EPA-R2-73-287
December 1973
Environmental Protection Technology Series
Investigation Of Treating
Electroplaters Cyanide Waste
By Electrodialysis
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
«l. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY series. This series
describes research performed to develop and
demonstrate instrumentation, equipment and
methodology to repair or prevent environmental
degradation from point and non-point sources of
pollution. This work provides the new or improved
technology required for the control and treatment
of pollution sources to meet environmental quality
standards.
For sate by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20*02 - Price 90 cents
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EPA-R2-73-287
December 1973
INVESTIGATION OF TREATING ELECTROPLATERS
CYANIDE WASTE BY ELECTRODIALYSIS
By
Sidney B. Tuwiner
Project 12010 DFS
Program Element 1B2036
Project Officer
Lloyd Kahn
Edison Water Quality Research Laboratory
Edison, New Jersey 08817
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
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EPA Review Notice
This report has been reviewed by the Environmental
Protection Agency, and approved for publication.
Approval does not signify that the contents
necessarily reflect the views and policies of the
Environmental Protection Agency, nor does mention
of trade names or commercial products constitute
endorsement or recommendation for use.
ii
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ABSTRACT
An electrodialysis procedure is developed whereby the dis-
charge of rinsewater is eliminated. The work, according
to this method, is rinsed in a sequence of two rinses; the
final rinse contains a concentration of cyanide of 1/10,000
of that of the plating.
These concentrations are maintained by the use of electro-
dialysis to transport cyanides continuously from the second
rinse solution back to the first rinse solution and also
from the first rinse back to the plating bath. In this
way, all cyanide is recovered and returned to the bath.
Design parameters are determined from the experiments of
this study and costs are estimated.
The experimental system used in- this study was a prototype
of a commercial size electrodialysis unit operated continu-
ously under conditions which simulated those of the projected
two-stage commercial system using a cyanide copper plating
bath.
iii
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CONTENTS
Section
I
II
III
IV
V
VI
VII
VIII
DC
X
Conclusions
Recommenda tions
Introduction
Experimental
Results and Discussion
Acknowledgments
References
Publications and Patents
Appendix
Glossary
Page
1
5
7
15
19
35
37
39
41
47
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FIGURES
Page
SCHEMATIC - ELECTRODIALYSIS
SYSTEM 13
SCHEMATIC REPRESENTATION OF
LABORATORY ELECTRODIALYSIS
SYSTEM 16
vi
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TABLES
Table page
1. Cyanide Recovery by Eleetrodialysis —
Summary of Performance
A. Experimental Results 22
B. Derived Parameters 23
2. Electroosmotic Transfer of Water 27
vii
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SECTION I
CONCLUSIONS
The experimental work was performed on a simulated cyanide
copper plating bath. The conclusions reached are considered
applicable to cyanide baths of zinc, cadmium, silver and
gold.
1. An 8 cell-pair stack, with 12.8 sq ft of total effec-
tive membrane surface, was successfully operated continu-
ously for one week. Cyanide was recovered from a simulated
rinse solution containing 406 mg/1 of cyanide by returning
it to a simulated plating bath containing 52,100 mg/1. A
simulated dragout rate of 0.87 gallons per hour was employed,
furnishing the cyanide which was recovered.
2. The stack successfully maintained the same concentra-
tion ratio, of 128:1, between solutions of lower concentra-
tion simulating a first and second rinse. Thus in a two
stage rinse system with two electrodialysis stacks, assuming
the same ratio of 128:1, the second rinse would contain
3.2 mg/1 of cyanide.
3. Such a two stage system, with a total effective membrane
area of 25.6 sq ft, is capable of handling a dragout of
0.87 gph without the discharge of rinse solution.
4. Essentially all cyanide and metal values are returned
to the plating bath or to a solution of comparable concen-
tration. Electrolyte impurities are returned also and are
consequently accumulated. The bath requires, therefore,
either periodic or continuous withdrawal and replacement,
or periodic or continuous purification. The requirement
of withdrawal or purification depends on the conditions of
the plating system and on the limits of impurity concentra-
tions which are tolerated.
-------
5. The handling and treatment of rinse waters is auto-
matic and self-regulating.
6. The membranes, lonac MA3475 and MC3470, 10 mils nominal
thickness, showed no evidence of deterioration after 800
hours of operation. Experience from application of these
membranes to other alkaline systems suggests a life expec-
tancy of from one to several years.
7. The system requires 30 sq ft of total effective area
for each gph of bath dragout when the final rinse concentra-
tion is 1/16,500 of that of the bath concentration.
8. The stack which was used in this study included no
separate electrode cell rinse solutions. There was conse-
quently a deposit of metallic copper on the cathode. This
deposit had to be removed after stack operation of one week.
9. For a typical small plating establishment with 1 gph
of bath dragout, the manufacturing cost of the eleetrodialysis
system is $1,600, exclusive of rinse vessels, installation
and space requirements.
10. The total power requirement for this capacity is 4 KW
for rectifier and pumps, based on 6 volts per cell-pair for
the stack.
11. The maximum concentration at which the cyanide may be
recovered is about 11.5 oz/gal of copper cyanide with an
appropriate concentration of about 13 oz/gal of sodium
cyanide. The reason for this limitation is the transport
of water, together with that of ions, in electrodialysis.
If the plating bath contains 16 oz/gal of copper cyanide
and 18 oz/gal of sodium cyanide,the return of the cyanide
and copper values requires an evaporation of 0.4 gallons
for each gph of dragout. Normally the plating solutions
which are from 50-70°C suffer evaporation losses which are
in excess of this amount.
-------
12. When the cyanide plating bath contains brighteners
which are anionic substances of high molecular weight, some
special provision is required to avoid membrane fouling.
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SECTION II
RECOMMENDATIONS
It is recommended that a second phase be undertaken to
design a modular unit for small to medium size plating
shops and to provide a demonstration and evaluation of the
system in a typical shop.
The stack design should provide for two stages combined in
a single stack, with internal stream flow between stages.
The design of the manifolds, frames and spacers used in
commercially available equipment should be modified to pro-
vide greater simplicity, insurance against internal leakage
and greater volume of solution flow than is now possible
with the Aquachem WD6-2 unit. This will optimize capacity
and efficiency and eliminate all but routine scheduled
maintenance. Provision should be made in the design modi-
fication for removal and replacement of cathodes without
breaking the remainder of the stack assembly.
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SECTION III
INTRODUCTION
Cyanide waste constitutes a major source of pollution.
