|
De-inking
Before printed
paper, such as office waste and newspapers, can be recycled the ink needs to be
removed, otherwise it will be dispersed into the pulp and a dull grey paper
would result.
There are two
main processes for de-inking waste paper - these are known as washing and
flotation.
Washing
The waste paper is placed into a pulper with large quantities of water and
broken down into a slurry. Contraries -such as staples - are removed using
centrifugal screens. Most of the water containing the dispersed ink is drained
through slots or screens that allow the dispersed ink particles through, without
taking the pulp. Adhesive particles, known as 'stickies' are removed by fine
screening.
Flotation
Again the waste is made into a slurry and contaminants removed. 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 inks 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.
|
