Facile preparation and characterization of lanthanum oxide powders by the calcination of lanthanum carbonate hydrate in microwave field

Facile preparation and characterization of lanthanum oxide powders by the calcination of lanthanum carbonate hydrate in microwave field

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Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Facile preparation and characterization of lanthanum oxide powders by the calcination of lanthanum carbonate hydrate in microwave field Kaihua Chena,b, Shenghui Guoa,b, Yangqing Zengc, Weichao Huangc, Jinhui Penga,b, Libo Zhanga,b, Shaohua Yina,b,∗ a

State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China c Chinalco Guangxi Nonferrous Jinyuan Rare Earth Co., Ltd., Wanggao Industrial Zone in Hezhou, Guangxi, 542603, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Lanthanum carbonate hydrate Lanthanum oxide Microwave heating Conventionally calcination Comparison

Micron-sized lanthanum oxide powders are prepared by the calcination of lanthanum carbonate hydrate in microwave field. The decomposition process of lanthanum carbonate hydrate was analyzed by TG-DSC and indicates the reaction undergoes three stages, resulting in the generation of lanthanum oxide at 770 °C. For microwave assisted calcination, XRD patterns demonstrate that hexagonal La2O3 structure is initially formed after calcination at 650 °C for 2 h, and FT-IR analyses confirm the decomposition of precursor is complete after calcination at 750 °C for 2 h. SEM investigations reveal that 800 °C is the optimal calcination temperature to generate La2O3 powders with uniform morphologies. In comparison, conventionally calcination experiments are carried out in electrical furnace. Both XRD and FT-IR analyses are in consistence with TG-DSC, which indicate the temperature required for fully decomposition of lanthanum carbonate hydrate by conventional heating is higher than that of microwave heating. SEM images present irregular morphologies and wide particle size distribution of conventionally prepared samples. All the techniques are utilized to prove the feasibility of decomposing La2(CO3)3 to generate La2O3 in microwave field and highlight the advantages of microwave heating.

1. Introduction Lanthanum is the first element of lanthanide series also a kind of light rare earth element (LREE). It is actually not rare, which makes up 39 mg/kg of earth's crust, ranking in the third place among all the REEs [1]. It also has unique electronic configuration, like other REEs, giving lanthanum compounds attractive chemical and physical properties. In the modern society, lanthanum is mainly consumed in the form of lanthanum oxide compound (La2O3) [2], and the consumption of La2O3 could be broadly divided into indirect utilization and direct utilization. For the indirect application, specific process would be used, such as impregnation, precipitation, polymeric, or MOCVD, to introduce La2O3 into a series of catalysts [3–6], onto battery cathode as coatings [7,8] or onto Si substrate as dielectric thin film [9]. In other cases, La2O3 fine powders could be directly applied as catalysts [10–12], starting material in the synthesis of specific glasses [13,14] or functional materials [15–20]. Given lanthana such diverse industrial and technological applications, considerable attention has been attracted to the preparation of high purity and fine-particle-sized lanthana powders.

Currently, precipitation technique is widely utilized in rare earth oxide (REO) industry to yield lanthana fine powders, which primarily refers to the preparation of precursor in mixer-settler followed by the calcination of as-prepared precursor in tunnel kiln. However, the manufacture of micro/nano scaled lanthana powders is hindered since the possibility of controlling precursor's crystal size and shape in industrial mixer-settler is limited. Furthermore, in the following calcination process, approximately 1.2 tons of coal should be consumed to supply the energy for the production of one ton of REO powders, and the manufacturing cycle time could be up to 24 h. A group of studies had attempted to tackle this issue, most of which focus on improving the precipitation step by introducing ultrasonic aid [21,22], switching to carbonation process [23], hydrothermal process [24] or sol-gel technique [25]. Few studies have yet mentioned the upgrade of the energy intensive and lengthy calcination procedure. Microwaves are electromagnetic waves with the frequency ranges from 300 MHz to 300 GHz. They were originally used for communication until Percy Spencer discovered their heating effect in 1946 [26]. After that discovery, microwave heating has been extensively utilized in

∗ Corresponding author. State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan, 650093, China. E-mail address: [email protected] (S. Yin).