Under Federal "drinking water standards" for cyanide, the
recommended maximum concentration is 0.01 ppm, and 0.2 ppm
is considered the level for rejection. This standard has
served as a guideline for the states and municipalities,
most of which have adopted limits ranging from 0.05 to
1 ppm. 1
There are at least 20,000 plating establishments in the
U.S., most of them involved with one or more cyanide
plating operations, and most of them are very small. Among
the major industries discharging cyanide-bearing wastes,
aircraft engines and parts account for 293,760 gpd, missile
parts 32,OOQ gpd at 80 ppm,^ instrumentation 13,000 gpd,
electronic hardware 259,000 gpd at 200-1500 ppm,6 home
appliances 108,000 gpd,' television antennae 11,000 gpd,
silverware 165,000 gpd at 172 ppm,9 automobile manufacture
410,000 gpd at 204,I0 and "unspecified," 250-400 gpd at
40-130 ppm.11
Cyanide at a concentration of 0.05 ppm is fatal to many
species of fish and other marine life. ^ In municipal
biological sewage treatment plants, a concentration of 1-2 ppm
of copper cyanide is detrimental, affecting activated sludge
formation adversely.1-*
All cyanide wastes may be treated with one or another form
of available chlorine for conversion of the cyanide to
cyanate or for its complete destruction. The latter re-
quires a holding time of 24 hours.^ Metallic elements
associated with the cyanide are converted to precipitates
of the hydroxides or basic salts. To reduce the metal con-
tent of the treated water to permissible concentration
levels it is necessary to clarify the effluent. It may be
necessary also to remove excess chlorine and to neutralize.
-------
Chlorination is relatively expensive in equipment and re-
agent costs as well as in labor for control and monitoring.
It requires 7 to 9 Ibs available chlorine per Ib cyanide. *-b
Chlorination with chlorine gas is practical only in large
installations. Usually it is preferable to chlorinate using
calcium hypochlorite (70 per cent as available chlorine).
The cost of this reagent is about 35 cents per pound of
available chlorine, or about $3 per pound of cyanide.
An alternate method, requiring about 3.5 Ibs available
chlorine per pound cyanide, is to chlorinate only to the
extent of converting cyanide to cyanate at pH 10.0. The pH
is then reduced to 3.5 and held for at least 5 minutes to
hydrolyze the cyanate. The control procedure for this method
is much more demanding than for complete chlorination.^5
Cyanide values are lost in both methods and metal removal is
very costly.
Other methods, involving evaporation of rinse solutions, have
been used or proposed. These eliminate the need for chlori-
nation but require even more expensivei equipment for evapo-
ration, condensation and steam generation. The simplest and
oldest of these methods involves a two-stage rinse, first in
a "still" rinse and finally in a "running" rinse. Periodi-
cally the still rinse solution is concentrated by evaporation
and returned to the plating bath. Still rinse solution which
is withdrawn to the evaporator must be replaced with water.
Although most of the dragout of cyanide is recovered in this
method, there is a portion which is discharged from the
running rinse solution. A more satisfactory approach to
abatement of cyanide pollution is represented by the evapora-
tive system which ideally recovers all of the cyanide and
metal values at the expense of considerable installation and
operating costs. This is discussed on pages 31-33.
The evaporation system is more expensive than Chlorination
for treating cyanide waste, except where the values
8
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recovered represent a substantial credit as is the case in
cyanide cadmium and precious metal baths in larger plating
establishments.
Electrodialysis is an industrial process in which ions are
transported from one to another solution while under the
influence of an electric field imposed across membranes
which separate the solutions. The commercial equipment
which provides the solution flow channels, membranes, cells
and electrodes is called a stack.
Commercial stacks differ from one another in numerous
details but all of them bear some resemblance to a filter
press with the membranes corresponding to filter cloths and
with spacers corresponding with the filter press frames.
There are also flow manifolds for the two solution streams
in electrodialysis, and ports connecting these manifolds
with the cells which are defined by the membranes and
separators. There are two types of cells which alternate
in sequence; one is for the solution which is being diluted
by the transport and the other is for the solution which is
being concentrated. The port connections are designed to
provide communication of each cell with the appropriate
pair of solution manifolds, one of them for the feed and the
other for the product solution. Thus there are four solu-
tion manifolds for each stack.
A stack includes end plates similar to those of a filter
press, and some means, such as a frame or tie rods, for
compressing the stack components between these plates.
This provides for ease of assembly and disassembly for
cleaning or replacement of membranes or other stack compo-
nents. When the stack is in operation these are held in
compression to insure against leakage internally between
the two solution streams, or externally from between the
frames.
There are several points of difference between an electro-
dialysis stack and a filter press. The space in the cells
between the membranes is occupied by plastic separators,
-------
which serve to prevent physical contact between the mem-
branes of each cell. These separators are foraminous in
construction so that there is a minimum of area of membranes
which is blanked out or covered and so that there is the
least obstruction to the flow of ionic current across the
cells.
The details of stack construction and the principles of
this method have been exhaustively described and discussed
elsewhere. Two types of membrane are used, one of them
is cation-selective while the other is anion-selective.
They alternate in sequence and each pair defines either a
concentrating or a diluting cell, depending on the direction
of the electric field in relation to the order of the mem-
brane sequence.
Thus the electrical field is induced by the end electrodes
of the stack and the ionic current flows from one electrode
to the other along all of the cells in series. With respect
to solution flow the cells are in parallel between each
manifold of the pair for each of the solution streams.
Except in the end cells, each bounded on one side by an
electrode, the only electrochemical process is that of ion
transport through the solutions and across the membranes.
Electrolysis occurs in these end cells. Most often in
commercial electrodialysis, solution streams are circulated
through these end cells which are apart from and in addition
to the two main solution streams through the other cells.
This is done to separate the products of electrolysis from
the contents of these main streams. This separation re-
quires additional pump and circulation means for the so-
called electrode rinse streams.
In all of the cells anions are transferred through anion-
selective membranes while cations move through cation-
selective membranes. Both types of ions are driven by the
electrical field in opposite directions. By virtue of ion-
selectivity of the membranes there is an accumulation of
electrolyte in one group of cells, the concentrate stream
cells, and a depletion in the other, the diluting stream cells
10
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The degree of ion-selectivity, expressed as a ratio of the
current carried by the favored ions to the total current
through a membrane, may vary up to unity for perfect ion-
selectivity. In this case of a perfect membrane the trans-
fer of each gram equivalent of electrolyte requires pre-
cisely one faraday of current for each membrane-pair in
the stack. This would represent a current efficiency of
100 per cent. In practice membranes are never perfectly
ion-selective. Some of the current is therefore carried
by counter-ions moving in the direction opposite to that
of the preferred ions. The effect of membrane ion-selectiv-
ity on current efficiency is expressed by the following:
E = ta + tc - 1
where E is the current efficiency as a fraction, and ta
and tc are the ion selectivities of anion-, and cation-
selective, membranes, respectively.
Current efficiency is an important parameter in an evalua-
tion of an electrodialysis system inasmuch as it is closely
related to unit power consumption as well as to unit membrane
capacity. Inasmuch as the current efficiency is a function
of the ion-selectivities of both anion- and cation-
selective membranes, it is impossible to assign values to
these individually. However, in a solution containing
polyvalent complex cyanide anions it is likely that a very
low current efficiency is ascribable to the "poisoning"
effect of these ions on the anion-selective membranes.
91
De Korosy has presented experimental data which indicates
that polyvalent anions, including complex cyanides, may be
immobilized in the membrane polymer and may promote the
backward eleetremigration of cations. There was some con-
cern lest this propensity might limit the concentration
ratio and also the current efficiency. On the other hand it
is known that the metal and cyanide values in rinse solutions
from the cyanide leaching of copper concentrates and gold
ores are recoverable by electrodialysis and that a patent1^
has issued in which this is claimed.
