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

Abstract

Ilmenite is a vital raw material for the production of metal titanium and titanium-containing 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 Fe₂O₃, TiO₂ and the inside mixture layer of Fe₂O₃ and TiO₂ in turn. Subsequently, it was roasted at 900 °C, and Fe₂Ti₃O₉ was firstly developed between Fe₂O₃ and TiO₂ layers. With the increase in the roasting time, the Fe₂Ti₃O₉ layer was decomposed into Fe₂TiO₅ and TiO₂, and Fe₂Ti₃O₉ continued to be formed along the diameter direction toward the center of the particle until Fe₂TiO₅ and TiO₂ were formed as the final products in the center of particles. Pseudorutile in natural ilmenite was directly decomposed into TiO₂ and Fe₂O₃ in the roasting process.

1. 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].

4FeTiO₃ (s) + O₂ (g) → 2Fe₂O₃ (s) + 4TiO₂ (s)

(1)

12FeTiO₃ (s) + 3O₂ (g) → 4Fe₂Ti₃O₉ (s) (H239) + 2Fe₂O₃ (s)

(2)

4FeTiO₃ (s) + O₂ (g) → 2Fe₂TiO₅ (s) + 2TiO₂ (s)

(3)

Fe₂Ti₃O₉ (s) (H239) → Fe₂TiO₅ + 2TiO₂ (s)

(4)

TiO₂ (s) + Fe₂O₃ (s) → Fe₂TiO₅ (s)

(5)

Fe₂Ti₃O₉ (s) (H239) + 2Fe₂O₃ (s) → 3Fe₂TiO₅ (s)

(6)

4FeTiO₃ (s) + 2Fe₂O₃ (s) + O₂ (g) → 4Fe₂TiO₅ (s)

(7)

4FeTiO₃ (s) + O₂ (g) → 2Fe₂Ti₂O₇ (s)

(8)

Fe₂Ti₃O₉ (s) (pseudorutile) → Fe₂Ti₂O₇ (s) + TiO₂ (s)

(9)

Fe₂Ti₂O₇ (s) → Fe₂O₃ (s) + 2TiO₂ (s)

(10)

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.

2. Experimental

2.1. Materials

Ilmenite, in this study, was from Liaoning, China, whose chemical composition is shown in

Table 1. Chemical composition of ilmenite (mass %).

TFe	FeO	CaO	SiO2	MgO	Al2O3	TiO2	MnO	V2O3	P	S
34.52	26.85	0.86	5.11	0.95	1.06	44.44	0.65	0.28	0.024	0.006

. 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.

2.2. 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.

3. Results and Discussion

3.1. 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.

3.2. 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).

3TiO₂ (s) + Fe₂O₃ (s) → Fe₂Ti₃O₉ (s) (H239)

(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)–(7).

3.3. Microstructure Transition of Natural Ilmenite above 800 °C

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.

Figure 9 illustrates microstructure of oxidation roasting ilmenite, in which points A, C and F are Fe₂O₃; points B, E and H are TiO₂; D and G are iron–titanium oxides by analyzing the atomic percentage in

Table 2. Atomic percentage of points in Figure 9.

Point	Fe	Ti	O
A	28.67	4.01	67.32
B	5.38	30.00	64.62
C	33.84	6.84	59.32
D	15.15	20.89	63.96
E	3.63	31.19	65.19
F	31.67	5.60	62.72
G	22.35	16.03	61.62
H	6.59	32.23	61.18

. 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.

3.4. 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.

Fe₂Ti₃O₉ (s) (pseudorutile) → 3TiO₂ (s) + Fe₂O₃ (s)

(13)

4. 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.