The present invention relates to a process for the bleaching chemical pulp, process which comprises at least one bleaching step and at least one alkali treatment step and in which the pulp is treated with an enzyme.

Especially the chemical pulp obtained' y sulfate cooking is brown in color, which is mainly due to lignin remaining in th pulp. Lignin is removed by bleaching, which is usually a multiple-step process in which the pulp is treated alternatel with oxidizing chemicals which decompose lignin and with chem icals which dissolve the substances produced as products of t decomposition. The most commonly used oxidizing chemicals hav been chorine-containing substances and oxygen, whereas alkali solutions have been used as chemicals removing the decomposi¬ tion products.

When reacting, the chlorine-containing bleaching chemicals convert lignin present in the pulp into organic chlorine com¬ pounds, which will pass into the bleaching effluent removed from the pulp or remain in low concentrations in the product. Such detrimental compounds are produced in largest quantities in the first chlorination step of the bleaching process.

Efforts have been made by various methods to decrease the env ronmental load due to the bleaching effluent, for example by replacing the chlorine gas with chlorine dioxide or by using oxygen as the oxidizing chemical in the first step. However, these methods have not alone sufficed to provide a satisfacto solution to the problem.

In recent investigations it has been observed that by using enzymes as an aid in bleaching it is possible to separate lig nin and/or hemicellulose from cellulose and to make the pulp

this manner easier to bleach in the subsequent bleaching steps. By means of hemicellulase enzymes, the consumption of chlorine chemicals can be reduced by approx. 30 %, which has to the corresponding degree decreased the effluent problem due to organic chlorine compounds. According to FI Patent Application 881192, the residual chlorine content of a pulp bleached by using chlorine chemicals can also be decreased by an enzyme treatment.

The object of the present invention is to provide a new method, based on an enzyme treatment of the pulp, by which the pulp can be bleached so that especially the consumption of chlorine chemicals is further decreased and at the same time the need for purifying the bleaching effluent is decreased. It is char¬ acteristic of the invention that the enzyme used is lipase, which digests fats present in the pulp.

Lipases are enzymes which break down lipids, i.e. fat mole¬ cules, into di- or monoglycerides or into free fatty acids. Such enzymes are produced with the help of fungi and yeasts, and they are available as commercial products, although their use has been limited.

The use of lipases for promoting the bleaching of chemical pul is based on a notion of their action on the so-called extrac¬ tives of wood, which remain in the pulp even after the cooking. The extractives comprise fats, terpenes and phenols, and fats are present in large quantities especially in birch, but also in coniferous woods, such as pine, in which the amount of ex¬ tractives is in general higher. In pulp to be bleached, ob¬ tained from cooking, the extractives are adhered to the fibers and for their part consume chemicals used for the bleaching. However, if the pulp is treated, according to the invention, with lipase, this treatment removes fatty extractives from the fibers so that they will not hinder the removal of lignin in the subsequent oxidation step. If chlorine-containing oxidatio

chemicals are not unnecessarily consumed in the removal of fats, their requirement will respectively be lower and the quantity of detrimental organic chlorine compounds produced will be smaller.

In order that the fatty acids and the mono- and diglycerides produced in the breaking down of fat could be effectively re¬ moved from the pulp, the pulp should be treated with an alkali immediately after the lipase treatment. Under the action of alkali, the said compounds become saponified and pass in the form of alkali salts into the washing liquid.

It should be emphasized that in the description of the mecha¬ nisms of lipase action above the question is of a theory which in itself does not limit the invention. It is, however, essen¬ tial that, according to preliminary experiments, the promoting action of lipases on bleaching is considerable. Especially the chlorine load of the first chlorination step of bleaching can be substantially decreased, and especially if lipase is used together with some other enzyme, such as hemicellulase, better results can be achieved with respect to the consumption of bleaching chemicals and the resulting bleaching effluent than can be achieved with the said other enzymes alone.

