Phase transformation and reduction kinetics during the ... - Springer Link

International Journal of Minerals, Metallurgy and Materials Volume 19, Number 5, May 2012, Page 384 DOI: 10.1007/s12613-012-0568-4

Phase transformation and reduction kinetics during the hydrogen reduction of ilmenite concentrate Xin-guo Si1,2), Xiong-gang Lu1), Chuan-wei Li1), Chong-he Li1), and Wei-zhong Ding1) 1) Shanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai University, Shanghai 200072, China 2) Hebei Iron and Steel Group Tangshan Steel Company, Tangshan 063020, China (Received: 26 May 2011; revised: 18 July 2011; accepted: 12 September 2011)

Abstract: The reduction of ilmenite concentrate by hydrogen gas was investigated in the temperature range of 500 to 1200°C. The microstructure and phase transition of the reduction products were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM). It was found that the weight loss and iron metallization rate increased with the increase of reduction temperature and reaction time. The iron metallization rate could reach 87.5% when the sample was reduced at 1150°C for 80 min. The final phase constituents mainly consist of Fe, M3O5 solid solution phase (M=Mg, Ti, and Fe), and few titanium oxide. Microstructure analysis shows that the surfaces of the reduction products have many holes and cracks and the reactions take place from the exterior of the grain to its interior. The kinetics of reduction indicates that the rate-controlling step is diffusion process control with the activation energy of 89 kJ·mol−1. Keywords: ilmenite; phase transformations; kinetics; hydrogen; reduction; metallization

[This work was financially supported by the Postgraduate Innovative Foundation of Shanghai University (SHUCX091031), the National Natural Science Foundation of China (No.51074105), and the National Basic Research Priorities Program of China (No.2007CB613606).]

1. Introduction There exist abundant mineral resources of titanium in China; more than 90% of them are vanadium-titanium magnetite ores deposited in Panzhihua district in southwestern China [1]. Ilmenite concentrate is often used for producing metallic titanium and titanium-containing compounds [2]. Therefore, the ilmenite concentrate has attracted considerable attention [2-4]. Kucukkaragoz and Eric [2] investigated the reduction of ilmenite concentrate with graphite under argon gas between 1250 and 1350°C. Their study indicated that the reduction process consists of two stages: (1) the reduction of Fe3+→Fe2+→Fe and Ti4+→Ti3+ as well as the formation of Fe3C until a 50% reduction level; (2) the reduction of Ti3+→Ti2+ and eventually the formation of TiO1−x after a 50% reduction level. Wang and Yuan [3] studied the reduction kinetics of a natural ilmenite with carbon and reported Corresponding author: Xiong-gang Lu

that the rate-controlled steps are different in different reduction temperature ranges. Merk and Pickles [4] dealt with the reduction of ilmenite with carbon monoxide from 500 to 1100°C and demonstrated that the rate and the degree of reduction depend on the formation of a metallic shell of iron and the reduction rate increased with increasing temperature. Pouraboli et al. [5] found that when the ilmenite concentrate was smelted by electro-slag crucible melting (ESCM), the TiO2 content in slag and the iron recovery were 70wt% and 84% under the optimum condition, respectively. Recently, on the basis of Fray-Farthing-Chen (FFC) Cambridge process [6-7], a solid-oxide-oxygen-ion conducting membrane (SOM) process was developed [8]. The SOM process succeeded in preparing a Ti-Fe-based hydrogen storage alloy from the multimetallic complex oxides of Fe2O3, TiO2, and MnO [9]. Hydrogen as a reduction reagent is a clean energy carrier that could realize zero carbon dioxide emission during the

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© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2012

