JP2010132522A - Method of manufacturing calcium carbonate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of producing calcium carbonate having high whiteness which can be used as a pigment for coated paper and in which impurity is not contained, from lime sludge formed when unslaked lime or slaked liquid produced by making unslaked lime react with a sodium hydroxide containing liquid is added to a green liquor produced by a pulp manufacturing process by a kraft process to be causticized. <P>SOLUTION: The method of manufacturing calcium carbonate from lime sludge formed by causticizing a green liquor produced by a pulp manufacturing process by a kraft process by unslaked lime or slaked liquid prepared by making unslaked lime react with a sodium hydroxide containing liquid is characterized in that lime sludge is passed through a white liquor recovery process, thereafter the lime sludge is dried by being passed through a flash dryer using an elevated temperature hot blast, and is dry ground. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a lime sludge produced when adding caustic liquid obtained by reacting quick lime or quick lime with a sodium hydroxide-containing liquid to the green liquor obtained in the pulp manufacturing process by the kraft method and causticizing it. The present invention relates to a method for obtaining high whiteness calcium carbonate that can be used as a pigment for coated paper and has good slurry dispersibility.

  Calcium carbonate has been widely used as a pigment for coated paper or as a filler for papermaking. This type of calcium carbonate is produced from light calcium carbonate obtained by chemical synthesis and from nature. It can be roughly classified into heavy calcium carbonate obtained by wet pulverization of limestone. Light calcium carbonate is generally produced by the carbon dioxide method, and has the advantage that the particle diameter and shape can be controlled by changing the reaction conditions, but it is more expensive than heavy calcium carbonate. On the other hand, heavy calcium carbonate is not only cheaper than light calcium carbonate, but it is frequently used as a pigment for coated paper because it does not hinder the coating operation even if it is highly blended in the coating composition. .

  By the way, as a calcium carbonate source that does not rely on synthetic methods and natural minerals, such as the carbon dioxide method as a representative example, lime produced when causticized green liquor by-produced in the pulp manufacturing process by the kraft method is calcined with quick lime. There is sludge. This lime sludge is a lump in which irregular shaped calcium carbonate particles are agglomerated, and the main component is calcium carbonate. Therefore, if it can be purified as lime sludge containing little or no impurities, it can be purified. By appropriately pulverizing, high whiteness calcium carbonate that can be used as a papermaking filler or as a pigment for coated paper can be obtained by pulverizing at a high concentration.

  Prior art to prevent contaminants from being introduced into the lime sludge produced when causticized green liquor with quick lime is free carbon that is contaminated by blowing air into the green liquor prior to causticization of the green liquor. A method for clarifying the green liquor by aggregating black suspended matters such as the above has already been proposed (see Patent Document 1). However, since the components that can be aggregated by blowing air are only a part of the impurities in the green liquor, this method has certain limitations in recovering high-quality calcium carbonate.

  In addition, in Patent Document 2 below, as a method for producing calcium carbonate that can be used as a filler for papermaking from the above lime sludge, the content of silica and insoluble substances in the lime sludge is set to a predetermined amount or less. It is disclosed to grind lime sludge with low insoluble material. And as a method of acquiring lime sludge with few silica and insoluble substances, clarification of green liquor by standing or filtering means is described. However, this patent document 2 does not specifically teach how much clarification of the green liquor can be achieved to obtain lime sludge composed of high-quality calcium carbonate with less colored impurities.

  Furthermore, the green liquor obtained in the pulp production process is passed through 200 g of green liquor through a glass fiber filter paper having a pore diameter of 1 μm, and the lightness of the filtered residue remaining on the filter paper measured by a spectral whiteness colorimeter is maintained at 50 or more. The clarification treatment step and the pulverized lime sludge slurry are supplied to a water washing step for the purpose of removing alkali components until the pH of the discharged filtrate becomes 11.0 or less. A method for purifying coated paper pigments by combining washing and dehydration processes has been proposed, but the causticizing process is complicated, and impurities are mixed in from the subsequent processes. However, it is difficult to reach the whiteness required as a pigment for coated paper (see Patent Document 3).

