SYSTEM FOR MANAGING SOLIDS IN PAPERMAKING WHITEWATER

Abstract

Methods and systems for reducing the loss of usable fibers from whitewater while simultaneously increasing paper making machine performance and paper quality are provided. The methods and systems utilize a fractionating saveall to separate whitewater into a dilute wastewater fraction that contains unwanted fine particles and a thicker consistency reusable fraction containing paper-forming fibers. The reusable fraction is recirculated back into the whitewater stream, while the wastewater is routed away from the whitewater stream.

Claims

1. A method for recovering usable pulp fibers in a paper making system, the method comprising: passing whitewater from a wet-end sump pit at the wet-end of a paper making system into a fractionating saveall, whereby the fractionating saveall separates the whitewater into a wastewater fraction in which particulate matter is concentrated and a reusable fraction in which usable pulp fibers are concentrated; discharging the wastewater fraction from the paper making system; and circulating the useable fraction directly back into whitewater in the wet-end of the paper making system.

2. The method of claim 1, wherein circulating the useable fraction directly back into whitewater comprises circulating the useable fraction directly back into whitewater in a wire pit.

3. The method of claim 1, wherein circulating the useable fraction directly back into whitewater comprises circulating the useable fraction directly back into whitewater in a silo.

4. The method of claim 1, wherein circulating the useable fraction directly back into whitewater comprises circulating the useable fraction directly back into whitewater in the wet-end sump pit.

5. The method of claim 1, wherein circulating the useable fraction directly back into whitewater comprises circulating the useable fraction directly into a whitewater chest.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0011] Illustrative embodiment of the invention will hereafter be described with reference to the accompanying drawing, wherein like numerals denote like elements.

[0012] FIG. 1 is a schematic representation of an embodiment of a paper making system incorporating a Fractionating Saveall as a mechanical drainage aid and usable fiber recovery device back into the wet-end whitewater sump or elsewhere in the whitewater system. Positioning the Fractionating Saveall so as to recover fiber into the whitewater system has several important benefits. Although FIG. 1 shows the recovery of usable fiber with a Fractionating Saveall into the wet-end sump, other vessels containing whitewater can be used as recovery points (for example, the whitewater chest, wire pit, or silo). When fiber is recovered from whitewater, it is conventionally returned into either a stock chest already containing stock, a chest designated specifically for the recovered fiber stream, or a hydrapulper. It is not returned into the whitewater system. This is because whitewater is difficult to contain given the volumes involved. It makes little sense to recover usable fiber into a whitewater system at one point only to have it overflow to the sewer somewhere else.

DETAILED DESCRIPTION

[0013] For the purposes of this disclosure, the following terms have the following definitions. The wet-end of the machine refers to the end of the machine where the sheet is formed. They include the wire pit, fan pump, pressure screen, head box, forming area, and forming fabric. The dry-end generally refers to the pressure roll, dryer, and reel section where the paper is dried and wound up into a roll.

[0014] Whitewater refers to water that has been drained out of the sheet during formation. It also includes water coming from showers, chemical solutions, and the like which are needed to maintain the operation of wet-end components. The water from these systems then mixes continually with the contents of the wire pit. Usable fiber refers to pulp fibers that can be easily incorporated into a paper product (e.g. fibers longer than 0.2 mm in length). It does not include the fines.

[0015] Stock refers to an aqueous slurry of pulp fibers intended to supply the raw material for a papermaking process. In contrast, fines refer to small particles of particulate matter (usually fiber fragments less than 0.2 mm in length) which are found in both the stock and whitewater systems. These substances naturally develop when pulp fibers are manipulated. In high concentrations, they are undesirable because they degrade the quality of the paper produced and can attack the systems, adversely affecting chemical efficiency, brightness, clothing life, drainage, machine speeds, production levels, etc.

