The present invention relates to a method for bleaching pulp, whereby an oxidizing enzyme and a chlorine-containing bleaching chemical are used in the method.

Pulp from sulfate cooking in particular has a brown colour which is mainly related to residual lignin in the pulp. Lignin is removed by bleaching that generally is a multistage process in which the pulp is alternately treated by oxidizing, lignin-degrading chemicals and the compounds resulting from lignin degradation are further treated by dissolving chemicals. Oxidizing chemicals most generally applied are chlorine-containing compounds, while the chemicals employed for removing the degradation components typically are alkaline solutions.

As a chemical process, the bleaching of pulp initially starts with the removal of lignin, while the final stage more closely approaches modification of lignin than its removal.

Chlorine-containing bleaching chemicals convert in the bleaching reactions the lignin contained in the pulp into organic chlorine compounds, which are carried away along with the waste water from the bleaching process. Due to the possible toxicity of the organic chlorine compounds contained in the waste waters of the bleaching process, these waste waters pose a problem in terms of environmental protection; and furthermore, the waste waters of the bleaching process have an intolerably high level of chemical oxygen demand. Because the measures taken by sulfate pulp plants for reduction of pollutant discharges to the environment have so far mainly been focused on other stages of the pulping process than bleaching, the relative

proportion of bleaching in the discharges has grown steadily.

Those waste waters which present the highest impact on the environment come from the washing stages, subsequent to the first chlorination stage and the first alkaline extraction stage. Therefore, an essential reduction of the amount of chlorine gas used in the bleaching process is being pursued by means of developing novel approaches to the bleaching process. Reductions in the load imposed by organic chlorine compounds in waste waters has been attempted, i.a., by using oxygen in the first bleaching stage, increasing the proportion of chlorine dioxide in the first chlorination stage or by improving the alkaline extraction stages through hydrogen peroxide or oxygen addition. Patent appli¬ cations FI 881192, FI 890700, FI 893338, FI 900549 and FI 901711 have shown that enzymes can be effectively used for reducing the need of chlorine-based chemicals in bleaching.

There remains, however, needs for developing ever more effective methods for reducing the amount of chlorine-based chemicals in bleaching.

The most commonly used enzymes in bleaching are hydrolytic enzymes such as xylanases, which as such do not entail any inherent bleaching capability, but rather, can modify cellulosic pulp into a form which is easier to bleach by chemical methods. Less positive results have been obtained from enzymes which are oxidizingly active on lignin such as ligninases and laccase. Patent application FI 892803 discloses a method in which lignin peroxidase can bleach cellulosic pulp in closely controlled conditions of a reaction solution containing a plurality of expensive chemical compounds. In practice, however, such a process is impossible to utilize due to its economic constraints. The

enzyme, lignin peroxidase, used in the process cannot be produced in sufficient quantities so as to make an industrial application possible. Therefore, the present invention seeks to achieve a method in which pulp can be bleached with the help of an enzyme produced on an industrial scale by applying said enzyme under conditions which are feasible on an industrial scale, thus avoiding the use of costly chemicals.

It is an object of the present invention to achieve such a method for bleaching cellulosic pulp that attains an essential reduction in the use of chlorine chemical in respect to the prior-art methods. Simultaneously, the amount of organic chlorine compounds in the waste water from the bleaching process is diminished. The invention is characterized in that said method makes an additional use of a transitional element metal and if necessary, a control of redox potential in the bleaching process.

When an oxidizing enzyme is used for the delignification of cellulosic pulp, it is essential to control the progress of the delignification process by adjusting the redox potential level. Depending on the redox potential level, the oxidizing enzyme either delignifies or polymerizes the lignin of the pulp. Due to the physical character of pulp, however, the measurement of redox potential level in practice has turned out to be difficult. To avoid this problem, it has been found in conjunction with the use of a transitional element metal in the oxidizing enzyme treatment that it is possible to omit, when desirable, the control of the redox potential level, or alternatively, it can be much coarser than is possible without the utilization of a transitional element metal.

