RU2243304C1 - Process of chlorine-free bleaching of sulfate cellulose - Google Patents

Abstract

FIELD: paper-and-pulp industry.

SUBSTANCE: process comprises tow-stage treatment of kraft pulp by alternation of acid and alkali treatments. In the first step, 5-20% kraft pulp is treated with solution of ammonium persulfate in 40-50% sulfuric acid at ratio of ammonium persulfate 0.5-2 weight parts to 1 weight part of absolutely dry cellulose at about 22-24^оС for 1-4 h. Resulting 5-10% pulp is then treated for 1 h with 3-4% sodium or potassium hydroxide solution at temperature close to boiling point of water.

EFFECT: excluded use of explosive and expensive hydrogen peroxide, reduced number of stages so reducing consumption of power and materials, and minimized destruction of cellulose fibers so increasing yield of bleached pulp.

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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.

A known method of bleaching unbleached sulfate pulp [Fedorova E.I., Smoleva L.L., Zakharova A.V., Politova N.K. Patent for invention N 2164572 dated 03/14/2000], including a five-stage treatment by alternating alkaline and acid treatments, while in the first stage, cellulose with a concentration of 10-20% is treated with hydrogen peroxide for 1 hour in an alkaline medium (pH 9.0- 10.9) at a temperature of 75-85 ° C with a flow rate of hydrogen peroxide of 2% by weight of absolutely dry cellulose. In the third and fifth stages, the pulp is treated at the same mass concentration for 2 hours each and at a temperature of 70-100 ° C with a flow rate of hydrogen peroxide of 0.5% and 1.0%, respectively. The second and fourth stages are carried out for 2 hours each at a temperature of 70 ° C with chlorine dioxide with a concentration of 0.7-1.0% and 0.3%, respectively, depending on the initial hardness of the cellulose. Since in the examples (1-10) the time for the individual stages varies, the total time for the bleaching process is 7-9 hours. The resulting product has a good yield of 95.3-98.6%, whiteness values of 80-88%.

The disadvantages of this bleaching method include the use of chlorine-containing (chlorine dioxide), explosive and fire hazardous and expensive (hydrogen peroxide) reagents, as well as a multi-stage process, which is associated with additional energy, time and material costs.

Traditional bleaching methods are a multi-stage technological process (from 5 to 8 steps), in which at each separate stage various bleaching agents are used for pulp processing. Between the stages of bleaching, the cellulose is washed with water to remove waste solutions from the mass. In addition, the use of chlorine and its compounds as bleaching agents leads to the appearance of highly toxic organochlorine compounds (dioxins, chlorophenols) in wastewater, as well as in the production itself. The large resource and energy consumption of traditional technologies is mainly due to the relatively long bleaching, rinsing processes, the large volume of water used and the high energy costs of both the main technological process and wastewater treatment.

The objective of the invention is to develop a method of bleaching sulfate cellulose without the use of chlorine-containing reagents.

Bleaching is carried out by oxidation of residual lignin, followed by extraction of the oxidation products with an alkali solution.

A two-stage method for bleaching unbleached sulfate pulp by alternating acid and alkaline treatments is proposed. In the first stage, unbleached pulp with a mass concentration of 5-20% is treated with a solution of ammonium persulfate in 40-50% sulfuric acid with a ratio of ammonium persulfate consumption from 0.5 to 2 to a unit mass of absolutely dry cellulose at a temperature of about 22-24 ° C for from 1 to 4 hours. At the second stage, alkaline treatment of the pulp with a concentration of 5-10% sodium hydroxide or potassium hydroxide solution with a concentration of 3-4% is carried out at a temperature close to the boiling point of water for 1 hour. After each step, the pulp is thoroughly washed with water. The technical result is a bleached pulp with a yield of 80-96% and a cellulose content of 84-94%.

