Note: Descriptions are shown in the official language in which they were submitted.
2~ J~
TREATMENT OF BLEAC~ PLANT FILTRATES
BACRGROUND AND ~UMMARY OF T~E INVENTION
It is desirable to minimize adverse environmental
effects of a kraft mill by "closing" a bleaching system of
a kraft pulping facility so that there is no or very small
discharge of washing filtrates, or the like, into the
environment; or it is possible to substantially Glose the
bleaching system so that the only discharge from the
washing/bleaching system is a small stream of wash
filtrate used in smelt dissolution (or effectively treated
so that there is essentially zero pollution associated
with it when discharged).
Perhaps the most promising bleaching chemical for
allowing chlorine to be eliminated while still achieving
effective bleaching, is ozone. ~hile oæone has been known
as a bleaching agent for a long time, it has not been
commercially used on an industrial scale in pulp
production until very recently in view of recent rapid
developments in machine and process technologies, because
of long standing problems.
A major problem encountered in utilizing ozone is
high ozone consumption. Ozone consumption may be
unreasonable high if there are too many metallic ions,
which react with the ozone, or if the ozone reacts with
organic material in the liquid phase of the pulp rather
than reacting with the pulp fibers. It is highly
desirable to have the ozone react only with fibers so that
the ozone consumption is minimized, making ozone bleaching
practical as well as effective, and allowing "closing" of
a bleach plant.
One Way in which the volume of metallic ions can be
decreased is by improving the washing after the acid
stage, where the majority of the metallic ions are
dissolved. Another way to reduce the load of metallic
ions is to utilize an electrical potential. The
electrical potential can be applied over the washer, or
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the filtrate supplied to the washer, or even to the pulp
prior to the ozone stage. A preferred way to apply the
electric field is to have the filtrate being used as
washwater to go through a chamber where there is a
suitable electric potential. In this way the filtrate is
cleaned of metallic ions before being used as wash water.
The electrical potential can effect sufficient removal of
ions ~o as to allow complete closing of the bleaching
plant, or to provide that the amount of wash filtrate from
the acid stage is so small that it is completely consumed
in smelt dissolution, and cooking liquor preparation.
According to the present invention, the amount of
organic material in the liquid phase is made
non-attractive to the ozone by oxidizing the organic
material. Since ozone is a strong oxidant, it is more
likely to react with the fibers than with the already
oxidized organic material in the liquid phase. It is
perhaps not possible to oxidize all dissolved material in
the liquid phase, but a considerable part can be oxidized
with oxygen prior to applying the ozone. However there is
always a significant amount of organic material in the
wash filtrates used during washing of the pulp prior to
the ozone stage so that if that organic material is
oxidized, the ozone consumption in the following ozone
bleaching stage can be significantly reduced.
According to one aspect of the present invention, a
method of treating filtrates from bleach plants for
bleaching cellulosic pulp, including utilizing at least
one ozone, H202, or oxygen bleaching stage, is provided.
The method comprises the steps of: (a) Washing the pulp
in a washer with a wash liquid prior to the ozone, H202,
or oxygen bleaching stage, the wash liquid including
filtrate from another washing stage of a bleaching stage.
(b) Oxidizing the organic material in the filtrate used as
wash liquid in step (a) prior to its use in step (a) in
order to make the organic material less attractive to
bleaching chemical in the bleaching stage. And, (c)
ozone, H202, or oxygen bleaching the washed pulp in the at
: ., ,. . ,., , , ,.................... . . :
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least one ozone, H202, or oxygen bleaching stage. Step
(b) ma~ be practiced by indirectly heating the filtrate
(as with already oxidized filtrate in the heat exchanger),
and then reacting the heated filtrate with oxygen
containing gas, such as oxygen carrier gas with residual
ozone discharged from an ozone bleaching stage. Alkali
can be added to the filtrate and oxygen during the
reacting step if desired.
According to the invention, there is also the further
step (d) of removing metallic ions from the filtrate so as
to minimize the consumption of ~leaching chemical in step
(c), by passing the filtrate through a magnesium filter,
such as a magnesium chip filter. Such a filter especially
removes Mn, Fe, and Cu all of which consume bleaching
chemicals. Mg, on the other hand, has a positive effect
on bleaching.
