RU2797173C1 - Method for making cellulose for chemical processing - Google Patents

Abstract

FIELD: pulp and paper industry.

SUBSTANCE: invention relates to a technology for production of cellulose for chemical processing, which can be used to produce cellulose nitrate esters. According to the method, sulphite pulping of birch wood is carried out in one stage with sulphite cooking acid, then fine fibres are separated from unbleached pulp in an amount of 7-8%, and then bleaching of unbleached pulp is carried out according to a scheme that includes delignification with hydrogen peroxide in an acidic medium in the presence of a sodium molybdate catalyst, alkaline treatment with the addition of sodium hypochlorite, acid treatment, hot alkaline refining. After hot refining, bleaching is carried out with sodium hypochlorite.

EFFECT: invention makes it possible to simplify the process of obtaining unbleached cellulose and to impart qualitative characteristics to bleached refined cellulose, allowing it to be used for the production of cellulose nitrate esters.

9 cl, 3 ex, 1 tbl

Description

The invention relates to the pulp and paper industry, in particular to a technology for the production of cellulose for chemical processing, which can be used to produce cellulose nitrate esters.

A known method for producing pulp for chemical processing, according to which unbleached pulp is obtained from spruce chips by the sulfite cooking method according to the following mode: welding at a temperature of 110 ° C - 1 h 30 min, impregnation at 110 ° C - 2 h, rise to a final temperature of 150 ° C - 1 h 30 min, cooking at final temperature - 1 h 30 min. After cooking, the spent cooking solution is removed by displacing it with the spent solution from the alkaline treatment, after which the alkaline treatment is carried out directly in the cooking apparatus at the ratio of the spent solution and fresh alkaline solution, respectively, wt. %: 50-70 and 30-50. This alkaline treatment can be carried out in the presence of hydrogen peroxide, which is introduced 80-90 minutes after the start of this stage in an amount of 2.0-2.5% by weight of absolutely dry fiber. The cooked pulp mass is subjected to multi-stage bleaching according to the G-D-G-K or G-D-K schemes (see copyright certificate SU No. 1730299, IPC D21C 3/06, 9/00, 9/16, publ. 04/30/92. Bulletin No. 16).

The disadvantages of this method are the use of scarce and expensive spruce wood, high cooking temperature, low pulp yield after cooking, which requires increased consumption of wood for cooking. These factors lead to an increase in the cost of the cooking process. The disadvantages also include the use of explosive chlorine dioxide for bleaching pulp and reduced viscosity of bleached pulp (13-16 mPa⋅s).

The closest to the proposed method is a method for obtaining viscose pulp, according to which birch chips are pre-treated with an aqueous solution of SO ₂ with a concentration of 2-7% at a temperature of 100-120 ° C before cooking, cooking is carried out with a sulfite-bisulfite solution at pH 5.0-6 ,5, at a temperature of 150-170°C, and subsequent treatment with an aqueous solution of SO ₂ is carried out at a concentration of the latter of 5-8% and a temperature of 145-165°C.

After cooking, the pulp is bleached with hot refining at an alkali consumption of 5.7 and 9% (see copyright certificate 794101, M.Cl. D21C 3/06, publ. 07.01.81. Bull. No. 1). This method is taken as a prototype.

The disadvantages of this method, taken as a prototype, is the complexity (multi-stage) of the cooking process, low viscosity of bleached wood pulp (20-23 MPa⋅s), which does not meet the quality requirements for bleached sulphite pulp to obtain cellulose nitrate esters; lack of data for bleached pulp on the mass fraction of resins and fats, ash, as well as information on the bleaching agents used.

Signs of the prototype, which are common with the proposed solution, obtaining pulp from birch wood by sulfite cooking method; bleaching of unbleached pulp with a hot refining stage.

The objective of this invention is to simplify the process of obtaining unbleached cellulose and imparting quality characteristics to bleached refined pulp, allowing it to be used for the production of cellulose nitrate esters.

The problem is solved due to the fact that in a known method, including sulfite pulping of birch wood and subsequent bleaching with hot refining, according to the invention, sulfite pulping of birch wood is carried out in one stage with sulfite cooking acid, then fine fibers are separated from unbleached pulp in the amount of 7-8 %, and then bleaching of unbleached pulp is carried out according to the scheme, including delignification with hydrogen peroxide in an acidic medium in the presence of a sodium molybdate catalyst, followed by alkaline treatment with the addition of sodium hypochlorite, acid treatment, hot alkaline refining, after which bleaching is carried out with sodium hypochlorite.

In addition, sulfite cooking acid of the composition is used - total SO ₂ 8.0-8.5%, bound SO ₂ 0.75-0.80%.

