US EPA Office of Research and Development
United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
EPA/625/R-99/009
December 2000
v>EPA Capsule Report
Managing Cyanide in
Metal Finishing
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EPA625/R-99/009
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
80% Recycled/Recyclable
Printed with vegetable-based ink on
paper that contains a minimum of
50% post-consumer fiber content
processed chlorine free
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Notice
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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Foreword
The U.S. Environmental Protection Agency is charged by Congress with
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
to a compatible balance between human activities and the ability of natural systems
to support and nurture life. To meet this mandate, EPA's research program is provid-
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
risks in the future.
The National Risk Management Research Laboratory is the Agency's center
for investigation of technological and management approaches for preventing and
reducing risks from pollution that threatens human health and the environment. The
focus of the Laboratory's research program is on methods and their cost-effective-
ness for prevention and control of pollution to air, land, water, and subsurface re-
sources; protection of water quality in public water systems; remediation of contami-
nated sites, sediments and ground water; prevention and control of indoor air pollu-
tion; and restoration of ecosystems. NRMRL collaborates with both public and pri-
vate sector partners to foster technologies that reduce the cost of compliance and to
anticipate emerging problems. NRMRLs research provides solutions to environmen-
tal problems by: developing and promoting technologies that protect and improve the
environment; advancing scientific and engineering information to support regulatory
and policy decisions; and providing the technical support and information transfer to
ensure implementation of environmental regulations and strategies at the national,
state, and community levels.
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
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Acknowledgments
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-
sign:
Dr. David Szlag U.S. Environmental Protection
Agency, NRMRL
Paul Shapiro U.S. Environmental Protection
Agency, Office of Research and
Development, Office of Science
Policy
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
IV
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Contents
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
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Contents (continued)
References 21
Appendices
A. Optimizing Operating Procedures 22
B. Best Management Practices 23
VI
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Tables
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
VII
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Figures
1. Open process showing solution maintenance using periodic dump or bleed and
countercurrent rinsing with a continuous wastewater discharge
13
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
VIII
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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 .
Background
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.
1
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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.
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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]
x-y
[ME(CN)y]
x-y
(1)
(2)
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-
nide.
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Table 1. Metal and Complexed Metal Electrode Potentials
Metal Electrode Potential Cyanide Complex
Source: Lange's Handbook of Chemistry 13th Ed.
Electrode Potential
Ni'2
Cu*1
Ag*1
Au*1
Zn'2
-0.25V
+0.52V
+0.80V
+1.68V
-0.76V
[Ni(CN)«]-2
[Cu(CN)3]-2
[Ag(CN)J-'
[Au(CN)J-1
[Zn(CN)J-2
-0.80V
-1.17V
-0.31V
-0.67V
-1.28V
Table 2. Cumulative Formation Constants for Cyanide Complexes
Matal LogK, Log Kj Log 1C,
LogK4
Cadmium 5.48 10.6
Copper (I) 24.0
Gold (I) 38.3
Iron (II)
Iron (III)
Mercury (II)
Nickel
Silver (I) 21.1
Zinc
15.23 18.78
28.59 30.30
41.4
31.3
21.7 20.6
16.7
35
42
Source: Lange's Handbook of Chemistry IS"1 Ed.
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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
sludge.
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-
nide.
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-
ishing.
Table 3. Toxicity of Various Cyanide Compounds
Compound
Hydrogen cyanide
Potassium cyanide
Sodium cyanide
Formula
HCN
KCN
NaCN
Physical Form
Gas
Solid
Solid
TLV
5 mg/m3
5 mg/m3
5 mg/m3
LDso
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
CNCI
NaCNO
KCNO
KJFefCNU
Gas
Solid
Solid
Solid
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
LDH
worker may be repeatedly exposed without adverse effect.
lethal dose to 50% of a specified population.
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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
Metal
Cyanide
Zn
Cu
Ni
Fe
4°C
30
400
1,700
22,000
pH7
20°C
14
130
700
7,700
PH
4°C
700
10,000
13,600
23,000
10.5
20°C
300
3200
5800
71,000
Source: Environment Canada
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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,
1997):
[HCN]air=[(1470/T)e(9-275-2992/T>]/[1+10(PH-9-3)]
(3)
where
HCN
T
= mg HCN/m3 air per ppm NaCN in solution.
= temperature, Kelvin.
