Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus
for continuously treating fibrous pulp with a treatment fluid
in the liquid or gaseous phase.
DESCRIPTION OF THE PRIOR ART
In the production of cellulose fibrous pulp, it
is conventional practice to subject the pulp to a variety of
treatments such as bleaching and washing, for example.
Apparatus has been proposed heretofore for treating
fibrous pulp by exchanging one liquid contained in the pulp
for another to be used in treating the pulp. For example,
U.S. patent No . 3, 564, 631 describes apparatus in which liquid
exchange takes place from both sides of a fibrous cake disF
posed between two feeder bands, the treatment being carried
15 out with a relatively low (estimated maximum 12%) pulp con-
centration. The treatment liquid is fed into the ~ibrous
cake and mixed with liquid contained therein, whereupon the
liquid mixture is removed from the cake. The treatment
causes a dilution of the liquid contained in the fibrous cake
and the diluted liquid is partly removed. Web dewatering
and rinsing apparatus has been devised in which the web is
first rinsed, then compressed between an outer screen drum and
an inner perforated drum, after which the web is again rinsed,
as shown in U.SO patent No. 3,342,124. Alternatively, the
web may be compressed at successive points on a drum by a
plurality of angularly spaced apart rolls and sprayed with
washing liquid at locations between adjacent rolls, as shown
in German application No. 1,054,826.
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Web treatment apparatus having multiple web washing
zones in the form of diverging and converging duct portions
for treating a pulp mat with like or different dilution liq-
uids has also been proposed, as shown in U.S. patents Nos.
3,564,631 and 3,980,518 and Russian Author's Certificate No.
427,117. Such apparatus is relatively inefficient, however,
and has a tendency towards plug formation.
U.S. patent No. 3,772,144 discloses a pressure
filter or wash press including two rotatable filter drums
enclosed in a positive pressure vat with a nip therebetween
and doctor blades above the nip to remove solid material
from the drum surfaces.
It has also been proposed to dewater a web to a
dryness in the range from 30% to 70~ after a washing operation
by passing it into the nip between a press roll and a rotat-
able drum on which the web is supported, as shown in U.S.
patent No. 3,878,698.
A typical pulp bleaching treatment is described in
U.S. patent No. 3,630,828 which gives comparative date for
high concentration bleaching and low concentration bleaching.
Also, U.S. patent No. 1,957,937 discloses a paper
pulp bleaching technique in which paper pulp in the dry state
is introduced into a closed vessel, a vacuum is created in
the vessel to withdraw air held in the interstices of the
fibrous material, and then a suitable gaseous bleaching agent
is introduced into the vessel. For continuous operation, a
sheet or web of cellulose material in a substantially dry
state is fed successively through a plurality of enclosed
chambers in each of which air is evacuated and the web is
then exposed to the action of a suitable bleaching agent in
gaseous form. Before and after passage of the web through
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the treatment chambers, however, the web is fully expos~d to
air, the known detrimental effects of which in a fiber sus-
pension are discussed in the publication TAPPI 58 (1975): 2,
p. 90-94 (May, O.W. and Buchman, S.J., "Practical Effects of
Air in Paper-making").
Heretofore, it has usually been the practice in web
treatment apparatus to remove the fibrous web from its carrier
in the presence of air. In certain suction filters of the
type disclosed in U.S. patent No. 2~689J653 and in Swedish
patents NosO 194,958 and 194,959, however, the web is subject-
ed to a vacuum in the web pick-off apparatus, with all of the
problems that that entails.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a new
and improved method and apparatus for treating a fibrous
pulp in which a treatment fluid in liquid or gaseous phase
can be utilized to impregnate a high concentration (18 - 5~%)
fibrous pulp in a rapid and highly effective manner.
Another object of the invention is to provide a new
and improved method and apparatus of the above character
which are capable of feeding a treatment fluid in liquid or
gaseous phase rapidly into a high concentration fibrous pulp
and distributing the fluid in the pulp in such fashion that
it is brought into contact with the individual fibers.