X.G. Si et al., Phase transformation and reduction kinetics during the hydrogen reduction of ilmenite concentrate

reduction process [10]. The main difference between hydrogen and carbon as a reducing agent is that the rate of reduction with hydrogen is much higher than that with carbon monoxide or carbon [11]. Furthermore, hydrogen can be obtained widely. For instance, it can be produced from the steam reforming of natural gas, electrolysis of water, cracking of oil and coal, and industrial waste gas [12]. Therefore, lots of studies focus on developing the reduction technology of hydrogen and hydrogen-rich gas. From the reduction reaction between ilmenite and carbon, it is known that the reduction degree of ilmenite decreases due to the presence of impurities; especially, manganese and magnesium oxide-rich zones prevented the complete reduction of Fe2+ [3]. Therefore, the phase transformation of impurities is one of the important parts in studying the hydrogen reduction of ilmenite concentrate. The aim of the present work is to investigate the parameters and characteristics of the reduction process by hydrogen and hydrogen-rich gas, including weight loss, metallization rate, phase transformations, and morphology. The activation energy of reduction reaction was calculated according to the Arrhenius equation, and the reduction mechanism of ilmenite concentrate was discussed based on experimental results and theoretical calculations.

2. Experimental 2.1. Raw materials and gases Ilmenite concentrate as a raw material was provided by Panzhihua Iron and Steel Research Institute, Pangang Group, and its chemical composition is listed in Table 1. Compared with ilmenite concentrate in other districts, it contains a higher level of MgO. In this experiment, the ilmenite concentrate was used as a feeding material with an average particle size of 70 μm. High-purity hydrogen and argon gas (99.99vol%) were used as reducing gas and shielding gas,

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Table 1. Chemical composition of ilmenite concentrate wt% Total Fe

TiO2 Fe2O3

FeO

32.62 45.24 24.17 20.19

MgO CaO MnO Al2O3 SiO2 6.60

1.19

0.74

1.08

3.73

respectively. 2.2. Reduction equipment and experiment The experiments were carried out in a SiC furnace with a horizontal quartz tube fitted with a platinum-rhodium thermocouple, which was controlled by a Lindberg UP150 temperature programmed control instrument. The length of the isothermal section was ~20 mm, where the temperature accuracy was controlled within ±3°C. The gas flow rate of 100 mL·min−1 was controlled by a mass flow meter. The schematic of the apparatus is shown in Fig. 1. In order to ensure that the reduction process proceeds in pure hydrogen atmosphere, a alumina boat filled with the sample was first located at the left side of the quartz tube, and hydrogen gas was blown into the tube at a flow rate of 100 mL·min−1 at room temperature for 1 h. The temperature was then increased to a prescribed temperature at a heating rate of 5°C·min−1. Meanwhile, the alumina boat containing the sample was relocated at the center of the isothermal section and the temperature was maintained steady for the required time. Afterwards, the boat was quickly moved to its original position to make sure that the reduction products could cool rapidly. The argon gas was started to continuously blow into the furnace until the temperature declined to room temperature at a cooling rate of 5°C·min−1. Finally, the reduced sample was taken out from the boat. The weight loss, phase composition, and microstructure were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM). 2.3. Determination of weight loss and metallization rate The weight loss of the sample during the isothermal re-

Fig. 1. Schematic of the apparatus.

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Int. J. Miner. Metall. Mater., Vol.19, No.5, May 2012

duction is mainly attributed to the loss of oxygen. This loss can be calculated by Weight loss=

M0 −Mt ×100 % M0

(1)

where M 0 and M t are the weights of the sample before and after reduction for t time, respectively. In this study, element Fe is used for representing the metallization rate. Therefore, metallization rate may be represented by Metallization rate=

M Fe t ×100% M Fe

3.2. Effect of reduction time on metallization rate at different temperatures The relationship between metallization rate calculated by Eq. (2) and reduction time at different temperatures is shown in Fig. 3. This relationship is similar to that between weight loss and reduction time at different temperatures. The metallization rate reaches its maximum value (87.5%) within 80 min at 1150°C. Fig. 3 shows that the metallization rate keeps unchangeable after 80 min at 1150°C as well as at 1200°C.

