JP7085325B2 - Aragonite-type light calcium carbonate and its manufacturing method - Google Patents

Description

The present invention relates to light calcium carbonate having a high BET specific surface area and a high dispersibility and a high aragonite content.

Light calcium carbonate produced by reacting slaked lime with carbon dioxide can control characteristics such as crystal system, particle size, specific surface area and high dispersibility by controlling the production conditions, depending on the purpose and application. Various light calcium carbonates are provided. As for the crystal structure, trigonal calcite is the most stable, but orthorhombic aragonite has a higher refractive index and superior strength than calcite, and is therefore applied to various applications.

As a method for producing aragonite-type calcium carbonate, for example, in Patent Document 1, a calcium carbonate seed having a high content of aragonite is produced in the presence of strontium hydroxide, and a carbonation reaction is carried out using this seed to carry out an aragonite-type calcium carbonate. Methods for producing calcium have been proposed.

Special Table 2016-528156 Gazette

In recent years, strontium carbonate, which can control optical characteristics such as birefringence, has been attracting attention as an inorganic filler for optical films such as retardation films. Since aragonite-type calcium carbonate has a large birefringence (nδ) and similar optical properties to strontium carbonate, it may be a substitute for strontium carbonate.

When aragonite-type calcium carbonate is used for optical applications, high dispersibility is required from the viewpoint of ensuring transparency, but in the conventional production method, a crystalline system is used in the process of dispersing the aragonite-type calcium carbonate produced by the carbonation reaction. Is changed, calcium carbonate is produced, and there is a problem that the aragonite ratio in the final product calcium carbonate is lowered. It is considered that this is because a part of aragonite-type calcium carbonate dissolves and recrystallizes in the dispersion process and changes to calcite.

On the other hand, important properties required for various uses of light calcium carbonate are BET specific surface area and pore volume. For example, when used as a filler in an inkjet material, pore volume is an important property that determines ink absorbency. However, the aragonite-type calcium carbonate obtained by the conventional production method does not always satisfy these characteristics. For example, the BET specific surface area of aragonite-type calcium carbonate disclosed in Patent Document 1 is 30 m ² / g or less.
The present invention solves the above-mentioned problems, and an object of the present invention is to provide aragonite-type calcium carbonate having high dispersibility and excellent various properties.

The aragonite-type calcium carbonate of the present invention has an aragonite ratio of 95% or more, a pore volume of 0.15 cm ³ / or more, and a specific surface area of more than 30 m ² / g. The maximum particle size (d ₁₀₀ ) of the calcium carbonate particles is 1.1 μm or less.

In the present invention, the pore volume is a value measured by the gas adsorption method, and the details of the measuring method will be described later. The particle size is a value obtained from the particle size distribution meter of the laser analysis method.

The method for producing aragonite-type calcium carbonate of the present invention is a method for producing calcium carbonate by blowing a carbon dioxide-containing gas into a slaked lime cloudy liquid and performing a carbonation reaction, in which strontium ions and organic acids coexist in the reaction liquid. It is characterized by reacting. When the strontium compound is strontium hydroxide octahydrate (Sr (OH) _{28H 2 O} ), the ratio of the strontium compound to the organic acid is preferably 35: 1 to 55: 1 by weight.

It is known that an organic acid is used for the carbonation reaction as a conventional method, but the calcium carbonate produced when the organic acid is used is a calcite type. On the other hand, in the present invention, by using an alkaline earth metal compound and an organic acid in a predetermined ratio, the formation of calcite is suppressed and calcium carbonate in which aragonite is dominant can be obtained.
The present invention also provides a material containing such aragonite-type calcium carbonate. Specifically, as a filler or a coating pigment or a carrier, it is a paper material containing the above-mentioned aragonite-type calcium carbonate, an ink, a paint, a plastic, an adhesive, a food, or a pharmaceutical product. Further, it is a film containing the above-mentioned aragonite type calcium carbonate, particularly an optical film.