In the proposed oxidizing system, consisting of 40-50% sulfuric acid and ammonium persulfate, a number of oxidizing agents are formed, the strongest of which is Caro acid [Zakis G.F., Neiberte B.Ya. Effect on lignin persulfates. // Chemistry of wood, 1971, No. 9, - p.109-117]. As a result of the oxidation process in the cellulose fiber, sulfolignin is formed, which is poorly soluble in the oxidizing system and slightly better in wash water. Processing with a hot alkali solution promotes an intensive transition of the products of the interaction of sulfolignin with alkali (quinoid structures) into the solution. The bleached pulp mass freed from oxidation products is washed many times with water until neutral.

As a result of the fact that the oxidation system does not dissolve the oxidation products (sulfolignin, etc.) poorly and is not fully developed, it can be used repeatedly with a small addition of fresh ammonium persulfate and a solution of sulfuric acid, if necessary.

The oxidation process is carried out at a temperature of about 22-24 ° C with a constant removal of the released heat, which is necessary, since with increasing temperature the content of Caro acid in the oxidizing system decreases, which negatively affects the lignin oxidation process. Also, preventing the heating of the mass minimizes the process of degradation of cellulose fiber.

The resulting bleached pulp was analyzed for cellulose, α-cellulose (as an indicator of degradation) and stiffness. The effect of the mass of the oxidizing agent and the time of oxidative treatment on the pulp bleaching process was investigated. The results obtained are summarized in tables 1, 2.

Example 1

Unbleached sulfate cellulose with a hardness of 85 permanganate units with a mass concentration of 10% (10 grams) is treated with a solution of ammonium persulfate in 50% sulfuric acid at a ratio of the consumption of ammonium persulfate and absolutely dry cellulose 1: 1 at a temperature of about 22-24 ° C for 1 hour . Then the cellulose is thoroughly washed with water and subjected to alkaline treatment with a solution of sodium hydroxide with a concentration of 4% and a concentration of pulp 10% at a temperature close to the boiling point of water for 1 hour. After washing, the cellulose is dried.

The technical result is a bleached pulp with a yield of 96%, a cellulose content of 84.3%, an α-cellulose content of 81.5% and a hardness of 53 permanganate units.

Example 2

Unbleached sulfate cellulose is treated with a solution of ammonium persulfate in sulfuric acid for 2 hours (the rest is the same as in example 1).

The technical result is a bleached pulp with a yield of 89%, a cellulose content of 87.8%, an α-cellulose content of 84.3% and a hardness of 39 permanganate units.

Example 3

Unbleached sulfate cellulose is treated with a solution of ammonium persulfate in sulfuric acid for 3 hours (the rest is the same as in example 1).

The technical result is a bleached pulp with a yield of 84%, a cellulose content of 91.4%, an α-cellulose content of 87.5% and a hardness of 26 permanganate units.

Example 4

Unbleached sulfate cellulose is treated with a solution of ammonium persulfate in sulfuric acid for 4 hours (the rest is the same as in example 1).

The technical result is a bleached pulp with a yield of 80%, a cellulose content of 93.7%, an α-cellulose content of 89.1% and a hardness of 15 permanganate units.

A positive technical result of the proposed two-stage method of chlorine-free bleaching of unbleached sulfate pulp is bleached pulp with a yield of 80-96% and a pulp content of 84-94%.

Claims (1)

The method of chlorine-free bleaching of sulfate cellulose, comprising a two-stage treatment of unbleached sulfate cellulose by alternating acid and alkaline treatments, characterized in that in the first stage, pulp with a concentration of 5-20% is treated with a solution of ammonium persulfate in 40-50% sulfuric acid with a ratio of the flow rate of ammonium persulfate from 0.5 to 2 to a unit mass of absolutely dry cellulose at a temperature of about 22-24 ° C for 1 to 4 hours, in the second stage, alkaline treatment of the pulp with a concentration of 5-10% solution m of sodium or potassium hydroxide with a concentration of 3-4% at a temperature close to the boiling point of water for one hour.