JP6304024B2 - Method for producing deinked pulp - Google Patents

Description

  The present invention relates to a method for producing waste paper pulp, and more particularly, to a method for producing deinked pulp excellent in maintaining high ash content and excellent ink removability from printed waste paper.

Conventionally, various yield agents and yield systems have been studied and adopted for improving the yield of fillers and fine fibers in the papermaking stage. The most widely adopted system is a single polymer formulation using a cationic polymer. Even in the used paper recycling process, in order to improve the yield rate of fine fibers and fillers, earnest research has been conducted on the collection of ash in the floss and the collection of fine fibers, and a certain yield improvement is possible. However, it was difficult to further improve the yield while maintaining the ink removal property of the finished deinked pulp.

For example, in the used paper recycling treatment process, after passing through the current alkaline region flotation, a method of adding a coagulant to the pulp slurry before the washing step and the dewatering step (for example, Patent Document 1), as well as the current alkaline After passing through the flotation of the region, a method of adding a water-soluble amphoteric copolymer obtained by polymerizing (meth) acrylamide + cationic monomer + anionic monomer as a constituent component (for example, Patent Document 2), and (meth) acrylamide and A cationic monomer as a constituent component, or (meth) acrylamide, a cationic monomer, and (c) an anionic monomer as a constituent component, and (d) a crosslinkable monomer and / or (e) a chain transfer agent. There is a method of adding a water-soluble ionic polyacrylamide copolymer polymerized by using (Patent Document 3). However, the flotation in the alkaline area currently used in the used paper deinking process does not completely remove the dirt and printing ink derived from the used paper, so the coagulant is used to improve the yield of deinked pulp afterwards. When the paper strength agent and the retention agent are used, there is a problem that the ink removal property of the finished deinked pulp is remarkably inferior.

Japanese Patent No. 4809585 Japanese Patent No. 4521578 Japanese Patent No. 4824376

  The conventional deinked pulp manufacturing method removes the ink contained in the deinked pulp together with ash in the screening process, flotation process, washing process, dehydrating process, etc. Manufacture. However, although the ink has an adverse effect on the quality of the finished deinked pulp, the ash is relatively white, and it can remain in the pulp slurry as a constituent of the finished deinked pulp in terms of cost and waste. From the viewpoint of reducing the environmental impact, and further, from the aspect of equipment maintenance, the ash discharged in the filtrate without remaining in the deinked pulp accumulates as a scale in the piping of the inflow. It is advantageous.

  In view of the above-mentioned conventional problems, the present invention provides a deinked pulp having high ash content and excellent residual ink removal properties from used printed paper, and is capable of reducing sludge ash and reducing the amount of filler used. It is an object to provide a manufacturing method.

Conventionally, in a method for producing deinked pulp from used printed paper, in a pulping process of used paper consisting of a combination of a disaggregation process, a screening process, a bleaching process, a kneading process, a flotation process, and a washing process, a bleaching process performed in an alkaline manner, The flotation process after the kneading process has been performed in an alkaline region in the range of more than pH 9 to about pH 11. As a result of repeated investigations, the inventors have sufficiently removed the ink by performing the flotation process in the neutral region of H6 to 9, and the coagulant, the paper strength agent, It has been found that adding one or more chemicals as a retention agent can provide a deinked pulp with high yield and excellent ink removability, and the following invention has been completed.

In a method of pulping used printed paper through a series of steps including at least a disaggregation step, a dust removal step, a bleaching step, a dispersion step, a flotation step, and a washing step, the pulp slurry that has undergone the bleaching step and the dispersion step has a pH of 6 in the flotation step. It is characterized by performing deinking treatment in a neutral region of ˜9, and adding one or more selected from a coagulant, a paper strength agent and a retention agent to the pulp slurry immediately before the subsequent washing step. A method for producing deinked pulp.

The said deinking pulp manufacturing method whose addition amount of 1 or more types of chemical | medical agents chosen from the coagulant | flocculant, a paper strength agent, and a retention agent in the said washing | cleaning process is 100 ppm-3000 ppm.

Manufacture of one of the above deinked pulps, wherein the series of steps is a step of manufacturing deinked pulp from waste printed paper by sequentially performing a disaggregation process, a dust removal process, a dispersion process, a bleaching process, a flotation process, and a washing process. Method.

