JP2009203137A - Method for producing calcium carbonate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently obtaining calcium carbonate having good grindability and suitable as a coating pigment, in a method for producing calcium carbonate in a causticizing process of a pulp production process by a sulfate method or a soda method. <P>SOLUTION: The causticized light calcium carbonate having improved grindability can be obtained by using the mixture of quicklime produced in a causticizing process and quicklime supplied from the outside or only the quicklime supplied from the outside as a raw quicklime, and (i) blending the quicklime so that the blending ratio of the quicklime supplied from the outside is 50-100 wt.% of the total blending weight of the quicklime, and (ii) defining the average primary particle diameter of the quicklime supplied from the outside as ≤3 μm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing calcium carbonate. More specifically, the present invention uses a causticizing step of a pulp manufacturing process, has an excellent white liquor separating property and cleaning property, and is excellent in grindability and is an inexpensive carbon dioxide suitable for use as a papermaking pigment. It relates to a method for obtaining calcium.

  In recent years, there is a high demand for high-quality lightweight coated paper having high whiteness, high opacity, and high gloss. In order to meet these requirements, expensive titanium dioxide, plastic pigments, and the like are blended in the coating pigments of high-quality lightweight coated paper, in addition to inorganic pigments such as kaolin, heavy calcium carbonate, and light calcium carbonate.

  Calcium carbonate conventionally used for coating pigments is very inexpensive, and although whiteness and opacity of coated paper can be improved by increasing the blending ratio in the paint, the glossiness of blank paper is significantly reduced. For this reason, in order to make calcium carbonate highly blended, it is common to reduce the particle size of calcium carbonate by wet grinding in order to improve the white paper glossiness. However, in order to reduce the particle size, it is necessary to increase the amount of dispersant added and pulverize for a long time, which increases the pigment production cost.

  Calcium carbonate is important for industrial materials such as rubber, plastics, paints, sealants, adhesives, fertilizers, etc., as well as heavy calcium carbonate obtained by mechanically pulverizing natural limestone dry or wet, There is precipitated calcium carbonate (synthetic calcium carbonate) obtained by chemical methods.

Patent Document 1 (Japanese Patent Laid-Open No. 10-226517) describes a method for producing calcium carbonate in a causticizing step of a pulp production step by a sulfate method or a soda method.
[Causticization process]
As one of the precipitated calcium carbonates obtained by chemical methods, causticizing reaction with green liquor after quick lime is neutralized with water or weak liquor in the causticizing step of pulp manufacturing process by sulfate method or soda method Causticized light calcium carbonate produced by the above (causticizing method).

  The pulp manufacturing process by the sulfate method or the soda method generally includes the following steps. That is, using a “white liquor” in which sodium hydroxide or sodium sulfide is dissolved, wood is digested at high temperature and pressure to isolate fibrin from the wood. Fibrin is separated and purified as a solid phase into pulp, and cooking waste liquid (“black liquor”) is concentrated and burned in a recovery boiler. At that time, the eluted component from the wood is recovered as a heat source, and the inorganic substance in the chemical solution is recovered as a smelt mainly composed of a mixture with sodium carbonate or sodium sulfide. The smelt dissolves in a calcium carbonate washing solution in which a white liquor component called a weak solution is partially dissolved to form a “green liquor”.

  This green liquor and quicklime are mixed, and sodium carbonate is converted into sodium hydroxide useful for cooking chemicals by the soothing and causticizing reactions shown by the following formulas [1] and [2]. At the same time, calcium carbonate is by-produced. In conventional pulp mills, green liquor and quicklime are mixed in a reaction tank called a slaker. In practice, this two-stage reaction proceeds considerably overlappingly, and soaking and causticization occur almost simultaneously.

[Calcium carbonate produced in the causticizing process]
Calcium carbonate produced in the papermaking process is a by-product in producing white liquor, which is the main product, and therefore can be used at a very low cost when used as a coating pigment. In addition, the calcium carbonate produced here is usually partly or wholly decarboxylated in a calcining furnace called a rotary kiln to become quick lime again and reused in the causticizing reaction.

