Note: Descriptions are shown in the official language in which they were submitted.
267
-- 1 --
This application is a divisional application of co-
pending Canadian Patent Application Serial Number 254,340 filed
June 8,1976.
The subject of the`present invention is a continuous
pulp-cooking method and a digester the purpose of which is, with
a high yield, within the range of yields of 55 to 80 per cent of
wood, to produce pulp with good paper-technical properties, e.g.
tear 2.5 m2 brightness 60 to 70 SCAN, the yield being 65 per cent
of wood, the pulp moreover having a sufficiently low content of
fibre bunches and screening residue in order that the running
quality of the pulp both in the paper machine and in particular
in the printing machine should be sufficiently good so that it
could replace the sulphate pulp, which has been used so far
primarily, as a component of groundwood newsprint or printing
paper, either fully or in part.
The methods so far known have usually been batch cooking
methods, in which one of the greatest drawbacks has been the in-
sufficiently precise control of the variables in this in itself
difficult process. The quality of the pulp to be manufactured
must be sufficiently uniform and good in order that undisturbed
functioning of the modern high-speed paper and printing machines
should be possible. Moreover, the heat consumption in batch cook-
ing is less advantageous than in continuous cooking. On the other
hand, in the continuous digesters so far constructed, a drawback
has consisted in uneven impregnation, which is seen in the quantity
of the screening fraction and in the paper-technical properties
of the pulp. Said uneven, or more appropriately, insufficient
impregnation has made it uneconomical to use, for example, chips
mainly consisting of pine as the ~awmaterial.
In the continuous processes so far known, another draw-
back has consisted in the uncertain feeding of the chips into the
~ `
~.
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267
- 2 -
cooking process especially when the pH of the cooking liquor be-
comes so low that free SO2 has been separated from the cooking
liquor, for example, in the case of sodium base at the level of
pH = 3.5 to 4.0, which would, on the other hand, be otherwise
suitable for the process. Moreover, in the continuous digesters
known at present, it is impossible to restore the secondary
- vapour expanded from the waste liquor separated from the digester
back to the preheating of the chips, because, owing to insuffi-
cient impregnation, it has been necessary to keep the concentra-
tion of the cooking liquor high right to the end of the cooking.
Thus, the expansion vapours have, among other things, contained
free SO2, which, as passed to the chips before impregnation, as
is known, causes deterioration in the brightness of the pulp.
This has meant an unnecessarily poor heat economy in
the prccess, on one hand because the expansion vapours haYe been
used for secondary purpos~s and, on the other hand~ as an un-
necessarily large quantity of chemicals in the recovery cycle
of chemicals.
A further drawback in the present digesters has been
the insufficient degree of fibration of the pulp after the
digester, which is seen as an unnecessarily high power consump-
- tion of the further fibration, on one hand, and in the strength
qualities of the pulp, on the other hand. In other words, the
best obtainable properties have not been obtained from the raw
material.
The present invention attempts to eliminate the draw-
backs listed above.
The invention is mainly characterized by what is
stated in claim 1 as well as in the sub-claims.
An embodiment of the invention will be described in the
following specification with reference to the attached figures
267
-- 3 --
1 and 2.
Figure 1 shows a digester in accordance with the
invention as a schematical presentation. According to an
embodiment, the chips are blown pneumatically through the pipe
41 into a chips cyclone 11. From the cyclone, the chips fall
into a chips hopper 12, and further onto a measuring device 13,
which, like in figure 1, may be a twin-screw feeder and has the
duty to maintain the chips flow as constant. From the twin-
screw feeder the chips fall onto a pocket-type feeder 14, which
has the function of constituting a pressure lock between the
pre-steaming vessel lS and the pressure of the open air. From
the low-pressure feeder the chips fall into said pre-steaming
~ vessel 15, which is provided with a conveyor screw for the
- purpose of feeding the chips further in the direction of the
arrow a to the high-pressure feeder 16 and from there further
through a bellows equalizer 17 and a pipe 18 to the impregnation
device 19. In the embodiment shown this device is a cylindrical,
diagonally positioned vessel 19, which is provided with a
conveyor screw 61 so as to move the chips in the direction of
the arrow d. The height of the impregnation solution in the
impregnation vessel is, for example, in agreement with the levels
b and c, of which b assumes the level of the exhaust opening
of the impregnation vessel and the position of c is determined
by the difference in pressures between the vapour spaces ~ and B.
