Synthesis of aragonite by carbonization from dolomite without any additives

International Journal of Mineral Processing 123 (2013) 25–31

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Synthesis of aragonite by carbonization from dolomite without any additives Ge Li a, Zenghe Li a,⁎, Hongwen Ma b a b

College of Science, Beijing University of Chemical Technology, Beijing 100029, China National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 21 November 2012 Received in revised form 24 March 2013 Accepted 31 March 2013 Available online 15 April 2013 Keywords: Calcium carbonate Aragonite Rod-like Carbonization Dolomite

a b s t r a c t A simple carbonization method was developed to synthesize rod-like aragonite in CaCl2-NH4Cl solution prepared from dolomite. Scanning electron microscopy and X-ray diffraction were used to characterize the morphology and crystal structure of the aragonite produced. The influences of carbonization time and temperature and aging time on the morphology and polymorph of the precipitated CaCO3 particles were investigated. The aragonite was obtained at room temperature by carbonization for 0.5 h without any additives and aging for 12 h. Aragonite rods with a length of 2.5–3 μm, a width of 0.3–0.4 μm, and an aspect ratio of 6–10 can be synthesized. The mechanism of the phase transformation from calcite to aragonite is also discussed. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Calcium carbonate (CaCO3) is a chemical industrial product that is extensively used as a novel functional material in fields such as plastics, rubber, paint, printing ink, weaving, toothpaste, make-up, and foodstuffs. It has three polymorphs, calcite, aragonite, and vaterite, which have trigonal, orthorhombic, and hexagonal crystal system, respectively. The morphologies of CaCO3 include chains, cubes, spheres, spindles, sheets (or plates), needles, and amorphous (Matsumoto et al., 2010; Schmidt et al., 2010). Different polymorphs of CaCO3 can have different functions as additives. For example, dispersion can be increased if cubic CaCO3 is added as an addition in paint; acicular or rod-like CaCO3 has a reinforcing effect on rubber and plastics; and spherical CaCO3 has a significant impact on the brightness and transparency of ink (Matsumoto et al., 2010). Therefore, controlling the structure and morphology of CaCO3 is an important subject for research and development scientists. Many approaches have been studied to control the phases and morphologies of CaCO3 to meet the demands of practical applications (Hu and Deng, 2004; Hu et al., 2009; Konno et al., 2002; Skapin and Sondib, 2010; Wang et al., 2009). By controlling the initial concentration of CaCl2-NH4Cl solution, stirring speed, pH, type and amount of additives, and other reaction conditions, CaCO3 with different polymorphs, morphologies, and grain sizes can be obtained. However, studies have mainly focused on the effects of organic additives on the crystallization of CaCO3 in batch reactors by carbonization at

⁎ Corresponding author. Tel.: +86 10 13511052617; fax: +86 10 64449548. E-mail address: [email protected] (Z. Li).

high temperature (30–70 °C) (Galai et al., 2007; Huang et al., 2007; Nan et al., 2008; Yan et al., 2009). In this paper, calcium is extracted simply from dolomite after preparation of magnesium oxide. Rod-like aragonite CaCO3 is successfully prepared at room temperature by carbonization in the absence of any additives. The effects of carbonization time, aging time, and carbonization temperature on the aragonite crystal morphology are explored. The experimental conditions used to prepare rod-like CaCO3 are discussed. In addition, the phase transition mechanism from calcite to aragonite is elucidated. 2. Experimental 2.1. Experimental procedure The raw ore used in this work was collected from Luonan county of Shaanxi province, China. The mineral phase content of the raw ore was calculated to be 30.1% talc, 62.1% dolomite, 3.9% quartz, and 3.9% other minerals (Li et al., 2011). The procedure used to prepare CaCO3 from the raw material was as follows. The raw materials were ball-milled for 1 h until the particle size was less than 0.074 mm. The milled material was leached with 20 wt% hydrochloric acid by mechanical stirring at room temperature for 30 minutes to dissolve all of the dolomite grains. The main chemical composition of the solution was as follows (in mass): C(CaO) = 62.43 g/L = 1.115 mol/L, C(MgO) = 55.97 g/L = 1.399 mol/L. The chemical reaction that occurred mainly included CaMg½CO3 2 þ 4HCl→CaCl2 þ MgCl2 þ 2H2 O þ 2CO2