[Dechlorination]
When the black liquor is burned in the recovery boiler, the chlorine derived from the chips tends to enter the black liquor and the boiler tends to corrode. Therefore, measures are taken to prevent boiler corrosion by discharging fly ash, which is part of the black liquor combustion product, out of the system and reducing the amount of chlorine. However, when a part of the smelt is discharged out of the system, the sulfur content derived from Na ₂ S and the like are discharged out of the system simultaneously with chlorine. Therefore, the sulfidity of the green liquor (NaOH + 1 / 2Na ₂ S) decreases, and the effective alkali (NaOH + 1 / 2Na ₂ S) of the white liquor after the causticizing step decreases. In order to increase the effective alkali of the white liquor in order to make the pulp production constant, it is necessary to increase the amount of quicklime added to the green liquor. For this purpose, it is necessary to increase the replenishment amount of lime to be fired in the kiln (hereinafter referred to as “kiln fired lime”) into the system.
Japanese Patent Laid-Open No. 10-226517

  However, in order to prevent boiler corrosion in this way, part of the fly ash of the black liquor combustion product is discharged out of the system for desalination, and as a result, when the amount of calcinated calcined lime increases, the causticizing process A new problem arose in the quality of the causticized light calcium carbonate obtained in

  That is, when the amount of quicklime (kiln calcined lime) generated in the causticizing process increases, the grindability of the causticized light calcium carbonate obtained in the causticizing process decreases, and the pulverized product with the target particle size is obtained in a normal grinding time. There was a problem that it could not be obtained. If the grindability of the causticized light calcium carbonate decreases, the particle size of the causticized light calcium carbonate pigment increases, the glossiness of the coated paper decreases, and the calender linear pressure is reduced to obtain the desired glossiness. Increasing the height causes problems in paper quality such as a decrease in paper thickness and a decrease in opacity.

  The object of the present invention is to overcome the drawbacks of the prior art, and to produce calcium carbonate produced by a causticizing process as an inexpensive coating pigment that has good white liquor separation / cleaning properties and good grindability after production. It is to provide a method of obtaining.

  As a result of intensive studies to solve the above problems, the inventors of the present invention, in the method for producing calcium carbonate in the causticizing step of the pulp production process by the sulfate method or the soda method, Using the mixture of quicklime generated in the chemical conversion step and quicklime supplied from the outside, or only quicklime supplied from the outside, (i) the blending ratio of quicklime supplied from the outside is 50 to 100 of the total weight of quicklime And (ii) by making the average primary particle diameter of the quick lime supplied from the outside 3 μm or less, a surprising effect that causticized light calcium carbonate with improved grindability can be obtained was discovered. The present invention has been completed.

  According to the present invention, in a causticizing step of a pulp manufacturing process by a sulfate method or a soda method, a large amount of calcium carbonate suitable for a coating pigment having high white liquor separation and washing properties and excellent grindability is produced at low cost. Be able to get.

  The present invention relates to a method for producing calcium carbonate in a causticizing step of a pulp manufacturing step by a sulfate method or a soda method, and as a raw quicklime, a mixture of quicklime generated in the causticizing step and quicklime supplied from the outside, or Only quick lime supplied from the outside can be used. In the present invention, quick lime generated in the causticizing step is quick lime obtained by decarboxylating calcium carbonate generated in the causticizing step in a kiln or the like.

  In the present invention, externally supplied quicklime is used as at least a part of the quicklime in the causticizing step. Calcium in the causticizing process (quick lime, slaked lime, calcium carbonate) constitutes a circulation system. By supplying quick lime from outside the system, cleanliness in the system and high purity of the circulation lime are achieved, and causticization is achieved. It can be expected to improve the process reactivity, improve the clarity of white liquor, and reduce waste.

[Calcination of calcium carbonate]
The calcium carbonate-derived quicklime (also referred to as kiln calcined lime) generated in the causticizing process used in the present invention converts sodium carbonate into sodium hydroxide in the causticizing process of the pulp manufacturing process by the sulfate method or the soda method. What is necessary is just to calcinate the calcium carbonate produced | generated in the case. Regarding the baking equipment at that time, Beckenbach furnace, Melz furnace, rotary kiln, Kunii type furnace, KHD (Kerhardy) furnace, Koma type furnace, Kalmatic furnace, fluidized baking furnace, mixed baking furnace, etc., calcium carbonate If it is an apparatus which converts lime into quicklime (calcium oxide), it will not be restricted in particular.

