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Synthesis of dispersive CaCO3 in the presence of MgCl2
Materials Chemistry and Physics 98 (2006) 236–240
Synthesis of dispersive CaCO3 in the presence of MgCl2 L. Xiang ∗ , Y. Wen, Q. Wang, Y. Jin Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 16 December 2004; received in revised form 19 August 2005; accepted 6 September 2005
Abstract Dispersive calcium carbonate (CaCO3 ) particles were synthesized at 40 ◦ C via the modified carbonation route developed in this paper. The influence of MgCl2 on the dispersion behavior of the slaking slurry and the carbonation product were investigated using a laser particle size analyzer and a zeta potential analyzer, the corresponding changes of the morphology and the structure were identified by the field scanning electron microscopy (FSEM) and X-ray diffractometry (XRD), respectively. The experimental results indicated that the addition of MgCl2 in the Ca(OH)2 slurry converted the Ca(OH)2 agglomerates to the dispersive Mg(OH)2 particles, the carbonation of the slaking slurry containing Mg(OH)2 and CaCl2 led to the formation of the dispersive CaCO3 (calcite) cubic particles with a particle size of 0.3–0.8 m and a mean agglomerate size of 1.3 m. © 2005 Elsevier B.V. All rights reserved. Keywords: Calcium carbonate; Dispersion; Carbonation; Magnesium chloride
1. Introduction Calcium carbonate (CaCO3 ) has elicited much interests because of its many industrial applications in fields as diverse as paper, rubber, plastics, paints and so on [1–5]. The advanced applications of this solid are actually related to its characteristics, such as the morphology, the structure, the dispersion, the particle size, etc. CaCO3 used as fillers in paper, rubber, plastics and paints usually requires particles with perfect dispersion property and uniform morphology. For example, the CaCO3 particles with an original size of about 0.1–1.0 m and an agglomerate size smaller than 2.0 m is usually expected in paper industry. Even though a lot of work has been done on the controllable synthesis of CaCO3 , most of them concerned with the control of the shape, the particle size or surface modification, little work has been reported directly on the commercial synthesis of dispersive CaCO3 . It is well known that the synthesis of CaCO3 can be conducted by either liquid–liquid or gas–liquid reactions, the former use the soluble calcium and carbonate salts as the reactants and the later use CO2 and Ca(OH)2 as the raw materials. The most widely used industrial process of obtaining CaCO3 is based on the gas–liquid reaction due to the low cost and the availabil∗
Corresponding author. Tel.: +86 1062 788984; fax: +86 1062 772051. E-mail address: [email protected] (L. Xiang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.09.015
ity of the raw materials and involves the following steps [6,7]: (a) calcinations, in which limestone (natural CaCO3 rocks) was decomposed to produce quicklime (CaO) and carbon dioxide (CO2 ), (b) the slaking, in which the quicklime is transformed to the slaked lime slurry (a Ca(OH)2 suspension) by controlled addition of water: CaO + H2 O = Ca(OH)2
(1)
and (c) the carbonation, in which the CO2 is bubbled through an aqueous slurry of slaked lime in a batch process: Ca(OH)2 + CO2 → CaCO3 + H2 O
(2)
The slaking and the carbonation are the crucial steps controlling the dispersion and morphology of the precipitated CaCO3 . In the normal slaking process, the agglomeration of the Ca(OH)2 particles is inevitable due to the poor dispersive property of Ca(OH)2 in water. The carbonation of the normal Ca(OH)2 slurry usually results in the formation of the CaCO3 agglomerates since the carbonation is a complex reaction process, including the dissolution of Ca(OH)2 to Ca2+ and OH− , the absorbing of CO2 in water to form CO3 2− , the reaction of Ca2+ and CO3 2− to form CaCO3 , some of the CaCO3 particles preferred to be formed on the surface of Ca(OH)2 agglomerates via the heterogeneous precipitation route [8]. In the literature, there are several references concerning with the dispersion of the precipitated CaCO3 . It was reported that the decrease of the
L. Xiang et al. / Materials Chemistry and Physics 98 (2006) 236–240
237
Fig. 1. Structure of the sparger.
