Deinking in the Pulp and Paper Industry is the industrial process of eradicating printing ink from paper fibers of recycled paper to make it a deinked pulp. It can be achieved by a combination of mechanical action and chemical means.

Depending on the kind of paper and the needs of the end product, a range of deinking technologies can be used. The most common types of deinking are as follows:

Flotation Deinking:

In flotation deinking, ink is eliminated with the addition of chemicals like surfactants and subsequent air introduction into the recovered pulp. The hydrophobic components like ink particles are then particularly floated & these components are then taken off in the resulting foam.

In a flotation deinking plant, the majority ink removal happens in the flotation process step. The ink particles are already detached from the fibers before to this stage. Flotation utilizes small bubbles which elevate the inks to the surface in a froth which is later skimmed away. The left-over pulp fibers are then at times bleached in order to increase their brightness levels.

Based on the deinked pulp specifications, sometimes a second flotation loop is also utilized for supplementary ink removal. Because of the range of papers collected, the mix of grades used as source papers, automated process control is frequently used to continuously supervise and alter the quality of the deinked pulp.

Bleaching:

In the bleaching type of deinking, the destruction of colorants in inks and the brightening of the pulp is done by using the bleaching agents like hydrogen peroxide and sodium dithionite. They i.e. the agents are added during the pulping step and/or in a completely different bleaching step.

Alkaline hydrogen peroxide is the most regularly used bleaching agent for mechanical pulp whereas Sodium dithionite also called as sodium hydrosulfite is another important reagent used for brightening mechanical pulps.

Washing:

Washing is the removing of inks and other unnecessary components in the recovered pulp like mineral fillers by washing the pulp in a diluted form on a wire screen. The fibers are got back on the screen and the filtrate is treated to eliminate the unwanted solid material. This type of deinking is effective only in the removal of small particle size inks.