CA1083870A - Method for treating cellulose containing pulp - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
In a method for treating cellulose containing pulp, the pulp is fractionated in two or more fractions according to particle size before treating one or more of the fractions with ozone. The fractionation can be carried out in a fractionating device which is included in a complete treatment process, the fractionating device constituting a series of screens.
Depending on the quality of the finished paper product, various ozone treated and untreated fractions of the pulp are brought together at various stages in the treatment operation.

Description

The present invention relates to a method for treat-ing aqueous cellulose containing pulp.
The object of the present invention is to provide instructions for a method for treating cellulose containing pulp, which permits an effective utilization of the agents in-cluded in the treatment, and which renders an optimum relation-ship between the strength properties of the finished treated pulp independent of the fields of application to be covered by the pulp.
In accordance with the invention there is provided a method for treating cellulose containing pulp, in which the pulp is fractionated in two or more fractions according to particle size (fibre length), characterized by treating one or more of the fractions with ozone, and by bringing at least one , of the ozone treated fractions together with one or more other - treated and/or untreated fractions to make a finished treated p u l p .
By such a method there is achieved a finished treated pulp which in the manufacturing of paper can give the 20 paper an optimum combination of high tear strength and high tensile strength. The method according to the invention also renders a more effective utilization of the ozone, the total ozone consumption in the treatment of the separate fractions being smaller than if the entire pulp was concurrently treated with ozone. This is due to the fact that the fractions com~
prising fine particles usually constitute a very large part of the total surface of the entire pulp and accordingly consume far more ozone than necessary relative to the quality improve-ments compared with the case when both the fine and coarse fractions of the pulp are ozone treated together.
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A further feature of the method according to the invention consists in that the fractions prior to the ozone treatment have their degree of concentration altered, the fractions having coarse particles being concentrated by the separation of liquid and treated with ozone at a relatively high solids content. The liquid which is separated from the coarse fraction , .

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~083870 is substantially added to fractions having fine particles, so that the fine particle fractions are treated in a very diluted condition.
Preferably, the fraction or fractions to be treated with ozone can be sorted in a number of screens for the separation of pulp particles of various sizes, the reject being ozonized prior to being subjected to a fur ther disintegration and returned to the screens for re-screening.
The purpose of subjecting wood pulp having a large content of chips to a screening is to separate the large chip particles therefrom, the large chip particles not being suitable as a pulp which is to be further treated, for example in a paper manufacturing machine. When subjecting these larger particles to an ozone treatment before they are divided into smaller particles and returned!to the screens for rescreening, a further effective utilization of the reject is achieved, the ozone treatment being easily adaptable to existing strainer or screening plants in which a rejecttreatment of cellulose containing pulp isaccomplished.
The ozonized reject pulp is easier to disintegrate, and the method according to the invention thus also contributes in unloading the disintegra-tion process in for example an exisiing strainer plant.
For a screening or strainer plant the problem underlying the present invention is resolved in that a first fraction of the coarsest reject is first subjected to ozone treatment and thereafter subjected to disintegra-tion in a first disintegrator before it is supplied to a step screen different from the screening steps, and that a second fraction of the reject from the coarsest screening steps is supplied to a se~ond disintegrator, whereafter the second disintegrated fraction is supplied to the step screen, the accept from said step screen being transferred to one or more of the other strainer ., steps for further screening and the reject being returned to the second dis-integrator.

