BG112971A - Method of bleached microcrystalline cellulose production from unbleached pulp - Google Patents

Abstract

The invention relates to a method of bleached microcrystalline cellulose production from unbleached pulp by two-stage acid hydrolysis and washing and removal of the dissolved products after each stage, both, during the hydrolysis, and during the subsequent bleaching of the microcrystalline cellulose that has been obtained. This method allows using unbleached paper grade pulp with high content of hemicelluloses in order to produce bleached microcrystalline cellulose with low pentosan content and properties suitable for use in the pharmaceutical, cosmetic and food industries.

Description

Field of technology

The present invention relates to a process for the preparation of microcrystalline cellulose from unbleached cellulose by two-step acid hydrolysis with intermediate washing with water and removal of dissolved pentose sugars, as well as subsequent bleaching of the resulting microcrystalline cellulose. This method allows unbleached pulp for paper, which contains a significant amount of hemicelluloses, to obtain bleached microcrystalline cellulose with a low content of pentosans and indicators suitable for use including in the pharmaceutical, cosmetic and food industries.

BACKGROUND OF THE INVENTION

It is known from the literature and practice that microcrystalline cellulose (MCC) is partially depolymerized cellulose with a low degree of polymerization (DP). When free of organic and inorganic contaminants, microcrystalline cellulose has a white, tasteless and odorless powder structure, insoluble in water, dilute acids and in most organic solvents.

Its production from plant fibers, its lack of toxicity, its high hygroscopicity and physiological inertness provide not only harmlessness, but rather a beneficial effect on the human body as an excipient for healthy foods and a source of dietary fiber. As a non-absorbable and indigestible ballast substance, it helps to reduce the caloric content of food products without affecting their organoleptic properties, has been shown to normalize metabolic processes in the body and even reduces the risk of neoplasms.

Due to these and other qualities due to its chemical nature, microcrystalline cellulose is a multifunctional product with extremely wide and diverse application in the food industry, pharmaceutical industry and biotechnology, cosmetics, chemical industry and agriculture.

Various methods are known in the literature for the preparation of MCC by a combination of acid hydrolysis and various other processes, usually using bleached cellulose for chemical processing, which is characterized by a low content of pentosans and which is subjected to bleaching with specific oxidants to achieve of high whiteness.

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U.S. Pat. No. 994,69695B2 describes a method for producing microcrystalline cellulose comprising acidifying and purifying fibrous cellulosic material from heavy metals, followed by washing and hydrolysis of the washed cellulosic material at elevated temperature to obtain a mixture of microcrystalline cellulose and hydrolyzate. followed by separation of the microcrystalline cellulose from the hydrolysis, wherein the mixture or the separated hydrolyzate or the microcrystalline cellulose can be neutralized. The proposed method does not take into account the whiteness of microcrystalline cellulose, which decreases significantly at high hydrolysis temperatures.

From BG19462, analogous to US patent 3,954,727, a method for producing microcrystalline cellulose is known, in which the hydrolysis of cellulose is carried out with dilute sulfuric acid at a temperature of 120 to 160 ° C, while simultaneously chemically disaggregating the product, which after this is subjected to additional bleaching, cleaning, washing and drying.

International patent application WO2016182867 describes a method which makes it possible to obtain bleached microcrystalline cellulose from both chemical processing pulp and paper pulp, which comprises neutralizing or alkalizing a suspension of an acidic microcrystalline reaction mixture, usually obtained by acidic hydrolysis or by electron beam irradiation followed by the addition of an oxidant to such a reaction mixture.

From patent RU2298562 C1 is known a method for production of microcrystalline cellulose from unbleached cellulose with initial whiteness up to 65%, which is subjected to single-stage hydrolysis with sulfuric acid and subsequent bleaching of the obtained microcrystalline cellulose with chlorine dioxide in one step. As a result, a maximum whiteness of 80% is achieved. The problem with the whiteness of the microcrystalline cellulose in this method is mainly due to the absorption of chromophore structures on the cellulose in the process of one-step acid hydrolysis.

