JP2009179520A - Method for producing calcium carbonate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for obtaining high-whiteness calcium carbonate usable as a pigment for coated paper from lime sludge produced when quicklime or a slaking liquid obtained by reacting quicklime with a sodium hydroxide-containing liquid is added to green liquor obtained in a pulp making process by the kraft process to causticize the green liquor. <P>SOLUTION: In a method for producing calcium carbonate from lime sludge produced by causticizing green liquor obtained in a pulp making process by the kraft process with quicklime or a slaking liquid prepared by reacting quicklime with a sodium hydroxide-containing liquid, the lime sludge is passed through a flash drier using high-temperature hot air after passing through a white liquor recovery step, the resulting dried lime sludge is mixed with hydrous lime sludge at a filter outfall, and the mixture is dry-ground. The high-temperature hot air used is preferably hot air utilizing exhaust heat from a kiln rear end. <P>COPYRIGHT: (C)2009,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 producing high-whiteness calcium carbonate which can be used as a pigment for coated paper, has good slurry dispersibility, is easy to handle and is economically inexpensive.

  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 of agglomerated amorphous calcium carbonate particles, and its 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 agglomerated by blowing air are only a part of the contaminants 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 known document 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 little 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 impure components are mixed in from the subsequent processes. Therefore, 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% by mass, the concentration is too low to be used for coating, and dehydration and drying steps are added as post-processes. Then, 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 another line from the causticizing process, and at the same time, when used as a pigment for the coating layer, it is necessary to increase its concentration, so dehydration, It was necessary to add a drying step, and this was an industrially lacking method (see Patent Documents 6 to 8).

Furthermore, 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 it is conceivable to use this dried one. Because the cohesive force is stronger than normal water-containing sludge before drying, the dispersibility in the slurry is poor, and because it is at a high temperature of 200 ° C or higher, it is cooled to a temperature that can be handled during handling such as transportation and dry grinding. There are disadvantages such as the necessity of cooling equipment, the amount of dust scattering due to dry powder, direct placement in the raw material warehouse, etc., and the need for storage silos and tanks .
JP 61-53112 A JP-A 61-179398 JP 2004-26639 A 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 can be used as a pigment for coated paper, which has good slurry dispersibility, is easy to handle, and is economically inexpensive when dry pulverization is performed. Another object of the present invention is to provide a method capable of easily producing high whiteness calcium carbonate.

  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). So far, the lime sludge by-product from the causticization process that has been proposed so far depends only on the dilution or replacement cleaning method. However, impurities such as carbon are essentially insoluble. High whiteness calcium carbonate used as a pigment cannot be purified.

  In the method for producing calcium carbonate of the present invention, the green liquor obtained in the pulp production process by these kraft methods is causticized with slaked quick lime liquor, and the produced lime sludge is solid-liquid separated to produce calcium carbonate. After the white liquor recovery process and the alkali component recovery process, the lime sludge is dried by a flash dryer using exhaust heat from the kiln kettle bottom, and the lime sludge containing water at the filter outlet is mixed. This is characterized in that it is pulverized dry.

  The lime sludge produced as a by-product in the causticization process is dried by passing it through a flash dryer using the exhaust heat from the kiln kettle bottom. High concentration that enables high-efficiency coating by obtaining high-quality calcium carbonate, which is superior in terms of handling and economy, by mixing hydrated lime sludge in the mouth, and then grinding it dry It can be used as a coating pigment.

  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, so by separating it into solid and liquid, Pulp fibers that are the raw material of the paper are 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 produced 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, it is considered as a source of calcium carbonate having a high whiteness necessary for use as a filler and pigment for paper. In some cases, the color of unburnt carbon and metal salts that are considerably contained in the dregs component greatly affects the whiteness of the calcium carbonate, so the amount of the dregs component contained in the lime sludge is extremely important. It has been proposed to separate and select this dregs component from lime sludge by a method such as washing and regenerate it as calcium carbonate. However, the dregs component is basically insoluble, and methods such as dilution washing and displacement washing are 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 fed to a double paddle mixer by a conveyor, and is thoroughly mixed and stirred together with dry lime having a moisture content of 0 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 cascade. The lime sludge sent to the cage mill is crushed into fine particles and at the same time sufficiently stirred with the 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 allows the lime sludge to be dried by passing through a flash dryer operated using exhaust heat from the kiln kettle bottom, and separates and cleans impurities such as unburned carbon contained in the sludge, and It is a requirement to provide a method for producing calcium carbonate that has good slurry dispersibility, is easy to handle, and is economically inexpensive.

  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. The biggest feature of the flash dryer used in the present invention is that it is possible to remove impurities while drying lime sludge using the exhaust heat generated in the kiln kettle bottom, so hot air is used in a normal cyclone. Any cyclone that can be classified while drying lime sludge can be used without problems.