From the impregnation vessel 19 the chips as well as
a quantity of impregnation solution that corresponds the quantity
of solution that has been passed to the impregnation vessel
falls through the bellows equalizer 20 of the digester and
through the normally open closing valve 21 into the digester.
In the digester, the chips constitute a chips column whose
upper surface is denoted by the broken line e. The liquid
",f~:, `Z
.. ...
\
267
assumes such a position that the upper surface constitutes the
level f, The level of the chips surface is measured by a
radioactiye measuring device 60, and according to a reading
given by this device, the quantity of pulp to be removed from
the digester is controlled so that the level of the surface
e remains constant. The liquid leyel f,which can also be lower
than the level shown, e.g. f'(shown as broken line) is made
constant by controlling the exhaust flow through the pipe 52.
Of course, the quantity of chips and/or cooking liquor fed into
the digester 22 can also be adjusted. The pulp is removed
through the bottom of the digester either through the pipe 42
or through the pipe 44. In order to facilitate the removal of
the pulp, a rotary bottom plough is arranged at the bottom of
the digester, the arms 24 of which plough move the pulp to the
- 15 middle of the digester and from there further-into the blow line
42 or 44. Into the blow line 42 or 44, respectively, a thickener
27 or 28, respectively, is arranged which has the function of
removing liquid out of the pulp and thereby to increase the
consistency. As to its construction, the thickener may be, for
example, a tubular or cylindrical object 27, 28. Inside this
outer mantle is placed another tube, i.e. inner mantle, made of
screen plate and placed at a certain distance from the inner sur- -
face of the outer mantle, into which inner mantle the pulp is
passed from the blow line 42, 44 and from there further into the
defibrator 29, 30. As the screen ~be and the thickener are
otherwise closed at their ends, the liquid that has passed
through the slits in the walls of the screen tube can be removed
through pipe connections 62, 63. Said inner mantle may be ad-
vantageously made of slit-type screen plate whose lengthy slits
are placed in the direction of movement of the pulp.
In the thickener, the space between the outer
. ..
267
and the inner mantle may be di~ided into two parts or chambers,
for example, by means of a partition wall parallel to the radius
of the tube 28, 29. Then, by means of timer etc. equipment(not
shown in the drawing), it is possible to take the escaping
liquid at certain periods of time only from one of the chambers,
for example, in accordance with the drawing, from the left
chamber and through the left pipe connection 63. The right
chamber is then at the rest stage and the slits of the inner
mantle therein can become clean. At the next stage, on the
other hand, the left chamber is at the rest stage and liquid is
taken from the right chamber and from the right pipe connection
62, 63. In this way the stages of work and rest can alternate
further. Moreover, a defibrator 29 or 30, respectively, is
arranged into the blow line,throuqh which defibrator the pulp is passed
lS as defibrated into the blow line 43 or 45, respectively, and
further into the blow tank (not shown in the drawing). As the
pulp flows in the blow line continuously, then the defibrator
can be considered as, from the point of view of the processing
of the pulp, being practically immediately after the digester,
in spite of the rather short blow line 42 or 44, respectively,
because there is no blow tank between the digester and the
defibrator Then, the defibrator can be, for example, of the
so-called disc-refiner type, which is provided with means for
- adjusting the distance of the discs/or the disc clearance.
In accordance with figure 2, the impregnation tank
may also be provided with a screw 31 for the purpose of sinking
the chips in the impregnation solution if the chips are so dry
that they do not sink otherwise by their own weight.
When the blow line 42, 43, the thickener 27, and
the defibrator 27 are in use, the blow lines 44, 45, the thick-
ener 28, and the defibrator 30 may be out of use and they may
267
- 6 -
be taken into use, for example, if a disturbance of operation
occurs in the blow lines 42,43, in the thickener 27 and/or in
the defibrator 29 and maintained in operation until the dis-
turbance has been eliminated. In this way, the operation of
equipment in pairs can be alternated if required, or they can
` also be used at the same time.
