EPA-600/2-75-028
September 1975
Environmental Protection Technology Series
ELECTROLYTIC TREATMENT OF
JOB SHOP METAL FINISHING WASTEWATER
Municipal Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
-------
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 environ-
mental 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 environ-
mental quality standards.
This document is available to the public through the National
Technical Information Service, Springfield, Virginia 22161.
-------
EPA-600/2-75-028
September 1975
ELECTROLYTIC TREATMENT OF
JOB SHOP METAL FINISHING WASTEWATER
by
Bruce E. Warner
New England Plating Company, Inc
Worcester, Massachusetts 01605
Grant No. 12010 GUG
Program Element No. 1BB036
Project Officer
John Ciancia
Industrial Environmental Research Laboratory
Edison, New Jersey 08817
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
-------
DISCLAIMER
This report has been reviewed by the Municipal Environ-
mental Research Laboratory, 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.
11
-------
FOREWORD
Man and his environment must be protected from the
adverse effects of pesticides, radiation, noise, and other
forms of pollution, and the unwise management of solid
waste. Efforts to protect the environment require a focus
that recognizes the interplay between the components of
our physical environment--air, water, and land. The
Municipal Environmental Research Laboratory contributes
to this multidisciplinary focus through programs engaged
in
• studies on the effects of environmental
contaminants on the biosphere, and
• a search for ways to prevent contamina-
tion and to recycle valuable resources.
The studies for this report were undertaken to demon-
strate the reliability and economics of a new electrolytic
technique for treating metal finishing rinse waters in a
full scale system under actual plant conditions. The
electrochemical process employs a semi-conductive bed of
carbon particles between the electrodes for the reduction
of hexavalent chromium or the oxidation of cyanides commonly
found in metal finishing wastes.
111
-------
ABSTRACT
Full scale in-plant production studies demonstrated the
reliability and economics of electrolytic cells containing
beds of conductive particles between cathodes and anodes
for reduction of hexavalent chromium and oxidation of
cyanide in plating rinse water. The heavy metals are
subsequently removed from the waste water by alkali
precipitation.
Seventy-five liter/min (20 GPM) sized cells were employed
for chromium and cyanide rinses. Chromium concentrations
to 250 mg/liter and cyanide concentrations to 150 ing/liter
were processed. Data were obtained with parallel equipment
using chemical treatment for cost comparison.
At low chromium concentrations (less than 25 mg/liter) ,
chemical reduction was more economical. At higher concentra-
tions (150 mg/liter) , electrolytic reduction became the more
economical process. In cyanide oxidation, the electrochemi-
cal process reduced direct costs associated with chlorina-
tion.
Waste treatment costs, capital and operating, for the job
shop are provided with an assessment of total costs on the
price of services provided. Water conservation techniques
are described. Experiences with tube settling equipment for
removal of suspended solids and centrifuge for sludge concen-
tration is provided.
This report was submitted in fulfillment of Project Number
12010 GUG by New England Plating Company, Inc., 31 Garden
Street, Worcester, Massachusetts 01605, under the partial
sponsorship of the Environmental Protection Agency.
IV
-------
CONTENTS
Page
Foreword iti
Abstract iv
List of Figures vi
List of Tables viii
Acknowledgments x
Sections
I Conclusions 1
II Recommendations 4
III Introduction 7
IV Source of Waste Water 12
V Water Conservation 18
VI Waste Treatment Facility 39
VII Demonstration of Treatment 63
VIII Solids Handling Program 101
IX Discussion 115
-------
FIGURES
No. Page
1 Surface Preparation Process 14
2 Post-Treatment of Zinc Plate 15
3 Extended Counterflow Rinsing-Cleaning 24
4 Extended Counterflow Rinsing-Plating 25
5 Extended Counterflow Rinsing-Hot Water Rinse 27
6 Chromium Rinse Water Treatment Schematic 44
7 Electrochemical Reduction Installation at 46
New England Plating Company
8 Chemical Reduction of Chromium Schematic 47
9 Cyanide Rinse Water Treatment Schematic 49
10 Electrochemical Oxidation Installation at 52
New England Plating Company
11 Semi-conductive Bed Electrolysis 54
12 Bed Particles 54
13 Electrochemical Treatment Cell 55
I1)- Batch Treatment Schematic 58
15 Hexavalent Chromium in Raw Waste 66
16 Concentration of Hexavalent Chromium in Raw 67
Waste - Probability Curve
17 Power Consumption for Chromium Reduction as 75
a Function of Feed Concentration
18 Cyanides Amenable to Chlorination in Raw 8l
Waste
19 Concentration of Cyanides Amenable to 83
Chlorination in Raw Waste - Probability
Curve
VI
-------
FIGURES (continued)
No.
