Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With washing
Reexamination Certificate
1998-04-06
2001-05-08
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With washing
C162S204000, C068S044000, C210S783000, C210S928000
Reexamination Certificate
active
06228215
ABSTRACT:
BACKGROUND OF THE INVENTION
In a conventional papermaking operation, wood chips are subjected to severe conditions of acidity or alkalinity at high temperatures in a pulping operation in order to dissolve the more soluble lignin, but not the less soluble cellulosic fibers. After pulping, the cellulosic pulp is subjected to a cleaning operation to remove suspended solids or particulate material. In a typical cleaning operation, debris and heavy material, such as stones, metal, glass, and the like, are initially removed from the pulp by a centrifugal cleaner and the pulp is then subject to coarse screening followed by secondary finer screening to remove large and small sized contaminants. Following this, the pulp is subjected to a forward cleaning operation to remove sand, small fiber bundles, ink and the like, followed by reverse cleaning to remove lighter materials, such as wax, latex, hot melt, and other materials. Following the cleaning the pulp undergoes a washing operation which acts to remove dissolved organic and soluble inorganic material present in the pulp mass. It is necessary to remove dissolved solids because the dissolved solids interfere with bleaching and with the papermaking operation. Further, dissolved solids are a source of increased biological oxygen demand, chemical oxygen demand and color when the dissolved solids are discharged into an effluent system.
After washing, the pulp undergoes stock preparation and the refined and diluted pulp is then utilized in the papermaking machine to produce a paper sheet in the conventional manner.
A common method of washing pulp is a countercurrent system in which fresh wash water is added to the pulp in the final stage of washing and the wash water is then recycled in an upstream direction toward the first stage of the washing operation. With the countercurrent system, the dissolved solids concentration of the wash water increases as it moves from the final stage to the first stage, while the concentration of dissolved solids in the pulp decreases as it travels from the first stage to the final stage.
United States patent application Ser. No. 08/697,271, filed Aug. 21, 1996, now U.S. Pat. No. 5,753,074 describes an improved pulp washing process in which a thin mat of pulp slurry is positioned between a pair of porous endless fabrics or belts to provide a composite structure. The composite structure is then moved upwardly through a washing zone over a series of vertically spaced rolls in a generally sinuous path of travel, such that a first of the belts is in direct contact with one roll, and the second belt is in direct contact with an adjacent roll. As the pulp mat travels through the washing zone, wash water is directed against the mat at the upper end of the washing zone and is cascaded downwardly through the washing zone by a series of baffles. As the pulp mat travels between adjacent rolls, the mat is impregnated with wash water and as the composite structure travels around the rolls, the tension in the belts increases, creating a dewatering action, so that the pulp mat is alternately showered with water and then dewatered by the belt tension.
Thus, the washing process as described in the aforementioned patent removes dissolved solids, as well as suspended fillers, while utilizing a minimum quantity of water, as compared with conventional countercurrent pulp washing operations.
SUMMARY OF THE INVENTION
The invention is directed to an improved method and apparatus for the countercurrent treating of slurries or sludges and has particular application to the treatment of residual slurries from a papermaking operation
In every papermaking process there is a need to purge the system of contaminants or undesirable materials. Typically, a pulp cleaning operation produces undesirable materials that fall into two categories: the first being a sludge or slurry containing a high concentration of suspended solid contaminants and the second being a liquid stream containing a minor portion of suspended solids and assortment of colloidal and dissolved solids.
In accordance with one aspect of the invention, the sludge or slurry containing the high concentration of solid contaminants is positioned between a pair of porous endless belts as a sludge mat, to provide a composite structure. The composite structure, consisting of the pair of belts with the intermediate sludge mat, is moved through a treating zone where the mat is subjected to alternate stages of compression and relaxation. In a preferred form of the invention, the compression stages are achieved by passing the composite structure over a series of spaced rolls in a generally sinuous path of travel.
The residual liquid stream from the pulp cleaning operation is concentrated to increase the concentration of dissolved solids and the concentrated stream is then cascaded over the sludge mat in a countercurrent fashion as the composite structure passes through the treating zone, causing the liquid stream to impregnate the mat when the mat is subjected to relaxation as it moves between adjacent rolls. This results in the dissolved solids in the highly concentrated liquid stream being dissolved into the aqueous phase of the sludge mat, to thereby reduce the concentration of dissolved solids in the liquid stream and correspondingly increase the proportion of dissolved solids in the sludge mat. In addition, suspended solids in the liquid stream are entrapped in the sludge mat.
As a result of the process of the invention, the concentration of dissolved solids in the liquid stream is substantially reduced, so that the liquid stream being discharged from the treating zone can be recycled in the pulp processing or paper-making operation. The sludge mat having the increased concentration of dissolved solids and suspended solids, can the compacted and the resulting solid residue can either be landfilled or incinerated.
The process of the invention has particular application for use in a zero liquid discharge papermaking operation, in that it eliminates the discharge of liquid having a high concentration of dissolved solids to the environment.
In a modified form of the invention, a cellulosic pulp slurry is positioned between a pair of porous endless belts as a mat to provide a composite structure, and the composite structure, including the pulp mat, is moved through a treating zone rolls where the mat is subjected to alternate stages of compression and relaxation.
A liquid stream containing a material, such as dye or bleach, which will react with the cellulosic fibers of the pulp is passed countercurrently through the treating zone and impregnates the pulp mat, when the mat is subjected to relaxation causing the reactive material to react with the cellulosic fibers. The pulp mat leaving the downstream end of the washing zone is then compacted to remove excess liquid. This process provides a convenient and efficient method of reacting a reactive substance with suspended particulate material in a slurry.
In a further modified form of the invention, a sludge or slurry containing a high concentration of suspended solids is positioned between a pair of endless porous belts as a thin mat to provide a composite structure. As in the case of the prior embodiments, the composite structure is moved through a treating zone and subjected to alternate stages of compression and relaxation, as for example by moving the composite structure in a generally sinuous path over a plurality of spaced rolls. A liquid stream containing suspended solids is directed against the composite structure at locations between adjacent rolls and the liquid is cascaded over the structure in a countercurrent manner, causing the suspended solids in the liquid stream to be entrapped in the mat. As a result of the process, the concentration of suspended solids in the liquid stream is reduced, while the concentration of suspended solids in the mat is correspondingly increased.
REFERENCES:
patent: 3906853 (1975-09-01), Wohlfarter
patent: 4113556 (1978-09-01), Kankaap{umlaut over (aa)}
patent: 4501040 (1985-02-01), Biondetti et a
Hoffman Enviornmental Systems, Inc.
Nguyen Dean T.
Weiss Philip M.
Weiss & Weiss PC
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