The present invention relates to a method for bleaching pulp, whereby oxygen, peroxide and enzymes 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 treate 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.

Chlorine-containing bleaching chemicals convert in their 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 possibl toxicity of the organic chlorine compounds contained in th waste waters of the bleaching process, these waste waters pose a problem for environmental protection; and furthermore, the waste waters of the bleaching process hav 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 been mainly 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. Novel inventions (refer to patent applications FI 881192,

FI 890700, FI 892151, 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-base chemicals in bleaching.

Delignification by oxygen is the standard approach today i attempts to reduce the environmental impact imposed by waste water discharges from the bleaching process of sulfate pulp manufacturing. The meaning of this process is believed to increase in the future through its application in new investments. The oxygen treatment stage makes it possible to reduce the lignin content of pulp received fro the sulfate cooking stage by approx. 40...50 %. In additio to its use immediately after the cooking stage, oxygen can be used in the alkaline extraction stages of the bleaching sequence to improve the removal of lignin. Problems involved with oxygen treatment are reductions in the pulp yield and quality of the product if the treatment with oxygen is extended beyond a certain limit.

Use of peroxide as an improver of complete bleaching of sulfate pulp has strongly increased during the two last years. In comparison with oxygen, peroxide provides severa

benefits that make peroxide a preferred alternative in a majority of cases. The most important of these benefits ar reduced need for investments, greater possibilities of energy savings and higher bleaching potential. Combination use of both these bleaching chemicals is possible, however in either the same stage or different bleaching stages.

The Swedish manufacturer Aspa Bruk has developed the Ligno bleaching process, in which the pulp is subjected after a pressurized pretreatment with oxygen to a treatment with EDTA (ethylene diaminetetraacetic acid) followed by a subsequent peroxide treatment. This process makes it possible to achieve completely bleached pinewood sulfate pulp without the use of gaseous chlorine. EDTA, however, i not a biodegradable material, and its circulation in the described process has not yet been solved.

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 the bleaching process proceeds stagewise from a pretreatment with oxygen subsequently followed by one or more treatments with hydrogen peroxide and chlorine, and that the pulp is subjected to an enzyme treatment in the process either before or after one or more of the afore-mentioned treatment stages.

According to a preferred embodiment of the invention, coniferous wood pulp is subjected after the cooking to an enzyme treatment, which is subsequently followed by oxygen delignification. After the oxygen treatment stage, the pulp

is subjected to chlorine dioxide bleaching, subsequently followed by an enzyme treatment and peroxide treatment stage. After the peroxide treatment stage, the pulp is further subjected twice to chlorine dioxide treatment.

The enzyme treatment is advanvatageously carried out according to the present invention over the temperature range 10...90 °C, preferably 40...80 °C and using a pH of 3.0...11.0, preferably pH 4.0...10.0. The enzyme utilized can be hemicellulase, cellulase, pectinase, esterase, ligninase, phenol oxidase, lipase, or a mixture thereof.

The invention also concerns the application of enzymes to the reduction of chlorine contents in waste waters from the bleaching stage, said application comprising a prebleaching stage with oxygen, and a sequence of treatments performed in a desired order, said sequence comprising one or several treatments with hydrogen peroxide, chlorine and enzyme. The enzyme is advantageously hemicellulase, cellulase, pectinase, esterase, ligninase, phenol oxidase or a mixtur thereof. The use of the enzyme takes place essentially in the manner described for the bleaching method above.

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

Example 1.

300 g of pulp solids obtained from pinewood sulfate cookin (pulp solids content 30 %) was mixed with diluted xylanase enzyme (Biocon, alkaline hemicellulase) so as to achieve 10 % consistency of the mixture and xylanase activity of 5 U per 1 g of pulp solids. The temperature during the enzyme treatment was held at 55 °C and the treatment time was 2 h.

Next, the pulp was subjected to oxygen delignification, whereby NaOH was added to the mixture by 3 % wt. and oxygen was added so as to achieve a reaction pressure of 3 bar. The reaction temperature was 95 °C and reaction time 45 min. After the treatment, the pulp was washed with 20-fold volume of water.

After this, the bleaching was continued by subjecting the pulp to chlorine oxide treatment in the following conditions: 2.5 % wt. addition of chlorine dioxide, reaction temperature 70 °C and reaction time 3 h. After the treatment, the pulp was washed with 20-fold volume of water.

Next, the pulp was subjected to a second enzyme treatment with Pulpzyme hemicellulase (Novo-Nordisk) . The conditions of the enzyme treatment reaction were: Xylanase 5 U per 1 g pulp, pH 4.8, reaction time 90 min and reaction temperature 55 °C.

After the enzyme reaction the pulp was subjected to peroxide treatment in the following conditions: 0.5 % wt. hydrogen peroxide, 1 % wt. NaOH, reaction temperature 70 °C and reaction time 2 h. After the treatment, the pulp was washed with 20-fold volume of water.

