JPH01226719A - Production of calcium carbonate for making paper - Google Patents

Abstract

PURPOSE:To industrially produce cheap CaCO3 having high whiteness, by adding a specified amt. and then, the remainder of quick lime in two steps in a causticizing stage of green liq. in production of kraft pulp. CONSTITUTION:Quick lime is added in two steps in a process adding quick lime to the green liq. produced in a process for recovering chemicals in production of kraft pulp to slake and causticize. In a first step, 0.5-50% the amt. of total quick lime to be added is added in advance and after removing the produced lime sludge, the remainder of the quick lime is added to cause causticizing reaction. In this case, the purpose of addition of quick lime in the first step is to remove impurities suspended in the green liq. by co- precipitating with CaCO3 produced in the causticizing reaction and therefore, further causticizing reaction is not needed. The light CaCO3 having high whiteness is obtd., because in a second step the quick lime is added to the green liq. clarified in the first step.

Description

Detailed Description of the Invention "Industrial Application Field" The present invention relates to a novel method for producing calcium carbonate for paper making, in which quicklime is added in stages in the chemical recovery process of the kraft pulp (hereinafter referred to as KP) production process. The present invention relates to a method for producing calcium carbonate having a high degree of whiteness and useful as a filler for paper making.

"Prior Art" Currently, calcium carbonate is used in large quantities as a filler for paper, plastics, rubber, etc., and as a white pigment for paints, inks, etc.

Incidentally, in recent years, library books and important documents that have been stored for long periods of time have been degraded by acid, causing controversy over their preservation, and there has been a strong demand for the paper industry to produce paper products that have a permanent shelf life. It's here.

Due to these circumstances, the paper manufacturing industry has changed from the conventional acidic paper making to neutral to
So-called neutral papermaking, in which paper is made under slightly alkaline conditions, is rapidly growing. Calcium carbonate is generally used as a filler for papermaking in neutral papermaking, and mainly heavy calcium carbonate, which is an inexpensive crushed product of natural stone and is abundant in Japan, has been used.

However, the metal water squeezing nets that have been used for many years in the dewatering part of paper machines are now being replaced with plastic wires due to the trends of the times. It has poor wear resistance, and has drawbacks in terms of operability and economy (short wire life). Therefore, recently there has been a shift to light calcium carbonate (synthetic calcium carbonate), which has low abrasion properties.

However, light calcium carbonate in -S is quicklime (C
Since slaked lime (Ca(OH)z) is produced by slaked aO) and then reacted with carbon dioxide, it results in an expensive product and increases the product cost.

Therefore, the amount of calcium carbonate used is currently limited mainly due to economic reasons, and inexpensive light calcium carbonate is strongly desired.

``Problems to be Solved by the Invention'' In view of the above-mentioned circumstances, an attempt was made to consider the application of calcium carbonate, which is produced as a by-product in the causticizing process of green liquor in the KP manufacturing process.

Calcium carbonate produced in the causticizing process is produced as follows.

That is, calcium carbonate is produced from soda carbonate in the green liquor by adding quicklime or slaked lime to the green liquor.
The calcium carbonate produced from the manufacturing process is light calcium carbonate. However, the light calcium carbonate obtained in this way usually contains compounds such as iron and manganese and unburned carbon, and its whiteness is extremely low.
It was completely unsuitable as a filler for paper making.

Here, the green liquor is made by craft cooking wood chips (
It is generated during the process of boiling chips in a mixture of caustic soda and soda sulfide, which are cooking chemicals, and then separating the resulting pulp from the cooking waste liquid and recovering the chemicals.It mainly contains soda carbonate and soda sulfide. However, the aqueous solution is colored green (black) because it contains impurities such as iron compounds (usually called Doresog). Note that the causticizing reaction refers to a reaction in which quicklime is added to green liquor to produce caustic soda, which is the main component of cooking chemicals.

For the reasons mentioned above, the inventors of the present invention have made extensive studies and have found that by providing simple operations and some incidental equipment, paper can be made from green liquor that has high whiteness and is excellent as an internal filler. They have discovered a method that makes it possible to industrially produce calcium carbonate for commercial use, and have finally completed the present invention.

"Means for Solving the Problems" The present invention divides the addition of quicklime into two stages in the process of adding quicklime to slake and causticize the green liquor generated in the chemical recovery process of the kraft pulp manufacturing process. In the first stage, 0.5 to 50% of the total amount of quicklime is added, and after removing the lime sludge, the remaining quicklime is added to carry out a causticizing reaction. This is a method for producing calcium.

