Note: Descriptions are shown in the official language in which they were submitted.
~049951
A process for treating a fiber pulp with a chemical solution
The present invention relates to a continuous-working treatment
of a fiber pulp with a chemical solution by forming from a pulp
suspension a pulp bed on a filter surface and by feeding the
bed through one of several chemical treatment steps in which a
chemical solution is displaced through the bed. When a process
of this type is used for the bleaching of a cellulose material
it is in general called displacement bleaching or dynamic
bleaching.
When it is desired to produce a cellulose fiber having a high
degree of brightness, the fiber has to be bleached after the
actual defibration process. For example, in order to obtain a
sulfate pulp with a high degree of brightness, it normally has
to be bleached in five steps by using chlorine and chlorine
dioxide as the bleaching chemical and sodium hydroxide as the
extraction chemical.
So far the most common process in this connection has been
first to mix the chemical solution with the pulp suspension,
thereafter to direct the mix (pulp + chemicals) through the re-
tention tank in order that the chemicals have time to diffuse
sufficiently and to react with the fiber material. Thereafter
the reaction products which have formed are washed off.
In order to obtain sufficient diffusion, depending on the chem-
ical, a relatively long retention time is used. For example,
when chlorine dioxide is used, the normal retention time is 3 -
5 hours. For this reason the apparatus required is relatively
large and expensiveO
It was observed long ago that by using the displacement method
instead the mixing method it is possible substantially to
shorten the retention time required. In the l9~0s, industrial
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equipment suitable for this process began to be developed. Some
of the expected advantages were indeed achieved, for example it
was possible to decrease substantially the heat requirement and
water consumption of the bleaching process as compared with the
technology which had been used until then. On the other hand,
the consumption of bleaching chemicals increased somewhat.
However, the conventional technology was also developed at the
same time, especially the mixers used in it. For this reason
the system based on the displacement technology was no longer
as competitive as had been expected. Today, displacement
bleaching is used primarily when a fiber material easy to
bleach is concerned.
The bleaching process based on the displacement method, used so
far, is briefly as follows:
Through a pulp bed which has been formed there is displaced a
chemical solution having a li~uid volume approximately equal to
the liquid content of the bed. Normally a quantity coefficient
1 - 1.1 (quantity coefficient ~ volume of chemical solution
used, divided by the liquid content of the bed). The concentra-
tion of the active chemical in the displacing chemical solution
is adjusted so that most of the active chemical is consumed in
the treatment of the pulp bed. Before the removal of the above-
mentioned chemical solution, and together with it the formed
reaction products, from the bed by displacing them with the
chemical solution used in the subsequent step, or in the case
of the last bleaching step with washing water, the pulp bed is
directed through a so-called stationary step, with a sufficient
retention time, in order that the concentration profile pro-
duced in the bed after the displacement should have time to
level out under the effect of diffusion.
It has been noted that, in order to achieve by this method a
result as good as that achieved by the conventional method, the
2~4~951
retention time of the above-mentioned stationary step should be
in the same order as the retention times in the conventional
method.
The object of the present invention is to avoid the above-
mentioned disadvantage by using a displacement process which
requires no stationary step for achieving an even chemical
treatment throughout the pulp bed.
The ~leaching process based on the displacement method is in
many respects similar to the washing process based on the dis-
placement method, but there are between them also great dif-
ferences which it is important to take into account expediently
in the further development of the bleaching process. In both
processes there occurs a similar mixing of liquids due to dis-
persion and to the fact ~hat the liquid flow in the fiber bed
is laminar. For this reason, when the liquid content of the
pulp bed is displaced with another liquid, the displacement
never occurs ideally as a plug, but there always occurs mixing
between the liquids, which is disadvantageous in both the wash-
ing process and the bleaching process. This disadvantageous
mixing can be reduced in both processes by carrying out several
successive displacements according to the countercurrent prin-
ciple. However, the higher the number of displacement steps
used, the higher the respective equipment costs.
In the washing of pulp there occur no chemical reactions, and
the principal purpose of the process is to separate from the
pulp, as carefully as possible, the chemicals present in the
solution in the pulp bed. In the washing of pulp, when the
quantity coefficient of the displacement increases, the re-
covery of chemicals improves, and for this reason a relatively
high quantity coefficient is in general used, although the re-
covered liquid content therefore increases and respectively the
evaporation requirement of the recovered solution increases.
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In displacement bleaching, on the other hand, in the manner it
has been implemented so far, it is not advantageous to use a
high quantity coefficient, since in that case the active chemi-
cal losses in bleaching respectively increase, owing to the
fact that the active chemicals displaced through the pulp bed
are not recovered in this case.
