English

• 1. INTRODUCTION

  As a kind of important inorganic chemical products, basic magnesium carbonate(4MgCO₃·Mg(OH)₂· 4H₂O, BMC) has been widely used in various industrial fields, such as pharmaceuticals, insulating material, rubber industry and lithographing inks and as precursors for other magnesium-based chemicals^[1-4]. In recent years, there have been a large number of different morphologies and sizes like needle-, tube-, rod-, rosetteand spherical-like structures. For BMC, the preparation method of surfactants and templates has been of growing interest^[5–9]. Among all the preparation methods, direct synthesis tends to obtain flat laminated coalescence particles due to the porous morphology, and the product is easily contaminated by the mother liquid, while precursor method can get high pure product without any entrainment^{[10, 11]} because the conversion reaction gives BMC via the formation of needlelike or columnar particles of metastable magnesium carbonate trihydrate (MgCO₃·3H₂O) in the synthetic process of the basic salt. Thus, high pure BMC can be obtained by high pure MgCO₃·3H₂O through thermal decomposition^[12-17].

  In our previous study, an effective method was investigated to synthesize needlelike particles of MgCO₃·3H₂O with bittern and alkali liquor below 55 ℃^[18]. In this study, BMC was synthesized by the precursor method. The effects of thermal decomposition temperature and time, liquid initial concentration and stirring time are investigated comprehensively. It offers a new approach for magnesium resource utilization in brine.

  2. MATERIALS AND EXPERIMENT

  2.1 Materials

  The precursor for preparing BMC was MgCO₃·3H₂O. The materials used in previous study could be described as bittern (Tianjin tanggu saltern, the concentration of Mg²⁺ was 185 g/L), ammonium bicarbonate (reagent grade, Tianjin Guangfu Superfine Chemical Research Institution), and ammonium hydroxide (reagent grade, Tianjin Beifang Tianyi Chemical Reagent Co., Ltd).

  2.2 Preparation of MgCO₃·3H₂O precursor

  Our research group had already designed orthogonal experimental and got the optimal process conditions of MgCO₃·3H₂O. The optimum conditions were as follows: initial concentration of Mg²⁺ was 0.5 mol/L, the concentration of NH₄HCO₃ was 1 mol/L, reaction temperature was 40~50 ℃, n(Mg²⁺): n(HCO₃^-) = 1:2.2, pH value was 8.9~9.2, stirring speed was 130 r/min, aging time was 3 h and dropping speed was 1 mL/min. Product synthetized was one-dimensional acicular with good settling performance. Average aspect ratio could be up to 29.60^[18].

  2.3 Preparation of BMC

  In a typical procedure, 250 mL deionized water was put in a glass jacket reactor and kept at the desired temperature. Subsequently, 0.345 g MgCO₃·3H₂O precursor was dissolved into the aqueous solution with continuous stirring. When the MgCO₃·3H₂O thermal decomposition completed, a white precipitate was collected and filtered, washed with deionized water for several times and dried at 60 ℃ for 6 h.

  2.4 Characterization

  The crystal structure of the synthesized sample was examined using X-ray diffraction (XRD, Rigaku D/max 2500v/pc, using CuKa radiation). The morphology and particle size of the synthesized sample were subjected to scanning electron microscopy (SEM). The pore size distribution of the particle was measured with laser particle size analyzer (Microtrac, S3500). The purity was analyzed by general test method in salt industry-determination of calcium and magnesium ions (GB/T13025.3-91).

  3. RESULTS AND DISCUSSION

  3.1 Effects of thermal decomposition temperature and time

  Phase transfer process of MgCO₃·3H₂O precursor thermal decomposition in pure water was respectively observed at 60, 70, 80, 90 and 100 ℃. Stop the experiment when the Mg²⁺ concentration tends to balance. Figs. 1 and 2 give the variation tendency of Mg²⁺ concentration and pH value against time at different temperature.

  Figure 1

  

  Figure 1.  Mg²⁺ concentration changing with different thermal decomposition temperature

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  Figure 2

  

  Figure 2.  PH value changing with different thermal decomposition temperature

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  As shown in Figs. 1 and 2, phase transfer process of MgCO₃·3H₂O precursor could be divided into three stages:

  In the first stage, the Mg²⁺ concentration sharply raised but the pH value increased slowly, which could be speculated as a decarbonation process. MgCO₃·3H₂O was decomposed, CO₃^2- ions were hydrolyzed into HCO₃^- ions and then the Mg(HCO₃)₂ solution was formed. The whole system was weak alkaline. This stage could be regarded as "induction period" of thermal decomposition, and it was shortened as the temperature increased. Induction period was 80 min at 60 ℃, 40 min at 70 ℃, 20 min at 80 ℃, and without at 90~100 ℃.

