FI88315B - FOERFARANDE FOER BLEKNING AV CELLULOSAMASSA - Google Patents

Description

88 31 5

The present invention relates to a process for bleaching cellulose pulp 5, which process comprises at least one oxidation step and at least one pre-treatment step, in which the pulp is treated with an enzyme.

In particular, the cellulose pulp obtained by sulphate cooking is brown in color, mainly due to the lignin remaining in the pulp. Lignin is removed by bleaching, which is usually a multi-step process in which the pulp is treated alternately with oxidizing, lignin-degrading chemicals and solubilizing chemicals. The most commonly used oxidizing chemicals have been chlorine-containing substances as well as oxygen, while alkali solutions have been used as decomposition chemicals.

20 Chlorine-containing bleaching chemicals react to convert lignin in the pulp into organochlorine compounds, which end up in the bleaching effluent leaving the pulp or remain in small concentrations in the product. Most of these harmful compounds are generated in the first 25 chlorination steps of the bleaching process.

Efforts have been made to reduce the environmental impact of bleaching effluents by various methods, such as replacing chlorine gas with chlorine dioxide or using oxygen as the oxidizing chemical in the first bleaching stage. However, these means alone have not been sufficient to solve the problem satisfactorily.

Recent studies have found that by using enzymes as an aid to bleaching, it is possible to remove lignin and / or hemicellulose from the cellulose and thus make the pulp more easily bleached in subsequent bleaching steps. With the help of hemicellulase enzymes, the consumption of chlorine chemicals 2 ο 8 6 I 5 in bleaching has been reduced by about 30%, which has correspondingly alleviated the problem of wastewater caused by organochlorine compounds. According to FI patent application 881192, the residual chlorine content of the pulp bleached with chlorochem-5 cabbages has also been reduced by enzyme treatment.

The object of the present invention is to provide a new solution based on the enzyme treatment of pulp, with which the pulp can be bleached in such a way that the consumption of chlorine chemicals in particular is further reduced and at the same time the need for bleaching wastewater treatment is reduced. The invention is characterized in that a lipase which degrades the fats present in the pulp is used as the enzyme.

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Lipases are enzymes that break down lipids, i. fat molecules into di- or monoglycerides or free fatty acids. These enzymes have been produced by fungi and yeasts and are available as commercial products, although their use has been limited.

The use of lipases to promote the bleaching of cellulose pulp is based on the idea of their effect on the so-called to the extractants which remain in the pulp even after cooking.

25 Extractants include fats, terpenes and phenols, and fats are abundant, especially in birch, but also in conifers, such as pine, where the amount of extractants is generally higher. The extractants are adhered to the fibers in the bleachable pulp obtained from the soup and contribute to the consumption of the bleaching chemicals.

However, if the pulp according to the invention is treated with lipase, this removes the fatty extracts from the fibers so that they do not impede the removal of lignin in the next oxidation step. When chlorine-containing oxidizing chemicals are not unnecessarily consumed for degreasing, their need for use is correspondingly lower and less harmful organochlorine compounds are generated.

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In order to effectively remove fatty acids and mono- and diglycerides from fat breakdown, the pulp should be treated with alkali immediately after lipase treatment. Under the action of alkali, said compounds 5 are saponified and transferred as alkali salts to the washing liquid.

It should be emphasized that the mechanism of action of the lipases described above is a matter of theory which does not per se limit the invention. However, it is essential that preliminary experiments 10 show that the lipase-promoting effect of lipases is considerable. In particular, the chlorine load of the first chlorination step of bleaching is considerably reduced and, especially when used in combination with another enzyme such as hemicellulase, lipase can achieve better results in terms of consumption of bleaching chemicals and the resulting bleaching effluent than said other enzymes alone.

According to a preferred embodiment of the invention, the lipase treatment constitutes the first treatment step of the bleaching process. After the lipase treatment, the pulp can be subjected to an oxygen alkali wash, which can be followed by the first chlorination step of the process using e.g. chlorine dioxide. However, chlorine gas, oxygen or peroxides can also be considered as oxidizing chemicals.

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In the bleaching process of the invention, the lipase may be the only enzyme used. However, it is possible that in the enzyme treatment involved in the process, in addition to lipase, another enzyme is used at the same time, such as, for example, hemicellulase. It is also possible that the process comprises several separate enzyme treatment steps using different enzymes, one of which is a lipase according to the invention. Thus, for example, the process may begin with a lipase treatment or a combined lipase and hemicellulase treatment, followed by an oxygen alkali wash, a chlorine dioxide treatment and then a second enzyme treatment using hemicellulase alone. Alternatively, the pure hemicellulase treatment may be the first step in process 8831, whereby the subsequent steps consisted of, e.g., treatments with chlorine dioxide followed by lipase or a mixture of hemicellulase and lipase.