20 Total Cyanide vs Cyanides Amenable to 84
Chlorination
21 Cyanide Oxidation Efficiency as a Func- 88
tion of Applied Voltage
22 Cyanide Concentration during Batch 94
Electrolysis
23 Solids Handing Schematic 106
24 Settler Effluent under Solids Input 109
Variations
25 Extent of Chromium Reduction with 120
Initial Concentration Variations
26 Chromium Reduced per Kilowatt Hour 122
vs Hexavalent Chromium Concentration
VII
-------
TABLES
No. Page
1 Typical Analysis of Untreated Combined Waste 13
2 Pollution as a Function of Production 13
3 Anticipated Effluent 42
4 Hexavalent Chromium in Isolated Raw Waste 68
during Production Day
5 Hexavalent Chromium in Electrochemical Cell 74
Discharge during Production Day
6 Cyanide in Isolated Raw Waste during Pro- 85
duction Day
7 Discharge after Treatment 98
8 Treated Discharge as a Function of 99
Production
9 Electrochemical Reduction System Equipment 123
Costs
10 Chemical Reduction System Equipment Costs 124
11 Operating Costs for Electrochemical 126
Reduction and Precipitation
12 Operating Costs for Chemical Reduction and 127
Precipitation
13 Electrochemical and Chemical Treatment 132
Equipment Costs for Cyanide Removal
14 Operating Costs for Electrochemical/ 134
Chemical Oxidation
15 Operating Costs for Chemical Oxidation 135
16 Major Contaminants Before and After 137
Treatment
17 Capital Costs for Total Plant 139
Vlll
-------
TABLES (continued)
No. Page
18 Direct Operating Expense for Total Plant
19 Summary of Waste Treatment Costs 141
IX
-------
ACKNOWLEDGMENTS
The financial support of this project by the Office of
Research and Development, Environmental Protection Agency,
is acknowledged with sincere thanks. Special appreciation
is given to Mr. William Lacy, Director of Industrial Pollu-
tion Control Division, and to Mr. John Ciancia of the Indus-
trial Waste Treatment Research Laboratories in Edison, New
Jersey, for their valuable administrative and technical sup-
port as well as their moral support and cooperation.
Joe H. Shockcor, P.E., provided technical direction and
assistance in program management during this project. His
interface with suppliers of the electrochemical technology
was effective in drawing the project to completion. His
support in the preparation of this report is likewise ac-
knowledged with sincere thanks.
Mr. Robert Potter and Mr. Dale Roemer of New England Plating
Company contributed significantly to the installation, super-
vision and operation of the treatment facility.
Other personnel who contributed in part to the project under
the direction of Dr. Nicholas B. Franco are Mr. Robert Hess,
Mr. William Litz and Mr. Robert Balouskus.
The cooperation of the management of Resource Control, Inc.
and W. R. Grace & Co. is also to be recognized.
-------
SECTION I
CONCLUSIONS
The use of the electrochemical process employing a semi-
conductive bed of carbon particles has been demonstrated
in full-scale equipment under actual plant conditions for
the reduction of hexavalent chromium and the oxidation of
cyanides commonly found in metal finishing waste water .
While the process has successfully been demonstrated to be
an effective and stable process for oxidation and reduction,
the specific process equipment used in the demonstration is
recognized to have some shortcomings.
The cells employed are larger than required for the low
chromium concentration normally seen in the rinse waters
after chromate conversion coatings on zinc and cadmium.
This fact increased the operating and capital costs to the
point where chemical reduction with acidic sodium bisulfite
is economically more attractive. However, when concentrations
of hexavalent chromium rise to the level normally experienced
after chromium plating, the economics advantage shifts to the
electrochemical approach.
In the reduction of hexavalent chromium, pH plays an impor-
tant part. Lower pH values are required as the chromium
concentration increases .
-------
The same type and size cells used for cyanide destruction
were smaller than required for the level of cyanide
experienced in rinse waters during this demonstration.
Higher voltages, together with increased cost of electricity,
produced a cost reduction that was lower than anticipated.
The operating cost advantage of this electrolytic process,
in conjunction with the chemical destruction of cyanides,
has been demonstrated. Final acceptance of the process by
industry will undoubtedly depend upon improvement in cell
design to improve yield and lower power costs.
There is adequate evidence that the electrochemical process
does not produce cyanates and ammonia, which can result from
chlorination. There is also a significant contribution to
reduction in total dissolved solids when the electrolytic
process is employed.
When used in the oxidation of cyanide, the process appears to
be volt age-dominated, but in the ranges studied, no improve-
ment in chromium reduction results from higher applied
potentials.
During the demonstration of the electrochemical process,
there was little, if any, destruction of the iron and nickel
cyanide complexes. Parallel work shows that these complexes
can be destroyed electrolytically. It is believed the pres-
ence of free cyanide along with these complexes inhibited
the attack on the complexes themselves.
-------
Considering the results obtained during the demonstration,
and taking into account the state-of-the-art of hardware
development, the electrochemical process offers economic in-
centives in operating costs when employed as a pretreatment
prior to chemical oxidation or reduction to lower the amount
of chemicals required for final treatment.
The demonstration project has provided information useful in
reducing water consumption. The extended counterflow tech-
nique will provide a new approach for others to consider .
The re-use of partially treated waste water has been demon-
strated under rather selected applications as a further
method to conserve water.
In solids handling, the tube settler approach to reducing
capital and space requirements by 75 per cent has been
demonstrated. A low-volume centrifuge, operating in a
continuous-intermittent cycle, has been shown to be an aid
in reducing the water content of sludges coming from waste
treatment practices. Sludges having 0.5 per cent solids
were concentrated to 30 percent solids.
-------
SECTION II
RECOMMENDATIONS
In order to improve the economics of the electrochemical
treatment, it is recommended that attention be directed to-
wards cell configuration so that:
1. anode life can be extended,
2. with more acid resistant anodes, lower pH
values may be used for greater chromium
reduction efficiences, and
3. improvements can be made in bed cleaning
techniques so as to promote bed fluidity
and eliminate filtration requirements
and labor attendant to these areas of
operating expense.