  Further, in Patent Document 4 below, the green liquor is filtered at a temperature of 65 ° C. or higher to remove solid impurities contained in the green liquor in advance, and then quick lime is added to the green liquor corresponding to the filtrate to eliminate the liquid. To remove unreacted substances, and then recover the calcium carbonate in the liquid without crushing or crushing, washing it with warm water of 80 ° C or higher, and then crushing and bleaching with an oxidizing agent A method for recovering calcium carbonate is described. However, this method has a drawback that the particle size of the calcium carbonate to be recovered is too fine, and the main purpose of causticization, that is, the production of white liquor, is very difficult. For this reason, its practical use is difficult.

  Furthermore, in the following Patent Document 5, in the production of kraft digested pulp, massive lime sludge obtained from the causticizing step is dispersed in water, adjusted to a concentration of 20 to 70% by mass, and then wet pulverized. Although described, if dispersed in water to a concentration of 20 to 70 mass%, the concentration is too low to be used for coating, and dehydration and drying steps are added as post-processes. It was an economic burden.

  In addition, in the part of the soothing and causticizing reaction in the process, a separate line for reuse is formed, and the reaction is controlled to obtain fine calcium carbonate, which is used for filler etc. However, it requires a large capital investment to make a separate line from the causticizing process, and at the same time, if it is used as a pigment for the coating layer, its concentration needs to be increased. It was necessary to add a process, and it was an industrially lacking method (see Patent Documents 6 to 8).

  Furthermore, after passing through the white liquor recovery process, the lime sludge can be dried by passing it through a flash dryer using high-temperature hot air, and this dried product can be used. In this method, the dried sludge is agglomerated. Since the force is stronger than the normal water-containing sludge before drying, there is a drawback that the dispersibility in the slurry is poor.

JP 61-53112 A JP-A 61-179398 Japanese Patent Laid-Open No. 2004-26639 JP 61-183120 A JP 2001-98482 A Japanese Patent Laid-Open No. 10-226517 Japanese Patent Laid-Open No. 10-226974 JP-A-10-292283

  The object of the present invention is to consider the above-mentioned problems of the prior art, and the properties of lime sludge produced as a by-product in the pulp manufacturing process by the kraft method, in particular, the type and amount of components contaminated therewith, Even if the type and amount of coloring components contaminated in lime sludge change, considering the type and the type of pulp black liquor, the combustion condition when burning the concentrate, and the type of auxiliary fuel. It is possible to always collect lime sludge with little colored impurities, and further, it can be used as a pigment for coated paper by drying this process in the waste heat recovery process equipment of the process and subsequently subjecting it to dry pulverization. An object of the present invention is to provide a method capable of easily producing high whiteness calcium carbonate having a whiteness of 90% or more and good slurry dispersibility.

The present invention relates to a method for producing calcium carbonate from lime sludge produced by causticizing and producing green liquor obtained in a pulp production process by a kraft method with a slaked liquid obtained by reacting quick lime or quick lime with a sodium hydroxide-containing liquid. In the method, the lime sludge is passed through a white liquor recovery step and then passed through a flash drier using high-temperature hot air to dry the lime sludge and dry pulverize the lime sludge.
The lime sludge is preferably dry pulverized to a standard deviation of particle size distribution of 4.0 μm or less. Moreover, when drying the said lime sludge with a flash dryer, it is preferable to dry at the drying temperature of 300 degrees C or less.
Furthermore, it is preferable that the whiteness of the calcium carbonate is 90% or more, a dispersant is added to the calcium carbonate, the dispersion is dispersed at a solid concentration of 75% by mass, and the viscosity after dispersion is less than 650 mPa · s. .

  According to the present invention, high-quality calcium carbonate is obtained from lime sludge produced as a by-product in the causticizing process, and can be used as a high-concentration coating pigment that enables high-efficiency coating. Furthermore, by dry pulverization to a standard deviation of the particle size distribution of 4.0 μm or less, the dispersibility of the slurry is improved, and higher quality calcium carbonate superior in terms of handling and economy can be obtained.

