EPA-600/2-76-296
December 1976
Environmental Protection Technology Series
At REMOVAL AND CYANIDE DESTRUCTION IN
PLATING
WASTEWATERS USING PARTICLE
BED ELECTRODES
Industrial Environmental Research Laboratory
Office of Rtsearch and
U.S. Environmental Proteetian
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. 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
4 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.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161
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EPA-600/2-76-296
December 1976
METAL REMOVAL AND CYANIDE DESTRUCTION
IN PLATING WASTEWATERS USING
PARTICLE BED ELECTRODES
by
W. Chen
H. L. Recht
G. P. Hajela
Atomics International Division
Rockwell International Corporation
Canoga Park, California 91304
Contract No. R-803342-01
Project Officer
Fred Ellerbusch
Industrial Pollution Control Division
Industrial Environmental Research Laboratory
Edison, New Jersey 08817
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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REVIEW NOTICE
This report has been reviewed by the Industrial Environmental
Research Laboratory - Cinn., U.S. Environmental Protection Agency, and
approved for publication. Approval does not signify that the contents
necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
Neither the United States, the U.S. Environmental Protection
Agency, the Metal Finisher's Foundation, nor the Rockwell International
Corporation, nor any of their contractors, subcontractors or their
employees, makes any warranty, expressed or implied, or assumes any
legal liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, or process disclosed, or
represents that its use would not infringe privately-owned rights.
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FOREWORD
When energy and material resources are extracted, processed, converted,
and used, the related pollutional impacts on our environment and even on our
health often require that new and increasingly more efficient pollution con-
trol methods be used. The Industrial Environmental Research Laboratory -
Cincinnati (lERL-Ci) assists in developing and demonstrating new and improved
methodologies that will meet these needs both efficiently and economically.
This report concludes that electro-chemical techniques will be a vital
element in the control of pollution from the metal finishing industry. The
report describes the method, operational tests, and optimization data for the
treatment of specific metal-cyanide wastewaters. For further information
than what is described in this document please contact Fred Ellerbusch of
the Industrial Pollution Control Division, lERL-Ci.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
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ABSTRACT
A small (0.5 gpm) pilot-plant unit for metal and cyanide removal was
constructed and demonstrated by operation on cadmium and zinc rinsewaters at
a plating plant. With single-stage operation, 73% of an initial 22 ppm
cadmium and 79% of an initial 44 ppm cyanide were removed. Retreatment of
the effluent (equivalent to two-stage operation) gave overall removals of 95%
of the cadmium and 94% of the cyanide. Similar two-stage treatment of zinc
rinsewater gave overall removal of 75% of an initial 57 ppm zinc and 78% of
an initial 195 ppm cyanide.
The heart of the unit is an electrolytic cell with tin cathode and
graphite anode particle bed electrodes and a cellophane separator. Sequential
water flow through the electrodes gives concurrent removal of metal and
cyanide under action of applied D.C. voltage. Design of the unit was based
on design of and test results with a laboratory scale apparatus. Best re-
movals were obtained with added NaCl as supporting electrolyte and anode bed
effluent recirculation. Estimated total treatment costs, derived from a
subsequent corporate-funded study, are $1.64/1000 ft2 plated, with cadmium
plating rinsewater use at 3.5 gpm for 2500 ft2/hr and 40 hr/week operation.
iv
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CONTENTS
Page
Forward -j i i
Abstract -j v
I. INTRODUCTION 1
11. SUMMARY 2
III. CONCLUS IONS 5
IV. RECOMMENDATIONS 5
V. THE ATOMICS INTERNATIONAL PBE PROCESS FOR REMOVING METALS
AND CYANIDE FROM WASTEWATER 6
VI. DESIGN OPTIMIZATION TESTS 9
A. Introduction 9
B. Analytical Methods 9
C. Cell Design 9
D. Optimization Test Results 11
1. Water Conductivity 11
2. Solution pH 11
3. Flowrates 11
4. N2 Gas Agitation 11
5. Recirculation 13
6. Cathode Current Collector Spacing 13
7. Anode Collector Material 13
8. Separator 13
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CONTENTS (Continued)
Page
VII. DESCRIPTION OF THE PILOT-PLANT UNIT 14
VIII. RESULTS 20
A. Operation of Pilot-Plant Unit with Synthetic Solutions 20
1. Test Conditions 20
2. Preliminary Testing for Design Verification
(Tests with Multiple Anode Collector Rods) 22
3. Results with Final Cell Design 22
B. Operation of the Pilot-Plant Unit at the Plating Plant 32
1. Cadmi inn Wastewater 32
2. Tests with Zinc Wastewater 40
3. Conclusions 46
IX. APPLICATION OF THE PARTICLE BED ELECTRODE SYSTEM TO PLATING
PLANT WASTEWATER TREATMENT 47
A. Plating Wastewater Effluent Requirements 47
B. Method of Application of AI's PBE System to Wastewater 47
1. Multistage Operation 48
2. Reuse of the Treated Wastewater 48
3. Preliminary Cost Estimation 48
X. REFERENCES 50
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TABLES
Page
1. Summary of Plating Metal and Cyanide Removal 3
2. AI Process Metal Removal Capability 8
3. Selected Test Results - Laboratory Test Unit 12
4. Results of Pilot-Plant Unit Test Runs at AI - Initial Test Runs
with Seven Anode Col 1 ectors 23
5. Results of Pilot-Plant Unit Test Runs at AI - Using One 2-1/4 in.
Graphite Rod as Anode Collector and Sodium Sulfate as Supporting
El ectrolyte 24
6. Results of Pilot-Plant Unit Test Runs at AI - Using 2-1/4 in.
Graphite Rod as Anode Collector and Sodium Chloride as Supporting
Electrolyte 26
7. On-Site Heavy Metal and Cyanide Removal Demonstration Results -
Preliminary Test Runs 34
8. On-Site Heavy Metal and Cyanide Removal Demonstration Results -
With New Separator and ^$04 as Supporting Electrolyte 36
9. On-Site Heavy Metal and Cyanide Removal Demonstration Results -
With New Separator and NaCl as Supporting Electrolyte 37
10. On-Site Heavy Metal and Cyanide Removal Demonstration Results -
Cadmium Line Wastewater Test Runs After Bed Regeneration 38
11. Effect of the Supporting Electrolyte on Cadmium and Cyanide
Regeneration 40
12. On-Site Heavy Metal and Cyanide Removal Demonstration Results -
Zinc Line Wastewater Test Runs After Bed Regeneration 45
13. Preliminary Cost Estimation for the AI PBE System for In-Process
Water Recycle Application 49
vii
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FIGURES
Page
1. Atomics International PBE Metal-Cyanide Removal Unit
(Schematic) 7
2. Laboratory Metal-Cyanide Removal Cell (Schematic) 10
3. Schematic of Pilot-Plant Unit Cell 15
4. Schematic Cross-Section of the Pilot-Plant Unit Cell Showing
Electrode Collector Configurations 16
5. Photo of Partly Assembled Pilot-Plant Unit 17
6. Original Pilot-Plant Unit Configuration 18
7. Pilot-Plant Unit On-Site at Plating Plant 19
8. Heavy Metal and Cyanide Removal Pilot-Plant Unit Flow Diagram... 21
9. Effects of the Supporting Electrolytes on Cyanide Removal 28
10. Effects of the Supporting Electrolytes on Cadmium Removal 29
11. Effects of Flow Rates on Cadmium and Cyanide Removal 30
12. Effects of Recirculation Through the Graphite Bed on Cadmium
and Cyanide Removal 31
13. Simplified Flow Diagram for In-Site Demonstration of the
Atomics International PBE System 33
14. Effects of Applied Current on Cyanide Removal - 2000 ppm Nad
Added 41
15. Effects of Applied Current on Percent Cadmium Removal - 2000 ppm
NaCl Added 42
16. Effects of Initial Cyanide Concentration on Cyanide Removal -
2000 ppm Na2S04 Added 43
17. Effects of Initial Cyanide Concentration on Cyanide Removal -
2000 ppm NaCl Added 44
vm
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I. INTRODUCTION
The wastewaters from metal finishing operations contain various metals
including Cr(VI), N1, Cu, Cd, and Zn. These wastewaters originate in rinsing
and related operations throughout the plating process. In these wastewaters,
the metal may be in soluble form, usually complexed, e.g., with cyanide, or
as solTd hydroxides or oxides formed by hydrolysis in the diluted plating
solution.