11
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There have been still other proposals to recover cyanide
from waste rinse streams by eleetrodialysis. These have
been economically unsuccessful, or at best of marginal
utility because such proposals have been for multistage
removal of cyanides from an existing effluent stream.
Since each stage in electrodialysis can conveniently remove
at best not more than about one-half of the cyanide in the
entering stream, the number of stages required to bring the
effluent to acceptable levels of cyanide concentration is
prohibitively great. On the other hand, treatment of the
stream in only two or three stages, followed by chlorination
serves only to complicate the system. Especially for
smaller establishments, the system must remain simple if it
is to be practical.
The present investigation was initiated to apply electro-
dialysis to a rinse system, so that compositions of the
rinse solutions are maintained and the cyanide and metal
values are returned and recovered. The basis of the study
is a concept, illustrated schematically in Figure 1, in
which electrodialysis is incorporated within the rinse
system in such a manner that the transport of electrolyte
in each stage of electrodialysis is precisely equal to the
transport of electrolyte in the opposite direction by drag-
out. According to this concept the combined system ideally
is closed and there is no discharge of solution nor any
requirement for plating solution makeup. Practically this
ideal is not fully achieved owing to accumulation of im-
purities and the requirement of some evaporation and com-
pensating water addition in the case of highly concentrated
plating solutions.
12
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•FIGURE #1
COMMERCIAL SYSTEM CYANIDE RECOVERY
1
1
1
A
Plating Tank
V
_
Rinse #1
Electrodialyzer Stack
__J ll
-1
I
I
Dragout
I
I
I
\7
Rinse #2
Pumps
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SECTION IV
EXPERIMENTAL
The electrodialysis stack, power supply, solution storage
containers, tubing connections, pumps, flowmeters, and
electrical instruments are components of Aqua-Chem Model
WD 6-2, Aquachem Division, Coca Cola Inc., Waukesha, Wise.
The solutions are stored in two plastic bottles, each of
5 gallon capacity, one holding the concentrating stream
solution, the other holding the diluting stream solution.
Each bottle is provided with a bottom discharge nipple and
hose connection to one of two circulation pumps. Each
pump circulates a flow through a rotometer and the electro-
dialyzer stack. A hose connection from the stack leads
the stream back to the storage bottle.
Each stream circulates continuously between the storage
vessel and the stack. Within the stack the two streams
are separated by the membranes. Transfer of the stream
components is negligible until the electric current is
applied. This current is maintained continuously, effect-
ing a proportionate rate of electrolyte transfer.
The dragout is simulated by means of a constant displacement
pump discharging a portion of the concentrated stream from
the storage bottle to the diluting stream which it enters
through the side arm of a "tee" connection in the line which
returns the diluting stream from the stack. There is pro-
vision for an overflow from the diluting stream storage
vessel to the concentrating stream. Volume changes of the
two solutions are observed and measured to determine the
electroosmotic flow rates under controlled experimental
conditions. Provision is also made for representative
sampling of both solutions.
15
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FIGURE #2
LABORATORY SYSTEM
Centrifugal
Pumps
Concentrating PIT
Solution / N
Bottle
Diluting
Solution
Bottle
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The composition of the bath used in this investigation is
as follows :
Copper Cyanide 16 oz/gal 120 gpl
Sodium Cyanide 18 oz/gal 135 gpl
Sodium carbonate 2 oz/gal 15 gpl
Sodium hydroxide 4 oz/gal 30 gpl
This bath is typical of those employed for heavy copper
plating.19 por improved speed and plating quality, the
bath is maintained at about 145°F. At the normal bath
temperature, the evaporation rate is about 5 gallons for
each sq ft of exposed bath surface in 24 hours. Some water
transport accompanies the cyanides when using electrodi-
alysis to recover them from the rinse. From the very dilute
solution of the second rinse, this electroosmotic flow of
solution is far less than the dragout rate, and the effect
on the water balance of the system is consequently very
slight; but, from the first rinse to the plating bath the
electroosmotic flow may exceed the dragout rate.
Specifications of the Aqua-Chem WD 6-2 electrodialysis stack
are as follows :
Number of Cell-Pairs 50
Effective Area per Membrane 0.8 sq ft
Rated Flow, Concentrating Stream 600 gal/hr
Rated Flow, Diluting Stream 600 gal/hr
Per cent removal, NaCl 5-77o
Stack voltage 50-120 volts
Concentration, NaCl Up to 4%
In this study the number of cell-pairs was reduced to eight.
This reduced the rated flow proportionately for both streams.
The flow rate in these experiments was actually approxi-
mately three times the manufacturer's rating and the
removal of cyanides was, under optimum conditions, as high
as 50 per cent when the stack was operated at 50 volts
compared with the rated 5-7 per cent for sodium chloride.
17
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This indicates an approximate ten-fold increase in unit
capacity from that which is normally assumed as a basis
for design. It suggests also that an even greater increase
in unit capacity may be possible if the stack components
are redesigned for a greater flow rate.
The experimental data was obtained under steady state
conditions. This required no more than 30 minutes for
attainment after a change in one of the system parameters.
Constancy of solution composition was confirmed by period-
ically monitoring the solution conductivities.
After a steady state was achieved in any run, the following
data were obtained:
1. flow rates of concentrating, and diluting, streams
to the stack,
2. flow rate in dragout stream by measuring the volume
delivered in 30 seconds by the metering pump,
3. concentration of copper and total cyanide in each
of the solution holding vessels.
The copper concentrations were determined by atomic ab-
sorption spectroscopy after decomposing the cyanides with
strong sulfuric acid, neutralizing with ammonia and acidifi-
cation with acetic acid. Total cyanide was by colorimetric
determination of cyanogen bromide with benzidine and pyridine
in amyl alcohol according to the procedure described in the
appendix.
For a period of an hour after attainment of a steady state,
the solution levels in the two vessels were noted and from
this data an estimate was made of the net volume transport
from one solution to the other. Volume transport in the
simulated dragout stream was known from the pump calibration
as described above. Except for internal leaks the sole re-
maining volume transport is by the electroosmotic transport
within the stack. This rate is estimated from the algebraic
difference between the measured net volume flow rate and the
dragout rate.
18
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SECTION V
RESULTS AND DISCUSSION
The purpose of this study was to obtain design parameters
for the system. The most important of these is the unit
transport capacity for cyanides. It is well known that,
for solutions which are sufficiently dilute, the transport
capacity of a stack, expressed as equivalents per hour, is
directly proportional to the concentration of the diluting
stream. This implies that this capacity may be expressed
as a parameter which denotes the number of gallons per hour
of diluting solution the electrolyte content of which is
transported per square foot of membrane.
As indicated in Section IV, we have determined the quantity
of cyanides transported per hour in each run. We know also
the concentration in the diluting stream to the stack.
Dividing the transport rate, in moles per hour, by the con-
centration, in moles per gallon, yields a parameter for the
stack, in units of gallons per hour of diluting solution.
When this is divided by the total number of square feet of
membrane surface, we obtain the parameter in gph/sq ft,
applicable to the design of any stack having the performance
characteristics of the stack used in this study.
