JP4813075B2 - Method for producing aragonite acicular calcium carbonate - Google Patents

Description

  The present invention relates to a method for producing acicular calcium carbonate having a high aragonite content by mixing slaked lime having a specific BET specific surface area, crystallinity, and particle size obtained by dry digestion of quicklime and green liquor. is there. Since acicular calcium carbonate obtained by the present invention uses quick lime as a starting material, it can be manufactured at a lower cost than conventional acicular calcium carbonate using slaked lime as a starting material, and has excellent whiteness, opacity, and wire wear resistance. It can be used as a raw material for papermaking.

  Paper used for printing or writing usually contains a filler for the purpose of improving whiteness, opacity, smoothness, writing property, touch, printability and the like. As this paper making method, so-called acidic paper making paper at a pH of around 4.5 using talc, clay, titanium oxide or the like as a filler, and so-called medium paper making a paper in a neutral to weakly alkaline range of pH 7 to 8.5. There is sex paper. In neutral papermaking, instead of talc and clay, inexpensive self-made light calcium carbonate having a high white and high specific scattering coefficient can be used as a filler.

  In recent years, neutral paper obtained by neutral papermaking has attracted attention due to problems such as storage stability of the paper. In addition, there are many advantages in terms of paper quality, cost, environmental measures, etc. The transition to papermaking is progressing, and the spread of the paper is in the future.

  Looking at recent papers from the demand side, the fields of flyers, catalogs, pamphlets, direct mail, etc. in commercial printing, and in publication printing, with the progress of the information society, computers, multimedia, NES related books, magazines, photographs The collection, mook, and comic paper fields are characterized by significant growth. Against this background, paper users are increasingly thinking about reducing costs, and there is a demand for lower cost and lighter paper.

  As described above, the positioning of calcium carbonate as a filler is very important as the demand for inexpensive and lightweight neutral paper increases. Calcium carbonate used as a filler in this neutral papermaking includes heavy calcium carbonate obtained by mechanically grinding natural limestone dry or wet, and precipitated calcium carbonate (light calcium carbonate) obtained by a chemical method. is there.

  However, heavy calcium carbonate obtained by pulverizing natural limestone with a pulverizer such as a ball mill produces sharp edges on the surface of the pulverized particles, and when used as an internal filler, it causes severe wear on the plastic wire during papermaking. End up. Furthermore, when pulverizing the limestone starting material, the particle size distribution is wider than that of precipitated calcium carbonate produced by controlling the reaction conditions. When manufactured paper is manufactured, the quality such as bulk, whiteness, opacity, smoothness, writing property, touch, and printability is insufficient.

  Furthermore, the above problem will become more serious as the weight of paper has been reduced. Conventional means for improving the opacity of conventional lightweight printing paper include fillers with a large specific surface area (for example, finely pulverized silica and white carbon), fillers with a high refractive index (for example, titanium dioxide), or sedimentation properties. Calcium carbonate has been used.

  As a method for producing the precipitated calcium carbonate, (1) a reaction between carbon dioxide-containing gas obtained from a lime baking apparatus and others, and (2) a reaction between ammonium carbonate and calcium chloride in the ammonia soda method. (3) The reaction of lime milk and sodium carbonate for producing sodium hydroxide by causticization of sodium carbonate is known. Among these methods, in (2) and (3), the production method for obtaining the main product has been changed to a new method, or because calcium carbonate is a byproduct, the content of impurities is high. Has not been studied much.

  On the other hand, (1) has a relatively simple reaction system (water, slaked lime, carbon dioxide), and extensive research has been conducted on methods for producing calcium carbonate suitable for various purposes. Some products are also seen. However, the direct purchase from the manufacturer has the disadvantage that the transportation cost is high and the total cost is high. Further, in the on-site carbon dioxide method, if the kiln exhaust gas is used, calcium carbonate can be produced at a low cost, but there is a problem that a large amount of capital investment is required.