0301-7516/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2013.03.006

ð1Þ

26

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After leaching and filtering, the second step was amination, which was performed by adding 18 wt% ammonium hydroxide dropwise into the solution to control the pH. Precipitation of magnesium hydroxide (Mg(OH)2) occurred as the solution was mechanically stirred at room temperature. The Mg(OH)2 precursor was calcined in air to produce MgO nanoballs. These results were reported in our previous paper (Li et al., 2011). The following equilibrium reactions occurred in the system: MgCl2 þ 2NH3  H2 O→MgðOHÞ2 ↓ þ 2NH4 Cl

ð2Þ

MgðOHÞ2 →MgO þ H2 O

ð3Þ

In this article, the preparation of CaCO3 was studied, as shown in dashed boxes in Fig. 1. The reaction involved in the course of carbonization is given as Eq. (4).The reaction of the Gibbs free energy is − 144.12 kJ/mol at 298 K. It is evident that the carbonation reaction can occur at 25 °C. −

CaCl2 þ 2OH þ CO2 →CaCO3 ↓ þ H2 O þ 2Cl

−

ð4Þ

A calcium-rich solution was obtained after filtration of the ammoniated mixture. CO2 with a concentration of 40% was bubbled through the solution. The effects of carbonization temperature and time and aging time on the morphology of the resulting product were investigated.

Fig. 2. XRD patterns of the CaCO3 products prepared after various carbonization times.

when the carbonization time was extended to 1–1.5 h. When the carbonization time was extended to 2 h, aragonite was the sole product. Based on the characteristic diffraction peaks of each polymorph in the XRD spectra, the content of each polymorph was calculated. The formulas are as follows (Chen, and Xiang, 2009): If only calcite and aragonite are present: X A ¼ 3:157I A

2.2. Analysis methods The chemical compositions of the prepared samples were determined by wet chemical analysis. The solid products were identified by powder X-ray diffraction (XRD), which was performed on a Rigaku D/Max-2500 diffractometer equipped with Cu Kα radiation. Diffraction data were collected over the 2θ range of 3–70 , with a step size of 0.01 . Surface morphology was investigated using a Hitachi S-4800 scanning electron microscope (SEM).

3.1. Effect of carbonization time CO2 gas with a concentration of 40% was bubbled through the CaCl2-NH4Cl solution. The precipitated CaCO3 was obtained after different carbonization times by filtration, and then washed three times with deionized water. A CaCO3 cake was obtained by drying the precipitate at 105 °C overnight. The XRD patterns of CaCO3 products prepared after different carbonization times are shown in Fig. 2. When the carbonization time was 0.5 h, calcite (PDF05-0586) and vaterite (PDF33-0268) were obtained. However, the calcite phase transformed into aragonite (PDF41-1475) and vaterite (PDF33-0268)

  104 221 ; = IC þ 3:157IA

X C ¼ 1−X A

ð5Þ

If only vaterite and calcite are present: X V ¼ 7:691I V

110

  104 110 ; = IC þ 7:691I V

X C ¼ 1−X V

ð6Þ

If calcite, aragonite, and vaterite are present: X A ¼ 3:157I A ¼ IC

3. Results and discussion

221

104

221

  104 221 110 ; = IC þ 3:157IA þ 7:691I V

 X A =3:157IA

221

; X V ¼ 1−X A −X C

XC ð7Þ

where XA, XV, and XC, are the percentage content of aragonite, vaterite, and calcite, respectively; and IA 221, IC 104, and IV 110 represent the diffraction intensities of the aragonite (221) plane, calcite (104) plane, and vaterite (110) plane, respectively. Fig. 3 shows the calculated contents of the three phases for different carbonization times obtained using formulas (5)–(7). The aragonite content gradually increased, the vaterite content first increased and then decreased, and the calcite content decreased with increasing carbonization time. When the carbonization time reached 4 h, the aragonite content was 81.35%, calcite was 4.56%, and vaterite was 14.09%. Extending the carbonization time aids the formation of aragonite.