As indicated in Section IV, the primary solution assays
were for copper and total cyanide. Accordingly, the trans-
port parameter is calculated as TCu from the copper assays
and also as TQV from total cyanide. These appear in Table
I. There is no reason to assume that there should be any
difference between the two parameters inasmuch as the amount
of uncomplexed copper is negligible and that of uncomplexed
cyanide is small. Differences which occur may therefore be
ascribed to experimental error.
19
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The principal parameters studied were concentrations,
voltage and simulated dragout rate. Within each group
of tests, viz., 1A-1F, 2A-2E, 3A-3D, 4A-4C, the prin-
cipal variable is the dragout rate. Comparison of each
groxp with the others reflects the effect of stack volt-
age and solution composition. The final series, 5A-5B,
was primarily to determine the maximum solution concen-
tration and the rate of water transport.
TRANSFER RATE CALCULATION
The transfer rate of the cyanides is measured in the
experimental system by operating under a steady rate,
i.e., one at which the solution concentrations do not
change significantly. The rate of electrolyte transfer
by electrodialysis is then equal to the reverse transfer
by the metering pump, which simulates the dragout.
In order to determine the transfer rate and the transfer
parameters, let:
Ci = the concentration of component i in the concentrating
stream in moles/gal
ci = the concentration in the diluting stream in moles/gal
D = the simulated dragout rate, in gals, per hour
Ri = C/c the ratio of concentrations
Qi = the transfer rate in moles/(sq. ft.)(hr.)
TŁ = the transfer parameter in gal/(sq. ft.) (hr.)
A *= the membrane area in sq. ft.
Then we have:
Qi =C
TI - Q
Subscript i designates the component, for example, copper
or total cyanide.
20
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To obtain the most precise determination of the transfer
parameter, T^, it is necessary to determine precisely the
concentration ratio, C^/c^.
TRANSFER RATES AND CAPACITY
Table 1 shows that a combination of temperatures and high
stack voltages favor maximum transfer. Note for example,
that in Runs 1 and 2, the third measured value of each run
was at the lowest temperature and each yielded the lowest
transfer parameter for that run. The transfer rate for
Run 2, at 40 volts, is greater than that of Run 1 at 30 volts
However, this is not necessarily ascribable to voltage
inasmuch as the temperature was also higher in Run 2 than
in Run 1.
In Run 3 we note the effect of higher solution concentration.
There is some indication that the transfer capacity is lower
at the higher solution concentration at 40 volts for the
stack. This is confirmed in Run 5, at 40 and at 30 volts.
With the solution in this run at from 52 to 66 per cent of
that of the full strength plating solution, the transfer
parameter is somewhat lower than it is at lower concentra-
tions for comparable voltage and temperature conditions.
For design purposes consider, as in Run 4, a stack voltage
of about 6 volts per cell-pair (allowing about 2 volts for
electrode potentials) and a transfer parameter of 8.82
gallons/ (sq. ft.) (hr,). The second set of data in this
run was obtained at 75°C., which is recommended as the
operating temperature. Inasmuch as this data was obtained
in concentrated solutions, the transfer parameter is con-
servative when applied to the more dilute second rinse
s ta ge.
The only component concentration, other than copper and
cyanide, which was determined in this study, was hydroxyl
ion. The activity of this ion is indicated by pH
21
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TABLE 1
CYANIDE RECOVERY BY ELECTRODIALYSIS - SUMMARY OF PERFORMANCE
A. EXPERIMENTAL RESULTS
Flow in GPH
Total Cyanide, molar Copper, atoms/I
PH
Run
1A
6
C
D
E
F
2A
B
C
D
E
3A
B
C
D
4A
B
C
5A
B
Temp°C
30
40
30
40
40
40
45
42
40
47
46
50
54
50
50
65
75
65
33
45
Amps
0.78
2.6
2.4
3.2
3.7
4.0
1.8
2.3
2.3
4.2
5.5
7
11
16
6.5
27.5
30.0
12
30
34
Volts
30
30
30
30
30
30
30
30
30
40
43
40
40
40
40
50
50
50
40
30
Dragout
0.31
.83
.73
.71
.70
.70
.65
.66
.66
.68
.82
.66
.86
.84
.33
.68
.86
.24
.86
.86
Dil. Stream
300
300
300
300
300
300
300
300
303
297
300
246
249
240
240
261
225
240
261
264
Concent. Sol'n
0.0473
.0960
.0757
.1475
.1771
.1913
.1199
.2075
.1637
.1992
.1453
.498
.698
.384
.181
2.210
2.006
2.292
3.40
2.60
Dilute
0.00025
.00153
. 00334
. 00169
.00234
. 00238
. 00125
. 00146
. 00312
.00233
. 00120
.0054
.0137
.0091
.0010
.0133
.0156
.0057
.1310
.1360
Concent. Sol
0.0120
.0353
.0241
.0467
.0554
.0584
.0458
.0678
.0482
.0493
.0530
.1164
.1324
.0795
.0511
.700
.586
.546
.887
.690
'n Dilute
0.000076
.000559
.001030
.000585
.000928
.000873
.000373
.000551
.000924
.000566
.000771
.0011
.0022
.0029
.0003
.0049
.0044
.0009
.0284
.0384
Concent.Sol'n
10.8
10.5
10.5
10.55
11.05
11.35
11.6
10.8
10.6
12.0
12.35
9.96
9.83
9.80
9.80
13.0
14.1
12.93
14+
14+
Dilute
7.6
9.85
9.6
9.6
9.76
9.93
9.4
10.0
9.8
9.6
9.75
___
9.35
9.79
9.64
9.23
10.23
9.20
.._
13.90
-------
B. DERIVED PARAMETERS
Ratio: Total Cyanide /Cu
Run
U
B
C
D
E
F
2A
B
C
D
E
3A
B
C
D
4A
B
C
5A
B
Concent.
2.37
2.72
3.14
3.16
3.20
3.28
2.62
3.06
3.40
4.04
2.74
4.25
5.27
4.83
3.54
3.16
3.42
4.20
3.83
3.82
Dilute
3.31
2.74
3.24
2.89
2.52
2.73
2.74
2.65
3.38
4.12
1.56
4.91
6.23
3.31
3.33
3.25
3.55
6.0
4.61
3.54
Concentration Ratio
CN
189.2
62.7
26.7
87.3
75.7
80.4
95.9
142.1
52.5
82.7
121.1
92.2
50.9
63.0
181.0
166.2
128.6
402.1
26
19
Cu
265
63
23
68
60
67
122
123
52
87
69
105.8
60.2
27.5
170.5
143
133.4
607
31.2
17.95
OH"
3000
5
8
9
20
23
158
6
6
25
398
-. —
3
1
1
589
7420
5375
-_,-
70 Removal, Dilute
CN
18.9
17.3
6.4
26.2
17.7
18.8
20.7
30.7
11.5
18.3
33.0
19.3
17.5
22.1
19.3
34.1
50.1
39.7
8.6
6.2
Cu
27.4
17.4
5.6
16.1
14.0
15.7
26.4
27.2
11.4
19.8
18.8
28.5
20.7
9.7
23.3
37.4
51.1
60.1
10.35
5.9
Transport-GPH/Sq Ft
CN
4.53
4.05
1.51
4.83
4.14
4.29
4.85
7.18
2.70
4.40
7.72
4.76
3.42
4.15
4.62
8.83
8.82
7.45
1.75
1.23
Cu
6.40
4.07
1.30
3.76
3.28
3.66
6.17
6.35
2.68
4.62
4.40
5.46
4.04
1.81
4.36
7.64
9.00
11.28
2.11
1.21
Current
Efficiency
7,
21.8
29.8
27.0
38.4
39.4
39.2
50.7
70.0
55.2
37.7
25.3
55.1
64.0
23.8
11.7
64.5
68.0
53.3
115
79
-------
from measurements using a glass electrode system. The
ratio of hydroxyl activity is calculated from the pH
values of the two streams and this ratio is shown in a
column of Table 1. It is notable that in Run 4, in
which there was free caustic alkali in the concentrating
solution, the ratio for hydroxyl is substantially greater
than the concentration ratios of copper and total cyanide.