All of the steps are preferably practiced with the
pulp at a medium consistency, e.g. of about 5-18%
(preferably about 8-15%).
Practicing the present invention, a method of
continuously kraft pulping and then bleaching comminuted
cellulosic fibrous material may be provided which
comprises the following steps: (a) Continuously digesting
the material to produce pulp having a Kappa Number
comparable to about 20 or below for softwood. (b)
Subjecting the digested pulp to hot alkali extraction
and/or oxygen bleaching so as to produce pulp having a
Kappa Number comparable to about 10 or below for
softwood. (c) Effecting bleaching of the pulp with
non-chlorine containing bleaching chemicals to obtain a
bleached pulp having properties comparable to CK pulp
bleached with a DEDED se~uence. (d) Effecting washing of
the pulp With wash filtrates, and to produce wash
filtrates, during the practice of step (c). And, (e)
treating and recirculating the wash filtrates from step
(d) so as to discharge no, or a small amount of, wash
filtrates into the environment. Step (c) includes at
least one ozone bleaching stage, and step (d) includes
:
~ ~ f;~ 2 ~ ~-; ',~
washing the pulp with wash filtrate just prior to the
ozone bleaching stage; and comprising the further step (f)
of oxidizing organic material in the wash filtrate prior
to the washing of the pulp just prior to the ozone
bleaching so as to make the organic material less
attractive to ozone in the ozone bleaching stage. The
bleaching sequences may be as described in the parent
application, e.g. they may be selected from the group
consisting essentially of OOAZEZPZ, OAZEZPZ, OOAZEZP, and
OAZEZP, or AZEZPZ or AZEZP if oxygen bleaching is used to
lower the Kappa Number to about 10.
Alternatively, step (e) may be the step of treating
and recirculating all of the wash filtrates in step (d) so
that the only wash filtrate discharged from the washing of
step (d) is a small stream used for smelt dissolution
(cooking liquor production). If all of the small stream
cannot be taken to cooking liquor preparation (smelt
dissolution and other associated processes), it is taken
to black liquor evaporation. It can be taken to black
liquor evaporation directly or by taking it to the brown
stock washing plant.
It is the primary object of the present invention to
minimize ozone consumption in the ozone bleaching of
cellulosic pulp, and/or to produce a closed, or
substantially closed, bleach plant in a pulp mill. This
and other objects of the invention will become clear from
an inspection of the detailed description of the invention
and from the appended claims.
BRIEF DESCRIPTION OF 1~ DRAWINGS
FIGURE 1 is a schematic representative of an ozone
bleaching stage in a cellulosic pulp bleaching plant;
FIGURE 2 is a schematic view of the wash aspects of a
typical non-chlorine bleaching sequence showing the liquid
flow interconnections between the various stages, and
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schematically illustrating treatments of the filtrates as
according to the invention;
FIGURE 3 is a schematic representation of one of the
particular filtrate treatments shown only as a block in
FIGURE 2;
FIGURE 4 is a schematic view like that of FIGURE 2
only showing the utilization of magnesium filters
according to the invention;
FIGURE 5 is a detail view of a treatment of filtrate
between wash stages of a system that can be utilized in
FIGURE 4;
FIGURES 6 and 7 are two exemplary bleaching sequences
that may be utilized in the practice of the invention; and
FIGURE 8 is a schematic perspective view, with
portions cut away for clarity of illustration, of an
exemplary magnesium filter that could be utilized
according to the invention.
DETAILED DESCRIPTION OF T~ DRAWINGS
FIGURE 1 schematically illustrates a typical ozone
bleaching stage, such as utilized in the chlorine free
pulping and bleaching sequence of the parent application,
for the practice of the invention. Pulp from a previous
stage 10 in the pulping sequence is subjected to acid
washing 11, and then fed to a retention vessel 12 from
which it is pumped by pump 13 through mixer 14 to the
reactor 15. The pulp may be a medium consistency (e.g.
about 5-18%), or low consistency. If a medium
consistency, a MC~ mixer, sold by Kamyr, Inc. of Glens
Falls, New York, may be utilized to intimately mix the
ozone with the pulp.