~5 In general terms, the present invention provide~
in a method for continuouslv treating fibrous material
comprising moving a liquid permeable screen element in an
enclosed endless path including a web forming zone and a first
treatment zone in that order, said first treatment zone being
defined by an enclosure overlying and confining a Eirst area
of the web, supplying a fibrous suspension of cellulosic
material to the screen element and draining suspension liquid
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therethrough to form a fibrous web on said screen element
in said web forming zone, subjecting said first confined
area of the web to a first treatment by introducing a first
treatment liquid under pressure into said enclosure to
displace liquid contained in the web through said screen
element, moving the web after treatment in said ~irst
treatment zone through the nip formed between the screen
element and a rotatable compression roll having an
impermeable surface, compressing the web in said nip to a
degree sufficient to result in weh dry content in the ran~e
of about 30% to 70% so as to produce a high concentration
of fibrous material, moving the web as it leaves said nip
directly through a second treatment zone defined hv a
second enclosure overlying and confining a second area of
the moving web, the high concentration of fibrous material
in said nip forming a seal between the first and second zones
substantially preventing the flow of treatment fluid from
the latter to the former, permitting the compressed web
to expand as it leaves said nip and enters said second
enclosure, and subjecting the expanding portion of said web
in said second enclosure to a second treatment hy introducing
into said second enclosure a treatment fluid different from
said treatment liquid, under pressure and in liauid or
gaseous phase, whereby the treatment fluid penetrates into
the web in a rapid, uniform and highlv effective manner
and displaces fluid contained therein through said screen
element.
Preferably, the treatment fluid is a fiher
bleaching agent. According to another feature of the present
invention, a method is provided as recited above, in which
the web, directly after treatment in said second treatment
zone, is passed through a press zone in which it is subjected
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to compression in the nip between a second compression roll
and said screen element so as to cause the treatment fluid
impregnating the web to displace any treatment liquid remaining
therein.
In another asDect, the present invention provides,
in apparatus for continuously treating fibrous material
comprising a liquid permeable drum rotatably mounted within
a housing shaped to form with the drum an enclosed perinheral
treatment duct, said duct having a first duct portion including
means for supplying a fibrous suspension to an inlet end
thereof to form a fibrous web on the surface of said drum by
drainage of suspension liquid through said surface into said
drum, and a second duct portion overlyi.ng and confining a
portion of the web and including first means for introducing
a treatment liquid under pressure thereinto to displace
suspension liquid remaining in the web into the drum, a
rotatable compression roll mounted downstream of said second
duct portion for rotation about an axis parallel to said drum
and having an impermeable surface forming a closure for an
opening in said housing and cooperating with the surface of
the drum to form a nip for applying pressure to the web to
compress it to a dry content in the ranqe of about 30~ to 70~,
means forming in said housing a third duct nortion overlving
and confining a portion of said weh as it leaves said nip,
and means for introducing into said third duct portion a
treatment fluid other than said treatment liquid, under
pressure, in liquid or gas phase, to displace treatment liquid
in the web into the drum, the high concentration of fihrous
material produced by the pressure of said nip on the web forming
a seal substantially preventing the flow of treatment fluid
from said third duct portion past said compression roll, and
the radial distances between the drum surface and the housinq
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portions immediately preceding and following the compression
roll exceeding the shortest distance between the drum surface
and the compression roll.
In a still further aspect of the present
invention, the present invention provides, in a method for
continuously treating a fibrous weh of cellulosic material
having liquid containing cavities between the -fihers in which
the web is compressed in the nip between a movable, li~uid
permeable screen element and a rotatable roll to press the li~uid
out of said cavities, the improvement comprisin~ allowing the
web, after compression in said nip and in the course of its
removal from the screen eiement, to expand in an enclosure
supplied with a pressurized gaseous medium other than air,
whereby said fiber cavities are impregnated with the gaseous
medium and absorption of air by the web is prevented.