In order to widen the application of electrochemical treat-
ment, it is recommended that pilot studies be conducted in
some of the more promising areas in the metal finishing in-
dustry:
1. oxidation of cyanide complexes common to
metals normally considered lower in the
electromotive series (copper, silver, gold),
2. extraction of metals from chelates commonly
-------
used in cyanide free or low cyanide plating
solutions,
3. extraction of metals from acid wastes
(especially copper from etchants and
pickling rinses), and
4. destruction of iron and nickel cyanide
complexes found in strips and descalers.
In order to determine the extent of water conservation which
can be achieved through the use of extended counterflow
rinsing, in-plant studies to obtain production experience
are required for plating solutions other than zinc and
cadmium.
In order to further improve and more fully understand the ad-
vantages of tube settlers, it is recommended that:
1. various shapes of tubes be fully
evaluated in a manner permitting
comparison,
2. evaluations be conducted on various
types of additives to enhance settling,
3. the role of recycling be fully evaluated,
4. attention be directed towards improvements
in removal of solids from the settler after
precipitation, and
5. studies be conducted on the role played
by surface active chemicals and wetters
in the settling rate of suspended solids.
-------
In the use of centrifuges for sludge concentration, it is
desirable to determine the optimum operating conditions when
centrate recycling is employed.
-------
SECTION III
INTRODUCTION
GENERAL
The Metal Finishing Industry is divided into two basic
groups: captive shops and job shops.
The captive shops are found in manufacturing plants where,
for one reason or another, it was deemed desirable to provide
in-house finishing facilities for the various items being
fabricated. The size of the captive shop will vary with the
industry and with the specific plant itself.
Job shop metal finishing plants provide added-value services
to manufacturers on a contract basis. The job shop size is
as varied as the captive group.
Accurate statistics on the magnitude of the Metal Finishing
Industry are virtually impossible to obtain. It is generally
agreed that there are about 20,000 firms in the United States
that can be included within this industry. Of this number,
at least 3,500 are classified as job shops. It is believed
that the annual dollar-added value for the industry exceeds
$2 billion.
-------
The geographic distribution is approximately 15 percent in
southern and central New England, 22 percent in New York,
New Jersey and eastern Pennsylvania, 35 percent in the mid-
western states of Ohio, Indiana, Illinois, Michigan and Wis-
consin, and 9 percent in California.
More detailed information on the scope of the industry can be
found in the literature for those interested in the categori-
zation and quantification of the industry.
NEW ENGLAND PIATING COMPANY
New England Plating Company, 32 Garden Street, Worcester,
Massachusetts, is considered a large metal finishing job
shop by this industry's standards. It is within the top 10
percent of the number of firms when based upon the number of
employees (74) . Annual sales volume is considered average for
this type of plant when one considers the number of employees
and the types of operations performed.
The 3,3^0 square meter (36,000 square feet) plant is located
on two floors. The greatest volume of work is done on five
fully automatic processing machines. Four are for rack
plating and one is for barrel plating. The finishes pro-
vided on these machines are: zinc, nickel and nickel-
chrome. In addition, hand-operated hoist lines are used for
barrel finishing of cadmium, copper and tin, as well as a
hand-operated rack line for copper and tin. Most zinc
plating receives chromate conversion coatings.
-------
The base metal normally encountered in this finishing plant
is steel. An insignificant amount of copper and copper al-
loys is occasionally processed. The results of this special-
ization are reflected in the cleaning cycles employed, the
near absence of copper in the untreated waste water, and in
the water re-use program developed.
The Metal Finishing Industry and its effect upon the environ-
ment has been well documented. New England Plating Company
had been discharging untreated waste into a small stream ad-
jacent to its plant. Other firms in the town used the sani-
tary sewerage system for discharge. New England Plating
elected to provide complete treatment and continue to use the
stream for discharge. The small stream was converted into an
underground storm sewer by the city of Worcester and ulti-
mately enters the Blackstone River. This same river receives
the discharge from the sanitary sewage treatment plant.
Prior to the installation of treatment facilities and adop-
tion of water conservation practices, the discharge could be
considered typical for the industry: too much water contain-
ing objectionable concentrations of materials considered
harmful to the stream ecology.
New England Plating Company, as is true for many of the
larger job shops, is continually changing its finishing re-
quirements to reflect customer requirements and process im-
provements. During the course of this demonstration project,
many process changes were made which varied the loading on
the treatment facility. A few process changes were made to
-------
remain compatible with the treatment plant, but in large, the
treatment system had to respond to the metal finishing changes
OBJECTIVES
This demonstration project provides for the installation and
operation of a waste treatment facility in a metal finishing
job shop to:
(a) re-use untreated, partially treated and fully
treated waste water to reduce the waste water
volume by at least 50 percent;
(b) study and report on the effectiveness and
economics of the semi-conductive bed elec-
trolytic cell method of:
- reducing hexavalent chromium bearing
rinse waters, and
- oxidizing cyanide bearing rinse
waters, and
(c) study and report the effectiveness of the
use of tube settling equipment for the
removal of suspended solids, followed by
sludge concentration in a centrifuge.
REFERENCES
1. Olson, A. E. Upgrading Metal-Finishing Facilities to
Reduce Pollution: In-Process Pollution Abatement.
Environmental Protection Agency Technology Transfer
Program. U. S. Government Printing Office, Washington,
D. C. Publication Number 0-503-729. 197?. p. 2.
2. Economic Analysis of Proposed Effluent Guidelines for
the Electroplating Industry. Environmental Protection
Agency, Washington, D. C. Publication Number 230/1-
73-007. Sept. 1973. P. IH-1 to IV-2.
