RU2128261C1 - Method of cellulose bleaching - Google Patents

Abstract

FIELD: production of cellulose; may be used in production of bleached cellulose without molecular chlorine. SUBSTANCE: method includes treatment of nonbleached cellulose with solution of potassium permanganate in acid medium at pH 1.2-2.1. Acid medium is used in the form of sulfuric acid or acid reactor residues of chlorine dioxide production. Potassium permanganate is taken in quantity of not more than 1% of weight of absolutely dry cellulose. Pulp is held at room temperature up to complete absorption of potassium permanganate with cellulose mass separated from solution and neutralized. Then, it is subjected to four-stage bleaching. The first and third stages are carried out in the presence of hydrogen peroxide in alkali medium at temperature of 70-100 C with consumption of hydrogen peroxide in the amount of 1.5-2.0% of absolutely dry cellulose weight. At the second and fourth stages, the bleached cellulose is treated with solution of chlorine dioxide at temperature of 50-70 C with total consumption of chlorine of 1.1-1.3% of absolutely dry cellulose mass. EFFECT: higher degree of whiteness of bleached cellulose. 3 cl, 1 tbl

Description

 The invention relates to the field of pulp production and can be used to produce bleached cellulose without molecular chlorine (ECF-cellulose).

 A known method of bleaching using potassium permanganate, ozone of chlorine dioxide [1]. In this case, sulfate cellulose from eucalyptus is exposed to potassium permanganate after preliminary oxygen delignification with a cellulose hardness of 7-8 Kappa units according to the scheme: potassium permanganate-ozone-chlorine dioxide. Consumption of chemicals (in% by weight of absolutely dry cellulose): ozone - 1.2; chlorine dioxide-1.0; potassium permanganate-2-4.

 The disadvantage of this method is the high consumption of oxidizing agents (more than 9% active chlorine per 8 Kappa units). In addition, the high consumption of oxidizing agents causes a significant destruction of cellulose fiber and a decrease in such mechanical indicators as tear resistance, which will affect the quality of the paper.

A known method of bleaching sulphate pulp, taken as a prototype, including a two-stage treatment of pulp. In the first stage, the treatment is carried out with chlorine dioxide at a mass concentration of 2-10% until the ClO _{2 is} completely absorbed by the pulp at a temperature of 50-100 ^o C for 30-120 minutes, while the flow rate is 0.07-0.12% ClO ₂ per unit Kappa, and in the second stage, bleached cellulose with a mass concentration of 10-12% is treated with hydrogen peroxide at a pH of 9.5-10.5 in the presence of a stabilizer at a temperature of 70-100 ^o C for 60-240 minutes, at a flow rate of 0.5- 3.0% by weight of absolutely dry cellulose. After each step, bleached cellulose is separated from the solution and neutralized [2].

The disadvantage of this method is not high enough whiteness in the range of 80.0-85.9% at a concentration of pulp 12-20% (at a temperature of 70-80 ^o C when treated with hydrogen peroxide, whiteness is 75-80%), while the consumption of reagents is higher than in the proposed method.

 The objective of this study is to develop a more efficient method of bleaching pulp.

 The result is achieved by introducing the stage of treatment with potassium permanganate in an acidic medium, redistributing the costs of hydrogen peroxide and chlorine dioxide, and bleaching each of these reagents in two stages.

The proposed method differs from the known one in that before bleaching the pulp is treated with a solution of potassium permanganate (KMnO ₄ ) in an acidic medium at a pH of 1.2-2.1, and instead of a solution of mineral acid [1] it is proposed to use acid reactor residues (KRO), generated as waste in the production of chlorine dioxide, which leads to significant cost savings and improves the environmental performance of the production as a whole, since these residues (KRO) fall into wastewater, and this allows to increase the brightness of the cellulose vines up to 85-88% at a lower temperature and pulp concentration, in addition, the alternation of acid and alkaline stages of bleaching provides better diffusion of bleaching reagents, and the implementation of bleaching in four steps with sequential exposure to nucleophilic and electrophilic reagents is more effective than the level of alkalization between bleaching chlorine dioxide.

 This is the technical result, which is in a causal relationship with the essential features of the invention.