According to theoretical considerations, and as also corroborated by experimental results, it is improbable due

to the high molecular weight of an enzyme that the lignin and the enzyme would react directly mutually on a greater scale. The interaction must rather be based on a radical reaction mechanism in which a transfer ion operates between the enzyme and the lignin that performs the oxidation potential transfer from the enzyme to the lignin, and thus accomplishes its role in the progress of the oxidation- reduction reaction between the enzyme and the lignin. In this manner, the use of a transitional element metal in conjunction with an oxidizing enzyme in the method according to the invention facilitates delignification with improved efficiency, and thus makes it possible to reduce the consumption of chlorine chemicals in the bleaching of cellulosic pulp and the residual content of organic chlorine compounds in waste waters from the bleaching stage.

The transitional element metal advantageously used in the method according to the invention is manganese which a relatively abundant metal and thus advantageous to use on an industrial scale nor has any negative effect on the product itself. The aforementioned does not, however, exclude the use of other similar element metals either alone, or alternatively, in combinations by way of the method according to the present invention. The oxidizing enzyme used in the above-described method is advantageously phenol oxidase, also called laccase, which so far is the only lignin-oxidizing enzyme that can be manufactured at reasonable cost on an industrial scale in sufficient quantities.

According to a preferred embodiment of the invention, softwood pulp is subjected in the middle of the bleaching sequence, after the first chlorination, the first alkaline treatment and the first chlorine dioxide treatment stages to a laccase treatment in the presence of manganese. After

the treatment the pulp is washed with water and alkaline chemicals.

According to the invention, the enzyme treatment is advantageously carried out at 10...90 °C temperature, preferably at 40...80 °C, and at a pH of 2.0...11.0, preferably at pH 3.0...6.0. The manganese compound advantageously used is manganese sulfate in concentrations from 10 g to 10 kg per 1 ton dry pulp, preferably from 100 g to 5 kg per 1 ton dry pulp.

The invention also concerns the use of a transitional element metal, preferable manganese, in conjunction with an oxidizing enzyme, for the purpose of reducing the content of chlorine-containing bleaching chemical in the pulp being bleached and the content of organic chlorine compounds in the waste waters from the bleaching stage.

In the following the invention is described by way of exemplifying embodiments based on laboratory tests.

Example 1.

The birch pulp used in the test (reference test 1) was commercial pulp which was semibleached in a sequence D0-E1- E0-D1, washed with water and centrifuged to a 30 % solids content.

23 g of absolutely dry pulp, 2855 ml of distilled water, 3.8 ml of 0.1 M manganese sulfate solution and 4 ml of laccase enzyme (activity 670 U/ml) per 1 kg dry pulp were mixed and reacted. The reaction time was 1 h, temperature 37 °C and mixer speed 60 r/min. The total volume of the reaction vessel was 5 1. After the completion of the reaction time, 580 ml of 0.5 M sodium hydroxide was added to the reaction vessel, and the mixture was poured into a

plastic filter funnel equipped with a filter paper (Schleicher & Schuell, Black ribbon, 0.2 μm) . The empty reaction vessel was additionally rinsed with 575 ml of 0.5 M sodium hydroxide and the rinsings were poured into the filter funnel. The pulp was recovered by vacuum filtration, suspended in approx. 5 1 of distilled water, and the suspension was acidified with 2 N sulfuric acid to a pH of 4...5 and filtered by vacuum filtration. After these steps, the brightness of the pulp was determined.

The results are presented in Table 1 below. In addition to the test (Test 1) performed to elucidate the invention, reference tests (Test 2), (Test 3) and (Test 4) were also carried out, and their results are correspondingly presented in Table 1 below. The tests were performed as follows:

Test 2 (reference test 2): The pulp was not treated with laccase enzyme nor manganese sulfate, but instead, these components were replaced by distilled water. For other parts, the test was similar to the test (Test 1) described above.

Test 3 (reference test 3): The pulp was subjected to laccase enzyme treatment alone. For other parts, the test was similar to the test (Test 1) described above.

Test 4 (reference test 4): The pulp was subjected to manganese sulfate treatment alone. For other parts, the test was similar to the test (Test 1) described above.

Table 1

TEST 1 TEST 2 TEST 3 TEST 4

The test verifies that the pulp can be bleached by a treatment in accordance with the present invention by virtue of the employed enzyme and transitional element metal in industrially feasible conditions of low environmental impact.

For those versed in the art it is evident that the different embodiments of the invention are not limited to those described in the presented examples, but rather, can be varied within the scope of the enclosed claims.