This is a consequence of the greater mobility of hydroxyl
ions in the anion-selective membranes compared with the
mobility of the other anions. This very high ratio of
hydroxyl activity is not observed in the other runs of
this study, owing probably to buffering effect of the
cyanides.
All of the experimental data of Tables 1 and 2 were from
analyses of solution samples taken under steady state
conditions. Under a steady state the membrane transport
of copper and total cyanide is equal to the rate at which
these components are transported in the simulated dragout
solution. If the volume rate and concentrations are known
for this stream which flows from the concentrating to the
diluting solution vessel, then we may assume that the
transport rates thus determined are equal and opposite
to the transport rates of copper and cyanide within the
stack.
The system illustrated in Figure 3 performed substantially
as anticipated and yielded the data which is summarized
in Table 1. Happily the unit capacity for cyanide trans-
port turns out to be markedly improved when the stack
voltage is increased. The size of a system is consequently
reduced by an order of magnitude below that which was
anticipated. This is achieved at the cost of a somewhat
greater power requirement which is, however, not excessive
in relation to the cyanide and metal values recovered, nor
to the saving of capital cost. The economics of the system
is discussed below.
There was no difficulty in operating the system continu-
ously and without attention over periods of one week after
which it was necessary to remove the copper deposited at
24
-------
the cathode. The only mechanical difficulty was from a
tendency to a slight internal leakage between the two
systems. It was determined that this was from a faulty
design of the manifold gaskets which tended to collapse
around the ports which connect the manifolds and the cells.
By control of the pressure differential it was found pos-
sible to insure that the slight leakage was always from
the diluting stream to the concentrating stream. The rate
of this leakage was determined by observing the volume
changes of the two solutions without electrical current.
A suitable correction was made for this leakage in esti-
mating the electroosmotic flow as described in Section IV.
The differential pressure was controlled by control of
the solution flow rates. In all runs the concentrating
stream rate was 300 gph and the diluting stream flow rate
was then adjusted until the required pressure differential
was obtained.
CURRENT EFFICIENCY CALCULATION
The current efficiencies are estimated from the total
electrolyte transport in equivalents per hour. Inasmuch
as the analytical determinations were for the major com-
ponents only, it has been assumed arbitrarily that, for
purposes of estimation, the other electrolyte components
are transported at a proportional rate to that of the
copper, and that the total sodium content of all solutions
is the same in proportion to the total cyanide as it is in
the starting solution. Sodium is the only cation in this
system, and the total number of gram-equivalents of sodium,
estimated in this manner, is therefore a measure of the
current efficiency. It is recognized that this method of
estimation is inexact, particularly because there is a
moderate amount of cyanide loss from the solution because
of anodic oxidation. The current efficiencies, when esti-
mated in this way, are therefore being stated conserva-
tively. The basis of calculation of sodium, in epl (equi-
valents per liter), is as follows:
25
-------
From NaCN 18 oz/gal x 7.489/49.08 - 2.750
Na2C03 2 oz/gal x 7.489/53 - .283
NaOH 4 oz/gal x 7.489/40.01 - .749
3.782 epl
The total cyanide in the starting solution is:
From NaCN 18 oz/gal x 7.489/49.08 = 2.750
CuCN 16 oz/gal x 7.489/89.56 - 1.338
4.088 epl
From this the sodium is 0.925 times the total cyanide in
epl in the simulated dragout. The product of this factor
and the cyanide molarity and d ragout rate in liters per
hour represents the total electrolyte transport rate.
Each gram-equivalent of transport per hour requires 26.8
amperes. The actual number of amperes multiplied by the
number of cell-pairs in the stack, 8 in this investigation,
is always greater than the electrolyte transport equivalent,
and the ratio of these quantities represents the current
efficiency.
As efficiency of 100 per cent is achieved only if all
membranes are perfectly ion-selective and if there is no
leakage of current through the solutions within the mani-
folds and ports of the stack. In actuality there are
current losses from both imperfect ion-selectivity and
current leakage. The data obtained in this study does
not permit an analysis of the current losses for the
present system. The efficiencies are, however, acceptable,
particularly in the eleetrodialysis of solutions in the
higher range of concentration. It is in this range that
the efficiences are most significant as an index of the
power requirement.
26
-------
ELECTROOSMOTIC TRANSFER OF WATER
The rate of transfer of water by electroosmotic flow is a
very important parameter in the first stage of eleetrodialysis
in this system. This is for the reason that the ratio of
transport of copper and cyanide to that of water determines
the maximum concentration of these components in the solu-
tions which are recovered, or if the concentrating system
is from the plating bath, it determines the amount of
dilution of the bath from the rinse recovery system. In
the second stage of eleetrodialysis the water transport is
negligible because of the much lower concentrations. Results
are shown in Table 2.
Table 2
Electroosmotic Transfer of Water
Run No. Gals/1000 amp-hrs.
3 B 3.2
3 C 8.1
3 D 14.8
4 B 11.5
6 A 5.6
6 B 6.7
6 C 9.8
These results show considerable scatter probably from the
difficulty in maintaining the hydraulic balance over the
time necessary for measurement with and without current
flow. There are also several process variables such as
temperature and solution composition which are known to
affect the electroosmotic transfer and contribute to the
scatter.
27
-------
Our concern is that if the water which is returned exceeds
the combined volume of both evaporation and dragout there
will be a problem of controlling the water balance. Within
the precision limits of these experiments it appears that
for the plating bath containing 16 oz/gal of copper cyanide
and 18 oz/gal of sodium cyanide the amount of water re-
turned to the plating bath is about twice the volume of
dragout. Evaporation from the surface of the bath in the
plating tanks is usually more than sufficient to eliminate
this amount of added water. If it is not sufficient some
external evaporation may be required.
SYSTEM CHARACTERISTICS AND DESIGN PARAMETERS
The system is fundamentally simple, flexible and stable
with respect to process disturbances. The only require-
ment placed on the operator is to maintain the minimum
stream flow rates and the mini mum stack voltage. Fail-
safe system requirements are satisfied by thermal overload
relays for the pump motors, a current-limiting relay on
the power line, and protection against excessive tempera-
ture rise of the solution.
Because of the very high unit capacity, it is possible to
design modular stack components suitable for both small and
medium size establishments. The smaller systems may be
considerably overdesigned at a relatively low cost penalty.