[Externally supplied quicklime and mixing ratio]
As a result of various investigations, quick lime supplied from the outside used in the present invention (also referred to as purchased quick lime) preferably has an average primary particle size of 3 μm or less. Whole quick lime (quick lime generated in the causticizing process + external supply) The ratio of the externally supplied quicklime in (quicklime) is preferably 50 to 100% by weight. In the present invention, the average primary particle size of the externally supplied quicklime is more preferably 2.0 μm or less, and even more preferably 1.5 μm or less. The blending amount of externally supplied quicklime is more preferably 50 to 85% by weight, and further preferably 55 to 80% by weight. The average primary particle diameter of quicklime can be measured by taking the average of the primary particle diameters of 20 quicklimes observed with an electron microscope.

  In addition, quick lime sinters when fired at high temperatures, and the crystals tend to melt and the primary particle size increases (C. Hanson and H. Theliander, “Properties and quality of lime, Part 1. The influence. of conditions during reburning ”, Nordic Pulp and Paper Research Journal no. 3 p.161-166 (1994)), as the externally supplied quicklime, quicklime having a firing temperature of 900 to 1000 ° C. is preferable.

  The term “primary particles” as used herein refers to, for example, the grains shown in FIG. 2 in the above-mentioned C. Hanson document, and an aggregate of these primary particles is a particle, which is expressed here as a secondary particle. The size of the secondary particles is usually 10 to 30 μm.

[Quicklime particle size]
Although there is no restriction | limiting in particular regarding the particle size of quicklime, The thing with an average particle diameter of 0.01 mm-10 cm, Preferably 0.01 mm-10 mm is suitable in terms of uniform mixing in the stirring at the time of decontamination. When the particle size is 0.01 mm or less, the cost for pulverization is high, and at the same time, generation of dust and troubles in the transfer device are undesirable.

[Green liquid]
The green liquor used in the present invention can be carried out at a total alkali concentration of 80 to 160 g / L (converted to Na ₂ O: the same applies hereinafter), preferably 100 to 150 g / L. When it is lower than 80 g / L, the concentration of the final white liquor is lowered, so that it is necessary to adjust the concentration when used for cooking. On the other hand, when it is higher than 160 g / L, it is not preferable because crystals such as sodium carbonate are easily generated in the green liquor before causticizing, causing trouble in the process.

[Speed of adding green liquor to quicklime]
In the present invention, the mixing method of quicklime and green liquor is not particularly limited. In a preferred embodiment, the addition rate of green liquor to 1 g of quicklime is 0.02 to 50 cc (green liquor) / min / g (quick lime), preferably 0.02 to 30 cc (green liquor) / min / g (quick lime). It can be carried out. At an addition rate of less than 0.02 cc (green liquor) / min / g (quick lime), the productivity is inferior and impractical, while an addition rate greater than 50 cc (green liquor) / min / g (quick lime) is very In addition, a large capacity pump is required, which is not practical.

[Mixer]
For mixing quicklime and green liquor at the time of soaking, select from the general stirring blade type, pump type, extruders, kneaders, and kneaders according to the viscosity of the liquid or slurry at the time of mixing. Can be selected and used as appropriate. References such as “Chemical Engineering Handbook” (published by Maruzen Co., Ltd., March 18, 1988) can be referred to as necessary.

[Reaction temperature]
Regarding the reaction temperature during soaking and causticization, the reaction temperature is preferably 30 to 105 ° C. When the temperature is higher than 105 ° C., the boiling point is exceeded in the simmering device or the causticizing tank, so that a pressure-type sunk device or a causticizing device is required, which is uneconomical. On the other hand, when the temperature is lower than 30 ° C., the reaction rate becomes extremely slow, so that a causticizing tank having a large volume is required. Furthermore, a large cooling device is required for the green liquor cooling step before the reaction, and heating of the white liquor is necessary to use the white liquor for high-temperature cooking, which is expensive and uneconomical.

[White liquor separator]
Separation of white liquor and calcium carbonate, and washing and removal method of white liquor components accompanying calcium carbonate, ordinary solid-liquid separation device, for example, gravity sedimentation separation method by clarifier, vacuum filtration separation method represented by Oliver filter, Any separation device can be used as long as the requirements can be achieved, such as a pressure filtration separation system represented by a leaf filter and a centrifugal filtration system using centrifugal force.

