Fractionation Prior to Flotation - A New Approach for Deinking Technology, 1990 Pulping Conference Proceedings

High consistency repulping and flotation are considered as state-of-the-art technology for the deinking process. Washing (ash removal), additional bleaching stages, or hot dispersion combined with bleaching and postflotation are established technologies to improve the quality and value of deinked pulp.

More recently the fractionation of deinked pulp into a long fiber fraction and a fines fraction are discussed as an ultimate process for upgrading of wastepaper. The long fiber fraction can be bleached very efficiently at high consistency with hydrogen peroxide. The fines fraction can be bleached only with reductive bleaching agents such as sodium dithionite (hydrosulfite) or FAS. The two different pulp qualities resulting from this procedure result in flexibility in papermaking and lower bleaching costs compared with conventional peroxide post-bleaching of the unfractionated deinked pulp.

Conventional flotation of printing ink from waste paper pulp requires a relative high amount of energy input for optimum air dispersion in the fiber suspension. However, ink removal is still not complete. The fiber yield is also lowered primarily by coflotation of the longer fibers.

These disadvantages of conventional flotation deinking can be overcome. Combined with the advantages of a fiber fractionation this results in a new process design with wastepaper repulping, fractionation/washing before flotation, and separate treatment of the long fiber and fines fraction. The long fiber fraction, which is almost similar to an extremely washed pulp, has a higher brightness than conventional deinked pulp.

Brightness can be further increased with a subsequent high-consistency hydrogen peroxide post-bleaching stage.

The fines fraction is cleaned of printing inks by froth flotation and subsequently post-bleached with a reductive bleaching agent such as sodium dithionite or FAS. The advantages of the new process are: smaller flotation capacity, energy savings, higher yield and optimum bleaching conditions for H201 bleaching in the long fiber line. Additionally, greater flexibility for the separate or combined use of the long fiber and the fines fraction is achieved.