RU2142529C1 - Method of bleaching cellulose - Google Patents

Abstract

FIELD: pulp industry; production of bleached cellulose without molecular chlorine and chlorine dioxide. SUBSTANCE: cellulose having rigidly of permanganate number 83 is subjected to alternating treatment by oxygen-containing reagents at five stages: hydrogen peroxide in alkaline medium and potassium permanganate in sulfate solution at stages 11 and IV; alkaline medium at stages I, III and V. Bleaching of sulfate cellulose by hydrogen peroxide is performed at 70 to 100 C at consumption of 2% and concentration of cellulose mass of 2 to 7%. Treatment with potassium permanganate is performed at consumption of 0.01 to 0.8% at 18 to 25 C till complete absorption. EFFECT: possibility of obtaining whiteness of cellulose of 83 to 85 % by ecologically clean method without using chlorine-containing reagents. 1 tbl, 8 ex

Description

 The invention relates to the field of cellulose production and can be used to produce bleached pulp without the use of chlorine-containing reagents, both molecular chlorine and chlorine dioxide (TCF-cellulose).

 A known method of bleaching unbleached pulp using potassium permanganate, ozone and chlorine dioxide using high costs of oxidizing agents, causing significant destruction of the cellulose fiber [Igt Pulp Bleach Conf., Vancouver., June 15,1994: Pousters. - Monreal, 1994, c 141-145 .- Kojim Yasuo. Cjorine free bleaching og KR wich permanganatea].

A known bleaching method, including pre-treatment with a solution of potassium permanganate for 30-90 minutes, in an acidic environment at a pH of 1.2-2.1 and at a molar ratio of acid: permanganate (50-100): 1 with a flow rate of KMnO ₄ 0.1 -4% by weight of absolutely dry cellulose and sequential treatment with chlorine dioxide and hydrogen peroxide (RU patent N 2129179, CL D 21 C 9/16, published on 04/20/1999).

 The disadvantage of this method is the high consumption of potassium permanganate, which results in a decrease in the mechanical properties of cellulose.

A known method of bleaching unbleached pulp, taken as the closest analogue, comprising pre-treating unbleached pulp with a solution of potassium permanganate in an acidic medium at a pH of 1.2-2.1, with a molar ratio of acid: permanganate (12.5-100): 1 and mass concentration of 2-7% (sulfuric acid contained in the acid reactor residues of chlorine dioxide production can be used as an acid), and with a consumption of KMnO _{4 of} not more than 1% by weight of absolutely dry cellulose, which was kept in solution at a temperature of 15-25 ^o C to full sweep potassium permanganate pulp and pulp was subjected to four-stage bleaching according to the scheme: P-D-P-D, where P is the treatment with hydrogen peroxide in an alkaline medium with a flow rate of the last 1.5-2% of the mass of absolutely dry cellulose and using a stabilizer at pH = 10-10.5, D - chlorine dioxide with a flow rate of 1.1-1.3% by weight of absolutely dry cellulose. After each step, the bleached mass is separated from the solution and neutralized (RU patent N 2128261, CL D 21 C 9/16, publ. 03/27/1999). The disadvantage of this method is the use of a chlorine-containing reagent - chlorine dioxide, the production of which is associated with environmentally unsafe sulfur dioxide, waste, as well as the formation of chlorine-containing organic compounds in the bleaching process. Modern requirements for bleaching provide for the elimination of any chlorine-containing reagents from this process [Muller L.N. Trends in bleaching technology // Pulp, paper, cardboard, - 1997 - N 3-4, - p.24].

 The present invention is the development of TCF-technology for the production of cellulose without the use of chlorine-containing reagents.

The result is achieved through the use of alternating alkaline and acid treatment under specified conditions, while in stage 1, cellulose with a mass concentration of 10-20% is treated with H ₂ O ₂ ; in an alkaline medium (pH 9.5-10.5) at 70-100 ^o C for 120-180 min, at a flow rate of H ₂ O ₂ 2% by weight of absolutely dry cellulose, at stages II and IV, cellulose with a concentration of mass 2 -7% is treated with a solution of KMnO ₄ in an acidic medium with a flow rate of KMnO _{4 of} 0.01-0.8% at 18-25 ^o C until complete absorption of the reagents KMnO ₄ , III and V stage is carried out under conditions and costs of stage I, given the initial hardness cellulose.