According to one 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 may be followed by the first chlorination step of the process, for example by using chlorine dioxide. However, chlorine gas, oxygen or perox¬ ides are possible as the oxidizing chemicals.

In the bleaching process according to the invention, lipase may constitute the only enzyme used. It is, however, possible that in the enzyme treatment belonging to the process some other enzyme, e.g. hemicellulase, is used simultaneously besides

lipase. It is also possible that the process includes a plural¬ ity of separate enzyme treatment steps using different enzymes, of which one is lipase, in accordance with the invention. Thus, for example, the process may start with a lipase treatment or with a combined lipase and hemicellulase treatment, which is followed by an oxygen-alkali wash, a chlorine dioxide treat¬ ment, and thereafter by another enzyme treatment, in which only hemicellulase is used. Alternatively, a pure hemicellulase treatment may be the first step of the process, in which case the subsequent steps would consist, for example, of treatments with chlorine dioxide and thereafter with lipase or a mixture of hemicellulase and lipase.

Except for the enzyme treatment, the pulp bleaching according to the invention may be carried out in the conventional manner so that the oxidation and alkali steps alternate. After the various steps the pulp can be washed in order to remove the bleaching chemicals and the broken down constituents. The wash¬ ing liquids can be directed to burning so that they will not cause an effluent load but the energy present in them can be exploited in the energy supply of the mill.

The treatment with lipase takes place according to the inven¬ tion preferably within a temperature range of 10-90 °C, most preferably 40-70 °C, and at a pH of 3.0-11.0, most preferably 7.0-10.0.

The invention also relates to the use of fat-digesting lipase in an enzyme treatment which belongs to a process for bleaching chemical pulp. Lipase can be used alone or, in accordance with what has been stated above, hemicellulase can be used in addi¬ tion to it, either simultaneously or in a number of successive enzyme treatment steps.

The invention is described below in greater detail, with the help of embodiment examples based on laboratory experiments.

Example 1

A diluted lipase mixture (Novo-Nordisk) was added to 220 g of pulp dry matter (solids content of the pulp 30 %) so as to bring the consistency of the mixture to 10 % and its lipase activity to 5000 U/g of the solids of the pulp. The temperatur of the enzyme treatment was 40 °C, the pH 8.8, and the treat¬ ment period 2 h.

After the enzyme treatment, the pulp was sucked dry in a suc¬ tion funnel and was diluted to a consistency of 5 % (w/v) with an NaOH solution. In the bleaching steps carried out as far as the intermediate kappa, the washing took 10 minutes at the enzyme incubation temperature. In the NaOH wash the concentra¬ tion of NaOH was 2 % (w/w) of the pulp. After the wash the pul was sucked dry in a funnel and was rinsed with a twenty-fold amount (in grams) of cold water in relation to the pulp. The pulp was sucked dry and was weighed into a plastic bag for bleaching with chlorine.

Next, the pulp was subjected to an oxidizing bleaching treat¬ ment by using a mixture which contained 90 % chlorine dioxide and 10 % chlorine gas. The mixture was batched in an amount which was 1.6 x the kappa number of the pulp after the oxygen- alkali wash. The treatment temperature was 40 °C and the treat¬ ment period 45 min. After the oxidation step the pulp was washed in a Buchner funnel with a 20-fold amount of water.

Next, the pulp was subjected to an alkali treatment by using a 5 % sodium hydroxide solution the batching of which was 0.9 x kappa. The consistency of the mixture was 10 % and the treat¬ ment temperature was 60 °C and the. treatment period 90 min. After the alkali treatment the pulp was washed in the same manner as after the oxidation step.

Thereafter the intermediate kappa of the pulp was analyzed, the

intermediate kappa indicating the amount of remaining lignin. By means of the intermediate kappa it is possible in advance to estimate the chlorine chemical amount needed when aiming at a greater whiteness.

