JP2009001438A - Method for producing calcium carbonate - Google Patents

Abstract

From lime sludge produced when caustic liquid is obtained by adding quick lime or a slaked liquid obtained by reacting quick lime with a sodium hydroxide-containing liquid to the green liquor obtained in the pulp manufacturing process by the kraft method. Another object of the present invention is to provide a method for obtaining high-whiteness calcium carbonate containing no impurities that can be used as a papermaking filler or a coated paper pigment.
A method for producing calcium carbonate from lime sludge produced by causticizing and producing green liquor obtained in a pulp production process by the kraft method with a lime or a slaked liquid obtained by reacting quick lime with a sodium hydroxide-containing liquid. The lime sludge is subjected to a white liquor recovery step and then passed through a cyclone using high-temperature hot air to dry the lime sludge and to obtain a high-whiteness lime sludge from which impurities are separated. A method for producing calcium. Moreover, it is preferable that the high temperature hot air used for a cyclone utilizes the exhaust heat from a kiln.
[Selection figure] None

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 free of impurities that can be used as a papermaking filler or a coated paper pigment.

  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, the main component of which is calcium carbonate. Therefore, if the lime sludge can be purified as lime sludge with 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 this 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 impurities are mixed in from the subsequent processes. However, it is difficult to reach the whiteness required as a pigment for coated paper (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, when dispersed in water to a concentration of 20 to 70% by mass, the concentration is too low to be used for coating as well as for filler, and the post-process As a result, adding a dehydration and drying process would require a large financial burden.

In addition, there is a description that a separate line for reuse is formed in the part of the process of soothing and causticizing, and the reaction is controlled to obtain fine calcium carbonate, which is used for fillers and the like. However, a large capital investment is required 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. It was necessary to add a process, and it was not economically suitable (patent documents 6 to 8).
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 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 collect lime sludge with little colored impurities, and this process is performed by using the waste heat recovery process equipment of the process, which is economically superior, for papermaking filler or coated paper It is an object of the present invention to provide a method capable of easily producing high whiteness calcium carbonate having a whiteness of 90% or more that can be used as a pigment.

  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 method of selecting lime sludge produced as a by-product from the causticization process is based solely on dilution or replacement cleaning methods, and impurities such as carbon are inherently insoluble. In the above method, paper fillers and pigments are used. The high-whiteness calcium carbonate used as can not be purified.

  One of the methods for producing calcium carbonate according to the present invention is to causticize the green liquor obtained in the pulp production process by these kraft methods with slaked quick lime liquid, and separate the produced lime sludge into solid and liquid to obtain calcium carbonate. In the manufacturing method, the lime sludge is subjected to a white liquor recovery process and an alkali component recovery process, and then an unburned carbon and other impurities are separated from the exhaust gas by a cyclone using exhaust heat from the kiln and simultaneously dried. It is characterized by obtaining lime sludge.

  The method of the present invention is difficult to obtain by the conventional method by separating and removing impurities such as unburned carbon from a lime sludge by-produced from causticization by passing a cyclone using high-temperature hot air. High quality calcium carbonate can be easily produced.

  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 by a multi-effect can etc. for the purpose of chemical recovery and heat recovery, and burned in a black liquor recovery boiler.

  A solubilized 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. 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 present invention, the lime sludge is dried by passing a cyclone using high-temperature hot air, and impurities such as unburned carbon are completely separated and purified, and can be used as a paper filler or a coated paper pigment. It is a requirement to improve the whiteness of.

  The cyclone as used in the present invention refers to a separation device using so-called centrifugal force. The greatest feature of the cyclone used in the present invention is to remove the impurities while drying the lime sludge by utilizing the exhaust heat generated in the kiln, so that the hot air is used in the normal cyclone and the lime is removed. Any cyclone that can be classified while drying sludge can be used without any problem. For example, what is called by the name of a flash cyclone, a hot cyclone, etc. can be used suitably.

  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 cleaning (mainly a weak liquid) for the recovery of alkali components cannot sufficiently remove impurities such as unburned carbon associated with lime sludge, so pigments and fillers with high whiteness In the present invention, which aims to obtain calcium carbonate for coating or coated paper, the lime sludge is dried and unburned by passing a cyclone using exhaust heat from the kiln through the lime sludge separated from the weak liquid. A process for completely separating and cleaning impurities such as carbon is added. By adding this cyclone, impurities can be removed, the combustion load in the kiln can be reduced, and since the waste heat of the kiln is used, the economic load required for the cyclone is also considerable. It becomes possible to reduce.