 The proposed method differs from the known one in that it does not require thorough washing of the pulp after sulfate cooking from residual alkalinity, since the first stage uses a bleaching reagent - hydrogen peroxide in an alkaline medium. Carrying out stabilized peroxide bleaching of sulfate cellulose in the first stage is possible due to a significant decrease in the degree of delignification of cellulose, for example, when the consumption of hydrogen peroxide is 2%, the degree of delignification is about 50%, and the selectivity of this reagent is more pronounced compared to oxygen-alkaline treatment.

 Subsequent treatment with potassium permanganate is carried out under the conditions specified in [Method for pulp bleaching / Fedorova E.I., Smoleva L.L., Demin V.A., Kulkova E.V .; RU patent N 2123261, class D 21 C 9/16, op. 27/03/1999], after which the cellulose is again subjected to treatment with hydrogen peroxide under conditions identical to the first stage of bleaching. At the same time, to reduce the consumption of hydrogen peroxide, oxidative alkali (ASC) can be used.

 The consumption of potassium permanganate in the next stage of bleaching depends on the hardness of the cellulose and amounts to 0.01-0.2%, after which the final bleaching is again carried out with hydrogen peroxide in the same mode. Thus, the bleaching scheme includes sequential treatment of cellulose with hydrogen peroxide and permanganate treatment in an acidic medium according to the scheme: P-Pm-P-Pm-P, where Pm is permanganate in an acidic medium. The considered scheme provides for the replacement of potassium permanganate with other oxygen-containing reagents, such as ozone, which expands the possibilities of TCF technology.

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

The essential features of the invention:
a method of bleaching cellulose, comprising treating cellulose with a mass concentration of 10-20% hydrogen peroxide in an alkaline medium (pH = 9.5-10.5) in the presence of a stabilizer (sodium silicate) and while maintaining the cellulose suspension at 70-100 ^o C for 120-180 min (consumption of hydrogen peroxide is 2.0%, sodium silicate 2.0-3%, and caustic soda - 1.3-1.5%), separation of the mass from the solution, its neutralization and subsequent processing of cellulose with a solution of permanganate potassium in an acidic environment with a permanganate flow rate of 0.01- 0.8% by weight of absolutely dry cellulose at a molar ratio acid: potassium permanganate (25-100): 1, at t = 18-25 ^o C until it is completely absorbed, at a mass concentration of 2-7%, separation and washing of the mass again with hydrogen peroxide under the conditions and costs discussed above, and then permanganate treatment is already carried out at low costs of this reagent 0.01-0.2% (pH 1.2-2.1) until it is completely absorbed at room temperature, final bleaching is carried out similarly to the 1st and 3rd stages, peroxide consumption hydrogen at this stage depends on the rigidity of the cellulose, 1-2% by weight of absolutely dry cellulose. The bleached mass is separated and washed thoroughly.

 Example 1 (prototype).

Unbleached pulp from hardwood with a hardness of 95 permanganate units, a viscosity of 28 cps is subjected to pretreatment with potassium permanganate in a sulfuric acid medium at a rate of 0.8% by weight of absolutely dry cellulose (molar ratio sulfuric acid / potassium permanganate 100: 1), suspension concentration 5% , at room temperature for 45 minutes (pH = 1.4). Then the cellulose is thoroughly washed and subsequently subjected to treatment with hydrogen peroxide and chlorine dioxide under conditions: mass concentration of 7 and 10%, temperature of 70 and 80 ^o C, duration of 120 and 180 minutes, respectively. The bleaching was carried out in four stages, with the following costs: the second stage - 1.5% H ₂ O ₂ , 1% NaOH, 3.5% Na ₂ SiO ₃ , the 4th - 0.5% H ₂ O ₂ , 0.8% NaOH, 2% Na ₂ SiO ₃ , on the 3rd and 5th 0.6 and 0.7% ClO _2, respectively. After each stage, the pulp is washed, after the last - dried.

