JPH05163018A - Production of calcium carbonate - Google Patents

Abstract

PURPOSE:To provide a method for producing calcium carbonate slightly affected by the change in the conditions for the production of the calcium carbonate, reduced in the change in the shape of the crystal and having excellent water dispersibility. CONSTITUTION:The exhaust gas of a lime kiln 1 of a pulp-producing plant is cooled with a wet type scrubber, carried to the shell side of a shell-and-tube type heat exchanger combined with the wet type scrubber, and subsequently cooled and subjected to the removal of dust. The treated exhaust gas is charged into a carbonization reaction tank 6 through a blower 5 to perform a carbonization reaction therein while forming fine gas bubbles. The carbonization reaction tank 6 contains a calcium hydroxide aqueous suspension having a concentration of 5-15wt.% therein and has a gas reservoir mounted at its lower part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing calcium carbonate, more specifically, fluctuations in concentration of aqueous calcium hydroxide suspension, temperature at the start of reaction, amount of supplied carbon dioxide, concentration of carbon dioxide, etc. The present invention relates to a method for producing calcium carbonate which is less affected by the above and has excellent water dispersibility.

[0002]

2. Description of the Related Art A typical industrial production method of calcium carbonate is to blow carbon dioxide gas into a suspension of calcium hydroxide having a concentration of about 5 to 20% by weight from a fixed jet such as a nozzle or a straight pipe. It is a method of carrying out a carbonation reaction.

In the method for producing calcium carbonate, when carbon dioxide gas is continuously blown into the calcium hydroxide aqueous suspension,
The viscosity of the suspension increases, and on the contrary, as the carbonation progresses, the viscosity decreases, and eventually becomes lower than that of the original suspension. Further, carbonation is advanced, and when the pH of the suspension reaches 7.0 to 8.0, the end point of carbonation is set, and then the produced calcium carbonate is filtered, dried and crushed.

That is, in the above-mentioned method, the average particle diameter (long side length) 1 to 1 by observation with an electron microscope is controlled by appropriately adjusting the concentration, temperature, carbon dioxide gas injection amount, etc. of the suspension of calcium hydroxide. Spindle particles of about 5 μm, light calcium carbonate consisting of columnar particles of calcite crystals or aragonite crystals, or average particle diameter (length of one side of cube) 0.03 to 0.
Colloidal calcium carbonate consisting of calcite crystals of cubic particles of about 3 μm is produced.

[0005]

However, in the above method, when the concentration, temperature, amount of carbon dioxide gas blown, and carbon dioxide concentration of the calcium hydroxide suspension are changed, the crystal form of calcium carbonate produced and the particle size thereof are changed. The variation is enormous.

Further, the reaction time is determined by the blowing amount of carbon dioxide gas or the specifications of the apparatus, but the conventional method has a drawback that the production efficiency is low because the reaction time is usually long at 5 to 24 hours.

Further, even if the amount of carbon dioxide gas is increased for the purpose of shortening the reaction time, the dispersibility of bubbles of carbon dioxide gas is poor, the particle size becomes large, and the effective contact area with the aqueous calcium hydroxide suspension is reduced. In addition, the bubbles are fused while rising, and the particle size further increases and floats rapidly on the liquid surface, which shortens the residence time in the calcium hydroxide aqueous suspension, etc. The proportion of carbon dioxide gas that is involved in the carbonation reaction (hereinafter referred to as the carbon dioxide gas utilization rate) decreases, so the facility capacity of the carbon dioxide gas blower needs to be increased more than necessary, and its power consumption also increases. However, there is a drawback in that the manufacturing cost increases because of the increase.

Exhaust gas from a lime firing kiln of a pulp mill, for example, an alkali pulp or kraft pulp manufacturing plant as a carbon dioxide gas source (hereinafter referred to as lime kiln exhaust gas).
When using, the lime kiln exhaust gas is normally heat-recovered in the dehydration process of the circulating lime mud, but due to the leaking air at that time, the carbon dioxide concentration in the subsequent process is 10 to 20% by volume.
May go down to.

Conventionally, in the production of calcium carbonate, the reaction is carried out at a carbon dioxide gas concentration of 25 to 35% by volume in the lime kiln exhaust gas, but when the concentration becomes less than 25% by volume, the amount of gas is reduced by the conventional method. Blowing carbon dioxide in the same way,
The reaction time is extended and the production capacity is reduced. Further, if a large amount of carbon dioxide gas is blown, there is a drawback that the carbon dioxide gas utilization rate decreases as described above.