  The lime sludge produced by adding quick lime or slaked liquid to the green liquor in the kraft method is usually from white liquor (sodium hydroxide, sodium sulfide, sodium carbonate, etc. as the main component) used in the pulp manufacturing process. It is usually separated and then subjected to a one-stage dilution dehydration washing step for the purpose of recovering a weak liquid (alkali component). The selection method of lime sludge produced as a by-product from the causticization process proposed so far depends only on the dilution or replacement cleaning method. Impurities such as carbon are inherently insoluble, and the above method is used for coated paper. High whiteness calcium carbonate used as a pigment cannot be purified.

  The method for producing calcium carbonate according to the present invention is a method for producing calcium carbonate by causticizing the green liquor obtained in the pulp production process by the kraft method with slaked quick lime liquor, and separating the produced lime sludge into solid and liquid. In the method, the lime sludge is subjected to a white liquor collecting step and an alkali component collecting step, and then dried lime sludge is obtained by a flash dryer, and this is pulverized dry.

Furthermore, the lime sludge is dry pulverized to a standard deviation of the particle size distribution of 4.0 μm or less. Here, the standard deviation is generally used as an index indicating the spread of the particle size distribution of the powder. The particle size distribution is regarded as a normal distribution, and from the 84% particle diameter (D ₈₄ ) of the integrated distribution to 16 % Particle diameter (D ₁₆ ) is reduced and divided by 2. That is, SD (standard deviation) = (D84−D16) / 2.

  First, an overview of the process for producing pulp fibers, which are used as paper materials, from wood chips will be given. In the kraft pulp production process, wood chips are cooked at high temperature and pressure in a digester that contains a cooking chemical mainly composed of caustic soda and sodium sulfide. By this cooking, components such as lignin contained in the wood are eluted in the cooking chemical, and the target pulp fiber is taken out from the cooking kettle in a state dispersed in this chemical. The pulp fiber used as a raw material is acquired. Then, the pulp waste liquid (referred to as black liquor) separated from the pulp fiber by solid-liquid separation is concentrated in a multi-effect can etc. for the purpose of chemical recovery and heat recovery, and burned in a black liquor recovery boiler.

  Smelt produced by burning concentrated black liquor (inorganic melt mainly composed of sodium carbonate and sodium sulfide) dissolved in a weak liquid described later is a so-called green liquor, and in a normal pulp manufacturing process, The green liquor from which the dregs (insoluble impurities) contained in this green liquor are settled and separated, and the dregs are separated and removed is subjected to a causticizing step in which quick lime is added to the green liquor. Lime sludge is generated by the chemical reaction. In a normal process, the soothing reaction and the causticizing reaction are performed simultaneously.

  Next, the slurry containing the lime sludge is subjected to solid-liquid separation, and sodium hydroxide and sodium sulfide contained in the liquid component are reused as a white liquor for cooking wood chips. On the other hand, the lime sludge separated from the liquid component is baked by a rotary kiln, a calsiner or the like and converted into quick lime, and the quick lime is circulated and used in the causticizing process of the green liquor. The liquid component obtained when the separated dregs or lime sludge is washed is used as a weak solution for dissolving the above-described smelt.

  In the normal pulp production process that is operated continuously, for example, sodium sulfate is added to compensate for sodium and sulfur lost in the cooking process, and there is not enough quick lime to causticize the green liquor. In this case, the chemicals required for cooking are covered with white liquor collected during the causticization process of the green liquor, and the quicklime required for causticization of the green liquor, Generally, it is covered with a calcined product of lime sludge produced in the causticizing step.