In figure l the cooking liquor, which is in the case
of the present invention an aqueous solution of some soluble
base (ammonium, sodium or magnesium) and sulphurdioxide, is
passed from the recovery and chemical preparation or regenera-
- tion plant (not shown in the drawing~through the pipe 4~ into the
cooking liquor tank 32, which may also be a pressurized tank,
depending on the properties of the cooking liquor at the
temperature concerned. From the tank 32 the cooking liquor is
lS passed by means of a pump (not shown in the drawing) through
pipe 47, heat exchanger 33, and further through pipe 48 into the
impregnation tank 19, below the surface c. The cooking liquor
is hereby heated up to the impregnation temperature, suitably
90C to 130C and preferably 100C to 120C, immediately or
practically immediately before it enters the impregnation space,
while the solution flows continuously through the heat exchanger
33 and from there through the pipe 48 into the vessel l9. In
this case, the cooking liquor does not have to stay a longer
time as hot in the pipe system or in some intermediate tank,
which would have harmful effects on the composition of the cook-
ing liquor, which, when hot, easily tends to be changed becauseof chemical reactions or corresponding phenomena then taking
place in the liquor.
The used cooking liquor, the so-called waste liquor,
is removed from the digester through screen devices 23 provided
with screen surfaces into the pipe 52 and further into the
~ .
P267
-- 7 --
expansion tank 34 where it is allowed to expand. The produced
vapour flows through the pipes 53 and 54 or 53 and 55 into the~ -
pre-steaming vessel 15, where it is used for heating the chips. ;
Of the pipe 55, the portion at the feeding device 14 has not
been shown in the drawing in order to clarify the drawing.
The separated waste liquor can, if required, be passed
through the pipe 56 to the next expansion tank 35, from which ~--
the expanded vapour flows through the pipes 57 and 57a, for
example, to the heating of the washing water (not shown in the
drawing). Alternatively, the vapour can also be passed to the
heating of the chips through the pipes 57 and 57b. The separated
waste liquor is passed through the pipe 58 to the evaporation
plant (not shown in the drawing) for further processing.
The vapours necessary for maintaining the pressures
and temperatures in the digester can be passed into the equip-
ment through the points indicated by the arrows 49, 50, and 51through pipes and valves` (all of which have not been shown in
the drawing). In order to cool the pulp down to an appropriate
blow and defibrating temperature, coo~ed waste liquor is passed
to the bottom of the digester from the washing plant (not shown
in the drawing) through the pipe 59, which is connected to~the
arms of the bottom plough 24. From nozzles placed in these
arms, the liqour is spread to the bottom part of the digester.`~
The equipment and components denoted above and below by refer-
ence numerals can be in themselves previously known equipment
and components in use in pulp industry. Therefore, all ofthem are not described more in detail here. ~
In addition to what was described above, a great number
of closing and controlling valves, controlling equipment, meters,
and other auxiliary equipment and pipe systems, pumps, etc. are``
required in the system, which are, however, not essential from
267
the point o~ view of the invention and which are therefore not
described in the present specification.
By means of the method and equipment in accordance
with the invention, pulp has been produced on the industrial
scale, which pulp is mainly used as the chemical pulp for news-
print and printing papers, in which use it replaces the sulphate
pulp, which has been used until now. Among the results obtained,
the following should be mentioned by way of example:
rawmaterial: factory chips, 60 % pine, 40 % spruce
. 10
cooking liquor: SO2-concentration 4 %, pH 4.3
Pulp: Kappa-number 95
yield 66 ~
brightness 61.5 SCAN
extract 0.4 dichloromethane
tear 2.3 m2
degree of grinding 26 / SR
When the pulp meant above is used as an admixture of
paper instead of sulphate pulp, it has been noticed that the
paper machines and the printing machines function normally, and -
no disturbances have been noticed. This involves a remarkable
saving in the c~nsumption of wood when, for example, a sulphate
pulp produced with a yield of 45 per cent and a bisulphite pulp
with corresponding properties and produc~d with a yield of 65
per cent are compared with each other.
The good properties of the pulp mentioned above result,
among other things, from the following factors. From the high-
pressure pre-steaming taki~g place before impregnation, at which
pre-steaming the air is removed from the chips as completely as
3 possible, from resulting proper impregnation before cooking,
which guarantees a low content of reject, from sufficiently
uniform cooking circumstances, as well as from proper defibrating
67
g
that takes place under correct circumstances.