In the figure which shows the TEM photograph of calcium carbonate of Example 1, (a) shows before dispersion, (b) shows after dispersion. (A) is a diagram showing the XRD of calcium carbonate after dispersion of Example 1, and (b) is a diagram showing XRD of calcium carbonate after dispersion of Comparative Example 1. In the figure which shows the particle size distribution before and after the dispersion of calcium carbonate of Example 1, the solid line shows after dispersion and the dotted line shows before dispersion.

Hereinafter, an embodiment of the method for producing aragonite-type calcium carbonate of the present invention and calcium carbonate obtained by the embodiment will be described.

<Manufacturing method of calcium carbonate containing high aragonite>
First, a method for producing calcium carbonate will be described. Slaked lime milk is used as a raw material for synthesizing calcium carbonate. Slaked lime milk can be prepared, for example, by suspending slaked lime powder in water, but is not limited thereto. The concentration of heavy calcium carbonate in slaked lime milk is 180 to 250 g / l, preferably 200 to 220 g / l. If the concentration is too high, the contact between the particles is promoted and the particle size increases. On the other hand, if the concentration is too low, aragonite will not be produced.

Before or after the start of the reaction, the strontium compound and the organic acid are added, and the reaction is carried out in the presence of strontium ions and in the presence of the organic acid. The strontium compound may be any compound that releases strontium ions in water, and a carbonate or hydroxide thereof can be used.

By coexisting with strontium ions, the formation of calcite can be suppressed and the aragonite ratio can be increased. The amount of the strontium compound added is 0.005 mol or more, preferably 0.015 mol or more, and more preferably 0.03 mol or more with respect to 1 mol of calcium hydroxide in the raw material. When strontium hydroxide octahydrate is used, it is preferably 5% by weight or more, preferably 10% by weight or more, and preferably 20% by weight or less of calcium hydroxide.

On the other hand, as the organic acid, an organic acid having two or more carboxyl groups or hydroxyl groups, such as a dicarboxylic acid or a hydroxycarboxylic acid, is preferable. Specifically, citric acid, tartaric acid, phthalic acid and the like can be used, and citric acid is particularly preferable.

The amount of the organic acid added is preferably 0.1% by weight or more, more preferably 0.2% by weight or more and less than 0.4% by weight of calcium hydroxide in the raw material. The ratio of the strontium compound to the organic acid is preferably 35: 1 to 55: 1 by weight. By setting the ratio of the organic acid to the strontium compound to 35: 1, the formation of calcite can be effectively suppressed. Further, by setting the same ratio to 55: 1, aragonite having excellent dispersibility and other properties can be produced.

The reaction may be started after mixing all the materials at once, or may be carried out while adding the materials in multiple steps, and the carbon dioxide-containing gas is blown into the reaction system at a predetermined gas blowing rate. However, the process is carried out under predetermined stirring conditions. The carbon dioxide-containing gas may be any gas containing 50% or more of carbon dioxide, and may contain an inert gas such as oxygen or nitrogen. The gas blowing rate is set to a low blowing amount of about 0.05 to 0.2 liter / min at the initial stage of the reaction. By controlling the amount of carbon dioxide gas supplied to a small amount in the initial stage of the reaction, it is possible to promote the formation of aragonite seed crystals. After sufficient seed crystals of aragonite are formed, the gas blowing rate can be increased (for example, to about 0.3 liter / min), whereby the reaction is promoted. In addition, it is possible to suppress the formation of crystals having a large particle size. The initial reaction stage limited to a low blowing amount varies depending on the charged amount of the material and the temperature, but is preferably from the start of the reaction to about 60 to 100 minutes.

The reaction temperature is not particularly limited, but is higher than room temperature, for example, 30 to 50 ° C, preferably 35 to 45 ° C. The end of the reaction is monitored by pH, and the reaction is terminated when the pH reaches about 7.

