TM 5-814-8
reused because the iron is a contaminant in the
(3) Cyanide destruction. Cyanides are found
fixing process. The high residual concentration of
principally in metal plating wastes (including
those wastes from metal-renovation operations)
potentially toxic metal also requires that bench
and/or pilot scale studies be used to establish the
and photographic processing wastewaters. The
treatability of the waste by conventional biologi-
most toxic form of cyanide is hydrogen cyanide
(HCN), while the complex iron cyanides (Fe(CN6)-4
cal systems.
and (Fe(CN)6)-3 and the cyanate (CNO)- are less
(c) Electrodeposition. Like metallic replace-
ment, electrolytic recovery is used to recover
toxic by several orders of magnitude. The most
valuable metals such as silver or copper from
widely used cyanide destruction process is alka-
line chlorination. Other treatment processes which
photographic processing, brass pickling or copper-
have been used in actual practice include oxida-
plating wastes. When a direct electrical current of
tion using hydrogen peroxide (including the pro-
t h e proper density is passed through the
wastewater solution, the metal in solution plates
prietary "Kastone" process), and ion exhange
out in a pure form on the cathode. The electro-
(32)(33)(34).
lytic method may be operated continuously or
(a) Alkaline chlorination. Alkaline chlorina-
batch-wise, is effective over a range of 1000 to
tion involves oxidation of the cyanide to carbon
100,000 mg/L of influent metal and may produce
dioxide and nitrogen gas using chlorine in a high
an effluent as low as 500 mg/L of metal. How-
pH solution. This is normally a single-step reac-
ever, close supervision is required in order to
tion requiring about 4 hours with a solution pH
maintain proper current density (30)(86)(130).
of 11. A two-step operation consists of cyanide
Again, the residual metal concentrations are high
conversion to cyanate at pH of 11, requiring
about 30 minutes, followed by complete destruc-
enough to limit biological treatment of the waste.
tion of cyanate to carbon dioxide and nitrogen
(d) Ion exchange. Ion exchange technology
has been developed for treating chromium wastes
gas at pH of 8, requiring another 30 minutes.
About 5 mg/l of excess chlorine is maintained
from plating processing to include chromium
(129). Vigorous agitation is required, especially
detoxification or recovery, water reuse and heat
when metal-cyanide complexes are present, to
recovery from hot rinses. This is normally a
prevent precipitation of untreated cyanide salts
continuous flow process rather than a batch-type
(34)(130). Generally, flows smaller than 20,000
operation. Mixed wastes of chromium and cya-
gallons per day use batch treatment in two tanks,
nides can be treated first by a cation exchanger
in which one tank of waste is treated while the
to remove metals from complex metal cyanides
other is filling. A continuous treatment scheme
generating hydrogen cyanide, and then by an
requires instrumentation to control the chemical
anion exchanger to remove the liberated cyanide.
additions, and is normally uneconomical for small
The concentrated solution formed by regenerating
flows. Either chlorine gas or hypochlorites may
the exchange resins can be a source of recoverable
be used as the chlorine source, depending on
product in many cases (34). Ion exchange is also
economics and particular preference. Either so-
being investigated for the recovery of silver from
dium hydroxide or lime is used to raise the pH
photographic processing wastes, chromate from
(34)(109).
cooling water system blowdown (115) and cad-
mium from plating solutions.
may be oxidized to cyanate by hydrogen perox-
ide. This process is used in Europe and has the
recover heavy metals particularly chromate from
advantage of not introducing an additional pollut-
some plating solutions. Evaporation by applying
ant (residual chlorine) into the water (33). The
heat or vacuum to the solution may be employed.
proprietary "Kastone" process is basically a hy-
The distilled water from evaporation is reused as
drogen peroxide-formaldehyde method of cyanide
process rinse water (129). Rinsing with high
oxidation. Formaldehyde reacts with the cyanide
to form formaldo-cyanohydrin which is readily
purity water results in low rinse water use.
oxidized by the hydrogen peroxide. This process
(f) Reverse osmosis and ultrafiltration. Re-
is particularly advantageous for plating waste
verse osmosis and ultrafiltration processes have
been rapidly improved in recent years, and are
treatment because the hydrogen peroxide also
precipitates bevy metals as oxides (124).
used in several cases to treat plating rinse waters.
Use of membrane processes for treatment of
(c) Ion exchange. Ion exchange using a
cooling water blowdown for dissolved solids and
strong base anion exchange resin can remove
c h r o m a t e removal has also been reported
cyanides effectively from plating wastes, although
not always from photographic wastes due to resin
(45)(50)(92).
6-29
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