[Crusher]
On the other hand, since the separated and recovered calcium carbonate is generally an amorphous calcite crystal having a large particle diameter, it is often impossible to obtain sufficient quality for use as a coating pigment as it is. Therefore, it is desirable to adjust the average particle size to 0.2 to 10 μm according to the desired quality requirement. The particle size can be adjusted by using a pulverizer such as a medium stirring pulverizer represented by a sand mill or a ball mill pulverizer.

[BET specific surface area]
The calcium carbonate obtained by the present invention can provide light calcium carbonate at low cost because the white liquor as the main product can be produced on an industrial scale as well as excellent quality as a coating pigment. In a preferred embodiment of the present invention, calcium carbonate having a BET specific surface area of 1.4 to 3.0 m ² / g, more preferably 1.4 to 2.0 m ² / g can be obtained.

[mechanism]
The details of the mechanism by which high-quality calcium carbonate is obtained by the present invention are not clear, and the present invention is not limited to the following, but the mechanism of the present invention is presumed as follows.

Generally, quicklime generated in the causticizing process contains a relatively large amount of impurities such as Na, Cl, and S. When impurities ₄ such as NaCl and CaSO much to be quick lime, it has been reported that an average primary particle of quicklime increases (DR Glasson, "Reactivity of lime and related oxides, XVI. Sintering of lime" , J. appl. Chem. Vol. 17 April (1967)). As described above, since the causticization reaction and the causticization reaction occur almost simultaneously, the shape of the quick lime is reflected in the shape of the causticized light calcium carbonate, and from the externally supplied quick lime having a small average primary particle size, It is presumed that causticized light calcium carbonate having a small average primary particle diameter is produced, and causticized light calcium carbonate having a large average primary particle diameter is produced from quick lime generated in the causticizing process.

  Therefore, when the ratio of the external quicklime having an average primary particle size of 3 μm or less is 50 to 100%, the average primary particle size of the causticized light calcium carbonate obtained by the reaction with the green liquor is reduced, and the subsequent grinding step It seems to be easy to be crushed.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the examples.
<Raw raw lime>
Table 1 shows the properties of quicklime used in the examples and comparative examples of the present invention. Quicklime A is kiln calcined lime produced in a causticizing process in a factory of Nippon Paper Industries Co., Ltd. The quick lime used as the externally supplied quick lime is obtained by subjecting quick lime commercially available from Ube Materials to screen processing with a 100 mesh sieve.

  The average primary particle size of the raw quicklime was determined by observing the particle size of 20 primary particles of quicklime with an electron microscope (JSM-840A, JEOL). The average primary particle size was 7.0 μm for quicklime A (kiln calcined lime), 1.1 μm for quicklime B (externally supplied quicklime), and 3.2 μm for quicklime C (externally supplied quicklime) (secondary particle size). Are about 20 μm).

<Measurement of average particle size and specific surface area of causticized calcium carbonate>
Measurement of average particle diameter: Using Microtrac MT3300 (Nikkiso Co., Ltd.), the particle diameter at the 50% point of the weight cumulative distribution was calculated as the average particle diameter.

  Specific surface area: Measured by the BET nitrogen adsorption method (Micromeritics Gemini 2360; Shimadzu). When the primary particle diameter of the causticized light cal sludge is small, the specific surface area becomes large, and thus the specific surface area is generally an index of the primary particle diameter.

<Evaluation of filterability>
The cake specific resistance was calculated | required with the following apparatus and method. This value is “the pressure difference required to filter a 1 kg / m / sec viscosity liquid at 1 m / sec when a 1 kg cake is formed on a 1 m ² filtration area”. , Separability and cleanability are poor.

・ Equipment: Constant pressure slurry evaluation equipment (IKABUST JT-F; Chuo Kako)
Sample: About 20 mL slurry (concentration 10%)
・ Decompression degree: 0.4 MPa
-Filtration area: 9.62 cm ²
[Example 1]
In an actual machine slaker of Nippon Paper Industries Co., Ltd., green liquor (composition: Na ₂ CO ₃ = 108 g / L, Na ₂ S = 21 g / L, NaOH = 6 g / L. All are Na ₂ O equivalent values, The same applies to the examples and comparative examples) and quick calcined lime A that is calcined lime is 137 t / day, quick calcined B that is externally supplied quick lime is 230 t / day, and this quick lime is mixed with 70 g of quick lime with respect to 1 L of green liquor. The reaction was carried out at a temperature of 90 ° C. for a residence time of about 3 hours to obtain causticized light calcium carbonate. The ratio of externally supplied quicklime was 63%.

