Control of Crystal Size and Morphology of Calcium Carbonate Crystal Polymorphism

Calcium carbonate, the main component of lime, has been widely used in in-dustry due to its stability and economy. Calcium carbonate has three types of crystalline polymorphism, calcite, aragonite and vaterite, each with different properties. Therefore, the control of crystal polymorphism is required for industrial applications. In addition, the control of crystal size and shape is si-milarly required for different applications. In this study, the effect of SrCO 3 on the size control of fine aragonite-type calcium carbonate crystals by uniform urea precipitation and the effect of SrCO 3 addition was investigated by adding solid strontium carbonate and dissolved strontium carbonate. The addition of solid strontium carbonate affected the crystal polymorphism and size of the calcium carbonate produced, depending on the properties of the solid particles and the amount of SrCO 3 added. Experiments on the addition of dissolved SrCO 3 showed that the supersaturation formation rate could be controlled to control the crystal polymorphism.


Introduction
Limestone is one of the few mineral resources that are self-sufficient in the country, and veins of limestone, the raw material of limestone, are scattered throughout the country. Calcium carbonate (CaCO 3 ), the main component of limestone, has good stability and economy and is widely used industrially in rubber, plastics, paints, and paper-making. The industrially used CaCO 3 is classified into two types: ground calcium carbonate (GCC), which is made by finely How to cite this paper: Nagaki, W., Doki, N., Yokota, M., Yamashita, K., Kojima duced through chemical synthesis. GCC is produced by the crushing and classifying process and has irregular particle size and shape, while PCC is mainly produced by the lime milk coal oxidation method, in which calcium carbonate is precipitated by blowing CO 2  Therefore, the control of crystal polymorphism by various methods has been studied [4]- [9]. The polymorphism of calcium carbonate crystal is also affected by the additives [6] [7]. For example, it has been reported that the addition of metal ions to the reaction system stabilizes aragonite by Ni 2+ addition, Mg 2+ promotes the transition to calcite, and Fe 3+ stabilizes the satellite [8]. In addition, calcium carbonate polymorphism is affected by the polymorphism present in the reaction system. When calcium bicarbonate saturated solution contains a small amount of calcite, the product is 100% calcite, and when a small amount of aragonite is present, only aragonite is obtained. In industrial applications, it is also desirable to control the size and morphology of the crystals according to the application. For example, fine crystals of CaCO 3 and crystals with large specific surface areas have the effect of increasing the strength of rubber and the glossiness of paper when added. In the past, most of the PCC used calcite, but recently, columnar aragonite has been attracting attention as a paper coating pigment.
Columnar calcium aragonite has good dispersibility in water. In addition, it has good flowability in a high particle density slurry state, improves the whiteness and opacity of coated paper, and shows good printing characteristics due to its good lubrication of the coated paper surface.
The purpose of the study was to control particle size of fine aragonite-type calcium carbonate crystals by uniform urea precipitation and to clarify the effect of SrCO 3 addition. Furthermore, the effect of SrCO 3 addition was investigated in detail by using two kinds of SrCO 3 , one is solid SrCO 3 and another is dissolved SrCO 3 .

CaCO3 Crystallization Experiments in the Presence of Solid SrCO3
The crystallization tank with a 500 mL interference plate contained 200 mL of 1 7.38 × 10 −2 and 2.95 × 10 −1 (g/100 mL). Three types of SrCO 3 with different particle sizes were used for the addition.
The mixture was kept at 80˚C and stirred at 300 rpm for 4 h with a stirring blade, and the resulting crystals were filtered, washed and dried and shown in Figure 1. The average grain size and aragonite content of the obtained crystals were evaluated. The average size was determined from the size distribution measured by optical microscope images. Aragonite content was calculated using calibration curve after PXRD measurement of the product crystals. In addition, elemental analysis was performed by EDX.

CaCO3 Crystallization Experiments in the Presence of Dissolved SrCO3
The

Effect of Adding SrCO3
Crystallization     Figure 5 shows the SEM and optical microscope images of CaCO 3 crystallized with 2.95 × 10 −1 g/100 mL of SrCO 3 . Figure 5(a) is an SEM image of the crystals obtained without the addition of strontium carbonate. The average particle size was 62.8 μm and the coefficient of variation (CV) was 0.38. Figures 5(b)-(d) show that the addition of 0.3 µm, 30 µm, and 100 µm solid SrCO 3 decreased the particle size of aragonite along with the decrease of strontium carbonate particle size, along with this average particle size. Furthermore, in Figure 5(b), cubic  particles were also observed, which also confirmed the formation of calcite.

Elemental Analysis of CaCO3 after Crystallization
Elemental analysis of CaCO 3 crystals obtained with 0.295 g/100mL of SrCO 3 crystals with the average grain size of 30 µm and 100 µm was performed by EDX mapping (Figure 6). In Figure 6, Ca is shown in red and Sr is shown in green,

The Effect of Dissolved SrCO3 Addition
The concentration of Ca ions in the initial solution and the rate of addition of the solution were changed and the images of the crystals formed are summarized in Figure 7.

Conclusion
In calcium carbonate crystallization using the homogeneous urea precipitation method, the addition of solid strontium carbonate affected the crystal polymorphism and size of the calcium carbonate crystals produced, depending on the characteristics of the solid particles and the amount added. It was found that finer aragonite-type calcium carbonate crystals with a higher content were