Dissolved-Air Ion Flotation of Industrial
Wastes Hexavalent Chromium
ROBERT B. GRIEVES, Professor and Chairman
Department of Chemical Engineering
University of Kentucky
Lexington, Kentucky
GREGORY A. ETTELT, Director of Laboratories
Metropolitan Sanitary District of Greater Chicago
Chicago, Illinois
J. THOMAS SCHRODT, Assistant Professor
Department of Chemical Engineering
University of Kentucky
Lexington, Kentucky
DIBAKAR BHATTACHARYYA, Fellow and Research Associate
Department of Chemical Engineering
University of Kentucky
Lexington, Kentucky
INTRODUCTION
Foam separation has been used successfully on a process scale for the removal
of ABS and other soluble organics from secondary sewage effluents and on a pilot-
plant scale for the concentration of radioactive cations from nuclear wastes. Ion
flotation, a foam separation process, has been applied to hexavalent chromium
(1,2,3,4,5) and to cyanide complexed by iron (6,7) in the laboratory and has
been used for the first time (8) on a pilot plant scale.
The ion flotation process involves the addition to the waste of a cationic surface-active agent, oppositely-changed to the HCrO.~(Cr2p72") lon which is to be
separated. The resulting surface-active complex which is a colloidal suspension
of particulates produced by reaction between the surfactant and the dichromate,
is floated to the surface' of the suspension by gas bubbles at the interfaces of which
it is absorbed, and a froth is formed. A polymer may be useful in promoting the
aggregation of the collodial particulates, providing more efficient flotation.
Dissolved-air flotation, based on the release of a pressurized flow in which air is
dissolved, generally produces bubbles of diameter one to two orders of magnitude
smaller than dispersed-air flotation, with a concommitant increase in interfacial
area.
The objective of this study is to evaluate the efficiency of a pilot plant scale,
dissolved-air ion flotation unit for the removal and concentration of dichromate.
The evaluation is based on the dependent variables: the fractional flotation (removal) of dichromate, the enrichment ratio for dichromate, and the fractional
flotation (removal) of surfactant Each dependent variable is related analytically
to five independent variables by a multiple regression analysis. Optimum values
of the independent variables are determined and a chemical cost estimate is carried out, based on an experimentally varified process for surfactant recovery and
reuse.
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