(2)

where M Fe is the total weight of Fe element in ilmenite concentrate before reduction and M Fe t is the weight of metallic iron in the reduction products, which was measured by chemical titration analysis.

3. Results and discussion 3.1. Effect of reduction time on weight loss at different temperatures The relationship between weight loss calculated by Eq. (1) and reduction time at different temperatures is shown in Fig. 2. It can be seen that the weight loss increases sharply with the increase of reduction time and reaches the maximum value (about 12%). In the temperature range from 1000 to 1200°C, the weight loss increases rapidly, afterwards becomes slow, and gradually reaches the maximum value within about 40 min. The curve of weight loss reaches approximately constant for 80 min at 1150 and 1200°C. At lower temperatures, the reduction of ilmenite concentrate is hardly to take place. The weight loss curves become flat and the reduction is negligible.

Fig. 3. Relationship between metallization rate and reduction time at different temperatures.

Therefore, the optimized process parameters of the Panzhihua ilmenite concentrate are to deoxidize with the hydrogen flow of 100 mL·min−1 at 1150°C for 80 min, and the metallization rate could reach 87.5%. Simultaneously, Figs. 2 and 3 show that the weight loss and metallization rate increase with the increase in temperature for the same reduction time. 3.3. Phase characteristics of reduction products The phase compositions of reduction products were characterized by XRD analysis and are shown in Figs. 4-6. All peaks of the samples match well with the standard XRD pattern for each phase.

Fig. 2. Relationship between weight loss and reduction time at different temperatures.

Reductive ilmenite concentrates obtained at 500, 600, and 700°C with different reduction times were characterized by XRD analysis, as shown in Fig. 4. Fe2O3 diffraction peaks vanish. Simultaneously, Fe and TiO2 diffraction peaks appear at 500 and 600°C, respectively; however, no FeTiO3 peaks can be found. Fe and TiO2 diffraction peaks are gradually broadened and their intensities are increased with the increase of temperature and time. However, the intensities of FeTiO3 decrease with the increase of temperature and time.


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M3O5 solid solution emerges at 1050°C and the mineral compositions of reduction products are principally metallic iron, the M3O5 solid solution, and little TiO2. The intensities of metallic iron and the M3O5 solid solution increase gradually, but that for TiO2 decreases further. A trace of titanium suboxide is not found at the beginning of 1050°C, indicating that titanium suboxide has been converted to the M3O5 solid solution by reduction with bivalent metal oxides. M3O5 solid solution phase in reduction products, in which most of M are Mg, Ti, and Fe, is remarkably complicated.

Fig. 4. XRD patterns of reduction products from 500 to 700°C (☆—FeTiO3; ●—Fe; ○—TiO2). A: raw material; B: 500°C, 30 min; C: 500°C, 120 min; D: 600°C, 30 min; E: 600°C, 120 min; F: 700°C, 20 min; G: 700°C, 120 min.

Fig. 5 shows XRD patterns of the sample after reduction with hydrogen at 800 and 900°C. TixOy diffraction peaks of reduction products appear; other peaks are the same as those in Fig. 1. The peaks of FeTiO3 are weak, and the peaks of metallic iron become stronger at 2θ=44.62° and are the strongest among the phases of reduction products reduce for 100 min at 800 and 900°C.

Fig. 6. XRD patterns of reduction products from 950 to 1200°C ( ☆ —FeTiO3; ●—Fe; ○—TiO2; ◎ —TixOy; △ —M3O5). A: raw material; B: 950°C, 100 min; C: 1000°C, 100 min; D: 1050°C, 100 min; E: 1100°C, 100 min; F: 1150°C, 100 min; G: 1200°C, 100 min.

3.4. Analysis of standard Gibbs free energy and phase transformation The Gibbs free energy changes ( ΔG Ο ) of chemical reactions among FeTiO3, Fe2O3, and H2 are calculated by using HSC software and are shown in Fig. 7. Numbers in the legend in Fig. 7 correspond to the following reaction equations.

