WO2021082306A1

WO2021082306A1 - Ionic-liquid-based bleaching protective agent for paper pulp

Info

Publication number: WO2021082306A1
Application number: PCT/CN2020/076725
Authority: WO
Inventors: 杨桂花; 陈嘉川; 齐乐天; 吉兴香; 吕高金; 彭建民
Original assignee: 齐鲁工业大学
Priority date: 2019-10-31
Filing date: 2020-02-26
Publication date: 2021-05-06
Also published as: CN110747681B; CN110747681A; KR102582865B1; KR20210056421A

Abstract

The present invention relates to an ionic-liquid-based ECF bleaching protective agent for paper pulp. The present application involves pre-treating eucalyptus kraft pulp with the ionic-liquid-based bleaching protective agent, followed by ECF bleaching. The addition amount of the bleaching protective agent is 5-15% of the paper pulp. This treatment can effectively dissolve the lignin in paper pulp fibers, and form a protective layer on the surface of the fibers, which improves the subsequent bleaching efficiency, and protects the fibers in the bleaching process. The pre-treatment can improve the cellulose content of the pulp, and also reduce the lignin content, and improve the physical strength of paper from the paper pulp, thereby broadening the application range of broad-leaved wood, such as eucalyptus. This treatment can improve the bleaching effect, can reduce the amount of a bleaching agent, improve the bleaching efficiency and reduce pollution. The present invention has a simple operation, a significant effect and strong practicality, pollution is low, and popularization is easy.

Description

An ionic liquid-based pulp bleaching protective agent

This application claims the priority of a Chinese patent application filed with the Chinese Patent Office on November 1, 2019, the application number is CN201911055224.1, and the invention title is "an ionic liquid-based pulp bleaching protective agent", the entire content of which is incorporated by reference Incorporated in this application.

Technical field

The invention belongs to the technical field of papermaking, and its specific design relates to an ionic liquid-based pulp bleaching protective agent.

Background technique

Disclosure of the background information is only intended to increase the understanding of the overall background of the present invention, and is not necessarily regarded as an acknowledgement or any form of suggestion that the information constitutes the prior art known to those of ordinary skill in the art.

With the large-scale, automation, and high-efficiency papermaking technology and equipment, and the paper industry's gradual emphasis on cleaner production, circular economy, reducing environmental pollution, and promoting sustainable development, the elemental chlorine-free (ECF) bleaching technology has become more mature. At present, in order to reduce the generation of pollutants, paper mills mostly adopt deep delignification, oxygen delignification and multi-stage bleaching with low chlorine consumption. Element-free chlorine (ECF) bleaching technology uses chlorine dioxide instead of chlorine to bleach, avoiding the emission of dioxins and greatly reducing the content of organic chlorides in wastewater. However, ECF is mainly bleached through oxidation reaction, which causes the degradation of carbohydrates, reduces the degree of polymerization and damages the strength of the fiber to a certain extent, resulting in lower paper strength properties, especially lower tensile strength.

Commonly used cellulose protective agents are anhydrous magnesium sulfate, magnesium carbonate, sodium silicate, borax, ethylene glycol, glycerol and the like. Among them, the application of anhydrous magnesium sulfate is more common, but it has no effect on improving the bleaching efficiency. In addition, pulp carbohydrates are still greatly degraded during oxygen delignification after adding anhydrous magnesium sulfate, and when the oxygen delignification effect is higher than 50%, the protective effect will be greatly weakened, resulting in a sharp drop in the viscosity of the pulp fiber.

Ionic liquid is a low-temperature molten salt that is composed entirely of anions and cations and is liquid at room temperature. It has the characteristics of good thermal stability, recyclability, non-volatility and good solubility. Ionic liquids can dissolve and separate lignin from wood fibers. Through proper ionic liquid selection and solution preparation, lignin from wood fiber can be selectively extracted. The use of such ionic liquids in the pulp bleaching process can increase the bleaching efficiency and enhance the bleaching effect. At the same time, it effectively protects the pulp fibers and enhances the physical strength of the paper.

