FI75196C - Process for bleaching chemical cellulose pulp. - Google Patents

Description

75196 1 Method for eslvalkalsemelex of chemical cellulosic pulps Förfarande för förbleknlng av kemiska cellulosamassor 5

The invention relates to a process for bleaching a chemically obtained cellulose pulp, in which process the cellulose pulp is bleached by a single or multi-bleaching process known per se.

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Conventional pre-whitening of cellulosic pulps involves treatment with gaseous chlorine. However, the recovery and rational use of chlorine compounds from bleaching has not been resolved and therefore the release of chlorine into the environment as waste has been inevitable. There is a considerable need to replace chlor-bleaching with other less environmentally harmful methods for S1-15. Batch replacement methods include e.g. oxygen bleaching, which is hampered by high oxygen consumption, and nitrogen bleaching, which is burdened by the release of nitrogen compounds into the environment.

According to the invention, it has surprisingly been found that chlorine treatment as a first step in the bleaching process of chemical pulps can be replaced by phenol treatment. Phenol treatment offers numerous advantages over the use of chlorine. First, phenol can be recovered and reused, so that environmentally harmful bleaching residues do not occur in nature. In addition to phenol, the process according to the invention also enables the recovery of residual lignin detached from cellulose in a bleaching process.

In addition, the use of phenol gives the cellulose pulp itself additional useful properties. None of these benefits are possible with conventional chlorine side salt.

The use of phenol in the treatment of cellulosic plastics is not in itself new. Thus, for example, cellulose pulp has been made from wood by treating the chips with a phenolic solution containing a concentrated acid which acts as a catalyst, for example hydrochloric acid. Examples include e.g. DE patents 2 229 673 and 326 705. However, said patents 2 75196 1 relate to the dissolution of cellulose, such as wood, and not to the bleaching of cellulose. The use of phenol in the escalation of chemically prepared cellulose is not previously known. On the contrary, it has been found by NMR study that when wood is treated with a mixture of phenol and hydrochloric acid, condensation of the wood between naphthalene and phenol occurs and thus darkening of the pulp, suggesting that phenol would not be suitable as a bleaching agent for cellulose at all.

The process according to the invention is characterized in that before said bleaching, said cellulose pulp is treated with a phenol solution at an elevated temperature.

Other features of the method according to the invention are set out in claims 2-6.

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In the process according to the invention, the phenol treatment is applied to the precursor of the chemical pulp. In this case, it is already a cultured mass in which the residual residue has changed under the action of the yellow chemicals so that condensation no longer occurs with the phenol. Thus, a substantially lower catalyst density and lower temperature are sufficient to remove lignin from the cultured cellulose. In addition, the actual cooking process, for example the sulphate process, can be interrupted earlier than usual without compromising the quality characteristics of the final pulp, and thus the yellowing capacity can be significantly increased.

Another advantage of the escalator process according to the invention is that the phenol also dissolves the chemical cellulose pulp from the extractant. This is particularly important for the quality of hardwood cellulose, as the extractants cause yellowing of the cellulose and a decrease in the absorbency of the fluff pulp, which is important for the fluff pulp, and a deterioration of the quality of the lock pulp.

The process according to the invention can be applied to the pre-whitening of chemical cellulose-35 pulps. By chemical pulps is meant herein those pulps obtained by a chemical calcination process. Such methods include e.g. sulphate method, various sulphite methods and soda method.

In practice, the pre-bleaching according to the invention is carried out simply by treating the chemical cellulose pulp obtained from the cooking with an aqueous solution of phenol for a sufficiently long time and at a sufficiently high temperature, after which the pulp is passed to further bleaching steps after removal of the treatment liquid.

The phenol content in the aqueous solution can vary so that the water: phenol ratio is between 3: 7 and 1: 1. The phenol solution preferably also contains a small amount of acid, preferably hydrochloric acid, the amount of which may vary between 0.1 and 0.5%, preferably between 0.2 and 1.0%, based on the amount of water in the solution. Too much acid causes hydrolysis of cellulose and hemicelluloses.

The treatment temperature is not critical to the invention, but since the phenol forms an azeotrope with water having a boiling point of about 100 ° C, it is preferred to perform the treatment at this temperature. Thus, a suitable processing temperature is e.g. 98-102 ° C. The treatment time will, of course, depend on the chemical concentrations and temperatures used, but it is normally sufficient for the temperature to be raised to a treatment temperature of a time of 20 to 30-60 minutes and that the treatment time at normal temperature may vary. between 1-60 minutes.

According to a preferred embodiment of the invention, the phenol treatment solution: is removed from the cellulose pulp after its temperature has been raised to the treatment temperature and replaced by a fresh solution. In this case, the pre-bleaching is enhanced because a larger amount of lignin can be removed without substantially affecting the viscosity properties.

The method according to the invention is suitable for both hardwood and softwood pulp. In the case of hardwood cellulose in particular, the process according to the invention has proved to be advantageous because it eliminates the problems caused by the extracts to the quality of the cellulose.

After treatment, the cellulose is washed in the normal manner free of phenol-35 compounds, which are returned to the beginning of the process for reuse. After possible washing, the cellulose is further passed to further bleaching steps, which thus do not have to be chlorine or hypochlorite bleaches, but, for example, chlorine dioxide or peroxide bleaches.