The reaction solution is dehydrated by a filter press or the like and, if necessary, dried to obtain aragonite-type calcium carbonate, which is a primary product. In this primary product, the primary particles of calcium carbonate are aggregated, and the content of aragonite-type calcium carbonate is 95% by weight or more. The primary particles constituting the aggregate are acicular particles having a minor axis of about 25 to 64 nm (average 50 nm) and a major axis of about 210 to 550 nm (average 330 nm). The minor axis and the major axis of the particles are values obtained from a transmission electron microscope (TEM) photograph (hereinafter, the same applies). The BET specific surface area (measured according to JIS Z8830: 2013) as the primary product is 30 m ² / g or more, and the pore volume (by the gas adsorption method) is 0.15 cm ³ / g or more.
In the present invention, the pore volume is a value measured by the following measuring method.
Weigh 0.2 g of sample and pretreat at 200 ° C. for 2 hours. Then, it is measured at −78 ° C. by a nitrogen gas adsorption method (equipment used: “BELSORP-mini” manufactured by Microtrac Bell).

<Manufacturing method of highly dispersed calcium carbonate>
Next, the dispersion treatment for obtaining highly dispersed calcium carbonate will be described. For the dispersion treatment, known methods such as stirring, a method using a shearing disperser, and wet pulverization using beads can be appropriately combined and used. Further, in the dispersion treatment, a known dispersant can be used. Hereinafter, an example of distributed processing will be described.

The aragonite-type calcium carbonate (before drying) obtained by the above-mentioned production method is dehydrated to a predetermined solid content, a polycarboxylate-based dispersant is added, and the mixture is first dispersed in an industrial mixer. As the polycarboxylate, a dispersant that is often used as a dispersant for calcium carbonate, such as sodium polycarboxylate, can be used.

Next, the slurry after the primary dispersion is passed through a horizontal bead mill filled with beads in a cylinder at a predetermined flow rate. This is repeated multiple times to complete the dispersion. The degree of dispersion can be confirmed by, for example, a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

The dispersed calcium carbonate thus obtained is a highly dispersed calcium carbonate having a high aragonite ratio of 95% by weight and almost no aggregation of aragonite particles even though it has undergone the dispersion step. It is considered that this is because the presence of strontium suppresses the formation of calcite when a part of the aragonite-type calcium carbonate dissolved in the dispersed slurry is recrystallized. Each of the dispersed particles is a needle-like particle having a minor axis of about 20 to 55 nm (average 32.5 nm) and a major axis of about 30 to 500 nm (average 165 nm), and is a fine particle containing no particles of 1 μm or more. Further, by such a dispersion treatment, the BET specific surface area becomes higher than that before the dispersion, and can be, for example, 40 m ² / g or more.

Next, the aragonite-type calcium carbonate of the present invention will be described.
The aragonite-type calcium carbonate of the present invention is a light calcium carbonate that can be typically produced by the above-mentioned method for producing calcium carbonate, has an aragonite content of 95% or more, and has a pore volume of 0. .15 cm ³ / g or more and a novel aragonite-type calcium carbonate having a BET specific surface area of 32 m ² / g or more. The light calcium carbonate of the present invention is basically composed of needle-shaped aragonite particles, and the needle-shaped particles are nanoparticles having a minor axis of 65 nm or less and substantially free of particles of 1 μm or more. Such aragonite-type calcium carbonate has high transparency, and even if it is added in an amount of 30% by weight in the resin, the light transmittance and hue of the resin are not changed.

It is considered that at least a part of the light calcium carbonate of the present invention is in the form of a complex salt with strontium carbonate, the change in the aragonite ratio is small before and after the dispersion, the high aragonite ratio is maintained, and the BET ratio after the dispersion. The surface area is 40 m ² / g or more.

The light calcium carbonate of the present invention having such characteristics can be used as a filler for paper materials, inks, paints, etc., which are general uses for calcium carbonate, as additives such as plastics and adhesives, or as foods, pharmaceuticals, etc. It can be used as a carrier.

For example, in inkjet recording paper, porous fine particles are used to fix the ink on the paper. A typical material thereof is silica, which has a pore volume of 0.3 cm ³ / l or more and an oil absorption of 100 to 400 ml / 100 g. The light calcium carbonate of the present invention has a pore volume close to that of silica and an oil absorption amount as compared with the pore volume and oil absorption amount of calcite-type calcium carbonate, and is therefore effective as a partial substitute for silica.

