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.
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.
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.