10
-------
Development Document for Proposed Effluent Limitations
Guidelines and New Source Performance Standards. En-
vironmental Protection Agency, Washington, D. C. Publi-
cation Number 440/1-73-003. Aug. 1973- P- 11-13-
A State-of-the-Art Review of Metal Finishing Waste
Treatment. Battelle Memorial Institute. U.S.Govern-
ment Printing Office, Washington, D. C. Publication
Number 0-400-817, FWQA Water Pollution Control Research
Series Number 12010 EIE 11/68. Co-sponsored by the
Metal Finishers' Foundation of the National Associaton
of Metal Finishers. 1970. p. 1.
Brummet, R. L., and Arnett, H. E. Business and
Economic Evaluation of the Metal Finishing Industry.
Bureau of Business Research. University of Michigan,
Ann Arbor, Michigan. Business Report Number 52.
Sponsored by the Metal Finishers1 Foundation of the
National Association of Metal Finishers. 1967. p. 2.
11
-------
SECTION IV
SOURCES OF WASTE WATER
GENERAL
At New England Plating Company, metal finishing operations
generate three categories of waste water: continuously
flowing rinses having relatively low concentrations of con-
taminants; periodic discharges of spent process solutions
having relatively high contamination levels; and accidental
discharges. The waste water may also be segregated and clas-
sified as cyanide bearing rinses and batch dumps, chromium
bearing rinses and batch dumps, and those rinses and batch
dumps that are free of cyanide and chromium.
Prior to waste treatment considerations, all waste water was
intermixed and collected in trenches and drains leading to
the adjacent stream. A typical analysis of the combined
rinse water at a flow rate of 757 liters/minute (1/min) (200
GPM) is shown in Table 1. There would be minor variations
from day to day depending upon the dragout of individual
parts being processed. There would be significant changes in
concentration for short periods when spent process solutions
were discharged or in the case of an accidental discharge.
12
-------
Table 1. TYPICAL ANALYSIS OF UNTREATED COMBINED WASTE
(milligrams/liter)
Cyanide, amenable 4
Hexavalent chromium 24
Trivalent chromium 11
Copper less than 0.5
Zinc less than 0.2
Nickel 4
Iron 2
Dissolved solids 10^6
Suspended solids 46
The rinse water at individual process centers or plating
steps was not analyzed and cannot be shown as a function of
production volume. However, it is believed that others will
be interested in the relationship of this typical analysis to
the overall volume of production. This relationship of pol-
lution to the amount of area being plated is shown in Table 2.
Table 2. POLLUTION AS A FUNCTION OF PRODUCTION
Cyanide
Hexavalent chromium
Trivalent chromium
Copper
Zinc
Nickel
Iron
Dissolved solids
Suspended solids
mg/m2
920
5,500
2,520
112
44
900
460
238,000
10,500
lb/106ft2
188
1,125
516
23
9
188
9^
48,600
2,160
13
-------
PROCESS DESCRIPTION
The process sequences vary slightly from one process center
to the next depending upon the finishes being applied. How-
ever, the basic steps of surface preparation through cleaning
and pickling of base metal, plating and post treatment of
plated surfaces remain constant.
The sequence of steps in preparing the surfaces for plating
is shown in Figure 1. On most cycles, counterflow rinsing is
employed after these cleaners or pickles.
ALKALI CLEANER
WATER RINSE
HCL, 50# BY VOLUME
WATER RINSE
ELECTROLYTIC CLEANER
WATER RINSE
Figure 1. Surface preparation process
Prior to nickel plating, a dilute acid dip and water rinse
follow the electrocleaner to provide a slightly acidic film
on the surface as the parts enter the nickel plating tank.
Following the surface preparation steps, various plating so-
lutions are used to meet the needs of customers. Zinc and
cadmium plating are done in conventional cyanide plating so-
lutions having bath compositions most favorable to rack or
barrel plating. Nickel plating is accomplished in proprie-
14
-------
tary bright nickel solutions common to the industry. The
chromium plating solution is a 240 g/1 (52 oz/gal.) bath
using proprietary self-regulating and foam blanket producing
additives. Copper plating from conventional cyanide solu-
tions is normally provided only as a strike prior to nickel
or tin plating. Tin is plated from an alkaline stannate so-
lution. Typical plating solution composition can be found in
a
the references.
Rinsing following plating is as varied as one would expect in
a widely diversified job shop. In general, the types of
7
rinses used are similar to those described by Olson. Drag-
out recovery tanks are employed after nickel and chromium
plating. Counterflow rinses are used extensively following
all plating steps. A more detailed discussion on rinsing is
provided later in the report in Section V, Water Conserva-
tion.
The sequence of operations following zinc plating is shown
in Figure 2. Again space requirements on individual machines
regulate the use of rinsing techniques.
WATER RINSE
DILUTE HN03 DIP
WATER RINSE
CHROMATE DIP
WATER RINSE
DRY
Figure 2. Post-treatment of zinc plate
15
-------
Where practical, analytical control provides for long solu-
tion life, and plating solutions themselves are seldom if
ever dumped. Other process solutions when spent are dis-
charged on a periodic basis which varies from once in several
months to once per day. On a monthly basis, the following
volumes are dumped:
cyanide bearing solutions - 3,800 1 (1,000 gal.)
chromium bearing solutions - 58,000 1 (10,000 gal.)
non-CN/CR bearing solutions - 34,000 1 (9,000 gal.)
OTHER WASTE WATER SOURCES
In addition to the main sources of rinses and batch dumps,
there were other sources of waste water that contributed to
the pollution loading. Steam condensate, while originally a
problem, was collected and returned to the boiler for re-use.