When the defibrator is placed immediately or practi-
cally immediately after the digester, the pulp can advantage-
ously be fed into the defibrator by means of the pressure of
the digester. Then the pulp comes immediately after the cook-
ing in a more favourable form to the defibrator which requires
less energy at the defibrating stage. By thickening the pulp
into an appropriate consistency, it is possible to keep the
disc clearance of the defibrator higher than otherwise. Thls
also results in the fact that a larger part of the fibres
become defibrated when they are just ground against each other.
Then the disc of the defibrator meets fewer fibres from the
pulp that has passed through the defibrator and cannot damage
them.
The properties of the pulp, for example its brightness,
are also improved by the fact that the sulphurdioxide that is
gasified from the cooking zone cannot enter through the
blocking liquid in the impregnation zone 19 and come into
contact with unimpregnated chips.
- 20 The decomposition of the cooking liquor is promoted
by organic substances brought into same, which deteriorates the
properties of the pulp to be cooked. Consequently, it has
usually not been considered as desirable to re-circulate any
waste products obtained from cooking back to the cooking. The
waste vapour obtained in accordance with the present invention
is, however,to such an extent pure, especially rather free from
detrimental sulphurdioxide, that it can be re-circulated back
to the process. This means a considerable economy in the heat-
ing expenses. Out of the above reasons, according-to the inven-
tion, however, only vapour is re-circulated. The waste liquor
etc. products are hereby not re-circulated.
'~
.
ilai~267
-- 10 --
0f course, the example given above is only one
embodiment of the invention, which can be Yaried extensively
within the scope of the patent claims. For example, one is not
restricted to the indicated pH range of 3.5 to 4 alone, but the
pH may also be lower.
The equipment putting the invention into effect can
also be of a different construction, for example, the feeder 13
-may be of the pocket type instead of twin-screw, the feeders 14
and 16 of the pocket type may be plug-screws instead of the
pocket feeder, the screen device 23 may be a mere screen plate
that is arranged in the wall of the digester instead of a
self-cleaning screen, etc.
The invention described in the above specification
can be used either as such or as applied in one or some of the
following methods 1, 2, 3, 4, and 5, in their processes, in
equipment or sets of equipment connected with them, and~or in
connection with one or some of them.
Method 1
A continuous cooking method, i.e. flow cooking, for
pulp, in which method pulp-containing fibrous material is contin-
uously fed into a digester under elevated pressure and in which
between the digester itself 22 and the block feeder 16, a zone
appropriately an impregnation zone 19, is arranged which zone
contains blocking solution that prevents the sulphurdioxide
gasified from the cooking zone 22 from entering onto the feeder
- 14 of the digester and from thereby coming into contact with
unimpregnated chips.
In this method the blocking solution may consist or
the cooking liquor, which is passed to the blocking zone of the
llC~f~267
digester, appropriately to the impregnation zone, at a tempera-
ture lower than the cooking temperature, appropriately at 90C
to 130C and preferably to 100 to 120C. The blocking solution
may at the same time also function as the impregnation solution,
and its chemical content is then adjusted so that the quantity
of chemicals absorbed into the pulp-containing fibrous material
is suitable for performing the cooking in liquid or vapour phase.
The difference in the levels of the surfaces b, c - -
of the blocking solution can be controlled to the desired value
either by means of the screw feeder 61 feeding the pulp-contain-
ing fibrous material, such as chips, or on the basis of the
difference in pressure between the vapour space of the digester
22 and the vapour space 18 before the blocking-liquid zone.
In order to put the method into effect, it is
possible to construct a continuous digester 22 with related equip-
ment, which comprises a vapour space 15 under elevated.pressure
and mainly containing water vapour for the purpose of passing
the pulp-containing fibrous material into same and for passing
the material from there further into the space 19 provided with
2~ a screw feeder and partly filled with blocking liquid and from
this space further into an appropriately vertical digester 22,
whereby said blocking liquid separates the.vapour spaces before
and after the digester from each other.
