The barium carbonate (BaCO3) powder is one of the most important inorganic chemical products [1–4], which is widely used in the manufacture of barium salt, kinescope, magnetic materials, ceramic, enamel, optical glass, pigment, paint, rubber, paint, electrode, water treatment, steel carburizing, radio components and high-rise buildings with brick [5–9].
In recent years, there are varieties of methods to prepare ultra-fine barium carbonate powder body, including high-gravity, liquid-phase precipitation, micro-emulsion method, homogeneous precipitation [10, 11], template method and low-temperature solid-phase synthesis. The study on preparation of ultra-fine high-purity barium carbonate powder has a variety of methods, and its application in practical production has made some preliminary results, but there are still many problems, including high requirements for equipment, high-cost, low-production efficiency, and product quality is not stable. Therefore, the research and exploration method and its application in preparation of high-purity barium carbonate powder system has become a research hot topic.
The present paper attempts to synthesize BaCO3 powders by microwave homogeneous precipitation. Microwave homogeneous precipitation is a new method to prepare nanoparticles, which combines the advantages of microwave heating and homogeneous precipitation. Specifically, in comparison with the conventional heating, microwave heating has numerous advantages [12–15], such as selective heating, higher rate of heating, higher heating efficiency, lower reaction temperature, shorter reaction time and better controllability. Compared with the methods reported in literature, the present method is efficient and economical [16–18]. To the best of our knowledge, the present work is the maiden attempt to synthesize BaCO3 powders, using microwave homogeneous precipitation.
Based on the concept mentioned above, barium chloride dihydrate, sodium hydroxide, and carbamide were used to prepare BaCO3 powders. Effect of Ba2+ concentration, (NH2)2CO concentration, NaOH concentration, reaction temperature and reaction time on the particle size and the yield of product were mainly researched. The crystal and the structures of BaCO3 powders were analyzed using X-ray diffraction (XRD) and scanning electron microscope (SEM). The particle size distributions of BaCO3 powders were also characterized using laser particle size analyzer.
Barium chloride dihydrate (BaCl2 · 2H2O), sodium hydroxide (NaOH), carbamide ((NH2)2CO), ethylenediaminetetraacetic acid (EDTA), sodium triphosphate (STPP) and citric acid (CA) with analytical grade were used as raw materials and were purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd.
A diagram of the microwave heating apparatus, which was employed in the preparation of BaCO3 powders, was shown in Figure 1. Microwave heating apparatus was carried out in a 2.45 GHz of microwave frequency and 1.5 kW of microwave power. The microwave heating apparatus has power controller to select continuous different power levels and timer for various microwave heating times at set microwave power level. The reaction of BaCO3 powders synthesized was carried out in a three-necked flask fixed in the chamber of the microwave heating oven. The temperature of the sample in the microwave heating processing was nearly impossible to measure accurately, which was measured using a thermocouple placed in the lower part of the reactor.
The phase composition of the BaCO3 particles prepared by microwave homogeneous precipitation method was characterized by XRD (D/Max2200, Rigaku, Japan) using CuKα radiation (λ=1.5406 Å) and a Ni filter, in the 2θ range 5–100° with the scanning rate of 0.25deg/min. The voltage and the anode current operated were 35 KV and 20 mA, respectively. The morphological aspects of the BaCO3 particles prepared by microwave homogeneous precipitation method were investigated by SEM (XL30ESEM-TMPG-TMP, Philips, Holland). The SEM instrument was operated at 20 kV in a low vacuum. Laser particle size analyzer (Rise 2002, Jinan Science and Technology Co., Ltd) was utilized to measure particle size distribution data of particle group. Thermogravimetry (TG) and differential scanning calorimetry (DSC) (STA449F3, Netzsch, Germany) were utilized to measure the quality changes and rate changes of materials accurately and were performed to determine the characteristic temperature in the thermal reaction and the heat absorbed and released of materials.
Method for determination of yield
The Ba2+ concentration was calculated by equation: (1)where is the volume of reaction solution and is the volume of consumption for the EDTA standard titration solution.
The yield of product was calculated based on the following equation: (2)where is the concentration of Ba2+ of initial reaction solution.
The particle size and the yield of product were chosen as the dependent variable, while Ba2+ concentration, (NH2)2CO concentration, NaOH concentration, reaction temperature and reaction time were chosen as independent variables.