  According to the present invention, in a method for producing deinked pulp from used printed paper, in a pulping method of used paper by a combination step having at least a disaggregation step, a screening step, a bleaching step, a kneading step, a flotation step, and a washing step, By removing the ink sufficiently by carrying out the flotation process in the neutral region of H6-9, adding one or more agents of a coagulant, a paper strength agent, and a retention agent to the subsequent pulp slurry is excellent. The yield can be greatly improved while maintaining the ink removability. Therefore, it is possible to improve the yield of ash containing fillers derived from waste paper, to highly blend deinked pulp cheaper than kraft pulp, mechanical pulp, etc. into paper products, and to reduce the environmental burden, The range of used paper is expanded.

Hereinafter, embodiments of the method for producing deinked pulp of the present invention will be described in detail.
Examples of the used printing paper as a raw material include used paper containing 7% to 40% ash, such as newspaper, fine coated paper, high ash coated paper, and non-coated paper. The present invention is also effective for used printed paper including some neutral newspapers having poor deinking properties due to a large amount of surface treatment agent.

There is no particular limitation on the disaggregation step in the method of the present invention, but as a preferable treatment, first, the raw printed paper is put into a disaggregator, diluted water is added so that the solid content concentration becomes 12 to 18%, and further chemicals (hydroxylation) Sodium) is added to the pulp in an amount of 0 to 3.0% by mass, preferably 0.1 to 2.5% by mass. In the case of adding a deinking agent, those having strong permeability to pulp fibers and strong ink peelability are preferable, and the deinking agent is 0.01 to 0.5% by mass, preferably 0.03% by weight of the pulp. Add ~ 0.3 wt%. The disaggregation time is 10 to 30 minutes, preferably 10 to 25 minutes, more preferably 10 to 18 minutes, and the disaggregation temperature is 10 to 50 ° C., preferably 30 to 50 ° C.

Examples of deinking agents with strong ink peelability include higher alcohol deinking agents and amphoteric deinking agents, such as DI-7020, DI-7030, DI-767, and DI-7282 manufactured by Kao Corporation. DIA-Z-100, DIA-Z-5000 manufactured by Nissin Chemical Laboratory, Neoscore FW-780, Neoscore FW-790, Neoscore FW-795, FT-467, FT manufactured by Toho Chemical Co., Ltd. -470, FT-487, FT-511, FT-513, FT-514, FT-515, BB agent, Daiho Kogyo Co., Ltd., Daihope 940, Daihope 960, Nikka Corporation Lipobright There is DP-810 etc., but it is not limited to these.

There is no restriction | limiting in particular in the apparatus used at the dust removal process in the method of this invention. Although it is sufficient that foreign substances in the raw material can be removed with a screen cleaner, it is preferable to use a slit screen (first stage 0.15 mm slit or less, second stage 0.15 mm slit or less). Any cleaner may be used as long as it can efficiently remove heavy foreign matter.

Hydrogen peroxide, hydrosulfide, thiourea dioxide, hypo, etc. are used as bleaching chemicals for recycled used paper. In the present invention, hydrogen peroxide is used. Further, in the method of the present invention, alkaline hydrogen peroxide bleaching is performed, and hydrogen peroxide is added in an amount of 0.5 to 5.0% by mass with respect to pulp. Even if the amount added is further increased, the increase in whiteness tends to saturate. Caustic soda is added to 1.5 to 3.0% by mass of pulp, and sodium silicate is added to 1.5 to 3.5% by mass of pulp. Bleaching time is 10 minutes to 5 hours, preferably 1.5 to 3 hours. To do. If the bleaching time is too short, hydrogen peroxide will not react sufficiently, which is not preferable. Also, if the bleaching time is longer than 5 hours, the consumption of hydrogen peroxide will increase, and the yellow color of the pulp will start from the point when there is no remaining hydrogen peroxide. It is not appropriate because it will occur. The bleaching pulp concentration is 15 to 35%, preferably 25 to 30%, and when the concentration is lower than 15%, the reactivity of hydrogen peroxide is deteriorated. On the other hand, if the concentration is higher than 35%, it is not preferable because the mixture with the chemical in the disperser is uneven. Further, the pH during bleaching is preferably 10.5 to 12.0. If the pH is outside this range, the bleaching property of hydrogen peroxide is inferior. It is effective to carry out the bleaching temperature at 50 to 120 ° C.