2 ~
\
Oxygen from source 16, which is essentially pure
oxygen (e.g. over 90% oxygen) may be treated by ozonator
17, and then pumped by a pump 18 to the mixing element 14
to be mixed with the pulp prior to reactor 15. The
element 14 may be a valve or an MC~ mixer. The ozone
bleached pulp is discharged in line 20, while carrier gas
(oxygen), with residual ozone, is discharged at 21. The
pulp passes to one or more tanks or vessels 22,
subsequently being discharged in line 23, under the
influence of pump 24, to a washer and then to the next
pulping stage. Oxygen gas is discharged from vessel 22
into line 25, combined with the discharge gas in line 21
in line 26, and it may be purified in the gas treatment
stage 27 so that the gas in line 28 consists essentially
of oxygen, perhaps with a small amount of residual ozone.
EIGURE 2 illustrates details of the washing sequences
that may be associated with a typical non-chlorine
bleaching sequence. The diffusion washer 30 receives pulp
in line 31 from brown stock washing. The pulp in line 31
has already been continuously digested, as with the known
Kamyr modified continuous cooking process, so that the
Kappa Number has been reduced to about 20 or below for
softwood, and has already been subjected to hot alkali
extraction and/or oxygen bleaching so that it has a Kappa
Number for softwood of about 10 or below. Typically one
or more oxygen delignifi~ation (bleaching) stages have
also been utilized, and ultimately the pulp will be
bleached with non-chlorine containing bleaching chemicals
to obtain a bleached pulp having properties comparable to
conventional kraft pulp ("CK") bleached with a DEDED
sequence. From the diffusion washer 30, filtrate is
returned in line 32 to brown stock washing.
The acid washing stage 11 is supplied with fresh
water in line 34, and filtrate from other washing stages
of the bleaching seguence in line 35. For example line 35
is connected up to the washer 36 for the first ozone
stage. According to the present invention, metallic ions
can be removed from the system by utilizing the apparatus
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38 in FIGURE 1, and/or apparatus 40 and/or 41 in FIGURE
2. As illustrated in FIGURE 1, the electrical potential
can be applied by the mechanism 38 directly to the pulp
between the acid washing stage 11 and the vessel 12 (at
the start of the ozone stage) in addition to, or instead
of, the mechanisms 40, 41. Apparatus 40 applies an
electrical potential to the filtrate in the line 35, while
apparatus 41 applies an electrical potential across the
entire washer 11, the electrical potential effecting
removal of a substantial number of metallic ions from the
system, and thus the pulp being fed to the ozone stage.
The filtrate discharged from washer 11 may pass
through a valve 42 to go into the line 43 for smelt
dissolution (or other part of the cooking liquor
production), or treatment, while at least part is
recirculated to the pulp being fed to the washer 11.
Depending upon the effectiveness of the bleaching and
washing stages, the removal of metallic ions, and the
like, the valve 42 may be adjusted so that the stream in
line 43 is either just a small stream only sufficient for
cooking liquor preparation, or so that the line 43 is
completely closed off so that no filtrates from the
washing system associated with the bleaching sequence of
FIGURE 2 go out from the fiberline. It is also possible
to take filtrate to the black liquor evaporation plant or
some part of the brown washing system and thus ultimately
burn the contaminants in the recovery boiler.
According to the present invention, apparatus 44 is
utilized in association with the washing stage prior to
each ozone bleaching stage to make organic material in the
liquid phase less attractive to the ozone. The structure
44 treats the filtrate in the washer prior to the ozone
stage, and after the bleaching stage prior to the ozone
stage. A typical apparatus 44 is illustrated in more
detail in FIGURE 3.
Unoxidized filtrate in line 35' (part of 35) passes
to the heat exchanger compartment 48 where it is brought
into heat exchange relationship with oxidized filtrate in
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line 35 " . In order to improve oxidation and heat
economy, in addition to using the heat exchanger 48
another indirect heat exchanger 49 may be utilized which
has line 50 leading into it and line 51 leading away from
it. Hot water or steam may be supplied to the line 50 so
that the unoxidized filtrate is heated. The unoxidized
filtrate is then fed to a reactor 52 where oxygen
containing gas, typically consisting essentially of oxygen
(per~aps with some residual ozone) is added. The oxygen
may be from the line 28 (see FIGURE 1).