In a yet another aspect, the present invention
provides, an apparatus for continuouslv treating a fibrous web
of cellulosic material having liquid containing cavities
between the fibers comprising a liquid permeable screen element
movable in an endless path, and a rotatable roll mounted to
cooperate with said screen element to form a nip for compressing
said web to press the liquid out of said cavities, the improvement
comprising cover means substantially completely enclosing said
web as it leaves said nip and expands with reformation of said
cavities, and means for supplying a pressurized gaseous medium
other than air to the interior of said cover means to refill
said cavities and thereby ~revent the web from absorbing air.
DESCRIPTION OF PREFERRED
EMBODIMENTS
.
Additional features and advantages of the invention
will be apparent from the following detailed description of
several representative embodiments, taken in conjunction with
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the accompanying drawings, in which:
Fig. l is a schematic view in side elevation, partly
in section, of web treating apparatus constructed according
to the invention;
Fig. 2 is an enlarged view of a portion of the ap-
paratus shown in Fig. l illustrating the progress of the dis-
placement of the washing liquid and of the treatment liquid
at different locations in the treatment path.
Referring now to Fig. 1, fibrous pulp treatment
apparatus embodying the invention comprises a rotatable drum
11 having a substantially cylindrical, li~uid permeable
screen-like wall or shell 13. The drum 11 is mounted on a
shaft 15 journalled in a machine stand (not shown) and is
provided with conventional drive and speed control means 17.
Surrounding most of the drum 11 is a shell or casing
19 defining a peripheral treatment path 21 about the drum ll,
the radial distance from the shell 13 to the casing 19 dimin-
ishing in the direction of rotation of the drum ll from a
maximum value at an inlet end of the treatment path 21 to a
2~ lesser value at the outlet and thereof. Controllable supply
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devices 23 are provided in the casing 19 for introducing
a substantially uniform flow of fibrous suspension to the
inlet end of the treatment path 21 over the whole width of
the shell 13.
The treatment path 21 includes an initial web
forming zone 25, followed in the direction of rotation of
the drum 11 by a washing zone 27, a compressing or compact-
ing zone 29, a treatment zone 31 and a press zone 33. A
washing liquid chamber 35 located outside the casing 19
supplies washing liquid through perforations 30 in the lat-
ter to the washing zone 27. Similarly, another chamber 37
also located outside the casing 19 contains a treatment
fluid which is adapted to be supplied through perforations 38
in the casing 19 to the treatment zone 31.
The washing liquid chamber 35 is provided with
liquid supply means 39 including at least one servo-governed
control valve 41. Also, one or more supply conduits 43 pro-
vided with servo-governed valves 45 are connected to supply
treatment fluid to the chamber 37.
Mounted between the washing zone 27 and the treat-
ment zone 31 is a rotatable press roll 47 having a liquid
impermeable surface defining with the adjacent portion of
the shell 13 of the drum 11 the compressing or compacting
zone 29. A second press roll 49, also havi.ng a liquid im-
permeable surface is mounted adjacent the shell 13 just be-
yond the treatment zone 31. The rolls 47 and 49 are mounted
onshafts 51 and 53, respectively, journaled in the machine
stand (not shown) with the drum 11 and define with the shell
13 of the drum 11 press nips 55 and 57, respectively, which
:: 30 have substantially constant gap widths during operation.
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The press roll 49 is provided with conventional
pressure gauge means 59 such as strain gauges, for example,
for producing signals indicative of the linear pressure in
the press nip 57.
From the press nip 57, the treatment path 21 leads
to a web removal and feeding device 61 consisting of a
cover 63 enclosing a combined shredding and feeding screw
65. The cover 63 is provided with a doctor blade 67 forming
a closure against the surface 13 of the drum 11 and a sealing
device 69 forming a closure against the press roll 49.
One or more steam supply conduits 71 provided with
valves 73 are connected to supply steam to the space inside
the cover 63. Also, the fiber feeding device 61 m~y be pro-
vided with means such as a flange (not shown) to connect
its output end to other treatment equipment. Within the
drum 11 is disposed an outlet 85 for discharging liquid
drained into the drum during the treatment process.