Finally, the pulp was twice subjected to chlorine dioxide treatment, whereby the chlorine dioxide content in the first treatment was 0.7 % and 0.3 % in the the second. The reaction time in both treatments was 3 h and the reaction temperature was 70 °C. Between these treatments the pulp was washed with 20-fold volume of water.

The bleached pulp was analyzed. The results are presented in the Table 1 below. In addition to the test (Test 3) described above, which was performed to illustrate the

operation of the invention, a parallel reference test (Test 1) was performed whose results are equally presented in the Table 1 below. The test arrangements were as follows:

Test 1 (reference test 1) : Pulp was not subjected to second enzyme treatment. Otherwise the test was similar to that described in the foregoing text (Test 3) .

Test 2 (reference test 2) : Pulp was not subjected to the first nor the second enzyme treatment.

TABLE 1

Initial Kappa number after cooking: 25.3

Treatment Test 1 Test 2 Test 3

B-^ treatment (alkaline xylanase)

O treatment Kappa 12.0 14.0 12.0

D-L treatment Kappa 3.8 5.9 38

B ₂ treatment (acid xylanase)

D ₂ treatment brightness % 83.0 79.0 84.6

D ₃ treatment brightness % 85.5 82.0 86.7

C10 ₂ consumption was 35 kg active chlorine per 1 ton pulp, while:

- elemental chlorine consumption was 7 kg per 1 ton pulp

- waste water load as AOX was 0.7 kg per 1 ton pulp.

Example 2 .

300 g of pulp solids obtained from pinewood sulfate cookin was subjected to oxygen delignification, whereby the content of NaOH was 3 % wt. and oxygen pressure 3 bar. The reaction temperature was 95 °C and reaction time 45 min. After the treatment, the pulp was washed with 20-fold volume of water.

After this, alkaline lipase (Resinase A, Novo-Nordisk) was added to the pulp by 1 1 per 1 ton dry pulp and the mixtur was incubated for 90 min at 40 °C. The consistency of the reaction mixture was 10 % and its pH 10. After the reaction, the pulp was washed.

Next, the bleaching was continued by subjecting the pulp t chlorine dioxide treatment in the following conditions: 2.5 % wt. C10 ₂ , reaction temperature 70 °C and reaction time 3 h. After the treatment, the pulp was washed with 20-fold volume of water.

Next, the pulp was subjected to a second enzyme treatment with Pulpzyme hemicellulase (Novo-Nordisk) . The conditions of the enzyme treatment reaction were: Xylanase 5 U per 1 pulp, pH 4.8, reaction time 90 min and reaction temperatur 55 °C.

After the enzyme reaction the pulp was subjected to peroxide treatment in the following conditions: 0.5 % wt. hydrogen peroxide, 1 % wt. NaOH, reaction temperature 70 ° and reaction time 2 h. After the treatment, the pulp was washed with 20-fold volume of water.

Finally, the pulp was subjected to chlorine oxide treatme in the following conditions: 0.5 % wt. C10 ₂ , reaction temp

erature 70 °C and reaction time 3 h. After the treatment, the pulp was washed with 20-fold volume of water.

The bleached pulp was analyzed. The results are presented in the Table 2 below. In addition to the test (Test 2) described above, which was performed to illustrate the operation of the invention, a parallel reference test (Test 1) was performed whose results are equally presented in the Table 2 below. The test 1 was performed as ollows:

Test 1 (reference test) : Pulp was not subjected to enzyme treatments. These stages were performed without the enzyme by adding an equal volume of water to the mixture. Otherwise the test was similar to that described in the foregoing text (Test 2) .

TABLE 2

Initial Kappa number after cooking: 25.3

Treatment Test 1 Test 2

0 treatment Kappa 13.9 13.9-

B-L treatment (lipase)

D-L treatment Kappa 5.9 4.4

B ₂ treatment (xylanase)

P treatment brightness % 68.1 74.1

D ₂ treatment brightness % 77.2 82.6

C10 ₂ consumption was 30 kg active chlorine per 1 ton pulp, while:

- elementary chlorine consumption was 6 kg per 1 ton pulp

- waste water load as AOX was 0.6 kg per 1 ton pulp.

Other advantageous bleaching sequences are, e.g.,

B(lipase) -O-B(xylanase) -P-D-D, B(alkaline xylanase) -O-

B(xylanase) -P-D-D, B(alkaline xylanase) -O-B(lipase) -P-D-D,

B(lipase) -O-D-B(xylanase) -P-D, B-O-B-D-B-P and B-O-B-D-B- -

B-D. Variations within the above-described .bleaching sequences are possible, because the position of the enzyme

reaction in the sequence is not decisive for the function of the enzyme.

The performed tests indicate that the bleaching sequences in accordance with the present invention make it possible to attain a fully bleached product with the use of essentially lower consumptions of chlorine chemicals than those used in conventional bleaching sequences. For reference it should be noted that the waste water load fro conventional bleaching sequences of coniferous pulps is in the order of 4 kg (as AOX) per 1 ton pulp.

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.