The Particle Bed Electrolysis (PBE) Metal-Cyanide Removal Process is
designed to treat these wastewaters by removing the metals present in soluble
form and destroying the cyanide ion. A small amount of salt is required in
the removal process to impart electrolytic conductivity to the wastewater.
No sludges are produced. The operating cost is relatively small. The
metals may be recovered for re-use. Accordingly, this project was undertaken
to demonstrate the process by designing and building a small metal-cyanide
removal pilot plant unit, and operating it on-site at a plating plant.
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II. SUMMARY
The Atomics International (AI) metal-cyanide removal process using
particle-bed electrodes has been successfully demonstrated by operation
of a pilot plant unit on cadmium and zinc rinsewaters at a plating plant.
Using single-stage operation at a 0.5 gpm flowrate, with initial concen-
trations of 22 ppm Cd and 44 ppm CN~, 73% cadmium and 79% cyanide removal
were obtained. Retreatment of the effluent (equivalent to two-stage opera-
tion) yielded additional, comparable results, for overall removals of 95% of
the cadmium and 94% of the cyanide. For the zinc plating-line rinsewaters,
with initial concentrations of 57 ppm Zn and 195 ppm CN~, single-stage treat-
ment gave 49% Zn removal and 41% CN removal; with a second treatment, addi-
tional removal was effected for overall removals of 75% of the Zn and 78%
of the CN".
The heart of this system is an electrolytic cell with a tin particle-
bed cathode, a graphite particle-bed anode, and a cellophane separator in
which relatively low voltage DC is used to remove soluble forms of metals and
cyanide. With wastewater flow in sequence through the cathode bed and the
anode bed, concurrent removal is obtained by deposition of the metal (or
metals) in the cathode bed and destruction of the cyanide in the anode bed.
No costly chemicals are required for the treatment, and no sludges are pro-
duced requiring disposal.
To accomplish the goals of this program, a metal-cyanide removal test
unit was designed and fabricated, and after preliminary testing at Atomics
International, was installed and operated at a plating plant. Simultaneous-
ly, laboratory studies and testing were carried out using a small two-chamber
cell of design similar to the pilot plant unit cell. The objectives of
this laboratory study were to improve the cell design and define the effect
of process parameters (current, pH, water conductivity, flowrate, electrode
collector spacing) on the removal process.
Laboratory tests showed that (within limits) removals of both metal
(cadmium only tested) and cyanide were greater at higher current, pH, water
conductivity, and (per unit of time) flowrate. Gas bubbling through the cell
compartment also enhanced removal. Removals of 98% Cd and 84% CN" were
obtained under one set of test conditions with the (standard) initial concen-
trations of 35 ppm Cd and 125 ppm CN". Addition of a small amount of salt
(e.g., 1000 ppm of Na2S04) as supporting electrolyte was required for high
removals. While better results were obtained at pH =" 11, good removals
(66% Cd and 49% CN") were obtained at a pH of -9.5.
The pilot plant unit cell resembled the laboratory cell, but was of 0.5
to 2.0 gpm nominal flow capacity. It was mounted on a transportable rack,
along with required pumps, flowmeters, flow control devices, and filters,
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for transport as a unit for on-site testing. During preliminary tests at
Atomics International, using the same composition synthetic wastewater as
used in the laboratory cell tests, it was found that better results were
obtained using Nad as supporting electrolyte than with ^SO^. While a high
(81%) Cd removal was obtained with ^SCty the best CN~ removal was only 34%.
With NaCl, the best removals were obtained with recirculation through the
graphite (anode) bed. In this case, 97% removal of the Cd and 77% removal of
the CN~ at a 0.5 gpm flowrate were obtained.
In on-site tests, using filtered cadmium plating-line rinsewater, results
similar to those on synthetic wastewater were obtained. After several modifi-
cations and repairs of transport damage, the unit was operated in numerous
tests. Some of the results on both cadmium and zinc plating-line rinsewaters
obtained with the final unit configuration are given in Table 1.
TABLE 1.
SUMMARY OF PLATING PLANT METAL AND CYANIDE REMOVAL RESULTS
Plating
Line
Cd
Zn
Supporting
Electrolyte
(amount)
NaCl
(2000 ppm)
Na2S04
(2000 ppm)
NaCl
(2000 ppm)
Na2S04
Initial Concentrations
(ppm)
Metal
M
22
56
57
63
Cyanide
CN
44
90
195
120
1st
M
73
27
49
70
Percent
Removals Effected
Pass
CN
79
44
41
33
2nd
M
91
47
58
46
Pass
CN
69
41
63
36
Overall
M
95
58
75
81
CN
94
67
78
55
(2000 ppm)
The process may be used for treatment and disposal of wastewaters, or it
may be used at lower cost in processing rinsewater for reuse, where the
rinsewater concentrations of metal and cyanide are maintained at or below
specified levels. With a three-stage system to treat the rinsewater for
recycle with the cyanide level down to 13 ppm from an initial 110 ppm, about
7 kwh/1000 gal would be required. With electricity at 4<£/kwh, and savings in
water (at 25<£/1000 gal) total costs (capital and operating) of 1000 gal of
wastewater are estimated for a 5 gpm recycle system at $2.11/1000 gal.
These cost estimates were based on pilot-plant unit design. A more
accurate cost estimation obtained subsequent to the conclusion of the project
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is reported. Under corporate funding, AI derived a detailed design for a
commercial-type unit and corresponding costs. With rinsewater use at 3.5 gpm
for 2500 ft /hr cadmium plating, with 40 hr a week operation, total costs of
$1.64/1000 ft plated were derived.
In summary, the Atomics International PBE metal-cyanide removal system
has been successfully demonstrated in on-site operation, and appears to
offer a technically and economically feasible method for treating metal
finishing wastewaters.
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III. CONCLUSIONS
The Atomics International particle bed electrode (PBE) system has been
shown by on-site demonstration at a plating plant to be an effective method
for removing both, heavy metals and cyanide from wastewater. A preliminary
economic assessment indicated that the process could be utilized at reason-
able costs. This was confirmed in a subsequent, considerably more detailed
corporate-funded design and economic evaluation.