' ` ~" ^ ' ' The method according to the invention is easily adaptable to e~ist-ing screening plants without subjecting the plant to anything else than relative small modifications.
It is to be understood that in association with the ozone treatment of the pulp there is also accomplished a dewatering operation and a fluffing operation. These operations are carried out before the pulp is subjected to the ozone treatment, subsequent to which the pulp goes through a slurry step, i.e. a step in which lye ~nd water are added to the ozonized pulp.
The ozone treatment of coarse fibre pulp usually takes place at a high degree of concentration, i.e. at a solids content of approximately 30-50%, and the splitting of the aqueous pulp in one or more fractions having coarse particles, facilitates the dewatering of these fractions, a suspended pulp being easier to dewater the coarser the suspended particles (fibres) are. As regards cellulose containing high yield pulps there is therefore according to the present invention, gained a substantially reduced dewatering time for the fractions having a low content of fine material, a fact which entails a direct energy saving in connection with the upgrading of the con-centration of the pulp suspension prior to the ozone treatment.
In the following the invention will be further described, reference being had to the drawings which depict various embodiments of the present invention.
Figure 1 is a flow diagram of a first embodiment of the method , according to the invention.
Figure 2 is a flow diagram of a variant of the embodiment shown ` in Figure 1.
;I Figure 3 is a flow diagram of a further embodiment of the invention.
Figure 4 is a flow diagram of a still further embodiment of the present invention.
Figure S is a flow diagram of an embodiment of the present inven--' , ' ~ , ~083870 tion as applied in a screening plant.
In the following the invention will be described in connection with the treatment of mechanical pulp which also is referred to as cellulose con-taining high yield pulp. However, it is to be understood that the method according to the invention can also be applied in connection with the treat-ment of cellulose pulp generally which, aside from mechanical pulp, can also comprise chemical mechanical pulp (semi-chemical pulp) and chemical pulp (sulphite or sulphate cellulose).
In Figure 1, which is a flow diagram of a first embodiment of the method according to the invention, 1 designates a grinding apparatus 6rra refiner which works a raw material such as shortwood or chips to a ground pulp or refined pulp. The pulps can in common be designated as mechanical pulp or cellulose containing high yield pulp.
From the apparatus 1 the refined mechanical pulp is conveyed to a fractionating device 2 in which the pulp is separated in a coarse fraction which i9 supplied to a dewatering/pressing device 3, and a fine fraction which partly is passed to a first ozonizer 4 and partly to a mixing vessel 5.
As indicated at 3~ back-water from the dewatering/pressing device is brought to the ozonizer 4 together with a portion of the fine fraction, in which ozonizer the fine fraction is diluted to a solids content of approximately 0,1-0,5%. A portion of the back-water from the dewatering/pressing device 3 is passed direct to a mixing vessel 5, as this is indicated by the dashed line 6. As indicated by the dashed line 4~, a portion of the untreated fine fraction from the fractionating device 2 may be passed direct to the mixing vessel 5.
With a solids content of approximately 35-50% the coarse fraction is conveyed from the dewatering/pressing device 3 to a fluffer 7 in which it is given a light and fluffy consistency, whereafter it is passed to a second ozonizer 8 with approximately the same solids content as when leaving the ~, .

, ''-~0~3870 dewatering/pressing device 3.
After the ozone treatment in the ozonizer 8 the treated coarse fraction is passed either direct to the mixing vessel 5 as this is indicated by the dashed line 9', or to a re-grinding apparatus 9, for example a refiner, in which the treated pulp is subjected to crushing by inner friction~ If desired, a portion of the ozone treated coarse fraction may be re-ground in the apparatus 9, whereas the rest is passed direct to the mixing vessel 5.
Such a distribution of the coarse fraction will depend on the quality of the final pulp or the paper products for which the pulp is to be used.
In the mixing vessel 5 the individually treated fine fractions and coarse fractions are brought together. The finished treated and mixed pulp is thereafter passed to a subsequent, not illustrated apparatus for being processed to paper or card board products or the like.
For the cellulose containing high yield pulp9, i.e. mechanical pulp, and to a certain degree also chemical mechanical pulp, it is primarily the increa~e of strength which is the most prominent result from the ozDne treat-ment, but within these pulp categories there is also achieved an increase in brightness, i.e. a bleaching, especially in connection with pulps from leaf wood. Cellulose containing high yield pulps treated with ozone are primarily used as a substitute for chemical pulps in various paper qualities. However it is to be understood that the described method can also be used for treat-ing chemical pulps, i.e. sulphite or sulphate cellulose.
The strength properties of the cellulose containing high yield pulps~ especially of the mechanical pulps, are primarily characterized by the ~`~ long fibre part, the so-called L-factor, and second~y by the specific surface - (the total surface per weight unit~ of the intermediate fraction - the so-called S-factor. With the intermediate fraction it is then meant the part of the pulp which has a mean fibre length. The L,factor can be said to give a mea9ure of the di9tribution of the fibre length of the pulp, whereas the . :