Exhaust gas from the lime kiln of the pulp manufacturing plant contains a small amount of sulfur oxides, unburned carbon or lime dust, etc. If these dusts are mixed in during the carbonation reaction, calcium carbonate is produced. However, there is a problem in that the whiteness is reduced and calcium carbonate grows by using the dust as a nucleus, which causes variation in particle size and non-uniform calcium carbonate is obtained.

An object of the present invention is to improve the above-mentioned conventional drawbacks, and particularly to provide a method for producing calcium carbonate which effectively utilizes exhaust gas from a lime kiln.

[0012]

A first aspect of the present invention is a method for producing calcium carbonate by reacting an aqueous suspension of calcium hydroxide with carbon dioxide, wherein a large number of openings are formed in the lower part of the tank. A reaction vessel equipped with a gas reservoir, which is loaded with a calcium hydroxide aqueous suspension having a concentration of 5 to 15% by weight, supplies fine bubbles of lime kiln exhaust gas from the gas reservoir to cause a carbonation reaction. Of the manufacturing method.

The second invention uses the lime kiln exhaust gas from a pulp manufacturing plant as the carbon dioxide gas source in the production of calcium carbonate according to claim 1, and the dust content in the lime kiln exhaust gas is 0.030. This is a method for producing calcium carbonate using lime kiln exhaust gas, which is said to be g / Nm ³ dry or less.

Further, a third invention is a combination of a lime kiln exhaust gas with a wet scrubber and a shell-and-tube heat exchanger, and the exhaust gas is shell-and-tube.
It is a manufacturing method of calcium carbonate which uses what was processed by passing through the shell side of a tube type heat exchanger.

[0015]

The present invention has the above-mentioned constitution, and the concentration of the aqueous calcium hydroxide suspension used in the reaction is 5 to 15% by weight. If it exceeds 15% by weight, the viscosity of the suspension becomes high, so that the fine bubbles of the lime kiln exhaust gas are not sufficiently dispersed, and large bubbles are formed and float up, so that the utilization rate of carbon dioxide gas decreases.

If it is less than 5% by weight, the carbon dioxide gas is well dispersed and the carbon dioxide gas utilization rate is good, but the production capacity is lowered,
Many of the calcium carbonate crystals formed are coarse, and are mixed with fine crystals to form an inhomogeneous mixture, which is not preferable.

Further, the lime kiln exhaust gas containing carbon dioxide which reacts with the calcium hydroxide is supplied as fine bubbles. As means for forming fine bubbles of the lime kiln exhaust gas, a gas reservoir having a large number of small holes may be attached to the lower part of the carbonation reaction tank, and the lime kiln exhaust gas may be bubbled through the large number of small holes.

The shape of this gas reservoir is not particularly limited, but it is desirable to make the central portion of the reaction tank high and incline downward toward the inner peripheral surface of the reaction tank. Further, the shape at this time may be any of a conical shape, a polygonal pyramid shape, a columnar shape, a semi-spherical shape, and the like. Further, the shape of a large number of openings may be any of circle, square, ellipse, rectangle, etc., but a circular shape without corners where the adherence of calcium carbonate is easy to fall off is preferable, and the opening ratio of the holes is 2 to 15%. It is preferably 3 to 13%.

By bubbling the fine bubbles of the exhaust gas as described above, the fine bubbles can be brought into sufficient contact with the calcium hydroxide aqueous suspension and the floating time to the liquid surface is prolonged, so that the utilization rate of carbon dioxide gas is improved. Can be achieved.

Further, the lime kiln exhaust gas from the pulp manufacturing plant as a carbon dioxide gas source has a high temperature and contains a dust content, so that cooling and dust removal are required during the carbonation reaction.

In the present invention, as means for cooling and removing dust, at least one means is a gas-liquid contact type scrubber. With this gas-liquid contact type scrubber, the high temperature gas of about 200 ℃ is primarily cooled to about 60 ℃,
In addition, the dust contained in the exhaust gas is removed.