Open-ended cooling water, used to maintain temperature in
zinc plating, was replaced with a recirculating coolant sys-
tem chilled by refrigeration. Most of these other pollution
problems were resolved prior to the flow measurements used to
define the raw waste water magnitude given earlier in this
report.
Prior to waste treatment and water conservation concern,
rinse waters contributed approximately 60 kg (130 Ib) of cy-
anide and 90 kg (200 Ib) of hexavalent chromium per month.
The batch discharge of spent solutions introduced an addi-
tional 200 kg (440 Ib) of cyanide and 35 kg (77 Ib) of hex-
avalent chromium.
16
-------
REFERENCES
6. Graham, A. K. Plating Bath Compositions and Operating
Conditions. In: Electroplating Engineering Handbook,
Graham, A. K. (ed.). New York, N. Y., Reinhold Pub-
lishing Corp. , 1955. p. 204-214.
7. Olson, A. E. Upgrading Metal-Finishing Facilities to
Reduce Pollution: In-Process Pollution Abatement.
Environmental Protection Agency Technology Transfer
Program. U.S. Government Printing Office, Washington,
B.C. Publication Number 0-503-729. 1973. p. 16-18.
17
-------
SECTION V
WATER CONSERVATION
GENERAL
Most waste treatment systems are significantly affected by
the volume of waste water produced by rinsing operations in
the various metal finishing processes. As the volume de-
creases, with a corresponding increase in contamination
level, invariably the capital cost is reduced. When water
was considered a readily available and low cost material, the
Metal Finishing Industry used excessive quantities. Over the
years, as increasing attention was focused on industrial
waste treatment, more and more attention has been paid to
rinsing itself. Previously the only criterion for rinsing had
been the one of maintaining quality. The results of consid-
erable effort by many concerned with rinse water volume re-
duction have been published during the last fifteen years.
Tallmadge, et al and Kushner have provided both the
metal finisher and the waste treatment designer with formula-
tion for determining contaminant level at various flow rates.
These theoretical relationships are based upon the assumption
that there is instantaneous and complete mixing. As a
18
-------
result, the calculated values are seldom realized in either
experiments or in production unless mechanical aids to rin-
sing are employed. Kramer16 reports good correlation between
theoretical rinsing and production rinsing for rack plating,
and poor comparison for barrel plating. The concept of theo-
retical rinsing to predict contaminant levels and flow rates
has never been fully accepted. This is because the plater
recognizes that the predicted values represent ideal con-
ditions and therefore are only a fraction of the observed
values.
Many excellent articles on the mechanical aspects of rinsing
have been published. Pinkerton16, Gary17, Lancy and
Ceresa18"30 as well as Spring21 and Tween3S offer advice on
techniques to improve rinsing and to reduce watef consumption.
Drip stations, dragout recovery tanks, counterflow rinses
and spray rinses are important techniques to reduce both
the volume of rinse water and the contamination level. In
some facilities, air blow-off, fog or mist sprays, and
drainboards offer additional methods of reducing the prob-
lems attendant to waste treatment. Circulation within the
rinse tank caused by bottom entry of incoming water and air
agitation as well as agitation of the work pieces being
rinsed assist in obtaining the desdred results predicted by
the theoretical rinsing formulae.
Delos33 and many others recommend the use of physical means
to restrict the flow of water in any given set of circum-
stances. Both simple and sophisticated devices are readily
19
-------
available to remove from the operators' control the means of
changing the water flow rate.
The plater has been slow to accept reduction in water con-
sumption. He realizes the advantages of many of the physi-
cal or mechanical aids to rinsing, but has been reluctant to
trust any significant reduction in water flow. His main con-
cern has been that he has no guidelines as to how contami-
nated the rinse water may become before the quality of his
34
finishing operations suffer. Barnes has reported statis-
tics on the range of contamination level and frequency of
variation of concentration. He also reports tolerances for
contaminants in rinsing to produce clean work at this instal-
lation. However, how these maximum contamination levels were
JJ K
established is not given. Graham and Pinkerton use 1000:1
dilution as being borderline rinsing and 5000:1 dilution as
being good rinsing. It must be realized that these figures
are suitable for some operations but not all. The industry
can only use the term "adequate rinsing" without defining it in
other than general terms. These same authors point out the
importance of reducing volumes only by the application of
plating know-how.
Until the Metal Finishing Industry determines how dirty rinse
water may become before process solution and product quality
degradation are unacceptable, the plater will continue to use
excess water and approach water use reduction with extreme
reluctance and caution. It is the intent of this study to
20
-------
provide additional operating data so that further confidence
will be gained that significant volume reductions are possi-
ble.
WATER CONSERVATION AT NEW ENGLAND PLATING COMPANY
Prior to considerations for waste treatment, excessive water
was used for rinsing operations. The main concern was to
maintain an adequate flow to prevent the contamination of
process solutions and to maintain a high quality of work be-
ing processed. Although water costs were low, effort had
been put forth to minimize water costs. Consequently, while
positive steps towards reducing water use were underway, the
quantity used was still well in excess of that required as a
minimum.
During the past three years as plans were put into effect
for treatment of waste water, additional modifications of
existing metal finishing equipment were made to reduce
water consumption. As new equipment was installed, many of
the mechanical and physical aids to rinsing and water volume
reduction mentioned earlier were incorporated. Most of the
techniques mentioned earlier in this report were employed.