With the development of the world economy and the continuous improvement of people’s living standards, people’s requirements for the quality of paper and cardboard are getting higher and higher. In the traditional pulp and paper industry, the tensile strength, burst resistance, and tearing strength of paper fibers after ECF bleaching The low-degree problem needs to be solved urgently.

The inventor’s previous patent CN201811466096.5 discloses a method for preparing paper-based materials from ionic liquid and ultrasonic treatment of ECF bleached kraft pulp. The unbleached kraft pulp is pretreated with ionic liquid and ultrasonic, and then the pretreated sulfuric acid The salt pulp is ECF bleached. However, in the follow-up research, the inventor found that: due to the addition of ultrasonic technology, the effect of ionic liquid is to enhance the lignin removal effect, and the lignin content in the pulp after treatment is low, but the treatment is not conducive to the enrichment of lignin on the surface of the pulp. Therefore, it cannot effectively protect.

Summary of the invention

In order to overcome the above-mentioned problems, the present invention provides an application of ionic liquid as a protective agent for bleaching pulp.

The present invention also provides an ionic liquid-based pulp bleaching protective agent, which is composed of an ionic liquid and a second component, and the second component is composed of magnesium carbonate, sodium silicate, borax, ethylene glycol, glycerol and no One or several components of hydromagnesium sulfate.

The present invention provides an ionic liquid-based bleaching protective agent for pulp. The pulp bleaching protective agent is used to pretreat the pulp, and then ODP bleaching is performed. The pretreatment of the bleach protector can dissolve the lignin in the pulp and protect the fiber during the ODP bleaching process. In this way, the bleaching effect of the pulp is improved, and the physical strength of the pulp into paper is improved.

In order to achieve the above technical objectives, the technical solutions adopted by the present invention are as follows:

An ionic liquid-based pulp bleaching protective agent, consisting of an ionic liquid and a second component, the second component being one of magnesium carbonate, sodium silicate, borax, ethylene glycol, glycerol and anhydrous magnesium sulfate Kind or several kinds of composition.

Preferably, the anion of the ionic liquid comprises one or more of hydrogen sulfate ion, formate ion and chloride ion;

Or the cation of the ionic liquid contains one or more of 1-butyl 3-methyl-imidazolium ion, 2-hydroxyethyltrimethylammonium ion and triethylammonium ion.

Preferably, the ionic liquid includes triethylammonium hydrogen sulfate ionic liquid or 1-butyl 3-methyl hydrogen sulfate ionic liquid.

Preferably, when the second component of the pulp bleach protective agent is anhydrous magnesium sulfate, the content of the anhydrous magnesium sulfate in the pulp bleach protective agent is 0.6%.

The present invention also provides a method for improving the strength of the paper by the ionic liquid-based pulp bleaching protective agent, which includes:

Use the pulp bleaching protective agent described in the above technical scheme to pretreat the pulp;

ECF bleaching the pretreated pulp, the bleaching process is ODP bleaching sequence;

Make paper and get it.

Preferably, the amount of the bleach protection agent is 5-15% of the absolute dry mass of the pulp.

Preferably, the O-stage oxygen delignification process in the ODP bleaching sequence is: pulp concentration 10%, NaOH dosage 3%, oxygen pressure 0.5 MPa, temperature 100°C, time 60 min, and MgSO ₄ dosage 0.6%.

Preferably, the D-stage chlorine dioxide bleaching process in the ODP bleaching sequence is as follows: the amount of chlorine dioxide is 0.7%, the pH value is 2 to 3, the temperature is 70° C., the pulp concentration is 10%, and the time is 30 minutes.

Preferably, the P-stage hydrogen peroxide bleaching process in the ODP bleaching sequence is as follows: the amount of hydrogen peroxide is 1%, the pulp concentration is 10%, the pH value is 11-12, the temperature is 90° C., and the oxygen pressure is 0.4 MPa.

Preferably, the pulp is kraft pulp, and the cooking process is to take wood chips with a length of 15-25 mm, a width of 10-20 mm, and a thickness of 3-5 mm. After natural air drying, they are cooked to obtain kraft puree. The cooking conditions are The amount of alkali used is 21%, relative to the absolutely dry raw material, as Na ₂ O; the sulfidity is 25%, the liquid ratio is 1:5, the maximum cooking temperature is 170 ℃, and the gas is deflated at 105 ℃. The gas is deflated for 15 minutes, and the heating time is 90 minutes. , The holding time is 90min; after cooking, the pulp is fully washed and screened to obtain the pulp.

