Note: Descriptions are shown in the official language in which they were submitted.
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METF~OD FOIL TltF~rTl~I~TT OF PULP
'fhe present invention relates to a method for tme~ of pulp. The invention is
particularly well applicable in intensifying the vof fiber susper~ions in the
paper
manufacturing industry in applications, which utilize liquid recycled from
later v~ras~hing
stages of the process as wash liquid. According to s preferred embodiment of
the
invention, a part of the liquid to be fed to a washer is separated to a flaw
of its own and
divided into a cleaner and a fouler fraction which are th,~ z,ecycled to
appropriate
locations in the process. According to another preferred embodiment,
condensate fx~pm
evaporation or liquid cleaned otherwise is fed to a point of the process,
which aims at
increasing the purity of the pulp as much as possible. The liquid obtained for
the
purification is f ltrate from the feed of an evaporation plant, i.e. chcmi~
recovery, from
the liquid circulation of the chemical r ecovery, from the liquid circulation
of the digestion
plant, from the circulation of the washing pleat, from the wash circulation of
an oxygen
stage, or from the wash circulation of the bleach plant.
The tendency is the wood processing industry has for decades been to reduce
the water
const:mption of the pulp bleaching process and the related washing states.
This has
resulted in the introduction of the so-called couattr-currant yvashing. rn
counter-current
washing, clean wash water is i~oducod into the last bleaching stage of a pulp
tre~ent
line for use as the wash liquid and filtrate obtained from this washing stage
is brought to
the preceding washing stage for use as the wash liquid, and so on. At its
best, #ha process
enables the liquid to circulate through the whole process and end up via the
digester to the
chemical recovery in the evaporation plant. In other words, the most modern
plaraz may
have a wash liquid circulatjon into which no clean liquid need be introduced
from outsido
the plant and no liquid is discharged from the washing circulation before the
digestion
process.
However, it has been discovered in the newest plants that the quality of pulp,
for example
in teams of pulp strength, tends to deteriorate in various treatment stagers
more than in
rraills using more liquid. This has been notietd first in connection with the
so-called
oxygen bleacliing stage when the oxygen bleaching stage follows brown stock
washing.
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Therefore, it has been concluded that impurities dissolved from the pulp to
the liquid
phase come to the oxygen stage from the digestion plant, from the oxygen stage
itself
because of the recirculation of filtrates, and also from later bleaching
stages. This
impurity may be measured for example based on dry solids, sodium, COD or other
known
parameters.
These problems, i.e. primarily the decrease in the strength of the pulp and
secondarily the
high dry-solids content in the wash liquid and thus in the pulp, have been
solved by
developing a solution according to which a part of the liquid circulated
counter-currently
is separated to form a partial flow which is treated in order to at least
partly separate dry
solids from the liquid, in other words to divide the liquid into a cleaner and
a fouler
fraction. The fouler one of these fractions is returned to the pulp at a point
where the dry
solids content of the liquid phase of the fiber suspension is at least the
same as that of the
fouler fraction to be introduced. This point may be somewhere in the process
between the
digester and the recovery, in the digester itself, or after the digestion
plant.
Correspondingly, the cleaner fraction is returned to a point in the process
where it is most
beneficial. The volume of the dissolved inorganic and organic material, i.e.
impurities,
introduced into the oxygen stage may be reduced by extracting foul wash
filtrate and
introducing cleaner liquid or by introducing more cleaner liquid before the
oxygen stage.
According to a preferred embodiment of the invention the greatest advantage is
to be
gained by introducing the cleaner fraction straight to the point from which
the partial flow
to be cleaned was separated.
According to another preferred embodiment of the invention the cleaner
fraction is
returned to a washing stage as late in the process as possible whereby its
purity (compared
to the rest of the wash liquid) has effect in as many washing stages as
possible.
According to a third preferred embodiment of the invention the cleaner
fraction is
returned to the point in the process where its effect is desired to be the
strongest.
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According to a fourth preferred embodiment of the invention the cleaner
fraction is
divided so that it is distributed to several points in the process.
Further, according to a preferred embodiment of the invention the fouler
fraction
(concentrate) is returned to a point of the process in which the dry solids
content
(foulness) of the liquid phase is at least the same as that of the concentrate
returned.
The method of the invention may employ for example one or more single-step or
multi-
step evaporators, membrane separators or any other separator suitable for the
purpose.