The studies conducted under this demonstration grant deal
mainly with the re-use of rinse water. The results of the
water conservation program at New England Plating are being
reported under:
21
-------
- Re-use of Untreated Water
- Re-use of Partially Treated Water
- Re-use of Fully Treated Water
- Other Techniques
Re-use of Untreated Rinse Water
Fresh water when introduced into a rinse tank becomes
contaminated with the process chemicals on the work pieces.
In many cases, this contamination level is low enough to per-
mit re-use of the water for less demanding rinsing. One well
versed with metal finishing technology has little trouble
finding other uses for these untreated waste waters.
Counterflow Rinsing -
The most widely employed technique for re-use of untreated
rinse water is known as counterflow rinsing. In this appli-
cation, the water is used in two or more adjacent rinse sta-
tions with the flow of water being counter to the flow of the
2 &>
work pieces. Olson provides an excellent discussion of
this technique. This technique is used extensively at New
England Plating Company where:
- the concentration of the process solution is high,
- extremely good rinsing is essential for quality,
- the solution being removed is difficult to rinse,
and
- space permits.
22
-------
Extended Counterflow Rinsing -
The counterflow rinse system described in the preceding para-
graph is well documented in the literature. In most uses,
it is limited to a single process solution. A logical exten-
sion of the counterflow technique is to re-use the water
still another time. This "Extended Counterflow11 can be ap-
plied successfully in cleaning cycles and with many plating
processes themselves. With this technique, the water used
for rinsing following a given process solution is the source
of water for rinsing prior to that given process solution.
For example, the rinse water following zinc plating is used
as the rinse water prior to zinc plating and after the pre-
ceding cleaning step.
Figure 3 shows in a schematic diagram how this extended
counterflow rinsing was accomplished in cleaning operations
on automatic plating machines at New England Plating. On
one zinc plating machine, the flow was reduced from 39 1/min
(10.3 GPM) to 10.6 1/min (2.8 GPM) after the plumbing change.
On other machines, flow reductions were 30 1/min (7-9 GPM) to
9.8 1/min (2.6 GPM) and 30 1/min (7-9 GPM) to 6.4 1/min
(1.7 GPM).
These results provided the basis for similar plumbing changes
throughout the facility. Since the rinse water leaving the
pickling rinse station is acidic, it should be obvious that
the work pieces are experiencing "chemical rinsing" in the
cleaner rinse tank. The data shows that enough acid is
-------
BEFORE
AFTER
ALKALI CLEAN
ALKALI CLEAN
RINSE
ACID PICKLE
RINSE
RINSE
RINSE
ACID PICKLE
P
RINSE
RINSE
SEWER
SEWER
Figure J. Extended counterflow rinsing - cleaning
24
-------
carried into the cleaner rinse tank to neutralize all cleaner
dragout. In this manner no alkali is being carried over on
the work pieces when they are immersed in the pickle, and
longer pickle life is realized.
Figure 4 illustrates how this extended counterflow rinsing
was accomplished in the plating process itself to reduce the
hydraulic loading on the waste treatment facility. At New
England Plating, this approach was used with zinc plating.
The flow rate is adjusted to provide adequate rinsing follow-
ing the plating process. The same water is then used prior
to plating. Fresh water previously used in this rinse tank
was discontinued. A typical flow rate of 6.5 1/min (1.7 GPM)
produces cyanide concentrations in the range of 125-300 mg/1
in the final overflow to the sewer.
BEFORE
AFTER
-»
— »
RINSE
PLATING
RINSE
RINSE
-»
r
— «
RINSE
PLATING
RINSE
RINSE
J
J
SEWER
SEWER
Figure ^. Extended counterflow rinsing - plating
-------
Analysis of the data shows that the work pieces prior to
plating are being rinsed with dilute plating solution. A
slight amount of plating solution is being returned to the
plating tank as the work pieces carry in this rinse water.
More significant is the fact that the conventionally employed
cyanide soak prior to plating is eliminated together with
its periodic neutralization.
Figure 5 shows how the extended counterflow rinsing technique
can be applied to hot water rinses. Prior to the concern for
waste treatment, this plant had stagnant final hot water
rinses in virtually every process line to aid in drying of
work pieces, as is common in the industry. To minimize water
stains on the plated parts, these stagnant rinses were dumped
at least once a day. At the time of dumping, the water was
still low in contamination level and in many ways suitable
for less demanding rinses. At New England Plating, it was
decided to convert these stagnant rinses to flowing rinses,
with the discharge directed to the previous rinse station.
Complete volume replacement would occur during the normal
life of the hot water rinse between dumps. That is, a
1800 1 (480 gal.) stagnant hot water rinse tank, which had
normally been discharged every eight hours, was converted to
a continuous flow rinse station with water flowing at a rate
of 3.8 1/min (1 GPM) so that total volume replacement would
take place in the same eight-hour period.
It was realized that while the rinse tank would never be as
clean as in previous practices, it would also never be as
26
-------
BEFORE
AFTER
n
RINSE
RINSE
HOT RINSE
-txj
n
RINSE
RINSE
HOT RINSE
U
SEWER SEWER
Figure 5. Extended counterflow rinsing - hot water rinse
contaminated. To keep this from becoming an additional hy-
draulic burden on the waste treatment facility, a correspond-
ing reduction of fresh water was made in the rinse station
prior to the hot water rinse. No change in the volume of
flowing rinse water was realized by this re-use of untreated
rinse water. The treatment facility actually received its
benefits from the plumbing change, as there were no longer
any surges of hot contaminated rinse water to drain or treat.
Plating quality was slightly better, since the incidence of
hard water stains was reduced. The additional steam required
to heat the small amount of water flowing into the final hot
water rinse tank is considered negligible when compared to
the demand for heating a complete tank of fresh cold water on
a daily basis.