The stack, power supply and control panel may be premounted
and assembled. Electrical connection to a three phase
power source and piping to the three solution tanks is all
that is required for installation.
In any application of this system it is necessary to know
the initial bath composition and the maximum concentration
allowable in the final rinse. For a two stage rinse
system with an equal concentration ratio between stages,
this ratio is the square root of the ratio of the concen-
tration of cyanide in the final rinse to that in the
plating solution. For example, if the concentration of
28
-------
cyanide is 100,000 ppm in the plating bath and 10 ppra in
the final rinse this, ratio is 10,000:1 and the ratio
between stages is 100:1.
It is also necessary to know the rate at which a solution
is carried with the work as it enters each rinse tank.
Solution carried out of a vessel is termed dragout and
solution carried into a tank is termed drag-in. The drag-
out of one stage is the drag-in of the next. It may be
assumed that the dragout from the plating bath and from
each rinse stage are all approximately the same volume.
The dragout rate depends on the amount of work handled, the
configuration of the working surface, and the drainage time
which is allowed. Plating establishment operators do not
generally know exactly what this rate is. It varies some-
what from time to time during operating. Dragout may be
estimated from the amount of cyanide required to maintain
the bath composition or else by monitoring accumulation of
cyanide in the rinse solutions.
These are rules of thumb which may serve as a guide in
estimating the dragout if the plating workload is known.
Typical amounts of solution removed with each 1000 sq ft
of work are as follows :^0
Vertical parts, well drained 0.4 gallons
Vertical parts, poorly drained 200 gallons
Vertical parts, very poorly drained 4.0 gallons
Horizontal parts, well drained 0.8 gallons
Horizontal parts, very poorly drained 10.0 gallons
Cup-shaped parts, very poorly drained 8-24 gallons
If the system is designed for a small or medium-sized
plating establishment, it is advantageous to utilize
standard power transformers. For example, a standard 208
volt, 3 phase secondary winding, with 120 volts from each
leg to ground, may be adapted, by including diodes in the
circuit, to deliver 135 volts DC. Allowing for the electrode
potentials and for ohmic losses within the stack, each stage
will contain 21 cell-pairs.
29
-------
A stack, such as the WD 6-2, used in this study, contains
0.8 sq ft of effective membrane per cell, or 1.6 sq ft per
cell-pair. With the 135 volt DC power supply, such a stack
contains 21 x 1.6, or 33.6 sq ft per stage, and it can
handle 2.3 gallons per hour of dragout, maintaining a
cyanide concentration ratio of 128:1 between the streams.
This estimate is based on a transfer parameter of 8.82
gallons/(sq. ft.)(hr.) which is obtained from the data
summarized in Table 1, and the calculations, 8.82/128 -
0.069 gallons/(sq. ft.)(hr.), and 33.6 x 0.069 - 2.3 gph.
Based on these assumptions of stack performance and of
128:1 dilution ratio per stage, a membrane area of 33.6/2.3,
or 14/6 sq ft, is required for each stage, 29.2 sq ft in
all, for each gallon per hour of dragout. With the provi-
sion of this amount of stack membrane capacity, the concen-
tration of cyanide and the other substances is sufficiently
low in the second rinse solution so that the quality control
of the rinse is satisfactory.
The volume of rinse solution in each stage must be suffi-
cient to avoid excessively abrupt changes of concentration.
This represents nothing more than normal plating shop prac-
tice in counter-current rinsing. In the system which has
been developed in this study, the current in each stage of
eleetrodialysis is directly proportional to the solution
concentration at that stage. An ammeter in each stage may
therefore serve to monitor the concentration and to provide
a warning of any tendency to overload the system.
POWER REQUIREMENT AND COSTS
The AC power requirement, allowing for current efficiency,
and rectifier and transformer efficiency, is 4.5 KW for
each gph of solution dragout of the assumed concentration.
Additionally, power is required for three fractional horse-
power pump motors. The rectifier energy requirement depends
on the total number of equivalents of electrolyte recovered
30
-------
(plating solution concentration times dragout rate) and on
the current efficiency; this is assuming a constant 6 volts
per cell-pair. It does not depend greatly on the dilution
ratio per stage.
Operating costs for the electrodialysis system, other than
those for power, are nominal if the system is well designed.
The only moving parts are in the three pumps, and the only
normal maintenance is to remove the copper cathodes
periodically. Costs, on the basis of one gph dragout at
16 oz/gal copper cyanide, are as follows:
Cost per day, 1 gph dragout, 24 hour operation
Power 120 KWH @ $0.03 $3.60
Labor, operating and maintenance,
1 hr. 4.00
Membrane replacement after 2 yrs.
@ $5/sq. ft. .50
Cost per day $8.10
The reagents which are recovered represent, at 1 gph for
24 hours, a value of about $49, assuming that there is
no bleedoff to maintain the impurity levels. By way of
comparison, the cost of chlorination represents a reagent
cost of about $2.50 per gallon dragout, or $60 per day at
the 1 gph rate which is assumed. Chlorination is, of
course, a method of treatment which provides no economic
recovery.
The evaporation system of the Pfaudler Division of Sybron
Corp., which achieves the same recovery of cyanide as the
electrodialysis system developed in this study, costs
approximately $30,000 including installation and steam
generation facilities. Operating cost of the Pfaudler
system, for 1 gph of dragout, using a two-stage rinse system
with 100tl rinse dilution ratio between stages as assumed
for the electrodialysis system, is as follows:
31
-------
Operating Costs per 24 hours
Utilities (steam, power, cooling)
@ $0.01 per gallon, 100 gph $24.00
Labor, $3.00 per man-hour x 2.5 hrs. 7.50
$31.50
The requirement of steam and cooling water is greatly
reduced in the Pfaudler system if additional rinse stages
are employed. Estimates such as those above are for pur-
poses of illustration, for comparison of the two systems,
and not to indicate the evaporative system which would be
utilized in practice.
Whether by electrodialysis or evaporation, the return of
all of the cyanide which is removed by the dragout also
returns impurities. These must not be allowed to accumulate
beyond a harmful level. Sodium carbonate is controlled by
occasionally cooling electrolyte, preferably to below
ambient temperature and removing crystals. Other impurities
are controlled by occasional bleedoff in accordance with
plating conditions and requirements. Disposal of concen-
trated bleedoff may be by electrochemical treatement,
chlorination, or by off site treatment. The cost of such
treatment is the same for the Pfaudler system as for the
electrodialysis.
Both of these systems possess the virtue of eliminating the
discharge of dilute cyanide waste and the need for monitor-
ing a waste stream. In the electrodialysis system the stack
electric current is proportional to the electrolyte concen-
tration in the diluting stream of each stage. If ammeters
are provided, it becomes a simple matter to estimate the
dragout load on the rinse system and to control the rinse
efficiency.
In addition to the saving in the labor of monitoring a
waste stream, there is a saving in the cost of rinse water
and of water treatment. However, inasmuch as the impurities
contained in evaporation make-up water are retained in the
32
-------
system and inasmuch as calcium and magnesium in this water
tend to produce a sludge with carbonate, it is preferable
to use demineralized water for make-up.
