| Before printed paper, such as office waste
and newspapers, can be recycled into high quality paper products the ink
needs to be removed, otherwise it will be dispersed into the pulp and a dull
grey paper will result. There are two
main processes for de-inking waste paper known as washing
and flotation.
The waste paper is put into a pulper with a
large quantity of water and broken down into a slurry. Contaminants or
'contraries' such as staples and plastic are removed by wire mesh machines
and a mechanical action. Most of the water containing dispersed ink is
drained off from the pulp through slots or screens that allow small
particles through, but not the pulp. Water can be added to rinse the fibres
and drained to remove more of the ink. Adhesive particles known as 'stickies'
are removed by fine screening.
About 80% of the original fibre is recovered
by this process (though it will depend on the type of washing equipment
being used) with the remaining 20% of ink, clay, filler, plastics etc. left
behind.
De-inking by washing has been used with
great success on 'woodfree' waste grades to produce pulp for writing papers
and tissue; and on old newspapers to produce a stock for newsprint
manufacture. It is more effective than the flotation process at removing
smaller ink particles.
Again the waste paper is made into a slurry
and the contaminants are removed. Then special surfactant chemicals are
added which makes a sticky froth on the top of the pulp. Air bubbles are
blown through the pulp and these carry the ink to the surface. As the
bubbles reach the top a foam layer is formed that traps the ink. The foam
must be removed before the bubbles break or the ink will go back into the
pulp. Because the ink is removed from the flotation machine in a
concentrated form, the flotation system does not require a large water
treatment plant.
When the flotation method is used to de-ink
old newspapers, around 30% used magazines are usually added. The clay
present in coated papers can improve de-inking efficiency as the ink
attaches itself to the clay particles before floating to the surface. The
flotation method is more able, than the washing method, to remove larger ink
particles.
Yields from flotation de-inking are quoted
as 90 - 95% but filler is not removed to the same extent as in the washing
process.
Once the pulp has been de-inked it is ready
to be made into paper.
The most common performance measurements of
the de-inking process are paper brightness (measured by a brightness meter)
and the number of ink specks on a sheet surface (measured by visual
inspection using a magnifying lens). However, the mechanical properties of
the paper, such as tear strength, also need to be determined.
Paper recycling is increasing, there are
approximately 420 de-inking mills world-wide (120 having been completed in
the past 4 years).
Newsprint, writing and office papers, and
tissue are made from de-inked recycled paper and research is continuing to
further improve the de-inking process. |
