Preparation of BaAl₂O₄ by microwave sintering

ZHANG Nian-bing(张念炳)^{1, 2}, BAI Chen-guang(白晨光)¹, MA Mang-yuan(马莽原)¹, LI Zhi-ying(黎志英)³

1. College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China;

2. College of Material and Civil Engineering, Guizhou Normal University, Guiyang 550001, China;

3. College of Materials and Metallurgy, Guizhou University, Guiyang 550003, China

Received 26 October 2009; accepted 13 April 2010

Abstract: The desulfurater (BaAl₂O₄) was successfully synthesized with BaCO₃ and Al(OH)₃ powders as raw materials by microwave sintering method. The mass loss of raw materials and the characterization of the outcome were investigated by means of TG-DSC, XRD and optical microscopy. The reaction mechanism was discussed. The experimental results show that synthesized BaAl₂O₄ by microwave sintering method is feasible. Compared with conventional sintering method, microwave sintering is a better way to synthesize BaAl₂O₄ with advantages of low temperature sintering, short time sintering and high synthesis rate.

Key words: BaAl₂O₄; microwave sintering; conventional sintering; synthesis rate

1 Introduction

China possesses about 150 million tons of high-sulfur bauxite, more than 57.2% of which is of high-grade[1]. This kind of bauxite is suitable for the Bayer process, but the problem of desulphurizing should be solved before it can be used. The desulphurizing methods include floatation[2-3], roasting[4-5] and wet process[6-9] at present. In a flotation or reverse flotation process, flotation reagents and washing water have an adverse effect on the Bayer process, and contamination will be caused by the tailing ore containing aluminum minerals. In a roasting process, there are still some serious problems such as high energy consumption and the release of SO₂ gas. In contrast, the alumina production capacity of Bayer process can be strengthened because the sodium aluminate solution can be purified in the wet desulfurization process. With the advantages of simple device, little pollution and high efficiency, the wet desulfurization process is the best way for desulfurization.

The barium salt is more effective in desulphurizing than other salt in wet process. In order to reduce desulphurizing cost and raise the efficiency, BaAl₂O₄ (BALO)[8-9] was used as desulfurater other than BaO[6] and Ba(OH)₂[7]. But BALO has only about 50% cost of BaO and Ba(OH)₂. Up to now, the report about preparation methods of BALO has not been seen yet. The single-phase BALO body is very difficult to obtain by conventional solid-phase sintering method, since it requires high temperature (1 300 °C) and long duration time (90 min). According to some reported results[10-15], microwave heating had a significant non- thermal effect in the stage of crystal growth compared with muffle heating.

In this work, the BaAl₂O₄ was successfully synthesized with BaCO₃ and Al(OH)₃ powders as raw materials by microwave sintering method. The experimental procedure and results analysis will be described in detail, and reaction mechanism will be discussed.

2 Experimental

The BALO was prepared by BaCO₃ (analytically pure, 99%) and Al(OH)₃ (micro-powder).The raw materials were weighed according to the mole ratio of 1.1?1 of Al₂O₃ to BaO. The powders were mixed for 30 min in ceramics mortar, and then subjected to sintering at 1 200 °C for 60 min using a muffle furnace or 1 300 °C for 90 min, respectively. In this process, 20 g mixed powders were put in alumina crucible which was located in a microwave oven (Fig.1). This device is capable of generating microwave power from 0 to 20 kW at operating frequency of 2.45 GHz. The temperature was measured by infrared pyrometer and controlled through adjusting input power. The sintering conditions were 800 °C for 20 min and 900 °C for 5 min, respectively.