The present invention also provides the application of the ionic liquid-based pulp bleaching protective agent described in the above technical scheme in the manufacture of relief printing paper, newsprint, offset printing paper, art paper, book cover paper, dictionary paper, copy paper or board paper.

The beneficial effects of the present invention are:

(1) The present invention pretreats eucalyptus kraft pulp with an ionic liquid-based protective agent to form a surface protective layer during the ODP bleaching process, which reduces the degree of fiber degradation and protects the fiber backbone.

(2) The paper-based material prepared by pretreatment with an ionic liquid-based protective agent has the advantages of high folding resistance, high tensile strength, high burst resistance, high tearing degree, etc., and is suitable for preparing various types of paper-based materials.

(3) The pretreatment of ionic liquid-based protective agent can selectively dissolve lignin, which significantly reduces the lignin content of the fiber, which is beneficial to the effect of bleaching agent. This treatment can improve the bleaching effect, can reduce the amount of bleaching agent, improve the bleaching efficiency, and reduce pollution.

(4) The ionic liquid used in the present invention has a stable structure, zero vapor pressure, can be recycled, and is environmentally friendly.

(5) The processing method of the present invention is simple, low in cost, strong in practicability, and easy to popularize.

Detailed ways

It should be pointed out that the following detailed descriptions are all illustrative and are intended to provide further explanations for the application. Unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs.

It should be noted that the terms used here are only for describing specific embodiments, and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should also be understood that when the terms "comprising" and/or "including" are used in this specification, they indicate There are features, steps, operations, devices, components, and/or combinations thereof.

As introduced in the background art, it is aimed at the current problems of low tensile strength, burst resistance and tearing of paper fibers after ECF bleaching. Therefore, the present invention proposes an ionic liquid-based bleaching protective agent for pulp, which is subjected to ODP bleaching after pretreatment of the pulp. The pretreatment of the bleach protector can dissolve the lignin and protect the fiber during the bleaching process. In this way, the bleaching effect is improved, and the physical strength of the pulp into paper is improved.

In order to achieve the above objectives, the technical solutions adopted by the present invention are as follows: the concentration of the slurry involved in the following are mass percentages unless otherwise specified; the dosage of the reagents and drugs involved, unless otherwise specified, are relatively dry In terms of pulp.

In order to overcome the problem that the current ionic liquid coordinated ultrasonic treatment is not conducive to the enrichment of lignin on the surface, this application has conducted a systematic study on its treatment method and effect law, and found that the combination of ionic liquid and specific bleaching sequence can achieve similar The enhancement effect of paper strength and whiteness in the treatment of medium-intensity ultrasound with ionic liquid is slightly lower than that of high-intensity ultrasound with ionic liquid treatment.

The present invention provides an ionic liquid-based pulp bleaching protective agent, which is composed of one or more of ionic liquid, magnesium carbonate, sodium silicate, borax, ethylene glycol, glycerol and anhydrous magnesium sulfate.

In some embodiments, the anion of the ionic liquid contains one or more of hydrogen sulfate ion, formate ion or chloride ion; the pulp bleaching protectant developed in this application can selectively dissolve lignin and improve bleaching. The efficient and at the same time soluble cellulose is concentrated on the surface of the pulp fiber to protect the fiber from excessive oxidative degradation during the bleaching process.

In some embodiments, the cation of the ionic liquid comprises one or more of 1-butyl 3-methyl-imidazolium ion, 2-hydroxyethyl trimethylammonium ion and triethylammonium ion, wherein 1 The structural formulas of -butyl 3-methyl-imidazolium ion, 2-hydroxyethyl trimethylammonium ion and triethylammonium ion are:

It should be noted that although the processing method of this application is similar to the previous application, according to its effect, the two applications are applicable to different pulp products. The reasons are as follows:

In this application, ionic liquid is used to selectively dissolve lignin and enrich it on the surface of the fiber, so that the glycan structure is less damaged and the degree of fiber polymerization is high. The pretreatment can loosen the fibers and increase the degree of swelling, which is conducive to the subsequent penetration of the bleaching agent, the destruction of the chromophore groups and the diffusion and dissolution of the reactants. At the same time, pretreatment can improve the morphology of fine fiber components, enhance cross-linking between fibers, thereby improving bleaching efficiency and enhancing the performance of paper-based materials.

