Note: Descriptions are shown in the official language in which they were submitted.
1083870
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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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
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steps for further screening and the reject being returned to the second dis-
integrator.
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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-
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~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
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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
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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
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1083870
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
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particle surfaces, and it is therefore desirable to make this contact surface
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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-
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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
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invention in the form of a flow diagram containing the same blocks as
~i discussed in connection with Figure 1. What discriminates the diagram
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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
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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
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1083870
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
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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
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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
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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.