Fig.1 Arrangement of microwave furmace

The raw material powders of BaCO₃ and Al(OH)₃ were analyzed by thermogravimetric-differential thermal analysis (TG-DTA) on a Netzsch (Luxx, STA, 449C, Germany) analyzer using Al₂O₃ as the reference material in the temperature range of 25–1 450 °C with a heating rate of 10 °C/min in Ar atmosphere. The samples were also analyzed by XRD (D/max-2500PC) for determination of phase structures. The micro- morphologies of the samples were dispersed in ethanol using ultrasonic and analyzed by digital polarization microscope (AXIOSKOP 40, HAL 100, Czech Republic).

3 Results and discussion

3.1 TG-DSC analysis

TG-DSC was used to assess the degree of reaction occurred and stability of the powders with respect to thermal as well as chemical composition. It can be noted from TG and DSC curves (Fig.2), the first major endothermic and mass loss peaks observed below 600 °C are mainly caused by the evaporation of planar water and some crystallization water. The next major change occurs in the temperature range of 900-1 300 °C, which could be attributed to the decomposition of BaCO₃ and the continuous evaporation of crystallization water.

3.2 X-ray diffraction analysis

The sintered powders were examined by XRD in order to investigate the phase formation of BALO. The results are shown in Fig.3. Fig.3(a) shows the XRD

Fig.2 DSC (a) and TG (b) curves of BaCO₃ and Al(OH)₃ mixed powders

Fig.3 XRD analysis results: (a) Microwave sintered at 800 °C for 20 min; (b) Microwave sintered at 900 °C for 5 min; (c) Convention sintered at 1 200 °C for 60 min; (d) Convention sintered at 1 300 °C for 90 min

spectrum of mixed Al₂O₃ and BaO powder with mole ratio of 1.1 by microwave sintering (MS) at 800 °C for 20 min. It is found that the main phase is Ba₂Al₂O₅·5H₂O and the minor phases are the raw materials of BaCO₃ and the residue of AlO(OH). By increasing sintering temperature to 900 °C, it is found that the BALO is the main phase (Fig.3(b)), and the other peaks are formed due to the raw materials of BaCO₃ and the reaction residue of Ba₂Al₂O₅·5H₂O and AlO(OH). For comparing phase formation, the conventional sintering powders were examined by XRD and the results are shown in Fig.3(c) and Fig.3(d). Fig.3(c) shows that the main phases are Ba₂Al₂O₅·5H₂O and BaCO₃. BALO is not formed completely because of only one small peak corresponding to BALO. By increasing the conventional sintering temperature to 1 300 °C and prolonging exposure time to 90 min, it is found that the BALO is the main phase, and the other peaks are the reaction residue of Ba₂Al₂O₅·5H₂O and BaCO₃.

3.3 Micro-morphological characteristics

Digital polarization microscope images of microwave sintered samples at 800 °C and 900 °C and conventionally sintered samples at 1 200 °C and 1 300 °C are given in Fig.4. It can be seen from Figs.4(a) and (b) that there is a great difference in grain size between BaCO₃ and Al(OH)_3. The conventionally sintered samples at 1 300 °C have smaller size and rougher surface than that at 1 200 °C, which may be caused by the decomposition of Ba₂Al₂O₅·5H₂O. Floccus grains have already appeared in microwave-sintered sample at 800 °C, but there is still a large number of non-reacted barium carbonate. From the results mentioned above, it is obvious that the reaction has begun, but the reaction tendency is small. When the samples are sintered at 900 °C for 5 min, there are a large number of small translucent grains of BALO generated. This shows that the reaction has begun with a great composition dynamics. At the same time, the phenomenon of grain aggregates exists, and shortening sintering time can lower aggregation tendency. It is concluded that microwave irradiation has a significant influence on the microstructure on account of its high energy[14].