  One possible method is to use calcium carbonate, which is a by-product when producing white liquor in the causticizing process for recovering and regenerating cooking chemicals, in the pulp manufacturing process using the sulfate method or soda method. is there. This method has the advantage that existing facilities can be used and the capital investment is minimized.

  In the pulp manufacturing process by the sulfate method or the soda method, in order to isolate fibrin from wood, cooking is performed at high temperature and pressure using white liquor in which sodium hydroxide or sodium sulfide is dissolved. Fibrin is separated and purified as a solid phase into pulp, and cooking waste liquid (black liquor) is concentrated and burned. 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. Smelt is dissolved 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 as a cooking chemical solution by a two-step reaction of soaking and causticization expressed by the following formulas [1] and [2] to obtain a white liquor. At the same time, calcium carbonate is by-produced. Conventionally, in a pulp mill, green liquor and quicklime are mixed in a reaction tank called a slaker.

CaO + H ₂ O → Ca (OH) ₂ [1]
Ca (OH) ₂ + Na ₂ CO ₃ → CaCO ₃ + 2NaOH [2]
The calcium carbonate produced here is a by-product in producing white liquor, which is the main product, so when used as a papermaking raw material, it can be used at a very low cost, but it is causticized in a conventional closed system. By extracting calcium carbonate out of the system from the cycle of calcium (quick lime, slaked lime, calcium carbonate) in the process, purification in the system and high purity of the circulating lime are achieved, and the reactivity of [1] and [2] above It can be expected to improve, improve the clarity of white liquor, and reduce waste.

  However, since the reactions [1] and [2] occur almost simultaneously in this conventional method, the obtained calcium carbonate has various shapes such as a dice and a hexahedron, and has a large particle size and most of them. It was in the form of a lump and was close to conventional heavy calcium carbonate. Therefore, when this is pulverized and normal high-quality paper and coated paper are produced as a filler, the performance of the plastic wire is large especially in recent large-scale and high-speed paper machines. For the same reason as when finely pulverizing the powder, there were problems such as insufficient whiteness, opacity, smoothness, writing property, touch, printability and the like.

In this way, the conventional method has poor plastic wire wear during papermaking, and when this is used, while maintaining printing quality, a filler or pigment for obtaining high-quality opaque paper or coated paper can be obtained. It was difficult to efficiently and inexpensively produce calcium carbonate.
On the other hand, recently, the following publications of Patent Documents 1, 2, and 3 have solved the above problems by specifying conditions for digestion reaction and causticization reaction of quicklime, and are excellent in wire yield and wire wear during papermaking. A method for producing an inexpensive aragonite-based crab-like calcium carbonate useful for papermaking using a causticizing process with high opacity is disclosed. However, these methods cannot obtain a predetermined aragonite-type squirrel-like calcium carbonate unless many steps are performed, and the addition rate of the green liquor in the final stage is controlled within a limited range. Had to do.

  Aragonite needle-like calcium carbonate has a higher refractive index than other calcite-type rice grains and spindle-like calcium carbonate, and has almost no sharp edges. It has excellent properties in improving the properties and opacity.

In addition, the quality of slaked lime suitable for the production of acicular causticized light calcium carbonate with a high aragonite content of 75% or more, and the dry digestion conditions of quick lime necessary for obtaining the slaked lime and the quality of quick lime are completely different. The situation was not reported.
JP 2000-264628 A JP 2000-264629 A JP 2000-264630 A

  In view of the situation as described above, in the conventional pulp manufacturing method by the sulfate method or soda method, compared with the method of manufacturing calcium carbonate when manufacturing white liquor in the causticizing step of recovering and regenerating cooking chemicals, papermaking Excellent wire wear resistance, and when used for paper production, has higher refractive index and is more advantageous for improving opacity than calcite rice granular and spindle-like calcium carbonate, and has fewer steps An object of the present invention is to provide a method for producing aragonite needle-like calcium carbonate for papermaking, which can produce white liquor similar to the conventional causticizing method.

