US EPA Office of Research and Development
          United States
          Environmental Protection
Office of Research and
Washington, DC 20460
December 2000
v>EPA   Capsule Report
          Managing Cyanide in
          Metal Finishing


                                                         December 2000
                Capsule Report

Managing  Cyanide in  Metal  Finishing
                U.S. Environmental Protection Agency
                 Office of Research and Development
             National Risk Management Research Laboratory
               Technology Transfer and Support Division
                      Cincinnati, OH 45268
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The U.S. Environmental Protection Agency (EPA) through its Office of Research and
Development funded and managed the research described here under contract num-
ber 8C-R520-NTSX to Integrated Technologies, Inc. It has been subjected to the
Agency's peer and administrative review and has been approved for publication as an
EPA document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.

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 protecting the Nation's land, air, and water resources. Under a mandate of national
 environmentai laws, the Agency strives to formulate and implement actions reading
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 ing data and technical support for solving environmental problems today and building
 a science knowledge base necessary to manage our ecological resources wisely,
 understand how pollutants affect our health, and prevent or reduce environmental
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 for investigation of technological and management approaches for preventing and
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       This publication has been produced as part of the Laboratory's strategic
 long-term research plan. It is published and made available by EPA's Office of Re-
search and Development to assist the user community and to link researchers with
their clients.
                             E. Timothy Oppelt, Director
                             National Risk Management Research Laboratory

This guide was prepared by Peter A. Gallerani, Integrated Technologies, Inc., Jeff
Lord, Black Company Environmental, and Kevin Klink, CH2M Hill. Douglas Grosse,
U.S. Environmental Protection Agency, Office of Research and Development, Na-
tional Risk Management Research Laboratory (NRMRL), was the project officer, and
performed technical review and editorial assistance. Dave Ferguson, NRMRL, served
as the technical consultant.

The following people provided technical review, editorial assistance, and graphic de-
       Dr. David Szlag                  U.S. Environmental Protection
                                      Agency, NRMRL
       Paul Shapiro                    U.S. Environmental Protection
                                      Agency, Office of Research and
                                      Development, Office of Science
       Joseph Leonhardt                Leonhardt Plating Co.
       Dr. John Dietz                   University of Central Florida
       Carol Legg                      U.S. Environmental Protection
                                      Agency, NRMRL
       JohnMcCready                 U.S. Environmental Protection
                                      Agency, NRMRL

 Notice	jj
 Foreword	jjj
 Acknowledgments	iv

 1.0 Introduction	1
    Background	1
 2.0 Cyanide Plating Chemistry	3

 3.0 Cyanide Toxicity	5

 4.0 Cyanide Safety	6

 5.0 Wastewater Treatment of Cyanide	8
    Alkaline Chlorination	8
    Metal Cyanide Complexes	9
    Oxidation of Cyanide with Hydrogen Peroxide	9
    Oxidation of Cyanide with Ozone	10
    Ultraviolet (UV) Oxidation	10
    Electrochemical Oxidation of Cyanide	10
    Thermal Oxidation	11
    Acidification and Acid Hydrolysis	11
    Other Cyanide Treatment....	11

 6.0 Source Reduction	12
    Carbonate Chemistry	12
    Other Contaminants	12
    Recovery Technologies	13
       Vacuum Evaporation	13
       Reverse Osmosis	13
       Ion Exchange	13
       Electrowinning	14

7.0 Cyanide Alternatives	16

8.0 Cyanide Monitoring and Analysis	17
    Wastewater Compliance Monitoring	17
    Cyanide Analysis	17
       "Standard Methods," Method 4500-G	\ """"."". 18
       EPA Method 335 Cyanide Amenable to Chorination	19
       ASTM D 2036 B	19
       EPA Method OIA-1677	19

9.0 Summary	20

                      Contents (continued)

References	21
A. Optimizing Operating Procedures	22
B. Best Management Practices	23


1.   Metal and Complexed Metal Electrode Potentials	4
2.   Cumulative Formation Constants for Cyanide Complexes	4
3.   Toxicity of Various Cyanide Compounds	5
4.   Cyanide Half-life Under Natural Degradation	6
5.   Concentrations of Free Cyanide in Solutions of Various Concentrated Metal
    Cyanide Complexes	g
6.   Cyanide and Non-cyanide Plating Processes	16

1.   Open process showing solution maintenance using periodic dump or bleed and
    countercurrent rinsing with a continuous wastewater discharge	
2.   Closed-loop process showing continuous solution maintenance and rinsewater
    recovery with natural evaporation	14

3.   Closed-loop process showing continuous solution maintenance and rinsewater
    recovery with reverse osmosis or vacuum evaporation	14

 4.  Open-loop process showing continuous solution maintenance and rinsewater
    recovery with natural evaporation and ion exchange/electrowinning	15

5.   Distillation apparatus for evaluating cyanide samples	18

6.   Flow injection analysis schematic	19

                                         1.0   Introduction
 The purpose of this document is to provide guidance to
 surface finishing manufacturers, metal finishing decision
 makers and regulators on management practices and con-
 trol technologies for managing cyanide in the workplace.
 This  information can benefit key industry stakeholder
 groups for implementing "cleaner, cheaper and smarter"
 environmental management of cyanide in the metal finish-
 ing industry. Key stakeholder groups  include the Ameri-
 can Electroplaters and Surface Finishers Society , the National Association of Metal Finish-
 ers , the Metal Finishing Suppliers
 Association  and the USEPA. It is
 important to understand existing practices as well as bold
 innovative ideas that enhance environmental performance
 in the metal finishing  industry. For  more information on
 new ideas in the metal finishing program, see .

 Cyanide has been used extensively in the surface finish-
 ing industry for many years; however, it is a hazardous
 substance that must be handled with caution. The use of
 cyanide in plating and stripping solutions stems from its
 ability to weakly complex many metals typically used in
 plating. Metal deposits produced from cyanide plating so-
 lutions are finer grained than those plated from an acidic
 solution. In addition, cyanide-based plating solutions tend
 to be more tolerant of impurities than other solutions,  of-
 fering preferred finishes over a wide range of conditions:
 (1) cyanide-based strippers are used to selectively remove
 plated deposits from the base metal without attacking the
 substrate,  (2) cyanide-based electrolytic alkaline descalers
 are used to remove heavy scale from steel and (3) cya-
 nide-based dips are often used before plating or after strip-
 ping processes to remove metallic smuts on the surface
 of parts. Cyanide-based metal finishing solutions usually
 operate at basic pH levels to avoid decomposition of the
 complexed cyanide and the formation of highly toxic hy-
 drogen cyanide gas.

 Cyanide complexes and free cyanide exist  in equilibrium
 depending on the pH of the solution. As a rule, lowering
the pH shifts the equilibrium forming hydrogen cyanide
 gas that can escape from the solution. Raising the pH
forces  a shift in the equilibrium that prevents hydrogen
cyanide formation and minimizes the loss of cyanide from
the plating solution. One exception is the strong cyanide
complex formed with gold. The potassium gold cyanide
complex is stable at acidic pH, and gold plating can effec-
 tively take place from a solution with a relatively low pH.
 Some complexes of cyanide are highly stable, such as
 iron, nickel or cobalt, and these complexes can cause prob-
 lems in effluent discharges, since they are stable and dif-
 ficult to destroy.

 Cyanide-bearing materials, solutions and wastestreams re-
 quire special handling and  management. Cyanide com-
 pounds are readily absorbed through the skin or lungs from
 dust or vapor. Fish populations are especially sensitive to
 cyanide, and fish kills can occur at levels less than one
 part per million (US EPA, 1979). Cyanide can also cause
 upsets at municipal wastewater treatment plants by dis-
 rupting biological treatment units. For these reasons, it is
 critical to limit cyanide compounds entering municipal
 waste treatment systems and the environment. In a typi-
 cal metal-finishing facility, cyanide-bearing wastestreams
 are segregated from other metal-finishing wastestreams
 and are pretreated using alkaline chlorination prior to other
 wastewater treatment.

 Cyanide use in metal finishing has become a focus area
 for governmental and non-governmental organizations.
 Though cyanide-related incidents in the metal finishing
 industry have been few, cyanide  use in the industry has
 been significantly reduced. Many facilities have turned to
 non-cyanide alternatives. Non-cyanide processes have
 been developed for copper, cadmium, indium and zinc plat-
 ing. Non-cyanide silver and  gold-plating processes have
 also been developed but are generally not well accepted.
 More effective substitutes for brass, bronze, silver, gold
 and other less common plating processes are still being
 developed. Non-cyanide alkaline descaling and metal-strip-
 ping processes are common and  utilize other metal
 complexers such as ethylene diamine triacetic acid (EDTA).
 Cyanide is usually replaced by strong chelating or
 complexing compounds, creating new process control and
 wastestream challenges. Furthermore, most non-cyanide
 replacements tend to be proprietary processes, with many
 of the technical process details concealed from potential
 users. This makes solution and rinsewater management
 more difficult.