  This causticized light calcium carbonate was concentrated to about 75% with a mud filter, and 1.0 part by weight of a polyacrylic acid dispersant (manufactured by Nippon Paper Chemicals Co., Ltd.) and water were added together to prepare a concentration of 73% by weight. . The causticized light cal sludge was pulverized from about 20 μm to about 6 μm with a coarse pulverizer to form a slurry, and then pulverized with a sand grinder (RL250V-SP) at a processing rate of 330 L / hour.

[Example 2]
The same procedure as in Example 1 was performed except that the quicklime used was a mixture of quicklime A of 136 t / day and quicklime B of 249 t / day. The proportion of purchased quicklime was 65%.

[Example 3]
The same procedure as in Example 1 was conducted except that the quicklime used was a mixture of quicklime A of 155 t / day and quicklime B of 226 t / day. The ratio of purchased quicklime was 59%.

[Example 4]
The same procedure as in Example 1 was performed except that the quicklime used was a mixture of quicklime A of 66 t / day and quicklime B of 228 t / day. The ratio of purchased quicklime was 78%.

[Comparative Example 1]
The same procedure as in Example 1 was repeated except that the quicklime used was a mixture of quicklime A at 188 t / day and quicklime B at 175 t / day. The proportion of purchased quicklime was 48%.

[Comparative Example 2]
The same procedure as in Example 1 was performed except that the quicklime used was a mixture of quicklime A of 179 t / day and quicklime B of 163 t / day. The proportion of purchased quicklime was 48%.

[Comparative Example 3]
The same procedure as in Example 1 was conducted except that the quicklime used was a mixture of quicklime A of 181 t / day and quicklime B of 177 t / day. The proportion of purchased quicklime was 49%.

[Comparative Example 4]
The same procedure as in Example 1 was conducted except that the quicklime used was a mixture of quicklime A of 136 t / day and quicklime C of 225 t / day. The ratio of purchased quicklime was 62%.

As shown in Table 1, when the average primary particle system is 1.1 μm quicklime B (externally supplied quicklime) and the ratio of externally supplied quicklime is 50 to 100%, causticized light carbonic acid before pulverization The specific surface area of calcium became 1.5 m ² / g or more, and high-quality calcium carbonate was obtained. In addition, when compared with the same treatment amount, calcium carbonate obtained from quick lime containing 50% or more of quick lime B gives a relatively fine causticized light calcium carbonate pigment of 1.1 μm or less by pulverization. It was.

On the other hand, when the ratio of quicklime B (externally supplied quicklime) was 50% or less, the specific surface area of causticized light calcium carbonate before pulverization was smaller than 1.4 m ² / g. Moreover, when compared with the same processing amount, even if the obtained calcium carbonate was pulverized, it became a causticized light calcium carbonate pigment of 1.2 μm or more, and the pulverizability was poor.

  In addition, when quick lime C (externally supplied quick lime) having an average primary particle size of 3.2 μm is used, even if the obtained calcium carbonate is pulverized even if the ratio of quick lime C is 50% or more, the same processing amount is obtained. , The causticized light calcium carbonate pigment of 1.2 μm or more was obtained, and the grindability was poor.

  Moreover, about cake specific resistance, Example 1-4 was smaller than Comparative Examples 1-4, and became a result with white liquor separability and good washability.

Claims (3)

A method of producing calcium carbonate in a causticizing step of a pulp manufacturing process by a sulfate method or a soda method,
The raw lime is a mixture of quick lime generated in the causticizing step and quick lime supplied from the outside, or only quick lime supplied from the outside,
(I) The blending ratio of quicklime supplied from the outside is 50 to 100% by weight of the total blended weight of quicklime,
(Ii) The average primary particle diameter of quick lime supplied from the outside is 3 μm or less.
The manufacturing method of the said calcium carbonate.   The manufacturing method of the calcium carbonate of Claim 1 whose compounding ratio of the quick lime supplied from the outside is 50 to 85 weight% of the total compounding weight of quick lime. The method for producing calcium carbonate according to claim 1 or 2, wherein a causticizing step is performed at a reaction temperature of 30 to 105 ° C to obtain calcium carbonate having a BET specific surface area of 1.4 to 3.0 m ² / g.