  However, when lime sludge produced as a by-product in the pulp manufacturing process is considered as a source of calcium carbonate, especially when it is considered as a source of calcium carbonate having a high degree of whiteness necessary for use as a filler or pigment for paper In this case, since the unburned carbon and metal salts that are considerably contained in the dregs component greatly affect the whiteness of the calcium carbonate, it is extremely important to remove the dregs component contained in the lime sludge. A proposal has been made to separate this dregs component from lime sludge by a method such as washing and regenerate it as calcium carbonate, but the dregs component is basically insoluble, and dilute washing or displacement washing can be used to separate the dregs component. It is powerless for careful selection.

  In the paper and paper industry, a short rotary kiln having a low equipment installation load is generally installed for a long rotary kiln. In this short rotary kiln, the exhaust gas temperature reaches a high temperature of 300 to 450 ° C. From the viewpoint of thermal economy, a flash dryer is installed at the bottom of the kettle for the purpose of lowering the exhaust gas temperature and increasing the thermal efficiency as well as the long rotary kiln and increasing the firing capacity of the rotary kiln. A lime sludge cake having a moisture content of 35 to 40% by mass is sent to a double paddle mixer by a conveyor, and is thoroughly mixed and stirred together with dry lime having a moisture content of 1 to 2% by mass sent from a flash dryer. To be dehumidified. The lime sludge dehumidified in this way is distributed by a divider, a part is supplied and fired to a kiln by a screw conveyor, and the rest is sent to a cage mill.

  The cage mill is a kind of crusher with a rotating blade row. The lime sludge sent to the cage mill is crushed into fine particles and at the same time sufficiently stirred with high-temperature exhaust gas (300-450 ° C) from the kiln, and the lime moisture rapidly increases. Removed. This lime passes through the drying tube from the inside of the mill together with water vapor and gas, and water vapor and gas are collected and separated by a flash dryer. The lime collected by the flash dryer returns to the mixer through the air lock feeder. The exhaust gas from the flash dryer is discharged to the atmosphere through the exhaust fan and the dust collector. The dust collected by the dust collector is returned again to the lime sludge washing tank as lime sludge.

  The present invention focuses on the property that lime sludge is dried by passing through a flash dryer using hot hot air, and impurities such as unburned carbon contained in the sludge are completely separated and cleaned. However, it must be used as a pigment for coated paper.

  The flash dryer referred to in the present invention is an exhaust heat recovery apparatus provided with an airflow type cyclone separator utilizing a so-called centrifugal force. As the flash dryer used in the present invention, generally used one can be used without any problem as long as it can perform classification while drying lime sludge using hot hot air. At this time, the supply source of the high-temperature hot air is not particularly limited, and any device can be used. In addition, waste heat from the kiln Kamajiri can be used.

  For causticization of green liquor, reaction conditions commonly used in the pulp manufacturing process can be adopted, and this causticization produces lime sludge. The lime sludge dispersion (lime milk) obtained here is solid-liquid separated for the purpose of recovering white liquor, and then the solid-liquid separated lime sludge is re-dispersed in water for the purpose of recovering alkali components. The filtrate obtained here is reused as a weak liquid. Examples of the dehydrator for this purpose include a belt filter, a drum filter, a disk filter, a filter press, a cylinder press, a decanter, or an apparatus that performs these under pressurized conditions. The conventional practice is to recover the target white liquor and alkali components by using one kind of apparatus appropriately selected from these dehydrators in a single stage.

  However, dehydration and cleaning (mainly to obtain weak liquids) for the recovery of alkali components cannot sufficiently remove impurities such as unburned carbon associated with lime sludge, so coated paper with high whiteness The present invention aimed at obtaining calcium carbonate for use is to dry lime sludge separated from weak liquid by passing it through a flash dryer using high-temperature hot air containing an airflow type cyclone in the system, and impurities such as unburned carbon. Is completely separated and cleaned.

  The lime sludge after causticizing treatment which separated white liquor and washed with fresh water usually has a concentration of 80% by mass or more. The causticized lime sludge washed with fresh water is dehydrated with a vacuum filtration device such as a drum filter and concentrated to a concentration of 80% by mass or more, but it is considered to be used as calcium carbonate for coated paper. It is necessary to dilute in the process of removing impurities, and if so, it must be concentrated again to a certain level.

