Foam Separation of Non-Surface-Active
Anions from Industrial Wastes
ROBERT B. GRIEVES, Professor and Chairman
DIBAKAR BHATTACHARYYA, Research Associate
Chemical Engineering Department
University of Kentucky
Lexington, Kentucky
INTRODUCTION
Inorganic and organic ions have been removed and concentrated from aqueous
solution by foam separation process, with most of the work referenced in three
excellent reviews (1,2, 3). An ionic surface-active agent with the long-chain ion of
opposite charge to the ion (colligend) to be foam separated is added to the solution;
surfactant plus colligend are accumulated at the gas-liquid interfaces associated with
generated gas bubbles and are separated in a foam or froth. Before foam separation,
the colligend may be complexed or precipitated. In foam separation, if no insoluble
product between surfactant and colligend is formed, the process may be called foam
fractionation; if an insoluble product results from interaction between the surfactant
and the colligend, the process may be called ion flotation; if the ion is first
precipitated and then the precipitate is floated (with or without the addition of a
surfactant), the process may be called precipitate flotation (4, 5, 6, 7).
In spite of the extensive work done on the foam separation of ions, including
recent studies which include the discussion of possible mechanisms (8, 9, 10, 11),
very little attention has been paid to the relative rates of flotation of the surfactant
and the oppositely-charged colligend. Such information is vital to the establishment
of the stoichiometry of the product that is foam separated and to at least a
qualitative understanding of the mode of interaction between the surfactant and the
colligend. In the large majority of the studies which have been made, the
concentration of the surfactant during the course of a foam separation experiment
has not even been monitored. Limited consideration has been given to relative
removals of surfactant and colligend (12, 13, 14, 15). However, these investigators, in
the development of a "relative fractionation parameter", did not use rate data, but
instead relied on data at long foaming times, corresponding to no further foam
formation. Such data are not ideally suited to the establishment of the stoichiometry
of foam separation, because they yield only the approximate rates of flotation
averaged over long time periods.
In this study, the instantaneous rates of foam separation of both surfactant and
colligend are presented for a cationic surfactant and each of the anions: I", HCr04",
S2032", and Ag(S203)^-(Ag(S203)~). The stoichiometry of the foam separated
product is established in"each case and the data are interpreted in terms of the mode
of interaction between surfactant and colligend.
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