'Reductive' Deinking: An Alternative to Peroxide for the Deinking of Wood-Based Papers, 1994 Recycling Symposium Proceedings

Maurice J. A. Hache

Conventional deinking of wood based papers utilizes sodium hydroxide in a pulper to swell and deink (i.e. saponification of resinous ink vehicles) the recycled pulp fibers. Because of the high pH’s encountered by the pulp due to the sodium hydroxide, some alkaline darkening of the pulp is usually observed. To off-set this darkening, an oxidizing chemical such as hydrogen peroxide is added to the pulper along with sodium silicate and achelant such as DTPA.

We have discovered that certain reducing agents can be substituted for hydrogen peroxide and results in ink removal and pulp brightnesses that are equivalent to that obtained by conventional ("oxidative")deinking at a lower chemical cost. Laboratory and pilot plant scale testing has demonstrated that "reductively" deinked pulp is whiter (i.e., lower yellow-blue axis, b* values) than conventionally deinked pulp. In addition, bleaching tests indicate that the brightness response of "reductively" deinked pulp to hydrosulfite, peroxide and two-stage peroxide-hydrosulfite (PY) bleaching is comparable to that obtained with conventionally deinked pulp.