Among them, leaching out lignin and enriching it on the fiber surface plays an important role in protecting the inner cellulose fiber backbone. However, due to the addition of ultrasonic technology in the previously applied patent, the effect of the ionic liquid is to enhance the removal of lignin, and the lignin content after treatment is low, which is not conducive to the enrichment of surface lignin. Therefore, it cannot effectively protect the fiber. Therefore, in some embodiments, the cation of the ionic liquid contains one or more of 1-butyl 3-methyl-imidazolium ion, 2-hydroxyethyl trimethylammonium ion and triethylammonium ion, which improves Improve the bleaching efficiency and enhance the performance of paper-based materials.

The present invention also provides a method for improving the strength of pulp into paper by an ionic liquid-based bleaching protective agent for pulp, which includes:

Make paper and get it.

In some embodiments, the present invention provides a method for improving the strength of pulp into paper by an ionic liquid-based bleaching protective agent for pulp. The experiment includes the following steps:

(1) Cooking: Take wood chips, put them in a cooking pot and cook to obtain puree. Then carry out sufficient washing and screening to obtain the sulphate slurry. The kappa number of the slurry is 12-16;

(2) Ionic liquid-based bleach protection agent pretreatment;

(3) ECF bleaching: The above-mentioned slurry treated with ionic liquid-based body-based bleaching protective agent is subjected to ECF bleaching, and the bleaching process is ODP bleaching sequence:

First, perform O-stage deoxygenation lignin on the pulp treated with ionic liquid body-based bleaching protective agent;

After the O-stage oxygen delignification is completed, the pulp is washed, and then the D-stage chlorine dioxide bleaching stage is carried out;

After the D stage chlorine dioxide bleaching is completed, the pulp is washed, and then the P stage hydrogen peroxide bleaching is carried out;

After the hydrogen peroxide treatment in the P stage is completed, the pulp is washed and completed.

The final pulp obtained after bleaching is used in the papermaking process.

Although the above technical solutions do not involve specific process parameters, according to the inventive concept of the present invention, the pretreatment with ionic liquid-based bleaching protective agent is particularly aimed at protecting the fiber during the oxidation reaction of the bleaching process, reducing the degree of fiber oxidative degradation, and improving Bleaching effect, you can get high-performance pulp.

In some embodiments, the specific method of cooking in the step (1) is: taking a relatively uniform wood chip with a length of 15-25 mm, a width of 10-20 mm, and a thickness of 3-5 mm, and after natural air-drying, cooking is performed to obtain sulfate Puree, cooking conditions are 21% alkali, relative to the absolute dry raw material, Na ₂ O; sulfidity 25%, liquid ratio 1:5, the highest cooking temperature is 170 ℃, venting at 105 ℃, release Air 15min, heating time 90min, heat preservation time 90min; after cooking, full washing and screening are carried out to obtain pulp, so as to improve the cooking efficiency and the strength performance of the paper.

Sulphate wood pulp uses a mixture of sodium hydroxide and sodium sulfide as a cooking agent. During the cooking process, because the effect of the liquid medicine is relatively mild and the fiber is not strongly corroded, the fiber is strong and strong, with high tensile strength, burst resistance, tearing and other mechanical strength indicators, and heat resistance and durability High sex.

In some embodiments, the specific step of the pretreatment in step (2) is: the slurry obtained in step (1) is treated under the condition of a bleach protectant amount of 5-15%, and then under a fixed condition Under ODP bleaching.

With the increase in the amount of bleach protectant, bleaching efficiency and pulp performance are improved, but when the amount of bleach protectant reaches a certain value, continue to increase the amount of bleach protectant, which will not greatly improve the pulp performance and increase the bleaching cost.