  Lime sludge after causticizing treatment in which white liquor is separated and washed with fresh water usually has a concentration of 80% or more. The causticized lime sludge washed with fresh water is dehydrated by a vacuum filter device such as a drum filter and concentrated to a concentration of 80% or more as described above, but it is always concentrated to a constant concentration. Not exclusively. When fluctuations occur in the concentration in this way, for example, when passing through a wet separation step, it is necessary to adjust the concentration from time to time, and when there is a need to dilute in the step of removing impurities, Considering the use as calcium carbonate for fillers or coated paper, it must be concentrated again to a certain level.

  From this point of view, drying once and then separating impurities is advantageous for the subsequent process, but the causticized lime sludge washed with fresh water usually has a concentration of 80% or more. Therefore, considerable energy costs are required for complete drying, which is economically undesirable.

  In the present invention, in order to reduce such energy costs, by using a cyclone utilizing exhaust heat generated from the kiln, it is possible to dry lime sludge and to separate impurities.

  There are other important reasons for using a cyclone that uses exhaust heat from a kiln. As a factor that reduces the whiteness contained in the lime sludge after the causticizing treatment, we investigated and found that it is unburned carbon and inorganic compounds generated in the cooking process. Since unburned carbon is black, it is necessary to reduce the whiteness because it is black, and although inorganic compounds have not been analyzed in detail, there are probably inorganic compounds mainly composed of green or black sulfur compounds. It is thought that the whiteness is lowered.

  As a result of our study, it was found that unburned carbon has a small particle size and a specific gravity smaller than that of calcium carbonate, so that it can be easily separated by passing a cyclone. In addition, it was found that the inorganic compound is not perfect, but is decomposed by heat treatment, and almost no color develops. That is, by using a cyclone that uses exhaust heat generated from the kiln, unburned carbon can be easily separated from the specific gravity difference, and by using exhaust heat, heat treatment is performed at the same time. It has been clarified that the whiteness is improved by decomposing black inorganic compounds to become colorless inorganic compounds. In addition, impurities and the like adhering to the surface of the lime sludge are also removed together with moisture in the dehydration step to be dried, so that calcium carbonate with higher purity can be obtained.

  The temperature of the exhaust heat from the kiln is not particularly limited. However, if the temperature is too high, calcium carbonate undergoes a decarboxylation reaction and is decomposed into calcium oxide. The decomposition of the inorganic compound that lowers the whiteness as described above is not promoted, which is not preferable. As temperature of the exhaust heat of the kiln used with a cyclone, the range of 150-300 degreeC is preferable. 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 impurities are removed. It becomes possible to do.

  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,% in an example shows 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 collecting 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 85%.

  The resulting lime sludge was then passed through a small laboratory cyclone. The cyclone treatment was carried out so that 2.5% of the total amount of impure solids separated as floss was rejected, and the temperature of the hot air at this time was 200 ° C.

Impurities were removed before and after the cyclone treatment, and the whiteness and water content after the cyclone treatment to obtain a dried lime sludge were measured by the following methods.
[Whiteness]:
The obtained calcium carbonate powder is put into a brass mold, a mirror-treated flat plate is placed on the mold, pressed at 70 kg / 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.

Example 2
The same procedure as in Example 1 was performed except that the temperature of the hot air of the cyclone was 250 ° C.

Example 3
The same procedure as in Example 1 was performed except that the temperature of the hot air of the cyclone was changed to 300 ° C.

Example 4
The same procedure as in Example 2 was performed except that the solid content of impure components separated as floss was 5.0% of the total amount charged.

Example 5
The same procedure as in Example 2 was performed except that the solid content of impure components separated as floss was 7.5% of the total amount charged.

Example 5
The same procedure as in Example 2 was performed except that the solid content of impure components separated as floss was 10.0% of the total amount charged.

Comparative Example 1
The same procedure as in Example 1 was performed except that the temperature of the hot air of the cyclone was set to 100 ° C.

  Table 1 shows the whiteness and moisture value before and after the cyclone of the obtained calcium carbonate in each of the above Examples and Comparative Examples. For the purpose of obtaining high whiteness calcium carbonate free of impurities that can be used as a papermaking filler or a coated paper pigment, calcium carbonate having a whiteness of 90% or more was used in the present invention.

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 the white liquor recovery step, the lime sludge is dried by passing a cyclone using high-temperature hot air, and a high whiteness lime sludge from which impurities are separated is obtained. .   The method for producing calcium carbonate according to claim 1, wherein the high-temperature hot air of the cyclone utilizes exhaust heat from the kiln.