https://doi.org/10.1016/j.ceramint.2019.08.244 Received 8 July 2019; Accepted 26 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Kaihua Chen, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.244

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numerous fields [27–29] because its unique heating mechanism could take positive effect on the heating process. In general, microwave irradiation would generate dielectric, magnetic, and conduction loss in the target material [30]. Among these mechanisms, dielectric loss, more specifically speaking dipolar loss, plays the dominant role in the interaction between microwave and electrical insulators [26]. When microwave impinge on dielectric material, the dipoles would continuously realign their orientations with the oscillation of electric field, resulting in ‘friction effect’ and random collision so that transfer the kinetic energy into heat [26–30]. Due to this uncommon heating characteristic, microwave heating technique features non-contact, rapid, and volumetric heating, and quick start-up and stopping leads to better control of heating process [28]. Thereby, applying microwave heating in the calcination procedure has the potential to be a highly efficient and environmentally friendly technique which is beneficial to the sustainable development of REO industry. The present work focuses on the feasibility of preparing lanthanum oxide fine powders by decomposing the lanthanum carbonate hydrate in microwave field. Additionally, the literature had reported that the dielectric constant of La2O3 ranges from 20 to 27 [31], indicating the fine capacity of electrical energy of La2O3. This property could be an advantage in the microwave assisted calcination process. In this paper, the effects of calcination temperature on the molecular change of lanthanum carbonate in microwave field are discussed in detail. Thermogravimetric and differential thermal analysis (TG-DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) are used to characterize the decomposition process. In comparison, conventionally calcination experiments are carried out to substantiate the advantage of microwave heating.

same calcination temperatures (from 600 °C to 850 °C) and same holding time fixed at 2 h. 2.3. Characterizations The thermogravimetric (TG) and differential scanning calorimetric (DSC) curves were recorded on a simultaneous thermal analyzer (Netzsch STA449F3). The products calcined at various temperature were analyzed by XRD (PANalytical EMPYREAN) with Cu-Kα radiation at a scan rate of 10°/min in the range of 2θ = 10–90°. The Fourier transform infra-red spectroscopy (FT-IR) spectra were recorded using Nicolet iS50 FT-IR spectrophotometer (Thermo Nicolet, USA) with KBr pellets. Morphology and microstructure of the powders were investigated by using a backscattering scanning electron microscope (PHENOM PROX). 3. Results and discussion 3.1. Temperature characteristic The temperature curve of lanthanum carbonate hydrate during microwave heating process in air is shown in Fig. 1. The sample temperature could be elevated from 20 °C to 950 °C within 38 min (average heating rate: 24.47 °C/min), and the heating rate could be divided into three stages. In the first stage, the sample temperature increases rapidly from 20 °C to 100 °C within 2 min. Considering the surface hydration and the excellent microwave absorption property of water molecule, this stage is mainly assigned to the evaporation of adsorbed water. The second stage ranges from 100 °C to 542 °C with heating rate decreases to approximately 13.55 °C/min. This stage might be the decomposition of lanthanum carbonate (La2(CO3)3) to generate lanthanum dioxide carbonate (La2O2CO3). The plausible explanation for the decreased heating rate could be the insufficient microwave absorption property of La2(CO3)3. The third stage corresponds to an exponentially increase of heating rate with the temperature rises from 542 °C to 950 °C in 3 min. During the last stage, a break could be observed at 657 °C which might be triggered by the decomposition of La2O2CO3 to yield La2O3. In this case, the markedly rapid heating rates before and after the break might be the heating of lanthanum dioxide carbonate and lanthanum oxide, respectively.

