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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.1 pp.1-7
DOI : https://doi.org/10.3740/MRSK.2021.31.1.1

Formation Characteristics of Precipitated Calcium Carbonate by Carbonation Process

Chiho Kim, Mingwang Seok, Yangdo Kim†
Department of Material Science and Engineering, Pusan National University, Busan 46241, Republic of Korea
Corresponding author E-Mail : yangdo@pusan.ac.kr (Y. Kim, Pusan Nat'l Univ.)
November 26, 2020 December 11, 2020 December 11, 2020

Abstract


The characteristics and morphology of precipitated calcium carbonate (PCC) particles produced by carbonation process with various experimental conditions are investigated in this study. The crystal structures of PCC formed by carbonation process are calcite and aragonite. The crystal structure of PCC particles synthesized without adipic acid additive is calcite only, regardless of the reaction temperature. Needle-like shape aragonite phase started to form at reactor temperature of 80°C with the adipic acid additive. Particle size of the single phase calcite PCC synthesized without adipic acid additive is about 1 ~ 3 μm, with homogenous distribution. The aragonite PCC also shows uniform size distribution. The reaction temperature and concentration of adipic acid additive do not show any significant effects on the particle size distribution. Aragonite phase grown to a large aspect ratio of needle-like shape showed relatively improved whiteness. The measured whiteness value of single calcite phase is about 95.95, while that of the mixture of calcite and aragonite is about 99.11.



초록


    Pusan National University(PNU)

    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    Calcium carbonate is the main component of limestone, which is the most abundant among non-metallic minerals, and there are three anhydrous crystalline polymorphs such as calcite, aragonite, and vaterite.1,2) The crystal structure of the calcite is most stable phase at normal surface temperature and pressure. Calcite is a trigonal structure with typical crystal such as cubic and easily synthesize in the natural conditions. Aragonite is an orthorhombic structure with twinned hexagonal prismatic crystals, such as needle-like crystals with a very large aspect ratio. Aragonite and vaterite are metastable phases in the natural conditions.3) The diversity in its morphology is one of the important properties of the calcium carbonate. PCC has more effective properties than other calcium carbonates with non-uniform shapes or sizes, so it has been applied to various industrial applications such as paper, rubber, and plastics.4) Especially, aragonite precipitated calcium carbonate (PCC) can be used as a filler in rubber and plastic products since it is needle-like shape, which has high aspect ratio with high bending and impact strength. Due to these effects, it can be app1icable to the car bumper and the dashboard as a filler material mixed with thermoplastic resin and polypropylene resin.5,6) However it is very difficult to synthesize aragonite phase due to metastable characteristics in natural and its easy transfer to more stable calcite phase. The calcium carbonate with a wide variety of applications as industrial raw material, the shape of crystallization and the size of the particles are very important factors. The characteristics of calcium carbonate are determined by their shape, morphology, particle size, particle size uniformity, whiteness, and purity. The calcium carbonate used in industries can be classified according to the manufacturing methods as ground calcium carbonate (GCC) produced by crushing limestone physically and PCC chemically formed through the calcination, hydration and carbonation of limestone. GCC production is cost effective and simple process, however, it is difficult to produce high quality GCC because the quality of the GCC is dependent on crushed stone technology and/or quality of limestone. Therefore, chemically formed PCC is more advantageous compared with GCC, since the control of such characteristics of crystallization shape, size, purity, and size distribution are more favorable.7) Methods for synthesizing PCC are carbonation method, which synthesizes calcium hydroxide by depositing calcium hydroxide in water and then injecting carbon dioxide, and aqueous solution method, which utilizes the reaction of calcium hydroxide and sodium carbonate. The carbonation method has the advantage of being able to design particle shapes and crystalline forms suitable for use by properly controlling the reaction conditions. For this reason, carbonation method is the most widely used method for synthesizing PCC on an industry scale.8-10)

    This study was performed to investigate the characteristics of PCC particles which produced by carbonation process under various experimental conditions. The effects of the reactor temperature, the additives and concentration of the additive on the formation of calcium carbonate were discussed.