calcination temperature, the rise of the slaking temperature, the aging of the Ca(OH)2 slurry or the use of the surfactants were helpful to improve the dispersion of Ca(OH)2 slurry [6,9]; the addition of chemicals, such as polyacrylic acid [10], polymaleicanhydride or poly(sodium 4-styene-sulfonate) [11], sodium stearate [12], alkyl polyglycoside [13], polystyrene sulfonate [14], ionic and nonionic dextrans [15] and the divalent cations (Mg2+ , Fe2+ , Ni2+ and Zn2+ , etc. [16–18]) in the liquid–liquid homogeneous precipitation system can improve the dispersion of the precipitated CaCO3 particles. Nevertheless, little studies have been reported, to the best of our knowledge, about the dispersion of the precipitated CaCO3 produced by the commercial carbonation route due to the difficulty in producing dispersive slaking slurry and the complexity in the carbonation process. The synthesis on a large scale of the dispersive CaCO3 particles is still a challenge. The present work developed an advanced method to produce dispersive CaCO3 particles via a modified carbonation route to meet the demand of paper industry. MgCl2 , which is a byproduct of sea-salt industry, was added in the slaking step to convert the Ca(OH)2 agglomerates to the dispersive Mg(OH)2 slurry. The carbonation of the resulting suspension led to the formation of the precipitated CaCO3 cubic particles with perfect dispersion. The influence of MgCl2 on the dispersion, composition and morphology of the slaking slurry and the carbonation product was investigated. 2. Experimental
2.2. Analysis The pH in solution was recorded by pHS-3C digital pH meter. The morphology of samples was observed by the field scanning electron microscope (FSEM, model JSM-6301F, JEOL, Japan). The crystal structures of the samples were characterized by X-ray diffraction (XRD, model D/Max-RB, Rigaku, Japan). The mean size of the particles was determined from SEM images of 50–100 particles, as previously suggested by Jung et al. [19]. The size distribution of the agglomerates of the slaking or the carbonation slurry was investigated with a laser particle size analyzer (model Micro-Plus, Malven, British). The zeta potential of the particles was detected in suspension containing known amount of particles (HCl and NaOH were used to adjust the pH to the desired values) by the zeta potential analyzer (model ZetaPLAS, Brookhaven, American). The values of the zeta potential were reproducible within ±2 mV. The contents of Ca and Mg in the carbonation product were detected by EDTA titration method.
3. Results and discussion 3.1. Morphology and dispersion of the slaking product Fig. 2 reveals the XRD patterns of the slaking products in the absence and presence of MgCl2 . The slaking slurry with the Mg/Ca molar ratio of 1.0 was synthesized by adding same molar amount of MgCl2 to the Ca(OH)2 slurry. The addition of MgCl2 in the Ca(OH)2 slurry led to the disappearance of Ca(OH)2 phase and the occurrence of Mg(OH)2 phase. The conversion of Ca(OH)2 to Mg(OH)2 was attributed to the difference of the solubility product for Ca(OH)2 and Mg(OH)2 at 25 ◦ C: Ca(OH)2 = Ca2+ + 2OH− ,Ksp = 4.8 × 10−6
(3)
2.1. Experimental procedure The carbonation experimental set-up consisted of a thermostated double-wall cylindrical plastic reactor with a diameter of 9.0 cm and a height of 25.0 cm. A radial-shaped cupper sparger with 24 nozzles of 1.0 mm diameter was used in the experiments (Fig. 1). All of the nozzles kept a same distance from the gas entrance and were distributed uniformly to ensure the uniform gas dispersion. Certain amount of MgCl2 was added to 500 ml slurry containing 26 g l−1 of Ca(OH)2 . After stirring (450 min−1 ) at room temperature for 30 min, the stirring was stopped and the slurry was aged for 120 min. Then the carbonation experiments were performed by bubbling a gas mixture (25%, v/v, of CO2 in air) into the slurry at a flow rate of 1.6 l min−1 . The temperature of the carbonation system was maintained at 40 ± 1 ◦ C by manipulating the temperature of the thermostat. The homogenization of the system was attained at the stirring rate of 450 min−1 . The flow rates of air and CO2 were measured with the calibrated rotameters. The carbonation reaction was stopped as soon as the pH decreased to lower than 7.0. An aliquot of the final suspension was used for the analysis of the particle size distribution; the rest of the suspension was filtrated, washed with distilled water and dried in an oven with the circulating air at 105 ◦ C for 24 h. The final white precipitate was used for analysis.
Fig. 2. XRD patterns of the slaking products in the absence (a) and presence (b) of MgCl2 . Molar ratio of Mg/Ca: 1.0; slaking time: 30 min; () Ca(OH)2 , (䊉) Mg(OH)2 .
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Fig. 3. Morphology of the slaking products in the absence (a) and presence (b) of MgCl2 . Molar ratio of Mg/Ca: 1.0, slaking time: 30 min.