The refractive index of aragonite-type calcium carbonate (α = 1.530, β = 1.681, γ = 1.685, δ = -0.155) and the refractive index of strontium carbonate (α = 1.520, β = 1). 667, γ = 1.669, δ = -0.149), and the calcium carbonate of the present invention has a nano-sized particle size and high transparency, so it is used in place of strontium carbonate used in optical films. be able to. In particular, since the birefringence index δ (calculated from α, β and γ) has a negative value, a retardation film combined with a polymer having a positive birefringence index such as polyphenylene ether, polycarbonate, polyethylene terephthalate and the like can be used. The application is valid.

Hereinafter, examples of the method for producing calcium carbonate of the present invention will be described. In the following description, "%" means "% by weight" unless otherwise specified.

[Manufacturing]
<Example 1>
Water was added to 88 g of commercially available slaked lime (JIS special item slaked lime) to prepare 400 ml of slaked lime slurry (milk concentration: 220 g / l). This slurry is placed in a stainless steel container and heated to 40 ° C. to add 8.8 g of strontium hydroxide octahydrate (10% with respect to slaked lime) and 0.16 g of citric acid (0.2% with respect to slaked lime). Added. After stirring with a stirrer for 3 minutes, stirring was continued while blowing carbon dioxide gas at a flow rate of 0.10 l / min. After 90 minutes, the gas flow rate was changed to 0.3 l / min, stirring was continued, and the blowing of carbon dioxide gas was stopped when the pH reached about 7.

Next, the primary product slurry obtained by the above production method was dehydrated using a filter press and a belt press to a solid content of 55%, and then a polycarboxylic acid-based dispersant (manufactured by San Nopco) was added to calcium carbonate having a solid content of 55%. : OT504) was added in an amount of 2% by weight and dispersed in an industrial mixer for 3 minutes to obtain a primary dispersion slurry of calcium carbonate. Next, 2 L of the primary dispersion slurry was continuously dispersed 6 times at a flow rate of 180 ml / min in a horizontal disperser (Dynomill) using a polyurethane container containing 1.1 L of ceramic beads. The stirring speed (peripheral speed) during the dispersion processing using the horizontal disperser was 5 m / sec, and the rotation speed (rpm) was 1200. The outlet temperature was 45 to 60 ° C.

<Examples 2 and 3 and Comparative Examples 1 to 4>
Aragonite-type calcium carbonate (primary product) was produced in the same manner as in Example 1 except that the amounts of strontium hydroxide octahydrate and citric acid added to the reaction system were changed as shown in Table 1. Further, the primary product was dispersed by the same method as in Example 1 to obtain a post-dispersion product.

[Confirmation of aragonite generation]
The reaction slurry, which is the primary product, was observed with a TEM (transmission electron microscope: JEM-1400 manufactured by JEOL Ltd.), and the formation of aragonite-type calcium carbonate was confirmed. The TEM of calcium carbonate (before dispersion) of Example 1 is shown in FIG. 1 (a).

Further, the powder obtained by drying the primary product slurry was analyzed by XRD (X-ray diffractometer: "MultiFrey" manufactured by Rigaku), and the peak of aragonite and the peak of calcite were confirmed. FIG. 2A shows the result of XRD of the primary product of Example 1, and FIG. 2B shows the result of XRD of the primary product of Comparative Example 1. From the integrated value of the peak of this XRD, the aragonite ratio was calculated by the following equation.
[Aragonite rate] = 100 × [Aragonite first peak intensity] × 3 / ([Aragonite first peak intensity] × 3 + [Calcite first peak intensity])
In the above equation, the coefficient 3 multiplied by [the first peak intensity of aragonite] is a coefficient for normalizing the intensities of the peaks of aragonite and the peak of calcite, and is a value obtained from a calibration curve prepared in advance. Is.