2. Materials and methods 2.1. Materials The lanthanum carbonate hydrate powders (purity > 99%) were supplied by Ganzhou Zhanhai Industrial and Trade Co., Ltd., China. The commercial lanthanum oxide (purity > 99.9%) powders were supplied by Jiangsu Guosheng Rare Earth Co., Ltd., China. All other reagents are of analytical grade. 2.2. Preparation process

3.2. Thermal analysis of lanthanum carbonate

The microwave assisted calcination experiments were carried out in a lab-made multi-mode microwave box furnace, which is equipped with two magnetrons operating at 2450 MHz. Each magnetron has the output up to 1500 W and both are cooled by water circulation. Before the experiments, the lanthanum carbonate hydrate powders were uniformly compacted in an alumina crucible (Ф60mm × H90mm), followed by transferring the crucible into microwave resonance cavity. After the placement of crucible, the calcination process was started at the fixed microwave power of 1000 W (500 W output for each magnetron). The sample temperature was monitored by a thermalcouple placed at the closest proximity to the material, and the temperature change was recorded by a computer system. The collected data were used to construct the heating curve. To study the effect of calcination temperature on the molecular change of the precursor, samples were microwaved to attain a range of temperatures (600 °C–850 °C) followed by holding at that temperature for 2 h. Finally, the products were cooled naturally to room temperature and stored in sealing bags for subjecting them to characterization. In order to demonstrate the advantage of microwave heating, conventionally calcination experiments were carried out in an electrical resistance furnace (YFX9/13Q-YC, Shanghai Y-Feng Electrical Furnace Co., Ltd.). Except for the heating rate fixed at 15 °C/min, other conditions are same as that of microwave heating experiments, including the

The thermal analysis is conducted in flowing air, and the results are

Fig. 1. Temperature characteristic of lanthanum carbonate by microwave heating. 2

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Fig. 2. TG-DSC curves of lanthanum carbonate.

presented in Fig. 2. Based on the calculation, the chemical formula of the raw material corresponds to La2(CO3)3·9.7H2O. From the TG curve, the decomposition of lanthanum carbonate hydrate can be divided into three stages (as shown in Table 1). The first stage starts at 25 °C and ends at 465 °C with 26.47% weight loss, wherein a break is given at 45 °C. The composition of the compound at 465 °C closely corresponds to La2(CO3)3·0.5H2O. Therefore, the reaction of first stage is mainly assigned to the release of adsorbed water and hydrate water. The second stage begins with the rapid decomposition of intermediate hydrate, but the weight loss is significantly slower after 535 °C, and this decreased mass loss trend continues until the formation of lanthanum dioxycarbonate at 670 °C. Beyond this temperature, La2O2CO3 decomposes to give the lanthanum oxide level at 770 °C.

Fig. 3. XRD patterns of the samples prepared at different temperatures in microwave field.

cannot be free of the contamination of atmospheric water until elevate the calcination temperature to 850 °C. 3.4. FT-IR analyses of samples prepared in microwave field The Fourier transform infrared (FT-IR) analyses of the precursor and samples thermally treated at diverse microwave calcination temperatures are shown in Fig. 4 (a) and (b), respectively. In Fig. 4(a), the absorption peak at 3386 cm−1 is attributed to the stretching vibration of the O–H bond and the bending vibration of H–O–H from water molecules on the external surface of the samples [22]. Carbonate group is observed at 1473 cm−1 and 1374 cm−1, which are attributed to the asymmetric stretching mode (υ3) of CO32− group [23]. The absorption bands at 1076 cm−1, 850 cm−1, and 746 cm−1 are assigned to the υ1, υ2 and υ4 modes of the carbonate ion, respectively [21]. In Fig. 4(b), the peak appeared at 3609 cm−1 is assigned to the stretching mode of OH-, indicating the formation of La(OH)3 [32]. The absorption band at 647 cm−1 could be attributed to the stretching mode of La–O [24]. Conclusively, the FT-IR data and XRD analyses both conclude that La2O3 is initially generated after microwave assisted calcination at 650 °C for 2 h, and the spontaneous hydroxylation of La2O3 leads to the presence of La(OH)3. Moreover, FT-IR patterns confirm that the decomposition of lanthanum carbonate hydrate is complete after microwave assisted calcination at 750 °C for 2 h. As for the absence of La (OH)3 phase in the 850 °C XRD pattern, it is probably due to the low abundance of La(OH)3 which is lower than XRD's detection limit.