Mg(OH)2 = Mg2+ + 2OH− ,Ksp = 5.8 × 10−12
(4)
Compared with Ca(OH)2 , Mg(OH)2 is more thermodynamically stable owing to its lower solubility product. The influence of MgCl2 on the typical morphology and size distribution of the slaking products is depicted in Figs. 3 and 4, respectively. In the absence of MgCl2 , the slaking product was composed of the un-regular particles with a diameter of about 1–3 m and the small granules with a diameter of about 0.1–0.2 m (Fig. 3a), corresponding to a triplicate modality pattern of the agglomerate size distribution and a mean agglomerate size of about 5.8 m (the curve a in Fig. 4). The first peak of the dotted curve was located at about 0.3 m, quite similar with the individual small granular size; the second peak occurred at 3.5 m was related to the existence of the un-regular Ca(OH)2 particles; the third peak located at 30 m may connect with the agglomeration of the Ca(OH)2 particles. It appeared that the particle agglomeration brought about the triplicate modality in the size distribution. The slaking product changed to dispersive particles after the addition of MgCl2 into Ca(OH)2 slurry (Fig. 3b). The particle size was in the range of 0.1–0.2 m and the corresponding mean agglomerate size was 1.1 m (the curve b in Fig. 4). The sharp decrease of the agglomerate size and the disappearance of the triplicate modality phenomenon in the size distribution curve
Fig. 4. Size distribution of the slaking products in the absence (a) and presence (b) of MgCl2 . Slaking time: 30 min.
indicated that the addition of MgCl2 in the slaking step was favorable for the formation of the dispersive slaking slurry. 3.2. Morphology and dispersion of the precipitated CaCO3 The chemical reactions involved in the carbonation of the Mg(OH)2 –CaCl2 –H2 O system are listed as follows: Mg(OH)2 = Mg2+ + 2OH−
(5)
CO2 + H2 O = CO3 2− + 2H+
(6)
Ca2+ + CO3 2− = CaCO3
(7)
H+ + OH− = H2 O
(8)
Fig. 5 displays the influence of the molar ratios of Mg/Ca on the morphology of the completely carbonated CaCO3 powders. Spherical agglomerated particles with a mean diameter of about 0.2 m were formed in the absence of MgCl2 . The morphology of the carbonation products changed to cubic shape with a mean diameter of about 0.3–0.8 m after the addition of MgCl2 into the Ca(OH)2 slurry. The cubic particles became more regular and dispersive with the increase of the Mg/Ca molar ratios from 0.75 to 1.0. Fig. 6 shows the variation of the XRD patterns of the carbonation products with the reaction time when the Mg/Ca molar ratio was controlled at 1.0. Before the carbonation, only Mg(OH)2 phase was detected; the peaks for Mg(OH)2 phase weakened gradually and the peaks for CaCO3 (calcite) phase appeared when the carbonation time increased from 10 to 30 min. After 45 min of carbonation, all of the detectable peaks consisted with those of CaCO3 (calcite) phase, no MgCO3 phase was detected even though the solubility for CaCO3 (1.34 × 108 ) and MgCO3 (1.74 × 108 ) are quite similar at 40 ◦ C, indicating the sole existence of CaCO3 crystals. Chemical analysis indicated that 37.6 wt% of Ca (or 94.0 wt% of CaCO3 ) and 1.7 wt% of Mg were detected in the final carbonation product, corresponding to a molar ratio of Ca to Mg 13.5:1. The above phenomena may be explained by the partial substitution of the Ca atoms in CaCO3 by Mg atoms. No Mg was detected in the carbonation product if the initial molar ratio of Mg/Ca was equal or smaller than 0.75.
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Fig. 5. Morphology of the carbonation products formed at various molar ratios of Mg/Ca. Molar ratio of Mg/Ca: (a) 0; (b) 0.75; (c) 1.0; carbonation time: (a) 36.4 min; (b) 73.0 min; (c) 87.8 min.
Fig. 7 presents the influence of the molar ratios of Mg/Ca on the pH of the carbonation solutions. With the increase of the molar ratio of Mg/Ca from 0 to 1.0, the initial pH decreased from 12.8 to 9.3 and the corresponding carbonation time prolonged from 36.4 to 87.8 min, indicating that the addition of MgCl2
Fig. 6. Variation of the XRD patterns of the carbonation products with reaction time. Molar ratio of Mg/Ca: 1.0; carbonation time: (a) 0 min; (b) 10 min; (c) 20 min; (d) 30 min; (e) 45 min; (䊉) Mg(OH)2 , () CaCO3 (calcite).