  As a result of intensive research in order to solve the above problems, the present inventors have utilized a causticizing step of a pulp manufacturing process by a sulfate method or a soda method, and a pulp manufacturing process by a sulfate method or a soda method. Slaked lime obtained by dry digestion of quick lime when starting the causticization reaction by mixing the same amount of green liquor and quick lime as required in the conventional operation to produce white liquor in the causticization process It was found that by specifying the BET specific surface area, crystallinity, and particle size, an aragonite content equivalent to acicular calcium carbonate obtained by a conventional causticizing method can be achieved. The calcium carbonate obtained by the above causticizing method is highly white and highly opaque, and an excellent papermaking raw material can be obtained by pulverizing.

In view of these problems, the present inventors have studied in detail the relationship between the quality of quick lime as a raw material and the quality of causticized light calcium carbonate produced by causticization, and as a result, obtained by dry digestion of quick lime. Correlation was observed between the BET specific surface area, crystallinity and particle size of slaked lime and the aragonite content of the acicular causticized light calcium carbonate. Furthermore, it has been clarified that slaked lime capable of achieving a high aragonite content can be produced with quick lime having a low residual CO ₂ content, and an inexpensive and high quality acicular calcium carbonate production method in the causticizing process has been completed.

That is, according to the present invention, a method for producing acicular calcium carbonate useful as a papermaking raw material in a causticizing step of a pulp production step by a sulfate method or a soda method, wherein a BET specific surface area is 20 m ² / g or less, X-ray diffraction 2θ = 18.0 ([001] crystal plane) peak intensity / 2θ = 34.1 ([101] crystal plane) peak intensity x 100 is 60 to 80, 100 mesh pass is 70% The slaked lime and 20-100 ° C. water are mixed to produce a slaked lime slurry, the slaked lime slurry is mixed with green liquor, and causticized while stirring or kneading to produce calcium carbonate, Provided is a method for producing acicular calcium carbonate useful as a papermaking raw material, comprising recovering calcium carbonate.

  With the completion of the present invention, the production cost of high-quality acicular causticized light calcium carbonate produced by causticization is greatly reduced, so the range of use of causticized light calcium carbonate as a papermaking raw material is greatly expanded, and as an accompanying effect The amount of causticized light calcium carbonate extracted from the white liquor production process can be increased, impurities easily accumulated in lime circulating in the process can be reduced, and the load on the kiln for firing can be reduced. Further, depending on the amount of calcium carbonate extracted from the process, the kiln can be stopped, and the production cost of white liquor, which is the main product in the causticizing process, can be greatly reduced.

As described above, according to the present invention, the BET specific surface area of slaked lime obtained by dry digestion of quicklime is 20 m ² / g or less, the peak intensity of 2θ = 18.0 ([001] crystal plane) of X-ray diffraction / 2θ = If the value of 34.1 ([101] crystal plane) peak intensity x 100 is 60 to 80 and the 100 mesh pass is 70% or more, the needle shape is generated by the carbonation reaction with green liquor in the subsequent causticizing process. The aragonite content of calcium carbonate can be achieved at 75%, and at the same time the manufacturing cost can be greatly reduced.

  The quick digestion lime used in the causticizing step of the present invention may be any calcined natural limestone mainly composed of calcium carbonate. Regarding the firing apparatus at that time, carbon dioxide such as Beckenbach furnace, Melz furnace, rotary kiln, Kunii type furnace, KHD (Kerhardy) furnace, Koma type furnace, Kalmatic furnace, fluidized firing furnace, mixed baking furnace, etc. If it is an apparatus which converts calcium into quicklime (calcium oxide), it will not be restricted in particular.

The residual CO ₂ content in quicklime is preferably 1% by weight or less based on the weight of quicklime. When the amount exceeds 1% by weight, the slaked lime having the quality according to claim 1 cannot be obtained even if the digestion water added at the time of dry digestion is reduced, and the aragonite content of the generated calcium carbonate is reduced and used as a paper filler This adversely affects the opacity of the paper, and the target paper quality cannot be obtained.

