Ink removal

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The printing ink removal or deinking (from English ink = " printing ink ", "ink") is the key process in paper recycling for removing the printing ink from printed waste paper . The waste paper is bleached using mechanical and chemical methods. In Germany, waste paper is deinked to produce newsprint, office paper (copy paper) and hygiene paper. Here it is primarily the optical properties that determine the utility value. Dark types of waste paper (packaging, cardboard) cannot be deinked, so they must be sorted out beforehand. Such types are used again for the production of cardboard or corrugated cardboard.

history

The origin of the deinking process can be traced back to a discovery by Justus Claproth in 1774, in which the printing ink was washed out of printed paper in order to produce rewritable paper. Fibers from old clothing were used for recycled paper until 1843, when wood pulp was used as the raw material. With the use of flotation for ink removal from paper in the 1950s, paper fibers could be used as a raw material in paper production. Previously, the waste paper was only suitable for the production of cardboard. The whiteness that could now be achieved enabled the production of newsprint and hygiene papers.

process

Before the mid-1970s, deinking systems consisted of disintegration (adding deinking chemicals to the pulper drum), sorting and flotation to remove the printing ink. In addition, there was thickening (separation of the stock preparation water circuit from the paper machine water circuit), hot dispersion for the fine division of the remaining contaminants and fine sorting using very fine slots. As a result, so-called stickies (sticky contamination) can be removed in addition to the printing ink. Today's deinking process usually includes a second flotation and thickening stage and the cleaning of the circulating water (DAF = Dissolved Air Flotation) in order to achieve a higher quality level. In addition to the flotation deinking common in Europe, there is also washing deinking (more common in North America). Flotation delivers higher yields and requires considerably less water than washing. The laundry is used in the manufacture of hygiene papers, where especially the long fibers without fines or fillers are screened out.

Flotation removes the printing ink from the waste paper used, thereby increasing the degree of whiteness. The hydrophilic (water-attracting) property of the fibers is used. These are wetted by water, while the hydrophobic (water-repellent) printing ink remains largely unwetted. The requirements for recovered paper suitable for deinking are derived from this process. This deinking product is defined in a list of types of waste paper.

The printing ink cannot be removed by flotation from newspapers produced by flexographic printing , which are common in England and Italy. This creates considerable problems for recycling. Even small proportions of flexographic newspapers in the waste paper mix lead to a poor degree of whiteness. The binder of the flexographic printing ink dissolves in a weakly alkaline environment and releases the primary pigments of the ink. These particles are very small and not hydrophobic, which is why the flotation process does not work with them. Pigmented inkjet inks, digital printing processes with polyethylene liquid toner and UV varnish cause similar problems with flotation. However, an optimization of the deinking chemicals used could help solve the problem. In contrast, newly developed polyester-based liquid toners can be deinked just like classic dry toners.

Deinkability is now recognized as a criterion for the recyclability of a paper product. First of all, the Nordic Swan ( Nordic Ecolabel ) and the Austrian Ecolabel for printed products included deinkability as a criterion. The European eco-label for printed products (EU Ecolabel for printed paper) adopted in 2012 also includes deinkability as an important requirement.

Recycled paper made from waste paper is deinked, so-called environmental paper is not used. In the treatment of waste paper to be recycled paper no chemicals are added, inks are not removed from the recycled fiber pulp and a bleach is omitted. The recycling process cannot be repeated as often as required, as the paper not only loses its brightness due to remaining ink particles, but also no longer meets the quality requirements of modern printing and reproduction processes with more intensive recycling. One consequence is that these very dark types of paper find fewer buyers and end up in the trash more quickly.

Theoretically, waste paper fibers can be recycled 4 to 6 times, as they are weakened and shortened with each recycling cycle. Small fragments are washed out, however, and fresh fibers from magazines, for example, compensate for the losses. With the current composition of waste paper from paper that has already been recycled (e.g. newspapers) and fresh fiber (e.g. from magazines), no “recycling collapse” is to be feared.

Other types of deinking that are occasionally also used are enzymatic deinking or "DAF" (dissolved air flotation).

