RU2735263C1 - Method of producing bleached hempen cellulose - Google Patents

Abstract

FIELD: pulp and paper industry.

SUBSTANCE: invention relates to pulp and paper industry, in particular to production of cellulose intended for production of cardboard, paper of various purposes from annual vegetal raw materials. Method involves pre-hydrolysis of hemp fiber, delignification in an alkaline medium, bleaching in an alkaline solution of hydrogen peroxide, intermediate washing, squeezing and drying. Prehydrolysis of raw material is carried out at temperature 85–95 °C in solution containing 0.5–2.0 wt% sodium hydroxide, 0.2–0.3 wt% synthanol and 0.3 wt% sodium thiosulphate for 60 min. Delignification is carried out in solution containing 1.5–3 wt% sodium hydroxide, 0.25–0.5 wt% sodium sulphide, 0.6–1.0 wt% sodium thiosulphate and 0.05–0.06 wt% synthanol, at temperature 125–140 °C for 90–130 minutes. Bleaching is carried out at temperature 85–95 °C in an alkaline solution of hydrogen peroxide, additionally containing 0.3–0.5 wt% sodium metasilicate, 0.02–0.05 wt% triethanolamine and 0.05–0.1 wt% chelating agent Trilon B at concentration of hydrogen peroxide of 0.5–0.6 wt%, for at least 90 minutes.

EFFECT: proposed method ensures production of bleached hemp cellulose with high molecular weight (medium degree of polymerization), lower content of lignin, satisfactory content of α-cellulose, ash and whiteness; obtained cellulose can be used for production of paper for various purposes, cardboard, as well as for chemical processing.

1 cl, 2 tbl

Description

The invention relates to the pulp and paper industry and can be used for the production of cellulose from cellulose-containing annual plant raw materials, intended for the manufacture of paper for various purposes, cardboard, chemical processing.

The quality of cellulose largely determines the technological, physical, mechanical and other properties of manufactured products, such as paper for banknotes, airmail, ultrathin paper, etc., which depend on the molecular weight (average degree of polymerization) of the cellulosic material. Paper for various purposes is produced mainly from wood and cotton, but due to the decrease in the world's forest area, as well as the lack of forests and cotton in many regions, the problem of finding alternative sources of raw materials remains urgent.

One of these alternative sources of raw materials is hemp, as an extremely highly profitable natural source of cellulose: the content of cellulose in hemp is 5-7 times higher than in wood, and unlike fibrous wood cellulose, hemp fiber has a fairly high strength.

One of the important indicators of the quality of cellulose is its molecular weight (average degree of polymerization). This is due to the fact that in the process of separating cellulose from the original fiber with a lignin content of no more than 13% in an alkaline medium, in parallel with the destruction of lignin and carbohydrate impurities, the destruction of the cellulose macromolecule occurs. It is known that the molecular heterogeneity of cellulose due to the presence of fractions with a low degree of polymerization reduces the mechanical (strength) properties of materials based on it. The high lignin content gives the cellulose a dark gray tint, which, depending on the purpose of the cellulose product, may require additional bleaching operations. Obtaining cellulose with a high molecular weight and a lower lignin content from hemp raw materials is an urgent task.

A known method of producing cellulose from the stems of industrial hemp. Raw materials from hemp are subjected to alkaline cooking at 125-145 ° C by adding anthroquinone as a chelator in an amount of not more than 0.1% by weight of raw materials. The resulting cellulose is bleached with oxygen and / or ozone (RU 23630055, D21C 3/02, publ. 19.08.2008).

The resulting cellulose by the known method is characterized by a relatively high content of lignin, which indicates the low efficiency of the technology.

There is also known a method of obtaining cellulose from technical hemp, including drying the raw material, grinding it and classifying the particles obtained, cooking using an alkaline cooking liquid with a concentration of not more than 18% wt. sodium hydroxide in a digester at high temperature, separation of the resulting cellulose from the cooking liquid (CORREIA, F., DN ROY and K. GOEL, Chemistry and Delignification Kinetics of Canadian Indastrial Hemp (Cannabis sativa L.) // J. Wood Chem. Technol 2001.21 (2) 97-111).

The disadvantage of this method is the choice of the technological regime, according to which the resulting cellulose is characterized by low values of molecular weight due to the destruction of oxidized macromolecules of cellulose fragments due to the high concentration of alkali and temperature.

