Limestone is the raw material for precipitated calcium carbonate. Limestone is the only mineral resource in which Japan is completely self-sufficient 1).
Japan mines the third largest amount of limestone in the world after the United States and China2). About 3% of the limestone mined in Japan is exported. In addition, as shown in Figure 1, limestone deposits are spread throughout all of Japan, from Hokkaido to Kyushu.
Table 1 shows example measurements for the concentration of impurity elements in limestone from Japan, the United States, and Europe. As you can see, Japanese limestone has fewer impurities and is of higher quality than limestone from overseas.
Japan is a country blessed with high quality limestone.
Figure 1. Location of limestone deposits 3)
Calcium carbonate is divided into two industrial categories: Ground Calcium Carbonate (GCC) and precipitated calcium carbonate (PCC).
The two categories use different manufacturing methods.
- GCC is manufactured by physically grinding and classifying limestone.
- PCC is manufactured through chemical synthesis using limestone as a raw material.
As shown in Figure 1, both GCC and PCC appear as the same white powder material.
Figure 1. GCC and PCC powder.
However, when viewed at the micro level as shown in Figure 2, GCC is composed of irregularly shaped particles of 0.5 to 10 µm, whereas PCC consists of homogeneous particles (rhombohedral particles, etc.) of 20 to 300 nm.
Figure 2. Scanning electron microscope (SEM) images of GCC and PCC.
In recent years, the importance of creating a low-carbon society has become a major issue throughout the world. Promoting environmental protection through the appropriate use of resources is now an important mission of companies.
As shown in Figure 1, Tsuneji Shiraishi, the founder of Shiraishi Kogyo Kaisha, Ltd., established a manufacturing method that does not dispose of the carbon dioxide (CO2) gas generated during limestone calcination, but instead recycles the gas for use in synthesizing calcium carbonate in 1909 (Figure. 2) 1).
Calcine limestone and obtain quicklime (calcium oxide) via decarbonation.
CaCO3 → CaO + CO2
Obtain lime milk (calcium hydroxide slurry) by hydrating quicklime with a sufficient amount of water.
CaO + H2O → Ca(OH)2
Precipitate calcium carbonate fine particles in liquid by bubbling the CO2 gas generated during calcination into the lime milk. *Reuse of gas generated in calcination (Step (1))
Ca(OH)2 + CO2 → CaCO3 + H2O
Figure 1. Manufacturing of precipitated calcium carbonate (Shiraishi method).
Even among numerous other inorganic materials, the manufacturing method for calcium carbonate synthesized in water is an environmentally-friendly manufacturing process.
Figure 2. Examples of patents during our founding period (filed in 1911).
Calcium carbonate has three crystalline polymorphs: calcite, aragonite, and vaterite. In the carbonation process, these crystals can be produced by changing various conditions such as concentration and temperature of lime milk, introduction rate of CO2 gas, and whether or not chemicals are added.
All reaction formulas are expressed as Ca(OH)2 + CO2 → CaCO3 + H2O. However, as shown in Figure 1, a high degree of control can be exerted on the particle morphologies and agglomeration states. When these particles are mixed into a matrix as fillers, several properties of the resulted composites can be enhanced.
Calcite colloidal particles
Calcite spindle-shaped particles
Aragonite needle-like particles
Vaterite plate-like particles
Aggregates (colloidal particles)
Aggregates (needle-like particles)
Aggregates (needle-like particles)
Figure 1. Precipitated calcium carbonate in various particle shapes and agglomeration states.
In addition to the particle shape, stabilizing the crystal and controlling the particle size are also important to achieve the desired filler characteristics.
For example, in the case of colloidal particles, the Shiraishi Group can produce calcium carbonate particles ranging from 20 to 300 nm (Figure 1). This manufacturing of homogeneous particles makes it possible to impart physical properties to composite materials with high reproducibility.
(1) Hakuenka O
(3) Hakuenka CC
(4) Hakuenka CC-R
Figure 1. Colloidal calcium carbonate with various particle sizes.
Treating the particle surface of calcium carbonate with various chemicals such as fatty acids and resin acids improves affinity with the matrix and achieves an even higher level of physical properties of the resulted composite materials.
Figure 1 is a graph showing the change in viscosity of two-component polyurethanes containing without PCC, with untreated PCC, and with PCC treated with fatty acids. The graph shows that surface modification using fatty acids is effective in improving the workability of sealants.
Figure 1. Viscosity of two-component polyurethanes without PCC addition, with untreated PCC, and with PCC treated with fatty acids.
Figure 2 shows transmission electron microscope (TEM) images of particle surfaces of an untreated particle and a particle treated with fatty acids. In the untreated product, the crystal lattice of calcite is observed to exist up to the vicinity of the particle surface. On the other hand, in the particle treated with fatty acids, a layered contrast with a thickness of about 2 nm is observed on the particle surface. This contrast is due to the presence of fatty acids.
Performing this homogeneous surface treatment makes it possible to achieve stable filler characteristics.
Figure 2. Transmission electron microscope (TEM) images of particle surfaces of an untreated particle and a particle treated with fatty acids.