De-inking is the process of removing the dried printing ink when paper is recycled.
There are two important processes for deink-ing pulp: washing and flotation. Both processes use the same chemicals. Washing, more common in the US, is easier, but up to 40 % of the fibres are lost. Very small ink particles and soluble inks can be removed. Flotation de-inking, frequent in Europe, relies on air bubbles producing a foam, which carries ink particles to the surface. It is more efficient than washing, but only for particles with more than 10 µm in diameter.
So the ink employed earlier is of crucial importance to the flotation de-inking process: Inks that give relatively big flakes to remove are more welcome; inks which dry chemically are hard to remove if they came into contact with the fibre before drying; coated papers are easier to de-ink than uncoated ones.
Principle of flotation
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(Source: Fonds der Chemischen Industrie, Germany; imageseries "Tenside")