Further, as long as the deinking agent can be uniformly mixed with the pulp before flotation, the deinking agent may be added to any of them before and after the bleaching step. Deinking agents having strong ink cohesion include fatty acid or fatty acid derivative-based deinking agents. For example, in the case of fatty acids, DI-254 (oleic acid), DI-268, Daiichi manufactured by Kao Corporation. K-4004-D manufactured by Kogyo Seiyaku Co., Ltd. is available. In the case of fatty acid derivatives, DI-1120 and DI-1050 manufactured by Kao Corporation, DIY-23543 manufactured by Nissin Chemical Laboratory Co., Ltd., Paper Aid W manufactured by Daiichi Kogyo Seiyaku Co., Ltd. Yes, but not limited to these.

A low-speed, high-concentration shaft type disperser or a disk-type disperser is suitable for the processing by the disperser performed in the dispersion step (kneading step) of the present invention. As the low-speed and high-concentration shaft type disperser, a uniaxial or biaxial kneader type disperser is preferable. It has a rotary blade attached to a shaft-shaped rotor and a fixed blade attached to a casing, and has a processing concentration of 20% to 50% (hereinafter, pulp concentration% means mass%) at a low speed of 50 to 300 rpm. ), Preferably at 25% to 40%, and at a temperature of 25 ° C to 100 ° C, preferably 40 to 90 ° C. In the shaft type disperser, the frictional action between the fibers is the main, and the ink peeling and dirt dispersion occur. If the treatment concentration is less than 20%, it is difficult to apply a mechanical load, and ink dispersibility / dirt dispersibility is lowered, and energy required for temperature increase is enormous, which is not suitable. Further, if the treatment concentration exceeds 50% and the concentration is high, it is difficult to squeeze water mechanically.

In general, as a single-shaft kneader, kneading disperser KD (trade name: manufactured by IH IFOIT Paper Technology), disperser (trade name: manufactured by Aikawa Tekko), disperser (trade name: ASEC), disperser (trade name: manufactured by Sanei Regulator Co., Ltd.), CCE type kneading machine (trade name: manufactured by Niihama Pump Manufacturing Co., Ltd.), kneader (product name: manufactured by Hyakuma Yamamoto Manufacturing Co., Ltd.), etc. In addition, as the biaxial kneader, those manufactured by Niihama Pump Manufacturing Co., Ltd., Yamamoto Hyakuma Manufacturing Co., Ltd., and the like are used, but are not limited to specific models.

Further, the disk type disperser is not particularly limited as long as it is a disk type disperser or a conical type disperser. Structurally similar to a disc refiner, but the disc plate has a different structure. Further, the conical type disperser has a rotating blade in a conical shape. Processing is performed at a rotational speed of 300 rpm to 2500 rpm and a processing concentration of 20% or more. The difference from the shaft type disperser is that the ink and the dirt are dispersed mainly by the collision action of the fiber and the blade. Generally, as a disk-type disperser, a disperser HTD (trade name: manufactured by IH / IFOIT PAPER TECHNOLOGY), a KRIMA hot dispersion facility (trade name: manufactured by Cellwood), etc. are used. As conical type dispersers, conical discs (trade name: manufactured by Aikawa Tekko Co., Ltd.), conical dispersion systems / HI pre-heaters / Optifiner dispersers (product name: manufactured by Metso SHI), etc. are used. It is not limited.

A particularly preferable combination of the dispersing machines is efficient and effective to perform processing in this order twice with a low-speed / high-concentration shaft type dispersing machine and once with a disk-type dispersing machine. The dispersion process by the low-speed / high-concentration shaft type disperser which is the first or second dispersion process is a high concentration of treatment concentration 20% to 50%, preferably 25% to 40%, and the temperature is 25 ° C. The treatment is performed at 100 ° C, preferably 40 to 90 ° C. In the shaft type disperser, the frictional action between the fibers is the main, and the ink peeling and dirt dispersion occur. If the treatment concentration is less than 20%, it is difficult to apply a mechanical load, and ink dispersibility / dirt dispersibility is lowered, and energy required for temperature increase is enormous, which is not suitable. Further, if the treatment concentration exceeds 50% and the concentration is high, it is difficult to squeeze water mechanically. In order to enhance the bleaching effect in the subsequent hydrogen peroxide bleaching step, the temperature needs to be raised to 25 ° C. or higher. However, if it exceeds 100 ° C., hydrogen peroxide is decomposed, which is not preferable.