S-factor gives a measure of the shape of the fibres. The tear strength primarily depends upon the L-factor and increases therewith, i.e. with an increasing long-fibre part. However, the so-called stretch qualities as - tensile strength, burst strength and wet strength are primarily depending on and partly proportional to the S-factor.
The mechanical exposure of fibres by grinding or refining will entail a cutting of a part of the fibres, so that both a high tear strength and a high tensile strength cannot be achieved for one and the same pulp.
However, compared with a common grinding process the disc refiners and especially the pressure refiners render a favourable relation between the t~ar strength and the tensile strength of a wood pulp.
In the ozone treatment of cellulose containing high yield pulps a greater increase in the tear strength is achieved the coarser the pulp is prior to the ozone treatment, i.e. the tear strength increases with the so-called "freeness~ (the dewatering property~ of the pulp. On the other side a coarse pulp will give a crude surface and accordingly be unsuited for the manufacturing of paper to be used for example in newspapers and magazines.
For such products the pulp must contain a certain portion of finer particles to give the paper product a smooth surface.
In the present invention, as described in connection with the embodiment of Figure 1, in which the cellulose containing pulp is separated in two or more fractions depending on the particles size and a subsequent individual treatment of either of the fractions is accomplished, a very favourable relation between the tensile strength and the tear strength of the ultimate, finished treated pulp is achieved. Aside from a more optimum strength relation which is characterized in a combination of high tear strength and high tensile strength, there is also in the method according to the invention achieved a considerably more effective utilization of the ozone, the total ozone consumption being lower than if the entire pulp - for the ,.

achievement of the same properties - was treated with ozone without a previous fractionation. The larger o~one consumption in conventional ozone treatment is due to the fact that the fine material part constitutes a very large part of the total surface of the pulp and correspondingly consumes far more ozone than necessary as when the fine and coarse fraction is individually treated with ozone.
The fractionation of the refined pulp in two or more fractions according to the particle size also entails the advantage that the dewatering/
pressing treatment of the coarse fraction or -fractions is facilitated, a pulp suspension being simpler to dewater the coarser the suspended particles or fibres are. Thus, in connection with cellulose containing high yield pulps there i9 achieved a substantial reduction of the dewatering time, i.e.
the residence time in the dewatering/pressing device 3 is reduced to a mini-mum.
In connection with pulp qualities where it is necessary to treat - both the coarse and the fine fraction with ozDne, the ozone treatment of the fine fraction at a very low consistency will also result in a net saving of energyJ the low consistency of the fine fraction being achieved within the water balance of the basic pulp. me reason for not accomplishing the ozone treatment of the entire pulp at low ccn~ency is that in order to achieve a good reaction course the individual particles must be completely separated from each other in the water phase and be freely available for the ozone which is added to the water phase. In connection with a non-fractionated pulp this means a consistency in the order of 0.1~, i.e. one ton of water per kilogram dry pulp, a fact which required large space and not least is connect-ed with energy consuming transport.
The increase of strength achieved in the treatment of ozone is due to a surface reaction of the individual fibres, i.e. a modification of the . ..

particle surfaces, and it is therefore desirable to make this contact surface .

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~Q83870 as large as possible and as ava~lable as possible for ozone. This can be done by adding a swelling chemical, and in connection with some types of lignine there has been achieved a three-fold increase of the volume when adding small quantities of lye (NaOH). This chemical is added to the various pulp fractions when they reside in the ozonizers 4 and 8, respectively. To make the surface as available for ozQne as possible a dispersing agent-may be used, so that the particles are distributed most favourably in the liquid face. The addition of a dispersing agent takes place preferably during the ozone treatment of the fine fraction, as this is indicated by the ozonizer 4. If~desired, a surface active agent or a solvent may be added to the pulp fractions.
Pure process economically it may be most favourable only to treat the coarse fraction with ozone. In connection with a plurality of final products such as multilayer paper, card board or paper board, this may ad-vantageously be accomplished in keeping the various pulp portions or fractions ,, separate, for example in three parallel layers, wherein the two outer layers preferably are substituted by fine material fractions rendering favourable optical and printing properties, whereas the intermediate layer is substituted by the coarse fraction which renders the finished paper product ~ appropriate strength.

