Recovery of fibers from a fiber processing waste sludge

Details Recovery of fibers from a fiber processing waste sludge Recovery of fibers from a fiber processing waste sludge

1998-12-18

2002-04-16

Nguyen, Dean T. (Department: 1731)

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

Waste paper or textile waste

C162S021000

Reexamination Certificate

active

06372085

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a process for recovering a usable population of fibers and fines from the waste sludge of a fiber production or fiber handling facility.
In particular, this invention provides a steam explosion process and a resulting product which permits the separation of a useful population of fibers and fines from the waste streams of fiber production processes such as a waste paper recovery operation or from the waste stream of a paper making process. The process is useful in fiber and fine recovery from white water waste streams from paper making operations and waste sludge from the de-inking and processing of waste paper. The present invention not only increases the amount of usable fibers and fines recoverable from the waste stream, but increases the fiber quality of the recovered fibers. Additional benefits include a concomitant reduction in the solid volume of the waste stream and increases the usable fiber content available from the initial raw material. The process steps of the present invention yield a fiber of improved quality suitable for making tissue products such as toilet and facial tissues, paper towels, and napkins.
DESCRIPTION OF THE PRIOR ART
Waste sludge from fiber production and paper making facilities typically contain numerous fibers and fines. In particular, efforts to recover the fibers and fines from the waste sludge streams of paper fiber recovery plants have been limited, in part, by the high ash content of the sludge. Conventional fiber screening techniques also retain the ash particulates. The high ash content renders the recovered fibers and fines undesirable for quality end products.
Paper is conventionally made by draining a low consistency dispersion of cellulose fiber pulp, fillers, and additives through a paper machine “wire” (essentially an endless mesh or sieve). A certain amount of solid material passes through the wire with the suspending water and is, thus, not retained in the wet paper web formed on the wire. The drained liquid suspension, known generally in the industry as “white water,” carries entrained solid material. White water from which the suspended particles have been removed is reused in the paper making process to the extent possible.
Obviously, wastepaper, if it can be recycled, is a much cheaper and environmentally friendly source of wood pulp for making paper. Before wastepaper can be reused as recycle material, however, the wastepaper must be de-inked. De-inking processes remove inks and coating materials from the wood fibers. Thus, when recycled fibers, as opposed to virgin pulps, are used in the paper making process, the drained liquid suspension will contain additional types of waste materials such as inks and hot melt adhesives.
Unused white water and de-inking effluents must be treated before being discharged from the paper mill. Treatment normally involves passing the effluent through a clarifier, prior to which flocculated are added to promote sedimentation of solid material suspended in the water. A biological treatment with microorganisms is also commonly performed to reduce the biological oxygen demand (BOD) of the liquid effluent before it is discharged. As can be expected, disposal of the unused white water and de-inked effluents results in costs to the paper making facility.
The sediment accumulated in the clarifier is a sludge composed of pulp fibers, fiber particles or fines, fillers, pigments, and other miscellaneous debris such as grit, sand, plastic particles, general dirt. Many of the sludge components are fillers, pigments and the like that were added to the pulp during the sheet-forming process for the purpose of producing desired properties in the finished paper. Such properties include proper surface, opacity, strength and brightness. For example, finely ground inorganic fillers, such as talc, certain clays, calcium carbonate, blanch fixed, and titanium dioxide may be added to papers to improve surface smoothness, whiteness, printability and opacity. Sizing agents, such as soaps, gelatins, and rosins (with alum), wax emulsions and starches, may be added to papers for improving resistance to penetration by liquids. In addition, coloring agents, such as acid, basic, direct and sulfur dyes and natural and synthetic pigments may be added for coloring purposes. Any of such products may ultimately end up in the clarifier as part of the sludge. In addition, because the clarifier is usually a large open air tank, other debris such as leaves, branches, insects, etc. Can also become part of paper sludge. The major constituents of the sludge, however, are generally fiber/fines and the inorganic fillers calcium carbonate and clay.
Most de-inking processes involve the use of flotation and washing. In de-inking processes, wastepaper is first washed and then pulled with dilute sodium hydroxide or surfactants in a pulpier tank to cause the fibers to swell and loosen the ink and coating material particles contained thereon. (These coating materials include the previously mentioned clays, talc, etc.) After pulling, the pulp stocks go through screening, cleaning, washing, floatation, and bleaching to further remove trash, stickiest, inks, ash, and short fiber fines. During the washing and floatation stages, most ash, stickiest, and short fiber fines are separated from the pulp stock. Thus, when the sludge comes from a mill using recycled waste paper, this sludge may also have accumulations of adhesives (otherwise known as “stickiest”), foreign bodies (such as pieces of plastic material or metal, otherwise known as “contraries”) in very small quantities, and other additives, such as those described above, that are used in the paper making process.
Normally, the sludge is drawn off from the clarifier at about 2.5 percent consistency (or “percent dry solids content”) and is then dewatered to a consistency of around 20 to 55 percent, for example, by means of rotary vacuum filters, screw presses, or belt presses. Dewatering reduces the weight of material going to the landfill and reduces the charges for landfill disposal because these are typically based on weight. Since the majority of the weight in the sludge comes from water, it behooves the sludge processor to remove as much water as possible. The dewatered sludge is in a semi-solid state and usually contains about 40 percent to about 80 percent by dry weight relatively fine wood fibers and from about 20 percent to about 60 percent inorganic (also referred to as “ash”) and the additives mentioned above. The material typically is a crumbly, not very cohesive material that appears to be dry. At thirty percent consistency, most sludges are more like dry solids as opposed to a suspension or dispersion. Because of the non-cohesive character of sludge, the materials handling equipment for moving, storing and transporting are generally the same as for dry materials. Once in this state, the sludge is then capable of being collected and transported for disposal in landfills.
According to some sources, it is estimated that the amount of dry waste (waste sludge with substantially all of the residual water removed) produced due to paper processing exceeds 4.6 million tons per year. This sludge is produced by both paper making from virgin pulp and paper making from recycled fibers. A typical de-inking plant employing recycled fibers processes about 100 dry tons of waste paper into about 65 to about 80 dry tons of recycled (reusable) fiber. The remaining 20 to 35 tons of waste paper is unusable, and becomes part of the sludge produced by the deinking plant. After recycled fiber sludge is dewatered with various suitable dewatering devices, including, for example, a belt press or screw press, 100 dry tons of waste paper still produce from about 70 to about 120 wet tons of sludge which must be disposed.
Moreover, the sludge produced during the making of tissue from an integrated mill with a recycled fiber plant produces 10 times the amount of sludge produced during the making of tissue from virgin pulp. The typical virgin pulp tissue making proces

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