in Figure 1. Figure 2 revealed microstructure of ilmenite and synthesis pseudorutile, and SEM-EDS of pseudorutile. From Figure 2a, the surface of ilmenite is smooth and dense. The incomplete separation of gangue and ilmenite leads to the high SiO2 content, with a mass fraction of 5.11%. For the synthesis pseudorutile particles, their size is below 10 μm, as shown in Figure 2b. By the analysis of SEM-EDS presented in Figure 2c,d, it is found that the particles are pseudorutile phases in accordance with the analysis results of Figure 1.

Experimental Methods
Ilmenite or pseudorutile powder, approximately 2 g, was placed flat on the bottom of a porcelain boat (3 cm × 6 cm × 1.5 cm). When muffle furnace reached the required temperature, the loaded porcelain boat was put into the constant temperature zone for a prepared time. Meanwhile, 1 L/min air was injected into the furnace. After oxidation roasting, it was taken out, and cooled to room temperature in air.
The chemical composition of ilmenite was analyzed by chemical analysis and X-ray fluorescence (XRF; ZSXPrimus-II, Rigaku, Tokyo, Japan), and the particle size composition of ilmenite was obtained by a Laser Diffraction Particle Size Analyzer (Mastersizer 3000, Malvern Panalytical, Malvern, UK). The X-ray diffractometer (XRD; X'Pert Pro, PANalytical, Almelo, the Netherlands) was used to measure phases both before and after oxidation roasting. In addition, SEM linked with EDS (SEM-EDS; Ultra Plus, Carl Zeiss GmbH, Oberkochen, Germany and MIRA3 XMH, TESCAN, Brno-Kohoutovice, Czech Republic) was applied to observe microstructure and obtain the compositions of different phases.

Confirmation of Oxidation Roasting Products of Natural Ilmenite
As is known to us, TiO2, Fe2O3, Fe2Ti2O7, H239 and Fe2TiO5 are the oxidation roasting products of ilmenite. There is no debate for TiO2, Fe2O3 and Fe2TiO5. With regard to Fe2Ti2O7 and H239, they do not emerge simultaneously in the previous studies. When oxidation temperature is below 800 °C, some researchers found H239 is one of the oxidation roasting products of FeTiO3 [19,24,25], while others found Fe2Ti2O7 is one of the oxidation roasting products of FeTiO3 [22,23]. In order to confirm which one is the oxidation product in this work, the ilmenite A1 which is oxidized at 600 °C for 180 min and then is oxidized at 800 °C for 60 min is chosen as the analysis object due to less overlap of peaks, as presented in Figure 3. Reference pattern of Cr2Ti2O7 is chosen as the reference pattern of Fe2Ti2O7 because of the similarity [23,26]. The three diffraction peaks at 21.7°, 39.1°, and 56.0° are only matched with H239; the diffraction peak of Fe2Ti2O7 at 46.7° does not appear. As a result, H239 is one of the oxidation products of FeTiO3, not Fe2Ti2O7 in this work. In addition, the main diffraction peak of Fe2TiO5 at 25.6° does not match with the XRD pattern, so Fe2TiO5 is not formed at 800 °C.