With the exception of the enzyme treatment, the pulp bleaching according to the invention can take place in a conventional manner with alternating oxidation and alkali steps. After the various steps, the pulp can be washed to remove bleaching chemicals and degraded ingredients. The washing liquids can be led to incineration so that they do not cause a wastewater load, but the energy they contain can be utilized in the factory's energy supply.

According to the invention, the lipase treatment preferably takes place in a temperature range of 10 to 90 ° C, preferably in the range of 40 to 70 ° C, and in a pH of 3.0 to 11.0, most preferably 7.0 to 10.0.

The invention also relates to the use of a fat-degrading lipase in the enzyme treatment of the cellulose bleaching process. Lipase may be used alone or in addition to hemicellulase, as described above, either simultaneously or in several successive enzyme treatment steps.

The invention will now be described in more detail by means of exemplary embodiments based on laboratory experiments.

Example 1 To 220 g of pulp dry matter 30 from pulp sulphate soup (pulp dry matter content 30%) was added a dilute lipase mixture (Novo-Nordisk) so that the mixture had a consistency of 10% and a lipase activity of 5000 U / g pulp dry matter. The temperature of the enzyme treatment was 40 ° C, pH 8.8 and the treatment time was 2 h.

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After enzyme treatment, the pulp was sucked dry in a suction funnel and diluted to a consistency of 5% (w / v) with NaOH solution. For bleaches up to the intermediate, washing lasted 10 5 · '8. · I b minutes at enzyme incubation temperature. In the NaOH wash, the NaOH content was 2% (w / w) by mass. After washing, the mass was sucked dry in a funnel and rinsed with twenty times the amount of cold water (in grams).

5 The pulp was sucked dry and weighed in a plastic bag for chlorine bleaching.

The pulp was then subjected to an oxidative bleaching treatment using a mixture of 90% chlorine dioxide and 10% chlorine gas. The mixture was metered in an amount equal to 1.6 x the number of pieces after the oxygen alkali wash. The treatment temperature was 40eC and the treatment time was 45 min. After the oxidation step, the pulp was washed in a Buchner funnel with 20 times the amount of water.

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The pulp was then subjected to alkali treatment using 5% sodium hydroxide solution at a dose of 0.9 x kappa. The consistency of the mixture was 10% and the treatment temperature was 60 ° C and the treatment time was 90 min. After the alkali treatment, the pulp was washed in the same manner as after the oxidation step.

The pulp spacer, which describes the amount of lignin remaining, was then analyzed. The intermediate cap can be used to estimate in advance the amount of chlorine chemical required to achieve higher brightness.

The intermediate results are shown in Table 1 below (Experiment 2).

In addition to the experiment (Experiment 2) illustrating the invention described above, a comparative experiment (Experiment 1) was performed, the result of which is also shown in the following Table 1.

The comparative experiment was performed as follows: 35 The pulp was not subjected to enzyme treatment. Instead of the enzyme treatment, the pulp was otherwise treated in exactly the same way as in Experiment 2, but instead of the enzyme, water was added to the pulp.

TABLE 1

Lipase pH Yield Kappa (U / g) (%) 5 ______ B 8 6 ί 5 b

Experiment 1 0 8.8 97.3 8.8 10

Experiment 2 5000 8.8 96.4 7.4

The experiment shows that the number of pieces of the lipase-treated pulp is clearly lower than that of the control pulp. It follows that 15 chlorine chemicals are needed in further bleaching for less enzyme-treated pulp.

Example 2 To 220 g of pulp dry matter 20 (pulp dry matter content 30%) obtained from birch sulphate soup was added a dilute lipase mixture (Novo-Nordisk) so that the mixture had a consistency of 10% and a lipase activity of 5,000 U / g pulp dry matter. The temperature of the enzyme treatment was 40 ° C, pH 8.8 and the treatment time was 2 h.

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After enzyme treatment, the pulp was sucked dry in a suction funnel and made up to 5% consistency with NaOH solution (2% NaOH / dry pulp). Washing after enzyme treatment was also performed at 40 ° C.