Control of the web forming, washing and treatment
operations may be effected by a control system comprising a
controller 75 connected by the conductors 77, 79, 81 and 83
to the pres~ure gauge means 59, the drum speed control de-
vices 17, the control valve 41 for the washing liquid cham-
ber 35 and the control valve 45 for the treatment liquid
chamber 37, respectively. The specific type and construc-
tion of the components of the control system and the system
as a whole may be selected by one skilled in the art, as a
matter of ordinary engineering skill, from a variety of elec-
trical, hydraulic, pneumatic or mechanical components and
systems that are well known per se.
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In operation, a suspension of fibrous material
under pressure and at a constant rate of flow is fed through
the supply devices 23 into the web forming zone 25 of the
treatment path 21 while the drum 11 is rotated at a substan-
tially constant speed. The pressure differential betweenthe treatment path 21 and the interior of the drum 11 causes
the suspension liquid to flow through the shell 13 and a
fibrous web 91 to be formed on the latter. As the web 91
is moved through the web forming zone 25 its thickness grad-
ually increases until it completely fills the space betweenthe shell 13 and the casing 19, as shown in Fig. 1.
Since the treatment path 21 converges in the direc-
tion of rotation of the drum 11, the fibrous web 91 is sub-
jected to continued increasing compression as it is moved
up to the compression zone 29 and suspension liquid continues
to be drained from it through the shell 13.
As the fibrous web 91 is moved through the washing
zone 27, washing liquid under pressure and at a constant
rate of flow is supplied through the perforations 30 in the
casing 19 and is forced into the web 91. The washiNg liquid
93 entering the web 91 displaces the suspension liquid 89
remaining therein to the level of a boundary layer defined
by the dot-dash line A -A' in Fig. 2, the displaced liquid
being drained through the shell 13. As the web 91 passes
from the washing zone 27 to the compressing zone 29, only
the web layer closest to the shell 13, i.e., below the line
A - A', contains suspension liquid, the web portion there-
above containing only washing liquid 93.
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As the web 91 continues along the treatment path
21 it is compressed in the nip 55 which forms the compress-
ing zone 29, preferably to a concentration of 30 - 70~ and
to a thickness less than the thickness of the web portion
which contains only washing liquid 93 at the location where
the web first enters the zone 29. The surface of the press
roll 47 being liquid impermeable, all liquid expressed from
the web 91 in passing through the nip 55 is drained through
the shell 13 of the drum 11. Consequently, all of the sus-
pension liquid 89 remaining in the fibrous web 91 is firstdrained through the shell 13 of the drum 11 and then part of
the washing liquid 93 is also drained through the shell 13
and is mixed with the suspension liquid 89 in the interior
of the drum 11.
Upon leaving the press nip 55 in the compression
zone 29, the web 91 is allowed to expand freely in the treat-
ment zone while treatment fluid 95 under pressure and at a
substantially constant rate of flow is forced through the
perforations 30 in the casing 19 into the web 91. Expansion
of the web 91 creates cavities therein which are filled by
the treatment fluid 95 so that penetration of the latter into
the fibrous web occurs very rapidly and effectively.
In the downstream part of the treatment zone 31,
taken in the direction of rotation of the drum 11, the
treatment fluid 95 in impregnating the web 91 displaces any
washing liquid therein. Such washing liquid will consist,
to a large extent, of washing liquid that has rewet the web
91 after the latter has passed through the nip 55. The
boundary layer between the treatment fluid impregnating the
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web and the washing liquid therein is represented by the
dot-dash line B - B' in Fig. 2. The displaced liquid drains
through the shell 13.
The displacement of the liquid in the web reaches
its culmination in the press nip 57, where the washing liq-
uid 93 is pressed out and part of the treatment liquid im-
pregnating the web penetrates the latter and drains into
the interior of the drum ll. Because of the high compression
of the web 91 in the press nip 55, the web itself serves as
a seal between the washing zone 27 and the treatment zone 31
preventing the flow of treatment fluid from the treatment
zone 31 into the washing zone 27.