From international application WO2019095024 there is a known method for producing microcrystalline cellulose by two-stage acid hydrolysis and washing with water after each of the stages of hydrolysis, which allows from bleached paper pulp to obtain microcrystalline cellulose with sufficiently high whiteness, (ie without need for additional bleaching) and with indicators suitable for use in the pharmaceutical, cosmetic and food industries, in agriculture and the chemical industry.

Based on the above, it can be concluded that most of the known methods for producing microcrystalline cellulose by acid hydrolysis of cellulose are not suitable for the treatment of cellulose with a high content of hemicelluloses. Carrying out acid hydrolysis to some extent leads to a significant absorption of chromophore structures on the cellulose and is accompanied by a significant increase in the Kappa number and a decrease in the whiteness of the cellulose. All this determines the need to develop efficient production processes to obtain from unbleached cellulose microcrystalline cellulose with high whiteness and low content of pentosans, which is suitable for use in the pharmaceutical, cosmetic and food industries.

BRIEF DESCRIPTION OF THE TECHNICAL SUMMARY OF THE INVENTION

A problem of the present invention is to develop an efficient method for producing microcrystalline cellulose by acid hydrosis of unbleached paper pulp, so as to obtain as a final product bleached microcrystalline cellulose with a low content of pentosans, characteristic of cellulose for chemical processing.

The problem of the invention is solved by a method for producing bleached microcrystalline cellulose from unbleached pulp for paper, which includes two separate stages of acid hydrolysis, each of which is followed by washing with water and removal of dissolved pentose sugars, and subsequent bleaching of microcrystalline cellulose to a high degree of whiteness.

The method according to the invention includes the following operations:

(a) First stage of acid hydrolysis, in which mainly hemicelluloses and to a lesser extent cellulose are dissolved,

b) Washing the cellulose after the first stage of acid hydrolysis, removing the sugars passed into solution;

(c) Second degree of acid hydrolysis, in which the degree of polymerisation of the cellulose is reduced to the particle size characteristic of microcrystalline cellulose and the content of pentosans is reduced to levels typical of cellulose for chemical processing;

d) Washing of the microcrystalline cellulose obtained after the second stage of acid hydrolysis;

e) Bleaching of microcrystalline cellulose obtained after two-stage acid hydrolysis with oxidative bleaching reagents.

f) Washing after each degree of bleaching and isolation of the resulting high whiteness microcrystalline cellulose.

DETAILED DESCRIPTION OF THE TECHNICAL SUMMARY OF THE INVENTION

The essence of the method according to the invention consists in the fact that as a first stage of processing the starting material - unbleached cellulose (as absolutely dry) with a concentration of 5% to 30% relative to the total weight of the reaction mixture, is subjected to acid hydrolysis. the acid is added in an amount of 1.0 to 5.0 weight percent, based on the weight of absolutely dry cellulose. The hydrolysis is carried out at temperatures of 125 ° C to 180 ° C, preferably at a temperature of 130 ° C to 160 ° C and even more preferably from 135 ° C to 150 ° C. The reaction time in the first step of the acid hydrolysis is from 10 to 180 minutes, preferably from 20 to 150 minutes and even more preferably from 40 to 120 minutes.

Between the two stages of acid hydrolysis, an intermediate wash of the cellulose with distilled water is carried out without the need for neutralization.

In the second stage of the process according to the invention, acid hydrolysis of the cellulose washed after the first stage (as absolutely dry) is carried out with a concentration of 5% to 30% by weight of the total weight of the reaction mixture. The acid is added in an amount of 0.5 to 5.0 weight percent, based on the weight of the absolutely dry cellulose. The hydrolysis is carried out at temperatures from 100 ° C to 180 ° C, preferably at a temperature from 120 ° C to 160 ° C and even more preferably from 130 ° C to 150 ° C. The reaction time in the second stage of hydrolysis is from 10 minutes to 180 minutes, preferably from 20 minutes to 150 minutes and even more preferably from 40 to 120 minutes.

The washing of the unbleached microcrystalline cellulose obtained after the second stage of acid hydrolysis is carried out with distilled water.