IV. RECOMMENDATIONS
It is recommended that a large test unit based on the commercial-type
design for a complete system and with complete instrumentation and controls
be designed, built, and tested on several suitable plating lines. These tests
will involve continuous operation with minimal operator attention. At the
same time, optimization studies on a laboratory unit should be carried out
for cathode bed regeneration and metal recovery.
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V. THE ATOMICS INTERNATIONAL PBE PROCESS FOR REMOVING
METALS AND CYANIDE FROM WASTEWATER
The Atomics International PBE Metal-Cyanide Removal Process employs an
electrolytic cell to treat waters containing soluble forms of heavy metals
and cyanide. Figure 1 shows the basic components of such a removal unit in
schematic fashion. The cell consists of a particle bed anode and cathode
whose bed and current collector materials are fairly good electronic con-
ductors. A separator (electronic nonconductor) which permits passage of ions
while preventing bulk flow of water is interposed between the anode and
cathode beds.
In operation, a DC potential is applied to the cell current collectors,
as shown in Figure 1. The water to be treated is passed through the cell, so
that it flows first through one bed and then the other. In the illustration,
the flow is first through the cathode compartment (bed), then through the
anode compartment (bed). In the cathode bed, electrochemical reduction
occurs, resulting in metal deposition from solution. In the anode, oxidation
occurs, resulting in the destruction of cyanide ion. Thus, the net result is
concurrent metal and cyanide removal.
The reactions occuring at the cathode are:
M+n + neM°
for uncomplexed soluble metallic ions, or,
M(CN)(x-n)- + ne^_Mo + xCN-
/\
for metallic ions in cyanide complex form. Also at the cathode, hydrogen
evolution occurs to an extent depending on (1) rate of transport of metallic
species to the cathode surface, (2) pH, (3) current, (4) applied potential,
and (5) nature of the surface, according to the equation:
2H20 + 2e~-H2 + 20H"
A certain minimal electrolytic conductivity is required of the waste-
water to be treated depending primarily on cell geometry. Additions of
small amounts of salt (e.g., 500 to 2000 ppm of Na2S04 or NaCl) may be made
to the incoming stream to obtain the required conductivity.
Tin had been found during previous studies at Atomics International to
be a superior material for the cathode bed. ' Its combination of corrosion
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CATHODE
ANODE
PLATING
WAST6WATEH
PURIFIED
EFFLUENT
AT CATHODE
CATHODE BED
(TIN)
CYANIDE DESTRUCTION
METAL REMOVAL
Figure 1. Atomics International PBE Metal-Cyanide
Removal Unit (Schematic)
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resistance and high hydrogen overvoltage is desirable. For the anode bed,
graphite appears to be superior to most materials. It resists oxidation,
even by Cl2, and has a high overvoltage for oxygen evolution. Stainless
steel was suitable and was used as a cathode collector; graphite was used
as the anode collector.
A variant of the present process for removal of heavy metals only was
developed earlier at AP ' and studied for the State of California.(2) in
the metal removal process, the nature of the anodic process is different
so that different cell designs may be required. The metals which can be re-
moved as well as those whose removal is unknown or is not possible are shown
in Table 2. All metals are removed as the metal itself, except Cr(VI) which
is removed by formation of Cr(III) with subsequent hydrolysis and formation
of Cr(OH)3. Cr(OH)3 is then removed by filtration. Thus, although only the
removal of cadmium and zinc as cyanide complexes was tested with the pilot-
plant unit, the PBE system will remove any or all of the metals listed in
the column of Table 2 if they are present in soluble form.
TABLE 2
AI PROCESS METAL REMOVAL CAPABILITY ^1>2^
Will Remove May Remove Will not Remove
Hg
Pb
Au
Ag
Cd
Cu
Zn
Fe
Cr
Ni (in Cl")
Sn
Sb
Bi
Ta
Co
As
Ti
Mn
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VI. DESIGN OPTIMIZATION TESTS
A. INTRODUCTION
A series of laboratory tests was carried out to explore the effect of
cell design features and of several key operating factors in determining the
rate of metal and cyanide removal, so that the results could be used in the
design of the on-site pilot-plant unit. After several discussions with
representatives of the local metal finishers and visits to several sites, a
plating plant was selected for the demonstration site. Samples were taken
and analyses were made of several of their wastewaters. A decision was then
made to initiate the design optimization testing program using a synthetic
wastewater based on cadmium line drag-out rinsewater which contained about
35 ppm cadmium and 125 ppm cyanide. Testing with this synthetic wastewater
would provide results representative of the system for removal of cyanide and
cadmium ions from the actual wastewater.
B. ANALYTICAL METHODS
A procedure was worked out for use of a cyanide specific electrode in
conjunction with an ion meter for rapid analyses of cyanide ion in the
treated solutions. Cadmium interference was not evident if pH was adjusted
to 13 for measurement. Atomic absorption spectrometry, AAS, was used for the
analyses of Cd+2 ions and Zn+2 ions.
C. CELL DESIGN
In cyanide removal tests in a laboratory cell, a Union Carbide UCAR
Grade BB-7, 3x20 mesh granular graphite gave results comparable to those with
a UCAR BB-6 8x20 mesh granular graphite and was judged to be physically more
suitable than the finer material. The BB-7 material was accordingly selected
for further laboratory tests and for use in the pilot plant unit.
+2
A_smal1 cell was designed and constructed for testing concurrent Cd
and CN~ removal. It consisted of a 22 in. long cylindrical Plexiglas outer
shell with suitable fittings, covers, etc. This is shown schematically in
Figure 2. A central, cylindrical anode compartment is separated from the
outer cathode compartment by a separator. The separator is a 1.5 mil thick
Cellophane sheet protected by an inner stainless steel screen and an outer
plastic screen. Tin shot (8x20 mesh) was used in the cathode bed with eleven
1/8 in. stainless steel rod collectors and the Union Carbide 3x20 mesh
granular graphite was used in the anode bed with a single 3/8 in. graphite
rod collector.
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CATHODE
COLLECTORS
ANODE COLLECTOR
SEPARATOR
CATHODE BED
ANODE BED
». SECTION A A
ANODE
COLLECTOR
b CELL ARRANGEMENT SCHEMATIC
Figure 2. Laboratory Metal-Cyanide Removal Cell
(Schematic)
10
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D. OPTIMIZATION TEST RESULTS
A series of laboratory tests was performed to evaluate the effect of
several critical design and operating parameters on the concurrent removal of
cadmium and cyanide from a synthetic plating wastewater. The important
parameters investigated included 1) water conductivity (1.0 to 3.0 millimhos/
cm), 2) pH (9.5 and 11.0), 3) flowrate (140 to 1000 ml/min), 4) N2 gas agita-
tion, 5) recirculation, 6) cathode (tin bed) current collector spacing (11,
6, or 3 collectors), 7) anode (graphite-bed) collector material (graphite and
stainless steel). A selected group of data from these laboratory tests
indicates the effects of various parameters on Cd+2 and CN~ removal. These
are given in Table 3. The pertinent observations were as follows:
1. Water Conductivity
Water conductivity was found to play a significant role in removal
effectiveness: this may be seen by comparing the results of Run No. 20 with
those of Run No. 8. It was also found that removal of both Cd+2 and CN~ in-
creased with increased current to a limiting value. This value, and the
corresponding percent removal, were higher with the more conductive water.