Other characteristic features of the method and the apparatus are disclosed in
the
appended patent claims.
The method of the invention is described more in detail below with reference
to the
accompanying drawing figures of which
Fig. 1 illustrates a so-called prior art fiber line for treatment of chemical
pulp;
Fig. 2 illustrates a solution according to a preferred embodiment of the
invention for
treatment of filtrate/wash liquid;
Fig. 3 illustrates a solution according to a second and a third preferred
embodiment of the
invention for treatment of filtrate/wash liquid in connection with a
continuous digester;
Fig. 4 illustrates a solution according to further six preferred embodiments
of the
invention for treatment of filtrate/wash liquid in connection with a
continuous digester;
Fig. 5 illustrates a solution according to further four preferred embodiments
of the
invention for treatment of filtrate/wash liquid in connection with a batch
digester;
Fig. 6 illustrates a solution according to further six preferred embodiment of
the invention
for treatment of filtrate/wash liquid in connection with a batch digester;
Fig. 7 illustrates the influence of the COD content on the consumption of
bleaching
chemical;
Fig. 8 illustrates the influence of the COD content on the decrease of
viscosity in the
oxygen stage; and
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Fig. 9 illustrates the solubility of soap as a ibnction of the dry solids
content of the black
liquor.
As illustrated in Fig. I, a prior art $ber line, i.c. a line used for
producing s~ bleaching
pulp, comprises the following components. The first component on the left is a
digestion
plant referred to by the reference number 2 which may comprise one ar more
digesterls.
If the process is the so-celled continuous aig~on as in Fig. 1 there is only
one digester,
and if the process is the so-called batch digestion (illustrated in Fig, s and
6 in connection
with some preferred embodiments of the inve~ion). there are several digesters,
usually in
the order of S - I0. The digestiozz pit is usually followed in both cases by a
so-called
blow tank 4. In a batch digestion process the blow tank is indispensable as
the digesters
of a batch digestion process art discharged one at a time to a blow teak,
fraxn A,]u~ a
continuous and even pulp flow is taken to the subsequent process. In the
pzpcess, ~e
blow tank 4 is usually followed by a screening plant 6 in which the particles
not
I5 acceptable in the produced pulp are separated from the pulp. The screening
plant may be
located also somewhere else in the process as will be described later. The
screening pit
6 is (allowed by so-called brown stock washing 8, which may ~ per formed with
a
DRUMDISPLACER'a washer (illustrated in Fig. 1), a diffuser, a pressure
diffuser, one or
several suction dnrm filters, one or several pressure filters, presses, other
equipment
available m the market intended for washing pulp or any combination of those.
In some
processes, the screening plant 5 may be arranged to follow the brown stock
washing.
The following stage in the process illustrated in the figure is oxygen
deligr~if~cation I0,
which today more and morn often is performed in a two-vessel reactor, i.e, in
two steps,
as illustrated in the figure, and it is fbllowed by an oxygen singe wig 1~.
~~. ~s
the process ~rrtinues in alternating dit~erent bleaching stages and washes
separating these
until the pulp is bright adequate for the purpose intended.
The process works so that wood material, is most cases chips, is introduced
into a
digesterldigesters 2 and the chips arc at Least partly disintegrated by the
cooking
chemicals already in the digester 2 into fibers. This disintegration is based
oar the
dissolution of the substances binding the fibers to each other, i.e. mainly
lignin, into the
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cooking solution. Today, a so-callod digester wash is performed in most cases
towards
the end of the digestian process which aims at sepa~.at~g t~ ~~,,g chemicals
and the
substaaccs dissolved during the digestion process iirto the Liquid phase, such
as the lignin
mentioned, from the pulp discharged faCOm the digester 2. This kind of a wash
is,
however, not even close to complatt but large volumes of cooking chemicals and
substances mentioned shave remain in the pulp. These are further removal
mainly is the
brown stock washing 8. The dul result is that the dry-solids contact of the
pulp declasses
relatively evenly from tlx digester 2 to the oxygen stage 10.