Fig. 7. Influence of the molar ratio of Mg/Ca on pH. Molar ratio of Mg/Ca: (a) 0; (b) 0.75; (c) 1.0.
in the Ca(OH)2 slurry inhibited the carbonation reaction, leading to the formation of CaCO3 crystals with bigger size and regular shape. Fig. 8 shows the influence of the molar ratio of Mg/Ca on the size distribution of the completely carbonated CaCO3 powders. Bimodality in size distribution was observed in all of the samples. In the bimodal distribution patterns, the small size peak (first peak) was about 0.3–0.5 m, corresponding closely to some of the individual CaCO3 particle size as shown in Fig. 5, whereas the large peak (second peak) ranged from 3 to 10 m, depending on the molar ratio of Mg/Ca, and was related to the agglomeration of the individual CaCO3 particles. With the increase of the Mg/Ca molar ratio from 0 to 1.0, the fraction of the first peak in the distribution was increased while the fraction of the second peak was decreased, leading to the sharp decrease of the mean agglomerate size from 7.8 to 1.3 m. Fig. 9 shows the influence of the molar ratio of Mg/Ca on the zeta potential of the precipitated CaCO3 particles. In the pH range of 7–12, the absolute values of the zeta potential were lower than 15 mV in the absence of MgCl2 and increased up to 15.2–32.6 mV when the Mg/Ca molar ratio was controlled at 1.0, the increase of the zeta potential may be attributed to the existence of the minor amount of Mg (or the partial substitution of Ca by Mg) and was favorable for the dispersion of the precipitated CaCO3 particles.
Fig. 8. Influence of the molar ratio of Mg/Ca on the size distribution of CaCO3 . Molar ratio of Mg/Ca: (a) 0; (b) 0.75; (c) 1.0; carbonation time: (a) 36.4 min; (b) 73.0 min; (c) 87.8 min.
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Fig. 9. Zeta potential of CaCO3 . Molar ratio of Mg/Ca: (a) 0 and (b) 1.0; carbonation time: (a) 36.4 min and (b) 87.8 min.
4. Conclusion An advanced method was developed to synthesize dispersive CaCO3 particles via the modified carbonation route at 40 ◦ C, using CaO and MgCl2 as the slaking raw materials. The addition of MgCl2 to the Ca(OH)2 slurry converted the Ca(OH)2 agglomerates to the dispersive Mg(OH)2 particles. The carbonation of the slaking slurry containing Mg(OH)2 and CaCl2 led to the formation of the dispersive CaCO3 (calcite) cubic particles with a particle size of 0.3–0.8 m and a mean agglomerate size of 1.3 m. Acknowledgement The FSEM and XRD analysis is supported financially by the Open Foundation of Tsinghua University. References [1] W.C.J. Zuiderduin, C. Westzaan, J. Huetink, R.J. Gaymans, Toughening of polypropylene with calcium carbonate particles, Polymer 44 (2003) 261–275. [2] C.M. Chan, J.S. Wu, J.X. Li, Y.K. Cheung, Polypropylene/calcium carbonate nanocomposites, Polymer 43 (2002) 2981–2992. [3] C. Kugge, J. Daicic, Shear response of concentrated calcium carbonate suspensions, J. Colloid Interface Sci. 271 (2004) 241–248.