    2. Experimental Procedure

    The starting material was Ca(OH)2 (assay 95 %, Junsei Chem. Co.) aqueous solution using primary distilled water. The starting materials was prepared by mixing of 0.4 mol Ca(OH)2 solution with distilled water in a triangular flask. The adipic acid (assay 99 %, Junsei Chem. Co.) was applied as additives and the reaction temperature was kept constant at to be 20, 40, 60 and 80°C by using water bath. By separating the unused as that used the additives were tested at each temperature. The concentration of adipic acid was varied from 0 ~ 0.8 M to investigate the effects of additive concentration on the formation of calcium carbonate.

    The CO2 gas (assay 99.99 %) was introduced at a flow rate of 200 cc/min. until reaction was terminated. A thermometer and a pH meter were equipped in the reactor to measure the temperature and the pH of calcium hydroxide, respectively. The solution was continuously stirred by magnetic stirrer at a rotation rate of 500 RPM during the process. The pH decreased continuously with the proceeding of precipitation until it is completed and the reaction finished approximately at pH 7. The obtained precipitates were filtered and dried at 80 °C for 12 h using dry oven. Morphology and microstructure were examined using scanning electron microscopy (SEM; MIRA3, Tescan). X-ray diffraction (XRD; ultima4, Rigaku) analysis was conducted to identify the crystalline phases of the precipitates. Fig. 1 shows the experimental procedure of the synthesis of precipitated calcium carbonate by carbonation process.

    3. Results and Discussion

    X-ray diffraction patterns were recorded on an X-ray diffractometer (XRD, UltimaIV, Rigaku) using Cu Kα radiation. Fig. 2 shows the XRD patterns of synthesized PCC after carbonation reaction at various temperatures with and without adipic acid additive. The crystal structure of PCC particles synthesized without adipic acid additive was calcite only regardless of the reaction temperature. The PCC particles synthesized with adipic acid additive also shows calcite structure at relatively lower temperatures at blow 60 °C. However, PCC particles synthesized with adipic acid additive at 80 °C showed mixed phase of calcite and aragonite as shown in Fig. 2(h). The amount of calculated calcite and aragonite phase ratio is about 30:70. Table 1. summarized the details of experimental conditions synthesized at various temperatures with and without adipic acid additive. The surface morphology and composition was observed by field-emission scanning electron microscope (FE-SEM, CZ/MIRAI LMH, TESCAN). Fig. 3 and 4 show the FE-SEM images of synthesized PCC after carbonation reaction at various temperatures without and with adipic acid additive, respectively. All of the cubic crystal form of calcite was produced without adipic acid additive regardless of the reaction temperature as shown in Fig. 3. However, two kinds of particles shapes were confirmed when synthesized at 80 °C with adipic acid additive. One is calcite of cubic shape and the other is aragonite of needle-like shape. Particle size of the single phase calcite PCC synthesized without adipic acid additive was about 1 ~ 3 μm with homogenous particle size distribution. The aragonite PCC synthesized with adipic acid additive also showed uniform size distribution. SEM analyses also revealed that the particle size of PCC remains relatively constant regardless of reaction temperature. According to previous studies on the synthesis of PCC by carbonation method, the general trend has been reported that the calcite is precipitated at room temperature, but the precipitation rate of aragonite is increased as temperature increased. The reason for that is explained by nucleation rate and the crystal growth rate.11,12) In other words, the aragonite form is better in a supersaturated solution. It means that the crystallization rate of metastable and high solubility of aragonite is faster than stable of calcite. Calcite has a rhombohedral structure in which 6 O2− ions are coordinated around the Ca2+ ions, and aragonite has a orthorhombic structure which is coordinated to 9 O2− ions. The reason for fast crystallization rate of aragonite is the number of more O2− ions which surrounding the Ca2+ ion than calcite and bonding with 9 O2− atoms. Therefore, calcium ions are easily captured than calcite by aragonite nuclei. In addition, thermal vibration of the CO3 groups are more restricted in aragonite than calcite. Nucleation of calcite and aragonite is affected by the dipole of the water which attached to the cation of constituting the nucleus.13) Water dipole attached to the constituting the nucleus of cation is easily broken from calcite nucleus which cohesion is relatively low. Ion-dipole bonds get loosing accordance with increasing temperature by thermal motion, thus, aragonite nuclei are formed easily at high temperatures. Fig. 5 shows the XRD patterns of synthesized PCC after carbonation reaction with various concentrations of adipic acid additive at 80 °C. The crystal structures of PCC particles synthesized with adipic acid additive were mixed phase of calcite and aragonite. As more adipic acid is added, the pH value of mixed solution reduced slowly from 12.5 to 11.4 and saturated with about more than 0.1M adipic acid additive. The Ca(OH)2 phase also started to appear with more than 0.2 M adipic acid additive and increased as increasing the amount of adipic acid additive. This indicates that the unreacted Ca(OH)2 due to the relatively low dissolution rate compare to that of the calcium carbonate formation rate governs the overall reaction rate.14,15) The formation of calcite also revealed that Ca2+ ions supplied fast enough to react with CO32− ions. For these reasons, it is necessary to lower the solubility of Ca(OH)2 and/or to precisely control the concentration of CO32− ions to have the aragonite phase formation. Table 2. summarized the details of experimental conditions synthesized at various concentrations of adipic acid additive at 80 °C. Fig. 6 shows the SEM images of synthesized PCC after carbonation reaction with various concentrations of adipic acid additive at 80 °C. Aragonite crystal of needle-like shape was observed with homogenous particle size distribution. SEM analyses also showed that the particle size of aragonite crystals remains relatively constant regardless of the concentration of adipic acid additive. The measured whiteness value of single calcite phase was about 95.95 while that of mixture of calcite and aragonite was about 99.11.