In addition, sulfite pulping of birch wood is carried out according to the following regime: welding at a temperature of 105-110 ° C - 1 hour 30 minutes, impregnation at 105-110 ° C - 3 hours, rise to a final temperature of 138-140 ° C - 3 hours, cooking at the final temperature - from 40 minutes to 1 hour.

In addition, delignification with hydrogen peroxide is carried out at a temperature of 60-65°C for 120-130 min. H₂O₂3.0-3.2% H₂SO₄0.25-0.30% Na₂MoO₄0.1% by weight of absolutely dry pulp.

In addition, alkaline treatment with the addition of sodium hypochlorite is carried out at a temperature of 50-55°C for 60 min, NaOH consumption 1.8-2.0%, NaOCl 0.3-0.5% by weight of absolutely dry cellulose.

In addition, the acid treatment of the mass is carried out at a temperature of 20°C for 40-50 minutes and the consumption of hydrochloric acid is 1.2-1.5% by weight of absolutely dry cellulose.

In addition, hot refining is carried out at a temperature of 100-105°C for 120-150 min, NaOH consumption is 12-13% by weight of absolutely dry cellulose.

In addition, the addition of protein with sodium hypochlorite is carried out at a temperature of 38-40°C for 180 minutes and the consumption of sodium hypochlorite is 0.5-0.6% by weight of absolutely dry cellulose.

In addition, the mass concentration of the acid treatment stage is 5%, at the other stages - 10%.

Signs that are different from the prototype - cooking birch wood with sulfite cooking acid composition - total SO₂ 8.0-8.5% bound SO₂ 0.75-0.80% is carried out in one step according to the regime: welding at a temperature of 105 - 1 hour 30 minutes, impregnation at 105-110 ° C - 3 hours, rise to a final temperature of 138-140 ° C - 3 hours, cooking at the final temperature - from 40 minutes to 1 hour; after cooking, fine fibers are separated from unbleached pulp in the amount of 7-8%; further bleaching and refining of cellulose is carried out; moreover, delignification with hydrogen peroxide (PK stage) is carried out at a temperature of 60-65 ° C for 120-130 min. H₂O₂ 3.0-3.2% H₂SO₄0.25-0.30% Na₂MoO₄0.1% by weight of absolutely dry pulp; alkaline treatment with the addition of sodium hypochlorite (stage SH) is carried out at a temperature of 50-55°C for 60 minutes, NaOH consumption 1.8-2.0%, NaOCl 0.3-0.5% by weight of absolutely dry cellulose; acid treatment of the mass is carried out at a temperature of 20°C for 40-50 minutes and the consumption of hydrochloric acid is 1.2-1.5% by weight of absolutely dry cellulose; hot refining (stage GO), carried out at a temperature of 100-105°C for 120-150 min, the consumption of NaOH12-13% by weight of absolutely dry cellulose; after hot refining, whitening is carried out with sodium hypochlorite at a temperature of 38-40°C for 180 minutes and a consumption of sodium hypochlorite of 0.5-0.6% by weight of absolutely dry pulp. The mass concentration of the acid treatment stage is 5%, at the other stages - 10%.

The developed method for producing cellulose for chemical processing makes it possible to obtain soluble sulfite bleached refined cellulose, suitable for the production of cellulose nitrate esters. Traditionally, spruce or fir wood is used to produce pulp used for chemical processing (CP). The stocks of these wood species are greatly reduced and are gradually depleted. In addition, spruce wood is the only and demanded raw material for the production of high-yield fibrous semi-finished products (ground wood pulp and thermomechanical pulp) in paper production.

In Russia, in terms of hardwood reserves, birch wood ranks, the reserves of which have not decreased over the past decades, but it can be used to produce high-quality sulfite pulp, including for chemical processing.

In the present invention, birch chips are used for pulping.

The cooking mode of birch wood differs from the cooking mode of spruce due to the high density of birch - longer processes at the impregnation temperature (parking at a temperature of 105 ° C) and raising the temperature to the final one.

For cooking birch wood, sulfite cooking acid is used with a total SO ₂ content of 8.0-8.5% and bound SO ₂ 0.75-0.80%. Cooking acid with a total SO ₂ content of 8.0-8.5% leads to an acceleration of cooking and this value is easily achievable in the production of sulfite acid for pulping. The content of bound SO ₂ in sulfite acid can be from 0.7 to 1.3%. When obtaining pulp for chemical processing, the optimal content of bound SO ₂ is 0.75-0.80%, since in this case the pulp after cooking is obtained with a minimum content of non-cellulosic impurities, in particular hemicelluloses, the proportion of which in hardwood pulp is quite large.