Fig. 1. Experimental procedure used to produce aragonite.

G. Li et al. / International Journal of Mineral Processing 123 (2013) 25–31

27

is produced (aragonite). As carbonization is continued, amorphous CaCO3 gradually disappears. After carbonization for 2 h, the CaCO3 is solely needle-like aragonite, which clusters together in daisy-like formations. These results confirm the findings of the XRD patterns. The Ca2+ concentration of the remaining solution after filtration obtained different carbonization times was determined to be 0.004 mol/L by titration with ethylenediamine tetraacetic acid. This shows that carbonization is completed after 0.5 h. At that time, the precipitation rate of CaCO3 reached 99.64%. Because aragonite is metastable in nature, it should transform into steady-state calcite during the preparation of CaCO3 (Schmidt et al., 2010). However, in our carbonization system, the opposite occurs; steady-state calcite is transformed into metastable aragonite at room temperature without any additives. A possible reason for this may be the influence of NH4+ ion impurities in solution or the extended carbonization time. Therefore, the following experiments were designed to study the influence of aging time at room temperature after carbonization for 0.5 h to explore the phase changes of CaCO3. Fig. 3. Contents of aragonite, calcite, and vaterite after various carbonization times.

3.2. Effect of aging time To understand clearly how the morphologies of CaCO3 changed at different carbonization times, several experiments were performed. Fig. 4 reveals the process of how the calcite transforms into aragonite. When the carbonation time is 0.5 h, the CaCO3 product is predominantly cubic with a small amount of amorphous material, which corresponds to calcite and vaterite, respectively. As the carbonation time is extended to 1 h, the cubic CaCO3 disappeared, and needle-like CaCO3

0.5 h

1.5 h

In these experiments, CO2 gas with a concentration of 40% was bubbled through the CaCl2-NH4Cl solution for a fixed time of 0.5 h. The suspension was aged at room temperature for 1, 2, 4, 6, 8, 10, and 12 h. The resulting CaCO3 precipitate was filtered, washed, and then dried at 105°C overnight. XRD spectra obtained for the CaCO3 products prepared after different aging times are presented in Fig. 5.

1h

2h

Fig. 4. SEM images of the transformation process of CaCO3 at various carbonation times from 0 h to 2 h.

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Fig. 5 indicates that when the carbonization time is 0.5 h and the solution is not aged, the product is solely calcite. As the aging time at room temperature is increased, the intensities of the calcite diffraction peaks weakened. After aging for 4 h, the characteristic diffraction peaks of aragonite appeared in the system. As the aging time lengthened further, the intensities of the aragonite diffraction peaks increased. It can be concluded that the longer aging time, the increased content of aragonite in the system. Fig. 6 shows the contents of the three phases of CaCO3 for different aging times calculated using formulas (5)-(7). The content of aragonite increased as the aging time lengthened. Meanwhile, the content of vaterite first increased and then decreased, while that of calcite decreased significantly. When the aging time is 4 h, the content of aragonite was 88.81%, that of calcite was 6.83%, and 4.36% of the sample was vaterite. After 4 h, the aragonite content decreased slightly with aging time. The aragonite content reached 80.70% when the aging time was 12 h. SEM images of the calcium carbonates generated as the aging time increased from 1 h to 12 h are presented in Fig. 7. When the aging time is 1 h, all of the precipitate is irregular spherical calcite. Spherical CaCO3 disappeared gradually as the aging time increased, and rod-like CaCO3 was generated. The rods were generated radially and had a length of 2.5–3 μm, width of 0.3–0.4 μm, and aspect ratio of 6–10. 3.3. Effect of carbonization temperature The effect of carbonization temperature on the CaCO3 product was investigated by bubbling CO2 gas with a concentration of 40% through the CaCl2-NH4Cl reaction system for 0.5 h at 25 °C, 40 °C, 60 °C, and 80 °C. The mixture was aged for 12 h. The CaCO3 was isolated by filtration, washed three times, and then dried at 105°C overnight. XRD patterns of the CaCO3 products prepared at different carbonization temperatures are presented in Fig. 8. At 25 °C, a mixture of aragonite and calcite was present. The intensity of the diffraction peaks and content of calcite increased as the carbonization temperature was increased. It can be concluded that some of the aragonite was transformed into calcite as the carbonization temperature increased. Fig. 9 shows the contents of the three phases of CaCO3 obtained at different carbonization temperatures calculated using formulas (5)-(7). The aragonite content decreased, the vaterite content increased slightly, and the calcite content increased then decreased as the carbonization

