Ion Flotation for Industrial Wastes:
Separation of Hexavalent Chromium
R. B. GRIEVES, Associate Professor of Civil Engineering
K. Y. SHIH
T. E. WILSON, Graduate Student
S. M. SCHWARTZ, Graduate Student
Illinois institute of Technology
Chicago, Illinois
INTRODUCTION
Ion flotation (foam separation) has been employed for the removal of inorganic anions and cations from aqueous solution. The most extensive application
has been the concentration of radioactive cations such as strontium and cesium
from nuclear waste solutions (1,2). The process involves the formation of a surface-active complex by means of the addition of a surface-active ion of opposite
charge to the ion to be separated. The complex is then floated to the surface of
the solution by means of generated gas bubbles, and a foam is formed. If the
complex is soluble and further enrichment is obtained in the foam, or if the process involves electrostatic ion attraction, it may be termed foam separation. Two
excellent reviews have been presented in the literature(3,4). If the complex is
insoluble and is floated from solution in the form of colloidal-size particulates,
the process may be termed ion flotation. An extensive discussion and review of
applications has been presented by Sebba (5).
A possible application of ion flotation would be for the removal of heavy
metals from wastes from the metal finishing industries. Hexavalent chromium
appears in the wastewaters from industries concerned with the cleaning of metals
or with the coating of them with chromium. Chromic acid is used in plating
plants for several purposes, including plating solutions, anodizing solutions, chro-
mating solutions, and paint pretreatment solutions. Although the concentrated
solutions are disposed of infrequently, the rinses following the baths are continually discharged to rivers or sewers. Since many states require effluent chromium
concentrations not to exceed 2.0 mg/1 and since the U. S. Public Health Service
drinking water standard is 0.05 mg/1, extensive in-plant treatment is provided.
The recovery of hexavalent chromium into concentrated streams is a benefit resulting from the waste treatment processes. A number of techniques are used for
chromium separation and recovery, including ion exchange and precipitation (6).
New methods are constantly being sought to improve the economics of the recovery process.
The objective of this investigation is the establishment of the feasibility of
ion flotation for the separation of hexavalent chromium from aqueous solution,
using a cationic surfactant which would form an insoluble complex with dichro-
mate ion. Batch and continuous flow experiments are included, with major emphasis placed upon the effects of dichromate concentration and surfactant concentration. The influence of pH and possible interferences from other anions are
taken into account.
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