In some embodiments, in order to obtain the best effect, the amount of the protective agent is 10%.

In some embodiments, the specific method of the ODP bleaching process in step (3) is: O-stage oxygen delignification process: pulp concentration 10%, NaOH dosage 3%, oxygen pressure 0.5MPa, temperature 100°C, time 60min, MgSO ₄ dosage is 0.6%; D stage chlorine dioxide bleaching process is: chlorine dioxide dosage 0.7%, pH value 2～3, temperature 70℃, pulp concentration 10%, time 30min; P stage hydrogen peroxide bleaching process is ：Hydrogen peroxide dosage is 1%, slurry concentration is 10%, pH value is 11-12, temperature is 90℃, oxygen pressure is 0.4MPa.

The oxygen delignification process can reduce the lignin content in unbleached pulp, reduce the amount of bleaching agent, and reduce the pollution load of bleaching wastewater. In some embodiments, the O-stage oxygen delignification process is: pulp concentration 10%, NaOH dosage 3%, oxygen pressure 0.5MPa, temperature 100°C, time 60min, which improves the efficiency of oxygen bleaching and pulp performance.

Chlorine dioxide has strong oxidizing properties, strong delignification ability and delignification selectivity. Under the same effective chlorine dosage, the adsorbable organic halide (AOX) produced by chlorine dioxide bleaching is only 1/5 of that of chlorine bleaching, without affecting the removal of lignin. During the ClO ₂ bleaching process, the establishment of a buffer system is beneficial to keep the pH stable. In some embodiments, the D-stage chlorine dioxide bleaching process is as follows: the amount of chlorine dioxide is 0.7%, the pH value is 2 to 3, the temperature is 70°C, the pulp concentration is 10%, and the time is 30 minutes, which improves the bleaching efficiency of pulp and reduces The production of organic halides (AOX).

The hydrogen peroxide bleaching process has less damage to the fiber, high yield, and strong process adaptability. The bleaching waste water contains organic chlorine compounds, which is easy to handle, and can realize the full recycling of bleaching wastewater. In some embodiments, the P-stage hydrogen peroxide bleaching process is as follows: the amount of hydrogen peroxide is 1%, the pulp concentration is 10%, the pH value is 11-12, the temperature is 90°C, and the oxygen pressure is 0.4 MPa to effectively protect the pulp strength , Improve its whiteness.

The research of this application found that the use of ODP bleaching sequence not only has a shorter process, but also a lower cost. At the same time, through the combination of ionic liquid-based bleach protectants, the pulp whiteness and pulp strength similar to the _{previous OAD 1} ED _{2 process can be achieved.}

The design concept of the present invention is: in view of the problem of low pulp strength caused by oxidative reaction damage to the pulp fiber during the bleaching process, the present invention proposes a pretreatment method using an ionic liquid-based protective agent to dissolve the lignin in the pulp and attach it to the surface of the fiber. Make the bleaching agent and lignin fully function, avoid the oxidative damage to the fiber during the bleaching process, and ensure the excellent physical strength of the paper material.

The present invention will be further described below in conjunction with specific embodiments.

The analysis method adopted in the embodiment of the present invention is as follows:

Determination of whiteness: After the pulp is decomposed by a fiber decomposing machine, it is copied into paper on a Kaiser copy machine made in Austria, and then measured with a YQ-Z-48B whiteness meter;

Folding resistance times: Measured with the MIT folding resistance tester made in China;

Bursting Index: Measured with a Bursting Tester produced by L&W, a Swedish company;

Tensile index: Measured with the XLWA(B) intelligent electronic tensile testing machine made in China;

Tear index: Measured with the MIT tear tester made in China.

Fiber surface lignin content: measured with XPS produced in the United States.

The remaining experimental methods that are not described in detail are conventional experimental methods in the field.