27
-------
Re-use of Partially Treated Water
Partially treated water is considered that water that has
been rendered innocuous as far as dissolved salts are con-
cerned, yet does not have the suspended solids removed. At
New England Plating Company, partially treated water consists
of a combination of: (1) rinses following alkaline cleaners
and acid pickles with pH adjusted to near neutral; and (2)
the discharge from the cyanide treatment system with its pH
also adjusted to the same range. There are many uses for
this second class water in a diversified plating shop, for
example, maintenance cleaning or after stripping of rejects.
On the other hand, one would rule out its application in such
critical points as final hot water rinses (because of the
high solids content) or for process solution make-up water.
Partially treated rinse water has proven an effective substi-
tute for fresh water at New England Plating. In referring to
Figure 1, Surface Preparation Process, it will be noted that
rinse water is required following the hydrochloric acid
pickle. Under normal production conditions, this rinse tank
is at a pH of about 2. Neutral water supplied to this tank
would accomplish the desired result of rinsing the acid from
the work pieces. If the source of the rinse water is rela-
tively high in suspended solids (as in the case of the sec-
ond class water), one would expect the metal hydroxides to
redissolve. At New England Plating the make-up water for
most rinses after acid pickling has been converted to par-
tially treated waste water without any adverse effect on
plating quality or process solution life.
28
-------
The re-use of partially treated waste water does not reduce
the expense of the pretreatment that is required to establish
an adequate quality of secondary water. The advantage is de-
rived from the reduction in hydraulic capacity of equipment to
remove suspended solids. At New England Plating, the re-use
of partially treated waste water will result in a reduction
of 33 percent in the size of solids separation equipment,
as the amount of partially treated waste water that does not
have to pass through the settler will be reduced by that
amount. The total amount of solids removed remains the same.
Re-Use of Fully Treated Rinse Water
Fully treated water is considered to be water that has been
rendered innocuous as far as dissolved salts are concerned
and has had the suspended solids removed to practical limits.
At the time this report is being prepared, the complete
solids handling facility has not been used extensively. Data
obtained under the Solids Handling Section of this report in-
dicate good solids removal from the tube settler. However,
based upon the success of the re-use of partially treated
waste water, one can only speculate at this point as to the
advisability of replacing the secondary water source with a
tertiary source (namely, water treated for solids removal).
The primary concern involving the re-use of fully treated
rinse water is one of accidental contamination with ions that
are not compatible with the projected use. At New England
Plating, waste water that passes through the solids removal
equipment includes waste water from the chromium treatment
29
-------
process. It should be noted that the chromium bearing stream
is excluded from the partially treated waste water. While
the waste treatment engineer might not be aware of the fol-
lowing, the plater knows from experience that trace contami-
nation with hexavalent chromium can cause severe plating
problems. For this reason alone the re-use of fully treated
rinse water may never reach full fruition. It is concluded,
therefore, that there are no advantages to using fully treat-
ed waste water over partially treated waste water.
Other Techniques
While most of the attention during this program was directed
towards the re-use of water for conservation purposes, a com-
plete picture should include a discussion of other pertinent
techniques directed towards reducing the amount of water.
Dragout Recovery -
Dragout recovery tanks are used following several plating
solutions. Only those plating baths that are heated can use
this approach to reducing the amount of solution that is car-
ried into the flowing rinses. The stagnant rinse following
the plating solution is used as make-up to replace the water
lost from the bath by evaporation. Fresh water is added only
to the dragout recovery tank to maintain its level.
To insure that the recovered solution will be used for make-
up, provisions must be made so that it can be readily re-
turned to the process tank in an easy or uncomplicated man-
ner. The operator must prefer to use the recovery system
30
-------
instead of fresh water. Manual transfer is perhaps the poor-
est way to provide for the return of solution. A bucket for
transfer will not be used in place of opening a water valve
to add water. Automatic provisions for solution transfer are
the only way to maximize dragout recovery systems.
Draining -
Where automatic transfer of work pieces is being accom-
plished, a definite pause in the cycle after the racks are
withdrawn from the plating solutions will show a reduction in
contamination level in rinses and permit less water to be
used. In a similar manner, a slowing of the transfer mecha-
nism during the withdrawal cycle is effective. Blow-off of
solution as the racks pass through an air knife above the
process tank can return as much as 50 percent of the solu-
tion. When manual processing of work pieces is being done,
it is more difficult to obtain the same advantages of drain-
ing -- even though thorough employee indoctrination is used.
Control of Flow -
It is believed that sufficient flow controls have been estab-
lished at New England Plating Co., yet enough flexibility has
been built into the system to adjust for the variability re-
quired by the quality of parts being plated. All attempts
were made to provide "convenient" controls at each process
center to obtain the maximum cooperation from the operators.
Each process center is provided with its own water distribu-
tion system, including a main shut-off valve and individual
31
-------
flow control valves installed at each rinse tank. The main
control valve minimizes the need for operator adjustment of
flow control valves at start-up. One place to turn water on
or off simplified start-up and shut-down procedures. Now the
readjustment of flow control valves is only to accommodate
the needs of work in process.
When one or more process centers is brought on line or is shut
down, there can be a significant change in water pressure in
the main supply pipes distributing water to each process cen-
ter. An increase in pressure to a given process center will
result in an increase in water flow. In a like manner, a re-
duction in water pressure will result in a reduction in water
flow. Prior to the concern for water conservation, flow con-
trol valves were set to accommodate the lowest water pres-
sure. When other process centers were shut down, the pres-
sure would increase and more than required water would flow.