Fig.4 Microscope images (dark field) of sintered powders by different sintering methods: (a) Al(OH)₃ powders; (b) BaCO₃ powders; (c) Microwave sintering at 800 °C for 20 min; (d) Microwave sintering at 900 °C for 5 min; (e) Convention sintering at 1 200 °C for 60 min; (f) Convention sintering at 1 200 °C for 90 min

3.4 Synthesis mechanisms

Fig.5 shows the schematic plan of the synthesis mechanisms of BaAl₂O_4. It is assumed that BaCO₃ and Al(OH)₃ are spherical. From Fig.5 we can see that the reaction mechanism can be explained by un-reacted core model. Al(OH)₃ particles wrapped by a layer of small barium carbonate particles are composed of different-size crystals, crystals gap filled with small crystals and closely combined inter-grain. Al(OH)₃ particles can rapidly dehydrate and have nanometer cracks on the original grain surface at 200-400 °C[16]. These cracks result in the phenomenon that the crystal surface area increases, which benefits the reaction.

With the temperature rising, the reaction scheme can be presented as follows:

                    (1)

 (2)

                      (3)

               (4)

Reaction (2) has lower reaction temperature than reaction (3). As shown in Fig.3, the reaction (2) has happened under the microwave sintering conditions or under the conventional sintering conditions at 800 °C or 1 200 °C, respectively, but the tendency is small. When the temperature are 900 °C (microwave sintering) or    1 300 °C (conventional sintering), reaction (3) obviously occurs.

3.5 Synthesis tests

According to the chemical reaction equation, the

synthesis mechanisms can be described briefly using reaction (5)[17]. At the same time, the synthetic rates of BALO can be calculated using reaction (6)[17]:

  (5)

                 (6)

where R is the molar ratio of Al₂O₃ to BaO, m is the mass of Al(OH)₃ before sintering, ?m is the mass difference after sintering. The heating-up curves of synthesis tests are given in Figs.6-8. The BALO synthetic rate under different sintering methods is listed in Table 1. From Table 1, with increasing the conventional sintering temperature from 1 200 to 1 300 °C and the sintering time from 60 to 90 min, the synthetic rate has only increased by 11.27% from 74.60% to 85.87%. However, with increasing the microwave sintering temperature from 800 to 900 °C and decreasing the microwave sintering time from 20 to 5 min, the synthetic rate has increased by 14.47% from 78.26% to 92.73%. It can be concluded from Table 1 that microwave sintering has

Table 1 BALO synthetic rate under different sintering methods

Fig.5 Schematic plan of synthesis mechanisms of BaAl₂O₄

lower sintering temperature, shorter sintering time and higher BaAl₂O₄ synthetic rate than conventional sintering. The BALO synthetic rate under different microwave sintering time at 900 °C is shown in Fig.9. From Fig.9, the synthetic rate keeps rising with sintering time increasing, so that the synthesis mechanism is verified.

Fig.6 Heating-up curves at different conventional sintering temperatures

Fig.7 Heating-up curves at different conventional microwave sintering temperatures

Fig.8 Heating-up curves for different microwave sintering time at 900 °C

Fig.9 Synthetic rate of sintering powder at different microwave sintering time

4 Conclusions

1) BALO was successfully synthesized by microwave sintering at 900 °C for 5 min other than conventional sintering at temperature 1 300 °C for 90 min. Compared with conventional sintering method, microwave sintering is a better way to synthesize BaAl₂O₄ with advantages of low temperature sintering, short time sintering and high synthesis rate.

2) The first major mass loss of BaCO₃ and Al(OH)₃ are caused by the evaporation of planar water and part of crystallization water release below 600 °C. The decomposition of BaCO₃ and dewatering of remaining crystal occur in the temperature range of 900-1 300 °C.

3) The reaction mechanisms can be explained using un-reacted core model, and the intermediate product of  Ba₂Al₂O₅·5H₂O exists.

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(Edited by LI Xiang-qun)

Foundation item：Project(50264001) supported by the National Natural Science Foundation of China; Project(QKH-J-2008-2009) supported by Guizhou Science and Technology Department, China

Corresponding author: ZHANG Nian-bing; Tel: +86-13595147556; E-mail: zhangnianbing@126.com

DOI: 10.1016/S1003-6326(09)60411-8