In addition, the applied voltage to maintain a given current was lower with
the more conductive water. The addition of a small amount of salts (e.g.,
NaCl or Na2S04) to the plating wastewater before treatment would enhance the
removal results of a given cell design.
2. Solution pH
The effect of wastewater pH may be seen by comparing results from Run
No. 2 and Run No. 20 (Table 3). At otherwise similar conditions, a (14%)
higher Cd+2 and a (43%) higher CN~ removal were obtained at pH = 11 than at
pH = 9.5. Nonetheless, the removal effected at the lower pH was substantial.
3. Flowrates
Tests in the laboratory unit (see the results of Run Nos. 20 and 24,
Table 3), in which flowrates were increased up to sevenfold, showed both
cadmium and cyanide removal rates (amount removed per unit residence time)
were increased by factors of up to 3.5 and 2.5, respectively. As noted
above, removal has been found to have a limit with current, no further re-
moval being effected with further increase in current. The value of this
limiting current was found to increase with increasing flowrate.
4. NO Gas Agitation
As may be seen from the results of Run No. 25, gas bubbling with nitro-
gen, at a given flowrate and current, was found to decrease the required cell
voltage somewhat and to increase the extent of removal significantly (by 2X
for Cd+2 and 25% for CN~); these are both desirable effects. While this
approach may not be economically feasible using nitrogen, use of air bubbling
in the anode (cyanide removal) compartment may be feasible.
11
-------
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5. Recirculation
In principle, recirculation of a fraction of the water (i.e., the return
of part of the cathode effluent to the cathode influent and/or part of the
anode effluent to the anode influent) should improve removal* by increasing
transport of the species to be removed to the electrode bed surface. With
recirculation in both cathode and anode compartments, no evident improvement
was noted under test conditions (pH = 9.7, conductivity = 2.5 millimhos/cm,
current = 2 amp and 6 amp) over recirculation ratios of 1:1, 2:1, and 4:1.
It should be noted that the initial concentration of the species being re-
moved decreases with increased recirculation ratio. As the rate of removal
is a function of the concentration, these effects may have cancelled out in
this particular case. Further testing of recirculation in the pilot plant
unit was still warranted.
6. Cathode Current Collector Spacing
Reducing the number of cathode collectors from 11 to 6 resulted in a
higher voltage (7.1 v cf 6.1 v at 6 amp) under otherwise identical conditions.
This pointed out the need for adequate contacting of the current collectors
with the beds.
7. Anode Collector Material
Quite satisfactory cyanide removal results have been obtained using a
graphite anode collector. Improved cyanide removal was obtained by use of a
stainless steel anode collector. In a set of tests, 75% CN~ removal was
observed compared with 38% removal under similar conditions with a graphite
anode collector. To optimize the process, therefore, tests were run to see
if stainless steel would hold up under extended testing. A 1000 ppm NaCl
solution adjusted to pH = 9.5 was used at a 140 ml/min flowrate at a cell
current of 4 amp for 15 hr. The 304 SS rod was corroded generally and
showed some pitting attack. As part of an effort to find an improved anode
collector, a series of small scale short time screening tests were made with
metal rods as anodes in a beaker. A test solution of 1000 ppm NaCl + 1000 ppm
Na2$04 was used. Of the three dozen metals** and alloys examined, all,
without exception, either dissolved or developed a very high resistance when
made anodic. Based on these results, graphite remains the most satisfactory
anode collector available and was used in the demonstration unit.
8. Separator
The separator of the laboratory test cell was found to have suffered
accumulation of ion oxides evidently from the stainless steel screen serving
to support the separator. It was found that this corrosion problem can be
avoided by replacing the stainless steel screen with a plastic screen.
*No resolubilization of metal should occur while the required D.C. potential
is applied to the cell.
**These included Ti, Ta, Nb, various stainless steels, Hastelloys, Inconels,
Incoloys, and others.
13
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VII. DESCRIPTION OF THE PILOT-PLANT UNIT
The design of the pilot-plant unit metal-cyanide removal cell is shown
in overall schematic fashion in Figure 3. This cell is similar in configura-
tion to the laboratory test unit (see Figure 2). The cell is of cylindrical
shape, with an outer cathode compartment filled with tin shot, an inner anode
compartment filled with granular graphite and a cellophane separator protect-
ed with a plastic screen, and supported on an epoxy coated stainless steel
screen. This epoxy coated stainless steel screen was later replaced with a
plastic screen to prevent corrosion problems. In the pilot-plant unit cell
the 12 stainless steel rod cathode current collectors are evenly spaced at
the periphery of the compartment; seven graphite anode current collectors
were positioned with one central rod and an outer hexagonal array. These
seven anode collectors were later replaced with a single central collector of
2-1/4" diameter for better structure support and cell operation. Figure 4
shows the electrode collector configurations in the initial and final cell
designs. A photograph of the partly assembled cell is shown in Figure 5.
The stainless steel rod cathode current collectors are shown as a cage-like
structure to the right of the Plexiglas cased cell. The horizontal rings (of
rubber) on the cathode collector assembly and on the screened separator (in
the cell) are designed to baffle flow through the cathode compartment, in-
suring that all the water is properly treated. Instead of one-piece rod or
rods, the graphite anode collector or collectors were assembled from 1 ft
sections of graphite rods. The cell, in a 9 in. diam Plexiglas shell, has
anode and cathode chambers about 6 ft long. The removal unit cell design
permits either upflow or downflow in the cathodic (metal removal) and anodic
(cyanide removal) compartments. For ease of operation, only upflow was used
in these tests.
During initial operation at Atomics International, a number of changes
were made to improve the performance and reliability of the unit. Several
leaks were found between the electrode compartments; these were corrected by
calking the ends of the membrane separator. In anticipation of a potential
problem, a porous, cushioned outer covering was installed over the separator
assembly to prevent penetration of the cellophane separator membrane by
sharp-edged protrusions on the cathode tin particles.
The original plan was for all auxiliary equipment to be mounted on the
base or back board of the frame. These pieces of auxiliary equipment in-
clude the DC power supply, pump, flowmeter, regenerant solution storage tank,
and requisite valves and tubing. The original overall pilot plant unit con-
figuration planned is shown in Figure 6. With the installation of liquid
level control tank, a second pump, and associated gear, changes were made in
the position of several components. Also, the power supply was mounted ex-
ternal to the frame. The unit in final form and operating on-site at the
plating plant is shown in Figure 7.
14
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Figure 5. Photo of Partly Assembled Pilot-Plant Unit
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VIII. RESULTS
A. OPERATION OF PILOT-PLANT UNIT WITH SYNTHETIC SOLUTIONS
1. Test Conditions
Prior to the on-site demonstration, the Pilot-Plant Unit was tested at
Atomics International using a synthetic wastewater prepared by dissolving
59.4 grams of KCN and 15.4 grams of 3CdS04 . SH^O in 50 gallons of distilled
water. The resulting synthetic wastewater consisted of about 35 ppm of Cd++
and 125 ppm of CN~. Na2S04 or Nad was also added as the supporting elec-
trolyte. The pH of this synthetic wastewater was about 10.5. The conduc-
tivity of the synthetic wastewater varied from 2.3 to 4.9 millimhos per
centimeter, depending on the amount and type of the supporting electrolyte
used.