Next, the exhaust gas is cooled from about 60 ° C. to about 40 ° C. by a shell-and-tube heat exchanger (hereinafter referred to as shell heat exchanger). However, the cooling temperature at this time is a factor of the quality design of calcium carbonate, and is appropriately changed according to the quality design.

In the present invention, the exhaust gas is conducted to the shell side of the shell type heat exchanger, while the cooling medium is conducted to a large number of tubes mounted inside the heat exchanger.

In this case, since the exhaust gas is treated by the gas-liquid contact type scrubber, the relative humidity is almost 100%, and the cooling medium comes into contact with many outer surfaces of the tubes which are conducted. Condensation and dew condensation occurs, and the outer surface of the tube becomes wet due to the progress of this dew condensation, and eventually drops and drops.

At this time, the dust component in the exhaust gas of the lime kiln which is not treated by the scrubber is efficiently captured and removed by repeating contact with the moisture condensed on the outer surface of the dense tube.

Condensation of water on the outer surface of the tube, formation of drops,
The dropping behavior is continuously performed, so that dust does not adhere to the outer surface of the tube, the outer surface of the tube is always kept clean, and an excellent cleaning action can be exhibited.

Furthermore, since the outer surfaces of many tubes through which the cooling medium is conducted are kept clean without dust sticking and are always kept in a wet state, the overall heat transfer coefficient is 400 Kcal / Since it can be maintained at around m ² · hr · ° C, the heat exchange rate is extremely high and continuous operation is possible.

In the present invention, the dust content is removed by the above-mentioned method. However, when the dust content in the exhaust gas exceeds 0.030 g / Nm ³ dry, the amorphous dust is converted to calcium carbonate during the carbonation reaction. It becomes a nucleus during generation, the particle size of calcium carbonate is large, and the dispersion is large, so the specific scattering coefficient is small, the whiteness of calcium carbonate is low, and it is used as a coating pigment or filler during the production of paper. In that case, the whiteness is lowered and exceeds the preferable range. When the dust in the exhaust gas is 0.030 g / Nm ³ dry or less, suitable calcium carbonate can be obtained.

According to the present invention, the secondary particles of the produced calcium carbonate have a weak agglomeration state, and are uniform and relatively fine. Therefore, the suspension is used as a papermaking filler as a suspension without undergoing the steps of drying and crushing. Even when used, a paper having high whiteness and opacity is obtained.

In some cases, the suspension may be wet-milled before use, or if the concentration of calcium carbonate is dehydrated to about 60% by weight, concentrated, and then blended into a pigment for coated paper, it becomes smooth. A matte coated paper with good properties can be obtained.

On the other hand, the carbon dioxide gas in the exhaust gas of the lime kiln is usually 25 to 35% by volume, but if the fine bubbles of the exhaust gas are reacted as in the present invention, a carbon dioxide gas having a low carbon dioxide concentration, for example, Concentration after flash dryer process 10 ~
Exhaust gas of about 25% by volume can be used, and the reaction efficiency at that time does not decrease so much, so that the lime kiln exhaust gas can be effectively used and the utilization rate of carbon dioxide gas can be improved.

A stable quality can be obtained even if the carbon dioxide concentration is less than 10% by volume, but the reaction time is prolonged and the power consumption of the blower blower is increased. Therefore, the carbon dioxide concentration is preferably 10% by volume or more.

Since the calcium carbonate obtained in the present invention has a uniform crystal form, it can be used in addition to paper fillers and pigments in paints,
It can also be used to manufacture ink, plastic, rubber, etc.
Further, the calcium carbonate obtained in the present invention can be used as it is in the papermaking step without undergoing the steps of drying and crushing.

Further, the calcium carbonate obtained in the present invention has a good dispersion state and relatively fine secondary particles in a weak agglomeration state, and when used as a papermaking filler, the yield in the wire part of a paper machine is increased. It has a good quality, and has excellent scattering ability in the internally added paper, which can contribute to increasing the whiteness of the paper.

By the way, according to the conventional method, when the amount and concentration of carbon dioxide gas blown, the concentration of calcium hydroxide suspension and the reaction start temperature are the same, the reaction time becomes long and the particle size of the produced calcium carbonate is It tends to be slightly rough, and when added internally to paper, the effect of increasing whiteness and opacity is small.

[0036]

EXAMPLES The present invention will be described in more detail below with reference to the drawings and experimental examples and examples, but the present invention is not limited thereto.