  From this point of view, it is advantageous for the subsequent steps to dry once and then separate impurities, but the lime sludge after causticizing treatment washed with fresh water or the like usually has a moisture of about 20% by mass. Therefore, a considerable energy cost is required for sufficient drying, which is not preferable. The present invention solves the limitations of the methods proposed so far with new ideas.

  There are other important reasons for using a flash dryer using hot hot air. As a result of investigating the factors that reduce the whiteness contained in the lime sludge after the causticizing treatment, it was revealed that they are unburned carbon and inorganic compounds generated in the cooking process. The unburned carbon is black because it is black. Therefore, the whiteness is lowered, and the presence of an inorganic compound mainly composed of a sulfur compound exhibiting green or black is also a cause of lowering the whiteness.

  As a result of the examination, it was found that unburned carbon has a small particle size and a specific gravity smaller than that of calcium carbonate. Therefore, it can be easily separated by passing through a flash dryer having an airflow cyclone as a structure. In addition, it was found that inorganic compounds are decomposed by heat treatment and no color develops. In other words, unburned carbon can be easily separated from the difference in specific gravity by passing through a flash dryer using high-temperature hot air, and heat drying is achieved by using exhaust heat, while heat treatment is performed. It has been clarified that the whiteness is improved by carrying out simultaneously and decomposing an inorganic compound exhibiting green or black to become a colorless inorganic compound. In addition, unburned carbon or the like contained in the lime sludge is also removed together with moisture in the dehydration step to be dried, so that calcium carbonate with higher purity can be obtained. As a result, calcium carbonate having a whiteness of 90% or more is obtained.

  The temperature of the hot hot air is not particularly limited. However, if the temperature is too high, calcium carbonate undergoes a decarboxylation reaction and is decomposed into calcium oxide. Such decomposition of the inorganic compound that decreases the whiteness is not promoted, which is not preferable. The temperature of the hot hot air used in the flash dryer is preferably 500 ° C. or less, and more preferably in the range of 150 to 300 ° C. By using in this temperature range, the moisture of lime sludge can be completely eliminated, and completely dried calcium carbonate can be obtained, and at the same time, the decomposition of inorganic compounds that lower the whiteness is promoted, and unburned carbon Can be removed.

  As described above, the dried lime sludge thus obtained has various properties suitable for use as a pigment for coated paper, which is not found in conventional wet lime sludge. However, if it is used as it is, its range of use is limited to filler applications. This is because such lime sludge particles are porous particles formed as a result of the causticization reaction, and when lime sludge contains a small amount of calcium hydroxide, etc. This is because the viscosity of the liquid becomes high and a large amount of diluting liquid is required at the time of slurrying. Therefore, in recent years, it cannot be used as a pigment for high concentration coating with a small amount of mainstream adjustment water in paper coating.

  This was solved as follows. The lime sludge becomes porous particles of about 30 μm as a result of aggregation of small particles of about 2 μm. It is easy to pulverize this secondary agglomerated lime sludge into primary particles of 2 to 3 μm, and it is possible without applying a strong pulverizing force.

  By pulverizing lime sludge, it becomes possible to disperse easily at a solid content concentration of about 75% by mass when slurried. The ability to disperse at this concentration makes it possible to increase the concentration of the paper coating solution. The calcium carbonate of the present invention can be controlled to 650 mPa · s or less after adding a dispersant and dispersing at a solid content concentration of 75% by mass.

  Furthermore, by subjecting this secondary agglomerated lime sludge to dry pulverization to a standard deviation of the particle size distribution of 4.0 μm or less, the dispersibility in the slurry is greatly improved even with the same average particle size, and the amount of dispersant added is reduced. Reduction can also be achieved. In this regard, in the case of dried lime sludge obtained by passing through a flash dryer, when dried at high temperature, quick lime, sodium nitrate, etc. accompanying decarbonation are generated and hardened on the particle surface. Strength increases and dispersibility is poor. When pulverizing this dried lime sludge, the sharper particle size distribution, that is, the smaller the standard deviation, the stronger the cohesiveness, the less coarse particles that remain, and the more uniform the particles, the effect of the dispersant is exhibited. It is presumed that the slurry dispersibility is remarkably improved due to the ease.

