Deinking is the
industrial process of removing
printing ink from
paperfibers of
recycled paper to make
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
recycling. A concern about recycling
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.
History
Before the invention of the
paper machine in 1799 the most common fibre source was recycled fibres from used textiles, hence the name
rag paper. The rags were from
hemp
Hemp, or industrial hemp, is a botanical class of ''Cannabis sativa'' cultivars grown specifically for industrial or medicinal use. It can be used to make a wide range of products. Along with bamboo, hemp is among the fastest growing plants o ...
,
linen
Linen () is a textile made from the fibers of the flax plant.
Linen is very strong, absorbent, and dries faster than cotton. Because of these properties, linen is comfortable to wear in hot weather and is valued for use in garments. It also ...
and
cotton. It was not until the introduction of
wood pulp in 1843 that paper production was independent of recycled materials.
Recycling of used paper before the
industrialisation
Industrialisation ( alternatively spelled industrialization) is the period of social and economic change that transforms a human group from an agrarian society into an industrial society. This involves an extensive re-organisation of an econo ...
of paper production, rag paper was recycled to make low-grade
board
Board or Boards may refer to:
Flat surface
* Lumber, or other rigid material, milled or sawn flat
** Plank (wood)
** Cutting board
** Sounding board, of a musical instrument
* Cardboard (paper product)
* Paperboard
* Fiberboard
** Hardboard, a ty ...
. A process for removing printing inks from recycled paper was invented by
German jurist
Justus Claproth in 1774.
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
froth flotation technique was adapted for deinking
recycled paper. 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.
Deinking process
Sorting
Waste paper may contain a mixture of different
paper types made of mixtures of different paperfibers. These must be sorted before processed. Broke (paper waste from paper production) is normally used directly in the
papermachine.
* Office waste (OW)
* Old
magazine papers (OMP)
* Old
newsprint
Newsprint is a low-cost, non-archival paper consisting mainly of wood pulp and most commonly used to print newspapers and other publications and advertising material. Invented in 1844 by Charles Fenerty of Nova Scotia, Canada, it usually has an ...
(ONP)
*
Paperboard
Paperboard is a thick paper-based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker (usually over 0.30 mm, 0.012 in, or 12 Inch#equivalences, points) than paper and has certain ...
*
Corrugated fiberboard
Corrugated fiberboard or corrugated cardboard is a type of packaging material consisting of a fluted corrugated sheet and one or two flat linerboards. It is made on "flute lamination machines" or "corrugators" and is used for making corrugated ...
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
A conveyor belt is the carrying medium of a belt conveyor system (often shortened to belt conveyor). A belt conveyor system is one of many types of conveyor systems. A belt conveyor system consists of two or more pulleys (sometimes referred to ...
to the
pulper
In agriculture, a pulper is a machine designed to remove pulp (i.e. the soft flesh) from agricultural produce. For example, in coffee growing the ripe red cherries are picked from the coffee bushes and before fermentation and later drying the s ...
. 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.
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:
* pH control:
sodium silicate
Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate , sodium orthosilicate , and sodium pyrosilicate . The anions are often polymeric. These compounds are generally colorless transparent ...
or
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
* Bleaching:
hydrogen peroxide
*
Calcium ion source:
hard water,
lime or
calcium chloride
* Collector:
fatty acid, fatty acid
emulsion, fatty acid
soap or organo-modified
siloxane
After pulping, the mixture is a
slurry
A slurry is a mixture of denser solids suspended in liquid, usually water. The most common use of slurry is as a means of transporting solids or separating minerals, the liquid being a carrier that is pumped on a device such as a centrifugal pu ...
. The slurry goes to
screening
Screening may refer to:
* Screening cultures, a type a medical test that is done to find an infection
* Screening (economics), a strategy of combating adverse selection (includes sorting resumes to select employees)
* Screening (environmental), a ...
.
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
stickies, soft rubbery particles which can make deposits and contaminate the recycled paper. Stickies can originate from
book bindings,
hot melt adhesives, PSA
adhesive
Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.
The use of adhesives offers certain advant ...
s from paper labels,
laminating adhesives of reinforced gummed
tapes, etc.
Deinking stage
In the deinking stage the goal is to release and remove the
hydrophobic contaminants from the recycled paper. The contaminants are mostly
printing ink and
stickies. Several processes are used, most commonly flotation or washing.
Flotation deinking
Froth flotation was adapted from the flotation process used in the
mining industry in the 1960s. It is the most common deinking process in
Europe used to recover
recycled paper. Often most of the collector is added to the inlet of the flotation. The process temperatures are normally in the range 45 - 55 °C.
Air is blown into the pulp
suspension. The collector has
affinity both to the ink particles and air bubbles, causing them to attach. The air bubbles lift the ink to the surface and form a thick
froth that can be removed. Normally the setup is a two-stage system with 3, 4 or 5 flotation cells in series. Flotation deinking is very effective in removing ink particles larger than about 10 µm.
Wash deinking
Wash deinking consists of a washing stage where
dispersants are added to wash out the
printing inks. When the pulp slurry is
dewatered (thickened), the medium to fine particles are
washed out. This process is most useful for removing particles smaller than about 30 µm, like water-based inks, fillers, coating particles, fines and
micro stickies. This process is more common when making
deinked pulp for
tissue. The processing equipment are
belt filters, pressure belt filters,
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
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 that virgin
wood fiber, see
papermaking
Papermaking is the manufacture of paper and cardboard, which are used widely for printing, writing, and packaging, among many other purposes. Today almost all paper is made using industrial machinery, while handmade paper survives as a speciali ...
.
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
flexographic printing inks with
particle sizes below 5 µm and poor solubility in
alkaline
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
conditions may cause problems in deinking, especially in the flotation stage. The solution is to use an extra
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, 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.
See also
*
Dissolved air flotation
*
Environmental issues with paper
*
Froth flotation
*
German inventors and discoverers
*
Paper recycling
The recycling of paper is the process by which waste paper is turned into new paper products. It has a number of important benefits: It saves waste paper from occupying homes of people and producing methane as it breaks down. Because paper fi ...
*
Pulp & Paper chemicals
References
Further reading
*
{{Authority control
Papermaking
Industrial processes
Paper recycling
German inventions