    4. Conclusions

    This study was performed to investigate the characteristics of PCC particle formed by carbonation process at various experimental conditions. The crystal structures of PCC formed by carbonation process were calcite and aragonite. At low reactor temperature of 20 to 60 °C, the microstructure and crystal structure of PCC was a single cubic shape of calcite phase. Needle-like shape aragonite phase started to form at reactor temperature of 80 °C with the adipic acid additive. As increasing the concentration of adipic acid than the concentration of Ca(OH)2 solution, calcite crystal phase decreased and aragonite crystal phase getting increased. Particle size of the single phase calcite PCC synthesized without adipic acid additive was about 1 ~ 3 μm with homogenous particle size distribution. The aragonite PCC synthesized with adipic acid additive also showed uniform size distribution. The reaction temperature and concentration of adipic acid additive did not show any significant effects on the particle size distribution. Synthesized aragonite phase grown to a large aspect ratio of needle-like shape showed relatively improved whiteness. The measured whiteness value of single calcite phase was about 95.95 while that of mixture of calcite and aragonite was about 99.11.

    Acknowledgement

    This work was supported by a 2-Year Research Grant of Pusan National University.

    Figure

    MRSK-31-1-1_F1.gif

    Experimental procedure of the synthesis of precipitated calcium carbonate by carbonation process.

    MRSK-31-1-1_F2.gif

    XRD patterns of PCC synthesized at various temperatures with and without adipic acid additive. The detail of experimental conditions is summarized in Table 1.

    MRSK-31-1-1_F3.gif

    SEM images of PCC synthesized without adipic acid additive at (a) 20 °C, (b) 40 °C, (c) 60 °C, and (d) 80 °C.

    MRSK-31-1-1_F4.gif

    SEM images of PCC synthesized with 0.4 M adipic acid additive at (a) 20 °C, (b) 40 °C, (c) 60 °C, and (d) 80 °C.

    MRSK-31-1-1_F5.gif

    XRD patterns of PCC synthesized at 80°C with various adipic acid additive. The detail of experimental conditions is summarized in Table 2.

    MRSK-31-1-1_F6.gif

    SEM images PCC synthesized at 80°C with various adipic acid additive. The detail of experimental conditions is summarized in Table 2.

    Table

    Summary of experomental conditions.

    Experimental conditions according to changes of adipic acid concentrations at 80°C.

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