Chemical Transformation of Natural Ilmenite above 800 °C
From previous studies, ilmenite without pseudorutile phase is very explicit below 800 °C. H239 and rutile were formed by parallel reactions during oxidation process of ilmenite, and small particle size, low temperature and high oxygen pressure are favored for the formation of H239 [19]. However, chemical transformation of natural ilmenite is not very clear above 800 °C. Thus, the chemical transformation mechanisms of natural ilmenite are revealed above 800 °C in this part. As shown in Figure 4, the oxidation roasting products of ilmenite A0 are TiO2, Fe2O3, H239 and Fe2TiO5 at 900 °C. There is no doubt that TiO2 and Fe2O3 are produced by Reaction (1). Because low temperature is beneficial to the formation of H239 in the oxidation process [19] and there are no H239 diffraction peaks at 800 °C in Figure 5a, H239 is most likely to be formed by combination of TiO2 and Fe2O3 on the basis of the interdiffusion of ion [28]. In order to verify this possibility, ilmenite A0 is oxidized to TiO2 and Fe2O3 at 800 °C for 60 min firstly and then the oxidation products are roasted at 900 °C for 30 min, 60 min and 450 min. The phases of roasting products are shown in Figure 5b-d, in which H239 and Fe2TiO5 arise. After the oxidation at 800 °C for 60 min, FeO content of ilmenite, measured by chemical analysis, is only 0.29%. So H239 is formed by the combination of TiO2 and Fe2O3 rather than the oxidation of FeTiO3 at 900 °C, as shown in Reaction (12).
Owing to the same content of SiO2 in oxidation products, changes of objective oxides contents is revealed by the characteristic peak intensity ratios of objective oxides to SiO2 [29]. Figure 6 presents the changes of objective oxides of the oxidation products in Figure  5. Fe2TiO5 content increases with the decrease in Fe2O3 content and TiO2 content. However, TiO2 content has an increasing trend while Fe2TiO5 content continues to increase after roasting for 60 min. At the same time, H239 content decreases. So, it is certain that Reaction (4) works on the formation of Fe2TiO5, not Reaction (6). As shown in Figure 4, H239 is formed and Fe2TiO5 is not detected when the roasting time is below 30 min. Therefore, TiO2 and Fe2O3 are combined to generate H239 firstly at 900 °C, and then Fe2TiO5 is formed by Reactions (4). H239 and Fe2TiO5 are not formed simultaneously. H239 is an intermediate phase for the formation of Fe2TiO5. Therefore, the formation of Fe2TiO5 is owed to Reaction (4), not Reactions (3) and (5) Figure 5. Combination of TiO2 and Fe2O3 for (a) ilmenite A0 oxidized at 800 °C for 60 min, (b) ilmenite A0 roasted at 900 °C for 30 min after it was oxidized at 800 °C for 60 min, (c) ilmenite A0 roasted at 900 °C for 60 min after it was oxidized at 800 °C for 60 min, and (d) ilmenite A0 roasted at 900 °C for 450 min after it was oxidized at 800 °C for 60 min.  Figure 7 shows the XRD patterns of the oxidation products of A0 and A1 at 800 °C for 60 min. Compared with phases of A0, A1 has a new phase, H239, formed by the oxidation of natural ilmenite. The microstructure of A0 in Figure 8a displays that a dark TiO2 layer forms with the ferrous ions migrating to the surface of particles to form a ferric oxide layer. After full oxidation, ilmenite A0 forms three layers, which are Fe2O3, TiO2 and the inside mixture layer of Fe2O3 and TiO2 in turn. When particle size decreases, a H239 layer is formed instead of TiO2 layer, as shown in Figure 8b. The main reason is that oxygen diffusion rate decreases with the increase in the diameter of ilmenite particle [30,31]. So oxygen partial pressure increases with the decrease in particle size for a simple ilmenite particle, which is beneficial to the formation of H239.    Table 2. When ilmenite A0 oxidized at 800 °C for 60 min continues to be roasted at 900 °C for 30 min, an iron-titanium oxides layer is formed between Fe2O3 and TiO2 layers in Figure 9b. The schematic diagram is shown in Figure 10a,b. The iron-titanium oxides layer is Fe2Ti3O9 layer according to the atomic percentage of point D in Table 2. When the roasting time continues to increase, the outermost Fe2Ti3O9 begins to be decomposed into Fe2TiO5 and TiO2, and Fe2Ti3O9 continues to be formed along the diameter direction toward the center of the particle until Fe2TiO5 and TiO2 are as the final products in the center of particles, as shown in Figure 10c,d. The wider iron-titanium oxides layer also certifies it, as shown in Figure 9c.

Chemical Transformation Mechanisms of Pseudorutile
Pseudorutile is the characteristic phase of the weathering ilmenite, for which the chemical transformation mechanisms are not clear due to the interference of oxidation products of ilmenite. As mentioned in the introduction, the pseudorutile was able to decompose to Fe2Ti2O7. However, Fe2Ti2O7 is also one of the oxidation products of natural ilmenite. Thus, effect of temperature and roasting time on chemical transformation of pseudorutile is investigated in this section. Figure 11 shows the chemical transformation mechanisms of pseudorutile in roasting processes at 600 °C and 800 °C. The pseudorutile is directly decomposed to TiO2 and Fe2O3 in Figure 11a-d, as Reaction (13) shows and no new phase is formed with the increase in roasting time, like Fe2Ti2O7. When the roasting temperature increases to 800 °C, it can be seen that the decomposition processes of pseudorutile is the same, and the decomposed products, TiO2 and Fe2O3, combine to form H239 and Fe2TiO5 at 800 °C with increasing time, as showed in Figure 11f,g. Therefore, pseudorutile in natural ilmenite is directly decomposed into TiO2 and Fe2O3 instead of being decomposed into Fe2Ti2O7 firstly and then into TiO2 and Fe2O3 in the roasting process.

Conclusions
In this work, microstructure and chemical transformation of natural ilmenite during an isothermal roasting process in air atmosphere was investigated. H239 not only can be formed by the oxidation of FeTiO3, but it also can be generated by the combination of TiO2 and Fe2O3 above 800 °C. The oxidation ilmenite of A0 has three layers at 800 °C for 60 min, which are Fe2O3, TiO2 and the inside mixture layer of Fe2O3 and TiO2 in turn. When it is roasted at 900 °C, Fe2Ti3O9 is firstly developed between Fe2O3 and TiO2 layers. With the increase in roasting time, the Fe2Ti3O9 layer is decomposed into Fe2TiO5 and TiO2, and Fe2Ti3O9 continues to be formed along the diameter direction toward the center of the particle until Fe2TiO5 and TiO2 are as the final products in the center of particles. Fe2TiO5 is formed by the decomposition of H239, not by the oxidation of FeTiO3 and combination of TiO2 and Fe2O3. Pseudorutile in natural ilmenite is directly decomposed into TiO2 and Fe2O3 instead of being decomposed into Fe2Ti2O7 firstly and then into TiO2 and Fe2O3 in the roasting process.  Data Availability Statement: Data is contained within the article.

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