"Function" As described above, the green liquor used in the present invention is generated in the chemical recovery process of the kraft pulp manufacturing process, and mainly contains soda carbonate and sodium sulfide. The causticizing process of green liquor is a reaction in which quicklime is added to the soda carbonate in the green liquor to produce caustic soda and calcium carbonate as shown by the following equation (1): NazCO3+CaO+H,0=2NaOH+CaCO5 (1). At this time, when 1 mol of sodium carbonate is causticized, 1 mol of calcium carbonate is produced as a by-product. Impurities such as compounds such as iron and manganese and unburned carbon are suspended in this green liquor, and it is necessary to remove these in advance.

Generally, in the KP manufacturing process, a tallifier (sedimentation machine) is mainly used to clarify the green liquor, but in order to obtain calcium carbonate with high whiteness required as a filler for paper making. is not an appropriate clarification method.

In the method of the present invention, the purpose of adding quicklime in the first step is to remove impurities suspended in the green liquor by co-precipitation with calcium carbonate produced in the causticizing reaction. If there is enough calcium carbonate for coprecipitation to remove the oxidation, no further causticization reaction is necessary. Therefore, the amount of quicklime added depends on the amount of impurities mixed in the green liquor, but it is 0.5 to 0.5 of the total amount of quicklime added.
50% by weight, more preferably in the range of 2.5 to 15% by weight. That is, even if the amount of quicklime added in the first stage is less than the lower limit of the above range or more than the upper limit, the desired results of the present invention cannot be expected. Also,
In the first stage, it is also possible to add various flocculants to expect a further clarification effect.

Next, the causticizing time in the first stage is generally about 1 to 60 minutes, although it depends on the temperature during the reaction, and the causticizing time in the second stage is generally about 30 to 180 minutes.

The causticizing reaction of soda carbonate is an endothermic reaction and becomes significantly slower at low temperatures. When quicklime is added, the slaked reaction is an exothermic reaction, so causticization is usually promoted by the heat, but when slaked lime is used, causticization is performed by heating. The causticizing temperature is more preferably 90°C or higher.

Furthermore, two methods have been proposed for adding quicklime in the first stage.

The first method is to add quicklime as a dreg sedimentation accelerator in green liquor clarification equipment such as a green liquor clarifier, and simultaneously perform slaking, causticization, and sedimentation in the green liquor clarification equipment. .

According to this method, the efficiency of removing Dregs in green liquor clarification equipment such as a green liquor clarifier is significantly improved, and the concentration and dewatering properties of Dregs that settle due to the addition of quicklime are also significantly improved, and the processing of Dregs is facilitated. It comes with the benefit of being. Furthermore, compared to the case where only an expensive polymer flocculant is used as a sedimentation accelerator, this method has great economic advantages because quicklime can be used in the chemical recovery process and almost no incidental equipment is required.

The second method is to install a slaker, causticizing tank, and settling tank in addition to the conventional process after green liquor clarification equipment such as a green liquor clarifier. The clarification of the green liquor is carried out in two stages, which further improves the clarification effect of the green liquor compared to the first method, and requires less quicklime to be added in the first stage. It is possible to obtain a larger amount of light calcium carbonate with high whiteness, which is the object of the present invention.

On the other hand, according to the method of the present invention, in the first step, the existing KP
After treating all or part of the green liquor using either of the above two methods in the chemical recovery process, it is also possible to bypass a part of the green liquor and use it for producing calcium carbonate for papermaking. be. In this case, as the second step is to install a slaker, causticizing tank, and sedimentation tank for producing calcium carbonate for papermaking separately from the existing KP chemical recovery process, it is possible to change the causticizing reaction conditions variously, and as a result, The particle size of the calcium carbonate obtained can be selected within a more preferable range for use as a paper filler.

When the second stage causticizing reaction in the present invention is completed, the produced calcium carbonate is separated through a known filtration means such as a belt filter or a pressure filter. The obtained calcium carbonate has a high degree of whiteness, and after thorough washing, it can be used as a filler for paper making as it is, but if necessary, it can be processed by a commonly known wet or dry pulverization process and/or classification. Treatment to remove coarse particles, improvement of filler dispersibility, and measures to improve the abrasion resistance of plastic wires are carried out as appropriate.

Note that the obtained filtrate is returned to the cooking process as white liquor (cooking chemicals). The white liquor in this case has excellent clarity and a high causticization rate (suitable as a cooking chemical) compared to the conventional method of adding quicklime.

"Examples" The present invention will be described in more detail with reference to Examples below, but the present invention is of course not limited to these.

In addition, unless otherwise specified, parts and % in the examples indicate parts by weight and % by weight, respectively.

Example I One part of quicklime generated in the chemical recovery step of the KP manufacturing process was dissolved in one part of water to prepare a slaked lime emulsion with a solid content concentration of 50%. Next, green liquor 100 having a composition as shown in Table 1 below is prepared.
1 part of the slaked lime emulsion obtained as described above was added to 0 parts, the temperature was maintained at 90° C., and the mixture was allowed to stand for 5 minutes, after which the precipitate was separated and removed by the sedimentation gradient method. Obtained supernatant liquid 90
As a second step, 136 parts of slaked lime emulsion was added to 0 parts,
A causticizing reaction was carried out for 90 minutes while maintaining the temperature at 90°C.