FIG. 1 is a flow chart of the carbonation reaction using the lime kiln exhaust gas. The exhaust gas of the lime kiln 1 is sent to the wet scrubber 3 by the blower 2, and this exhaust gas is further combined with the wet scrubber 3 in the shell. The exhaust gas is sent to the type heat exchanger 4, and the exhaust gas is supplied to the carbonation reaction tank 6 by the blower 5.

As shown in FIG. 2, the shell type heat exchanger 4 of the present invention is a quadrangular prism type, and has a large number of small diameter tubes 8 connected in parallel in the shell 7 inside the shell 7.
Is attached. As an example, the cross-sectional area of the shell is about
2.2m ² , the tube diameter in this case is about 15.9m
m, the number of tubes is 3120.

An exhaust gas inlet 9 is formed on the lower side of the shell 7, and an exhaust gas outlet 10 is formed on the top of the shell 7, so that the exhaust gas is introduced to the shell 7 side of the shell type heat exchanger 4. And small tube 8
The cooling water is introduced from the cooling water inlet 11, is sequentially sent to the many tubes 8 connected and arranged, and is discharged from the cooling water outlet 12. Reference numeral 13 in FIG. 2 is a shower for cleaning the outer surface of the tube 8 when the shell heat exchanger 4 is stopped.

As shown in FIG. 1, the exhaust gas of the wet scrubber 3 is supplied into the shell 7 through the exhaust gas inlet 9 of the shell type heat exchanger 4 associated therewith, and the tube 8 of the shell type heat exchanger 4 is supplied. It is cooled by the cooling water supplied inside. Since the exhaust gas supplied from the wet scrubber 3 has a relative humidity of 100%, the moisture in the exhaust gas comes into contact with the tube 8 of the shell heat exchanger 4 to cause dew condensation, which drops as a drop. Therefore, the dust contained in the exhaust gas is captured and removed by the condensed water, and the dust is remarkably reduced.
And sent to the carbonation reaction tank 6.

Since the exhaust gas is made even lower by removing dust in the shell heat exchanger 4 of the present invention, the blower 5 for sending the exhaust gas to the carbonation reaction tank 6 has a small volume of shaft power. The type roots blower can be used to save power.

FIG. 3 is a sectional view of the carbonation reaction tank of the present invention. A blow valve 14 is attached to the lower part of the carbonation reaction tank (hereinafter simply referred to as the reaction tank) 6, and the reaction tank 6 has the above-mentioned structure. A gas reservoir 15 is attached to the upper portion of the blow valve 14. The gas reservoir 15 has a conical shape having an apex at the center of the reaction tank 6, and a large number of holes 16 are formed in the gas reservoir 15 as shown in FIG. This hole 16
Has an aperture ratio of 5% and an apex angle of 90 degrees.

When the calcium hydroxide aqueous suspension is supplied to the reaction tank 6 from the suspension supply port 18 and the exhaust gas is supplied from the exhaust gas supply pipe 17, the exhaust gas is supplied to the gas reservoir 15 with a large number of holes 1.
As a result, fine bubbles form fine bubbles in 6 and slowly float up in the suspension, and while floating, they sufficiently contact with calcium hydroxide in the suspension to achieve a carbonation reaction. In this case, although illustration is omitted, a pipe for supplying fresh water is provided below the exhaust gas supply pipe 17, and fresh water is supplied before the exhaust gas is supplied to remove the hydroxide suspension from the gas reservoir 15. If it is located at the upper part, partial unreacted can be prevented.

After the carbonation reaction is completed, blow valve 1
Open 4 and remove the calcium carbonate suspension produced. At this time, the calcium carbonate adhering to the many holes 16 of the gas reservoir 15 is washed away by the reverse flow of the calcium carbonate suspension, so that the many holes 16 are hardly clogged.

On the other hand, as a method for controlling the carbonation reaction, means such as measuring the pH of the suspension during the reaction and adjusting the amount of carbon dioxide gas to be introduced is used. In this method, the suspension in the middle of the reaction in the reaction tank is introduced from the lower part of the reaction tank 6 into the conduit 20 and the liquid feed pump 2.
After being sent to the pH measuring instrument 22 equipped with a pH electrode via 1 to measure the pH, and then returning to the reaction tank 6 through the conduit 23,
PH of acidic waste water or fresh water via conduit 24 and liquid delivery pump 21
After being sent to the measuring machine 22 and washed, it is discharged from the discharge port 25 to the outside of the system.

