Defluoridation of Wastewater
WALTER ZABBAN, Chief Engineer
ROBERT HELWICK, Chief Chemist
The Chester Engineers, Inc.
Coraopolis, Pennsylvania 15108
INTRODUCTION
Fluorides are discharged in significant quantities in effluents from glass
manufacturers, from electroplating operations, aluminum and steel producers and
processors, and fertilizer manufacturers.
In the glass manufacturing industry fluoride wastes originate either from chemical
frosting, etching or polishing operations, as well as from wet scrubbing processes. Among
the major constituents found in wastewater from frosting operations are, in addition to
fluorides, ammonia and siliceous compounds. The wastewater from etching and polishing
operations does not contain ammonia but may contain sulfates.
Where lead-alkali silicate or borosilicate glasses are manufactured in addition to the
more common soda-lime glass, other important constituents such as lead and boron will be
found in the fluoride-rich wastewater. Although other constituents may be found in the
wastewater these and those mentioned above are listed because they may and often do have
an effect on the efficiency of removal of fluoride by chemical treatment.
In the effluents from electroplating operations the presence of fluoride may be due to
surface preparation processes or to electroplating with metal fluoborate salts.
In the steel production operation the presence of fluoride is usually attributed to
pickling of stainless steel with mixtures of nitric and hydrofluoric acid, in which case
additional constituents present in the effluents would be iron, nickel and chromium.
In the aluminum production industry the presence of fluoride in wastewater is
attributed to the wet scrubbing of emissions from reduction furnaces in which aluminum is
separated from bauxite ore in the presence of cryolite, Na3AlF6.
In the manufacture of phosphate-rich fertilizers, fluoride which is initially present in
the phosphate rock in the form of a calcium fluorophosphate(fluorapatite, Ca5F(P04)3) is
liberated by acidulation with sulfuric acid in the form of silicon tetrafluoride, SiF4, which is
converted to fluosilicic acid, H2SiF6-nH20, by hydrolysis in water.
In addition to the above-mentioned sources of wastewater, fluoride is found in
discharges from certain organic chemical manufacturing processes where boron trifluoride
(BF3) catalyst is used; in discharges from the beneficiation of ore containing feldspar, where
the fluoride is present in the form of a complex ion; in the effluent from coke-oven plants,
and in the effluents from the manufacture of semiconductor and /or electronic components.
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