[4] H. Colfen, Precipitation of carbonates: recent progress in controlled production of complex shapes, Curr. Opin. Colloid Interface Sci. 8 (2003) 21–31. [5] T. Lozano, P.G. Lafleur, M. Grmela, C. Thibodeau, Effect of filler dispersion on polypropylene morphology, Polym. Eng. Sci. 44 (5) (2004) 880–890. [6] A. Moropoulou, A. Bakolas, E. Aggelakopoulou, The effects of limestone characteristics and calcination temperature to the reactivity of the quicklime, Cem. Concr. Res. 31 (2001) 633–639. [7] C. Domingo, J. Garcia-Carmona, E. Loste, A. Fanovich, J. Fraile, J. Gomez-Morales, Control of calcium carbonate morphology by precipitation in compressed and supercritical carbon dioxide media, J. Cryst. Growth 271 (2004) 268–273. [8] J.G. Carmona, J.G. Morales, J.F. Sainz, E. Loste, R.R. Clemente, The mechanism of precipitation of chain-like calcite, J. Cryst. Growth 262 (2004) 479–489. [9] B. Delfort, M. Born, A. Chive, L. Barre, Colloidal calcium hydroxide in organic medium: synthesis and analysis, J. Colloid Interface Sci. 189 (1) (1997) 151–157. [10] B. Cheng, M. Lei, J. Yu, X.J. Zhao, Preparation of monodispersed cubic calcium carbonate particles via precipitation reaction, Mater. Lett. 58 (2004) 1565–1570. [11] J.G. Yu, M. Lei, B. Cheng, Facile precipitation of monodispersed calcium carbonate spherical particles via a simple precipitation reaction, Mater. Chem. Phys. 88 (2004) 1–4. [12] M.G. Song, J.Y. Kim, J.D. Kim, Effect of sodium stearate and calcium ion on dispersion properties of precipitated calcium carbonate suspensions, Colloids Surf. A Physicochem. Eng. Aspects 229 (2003) 75–83. [13] M.G. Song, J.Y. Kim, J.D. Kim, The dispersion properties of precipitated calcium carbonate suspensions absorbed with alkyl polyglycoside in aqueous medium, J. Colloid Interface Sci. 226 (2000) 83–90. [14] A. Jada, A. Verraes, Precipitation and microelectrophoresis characterization of calcium carbonate particles in the presence of anionic polyelectrolyte, Colloids Surf. A Physicochem. Eng. Aspects 219 (2003) 7–15. [15] V.V. Hardikar, E. Matijevic, Influence of ionic and nonionic dextrans on the formation of calcium hydroxide and calcium carbonate particles, Colloids Surf. A Physicochem. Eng. Aspects 186 (2001) 23–31. [16] N. Wada, K. Yamashita, T. Umegaki, Effects of divalent cations upon nucleation, growth and transformation of calcium carbonate polymorphs under conditions of double diffusion, J. Cryst. Growth 148 (1998) 297–304. [17] M. Kitamura, Crystallization and transformation mechanism of calcium carbonate polymorphs and the effect of magnesium ion, J. Colloid Interface Sci. 236 (2001) 318–327. [18] E. Loste, R.M. Wilson, R. Seshadri, F.C. Meldrum, The role of magnesium in stabilising amorphous calcium carbonate and controlling calcite morphologies, J. Cryst. Growth 254 (2003) 206–218. [19] W.M. Jung, S.H. Kang, W.S. Kim, C.K. Choi, Particle morphology of calcium carbonate precipitated by gas–liquid reaction in a Couette-Taylor reactor, Chem. Eng. Sci. 55 (2000) 733–747.
Synthesis of dispersive CaCO3 in the presence of MgCl2 L. Xiang ∗ , Y. Wen, Q. Wang, Y. Jin Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 16 December 2004; received in revised form 19 August 2005; accepted 6 September 2005
Abstract Dispersive calcium carbonate (CaCO3 ) particles were synthesized at 40 ◦ C via the modified carbonation route developed in this paper. The influence of MgCl2 on the dispersion behavior of the slaking slurry and the carbonation product were investigated using a laser particle size analyzer and a zeta potential analyzer, the corresponding changes of the morphology and the structure were identified by the field scanning electron microscopy (FSEM) and X-ray diffractometry (XRD), respectively. The experimental results indicated that the addition of MgCl2 in the Ca(OH)2 slurry converted the Ca(OH)2 agglomerates to the dispersive Mg(OH)2 particles, the carbonation of the slaking slurry containing Mg(OH)2 and CaCl2 led to the formation of the dispersive CaCO3 (calcite) cubic particles with a particle size of 0.3–0.8 m and a mean agglomerate size of 1.3 m. © 2005 Elsevier B.V. All rights reserved. Keywords: Calcium carbonate; Dispersion; Carbonation; Magnesium chloride