  In the second stage, the Mg²⁺ concentration increased slowly while the pH value fell fast. As we known, MgCO₃·3H₂O was unstable and could transform into BMC. In this process, carbon dioxide escaped and the pH value decreased. The conversion rate of MgCO₃·3H₂O significantly sped up as temperature increased. The conversion time was not completely the same at different temperature: 60 ℃ for 140 min, 70 ℃ for 80 min, 80 ℃ for 60 min, 90 ℃ for 20 min and 100 ℃ for 10 min.

  In the third stage, it was noteworthy that the Mg²⁺ concentration decreased and then tended to balance. The pH value was stable. Fig. 1 showed that when the temperature increased, the conversion rate rose. Thus, the residual magnesium ion concentration became lower. Fig. 2 indicated that system had been a boiling state under 100 ℃ and CO₂ had been escaped with the pH vale at a higher level.

  Figs. 3 and 4 show typical SEM images and XRD patterns of the final product at different temperature, respectively.

  Figure 3

  

  Figure 3.  SEM images of product synthesized at different thermal decomposition temperature

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  Figure 4

  

  Figure 4.  XRD patterns of products synthesized at different thermal decomposition temperature

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  XRD spectra (Fig. 4b) of the reaction products match precisely with the diffraction patterns of BMC in the database (ICDD PDF-00-025-0513) provided by the Jade software, indicating the sole presence of 4MgCO₃·Mg (OH) ₂·4H₂O. The diffraction intensity of (011) face increased significantly with temperature rise. The shapes of the diffraction peaks illustrated that BMC sample was well crystallized. Ideal crystal morphology of MgCO₃⋅3H₂O and BMC^[19] are shown in Fig. 5.

  Figure 5

  

  Figure 5.  Simulated morphology of MgCO₃⋅3H₂O and 4MgCO₃·Mg(OH)₂·4H₂O in viewpoint of chemical bond

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  As shown in Fig. 3, the particle form and thermal decomposition temperature were closely interrelated. Flake or cluster crystals were obtained at 60~70 ℃. Spherical-like crystals were obtained at 80~100 ℃. This finding reveals that MgCO₃·3H₂O broke down and a supersaturated solution formed during the induction period. The concentration of CO₃^2- decreased rapidly while HCO₃^- ion increased fast. Crystallization center formed through addition and coordination due to the directionality and selectivity of the centripetal force, van der Waals force and hydrogen bond between the molecules^[20]. The thermal decomposition rate of Mg(HCO₃)₂ solution significantly became fast. According to crystal nucleation theory, the associating ion pair did not enter into lattice immediately when it reached the crystal surface. However, it lost a degree of freedom and moved in the surface of crystal freely. There was an adsorption layer at the junction of the crystal surface and solution which was combined by all sorts of ions. The adsorption layer and magnesium bicarbonate solution establish a dynamic equilibrium. Associating ion pair was prior to link with the position of large lattice attraction and then formed rod-shaped BMC with small concave and convex shape (Fig. 3b), which are the ideal conditions of sheet-like product formed. By taking the concave and convex parts as centers to two-dimensional diffusion, sheet-like BMC was obtained (Fig. 3c). If taking Mg²⁺ as the center, it could be divided into the compact and diffusion layers. The ions in compact layer were relatively stable and closely integrated with Mg²⁺. However, in the diffusion layer they are unstable and loose structure appear. That is to say, as the thermal decomposition temperature continued rising, the binding force between the ions of compact layer increased while the diffusion layer decreased. When the ions in the diffusion layer could not move freely, the particle radius was relatively stable, and then spherical-like BMC was gained (Figs. 3d~3f).

  To gain insight into the formation mechanism of BMC, time-dependent experiments were carried out. The typical SEM images of the particles synthesized at various reaction time at 80 ℃ are shown in Fig. 6.

  Figure 6

  

  Figure 6.  SEM images of the particles from various thermal decomposition time at 80 ℃

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  When the reaction time was 20 min, it was interesting that there were many obvious sphericallike particles on the surface of the needle-like particles, as shown in Fig. 6b, while the needle-like particles were still dominant in these particles. As reaction time extended to 40 min, these particles were all converted into spherical-like ones (Fig. 6c). When the reaction time increased to 90 min, all spherical-like particles are nearly uniform with a layered structure (Fig. 6d).

  We can speculate that the formation of spherical-like particles may undergo a complex process. According to the law of the Ostwald role of stage, for an unstable system, the momentary transformation trend is not arriving at the steadiest state immediately but the adjacent state has the lowest expense of free energy. As those needle-like particles are unstable at higher temperature and dissolve into the solution and then react with OH⁻ in solution, sheet-like particles self-assemble into spherical-like ones and finally form BMC. The possible process is shown in Fig. 7.

  Figure 7

  

  Figure 7.  Schematic diagram of nesquehonite phase change process

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  3.2 Effect of the stirring time and liquid initial concentration

  The stirring time and liquid initial concentration also play important roles in determining the final morphology and size of BMC. Size distribution of the particles with various stirring time and different thermal decomposition liquid initial concentration at 80 ℃ are shown in Figs. 8 and 9, respectively.