Weighed a certain amount of BaCl2 · 2H2O, NaOH and (NH2)2CO, preparing solution using ultrasonic pretreated method, and added dispersing agent. The solution was loaded on a three-necked flask, which was placed inside a stainless steel chamber of microwave heating apparatus, and heated to different reaction temperatures and different held reaction times at this temperature, in order to acquire optimal experimental conditions of preparing BaCO3 powders. After finishing the filtering, cleaning and drying process, the treated raw materials were naturally cooled to room temperature in the furnace. Then, the superfine high-purity barium carbonate powder was obtained.
Results and discussion
There are several types of experimental parameters in the synthesis of BaCO3 powders that would affect its microstructure and properties, some of which is Ba2+ concentration, (NH2)2CO concentration, NaOH concentration, reaction temperature and reaction time.
Effect of Ba2+ concentration on particle size and yield of product
Effect of Ba2+ concentration on average particle size and yield of product are shown in Figure 2, respectively. Figure 2(a) illustrates the average particle size of BaCO3 powders synthesized at different Ba2+ concentrations by microwave homogeneous precipitation. It indicates that with the Ba2+ concentration increasing, the average particle size of barium carbonate powder decreases, when the Ba2+ concentration is at 0.8 mol/L, there has the smallest average particle size, then the average particle size increased with increasing Ba2+ concentration. Figure 2(b) shows the yield of BaCO3 powders prepared using microwave homogeneous precipitation at different Ba2+ concentrations. As could be seen in Figure 2(b), the Ba2+ concentration has positive correlation with the yield of product, which increased with the Ba2+ concentration. Therefore, the optimization experimental condition of Ba2+ concentration is 0.8 mol/L.
Effect of (NH2)2CO concentration on particle size and yield of product
Effect of (NH2)2CO concentration on average particle size and yield of product are shown in Figure 3, respectively. Figure 3(a) shows the average particle sizes of BaCO3 powders synthesized at different (NH2)2CO concentrations by microwave homogeneous precipitation. Figure 3(a) indicates that the average particle size of barium carbonate powder samples has negative correlation with the amount of urea, in which with the urea content increasing, the average size of products decreases. When the molar ratio of urea with barium chloride is 2:6, the average particle size decreases rapidly with an increase of the urea content. When the molar ratio of urea and barium chloride is more than 6, the average particle size does not change significantly. Figure 3(b) illustrates the yield of BaCO3 powders prepared using microwave homogeneous precipitation at different (NH2)2CO concentrations. It shows that the yield has positive correlation with urea content, where the yield increases with the urea concentration increasing. Therefore, the molar quantity of urea is 6.
Effect of NaOH concentration on particle size and yield of product
Effect of NaOH concentration on average particle size and yield of product are shown in Figure 4, respectively. Effect of NaOH concentration on average particle size of BaCO3 powders samples prepared by microwave homogeneous precipitation is shown in Figure 4(a). It indicates that NaOH has little effect on average particle size of BaCO3 powder samples; the average size of samples is 2.2–2.4 µm. Figure 4(b) shows the yield of BaCO3 powders prepared using microwave homogeneous precipitation at different NaOH concentrations. It shows that NaOH has significant effect on the yield; the yield increases with the amount of NaOH increasing. We can know the excessive sodium hydroxide can promote chemical reactions; the molar quantity of sodium hydroxide is 2.5.
Effect of reaction temperature on particle size and yield of product
Effect of reaction temperature on average particle size and yield of product are shown in Figure 5, respectively. Figure 5(a) illustrates the average particle size of BaCO3 powders synthesized at different reaction temperatures by microwave homogeneous precipitation. It shows that the reaction temperature has little effect on average particle size of BaCO3 powder samples; the average particle size is 2.2–2.4 µm at different reaction temperatures (from 75°C to 93°C). Figure 5(b) show the yield of BaCO3 powders prepared using microwave homogeneous precipitation at different reaction temperatures. It shows that the reaction temperature has great effect on the yield of product. With an increase of the reaction temperature, the yield of product increases. Considering the influence of reaction temperature on the role of two parameters, the reaction temperature is 85°C.
