Method for bleaching of cellulosic pulp

A first portion of bleaching liquid is passed through cellulosic pulp in a bleaching zone at a first level with respect to the pulp feed direction and withdrawn through transversely spaced strainer means located at about said first level so that the passage of the first portion of bleaching liquid through the pulp is in a direction transverse to the feed direction of the pulp and a second portion of bleaching liquid is passed through said cellulosic pulp at a second level with respect to the pulp feed direction and withdrawn through transversely spaced strainer means located at about the second level so that the passage of the second portion of bleaching liquid through the pulp is in a direction transverse to the feed direction of the pulp and opposite from the transverse direction through said first level.

In order to accelerate the bleaching of cellulosic pulp it has proved 
advantageous to bring the bleaching agent to move relatively to the pulp 
during the bleaching reaction proper. Thereby the reaction products are 
displaced and new amounts of bleaching agents reach the separate fibers of 
the pulp, thereby maintaining a high reaction rate. 
When such, so called "dynamic" bleaching is effected upon pulp which is fed 
continuously or stepwise in a certain direction and when the bleaching 
agent is forced through the pulp transversely to the direction of feed of 
the pulp, certain drawbacks appear which the present invention has for its 
object to eliminate. 
Said drawbacks are connected therewith that the bleaching agent is 
gradually spent or weakened during its transverse passage through the pulp 
flow involving that the side of the pulp flow where the bleaching agent is 
introduced therein will be treated by bleaching agent of a higher 
concentration than the opposite side of the pulp flow where liquid is 
withdrawn therefrom. Accordingly, the bleaching effect will be unevenly 
distributed over the cross-section of the pulp flow. 
According to the invention said drawback is counteracted by the bleaching 
agent being forced through the pulp flow first in a certain transverse 
direction and then in another transverse direction, preferably a direction 
opposite to the first-mentioned direction. In this manner, equalization 
will take place on account of the transverse flow of bleaching agent 
changing its direction and different parts of the pulp will successively 
become the spots where the supplied bleaching agent is of its highest 
concentration and effects its strongest action. A portion of the pulp from 
which during a certain stage a more or less inactive liquid is withdrawn, 
therefore during a later stage will be treated by bleaching agent just 
supplied and having a strong bleaching action. 
When the invention is applied to bleaching of pulp which in a cylindrical 
upright bleaching vessel forms a vertically progressing cylindrical 
column, the transverse currents of bleaching agent are directed 
essentially radially and the directions at different levels of the pulp 
column are made opposite to each other, so that the moving pulp column 
after having been subjected at a certain level to a transverse current of 
bleaching agent directed radially inwardly, will later and at another 
level be subjected to a transverse current of bleaching agent which is 
directed radially outwardly, or vice versa. 
The invention also relates to a device for the performance of the 
above-mentioned method in an upright cylindrical vessel, in which 
cellulosic pulp is fed continuously or stepwise in the axial direction and 
which is provided with outlet orifices for supply of bleaching agent and 
with strainers placed at approximately the same level as said orifices and 
serving for withdrawal of liquid displaced by the bleaching agent 
supplied. Said device is essentially characterized in that the outlet 
orifices and the pertaining strainers at the various levels are offset 
axially relatively to each other in such a manner that the vertically 
moving pulp column is penetrated in succession by essentially horizontal 
bleaching agent currents of mutually different directions. Further 
characterizing feature of the device preferably applied when performing 
the method in a known vessel with strainers, is stated in the following 
patent claims.

The shown bleaching vessel forms a vertically standing tower with a 
cylindrical shell 11. Cellulosic pulp is charged continuously or as small 
batches into the lower end (not shown) of the tower, so that the pulp is 
caused to move as a coherent column upwardly through the vessel with a 
speed which is as uniform as possible over the cross-section. Arranged at 
the upper end of the vessel is a discharging device which diverts the 
rising pulp out through a lateral pulp outlet 15. Said discharging device 
may consist of a scraper composed of obliquely set plates 17 carried by 
radial arms 19. Said arms are attached to a tube located centrally in the 
vessel and forming a hollow core 22 which is connected to a rotary 
vertical shaft 21. At its upper end outside the vessel said shaft is 
journalled in a bearing 23 and driven by a motor 25. Preferably the vessel 
is closed so that it can be held under over-pressure. 
