Applications and Methods for Increased Liberation and Separation in the Deinking of Recycled Paper Fiber

Deinking flotation is an important part of the recycling process for used fibers, in which liberation is a key step in preparing the ink particles to be separated from fibers. The pigments of oil-based inks are hydrophobic and, therefore, can be readily separated from hydrophilic fibers by flotation, which is designed to selectively collect hydrophobic particles on the surface of air bubbles. On the other hand, the pigments in water-based inks are hydrophilic; therefore, they cannot be separated from fibers by flotation. Furthermore, the pigments in water-based inks are much smaller than those in oil-based inks and readily redeposit onto the fiber surfaces after they are liberated during the pulping process. Team members and I developed a novel method of measuring the degree of liberation of oil-based ink by ζ-potential measurement. The same method has been used to determine the degree of liberation of water-based ink. It has been found that adding a polyelectrolyte during the pulping stage greatly increases the liberation by increasing the disjoining pressure in the thin liquid film (TLF) of water confined between the pigment and fiber while at the same time preventing re-deposition. After the liberation, the pigments are then hydrophobized by adsorbing a surfactant on the surface to greatly improve flotation deinking. Finally, different types of flotation equipment were tested to identify the optimal flotation system to use in the deinking process.