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The pulp was then subjected to an oxidative bleaching treatment using a mixture of 90% chlorine dioxide and 10% chlorine gas. The mixture was metered in an amount of 1.2 x pulp number after oxygen alkali washing, calculated as active chlorine. The treatment temperature was 40 ° C and the treatment time was 45 min. After the oxidation step, the pulp was washed in a Buchner funnel with 20 times the amount of water.

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The pulp was then subjected to alkali treatment using 5% sodium hydroxide solution at a dose of 0.9 x kappa. The consistency of the mixture was 10% and the treatment temperature was 60 ° C and the treatment time was 90 min. After the alkali treatment, the pulp was washed in the same manner as after the oxidation step.

The pulp spacer, which describes the amount of lignin remaining, was then analyzed. Bleaching was continued with the final bleaching sequence: D1-E2-D2 (D = chlorine dioxide treatment, E = alkali treatment). In step D1, chlorine dioxide was used as active chlorine, calculated as 2.6% of the dry weight of the pulp, and in step D2, 1.3%, respectively. The incubation temperature of the D1 step was 70 ° C, duration 180 min and consistency 10% (w / w). After treatment, the pulp was sucked dry in a Buchner-15 funnel, diluted to a consistency of 5% (w / v) with 70 ° C warm water, sucked dry, rinsed with 20 times the amount of cold water and sucked dry. The alkali step E2 corresponded to the E1 step in terms of treatment conditions, except that NaOH was added to 1% (w / w) of the dry weight of the pulp.

Brightness (ISO brightness, standard SCAN-C11: 75) and yield were determined from the fully bleached pulp. The results are shown in Table 2.

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In addition to the experiment (Experiment 2) illustrating the invention described above, a comparative experiment (Experiment 1) was performed, the result of which is also shown in the following Table 2.

30 The comparative test was carried out as follows:

The pulp was not subjected to enzyme treatment. Instead of the enzyme treatment, the pulp was otherwise treated in exactly the same way as in Experiment 2, but instead of the enzyme, water was added to the pulp 35.

TABLE 2 8 8831 5

Lipase Spacer Total Brightness (U / g) Yield (%) 5 ---------------

Experiment 1 0 8.4 94.8 84.0 10

Experiment 2 5000 8.6 93.8 85.9

The experiment shows that the brightness of the lipase-treated pulp is clearly higher than that of the control pulp. It follows that in order to achieve a constant brightness, less chlorine chemicals are needed for the enzyme-treated pulp.

It will be apparent to those skilled in the art that the various embodiments of the invention are not limited to the examples set forth above but may vary within the scope of the appended claims. Thus, for example, a bleaching process may be considered, in which enzymatic treatment is first performed under neutral or slightly temporal conditions using lipase 25 or a mixture of about half lipase and half hemicellulose, followed by oxygen alkali washing, chlorine dioxide treatment, acid hemicellulase treatment, acid hemicellulase treatment. alternating oxidation and alkali treatment steps.

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Claims (10)

A process for bleaching a cellulosic pulp, which process comprises at least one oxidation step and at least one alkali treatment step, in which the pulp is treated with an enzyme, characterized in that a lipase which degrades the fats present in the pulp is used as the enzyme. Process according to Claim 1, characterized in that the lipase treatment is followed by an alkali treatment step in the process. Process according to Claim 2, characterized in that the lipase-treated pulp is subjected to an oxygen alkali wash. 15 Method according to one of the preceding claims, characterized in that the lipase treatment forms the first treatment step of the bleaching process. Method according to one of the preceding claims, characterized in that the pulp is subjected to an enzyme treatment in which at least one other enzyme, such as hemicellulase, is present simultaneously in addition to the lipase. Process according to Claim 5, characterized in that the pulp is treated simultaneously with lipase and hemicellulase in substantially equal amounts. Method according to one of the preceding claims, characterized in that the process comprises at least two separate enzyme treatment steps, one of which uses lipase or simultaneously lipase and hemicellulase and the other only hemicellulase. 35 Process according to one of the preceding claims, characterized in that the lipase treatment takes place at a temperature in the range from 10 to 90 ° C, preferably at 40 to 70 ° C, and at a pH of 3.0 to 11.0, preferably 7, 0 to 10.0. Process according to one of the preceding claims, characterized in that a chlorine-containing bleaching chemical, such as chlorine dioxide, is used in the oxidation step. 10. Use of a fat degrading lipase in an enzyme treatment of a cellulose bleaching process. 15 '6 8 3 Ί 5