3.3. X-ray diffraction of samples prepared in microwave field The XRD patterns of samples prepared at different microwave calcination temperatures are presented in Fig. 3, and the decomposition process of lanthanum carbonate could be revealed by XRD analyses. The pattern of sample thermally treated at 600 °C reflects typical diffraction peaks of lanthanum oxide carbonate (La2O2CO3, standard PDF card No. 48–1113). With the calcination temperature increases to 650 °C, the peak intensity of La2O2CO3 slightly decreases while several mild diffraction peaks of La2O3 (standard PDF card No. 74–2430) and La(OH)3 (standard PDF card No. 36–1418) are detected. The decreased peak intensity of La2O2CO3 could be attributed to the progress of decomposition, leading to the formation of hexagonal La2O3. Furthermore, the existence of La(OH)3 could be derived from the contamination of air, since Peter et al. reported that La2O3 has significant reactivity with atmospheric water in ambient condition [32]. Therefore, the hydroxylation would get started once the sample is freshly prepared and removed from the furnace, and it is unavoidable during the period of analyses. After the calcination temperature rises to 700 °C, the peak of La2O2CO3 vanishes while La2O3 becomes the only obviously observed phase. Further increase the calcination temperature to 750 °C and 850 °C leads to the continuing increase of the peak intensity of La2O3, indicating the growth of crystal size with the increasing calcination temperature. However, the sample prepared at 700 °C and 750 °C

3.5. SEM analyses of samples prepared in microwave field The morphologies of the samples obtained at various temperatures in microwave field are shown in Fig. 5. All images are collected at the same magnification. It could be seen in Fig. 5(a) that all particles present irregular morphologies with particle size ranges from 0.5 μm to 10 μm. Similar characteristics could also be recognized in Fig. 5(b).

Table 1 The decomposition process of lanthanum carbonate hydrate. Stages

Temperature range (°C)

Experimental mass loss (%)

Chemical reaction

1 3 4

25–465 465–670 665–770

26.47 15.01 6.97

La2(CO3)3·9.7H2O→La2(CO3)3·0.5H2O+9.2H2O La2(CO3)3·0.5H2O→La2O2CO3+2CO2+0.5H2O La2O2CO3→La2O3+CO2

3

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Fig. 4. FT-IR spectra of the precursor (a) and samples prepared at various temperature in microwave field (b).

However, with the calcination temperature increase to 700 °C (Fig. 5(c)), the particle sizes are significantly decreased with occasional exception of some particles size bigger than 3 μm. Taking XRD and FTIR analyses into consideration, this decreased particle size might be attributed to the decomposition of lanthanum dioxide carbonate. Fig. 5(d) reveals the slightly increased particle size when the calcination temperature is 750 °C, and this increase might derive from the growth of La2O3 crystals. In Fig. 5(e), desired features are observed that the particles calcined at 800 °C present smooth and uniform shapes with the particle size distribution narrows down. After the calcination temperature increases to 850 °C (Fig. 5(f)), a large amount of rod-shaped particles begin to agglomerate, resulting in the generation of a group of big particles. This observation indicates that increasing the temperature to 850 °C breaks the system's thermal equilibrium, which triggers the aggregation of small particles to reduce the overall surface energy. Consequently, controlling a reasonable calcination temperature is crucial to the preparation of La2O3 powders. In all, 800 °C is considered to be the optimal temperature to yield powders with desired particle size and uniform morphologies.

Fig. 5. SEM morphologies of the samples obtained at various temperatures in microwave field: (a) 600 °C, (b) 650 °C, (c) 700 °C, (d) 750 °C, (e) 800 °C, (f) 850 °C.

(standard PDF card No. 36–1418) caused by the hydroxylation of La2O3, which again highlight the importance of storing lanthana powders in water isolated condition. However, two differences could be observed: In Fig. 6, the phase transformation of lanthanum dioxide carbonate (from standard PDF card No. 48–1113 and No. 84–1963) could be recognized in the calcination temperature ranges from 600 °C and 700 °C, while only one kind of La2O2CO3 (standard PDF card No. 48–1113) exists in Fig. 3. Another difference is the calcination temperature required for the fully decomposition of lanthanum carbonate hydrate. In Fig. 6, the peaks of La2O2CO3 cannot be detected after the precursor calcined at 800 °C for 2 h in electrical furnace, while those peaks vanish in the 700 °C pattern (Fig. 3). Considering the TG-DSC analysis, the decomposition of lanthanum carbonate hydrate should be complete at 770 °C, which is in good agreement with the XRD analyses of the conventionally prepared samples. The decreased reaction temperature in microwave field is an attractive feature which has the potential to reduce the energy consumption in REO industry.