  As a means required for pulverizing lime sludge, a conventional pulverizer such as a jet mill, an attritor, or a roll mill that is operated in a dry manner can be used. In particular, in order to provide particles having a sharp particle size distribution, the jet mill method is effective and can be adjusted by adjusting a classifier incorporated in the pulverizer.

  EXAMPLES The present invention will be described more specifically with reference to the following examples. However, these examples do not limit the technical scope of the present invention. In addition,% display in an example shows the mass% except the numerical value of whiteness.

Example 1
(Making lime sludge)
In the pulp manufacturing process, smelt obtained by burning concentrated black liquor was dissolved in weak liquor to prepare green liquor, placed in a stainless beaker, and heated to 104 ° C. in an oil bath. Next, quick lime was mixed with the green liquor at a rate of 70 g per liter of green liquor, and a lime milk was obtained by performing a soaking and causticizing reaction at 104 ° C. for 2 hours. For the purpose of recovering white liquor, this lime milk was suction filtered through a polypropylene filter cloth to obtain a lime sludge having a cake solid content concentration of 80% by mass.

(Flash dryer treatment)
The resulting lime sludge was then passed through a small laboratory airflow flash dryer. The treatment of the flash dryer was performed so that 2.5% by mass of the total amount of impure solids to be separated was discarded, and the temperature of the hot air at this time was 330 ° C.

The whiteness and moisture value, average particle size, and standard deviation before and after the flash dryer treatment were measured by the following methods.
[Whiteness]:
The obtained calcium carbonate powder is put into a brass mold, a mirror-finished flat plate is placed on the mold, pressed at 70 kgf / cm ² for 30 seconds to form a pellet, and a spectral whiteness colorimeter ( SC-10WN, manufactured by Suga Test Instruments Co., Ltd.) was used to measure the whiteness of the pellet on the side that was in the mirror surface. In measuring the whiteness, the conditions of a D65 light source and a 10 degree visual field were used.
[Moisture value]:
10 g of the obtained calcium carbonate was dried with a dryer at 100 ° C. for 30 minutes, and the moisture value of calcium carbonate was measured from the mass difference before and after drying.
[Average particle diameter, standard deviation]:
50% average particle diameter and standard deviation were measured with MT3000 manufactured by Microtrack.

(Crushing process)
Next, the dried lime sludge was pulverized with a turbo mill (single track jet mill FS4, manufactured by Seishin Enterprise Co., Ltd.) to an average particle size of 6 μm and a standard deviation of 5 μm. 200 g of this lime sludge is added, and after adding 0.16% by mass of the dispersant (Aron T40, manufactured by Toagosei Co., Ltd.), the total solids concentration is adjusted to 75% by mass. After stirring and dispersing with a stirrer, the viscosity was measured with a B-type viscometer. As a result, the viscosity after dispersion was 320 mPa · s.

Example 2
In the pulverization process, the same process as in Example 1 was performed except that the lime sludge was pulverized to an average particle size of 10 μm and a standard deviation of 6 μm. As a result, the viscosity after dispersion was 380 mPa · s.

Example 3
In the pulverization process, the same process as in Example 1 was performed except that the lime sludge was pulverized to an average particle diameter of 16 μm and a standard deviation of 7 μm. As a result, the viscosity after dispersion was 410 mPa · s.

Example 4
In the pulverization process, a jet mill (single track jet mill FS4, manufactured by Seishin Enterprise Co., Ltd.) was used instead of a turbo mill, and lime sludge was pulverized to an average particle size of 6 μm and a standard deviation of 3.5 μm. Processed. As a result, the viscosity after dispersion was 280 mPa · s.