 Residual cyanide in metal finishing sludge has become an
 increasing concern for metal finishers as the disposal op-
tions for cyanide-bearing sludge are limited and costs are
 high. Many metal finishers have adopted advanced cya-
 nide destruction and segregated precipitation systems to
control cyanide residuals in metal finishing sludge.

Environmental, health and safety requirements, coupled
with competitive pressures, have forced metal finishers to
adopt better process management practices. Advances in
operating practices, process control, chemical recovery
and pretreatment make it possible to use cyanide without
increasing risk to workers or the public. To manage cya-
nide efficiently, its toxicity must be understood and inad-
vertent exposure tightly controlled. In addition, the chem-
istry of the cyanide system must be controlled and moni-
tored to prevent fugitive emissions from the system. Con-
trol technology should encompass plating process tanks,
rinse tanks, recovery systems, waste treatment and air
emission control devices to enable a facility to safely and
effectively use cyanide, while protecting workers from sig-
nificant exposure and minimizing environmental impacts
from water, solid waste and air emissions.

Many operators and decision makers inside and outside
the industry assume non-cyanide processes to be envi-
ronmentally and occupational^ safer than cyanide pro-
cesses. The issues are much more complex than that.

This report covers various aspects of cyanide chemistry,
use, toxicity, problems and control.

                               2.0 Cyanide  Plating Chemistry
 Cyanide readily joins with a variety of metals. Bonding
 between the metal ion and cyanide (a ligand) occurs quite
 readily. Electrochemically, the formation of a metal-cya-
 nide complex can alter the reduction potential, changing
 the required potential (voltage) for metal deposition to oc-
 cur in plating. (Lowenheim, 1953; Swartz, 1996). Table 1
 shows the shift of the electrode potential for several com-
 mon plated metals when complexed with cyanide. This
 shift may improve plating, prevent immersion deposits from
 forming or shift the potential of two different species to be
 nearly identical. In electrolytic plating cells, the metal with
 the lowest potential (most negative) will typically plate first.
 If the electrical potentials are close, alloy plating can oc-
 cur. Cyanide use in brass plating shifts the potentials of
 copper and zinc from a difference of greater than one volt
 to a difference of approximately 0.1V that allows brass
 plating to occur.

 Each cyanide ion attaches to a metal via a coordination
 site and exists in equilibrium between the complexed spe-
 cies and free cyanide ions. When the pH of the system is
 lowered, free cyanide will combine with the available hy-
 drogen ions and form hydrogen  cyanide (HCN) gas that
 has the propensity to escape from  solution. Some pro-
 cesses exhibit small releases of hydrogen cyanide during
 plating, such as acid cyanide gold plating. Many variables
 govern the quantity released; however, actual hydrogen
 cyanide measurements above the tank have shown HCN
 concentrations in the range of 3-5 ppm (California State
 University, 1990). Most cyanide plating solutions are oper-
 ated at alkaline pH to prevent the potential release of HCN.
 Alkaline operation causes the solution to slowly absorb
 carbon dioxide from the air, forming carbonates. Carbon-
 ates are generally not an interference at low concentra-
tions (below 60 g/L), but as the concentration increases,
they will begin to precipitate, which can interfere with the
quality of the plated deposit. Consequently, cyanide solu-
tions, and other alkaline solutions, are generally not air-
agitated since solution aeration would introduce more car-
bon dioxide to the system and increase the carbonate build-
 up rate.
 Common cyanide metal complexes encountered in metal
 finishing are shown in Table 2. The formation constant is
 derived from the equilibrium expression shown, generally,
 in equations 1 and 2. In equation 1, metal and cyanide
 ions react to form a metal cyanide complex. However, in
 most of these types of reactions, some reactant will re-
 main after the reaction ceases. Completeness of the reac-
 tion is measured by comparing the relative amounts of
 reactants and products, shown mathematically in equa-
 tion 2. The greater the concentration of the reaction prod-
 ucts, the higher the value of the formation constant.
       MEX+ +yCN~ <=>[Me(CN)y]
An unfavorable reaction (release of HCN) exhibits a nega-
tive log of the formation constant. Table 2 shows that the
stability of the respective metal cyanide complexes can
vary a great deal. Iron complexes are approximately 10 to
15 orders of magnitude more stable than copper or silver
complexes. Copper and cadmium form complexes where
the additions of the second, third or fourth ligands do not
significantly increase the solution stability, and plating can
readily occur. The formation constant for gold is quite high;
however, as shown in Table 1, if the electrode potential for
the gold cyanide complex has been lowered significantly,
plating can occur. In general, deposition appears  to take
place from the lowest coordinated form (Lowenheim, 1953).
The formation constants for cyanide complexes show why
iron cyanide as ferrocyanide or ferricyanide is difficult to
destroy and why incomplete cyanide destruction  is pos-
sible for a variety of metal cyanide complexes. Further-
more, complexing can greatly effect the toxicity of the re-
sultant compound. For example, ferrocyanide is less toxic
than copper cyanide, which is less toxic than sodium cya-

Table 1.    Metal and Complexed Metal Electrode Potentials

Metal                Electrode Potential               Cyanide Complex
Source: Lange's Handbook of Chemistry 13th Ed.
              Electrode Potential
Table 2. Cumulative Formation Constants for Cyanide Complexes

Matal             LogK,           Log Kj            Log 1C,
Cadmium 5.48 10.6
Copper (I) 24.0
Gold (I) 38.3
Iron (II)
Iron (III)
Mercury (II)
Silver (I) 21.1
15.23 18.78
28.59 30.30

21.7 20.6


 Source: Lange's Handbook of Chemistry IS"1 Ed.

                                       3.0 Cyanide Toxicity
Many forms of cyanide are toxic to humans. Toxicity can
be attributed to interactions with low pH (acidic) solutions
and some biological systems to produce hydrogen cya-
nide. Hydrogen cyanide has a time-weighted average ex-
posure limit of 10 ppm for 8 hours. (Sax, 1989). Most of
the inorganic salts have exposure limits of a few parts per
million. Exposure can occur by absorption through the skin,
by inhalation of dusts or gas, or by ingestion. Exposure to
minor amounts of cyanide on the skin can result in derma-
titis. Certain species of fish are extremely sensitive and
can be killed by low levels of cyanide (US EPA, 1979).
Bluegill, salmon and trout are killed by levels slightly over
0.1 ppm cyanide. Compound levels below 0.1 ppm can
functionally effect metabolic and reproductive cycles. Cya-
nide levels that kill fish often do not adversely impact lower
aquatic organisms like crustaceans and mussels. Toxicity
may extend to microorganisms that digest sewage and

Chlorination can result in the formation and release of cy-
anogen chloride, with the exposure limit for cyanogen chlo-
ride more than an order of magnitude lower than for cya-
                                               Cyanide exposure in metal-finishing shops usually occurs
                                               via skin absorption and inhalation. Poor personal hygiene
                                               or improper use of personal protective equipment (PPE)
                                               can lead to ingestion. Careful cleaning and storage of tools
                                               and PPE are necessary to avoid potential exposure to
                                               cyanide. Handling reagents, solutions and waste can lead
                                               to skin absorption. Exposure to hydrogen cyanide and/or
                                               related gases resulting from plating operations present an
                                               inhalation hazard. Cyanide emissions from cadmium, cop-
                                               per and gold cyanide plating are known to release hydro-
                                               gen cyanide gas at low levels during the plating cycle (Elec-
                                               troplating, 1996). Sound process control practices limit gas
                                               emissions from process solutions and wastewater treat-
                                               ment operations. Ventilation is recommended for all cya-
                                               nide processes. Human exposure toxicity is typically acute
                                               rather than chronic. Exceeding exposure levels can result
                                               in disorientation, dizziness and nausea. Cyanide poison-
                                               ing occurs by blocking blood oxygen transfer, which can
                                               result in death by asphyxia. Table 3 lists key exposure
                                               data for cyanide compounds commonly used in metal fin-
Table 3.   Toxicity of Various Cyanide Compounds
Hydrogen cyanide
Potassium cyanide
Sodium cyanide
Physical Form
5 mg/m3
5 mg/m3
5 mg/m3
1 mg/kg human
10 mg/kg rat
2.85 mg/kg human
6.44 mg/kg rat
2.85 mg/kg human
Cyanogen chloride

Sodium cyanate

Potassium cyanate

Potassium ferricvanide






0.3 ppm
                   260 mg/kg mice

                   320 mg/kg mice

                   1600 mo/kg rat
Sources: (Sax, Merck)
TLV    threshold limit value is the time time-weighted average concentration for an 8-hour workday and 40-hour workweek to which a
worker may be repeatedly exposed without adverse effect.

lethal dose to 50% of a specified population.