A problem, which has given rise to the invention, i. e. dcteriooration of the
pulp quality in
the oxygen stage, will be discussed in the following. Since the main purpose
of the
oxygen stage 1 Q is to decrease the Kappa number of the pulp, in otbes words
mainly to
dissolve the lignin still remaining in the fibers into the liquid phase, the
dry-solids content
of the liquid phase increases essectialty in the oxygen stage 1 Q. This dry-
solids oozrtent of
the liquid phase is decreased in the following wash 12 so that there would not
be much
extra impurities is the pulp in the bleaching stage following tht oxygen
stage. A,il the
impurities ending up in the blenching stage consume bleaching chemicals; thus
it is
profitable also in view of the chemical economy to separate these substances
effeiently
before the bleaching. For cxannple, if the oxygen stage is followed in the pby
~
ozone stage, ozone will react with all dry substance it meets, i.e. also with
nay organic
substance in the liquid phase. Thus all the ozone, which has reacted with any
other
material than with the lignin in the fibers has been consu~mcd in
inappropriate reactions
and thus has not been made use of. Naturally, the savm~ phenomenon applies
also to other
chenucals. Figure 7 illustrates the influence of COb on the consumption of the
trt~tment
chemical at different Kappa numbers of the pulp. The figure clearly indicates
an increase
in the consumption of the chemical when the COD increases, irrespective of the
Kappa
number. For this reason the washing stages between the bleaching stages,
particularly the
wash following the first oxygen stage, or in a broader sense the wash
subsequent to the
delignification or pre-bleaching stage, arc arranged very efficient so as to
miuitrtize the
consumption of bleaching chemicals in unnecessary ions. Correspondingly, all
bleaching stages are followed by one or more washerls which aim at washing the
reaction
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products of the bleaching stage from the pulp as completely as possible before
introduction of the subsequent bleaching chemical to the pulp.
However, primarily environmental factors, although in some cases also the
volume of
water available, have effectively limited the volume of water used in the
washing stages.
The ultimate goal is to provide a pulp mill with a closed water circulation.
This means a
situation where the mill uses the water it takes from a water system so
efficiently that
practically no water at all is discharged back to the water system but the
liquid is
circulated continuously inside the mill and fresh water is introduced only to
replace the
liquid evaporated in the process.
In order to accomplish the closed circulation, mills have almost without
exception
adopted the so-called counter-current wash method. This means that clean wash
liquid,
i.e. either water circulated inside the mill or water taken from a water
system or these
both are guided to be used as the wash liquid in the washing stage following
the last
bleaching stage of a fiber line. In other words "clean" wash liquid is
transported to a
point in which the pulp contains the smallest volume of dry solids or
chemicals to be
washed and in which the purity requirement is the greatest. From this point on
the wash
liquid is transferred counter-currently from one washer to another towards the
digester/digestion plant so that while in each washing stage the dry-solids
content of the
pulp decreases the dry-solids content of the wash liquid recycled counter-
currently
increases.
The volume of impurities in the wash liquid has been found particularly
problematic in
connection with the oxygen stage. The reason for this is that a modern oxygen
stage,
particularly the two-stepped oxygen stage which is more and more often used,
dissolves
dry solids from fibers so efficiently that large volumes of dry solids end up
in the filtrate
in the washer following the oxygen stage. When this filtrate is transported to
the washer
preceding the oxygen stage to be used as wash liquid, most-of the dry solids
dissolved in
the oxygen stage is returned to the pulp, and thus in the oxygen stage there
are dry solids
present both in the fibers and in the liquid phase surrounding the fibers. The
volume of
impurities increases in the circulation cumulatively until the volume reaches
a balance,
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which depends mainly on the efficiency a~the washers, the dilution factor and
the amount
of impurity dissolved This has bcxn found to have a detrimental i~~oe on the
quality
of the pulp. Primarily this means a di~~ decrease of the pulp str~th ~ t~
oxygen
stage. 'i tris prvpcrty is illust~ted in Fig. 8, which depicts the influence
of the COD
content of the pulp an the decrease of viscosity of the pulp in the oxygen
stage.
it is known from the prior art that a COD (Chenucal Oxygen Den~and~ content in
the
liquid phase decreases the selectivity of delignification and bleaching and,
as a
consequence, the delignifying and bleaching chemicals react both with the
lignin and the
cellulose which results in a decrease in the pulp strength. It is further
known from ~e
prior art that the COD in the liquid phase varies when determined ax different
stags of
the delignificatian. On one hand, it has been alleged that the type of the COD
has no
~fuificanec in view of the pulp strength but on the other hand, also the
opposite has been
~'8~a. Ln other words, it has been alleged that a COD which has passed t~~ a
I5 deligniFrcation stage has changed so that it has a stronger decreasing
izrfiuence on the
selectivity whereb~~ recycling this ltind of COD back to the deliguification
stage by
counter-current washing would be wrong.
