Cyanide Destruction on Trickling
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C. FRED GURNHAM
Head Department of Chemical Engineering
Michigan State University
East Lansing, Michigan
Cyanides are among the most toxic compounds that occur frequently
in industrial effluents. Cyanide wastes originate not only in the electroplating and metal-treating industries, but also in coke and gas manufacture and in the production of various chemicals, plastics, and synthetic fibers. Their principal sources are the metal industries, including
the electroplating of such metals as copper, zinc, brass, cadmium, and
silver, and various heat-treating operations such as cyanide hardening.
Because most metal-finishing industries are located in cities, their
wastes are usually discharged to municipal sewers rather than directly
to streams. The effects of cyanide and metal wastes on sewage treatment plant operation are often disastrous. A recent report of the American Public Works Association (1) lists "metals and metal plating"
third in a long list of most "objectionable" types of industrial wastes
discharged into municipal sewers. Only dairy and meat-packing wastes
were listed by a greater number of cities—probably because most cities
have these industries, and their organic load may be overwhelming in
small treatment plants.
Cyanide wastes can be treated by any of several techniques in order
to destroy or render harmless their cyanide content. Currently the most
popular method is chlorination, which oxidizes inorganic cyanide compounds to substances that are harmless to sewage treatment plants.
Other oxidation techniques that have been adopted or recommended
include ozonation, electrolytic oxidation, and conversion to thiocyanate.
Acidification and volatilization of hydrogen cyanide gas into the atmosphere has been used on a large scale, but presents the obvious problem
of atmospheric pollution. Complexation of cyanide with iron salts,
with formaldehyde, or with picric acid is technically possible, as is
hydrolysis at high temperature, but these methods are uneconomical or
unsatisfactory for other reasons.
Effects of Cyanide Discharged to Sewers
Cyanide waste discharges into municipal sewers may cause troubles
for any of several reasons.  Toxicity hazard to sewer workers and treat-
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