The pH measurement of the reaction solution and the in-system cleaning with acidic waste water or fresh water can be automated by sequence control, and the interval thereof can be appropriately selected, but the time required for pH measurement is 2 to 3 times. It is desirable to wash the double with acid drainage or fresh water.

The acidic wastewater generated from the pulp mill includes chlorinated wastewater in the bleaching step, sulfuric acid-containing wastewater in the chlorine dioxide manufacturing step, etc., but from the bleaching step the calcium-free precipitate is not formed. The chlorinated waste water etc.

Since the pH measuring device is outside the reaction tank 6 and the pH of the suspension in the middle of the reaction is measured, it is immediately washed and the pH electrode is always kept clean. Therefore, maintenance frequency can be reduced.

Experimental Example A reaction vessel was charged with 600 liters of a calcium hydroxide aqueous suspension having a concentration of 9.0 to 14.5% by weight, and while diluting carbon dioxide gas from a carbon dioxide gas cylinder with air, the flow rate of the dilution gas was 1500 liters / minute ( It was blown at a rate of 15 ° C).

As a means for forming carbon dioxide fine bubbles,
Micro bubbles of carbon dioxide gas were bubbled through a small hole (round shape, diameter 20 mm; opening rate 5%) formed in a conical gas reservoir (vertical angle 90 °) in the lower part of the reaction tank. Table 1 shows the reaction conditions at this time and the results of measuring the paper quality of the internally added paper. Also, carbon dioxide concentration
At 20% by volume, the reaction time is 120 minutes (No. 2) and 160 minutes (No.
3) Similarly, when the carbon dioxide concentration is 10% by volume, the reaction time is 22
It was 5 minutes (No.1).

[0051]

[Table 1]

The measuring methods adopted in Tables 1 to 4 are as follows. (Amount of dust) Based on JIS Z 8808 "Dust concentration measuring method in exhaust gas". (Average particle size) Shimadzu centrifugal sedimentation type particle size distribution analyzer SA-CP
It was measured by type 2. The sample was 3 g in an aqueous solution in which 0.015 g (solid content) of Aron T-40 (manufactured by Toagosei Co., Ltd.) was dissolved.
(Solid content) Melted and dispersed for 5 minutes with an ultrasonic disperser.
The average particle shape is indicated by the median cumulative value (median diameter). (Crystal grain size; major axis) Using a scanning electron micrograph,
The typical diameter of spindle-shaped or needle-shaped crystals was measured.

(Powder Whiteness) US TAPPI Standard Method T-646 OS-
Compliant with 75. The whiteness of the calcium carbonate suspension is
After dehydrating with a Buchner filter using No. 2 filter paper (manufactured by Toyo Roshi Kaisha, Ltd.), it was dried at 40 ° C. for 24 hours. A household mixer was used for crushing and homogenizing the sample.

(Dry pick) Printing was repeated (about 3 times) until paper stripping occurred on the RI-1 type printing tester (manufactured by Ming Seisakusho), and after transferring the pattern on the rubber roll to a blank sheet,
Ink density measuring machine (ink density meter DM-273
Type, manufactured by Dainippon Screen Mfg. Co., Ltd.).

(Wet Pick) After water is applied to the sample with a sleeve winding roll (containing water), printing is repeated to transfer the pattern on the rubber roll onto a blank sheet, and then the ink density is measured. (Ink set) After printing, overlay a blank paper on the printing surface, transfer the ink to the blank paper after 30 seconds and 60 seconds with the press roll, and measure the ink density. (Lose water) Pipette water between metal roll and rubber roll.
After placing 2 ml and separating the rolls a little, transfer the pattern on the paper and the rubber roll to a white paper, and then measure the ink density.

(Glossiness of printing surface) Printing is performed by changing the ink amount, and the glossiness of the printing surface when the ink density is 1.40 is measured. (Smoothness, air permeability) Measured with an Oken type air permeability, smoothness tester (manufactured by Asahi Seiko Co., Ltd.).