The most significant economic advantage of the system
developed in this study is the potentially low capital
requirement. At the present time the minimum cost, using
commercially available equipment, is about $5,000 for the
system, exclusive of rinse tanks. It is believed that a
much more satisfactory system is possible with stack, pumps,
and electrical gear designed specifically for a 2-stage
alkaline cyanide recovery in a single stack with six pipe
connections at one end plate. Such a system can be manu-
factured in quantities at a cost which should not exceed
$1600. The annual payout of recovered values, based on
240 working days with 1 gph dragout of solution containing
16 oz/gal of cuprous cyanide, after deducting operating
costs, is more than $10,000.
This system of electrodialysis is economically sound only
if the membrane life is adequate. Membranes used in this
study were lonac MA 3475 as anion-selective and MC 3470,
cation-selective membranes. These are products of lonac
Division of Sybron Corporation. They were used in this
study for 800 operating hours.
33
-------
SECTION VI
ACKNOWLEDGEMENTS
The author wishes to thank Messrs. William J. Lacy, Lloyd
Kahn, John Ciancia , Edward Dulany, and Dr. Murray P. Strier
of the Environmental Protection Agency for comments and
suggestions.
35
-------
SECTION VII
REFERENCES
1. Terry, L. "Public Health Service Drinking Water
Standards," Public Health Service Publication No. 962
(1962).
2. Private Communication, National Association of Metal
Finishers.
3. Salvatorelli, J. "Aircraft Engine Wastes Treated in
Continuous Flow Through Plant," Wastes Engineering,
Vol. 30, pp 310-314, 342 (1959).
4. Sweglar, C. "Plating Solutions," Industrial Wastes,
Vol. 4, No. 3, pp 40-42 (1959).
5. Anon., '"Waste Treatment Controls Cut Chemical Cost
Sharply," Chem. Processing, pp 83, 84 (October, 1959).
6. Kempson, N.W., "Alkaline Chlorination of Metal
Finishing Waste Waters," Wastes Engng., Vol. 22,
pp 646-652 (1951).
7. Gasper, W.L., "industrial Waste Treatment and Water
Reclamation - A Case Study," Tech. Proc. Amer.
Electroplater's Co., pp 63-67 (1958).
8. Weisberg, L., and Quinlan, E.J., "Recovery of Plating
Wastes," Plating, Vol. 42, pp 1006-1011 (August, 1955),
9. Dodge, B.F., and Walker, C.A., "Disposal of Plating
Wastes at a Silverware Plant," Plating, Vol. 41,
pp 1288-1294 (November, 1954).
10. Besselievre, Edmund B., "Pontiac Motors Treats Its
Wastes," Wastes Engng., p 642 (November, 1958).
37
-------
11. Mulcahy, E.W., "Pollution by Metallurgical Trade
Wastes," Metal Finishing Journal, p 289 (July, 1955).
12. Pickering, Q.H., and Henderson, C., "The Acute
Toxicity of Some Heavy Metals to Different Species of
Warmwater Fishes," Air and Water Poll. Int. J.,
Vol. 10, pp 453-463 (1966).
13. Maselli, J.W., et al, "Effect of Industrial Wastes on
Sewage Treatment," New England Interstate Water
Pollution Control Commission.
14. Graham, A.K., "Electroplating Engineering Handbook,"
Reinhold Pub. Co., New York (1955), p 295.
15. Serota, L., "Cyanide Disposal Methods," Metal Finishing,
p 75 (October, 1957).
16. Tuwiner, S.B., "Diffusion aid Membrane Technology,"
Reinhold Publishing Co., New York, pp 237, 239,
257-260, 269-286 (1956).
17. "Electrodialysis in Advanced Waste Treatment," U.S.
Dept. of Interior, FWPCA pub. WP-20-AWTR-18, p 44
(February, 1967).
18. Tuwiner, S.B., U.S. Patent 3,357,823.
19. Brohan, H.K., "Electroplating Engineering Handbook,"
Reinhold Publishing Co., New York, p 206 (1955).
20. Ibid, p 592.
21. De Korosy, F., and Zeigerson, E., Desalination 1968«
5(2) 185-99; J. Phys. Chem. 71 (11) 3706 (1967);
OSW Res/Der. Report No. 380 (Dec. 1968).
38
-------
SECTION VIII
PUBLICATIONS AND PATENTS
U.S. Patent 3,357,823 was awarded to the author, S.B. Tuwiner
for the recovery of copper and precious metals using electro-
dialysis. U.S. Patent 3,674,669 and corresponding applications
in Great Britain, Germany, Italy, Canada, Netherlands and
Australia is for the system of this report.
39
-------
APPENDIX
DETERMINATION OF CYANIDE
Cyanogen Bromide with Pyridine and Benzidine
in n-amyl Alcohol Method
DISCUSSION
In a weakly acid solution, cyanide is converted to cyanogen
bromide by bromine water. After removal of excess bromine,
the cyanogen bromide reacts with pyridine and benzidine to
form a red-orange color extractable into n-amyl alcohol.
The optical density of the n-amyl alcohol layer is then
determined on a spectrophotometer after 15 minutes.
KEA CENTS
Phosphoric Acid, 2.5%
Sodium Arsenite , 27o
Pyridine Solution, 30%
Dilute 15 ml. of concentra-
ted phosphoric acid (85%)
to 500 ml. with distilled
water.
Weight 2 grams sodium
arsenite and dilute to
100 ml. with distilled
water.
Dilute 60 ml. of pyridine
with distilled water, add
5.0 ml. concentrated
hydrochloric acid and dilute
to 200 ml. with distilled
water, (Add acid to approx-
imately 100 ml. of distilled
water in a 200 ml. volu-
metric flask, mix and add
pyridine, mix and dilute to
200ml.) Prepare daily.
41
-------
Benzidine Hydrochloride, 1.5%
Bromine Water, Saturated
MA II
Stock Cyanide Solution A
It-oil
B1
Weight 1.5 Gm. benzidine
hydrochloride and make to
100 ml. with distilled
water. Add a few drops of
1:1 HC1 to assist solution
of the material if neces-
sary. Prepare daily.
Add several drops of bromine
to 100 ml. distilled water
and shake vigorously for 10
to 15 seconds. Since the
solution loses strength
with time, due to volatil-
ization of bromine, it is
advisable to resaturate with
bromine just prior to use.
Store in a glass bottle in
a relatively cool area.
Dissolve 0.195 grams of
NaCN, reagent grade, in
distilled water made alka-
line with NaOH solution to
pH 11. Dilute to one liter
with distilled water.
0.1 gm. CN~ per liter
Dilute 10 ml. of Solution
"A" to one liter with
distilled water made alka-
line with NaOH solution.
0.001 gm. CN" per liter
Dilute 10 ml. of Solution
"B" to 100 ml. with dis-
tilled water made alkaline
with NaOH solution.
1 ml. = 0.1 ppm CN~
42
-------
n-amyl Alcohol, C.P.
Sodium Sulfate, Anhydrous,
Crystal
(Standard solutions should
be prepared fresh at fre-
quent intervals, maximum
weekly and stored in a cool
place.)
Used as received.
Used as received.
PROCEDURE
Measure a 100 ml. sample and place in a 250 ml. separatory
funnel.
Measure 100 ml. of distilled water made alkaline with NaOH
solution into a 250 ml. separatory funnel. (Blank)
Standards for preparation of a standard curve.