Furthermore, the precipitate formed was separated and removed in the same manner as in the first step to obtain a white liquor. At this time, the compositional analysis values, causticization rate, and SS amount of the green liquor and white liquor were measured, and the results are shown in Table 2.

The composition analysis value, causticization rate, and SS amount were measured according to the Scandinavian paper valve standard 5CAN-N2:63 and 5.
It was conducted in accordance with CAN-W6 Near 1.

On the other hand, the precipitate from the second stage was filtered, separated, collected, thoroughly washed with water, and then dried to obtain a calcium carbonate sample. Also,
The precipitate was filtered, separated and collected, thoroughly washed with water, and made into a slurry having a solid content and seismic intensity of 55%.The slurry was pulverized using a sand grinder, and then dried to obtain a calcium carbonate sample. Further, the precipitate from the second stage was filtered, separated and collected, thoroughly washed with water, dried, and classified using a powder air classifier (manufactured by Nippon Pneumatic Industries) to obtain calcium carbonate for testing. In addition, the precipitate B in the second stage was filtered, separated and collected, thoroughly washed with water, pulverized in the same manner as above, dried and classified in the same manner as above, and the calcium carbonate sample was obtained. Obtained.

The whiteness, average particle diameter, and wire abrasion test of each calcium carbonate sample obtained as described above were conducted, and the obtained results are shown in Table 3. The whiteness was measured by preparing tablets according to the American Pulp and Paper Technology Association standard T672os-76 using an El Rejo brightness meter, and the average particle size was measured using an X-ray transmission particle size distribution analyzer manufactured by Shimadzu Corporation. (Sedigraph 5000-01), and the wire abrasion test was performed using a Nippon Filcon type wire abrasion tester.

Table 1 [Analysis values of causticizing reaction leading edge fluid] Examples 2 to 7 Using the same slaked lime emulsion and green liquor as in Example 1, Table 2
The same procedure as in Example 1 was repeated except that the amounts of slaked lime emulsion added in the first and second stages were changed as shown in Table 3, and the results obtained are shown in Table 3.

In addition, the unclassified and unpulverized calcium carbonate obtained in Examples 2 to 6 and Comparative Example 1, commercially available heavy calcium carbonate SS-1200 (commercial product 1/manufactured by Shiraishi Kogyo), and commercially available light calcium carbonate TNC -C30 (commercial product 2/manufactured by Toyo Denka Co., Ltd.) was used as an internal filler to make paper under the following papermaking conditions, and a paper quality test was conducted on the paper obtained. The test results are shown in Table 4.

[Paper making conditions]

Pulp, LBKP 100% Beating degree; 400 mA' C3F filler; Ash content (Ca COz equivalent) 10% sizing agent;
Burcon 40 0.4% (Dick Bercules type) Fixing agent: Separan AP-300.04% (manufactured by Dow Chemical) Under the above preparation conditions, air-dried according to JIS-P 8209, approximately 60 g /r+? I obtained handmade paper.

Comparative Example 1 To 1000 parts of the same green liquor as in Example 1, 137 parts of slaked lime emulsion prepared in the same manner as in Example 1 was added at once, and 9
The causticization reaction was carried out for 90 minutes while maintaining the temperature at 0°C. Next, the resulting precipitate was separated and removed in the same manner as in Example 1 to obtain a white liquor.

In addition, the composition analysis, causticization rate, and SSI of the white liquor were determined in Example 1.
The results were shown in Table 2. Also,
The precipitate was obtained in the same manner as in Example 1 to obtain each calcium carbonate sample, and the physical properties of each powder were measured. The obtained results are shown in Table 3.
It was shown to.

"Effects" As is clear from the examples, the present invention adds quicklime to the green liquor in two parts, and the clarification of the green liquor in the first stage is more effective than the conventional treatment method using only a tallifier. In the second stage, quicklime is added to the clarified green liquor, resulting in highly white calcium carbonate.

Moreover, since the method of the present invention utilizes calcium carbonate produced as a by-product in the KP manufacturing process, it has great economic advantages.

Claims (1)

[Claims] In the process of adding quicklime to slaked and causticized green liquor generated in the chemical recovery process of the kraft pulp manufacturing process, the addition of quicklime is divided into two stages, and the quicklime is added in the first stage. A method for producing calcium carbonate for paper making, which comprises first adding 0.5 to 50% of the total amount of quicklime, removing the resulting lime sludge, and then adding the remaining quicklime to carry out a causticizing reaction.