                                       4.0 Cyanide  Safety
The hazards associated with cyanide use cannot be mini-
mized; however, the risks can be reduced through safe
handling practices. It is important to recognize potential
hazards and routes of exposure. Process operators, waste-
water treatment operators, maintenance personnel, labo-
ratory technicians, engineers, shipping and receiving clerks
and facility visitors can all be exposed to cyanide in differ-
ent ways and degrees. Cyanide exposure can occur through"
contact with solutions, rinsewater, wastewater, concen-
trated wastes, sludge, raw  materials, fumes, mists and
contaminated materials and equipment. Contaminated ma-
terials and equipment include filtration media, drums, buck-
ets, tanks, pumps, hoses, mixers, piping, ductwork, elec-
trodes, etc. Cyanide safety requires the development and
communication of procedures for the safe handling of cya-
nide reagents and residuals. Cyanide safety procedures
should include instructions for chemical storage, contain-
ment, piping, transportation, handling, use, protective
equipment, personal hygiene, monitoring and emergency
contingencies. All personnel who are exposed to cyanide,
including contractors and visitors, should receive appro-
priate training.

Managing cyanide safely requires effective segregation of
cyanide solutions, rinses, wastestreams, sludge, raw ma-
terials and other cyanide containing materials from acids
and other non-cyanide materials. The accidental mixing of
acids with cyanide causes a reaction that can quickly re-
lease dangerous amounts of hydrogen cyanide gas. Cya-
nide solutions (with the exception of gold plating) must be
maintained in an alkaline condition since even mildly acidic
conditions allow hydrogen cyanide gas to form and es-
cape. In addition, all sources of cyanide in the facility must
be identified and  controlled. Many surface finishing pro-
cess solutions can contain cyanide, including cleaners,
stripping solutions and chromating treatments.

Safe cyanide handling requires careful attention to per-
sonal hygiene. Workers must avoid skin and eye contact
through the use  of protective clothing and equipment.
Workers should keep a spare set of clothing at work in
case clothing becomes contaminated with cyanide. Ide-
ally, workers should shower and change  clothes at the
end of the work shift and workers should always wash up
before handling food or other items. Exposure to small
amounts of cyanide over a period of time can result in
dermatitis. Dermatitis, if left untreated, can develop into
sores and lead to infection, and provides an easy entry
point for cyanide into the body.
 Handling of solids should be limited to trained personnel,
 and solutions should be prepared in areas with adequate
 ventilation to prevent exposure to dust. Ventilation sys-
 tems designed for use in conjunction with solids handling
 should include dust collection. The appropriate dust col-
 lection technique will vary, depending on the quantity
 handled, and may include the use of dust masks for fur-
ther protection.

 Remote  exhaust systems on process and waste treatment
 tanks capture hazardous mists and fumes. Wet process
 ventilation may also require a scrubber to control air emis-
 sions. Air agitation of cyanide solutions should be avoided
 because it causes misting. Air agitation should also be
 avoided  since carbon dioxide in the air is acidic enough to
 liberate  hydrogen cyanide. Air agitation also enhances
 carbonate build-up by absorption of carbon dioxide in the
 alkaline  solution. Table 4 provides the half-life of cyanide
 at various temperatures.

 Alkaline chlorination treats cyanide wastewater. During this
 process, cyanide destruction occurs in two steps. Main-
 taining the proper pH is  essential to avoid the gaseous
 release of chlorine and cyanogen chloride and the forma-
 tion of hydrogen cyanide. Oxidation-reduction potential
 (ORP) devices measuring residual chlorine determine treat-
 ment endpoints. Complete destruction of cyanide requires
 adequate reaction time and excess chlorine. A residual
 concentration of free chlorine will be present after treat-
 ment, and it is important that the residual be reduced. An
 Table 4. Cyanide Half-life Under Natural Degradation

                          Half-life, hours

 Source: Environment Canada

excessive concentration of chlorine can result in the re-
lease of chlorine gas during pH adjustment. Other sources
of CI2, such as sodium hypochlorite, may be safer.

Some facilities have installed continuous monitors to en-
sure that hydrogen cyanide, cyanogen chloride and chlo-
rine exposure are kept well below minimal levels.

Where HCN limits are usually expressed in mg/m3, cya-
nide air emissions can be estimated on the basis of equi-
librium levels of cyanide in the air over solutions at vari-
ous temperature and pH values from the following (Menne,
=   mg HCN/m3 air per ppm NaCN in solution.
=   temperature, Kelvin.

                               5.0 WastewaterTreatment of Cyanide
Cyanide-bearing wastewaters usually require segregation
of cyanide wastestreams from other wastestreams. Pre-
treatment of cyanide prior to other treatment operations
prevents the formation of HCN in untreated wastewateror
primary treatment operations. Segregation also prevents
CN complexing of metals from non-cyanide-bearing waste-
water and minimizes overall wastewater treatment costs.
Cyanide pretreatment typically involves alkaline chlorina-
tion; however, acid hydrolysis, UV oxidation, electrolytic
decomposition and thermal destruction are also used. Con-
centrated cyanide wastestreams are typically treated us-
ing electrolytic decomposition or thermal destruction. Con-
centrated wastestreams are often bled into more dilute
wastestreams at a prescribed rate to facilitate treatment
with conventional technology.

Many oxidants are available for cyanide destruction  in-
cluding these: chlorine gas, sodium hypochlorite, calcium
hypochlorite,  ozone and hydrogen peroxide. Cyanide de-
struction using chlorine gas and sodium hypochlorite far
exceeds the use of other oxidants in industrial practice.
The effectiveness of cyanide destruction is usually mea-
sured by the  concentration of total residual cyanide re-
maining in the wastestream. Total cyanide has two com-
ponents: cyanide amenable to chlorination and non-ame-
nable cyanide. Cyanide amenable to chlorination can be
destroyed using conventional alkaline chlorination.

Alkaline Chlorination
Alkaline chlorination occurs at basic pH using hypochlo-
rite. Alkaline chlorination destroys cyanide in a two-step
process by oxidizing cyanide first to cyanate and second
to carbon dioxide and nitrogen. Hypochlorite is  produced
by sodium contacting chlorine with sodium hydroxide (equa-
tion 4). The reaction is reversible, with some free chlorine
left in solution. In cyanide destruction, chlorine reacts with
cyanide to form cyanogen chloride (equation 5). The cy-
anogen chloride reacts with available hydroxide to form
cyanate (equation 6). Cyanogen chloride is a gas with a
very high solubility in water (25 liters gas per liter of water)
and does not readily escape from solution. (Hartinger, 1994)
Then the cyanate (equation 7) is converted to the more
innocuous carbon dioxide and nitrogen.

In the first step, the reaction vessel is operated at a pH
between 10 and 12 to optimize the conversion of cyanide
to cyanate. Increasing the pH from 10 by one unit increases
the reaction rate ten-fold, to a pH of 12, where no addi-
tional change in rate is observed (Hartinger, 1994). During
the first step, cyanogen chloride, which is highly toxic, is
formed as an intermediate. If the pH is maintained in the
prescribed range and sufficient hypochlorite is available,
the intermediate cyanogen chloride is converted immedi-
ately to cyanate, preventing its release from solution. The
oxidation of cyanide to cyanate reduces the toxicity of the
compound significantly. Although this first step typically
requires a reaction time of between 1 and 20 minutes at a
pH > 10, a 40-60 minute retention time is required for con-
tinuous-flow systems. Longer retention times (up to 12
hours) are required for certain metal cyanide complexes.
Temperature for batch reactors can also affect the reac-
tion rate significantly where at 26°C and pH 10 the rate is
as fast as at pH 11.5 and 18°C (Hartinger, 1994).The va-
por pressure of cyanogen chloride increases rapidly with
temperature, and operation of cyanide treatment reactors
above 50°C is not recommended. In addition to controlling
pH, the ORP should be calibrated at +325 to  +400 milli-
volts during the first stage reaction to maintain the proper
chlorine dose.