The temperature in the case of performing the dispersion process by the disk type disperser in the method of the present invention is 90 to 130 ° C. If the treatment is less than 90 ° C., sufficient dart reduction effect and bleaching effect of thiourea dioxide cannot be obtained, while if it exceeds 130 ° C., yellowing of the pulp occurs, which is not suitable. In addition to hydrogen peroxide and thiourea dioxide, alkaline chemicals may be added to the disk type disperser. As the alkaline chemical, sodium hydroxide is preferable. The amount of hydrogen peroxide added is 0.05 to 3% by mass with respect to the mass of the absolutely dry pulp, and the amount of thiourea dioxide added is 0.1 to 2% by mass with respect to the mass of the absolutely dry pulp, particularly preferably 0. .2 to 1.0% by mass.

A flotation process is performed after the high-temperature disperser treatment. It is efficient to remove the ink that has been peeled off by the disperser treatment, particularly in the neutral zone flowator process. In the case of the method of removing in the washing step, it is not preferable because it circulates in the white water together with the washing solution and may affect the finished whiteness of the pulp. Although there is no restriction | limiting in the form of the flotator in a flotation process, it is preferable to carry out at a processing concentration of 0.7 to 1.5% and a flotator processing temperature of 10 to 55 ° C, preferably 30 to 50 ° C.

The treatment pH in the flotation step in the method of the present invention is suitably 6-9, more preferably 7-8.5. When the pH is lower than 6, corrosion of piping and equipment becomes a problem, and equipment costs are expensive. On the other hand, if the pH exceeds 9, the ink removal property of the deinked pulp is significantly deteriorated. Further, sulfuric acid, hydrochloric acid, caustic soda, sodium silicate, or the like is used as an agent for adjusting the flotation treatment pH, but is not limited thereto.

The method of the present invention includes a washing step of dehydrating and washing fine ink that has not been removed by a flotator after flotation in a neutral region of pH 6-9. Although there is no restriction | limiting in particular in a washing | cleaning apparatus, After diluting 0.6-1.5% pulp slurry with fresh water or the clear white water of a paper machine, it is preferable to carry out dehydration washing to 15-35%. The cleaning equipment used in the cleaning process includes extractors, fall washers (Eiko), and double nip thickeners (Ishikawajima Sangyo), but the cleaning equipment preferentially removes ink in the raw materials. However, it is preferable that the washing machine can minimize loss of fiber and ash. Among them, a wire washer is preferable, and the eye hole can be about 20 to 200 mesh, preferably 40 to 100 mesh, and more preferably 50 to 80 mesh.

In the method of the present invention, one or more of a coagulant, a paper strength agent, and a retention agent are added to the deinked pulp slurry immediately after the washing step after the flotation step. Specific examples of the coagulant, paper strength agent, and retention agent are commercially available as a paper strength agent, such as polyacrylamide, or as a retention agent. Although it is not always accurate to classify it clearly as either an agent or a retention agent, as a specific agent that can be used, it is usually a coagulant, a paper strength agent, or a retention agent. The following are used as:

One or more mixtures selected from drugs mainly composed of polyacrylamide, aluminum sulfate, polydadomac, polyamine, polyethyleneimine, 2- (methacryloyloxy) ethyltrimethylammonium chloride polymer, modified polyethyleneimine, etc. Or a drug composed of (meth) acrylamide, a water-soluble cationic monomer, and a water-soluble anionic monomer (generally referred to as amphoteric acrylamide, for example, “Hariup AC” manufactured by Harima Chemical Co., Ltd.) .) Other commercially available drugs such as Polystron 372, KW-387-20 manufactured by Arakawa Chemical Co., Inc. Acracac 208E manufactured by MT Aquapolymer Co., Accurac 100E, Fixage 621 manufactured by Kurita Kogyo Co., Hymolock NR-70 manufactured by Hymo Co., Ltd. Realizer R500, Realizer R600 manufactured by Somaar, Keith Lock RP-532 manufactured by Kyowa Sangyo Co., Ltd. Of these agents, any agent that improves the yield of ash and fibers can be preferably used. Among them, a drug containing amphoteric acrylamide as a component is particularly effective for improving the yield.

  The amount of the drug added is preferably 100 ppm to 3000 ppm, more preferably 1000 ppm to 2000 ppm. If it is less than 100 ppm, the effect may be low. On the other hand, even if it is added in excess of 3000 ppm, there is little improvement in the effect, which is uneconomical and may cause contamination in the system.