Fig. 5. XRD patterns of the CaCO3 products prepared after various aging times.

Fig. 6. The contents of the three phases of CaCO3 formed after various aging times.

temperature was increased. When the carbonization temperature was 25 °C, the aragonite content was 88.32%, and the calcite content was 11.68%. When the carbonization temperature was 80 °C, the content of the product was aragonite 56.96%, calcite 31.56%, and vaterite 11.49%. This shows that an increased carbonization temperature is not conducive to the formation of aragonite. SEM images of the CaCO3 structures generated at different carbonization temperatures are presented in Fig. 10. Rod-like aragonite structures were observed when the carbonization temperature was 25 °C. Spherical CaCO3 (vaterite) formed gradually as the carbonization temperature increased. The higher the carbonization temperature, the greater content of spherical morphology. It is consistent with the XRD results. This shows that reaction at room temperature is propitious to the formation of aragonite. Aragonite is a thermodynamically metastable crystalline phase. It can easily transform into the stable calcite crystal phase in aqueous solution. However, the current experimental results show that reaction at room temperature is not conducive to the formation of aragonite crystals. However, the reaction temperature is not the overall determining factor for the existence of aragonite. If the aging time is extended, initially formed aragonite crystals will grow. Aragonite and calcite possess different crystal structures and crystal growth patterns; calcite is hexagonal, and aragonite is orthorhombic. The symmetry of the former is higher than that of the latter. Considered from the crystal structure the formation of calcite nuclei is more difficult than that of aragonite nuclei. From the coordination number, Ca 2 + and O 2 − in calcite are 6-coordinate, and Ca 2 + and O 2 − in aragonite are 9-coordinate. According to the principle that the higher the coordination number the more stable a structure, the system tends to form aragonite growth units (Kitamura et al., 2002). When the concentration of CO32 − is very low during the initial stage of carbonization, the proportion of calcite growth units is the biggest. Although the stability of aragonite growth units superimposed on each nucleus is lower than that of calcite because the competition of calcite growth units is smaller, the nucleation of aragonite has priority. Once aragonite nucleation started, because of the small size of the nuclei, the driving force of nuclei disappearance is less than the minimum driving force of nuclei growth, so the nuclei can grow (Feng et al., 2007; Kitamura et al., 2002). Because aragonite is metastable, a certain number of dislocations (including the screw dislocation) can be produced during the crystal growth process, which is able to reduce the force field and reduce the free energy of the system (Matsumoto et al., 2010; Schmidt et al., 2010). Once the dislocation generated, its outcrop points can be used as a step source of crystal growth. No matter how crystal growth occurs,

G. Li et al. / International Journal of Mineral Processing 123 (2013) 25–31

1h

2h

6h

29

4h

10 h

8h

12 h A

12 h B

Fig. 7. SEM images of the CaCO3 transformation process during aging from 1 h to 12 h.

this step remains, which provides an endless source for growth. This completely eliminates the need for two-dimensional nucleation, so the crystal can grow far below the critical driving force required for

two-dimensional nucleation. Thus, aragonite crystals grow as the aging time increases. The proportion of aragonite increases as the growth of this phase continues (Wen et al., 2003).