The second step reaction involves conversion  of cyanate
to carbon dioxide (or carbonate) and nitrogen.  During the
second step, the pH is reduced to 8.5. It should never fall
below pH 8.0 since cyanogen chloride may be released
should the first-stage reaction be incomplete. The second-
stage reaction  rate is also pH dependent, starting rapidly
and decreasing speed as the pH is lowered to 8.5 where
no further rate increase is observed. This second step
requires a reaction time of between 30 and 60  minutes at
pH 8.5.The ORP should be controlled at +600  (typical) to
+800 millivolts  during the second stage reaction.
                                                     2 NaOH + CL  NaOCI +NaCI + H,O

 NaCN + CI2 => CNCI + NaCI
 2 NaCNO + 3 NaOCI
 => 2 CO2 + N2 + 3 NaCI + 2 NaOH

 Eventually, cyanide destruction results from the reaction
 of cyanate with hypochlorite (equation 7) forming nitrogen,
 carbon dioxide and regenerated sodium hydroxide. The
 combined reactions of equations 5 and 6 in the formation
 of cyanate from cyanide occur very rapidly. The final de-
 struction represented by equation 7 occurs more slowly.
 Cyanate will slowly hydrolyze to form ammoniacai spe-
 cies and carbon dioxide in the absence of hypochlorite
 (equation 8). Proper contact time in the reaction vessel is
 critical to ensure that complete conversion to carbon diox-
 ide and nitrogen has occurred.


 Interference with this reaction can occur in the presence
 of large concentrations of certain metal cyanide complexes
 (i.e., ferro-ferricyanide complexes). Each metal has a dis-
 sociation constant (Table 5), and very stable complexes
 such as the iron cyanide complexes will remain largely
 intact because the cyanide is not free to react. Conse-
 quently, alkaline chlorination is not effective in destroying
 iron cyanide  complexes.  Alkaline chlorination of nickel
 cyanide requires excess chlorine and additional retention
 time due to the competing reaction that forms black nickelic
 trioxide (Ni2O3).

 Metal Cyanide Complexes
 Destruction of metal cyanide complexes is dependent upon
 the dissociation constant. Table 5 provides a summary of
 these values.
                                  Metal cyanide dissociation is summarized by the follow-
                                  ing equation:

                                  [Me(CN)(z+y) ]    « Me(CN)z

       Destruction of the cyanide complexes containing cadmium,
       copper and zinc are readily destroyed with alkaline chlori-
       nation. Cyanide complexes containing cobalt, iron, gold,
       nickel and silver require alternative treatment techniques.
       The highly stable ferrocyanide complex reacts with chlo-
       rine only to the extent that the Fe++ ion is oxidized to Fe^
       with the slightly less stable ferricyanide complex gener-
       ated. Iron cyanide complexes are not amenable to chlori-
       nation and are considered relatively non-toxic. Destruc-
       tion of complexed nickel cyanide through alkaline chlori-
       nation requires much higher chlorine dosing (up to 1 0 times
       the stoichiometric dose) and much longer retention times
       (up to 12 hours). Kinetic rather than thermodynamic fac-
       tors may explain the slow oxidation rate of the nickel cya-
       nide complex, since the dissociation constant for nickel
       replicates the values for copper, which is easily oxidized.
       The process will also result in precipitation of black hy-
       drated nickel oxide.

       The silver cyanide complex is destructible with alkaline
       chlorination;  however, due to its very small dissociation
       constant, the reaction is very slow.

       Oxidation of Cyanide with Hydrogen
       Hydrogen peroxide provides another alternative in treating
       wastewaters  containing cyanide. In a reactor-based sys-
       tem, hydrogen peroxide has an electrode potential of +0.878
       V in alkaline solutions, which can be used as  an oxidizer
       for cyanide. Cyanide is oxidized to cyanate and hydrogen
       peroxide is reduced to water per the following equation:
                                                     CM- + H2O2 => CMC- + H2O  pH 9.5 -10.5
Table 5. Concentrations of Free Cyanide in Solutions of Various Concentrated Metal Cyanide Complexes
CNO- + 2H-.O
NH3 + CO3-
The cyanide oxidation rate is dependent on the cyanide
concentration, excess hydrogen peroxide concentration and
temperature. Introducing catalysts can also play an im-
portant role. For example, copper can greatly increase the
oxidation rate. However, copper reacts with ammonia to
form a tetrammino copper complex (Hartinger, 1994). The
cyanate is not further oxidized to carbon dioxide and nitro-
gen but is instead hydrolyzed to form ammonia and am-
monium ions. The reaction is very slow at alkaline pH and
increases as pH decreases.

Oxidation of Cyanide with  Ozone
Another oxidizer which has shown potential in oxidizing
cyanide is ozone. Ozone, with an electrode potential of
+1.24 V in alkaline solutions, is one of the most powerful
oxidizing agents known. Cyanide oxidation with ozone is a
two-step reaction similar to alkaline chlorination. Cyanide
is oxidized to cyanate, with ozone reduced to oxygen per
the following equation:
Then cyanate is hydrolyzed, in the presence of excess
ozone, to bicarbonate and nitrogen and oxidized per the
following reaction:
2CNO- + 3CX+I-UO
        ozone will absorb in this band. A major advantage of UV/
        peroxide and UV/ozone oxidation is that no undesirable
        byproducts (e.g., ammonia) are generated. UV oxidation
        has also been used in conjunction with Fenton's reagent
        and titanium dioxide.

        The following equations summarize the reaction of hydro-
        gen peroxide and ozone in the presence of UV light.
The reaction time for complete cyanide oxidation is rapid
in a reactor system with 10- to 30-minute retention times
being typical. The second-stage reaction is much slower
than the first-stage reaction. The reaction is typically car-
ried out in the pH range of 10-12 where the reaction rate is
relatively constant. Temperature does not influence the
reaction rate significantly.

The metal cyanide complexes of cadmium, copper, nickel,
zinc and silver are readily destroyed with ozone. The pres-
ence of copper and nickel provide a significant catalytic
effect in the stage one reaction but can reduce the rate of
the stage two reaction (oxidation of cyanate). Iron, gold
and cobalt complexes are very stable and are only par-
tially oxidized, unless a suitable catalyst is added. Ultra-
violet light (UV oxidation), in combination with ozone, can
provide complete oxidation of these complexes.

 Ultraviolet (UV) Oxidation
UV light causes metal complexes such as ferricyanide
and ferrocyanide to partially dissociate. UV oxidation, in
combination with hydrogen peroxide or ozone, can com-
pletely oxidize all metal cyanide complexes. UV oxidation
is limited to relatively clear solutions, since wastestreams
are passed through a light-transmitting chamber and ex-
posed to intense UV light. UV in combination with hydro-
gen peroxide results in the formation of OH- radicals, which
are strong oxidizing agents capable of oxidizing iron cya-
 nide complexes. Suitable light sources emit in the range
of 200 to 280 nanometers (nm). Hydrogen peroxide and
                              Hydrogen peroxide:
                                           H2O + O2H-
                              HA + OH' => H2O + OH-+ 02

                              OH + OH' -> HA
                              O2H + O2H':
         02H + II OH' => H20 + O2


         O3+llhv;=> O2 + O
                                     O + H2O







                              Electrochemical Oxidation of Cyanide
                              Cyanide can be oxidized electrochemically (anodically) in
                              chloride-based solutions. This is one of the most effective
                              treatments for concentrated cyanide wastestreams. The
                              reaction involves the formation of chlorine gas that dis-
                              solves in alkaline solution to form sodium hypochlorite, as
                              shown in equation 22:
                              2NaOH + CL <=> NaCI + NaOCI
                              During cyanide oxidation, hypochlorite reacts with the cya-
                              nide to produce cyanate and chloride. The chloride is oxi-
                              dized anodically to form hypochlorite in a closed loop.
         Cl- + 2 OH-<=> OCI- + H2O + 2 e-
                              Cyanide can also be oxidized anodically without chloride,
                              although the reaction is very slow. The theoretical energy
                              requirement is 2.06 amp-hr per kg of cyanide. At a cell
                              voltage of 2-4 volts, this would correspond to 4.1 to 8.2
                              kWh/kg of cyanide (Hartinger, 1994).The anodic reactions
                              are shown in equations below. Electron loss leads to the
                              formation of the unstable dicyanogen radical that immedi-
                              ately hydrolyzes to cyanate. The reaction is enhanced at
                              higher temperatures (125°-200°F) with ammonia produced
                              as an additional byproduct (Patterson, 1985).