1. Introduction Calcium carbonate (CaCO3 ) has elicited much interests because of its many industrial applications in fields as diverse as paper, rubber, plastics, paints and so on [1–5]. The advanced applications of this solid are actually related to its characteristics, such as the morphology, the structure, the dispersion, the particle size, etc. CaCO3 used as fillers in paper, rubber, plastics and paints usually requires particles with perfect dispersion property and uniform morphology. For example, the CaCO3 particles with an original size of about 0.1–1.0 m and an agglomerate size smaller than 2.0 m is usually expected in paper industry. Even though a lot of work has been done on the controllable synthesis of CaCO3 , most of them concerned with the control of the shape, the particle size or surface modification, little work has been reported directly on the commercial synthesis of dispersive CaCO3 . It is well known that the synthesis of CaCO3 can be conducted by either liquid–liquid or gas–liquid reactions, the former use the soluble calcium and carbonate salts as the reactants and the later use CO2 and Ca(OH)2 as the raw materials. The most widely used industrial process of obtaining CaCO3 is based on the gas–liquid reaction due to the low cost and the availabil∗
Corresponding author. Tel.: +86 1062 788984; fax: +86 1062 772051. E-mail address: [email protected] (L. Xiang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.09.015
ity of the raw materials and involves the following steps [6,7]: (a) calcinations, in which limestone (natural CaCO3 rocks) was decomposed to produce quicklime (CaO) and carbon dioxide (CO2 ), (b) the slaking, in which the quicklime is transformed to the slaked lime slurry (a Ca(OH)2 suspension) by controlled addition of water: CaO + H2 O = Ca(OH)2
(1)
and (c) the carbonation, in which the CO2 is bubbled through an aqueous slurry of slaked lime in a batch process: Ca(OH)2 + CO2 → CaCO3 + H2 O
(2)
The slaking and the carbonation are the crucial steps controlling the dispersion and morphology of the precipitated CaCO3 . In the normal slaking process, the agglomeration of the Ca(OH)2 particles is inevitable due to the poor dispersive property of Ca(OH)2 in water. The carbonation of the normal Ca(OH)2 slurry usually results in the formation of the CaCO3 agglomerates since the carbonation is a complex reaction process, including the dissolution of Ca(OH)2 to Ca2+ and OH− , the absorbing of CO2 in water to form CO3 2− , the reaction of Ca2+ and CO3 2− to form CaCO3 , some of the CaCO3 particles preferred to be formed on the surface of Ca(OH)2 agglomerates via the heterogeneous precipitation route [8]. In the literature, there are several references concerning with the dispersion of the precipitated CaCO3 . It was reported that the decrease of the
L. Xiang et al. / Materials Chemistry and Physics 98 (2006) 236–240
237
Fig. 1. Structure of the sparger.
calcination temperature, the rise of the slaking temperature, the aging of the Ca(OH)2 slurry or the use of the surfactants were helpful to improve the dispersion of Ca(OH)2 slurry [6,9]; the addition of chemicals, such as polyacrylic acid [10], polymaleicanhydride or poly(sodium 4-styene-sulfonate) [11], sodium stearate [12], alkyl polyglycoside [13], polystyrene sulfonate [14], ionic and nonionic dextrans [15] and the divalent cations (Mg2+ , Fe2+ , Ni2+ and Zn2+ , etc. [16–18]) in the liquid–liquid homogeneous precipitation system can improve the dispersion of the precipitated CaCO3 particles. Nevertheless, little studies have been reported, to the best of our knowledge, about the dispersion of the precipitated CaCO3 produced by the commercial carbonation route due to the difficulty in producing dispersive slaking slurry and the complexity in the carbonation process. The synthesis on a large scale of the dispersive CaCO3 particles is still a challenge. The present work developed an advanced method to produce dispersive CaCO3 particles via a modified carbonation route to meet the demand of paper industry. MgCl2 , which is a byproduct of sea-salt industry, was added in the slaking step to convert the Ca(OH)2 agglomerates to the dispersive Mg(OH)2 slurry. The carbonation of the resulting suspension led to the formation of the precipitated CaCO3 cubic particles with perfect dispersion. The influence of MgCl2 on the dispersion, composition and morphology of the slaking slurry and the carbonation product was investigated. 2. Experimental
2.2. Analysis The pH in solution was recorded by pHS-3C digital pH meter. The morphology of samples was observed by the field scanning electron microscope (FSEM, model JSM-6301F, JEOL, Japan). The crystal structures of the samples were characterized by X-ray diffraction (XRD, model D/Max-RB, Rigaku, Japan). The mean size of the particles was determined from SEM images of 50–100 particles, as previously suggested by Jung et al. [19]. The size distribution of the agglomerates of the slaking or the carbonation slurry was investigated with a laser particle size analyzer (model Micro-Plus, Malven, British). The zeta potential of the particles was detected in suspension containing known amount of particles (HCl and NaOH were used to adjust the pH to the desired values) by the zeta potential analyzer (model ZetaPLAS, Brookhaven, American). The values of the zeta potential were reproducible within ±2 mV. The contents of Ca and Mg in the carbonation product were detected by EDTA titration method.
