The present invention relates to a method of bleaching pulp. It relates especially to novel pro-environmental bleaching process types used in the wood-processing in¬ dustry, in which methods the chlorine chemicals are replaced by other bleaching chemicals involving dis¬ charges less harmful to the environment.

Cellulose pulp is traditionally bleached with chlorine- containing chemicals, such as chlorine gas and chlorine dioxide. The chlorine compounds generating in bleaching processes are unpleasent because they are malodorous and also because they cause problems in the purification of the effluents. Partially for the above mentioned reasons the environmental requirements on the cellulose industry have become more strict, and therefore there is a tendency to use more and more chlorine-free/non-chlorine bleaching chemicals, such as peroxide and ozone. Peroxide and ozone bleaching are both known bleaching processes. However, replacing chlorine chemicals with peroxide or ozone has not always led to final products of sufficient quality. By using the processes in accordance with the present invention it is possible to intensify peroxide and oxygen bleaching in such a way that no or hardly any difference may be seen in the quality of the final product, at least any negative, compared with chlorine bleaching.

The present invention relates to a method of bleaching pulp in such a way that peroxide and ozone bleaching is intensified with metal stripping, acidation and enzyme treatment.

Following abbreviations are used of the bleaching stages in the enclosed text:

0 = oxygen stage P = peroxide stage

CONFIRMATION COFY

E = alkali extraction stage

HAE = hot alkali extraction

Z = ozone stage

A = acidation stage

It is possible to use additional chemicals in different stages, and the following abbreviations are used to refer to them:

e = entzyme

Q = complex former The above symbols are used in such a way that, for example, A _Q refers to acid stage, in which a complex former, such as EDTA, is used for removing metal ions. Respectively A _eQ refers to acidation stage, in both the complex former and entzymes are used.

Complex formers are substances, which are used for removing metal ions from the pulp stock because the metal ions decompose peroxide and ozone and thus decrease the bleaching efficiency.

Entzymes are used for treating the fibers in such a way that the bleaching chemicals may more readily affect the lignin. Entzymes partly cause the fiber walls to swell and partly open pores.

HAE, i.e. hot alkali extraction stage is an alkali stage, in which the temperature is above 90°C, preferably above 100°C. In said hot alkali stage the pores of the fiber swell in such a way that the bleaching chemical is better able to react and also the residual lignin may be more readily extracted from the fiber walls.

It is a characterizing feature of the present bleaching method that in multistage bleaching in at least two points of the sequence the fibers are subjected to a treatment, which improves the bleachability thereof.

The invention will be described more in detail below with reference to the examples.

Example 1

It is known that by using bleaching sequence 0-A _Q -P-P and 0-A _e -P-P it is possible to bleach pulp to the kappa number 12 - 8 when the brightness is with birch about 75 - 80 and with pine 65 - 75.

The experiments have surprisingly shown that the addition of entzymes prior to the first bleaching stage when bleaching birch pulp affects the bleaching result only very little, but instead when used between two peroxide stages the entzymes significantly intensify the bleaching process. Thus the sequence 0-A _Q -P-A _e -P results in high brightness and low consumption of peroxide. It became apparent in research that a two-stage peroxide treatment with birch pulp including entzyme activation between the two P-stages, produces with the same peroxide consumption approximately 5 % higher brightness in the final product. Respectively, the same final brightness is achieved by about 10 kg H ₂ 0 ₂ /adt lower peroxide dosing.

The phenomenon illustrated in the above example 1 may be utilized in many ways. In the peroxide bleaching the brightness may further be increased by using ozone. An ozone stage is preferably added to follow the acid stage, because ozone requires acid conditions to act. Appropriate points for ozone bleaching are pointed out in the following sequence with (Z). Normally it is reasonable to add one or two ozone stages at most in a bleaching sequence. The sequences using ozone as an additional chemical for peroxide bleaching, are thus: 0-A _Q -( Z )-P-A _e -(Z )-P-(Z ) .

It is seldom useful to us such a long sequence, and therefore applicable sequences are, for example: 0-A _Λ -P-A Z-P or

0-A _Q -Z -P-A _e -P or 0-A _Q -P-A _e -P-Z .

The combination A _e Z means that there is no washing between the A- and Z- stages. The dash "-" means that there is either a washing or pressing stage between the stages.

The possibility to use a HAE-stage between the peroxide stages has not yet been tested more accurately in the laboratory. Since the peroxide stages are hot, it might be possible to position a HAE-stage between the P-stages. The purpose of the HAE-stage would correspond to that of A _e -stage. The temperature of the HAE-stage might be, for example, 120°C. A suitable bleaching sequence would therefore be, for example,

0-A _Q -Z-P-HAE-P or 0-A _Q -P-HAE-P-Z.

An alternative to replace the second P-stage is to add peroxide to the HAE-stage (HAE _p ), whereby the sequence is simplified considerably. The sequence might then be 0-A _Q - P-HAE _p -Z or 0-A _Q -HAE _p -P-Z, although the ozone stage may, if required, also be positioned subsequent to the A _Q -stage either to replace the ozone stage in the above described sequence or to add a second ozone stage to the sequence.

Example 2

The pulp was digested to a kappa number 20-25 prior to the oxygen bleaching, in which the kappa number decreased to 12 - 16. The bleaching sequence subsequent to the digestion was 0-A _eQ -EPO-A _e -P. EPO means peroxide stage, in which also oxygen is used.

In the test runs the final product obtained was pulp, the brightness of which was almost 80 and the kappa number at the end of the test run 6. The consumption of peroxide was only 30 kg H ₂ 0 ₂ . By adding an ozone stage to the

sequence the brightness might readily be raised above 85 and respectively the kappa number to about 2 - 4.

As may be seen from the examples and a number of different bleaching sequences described therewith, it is possible to obtain quality pulps with very high brightness and very low kappa number. The above given examples as well as the sequences described therewith must be considered only examples of the innumerous variations, which the process arrangement in accordance with the present invention enables. Therefore, the above examples and example sequences are by no means given to limit the invention from what is described in the attached claims which alone define the scope of invention. For example, although both in the description and in the claims a sequence seems to end at the ozone stage, it has no significance, since it is obvious that, if desired, it is possible to extend the sequence by adding a treatment stage, which, however, has no specific meaning in view of the present invention.