Part of the wash water of the second acid degree can be used in the intermediate wash.

The two stages of acid hydrolysis can be carried out in the same reactor or separately as a batch or continuous process.

Unobviously for the person skilled in the art, it was found that when carrying out the first stage of acid hydrolysis at the beginning of the process, but at different durations as a function of the reaction temperature, a moment passes from hemicelluloses and related chromophore structures. Completion of the first stage of hydrolysis at this time ensures their removal from the system by intermediate washing. Thus, in the second stage of acid hydrolysis, the destructive processes with dissolved sugars are significantly limited, leading to the formation of new chromophore structures and the possibility of their absorption on cellulose is minimized. The obtained unbleached microcrystalline cellulose has a significantly lower Kappa number than the original unbleached cellulose and that obtained only after one-step acid hydrolysis, which in turn is a prerequisite for more efficient and lower consumption of bleaching reagents. The content of pentosans is also significantly lower, which is comparable to the levels in cellulose for chemical processing.

The bleaching of the obtained unbleached microcrystalline cellulose is carried out with classical oxidative bleaching reagents such as: hydrogen peroxide in an alkaline medium; pressurized oxygen in an alkaline medium (alone or in combination with hydrogen peroxide); chlorine dioxide and peracetic acid. Bleaching conditions are standard for the respective reagents. The bleaching temperature is in the range from 50 ° C to 110 ° C, preferably at a temperature of from 60 ° C to 100 ° C and even more preferably from 70 ° C to 100 ° C. The duration of the bleaching in the individual stages is from 10 minutes to 240 minutes, preferably from 20 minutes to 180 minutes and even more preferably from 30 to 120 minutes. The consumption of bleaching reagent and added NaOH is from 0.2 to 5.0 weight percent, based on the weight of absolutely dry cellulose. The pressure during oxygen bleaching is from 0.2 to 7.0 atm. The number of bleaching steps and the consumption of bleaching reagents is less than that required to bleach the starting cellulose, which is determined by the significant reduction in the Kappa number as a result of the acid hydrolysis. After each degree of bleaching, rinse with water.

The final washing of the resulting bleached microcrystalline cellulose is performed with desalinated water. Some of the wash water can be used for rinsing after previous stages.

According to the invention, unbleached cellulose is used as a starting fibrous material, which can be: sulphate cellulose; sulfite cellulose; soda ash; cellulose obtained after boiling with organic solvents or after processing of waste paper.

The cellulose used as a starting material can be from coniferous wood, deciduous wood, from non-woody plant raw materials, such as straw, hemp, kenaf, ramie, bamboo and other cellulose-containing agricultural vegetable waste.

The cellulose used may be high in pentosans above 15%.

The hydrolysis in the first and second stages of acid hydrolysis in the process according to the invention can be carried out with dilute organic acids (oxalic, formic, acetic, etc.), solutions of sodium bisulphite and sodium bisulphate, with dilute inorganic acids, preferably mineral acids. such as hydrochloric acid, sulfuric acid and nitric acid.

The advantages of the method according to the invention are:

- production of high-whiteness microcrystalline cellulose by acid hydrolysis of unbleached cellulose and subsequent bleaching at significantly lower reagent consumption and at a reduced number of stages compared to those that would be necessary for the bleaching of the starting cellulose;

- carrying out the acid hydrolysis in two stages and washing the cellulose after the first stage leads to limitation of the destructive processes with dissolved sugars and the formation of chromophore structures, as a result of which the possibility of absorption of these structures on the cellulose is minimized;

- the obtained unbleached microcrystalline cellulose has a significantly lower Kappa number than the initial cellulose, which determines the more efficient course of the bleaching process;

- by appropriate temperature-time regime of the two-stage acid hydrolysis and after additional bleaching, bleached microcrystalline cellulose with low content of pentosans is obtained, characteristic for the cellulose for chemical processing and with indicators suitable for use in the pharmaceutical, cosmetic and food industries;

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- between the two stages of acid hydrolysis, the reaction product is washed without pH adjustment;

- An additional significant advantage of the two-stage control of the acid hydrolysis process is its much more uniform flow.