‘75196 1 The washing is preferably carried out with phenolic water and hot water.

The invention is described in more detail below by means of examples.

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Example 1

Unbleached birch sulphate cellulose, the properties of which are shown in Table I, was treated with phenol under the following conditions:

Cellulose: liquid 1: 8

Water: phenol 3: 7 HCl in water (%) 0.2 Treatment temperature (° C) 98-100 15 Lifting time to treatment temperature (min) 45 Treatment time (min) 10

Table I below compares the properties of the cellulosic pulp before and after the aforementioned phenol treatment.

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Table I

• j Starting cellulose Treated 25 cellulose

Chlorine number (KCL) 4.57 2.46

Viscosity (SCAN) dm3 / kg 1190 1180 30 DKM extract (%) 0.46 0.03

Ash content (%)

Yield (%) 94.9

Yield from wood (%) 52.8 50.1

Lignin removed in pre-bleaching (%) - 46.6 35 Average fiber length (mm) 0.93 0.86

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5 751 96 1 Table I shows e.g. the fact that the viscosity of the cellulose has remained almost unchanged, which means that the small molecule hemicellulose has not dissolved and the cellulose chains have not been cleaved. Maintaining the average fiber length unchanged indicates that 5 the acid hydrolysis has not cleaved the fibers.

Example 2

Example 1 was repeated, but the hydrochloric acid content was increased to 1.0%. The amount of lignin removed then increased to 79 Z, but the viscosity then decreased to 480, indicating that substantial carbohydrate cleavage has occurred.

Example 3 15

Example 1 was repeated with the water phenol ratio changed to 1: 1. In this case, only 32 Z of lignin was removed from the pulp. This indicates that switching to a larger amount of water reduces the lignin dissolution and the pre-bleaching efficiency. The viscosities of cellulose-20 ti remained at 1330.

: Example 4

Example 1 was repeated with an acid content of 0.12% of the amount of water. This gave a cellulose product with a chlorine number of 3.27 and a viscosity of 1330. The amount of lignin removed was only 29%.

Example 5 Unbleached birch sulphate cellulose, the properties of which are shown in Table II, was treated with phenol under the following conditions:

Cellulose: liquid 1:12

Water: phenol 3: 7 35 HCl in water (Z) 0.2 Treatment temperature (° C) 98-100

Lifting time to processing temperature (min) 45 Processing time (min) 60 6 75196 1 After the treatment, the properties of the cellulose were compared with the properties of the starting pulp. The results are shown in Table 11.

Table II

5 Starting cellulose Treated cellulose 10

Chlorine number (KCL) 5.23 3.31

Viscosity (SCAN) dm3 1340 1330 DKM extract (%) 0.41

Yield (%) 96.6 15 Yield from wood (%) 53.2 51.4

Lignin removed in pre-bleaching (%) - 37

Average fiber length (mm) 0.92 0.90 20

Example 6

Example 5 was repeated by changing the treatment solution to fresh solvent after raising the temperature to treatment temperature. After a treatment time of 30 minutes, the chlorine number was 2.46 and the viscosity was 1030, and the amount of lignin removed was 52%. When the same experiment was repeated with a treatment time of 60 minutes, a chlorine number of 1.85 and a viscosity of 910 and a lignin removal of 65% were obtained. The results show the advantage of changing the treatment solution.

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Example 7 Pine sulphate cellulose, the properties of which are shown in Table III, was treated with a phenol solution under the following conditions:

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75196 1 Cellulose: liquid 1:10

Water: phenol 3: 7 HCl in water (%) 0.16 Treatment temperature (° C) 98-100 5 Lifting time to treatment temperature (min) 45 After treatment, the properties of the cellulose were compared with those of the starting cellulose. The results are shown in Table III.

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Table III

Starting cellulose Treated 15 cellulose

Chlorine number (KCL) 6.2 4.50

Viscosity (SCAN) dm ^ 1070 850 20 Yield (%) 97.8

Yield from wood (%) 46.5 45.5

Lignin removed in pre-bleaching (%) - 24.5

Average fiber length (mm) 1.94 1.97 25

The example shows that the process according to the invention is also suitable for the pre-bleaching of softwood celluloses.

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Claims (6)

75196 A process for the bleaching of chemically obtained cellulose pulp, in which process the cellulose pulp is bleached by a single or multi-bleaching process known per se, characterized in that before said bleaching said pulp is treated with a phenol solution at an elevated temperature. 1 Claims Process according to Claim 1, characterized in that the phenolic solution used is an aqueous solution which additionally contains 0.2 to 0.5% by volume of water of an acid catalyst, preferably hydrochloric acid. Process according to Claim 1 or 2, characterized in that the treatment with the phenol solution is carried out at a temperature of 98 to 102 ° C. Process according to one of the preceding claims, characterized in that the phenol solution is replaced by a fresh phenol solution after its temperature has been raised to the treatment temperature. 20 Method according to one of the preceding claims, characterized in that the washing is carried out with an aqueous solution containing phenol and hot water. Process according to one of the preceding claims, characterized in that sulphate or sulphite or soda-boiled cellulose is used as cellulose. 30 35 II