A flow diagram illustrating ozone treatment of the coarse fraction alone is depicted in Figure 2. Here, the ozonizer 4 is omitted, the fine fraction from the fractionating device 2 being passed direct to the mixing vessel 5 in which it is brought together with the ozonized and re-ground coarse fraction. If desired, the pulp from the re-grinding apparatus 9 may be conveyed to a step screen and a further refiner.

In Figure 3 there is illustrated a flow diagram of a further embodiment of the method according to the invention, wherein also the same blocks as described in connection with Figure 1 are involved. What dis-:;

~ ~083~370 criminates the flow diagram of Figure 2 from thediagram of Fig~re 1, is the insertion of a further mixing vessel 10 between the ozonizer 4 and the mixing vessel 5, to which vessel 10 partly treated and partly untreated fine frac-tion pulp is conveyed together with other pulp suspension, for example untreated mechanical pulp, cellulose or similar, as this is indicated by the line 11. From the mixing vessel 10 the mixture of treated and untreated fine fraction and untreated arbitrary cellulose containing pulp are brought to-gether with the ozone treated coarse fractions in the mixing vessel 5, the coarse fractions being supplied either direct from the ozonizer 8 or via the re-grinding apparatus 9.
The reason for using a further mixing vessel 10 in the method according to Figure 3, is that the untreated fine material which is supplied via the line 4', has a ppor binding quality and contributes to a neutraliza-tion of the very binding active fibres of which the coarse fraction sub-stantially consists. In other words the fine material will be apt to 'Ibake in" the binding active fibres. In the flow diagram according to Figure 3 this condition is sought improved, in that the portion of the fine fraction which via the line 4' is removed from that part of the pulp which is to be treated with ozone, is mixed with further cellulose containing pulp, as indicated by the line 11 and is thoroughly mixed therewith in the mixing ve99el 10 before it together with the ozone treated fine particle material is added to the ozone treated coarse fraction in the mixing vessel 5. In such a method there is achieved a substantially reduced concentration of the i ~ fine material in relation to the pulp with which the coarse fraction is to i be mixed, and thereby a reduced possib;l;ty of baking in of the binding active ~, ozone treated fibres.

In Figure 4 there is illustrated still another embodiment of the .,~ ., invention in the form of a flow diagram containing the same blocks as ~i discussed in connection with Figure 1. What discriminates the diagram ., _ g _ ,:

~083870 according to Figure 4 from the diagram according to Figure l, is that the dashed lines 4~, 6 in Figure l are removed, whereas between the ozonizers 8 and 4 there is illustrated a transport path 12 for the transmission of organic compounds resulting from the ozone trea~ment of the high consisten~y coarse pulp in the ozonizer 8. These organic compounds are to a large degree surface active agents and they contribute to an increase of the reaction speea between the ozone and the low consistency fine fraction in the ozonizer 4.
To further improve the properties of the ozone treated puhps, suitable chemicals may be added to the pulps at appropriate steps prior to or subsequent to the ozonization.
In Figure 5 which is a flow diagram of a preferred embodiment of the method according to the invention, l' designates a supply path for a cellulose containing mechanical sorting pulp or raw material which inter alia consists of chips of various sizes.
The raw material pulp is supplied to a coarse screen 2~ which has for its aim to sort out the large particles of the pulp. The large particles al~o called ~knots~ are removed from the coarse screen as not accepted pulp or reject and may suitably be further treated in a rechipper or hammer mill as this is indicated by a block 3'.

The knots may be passed from the Coarse screen to the rechipper or the hammer mill 3~ in a water channel, on a rubber belt or in a screw conveyor.
A rechipper may for example comprise a shaft rotating in a housing, and a perforated plate defines the bottom of the housing. On the shaft there may ., ,~ .
be attached circular plates, the plates at their circumference being equipped with tags approximataly as on a saw blade. The knots are broken up between ;
the rotor and the housing before passing the perforated plate.
In a hammer mill the rotor in the housing may be provided with hammers which break up the knots to homogeneous small chips. These smaIl chips can pass the perforated plate of the hammer mill, whereafter they are _ 10 --:

