Method of washing a thin mat of cellulosic pulp

A method of washing cellulosic pulp to remove dissolved solids as well as dirt, inorganic fillers and fiber debris. A mat of cellulosic pulp having a weight of 3 to 100 lbs. of dry fiber per 1000 sq.ft. is positioned between a pair of porous endless belts to provide a composite structure. The composite structure is moved in a generally vertical path of travel through a vessel containing a plurality of superimposed rolls and the composite structure is passed over the rolls in a manner in which a first of the belts is in direct contact with one roll and the second of the belts is in direct contact with the next adjacent roll. A stream of water is directed against the composite structure as it travels in the upper portion of the vessel, and the water is cascaded downwardly through the vessel and directed into contact with opposite sides of the structure to thereby remove dissolved solids, as well as inorganic fillers. The tension of the belts as they pass over the rolls creates a dewatering action. After passing through the vessel, the washed pulp mat is removed from the belts, and the washing water is discharged through an outlet in the vessel and can be used in other portions of the papermaking process.

BACKGROUND OF THE INVENTION 
A challenge in developing a papermaking mill using recycled fibers, such as 
used corrugated containers wastepaper, and the like, is minimizing the 
environmental impact resulting from water and solid waste disposal. 
In a typical virgin pulp papermaking process, the pulp, after pulping, is 
subjected to a series of cleaning operations to remove debris and larger 
particulate material, and the pulp is then subjected to a washing process 
to remove the dissolved solids generated by the chemicals used in the 
pulping process. In the past, virgin pulp has been washed using either a 
drum washing process or a belt washing process. In a drum washing process 
a mat or layer of pulp is applied to the outer surface of a porous drum 
and is showered with water. A vacuum drawn on the inside of the drum draws 
the wash water through the mat. Generally three to five stages of washing 
are utilized with a conventional drum washing operation. 
In a belt washing process, a mat of virgin pulp is fed onto an endless 
porous belt or fabric, and wash water is sprayed onto the mat. The mat is 
dewatered by vacuum boxes located beneath the fabric. In a typical 
operation, five stages of washing may be employed in order to remove the 
dissolved solids from the virgin pulp. After washing, virgin pulp still 
maintains a significant quantity of dissolved solids, and because of the 
nature of washing utilizing very thick pulp mats, dirt, and fiber debris 
are also present in conventionally washed virgin pulp. 
Recycled pulp, composed of used corrugated containers, wastepaper, white 
office waste, and the like, cannot successfully be washed in the same 
manner as virgin pulp, because of the presence of inorganic fillers, such 
as clay and calcium carbonate, in the recycled pulp. In the conventional 
drum and belt washing, as described above, the pulp mats are very thick 
and it is difficult to remove small particles of the fillers, through a 
thick mat since the pulp acts like a filter and entraps the filler 
particles. Consequently pulp washing of recycled fiber has been done with 
a side hill screen, or with a unit known as a "Variosplit". With the use 
of a side hill screen, the pulp is diluted with a tremendous quantity of 
water and is flowed over the screen. While the side hill screen will act 
to remove dissolved solids as well as the fillers, unfortunately a 
substantial quantity of small fibers are carried through in the liquid 
stream, and to remove the inorganic fillers and small fibers from the 
liquid stream, a large floatation clarifier is normally utilized. In the 
clarifier, air and a coagulative polymer are added to the liquid, 
resulting in a sludge that floats to the top of the vessel and typically 
contains from 60% to 80% of potentially usable fibers, with the remainder 
being short fiber and filler. This sludge is further dewatered and 
traditionally has been landfilled. Thus, the present processes for washing 
recycled pulp not only utilize a tremendous quantity of water, but result 
in a substantial loss of good fibrous material. Additionally, washing of 
recycled pulp is very inefficient in removing dissolved solids since 
significant quantities of wash water are incorporated. 
SUMMARY OF THE INVENTION 
The invention is directed to a process for washing pulp and has particular 
application to washing recycled and Virgin pulp which contains dirt, 
inorganic fillers, and fiber debris. The process removes dissolved solids, 
as well as inorganic fillers, with a minimum quantity of wash water, and 
the wash water being discharged from the process can be re-used in the 
pulping operation. 
In accordance with the invention, a thin mat of pulp having a weight of 3 
to 100 lbs. of dry fiber per 1000 sq. ft. is applied to a porous endless 
fabric or belt, and preferably is sandwiched between a pair of porous 
endless fabrics or belts, to provide a composite structure. The composite 
structure is then passed over dewatering devices, such as foils, to 
partially remove water from the mat, and the composite structure then 
travels upwardly in a generally vertical path of travel through a vessel 
or tank which contains a series of vertically superimposed rollers or 
drums. The composite structure is passed over the rolls in a manner such 
that a first of the porous belts is in direct contact with one roll, and 
the second belt is in direct contact with an adjacent roll. 
As the composite structure travels upwardly through the vessel, wash water 
is directed again the structure and the water is cascaded downwardly 
through the vessel by a series of baffles that are attached to the inner 
wall of the vessel. The baffles are arranged so that the wash water will 
contact both faces of the composite structure, as it is moving through the 
vessel to thereby remove dissolved solids, as well as fillers, such as 
clay and calcium carbonate. 
The tension in the belts as they travel around the rolls, creates a 
dewatering action, so that the pulp mat is alternately showered with water 
and then dewatered by the belt tension. Thus, water is fed into opposite 
sides of the mat and is similarly drained from both sides of the mat, as 
the composite structure passes in its sinusoidal path over the rollers or 
drums. 
