Behavior of titanium dioxide in alumina carbothermic reduction-chlorination process in vacuum

ZHU Fu-long^{1, 2, 3}, YUAN Hai-bin^{1, 2, 3}, YU Qing-chun^{1, 2}, YANG Bin^{1, 2},

XU Bao-qiang^{1, 2}, DAI Yong-nian^{1, 2}

1. National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, China;

2. Key Laboratory of Nonferrous Metals Vacuum Metallurgy of Yunnan Province, Kunming University of Science and Technology, Kunming 650093, China;

3. Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China

Received 15 September 2010; accepted 6 April 2011

Abstract: Behaviors of TiO₂ in the alumina carbothermic reduction and chlorination process in vacuum at different temperatures were investigated experimentally by means of XRD, SEM and EDS. In the preparation of materials, the molar ratio of Al₂O₃ to C was 1:4, and 10% TiO₂ and excess AlCl₃ were added. The results show that TiC is produced by C and TiO₂ after TiO₂ transforms from anatase into rutile gradually. In the temperature range of 1 763-1 783 K, the compounds of Ti and Al are not found in slags and condensate. The purity of aluminum reaches 98.35%, and TiO₂ does not participate in alumina carbothermic reduction process and chlorination process in vacuum.

Key words: titanium dioxide; alumina; carbothermic reduction; chlorination

1 Introduction

It is known that aluminum is not produced from bauxite directly. The process producing aluminum industrially includes alumina refining and aluminum electrolysis. But the development of electrolysis process is limited by some factors such as expensive raw materials, serious environmental pollution and large electrical energy consumption [1]. In recent years, extracting aluminum from alumina or bauxite by subhalide decompositions process has been studied  [2-4]. The mechanism of this process includes carbothermic reduction, chlorination and subhalide decomposition [5]. The recovery rate and the content of aluminum may change obviously when bauxite is used as material [6]. Because of the presence of impurities such as silica, iron oxide and titanium dioxide, the content of alumina in bauxite is different (25%-80%). The content of titanium dioxide is less than 11% roughly [7]. The disproportionations of titanium dioxide in the aluminium extraction with subfluorides in vacuum have been studied [8]. If titanium dioxide less than 10% (mass fraction) was added in raw materials, the recovery rate of aluminum increases with increasing the content of titanium dioxide [6]. On the other hand, it is con?rmed that the reactions of TiO₂ and C could occur as [9]: TiO₂→Ti_nO_2n-1(n>4), Ti_nO_2n-1→Ti₄O₇, Ti₄O₇→Ti₃O₅, Ti₃O₅→Ti₂O₃, Ti₂O₃→TiC_xO_y and TiC_xO_y→TiC. Ti₃AlC₂, TiAl₃, Ti₃Al, Ti₂AlC, Ti₃AlC and Ti₃AlC₂ could be successfully obtained by the reaction of TiC, Ti, Al₄C₃ and Al [10-12]. But specific effects of titanium dioxide in alumina carbothermic reduction process at different temperatures in chlorination process have not been studied systematically.

The aim of the present work is to investigate the definite behavior of TiO₂ in alumina carbothermic reduction and chlorination process. The behaviors of TiO₂ at different temperatures were studied by XRD analysis. The surface morphology and composition of condensation product were examined by SEM and EDS.

2 Experimental

2.1 Raw material

Analytical grade alumina, graphite, titanium dioxide and anhydrous aluminum chloride were used as raw materials in the experiments.

2.2 Experimental method

Alumina (15 g) and graphite (6 g) as starting materials were mixed on a molar ratio of 1?4 of Al₂O₃to C with addition of 10% TiO₂. They were then made into pieces of d 20 mm × 5 mm under pressure of 2-6 MPa. The pieces and AlCl₃ were put in different graphite crucibles (shown in Fig. 1). The heating rate of the system was about 20 K/min. In the alumina carbothermic reduction process, the system needs to be kept at 993,   1 193, 1 393, 1 493, 1 593, 1 713 and 1 763 K for 30 min, respectively. Afterward, AlCl₃ sublimated at 373-403 K and was transported to the carbothermic reduction slag for 40-90 min, then chlorination process started. At high temperature, AlCl(g) was produced and moved into upper condensing towers. Meanwhile, AlCl(g) decomposed into AlCl₃(g) and Al(l) at low temperature. The schematic diagram of vacuum furnace is shown in Fig. 1.