Add distilled water made alkaline with NaOH solution
to pH 11.0 to three 250 ml. separatory funnels; 95 ml.,
90 ml. and 80 ml. Now add 5 ml., 10 ml. and 20 ml.
aliquots of the "Clf Standard Stock Solution to the
funnels by pipette. Equal to one part per billion per
ml. when diluted to 100 ml.
Carry standard samples, blank and unknowns, through the
whole procedure.
Add 4 ml. phosphoric acid solution to each funnel and mix.
Check the pH at this point. If the pH is above 3, using
pHydrion paper, add an additional milliliter of phosphoric
acid. Use a 25 ml. Mohr Pipette.
Add bromine water dropwise until a distinct pale yellow
color is seen and mix. Usually one to three drops are
sufficient, but more may be necessary for high samples.
Use a 2 ml. transfer pipette.
43
-------
Add 25 ml. n-amyl alcohol, preferably by buret, to each
flask.
Add 1 ml. benzidine solution to each flask, stopper and
shake vigorously for about 15 seconds.
Add 10 ml. pyridine solution to each flask. (Do not pipette
by mouth.)
Begin timer.
After 15 minutes, draw off the lower aqueous layer and
discard.
Now add the n-amyl alcohol layer to a 125 ml. erlenmeyer
flask containing roughly 20 grams anhydrous sodium sulfate
(one level tablespoon). Swirl to remove water.
Transfer to cuvettes and read on a spectrophotometer at a
wave length of 475 mu, using the blank as a reference blank.
NOTES ON TRACE CYANIDE ANALYSIS
CONTAMINATION - The concentrations of Cyanide in solutions
that this procedure is designed to detect and analyze are
in the 10 parts per billion range. Carlin mill solutions
contain 0.25% Cyanide. Any contact with mill solutions via
hands, or interchange of glassware would result in high
Cyanide assays (incorrect because of contamination).
Glassware used for this procedure must not be used for any
mill solutions under any circumstances.
PIPETTING - All pipetting must be done by use of the rubber
bulb. Remember that cyanide is a poison and mouth suction
on a pipette is a dangerous and hazardous technique.
44
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A stream of distilled water should be forced through each
pipette used, immediately after use. Do not permit solutions
of unknown or higher concentrations to dry on the inside of
pipette. Resulting contamination would give high or erratic
assays.
In pipetting standards or aliquots, one technique is very
helpful to avoid contamination. Fill the pipette with the
solution to be pipetted and discard three times. Fill the
fourth time, wipe the outside of the pipette with a tissue
and drain to the line. Then transfer into the flask the
aliquot is intended for.
PRESERVATION OF SAMPLES - Since most cyanides are very
reactive and unstable, analysis should be made as soon as
possible after sampling. If the sample cannot be analyzed
immediately, add NaOH to raise the pH to 11.0 or above and
store in a cool place.
CLEANING GLASSWARE - Clean pipettes in chromic acid cleaning
solution daily. Rinse well with distilled water and air dry
in dust-free area.
Glassware which is contacted by n-amyl alcohol should be
washed in hot alconox detergent solution. Includes 250 ml.
buret, separatory funnels, 125 erlenmeyer flasks, cuvettes
and beakers used to drain aqueous from separatory funnels.
Rinse well with distilled water.
STANDARDS - Standards are not necessary for each set of
unknown determinations. Three points on the curve are run
with each set for contamination control.
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SECTION X
GLOSSARY
Anion-selective membrane A membrane which is considerably
more permeable to anions than to cations. Ideally such a
membrane should prevent cation passage and permit free
electromigration of anions
Cation-selective membrane A counterpart of the above.
It permits cation electromigration and restricts the flow
of anions.
Cell The space, usually narrow, between two membranes
and enclosed within a frame
Concentrating solution, concentrating stream The solution
or stream which becomes more concentrated in electrodialysis
Diluting solution, diluting stream The diluting counter-
part of the above
Electrodes Electrically conducting sheets or plates
within, or a part of, the end plates of en electrodia lysis
stack. One is an anode and the other a cathode.
Electrode rinse cell, electrode rinse stream The cell,
or solution stream of an electrode. The electrode rinse
stream contains the products of the electrode reaction
Eleetrodialyzer A system for transport of electrolyte
from a diluting stream to a concentrating system by the
electromigration of cations through a cation-selective
membrane and of anions through an anion-selective membrane.
The term is applied either to the stack or stacks alone,
or to the entire system including pumps and rectifier.
47
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E lee t r od ia ly s i s A method based on the use of one or more
electrodialyzers
Electroosmotic flow Transport of water or solution
through ion-selective membranes under the influence of an
electric field
E lee t romi gra t ion Migration of ions through an ion-
selective membrane in an electric field
Membrane A barrier which is selectively permeable to ions
or molecules. Those of interest are polymer sheet materials
which are useful in eleetrodialysis
Polarization A phenomenon which limits the flow of ionic
current. One form is concentration polarization, which occurs
because of electrolyte depletion at a membrane-solution
interface .
Ratio of concentration or dilution A ratio of concentra-
tion of a component in the concentrating and diluting streams
or between rinse stages
Stac k., _ or e lee t r od ia ly ze r s ta c k An assembly of membranes,
frames and spacers with end plates and electrodes. The term
derives from the parallel stacked arrangement of the components
48
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
J. Report If o.
W
Title
Investigation of Treating Electroplaters Cyanide Waste by
Electrodialysis
Author(s)
Tuwiner, Sidney B., Ph.D.
'•'. Organization
RAI Research Corporation
36-40 37th Street
Long I Island City, New York 11101
12. spongi
tJS.2.
10, Project No.
12010 DPS
1}.
13. Type of Report and
Period Covered
Agency
J(f,. , .. SB
ry Nates
Environmental Protection Agency report number,
EPA-R2-73-287, December 1973.
16. Abstract .
An electrodialysis procedure Is developed whereby the discharge of rlnsewater 1s
eliminated. The work, according to this method, 1s rinsed in a sequence of two
rinses; the final rinse contains a concentration of cyanide of 1/10,000 of that
of the plating. .
These concentrations are maintained by the use of electrodialysis to transport
cyanides continuously from the second rinse solution back to the first rinse
solution and also from the first rinse back to the plating bath. In this way, all
cyanide is recovered and returned to the bath. Design parameters are determined
from the experiments of this study and costs are estimated.
The experimental system used in this study was a prototype of a commercial size
electrodialysis unit operated continuously untjer conditions which simulated those
of the projected two-stage commercial system using a cyanide copper plating bath.
17a. Descriptors
Effluents, Industrial Wastes, Membrane Processes
17b. Identifiers
Dialysis, Separation Techniques, Electro-Osmosis, Ion Transport, Liquid Wastes,
Cyanide, Electroplating Electrodialysis.
:7c. COW RR Fide! & Gr<,n
A-fe'.Vtrj'V'-;-^ftCs'*>*C-C2* rV-^^'O.' i1"*** -'7^''!\i
Send fo:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C. 2O24O
T Llovd Kahn
InK'itutinn
EPA-Reaion II
us. eovEKtMDrr morms OFFICE 1974-546-314/204
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