Fig. 5. XRD patterns of reduction products from 800 to 900°C (☆—FeTiO3; ●—Fe; ○—TiO2; ◎—TixOy). A: raw material; B: 800°C, 20 min; C: 800°C, 100 min; D: 900°C, 10 min; E: 900°C, 100 min.

XRD patterns of products reduced between 950 and 1200°C are listed in Fig. 6. The ilmenite disappears, but an

H2+FeTiO3=H2O(g)+Fe+TiO2 H2+3/4FeTiO3=H2O(g)+3/4Fe+1/4Ti3O5 H2+2/3FeTiO3=H2O(g)+2/3Fe+1/3Ti2O3 H2(g)+1/3Fe2O3=H2O(g)+2/3Fe H2(g)+3Fe2O3=H2O(g)+2Fe3O4 H2(g)+Fe3O4=H2O(g)+3FeO

(3) (4) (5) (6) (7) (8)

The ΔG Ο of each reaction decreases with increasing temperature, implying that titanium and iron suboxides may be prepared through chemical reactions between FeTiO3,

388


the Gibbs free energy change in Fig. 7, FeTiO3 was deoxidized to iron, TiO2, and fewer titanium suboxide, indicating that the reduction reaction is mainly controlled by reaction (3) and reactions (4) and (5) play significant roles.

Fig. 7. Change of standard Gibbs free energy with temperature.

Fe2O3, and H2 at elevated temperatures. At low temperatures (500-600°C), metallic iron occurs in reduction products without TiO2 phase, as shown in Fig. 4. This may imply that Fe was prepared from Fe2O3 other than FeTiO3 because Fe2O3 disappeared and FeTiO3 kept unchanged. Fe in Fe2O3 phase was partially converted to pure iron and ferrous suboxide because the iron content (calculated from Fig. 3) in the product is lower than that (calculated from Table 1) in the raw material. Therefore, reactions (6) and (7) could occur at this stage. With the increase of temperature from 700 to 1000°C, the diffraction peaks of Fe and TiO2 become stronger, and the reduction product has less titanium suboxide. Considering

Above 1050°C, the M3O5-type solid solution began to form and then other phases were replaced by iron and this solution phase, where M represents metal elements Mg, Ti, and Fe. In this experiment, most of Fe in the raw material has been converted to pure iron and suboxides (FeO) at high temperatures. According to the results of Refs. [13-14], it seems reasonable to form the M3O5 solid solution because of the presence of FeO, MgO, TiO2, and Ti2O3 at 1050-200°C. M3O5 solid solution formation may contribute to the enrichment of Mg, Ti, and Fe with bivalence in the product. 3.5. Microstructure of samples during the reduction process To study the characteristics of hydrogen reduction, the microstructures of the samples was analyzed by SEM and OM and are shown in Figs. 8 and 9. Fig. 8(a) shows that particles in the reduced ilmenite concentrate are compact and irregular on the outer margins. The particles become loose with pores, as shown in Fig. 8(b), after hydrogen was blown into the raw material. This is because hydrogen atoms have small size and high diffusivity. XRD results suggest that metallic iron phase starts to present

Fig. 8. SEM images of ilmenite concentrate reduced: (a) raw material; (b) 600°C, 120 min; (c) 1150°C, 20 min; (d) 1150°C, 100 min.