Salient features of the invention: a method for bleaching cellulose, comprising pretreatment of unbleached pulp with a mass concentration of 2-7% potassium permanganate solution in an acidic medium (pH 1.2-2.1) with a consumption of KMnO _{4 of} not more than 1% by weight of absolutely dry cellulose the molar ratio of acid: potassium permanganate (12.5-100): 1, the upper limit (100: 1) is limited by the amount of cellulose degradation (a decrease in the degree of polymerization of cellulose), and the lower (12.5: 1) region is formed by the formation of manganese dioxide (IV ), sulfuric acid or acid used as the acid eaktornye residues of chlorine dioxide production (KPO), extract the solution at a temperature of 15-25 ^o C until complete absorption KMnO ₄ pulp for 30-90 minutes, separating the mass from the solution to neutralize it, and the subsequent bleaching of pulp in four stages, with the first and the third stage of bleaching is carried out by treating cellulose (mass concentration of 10-20%) with a solution of hydrogen peroxide in an alkaline medium (pH 9.5-10.5) in the presence of a stabilizer, processing is carried out until complete absorption of hydrogen peroxide at a temperature of 70-100 ^o C for 120-240 minutes, with Consumption is H ₂ O ₂ constitutes 0.5-2.0% of the weight of bone dry pulp, and as a stabilizer is sodium silicate in an amount of 2,0-3,7% by weight of absolutely dry pulp, bleached mass separated from the solution and neutralize, the second and fourth stages of bleaching is carried out by treating bleached cellulose (mass concentration 5-8%) with a solution of chlorine dioxide (ClO ₂ ) until the ClO _{2 is} completely absorbed by cellulose at a temperature of 50-70 ^o C for 60-120 minutes, while the consumption of ClO ₂ is 1.1-1.3% by weight of absolutely dry cellulose, then the bleached mass is separated from solution and neutralize.

Example 1 (prototype)
Unbleached sulfate pulp from hardwood with a hardness of 75 permanganate units, 35% whiteness, 1080 polymerization degree is treated with chlorine dioxide at a rate of 1.3% (by weight of absolutely dry cellulose) at a concentration of 7% and a temperature of 70 ^o C. After 2 hours of treatment cellulose is thoroughly washed until neutral and the second stage is carried out, treated with hydrogen peroxide at a rate of 1.5% (by weight of absolutely dry cellulose), in the presence of sodium silicate 3.7% and sodium hydroxide 1%, at a pH value of 10.5 (in end of process). The concentration of the suspension of cellulose 10%, duration 3 hours, temperature 80 ^o C. After processing, the pulp is washed with water and dried. Cellulose whiteness 73.0%, SP = 890, yield 98.5% (table, p. 1).

Example 2 (prototype)
Unbleached sulfate cellulose is treated in example 1 with chlorine dioxide at a rate of 1.6% ClO ₂ by weight of absolutely dry cellulose. The second stage of treatment is carried out with hydrogen peroxide with a flow rate of 2.5%. Other conditions are the same as in example 1. Cellulose whiteness is 75.9%, SP = 880, yield 98.5% (table, p. 2).

Example 3
Unbleached sulfate cellulose from hardwood with a hardness of 95 permanganate units, a viscosity of 28 cPs is subjected to preliminary treatment with potassium permanganate in a sulfuric acid medium with a flow rate of 0.8% by weight of absolutely dry cellulose (molar ratio of sulfuric acid / potassium permanganate 100: 1), cellulose suspension concentration 5%, room temperature for 45 minutes (pHk = 1.4). Then the cellulose is thoroughly washed and sequentially treated with hydrogen peroxide and chlorine dioxide under the conditions specified in example 1. The redistribution of the flow of hydrogen peroxide and chlorine dioxide provides better diffusion of bleaching reagents, bleaching is carried out in four stages, with the following reagent costs: in the second stage 1, 5% H ₂ O ₂ , 1% NaOH, 3.5% Na ₂ SiO ₃ , in the fourth stage, 0.5% H ₂ O ₂ , 0.8% NaOH, 2% Na ₂ SiO ₃ , in the third and fifth stages 0.6 and 0.7% ClO _2, respectively. After each stage, the pulp is washed, after the last - dried. The whiteness of cellulose is 85.1%, SP = 780, yield 93% (table, p. 3).

