Pollution Prevention and Control Technologies for Plating
Section 6 - Wastewater Treatment
6.2 CONVENTIONAL TREATMENT TECHNOLOGIES
6.2.3 Cyanide Oxidation
Nearly all electroplating shops that generate dilute cyanide bearing
wastewaters employ alkaline chlorination treatment. This process,
which has been in commercial use for over 35 years, is suitable
for destroying free dissolved hydrogen cyanide and for oxidizing
all simple and some complex inorganic cyanides in aqueous media
(ref. 348). If properly designed, maintained and operated (good
pH and oxidation reduction potential control), the process will
oxidize cyanides which are amenable to chlorination (i.e., the
cyanide that can be oxidized by the alkaline chlorination process),
to less than 1.0 mg/l cyanide (ref. 39). Extensive sampling by
EPA showed that the average effluent concentration from a well
operated cyanide destruction system contains 0.18 mg/l of total
CN and 0.06 mg/l of amenable CN.
The cyanide in very stable complexes, such as ferrocyanides or
ferricyanides, is basically unaffected by chlorination (ref. 243).
Cyanide that is complexed with copper, nickel and precious metals
is amenable to chlorination, but reacts more slowly than free
cyanide and therefore requires excess chlorine for efficient cyanide
destruction. Concentrated cyanide wastes, such as spent plating
or stripping solutions, should not be reacted with hypochlorite
because the reaction can be violent, with the emission of chlorine
gas. These wastes can be batch treated by electrolytic oxidation
and thermal destruction (ref. 242).
A schematic of an alkaline chlorination process is shown in Exhibit
6-6. Most often the process is operated in two stages, with separate
tanks for each stage. A common exception is batch treatment systems,
where one tank is typically employed (ref. 38). Destruction of
dilute solutions of cyanide by chlorination can be accomplished
by direct addition of sodium hypochlorite (NaOCl), or by addition
of chlorine gas plus sodium hydroxide (NaOH) to the wastewater.
With direct chlorine gas addition, sodium hydroxide reacts with
the chlorine to form sodium hypochlorite. Selection between the
two methods is based primarily on economics and safety. The chemical
costs for chlorine gas treatment are less than half those of direct
hypochlorite addition (see Section 6.3.2), but handling is perceived
to be more dangerous and equipment costs are higher (ref. 39).
Relative to the safety issue, one expert source argues that modern
gas handling equipment is highly reliable and designed to be relatively
fail safe (ref. 38). The results of the Users Survey indicate
that 95% of the respondents with alkaline chlorination processes
use sodium hypochlorite and 5% use chlorine gas plus sodium hydroxide.
The shops that use chlorine gas tend to be the same shops that
select sulfur dioxide for chromium reduction (e.g., PS 025 and
PS 093). Generally, these shops have substantial cyanide wastestream
In the first stage of treatment, hypochlorite oxidizes cyanide
to cyanate. This reaction is accomplished most completely and
rapidly under alkaline conditions at pH 10 or higher (preferred
range is 11.0 to 11.5) (ref. 38). An oxidation period of 10 to
15 minutes is usually adequate (ref. 38); however retention times
up to 60 minutes are routinely used (ref. 39, 348). An ORP set-point
of approximately +325 millivolts is adequate for most operations.
A higher set-point may be needed, depending on the composition
of the wastewater. A set-point of +400 millivolts is considered
a maximum point (ref. 38). Potassium iodide-starch test paper,
which indicates residual chlorine, is sometimes used to determine
if the reaction is complete (ref. 243). To avoid producing solid
cyanide precipitates, which may resist chlorination, the wastewater
should be continuously and vigorously mixed during treatment (ref.
38, 39). The resulting cyanate is further oxidized to carbon dioxide
and nitrogen in the second stage. In this stage, the pH is lowered
to approximately 8.5 and additional hypochlorite is added. An
ORP reading of +600 to +800 generally signals a complete reaction.
The retention time of the second stage is typically 30 to 60 minutes;
however, times of 120 minutes are sometimes specified (ref. Delta
Pollution Control File).
Although a two-tank system is preferred, complete cyanide oxidation
to carbon dioxide and nitrogen can be accomplished in a single-stage
unit, provided close pH control is maintained (ref. 39).