3.6. Microwave versus conventional calcination 3.6.1. XRD analysis The XRD patterns of samples conventionally prepared at different temperatures are presented in Fig. 6. Compared with the XRD patterns shown in Fig. 3, similarities could be revealed in two aspects: The La2O2CO3 decomposes with the increase of calcination temperature, resulting in the generation of La2O3 (standard PDF card No. 74–2430); another issue is the simultaneous occurrence of La2O3 and La(OH)3

3.6.2. FT-IR The FT-IR spectra of samples prepared in electronic furnace are presented in Fig. 7. The absorption peaks appeared in Fig. 7 are similar to the observations in Fig. 4(b), including the stretching mode of OH- at 4

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Fig. 6. XRD patterns of samples prepared at various temperatures in electrical furnace.

Fig. 8. SEM of samples prepared at various temperatures in electrical furnace: (a) 600 °C, (b) 650 °C, (c) 700 °C, (d) 750 °C, (e) 800 °C, (f) 850 °C.

the calcination temperature has not significantly influence the products morphologies and particle size. All samples prepared in electrical furnace present irregular morphologies and wide particle size distribution. In comparison, the samples calcined at 800 °C for 2 h in microwave field possess uniform morphology, small particle size, and narrow size distribution. On the basis of XRD and FT-IR analyses, it could be inferred that in the conventional calcination process, increasing the calcination temperature would definitely lead to decomposition of intermediate compounds, but the effects of temperature on the products morphologies and particle size are not as much as microwave heating.

Fig. 7. FT-IR of samples prepared at various temperatures in electrical furnace.

3608 cm−1 [32], asymmetric stretching mode of CO32− group between 1500 cm−1 and 1400 cm−1 [22], υ1, υ2 and υ4 modes of the carbonate ion at 1086 cm−1, 845 cm−1, 732 cm−1, respectively [21], and the stretching mode of La–O at 661 cm−1 [24]. The results of FT-IR spectra are consistent with the XRD analyses that the decomposition of lanthanum carbonate hydrate in electrical furnace is complete after calcination at 800 °C for 2 h. As mentioned before, the TG-DSC result depicts that 770 °C is enough to decompose the lanthanum carbonate hydrate, but this temperature could be lowered to 750 °C when the calcination is carried out in microwave field. However, more detailed studies about the reduced reaction temperature are currently in process.

4. Conclusions Lanthanum oxide powders are successfully prepared from lanthanum carbonate hydrate by microwave assisted calcination. The precursor could be heated considerably rapid by microwave irradiation from 20 °C to 950 °C in 38 min. XRD and FT-IR analyses indicate the decomposition of precursor is complete after microwave assisted calcination at 750 °C for 2 h, giving hexagonal lanthanum oxide. The SEM analyses indicate that 800 °C is the optimal calcination temperature to yield lanthana powders with desired narrow particle size distribution. In comparison, lanthanum oxide powders are also prepared from lanthanum carbonate hydrate by conventionally calcination in electrical furnace. TG-DSC, XRD and FT-IR analyses demonstrate that the

3.6.3. SEM The SEM pictures of samples prepared at various temperatures in electrical furnace are shown in Fig. 8. It could be seen that increasing 5

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temperature for the completely decomposition of precursor in electrical furnace should be above 770 °C, while that of in microwave field could be lowered to 750 °C. The SEM analyses depict that microwave heating takes more positive effect on the particles morphology and size distribution than conventional heating. All the analyses conclude that microwave assisted calcination technique is feasible in the direct preparation of lanthanum oxide powders from lanthanum carbonate hydrate. Moreover, these data also suggest that special care should be taken in the preparation of lanthana powders since La2O3 would quickly reacts with atmospheric water to form La(OH)3 when they are exposed to ambient air.

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