An exemplary embodiment of the invention

The invention is illustrated by the following examples, which illustrate it without limiting its scope of protection.

The tests were performed with unbleached and oxygen-delignified broad-leaved cellulose, which is characterized by a content of pentosans in the cellulose over 18%.

The acid hydrolysis in the first and second stage is carried out in rotating 2 liter stainless steel laboratory autoclaves, which are isolated by a heat-resistant polymer from possible contact of the acid with the metal.

The autoclaves are loaded with a pre-disintegrated pulp containing 30 g of abe. dry cellulose, a mass concentration is determined and the required amount of dilute acid is added to it.

The autoclaves are placed in a bath of polyethylene glycol, which is preheated to the required temperature. The autoclave tempering time is 5 minutes. After completion of the hydrolysis process, the autoclaves are removed and cooled with cold water.

The resulting microcrystalline cellulose was washed on a Buchner funnel with a vacuum pump through a filter with a pore size of 2 - 3 pm. The first filtrate was separated for analysis for sugars and furfural by HPLC Dionex liquid chromatographic system according to NREL Technical Report (NREL / TP-510-42623). Each sample is washed with distilled water. A partial intermediate wash between the acid hydrolysis steps is also performed, in which case the mass is diluted with distilled water to a concentration of 5% and then filtered.

Bleaching with CH ₂ , hydrogen peroxide and peracetic acid is carried out under appropriate conditions in plastic bags placed in a water bath. Oxygen bleaching is performed under pressure in rotating autoclaves.

Rinsing after each degree of bleaching is done with distilled water, and the final rinsing with deeply desalinated water.

Particle size and size distribution were characterized by a Fritsch Analysette 22 Micro Tec Plus laser analyzer. The average degree of polymerization of DP cellulose was determined according to the pharmacopoeia standard and the whiteness according to ISO 2470. The content of pentosans in cellulose was determined according to TAPPI T223 cm-10 and the Kappa number according to ISO 302: 2004. The yield of IAS is determined by weight.

Example 1

Table 1 presents the results of one-stage and two-stage acid hydrolysis of oxygen-delignified cellulose, characterized by a Kappa number of 8.83% and a pentosan content of 19.7%. Acid hydrolysis was performed with dilute sulfuric acid at an acid consumption relative to absolutely dry cellulose of 3.0% for the first stage and 2.0% for the second stage. The reaction temperature is 150 C and the mass concentration is 12.5 Λ for both stages. The duration of the process is 60 minutes for the first degree and 60 and 90 minutes for the second. The cellulose is washed after the first acid stage with distilled water.

Under the conditions of the process before the second stage of acid hydrolysis, no absorption of caramels and breakdown products of sugars on MCC is observed. At this temperature after 60 minutes, the content of pentosans decreases to 11% and the Kappa number to 4.6. The average degree of polymerization is reduced to 460.

Table. 1 Acid hydrolysis of standard oxygen-delignified cellulose at

hydrolysis conditions yield,% glucose,% from asc. xylose,% of asc. FurFur,% from asc. DP by SCAN DP on the farm. pentosans% cap number First st epen acid hydrolysis of standard oxygen-delignis ppi consumption of k-na 3%; duration 60 min and temperatures target temperature 150 ° C улоза 3.0% H ₂ SO ₄ 60 min. 82.8 1.66 8.16 0.45 577 461 10.98 4.58 Second degree acid hydrolysis of standard oxygen-delignified cellulose at 2% times. duration 60 and 90 min and temperature 150 C ______________ under the k-na 60 min. 75.5 (1.66 + 2.33) 3.99 (8.16 + 2.62) 10.78 0.75 400 319 6.70 4.48 90 min. 71.9 (1.66 + 2.30) 3.96 (8.16 + 2.95) 11.11 0.79 377 301 6.42 3.89

After the second stage of hydrolysis, the pentosan content is further reduced to 6%. The average degree of polymerization is a little over 300. The kappa number after the second degree of acid hydrolysis reaches levels of 4 units, which is a prerequisite for more efficient bleaching.