3. Results and discussion 3.1. Morphology and dispersion of the slaking product Fig. 2 reveals the XRD patterns of the slaking products in the absence and presence of MgCl2 . The slaking slurry with the Mg/Ca molar ratio of 1.0 was synthesized by adding same molar amount of MgCl2 to the Ca(OH)2 slurry. The addition of MgCl2 in the Ca(OH)2 slurry led to the disappearance of Ca(OH)2 phase and the occurrence of Mg(OH)2 phase. The conversion of Ca(OH)2 to Mg(OH)2 was attributed to the difference of the solubility product for Ca(OH)2 and Mg(OH)2 at 25 ◦ C: Ca(OH)2 = Ca2+ + 2OH− ,Ksp = 4.8 × 10−6
(3)
2.1. Experimental procedure The carbonation experimental set-up consisted of a thermostated double-wall cylindrical plastic reactor with a diameter of 9.0 cm and a height of 25.0 cm. A radial-shaped cupper sparger with 24 nozzles of 1.0 mm diameter was used in the experiments (Fig. 1). All of the nozzles kept a same distance from the gas entrance and were distributed uniformly to ensure the uniform gas dispersion. Certain amount of MgCl2 was added to 500 ml slurry containing 26 g l−1 of Ca(OH)2 . After stirring (450 min−1 ) at room temperature for 30 min, the stirring was stopped and the slurry was aged for 120 min. Then the carbonation experiments were performed by bubbling a gas mixture (25%, v/v, of CO2 in air) into the slurry at a flow rate of 1.6 l min−1 . The temperature of the carbonation system was maintained at 40 ± 1 ◦ C by manipulating the temperature of the thermostat. The homogenization of the system was attained at the stirring rate of 450 min−1 . The flow rates of air and CO2 were measured with the calibrated rotameters. The carbonation reaction was stopped as soon as the pH decreased to lower than 7.0. An aliquot of the final suspension was used for the analysis of the particle size distribution; the rest of the suspension was filtrated, washed with distilled water and dried in an oven with the circulating air at 105 ◦ C for 24 h. The final white precipitate was used for analysis.
Fig. 2. XRD patterns of the slaking products in the absence (a) and presence (b) of MgCl2 . Molar ratio of Mg/Ca: 1.0; slaking time: 30 min; () Ca(OH)2 , (䊉) Mg(OH)2 .
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Fig. 3. Morphology of the slaking products in the absence (a) and presence (b) of MgCl2 . Molar ratio of Mg/Ca: 1.0, slaking time: 30 min.
Mg(OH)2 = Mg2+ + 2OH− ,Ksp = 5.8 × 10−12
(4)
Compared with Ca(OH)2 , Mg(OH)2 is more thermodynamically stable owing to its lower solubility product. The influence of MgCl2 on the typical morphology and size distribution of the slaking products is depicted in Figs. 3 and 4, respectively. In the absence of MgCl2 , the slaking product was composed of the un-regular particles with a diameter of about 1–3 m and the small granules with a diameter of about 0.1–0.2 m (Fig. 3a), corresponding to a triplicate modality pattern of the agglomerate size distribution and a mean agglomerate size of about 5.8 m (the curve a in Fig. 4). The first peak of the dotted curve was located at about 0.3 m, quite similar with the individual small granular size; the second peak occurred at 3.5 m was related to the existence of the un-regular Ca(OH)2 particles; the third peak located at 30 m may connect with the agglomeration of the Ca(OH)2 particles. It appeared that the particle agglomeration brought about the triplicate modality in the size distribution. The slaking product changed to dispersive particles after the addition of MgCl2 into Ca(OH)2 slurry (Fig. 3b). The particle size was in the range of 0.1–0.2 m and the corresponding mean agglomerate size was 1.1 m (the curve b in Fig. 4). The sharp decrease of the agglomerate size and the disappearance of the triplicate modality phenomenon in the size distribution curve
Fig. 4. Size distribution of the slaking products in the absence (a) and presence (b) of MgCl2 . Slaking time: 30 min.