Fig. 8. XRD patterns of the CaCO3 products prepared at various carbonation temperatures.

Fig. 9. Contents of the three phases of CaCO3 prepared at various carbonization temperatures.

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G. Li et al. / International Journal of Mineral Processing 123 (2013) 25–31

40 °C

25 °C

60 °C

80 °C

Fig. 10. SEM images of the CaCO3 transformation process at carbonation temperatures from 25 °C to 80 °C.

Our work has shown that the experimental conditions to prepare rod-like aragonite involve carbonization with 40% CO2 from CaCl2-NH4Cl solution at room temperature for 0.5 h, and then aging for more than 12 h. The chemical composition of the product meets the Chinese first-class industry standard (HG/T2226-91). The aragonite content reached 97.12% and a whiteness of 96.85 was achieved, so the product can be used as an addition in rubber, paper, and paint. Generally, crystallization accelerators are not added in industrial production of CaCO3 from direct carbonization of a Ca(OH)2 suspension. Therefore, the product is almost entirely calcite. To obtain aragonite, the crystallization process needs to be strictly controlled. Methods that involve adding two different types of crystallization accelerator such as soluble divalent metal salts and phosphate during the Ca(OH)2 carbonization process are also widely used. In our paper, aragonite was successfully prepared without any additives at room temperature. Our procedure provides a pathway for industrial production of aragonite from dolomite. The infrared spectra of aragonite product (aragonite-01) is shown in Fig. 11. Due to the different crystal structures of calcite, aragonite, and vaterite, the wave number of functional group in the infrared spectra also vary. Table 1 lists the wave numbers of different functional group in three crystals (Schmidt et al., 2010). It can be seen from Table 1 and Fig. 11 that all the functional group in infrared spectra is aragonite. That can be proved that no vaterite and calcite exist in final aragonite product. Since the rod-like aragonite in the rubber field of the best served due to the reinforcing effect, the aragonite prepared in this research can be used in rubber field widely.

Fig. 11. The IR spectra of aragonite (aragonite-01). Table 1 Wave numbers of different functional group in calcium carbonate (cm−1). Antisymmetric stretching vibration of the C-O Calcite 1421 Aragonite 1421 Vaterite 1421

Symmetric stretching vibration of the CO32−

External plane bending vibration of the CO32−

Internal plane bending vibration of the O-C-O

1082 1082 1082

876 853 870

713 705,700 750

G. Li et al. / International Journal of Mineral Processing 123 (2013) 25–31

4. Conclusion Rod-like aragonite was synthesized from CaCl2-NH4Cl solution by a simple carbonization procedure. Aragonite rods with a width of 0.3– 0.4 μm and an aspect ratio of 6–10 were formed by feeding CO2 gas into a CaCl2-NH4Cl solution at room temperature without any additives. The morphology of the CaCO3 is sensitive to the carbonization time, aging time, and carbonization temperature. Increasing the aging time and room temperature carbonization promotes the formation of CaCO3 with rod morphology. The chemical composition of the aragonite product prepared in this research meets the Chinese first-class industry standard, so it is suitable for use as an addition in rubber, paper, or paint. A facile route to fabricate aragonite with tuned morphology has been developed. Acknowledgements The authors gratefully acknowledge support from the Fundamental Research Funds for the China Central Universities (2011PY0178). References Chen, J., Xiang, L., 2009. Controllable synthesis of calcium carbonate polymorphs at different temperature. Powder Technol. 189, 64–69. Feng, B., Yong, A.K., An, H., 2007. Effect of various factors on the particle size of calcium carbonate formed in a precipitation process. Mater. Sci. Eng. A 445–446, 170–179.

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