EP0226114B1 - Process for bleaching and delignification of cellulosic products - Google Patents

Abstract

1. Process for the bleaching and delignifying of cellulose-containing products with peroxides and/or oxygen and/or ozone, characterised in that one additionally uses 0.01 to 2.5 wt. % of cyanamide and/or cyanamide salts, referred to the dry weight of the cellulose, and adjusts the pH value of the bleach to 4 to 13.

Description

The present invention relates to a method for bleaching and delignifying pulp-containing products such as Pulp, wood pulp, wood yield pulp and semi-pulp with peroxides and / or oxygen and / or ozone as bleach.

For the production of cellulose from wood, reeds, straw, bagasse, etc. vegetable raw materials, these raw materials are subjected to a digestion process (sulfite or sulfate process). After digestion or grinding, the pulp is processed by washing and bleaching. The main purpose of bleaching is to lighten the color of the pulp and to remove a large part of the lignin that is still present.

The most widely used method for delignifying and bleaching the pulp is treatment with chlorine and chlorine dioxide or hypochlorite. Standard bleaching generally consists of three stages, with the first stage being chlorination of the residual lignin with chlorine water or chlorine gas at pH 1-2. In the second stage the chlorine lignins are then washed out with water or sodium hydroxide solution, while in the third stage the pulp is subjected to an oxidizing and brightening bleaching with hypochlorite at pH> 6. The particularly gentle, but expensive bleach with chlorine dioxide can either be added to the standard bleach or replace the hypochlorite treatment.

A disadvantage of these processes is the high environmental impact, which is caused above all by the chlorination, because a large amount of chlorine lignins are produced here, which represent a high pollution of wastewater.

A much more environmentally friendly method is bleaching with peroxides (such as sodium peroxide or hydrogen peroxide) or oxygen or oxon. Peroxide bleaching, like chlorine dioxide treatment, is used in addition to standard bleaching levels to produce very white pulps. In addition, these compounds can also be used as sole bleaching agents, particularly in the case of semi-bleaching. A disadvantage of these processes is the limited bleaching effect of the peroxides or oxygen, so that the quality of a highly bleached pulp cannot be achieved economically. This is one of the main reasons why these bleaching processes have not yet become widespread.

From FR-A-2 560 898 a method for digesting wood waste from industry and agriculture using peroxide and cyanamide is known. Raw cellulose, which is in a crystalline form, is broken down by an alkaline oxidizing treatment under extremely drastic conditions in such a way that it can be digested by microorganisms. In this way, however, no bleached delignified pulp is obtained, but rather a cellulose which is at least partially broken down in such a way that it can be used as feed for ruminants.

From US-A 4,025,453 a method for activating peroxide-based bleaching agents is known, according to which a bleaching composition is obtained which consists of a bleaching agent, a buffer and cyanamide and which is suitable for bleaching fabrics in household washing machines at temperatures of 15 up to 70 ° C.

The present invention was therefore based on the object of developing a method for bleaching and delignifying cellulose-containing products with peroxides and / or oxygen and / or ozone, which does not have the disadvantages of the prior art mentioned, but which, despite good environmental compatibility, has an excellent bleaching effect .

This object was achieved according to the invention by additionally using 0.01 to 2.5% by weight of cyanamide and / or cyanamide salts, based on the dry weight of the pulp, and adjusting the pH of the bleach to 4 to 13.

Surprisingly, it has been shown that the addition of cyanamide and / or cyanamide salts can achieve significantly higher degrees of whiteness than without cyanamide.

Simultaneously with the improved bleaching effect, there is also a significantly higher degree of delignification of the pulp. In addition, the addition of cyanamide means that cellulose degradation is significantly less, which is synonymous with special care for the cellulose fibers and was also not predictable.

In the process according to the invention, as is customary in peroxide bleaching, hydrogen peroxide or sodium peroxide is used in an amount of 1 to 2.5% by weight, based on the dry matter content of the pulp, the H ₂ 0 ₂ preferably in a 30 to 35% aqueous solution is used.