Fig. 1 Schematic diagram of vacuum furnace

To identify the effect of TiO₂ on carbothermic reduction, another powder mixture without TiO₂ powders was prepared and kept at 1 763 K for 30 min in the same way. The molar ratio of Al₂O₃ to C in the mixture remained 1:4.

2.3 Analysis methods

The crystalline phase of the products was identified by X-ray diffraction instrument (Rigaku Corporation, D/max-3B). The structural features and chemical composition were characterized by scanning electron microscopy and EDAX (Philips, Holland), respectively.

3 Results and discussion

3.1 Behavioral analysis of TiO₂ in alumina carbothermic reduction process at different temperatures

The XRD patterns of the slags after alumina carbothermic reduction process at different temperature are shown in Fig. 2. Because the location and intensity of  diffraction peaks of γ-Al₂O₃, η-Al₂O₃, θ-Al₂O₃, κ-Al₂O₃ and δ-Al₂O₃ are similar, it is difficult to determine the specific crystalline forms of alumina. But there is obvious difference between corundum (α-Al₂O₃) and other alumina [13]. Only Al₂O₃ and α-Al₂O₃ are marked in Fig. 2.

Fig. 2 XRD patterns of slag after carbothermic reduction at different temperatures

It can be seen from Fig. 2(a) that the slag contains alumina, graphite and anatase at 993 K. Diffraction peaks of rutile already appear at 1 193 K. The experimental result is consistent with that in Ref. [14], which indicates that the phase transformation of TiO₂ occurs at 1 193 K or below. With increasing temperature from 1 193 to    1 393 K, the intensity of diffraction peaks of rutile increases, while that of significantly anatase becomes weaker than that at 993 K and finally disappears.

Besides, it is observed that the diffraction peak of α-Al₂O₃ appears and original alumina is consumed completely at 1 393 K. The experimental result is consistent with that in Ref. [15], which reaches the changing conditions of α-Al₂O₃. It was reported that the crystalline transformations of many materials may be affected by some elements and compounds [16-17]. But in this process, the crystalline transformations of TiO₂ and Al₂O₃ do not involve with each other.

In Fig. 2(b), graphite and α-Al₂O₃ remain unchanged at 1 493-1 713 K. Ti₂O₃ and TiC are generated, while rutile disappears at 1 493 K. The intensity of TiC increases with increasing temperature from 1 493 to 1 713 K. Ti₂O₃ reduces and disappears gradually. Only Ti₂O₃ and TiC are detected while other intermediates fail to be detected, according to Ref. [8], because the reactions of these intermediate products may be too fast, and the interval of temperature range of experimental research is too broad and other factors.

A graph of Gibbs free energy change (ΔG_T) related to temperature (T) under 120 Pa is drawn to clarify these processes (Fig. 3). Because of being short of thermo- dynamic data on Ti_nO_2n-1 (n>10) and TiC_xO_y, only TiO₂→Ti₄O₇, Ti₄O₇→Ti₃O₅, Ti₃O₅→Ti₂O₃, Ti₂O₃→TiC are analyzed in these processes:

4TiO₂+C=Ti₄O₇+CO(g)                       (1)

3Ti₄O₇+C=4Ti₃O₅+CO(g)                     (2)

2Ti₃O₅+C=3Ti₂O₃+CO(g)                     (3)

Ti₂O₃+5C=2TiC+3CO(g)                     (4)

As shown in Fig. 3, these process reactions begin at 1 250 K or below, but the experiments show that all these begin above 1 393 K. This is because the reaction rate of solid-solid phase reaction is not only relative with temperature, but also with activation energy, distribution of grain, contact area of reactant and etc [18].

Fig. 3 Relationship between Gibbs free energy change and temperature under 120 Pa

The XRD patterns of the slags kept at 1 763 K for 30 min with and without the addition of TiO₂ are shown in Fig. 2(c). The intermediates (Al₄C₃ and Al₄O₄C) which are very important to the chlorination process [6] are generated by reactions between graphite and alumina at  1 763-1 783 K in vacuum (15-120 Pa). As seen in Fig. 2(c), the intermediates are produced by graphite and alumina whether adding TiO₂ or not. No compounds of Ti and Al are detected (Ti₃AlC₂, Al₃Ti, Ti₂AlC and so on).