During the test runs, the synthetic wastewater from a 50-gal supply drum
was pumped at a fixed flowrate--e.g., 0.5 gpm, 1.0 gpm, or 1.5 gpm--into the
tin bed of the test cell. The effluent from the tin bed was collected in a
liquid-level control tank, and then pumped into- the graphite bed of the test
cell. A bypass valve, whose opening was adjusted by a floating arm in the
liquid-level control tank, maintained the liquid level in the control tank.
The effluent from the graphite bed went to a treated wastewater collection
drum. A portion of the effluent from the graphite bed could be diverted to
the liquid-level control tank for recirculation through the graphite bed.
During the test run, current was kept constant for at least 15 minutes prior
to sample collection to ensure that steady-state conditions had been at-
tained.
Feed samples were taken both at the beginning and just before the end
of each test run. Treated wastewater samples of the graphite bed effluent
were taken just before change in operating parameters. The operating para-
meters were current and its corresponding voltage, flowrate, and recircula-
tion ratio (if any) through the graphite bed. Prior to the actual test runs,
a dye test was made to determine if there were any significant cross-flow
through the separator of the test cell. This was carried out by recircula-
ting methylene blue solution through the tin bed and tap water through the
graphite bed. No color appeared in the tap water tank. A hydraulic test
showed that equal flow in both beds could easily be obtained. Flows varied
from 0.2 to 2.0 gpm. At 2.0 gpm, the pressure differential between the tin
and graphite beds exceeded 5 psi with the pressure drop in the tin bed being
greater. As this value may put undue stress on the cell separator, actual
test runs were limited to a maximum flowrate of 1.5 gpm. Details of the
hydraulic system are shown in Figure 8.
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2. Preliminary Testing for Design Verification (Tests with Multiple
Anode Collector Rods)
In the first series of tests, as described above, the cell configuration
with seven graphite anode current collector rods was used. The top view of
this test cell is shown in Figure 4. The results varied from 6 to 89% for
cadmium removal and 11 to 25% for cyanide removal. Details of the test
results are shown in Table 4.
It was observed in most cases that cadmium removal increased somewhat
with increased current, while cyanide removal decreased with higher currents.
It was concluded that relatively simple cell modifications should improve
cyanide and cadmium removals. Before any change was made, several tests were
run to check the effects of graphite anode collector rod spacing on the
cadmium and cyanide removals. These tests were carried out by rearranging
the contacts on the graphite rods. Three different rod configurations were
tested. These wereru) a central rod alone/2) one central rod and an outer
triangle array, and (3) six rods in an outer hexagonal array alone. The re-
sults obtained using various graphite rod configurations were not conclusive.
However, a simple theoretical evaluation of current and potential patterns in
the cell suggested that a single central rod of adequate current handling ca-
pacity was the most effective configuration. For this reason and for better
structural support, the test cell was modified by replacing the seven anode
rods with a single, central rod of 2-1/4" diameter. This modified test cell
resembled a scaled-up version of the small lab unit described in Section VI-C.
3. Results with Final Cell Design
a. Cell Design Changes
As noted above, the pilot-plant test cell was modified by replacing the
seven graphite rods with a single centrally located 2-1/4 in. diameter graph-
ite anode collector rod. Four 14 in. single segments were threaded together
to form this rod; a 1/2 in. diameter stainless steel rod at the top was
threaded into the graphite for electrical contact, while a short 1/2 in.
diameter graphite rod was threaded into the large graphite rod at the bottom
for positioning it in a slot in the plastic base. The top view of this cell
with the modified collector rod arrangement is shown in Figure 4.
b. Test Conditions
Tests of cadmium and cyanide removals were again carried out using the
synthetic wastewater described above. The test results obtained using Na2S04
as the supporting electrolyte are given in Table 5. Results were consider-
ably better with 2000 ppm of Na2S04 than with 1000 ppm. While cadmium
removal increased, or, in some cases, remained nearly constant with increased
current, in all cases, cyanide removal decreased with increased current.
To increase cyanide removal, test runs were made using NaCl as the sup-
porting electrolyte. It was believed that with $04, the CN" oxidation was
occurring only by direct electrode reaction at the graphite surface. Evolution
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of 02 was believed actually to be hindering the oxidation of cyanide by
blocking the graphite bed surface. Chlorine (or C10~) was expected to
supplement the electrochemical oxidation of CN~ with chemical oxidation. The
results confirmed the expectation that Nad was a better supporting electro-
lyte for cyanide oxidation. These are summarized in Table 6 and discussed
below.
c. Results with the Final Cell Design
As high as 60% of cyanide removal was obtained using Nad as the sup-
porting electrolyte while the highest removal obtained with Na^SO. was 34%.
In order to improve the transfer of CN~ to the graphite surfaces in the
particle bed, to distribute the electrolytically generated Cl2 (or C10~) more
uniformly, and to increase the contact time of this Cl2 with the water,
recirculation of part of the water flowing through the graphite bed (anode)
was tested. Approximately 75% of the flow was recycled so that at a net
flowrate of 0.5 gpm, the flow through the graphite bed was about 2.0 gpm.
With this recirculation through the graphite bed, cyanide removal of 77% was
obtained compared with 60% removal without recirculation. The corresponding
cadmium removal was 97% compared with the highest previous removal of 81%
without recirculation. These were the best results obtained in the testing
of the pilot-plant unit at AI using the synthetic wastewater.
d. Discussion of Results
(1) Introduction
The overall objective in operation of the pilot-plant unit at AI was
to test cadmium and cyanide removal using the synthetic wastewater and to
determine the effect of several operating parameters on these removals. This
objective was successfully achieved. Based on the results obtained the
effects on the cadmium and cyanide removal of, (a) the type and amount of the
supporting electrolyte, (b) flowrate, (c) graphite bed recirculation, and
(d) the applied current are summarized in the following sections.
(2) Type and Amount of the Supporting Electrolyte
For cyanide removal, better results were obtained with Nad rather than
with Na2S04 as the supporting electrolyte and at a higher concentration of
the supporting electrolyte, i.e., 2000 ppm y_s_ 1000 ppm. It is believed that
the cyanide concentration was reduced both by electrochemical oxidation, and
by chemical reaction with chlorine or hypochlorite ion generated in the anode
bed.
Chorine or hypochlorite generation involves the reaction:
2 Cl" -C12 + 2e~, possibly followed by the reaction:
H20 + CL2 -HC1 + HC10
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Cyanide oxidation then Involves the reactions:
CN" + 20H" -CNO~ t H20 + 2e~,
CN~ + NaClQ -CNO" + NaCl, or
20H" + CN" + Cl-Tj -CNO~ + 2C1" + H20
For cadmium, about the same % removals were obtained in the test runs
with 2000 ppm of Na2S04, and with 10QO and 2000 ppm of NaCl. With 1000 ppm
of Na2S04, lower cadmium removal was obtained. These data are shown graphic-
ally in Figures 9 and 10.