Both in washing and in bleaching, the displacement is based on
the cross-flow method, in which the liquid flow ~hrough the bed
during the displacement is perpendicular to the travel direc-
tion of the bed.
Since pulp fibers are compressible, the consistency of the pulp
bed is not constant throughout the bed; it increases exponen-
tially towards the filter surface. This profile is, of course,
dependent on the type of the pulp concerned and on how large a
pressure difference is used to produce the liquid flow. Normal-
ly it rises from a consistency value of approx. 2 % to approx.
20 %, being on average approx. 10 %.
From the viewpoint of the efficacy of the wash this phenomenon
does not have a great significance, but in bleaching, in which
it is important that all the fiber layers in the bed receive a
sufficient quantity of active bleaching chemicals, it is of
especially great significance, in particular since, in addi-
tion, the concentration of the active chemicals in the solution
decreases continuously as the solution flows toward the filter
surface. This is due to the above-mentioned dispersion phenome-
non and the fact that chemicals are consumed as they react with
the pulp layers through which they flow.
For this reason, in the displacement bleaching method so far
used, the pulp layers which are at a greater distance from the
filter surface are treated with a chemical solution which has a
great excess of chemicals as compared with the requirement.
Respectively, the layers close to the filter surface do not
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receive a sufficient amount of active chemicals, or receive no
active chemicals, during the displacing, and so the bleaching
of the pulp bed is quite uneven.
It has been observed, however, that the use of excess active
chemicals is not detrimental by causing undesirable secondary
reactions, such as disintegration of cellulose and fibers, if
the process conditions such as the temperature and the pH are
maintained at the correct values. However, the excess chemicals
must be removed from the pulp before the subsequent chemical
treatment, during which the process conditions will again be
different.
When the displacement technique is used for bleaching pulp, the
most important thing is that all the layers of the pulp bed are
treated with a sufficient amount of chemicals and that, in or-
der to ensure this, a solution is used in which the amount of
active chemicals is substantially greater than what is required
for complete treatment of the whole bed.
The object of the present invention is to provide this kind of
pulp bed treatment, but in such a manner that through losses of
active chemicals there is not caused a higher consumption of
chemicals than in a normal bleaching process.
The main characteristics of the invention are given in the ac-
companying claims.
Briefly, the process according to the invention is as follows:
A pulp bed is formed from a clean washed pulp to be bleached,
and the washing liquid present in it is displaced by using as
the displacing liquid the first bleaching solution (which con-
tains as the active chemicals, for example, chlorine and/or
chlorine dioxide~, which contains active chemicals in excess,
preferably approx. double the amount required in this step for
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the complete chemical treatment of the whole pulp bed. Im-
mediately, without a delay after this displacement, displace-
ment is continued with another solution or several solutions to
the degree that the solution used in the first displacement and
the active chemical quantity present in it has in its entirety
or almost in its entirety been displaced from the bed.
The filtrate displaced from the bed during these displacements
is divided into two fractions so that the first fraction con-
tains the above-mentioned washing liquid and the first dis-
placed bleaching solution which has reacted with the bed and
contains in the main only spent bleaching chemicals and bleach-
ing reaction products. The second fraction of the filtrate con-
tains most of the bleaching chemicals which have not been spent
during the displacement. This solution fraction is recovered
and reinforced with fresh bleaching chemicals in the same
amount as is the consumption of the active chemical in the
step, before it is reused in the same chemical treatment step.
In the process according to the invention, one bleaching step
contains not only one displacement but displacements with at
least two solutions. Respectively, the filtrates are divided
into at least two fractions.
The bleaching steps described above may be included several in
succession in the process.
In the manner described above it is possible to select freely
the quantity coefficient to be used in the displacement, and it
may be substantially higher than one without increasing the
losses of bleaching chemicals. On the contrary, as is the case
in the washing process, there is obtained a bleaching process
in which the efficacy and the use and recovery of chemicals are
improved as a higher quantity coefficient is used.
In this bleaching process, as was shown above, there may thus
~ 0 ~
be one or several liquid zones in the same bed cross section,
depending on how high the partial quantity coefficients are
which have been used for the different liquids. The quantity
coefficient of the recovered filtrate which contains the active
chemical determines how effectively the unused active chemicals
are recovered. The higher this quantity coefficient is, the
better the recovery. Through this there is also determined the
quantity coefficient for the chemical solution used for the
displacement, since it has to be the same as the quantity coef-
ficient of the recovered filtrate, added with the effect of the
quantity of fresh chemicals.
