Deinking

Source: Wikipedia, the free encyclopedia.

Deinking is the

deinked pulp
.

The key in the deinking process is the ability to detach ink from the fibers. This is achieved by a combination of mechanical action and chemical means. In Europe the most common process is froth flotation deinking.

Paper is one of the main targets for

wood pulp paper is that the fibers are degraded with each cycle and after being recycled 4–6 times the fibers become too short and weak to be useful in making paper.[1]

History

See also: History of paper

Before the invention of the

wood pulp in 1843 that paper production was independent of recycled materials.[2]

German jurist Justus Claproth in 1774.[2] He practiced together with German paper producer Johann Engelhard Schmid
. Today this method is called deinking.

First in the 1950s and 1960s the use of recycled fibres from paper made of wood pulp begun to increase, and was mainly used in packaging paper and paperboard. In the 1950s the

recycled paper.[3] Use of recovered paper increased in the 1970s mainly in graphic and hygienic papers, and accelerated in the 1980s. The annual growth in use of recovered paper increased by 6% between 1980 and 1996. The use of virgin fibres only increased 2% in the same period. In 1997 recovered paper production was 42% of the total paper production.[2]

Deinking process

Schematic layout of a deinking plant.

Sorting

Waste paper may contain a mixture of different

papermachine
.

Recycled paper can be used to make paper of the same or lower quality than it was originally. The sorted paper is baled and shipped to a

papermill
. The pulpmill uses waste paper grade according to the paper quality they want to make.

Debaling

The bales are opened and large foreign objects are sorted out on the conveyor belt to the pulper. Many extraneous materials are readily removed. Twine, strapping, etc. are removed from the hydropulper by a "ragger". Metal straps and staples can be screened out or removed by a magnet. Film-backed pressure-sensitive tape stays intact: the PSA adhesive and the backing are both removed together.[4]

Pulping

Pulpers are either batch, which uses a tub with a high shear rotor, or continuous, using a long, perforated drum. Drum pulpers are very expensive but have the advantage of not breaking up contaminants, thus giving cleaner end product.

The pulper chops the paper to smaller pieces; water and chemicals are added. Normally the pH is adjusted to 8.5 - 10.0. Normal deinking chemicals are:

After pulping, the mixture is a slurry. The slurry goes to screening.

Cleaning and screening

Centrifugal cleaning is spinning the pulp slurry in a cleaner, causing materials that are denser than pulp fibers to move outward and be rejected. Screens, with either slots or holes, are used to remove contaminants that are larger than pulp fibers.

Materials which are more difficult to remove include wax coatings on corrugated cartons and

laminating adhesives of reinforced gummed tapes, etc.[6][7][8]

Deinking stage

In the deinking stage the goal is to release and remove the

printing ink and stickies
. Several processes are used, most commonly flotation or washing.

Flotation deinking

Diagram of a froth flotation cell.

froth that can be removed. Normally the setup is a two-stage system with 3, 4 or 5 flotation cells in series.[9]
Flotation deinking is very effective in removing ink particles larger than about 10 μm.

Wash deinking

Wash deinking consists of a washing stage where

disk filters and static filters
. This stage is much more efficient than normal washing / dewatering stages.

Combined washing and flotation

High quality deinking of office wastes and other printing papers often commonly uses a combination of washing and flotation.

Enzymatic deinking

This method of deinking uses industrial or food grade enzymes in conjunction with flotation deinking to aid in the removal of inks in recycling mills. More efficient removal of ink increases fiber yield, decreases dirt count, and increases paper brightness. Often the use of enzymatic deinking helps mills reduce their bleach usage or use cheaper furnish.

Other deinking processes

Dissolved air flotation (DAF) is used by some mills in the deinking stage and will remove some ink and filler (ash); however, it is mainly used to clarify the process water.

Washing / dewatering

Washing / dewatering (thickening) is a filtration process. Small particles (< 5 μm) are removed by passing water through the pulp.

Bleaching

Main article: Bleaching of wood pulp

If white paper is desired, bleaching uses peroxides or hydrosulfites to increase the brightness of the pulp. The bleaching methods are similar for mechanical pulp, but the goal is to make the fiber brighter.

Papermaking

The deinked fiber is made into a new paper product in the same way as virgin

wood fiber, see papermaking
.

Byproducts

The unusable material left over, mainly ink, plastics, filler and short fibers, is called sludge. The sludge is buried in a landfill, burned to create energy at the paper mill or used as a fertilizer by local farmers.

Problems

Water based

acidic
washing stage.

Temperature control is important as this affects the stickiness of stickies.

Additional issues arise when taking into account the number of chemicals potentially present in paper for recycling. Studies have indicated that paper might contain as many as 10,000 different chemicals,[10] the fate of which in the deinking process still remains unknown. Polychlorinated biphenyls are often found in pigments which are used in newspaper and magazine paper.[11]

See also

References

  1. ^ "Paper Recycling Information Sheet". Waste Online. Archived from the original on October 18, 2007. Retrieved October 20, 2007.
  2. ^
    ISBN 978-952-5216-07-3
    .
  3. ISBN 978-3-527-30999-3
    .
  4. ^ Jensen, Timothy (April 1999). "Packaging Tapes: To Recycle of Not". Adhesives and Sealants Council. Archived from the original on 2007-11-09. Retrieved 2007-11-06.
  5. ^ WO 011717, Nellesen, Bernhard & Northfleet, Christina, "METHOD OF DEINKING", published 05.02.2004, issued 2004 
  6. ^ "Recycling Compatible Adhesives Standards". Tag and Label Manufacturers Institute. 2007. Archived from the original on 2007-11-09. Retrieved 2007-11-06.
  7. ^ "Voluntary Standard for Repulping and Recycling Corrugated Fiberboard" (PDF). Corrugated Packaging Alliance. 2005. Archived from the original (PDF) on 2007-12-03. Retrieved 2007-11-06.
  8. ^ Seiter, Pikulin (October 1998). "Environmentally benign USPS stamps" (PDF). TAPPI Pulping Conference. Archived from the original (PDF) on 2006-10-12. Retrieved 2007-11-08.
  9. ^ Voith EcoCell flotation plant "Archived copy" (PDF). Archived from the original (PDF) on 2009-08-24. Retrieved 2009-01-08.{{cite web}}: CS1 maint: archived copy as title (link)
  10. S2CID 205676176
    .
  11. PMID 23454657
    .

Further reading
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