Although TiC is produced by the reaction between graphite and TiO₂, this process is completed before alumina carbonthermic reduction. The intermediates (Al₄C₃ and Al₄O₄C) generated in alumina carbothermic reduction process are not apparently different with or without the addition of TiO₂. Compounds of Ti and Al are not detected in alumina carbothermic reduction process. Titanium-containing materials are not involved in this process.

3.2 Behavioral analysis of TiO₂ in chlorination process

Intermediates could be participated in chlorination process at 1 763-1 783 K in vacuum (15-120 Pa) [6]. Figure 4 shows the XRD pattern of the slags obtained after chlorination process during which TiO₂ is added. The materials contained in slags are the same as the materials after carbothermic reduction. The compounds of Ti and Al are also not detected after chlorination process, which implicates that TiC is not involved in chlorination process.

During the chlorination process, AlCl(g) is generated, which would decompose at low temperature, then aluminum is produced [6]. As shown in Fig. 5, aluminum appears in every condensing tower. Element C is detected in the condensate because the condensing towers are made from graphite. Al₄C₃ in condensing towers may be produced by the reaction between Al and C as:

4Al+3C=Al₄C₃                              (5)

Fig. 4 XRD pattern of slag after carbothermic reduction- chlorination process

Fig. 5 XRD patterns of condensates from different condensing tower

As shown in Fig. 6, the Gibbs free energy change (ΔG_T) of reaction (5) is below zero when the system temperature is below 1 773 K. Below and above the melting point of aluminum (993.4 K), the Gibbs free energy change of reaction (5) is evident. Because the temperature gradient in the condenser is obvious, the condensates in 2^# condensing tower contain less Al₄C₃ than those in 3^#. Aluminum in 1^# condensing tower is pure.

Figure 7 shows the SEM image and EDS analysis of condensates obtained from 1^# condensing tower. The product aluminum has a filar structure. As shown in the EDS image, it is obvious that the contents of aluminum and oxygen in condensates are 98.35% and 1.65% (mass fraction), respectively. The titanium-containing compound is not detected in the condensate, which indicates that it is not involved in the subhalide decomposition process.

Fig. 6 Relationships between Gibbs free energy change and temperature

Fig. 7 SEM image and EDS analysis of condensation product

4 Conclusions

1) In the carbothermic reduction process, after TiO₂ transforms from anatase into rutile gradually, TiC is produced by the reaction between TiO₂ and C. In the range of 1 763-1 783 K, Al₃C₄ and Al₄O₄C are generated by alumina and graphite with TiC. The compounds of Ti and Al in slag are not detected.

2) In chloride reaction process at 1 763-1 783 K, TiC remains unchanged and the intermediates participate in the reaction. Aluminum is generated and its purity is 98.35%. Compounds of Ti and Al in slag or titanium- containing materials in the condensate are not found.

3) The experimental results indicate that TiO₂ is not involved in the alumina carbothermic reduction- chlorination process.

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氧化铝真空碳热还原-氯化工艺中

二氧化钛的行为分析

朱富龙^{1, 2, 3}, 袁海滨^{1, 2, 3}, 郁青春^{1, 2}, 杨 斌^{1, 2}, 徐宝强^{1, 2}, 戴永年^{1, 2}

1. 昆明理工大学 真空冶金国家工程实验室，昆明 650093；

2. 昆明理工大学 云南省有色金属真空冶金重点实验室，昆明 650093；

3. 昆明理工大学 材料科学与工程学院，昆明 650093

摘  要：在不同温度下氧化铝真空碳热还原和氯化反应的过程中，利用XRD、SEM 和EDS检测手段分析TiO₂的行为。在制备材料时，Al₂O₃和C的摩尔比为1:4，并添加10% TiO₂和过量的AlCl₃。结果表明，TiO₂ 从锐钛矿型转化为金红石型后与C反应生成TiC。在1 763-1 783 K的温度区间，在残渣和冷凝物中没有发现Ti和Al的化合物。生成铝的纯度达到98.35%，且TiO₂不参与氧化铝真空碳热还原和氯化过程。

关键词：二氧化钛；氧化铝；碳热还原；氯化反应

(Edited by FANG Jing-hua)

Foundation item: Project (u0837604) supported by the Joint Funds of the National Natural Science Foundation of China and Yunnan Province; Project (20095314110003) supported by the Special Research Funds of the Doctor Subject of Higher School, China

Corresponding author: YANG Bin; Tel: +86-871-5161583; E-mail: kgyb2005@126.com, zfl279@163.com

DOI: 10.1016/S1003-6326(11)60941-2