Ion Flotation for Color Removal From
Kraft Mill Effluent
DONALD W. HERSCHMILLER, Project Engineer
Envirocon, Ltd.
Calgary, Alberta, Canada
RICHARD M.R. BRANION, Assistant Professor
Department of Chemical Engineering
University of British Columbia
Vancouver, B.C., Canada
INTRODUCTION
Pulp and paper wastewater treatment, in its present form, involves the removal of
settleable solids and the stabilization of the biodegradable organic fraction of the effluents.
The methodology for these two objectives has now been developed to a substantial degree.
This treatment does not, however, affect certain properties such as effluent color. This
underlies the need for at least another step in the conventional treatment scheme, a tertiary
stage to remove, or partially remove, the color containing, relatively non-biodegradable
portion of the effluent.
Historically (1), this fraction was considered to be relatively innocuous to aquatic life.
However, recent work (2,3) has shown that reduced light penetration and the altered
spectrum of the light which does penetrate to a measured depth may affect aquatic growth.
The soluble, non-biodegradable portion of the pulp mill effluent is composed primarily
of wood extractives and lignin degradation products which are formed during the pulping
and bleaching processes. A large portion of this material is chromophoric. The purification
technique which is the subject of this paper was for the most part, related to just the removal
of this chromophoric material.
Available information (4) indicates that the chromophores in pulp mill effluents are
largely, lignin derived. In the lignin degradation process quinonoid type compounds are
usually produced. Figure 1 illustrates a few of the general types of chromophoric structures
present in kraft pulping wastes. Figure 2 illustrates some possible sites of degradative attack
under the, comparatively, much less harsh conditions of the pulp bleaching operations. The
chemicals used in bleaching have been shown (3) to convert lignin to o- or p-quinoid type
structures which can then undergo further reactions. Figures 3 and 4 show both a specific
example and a generalized mechanism for this procedure.
An interesting property of kraft mill wastewater is exhibited by Figure 5 which is a plot
of total mill effluent color (or absorbance) versus the hydrogen ion concentration of the
effluent. This property, coupled with the fact that a portion of the chromophoric material
posses structures very similar to those of such common pH indicators as phenolphthalein,
congo, red, or phenol read lead the investigators (4) to the conclusion that "the effluent
behaves, in a sense, as a pH indicator" and thus "a substantial portion of the chromophoric
material" must posses a negative charge, at least under alkaline conditions. Based on this
conclusion the work reported in this paper was undertaken, using the ion flotation
technique with a cationic surfactant as the foundation upon which to build. It is interesting
to note, at this point, that other investigators (5), proceeding from a slightly different
vantage point, embarked on similar studies.
Preliminary experiments with this ion flotation technique for kraft pulp mill effluent
decolorization proved to be successful. In itself, this event presented a problem; would it be
better to investigate a few specific areas, in depth, or to explore the technique as widely as
possible? After reviewing all considerations, the latter course of action was chosen.
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