In terms of efficient exploitation of the bleaching chemicals,
the recovery of the above-mentioned active chemicals effective-
ly displaced through the bed is important in order that as
little as possible of the solution of the step, containing ac-
tive chemicals, should become mixed with the active chemical
solution used in the subsequent treatment step. For this reason
and owing to the fact that in a treatment step it is possible
to use a high total quantity coefficient without thereby in-
creasing the losses of active chemicals, the active chemical
solution of the directly subsequent treatment step is not used
for displacing the active chemical solution of the step con-
cerned, but a solution as neutral as possible with respect to
both treatment steps is used, for example preferably pure water
at a suitable temperature. If, for example, a chlorine solution
is displaced directly with a sodium hydroxide solution, owing
to dispersion the solutions become mixed with each other in the
boundary zone. Thus they partly cancel each other's positive
effect in terms of bleaching and, in addition, disadvantageous
secondary reactions are produced. Owing to the drastic pH
change, for example, the chlorine solution may be converted to
a hypochlorite solution.
In some cases it may, however, be advantageous to use a fil-
trate which contains only chemicals or reaction products which
99~1
have become spent from the viewpoint of the bleaching process.
The filtrate which is removed from the fiber bed through the
filter surface during the chemical treatment by displacing it
with the active chemical solution and the neutral solution used
in the step chanaes with respect to its chemicals content as
the pulp bed proceeds during the progress of the displacement.
Its first fraction contains mainly chemicals used and spent in
the previous step and the reaction products produced. The frac-
tion then changes more slowly or faster, according to the
strength of the dispersion caused by the bed, to a filtrate
which mainly contains the active chemicals used in the treat-
ment step in guestion, which chemicals have become non-active
when reacting with the fiber material of the pulp bed, and
reaction products which have been formed. This fraction then in
a corresponding manner changes to a fraction which in the main
contains active chemicals used in the treatment step which have
not been spent through reaction and correspond to the excess of
active chemical which is used in the treatment step. This frac-
tion for its part changes, when the bed proceeds, in a cor-
responding manner to a fraction which in the main contains
chemicals which are derived from the neutral solution which is
used as a displacement solution in this step immediately after
the active chemical solution. If this solution, as is normally
recommendable, is pure water, the above-mentioned fraction
which contains active chemicals changes to nearly pure water
before it again changes to a solution which contains spent
chemicals used in the subse~uent treatment step and the formed
reaction products, unless the step in question is the last
treatment step.
The third filtrate fraction above, which contains active chemi-
cals which have not been spent in the reactions in the treat-
ment step, are recovered as carefully as possible and are re-
used together with fresh added chemicals as a chemical solution
in the same treatment step.
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The portion of this solution which can be recovered is, of
course, the active chemical solution quantity of the step minus
the quantity of fresh chemicals.
Since the solution quantity to be recovered is limited in the
above-mentioned manner and, in addition, owing to the disper-
sion in the fiber bed, it is not possible to recover 100 per-
cent the total active chemical quantity which is displaced
through the bed; a small proportion of it ("tail") is always
left in the filtrate fraction displaced before it from the bed,
and respectively a second "tail" in the filtrate fraction dis-
placed after it.
The maximum recovery of active chemicals is obtained when it is
possible to adjust the removal of the filtrate fractions so
that the maximum concentration of active chemicals in both of
the "tails" is the same, i.e. the concentration of active chem-
icals in the filtrate is the same at both division points of
the filtrate fractions.
The filtrate could be divided into different fractions simply
by dividing the filtrate chamber behind the screen surface by
means of partitions sealing tightly to the screen. In this case
it would, however, not be possible in all operating situations
to achieve maximal recovery of chemicals, since the location of
the chemical profile of the filtrate in the equipment used
shifts when the ratio between the displacement liquid flow rate
and the filter surface travel speed changes. In addition ~o
this, the location of the chemical profile between the chemi-
cals spent in the reaction and the still active chemicals is
affected by the proportion of the used active chemicals spent
in the treatment of the bed. This proportion depends, among
other things, on how much residual lignin or a similar
chemicals-consuming constituent is present in the pulp bed
which is being treated.
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-~y making use of ~he-equip~t system;disclosed in cur copending Fmnish Patent
Application891661 for the implementation of the process dis-
closed in the present application it is possible also in the
above-mentioned varied operating situations to adjust the with-
drawing of the filtrate fraction which contains active chemi-
cals in such a manner that the recovery of the active chemicals
is always maximized.
The filtrate chamber is divided by means of partitions which
leave a uniformly wide slit in relation to the filter surface,
through which slit part of the filtrate fraction can pass into
the preceding or subsequent filtrate fraction. When the con-
centrations of active chemicals are measured at the slits of
both partitions separating the filtrate fraction which contains
active chemical from the other filtrate fractions and the mu-
tual withdrawing ratio of these other filtrate fractions is
adjusted according to it so that both measured concentrations
will be the same, an optimal recover~ of chemicals is obtained,
as shown above.