Peroxide bleaching was performed at a mass concentration of 10%, temperature 70 ° C, duration 120 min, peroxide consumption 1.0% and NaOH consumption 2.5%.

Bleaching with CY ₂ was performed at a mass concentration of 10%, temperature 70 ° C, duration 60 minutes, consumption of aCl 2.0% and 2.5%.

The microcrystalline cellulose obtained after two-stage acid hydrolysis of oxygen-delignified unbleached cellulose is bleached to high whiteness in two bleaching stages (Table 2), with an additional slight decrease in the content of pentosans and the average degree of polymerization of the cellulose.

The degree of bleaching and the consumption of bleaching reagents is less than that required for the bleaching of the starting cellulose, which is determined by the significant reduction in the Kappa number as a result of the acid hydrolysis.

It was found that carrying out the stages of bleaching with C1O ₂ in an acidic medium, the last stage providing the necessary low ash content of microcrystalline cellulose.

A higher bleaching rate of microcrystalline cellulose was observed than that of paper pulp bleaching.

Table. 2 Bleaching with _{C 2} O ₂ and H _{2 O 2} of MCC obtained after two-stage acid hydrolysis

nya styanpyaptnya [gislopopno-leligniLitsipana cellulose __________________________ Bleaching conditions DP by Faomakop. whiteness,% ISO content of ash,% pentosans,% average MCC size, μπι After in the forest smallpox acid hydrolysis of standard oxygen-delignified at .......... ..... pa mass 12.5%: temperature T 150 'C. duration 60 min and acid consumption 2% 1st floor bleaching 1.0% H ₂ O ₂ consumption 2.5% NaOH consumption 120 min, 70 ° C, 10% conc. II stage bleaching with С1О ₂ 2.0% consumption аС1 60 min. ΌΌ. 10% conc. 307 88.5 0.2086 6.27 31.3 After the second degree acid concentration of the ointment 12.5%: I degree bleaching 1.0% consumption H ₂ O ₂ 2.5% consumption NaOH 120 min., 70 ° С, 10% conc. hydrolysis of the standard oxygen-delignified at a temperature of ^T 150 P C. duration 90 min and consumption of vinegar. shna 2% II degree bleaching with С1О ₂ 2.5% consumption аС1 60 min., 70 ° С, 10% conc .______ 286 91.0 0.0449 5.88 31.6

The presented example shows oxygen-deligified unbleached cellulose, by two-stage acid hydrolysis and subsequent bleaching, microcrystalline cellulose can be obtained, meeting the requirements of the European Pharmacopoeia and characterized by a content of pentosans comparable to that of commercial microcrystalline cellulose products. bleached cellulose for chemical processing.

Example 2

Table 3 presents the results of single-stage and two-stage acid hydrolysis of unbleached broadleaf cellulose, characterized by Kappa number 18.01% and pentosan content 19.9%. Acid hydrolysis was performed with dilute sulfuric acid at an acid consumption relative to absolutely dry cellulose of 3.5% for the first stage and 2.0% for the second stage. The reaction temperature was 150-C and the mass concentration was 12.5% for both grades. The duration of the process is 60 minutes for the first degree and 60 and 90 minutes for the second. The cellulose is with distilled water.

the process before the second stage of acid washing after the first acid stage Under the conditions of hydrolysis, no absorption of caramels and breakdown products of sugars on MCC is observed. At this temperature after 60 minutes, the content of pentosans decreases to 10% and the Kappa number to 13. The average degree of polymerization decreases to 460.