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Fig. 9. OM images of ilmenite concentrate reduced: (a) raw material; (b) 600°C, 100 min; (c) 1150°C, 20 min; (d) 1150°C, 100 min.

at 600°C for 120 min, but the concentration of iron phase is too small to fill with holes or pores. More metallic iron particles appear at high temperatures for long time so that the holes are blocked, as shown in Figs. 8(c) and 8(d). Using OM, metal iron particles in the samples were identified as having a zoned texture with white and irregular structure. Fig. 9 shows that few metallic iron particles appear on outer rims or in cracks at low temperature and in the inside with the increase of temperature. It can be inferred that reduction reaction took place from the outside of the particle to its interior. The two main phases, which have been identified by XRD analysis, are metallic iron and M3O5 solid solution phases at 1150°C for 100 min. In Fig. 9(d), the dark area represents epoxy resin that is used to hold the sample, the grey part is M3O5 solid solution phase, and the bright point is metallic iron phase. The compositions of M3O5 solid solution phase are in the range between Mg0.6Ti2.4O5 and Fe0.5Mg0.5Ti2O5. Bessinger et al. [15] reported that the unreduced iron was mainly present in a divalent oxidation state. This unreduced iron in the M3O5 solid solution may result relatively in the slow rate of reduction at the final step for high temperature because the reduction of FeTiO3 is controlled by the reduction of FeO phase [3].

α=

M 0 −M t ×100% ΔM

(9)

where ΔM is the total potential loss in weight in the whole reduction process. From Fig. 3, it can be inferred that the weight loss is lower below 900°C and temperature has a critical effect on the weight loss from 1000 to 1200°C. Major reduction happens in the temperature range from 1000 to 1200°C; therefore, we focused on the reduction mechanism above 1000°C. The reduction degree is calculated according to Fig. 2 and Eq. (9), as shown in Fig. 10.

3.6. Reduction mechanism

Fig. 10. Effect of reaction time on the reduction degree of ilmenite concentrate from 1000 to 1200°C.

To analyze the reduction mechanism, define the reduction degree (α) as

According to solid-state kinetics [3], Eq. (10) is controlled by chemical reactions at the interface, Eq. (11) is

390


controlled by diffusion, and Eq. (12) is controlled by both chemical reactions and diffusion. 1− (1−α )1/3 = kt

(10)

1− 2α /3− (1−α ) 2/3 = kt

(11)

3k1[1− (1−α )1/3 ]+ 3/2⋅k2 [1− 2α /3− (1−α )2/3 ] = t

(12)

where k, k1, and k2 are the rate constants at different rate-controlling steps. It is calculated from Fig. 10 that at 1000-1200°C the diffusion process control is the rate-controlling step owing to the better linear correlation of 1− 2α /3− (1−α ) 2/3 to t , which is 0.999, 0.999, 0.998, and 0.998, respectively. ⎛ E ⎞ k = A exp ⎜⎜− a ⎟⎟⎟ ⎜⎝ RT ⎠

(13)

where A is frequency factor, T the reduction temperature (K), R the gas constant, and Ea the activation energy (kJ·mol−1). According to the Arrhenius equation as shown in Eq. (13), the natural logarithm is used to calculate the activation energy of reaction. The result in Fig. 11 allows the calculation of activation energy. The activation energy of the reduction reaction is 89 kJ·mol−1 at 1000-1200°C.

concentrate grain first and then propagate into its interior due to the large diffusivity of hydrogen. The reduced sample becomes loose with pores. (4) In the temperature range from 1000 to 1200°C, the diffusion process is the rate-controlling step, whose activation energy is 89 kJ·mol−1.

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[2] [3]

[4] [5]

[6]

[7]

[8]

[9]

[10] Fig. 11. Arrhenius plots of the reduction reaction.

[11]

4. Conclusions (1) The weight loss and iron metallization rate increase with the increase of temperature and reduction time. The iron metallization rate can reach 87.5% with the hydrogen flow of 100 mL·min−1 under the conditions of 1150°C and 80 min. (2) Hematite phase is deoxidized prior to ilmenite phase with hydrogen. The final reduction products mainly consist of metallic iron, TiO2, and the M3O5 solid solution (M=Mg, Ti, and Fe). (3) Metallic iron particles appear around the ilmenite

[12]

[13]

[14]

[15]

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