Oxidation also may be improved by adding alkali in
line 53 to the reactor 52. Adding alkali keeps the pH
high, and may be desirable since carbon dioxide which is
favored during oxidation lowers the pH. However if one
desires the pH to drop during oxidation and thus obtain an
acid filtrate, which can be used to lower the pH of the Z
stage following the washer 11, no alkali will be added in
line 53. Residual gases remaining after the oxidation
takes place in reactor 52 can either be removed in the
reactor 52, or after cooling the oxidized filtrate in heat
exchanger chamber 55 to which the oxidized filtrate is
supplied via line 54. The oxidized filtrate in line 35"
(part of 35) then proceeds to the washer 11 (e.g. through
the structure 40).
FIGURE 2 also illustrates various other washers and
lines, including the washer 60 for the E stage supplied
with filtrate from line 61, which is connected to the
washer 62 from the second Z stage, which in turn is ~-
supplied with filtrate from line 63, connected to the
washer 64 associated with the P stage. Fresh water is
supplied to the washer 64 via line 65, and the bleached
pulp that is produced is discharged in line 66. Filtrate
from line 67, connected to washer 60, and from line 68,
connected to washer 36, may be supplied to the diffusion
washer 30. It is noted that an oxidizing unit 44 is
associated with the washer 60 prior to the second ozone
stage (having washer 62) too.
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Utilizing the system and procedure described above, a
closed system, or a substantially closed system, may be
provided, in which there is no discharge of filtrates from
the bleaching plant, or a minimal discharge (in line 43).
An example of the filtrate volume associated with 1, 2, or
3 ozone stages is illustrated in the following tables.
As can be seen from Table 1, the amount of filtrate
going into the washers before the first and second ozone
stages (Zl and Z2) in the closed system is 6.7 - 2.5 = 4.2
m /ton/filter, and in a substantially closed system 6.7 -
4.0 = 2.7 m3/ton pulp/filter. In this case when the
consistency of the pulp during washing is 15%, about 6.7
m3 of total wash water is used on each filter. If a
washer consistency of 10% had been used, the use of total
wash water on the washer would have been about 10 m3/ton
pulp/washer, and the use of a filtrate would have been
somewhat higher than in the above calculations. These
relatively small recirculated filtrate flows ~e.g. 2.7 -
4.2 m /ton pulp/washsr) to the washers ahead of each of
the Z stages can easily be oxidized by the oxygen gas in
line 28. ~lso, Table 2 shows that the amount of metallic
ions is small enough so as to be effectively handled too.
.. .
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TABLE 1
Water Usage in Bleach Plant
Fre~h Water Usage 3Waste Water
System m /ton pulp m /ton pulp
Z1 Z2 Z3
Closed 2.5 2.5 6.7 4.3
Substantially Closed 4.0 4.0 6.7 7.3
Open 6.7 6.7 6.7 1207
The waste water streams in Table 1 can be taken to
white liquor preparation, particularly smelt dissolution.
About 2-4 m3 can be used for this purpose. The rest of the
waste water can be taken to brown stock washing, or black
liquor evaporation. Also some, especially in the open case,
may have to go to waste water treatment.
. ' - :' -~ " ', ' ',' ' ' ' ' '' . ' . ' -
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TABLE 2
Metallic Ions Distribution
Metallic ions
System % of total dissolved in A-stage
Z1 Z2 Z3 Waste Water
Closed 25% 2% 2% 75%
Substantially Closed 15% 1% 1% 90%
Open 10% ~1% ~1% 95%
TABLE 3
Accumulation of Organic Material
Dissolved Organic Material
SystemPresent in Stage kq/ton Pulp
Z1 Z2 Z3
Closed 9 10 2
Substantially Closed 4 6 2
Open 2 3 2
-
Practicing the present invention, a method of
continuously kraft pulping and then bleaching
comminuted cellulosic fibrous material may be
provided which comprises the following steps: (a)
Continuously digesting the material to produce pulp
having a Kappa Number comparable to about 20 or
below for softwood. (b) Subjecting the digested
pulp to hot alkali extraction and/or oxygen
bleaching so as to produce pulp having a Kappa
Number comparable to about 10 or below for
softwood. (c) Effecting bleaching of the pulp with
non-chlorine containing bleaching chemicals to
obtain a bleached pulp having properties comparable
to CK pulp bleached with a DEDED sequence. (d)
~ffecting washing of the pulp with wash filtrates,
- .' ' ~ ,' ........................................ : '
... . . .