(3) Flowrate
The effects of flowrate on the cadmium and cyanide removals are shown in
Figure 11. Almost the same percent of cadmium removal was obtained at a
1.0 gpm flowrate as at an 0.5 gpm flowrate.
For cyanide removal, a higher percent removal but a smaller net amount
of removal were obtained at the lower flowrate. For example, at 50 amperes,
52% (or 123 mg/min) and 37% (or 175 mg/min) of cyanide removals were obtained
with flowrates of 0.5 gpm and 1.0 gpm, respectively.
(4) Recirculation Through the Graphite Bed
Comparative results, with and without recirculation, are shown in Fig-
ure 12. Recirculation through the graphite bed was found to enchance both
cyanide and cadmium removals. For example, without recirculation with a flow
rate of 0.5 gpm and a cell current of 50 amperes, 67% cadmium removal and 52%
cyanide removal were obtained. With recirculation of approximately 3/4 of the
anode effluent at the same overall operating conditions, 93% cadmium removal
and 72% cyanide removal were obtained. These better results were probably
due to the following: (a) higher rate of transport of CN~ to the graphite
bed particles, (b) better contact for a longer time of the CN~ with the
evolved Cl2» and (c| additional Cd removal in the graphite bed by (1) release
of Cd+2 from Cd(CN)| upon oxidation of CN~, followed by (2) precipitation of
Cd(OH)2 and its removal by filtration in the bed.
(5) Applied Currents
Figures 9 to 12 show the effects of the applied current on percent
cadmium and cyanide removals at various operating conditions.
With NaCl as the supporting electrolyte, increasing the current beyond
about 50 amperes gave only marginally greater cadmium and cyanide removals.
With Na2SC>4 as the supporting electrolyte, the same pattern for cadmium
removal was observed, while cyanide removal decreased with increased current
beyond 10 amperes. This was explained earlier by the blocking action of
evolving oxygen. It should be noted that with higher concentrations of Cd
and CN~, the current beyond which increases in removal are marginal would be
greater.
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B. OPERATION OF THE PILOT-PLANT UNIT AT THE PLATING PLANT
The Atomics International Pilot-Plant Test Unit was shipped on a shock-
resistant platform to the plating plant selected for the on-site demonstra-
tion. The overall objective of the on-site operation was to demonstrate
metal and cyanide removal from actual plating wastewater. Most of the test-
ing was done on a cadmium line drag-out rinsewater, with some tests also made
with a zinc line rinsewater.
1. Cadmium Wastewater
a. Operating Procedures Adopted
Operating procedures used at the plating plant were essentially the same
as those used for synthetic solutions in tests at Atomics International.
Wastewater from the first (drag-out) rinse tank of the plating plant rinse
system was pumped to a 55-gallon drum. Supporting electrolyte, either NaCl
or Na2SO/i solution, was added. The resulting wastewater, which contained
1000 to 2000 ppm of the supporting electrolyte, was then used for the test
runs. Since the barrel plating operation of the cadmium line was a batch
system, the cadmium and cyanide concentrations in the drag-out rinse tank
varied from time to time. While efforts were made to obtain the wastewater
with high levels of impurities, variations of cadmium concentrations from 8
to 131 ppm and cyanide concentrations from 23 to 170 ppm were observed.
Minor changes were made to accommodate the on-site conditions. The
200 amperes, 20 volt Harrison Lab DC power supply was modified for 230 volt -
3 phase AC available at the plating plant (from 208 volt-3 phase AC available
at Atomics International). A third pump was installed for transferring
wastewater from the rinse tank to the pilot-plant unit, and after testing,
back to the wastewater line of the plating plant. A flow diagram for the on-
site demonstration of the system is shown in Figure 13.
b. Start-up and Check-out Test Results
After the pilot-plant unit was set up in the plating plant, several
preliminary test runs using NaCl as the supporting electrolyte were made.
The results are shown in Table 7. In general, these results were in agree-
ment with those obtained using the synthetic wastewater. A comparison be-
tween these results is given in Section VIII-B-1-f.
In these preliminary tests, some abnormal currents and corresponding
voltages were observed. Several bed regeneration techniques were tried in
order to restore the performance of the Pilot-Plant Unit, including NH^ wash
with and without air agitation, and dilute HC1 wash. Two probes were then in-
serted in several positions in the Pilot-Plant Unit to measure the potential
drop across the separator and beds. It was found that the potential drop
across the separator had increased over the original value. Based on this
result, the graphite and tin beds were removed in order to clean and inspect
the separator. It was then found that the epoxy-coated stainless steel
screen used for structural support in the test cell had been corroded. It
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was also found that a piece of small graphite rod, which was threaded to
connect the 2nd and 3rd segments of the central graphite rod from the bottom,
was broken. This small graphite rod was replaced. The test cell was then
reinstalled with a new separator using plastic screen instead of stainless
steel screen for structure support.
c. Results of Initial Test Operation
After the test cell was reassembled with the new separator, several sets
of test runs were made to investigate the cadmium and cyanide removals from
drag-out rinsewater at 0.5 gpm flowrate with 2000 ppm of Na2SO/i or NaCl as
the supporting electrolyte. Tests were also made using the effluent from
test runs as feed to simulate multistage operation, and to investigate the
effects of cadmium and cyanide concentrations on the extent of removal
obtained. Results from these tests with the new separator are shown in
Tables 8 and 9.
In runs with ^50^ as the supporting electrolyte (Table 8), both with
and without recirculation through graphite bed, cyanide removal remained the
same or, decreased at currents greater than about 10 to 25 amperes. Similar
results had also been obtained with synthetic solutions. With NaCl as the
supporting electrolyte, the cyanide removals increased as the current in-
creased over the range of current used. Also, the percent cyanide removal on
a second pass was the same as on the first pass at the same flow rate and
current.
The cadmium analyses on effluents from the test runs made after the cell
was reassembled suggested that cadmium deposited in the tin bed may have
become oxidized and was being released to the test solution. Tin bed regen-
eration was carried out to restore the test cell performance.
d. Bed Regeneration
Tin (cathode) bed regeneration was carried out using an aerated NH^
solution. Before the ammonia wash, fresh tap water was pumped through the
pilot-plant unit for two hours at 1.0 gpm. Then 15 gallons of ammonia solu-
tion, which contained 250 grams of Nh^Cl and 500 ml of 28.3% NH3 solution,
was recirculated through the tin bed at 0.5 gpm for about 3 hours. At the
end of the ammonia wash, a cadmium sample was taken from the resulting am-
monia wash solution. Later analysis showed that the ammonia wash sample con-
tained 9 ppm of cadmium. The ammonia wash solution was then drained from the
cell and dumped into the ammonia line of the plating plant.
After the ammonia regeneration, the pilot-plant unit was rinsed with
fresh tap water at 1.0 gpm for one hour. Further test runs made after the
ammonia regeneration and described in the next section, verified that the tin
bed had been cleaned and cadmium removal efficiency restored to the cell.
e. Test Results After Bed Regeneration
The results from the tests run after tin bed regeneration, are shown in
Table 10. Two-stage operation was simulated with the first stage using the
35
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actual wastewater, and the effluent from the first stage providing the feed
for the second stage of operation.