  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.

  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. There is a need to dilute in the process of removing impurities, and if so, it must be concentrated again to a certain concentration.

  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.

  In the present invention, in order to reduce such energy costs, the lime sludge is dried by using a flash dryer using exhaust heat generated from the kiln kettle bottom, and water-containing lime is used for cooling after drying. By mixing the sludge, the temperature can be reduced to a predetermined easy-to-handle temperature. Also, by mixing, the dispersibility in the slurry is greatly improved, and the amount of dispersant added can be reduced. In this regard, in the case of dried lime sludge obtained by passing through a flash dryer, when dried at a high temperature, powdery nitric acid is generated and hardened on the particle surface, and the cohesiveness is increased and the dispersibility is poor. . It is presumed that the slurry dispersibility is remarkably improved by a synergistic effect by mixing the hydrated lime sludge with the dried lime sludge. Furthermore, by mixing, dry lime sludge in the form of dry powder is almost adsorbed, dust scattering is eliminated, and handling such as direct placement in a raw material warehouse becomes easy.

  Even if the water content after mixing is high, it is possible to cope with it, but considering the subsequent handling and economy, the lower one is more efficient within the range that can solve the existing problems, and the water content after mixing is within 1% by mass. Is preferred.

  The temperature of exhaust heat from the kiln kettle bottom is not particularly limited, but if it is too high, it is not preferable because calcium carbonate undergoes a decarboxylation reaction and decomposes into calcium oxide. However, the decomposition of the inorganic compound that lowers the whiteness as described above is not promoted, which is not preferable. The temperature of exhaust heat from the kiln pot bottom used in the flash dryer is preferably in the range of 150 to 300 ° C.

  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, when such lime sludge is observed in detail, the particles are porous particles formed as a result of the causticization reaction, and further, a small amount of calcium hydroxide is contained in the lime sludge. If the liquid is to be made into a liquid, the properties of the liquid are poor, that is, the viscosity is high and a large amount of diluting liquid is required at the time of slurrying. .

  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. When this is pulverized in this way to make a slurry, it can be easily dispersed at a concentration of about 75% by mass. The ability to disperse at this concentration makes it possible to increase the concentration of the paper coating solution. As a means required for the pulverization, the object can be achieved by a conventional pulverizer such as a jet mill, an attritor, and a roll mill which are operated in a dry manner.

  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
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. Subsequently, quicklime 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 the white liquor, this lime milk was suction filtered with a polypropylene filter cloth to obtain a lime sludge having a cake solid content concentration of 80%.

  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% of the total amount of impure solids to be separated was discarded, and the temperature of the hot air at this time was 200 ° C. This dried lime sludge and the lime sludge before drying were mixed so that it might become predetermined | prescribed moisture, and it was set as the test raw material.

The whiteness, moisture value, and average particle size 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 field of view 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 size]:
The particle size was 50% average particle size measured with MT3000 manufactured by Microtrac.

  Next, this dried lime sludge was pulverized to an average particle size of 4 μm by a jet mill. 200 g of this lime sludge was sampled. On the other hand, after adding 0.16% of a dispersant (Aron T40), this lime sludge was put into an aqueous medium adjusted to a total concentration of 75%, and this was put into a stirrer. After stirring and dispersing, the viscosity was measured with a B-type viscometer. As a result, the viscosity was 190 cps.

Example 2
The same operation as in Example 1 was performed except that the average particle size was changed to 10 μm by pulverization with a jet mill. As a result, the viscosity after dispersion was 250 cps.

Example 3
The same procedure as in Example 1 was performed except that the raw material moisture was 1%. As a result, the viscosity after dispersion was 150 cps.

Comparative Example 1
The same procedure as in Example 1 was performed except that only dried lime sludge was used as a raw material. As a result, the viscosity after dispersion was 380 cps.

  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, the lime sludge concentration when dispersing it, and the viscosity after dispersion are summarized. Table 1 shows.

  In the method of the present invention, the lime sludge produced as a by-product in the causticizing process is passed through a flash dryer using exhaust heat from the kiln kettle bottom so that the lime sludge is dried and high quality superior from the viewpoint of handling and economy. Can be used as a high-concentration coating pigment that enables high-efficiency coating.

Claims (2)

  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 process, dry lime sludge obtained by passing through a flash dryer using high-temperature hot air and hydrated lime sludge at the filter outlet are mixed and dry-ground. A method for producing calcium carbonate.   2. The method for producing calcium carbonate according to claim 1, wherein the dried lime sludge and the hydrated lime sludge at the filter outlet are mixed under a condition within 1% by mass of water after mixing.