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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
(4)
-------
NaCN + CI2 => CNCI + NaCI
CNCI + 2NaOH
NaCNO+NaCI + H2O
2 NaCNO + 3 NaOCI
=> 2 CO2 + N2 + 3 NaCI + 2 NaOH
(5)
(6)
(7)
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.
CNO~+2H2O
H+
H+TiOH
OI-r=>NH
NH4++HCO3
(8)
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
z+
«Me+(z+y)CN
(9)
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
Peroxide
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
(10)
Table 5. Concentrations of Free Cyanide in Solutions of Various Concentrated Metal Cyanide Complexes
Metal
Cyanide
Complex
[Hg(CN)J-2
[Ag(CN)J-
[Fe(CN)e]-3
[Ni(CN)J2
[Cu(CN)J-3
[Zn(CN)
-------
CNO- + 2H-.O
NH3 + CO3-
(11)
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:
CNO-+O,
(12)
Then cyanate is hydrolyzed, in the presence of excess
ozone, to bicarbonate and nitrogen and oxidized per the
following reaction:
2CNO- + 3CX+I-UO
2HCO3-+N2
(13)
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
Ozone:
O3+llhv;=> O2 + O
O + H2O
20H-
(14)
(15)
(16)
(17)
(18)
(19)
(20)
(21)
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
(22)
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-
(23)
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).
10
-------
Anode reactions:
Cyanide
2CN-o(CN)2 + 2e-
(CN)2 + 4 OH' o 2 CNO- + 2 H2O + 2 &
Hydroxide
2OHH2O
- + OCNO-
(24)
(25)
(26)
(27)
(28)
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,
1985).
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
HCOO- + NH,
(29)
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,
4H2O
(30)
(31)
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:
Acid
Medium
Strongly
Alkaline
Medium
HOCN + H+ -
HOCN + H20
NCO- + 2H2O:
(very slow)
NH4+ + CO2 (rapid) (32)
:> NH3 + CO2(slow) (33)
NH,
+ HC03-
(34)
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,
1994)
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,
1987)
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.
11
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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-
tions.
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
(36)
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
(37)
2NaCN + C02 + H2O -» Na2CO3 + 2HCN
(35)
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.
12
-------
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-
Makeup
Chemistry
t
Evaporation
H2O
Process Solution
Dump/Bleed
i
Wastewater
Figure 1. Open process showing solution maintenance using periodic dump or bleed and Countercurrent rinsing with a con-
tinuous wastewater discharge.
13
-------
Makeup
Chemistry
Evaporation
Process Solution
Malnt.
Residual
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
electrowinning.
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
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
Makeup
Water
Figure 3. Closed-loop process showing continuous solution maintenance and rinsewater recovery with reverse osmosis or
vacuum evaporation.
14
-------
Makeup
Chemistry
Evaporation
Process Solution
Maint.
Residual
I
__ Regen.
Chemistry"
H2O
H20
I
Rinse 3
IX
EMR
Regen.
Wastestream
Metal/
residual
Figure 4. Open-loop process showing continuous solution maintenance and rinsewater recovery with natural evaporation and
ion exchange/electrowinning.
15
-------
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.
Metal
Brass
Bronze
Cadmium
Copper
Gold
Indium
Silver
Zinc
Cyanide and Non-cyanide
Cyanide
Proven
Proven
Proven
Proven
Proven
Proven
Proven
Proven
Plating Processes
Non-cyanide
No
No
Yes
Proven*
Developing
Yes
Developing
Proven
New alternatives in development for specific cyanide processes.
16
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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-
tive.
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
Register\r\~\998.
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-
17
-------
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
Allihn
water-cooled
condenser
Thistle-
tube
Water in
9-mm connecting tube
1000-mL
modified
Claissen
flask
Suction
Heating mantle
38-mm x 200-mm
test tube
Figure 5. Distillation apparatus for evaluating cyanide samples.
18
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chlorine to ensure a true determination of the amenable
cyanide.
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.
ASTMD2036B
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.
Acceptor
Carrier
Waste
Pump
Figure 6. Flow injection analysis schematic.
19
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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-
plored.
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.
20
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References
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.
http://users.wantree.com.au/~menne/cnmanage.htm.
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
1999.
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.
21
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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
pretreatment.
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-
posal.
Avoid air agitation or any other aeration of cyanide
solutions.
22
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Appendix B
Best Management Practices
Best management practices for cyanide include the
following:
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-
agement.
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
hygiene.
Dispense and weigh raw materials in a ventilated
area.
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
13).
23
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