There is also known a method of producing cellulose, including cleaning cellulose-containing fiber, cooking in an alkaline solution containing a surfactant, bleaching in a solution of hydrogen peroxide, acidification, intermediate washing and drying (RU 2507331, D21C 5/00, publ. 20.02.2014). As a cellulose-containing fiber, hemp fiber is used, which, after cleaning, is additionally subjected to acidification in a solution containing 1.0-1.5 wt% sulfuric acid, and cleaning is carried out with soda cooking, while cooking in an alkaline solution is carried out with the addition of sodium sulfide at the following ratio components, wt. %: sodium hydroxide 5-6, sodium sulfide 0.75-1.8, surfactant 0.05-0.1; moreover, cleaning, acidification and cooking in an alkaline solution are carried out at a temperature of 100-125 ° C and treated for 60 to 120 minutes.

The disadvantages of the known method is a severe technological regime (high values of the concentration of alkaline cooking liquor, process temperature and the presence of acidification operations), leading to a decrease in the yield and a decrease in the molecular weight of the resulting cellulose due to acid hydrolysis of cellulose and oxidative destruction of high-molecular fractions during alkaline cooking at high concentrations alkalis.

The closest analogue is a method for producing bleached bast cellulose from the down fraction obtained by cottonizing bast fibers (RU 2578586, D21C 5/00, publ. 03/27/2016). The method includes the following stages:

- cleaning (pre-hydrolysis) of bast fiber at a temperature of 95-110 ° C in an alkaline solution containing 10-20 g / l (1-2% wt.) sodium hydroxide and 2-5 g / l (0.2-0.5 % wt.) surfactant - synthanol for 40-60 minutes,

- acidification in a solution of sulfuric acid with a concentration of 3-8 g / l (0.3-0.8 wt%) at 100-120 ° C for 30-50 minutes,

- alkaline cooking (delignification of organic materials) in a solution containing 15-30 g / l (1.5-3.0% wt.) sodium hydroxide and 3.5-7.0 g / l (0.35-0, 7% wt.) Stabilizer at 95-120 ° C for 120-180 min,

- bleaching in a solution containing hydrogen peroxide 2-4 g / l (0.2-0.4% wt.), sodium hydroxide 1-9 g / l (0.1-0.9% wt.) at 100- 110 ° С for 90-150 min,

- intermediate rinsing and drying.

Moreover, bleaching is carried out immediately after alkaline cooking, without using the operation of washing the fiber, and sodium sulfide or sodium sulfite is used as a stabilizer.

The disadvantage of this method is the low molecular weight (medium degree of polymerization) of the resulting cellulose, the high content of lignin due to the acidification operation, as well as the severe operating conditions of the cleaning (prehydrolysis) and fiber bleaching process,

A technical problem is the creation of an effective method for producing bleached cellulose from hemp fiber, providing an increase in the molecular weight and a decrease in the lignin content in the target product, suitable for the production of paper for various purposes, cardboard and for chemical processing.

The technical problem is solved by a method for producing bleached hemp cellulose, including pre-hydrolysis of hemp fiber, delignification in an alkaline medium, bleaching in an alkaline solution of hydrogen peroxide with an alkali concentration of 0.15-0.17% wt., Intermediate washing, pressing and drying, in which pre-hydrolysis of raw materials lead at a temperature of 85-95 ° C in a solution containing 0.5-2.0% wt. sodium hydroxide, 0.2-0.3% wt. synthanol and 0.3% wt. sodium thiosulfate, for 60 minutes, delignification is carried out at a temperature of 125-140 ° C in a solution containing 1.5-3.0% wt. sodium hydroxide, 0.25-0.5% wt. sodium sulfide, 0.6-1.0% wt. sodium thiosulfate and 0.05-0.06% wt. synthanol, for 90-130 minutes, and the bleaching is carried out at a temperature of 85-95 ° C in an alkaline solution of hydrogen peroxide, additionally containing 0.3-0.5% wt. sodium metasilicate, 0.02-0.05% wt. triethanolamine and 0.05-0.1% wt. Trilon B, at a hydrogen peroxide concentration of 0.5-0.6% wt., for at least 90 minutes.

The technical result is an increase in the molecular weight of the resulting cellulose by an average of 2 times and a decrease in the lignin content by an average of 30% compared to the prototype. Also, compared with the prototype, the yield of the target product increases, while the resulting cellulose is characterized by satisfactory performance in terms of the content of α-cellulose, ash and whiteness. In addition, the proposed method provides a reduction in energy consumption for the process by reducing technological operations, reducing the temperature and time parameters of the process.

The essence of the proposed method is as follows.