Hereinafter, the manufacturing method of slaked lime is explained in full detail. The industrial digester for producing slaked lime is not limited to a batch system or a continuous system as long as it is a dry digester.
The quick lime used as a raw material is not particularly limited as long as it is dry pulverized, but it is preferable to use a quick pulverizer such as a cage mill, a vibration mill, a ball mill, a disk mill, etc. A pulverized powder or granular material is used. In the case of powder, the particle size is not particularly limited, but powder of 100 mesh or less is preferable.

The ratio of quicklime and digestive water is about 1.0 to 1.5 when converted to 32 to 48 parts by weight of digestive water and a molar ratio with respect to 100 parts by weight of quicklime. By digesting at such a ratio, quicklime can be digested without remaining, and slaked lime aggregation due to excess moisture can be prevented. The allowable range of this digestion water is affected by the residual CO ₂ content in quicklime. For example, the lower the residual CO ₂ content in quicklime, the higher the upper limit of the amount of digested water tends to be, so the permissible range becomes wider. If the digestion water is less than 32 parts by weight, undigested quick lime remains in the slaked lime after the digestion reaction. On the other hand, when the amount of digested water is more than 48 parts by weight, the BET specific surface area is 20 m ² / g or more, the crystallinity is high, the slaked lime is rich in agglomerated particles, and the content of aragonite is significantly reduced. However, the ratio of the quicklime and digested water can vary beyond this range depending on conditions such as the activity of quicklime and the type of digester.

The digestion reaction is usually carried out with stirring using a mixer. Agitation is preferably carried out at a relatively high speed. As the digestion water, tap water or industrial water is used. Moreover, although the temperature of digestion water is not specifically limited, 20-80 degreeC is preferable.
As the mixer, known mixers such as a plow blade mixer, a single paddle screw mixer, and a double paddle screw mixer can be used.

  The product after the digestion reaction is then aged by allowing it to stay in an aging machine for a predetermined time as necessary. Unreacted quicklime can be eliminated by aging. The residence time for aging is usually 10 to 180 minutes, preferably 30 to 60 minutes. The temperature of the aging machine is 70 to 120 ° C, preferably 80 to 110 ° C.

In the present invention, when slaked lime and green liquor are mixed, the BET specific surface area is 20 m ² / g or less, the peak intensity of 2θ = 18.0 ([001] crystal plane) of X-ray diffraction / 2θ = 34.1 ([101] crystal Slaked lime having a peak surface intensity × 100 value of 60 to 80 and a 100 mesh pass portion of 70% or more must be mixed with water at 20 to 100 ° C. in advance to generate a slaked lime slurry. Although the mechanism of the present invention has not been fully elucidated, generally, the higher the rate of formation of calcium carbonate, the easier it is to form aragonite crystals, and slaked lime that satisfies the above conditions has high solubility in water, and the carbonation reaction Because it is promoted, the aragonite content is expected to increase. The temperature of the water mixed with the slaked lime is 20 to 100 ° C, and preferably 50 to 80 ° C. If the temperature is below 20 ° C, the shape of the calcium carbonate produced will be indeterminate or agglomerated, resulting in inferior wire wear properties and the inability to obtain the desired paper quality. At temperatures above 100 ° C, the boiling point under atmospheric pressure Therefore, a pressurized causticizing device is required, which is uneconomical.
The green liquor used in the present invention is usually the one produced in the causticizing process, but the green liquor introduced from outside the above process, that is, the green liquor produced in another causticizing process can also be used. It is.

As a causticizing reaction method, in addition to the conventional causticizing method, a method for controlling the shape of the causticized light calcium carbonate to be produced can be used without any change.
The acicular calcium carbonate obtained by the present invention can be produced at a lower cost than the acicular calcium carbonate obtained in the conventional causticizing process, and the quality as a filler for papermaking is equivalent. By using this as a papermaking raw material for paper production, high-quality paper and coated paper having excellent whiteness, opacity, printability and the like can be produced at a lower cost than before.

