Deinking

A process for removing printing inks from recycled paper was invented by German jurist Justus Claproth in 1774.

The bales are opened and large foreign objects are sorted out on the conveyor belt to the pulper.

Film-backed pressure-sensitive tape stays intact: the PSA adhesive and the backing are both removed together.

[4] Pulpers are either batch, which uses a tub with a high shear rotor, or continuous, using a long, perforated drum.

Drum pulpers are very expensive but have the advantage of not breaking up contaminants, thus giving cleaner end product.

Materials which are more difficult to remove include wax coatings on corrugated cartons and stickies, soft rubbery particles which can make deposits and contaminate the recycled paper.

[6][7][8] In the deinking stage the goal is to release and remove the hydrophobic contaminants from the recycled paper.

The air bubbles lift the ink to the surface and form a thick froth that can be removed.

High quality deinking of office wastes and other printing papers often commonly uses a combination of washing and flotation.

Often the use of enzymatic deinking helps mills reduce their bleach usage or use cheaper furnish.

Dissolved air flotation (DAF) is used by some mills in the deinking stage and will remove some ink and filler (ash); however, it is mainly used to clarify the process water.

The unusable material left over, mainly ink, plastics, filler and short fibers, is called sludge.

The sludge is buried in a landfill, burned to create energy at the paper mill or used as a fertilizer by local farmers.

Water based flexographic printing inks with particle sizes below 5 μm and poor solubility in alkaline conditions may cause problems in deinking, especially in the flotation stage.

Additional issues arise when taking into account the number of chemicals potentially present in paper for recycling.