Example 4
Unbleached sulfate cellulose is subjected to preliminary treatment with potassium permanganate in a sulfuric acid medium with a flow rate of 0.8% KMnO ₄ by weight of absolutely dry cellulose at a molar ratio of sulfuric acid / potassium permanganate of 50: 1, the regimen and consumption of reagents are similar to example 2 (pHк = 1.8) . The whiteness of cellulose is 86.0%, SP = 815, yield 93% (table, p. 4).

Example 5
Unbleached sulfate cellulose is pretreated with potassium permanganate according to Example 2, pHk = 1.2 (sulfuric acid used in acidic reactor residues of chlorine dioxide-KPO production is used). The molar ratio of sulfuric acid / potassium permanganate is 100: 1. Then the cellulose is separated from the solution and washed with water until neutral and subjected to a four-step treatment with the reagents of Example 2. White pulp 88.3%, SP = 770, yield 93.5% (table, p. 5).

Example 6
Unbleached sulfate cellulose was treated with the reagents of Example 4 at a molar ratio of sulfuric acid (contained in KPO) / potassium permanganate of 50: 1, then the cellulose was separated from the solution and washed with water until neutral. Subsequent treatment with bleaching reagents is carried out according to example 2. White pulp 86.0%, SP = 780, yield 92.5% (table, p. 6).

Example 7
Unbleached sulfate cellulose with a hardness of 95 permanganate units is subjected to preliminary treatment with potassium permanganate with a consumption of 0.4% by weight of absolutely dry cellulose at a molar ratio of sulfuric acid / potassium permanganate equal to 12: 1. Then, the cellulose is thoroughly washed and subjected to treatment with bleaching reagents according to example 2. Cellulose whiteness 83%, SP = 800, yield 97% (table, p. 7).

Example 8
Unbleached sulfate cellulose with a hardness of 95 permanganate units is pretreated with potassium permanganate in a sulfuric acid medium at a rate of 2% KMnO ₄ by weight of absolutely dry cellulose (sulfuric acid / potassium permanganate ratio 100: 1) according to Example 1, pHk = 1.3. Then the cellulose is separated from the solution and washed with water until neutral. The washed cellulose is subjected to treatment with hydrogen peroxide with a consumption of 1.5% by weight of absolutely dry cellulose and treatment with chlorine dioxide (total consumption is 1.1%) with an intermediate alkalization stage. Modes of bleaching carried out correspond to example 1. After processing, the pulp is washed with water and dried. The whiteness is 86.1%, SP = 700, the yield is 96% (table, p. 8).

 Sources of information.

 1. Igt. Pulp Bleach. Conf., Vancouver, June 15, 1994: Pousters.-Monreal, 1994. -C. 141-145.- Kojim Yasuo. Cjorine free bleaching of KR wich permanganatea.

 2. RF patent N 2075566. Method for bleaching sulphate pulp. Publ. BI N 8.

Claims (3)

1. The method of bleaching cellulose, including the stepwise treatment of cellulose with a solution of chlorine dioxide and hydrogen peroxide in the presence of a stabilizer at a pH of 9.5 to 10.5 until the reagents are completely absorbed, separating the bleached pulp from the solution and neutralizing it, characterized in that the unbleached pulp is pretreated potassium permanganate solution in an acidic environment at pH 1.2 - 2.1, which is used as sulfuric acid or acid reactor residues of chlorine dioxide production with a potassium permanganate consumption of not more than 1% by weight of absolutely dry cellulose, kept in solution at a temperature of 15 - 25 ^o C until the potassium permanganate is completely absorbed by the pulp, and after separation of the pulp from the solution and neutralization it is subjected to four-stage bleaching, the first and third stages being carried out by treatment with a solution of hydrogen peroxide in an alkaline medium at a temperature 70 - 100 ^o C at a flow rate of hydrogen peroxide 1.5 - 2.0% by weight of oven-dry pulp, and the second and fourth stages of the bleached pulp is treated with chlorine dioxide at a temperature of 50 - 70 ^o C with a total flow rate of chlorine dioxide of 1.1 - 1.3% by weight of oven-dry pulp.  2. The method according to claim 1, characterized in that with a molar ratio of (12.5 - 100): 1, respectively.  3. The method according to claim 1, characterized in that the treatment with potassium permanganate is carried out at a mass concentration of 2 to 7% for 30 to 90 minutes

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