Table. Acid hydrolysis of standard unbleached cellulose at concentration and mass

12.5% and temp hydrolysis conditions era yield,% G 150 ° C glucose,% from asc. xylose,% of asc. furfur,% from asc. DP by SCAN_ DP on the farm. pentosans% cap number First degree of acid hydr < 3.5%; continued goes to standard unbleached cellulose at a consumption of, .., ™ .ope ap min and temperature T 150 ° C _____________ to-on 7.65 0.43 580 463 10.06 13.04 3.5% H ₂ SO ₄ 60 min. 79.2 1.44 Second degree acid hydrolysis of _____ continued standard and specific unbleached cellulose at a consumption of pa Tropot 60 I 90 min and temperature 150 ° С ----- (7.65 + 1.7 8) 9.43 0.60 410 328 5.85 12.28 60 min. 68.4 (1.44 + 1.39) 2.83 11.56 90 min. 69.0 (1.44 + 1.82) 3.26 (7.65 + 2.1 1) 9.76 0.67 369 295 5.73

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It can be seen that after the second stage of acid hydrolysis the content of pentosans is further reduced to levels below 6%. The average degree of polymerization is about 300. The Kappa number decreases after the second degree of acid hydrolysis to levels of about 12 units, which is a prerequisite for more efficient bleaching.

Bleaching with peroxide was performed at a mass concentration of 10%, temperature 70 ° C, duration 120 min, peroxide consumption 2.0% and NaOH consumption 2.5%.

Bleaching with C ₂ O 2 was performed at a mass concentration of 10%, temperature 70 ° C, duration 60 minutes, consumption of aCl 0.5%, 1.0% and 2.0%.

Table. 4 Bleaching is C1O ₂ and H ₂ O ₂ of MCC obtained after two-stage acid hydrolysis

of unbleached cellulose Bleaching conditions DP on gjapmjakop. whiteness,% ISO content of ash, ° / « pentosans,% medium size MCC, pm__ After the second degree of acid mass 12.5% ^ temperature T I degree of bleaching 2.0% consumption H ₂ O ₂ 2.5% consumption NaOH 120 min ,, 70 “C, 10% conc. II stage bleaching with C1O ₂ 2.0% consumption aC1 hydrolysis of a standard non-isolated whole 150 ° C, duration 60 min and flow rate choza at a concentration of • isslina 2% _______ III stage bleaching with C1O ₂ 1.0% consumption aC1 311 90.3 0.1094 5.52 31.8 ---— 60 min .. 0 (. Ю <> конн .___ After staring degree of acidity 1 hydrolysis of standard unbleached cellulose ιιρι mass concentration ιζ.θ / c- 1 tbsp bleach 2 .0% H ₂ O ₂ consumption 2.5% NaOH consumption 120 min., 70'C, 10% conc. II stage bleaching with C1O ₂ 2.0% consumption aC1 III degree bleaching with С1О ₂ 0.5% consumption аС1 60 min., 70 “С, 10% conc .______ 300 89.9 0.0293 ___ 5.04 32.4

High-whiteness microcrystalline cellulose with indicators meeting the requirements of the European Pharmacopoeia is obtained by bleaching microcrystalline cellulose, which is obtained after two-stage acid hydrolysis of unbleached cellulose (Table 4).

The degree of bleaching and the consumption of bleaching reagents is less than that required for the bleaching of the starting cellulose, which is determined by the significant reduction in the Kappa number as a result of the acid hydrolysis.

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It has been found that carrying out the blending steps with SO 2 in an acidic medium, the latter stage, provides the necessary low ash content of microcrystalline cellulose.

A higher bleaching rate of microcrystalline cellulose was observed than that of paper pulp bleaching.

Example 3

The studies were performed with unbleached broadleaf cellulose, characterized by a Kappa number of 16.62% and a pentosan content of 19.6%.

The acid hydrolysis was carried out with dilute sulfuric acid at an acid consumption relative to absolutely dry cellulose of 3.5% for the first stage and 2.0% for the second stage. The reaction temperature is 150 ° C and the mass concentration is 12.5 Λ for both stages. The duration of the process is 30, 60, 90, 120 and 150 minutes for the first stage and 60 and 90 minutes for the second (Table 5). The cellulose was washed after the first step of acid hydrolysis with distilled water, and a partial intermediate wash was performed.

In the course of the first stage of acid hydrolysis, a significant decrease in the average degree of polymerization of the cellulose and the content of pentosans is observed, and the process is accompanied by a corresponding decrease in the yield.