 The cellulose whiteness is 85.1%, the viscosity is 8 cPs, and the yield is 93%.

 Example 2

Unbleached hardwood pulp with a hardness of 83 permanganate units, a viscosity of 11 cPs is subjected to peroxide treatment at a mass concentration of 10%, a temperature of 80 ^o C for 120 min, with a flow rate of 2% H ₂ O ₂ from the mass of absolutely dry cellulose, 1.3% NaOH , 2% Na ₂ SiO ₃ , pH = 10.8. In the second stage, permanganate treatment is carried out with a consumption of KMnO _{4 of} 0.4%, a molar ratio of potassium permanganate / sulfuric acid 1: 100, with a mass concentration of 2.5%, pH = 1.1. After washing, the cellulose is subjected to peroxide treatment under conditions identical to the first stage of bleaching, and then, thoroughly washing the pulp, it is treated with potassium permanganate at a rate of 0.2% under conditions similar to the second stage. After washing, the cellulose is again treated with hydrogen peroxide under conditions identical to the peroxide bleaching stages (first and third), washed and dried.

 Whiteness 83.5%, yield 96.0%, viscosity 8.1 cps.

 Example 3

 Unbleached cellulose is treated according to example 2, with a consumption of potassium permanganate in the second stage of 0.8%. Other conditions are also similar to example 2.

 Cellulose whiteness 85%, viscosity 8.0 cps, yield 92.0%.

 Example 4

 Unbleached cellulose is treated according to example 2, with a potassium permanganate consumption of 0.4% and 0.1% in the fourth stage, respectively, with a molar ratio of potassium permanganate / sulfuric acid 1:50 (pH = 1.45). Other conditions are similar to example 2.

 Whiteness 76.8%, viscosity 9.6 cps, 95% yield.

 Example 5

 Unbleached cellulose is treated according to example 2, at a potassium permanganate consumption of 0.4% and 0.2% in the fourth stage, respectively, with a molar ratio of potassium permanganate / sulfuric acid 1:50. Other conditions are similar to example 2.

Whiteness 81.4, viscosity 9.3 cps, yield 95%
Example 6

Unbleached pulp with a hardness of 83 permanganate units was treated with potassium permanganate at a mass concentration of 2.5% in a sulfuric acid solution at a molar ratio of potassium permanganate / sulfuric acid 1: 100 (pH = 1.55) at a flow rate of 0.1% by weight of absolutely dry cellulose, room temperature until complete absorption of potassium permanganate. In the second and fourth stages, peroxide treatment was carried out under identical conditions as in Example 2 (total consumption of H ₂ O ₂ 4%). In the third and fifth stages, the cellulose was treated with potassium permanganate under the same conditions as in the first stage, at a flow rate of 0.2 and 0.01%, respectively.

 Whiteness 72-3%, viscosity 7.54 cps, yield 92%.

Example 7
Unbleached pulp with a hardness of 83 permanganate units was processed according to example 6 using 0.4% potassium permanganate under other identical conditions.

 Whiteness 77.2%, viscosity 7.4 cps, yield 90%.

Example 8
Unbleached pulp with a stiffness of 83 permanganate units was processed according to Example 6 using 0.8% potassium permanganate under otherwise identical conditions.

 Whiteness 77.7%, viscosity 7.1 cPs, yield 90%.

Claims (1)

The method of bleaching cellulose, including the processing of unbleached pulp by alternating alkaline and acid treatment under specified conditions, while in stage I, cellulose with a concentration of 10-20% is treated with H ₂ O ₂ in an alkaline medium (pH 9.5 - 10.5) at 70 - 100 ^o C for 120 - 180 min at a flow rate of H ₂ O ₂ 2% by weight of absolutely dry cellulose, at stages II and IV, pulp with a concentration of 2-7% is treated with a solution of KMnO ₄ in an acidic medium with a flow rate of KMnO ₄ 0 , 01 - 0.8% at 18 - 25 ^o C until complete absorption of the reagents KMnO ₄ III and V stage is carried out under conditions and costs of stage I, given the initial hardness of the pulp.

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