From Table 10, data on two-stage operation may be derived. With
as the supporting electrolyte, first stage operation (OD-31) showed 27%
cadmium (from 56 to 41 ppm) and 44% cyanide (from 90 to 50 ppm) removal. In
the second-stage operation (OD-32) 47% (from 43 to 23 ppm) cadmium removal
and 41% (from 51 to 30 ppm) cyanide removal were obtained. The overall
results of this two-stage operation were 59% (from 56 to 23 ppm) cadmium
removal and 67% (from 90 to 30 ppm) cyanide removal. With NaCl as the sup-
porting electrolyte, first-stage operation (OD-33) showed 73% (from 22 to
6 ppm) cadmium removal and 79% (from 44 to 9.4 ppm) cyanide removal. In the
second stage operation (OD-34), 91% (from 11 to 1 ppm) cadmium and 69% (from
9.4 to 2.9 ppm) cyanide removals were obtained. The overall results of the
two-stage operation were 94.6% (from 22 to 1 ppm) cadmium removal and 93.6%
(from 44 to 2.9 ppm) cyanide removal.
f . Discussion of the Test Results
The effects of the test parameters on the removal results obtained at
the plating plant may be summarized as follows:
1) Type of the Supporting Electrolyte
The effects of the types of the supporting electrolyte on cadmium and
cyanide removals are shown in Table 11. While the amount of data is limited,
it appears that a higher degree of removal of both cadmium and cyanide are
obtained with NaCl than with ^£$04. These results were also observed using
the synthetic wastewater. They are probably due to the more efficient oxida-
tion of cyanide, plus hydrolysis and filtration of cadmium freed from the
complex.
2) Applied Current
The effects of applied current on cadmium and cyanide removals, and
comparisons of the test results obtained using both the actual wastewater and
the synthetic wastewater with NaCl as the supporting electrolyte are shown
in Figures 14 and 15. Both cyanide and cadmium removal increased as the
current increased. One hundred amperes appeared to be the limiting current.
The cyanide removal results with the actual wastewater were in agreement with
those obtained with the synthetic wastewater. Cadmium removal results ob-
tained with the synthetic wastewater were better than those obtained using
the actual wastewater.
No data on the effect of applied current on cadmium removal was obtained
with Na2$04 as the supporting electrolyte. Cyanide removal appears to de-
crease with increased current. This was observed with synthetic wastewater
and is probably due to the same cause, namely, blocking of the graphite bed
by evolving oxygen.
39
-------
TABLE 11
EFFECTS OF THE SUPPORTING ELECTROLYTE ON
CADMIUM AND CYANIDE REMOVALS
Type of the
Supporting
Electrolye
2000 ppm
NaS04
2000 ppm
NaCl
Run
No.
OD-31
OD-32
OD-26
OD-28
Initial
Concentration,*
ppm
Cd+2
56
43
59
27
CN"
90
51
110
44
Final
Concentration
ppm
Cdf2
41
23
30
9
CN"
50
30
54
15
01
h
Removal
Cd CN
27
47
49
67
44
41
51
66
*Flow rate =0.5 gpm with recirculation through the graphite bed (tin bed =
0.5 gpm - graphite bed = 1.8-2.0 gpm). Current = 25 amperes.
3) Initial Wastewater Concentration
The effects of the initial wastewater concentrations on the cyanide
removals are shown in Figures 16 and 17. With both Na2S04 and NaCl as
supporting electrolytes, the percent cyanide removal decreased as the initial
cyanide concentration increased. However, the quantity of cyanide removed
increased with increased initial cyanide concentration.
2. Tests With Zinc Wastewater
Demonstration test runs using the wastewater from a zinc plating line
were also carried out at the same plating plant. The rinse operation in the
zinc line at the plating plant was a continuous system. Two 50 gal. samples
of the zinc wastewater were used for these tests. One had 120 ppm of cyanide
and 63 ppm of zinc, while the second had 195 ppm of cyanide and 57 ppm of
zinc. The test results are shown in Table 12.
As with the cadmium wastewater, a two-stage sequence of testing was
carried out. In the first set of tests with Na2SOa as the supporting electro-
lyte, the cyanide concentration was reduced from 120 to 80 ppm (33% removal)
in the first-stage operation (OD-35) and from 85 to 54 ppm (36% removal) in
the second-stage operation (OD-36) for an overall removal of 55%. Zinc con-
centration was reduced from 63 to 19 ppm (69.7% removal) in the first stage
(OD-35) and from 22 to 12 ppm (45.5% removal) in the second stage (OD-36)
for an overall removal of 81%. With NaCl as the supporting electrolyte,
cyanide concentration was reduced from 195 to 115 ppm (41% removal) in the
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first stage (OD-37) and from 120 to 44 ppm (63% removal) in the second stage
for an overall removal of 77%. Zinc concentration was reduced from 57 to
29 ppm (49% removal) in the first stage (OD-37) and from 33 to 14 ppm (57.6%
removal) in the second stage for an overall removal of 75%.
The higher overall cyanide removal obtained with NaCl (77%) compared to
that with Na£S04 (55%) can be attributed to the added action of free Cl2 (or
OC1~) produced at the higher current used at the anode. The somewhat lower
zinc removal (75% compared with 81%) with NaCl cf to Na2S04 may be due to
surface blocking by evolving H£ gas, or to a lower Zn deposition rate at the
higher |CN j with a corresponding lower concentration of free Zn .
3. Conclusions
The overall objective of the on-site testing at the plating plant was
to demonstrate the removal of heavy metals and cyanide from actual wastewater.
This objective was successfully achieved. Tin bed regeneration with aerated
ammonia solution was also demonstrated.
In a two-stage treatment of the cadmium line wastewater, cyanide concen-
tration in the wastewater was reduced from 110 to 7 ppm and cadmium concen-
tration was reduced from 59 to 7 ppm. These represent 94 and 88% removals of
cyanide and cadmium, respectively. In a similar two-stage treatment of zinc
line wastewater, cyanide concentration was reduced from 195 to 44 ppm and
zinc concentration was reduced from 57 to 14 ppm. These represent 77 and
75% removals of cyanide and zinc, respectively. This indicates that the
desired effluent concentration may be achieved by multistage application of
the removal process. The treated wastewater can, if desired, be reused as
rinsewater.
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IX. APPLICATION OF THE PARTICLE BED ELECTRODE SYSTEM
TO PLATING PLANT NASTEWATER TREATMENT
A. PLATING WASTEWATER EFFLUENT REQUIREMENTS
Interim regulations concerning the permissible limits on effluent waste-
waters are currently being established. It may be anticipated that the per-
missible federal or federal-approved state levels for heavy metals and for
cyanide will be in the range from 1 to 5 ppm for waters entering sewers.
Based on the on-site demonstration test results, a system of this type
can easily be operated to meet these wastewater effluent requirements. The
ultimate goals set by the EPA for wastewater requirements are likely to be
lower. Based on the removals effected during the demonstration tests, and
the results of prior tests^ ' where zinc and cadmium effluent concentrations
below 1 ppm were attained, it is judged that the AI PBE system should be able
to meet such limits.
B. METHOD OF APPLICATION OF AI's PBE SYSTEM TO WASTEWATERS
1. Multistage Operation
Multistage operation is a common practice used in various separation
processes. This results in effective utilization of equipments so that
capital and operating costs can be minimized. The items of equipment used in
the multistage operation may be identical in design, or may vary in detail to
accommodate changes in the feed.