Example 5
In the pulverization process, a jet mill (single track jet mill FS4, manufactured by Seishin Enterprise Co., Ltd.) was used instead of a turbo mill, and lime sludge was pulverized to an average particle size of 7 μm and a standard deviation of 4.0 μm. Processed. As a result, the viscosity after dispersion was 350 mPa · s.

Example 6
Except that the hot air temperature was set to 450 ° C. in the flash dryer process, and the pulverization process was performed by using a jet mill (single track jet mill FS4, manufactured by Seishin Enterprise Co., Ltd.) instead of a turbo mill to an average particle size of 6 μm and a standard deviation of 3.5 μm. The same processing as in Example 1 was performed. As a result, the viscosity after dispersion was 600 mPa · s.

Example 7
Except that the hot air temperature was set to 300 ° C. in the flash dryer process, and the pulverization process was performed using a jet mill (single track jet mill FS4, manufactured by Seishin Enterprise Co., Ltd.) instead of a turbo mill to an average particle size of 6 μm and a standard deviation of 3.5 μm The same processing as in Example 1 was performed. As a result, the viscosity after dispersion was 220 mPa · s.

Example 8
Except that the hot air temperature was set to 150 ° C. in the flash dryer process, and the pulverization process was performed by using a jet mill (single track jet mill FS4, manufactured by Seishin Enterprise Co., Ltd.) instead of a turbo mill to an average particle size of 6 μm and a standard deviation of 3.5 μm. The same processing as in Example 1 was performed. As a result, the viscosity after dispersion was 210 mPa · s.

Comparative Example 1
Lime sludge having a cake solid concentration of 80% by mass was obtained in the same manner as in Example 1, and the whiteness, moisture value, average particle diameter, and standard deviation were measured for this lime sludge in the same manner as in Example 1. Next, this lime sludge was added and adjusted so that the total solid content concentration became 75% by mass. After stirring and dispersing with a stirrer, the viscosity was measured with a B-type viscometer. As a result, the viscosity after dispersion was 3500 mPa · s.

  In each of the above Examples and Comparative Examples, the whiteness of the obtained lime sludge, moisture, the average particle diameter after pulverizing this dry, standard deviation, lime sludge concentration when dispersing it, after dispersion The viscosities are summarized in Table 1.

  Comparing Examples 1 to 8 with Comparative Example 1, the lime sludge of the present invention is dried and then dry pulverized to provide calcium carbonate having a whiteness of 90% or more and good dispersibility. You can see that

  Comparing Example 1 and Example 4, it can be seen that the dispersibility is improved by reducing the standard deviation of the particle size of the pulverized particles. This effect is presumed to make the particle distribution narrow, in particular, prevent the mixing of coarse particles and stabilize the dispersibility.

  Comparing Examples 4 and 5 with Examples 7 and 8, by suppressing the drying temperature during the drying treatment of lime sludge to 300 ° C. or less, calcium carbonate with better dispersibility can be obtained while maintaining high whiteness. It can be seen that it can be provided.

Claims (4)

  In the method of producing calcium carbonate from lime sludge that is formed by causticizing and producing green liquor obtained in the pulp manufacturing process by the kraft method, with caustic liquid obtained by reacting quick lime or quick lime with sodium hydroxide-containing liquid, lime sludge After passing through the white liquor recovery step, the lime sludge is dried by passing it through a flash dryer using high-temperature hot air, and this is dry pulverized.   The method for producing calcium carbonate according to claim 1, wherein the lime sludge is dry-pulverized to a standard deviation of a particle size distribution of 4.0 μm or less.   The method for producing calcium carbonate according to claim 1 or 2, wherein the lime sludge is dried at a drying temperature of 300 ° C or lower when the lime sludge is dried by a flash dryer.   The whiteness of the calcium carbonate is 90% or more, a dispersant is added to the calcium carbonate, the dispersion is dispersed at a solid content concentration of 75% by mass, and the viscosity after dispersion is less than 650 mPa · s. 3. The method for producing calcium carbonate according to any one of 3 above.