However, irrespective of the reason for the decrease in the pulp strength,
i.c. a high COD
content in the circulation waters, a wrong type of the COD in the oxygen stage
or in
general any physical,propcrty of the liquid phase, i.e. the amount of dry
Aids, the COD
or the alkalinity, the present invention efficiently removex these pmblcms.
Figure 2 illustrates a preferred emhodime~at of the invention for solving for
example the
2~ problem discussed above. As it has been suspected that the xg for the
problem is the
high dry solids or COD content of the filtrate circulated from the washer I2
following the
oxygen stage 10, a separator 114 has boon provided in the line transporting f
ltrate from
the washer 12 following the oxygen stage 10 to the washer 8 preceding the
oxygen stage
10, the separator sepaoating a partial liquid flow LI from the filtratelwash
liquid flow
between the washers 8 and 12 for further trea~em, ~ ~ og ~ separator 114 is to
divide 'the filtrate flow LI to be treated rote a cleaner fraction CC, i.e. a
fraction having a
lower dry solids or COD content, and into a fouler fraction CD, i.e. a
fraction having a
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higher dry solids or COD content, and to return the cleaner fraction CC to the
wash liquid
introduced into the washer 8 preceding the oxygen stage 10 as illustrated in
Fig. 2. Then
the dry solids or COD content of the filtrate in question, i.e. the wash
liquid of the washer
8 preceding the oxygen stage 10, may be reduced remarkably so that the amount
of dry
solids or COD introduced into the oxygen stage 10 is essentially lower than
before.
Performed tests have confirmed that the quality of the pulp improves when the
impurity
or COD content decreases. The fouler fraction CD, or the fraction having a
higher
content of dry solids or COD, from the separator 114 is transported counter-
currently so
far that the dry solids or COD content of the liquid phase at that point is
the same or
higher than that of the fouler fraction CD to be returned. Locations of this
kind may be
for example the filtrate BSF flowing from the brown stock washing 8 to the
digester wash
of the digester 2, or black liquor, suitable internal liquid circulations of
the digester 2, or
black liquor flowing from the digester 2 to the chemical recovery CR.
It has been found out during studies that the same or sometimes even better
effect on the
quality of the pulp is obtained when an appropriate filtrate or other liquid
containing
liquor is treated closer to the digester. In this case also alkaline in the
liquid phase comes
into question. Further, the circulation contains as impurity also for example
soap. It is a
known fact that local separation of soap may be intensified by increasing the
dry solids
content of the liquid to an adequate extent. Fig. 9 illustrates the influence
of the COD on
the solubility of soap. As increasing the COD decreases the solubility of
soap, soap is
separated more easily onto the surface of the concentrate from which it may be
removed
by known methods. The removal of soap from the process improves the
operability and
the controllability of the whole process. Thus the present invention also
provides a
solution for the various problems caused by soap in different stages of the
process.
Figure 3 illustrates a second and a third embodiment of the invention. In the
embodiments
of the figure, a separation device, in this embodiment an evaporator 214
changing a
physical property of the liquid phase, such as dry solids content, COD, sodium
or alkali
content, has been provided in connection with the brown stock washing 8. A
part LI of
the filtrate transported from the washer 12 following the oxygen bleaching 10
to the
brown stock washer 8 for use as the wash liquid is separated to the evaporator
214. In the
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embodiment of the figure, the washer 12 is a so-called fractionating washer,
which
separates two filtrate fractions FC and FD, either of which is treated in the
evaporator
214. Performed tests and calculations have proved that a maximum of 6 m3/adt
of clean
fraction may be used economically, usually about 1 - 5 m3, preferably 1 - 3.5
m3. Then
the exact volume of the liquid flow directed into the evaporator from each of
the flows
cannot be stated because the volume of the filtrate LI to be brought to the
evaporator
depends also on the dry solids, COD and alkali content of the filtrate LI in
question. The
filtrate part LI is treated in the evaporator 214 so that condensate CC and
concentrate CD
are obtained. The basic principle is, as already explained earlier, that the
concentrate CD
is brought counter-currently to a point of the process where the foulness of
the liquid
phase, the volume of impurities, the dry solids, COD, or alkali content are
the same as or
greater than that of the concentrate CD. Correspondingly, the condensate CC is
either
returned to the point of the process from which the flow to the evaporator was
extracted ,
or to some other later point (in the flow direction of the fiber suspension)
in the process
to be used as wash liquid.