  Figure 8

  

  Figure 8.  Size distribution images of the particles with various stirring time at 80 ℃ (Note: thermal decomposition liquid initial concentration was 0.1 mol/L and the whole reaction time was 90 min)

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  Figure 9

  

  Figure 9.  Size distribution images of the particles with various thermal decomposition liquid initial concentration at 80 ℃ (stirring time: 5 min, the whole reaction time: 90 min)

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  Fig. 8 indicates the size distribution of the particles was narrow but the grain diameter was large with the stirring time of 15 min. When the stirring time was increased up to 45 min, size distribution of the particles became wide obviously. If the stirring was continued in the whole process, size distribution of the particles distributed most widely with bad settling property.

  It is obvious that size distribution breadth of the particles is narrow with appropriate stirring compared with that constant stirring. The first batch of crystal nucleus dominated in the original fifteen minutes. And then stop stirring, collision between particles greatly reduced and crystal nucleation probability decreased, that is to say, no new crystal nucleus produced. It gave priority to crystal growth, grain diameter of the particles increased, and BMC with fine settling property obtained. While continuous stirring led to a large number of crystal nucleus produced and growth process shorten, so size distribution of the particles is the most extensive.

  Fig. 9 shows that grain diameter became small with increasing the concentration. Driving force of crystal phase transformation became large with high super saturation and transition period of amorphous turned short. At the same time, a large number of crystal nucleus occurred, and the variation coefficient was bigger, too. High degree of super saturation went against the growth of MgCO₃·3H₂O. What's more, crystal nucleation rate significantly increased while the growth rate kept invariable when the crystal pulp slurry density augmented, which was also not conducive to the crystal growth. So, in order to get big particles, the degree of super saturation and pulp slurry density should be reduced.

  3.3 Direct synthesis method

  Our research team also took the direct synthesis method as a comparison with the precursor one. BMC was obtained with brine and ammonium bicarbonate reacting directly at different temperature. Other operation conditions were the same as the precursor preparation process. The morphology of products was visualized by SEM, and the representative images are present in Fig. 10.

  Figure 10

  

  Figure 10.  SEM images of direct synthesized product at different temperature

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  It was observed that a few nest-like particles were being hatched on the surface of the needlelike particle at 55 ℃ (Fig. 10b). The morphology of product kept highly consistent with precursor. Sheet-like particles obtained at 70 ℃ were obviously built by nano-sheets with layer-by-layer (Fig. 10c). Rosette-like products with diameter of 40 µm were obtained at 80 ℃ (Fig. 10d). It was a great inspiration that rod-like BMC with a surface of "house of card" structure was easily obtained at low temperature and rosette-like product at high temperature by direct synthesis.

  3.4 Purity analysis

  5 g product was dissolved with HCl and then diluted with distilled water to 500 mL. 5 mL such solution was pipetted and the Mg²⁺ concentration was determined by titration using a standardized solution of 0.0209 mol/L EDTA. The consumption volume was 5.10 mL.

  The following formula was used to calculate the product purity:

  $ { purity }=\frac{\frac{0.0209 \times 5.10}{5} \times 0.5 \times 466}{5} \times 100 \%=99.34 \% $

  4. CONCLUSION

  In summary, BMC was prepared by MgCO₃·3H₂O precursor method. The influence of thermal decomposition temperature and time, stirring time and thermal decomposition liquid initial concentration was investigated respectively, suggesting many parameters are important for the morphology and size of BMC. Flake or cluster crystals were obtained under 60~70 ℃ and spherical-like crystals under 80~100 ℃ with thermal decomposition time of 90 min. For better governing the morphology of BMC, the conditions of thermal decomposition process should be precisely controlled under the stirring time of 15 min and liquid initial concentration of 0.1 mol/L. Moreover, the purity of BMC by precursor method was more than 99 percent. The influence of other inorganic salts in brine on the morphology of product will be further researched.

  
  
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  Figure 1  Mg²⁺ concentration changing with different thermal decomposition temperature

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  Figure 2  PH value changing with different thermal decomposition temperature

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  Figure 3  SEM images of product synthesized at different thermal decomposition temperature

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  Figure 4  XRD patterns of products synthesized at different thermal decomposition temperature

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  Figure 5  Simulated morphology of MgCO₃⋅3H₂O and 4MgCO₃·Mg(OH)₂·4H₂O in viewpoint of chemical bond

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  Figure 6  SEM images of the particles from various thermal decomposition time at 80 ℃

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  Figure 7  Schematic diagram of nesquehonite phase change process

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  Figure 8  Size distribution images of the particles with various stirring time at 80 ℃ (Note: thermal decomposition liquid initial concentration was 0.1 mol/L and the whole reaction time was 90 min)

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  Figure 9  Size distribution images of the particles with various thermal decomposition liquid initial concentration at 80 ℃ (stirring time: 5 min, the whole reaction time: 90 min)

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  Figure 10  SEM images of direct synthesized product at different temperature

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