The principle of the present invention is: using an ionic liquid-based pulp bleaching protective agent to selectively dissolve lignin and enrich it on the surface of the fiber, so that the cellulosan structure is less damaged and the fiber polymerization degree is high. The pretreatment can loosen the fibers and increase the degree of swelling, which is conducive to the subsequent penetration of bleaching agents, the destruction of chromophores and the diffusion and dissolution of reactants. At the same time, pretreatment can improve the morphology of fine fiber components, enhance the interweaving ability between fibers, thereby improving bleaching efficiency and enhancing the performance of paper-based materials.

Example 1:

Proceed as follows:

(1) Cooking: eucalyptus wood chips are used in the alkali amount of 21%, Na ₂ O; sulfidity 25%, liquid ratio 1:5, cooking at the highest temperature of 170 ℃, venting at 105 ℃, heating time 90 minutes , Holding time 90min. After the cooking, the pulp is fully washed and sieved to obtain the pulp.

(2) Pretreatment of protective agent: accurately weigh out three bags of 25g of the above eucalyptus sulphate pulp (calculated as dry pulp), and measure 10% triethylammonium hydrogen sulfate ionic liquid (TEA-HSO ₄ ) based bleaching protective agent (In addition to TEA-HSO ₄ , the bleach protectant also contains a small amount of anhydrous magnesium sulfate (0.6%)) is added to the slurry, the slurry concentration is adjusted to 10% in a polyethylene bag, and the mixture is kneaded until uniform.

(3) ECF bleaching: bleach the pulp after the above pretreatment, the bleaching process is ODP, and the bleaching process steps are: first perform O-stage oxygen delignification on eucalyptus kraft pulp, the process is: pulp concentration 10%, The amount of NaOH is 3%, the oxygen pressure is 0.5MPa, the temperature is 100°C, the time is 60min, and the _{amount of MgSO 4} is 0.6%; then the D-stage chlorine dioxide bleaching process is performed: the amount of chlorine dioxide is 0.7%, the pH value is 2 to 3, and the temperature is 70 ℃, pulp concentration 10%, time 30min; wash the treated pulp, and then carry out P-stage hydrogen peroxide bleaching. The process is: hydrogen peroxide dosage 1%, pulp concentration 10%, pH value 11-12, temperature 90°C, oxygen pressure 0.4MPa; wash the treated slurry.

(4) Papermaking: The slurry is beaten according to a beating degree of 40°SR, and then it is decomposed by a fiber decomposer to make it uniformly mixed, and then 80g/m ^{2 of} base paper paper-based material is copied on the paper sheet former.

Table 1 The influence of TEA-HSO ₄ -based protective agent pretreatment on slurry properties

Table 2 The influence of TEA-HSO ₄ -based protective agent pretreatment on the paper properties of ODP bleached pulp

Result: After testing the performance of the pulp into paper, it can be concluded from the data in Table 1 that the use of TEA-HSO ₄ ionic liquid-based bleaching protectant pretreatment makes the pulp performance substantially improved while the yield is basically unchanged. In the oxygen bleaching section (O): Kappa number decreased from 8.06 to 7.89, a decrease of 2%; pulp viscosity increased from 885mL/g to 999mL/g, an increase of 13%; degree of polymerization increased from 1313 to 1501 , An increase of 14%. After bleaching (ODP): Kappa number decreased from 1.54 to 0.48, a decrease of 69%; viscosity increased from 684mL/g to 806mL/g, an increase of 18%; degree of polymerization increased from 988 to 1184, an increase of 20 %.

After the paper performance test, it can be seen from the data in Table 2 that the use of TEA-HSO ₄ ionic liquid-based bleaching protective agent pretreatment makes the physical properties of the slurry substantially improved when the whiteness is basically unchanged. Among them: the tensile index of pulp into paper increased from 4.22kN·m ^-1 to 4.81kN·m ^-1 , an increase of 14%; the tearing index increased from 6.927mN·m ² ·g ^-1 to 7.866mN·m ² ·g ^-1 , an increase of 14%; the number of folds increased from 126 to 319, an increase of 2.5 times.