  Although acicular calcium carbonate is very excellent in performance as a papermaking raw material, the use and use amount as a papermaking raw material are limited because the price of the obtained calcium carbonate is higher than that of conventional self-made calcium carbonate.

  Compared to these conventional methods, in the method of the present invention, the causticizing raw material is quick lime, and the conventional quality of acicular calcium carbonate, which has been a problem by controlling well the quality and dry digestion conditions, is maintained, and the production cost is greatly increased. Reduced.

Action

Although the mechanism of the present invention has not been fully elucidated, the following can be considered as the cause of the high residual CO ₂ content in quicklime reducing the aragonite content of acicular causticized light calcium carbonate. Residual CO ₂ content in quicklime is thought to exist as calcium carbonate in calcite crystals, and in the carbonation of slaked lime derived from quicklime containing much residual CO ₂ content, calcite calcium carbonate in slaked lime It is presumed that calcite-based calcium carbonate is easily generated. Moreover, the BET specific surface area is 20 m ² / g or less, the peak intensity of 2θ = 18.0 ([001] crystal plane) by X-ray diffraction / the peak intensity of 2θ = 34.1 ([101] crystal plane) × 100 is 60. The reason why a high aragonite content (75% or more) can be achieved by carbonizing slaked lime with a -80 or 100 mesh pass content of 70% or more is considered as follows. In general, it is said that aragonite is more likely to be produced as the production rate of calcium carbonate during the carbonation reaction becomes faster. Slaked lime that satisfies the above conditions with high amorphousness and low agglomeration content easily dissolves in water, and as a result of promoting the carbonation reaction, a high aragonite content can be achieved.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is naturally not limited to the examples.
[Test method]
(1) Residual CO ₂ min: Measured with a carbon-in-metal analyzer (Horiba Seisakusho EMIA-110).
(2) BET specific surface area: Slaked lime was dried under reduced pressure at 50 ° C. for 8 hours, and then measured with a Micromeritics Gemini 2360 manufactured by Shimadzu Corporation.
(3) Crystallinity: After drying slaked lime under reduced pressure for 8 hours at 50 ° C., the peak intensity and 2θ of 2θ = 18.0 ([001] crystal plane) obtained by X-ray diffraction (XD-D1 manufactured by Shimadzu Corporation) measurement A peak intensity ratio of 34.1 ([101] crystal plane) was used as an index of crystallinity.
(4) Aragonite content: Samples having different aragonite contents were prepared in advance by mixing 100% aragonite calcium carbonate and 100% calcite calcium carbonate, and then measured with an X-ray diffractometer (XD-D1 manufactured by Shimadzu Corporation). The aragonite content of acicular causticized light calcium carbonate was calculated using a calibration curve obtained from the aragonite peak intensity of 2θ = 26.2 and the calcite peak intensity of 2θ = 29.4 of each sample.
[Example 1]

After putting 605 g of quick lime with a residual CO ₂ content of 0.6% into a dry digester (Proshear mixer WB-5 manufactured by Taihei Koki Co., Ltd.), the mixture was vigorously stirred with a spindle blade (frequency 60/120 Hz). Digested water (tap water) was dropped at a flow rate of 66 mL / min onto a chopper rotating at high speed (frequency 60/120 Hz) over 5 minutes. The ratio of quicklime and digested water is 1.7 mole of digested water per mole of quicklime. After completion of the digestion water addition, the chopper was stopped, and further digestion and aging were performed with stirring on the spindle for 55 minutes.

  A separable flask (volume 1 L) equipped with a stirrer (stirring speed 700 rpm, Kyoei Power Stairrer Type PS-2N) and a heating mantle heater was used as a causticizing reaction apparatus. 75 mL of hot water at 50 ° C. was poured into the flask, and then 50 g of slaked lime obtained by the above dry digestion was added to obtain a 40% slaked lime slurry.