In the proposed method, at the stage of prehydrolysis, a selective reducing agent, sodium thiosulfate, is additionally introduced into the alkaline solution. In the presence of sodium thiosulfate, the process of destruction of lignin-containing substances proceeds selectively, bypassing the polycondensation reaction of lignin-containing fragments, which makes it possible to reduce the lignin content by more than 2 times in comparison with the lignin content in the feedstock and reduce the prehydrolysis temperature to 85-95 ° C (in the prototype, 95- 110 ° C). Sodium thiosulfate prevents intense degradation of cellulose fiber, which, in combination with a lower process temperature, ensures the preservation of the molecular weight of cellulose.

In the process of delignification, the addition of sodium thiosulfate and the surfactant synthanol to the alkaline solution also contributes to the preservation of high molecular weight fractions of cellulose and a decrease in the lignin content. Synthanol, loosening the fiber structure, enhances its impregnation with an alkaline solution and ensures selective interaction of thiosulfate with lignin-containing compounds, preventing the development of the process of polycondensation of lignin fragments at high temperatures. At the same time, there is a high degree of destruction of the compounds formed between lignin and carbohydrates and a better penetration of the fiber while maintaining a high degree of resinification. The presence of the selective reducing agent sodium thiosulfate prevents the development of oxidative degradation of high molecular weight cellulose fractions.

At the bleaching stage, triethanolamine, sodium metasilicate and the chelating agent Trilon B are additionally introduced into the alkaline solution of hydrogen peroxide. Triethanolamine promotes loosening of the fiber structure (swelling) of cellulose and diffusion accessibility of the inner surface of cellulose fibers, ensuring efficient oxidation of lignin-containing substances at a lower temperature of 85-95 ° C (in the prototype 100-110 ° C) and during subsequent washes to remove them from the fiber. Sodium metasilicate, forming water-soluble complexes with heavy metals, prevents their accumulation in the cellulose fiber by removing the formed complexes in the course of further washing (ash content decreases). In addition, triethanolamine, as a reducing agent, together with the chelating agent Trilon B, prevents the reaction of oxidative destruction of high molecular weight fractions of cellulose, which, in combination with a lower process temperature, makes it possible to preserve high molecular weight fractions of bleached cellulose.

During intermediate and final washes, water-soluble low-molecular-weight products formed during the oxidative destruction of hemicellulose, pectin substances and lignin-containing compounds, including low molecular weight cellulose fractions, are removed. As a result, the molecular weight (average degree of polymerization) of the resulting cellulose increases, the lignin content decreases, the yield of the target product increases, while the cellulose is characterized by satisfactory parameters in terms of the content of * a-cellulose, ash and whiteness.

The proposed method eliminates the stage of acidification, which leads to a significant decrease in the molecular weight of cellulose due to acid hydrolysis [Valishina Z.T. Modeling the process of cellulose hydrolysis / Z.T. Valishina, A.V. Kostochko // Bulletin of Kazan Technological University. - 2010. - No. 11. - S. 132-13].

Changing the stated operating conditions and concentrations of chemical reagents does not lead to the achievement of the technical result of the invention.

Characteristics of raw materials and chemicals used.

1. Raw material - hemp fiber:

- Hemp short grade 1 and grade 2 GOST 9993-74 Specifications with a fire content not more than 13%, fiber breaking load - not less than 200 kgf, fiber length not less than 20 mm;

- Scutched hemp, grade 1 and grade 2 GOST 10379-76 Specifications;

2. Technical sodium hydroxide, grade RD - GOST 2263-79;

3. Surfactant synthanol ALM-10 - TU2483-003-71150986-2012;

4. Hydrogen peroxide technical - GOST 177-88;

5. Sodium sulfide - GOST 596-89;

6. Sodium sulfate (sodium thiosulfate) 5-water - GOST 27068-86;

7. Sodium silicate meta 9 aqueous (sodium metasilicate) - TU 6-09-5337-87;

8. Triethanolamine grade A - TU 2423-168-0020203335-2007;

9. Trilon B - disodium salt of ethylenediaminetetraacetic acid 2-water-GOST 10652-73.

Examples of specific implementation of the method.

Example 1.

A laboratory reactor was loaded with 250 g of fiber purified from mechanical impurities, crushed to a size of 8-10 mm and poured with a solution containing sodium hydroxide - 1.5% wt., Syntanol - 0.3% wt. and sodium thiosulfate - 0.3% wt., was heated to a temperature of 90 ° C and kept at this temperature for 60 min (hydromodule of the process 1:15). Upon completion of prehydrolysis and separation of the spent solution, the hemp product was washed twice in a container with softened water for 25 minutes at 85-90 ° C and for 20 minutes at 35-40 ° C.