Hardwood cellulose has a high content of fine fiber containing hardly saponifiable neutral substances and fatty acids. In order to reduce extractives in pulp, the requirements for the content of which in chemical processing pulp are very strict, it forces manufacturers of chemical processing pulp to separate fine fibers from pulp before bleaching.

With the separation of fine fibers in the amount of 7-8% by weight of absolutely dry unbleached pulp, the mass fraction of lignin and extractives decreases significantly. When the separation of fine fibers is less than 7-8%, the decrease in the indicators of the "mass fraction of lignin and extractives" is not enough, and when the separation is more than 7-8%, there is a significant decrease in the yield of bleached refined pulp.

In order to give the resulting cellulose properties that make it suitable for chemical processing, bleaching of cellulose is mandatory after cooking, with the inclusion of a hot alkaline refining stage in the scheme.

In accordance with modern environmental requirements, delignification of cellulose is adopted in an unconventional way by hydrogen peroxide in an acidic medium, followed by alkaline treatment. Catalytic process using sodium molybdate catalyst.

At the stage of alkaline treatment, the addition of sodium hypochlorite was used to reduce the viscosity of the cellulose.

To create effective conditions for hot refining, acid treatment of pulp is carried out in order to create an alternation of acidic and alkaline environments and provide a “pumping” effect when removing bleaching reaction products and waste chemicals, i.e. creating favorable conditions for the upgrading process.

To control pulp viscosity, bleaching with sodium hypochlorite was adopted as the last stage of bleaching.

The proposed method for producing cellulose is carried out as follows.

Birch wood chips are loaded into an autoclave. In order to approach production conditions, the sodium-based pulping solution is preheated to 70-75°C, poured into an autoclave, and the pulp is cooked. After cooking, the obtained unbleached pulp is washed with cold water and fine fibers are separated. Next, unbleached sulfite pulp, freed from fine fibers, is subjected to bleaching and refining according to the Pk-ShchG-K-GO-GK scheme.

Examples of specific implementation.

Example 1

Birch wood chips were loaded into an autoclave and filled with a sodium-based sulfite cooking solution, which was preheated to a temperature of 70°C. Next, pulp was boiled according to the following temperature regime:

- temperature rise to 105°C - 1 h 30 min,

- parking at a temperature of 105 ° C - 3 hours,

- temperature rise up to 140°C - 3 hours,

- cooking at final temperature - 40 min.

The hydromodulus during cooking was 5:1.

After cooking, the cellulose was washed with cold water and fine fiber was separated in the amount of 7-8% through a No. 28 sieve.

After separating the fine fibers, the unbleached sulfite pulp is subjected to bleaching and refining according to the Pk-SchG-K-GO-G-K scheme.

To do this, unbleached pulp was thickened on a Buchner funnel and placed in a porcelain glass, which was heated in a thermostat to maintain a constant temperature. The mass was heated to a temperature of 65°C and a delignifying solution was added containing H ₂ O ₂ 3.2%, H ₂ SO ₄ 0.3%, Na ₂ MoO ₄ 0.1% by weight of absolutely dry cellulose. Delignification with hydrogen peroxide in an acidic medium was carried out for 120 min. At the end of the specified time, the pulp mass was washed on a Buchner funnel and placed in a porcelain glass, which was heated in a thermostat. When the temperature reached 55°C, an alkali solution was added with a consumption of NaOH 2.0%, and sodium hypochlorite with a consumption of 0.5% by weight of absolutely dry cellulose. Alkaline treatment was carried out for 120 min. Next, the pulp mass was washed and acidification of the mass was carried out at a consumption of hydrochloric acid of 1.5% of the mass of absolutely dry pulp, at a temperature of 20°C for 40 minutes. After acidification, the pulp mass was washed with water until neutral. Next, they were placed in a porcelain glass, a NaOH solution was added at a rate of 12% by weight of absolutely dry cellulose, heated to a temperature of 100°C, and the contents of the glass were kept at this temperature for 120 minutes. Next, the pulp mass was washed on a Buchner funnel, placed in a porcelain glass, sodium hypochlorite solution was added at a rate of 0.6% of the mass of absolutely dry fiber, and subjected to bleaching at a temperature of 40°C for 180 minutes.

After completion of bleaching with sodium hypochlorite, the mass was washed with water and subjected to acidification, which was carried out with hydrochloric acid. The consumption of hydrochloric acid was 1.0% by weight of absolutely dry pulp. Sour conditions - room temperature, duration 30-60 minutes. After acidification, the pulp mass was again washed with water until neutral.

The mass concentration at the stages of acidity is 5.0%, at the other stages - 10%.

After each stage of bleaching, the pulp is washed with water until neutral.

Example 2

It is carried out similarly to example 1.

Cooking at final temperature - 50 min.