According to the method of the present invention, it is possible to produce a wide range of deinked pulps having an ISO whiteness of 50% to 85% from printed newspapers mainly composed of used newspapers. In any case, high yield removal with excellent ink removability is possible. It becomes possible to provide black pulp. On the other hand, it is possible to provide an environmentally friendly deinked pulp production method by reducing the amount of sludge ash generated, and also reduce the cost of chemicals because the filler used in the papermaking process can be reduced.

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. In the examples, comparative examples, and reference examples shown below, “%” means “% by mass” except for the whiteness.
Moreover, the usage-amount of the coagulant | flocculant containing a commercially available chemical | medical agent, a paper strength agent, and a retention agent was shown by the "ratio (mass%) with respect to the absolute dry pulp mass." In addition, “absolutely dry pulp mass” means that a certain amount of sample is taken each time in each step, solid content and moisture are separated by filtration using filter paper, and the mass of the solid content becomes constant. Until drying in a dryer at 105 ° C.
Examples 8 and 10 are reference examples.

The following three types were used as additive drugs.
Commercial drug I: Harima Kasei Co., Ltd., trade name Hariup AC (as a retention agent)
Commercially available drug II: manufactured by Arakawa Chemical Co., Ltd., trade name KW-387-20 (as a paper strength agent)
Commercially available drug III: manufactured by Kurita Kogyo Co., Ltd., trade name Fixage 621 (as a coagulant)

Example 1
<Recycled raw paper>
60% of printed newspapers with ash content of 13.1% in the 525 ° C combustion method specified in JIS P8251 and 9.8% in the 900 ° C combustion method specified in JIS P8252, 40% of leaflets The used printed paper consisting of The mass of the absolutely dry pulp occupying 1 kg of the used waste paper was 928 g.

<Disaggregation process>
Put the raw waste paper into a standard pulp disintegrator manufactured by Kumagaya Riki Kogyo Co., Ltd., add water at 30 ° C. to a solid content concentration of 3%, 0.4% caustic soda with respect to the absolute dry pulp mass A deinking agent (FT-515, manufactured by Toho Chemical Co., Ltd.) of 0.10% with respect to the mass of the absolutely dry pulp was added, and the disaggregation treatment was performed for 20 minutes according to JIS P8220.

<Dust removal process>
The disaggregation solution after the disaggregation treatment was subjected to a dust removal treatment using a flat screen (manufactured by Kumagai Riki Kogyo Co., Ltd.) equipped with a 6-cut screen plate having a slit width of 0.15 mm.

<Dispersing process>
The decontaminated solution after the dust removal treatment was concentrated and dehydrated with a 150 mesh wire until the solid content concentration became 30%. After dehydration, 1.5% caustic soda with respect to the absolute dry pulp mass, 1.5% sodium silicate with respect to the absolute dry pulp mass, and 1.5% peroxidation with respect to the absolute dry pulp mass Hydrogen and water at 30 ° C. were added to adjust the solid concentration to 25%. Thereafter, using a PFI mill manufactured by Kumagai Riki Kogyo Co., Ltd., 1000 rotation dispersion processing was performed according to JIS P8221-2.

<Bleaching process>
The dispersion after the dispersion treatment was placed in a plastic bag, and this was placed in a constant temperature water bath at 70 ° C. and held for 120 minutes for bleaching treatment.

<Flotation process>
The dispersion after the bleaching treatment is placed in a standard pulp disintegrator manufactured by Kumagai Riki Kogyo Co., Ltd., 40 ° C. warm water is added, and the pH of the pulp slurry is adjusted to 7.5 with sulfuric acid. 0.20% of a deinking agent (Kao, DI-3000) was added to the dry pulp mass. Then, the fiber was defibrated for 1 minute to thoroughly disperse the pulp in water, and the deinking agent and the pulp were mixed well to prepare a pulp slurry having a solid content of 1% and 40 ° C. Then, this pulp slurry was put into MT flowtater made by IHI Voith Paper Technology Co., Ltd. and subjected to deinking treatment by flotation for 5 minutes.

<Washing process>
Polyacrylamide is added to the pulp after the above flotation at a ratio of 0.05% with respect to the pulp, and 1000 ml is taken with a graduated cylinder, poured onto a 40 mesh wire, and a dedicated spatula until the squeezed amount reaches 850 ml. Was used to obtain a finished deinked pulp on the wire.

(Examples 2 to 10, Comparative Examples 1 to 3, Reference Example)
In the flotation step in the method of Example 1, the pH adjustment as shown in Table 1 was performed, respectively, and the type and addition amount of the drug as shown in Table 1 were added to the deinked pulp slurry before the washing step, respectively. A finished deinked pulp was obtained in the same manner as in Example 1 except that the addition was changed.