In this case, the space 19 partly filled with block-
ing liquid may consist of an oblong vessel that is positioneddiagonally and provided with a conveyor screw 61 and that may be
connected to a space A projecting from it for the purpose of
guiding the pulp-containing material, and this space may possibly
involve a separate conveyor screw 31 for sinking the pulp-
containing material into the blocking or impregnation liquid,whereby the latter conveyor screw 31 is positioned perpendi-
:
.'r267
- 12 -
:
cularly or substantially perpendicularly to the former conveyor
screw 61
Method 2
A continuous cooking method for pulp, in which method
chips of any conifer tree of mixture or conifer trees are dissol-
ved in a bisulphite cooking liquor for the manufacturing of pulp
with high yield and high quality of paper-technical properties
and in which method the chips are passed into an impregnation
solution under the pressure of the digester, which solution is
appropriately said bisulphite cooking liquor, through a vapour
space 15 and/or 18 under the pressure of the digester, in which
vapour space the chips remain from a few seconds up to 3 minutes,
however, appropriately no more than 2 minutes, and whereby the
chips can be continuously passed from the impregnation solution
further into the digester 22.
Hereby, the vapour in the space 15 and/or 18 ahead
of the impregnation solution may be water vapour o~r maximum
degree of saturation. The chips can be fed from the vapour
space 18 into the impregnation solution by their own weight or
by means of a screw 62.
In order to perform the method, it is possible to
construct a continuous digester 22 with related equipment, where-
by said vapour space and impregnation space are connected to a
substantially vertical tube 18 and to a cylindrical space 19
positioned diagonally and placed after said tube, a conveyor
screw 61 being placed inside said cylindrical space. In connec-
tion with said diagonal conveyor or so-called impregnation screw
61, a screw 31 directed substantially perpendicularly to the axis
of said former screw 61 may be arranged for the purpose of
feeding the chips into the impregnation solution. Into the
2~;7
- 13 -
yapour space 15 provided ahead of the impregnation space and
being substantially under the pressure of the digester, the
chips can be fed by means of an, in itself known, block feeder
13 of the pocket type or by means of an, in itself known, plug
conveyor screw 14.
.
Method 3
A continuous sulphite or bisulphite cooking method
for pulp, in which the cooking liquor is an aqueous solution of
one or some so-called soluble bases, such as ammonium, sodium,
and magnesium, and of sulphurdioxide and in which, before the
cooking proper, impregnation is performed substantially under
the pressure of the digester and at a temperature lower than
the cooking temperature, and in which the pulp-containing mat-
erial is any conifer tree or any mixture of conifer trees, withthe purpose of manufacturing pulp with a high yield and with
high-quality paper-technical properties, and whereby the
cooking liquor is heated to the impregnation temperature immedi-
ately before it is passed into the impregnation space. The
cooking liquor used as the impregnation solution can be passed
further, together with the chips, into the digester 22, and
when falling into the digester, it is heated by means of direct
vapour to the cooking temperature.
In order to perform the method, it is possible to
construct a continuous digester 22 with related equipment, in
which the chips are fed into the impregnation space 19 of the
continuous digester 22 through a vapour space 18 substantially
under the pressure of the digester and in which the chips are
passed into the impregnation space and from there further into
the digester 22, and whereby a heat-exchanger 33 is placed in -
the feeding pipe of the cooking liquor immediately or practically
267
- 14 -
in~mediately before the impregnation space. The impregnation
space may also be arranged so that a quantity of cooking liquor
corresponding the quantity of cooking liquor passed into the
impregnation space 19 is passed further, together with the chips
into the vapour space B placed at the top portion of the
digester 22, where the temperature of the cooking liquor is
raised to the cooking temperature by means of direct vap~ur.
~rom the impregnation space 19 the impregnation solution may
flow into the digester 22 as overflow.
Method 4
A continuous sulphite or bisulphite cooking method
for pulp, in which the cooking liquor is an aqueous solution of
one or some so-called soluble bases, such as ammonium, sodium,
and magnesium, and of sulphurdioxide and in which the fibrous
rawmaterial is a mixture of one or some conifer trees, with
the purpose of manufacturing pulp with a high yield and with
high-quality paper-technical properties, and in which the pulp
is removed from the digester through a defibrating stage, where-
by the defibrating takes place primarily under the pressure of
20 the digester and within the temperature range of 90 to 120 C and
fully or mainly without washing. The defibrating can then take
place at each occurring consistency while using a disc clearance
of the defibrator 29, 30 in which the degree of grinding of
the pulp is not changed essentially. The consistency of the
pulp during defibrating can also be maintained as high as
possible, for example by thickening the pulp.