indicated that the addition of MgCl2 in the slaking step was favorable for the formation of the dispersive slaking slurry. 3.2. Morphology and dispersion of the precipitated CaCO3 The chemical reactions involved in the carbonation of the Mg(OH)2 –CaCl2 –H2 O system are listed as follows: Mg(OH)2 = Mg2+ + 2OH−
(5)
CO2 + H2 O = CO3 2− + 2H+
(6)
Ca2+ + CO3 2− = CaCO3
(7)
H+ + OH− = H2 O
(8)
Fig. 5 displays the influence of the molar ratios of Mg/Ca on the morphology of the completely carbonated CaCO3 powders. Spherical agglomerated particles with a mean diameter of about 0.2 m were formed in the absence of MgCl2 . The morphology of the carbonation products changed to cubic shape with a mean diameter of about 0.3–0.8 m after the addition of MgCl2 into the Ca(OH)2 slurry. The cubic particles became more regular and dispersive with the increase of the Mg/Ca molar ratios from 0.75 to 1.0. Fig. 6 shows the variation of the XRD patterns of the carbonation products with the reaction time when the Mg/Ca molar ratio was controlled at 1.0. Before the carbonation, only Mg(OH)2 phase was detected; the peaks for Mg(OH)2 phase weakened gradually and the peaks for CaCO3 (calcite) phase appeared when the carbonation time increased from 10 to 30 min. After 45 min of carbonation, all of the detectable peaks consisted with those of CaCO3 (calcite) phase, no MgCO3 phase was detected even though the solubility for CaCO3 (1.34 × 108 ) and MgCO3 (1.74 × 108 ) are quite similar at 40 ◦ C, indicating the sole existence of CaCO3 crystals. Chemical analysis indicated that 37.6 wt% of Ca (or 94.0 wt% of CaCO3 ) and 1.7 wt% of Mg were detected in the final carbonation product, corresponding to a molar ratio of Ca to Mg 13.5:1. The above phenomena may be explained by the partial substitution of the Ca atoms in CaCO3 by Mg atoms. No Mg was detected in the carbonation product if the initial molar ratio of Mg/Ca was equal or smaller than 0.75.
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Fig. 5. Morphology of the carbonation products formed at various molar ratios of Mg/Ca. Molar ratio of Mg/Ca: (a) 0; (b) 0.75; (c) 1.0; carbonation time: (a) 36.4 min; (b) 73.0 min; (c) 87.8 min.
Fig. 7 presents the influence of the molar ratios of Mg/Ca on the pH of the carbonation solutions. With the increase of the molar ratio of Mg/Ca from 0 to 1.0, the initial pH decreased from 12.8 to 9.3 and the corresponding carbonation time prolonged from 36.4 to 87.8 min, indicating that the addition of MgCl2
Fig. 6. Variation of the XRD patterns of the carbonation products with reaction time. Molar ratio of Mg/Ca: 1.0; carbonation time: (a) 0 min; (b) 10 min; (c) 20 min; (d) 30 min; (e) 45 min; (䊉) Mg(OH)2 , () CaCO3 (calcite).
Fig. 7. Influence of the molar ratio of Mg/Ca on pH. Molar ratio of Mg/Ca: (a) 0; (b) 0.75; (c) 1.0.
in the Ca(OH)2 slurry inhibited the carbonation reaction, leading to the formation of CaCO3 crystals with bigger size and regular shape. Fig. 8 shows the influence of the molar ratio of Mg/Ca on the size distribution of the completely carbonated CaCO3 powders. Bimodality in size distribution was observed in all of the samples. In the bimodal distribution patterns, the small size peak (first peak) was about 0.3–0.5 m, corresponding closely to some of the individual CaCO3 particle size as shown in Fig. 5, whereas the large peak (second peak) ranged from 3 to 10 m, depending on the molar ratio of Mg/Ca, and was related to the agglomeration of the individual CaCO3 particles. With the increase of the Mg/Ca molar ratio from 0 to 1.0, the fraction of the first peak in the distribution was increased while the fraction of the second peak was decreased, leading to the sharp decrease of the mean agglomerate size from 7.8 to 1.3 m. Fig. 9 shows the influence of the molar ratio of Mg/Ca on the zeta potential of the precipitated CaCO3 particles. In the pH range of 7–12, the absolute values of the zeta potential were lower than 15 mV in the absence of MgCl2 and increased up to 15.2–32.6 mV when the Mg/Ca molar ratio was controlled at 1.0, the increase of the zeta potential may be attributed to the existence of the minor amount of Mg (or the partial substitution of Ca by Mg) and was favorable for the dispersion of the precipitated CaCO3 particles.
Fig. 8. Influence of the molar ratio of Mg/Ca on the size distribution of CaCO3 . Molar ratio of Mg/Ca: (a) 0; (b) 0.75; (c) 1.0; carbonation time: (a) 36.4 min; (b) 73.0 min; (c) 87.8 min.
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Fig. 9. Zeta potential of CaCO3 . Molar ratio of Mg/Ca: (a) 0 and (b) 1.0; carbonation time: (a) 36.4 min and (b) 87.8 min.
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