<Evaluation>
(Evaluation of yield improvement / Measurement of total yield)
1000g of pulp after the flotation process is dehydrated and washed so that the pulp is rolled on the wire until the squeezed mass reaches 850g with a 40 mesh wire, pulp before dehydration washing, pulp after dehydration washing, and pulp from 3 parts of squeezed water The mass and the concentration were measured according to JIS P 8225, the total yield was calculated from the quantity balance before and after the dehydration washing process, and the yield improvement was evaluated according to the following evaluation criteria. The results are shown in Table 1.

(Evaluation criteria)
A: The overall yield was 87.0% or more.
○: The total yield was 85.0% or more and less than 87.0%.
Δ: Total yield was 83.0% or more and less than 85.0%.
X: The total yield was less than 83.0%.

(Evaluation of free ink removability / Measurement of whiteness)
After completion of the flotation process, 15% aluminum sulfate is added to the finished deinked pulp obtained after the washing process and stirred well to fix the free ink peeled from the fiber to the fiber. It was. Thereafter, according to JIS P 8222, hand-made paper was prepared using a hand-making machine manufactured by Kumagai Riki Kogyo Co., Ltd. After producing the handmade paper, the whiteness was measured according to JIS P8148. Then, according to the following evaluation criteria, the free ink removal property was evaluated. The results are shown in Table 1.

(Evaluation criteria)
A: Whiteness was 68% or more.
○: Whiteness was 67% or more and less than 68%.
Δ: Whiteness was 66% or more and less than 67%.
X: The whiteness was less than 66%.
The “whiteness” here is an index indicating how much free ink that could not be removed mainly by the deinking process by flotation remains. The higher the whiteness, the less free ink remains. Means that.

(Evaluation of economic efficiency)
When adding a coagulant, a paper strength agent, or a retention agent, the cost increases. Therefore, economic evaluation was performed on the amount added. The results are shown in Table 1.

(Evaluation criteria)
A: The amount of drug added was 0 to 1000 ppm or less. O: The amount of drug added exceeded 1000 ppm and was 2000 ppm or less.
(Triangle | delta): The chemical | medical agent addition amount exceeded 2000 ppm and was 3000 ppm or less.
X: The amount of drug added was more than 3000 ppm.

(Comprehensive evaluation)
Based on the above evaluation results for yield improvement and ink removability, comprehensive evaluation was performed according to the following evaluation criteria.
(Evaluation criteria)
A: Among the above three evaluations, one or more were A, and the rest were B.
○: All of the above three evaluations were ○.
Δ: Among the above three evaluations, one or more were Δ, and the remaining were ○ or ◎.
X: One or more of the above three evaluations was x.

  From the results shown in Table 1, the yield improvement is recognized by adding any chemical before the washing step, and the yield improvement after deinking in the neutral region is remarkable. In addition, it can be seen that deinking is performed in a neutral region, so that ink removability is remarkably improved.

As can be seen from a comparison between Examples 2, 3, 6, and 7 and Comparative Examples 1 and 2, it can be seen that the ink removability is remarkably inferior when deinking is not performed in the neutral region. In addition, as the results of Comparative Example 3 and Reference Example show, the yield improvement cannot be expected if the amount of the chemical added before the washing process is too low, and the cost price for improving the yield is too high even if the addition rate is too high. It is extremely inferior and the economy is greatly deteriorated.

From the above results, the flotation step is performed in a neutral region of pH 6-9, and one or more of a coagulant, a paper strength agent, and a retention agent is added to the pulp slurry immediately before the subsequent washing step. In addition, it is possible to produce a deinked pulp having improved deinking and bleaching effects and excellent ink removability.

Claims (1)

In a method of pulping used printed paper through a series of steps including at least a disaggregation step, a dust removal step, a bleaching step, a dispersion step, a flotation step, and a washing step, the pulp slurry that has undergone the bleaching step and the dispersion step has a pH of 6 in the flotation step. Deinking treatment in the neutral region of ~ 9, adding one or more chemicals selected from a coagulant, a paper strength agent and a retention agent to the pulp slurry immediately before the subsequent washing step, and washing treatment , the coagulant, method for producing deinked pulp amount of one or more agents selected from paper strength agents and yield agent and wherein the Dearuko 500Ppm～2000ppm.