The amount of cyanamide or cyanamide salt used according to the invention depends essentially on the peroxide or oxygen or ozone content and is 0.01 to 2.5% by weight, preferably 0.1 to 1.0% by weight. , based on the dry matter content of the pulp.

At less than 0.01% by weight, the improvement in the bleaching effect starting from the cyanamide can practically no longer be determined, while if the upper limit of 2.5% by weight is exceeded, no significant improvement can be achieved and is therefore uneconomical. Based on the peroxide content, this results in a molar ratio of cyanamide: peroxide of 0.1 to 0.7.

The concentration of the cyanamide or cyanamide salt used can vary within wide limits will. Cyanamide itself can be used both as a solid and as an aqueous solution (for example in the form of a 10 to 60% strength aqueous solution).

The alkaline salts such as sodium hydrogen cyanamide, calcium cyanamide or magnesium cyanamide are preferably used as cyanamide salts, because a certain alkaline pH value adjustment can be carried out at the same time. The alkaline earth ions calcium and magnesium simultaneously have a stabilizing effect on the H ₂ 0 ₂ solution, as will be explained in more detail below.

Decomposition of the peroxide during the bleaching process is highly undesirable, since not only is the bleaching effect reduced, but at the same time there is also noticeable damage to the cellulose fiber. In order to prevent this decomposition, stabilizers such as e.g. Water glass or complexing agents such as Na-EDTA in an amount of 0.1 to 5 wt .-%, based on the dry content of the pulp. However, such an addition can generally be dispensed with if the heavy metal salts are removed by washing the pulp before bleaching.

In addition to water glass, the cellulose suspension can also be mixed with metal salts, which in addition to the stabilizing effect also have an activating effect on the peroxide. Here too, amounts of 0.1 to 5% by weight, based on the dry weight of the pulp, are recommended. The aluminum salts or alkaline earth metal salts such as magnesium, calcium and barium salts are preferred as the metal salts, while the oxides, hydroxides, sulfates, chlorides and nitrates are particularly suitable as anions due to their low cost.

With regard to the reaction conditions, it should be noted that the optimum pH is 4.0 to 13.0, preferably 5.0 to 10.0, and that temperatures of 20 ° C. to 150 ° C. are to be regarded as preferred. In these conditions, the best results are generally high degree of whiteness possible with a relatively short treatment time and under gentle conditions, whereby the exact process parameters depend on the type and pretreatment of the pulp used. In a preferred embodiment, the pulp suspension, which generally has a solids content of 3 to 20% by weight, is adjusted to the desired pH with the customary acidic or basic reacting substances, and the stabilizers or activators such as water glass or metal salts are then added . The peroxide or oxygen or ozone and cyanamide are then added. After bleaching at a certain temperature, which takes between 15 minutes and 4 hours, depending on the type of whiteness desired, the pulp is worked up. For this purpose, the pulp suspension is adjusted to a pH of 5 to 7 and then the pulp is dewatered or dried.

With the aid of the method according to the invention, it is possible to increase the whiteness (i.e. the bleaching effect) of unbleached pulp by up to 50% compared to pure peroxide bleaching, while the corresponding increase in pre-bleached pulp is still 20 to 30%.

The degree of delignification, which is defined by the kappa value (see examples), is about 50% cheaper for peroxide or oxygen bleaching with cyanamide than for bleaching without cyanamide. Finally, the viscosity, which is a measure of the degree of polymerization of the cellulose and thus a direct indication of the damage (decomposition) of the cellulose fiber, is significantly better in the process according to the invention than in the processes according to the prior art.

The method according to the invention can in principle apply to all cellulose-containing products such as Sulfite or sulfate pulp, CTMP pulp, needle pulp or wood pulp and can be used in all bleaching stages such as pre-bleaching or post-bleaching. It is also possible to combine different bleaching levels, e.g. Peroxide and chlorine dioxide bleach if this appears beneficial for any reason.