The chemical composition of the pulp was tested, and it was found that the _{pretreatment with TEA-HSO 4} ionic liquid-based bleaching protective agent significantly reduced the lignin content in the pulp. Among them, the oxygen bleaching stage (O) fiber lignin content was reduced from 6.76% To 3.52%, a decrease of 48%; after bleaching (ODP) fiber lignin content decreased from 1.97% to 1.62%, a decrease of 18%. The O/C ratio of the fiber surface decreased from 0.53 to 0.48 after the oxygen bleaching section was detected by XPS, indicating that a protective layer of lignin was formed on the fiber surface during the bleaching process.

Through XRD analysis, it was found that after ODP bleaching, _{pretreatment with TEA-HSO 4} ionic liquid-based bleach protectant increased the cellulose crystallinity from 36.2 to 46.0, an increase of 27%.

Example 2:

Proceed as follows:

(1) Cooking: The condition is 21% alkali, Na ₂ O; sulfidity 25%, liquid ratio 1:5, maximum cooking temperature 170 ℃, venting at 105 ℃, heating time 90 min, holding time 90 min ; Carry out sufficient washing and screening after cooking to obtain pulp.

(2) Accurately weigh three bags of 25g slurry (based on absolute dry pulp), and add 5%, 10%, and 15% 1-butyl 3-methyl hydrogensulfate ionic liquid (BMIM-HSO ₄ ) respectively In the slurry, adjust the slurry concentration to 10% in a polyethylene bag, and knead until it is evenly mixed

(3) ECF bleaching: bleach the pulp after the above pretreatment. The bleaching process is the ODP bleaching process. The steps are: first perform O-stage oxygen delignification on eucalyptus kraft pulp, the process is: pulp concentration 10%, NaOH Dosage is 3%, oxygen pressure is 0.5MPa, temperature is 100°C, time is 60min, and MgSO ₄ dosage is 0.6%; then D-stage chlorine dioxide bleaching is carried out. The process is: chlorine dioxide dosage of 0.7%, pH value of 2～3, temperature of 70°C , Pulp concentration 10%, time 30min; wash the treated pulp, and then carry out P-stage hydrogen peroxide bleaching. The process is: hydrogen peroxide dosage 1%, pulp concentration 10%, pH value 11-12, temperature 90 ℃, oxygen pressure 0.4MPa; wash the treated slurry.

Table 3 The effect of BMIM-HSO ₄ -based protective agent treatment on slurry properties

Table 4 The effect of BMIM-HSO ₄ -based protective agent pretreatment on the paper properties of ODP bleached pulp

Result: After the pulp paper performance test, it can be concluded from the data in Table 3 that the use of BMIM-HSO ₄ ionic liquid-based bleaching protectant pretreatment makes the pulp paper performance under the condition that the yield is basically unchanged. The oxygen bleaching section (O): the Kappa number decreased from 8.06 to 7.47, a decrease of 7%; the viscosity increased from 885mL/g to 905mL/g, an increase of 2%; the degree of polymerization increased from 1313 To 1346, an increase of 3%. After bleaching (ODP): Kappa number decreased from 1.54 to 0.67, a decrease of 56%; viscosity increased from 684mL/g to 721mL/g, an increase of 5%; degree of polymerization increased from 988 to 1047, an increase 6%.

After the paper performance test, it can be seen from the data in Table 4 that the use of BMIM-HSO ₄ ionic liquid-based bleaching protective agent pretreatment makes the physical properties of the slurry substantially improved when the whiteness is basically unchanged. Among them: the pulp tensile index increased from 4.22kN·m ^-1 to 4.57kN·m ^-1 , an increase of 8%; the tear index increased from 6.927mN·m ² ·g ^-1 to 7.598mN·m ² · g ^-1 , increased by 10%; the number of folding resistance increased from 126 times to 252 times, an increase of 2 times.

The chemical composition of the pulp was tested, and it was found that the _{pretreatment with TEA-HSO 4} ionic liquid-based bleaching protective agent significantly reduced the lignin content in the pulp. Among them, the oxygen bleaching stage (O) fiber lignin content was reduced from 6.76% To 5.96%, a decrease of 12%; after the bleaching (ODP) fiber lignin content decreased from 1.97% to 1.83%, a decrease of 7%. The O/C ratio of the fiber surface decreased from 0.53 to 0.51 after the oxygen bleaching section was detected by XPS, indicating that a protective layer of lignin was formed on the fiber surface during the bleaching process.