, . : ,-: , : : . . .
, ~ . , . , . . . : . .
and to produce wash filtrates, during the practice'~v ~ g
of step (~). And, ~e) treating and recirculating
the wash filtrates from step (d) so as to discharge
no, or a small amount of, wash filtrates into the
environment. Step (c) includes at least one ozone
bleaching stage, and step (d) includes washing the
pulp with wash filtrate just prior to the ozone
bleaching stage; and comprising the further step (f)
of oxidizing organic material in the wash filtrate
prior to the washing of the pulp just prior to the
ozone bleaching so as to make the organic material
less attractive to ozone in the ozone bleaching
stage. The bleaching sequences may be as described
in the parent application, e.g. they may be selected
from the group consisting essentially of OOAZEZPZ,
OAZEZPZ, OOAZEZP, and OAZEZP, or AZEZPZ or AZEZP if
oxygen bleaching is used to lower the Kappa Number
to about 10.
FIGURE 4 schematically illustrates a bleaching
sequence according to the present invention with
filter recirculation and with treatment of the
filtrate so as to minimize the accumulation of COD
and heavy metal ions in the system. Pulp in line 70
is preferably from an EMCC~ digester (from Kamyr,
Inc. of Glens Falls, New York) which passes to a T
stage, where there is treatment with EDTA or the
like (e.g. at a pulp consistency of about 10%, with
a dosage of EDTA of about 2 kilograms per ton of
pulp, a temperature of about 50C, and a treatment
time of about 30 minutes), after which the pulp
passes to a wash stage 72, then to a Z (ozone
bleaching) stage 73, then to another wash stage 74,
an (EP) stage 75, for caustic extraction and
peroxide bleaching, and then to a washer 76, the
final pulp 77 being produced.
In the Z stage, the target pH is about 3.0,
which may be achieved by adjusting the pH of
filtrate provided just before the stage 73 (in line
:
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13
85) with sulfuric acid to about 2.15. In the Z
stage 73 the pulp consistency is about 10%, the
temperature about 20 to 30C, and the ozone dosage
about 7 kilograms per ton of pulp.
In the T stage, the treatment may be with acid,
or EDTA and acid, and its primary purpose is to
remove metals.
In the (EP3 stage 75, the pulp consistency is
about 10%, the sodium hydroxide dosage about 15
kilograms per ton of pulp, the pero~ide dosage about
3 kilograms per ton of pulp, the temperature about
70C, and the treatment time about 60 minutes.
The pulp in line 70, as earlier indicated, is
typically oxygen delignified commercial soft wood
kraft pulp, having a consistency of about 28%, and
is diluted with water (filtrate) from washing stage
72, the filtrate passing in line 78 through a
magnesium filter 79 to the line 70. The wash liquid
for the wash stage 72 is supplied in line 80 from
wash stage 74, passing through magnesium filter 81.
The washer 74 is provided with wash water in the
form of filtrate in line 82 from third washer 76,
passing through magnesium filter 83. Typically
fresh water is used as the wash water for the third
washing stage 76.
In each of the filtrate lines 78, 80, 82, a
portion of the liquid may be removed so that the
consistency of the pulp being treated is maintained
at a desired level. The filtrate in line 84 could
be passed to disposal or treatment, while the
filtrate in lines 85 and 86 enters the pulp stream
prior to a bleaching stage. The pH of the fiber
suspension may be reduced by adding acid, such as
sulfuric acid although many other acids can be used,
to the suspension before a washer. The acid can be
added directly to the pulp or to dilution water
entering the fiber suspension, such as in line 85,
and the return line from filter 7g to line 70, since
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14 ' 2
the T and Z stages are acidic stages. The pH range
de~ired is about 2-6, preferably about 5. At a p~
of about 5, Mn and Cu dissolve, but any ma~nesium
remains at the fibers. Alkali may be added before
an alkali bleaching stage if desired; the ~EP) stage
(as would be a peroxide stage) of FIGURE 4 is
alkali, so that alkali could be added to the line 86
to adjust the pH.