The following examples are intended to explain the invention in more detail, but without restricting it thereto.

Examples Regulations:

Whiteness: irradiation of the specially prepared pulp samples with light and photoelectric measurement of the reflectivity. The reflection was carried out with a Gardner whiteness measuring device (measuring geometry 45 °). The reflection of magnesium oxide corresponds to the whiteness of 100. In the examples, the relative whiteness (A whiteness = whiteness with cyanamide = whiteness without cyanamide) is always given.

Kappa number (x): This parameter is a measure of the lignin content in the pulp. The determination is carried out according to ISO 302. The relative values (A _X = _X with cyanamide - _X without cyanamide) are also noted in the examples.

Viscosity: measure of the degree of polymerization of cellulose. Determination takes place according to CCA 16 (Zellcheming IV / 30/62). In the examples only the relative viscosities are given (Δ viscosity = viscosity with cyanamide - viscosity without cyanamide). Unit: mPa-sec.

The amounts of cyanamide in the examples are given in% by weight and, unless stated otherwise, relate to the dry content of the pulp.

Example 1 (comparison)

85 g of unbleached Magnefite needle pulp (dry weight approx. 10 g) were initially mixed with 0.5 g Sodium hydroxide solution (10%) and 0.5g g of commercially available water glass and then mixed with 0.44 g of hydrogen peroxide (35%). The bleaching was carried out at 60 ° C. and took about 2 hours. The pH of the suspension was 10.0 before bleaching and 8.1 afterwards. The pulp suspension was diluted to 500 ml with water, the pH was adjusted to 6.0 with sulfuric acid and the pulp was dried.

Example 2

The procedure was as in Example 1 but was carried out after the hydrogen peroxide addition another addition of 20 mg of cyanamide in the form of 0.2 g of a 10% cyanamide solution _(^ -_ 0.2 wt .-% based on the dry content of the Pulp).

Reaction conditions and workup cf. Example 1.

Example 3

The procedure was as in Example 2, but the addition of solid cyanamide was 50 mg (0.5% by weight).

The results of Examples 1 to 3 are summarized in Table I:

Example 4 (comparison)

50 g of unbleached Magnefite needle pulp (10 g dry weight) were mixed with 2 g sodium hydroxide solution (10%), 0.5 g water glass and 0.44 g hydrogen peroxide (35%), filled into the glass insert of an autoclave and filled with 20 ml Water heated to 120 ° C for 1 hour.

The pH after bleaching was 8.3, residual peroxide was not detectable with titanyl sulfate. Working up was carried out analogously to Example 1.

Example 5

The procedure is as in Example 4, but 0.5 g of a 10% strength aqueous cyanamide solution (0.5% by weight) were added after the H ₂ O ₂ addition. Refurbishment see Example 1.

The results of Examples 4 and 5 are summarized in Table 11:

Example 6 (comparison)

308 g of Magnefite needle pulp, which was chlorinated with a mixture of chlorine / chlorine dioxide (90:10) and then extracted alkaline (pH value of the pulp 9.7) was mixed with 2.64 g of hydrogen peroxide (30%) and mixed at 60 ° C bleached. Samples were taken and processed after 15, 30, 60 and 120 minutes. The pH after bleaching was 6.2 to 6.4.

Example 7

The procedure is as in Example 6, but with the addition of 4.0 g of a 10% strength aqueous cyanamide solution (1% by weight).

The results of Examples 6 and 7 are shown in Table 111.

Example 8 (comparison)

63 g of an unbleached spruce CTMP fabric (10 g dry weight) were mixed with 2.0 g sodium hydroxide solution (10%), 0.2 g water glass and 0.44 g hydrogen peroxide (35%) and 2 hours at 60 ° C. bleached. The pH was 12.7 before bleaching and 10.3 after bleaching. The workup corresponded to example 1.