 Anode reactions:


 2CN-o(CN)2 + 2e-

 (CN)2 + 4 OH' o 2 CNO- + 2 H2O + 2 &

    - + OCNO-




 Anode materials include graphite, platinized titanium,
 lithium platinite and nickel. Anodic oxidation without chlo-
 ride is highly dependent on anode materials. Dicyanogen
 formation improves progressively as steel is replaced by
 platinum, which is replaced by carbon and,  ultimately,
 nickel. Electrochemical oxidation becomes uneconomical
 at cyanide concentrations below several hundred ppm. In
 this case, conventional alkaline chlorination or other treat-
 ment procedures are used for final treatment (Patterson,

 Thermal Oxidation
 Thermal oxidation is another alternative for destroying
 cyanide. Thermal destruction of cyanide can be accom-
 plished through either high temperature hydrolysis or com-
 bustion. At temperatures between 140°C and 200°C and a
 pH of 8, cyanide hydrolyzes quite rapidly to produce for-
 mate and ammonia (Hartinger, 1994). Pressures up to 100
 bar are required, but the process can effectively treat
 wastestreams over a wide concentration range and is ap-
 plicable to both rinsewater and concentrated solutions.
CN- + 2H20
In the presence of nitrates, formate and ammonia can be
destroyed in another tube reactor at 150°C, according to
the following equations:
                             ; + 2H2O
3 HCOOH + 2 NO," + 2 H+ => SCO,

Acidification and Acid Hydrolysis
Direct acidification of cyanide wastestreams was once a
relatively common treatment. Cyanide is acidified in a
sealed reactor that is vented to the atmosphere through
an air emission control system. Cyanide is converted to
gaseous hydrogen cyanide, treated, vented and dispersed.

Acid hydrolysis of cyanates is still commonly used, fol-
lowing a first stage cyanide oxidation process. At pH 2 the
         reaction proceeds rapidly, while at pH 7 cyanate may re-
         main stable for weeks (Eilbeck, 1987). This treatment pro-
         cess requires specially designed reactors to assure that
         HCN is properly vented and controlled.

         The hydrolysis mechanisms are as follows:
            HOCN + H+ -
            HOCN + H20
            NCO- + 2H2O:
               (very slow)
 NH4+ + CO2 (rapid)   (32)
:> NH3 + CO2(slow)   (33)
      + HC03-
 Other Cyanide Treatment
 Additional cyanide treatment processes, which have been
 proposed or used in limited practice, include the following:

  •   Cyanide Precipitation with Ferrous Salts (Hartinger,

  •   Cyanide Adsorption on Catalyzed Activated Carbon
      (Patterson, 1985)

  •   Kastone Process (Patterson, 1985)

  •   Cyanide Destruction with Mono-Peroxy Sulf uric Acid
      or Caro's Acid (Eilbeck, 1987; Hartinger, 1994)

  •   Cyanide Destruction with Oxygen (Hartinger, 1994)

  •   Cyanide Destruction with Aldehydes (Eilbeck, 1987)

  •   Cyanide Destruction with SO/Air (I nco, 1993)

  •   Cyanide Destruction with Fenton's Reagent (Eilbeck,
 Proponents of cyanide destruction  have also proposed
 using bacteria, enzymes and natural clay.

 Cyanide treatment systems are usually designed to de-
 stroy cyanide in a pretreatment step. Treated wastewater
 is then directed to secondary treatment steps, which could
 include additional chemical treatment and/or metal pre-
cipitation. Another alternative is to install a precipitation
step immediately following cyanide destruction so that cya-
 nide treatment may include solids removal. This added
step can reduce  the concentration of complexed iron or
 nickel and effectively reduces cyanide levels in the waste-
water discharge (Martin, 1992). Although this approach may
optimize total cyanide removal, it will also increase the
capital and operating costs of the wastewater treatment
system. Identifying additional sources of cyanide and en-
suring that these  sources are routed through the cyanide
destruction system requires a thorough analysis of all so-
lution chemistries.

                                    6.0 Source Reduction
Cyanide plating processes can be operated effectively on
a closed-loop or modified closed-loop basis using coun-
tercurrent rinsing and evaporative recovery techniques.
Some cyanide-based processes must be operated at higher
than normal operating temperatures to maximize bath
evaporation and facilitate drag-out recovery. Many of these
processes operate at ambient temperature ranging from
60 to 100°F. Recovery opportunities can be dramatically
improved by operating in this temperature range. The im-
pact of higher operating temperatures can be offset by
adjusting the cyanide to metal ratio or by decreasing bath
concentration. Reduced bath concentration not only re-
duces the mass drag-out but may also reduce the volu-
metric drag-out due to reduced solution viscosity. Vacuum
evaporation, reverse osmosis and ion exchange can ex-
tend the application range of basic source reduction tech-
niques. Drag-out (or rinsewater) recovery can reduce oper-
ating costs through reduced material purchases, reduced
wastewatertreatment costs and water usage. Drag-out re-
covery also eliminates the principal contaminant purge as
contaminants are recovered with valuable materials.

Effective solution maintenance is important for high-
quality surface finishing, especially in closed-loop pro-
cessing. Solution maintenance requires basic operating
procedures, including filtration to control particulates and
treatment to control organics, carbonates and metallic im-
purities. Cyanide will slowly hydrolyze, producing ammo-
nia and the formate ion. Ammonia will readily escape the
alkaline solution, and formate is generally non-interferring.
Controlling carbonates is probably the most challenging
maintenance problem encountered in cyanide-based solu-

 Carbonate Chemistry
Most cyanide baths are alkaline resulting from the hydroly-
sis of sodium and potassium cyanide,  liberating sodium
hydroxide (NaOH) or potassium hydroxide (KOH) and hy-
drocyanic acid (HCN). Most cyanide baths contain car-
bonates of sodium or potassium that are largely a result of
the adsorption of carbon dioxide from the air and the even-
tual liberation of hydrocyanic acid. This reaction is accel-
erated by aeration of the solution and retarded by the addi-
tion of free hydroxide.
       anodes. An  intermediate product, sodium cyanate
       (NaCNO), may be formed.
2NaCN + 2H20 + 2NaOH + O2
                                      2Na2CO3 + 2NH3
        Cyanide may also decompose at high temperatures in
        nearly neutral solutions. This could occur with rinsewater
        in an atmospheric evaporator. The formic acid or sodium
        formate may then be oxidized to carbonate.
        HCN II + II 2'H20 -* HCOOH + NH3
 2NaCN + C02 + H2O -» Na2CO3 + 2HCN
 Another source of carbonate is through oxidation of cya-
 nide. This reaction is accelerated with the use of insoluble
Carbonates can contaminate cyanide plating baths when
they exceed their solubility and begin to precipitate in so-
lution, causing rough plating. Solution temperature is an
important variable, since the solubility of carbonates is
temperature dependent. Carbonate crystals can also be
introduced into the metal deposit. In addition, bath con-
ductivity, current efficiency and throwing power decrease
with increased carbonate concentration. Carbonates can
be controlled within an acceptable range (2 to 8 ounces/
gallon) through crystallization or chemical precipitation.
Either process can be operated in a batch or continuous
mode. Continuous treatment systems usually operate on
a slipstream and batch processes require a separate tank.
Both processes require separation of treated solutions from
the resulting solids (sludge or crystals).

Solution agitation is necessary in cyanide-based process
solutions to assure good mixing and allow for operation at
higher current densities. Solution agitation is provided by
a mechanical mixer or through solution pumping. Air agita-
tion should be avoided, since aeration will increase car-
bonate build-up. Similarly, atmospheric evaporators are not
normally used in cyanide-based processes, since signifi-
cant aeration of the circulated solution or rinsewater in-
creases carbonate build-up.

Other Contaminants
Other cyanide-based process solution contaminants are
organics, metals and chlorides. Organic contaminants are
typically removed from solution with activated carbon. Metal
contaminants can  be removed chemically or electrolyti-
cally. For example, sodium polysulfide is used to precipi-
tate zinc, cadmium and lead. Zinc dust is used to remove
copper by displacement. Hexavalent chromium can be elec-
trolytically reduced to trivalent chromium and plated out
using high-current density, low-efficiency dummy plating.

 Tin and copper can be plated out with low-current density,
 high-efficiency dummy plating. Chloride can also cause
 problems in cyanide plating processes by attacking (etch-
 ing) steel anodes or anode baskets to produce dissolved
 iron. Rinsing in straightforward processes should be con-
 trolled to avoid drag-in of chlorides, iron and other con-
 taminants from processes (such as pickling solutions).

 Countercurrent rinsing is preferred, following cyanide pro-
 cesses,  since low contamination of cyanide residual is
 essential in the final rinse to minimize cyanide drag-in to
 subsequent processes and to protect operators while han-
 dling parts. A cyanide residual of <5 ppm is recommended.
 Countercurrent rinsing will also reduce water usage sig-
 nificantly and reduce the  life  cycle costs of wastewater
 treatment, recycling  and/or recovery systems. Figure 1
 shows Countercurrent rinsing.