Unexpectedly for the person skilled in the art, it was found that the Kappa number decreases sharply by about 4 units at the very beginning of acid hydrolysis, and at this reaction temperature after the 60th minute of the first stage of acid hydrolysis increases significantly and reaches 150 minutes a value which exceeds by 2 units that of the initial cellulose. This effect can be explained by the absorption of caramels and sugar breakdown products on the IAS.

After the second stage of acid hydrolysis with duration of 60 and 90 minutes the content of pentosans is further reduced to 2.8% - 3.8%, and the average degree of polymerization is 247 - 281. The kappa number of microcrystalline cellulose practically does not change after the second stage of acid hydrolysis .

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Table. 5 Acid hydrolysis of standard unbleached cellulose at mass concentration

12.5% and temperature T 150 С glucose,% from asc. xylose,% of asc. FurFur,% of asc. DP by SCAN DP on the farm. pentosans% cap number hydrolysis conditions yield,% First step < gn sour 1na hydrol 12.5%; horses because of the standard unbleached whole entpation of H _{2 8O 4} 3.5% and temperg vine at 1tura T 1 concentration of 50 ° C I'm on 3.5% H ₂ SO ₄ 30 min. 78.8 1.69 7.95 0.55 590 472 8.91 12.39 3.5% H ₂ SO ₄ 74.6 2.64 9.22 0.98 543 434 7.34 12.81 3.5% H ₂ SO ₄ 72.1 3.21 9.31 1.29 458 366 5.42 14.71 3.5% H ₂ SO ₄ 70.9 3.86 8.96 1.81 416 333 4.66 17.35 3.5% H ₂ SO ₄ 67.7 4.90 8.68 2.52 360 288 3.60 18.32 Second c warm vinegar Hellenic hydrolysis of standard unbleached cellulose for 60 and 90 minutes and temperature 15 for at 0 ° C 2 ° course / 0, 60 min. 1 degree + 90 min. 65.3 (2.64 + 2.35) 4.99 (9.22 + 1.10) 10.32 1.23 351 281 3.78 12.82 90 min. I degree + 60 min. II degree ___ 64.5 (3.21 + 2.47) 5.68 (9.31 + 1.18) 10.48 1.43 309 247 2.82 15.18

Therefore, from the point of view of optimal bleaching of microcrystalline cellulose, it is desirable under the conditions of carrying out the two-stage acid hydrolysis with intermediate washing with water, the first stage of acid hydrolysis, at the indicated reaction temperature, to be an optimal duration of about 60 minutes. achieves a final Kappa number of microcrystalline cellulose 4 units lower than that of the original cellulose.

Bleaching with peroxide was performed at a mass concentration of 10%, temperature 70 ° C, duration 120 minutes, peroxide consumption 2% and NaOH consumption 2.5%.

Bleaching with Cl ₂ was performed at a mass concentration of 10%, temperature 70 ° C, duration 60 minutes, consumption aC12.0%.

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rn Μ η is MCC obtained after one-stage and two-stage acid Table. 6 Bleaching with C1O ₂ and H ₂ O ₂ of MCC obtained after

hydrolysis of unbleached target Bleaching conditions улоза ___ DP по whiteness,% ISO pentosans,% __ average MCC size, pm __ (p a r m k oil »1 ________ „„ ..τταητιπι unbleached cellulose ...... ap ...... 1 degree and 90 min. II degree sour ^^ --- - , I; —й __------- ----— τ- Ι st. Isoelvane 2.0% consumption of H ₂ O ₂ 2.5% consumption of NaOH ---- 120 min, 70 C, S / w. 11 st. Whitening with SU ₂ 2.0% consumption aC1 262 88.2 3.47 32.7 oo min .. 0 C. 10 °> conc. ________1 .................................................. . Merge 90 mhil] _degree_i60 _! * 1in P degree kis ^ inn ^ xt ^ ™ ^^^^ лу »юза— .- I st. Whitening 2.0% consumption of H ₂ O ₂ 2.5% consumption of NaOH _________— 120 min, 70 C, w / o conc. II stage bleaching with SU ₂ 2.0% consumption aC1 238 84.7 2.75 26.8 (10 М1111 .._ ⁽ н_. 10_о_КОП1 £ ____ After 150 min. 1 degree sour cellulose ____ "on --- 1 tbsp whitening 2.0% consumption of H ₂ O ₂ 2.5% consumption of NaOH _____ 120 min., 70 O, S / o conc ._____ II degree bleaching with SU ₂ 2.0% consumption aC1 60 min., 70 ° C, 10% conc. 273 _____ 81.2 3.46 35.4