(Specific Scattering Coefficient and Specific Absorption Coefficient of Filler) The specific scattering coefficient (s value) and the specific absorption coefficient (k value) of a sheet having a basis weight of 60 g / m ² and an ash content of 10% containing the filler are determined. On the other hand, the specific scattering coefficient (S _P value) and the specific absorption coefficient (K _P value) of a sheet of the same pulp with no filler internally added and a basis weight of 60 g / m ² are determined. It is assumed that the s-value and k-value of the internally added paper are additive between the S _P- value and K _P- value of the pulp and the specific scattering coefficient (S _A value) and specific absorption coefficient (K _A value) of the filler. Then, the following relational expression holds.

[0058]

[Equation 1]

[Equation 2]

The fillers S _A and K _A were obtained from this relational expression [see Paper and Paper Cooperative Journal Vol. 33, No. 4, (April 1979) pp. 34-45].

The particle size of the produced calcium carbonate (major axis of spindle-shaped crystals) is 2 to 5 μm, and the filler when the calcium carbonate is internally added to the paper as a suspension without the drying and crushing steps. Has a specific scattering coefficient of 2200 to 2600 cm ² / g, and the whiteness of a paper having a basis weight of 60 g / m ² containing 15% by weight of ash is higher than that of a paper having a basis weight of 60 g / m ² containing no calcium carbonate. The whiteness was 3-4 points, and the opacity was 9-10 points.

In general, the paper strength of the paper in which the filler is added has a tendency that the finer the particle size of the filler is, the more the interfiber bond of the pulp is inhibited, so that the tensile strength, the burst strength and the folding endurance are weakened. However, in the case of bubbling carbon dioxide gas in the form of fine bubbles as in the present invention, the generated calcium carbonate has a uniform shape and particle size, and fine particles are reduced, so that the paper strength of the internal paper tends to be slightly stronger.

Further, the light calcium carbonate (No. 3) obtained by bubbling carbon dioxide gas as fine bubbles has an average particle diameter of almost the same as that of commercially available light calcium carbonate (PCX-850 manufactured by Shiraishi Industry Co., Ltd.). Although equal, the paper strength of the present invention having a uniform shape and particle size is slightly higher.

Example 1 A reaction vessel having the same structure as in the experimental example was charged with 750 liters of an aqueous suspension of calcium hydroxide having a concentration of 5 to 11% by weight, and the lime kiln exhaust gas from the kraft pulp manufacturing process was subjected to gas-liquid contact type. Conducted through a scrubber, cooled from 210 ° C to 65 ° C and removed dust, then passed through the lime kiln exhaust gas to the shell side of the shell-type heat exchanger, conducted water into the tube as a cooling medium, and heated from 65 ° C Cooled to 45 ° C. The dust in the exhaust gas was 0.022 to 0.026 g / Nm ³ dry, and the exhaust gas flow rate was 1000.
Blow at a constant flow rate of ~ 2500 liters / min. The carbon dioxide concentration in the exhaust gas was about 16% by volume. Table 2 shows the reaction conditions and the paper quality of the paper internally added with the produced calcium carbonate.

[0064]

[Table 2]

The reaction time fluctuated between 80 and 225 minutes by changing the flow rate of the carbon dioxide gas or the concentration of the calcium hydroxide aqueous suspension, but the specific scattering coefficient of the calcium carbonate produced was
It was 2000 cm ² / g or more, and the increase in whiteness and opacity of the internal paper was 3 points and 10 points or more, respectively, and the improvement effect was satisfactory.

The concentration of the calcium hydroxide aqueous suspension was adjusted to 5.
When the temperature is raised from 8% by weight (No.4) to 10.2% by weight (No.6), the temperature at the start of the reaction is from 50 ℃ (No.8) to 30 ℃
When it was lowered to (No.7), the particle size of the generated calcium carbonate became smaller, but the gas injection amount was 1000 liters /
The particle size of the calcium carbonate remained almost unchanged even when the amount (No. 9) was increased to 2500 liters / minute.

Therefore, if the gas injection amount is further increased, the reaction time can be shortened without significantly changing the quality of the produced calcium carbonate, and the particle size and shape can be controlled by the calcium hydroxide suspension. This can be dealt with by changing the concentration of the suspension or the reaction start temperature.