In order to perform the method, it is possible to
construct a continuous digester 22 with related equipment, where-
by the defibrator 29,30 is placed in the pulp conduit leaving
the digester, i.e. in the so-called blow line 42,44, immediately
.lZ67
- 15 -
or practically immediately after the digester.
The defibrator 29,30 may be a defibrator of the so-
called disc refiner type, which is provided with means for
adjusting the disc clearance.
A thickener 27, 28 may be placed between the bottom
of the digester 22 and the defibrator 29, 30 for the purpose of
thickening the pulp to the desired consistency. As to its
construction, the thickener 27, 28 may be a tube, inside which
an inner mantle is placed, which inner mantle is made of screen
plate and in which tube necessary pipe connections 62, 63 are
arranged for draining the isolated liquid.
The inner mantle may be manufactured of slit-type
screen plate whose lengthy slits are positioned in the direction
of movement of the pulp.
Method 5
A continuous sulphite or bisulphite cooking method
for pulp, in which the cooking liquor is an aqueous solution of
one or some so-called soluble bases, such as ammonium, sodium,
and magnesium, and of sulphurdioxide and in which the fibrous
rawmaterial consists of a conifer tree or of a mixture of
conifer trees, with the purpose of manufacturing pulp with a
high yield and with high-quality paper-technical properties,
and in which the only substance that is re-circulated back to
the cooking is the low-pressure secondary vapour expanded or
isolated from the waste liquor separated from the digester,
the SO2-content of which vapour has been made so low, by means
of impregnation that has taken place before cooking, substan-
tially at the pressure of the digester but at a temperature
lower than the cooking temperature and by means of possible pre-
steaming at a lower pressure, so that such a vapour can be fed
3267
- 16
into the chips in order to steam and/or heat them, and, besides
that or instead of that, the vapour is also used for other
purposes at each particular time concerned in connection with
the pulp manufacturing.
Then, said expansion vapour can be used for steaming
the chips to be fed into the digester.
The waste liqour separated from the digester can be
expanded at two stages and the produced secondary vapour be
used for steaming the chips to be fed into the digester and~or
for heating the washing water to be used at the washing plant.
The secondary Yapour under eleYated pressure, obtained
from the first expansion, can be used for steaming the chips and
the secondary vapour obtained from the second expansion can be
used for heating the chips before said steaming. The waste
liquor to be separated from the digester can be separated at
two or more stages and the vapours o different pressures
obtained in this way can be used for steaming the chips and/or -
for heating the chips and/or for heating the pulp washing water and
or for heating the waste liquor to be passed to the evaporation
plant.
In order to perform the method, it is possible to
construct a continuous digester 22 with related equipment, the
bottom part of which digester is provided with liquid-separators
23,appropriately with a screen zone or with screen faces ~ --
separate from each other, whereby one or more expansion cyclones
25 34, 35 are placed in the exhaust pipe 52 of the screen zone for
the purpose of expanding the liquid ullder elevated pressure
and whereby the cyclone or cyclones, respectively, are provided
with pipe connections 53, 57, 58 for passing the vapour liberat-
ed and the liquid separated in the expansion further to the
desired ohject.
If the number o expansion cyclones 34,35 is higher
;a67
- 17 -
than or equal to two, they can be connected in series so that
the liquid separated from the first cyclone 34 is passed to the
second cyclone 35 for expansion and the liquid separated from
the second cyclone is passed to the third cyclone for expansion,
etc., whereby the separated secondary vapours are passed to
separate objects of use of their own. Both for heating the
washing water of pulp and for heating the liquid for the evapor-
ation plant or for the heating of either one alone (by means of
separated vapour) it is possible to use an indirect heat
exchanger.
Re-circulating of the low-pressure vapour expanded
from the waste liquor separated from the digester back to a
suitable stage of the cooking, such as for steaming and~or heat-
ing the chips to be fed, is, besides from the viewpoint of heat
control, even otherwise advantageous in view of correct balancing
of the circumstances of the cooking according to the present
application.