Furthermore, 540 mL of green liquor at 50 ° C (composition: Na ₂ CO ₃ = 95 g / L, Na ₂ S = 25 g / L, NaOH = 12 g / L, all in terms of Na ₂ O) in 2 hours It was causticized as a result. Calcium carbonate produced from the reaction solution was collected by suction filtration, thoroughly washed with ion-exchanged water, dehydrated, and dried in a blow dryer at 105 ° C. to obtain powdered calcium carbonate. Table 1 shows the measurement results of the aragonite content of calcium carbonate.
[Comparative Example 1]

Using dry lime with a residual CO of ₂ % 1.0%, after dry digestion in the same manner as in Example 1 except that the ratio of quick lime and digested water during dry digestion is 1.2 moles of digested water with respect to 1 mole of quick lime, Calcium carbonate was synthesized. Table 1 shows the aragonite content measurement results.
[Comparative Example 2]

Using dry lime with a residual CO of ₂ % 1.0%, after dry digestion in the same manner as in Example 1 except that the ratio of quick lime and digested water during dry digestion is 1.5 moles of digested water with respect to 1 mole of quick lime, Calcium carbonate was synthesized. Table 1 shows the aragonite content measurement results.
[Comparative Example 3]

Calcium carbonate was synthesized after dry digestion in the same manner as in Example 1 except that quick lime having a residual CO ₂ content of 1.6% was used. Table 1 shows the aragonite content measurement results.
[Comparative Example 4]

Calcium carbonate was synthesized after dry digestion in the same manner as in Example 1 except that quick lime having a residual CO ₂ content of 2.3% was used. Table 1 shows the aragonite content measurement results.
[Example 2]

Residual CO of quicklime is 1.0% for ₂ minutes, and after dry digestion in the same manner as in Example 1 except that the ratio of quicklime and digested water during dry digestion is 1.0 mole of digested water per mole of quicklime, Calcium was synthesized. Table 1 shows the aragonite content measurement results.
[Example 3]

The residual CO of quicklime is 1.0% for ₂ minutes, and after dry digestion in the same manner as in Example 1 except that the ratio of quicklime and digested water during dry digestion is 1.1 mole of digested water to 1 mole of quicklime, Calcium was synthesized. Table 1 shows the aragonite content measurement results.
[Comparative Example 5]

Residual CO of quicklime is 1.0% for ₂ minutes, and after dry digestion in the same manner as in Example 1 except that the ratio of quicklime and digested water during dry digestion is 2.0 moles of digested water with respect to 1 mole of quicklime, Calcium was synthesized. Table 1 shows the aragonite content measurement results.

From Table 1, it was found that the lower the residual CO ₂ content in quicklime, the higher the aragonite content (Example 1 and Comparative Examples 3 and 4). When quick lime with a residual CO ₂ content of 1.0% is used, the BET specific surface area is 20 m ² / g or less, the X-ray diffraction 2θ = 18.0 ([001] crystal plane) peak intensity / 2θ = 34.1 ([ 101] crystal plane) peak intensity × 100 is 60 to 80, and slaked lime with a 100 mesh pass is 70% or more, the aragonite content is 75% or more (Examples 2, 3 and Comparative Examples). 1, 2, 5).

Claims (2)

A method for producing acicular calcium carbonate that is useful as a raw material for papermaking in the causticizing step of the pulp production process by the sulfate method or soda method, and has a BET specific surface area of 20 m ² / g or less, X-ray diffraction 2θ = 18.0 ([001] crystal plane) peak intensity / 2θ = 34.1 ([101] crystal plane) peak intensity x 100 is 60 to 80, and 100 mesh passes are 70% or more. As a raw material for papermaking, comprising mixing -100 ° C water to produce a slaked lime slurry, mixing the slaked lime slurry and green liquor, and generating calcium carbonate by causticizing while stirring or kneading. A method for producing useful acicular calcium carbonate.   The method for producing acicular calcium carbonate according to claim 1, wherein the slaked lime is obtained by dry digestion of quicklime.