In the embodiment of Fig. 3, the condensate CC from the evaporator 214 is
returned to
the same point from which it was extracted, i.e. to the feed of the brown
stock washer 8.
As the washer in the embodiment of the figure is a so-called
DRUMDISPLACER°
washer which uses wash liquids of various different degrees of purity, the
condensate CC
is returned to the feed line of the cleaner wash liquid FC. The concentrate
CD, on the
other hand, is guided to the black liquor flowing from the digester to the
chemical
recovery CR.
The figure illustrates with a broken line as a third preferred embodiment of
the invention
also another point where the condensate CC from the evaporator 214 may be
returned.
This point is the wash liquid feed of the washer 12 subsequent to the oxygen
stage 10.
Further, since the washer is a DRUMDISPLACER° washer, as the figure
illustrates, the
condensate CC may be returned to the cleaner wash liquid to be supplied to the
washer
12.
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Figure 4 illustrates preferred embodiments of the invention as applied to the
digester of a
continuous digestion process. The common feature with the embodiments of
figure 4 is
that the evaporator 314 treats either black liquor BSF transferred from the
brown stock
washing 8 to the digester 2 or black liquor transferred from the digester to
the recovery
5 CR.
In other word there are two alternative liquors introduced to the evaporator
314. The first
one is to take a part LI from the filtrate transported from the brown stock
washing 8 to the
digestion plant to the so-called digester wash and to treat it in the
evaporator 314. When
10 a fractionating DRUMDISPLACER~ washer is used as the brown stock washer,
the
fouler wash filtrate is brought in the embodiment of the figure from the
washer 8 to the
digester and the cleaner wash filtrate is directed to the bottom dilution of
the blow tank 4.
The filtrate BSF is used in the digester 2 in the so-called digester wash or
in some other
liquid circulation of the digester. The other alternative is to take to the
evaporator 314 the
part LI from the black liquor transported from the digester to the recovery
CR. As it is
quite feasible to use for example the apparatus of the embodiment illustrated
in figure 3
combined with the apparatus of this embodiment, at least a part of the
concentrate to be
returned from the evaporator 214 of figure 3 may end up in the evaporator 314.
As regards alternative ways of returning the condensate from the evaporator
314 to the
process, figure 4 illustrates six alternatives. According to one embodiment
the
condensate is returned to the cleanest wash liquid of the washer 12 subsequent
to the
oxygen bleaching 10, of course only if the condensate is cleaner than the
fouled wash
liquid. The second alternative is to return the condensate to the cleanest
wash liquid of
the brown stock washer 8 or to the final dilution if a press is used there.
The third
alternative is to return the condensate to the digester 2 for digester wash
with the cleanest
filtrate from the brown stock washer 8. The fourth alternative is to return
the condensate
to the bottom dilution of the blow tank 4. The fifth alternative is to return
the condensate
to the point in the bleaching plant BL where cleanliness is needed most. The
wash
filtrates are recycled in counter-current wash from the bleaching plant BL to
the chemical
recovery. The sixth alternative is to divide the condensate flow into two or
more separate
flows and to guide them to the locations described above.
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Figure 5 illustrates in fact four fwrthcr preferred embodiments of the
invention as applied
to the batch digestion process. In the embodiment of figure 5, the liquid LI
supplied to
the evaporation plant is obtained from eitlxr of the filtrate/wash liquid FC
flowing from
the washer (DRr3IVlDISPLACER~ 12 following from the oxygen stage 10 to the
brow
stock washer 8. The feature dctcrmi.ning the choice of the filtrate as well in
this
embodiment as in the others is the COD and dry solids coatem of the; filtrate
at that point.
Ia most casts the filtrate having a higher content of COD andlos dry solids is
chosen.
Further, as illustrated in figure 5, the cvndcnsate CC is returned the wash
liquid flowing
to the brown stock waslier 8. P'rcferably to the same wash liquid FC frorn
which the
liquid LI to be supplied to the evaporator 414 is obtained. Another altcmative
of
returning the condensatc CC is to direct it to a location far in the process,
even after the
oxygen stage in the flow direct3oa of the fiber suspension. Figure 5
illustrates a process
where the screening plant 6 has bten positionat after the oxygen stage 10. In
this case it
has been considered advaixtageous to direct the condensate CC to the wash
liquid of the
washing stage 16 subsequent to the screening plant 6. Agar it may be stated
that, if wash
liquids of different concentrations are supplied to the washer, it is
advantag~us to return
the condensa~ CC to the cleaner wash liquid, i.e. in practice to the last wash
stage or
zone of the washer. It would even be advantageous to transport the condensate
CC as a
separaxe flow to the end of the washing stage or to the end dilution of a
press.