When the equi~t system-accordmg to our copending Finnish Patent Application
891661 is used for implementing the present bleaching process,
the most recommendable embodiment is that the principal vari-
ables of the process are in the same order as when the equip-
ment is used for washing pulp. The thickness of the pulp bed is
20 -100 mm, preferably approx. 50 mm. The pressure difference
for accomplishing the liquid displacement is 1 - 4 meters of
water head. The travel speed of the screen is 0,2 - 1 m/s,
preferably approx. 0.5 m/s. The total quantity coefficient of
the displacement of one treatment step is >1.5, preferably ~2.
The quantity coefficient of the active chemical solution is
approx. one~half of these values. On the basis of the above
values, the total retention time of the fiber bed in one treat-
ment step will be 10 - 50 seconds, normally approx. 20 seconds.
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Since the most preferred embodiment of the present invention is
to use in each displacement bleaching step a high quantity co-
efficient, i.e. a coefficient approx. twice that in the dis-
placement bleaching processes used so far, and in addition to
divide each step into at least two partial steps, the use of
this bleaching process requires that there be available a
method and equipment required for implementing the method, en-
abling several successive displacement steps to be implemented
in one and the same apparatus and that, when steps are added
their marginal costs are relatively low and also the equipment
costs calculated per effective screen surface are economical.
If it is not possible to carry out all bleaching steps in the
same apparatus and the pulp web has to be transferred from one
apparatus to another between the bleaching steps, this transfer
of the pulp causes an almost complete mixing of the pulp bed
and its liquid content. In order to be able in this case to
avoid the mixing of the different chemical solutions with each
other, the transfer must take place so that the liquid content
of the entire bed is in the main of only one chemical solution,
preferably water or a neutral solution. This means that the
partial quantity coefficient of this chemical solution must be
at minimum one, preferably higher.
In the present process the surface layer of the fiber bed is
treated with an approx. double amount of chemicals as compared
with the bottom layer. This non-uniform distribution does not,
however, affect the uniform chemical treatment of the hed,
since, if we examine the individual fiber layers of the bed,
through each layer there flow active chemicals the amount of
which approaches infinite in ratio to the amount that the fiber
layer concerned spends in order to achieve a complete reaction.
Thus a sufficiently high probability is achieved in each fiber
for a reaction between the bleaching chemical and the residual
lignin, the probability being independent of the treatment
~0~9~1
period, i.e. it is independent of the retention time of the bed
in the bleaching step. From this it also follows that the pulp
bed need not be thick; its optimal thickness is determined on
the basis of the fact that the liquid dispersion caused by the
bed is sufficiently small without an unnecessarily large filter
surface being required.
The above means that the forming and maintaining of a homoge-
nous pulp bed is advantageous for the present process. In addi-
tion, it is especially important for this process that it is
possible to separate from each other the two filtrate frac-
tions, the first of which contains mainly reaction products and
the second active chemicals, before they become mixed with each
other. In addition it is required that it is possible to mea-
sure the concentration of chemicals immediately behind the fil-
ter surface and accordingly to determine and adjust the fil-
trate rates in an optimal manner.
The present process requires a maximally rapid transfer of
chemicals, which is based on a forced, directed flow not only
around the fibers but also through them, because the retention
time is so short that no significant transfer of material can
take place through diffusion. For this reason it is also neces-
sary to aim at the system remaining a two-phase system, which
requires that the pressure in the system will not be below the
total equilibrium pressure of the gases and vapors. In this
case a maximum proportion of the gases and vapors remains dis-
solved in the liquid phase, and no disturbing gas bubbles or
gas phase is formed in the bed during the treatment.
The process and the apparatus developed for it, which well meet
these prereouisites~and conditions, are`aiscl~sed m ~r copendinq Finnish Pate~
Application 891661 By using the method and apparatus of the
said application for the present bleaching process, the follow-
ing advantages, among others, are gained over the conventional
processes which utilize mi~ing and over the displacement
~0~9~
13
bleaching processes used thus far:
In terms of process and equipment technology, the bleaching
process becomes simple, since it is possible to carry out
several bleaching steps in one apparatus.
The consumption of energy is low, since the need for pumping
liquid is small and there is no need for mixing.
The consumption of bleaching chemicals is less than in nor-
mal bleaching, since it is possible effectively to keep the
solution which contains active chemicals separate from the
solutions produced during the bleaching process which con-
tain reaction products which could otherwise further consume
bleaching chemicals if they were allowed to mix with them.
It is possible to maintain optimal process conditions in the
various bleaching steps and, also, fiber clusters which have
been delignified poorly before the bleaching will be treated
with a sufficient amount of chemicals. These factors provide
the preconditions for accomplishing a high and even bright-
ness of the pulp and a low dirt content, and good strength
properties are maintained.