Next, coated paper is produced from the calcium carbonate obtained in the present invention and commercially available calcium carbonate, and the properties of the coated papers are compared. Manufacture of coated paper: The calcium carbonate aqueous suspension (concentration 8.5% by weight) obtained in No. 7 of Example 1 was dehydrated to a concentration of 65% by weight, and 100 parts by weight of the pigment was added to the coating composition shown below. Was dispersed with a Cowles disperser to prepare a coating color having a concentration of 61% by weight.

Oxidized starch (Oji Ace B, manufactured by Oji Constarch) 5.0 parts by weight Styrene-butadiene latex (JSR 0691, manufactured by Japan Synthetic Rubber Co., Ltd.) 12.0 parts by weight Calcium stearate (Fco Disper, manufactured by ADEKA Fine Chemical Co., Ltd.) 1.0 part by weight Polyamide Polyurea-based resin (Sumiraz 636, Sumitomo Chemical Co., Ltd.) 0.5 parts by weight Sodium acrylate (Aron T-40, Toagosei Co., Ltd.) 0.2 parts by weight

Next, this color is applied to a base paper for coating (basis weight 64
g / m ² , thickness 90 μm), coating speed 30 m / min, solid content 13 g / m ²
Coating with a blade coater for testing so that
Dry at ℃ and pass through a super calendar twice (line pressure 190
Kg / cm) coated paper was obtained.

Separately, as a comparative example, a coating color having a concentration of 63% by weight was prepared in the same manner as above for the following commercial coating grade calcium carbonate and kaolin to obtain the same coated paper. Table 3 shows the paper quality of these various coated papers and the reaction conditions of calcium carbonate.

Escalon 2000 (dry ground calcium carbonate, manufactured by Sankyo Seiko Co., Ltd.) Carbital 90 (wet ground calcium carbonate, manufactured by Fuji Kaolin Co., Ltd.) Brilliant 15 (light calcium carbonate, manufactured by Shiraishi Industry Co., Ltd.) Ultra Coat (imported kaolin, Engelhard) (Made by the company)

[0073]

[Table 3]

The calcium carbonate produced according to the present invention belongs to light calcium carbonate, but unlike fine cubic crystals of Brillianto 15 (side length 0.15 μm) of the same light calcium carbonate, coarse spindle-shaped or needle-shaped crystals (major axis). Since it is 1.5 μm), the white paper glossiness and the print surface glossiness in Table 3 are low, but the smoothness is high, so it can be said that the pigment is suitable for matte coated paper.

On the other hand, heavy calcium carbonate Escalon 20
00 (average particle size 2.2 μm) and carbital 90 (average particle size 1.
8 μm), as the pulverization progresses and the particle size becomes finer, the white paper glossiness, the printing surface glossiness and the smoothness of the coated paper tend to be higher.

The calcium carbonate produced according to the present invention has low white paper glossiness and print surface glossiness like Escalon 2000, but gaps are formed between pigment particles during coating, and they are compressed and smoothed by a super calendar. Therefore, since the particle shape and the particle size are uniform, the smoothness is extremely high and is close to that of the ultra coat (kaolin). Higher smoothness is advantageous because printability is improved.

The wet pick and dry pick were inferior to Escalon 2000, but at a level that can be prevented by adding a binder (latex and starch) for the coating color or by mixing with other pigments. is there.

As described above, in order to compare the coating suitability of pigments, 100 parts of each pigment was blended and coated, and the paper quality of the coated paper was examined. The calcium carbonate produced by the present invention was matte-like. It has been found that when used in a coated paper pigment in the total amount or mixed with other pigments, it exhibits excellent properties.

Comparative Example Concentration in a reaction tank in which carbon dioxide gas is blown downward from a cylindrical blowing tube (hereinafter referred to as a simple reaction tank) and a reaction tank in which a stirrer is attached to the simple reaction tank (hereinafter referred to as a stirred reaction tank) 6.0
2000 liter of calcium hydroxide aqueous suspension of ~ 10.1% by weight was loaded, and lime kiln exhaust gas from the kraft pulp manufacturing process was treated with a wet scrubber and an electrostatic precipitator without passing through a shell type heat exchanger, and 2500 liter / It was blown in at the rate of minutes. At that time, the temperature of the exhaust gas was 60 ° C and the dust content was 0.042.
〜0.060g / Nm ³ dry, carbon dioxide concentration was about 16% by volume. Table 4 shows the results of comparison between the properties of calcium carbonate produced in the present invention and the simple reaction tank and the agitation reaction tank, and the properties of these internally added papers.