When sodium hypochlorite is used, the reaction in the first stage
NaCN + NaOCl - -> NaCNO + NaCl
and in the second stage,
2NaCNO + 3NaOCl + H2O - -> 3NaCl + N2 + 2NaHCO3
Sodium hypochlorite consumption is usually estimated to be 25
to 100 percent greater than the stoichiometric requirement (approximately
7 lbs of Cl2 or 7.5 lbs of NaOCl per lb of CN); where the excess
is consumed by oxidation of organics and raising the valences
of metals in the wastewater (ref. 39). The results of the Users
Survey indicate that most shops are using substantially higher
dosages (up to five hundred percent or more). Higher dosages by
respondents may be the result of the formation of metal complexes
(e.g., due to inadvertently combining cyanide and nickel or iron
bearing wastewaters, use of unlined steel tanks, use of steel
anode baskets and not retrieving fallen parts from tank bottoms)
and/or poor pH control during treatment. The latter reason is
especially apparent for several shops that operate single stage
cyanide destruction processes and do not add acid to lower the
pH. As a result, the second part of the oxidation reaction is
slow and operators probably add higher dosages of NaOCl to compensate
for the speed of the reaction. Shops operating under these conditions
include: PS 058, PS 135, PS 204 and PS 210. It should be noted
that acid additions must be made under the correct conditions
or a severe safety problem could arise. Dilute acid is sometimes
used in place of concentrated acid to reduce the danger of operating
this process (ref. 243). Also, some equipment vendors provide
a caustic feed capability that is controlled by an independent
set-point, usually at a pH of 7.5 (ref. 38). The use of acid in
the alkaline chlorination process is discussed by Roy (ref. 38).
Alkaline chlorination systems have generally proven reliable if
well maintained. Use of a well-designed ORP control system is
highly recommended. Most problems with the system focus on failures
of this element. Exhibit 6-7 shows the response of various electrodes
to the cyanide-to-cyanate reaction end point. The graph shows
that the gold-plated electrode, although more expensive, gives
much better reagent addition control (ref. 39).
There are several alternatives to the alkaline chlorination process.
These are discussed in the following paragraphs.
Ozone oxidation of cyanide has been practiced as a substitute
technology for alkaline chlorination. It is effective in destroying
cyanide to the levels required by EPA. The advantage of using
ozone lies in reduced operating costs. Ozone is generated on-site
(typically by the silent electrical discharge method) and is less
expensive than chlorine or sodium hypochlorite. The equipment
cost is significantly higher, however, owing to the expense of
an ozone generator. Another disadvantage of the process is that
it does not oxidize cyanide past the cyanate stage, unless excess
ozone is used. Ozone oxidation requires 1.8 to 2.0 lbs of ozone
per pound of CN to reach the cyanate stage and 4.6 to 5.0 lbs
to reach complete oxidation. An advantage of ozone oxidation is
the absence of chlorine that can combine with organics present
in the wastewater to produce toxic compounds (ref. 39). Another
advantage is the ability of ozone to destroy zinc, copper and
nickel cyanide complexes (ref. 243). The ozonation process has
also been combined with UV radiation for the treatment of halogenated
organics (ref. 348, 386). None of the respondents to the Users
Survey employ ozone oxidation of cyanides.
Other alternatives to alkaline chlorination include: alternative
chemistries; electrochemical oxidation; thermal oxidation; and
Alternative chemistries include hydrogen peroxide (PS 020 and
PS 273) (ref. 386) and calcium hypochlorite (PS 174).
Electrochemical oxidation is sometimes used to destroy concentrated
cyanide wastes (e.g., >50,000 mg/l CN). A description of the
required equipment, including a small commercial unit, is presented
in AESF literature (PS 243).
Thermal treatment of cyanide wastes is performed by two of the
respondents to the Users Survey (PS 142 and PS 245). Their equipment
is provided on a rental basis by Cyanide Destruct Systems (Canada).
The equipment can be purchased for $40,000 or rented for $600
per week. PS 142 also indicated that they required a facility
modification/ancillary equipment purchase costing of $700. The
equipment is intended for concentrated cyanide wastes (at both
shops dilute CN wastes are treated by alkaline chlorination).
The rental unit is essentially a heated pressure chamber (35 gal,
450 to 470°F, 600 psi) with an agitator. PS 142 used the
unit to treat cyanide wastes containing 100,000 mg/l CN to approximately
25 mg/l. Disposal costs for these wastes were previously $800
to $1,000 per 55-gal drum. In operation, CN wastes are transferred
into the vessel and treated on a batch basis for 10 hours and
then discharged to conventional treatment. Ammonia gas generated
by the unit is vented to the atmosphere. PS 142 reported operating
costs, excluding labor, of $4,000 per year and a labor requirement
of 750 man-hours. They also indicated that they experienced temperature
and pressure control problems due to a seal leak (N2 and oil seal)
at the point of agitator entry into the vessel. PS 142 indicated
that the unit had a downtime of 50% and that they have discontinued
Chemical precipitation of cyanide can be achieved using ferrous
sulfate. This process precipitates the cyanide as a ferrocyanide,
which can be removed in a subsequent sedimentation process (ref.
386). Results of EPA sampling of such a process showed that an
average influent concentration of 2.7 mg/l CN was treated to 0.023
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