[0016] Stock preparation (or stock prep) refers to the series of operations in which pulp is mechanically and/or chemically processed to tailor the pulp stock properties for a given paper product. Stock preparation typically includes beating (using, for example, a hydrapulper) to swell and soften the pulp fibers as they are mixed with water to produce the stock.

[0017] It should be noted that the systems described herein differentiate a whitewater system as compared to a stock system. For the purpose of distinguishing between the two, it should be noted that stock systems are intended to supply raw material (stock) to the paper making machine. Whitewater, on the other hand, is a lower consistency stream of water, recovered after the sheet is formed and later used to mix with more fiber in the process of generating more stock.

[0018] The term “Fractionating Saveall” refers to a saveall that concentrates usable fibers in a first fraction and concentrates the particulate matter in a separate fraction. Examples of Fractionating Savealls are described in U.S. Pat. Nos. 6,622,868 and 7,055,697. Thus, a Fractionating Saveall is distinguishable from a conventional saveall, such as a disc saveall, or from a dissolved air flotation clarifier in that the latter concentrate both the usable fibers and the fines together. What makes a Fractionating Saveall unique is that it separates fines from usable fiber, making it possible to recover the usable fiber without polluting the papermaking process with fines.

[0019] It is an object of the present invention to provide an effective method and system for reducing the loss of usable fibers from whitewater while simultaneously increasing paper making machine performance and paper quality. This is accomplished by utilizing the Fractionating Saveall to recover only the usable fiber left over following the sheet forming process. The usable fiber is recovered from and returned back into the whitewater system from the whitewater sump on the wet-end of the machine. The rest of the Fractionating Saveall stream is removed from the process loop. Water discharged this way only contains particulate matter (fines and ash). Purging it not only reduces the amount of undesirable material in the process but enables the operator to control total whitewater volumes simply by adjusting the rate of flow coming to the Fractionating Saveall. Volume control is key to managing the system and to avoid overflowing or otherwise spilling the enriched whitewater stream.

[0020] Recovering fiber with a Fractionating Saveall is effective whether the fiber goes into a stock stream or a whitewater stream. However, when the recovered fiber enters a thick stock stream, care must be taken to ensure that consistency in the chest is maintained. Otherwise, refining operations or end product basis weight control could be compromised. These problems are avoided when recovering fiber into a stock chest by using larger screening systems which are significantly more expensive (to purchase, install, operate, and maintain). Using the Fractionating Saveall at the wet-end sump can accomplish the same objectives (minimizing fiber loss and maximizing machine performance) at a fraction of those costs because the system is much smaller. When recovering into the whitewater system, consistency of the recovered fiber is of less importance. In fact, recovering fiber at lower consistencies is more desirable because these systems are designed to move water, not stock.

[0021] The Fractionating Saveall isolates and removes particulate matter from the whitewater system. As such, it serves to improve water drainage and machine efficiency. This means that the Fractionating Saveall can be used as a mechanical drainage aid.

[0022] Only a small portion of the total volume entering the Fractionating Saveall is recovered: the usable fiber. Most of the water (along with the fines) is isolated and can be removed from the process, giving the papermaker volume control. As a result, the usable fiber can be easily recirculated directly back into the whitewater stream without fear of overflowing the system elsewhere in the process.

[0023] In some embodiments of the methods and systems, the Fractionating Saveall is located in the whitewater system downstream of the wet-end sump pit, where the useful fraction is recovered into the whitewater chest directly. Others include recovery into the wire pit or silo, but the technique for using this device would be the same.

[0024] Fractionating Saveall technology which can be used in the present methods and systems include only that which is described in U.S. Pat. Nos. 6,622,868 and 7,055,697, the entire contents of which are incorporated herein by reference for the purpose of describing the components and functioning of that particular system. Water leaving the Fractionating Saveall will be very dilute with a low consistency. This fraction can be disposed of by, for example, routing it to the sewer or a wastewater treatment system. In contrast, the reusable fraction in which the useful paper making fibers are preferentially concentrated will have a thicker consistency. By way of illustration only, the reusable fraction may have a consistency in the range from about 0.5% to 0.75%, where consistency refers to the mass fraction (or percentage) of solid material in the fraction.