Calculations for an operating line were based on demonstration test
results in Runs No. OD-2, OD-26, OD-28 and OD-34, and are used here to illus-
trate the multistage operation of the AI PBE system. These calculations show
that the cyanide concentration can be reduced from 120 ppm to 3 ppm or less
in three stages. In the first stage, cyanide concentration is reduced from
120 to 39 ppm (67.5% removal). In the second stage, cyanide concentration is
reduced from 39 to 6 ppm (84.6% removal). In the third stage, cyanide concen-
tration is reduced from 6 ppm to considerably less than 3 ppm. Treated waste-
water from this three-stage operation may proceed to further treatment in a
one or two-stage unit to meet regulations of less than 0.5 ppm of cyanide in
the wastewater effluent. It should be noted that the removal efficiency,
i.e., amount of electricity required per unit amount of cyanide removed, is
low (about 10% coulombic efficiency) in the treatment of the very dilute
cyanide solutions, e.g., 5 ppm or less.
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2. Reuse of the Treated Wastewater
Instead of dumping the treated wastewater, which contains only a very
small amount of cyanide, this treated wastewater can be reused as rinse water.
By this reuse, the net tap water consumption can be dramatically reduced. The
cyanide concentration of the treated wastewater can be relaxed to some extent
in such recycle operation so that energy consumption in the wastewater treat-
ment can also be reduced.
3. Preliminary Cost Estimation
The technical feasibility of the AI PBE System has been demonstrated.
Based on the actual test results at the plating plant, a preliminary estimate
of process economics was made using a) 6-stage operation for wastewater treat-
ment, and b) 3-stage operation of wastewater treatment for in-process reuse of
the treated wastewater. Thtse are described in the following sections. It
should be noted that the economics of a commercial system would probably be
different from those derived here.
a. Electricity Cost for Wastewater Treatment for Discharge
It is assumed that the wastewater feed to the AI System contains 110 ppm
of cyanide. Calculations were made which show that cyanide concentration can
be reduced from 110 to 0.3 ppm with a 6-stage treatment. In the first stage,
cyanide is reduced from 110 to 55 ppm. In the second, third, fourth, fifth,
sixth stages, cyanide is reduced to 25, 10, 4, 1.6, and 0.3, respectively.
The power requirement was estimated at about 11 kwh per 1000 gallons of
wastewater treated.
b. Electricity Cost for In-Process Reuse of the Treated Wastewater
Assuming that the wastewater feed to the AI System contains 120 ppm of
cyanide, the cyanide concentration may be reduced from 120 to 13 ppm in three
stages. This treated wastewater would be reused as rinse water to pick up
cyanide from 13 ppm to about 120 ppm in the rinse tank. Per 1000 gallons of
wastewater treated, the power requirement is about 7 kwh. Comparing these two
modes of applications, it appears that the in-process reuse of the treated
wastewater provides an economic incentive in regard to both capital (3 stages
instead of 6) and operating costs. An overall preliminary appraisal of the AI
System process economics was made based on in-process reuse of the treated
wastewater.
c. Economics of Operation of the System with In-Process Reuse of the
Treated Wastewater
Two system sizes, namely 0.5 gpm and 5.0 gpm, were evaluated for the
cadmium line wastewater treatment. These are listed in Table 13. For the
0.5 gpm system, it is estimated that the cost for treating 1000 gallons of
wastewater is $6.28. For the 5.0 gpm system, the cost would be a little over
$2.00. These estimations of operating costs were based on the actual demon-
stration test results. The capital costs were based on (1) the design of the
pilot-plant unit; (2) the costs of material for this unit; and (3) a scaling
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down and modification of costs estimated previously for a larger (50 gpm) unit
of different design. It should be emphasized that these are only estimates.
Nevertheless, they do indicate that treatment using the AI system can be
carried out at reasonable costs.
TABLE 13
PRELIMINARY COST ESTIMATION FOR THE AI PBE SYSTEM FOR
IN-PROCESS WATER RECYCLE APPLICATION
0.5 gpm 5.0 gpm
Capital Investment $3,750 $12,500
Gross Cost $/1000 gallon of wastewater treated
Electricity at 4<£/kwh 0.28 0.28
Other operating cost nil nil
Capital charge at 30% 6.25 2.08
of capital investment ____
6.53 2.36
Less Credit
Water at 25^/1000 gallons 0.25 0.25
Net Cost 6.28 2.11
*Based on (1) 110 ppm of cyanide removal (from 120 to 10 ppm) and 50 ppm of
cadmium removal; (2) power requirement for pumping wastewater contributes
less than 0.3% of the total electricity cost; and (3) 250 working days
per year.
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XI. REFERENCES
1. U.S. Patent 3,899,405 (August 12, 1975), "Method of Removing Heavy
Metals from Water and Apparatus Therefor."
2. "Laboratory Demonstration of the Atomics International Process for
Removal of Heavy Metals from Water," Final Report to the California
Water Resources Control Board, October 1972, AI-73-13.
50
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 REPORT NO.
EPA-600/2-76-296
3. RECIPIENT'S ACCESSION>NO.
4. TITLE AND SUBTITLE
Metal Removal and Cyanide Destruction in Plating
Wastewaters Using Particle Bed Electrodes
5. REPORT DATE
December 1976 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
W. Chen, H. L. Recht, and G. P. Hajela
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Atomics International Division
Rockwell International Corporation
8900 DeSoto Avenue
Canoga Park, CA 91304
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
R-803342-01
12 SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Laboratory-Cin.
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
OH
13. TYPE OF REPORT AND PERIOD COVERED
Final Report 7/74-10-75
14. SPONSORING AGENCY CODE
EPA/600/T2
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A small (0.5 gpm) pilot-plant unit for metal and cyanide removal was constructed
and demonstrated by operation on cadmium and zinc rinsewaters at a plating plant.
With single-stage operation 73% of an initial 22 ppm cadmium and 79% of an initial
44 ppm cyanide were removed. Retreatment of the effluent (equivalent to two-stage
operation) gave overall removals of 95% of the cadmium and 94% of the cyanide. Simi-
lar two-stage treatment of zinc rinsewater gave overall removal of 75% of an initial
57 ppm zinc and 78% of an initial 195 ppm cyanide.
The heart of the unit is an electrolytic cell with tin cathode and graphite
anode particle bed electrodes and a cellophane separator. Sequential water flow
through the electrodes gives concurrent removal of metal and cyanide under action of
applied DC voltage. Design of the unit was based on design of and test results with
a laboratory scale apparatus. Best removals were obtained with added NaCl as support-
ing electrolyte and anode bed effluent recirculation. Estimated total treatment costs
derived from a subsequent corporate-funded study, are $1.64/1000 ft^ plated, with
cadmium plating rinsewater use at 3.5 gpm for 2500 ft^/hr and 40 hr/week operation.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATl Field/Group
Wastewater
Water Pollution
Plating Wastewater
Cadmium
Cyanide
Particle Bed Electrode
Electrolytic Cell
1302
1308
0701
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report}
Unclassified
21. NO. OF PAGES
59
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
51
OU.S, GOVERNMENT PRINTING OFFICE 1977-757-056/5509
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