[0080]

[Table 4]

The calcium carbonate produced in the simple reaction tank had a coarser particle size and a non-uniform shape as compared with the calcium carbonate produced in the present invention. Further, the calcium carbonate produced in the stirred reaction tank had a uniform shape as compared with the simple type, but the particle shape did not change so much.

On the other hand, in the calcium carbonate produced according to the present invention, the spindle-shaped calcite-type crystals were partially mixed with the acicular aragonite-type crystals, but the particle size was finer than that of the above two methods, and the filler The specific scattering coefficient was also equal or slightly higher. When used as a filler for paper, there is no problem even if aragonite type crystals are mixed.

[0083]

As described in detail above, according to the present invention, the concentration of the calcium hydroxide aqueous suspension is kept in a certain range, and the lime kiln exhaust gas is cooled and dust-removed by the wet scrubber and the shell heat exchanger to obtain fine bubbles. As a result of bubbling, the utilization factor of carbon dioxide gas is significantly improved, and as a result, calcium carbonate having excellent dispersibility and a uniform shape can be easily obtained.

In addition, when used as a filler for paper, the scattering power of the filler is large, so that the whiteness and opacity of the internally-added paper increases greatly, and the paper is agglomerated to an appropriate size. Since the binding was not hindered, the decrease in paper strength was smaller than that in the case where calcium carbonate obtained by pulverizing calcium carbonate or limestone obtained by the conventional method to a similar average particle size was internally added.

When the calcium carbonate obtained in the present invention is added to the coating pigment, the smoothness is superior to that in the case where the pigment calcium carbonate obtained by dry-milling limestone is added.
A matte coated paper with good printability can be obtained.

Furthermore, calcium carbonate can be efficiently produced by using the lime kiln exhaust gas from the alkaline pulp or kraft pulp manufacturing plant of the paper pulp factory, and the carbon dioxide concentration in the exhaust gas after the heat recovery or dust removal process becomes low. Even in the case of the conventional method, the reaction can be completed in a shorter time than the conventional method, and the remarkable effect that it can be internally added or coated on paper without drying and crushing is exhibited.

Further, according to the present invention, even if the concentration of the reaction solution or the temperature of the reaction solution changes, the effect of the change in the particle shape of calcium carbonate is small.

[Brief description of drawings]

FIG. 1 is a flow chart of a carbonation reaction using a lime kiln exhaust gas.

FIG. 2 is a cross-sectional view of a shell heat exchanger.

FIG. 3 is a sectional view of a carbonation reaction tank used in the present invention.

FIG. 4 is a plan view of a gas reservoir.

[Explanation of symbols]

 1: Lime kiln 2, 5: Blower 3: Wet scrubber 4: Shell heat exchanger 6: Carbonation reaction tank 7: Shell 8: Tube 9: Exhaust gas inlet 10: Exhaust gas outlet 11: Cooling water inlet 12: Cooling water outlet 13: Shower 14: Blow valve 15: Gas reservoir 16: Hole 17: Exhaust gas supply pipe 18: Suspension supply port 19: Gas discharge port 20, 23: Conduit 21: Liquid feed pump 22: pH measuring device 24: Conduit 25 : Vent

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location D21H 17/67

Claims (3)

[Claims] 1. A method for producing calcium carbonate by reacting an aqueous suspension of calcium hydroxide with an exhaust gas of a lime-calcining kiln, wherein a reaction tank equipped with a gas reservoir having a large number of openings in the lower portion of the tank. A calcium carbonate aqueous suspension having a concentration of 5 to 15% by weight is charged into the mixture, and fine bubbles of the lime-calcining kiln exhaust gas are supplied from the gas reservoir to carry out a carbonation reaction. Method. 2. The lime-calcining kiln exhaust gas is a lime-calcining kiln exhaust gas from a pulp manufacturing plant, and the dust content in the exhaust gas is 0.030 g / Nm ³ dry or less.
A method for producing the described calcium carbonate. 3. A lime firing kiln exhaust gas is a combination of a wet scrubber and a shell-and-tube heat exchanger, and the exhaust gas is treated through the shell side of the shell-and-tube heat exchanger. 2. The method for producing calcium carbonate according to claim 1, wherein the prepared one is used.