Through XRD analysis, it was found that after ODP bleaching, _{pretreatment with BMIM-HSO 4} ionic liquid-based bleach protectant increased the cellulose crystallinity from 36.2 to 48.8, an increase of 35%.

The description of the above embodiments is only used to help understand the method and core idea of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention. Various modifications to these embodiments are obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown in this document, but should conform to the widest scope consistent with the principles and novel features disclosed in this document.

Claims

An ionic liquid is used as a protective agent for bleaching pulp.
An ionic liquid-based pulp bleaching protective agent, which is characterized in that it is composed of an ionic liquid and a second component, and the second component is composed of magnesium carbonate, sodium silicate, borax, ethylene glycol, glycerol and anhydrous One or more components in magnesium sulfate.
The ionic liquid-based pulp bleaching protective agent according to claim 2, wherein the anion of the ionic liquid comprises one or more of hydrogen sulfate ion, formate ion and chloride ion;

Or the cation of the ionic liquid contains one or more of 1-butyl 3-methyl-imidazolium ion, 2-hydroxyethyltrimethylammonium ion and triethylammonium ion.
The ionic liquid-based pulp bleaching protectant according to claim 3, wherein the ionic liquid comprises triethylammonium hydrogen sulfate ionic liquid or 1-butyl 3-methyl hydrogen sulfate ionic liquid.
The ionic liquid-based pulp bleach protectant according to claim 2, wherein when the second component of the pulp bleach protectant is anhydrous magnesium sulfate, the pulp bleach protectant contains anhydrous magnesium sulfate The content is 0.6%.
A method for improving the strength of paper by an ionic liquid-based pulp bleaching protective agent, which is characterized in that it comprises:

Using the pulp bleaching protective agent according to any one of claims 2 to 5 to pretreat the pulp;

ECF bleaching the pretreated pulp, the bleaching process is ODP bleaching sequence;

Make paper and get it.
The method for improving the strength of pulp into paper by a ionic liquid-based bleaching protective agent for pulp according to claim 6, wherein the amount of the pulp bleaching protective agent is 5-15% of the absolute dry mass of the pulp.
The method for improving the strength of pulp into paper by a ionic liquid-based bleaching protective agent for pulp according to claim 6, wherein the O-stage oxygen delignification process in the ODP bleaching sequence is: pulp concentration 10%, NaOH dosage 3%, oxygen pressure 0.5MPa, temperature 100°C, time 60min, dosage of MgSO 4 0.6%.
The method for improving the strength of pulp into paper by a ionic liquid-based bleaching protective agent for pulp according to claim 6, wherein the D stage chlorine dioxide bleaching process in the ODP bleaching sequence is: the amount of chlorine dioxide is 0.7%, The pH value is 2～3, the temperature is 70℃, the slurry concentration is 10%, and the time is 30min.
The method for improving the strength of pulp into paper by a ionic liquid-based bleaching protective agent for pulp according to claim 6, wherein the P-stage hydrogen peroxide bleaching process in the ODP bleaching sequence is: the amount of hydrogen peroxide is 1%, The slurry has a concentration of 10%, a pH value of 11-12, a temperature of 90°C, and an oxygen pressure of 0.4MPa.
The method for improving the strength of pulp into paper by a ionic liquid-based bleaching protectant for pulp according to claim 6, wherein the pulp is kraft pulp, and the cooking step is: taking a length of 15-25mm, a width of 10-20mm, Wood chips with a relatively uniform thickness of 3～5mm. After natural air drying, they are cooked to obtain a sulphate puree. The cooking conditions are 21% alkali content, based on the absolute dry raw material, as Na 2 O; sulfidity 25%, liquid ratio 1:5, the highest cooking temperature is 170℃, deflate at 105℃, deflate for 15min, heating time 90min, holding time 90min; after cooking, fully wash and screen to obtain pulp.
The use of the ionic liquid-based pulp bleaching protective agent according to any one of claims 2 to 5 in the manufacture of relief printing paper, newsprint, offset printing paper, coated paper, book cover paper, dictionary paper, copy paper or board paper.