Provided in the vessel is a strainer device composed of two sets of 
strainer bodies, viz. an upper set consisting of two annular strainer 
bodies 29, 31 concentric to each other and to the axis of the vessel, and 
a lower set comprising two annular strainer bodies 33, 35 which likewise 
are concentric to each other and to the axis of the vessel. The strainer 
bodies 29, 35 each consist of two substantially cylindrical strainer 
plates 41, 43 and 42, 44, respectively, of slightly different diameters 
inserted into each other and having their upper and lower edges united. A 
cylindrical partition 46 may be inserted in the strainer bodies between 
the strainer plates. The strainer plates 41, 43, 42, 44 are generally 
vertical everywhere and are provided with perforation apertures or 
vertically extending strainer slits distributed over essentially their 
entire face. The remaining strainer bodies 31, 35 located close to the 
central core 22 of the vessel and close to its shell 11, respectively, are 
designed essentially in the same manner as the strainer bodies 29, 35 but 
are provided with strainer apertures or slits merely on the cylindrical 
face 48 and 50, respectively, that is facing the pulp moving past the 
same. Liquid withdrawn from the pulp penetrates into cavities 47, 49 
within the strainer bodies. The strainer bodies of the two sets are 
rigidly connected to horizontally and radially extending carrying arms 54, 
56. The outer ends of said arms extend through slots in the vessel shell 
11. Said slots extend in the vertical direction sufficiently far for 
allowing a restricted reciprocating movement of the carrying arms in the 
axial direction of the vessel. Plates 51 attached to the carrying arms 
cover said slots in order to reduce the leakage therethrough. The carrying 
arms are provided with interior passages 53, 55 for conveying the liquid 
withdrawn by the strainer bodies to exterior discharge conduits. The upper 
passages 53 of the carrying arms communicate with the cavities 47 of the 
strainer bodies 29, 31 of the upper set, and the lower passages 55 
communicate with the cavities 49 of the strainer bodies 33, 35 of the 
lower set. The strainer bodies are moved vertically in one and/or the 
other direction by an operating device located outside the vessel and 
arranged to act upon the outwardly extending ends of the carrying arms 54, 
56. Thus, for each carrying arm there is attached to the outside of the 
vessel a hydraulic or pneumatic cylinder 60 having a vertical piston 
connected to an attachment 64 upon the end of the carrying arm. 
The two sets of strainer bodies have different diameter measurements of 
their strainer bodies. Thus the strainer body 33 is of greater diameter 
than the strainer body 29, and the strainer body 35 is of greater diameter 
than the strainer body 31. Further the strainer body 35 is of a diameter 
which is intermediate and preferably differs equally much from the 
diameter measurements of the strainer bodies 29, 31 and the strainer body 
29 is of a diameter which is intermediate and preferably differs equally 
much from the diameter measurements of the strainer bodies 33 and 35. If 
the number of strainer bodies of each set is increased to three or more, 
the same applies to the added strainer bodies, viz. no strainer body of 
one set shall be of the same diameter as any strainer body of the other 
set. If the strainer bodies are considered in the order of increasing 
diameters, every second will be found in the upper set and every second 
inbetween in the lower set. The diameter measurements form a series the 
members of which show successive, preferably equally great increments, 
i.e. the strainer bodies of one set should be of diameters that differ 
equally much from the diameter measurements of the next greater and the 
next smaller strainer body of the other set. This applies to all strainer 
bodies except the greatest 33 which is situated close up to the shell of 
the vessel, and except the smallest 31 which is situated close up to the 
central core 22 of the vessel. In each strainer set the strainer bodies 
are located at the same level, but the two sets are mutually offset in the 
axial direction. Thus, the upper edges of the strainer bodies 33, 35 of 
the lower set are located in one or the same horizontal plane, and said 
horizontal plane preferably is situated somewhat lower than the horizontal 
plane extending through the lower edges of the strainer bodies of the 
upper set. 
The device according to the invention also comprises a system of bleaching 
agent supply conduits provided with bleaching agent outlets located at the 
same levels as the strainer body sets. To each set of strainer bodies 
there belongs a group of outlets provided in a number equal to the number 
of strainer faces of said set of strainer bodies, and said outlets are 
located within the range of levels covered by the corresponding strainer 
faces, preferably in the lower part of said range. Thus, to the upper set 
of strainer bodies there belong outlet orifices 61, 63 placed upon the 
lower ends of vertical tubes 65, 67 attached to the scraper arms 19 and 
extending vertically therefrom, said tubes being connected to supply 
conduits 69, 71 respectively. When the scraper arms are turned, said 
outlet orifices move in circular paths concentric to the strainer faces 
and to the central core 22. The tube 65 is situated straight above the 
outermost strainer body 33 of the lower set of strainer bodies and close 
to the vessel shell 11, and its outlet orifice 61 describes a circular 
path concentric to the strainer bodies and of approximately the same 
radius as the strainer body 33. The tube 67 with outlet orifice 63 is 
located straight above the strainer body 35 and moves in a path of the 
same radius as said strainer body. Preferably the outlet orifices are 
extended in the vertical direction so that the liquid ejected therefrom is 
spread as a cylindrical film. However, the extension in the height 
direction of said film is usually less than that of the strainer bodies. 
From said cylindrical distributing face the liquid flows in essentially 
horizontal and radial paths through the pulp towards the opposite strainer 
face, thereby displacing the liquid previously forming part of the pulp 
and expelling a corresponding amount of liquid through the respective 
strainer face. Thus, from the immaterial cylindrical face to which 
bleaching liquid is supplied by the outlet orifice 61, a flow which is 
evenly distributed over the circumference runs radially inwards towards 
the strainer face 41, and through said strainer face there departs liquid 
containing spent or residual bleaching agent and reaction products. 