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mixed with water and pumped on to the real reject refining which will be described below in more detail.
The accept from the coarse screen 2' is passed on for fine screen-ing, that is for the separation of smaller chips and large fibre bundles from the pulp. In the plant illustrated in the drawing the fine screening takes place in a first step screen 4', a second step screen 5' and a rescreen 6'.
The accepted pulp from the coarse screen 2~ is supplied to the first step screen 4~, and the accept from the step screen 4~ is supplied to the rescreen 6'. Reject from the first step screen 4' and the rescreen 6' are conveyed to an ozonizing and disintegrating step. In the drawing the apparatus in-cluded in this step are embraced by the dashed block 7l.
The reject separated from the first step screen 4' is not only chips or bundles of fibres, but also good fibres which should be utilized in the subsequent paper production.
The pulp accepted by the first step screen 4~ is not quite pure and is therefore rescreened once more in the rescreen 6~. Even if the pulp is screened several times, it will contain small chips, sand and bark particles. Theunwanted particles are removed from the pulp in a whirler separator 8I before leaving the screening plant.
Not only reject from the first step screen 4~ and the rescreen 6~ are supplied to the block 7~, but also the major part and in several in-stances 100% of the reject from the coarse screen 2~, i.e. the reject which is pretreated in the hammer mill or rechipper 3~. The possible remaining part of the pretreated ~knots-reject~ is passed direct to a disintegrator, ! for example a disc refiner 9~, in which the reject is ground to acceptable fibres which serve to increase the strength of the wood pulp. A1SG the pulp which has passed the blQck 7~, but which has not been accepted by the second step screen 5', is supplied to the refiner 9'.
The pulp leaving the disc refiner 9~, i.e. substantially pulp which :

has been supplied to the second step screen 5~ from the block 7~, and which has not been accepted by this step screen, is supplied to the second step screen 5~, and accept from the second step screen 5~ is supplied to the first step screen 41.
The pulp which is supplied to the block 7', that is the major part of the pretreated reject pulp from the coarse screen 21, and reject from the first step screen 41 and the rescreen 6', is first subjected to a dewatering process in the dewatering/pressing device lO' and is then conveyed to a fluffer ll1 giving the pulp a light and fluffy consistency.
From the fluffer ll1 the pulp is conveyed to an ozonizer l21 in which it is treated with ozone (03) and is mixed with lye (NaOH). Due to the ozone treatment there is achieved an increase of the strength of the pulp, and this increase of strength is due to a surface reaction of the indiridual fibres, i.e. a modification of the particle surface. It is there-; fore desired to make this contact surface as large as possible or as available as possible for ozone. This can be done by adding a swelling chemical, and in connection with some types of lignine there has been achieved a three-fold increase of the volume by adding small quantities of lye (NaOH). This chemical is added to the pulp when it resides in the ozonizer 121. If desired, a surface agent or a solvent may be added to the pulp.
Ozone treatment of coarse fibre pulp normally takes place at a high degree of concentration, i.e. at a dry substance content of approximately 30-50%, and since a pulp suspension is easier to dewater the coarser the suspended particles or fibres are, there is in the treatment of the coarse reject gained a s~bstantial reduction of the dewa~erIng time, i.e. the residence time in the dewatering/pressing device lO1 is reduced to a minimum.
-~ Subsequent to the ozone treatment the reject pulp is supplied to a disintegrator, for example a disc refiner 13' which refines the ozone treated pulp to a finer consistency. This after-refinement can be carried -, ' ' s .