As shown in the figure, the peak of calcite observed in Comparative Example 1 was not observed in Example 1, and it was confirmed that almost aragonite-type calcium carbonate was produced. The peak near 26 degrees derived from aragonite-type calcium carbonate had a wider integration range in Example 1 than the peak in Comparative Example 1. This is about the same width as the peak width of strontium carbonate, and it is presumed that a salt close to the complex salt of strontium carbonate and calcium carbonate is produced.
In Comparative Example 4 in which the amount of citric acid was large (0.4%), the aragonite ratio was low (about 10%), and it was confirmed from the SEM (scanning electron microscope) photograph that almost spindle-shaped calcite was formed. .. From this, it was confirmed that even if the strontium compound was added, the formation of aragonite was hindered and the formation of calcite became predominant when the amount of citric acid was large. This is also consistent with the fact that calcite is generally produced when organic acids are added.

The slurry after dispersion was also observed by TEM in the same manner as the primary product, the formation of aragonite-type calcium carbonate was confirmed, and the aragonite ratio was calculated from the peak of XRD. The TEM of calcium carbonate (after dispersion) of Example 1 is shown in FIG. 1 (b). As shown in the figure, it was confirmed that the aggregated particles were highly dispersed by the above dispersion step.

[Measurement of physical properties of primary products]
The primary products of aragonite-type calcium carbonate produced in Examples 1 to 3 and Comparative Examples 1 to 3 were dehydrated and then dried at 110 ° C. to obtain calcium carbonate particles. The particle size (minor diameter, major diameter), BET specific surface area and pore volume of the dried calcium carbonate particles were measured by the following methods. The aspect ratio was calculated from the minor axis and the major axis.

<Particle diameter>
The minor axis and the major axis were measured by TEM, and the average value was calculated. Further, the particle size distribution was obtained with a particle size distribution meter (manufactured by HORIBA, Ltd .: LA-950), and the median diameter (d ₅₀ ) of the particles was measured. The measurement result of the particle size distribution of calcium carbonate of Example 1 is shown in FIG.

<BET specific surface area and pore volume>
The specific surface area and pore volume were measured by a gas adsorption method according to JIS Z8830: 2013 using a specific surface area / pore analysis measuring device (manufactured by Microtrac Bell: BELSORP-mini). The measurement results of the physical properties are shown in Table 2-1.

From the particle size distribution of FIG. 3, it can be seen that the calcium carbonate of Example 1 is a highly dispersed aragonite-type calcium carbonate having a sharp particle size distribution including large aggregated particles by the dispersion treatment.

Further, as shown in Table 2, the calcium carbonates obtained in Examples 1 to 3 are all calcium carbonates having a high aragonite ratio of 95% or more, and acicular crystals having a small particle size and a high aspect ratio. was gotten. It was also confirmed that the BET specific surface area exceeded 30 m ² / g and the pore volume was large. The pore volume of Example 3 was not measured.

On the other hand, the calcium carbonate produced in Comparative Example 1 to which no additive was added had a large particle size, a low aragonite ratio, and a small specific surface area and pore volume. In Comparative Example 2 in which only the strontium compound was added and no organic acid was added, the aragonite ratio was relatively high, but the particle size was larger than that of the calcium carbonate of the example, and the specific surface area and the pore volume were smaller than those of the example. ..
Further, in Comparative Example 3 in which the amount of the organic acid added to the strontium compound was larger than that in the example, calcium carbonate having a small particle size and a large specific surface area was obtained, but the aragonite ratio was low. Further, the ratio of the amount of the organic acid added to the strontium compound is the same as that of Comparative Example 3, but in Comparative Example 4 in which the amount of the organic acid added to calcium carbonate is large, almost no aragonite is produced and spindle-shaped calcite is generated. , Specific surface area and pore volume were both low.

[Measurement of physical properties of post-dispersion product]
The aragonite ratio, particle size (minor and major diameters) and BET specific surface area of the dispersed calcium carbonate slurries of Example 1 and Comparative Example 1 were measured in the same manner as in the primary product, respectively. The results are also shown in Table 2-2.

As can be seen from the results in Table 2-2, the aragonite ratio of Comparative Example 1 was 82% before dispersion, whereas it decreased to 76% after dispersion, but the calcium carbonate of Example 1 was obtained. The aragonite rate was 98% before and after the dispersion, and it was confirmed that the aragonite rate did not change even with the dispersion. Further, the dispersed calcium carbonate of Example 1 had a smaller particle size and a significantly improved specific surface area.

