Foam Separations for Industrial Wastes:
Process Selection
ROBERT B. GRIEVES, Professor and Chairman
Department of Chemical Engineering
University of Kentucky
Lexington, Kentucky
INTRODUCTION
Foam separation processes have a great deal of potential for the treatment of
industrial and municipal wastes. Such processes can be subdivided into several
categories, including foam fractionation, ion flotation, microflotation, and precipitate flotation (1). In many cases, a surface active agent already present in the
waste can be utilized as a collector of soluble and/or particulates species, drawing and
attaching the species to generated air bubbles, and as a frother to produce a stable
foam in which the contaminant species is concentrated and removed. In other cases,
a surface active agent must be added to the waste, with the required charge of the
long chain surfactant ion determined by the nature and charge of the species to be
foam separated. Recent examples of foam separations for industrial wastes include
the foam fractionation of black liquor from sulfate pulping, the flotation of oily
iron-dust, the foam separation-scavenging of radioactive isotopes, and the foaming of
acid mine water combined with domestic sewage (2). Very recently, preliminary
work has been done on the foam separation of sour waters for H2S removal (3) and
on the precipitate flotation of cyanide complexed by ferrous iron (4).
PARAMETER SELECTION
In order to establish quantitatively the effect of each independent variable of
significance in the design of a foam separation process, a means of expressing the
extent of separation must be adopted. No single parameter will suffice. Instead, two
parameters must be utilized: one contains a concentration of the component of
interest in an effluent stream from the process and the other contains a quantity of
the component of interest. Consider the schematic diagram of a continuous flow
foam separation process presented in Figure 1: L, F, B are flow rates, X|, Xf, X^ are
surfactant concentrations, Z\, Zf, and ZD are contaminant species concentrations,
and G is the air rate. Material balances are also given in Figure 1.
Of the multitude of pairs of parameters which can indicate the extent of
separation, perhaps the best are 1-XD/Xi (1-Zb/Zi for the contaminant) and
l-XDB/XiL (l-ZDB/ZiL). It is desirable to minimize X\, (and ZD) while maximizing
the flow rate of the effluent stream, B. This produces a foam stream highly
concentrated in the surfactant and in the contaminant species. Thus for optimum
operation, 1-Xd/Xt —»1.0 and 1-XbB/XiL ■+ 1-XD/Xi. For simplicity, 1-Xb/Xi is
designated 1-X and 1-XDB/Xi L is designated 1-XS.
The effect of four independent variables on the foam fractionation of a
cationic surfactant, ethylhexadecyldimethylammonium bromide (EHDA-Br) from
distilled water solution (5) is given in Figures 2 and 3, as indicated by the parameters
1-X and 1-XS. Of course 1-XS > 1-X and therefore the top curve or line always
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