In the embodiment of figure 5, the concentrate CD is returned either to the
black liquor
#lowing from the digestGrldigestion pleat 2 to the chemical recovery Clt, or
to the Sltrate
BSF flowing from the brown stock washing 8 to the digestio~a wash of the
digester 2 or to
soma other liquid circulation of the digester.
Figure 6 illustrates six fiuther preferred embodiments of the invention as
applied to the
batch digesfion process. In the embodiment of the figure, the black liquor LI
supplied to
the evaporator S I4 is ob~n~ed either from the flow HSF flowing from the brow
stock
washing 8 to the digestion plant, from the flow from the brown stock washing 8
to the
chemical recovery CR, or from the flow from the digestion plant 2 to the
chemical
recovery CR. The condensate GC in turn is added either to the wash liquid
coming to the
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brown stock washer 8, in case of a DRUMDISPLACER~ washer to the cleaner wash
liquid FC coming to the washer 8, or in this embodiment even to the wash
liquid of the
washer subsequent to the oxygen stage 10 and the screening 6. The concentrate
CD from
the evaporator 514 in turn is returned directly to the flow to the chemical
recovery CR.
Yet another application of or an alternative way of using the method of the
invention is
the separation treatment of the white liquor coming from the chemical recovery
to the
digester. Quite like in the examples described above, the white liquor may be
separated
for example by evaporating into condensate and concentrate and the condensate
may be
brought to the same points of the process as in the previous embodiments. The
concentrate in turn is supplied to the digester as more concentrated white
liquor, i.e. to the
same place where it would be supplied anyway.
All the embodiments described above employ without exception a DRUMDISPLACER~
washer as the washer which has the typical feature that wash liquids of
several different
concentrations may be supplied to it and filtrates of several different
concentrations may
be obtained from it. Further, it is characteristic of the washer in question
that it may
comprise several wash stages whereby the liquid circulations between the wash
stages
have been arranged by connections within the washer as described in various
patents and
patent applications discussing the subject. The corresponding function may at
least partly
be effected for example by means of suction drum filters or presses which in
practice
means that several suction drum filters or presses are connected one after the
other. In
this case it is possible to extract liquid for the evaporation treatment also
from the
filtrate/wash liquid lines between the filters/presses connected in series. In
other words,
the DRUMDISPLACER° washer is not indispensable in carrying out the
invention but
the invention may be used in connection with all washing apparatus available
on the
market. Thus it is clear that the invention is applicable also in situations
where only one
kind of wash liquid can be supplied to the washer and only one kind of
filtrate can be
extracted from the washer.
It should be noted that although the invention has been described in
connection with an
oxygen stage used as a delignification or prebleaching stage, the invention
may be used
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13
in connection with any kind of treatment stages. Thus the invention may quite
well be
employed in connection with a delignification stage using peroxide and oxygen
together
or chlorine dioxide although the use of chlorine dioxide sets certain
limitations to
recycling of the concentrate.
Further, it is worth mentioning that although the above description speaks
about an
evaporator as the separating device in practice there are also other
possibilities.
For example in cases where absolute purity of the fraction CC to be recycled
is not
important, a membrane separator may be used which separates macromolecular dry
solids
and/or COD from the liquid to be recycled. Then dry solids and/or COD having
smaller
molecules will remain in the cleaner fraction CC but when this fraction CC is
recycled to
a suitable location in the process it does not cause essential problems.
Further, it should be remembered that the cleaner fraction obtained from the
separation
apparatus may be distributed not only to one location as described above but
also to
several locations. An example of the many alternatives is to take a portion of
the cleaner
fraction CC to the washer of the oxygen stage and another portion to the
washer
preceding a PO stage (PO = peroxide bleaching stage intensified with oxygen)
of the
bleaching plant. Then, in the end the cleaner fraction brought to the process
will end up
counter-currently to the digester. The test we have performed have shown that
the cleaner
fraction could be returned to a point from which it would have to travel
counter-currently
through as many washes/washing stages as possible.