FIGURE 5 schematically illustrates the same
type of treatment as in FIGURE 4 only for a "P"
stage per se, and showing the various components in
the filtrate return line in more detailr Pulp in
line 90 passes through washer 87 (as from a Z
stage), then to the P bleach stage 88, and to
another washing stage 89. The filtrate from stage
89 is used as the wash water in stage 87. First and
second magnesium filters 91, 92 are provided in the
recirculatory line 95 from the wash stage 89 to the
wash stage 87. Valves 93 are provided on opposite
sides of the filter 91, while valves 94 are provided
on opposite sides of the filter 92. Preferably a
fiber filter 95' is provided in line 95 before the
filters 91, 92 so as to remove fibers which might
otherwise have a tendency to clog the magnesium
filters 91, 92. The fiber filter 95' may be of any
conventional type, such as the Ahldecker fiber
filter sold by A. Ahlstrom Corporation of Helsinki,
Finland.
Utilizing apparatus like that of FIGURE 4, and
the conditions within each of the stages 71, 73, and
75 described, effectively bleached pulp can be
produced.
In utilizing the apparatus of FIGURE 5, the
filter 91 has the valves 93 associated therewith
open, while the valves 94 associated with filter 92
are closed. Once the effectiveness of the magnesium
filter 91 for removing heavy metals, such as Mn, Fe,
Zn, Co, and Cu, deteriorates, the valves 93 are
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closed at the same time that the valves 94 are
opened to put the filter 92 into service. Then the
filter 91 is either replaced or cleaned of
contaminants and put back in service. The filter 92
is used until it becomes contaminated, and then the
process is repeated.
In use of the filters 91, 92, some magnesium
will probably dissolve in the filtrate to form Mg
ion. While heavy (e.g., transition) metals such as
Mn, Fe, Zn, Co and Cu have a negative affect on
bleaching with peroxide or ozone, or delignification
with oxygen, because they consume bleaching
chemical, the magnesium ions have a positive
affect. Therefore the magnesium ions associated
with the magnesium chips in the filters 91, 92
attract heavy metals to the surface of the chips,
and if dissolved in the pulp have a beneficial
affect on bleaching.
FIGURES 6 and 7 illustrate two typical
bleaching seguences that can be used according to
the invention. In FIGURE 6 oxygen delignification
(O) is followed by an EDTA, acid, or like treatment
stage (T), followed by ozone bleaching (Z), followed
by a caustic extraction stage, preferably having
oxygen or peroxide associated therewith (Eop). In
FIGURE 7 oxygen delignification is followed by an
ozone stage and one or more peroxide stages.
FIGURE 8 schematically illustrates an exemplary
magnesium filter that can be utilized according to
the invention, shown generally by reference numeral
96. Magnesium chips for removal of copper, iron,
and like heavy metals from a liquid is known per se,
and is very advantageous according to the invention
for minimizing bleaching chemical use. The chips
are shown schematically ~y reference numeral 97 in
FIGURE 8, and although they may take a variety of
forms they preferably are in the shape of curled
sawdust and have a maximum dimension of a~out three
: .
.
millimeters or less (e.g. about one to three 9~ 8
millimeters or less). The chips 97 are disposed in
a housing 98 which may have one or more
compartments, and filtrate to be treated passes into
inlet 99 and passes through outlet 100 after
following a torturous path within the housing 98,
and for that purpose baffles and other flow
directing elements may be provided in the housing
98.
As the magnesium chips 97 react with components
in the filtrate within the filter 96, hydrogen gas,
or other explosive gases, may form. In order to
prevent the possibility of an explosion, a
ventilation system -- illustrated schematically at
101 in FIGURE 8 -- is provided to remove the gases.
The ventilation system 101 may include a small
vacuum pump, or other conventional elements, to
ensure proper removal of gases which form, or are
released from the filtrate, in the housing 98.
These gases are then passed to a safe disposal or
conversion area.
It will thus be seen that according to the
present invention ozone consumption can be minimized
in a non-chlorine bleaching sequence, so as to make
the non-chlorine bleaching sequence practical.
Also, utilizing the teachings of the invention and
the parent application a bleaching plant can be
closed, or substantially closed.
While the invention has been herein shown and
described in what is presently conceived to be the
most practical and preferred embodiment it will be
apparent to those of ordinary skill in the art that
many modifications may be made thereof within the
scope of the invention, which scope is to be
accorded the broadest interpretation of the appended
claims so as to encompass all equivalent methods and
procedures.
