De-inking is an industrial process of removing printing ink from the fibers of recycled paper to make deinked paper.

Flotation process in de-inking

Froth flotation is the most commonly used de-inking process. It is adapted from the flotation process typically used in the mining industry. The principle of the flotation process is simple: in the flotation tank, fine bubbles are dispersed in the pulp. Air bubbles will rise, collecting the hydrophobic ink particles. Thus, ink particles are lifted to the top of the tank from where they are skimmed off and de-inked pulp is collected from the bottom of the tank.

The dispersed air bubbles play a key role in the process. Their properties are strongly influenced by the composition of the fluid medium that consists of water and flotation reagents, such as frothers, collectors, depressants, etc. Of all these reagents, frothers are surface-active substances that affect the formation and behavior of the air bubbles. The frother molecules adsorb on the bubble surface creating an adsorption layer. During the adsorption, the surface tension decreases until it reaches an equilibrium value. Under dynamic conditions, the kinetics of adsorption and desorption of surfactants to and from solution have a major effect on the behavior of bubbles. Understanding the rheological properties of the adsorbed layers is important for the characterization of commercial surfactants.