1~83870 out with a minimum consumption of energy, an ozonized pulp being defibrated more easily than an untreated pulp. Prior to the defibration of the ozone treated pulp it is mixed with suitable quantities of water.
From the refiner 13~ the ozone treated and defibrated reject pulp together with the pulp from the refiner 9~ are passed to the second step screen 5', from which, as discussed above, the accept is passed to the first step screen 4~ and the reject to the refiner 9~.
The method according to the invention renders an improved treatment of reject pulp, the pulp which consists of relative coarse particles, being especially favourable for ozone treatment, because there is achieved a larger increase in the tear strength the coarser the pulp is prior to the ozone treatment. Besides, smaller quantities of ozone are required for gaining the same favourable result as compared with the treatment of fine particle pulp.
Further, a dewatering of the coar9e reject pulp require~ a minimum of energy, since a coarser pulp is more easily dewatered than a finer pulp.
Aside from a lower consumption of energy in the dewatering process, this also entails a shorter dewatering time and larger plant capacity.
A further advantage of the method according to the invention is that the process is easily adaptaable to existing plants, the cooperation between previous screening processes and the1lmethod according to the invention leading to a more effective utilization of the entire p~ant viewed as a whole.
Thus, an advantageous feature of such a plant consists in that the quantity of the coarse screen reject which is to be subjected to an ozone treatment anddisintegration, is decided by the chip size of the coarse screen reject and the total quantity thereof, a small total quantity of coarse screen reject and/or small chip size involving that a small portion of the coarse screen reject is subjected to ozonization and disintegration, whereas a large quantity of coarse screen reject and/or large chip size thereof involving ~ , that a large portion of the coarse screen reject is subjected to ozonization and disintegration for unloading the refiner.
However, it is to be understood that the disintegrator 13' may be substituted by a simple but powerful stirring means which lends to the ozone treated diluted pulp a po~erful stirring before it is supplied to the second step screen 5!. In the step screen the large particles, i.e. the reject, will be conveyed to the refiner 9', and this alone will accomplish the disintegration of the pulp which is necessary for the coarse pulp to be accepted by the second step screen and the remaining screening steps in the process.
In an existing plant the refiner 9' will then alone accomplish the necessary disintegration of the pulp, but the capacity of the system can nevertheless be increased, an ozone treated pulp being disintegrated more easily than untreated pulp.
Compared with usual methods for oæone treatment of pulps in the pulp and paper industry the method according to the invention entails a reduction of the necessary quantities of ozone of up to a thi~d, a fact which aside from reducing the operational costs, also involves a subst~ntial re-duction of the capital costs. Besides, thelmethod gives a pulp product which exhibits optimum strength properties, and the individual processing steps may be accomplished more easily and with a minimum consumption of energy.
This is so not only for the dewatering process of the pulp fractions having coarser particles, but also for a possible after-grinding thereof when the ozone treatment is c~mpleted, the ozonized pulp being far more easy to dis-integrate than an untreated pulp.

Claims (27)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for treating cellulose containing pulp, in which the pulp is fractionated in two or more fractions accord-ing to particle size (fibre length), characterized by treating one or more of the fractions with ozone, and by bringing at least one of the ozone treated fractions together with one or more other treated and/or untreated fractions to make a finished treated pulp.

2. A method as claimed in claim 1, characterized by altering the degree of concentration of the fractions prior to the ozone treatment, the fractions having coarse particles being concentrated by the separation of liquid and being treated with ozone at a relatively high solids content.

3. A method as claimed in claim 2, characterized in that the liquid separated from the coarse fraction is sub-stantially added to fractions having fine particles, so that the fine particle fractions are treated in a very diluted condition.

4. A method as claimed in claim 1, characterized in that a fine particle pulp fraction subsequent to being treated with ozone, is mixed with a fine particle fraction which has not been treated with ozone, and that the mixture is brought together with a coarse particle pulp fraction which has been treated with ozone.

5. A method as claimed in claim 4, characterized by adding to the mixture of ozone treated and untreated fine particle pulp fractions further quantities of arbitrary cellu-lose containing pulp before it is brought together with the coarser ozone treated pulp fractions.

6. A method as claimed in claim 1, characterized in that the organic compounds which are released during the ozone treatment of the coarse particle fractions, are brought together with the fine particle fractions before or during the treatment thereof with ozone.

7. A method as claimed in claim 1, characterized in that one or more of the fractions preferably the fractions con-taining coarse particles and having a high solids content, are exposed to a disintegration after the ozone treatment and before the mixing thereof with the other fractions.

8. A method as claimed in claim 1, characterized by sorting the fraction or fractions to be treated with ozone in a number of screening steps for the separation of pulp particles of various sizes, and by treating the reject with ozone before it is subjected to a further disintegration and returned to the screening steps for rescreening.

9. A method as stated in claim 1, characterized by passing the ozone treated reject pulp through a step screen separate from the screening steps, and by subjecting the reject from said step screen to a disintegration before it is returned to the step screen whereas the accept from the step screen is returned direct to the screening steps.