The wash water is discharged through an outlet in the lower end of the 
vessel and can be reused in the papermaking process, as for example, in 
the pulping operation. 
The composite structure after washing exits from the upper end of the 
vessel and the pulp mat is then removed from between the belts and 
subjected to further treatment and fed to a pulp chest. 
The washing process of the invention removes fillers and dissolved solids, 
while utilizing a minimum quantity of water, as compared with conventional 
recycled pulp washing operations. 
As the water being discharged from the washing vessel can be recycled and 
used in the pulping operation, this results in an overall decrease in 
water consumption in the papermaking process. 
Other objects and advantages will appear in the course of the following 
description.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
The invention is directed to a process for washing pulp and has particular 
use in washing recycled pulp composed of material such as used corrugated 
cartons, wastepaper, and the like, which contain dirt and inorganic 
fillers, such as clay and calcium carbonate, in addition to dissolved 
solids like alum and sodium salts. As shown in the drawing, a pulp, 
prepared by conventional pulping techniques and having a solids content of 
about 0.3% to 2.0% by weight, and preferably about 0.4% to 1.0% by weight, 
is fed from a head box 1 onto a porous belt or screen 2 in the form of a 
very thin mat 3 having a basis weight in the range of 3 to 100 pounds of 
dry fiber per 1,000 sq. ft. and preferably 5 to 30 pounds of dry fiber per 
1,000 sq. ft. Belt 2 preferably has a mesh size of about 25 to 70. 
Belt 2 carrying mat 3 then passes over a foil box 4, where a portion of the 
water is removed from the mat and then passes around roll 5. A second 
porous belt 6 or screen having a mesh size similar to belt 2 also passes 
around roll 6 on the outer surface of mat 3, thus sandwiching mat 3 
between the belts 2 and 6 and providing a composite structure. 
After leaving the roll 5, the composite structure passes across a second 
foil box 7 to remove additional water from the mat, and the composite 
structure then enters the lower end of a vessel or tank 8. Journaled for 
rotation within vessel 8 is a series of vertically superimposed rolls 9, 
10, 11 and 12. Rolls 11 and 12 can be drive rolls, while rolls 9 and 10 
are idler rolls. As belt 2 is in direct contact with roll 11, and belt 6 
is in direct contact with roll 12, both belts will be connected to the 
drive mechanism for synchronization. The drive for rolls 11 and 12 can be 
any conventional electric or hydraulic drive system, which will drive the 
composite structure at a speed in excess of 1000 ft. per minute and 
generally at a speed in the range of 300 to 4,000 ft. per minute. 
As the composite structure passes upwardly through vessel 8, wash water is 
directed against the structure through a suitable shower or jet 13. As 
shown in the drawing, the shower 13 is located between rolls 11 and 12, so 
that the wash water will be directed at the portion of the composite 
structure passing between rolls 11 and 12. 
The wash water is cascaded downwardly through vessel 8 by a series of 
baffles 14 which are mounted on the inner surface of vessel 8 on either 
side of the row of rolls 9-12. The baffles 14 serve to direct the 
cascading water against opposite faces of the pulp mat 3 which is 
sandwiched between belts 2 and 6, to thereby impregnate the mat with 
water. As the composite structure then passes over the rolls 9-12, the 
tension in belts 2 and 6 creates a dewatering action to extract or 
discharge the impregnated water. 
In practice, mat 3 may have a solids contents of 0.3% to 2.0% by weight, 
and preferably about 1.0%, as it is applied to belt 2 from headbox 1, and 
will be dewatered by foils 4 and 7, so that it has a solids content of 
2.0% to 6.0% and preferably about 4.0% as it enters the washing vessel 8. 
As wash water contacts and enters the mat, the solids content may decrease 
to 1.0% to 3.0%, and normally about 2.0%, and as the mat passes around the 
rolls, due to the tension in belts 2 and 6, the solids content may be 
increased to 2.0% to 6.0%, and generally about 4.0%. Thus, the solids 
content will alternately decrease and increase, as the mat moves in its 
path of travel through vessel 8. This intermittent squeezing action will 
act to effectively remove the dissolved solids, as well as inorganic 
fillers from the mat. 
The wash water being introduced into the vessel 8 through shower 13 can be 
water removed during the papermaking process, and the wash water 
discharged from vessel 8 through outlet 15 can be recycled back to the 
pulper, as makeup water. 
The composite structure consisting of belts 2 and 6 and the interposed mat 
3, exits from the top of the washing vessel, and the mat 3 is then 
separated from the belts. As shown in the drawing, the composite structure 
passes between vacuum box 16 and roll 17. Belt 2 passes upwardly around 
the roll 17, and vacuum box 16 serves to prevent the mat 3 from following 
the belt 2. Belt 2 then travels over rolls 18, 19 and 20, in returning to 
headbox 1. 
Belt 6 carrying mat 3 then passes around roll 21 and a doctor blade 22 
removes the mat from belt 6. The removed mat is deposited in a trough 23 
and can be conveyed by an auger 24 where it may typically be conveyed to a 
screw press to thicken the pulp for further treatment. Belt 6 then passes 
around rolls 25, 26, 27, 28 and 29 in returning to roll 5. 
As the mat passes over the rolls 9-12, it is subjected to alternate 
compression and relaxation, resulting in a pumping action in which water 
is introduced and discharged from both surfaces of the mat. This action 
effectively removes inorganic fillers, such as clay and calcium carbonate 
from the pulp, as well as dissolved solids with minimum loss of fiber. 
Various modes of carrying out the invention are contemplated as being 
within the scope of the following claims particularly pointing out and 
distinctly claiming the subject matter which is regarded as the invention.