Effect of reaction time on particle size and yield
Effect of reaction time on average particle size and yield of product are shown in Figure 6, respectively. Figure 6(a) shows the particle size of BaCO3 powders prepared using microwave homogeneous precipitation at different reaction times. Figure 6(a) indicates that the average size of products increases along with the extending of reaction time. The yields of BaCO3 powders synthesized at different reaction times by microwave homogeneous precipitation are represented in Figure 6(b). It shows that along with the extending of reaction time, the yield increases. Considering the influence of reaction time on the two parts of experiment, reaction time is 2 h.
The optimal experimental conditions
Considering the single-factor experiment research of ultra-fine barium carbonate powders prepared by microwave homogeneous precipitation, the optimal process conditions are selected as follows: the reaction temperature is 85°C, the reaction time is 2 h, the Ba2+ concentration is 0.8 mol/L, the amount of sodium hydroxide is 2.5 and the amount of urea is 6.
The XRD pattern of the BaCO3 powders prepared by microwave homogeneous precipitation is shown in Figure 7. It is clear that the XRD pattern is in agreement with the typical orthorhombic structure of BaCO3 (JCPDS No.05-0378). The result indicates that the product of barium carbonate prepared is orthorhombic structure of BaCO3. The sharp diffraction peaks indicate the ability of the present process to generate well-crystallized BaCO3 crystals. The absences of characteristic peaks from impurities indicate the high purity of the BaCO3 nanoparticles.
In the present study, the pretreated BaCO3 particles are characterized by SEM techniques, as shown in Figure 8. From the SEM image in Figure 8, the results indicated that the ultra-fine barium carbonate powder is prepared with smaller yet more uniform, and the typical structure of needle-like or column-like. The part of the column appeared after being treated by microwave irradiation, which is caused by stirring during barium carbonate crystal growth process.
The particle size distributions of the BaCO3 particles are illustrated in Figure 9. The results signify that the average particle size of prepared samples is 1.61 µm, and the particle size has a narrow range of distribution. Therefore, barium carbonate powder prepared by microwave homogeneous precipitation method has uniform particle size distribution and high quality.
The TG/DSC measurement of barium carbonate powder prepared is performed, and the results are shown in Figure 10. According to Figure 10, the DSC curve analyses show the three endothermic peaks at 805.6°C, 958.3°C and 837.2°C, respectively. An endothermic peak observed at 805.6°C on the DSC curve indicates that -type barium carbonate transforms into -type barium carbonate. Phase transformation reaction of -type barium carbonate transforms into -type barium carbonate in the second endothermic peak at 958.3°C. The barium carbonate decomposed in the three steps indicates weight reduction of 18.91% due to the decomposition reaction of -type barium carbonate, accompanied by a very sharp exothermic in the TG curve at 1,062.5°C. The TG curve of sample shows that the weight loss processing is observed from 1,100°C, and the weight loss rate is 22.54%, which is close to the theoretical weight loss rate of barium carbonate (22.30%). The thermoanalysis results showed that the preparation of barium carbonate powder has high purity.
Compared with conventional homogeneous precipitation methods, microwave homogeneous precipitation methods have the advantages of short apparent activation energy for the preparation of BaCO3 powders.
In this paper, BaCO3 powders have been successfully synthesized using microwave homogeneous precipitation method. The crystal, structure and the particle size distributions of prepared BaCO3 powders were characterized by XRD, SEM, and laser particle size analysis, respectively. The optimum conditions for preparation of BaCO3 powders were obtained with the reaction temperature of 85°C, the reaction time of 2 h, the Ba2+ concentration of 0.8 mol/L, the amount of sodium hydroxide of 2.5 and the amount of urea of 6. The preparation method of microwave homogeneous precipitation for preparing a powder material has not been reported in the literature and patents, which is more efficient and economical as compared with the literature-reported process techniques, and the quality standard of BaCO3 powders prepared was much higher than that of high-class product.
This work was supported financially from the International S&T Cooperation Program of China (No: 2012DFA70570), Yunnan Provincial International Cooperative Program (No: 2011IA004) and the Applied Foundation Fund of Yunnan Province of China (No: 2012FD015), and Yunnan Provincial Science and Technology Innovation Talents scheme – Technological Leading Talent (No: 2013HA002).
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About the article
Published Online: 2015-04-17
Published in Print: 2015-12-01