 Optimizing operating practices, as listed in Appendix A,
 can reduce process  and wastestream problems signifi-
 cantly. See Appendix B for Best Management Practices
 for treating cyanide-based plating processes.

 Recovery Technologies
 Vacuum evaporation, reverse osmosis and  ion exchange
 have all  been used very effectively to recover cyanide-
 based process drag-out and/or to recycle rinsewater. How-
 ever, effective use of these technologies  with cyanide
 wastewater requires some special considerations.

 Vacuum Evaporation
 Vacuum evaporation (VE) has been used extensively to
 concentrate cyanide-bearing rinsewater to  recover solu-
tion dragout. The distillate is normally reused as rinsewater.
The principle problems associated with the application of
 VE to cyanide-bearing rinsewater are fouling  of heat ex-
changers due to scaling and precipitation of solids and
pass-through of cyanide in the distillate. The first problem
can be partially controlled by process solution maintenance
to control particulate and carbonate levels in the process
solution  (and therefore the  rinsewater) and through
prefiltration of the evaporator feed. In addition, the operat-
                                   ing temperature must be controlled to avoid decomposi-
                                   tion of cyanide and the maximum concentration of the
                                   concentrate stream must be controlled to avoid precipita-
                                   tion of solids. The second problem requires selection of
                                   an evaporator with a well-designed separator to control carry
                                   over of cyanide in the condensate (distillate).  Figure 2
                                   shows a process that uses natural evaporation.

                                   Reverse Osmosis
                                   Reverse osmosis (RO) has also been used extensively
                                   with cyanide-bearing rinsewater. RO is also sensitive to
                                   fouling and requires prefiltration to less than one micron.
                                   Ultrafiitration (UF) is often used as an effective prefilter in
                                   this application. Membranes that are more resistant to foul-
                                   ing have been developed; however, effective prefiltration
                                   is the best assurance for efficient RO performance in cya-
                                   nide applications. Since the concentration of the RO per-
                                   meate is largely a function of the concentration of the RO
                                   feed stream, the RO permeate may not be suitable for use
                                   as makeup water in the final rinse. This problem can be
                                   avoided through proper sizing of the RO system to ensure
                                   an adequate rinsewater flow rate and an optimized feed
                                   concentration. Figure 3 uses RO or vacuum evaporation
                                   to recover rinsewater.

                                   Ion Exchange
                                   Ion exchange (IX) has been used most often with pre-
                                   cious metal plating (silver and gold) and requires a special
                                   configuration to recover the strongly complexed metal cya-
                                   nides. IX is commonly used in surface finishing to recover
                                   metal  and recycle rinsewater and is typically used in a
                                   cation-anion configuration. Cations are the cation resin (typi-
                                   cally strong acid) and anions, including free cyanide, are
                                   captured on the anion resin (typically strong base). The
                                   strong acid resin will break the weakly bound cyanide salts
                                   of sodium, potassium, cadmium, copper and zinc. Silver
                                   and gold plating applications require an anion-cation-anion
                                   configuration and the metal cyanide complex is then bound
                                   to the leading anion exchanger. The silver- or gold-bearing
                                   anion resin is generally sent off-site for recovery. Other
                                   anion resins may be regenerated with NaOH.The subse-
                                   quent dilute, metal-free cyanide stream may be concen-
                 Process Solution
 Figure 1.  Open process showing solution maintenance using periodic dump or bleed and Countercurrent rinsing with a con-
           tinuous wastewater discharge.

              Process Solution
Figure 2. Closed-loop process showing continuous solution maintenance and rinsewater recovery with natural evaporation.
trated for reuse or treated for discharge. The cation resin
may be regenerated on-site with sulfuric acid. The metal
may be recovered with electrowinning. In either configura-
tion, the rinsewater may be recycled as rinsewater makeup.
Figure 4 shows a process that uses ion exchange and

IX resins are highly sensitive to fouling from precipitates
and, like other technologies, prefiltration is important. Or-
ganic components in the feed stream can also foul resins.
Granular activated carbon (GAG) is often used before IX
                              to control organics. IX system design also requires con-
                              sideration to avoid mixing of cyanide and acids.

                              Electrowinning or electrolytic metal  recovery (EMR) has
                              been used extensively on cyanide-bearing rinsewater to
                              recover metal and to destroy cyanide in situ. Electrowin-
                              ning is not capable of recycling rinsewater; however, it can
                              minimally reduce water usage. It is an important second-
                              ary technology that can be deployed with ion exchange,
                              as described above.
                         Reverse Osmosis or
                         Vacuum Evaporator
Figure 3. Closed-loop process showing continuous solution maintenance and rinsewater recovery with reverse osmosis or
          vacuum evaporation.

              Process Solution

                                                              __ Regen.


                                                                                  Rinse 3

Figure 4. Open-loop process showing continuous solution maintenance and rinsewater recovery with natural evaporation and
           ion  exchange/electrowinning.

                                  7.0 Cyanide Alternatives
Non-cyanide-based surface finishing processes have be-
come increasingly important to the industry. At present,
over 70% of zinc plating is produced from non-cyanide
plating chemistry (Hajdu, 1999). Non-cyanide copper plat-
ing is significantly more common than cyanide copper plat-
ing, even though cyanide copper plating remains an im-
portant process for preparing substrates for further plat-
ing. Non-cyanide cadmium- and indium-plating processes
have been developed; however, the cyanide-based pro-
cesses remain the dominant chemistry. Non-cyanide sil-
ver- and gold-plating processes have also been developed
but are not generally well accepted. Effective substitutes
for cyanide-based brass, bronze, silver and gold processes,
as well as less common plating processes, are still in de-
velopment. Table 6 provides and overview of cyanide and
non-cyanide plating processes.

Non-cyanide alkaline descaling and metal stripping pro-
cesses are common and utilize other metal complexers
such as EDTA. Despite the effectiveness of cyanide-based
cleaners and strippers there are very few situations that
favor cyanide use in cleaning and stripping.
Table 6.
Cyanide and Non-cyanide
Plating Processes
New alternatives in development for specific cyanide processes.

                          8.0 Cyanide Monitoring and Analysis
Wastewater Compliance Monitoring
A well-designed and operated facility can handle cyanide
safely and avoid exposure to workers, but it may encoun-
ter compliance problems, usually concerning wastewater
discharges. There are two components to the cyanide dis-
charge: the total cyanide concentration and the quantity of
cyanide amenable to chlorination. The amenable cyanide
is a measure of the effectiveness of the cyanide destruc-
tion system in treating free cyanide and metal complexes
amenable to alkaline chlorination. As previously noted,
complexed iron cyanide is not destroyed by alkaline chlo-
rination. Nickel and silver cyanide require extensive reten-
tion times and excess chlorine dosage beyond stoichio-
metric. Discharge levels for total cyanide are very low, in
the range of 1 -2 ppm, depending on state regulations, and
are usually measured at the point of treatment rather than
at the point of discharge. Residual chlorine concentrations
are usually controlled at <10 ppm.

Many operations are permitted on  the basis of amenable
cyanide rather than total cyanide for wastewater discharges.
This alternative compliance method requires  less than 1
ppm amenable cyanide and requires an additional labora-
tory test; however, the standard is  generally much easier
to attain and can be achieved with conventional technol-
ogy. Challenges to the use of the amenable cyanide ap-
proach include difficulties with the sample analysis proce-
dures established by USEPA.

