Removal of Lignin by Foam
Separation Processes
THOMAS E. WILSON, Assistant Professor
MU HAO WANG, Graduate Student
Department of Civil and Environmental Engineering
Rutgers University
New Brunswick, New Jersey
INTRODUCTION
Lignin is a compound which is extremely difficult to remove from a waste
water stream. The form of lignin found in the black liquor waste stream of the Kraft
process is particularly difficult to remove (1). Lignin, and its decomposition products
are also considered to be the major source of color in natural waters (2). It is the
purpose of this paper to investigate one particular type of treatment process for the
removal of lignin: foam separation.
Previous investigators have studied various other methods for the removal of
lignin. These include biological processes (3,4), physicochemical processes (5,6) and
combinations of these two type processes (8). A partial review of these processes
may be found elsewhere (9). All of these processes have been shown capable of color
(lignin) removals in excess of 90 per cent.
The work presented herein is particularly concerned with two types of foam
separation processes: foam fractionation and ion flotation. The former process relies
upon the natural surface activity of lignin which causes the lignin to migrate to air
water interfaces (i.e. bubbles) and to be removed at the liquid surface in the resulting
foam. Ion flotation relies upon the soluble anionic lignin reacting stoichiometrically
with a cationic surfactant to form an insoluble precipitate. Further additions of
surfactant are then used to remove this lignin-surfactant precipitate by causing it to
be adsorbed onto the surface of air bubbles and subsequently removed in the resulting foam.
Several system parameters are known to be of importance in foam separation
processes (10). Thus the effects of pH, type and dosage of surfactant, and dissolved
as well as suspended solids concentration are all considered in this work. In order to
make this work applicable to a broad range of color problems, the processes are
studied at several lignin concentrations over a range of 20 to 2,000 units (as color).
APPARATUS
All experiments were performed using the apparatus shown schematically in
Figure 1. The actual flotation occured in a 24 in. tall lucite column which had a 4
in. diameter (600cc) BUchner funnel containing a fritted glass disc (prorsity: 40-60m)
as its base. The column was sealed at the top. A 1 in. diameter port which was
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