Example 3

It is carried out similarly to example 1.

Cooking at final temperature - 60 min.

Simplifying the pulping process to unbleached pulp will result in savings in chemicals, electricity and steam, which will reduce production costs.

Bleaching of unbleached pulp using an environmentally friendly technology eliminates the formation of chlorine-containing components in wastewater and gas emissions from the bleaching plant, which reduces the cost of wastewater treatment.

The indicators of bleached bleached sulfite pulp from birch wood are presented in the table.

Hardwood pulp fibers, unlike coniferous pulp, contain extractive substances that, during the bleaching process, can undergo saponification and release sticky fatty acids that contribute to the formation of resin deposits. A particularly high mass fraction of extractives is found in small fibers of hardwood.

It can be seen from the table that the separation of fine allows to significantly reduce the mass fraction of extractives, while the mass fraction of residual lignin in cellulose decreases, which is important for cellulose used for chemical processing.

Table
Characteristics of unbleached pulp after pulping and bleached bleached sulfite pulp from birch wood Pulp performance Brew number Norms for sulfite pulp TsA grade K 1 2 3 Unbleached pulp after pulping Pulp yield, % of wood 48.7 48.1 47.7 Mass fraction in cellulose, %: - residual lignin 2.13 2.07 1.93 - substances extractable with methylene chloride 2.50 2.51 2.48 Unbleached pulp after fine fiber separation Pulp yield after fine fiber separation, % from wood 45.3 44.8 44.5 Mass fraction in cellulose, %: - residual lignin 1.35 1.24 0.13 -substances extractable with methylene chloride 1.50 1.47 1.49 Cellulose after bleaching according to the scheme Pk-SchG-K-GO-G-K Yield of bleached pulp, % of unbleached pulp after fine fiber separation 82.1 81.0 81.3 Yield of bleached pulp, % of wood 37.2 36.3 36.2 Viscosity of 1% copper-ammonia solution, mPa*s 53 47 39 30-55 Mass fraction in cellulose, %: - alpha cellulose 93.5 93.3 93.3 At least 93 - residual lignin 0.28 0.24 0.19 Not more than 0.4 - ash 0.12 0.11 0.13 Not more than 0.16 - substances extractable with methylene chloride 0.35 0.37 0.34 Not more than 0.5

Claims (12)

1. A method for producing pulp for chemical processing, including sulfite pulping of birch wood and subsequent bleaching with hot refining, characterized in that sulfite pulping of birch wood is carried out in one stage with sulfite cooking acid, then fine fiber is separated from unbleached pulp in an amount of 7-8% , and then bleaching of unbleached pulp is carried out according to the scheme, including delignification with hydrogen peroxide in an acidic medium in the presence of a sodium molybdate catalyst, alkaline treatment with the addition of sodium hypochlorite, acid treatment, hot alkaline refining, after hot refining bleaching with sodium hypochlorite is carried out. 2. The method according to claim 1, characterized in that they use sulfite cooking acid of the composition - total SO ₂ 8.0-8.5%, bound SO ₂ 0.75-0.80%. 3. The method according to claim 1, characterized in that the birch wood sulfite cooking is carried out according to the mode: welding at a temperature of 105-110 ° C - 1 hour 30 minutes, impregnation at 105-110°C - 3 hours, rise to a final temperature of 138-140 ° C - 3 hours, cooking at a final temperature - from 40 minutes to 1 hour. 4. The method according to claim 1, characterized in that delignification with hydrogen peroxide is carried out at a temperature of 60-65°C for 120-130 min. H₂O₂ 3.0-3.2% H₂SO₄ 0.25-0.30% Na₂MoO₄ 0.1% by weight of absolutely dry pulp. 5. The method according to claim 1, characterized in that the alkaline treatment with the addition of sodium hypochlorite is carried out at a temperature of 50-55°C for 60 minutes, NaOH consumption 1.8-2.0%, NaOCl 0.3-0.5 % by weight of absolutely dry pulp. 6. The method according to claim 1, characterized in that the acid treatment of the mass is carried out at a temperature of 20°C for 40-50 minutes and the consumption of hydrochloric acid is 1.2-1.5% by weight of absolutely dry pulp. 7. The method according to claim 1, characterized in that hot refining is carried out at a temperature of 100-105°C for 120-150 minutes, NaOH consumption is 12-13% by weight of absolutely dry cellulose. 8. The method according to claim 1, characterized in that the additional protein with sodium hypochlorite is carried out at a temperature of 38-40°C for 180 minutes and the consumption of sodium hypochlorite is 0.5-0.6% by weight of absolutely dry cellulose. 9. The method according to claim 1, characterized in that the mass concentration of the acid treatment stage is 5%, at the remaining stages - 10%.