The intermediate kappa results are shown in the accompanying Table 1 (Experiment 2) .

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

The control experiment was carried out as follows: No enzyme treatment was carried out on the pulp. Instead of the enzyme treatment the pulp was subjected to a treatment other¬ wise fully corresponding to that in Experiment 2, but water instead of enzyme was added to the pulp.

TABLE 1

The experiment shows that the kappa number of a pulp treated with lipase is clearly lower than that of the control pulp. From this it follows that chlorine chemicals are needed in a smaller amount in the further treatment of an enzyme-treated pulp.

Example 2

A diluted lipase mixture (Novo-Nordisk) was added to 220 g of dry pulp matter (solids concentration in the pulp 30 %) ob¬ tained from birch sulfate cooking, so as to bring the pulp consistency to 10 % and the lipase activity to 5000 U/g of the solids of the pulp. The temperature of the enzyme treatment wa 40 °C, the pH 8.8, and the treatment period 2 h.

After a treatment with the enzyme, the pulp was sucked dry in suction funnel and its consistency was brought to 5 % by means of an NaOH solution (2 % NaOH/dry pulp). The wash after the enzyme treatment was also carried out at 40 °C.

Next, the pulp was subjected to an oxidizing bleaching treat¬ ment by using a mixture which contained 90 % chlorine dioxide and 10 % chlorine gas. The mixture was batched in an amount which was 1.2 x the kappa number of the pulp after the oxygen- alkali wash, calculated as active chlorine. The treatment tem¬ perature was 40 °C and the treatment period 45 min. After the oxidation step the pulp was washed in a Buchner funnel with a 20-fold amount of water.

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

Thereafter the intermediate kappa of the pulp, indicating the amount of remaining lignin, was analyzed. The bleaching was continued with the final bleaching sequence: D1-E2-D2 (D = chlorine treatment, E = alkali treatment). In step Dl, chlorin dioxide, calculated as active chlorine, was used at 2.6 % of the dry weight of the pulp, and in step D2 respectively at 1.3 %. The incubation temperature of step Dl was 70 °C, dura-

tion 180 min, and consistency 10 % (w/w) . After the treatment the pulp was sucked dry in a Buchner funnel, it was diluted with water at 70 °C to a consistency of 5 % (w/v) , was sucked dry, was rinsed with a cold water amount 20-fold as compared with the pulp, and was sucked dry. The treatment conditions in alkali step E2 corresponded to those in step El, except that NaOH was added at 1 % (w/w) of the dry weight of the pulp.

The whiteness (ISO whiteness, standard SCAN-C11:75) and the yield of the completely bleached pulp were determined. The results are shown in Table 2.

In addition to the experiment (Experiment 2) described above, illustrating the invention, a control experiment (Experiment 1) was also carried out, the results of which are also shown in the following Table 2.

The control experiment was carried out as follows:

No enzyme treatment was carried out on the pulp. Instead of an enzyme treatment the pulp was subjected to a treatment other¬ wise fully corresponding to that in Experiment 2, but water instead of enzyme was added to the pulp.

TABLE 2

The experiment shows that the whiteness of the lipase-treated

pulp is clearly higher than that of the control pulp. From thi it follows that, when the aim is a standard whiteness, chlorin chemicals are needed in a smaller quantity in the case of an enzyme-treated pulp.

For an expert in the art it is clear that the various embodi¬ ments of the invention are not limited to the examples de¬ scribed above but may vary within the scope of the accompanyin claims. Thus, for example, one possibility is a bleaching proc ess in which an enzyme treatment with lipase or a mixture con¬ taining approximately half lipase and half hemicellulase is first carried out under neutral or slightly alkaline condi¬ tions, and this treatment is followed by an oxygen-alkali wash, a chlorine dioxide treatment, an acid hemicellulase treatment, an oxygen-alkali wash, and, according to need, further alter¬ nating oxidation and alkali treatment steps.