10. A method as claimed in claim 9, characterized in that a first fraction of the coarsest reject is first subjected to ozone treatment and thereafter subjected to disintegration in a first disintegrator before being supplied to the mentioned step screen and that a second fraction of the reject from the coarsest screening step is supplied to a second disintegrator, whereafter it is supplied to the step screen, the accept from the step screen being transferred to one or more of the remain-ing screening steps for further screening and the reject being returned to the second disintegrator.

11. A method as claimed in claim 9, characterized in that the reject from one or more of the remaining screening steps is brought together with the reject from the coarse screening step and is ozonized and disintegrated together with the coarse re-ject, whereafter the ozone treated reject pulp is supplied to the said step screen.

12. A method for treating a cellulose containing pulp as stated in claim 8, wherein the pulp is passed through a coarse screen for the separation of coarse chips (knots), the finer chips which are accepted by the coarse screen being supplied to a first step screen which is connected to a subsequent rescreen connected to a whirler separator characterized in that ozonized chips are supplied to a second step screen, the accept from the second step screen being supplied to the first step screen, whereas the reject from the second step screen is supplied to a refiner, the output of which is connected to the input of the second step screen.

13. A method as claimed in claim 12, characterized in that the reject from the first step screen and the rescreen are subjected to ozone treatment together with the coarse chips.

14. A method for treating a cellulose containing pulp as stated in claim 8, wherein the pulp is first passed through a coarse screen for the separation of coarse chips (knots), which are supplied to a refiner for further disintegration in smaller chips, whereas finer chips which escape through the coarse screen are supplied to a first step screen which is connected to a subsequent rescreen, wherein the disintegrated chips from the refiner are supplied to a second step screen serving to fraction-ate finer chips and transfer these to the first step screen, the second step screen aside from receiving chips from the refiner also receiving chips which do not escape through the first step screen and the rescreen, and the second step screen transferring not acceptable chips back to the refiner for retreatment therein, and wherein the pulp accepted by the rescreen is conveyed to a whirler separator for cleaning of impurities, characterized in that a portion of the chip pulp which is not accepted by the coarse screen, is ozonized and disintegrated, whereafter it is conveyed to the second step screen, and that the chip pulp which is not accepted by the first step screen and the rescreen, is ozonized and disintegrated together with the reject from the coarse screen before being supplied to the second step screen.

15. A method as claimed in claim 14, characterized in that the quantity of the reject from the coarse screen which is to be subjected to ozone treatment and disintegration, is determined by the chip size of the coarse screen reject and the total quantity thereof, a small total quantity of the coarse screen reject and/or a small chip size thereof involving that a small part of the coarse screen reject is subjected to ozoniza-tion and disintegration, whereas a large quantity of coarse screen reject and/or a large chip size thereof involving that a large portion of the coarse screen reject is subjected to ozon-ization and disintegration to unload the refiner.

16. A method as claimed in claim 7, characterized in that the disintegration takes place in an alkaline environment.

17. A method as claimed in claim 16, wherein disintegra-tion is carried out with the addition of hydrogen peroxide (H2O2).

18. A method as claimed in claim 1, characterized by add-ing to the fractions small quantities of a swelling agent before or during the ozone treatment.

19. A method as claimed in claim 18, wherein the swelling agent is lye (NaOH).

20. A method as claimed in claim 1, characterized in that before or during the treatment of ozone a dispersing agent, a surface-active agent or a solvent is added to one or more of the fractions.

21. A method as claimed in claim 20, wherein the dispers-ing agent, surface-active agent or solvent is added to at least the fine particle fraction.

22. A method as claimed in claim 1, characterized in that the pulp is a mechanical pulp produced by the grinding of short-wood or crushing of chips with or without compressed steam.

23. A method as claimed in claim 1, characterized in that the pulp is a chemical mechanical (semi-chemical) pulp.

24. A method as stated in claim 1, characterized in that the pulp is a chemical pulp, i.e. sulphite or sulphate pulp.

25. A method as claimed in claim 1, characterized in that the pulp is a mixture of mechanical pulp, semi-chemical pulp and/or chemical pulp.

26. A method as stated in claim 1, characterized in that the various fractions are formed into separate webs which are brought together in layers to form a finished treated pulp web.

27. A method as stated in claim 26, characterized in that the fractions are formed into webs having a fine material fraction and a web having a coarse fraction, and that the webs are brought together in layers with the coarse fraction as an intermediate layer.