There are three official methods for wastewater compli-
ance monitoring: ASTM D 2036 Method B, EPA Method
355 and Standard Methods for the Examination of Water
and Wastewater, 19th Edition, Method 4500-G (1995). Each
allows the use of slightly different procedures and equip-
ment. Varying levels of residual chlorine are possible with
the procedures (Altmayer, 1997), and since the complexed
cyanide is in equilibrium with free cyanide, aging samples
with residual chlorine shows a marked decrease in cya-
nide concentrations (ASTM, 1998).  In addition, sulfides
and thiocyanate,  which are common chemicals found in
metal finishing wastestreams, can interfere with the analy-
sis. The main point of difficulty is that a separate test
must be used to determine the cyanide amenable to chlo-
rination, and a variety of experimental errors and method
imprecision can lead to a non-compliance determination.
An alternative method currently under consideration is the
"Modified Roberts-Jackson Method for Analysis of Simple
Cyanides" (Altmayer, 1997). A facility can petition the lo-
cal regulatory authority to use this method as an alterna-

Cyanide Analysis
Potential compliance problems can be compounded as a
result of several analytical methods being used, which can
vary from laboratory to laboratory. Methods used depend
on the character of the cyanide compounds in the waste,
their concentration and the presence of interfering agents.
In addition, the laboratory procedures allow the use of steps
which can make comparison of results difficult. Under-
standing the basics of the laboratory analyses for cyanide
can assist a facility in  maintaining better compliance of
the operation and control of cyanide plating processes.
This section covers the laboratory analyses used to de-
termine cyanide concentrations in wastewater, the condi-
tions where difficulties  arise and a new technique for de-
termining cyanide in wastewater, proposed in the Federal

The accepted methods currently in use for cyanide analy-
sis in wastewater are EPA method 335; Method 4500 G,
Standard Methods for the Examination of Water and Waste-
water, 19th Edition (Standard Methods); ASTM D 2036
Method B.The new method is EPA Method OIA-1677. In
general, the methods are most effective with cyanide spe-
cies that fully dissociate. These include cadmium and zinc.
Some of the species only partially dissociate and are not
fully recovered. Cyanide complexes in this category in-
clude copper, nickel, silver and mercury complexes. Some
cyanide complexes do not  dissociate appreciably and are
not measured effectively using these techniques. The cya-
nide compounds in this category include the more noble
metal complexes, including gold, platinum and cobalt. All
of the methods are subject to interference; controlling the
interfering sources is key to consistent results. Minimal
detection limits range from 0.005 mg/L for colorimetric
cyanide determination to 0.4 mg/L by titration.

Most methods are sensitive to sulfides and thiosulfate.
These compounds can lead to the formation of hydrogen
sulfide gas, which is collected with hydrogen cyanide gas
and interferes with measurement of the cyanide present.
Many organic compounds can also interfere, including fatty
acids, sugars and aldehydes. Fatty acids form soaps dur-
ing the distillation process, making determination of the
endpoint difficult. Aldehydes and sugars react with cya-
nide to form cyanohydrin, a compound not detected as
cyanide. In addition, the presence of oxidizers can de-

stroy the available cyanide. Sunlight and ultraviolet radia-
tion breakdown cyanide; all of the procedures contain steps
to shield the samples from sunlight. Dilution of the samples
can  lead to erroneous results as the dilution allows for
new equilibrium conditions to form and can effect the cya-
nide available for chlorination. Typically, dilution can result
in falsely inflated values for amenable cyanide.

Another key aspect of the available cyanide is the deter-
mination of excess chlorine during chlorination. Excess
chlorine is removed prior to cyanide analysis and is usu-
ally monitored with potassium iodide starch test papers. A
color change occurs in the presence of excess chlorine.
The starch test papers are not precise and can err by a
few ppm in the actual quantity of chlorine present, but are
an excellent qualitative  indicator. All cyanide analytical
methods include steps to track and treat chlorine residual.
However, the methods differ on the amount of excess re-
agent necessary to achieve chlorine removal, and the con-
dition can actually result in some residual chlorine. The
residual chlorine reacts  with cyanide during the cyanide
analysis in the gas scrubber to destroy additional cyanide
and can lead to inflated results for available cyanide.

"Standard Methods," Method 4500-G
In this method, the wastewater sample is split into two
parts. The samples must measure 500 ml and may  be
diluted to obtain this volume. One split is evaluated for
total cyanide. The second split is chlorinated to destroy
the available amenable cyanide and then evaluated for to-
tal cyanide. The difference between the two samples is
expressed as amenable cyanide. The chlorination reac-
tion is run for one hour with agitation at high pH (11 -12). A
chlorine residual of 50-100 ppm is maintained and the re-
sidual is monitored with potassium iodide test papers, which
turn blue in that concentration range. Following the reac-
tion, the  excess chlorine is removed by the addition of
sodium arsenate or by the combination of hydrogen perox-
ide and sodium thiosulfate. The chlorine  residual is tested
with potassium iodide starch  papers.  Chlorinated and
unchlorinated sample portions are then distilled using the
apparatus shown in Figure 5. Only samples containing less
than 10 mg/L cyanide may be tested this way. If higher
concentrations are expected, the sample must be diluted.
During the distillation, the dissolved cyanide is converted
to hydrogen cyanide gas by acidification and captured in
the gas dispersion tube. This tube contains an alkaline
solution to capture the cyanide gas. This  sample is treated
with chloramine-T, forming cyanogen chloride and chang-
ing to a red-blue color by the indicator. The color intensity
is then measured colorimetrically to determine the cya-
nide concentration.

Dilution can effect the concentration of cyanide detected
by allowing the equilibrium between the free and complexed
cyanide to be adjusted, releasing additional cyanide. In
addition, potassium iodide test papers  may not detect a
.few ppm chlorine. This condition can lead to additional
destruction of cyanide during the distillation step. An ex-
cess of reducing reagent is required to destroy excess

                            Water in
                                                          9-mm connecting tube
                                Heating mantle
                                               38-mm x 200-mm
                                                  test tube
                            Figure 5. Distillation apparatus for evaluating cyanide samples.


  chlorine to ensure a true determination of the amenable

  EPA Method 335 Cyanide Amenable to Chlorination
  This method is similarto"Standard Methods" method 4500-
  G. The wastewater sample is divided into two portions.
  One is tested for total cyanide. The other is chlorinated to
  destroy the amenable cyanide and then tested for total
  cyanide. The difference between the two concentrations is
  the cyanide amenable to chlorination. Depending on the
  cyanide concentration, this test is either accomplished by
  titration or spectrophotometry. Like the "Standard Meth-
  ods," the extent of chlorination is monitored with potas-
  sium iodide starch papers. After chlorination, the excess
  chlorine is removed by the addition of ascorbic acid, and
  the removal is checked with the potassium iodide test
  papers. Unlike the "Standard Methods," an excess of ascor-
  bic acid is necessary if a negative test for chlorine is de-
  tected with the starch test papers.

  The  sample volumes must be 500 ml and can be diluted
  to reach that volume. Dilution can affect the concentration
  of cyanide detected (Altmeyer, 1997) by allowing the equi-
  librium between the free and complexed cyanide to be
  adjusted, releasing additional cyanide. The concentration
 determined in the original sample includes the dilution fac-
 tor, an effect which sometimes results in inflated ame-
  nable cyanide concentrations. In addition, the method is
 susceptible to several interferences, many of which result
 in inflated values of amenable cyanide.

 This method is similar to the previous two. Amenable cya-
 nide  is again determined as the difference of two mea-
 surements. Sample size is 500 ml for the determination of
 the total cyanide and the cyanide amenable to chlorina-
 tion. Dilution of the sample is allowed either to make up for
 insufficient volume or to reduce the total cyanide concen-
 tration. Removal of excess chlorine is accomplished with
 sodium arsenate and hydrogen peroxide. Each of the two
 samples are then analyzed for total cyanide using the same
 distillation procedure previously described.

 EPA Method OIA-1677
 This method uses a two-step process: the sample is pre-
 treated, then injected into the cyanide detection cell. Pre-
 treatment consists of mixing the sample with ligand ex-
 change reagents, forming stable complexes of the transi-
 tion metals present,  and releasing the cyanide from the
 original complexes in the sample. Detection makes use of
 a flow injection analysis shown schematically in Figure 6.
 In the flow system, hydrochloric acid reacts  with the in-
 jected sample to release hydrogen cyanide  gas, which
 passes through the gas-permeable membrane. The gas is
 recaptured on the detector side of the membrane with an
 alkaline solution that converts the hydrogen cyanide gas
 backto dissolved cyanide ion. The cyanide concentration
 affects the current of the electrode system, made up of a
 silver working electrode, a silver/silver chloride reference
 electrode and a platinum/stainless steel counter electrode.
 The current change is proportional to the cyanide concen-
 tration. Analysis time is less than five minutes.

 This test was proposed in the Federal Register m the fall
 of 1998. The technique has several advantages over the
 different techniques currently used, including reduced pro-
 cessing time and its  ability to fully recover cyanide from
 complexes that only partially dissociate. This includes the
 nickel silver and mercury cyanide complexes. Like the other
 techniques, it is unable to process extremely stable com-
 plexes. The flow analysis system must be calibrated regu-
 larly, and the apparatus is expensive. Expense is impor-
 tant because this technique is  not being proposed as a
 replacement for the other techniques, but, rather, it is in-
tended to augment the existing techniques. Therefore, many
 laboratories may choose not to include this technique un-
til it has been further proven.
Figure 6. Flow injection analysis schematic.