Example 9

The procedure was analogous to Example 8, but with the addition of 0.5 g of 10% cyanamide solution (0.5% by weight). The pH values were 11.6 before and 10.5 after bleaching.

The results of Examples 8 and 9 are shown in Table IV.

Example 10 (comparison)

50 g of Magnefite needle pulp (10 g dry weight) were mixed with 6.0 g of sodium hydroxide solution (10%) and 0.01 g of magnesium oxide, filled into an autoclave and 5 bar of oxygen injected. The bleaching was carried out at 85 ° C. (duration 30 minutes). The pH was 11.8 before bleaching, then 11.4 afterwards. Working up was carried out according to Example 1.

Example 11

The procedure is as in Example 10, but with the addition of 0.5 g of a 10% aqueous cyanamide solution.

The results of Examples 10 and 11 are summarized in Table V.

Example 12 (comparison)

50 g of Magnefite needle pulp (10 g dry weight) were mixed with 0.1 g aluminum sulfate, 30 g chlorine dioxide water (1.2% by weight chlorine dioxide based on the dry weight of the pulp) and 0.44 g hydrogen peroxide (35%) and mixed for 2 hours bleached at 60 ° C. The pH was 4.8 before bleaching, then 6.4 (for working up see Example 1).

Example 13

The procedure was as in Example 12, but with the addition of 0.5 g of a 10% cyanamide solution (0.5% by weight).

The results of Examples 12 and 13 are shown in Table VI.

Example 14 (comparison)

67 g of a spruce wood pulp (10 g dry weight) were mixed with 1.5 g sodium hydroxide solution (10%), 0.5 g water glass and 0.43 g hydrogen peroxide (35%) and bleached at 40 ° C. for 2 hours. The pH was 10.9 before bleaching, then 9.1. Working up was carried out as described in Example 1.

Example 15

The procedure is as in Example 14, but with the addition of 0.5 g of a 10% cyanamide solution.

The results of Examples 14 and 15 are summarized in Table VII:

Claims (14)

1. Process for the bleaching and delignifying of cellulose-containing products with peroxides and/or oxygen and/or ozone, characterised in that one additionally uses 0.01 to 2.5 wt.% of cyanamide and/or cyanamide salts, referred to the dry weight of the cellulose, and adjusts the pH value of the bleach to 4 to 13.

2. Process according to claim 1, characterised in that one uses 0.1 to 1.0 wt.% of cyanamide and/or cyanamide salts.

3. Process according to claims 1 and 2, characterised in that the mole ratio of cyanamide:peroxide amounts to 0.1 to 0.7.

4. Process according to claims 1 to 3, characterised in that one uses the cyanamide in the form of a 10 to 60% aqueous solution.

5. Process according to claims 1 to 4, characterised in that one uses sodium hydrogen cyanamide as cyanamide salt.

6. Process according to claims 1 to 5, characterised in that one uses calcium cyanamide as cyanamide salt.

7. Process according to claims 1 to 6, characterised in that one also adds peroxide stabilisers to the cellulose suspension in amounts of 0.1 to 5 wt.%, referred to the dry content of the cellulose.

8. Process according to claim 7, characterised in that one uses waterglass as peroxide stabiliser.

9. Process according to claim 7, characterised in that, as peroxide stabiliser, one uses complex formers, such as e.g. sodium ethylenediamine-tetraacetate.

10. Process according to claims 1 to 9, characterised in that one mixes the cellulose suspension with metal salts in amounts of 0.1 to 5 wt.%, referred to the dry content of the cellulose.

11. Process according to claim 10, characterised in that one uses an aluminium salt as metal salt.

12. Process according to claim 10, characterised in that one uses an alkaline earth metal salt as metal salt.

13. Process according to claims 1 to 12, characterised in that one adjusts the pH value of the bleach to 5.0 to 10.0.

14. Process according to claims 1 to 13, characterised in that the reaction temperature lies at 20 to 150°C.