Chemicals

Similar to washing laundry, detergent is required for deinking . The prerequisite for removing the printing ink from the waste paper stock is the separation of the color particles from the fibers. For this purpose, the necessary chemicals are usually added when the waste paper is defibrated in the pulper or drum. Deinking recipes contain caustic soda , soda waterglass , hydrogen peroxide and a surfactant (soap). These chemicals help to dissolve the waste paper and, together with the mechanical stress, cause the printing ink to detach from the fibers. In order to fully utilize the effect of the chemicals, certain reaction times must be observed. For this reason, the dissolved waste paper stock is usually stored in large vats for some time after defibering before it is cleaned and deinked. Wood-containing waste paper in particular yellows slightly under the action of caustic soda, which is why peroxide is already added here. This bleaching agent counteracts yellowing of the fibers, supports the detachment of the printing inks and can also cause a bleaching effect (destruction of bleach-sensitive dyes). Peroxide is easily broken down by heavy metal ions, which is why a peroxide stabilizer is often added shortly beforehand in the pulper . This is done using water glass (sodium silicate), which is still the best peroxide stabilizer today; Complexing agents are practically no longer used in waste paper processing. Water glass also supports both ink detachment and ink removal during flotation. If the printing ink is detached from the fiber, a collector has to bind the printing ink particles to itself and ensure that they attach to the air bubbles during flotation. This is the job of the washing-active substances: mostly soaps (anionic surfactants), which experience has shown to achieve the best results with conventional deinking. Surfactants improve the removal of the printing ink. Therefore, they are usually added in the pulper, usually 0.5-1.0%. In contrast to washing laundry, surfactants need hard water, at least 1.8 mmol / l (10 ° dH ), for deinking . Calcium ions are added to soft water or synthetic surfactants are used. In the course of deinking, most of the chemicals are broken down (e.g. hydrogen peroxide turns into water). The surfactants are removed with the flotation foam.

Residues

The deinking residues, so-called deinking sludge, consist of fillers (calcium carbonate, kaolin, silicates), fibers, extracts (fats, soluble printing inks and coating binders) and fine substances (insoluble printing inks and coating components, adhesive components). Thermal treatment ( waste incineration ) plays a central role in the recovery of these materials . Almost all residues from the paper industry have a relatively low solid content, but due to the high content of organic components they usually have such a high calorific value that they burn without a supporting fire, i.e. energy is generated. That is why more than 55% of the deinking residues are burned as substitute fuel in the paper mills' own power stations or externally in order to generate energy. The non-combustible components remain as (possibly usable) ash , slag and filter dust.

Around 42% of the deinking residues are recycled. For example, because of their high fiber content, they are used in the manufacture of perforated brick . Here they are a welcome aid to porosity - when the bricks are fired, the fibers leave tiny cavities that improve the heat-insulating effect of the bricks. Deinking residues are also suitable for cement production because of the high aluminum content (from the kaolin , which is used, among other things, in paper coating). Depending on the clay used, adding residues can improve the quality of the cement. Organic substances such as paper fibers or residues from printing inks burn when the cement burns.

Web links

Wiktionary: Deinking  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ a b Jürgen Blechschmidt (Ed.): Altpapier. Regulations - recording - processing - machines and systems - environmental protection. Fachbuchverlag Leipzig in Carl-Hanser-Verlag, Munich 2011, ISBN 978-3-446-42616-0 .
  2. ↑ Lothar Göttsching, Heikki Pakarinen (Ed.): Recycled Fiber and Deinking (= Papermaking Science and Technology. A Book Series covering the latest Technology and future Trends. Vol. 7). Fapet Oy, Helsinki 2000, ISBN 952-5216-07-1 , pp. 12-141.
  3. ↑ Jürgen Blechschmidt (Ed.): Altpapier. Regulations - recording - processing - machines and systems - environmental protection. Fachbuchverlag Leipzig in Carl-Hanser-Verlag, Munich 2011, ISBN 978-3-446-42616-0 . P. 89
  4. ↑ Recovered paper: List of European (CEPI / BIR) standard grades and their qualities. ( Memento of the original from October 5, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: alpa-rohstoffhandel.de. Retrieved on July 4, 2017 (PDF; 22 kB). @1@ 2Template: Webachiv / IABot / www.alpa-rohstoffhandel.de
  5. ↑ INGEDE website for deinking digital prints : Significant damage from indigo prints.
  6. ↑ Bhattacharyya, Manoj K .; Ng, Hou T .; Mittelstadt, Laurie S .; Hanson, Eric G .: Deinking of Digital Prints: Effect of Near-Neutral Deinking Chemistry on Deinkability . In: Journal of Imaging Science and Technology . tape 56 , no. 6 , November 2012, p. 60503-1-60503-5 (5) , doi : 10.2352 / J.ImagingSci.Technol.12.56.6.060503 .
  7. ↑ Austrian Ecolabel for Print Products (UZ24) , long version (pdf).
  8. ↑ Decision of the Commission of August 16, 2012 laying down the environmental criteria for awarding the EU eco-label to print products , accessed on January 10, 2014.