Technological effect of the bleaching process after two-stage acid hydrolysis of unbleached cellulose at a reaction temperature of 150 ° C, with a duration of the first stage of 60 minutes and the second 90 minutes (Table 6) is the production of microcrystalline cellulose with high whiteness, low content of pentosans and with indicators meeting the requirements of the European Pharmacopoeia.

Higher values of Kana number in unbleached microcrystalline cellulose naturally one-step lead to lower whiteness after bleaching, and in the process of acid hydrolysis the whiteness of microcrystalline cellulose is 7% lower.

Claims (12)

A method for producing bleached microcrystalline cellulose from cellulose at a concentration of 5% to 30% by weight of the reaction mixture, by two-stage acid hydrolysis with dilute acid, comprising intermediate washing with water of the cellulose obtained after the first stage of acid hydrolysis, characterized in that unbleached cellulose is used as a raw material, the first stage of acid hydrolysis is carried out with dilute acid in an amount of 1.0 to 5.0, based on the weight of absolutely dry cellulose at temperatures from 125 ° C to 180 ° C for a time from 10 to 180 minutes, after which the cellulose washed after the first stage of acid hydrolysis at a concentration of 5% to 30% by weight of the total reaction mixture is subjected to a second stage of acid hydrolysis with dilute acid in an amount of 0.5 to 5.0% by weight, based on the weight of absolutely dry cellulose, in which the hydrolysis is carried out at temperatures from 100 ° C to 180 ° C, for a period of 10 minutes to 180 m followed by washing with water and bleaching the resulting unbleached microcrystalline cellulose, after each stage of bleaching the microcrystalline cellulose is washed with water and the final washing of the obtained bleached microcrystalline cellulose is carried out with desalinated water. Process according to Claim 1, characterized in that the temperature for carrying out the first step of acid hydrolysis is preferably from 130 ° C to 160 ° C and even more preferably from 135 ° C to 150 ° C. Process according to Claims 1 and 2, characterized in that the temperature for carrying out the second stage of acid hydrolysis is preferably from 120 ° C to 160 ° C and even more preferably from 130 ° C to 150 ° C. Process according to Claim 1, characterized in that the time for carrying out the first step of acid hydrolysis is preferably 20 to 150 minutes and even more preferably 40 to 120 minutes. Process according to Claims 1 and 4, characterized in that the time for carrying out the second stage of acid hydrolysis is preferably 20 to 150 minutes and even more preferably from 40 to 120 minutes. Process according to Claim 1, characterized in that the acid in the first and second stages of acid hydrolysis can be a dilute organic acid, an aqueous solution of sodium bisulphite, an aqueous solution of sodium bisulphate, a dilute inorganic acid, preferably a mineral acid. . Process according to Claims 1 and 6, characterized in that the dilute organic acid can be oxalic, acetic, formic acid. Process according to Claims 1 and 6, characterized in that the dilute mineral acid can be hydrochloric acid, sulfuric acid, nitric acid. Process according to Claim 1, characterized in that the cellulose can be unbleached sulphate cellulose; unbleached sulphite cellulose, soda ash, cellulose obtained by digestion with organic solvents or after processing of waste paper. Method according to claim 1, characterized in that the cellulose can be from coniferous wood, deciduous wood, from non-woody plant raw materials such as straw, hemp, kenaf, ramie, bamboo and other cellulose-containing agricultural vegetable waste. Process according to Claim 1, characterized in that the cellulose may have a pentosan content of more than 15% by weight relative to its total weight. Process according to Claim 1, characterized in that the two stages of acid hydrolysis can be carried out in the same reactor or separately as a batch or continuous process.