[0025] FIG. 1 illustrates an embodiment of a paper making method and system that uses a Fractionating Saveall as a mechanical drainage aid in combination with recirculation of recovered usable fibers directly back into the whitewater loop to reduce usable fiber loss and improve system performance. The paper making system shown in FIG. 1 is used for illustrative purposes. Not all of the components shown in this FIGURE are required, and other components that are conventionally used in paper making systems can be present even if they are not depicted in this FIGURE.

[0026] The paper making system illustrated in FIG. 1 includes a whitewater chest 1 that collects whitewater from the wet-end sump 6A of the paper making system. In the embodiment, whitewater from the whitewater chest 1 is fed into a hydrapulper 3 where it is mixed with dry pulp 2. In the hydrapulper 3, the pulp slurry undergoes hydrodynamic shear to loosen the pulp fibers as they are prepared into a high-consistency (“thick”) pulp stock. From hydrapulper 3, the prepared stock is introduced into a sequence of chests where it is agitated and sequentially diluted. As shown here, the prepared stock first passes into the primary stock chest 4 for storage. It then is sent on to machine chest 5 from which it is metered at a constant rate into the supply stream ahead of fan pump 7. Water from the wire pit 6 combines with this stock, mixing as they are pulled into the fan pump. The fan pump pushes this slurry into and through the pressure screen 8, then into head box 9, and finally into the forming area 10. This is where the sheet 12 is formed. As water passes through the forming fabric 11, the fibers lay down a matrix on top of it. This is how paper is made. Although the illustrative system shown in FIG. 1 includes two chests, more could be used.

[0027] After the sheet is formed, it is transferred from the forming fabric 11 onto the machine felt 13 at the couch 14. As the felt and fabric pass between the couch rolls 14, the sheet is pressed onto the underside of the felt 13. The sheet is then carried by the felt into the dryer 15 and pressure roll 16 nip where it is pressed onto the dryer. The paper is then dried and wound up into a roll 18 at the reel 17.

[0028] Following sheet formation, the whitewater is collected back into the wire pit 6 from which it is resupplied with fiber from the machine chest 5 and the process continues. As the whitewater supply in the wire pit grows, the excess overflows into the wet-end sump 6A. Normally this sump would serve to recover the whitewater and deliver it back to the whitewater chest 1. However, in FIG. 1 the Fractionating Saveall 1A is installed so that a portion of this stream can be diverted to the device. The accepts 1B are recovered into the wet-end sump 6A. The waste fraction 2B goes to the sewer. When recovering usable fiber this way, some of it will invariably pass back into the Fractionating Saveall more than once, but the nature of the screening system traps that fiber until it is carried beyond this point and into the whitewater chest 1. A large portion of the flow discharged from the wet-end sump 6A is not expected to reach the Fractionating Saveall. Volumes and flow rates will vary depending on process and machine requirements. The key to operating this system effectively is to control the volume entering the device so as to balance the needs of stock prep with the amount of whitewater recovered to it. The removal of particulate matter (fines) from the paper making system serves as a mechanical drainage aid since the fine particles being removed could otherwise impede water drainage during wet-end sheet formation. The paper making system shown in FIG. 1 recirculates the usable fibers from the Fractionating Saveall back into whitewater rather than into the stock system. This is made possible because excess whitewater (the fines laden fraction) is sewered rather than recirculated back into the process. In this way, volume control can be managed.

[0029] As also illustrated in FIG. 1, the solutions described herein (e.g., pulp stocks and whitewater, wastewater, and/or usable fiber-containing discharge) can be passed from one container (e.g., chest, tank, chamber, pit, etc.) to another through one or more connecting conduits (e.g., pipes, tubing, etc.).

[0030] The word “illustrative” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “illustrative” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, “a” or “an” means “one or more.”

[0031] The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.