Next, the washed and squeezed mass was placed in a laboratory autoclave with a capacity of 5 liters and filled with a solution containing sodium hydroxide - 1.5% wt., Sodium sulfide - 0.4% wt., Synthanol - 0.06% wt., And sodium thiosulfate - 0.6% by weight, heated to a temperature of 125 ° C and kept at this temperature for 130 minutes. Then unbleached hemp hemp cellulose was separated from the spent alkaline solution, washed twice in a 5-liter container with softened water for 25 minutes at 85-90 ° C and for 15-20 minutes at 35-40 ° C.

After wringing out excess water, the resulting pulp was loaded into a bleaching vessel and filled with a solution containing 0.15% wt. sodium hydroxide, 0.5% wt. hydrogen peroxide, 0.05% wt. triethanolamine, 0.4% wt. sodium metasilicate and 0.05% wt. trilon B, and heated to a temperature of 90 ° C, the processing time was 100 minutes. Bleaching hydronic module 1:15. At the end of the bleaching process, the cellulose was separated from the spent solution, washed with softened water in a 5 liter container, first for 25 minutes at 85-90 ° C, then for 20 minutes at 35-40 ° C, then washed with cold water at 18-22 ° С (hydraulic module at washing operations 1:10). The wet cellulose was pressed to a moisture content of at least 45%. The resulting cellulose was dried at room temperature, then dried at a temperature of 55-60 ° C to a residual moisture content of no more than 7%. The yield of bleached hemp pulp was 68.7%.

Examples 2-4 are similar to example 1, operating conditions are shown in table 1.

Example 5 (according to the prototype method).

Due to the absence in the description of the prototype of the necessary quality indicators of the obtained cellulose (degree of polymerization, lignin content) and for a correct comparison of the results, a cellulose sample was additionally obtained according to the prototype method. The acidification stage after fiber prehydrolysis was carried out in 0.8% wt. sulfuric acid solution at a temperature of 100 ° C for 50 minutes. Operating conditions at the stages of prehydrolysis, delignification and bleaching are shown in Table 1.

For the obtained samples of bleached hemp cellulose, the following parameters were determined:

- mass fraction of alpha cellulose according to GOST 595-794

- lignin content according to GOST 11960-79

- ash content according to GOST 18461-93;

- degree of polymerization (intrinsic viscosity) according to GOST 25438-82;

- whiteness according to GOST 595-79.

The results are shown in Table 2.

Analysis of the experimental data shown in Table 2 shows that, compared with the prototype, the use of the proposed method leads to an increase in the molecular weight of the resulting cellulose by almost 2 times, to a significant decrease in the lignin content (on average by 30%) and an increase in the yield of the target product. At the same time, the obtained cellulose is characterized by values of α-cellulose, ash and brightness that are satisfactory for further processing.

Thus, the proposed method, in contrast to the prototype, provides for the production of bleached hemp cellulose with a high molecular weight (medium degree of polymerization), a lower lignin content with a good yield and a content with satisfactory parameters for the content of α-cellulose, ash and whiteness. In addition, the proposed method provides a reduction in energy consumption for the process by reducing technological operations and reducing the temperature and time parameters of the process. The resulting cellulose can be used for the production of paper for various purposes, cardboard, as well as for chemical processing.

Claims (1)

A method of producing bleached hemp cellulose, including prehydrolysis of hemp fiber, delignification in an alkaline medium, bleaching in an alkaline solution of hydrogen peroxide with an alkali concentration of 0.15-0.17% wt., Intermediate washing, pressing and drying, characterized in that the prehydrolysis of raw materials is carried out at a temperature of 85-95 ° C in a solution containing 0.5-2.0% wt. sodium hydroxide, 0.2-0.3% wt. synthanol and 0.3% wt. sodium thiosulfate, for 60 minutes, delignification is carried out at a temperature of 125-140 ° C in a solution containing 1.5-3% wt. sodium hydroxide, 0.25-0.5% wt. sodium sulfide, 0.6-1.0% wt. sodium thiosulfate and 0.05-0.06% wt. synthanol, for 90-130 minutes, and the bleaching is carried out at a temperature of 85-95 ° C in an alkaline solution of hydrogen peroxide, additionally containing 0.3-0.5% wt. sodium metasilicate, 0.02-0.05% wt. triethanolamine and 0.05-0.1% wt. the chelating agent Trilon B at a hydrogen peroxide concentration of 0.5-0.6% wt., for at least 90 minutes.