                                           9.0 Summary
A well-designed and operated facility can handle cyanide
safely and protect its workers, manage cyanide-bearing
wastestreams and enjoy the benefits of cyanide-based pro-
cesses. Clearly, some non-cyanide process alternatives
provide surface finishing facilities with significant advan-
tages in specific circumstances. However, non-cyanide pro-
cesses are often more difficult to treat in conventional
wastewater treatment systems, more difficult to recover
or recycle and are, generally, more difficult to control. Cya-
nide use requires careful management, thorough operator
training and proper facility design.

Similarly, it is important for the regulatory community to
address deficiencies that exist in compliance testing. Cur-
rent protocols can lead to unwarranted additional sampling,
testing and corrective action. Improved analytical meth-
ods for detecting cyanide in wastestreams need to be ex-

While many facilities have already phased cyanide out of
their operations, other facilities have retained cyanide use
and have assumed responsibility for its control. It is likely
that effective non-cyanide alternatives will continue to be
developed to provide users with new alternatives in the
future. However, for those facilities that continue to use
cyanide, rigorous adherence to best management prac-
tices and effective use of technology are essential.

Section 1.0
1.   Development Document for Existing Source Pretreat-
    ment Standards for the Electroplating Point Source
    Category, EPA Document No. 440/1-79/003. USEPA.
    August 1979.

Section 2.0
2.   Modern Electroplating, 2nd Edition. Lowenheim, F.A.,
    ed. John Wiley & Sons. NY, NY. 1953.

3.   AESF Electroplating  Course. Schwartz, M. and D.
    Swalheim. AESF. Orlando FL. 1996.

4.   Treatment of Metal Waste Streams, 1st Edition. Cali-
    fornia State University. Sacramento, CA. 1990.

5.   Lange's Handbook of Chemistry, 13th Edition. Dean,
    J.A., Ed. McGraw-Hill. 1985.

Section 3.0
6.   Dangerous Properties of Industrial Materials, 7th Edi-
    tion. Sax, N.I. and R.J. Lewis. VanNostrand Reinhold.
    NY, NY. 1989.

7.   Development Document for Existing Source Pretreat-
    ment Standards for the Electroplating Point Source
    Category, EPA Document No. 440/1 -79/003. USEPA.
    August 1979.

8.   The Merck Index, 11th Edition.

9.   Electoplating, AP-42,  Section 12.20. July 1996.

Section 4.0
10.  Managing Cyanides in  Waste Discharges. Menne, D.M.
    May 1997.

Section 5.0
11.  Handbook of Effluent Treatment and Recycling for the
    Metal Finishing Industry, 2nd Edition. Hartinger, L. Fin-
    ishing Publications. 1994.

12.  Industrial Wastewater Treatment Technology, 2nd Edi-
    tion. Patterson, J.W., Butterworths. 1985.
13. Chemical Processes in WastewaterTreatment. Eilbeck,
    W.J.and G. Mattock. Ellis Horwood Limited. 1987.

14. Cyanide Destruction: The Inco SO2/Air Process. INCO.
    Inco Exploration and Technical Services. 1993.

15. Plating and Surface Finishing, "Chasing Those Elu-
    sive Cyanide Ions." Martin, T.H. AESF. Orlando, FL.
    November 1992.

Section 7.0
16. AESFSUR/FIN99 Proceedings. "Cyanide Replace-
    ment in Zinc Plating: A Case History." Hajdu, J. June

Section 8.0
17. Altmayer, 1997

18. Standard Methods for the Examination of Water and
    Wastewater.  19th Edition. Amer. Pub. Health Assn.
    Washington, D.C. 1995.

19. Annual Book of ASTM Standards, Method D 2036.
    ASTM. 1998.

20. Federal Register. Vol 63, No. 129, Proposed Rules, pp
    36809-36824. July 1998.

Additional References
21. Operation  and Maintenance of Surface  Finishing
    Wastewater Treatment Systems. Roy, C.H. American
    Electroplaters and Surface Finishers Society. 1988.

22. Principles of Industrial Wastewater Treatment.
    Gurnham, C.F. John Wiley & Sons. 1955.

23. Industrial  Water Pollution Control,  2nd Edition.
    Eckenfelder, W. W. Jr. McGraw-Hill. 1989.

24. Plating  and Surface Finishing,  "Advice & Council,"
    Altmayer, F. AESF. Orlando  FL. February 1997.

25. Handbook of Chlorination. White, G.C. Van Nostrand
    Reinhold. NY, NY. 1972.

                                           Appendix A
                            Optimizing  Operating Procedures
Good operating procedures can reduce process operating
and wastestream problems significantly. While cyanide pro-
cesses are relatively tolerant compared with many alter-
native surface finishing processes, good process control
is always cost effective over the long term.

Good operating practices for use with cyanide-based pro-
cesses include the following:

•  Use countercurrent rinsing to improve rinsing effec-
   tiveness and reduce water usage and wastewater gen-
   eration. Three rinse stages are generally most effec-
   tive with cyanide-based processes. Countercurrent
   rinsing may also reduce wastewater flow sufficiently
   to facilitate batch treatment of wastewater.

•   Establish water and rinsewater quality standards. A
   cyanide residual of <5 ppm is recommended in the
   final process rinse. Makeup water quality can affect
   process and product quality as water-based contami-
    nants accumulate in the process. Chloride, fluoride,
   iron, copper, zinc and lead, as well as other heavy
    metals, can lead to various process problems. Deion-
   ized water is recommended for solution makeup and
   for rinsewater makeup in closed-loop processes.

•   Control drag-in of chlorides, iron and other contami-
    nants from preplate processes by setting rinsewater
quality standards for these processes. Use counter-
current rinsing to improve rinsing effectiveness and
minimize water use.

Use drag-out or rinsewater recovery; it is cost effec-
tive with many cyanide-based processes. Closed-loop
rinsing may reduce or eliminate the need for cyanide

Solution maintenance should incorporate control of
contaminants, including particulates, organics, met-
als and carbonates. Concentration limits should be set
for contaminants, and analytical control procedures
should be set to monitor contaminants  as  well as
makeup chemicals.  Solution dumps  and/or bleeds
should be based on contaminant build-up and not on a
simple time basis.

Don't use cyanide-based plating processes as cleaner
and plating solution. Parts should have a water break-
free surface prior to plating.

Segregate cyanide wastestreams from other
wastestreams for wastewater treatment or off-site dis-

Avoid air agitation or any other aeration of cyanide

                                           Appendix B
                                Best Management  Practices
Best management practices for cyanide include the

  •   Develop a cyanide management plan, and integrate
     it with the facility environmental and safety man-
     agement plan.

  •   Establish initial and refresher cyanide management
     training for managers, workers and contractors.

  •   Establish well-defined personnel responsibilities and
     clear chains of command for cyanide use and man-

  •   Develop safe procedures for handling cyanide, i.e.,
     storage, transprtation, containment, spill manage-
     ment, production processes, raw material additions,
     solution sampling and analysis, solution  mainte-
     nance, waste treatment, waste disposal and equip-
     ment maintenance.

  •   Develop rinsewater quality standards for all cyanide
     processes and pre-cyanide processes. Generally 5
     ppm is a reasonable rinsewater standard for a final
     cyanide process rinse.

  •   Develop and implement an integrated pollution pre-
     vention strategy encompassing point source waste
     minimization, recovery, and recycling, waste treat-
     ment and off-site disposal.

  •   Conduct regular cyanide audits, with corrective ac-
     tion, and update the cyanide management plan on a
     regular basis.

  •   Maintain primary containment tanks, drums, piping,
     valves, pumps and other equipment to prevent leaks
     and spills.

  •   Segregate cyanide processes, pretreatment, stor-
     age and other operations from non-cyanide opera-
     tions in a separate secondary containment system.
Store cyanide-based raw materials in a secure, dry
and ventilated storage area.

Monitor work areas for hydrogen cyanide and waste-
water treatment areas for hydrogen cyanide and cy-
anogen chloride

Ventilate all cyanide process, wastewater treatment
and storage areas

Provide workers and visitors with proper protective
equipment including gloves, aprons, face shields,
goggles, safety glasses, respirators and other pro-
tective clothing.

Provide workers and visitors with access to lavato-
ries and showers to maintain appropriate personal

Dispense and weigh raw materials in a ventilated

Develop and practice emergency procedures for
cyanide spills.

Develop and practice emergency procedures for
human exposure (skin, eye, ingestion) to cyanide
mists and fumes.

Use the minimum amount of cyanide required for
adequate process or operation.

Avoid air agitation of cyanide solutions as carbon
dioxide can liberate hydrogen cyanide, (see page



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