Usually the amounts of supplied liquid and withdrawn liquid correspond to 
each other, but a certain controlled difference between said amounts may 
be maintained whereby the pulp is thickened or diluted during its passage 
upwardly past said strainer face. The bleaching agent ejected from the 
outlet orifice 63 is likewise spread over an imaginary cylindrical surface 
in the pulp. Said amount of bleaching agent is divided and flows in part 
radially inwards towards the strainer face of the strainer body 31 and in 
part radially outwards towards the strainer face 43 of the strainer body 
29. 
To the lower set of strainer bodies there belong outlet orifices 73, 75 
which are connected to conduits 77 and 79 for the supply of bleaching 
agent, which conduits extend within the core 22. The outlet orifice 73 is 
provided on the upper end of a vertical tube 81 situated straight below 
the strainer body 41 and connected to the rotary core 22 by means of a 
tube 83 extending horizontally and radially below the lower set of 
strainer bodies. Thus, from the outlet 73 bleaching agent is spread over a 
cylindrical face of the same radius as the strainer body 29. The outlet 75 
is situated close to the core 22 and when the core is turned, said outlet 
distributes bleaching agent over a cylindrical face of the same radius as 
the strainer body 31. 
By the strainer bodies the pulp which flows axially upwards in the vessel 
as a coherent column, is divided up into three concentric part flows each 
having the cross-section of an annulus. The innermost of said part pulp 
flows which passes between the core 22 and the strainer body 35 is treated 
first by the bleaching liquid which from the outlet 75 is forced 
transversely through the pulp column towards the strainer face 44 of the 
strainer body 35, through which face a corresponding amount of displaced 
liquid is drawn out of the pulp. The concentration of the bleaching agent 
which is highest at the outlet 75, decreases according as the bleaching 
agent penetrates farther out and is used up by the reaction. Accordingly, 
the bleachng reaction will be unevenly distributed over the cross-section 
of the pulp and will be considerably weaker at the outer portions of the 
part pulp flow under consideration. According to the invention said 
inconvenience is compensated thereby that said pulp flow, when it reaches 
the level of the upper set of strainer bodies, is penetrated by a 
transverse current of bleaching liquid which emanates from the outlet 
orifice 63 and thus will be most effective upon the radially outer portion 
of said part flow of pulp. 
In a similar manner the part flow of pulp which passes between the strainer 
bodies 35 and 29 will be treated first with bleaching liquid which from 
the outlet 73 is forced radially inwards towards the strainer face 42, and 
at a later stage with bleaching agent which from the outlet 63 is forced 
radially outwards towards the strainer face 43. Finally, the outermost 
part flow of pulp which passes between the strainer body 29 and the shell 
11 will be treated first with bleaching liquid which from the outlet 73 is 
forced radially outwards towards the strainer face 50 and then with 
bleaching liquid which from the outlet 61 is driven radially inwards 
towards the strainer face 41. 
The used bleaching agent which may be of any kind and e.g. consists of a 
water solution of chlorine, chlorine dioxide or hypochlorite, is supplied 
through a conduit 85 and is distributed by means of valves 87 in proper 
proportions upon conduits which are connected over a stuffing box 89 each 
with a separate one of the rotary conduits 69, 71, 77, 79 in the core 22. 
In the shown embodiment the bleached pulp is discharged through the outlet 
15 together with bleaching reaction products and possibly residual 
bleaching agent. However, it may be advantageous to perform a washing 
operation of the pulp already in the vessel 11, and to this end there may 
be interposed between the uppermost set of strainer bodies 29, 31 and the 
scrapers 17 a washing device likewise consisting of concentric strainer 
rings and nozzles for spreading water or other wash liquid, said nozzles 
being moveable between said strainer rings, e.g. a wash device of the kind 
shown in the Swedish Pat. No. 225,814. 
On account of the bleaching liquid moving relatively to the fibers of the 
pulp, the required time of the bleachng operation will be reduced and a 
desired bleaching stage will be completed during a comparatively short 
axial movement of the pulp through the vessel. Due thereto it is possible 
with bleaching towers of conventional heights to perform several 
successive bleaching stages in one and the same tower. Then one or more 
further double sets of strainer bodies and liquid spreaders of the same 
design as above described and shown in the drawing are arranged 
therebelow. If desired, the bleaching stages or one or more of them may be 
followed by a washing zone comprising concentric strainer bodies. The 
operating device 60 may be common to all strainer bodies of said bleaching 
and washing stages. However, in certain cases the device may function 
satisfactorily even if the strainer bodies remain stationary. The latter 
applies particularly if the pulp is fed stepwise and the pressure 
difference over the strainer faces is shut off during the advancement of 
the pulp. 
Of course, the above described embodiment may be modified as to its details 
and the invention may be applied also to bleaching plants of a completely 
different design.