The linear pressure in the press nip 57, the
speed of rotation of the drum 11, the flow of the washing
liquid 93 and the flow of the treatment liquid 95 are all
coordinated in a control system so as to secure the desired
displacements in the web of the washing liquid down to the
line A - A' and of the treatment fluid down to the line B -
B' (Fig. 2). Thus, with the gap width of the press nip 57
2~ and the supply of the fibrous suspension maintained substan-
tially constant by means known per se (not shown), the nip
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linear pressure responsive signals from the gauge means 59
are supplied to the controller 75. There they are compared
with a set point value to produce a correction signal re-
sponsive to any difference between the actual and set pointvalues. The correction signal is fed to the speed control
devices 17 for the drum 11 to change the speed of the latter
such that when the linear nip pressure decreases, the speed
of rotation of the drum is reduced.
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The controller 75 also produces, in dependence
upon the set point value for the rotational speed of the drum,
additional siynals for adjusting the servo-controlled valves
41 and 45 to correct the flow of washing liquid 93 to the
washing chamber 35 and the flow of treatment fluid 95 to the
chamber 37, respectively.
After the passage of the web 91 through the press
nip 57, the web is removed from the shell 13 by the doctor
blade 67 and is guided into the removal and feeding device
61. In the latter, the web is allowed to expand freely while
a gaseous medium, preferably water vapor, is supplied to the
device 61 under pressure through the supply conduit 71 In
expanding, cavities are formed in the web which are filled
with the gaseous medium, preventing the web from absorbing air.
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The method and apparatus of the invention enable
highly effective and eEficient washing of the web to be
achieved. For example, when pressing a wet fiber web of
ordinary pulp (e.g.~ softwood sulphate pulp) to a dryness
of at least about 30 percentage by weight, not only is the
bulk of the liquid existing between the fibers pressed out
but part of the liquid from the fiber lumen (the cavity inside
of the cell walls of a fiber) is also expressed. During the
subsequent relaxation, when the pressure is relieved, the
fiber lumen sucks in liquid from surronding free liquid
existing between the fibers. The liquid being sucked into
the fiber lumen will have a lower content of electrolytes
(e.g., substances dissolved out of the wood during the cook-
ing process which desirably are to be removed as far as pos-
sible by the washing process) than the average electrolyte
content in the surrounding liquid, since the cell walls act
as molecular screens (filters) or semi-permeable diaphragms.
Thus, the electrolyte content of the remaining free liquid,
i.e., liquid not contained in the fiber lumen, is increased.
A further pressing in a subsequent press nip, accordingly,
presses out the liquid having the increased electrolyte con-
tent. Such a washing process is of extremely great importance,
since the pressing sequence makes high wash efficiency pos-
sible.
If the expanding fiber web upon leaving the press
nip is treated with wash liquid such as water vapor, for
example, an increased effect is obtained and the subsequent
pressing becomes still more effective.
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Similarly, if the expandlng fiber web is treated
with a treatment fluid containing electrolytically undis-
sociated chemicals or other reagents, as in bleaching, for
example, an extremely good infiltration of -these chemicals
or reagents into the very vicinity of the fibers is obtained,
whereby the reaction times are greatly reduced.
The invention thus provides a highly effective
method and apparatus for the treatment of fibrous pulp. By
forming the pulp into a web of high fiber concentration and
impregnating the web with a reaction agent, both the reaction
time and the quantity of agent required can be substantially
xeduced.
Where in the following claims reference is made
to a fluid useful for the bleaching of fibrous pulp, this
includes in ad!dition to conventional bleaching agents such
as chlorine, chlorine dioxide, hydrogen peroxide, oxygen
and others, reaction agents and catalytic agents such as
caustic soda, ammonium hydrate, magnesium carbonate, alkali
metal borates and others, as well as combinations of such
agents.
The specific embodiments described above are
merely illustrative and are susceptible of modification in
form and detail within the scope of the following claims.