[Measurement of transparency]
In order to evaluate the transparency, the whiteness and Yi value of the post-dispersion product of Example 1 and the commercially available calcium carbonate (Comparative Example 5) (calcite-type calcium carbonate manufactured by Okutama Kogyo: TP-121) were determined by the following methods. Measured at.

Prepare a sample by adding the dispersed calcium carbonate slurry (solid content 35%) to styrene butadiene rubber (manufactured by JSR: 0692, solid content 50%) at a mixing ratio of 0.3%, 5% and 30%, respectively. Then, the sample was coated on the coated surface of the hiding ratio test paper with a # 04 bar. The one coated only with SBR was used as a reference example (Blank). The ISO whiteness of the test paper after the sample coating was measured using "CMS-35SPX" manufactured by Murakami Color Technology Laboratory. The results are shown in the table.

As can be seen from the results in Table 3, the calcite-type calcium carbonate (Comparative Example 5) showed a whiteness of 0.2 and a decrease in transparency at a blending ratio of 30%, but the dispersal calcium carbonate of Example 1 was the same. It was confirmed that the whiteness was 0 and the Yi value was 0 at a blending ratio of 30%, which was similar to that of the Blank sample and had high transparency.
As is clear from the above examples, according to the present invention, there is provided an aragonite-type calcium carbonate having a high specific surface area and a high pore volume, which is highly dispersible and whose aragonite ratio does not change even after dispersion.

Claims (12)

Light calcium carbonate produced by the reaction of slaked lime and carbon dioxide, containing strontium, aragonite content of 95% or more, pore volume by gas adsorption method of 0.15 cm ³ / g or more, BET Aragonite-type light calcium carbonate characterized by a specific surface area of more than 30 m ² / g. The aragonite-type light calcium carbonate according to claim 1, wherein 0.005 mol or more of strontium is contained with respect to 1 mol of calcium. The aragonite-type light calcium carbonate according to claim 1 or 2.
Aragonite-type light calcium carbonate composed of acicular particles having a minor axis of 60 nm or less and substantially free of particles having a particle diameter of 1 μm or more. The aragonite-type light calcium carbonate according to any one of claims 1 to 3.
Aragonite-type light calcium carbonate characterized by being substantially non-aggregated. A method for producing light calcium carbonate by blowing carbon dioxide-containing gas into a slaked lime slurry and reacting the slaked lime with carbon dioxide. The ratio of the organic acid to calcium hydroxide in the slurry is 0.1% by weight or more and less than 0.4% by weight, and the ratio (weight ratio) of the strontium compound to the organic acid is 33: 1 to 100: 1. A method for producing slaked slaked calcium hydroxide, which comprises adding the above-mentioned substance to allow the reaction to proceed. The method for producing light calcium carbonate according to claim 5.
A method for producing aragonite-type light calcium carbonate, which comprises adding 10 to 20% by weight of the strontium compound to calcium hydroxide in the slaked lime slurry. The method for producing light calcium carbonate according to claim 5 or 6.
A method for producing an aragonite-type light calcium carbonate, wherein the organic acid is one or more selected from citric acid, tartaric acid, and phthalic acid. The method for producing light calcium carbonate according to any one of claims 5 to 7.
The reaction step includes two reaction steps in which the amount of carbon dioxide-containing gas blown is different, and the amount of gas blown in the first step is smaller than the amount of gas blown in the subsequent steps, and is characterized by being 0.2 l / min or less. A method for producing aragonite-type light calcium carbonate. The method for producing light calcium carbonate according to claim 8.
A method for producing aragonite-type light calcium carbonate, which comprises adding a dispersant and dispersing in a bead mill after the reaction is completed. The paper material containing aragonite-type calcium carbonate according to any one of claims 1 to 4. The ink or paint containing aragonite-type calcium carbonate according to any one of claims 1 to 4. The film containing aragonite-type calcium carbonate according to any one of claims 1 to 4.

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