It was desirable to eliminate this variable. Pressure reduc-
ing valves were installed in each water line supplying each
center. The discharge pressure was set for the condition
when all process centers were in use (at the lowest pressure
in the main supply line). In this manner, at no time was the
pressure supplied to any given center greater than that used
to set the flow control valves, even though the main supply
line pressure increases.
Other techniques to restrict the flow at a given process cen-
ter were evaluated. While many provided good results, none
were more economical than the use of pressure-reducing valves.
32
-------
In one station, a wide range of flow is necessitated by the
process itself. The hand-operated barrel line consumes a
large volume of water for a short period of time when the
barrel is immersed in the rinse tank, and little or no flow
is required when the barrel is elsewhere in the cycle. At-
tempts to average the flow in the rinse tank reduced rinsing
effectiveness and quality and/or production suffered. A so-
lution was provided by the installation of automatic reset
timers and solenoid valves. When the barrel is immersed in
the rinse tank, the operator presses the timer button, the
valve opens and water is admitted to the rinse tank. After
the barrel is removed from the tank, the timer automatically
stops the water flow until the cycle is repeated. In this
manner, the turning off of a large volume of water was re-
moved from the operator's control, yet adequate rinsing is
available when needed.
Air Agitation -
As means to restrict the flow of water were installed, it be-
came increasingly important that the maximum use of existing
rinse water had to be made. Rinsing, per se, is the dilution
of the film on the surface of the work pieces to a lower con-
centration. Physical means to accelerate this dilution (or
mixing) will result in better rinsing and/or less water con-
sumption. In hand-operated lines, agitation of the plating
racks accelerates the film dilution. In automatic machines,
work piece agitation becomes expensive. When the volume of
rinse water was of less concern, excessive water flows in-
creased the rate of film dilution, as the agitation caused
33
-------
by the water flow aided in film dilution and approached the
theoretical rinsing discussed earlier.
Since excessive water flow was undesirable from a water
treatment standpoint, other attempts to displace the film of
solution on the work pieces were employed. The most effec-
tive technique was the use of air agitation in the rinse
tanks. Headers were designed and installed in the bottom of
all rinse tanks, so that the bubbles of air would increase
solution agitation as they rose to the surface of the tank.
Air agitation permitted the use of a minimum amount of water
for rinsing. In addition, it created the impression among
the operators that they were obtaining the same degree of
rinsing that was apparent before air agitation when large
water flows were standard operating procedure.
Proof of Rinsing Adequacy -
Prior to evaluations, water flows were set at "best estimate"
by operators. Based upon the successes in more important
rinse areas (final rinsing, cleaning, etc.) contaminant meas-
urement following other process solutions were done to see what
other water economies could be realized. Many cases showed
that good rinsing was possible with lower flows.
Following the "neutralizer dip" used after zinc or cadmium
plating and prior to chromate coating, water flows were cut
by 50 percent. The pH of the rinse water on the zinc plater
went from 7.0 to 6.0. The flow following the same dip on
another plater was then set to average at a pH of 6.0.
34
-------
With adequate documentation of contamination levels in other
rinse stations, confidence was established that less water
could be used.
Chemical Rinsing -
The use of neutralizing chemicals as an aid in rinsing is
common in the Metal Finishing Industry. Following cyanide
copper plating, for example, most platers use a dilute sul-
furic acid dip prior to nickel plating. This dip insures the
absence of any alkaline film on the surfaces of the work in
process and protects the pH of the nickel plating bath from
undue alkali being dragged into the tank. In a similar man-
ner, many employ dilute nitric acid dips after zinc plating
and prior to chromate conversion coatings.
At New England Plating, an example exists wherein the addi-
tion of a chemical rinse had a significant effect on waste
water volume reduction. The unit processes on a cadmium
plating automatic were compared before and after the water
conservation program began. Before recent concern
for water use reduction, 48 1/min (12.5 GPM) of rinse
water was required to obtain adequate rinsing prior to chro-
mate coating. A counterflow rinse following plating, to-
gether with the addition of an acid dip and its rinses re-
duced the flow to 11.5 1/min (J.O GPM). This machine was
converted at a later time to zinc plating.
35
-------
Personnel Factors
The best of intentions in water conservation efforts are
meaningless unless those operators responsible for production
and quality are well aware of the ultimate desires of manage-
ment. Educational sessions directed towards keeping the op-
erators informed of the need for water conservation and indi-
vidual briefing for each process center as changes were ini-
tiated improved the picture. If the operator is not aware of
the objectives for his individual area, he will develop meth-
ods to overcome water conservation techniques that he feels
are forced upon him. The need for maintaining operator
awareness is necessary on a continuing basis.
REFERENCES
8. Walker, C. A., and J. A. Tallmadge. Metal Finish-
ing Waste Reduction. Chemical Engineering Progress.
55:73-78, May 1959.
9- Tallmadge, J. A., and B. A. Buffham. Rinsing Effective-
ness in Metal Finishing. Journal Water Pollution Con-
trol Federation. 33:817-828, August 1961.
10. Tallmadge, J. A., B. A. Buffham and R. R. Barbolini.
A Diffusion Model for Rinsing. A.I.Ch.E. Journal.
8(5):649-653, November 1962.
11. Tallmadge, J. A. and R. R. Barbolini. Rinsing Effec-
tiveness - Cube and Other Shapes. Journal Water Pollu-
tion Control Federation. 38:1461-1471, September 1966.
12. Tallmadge, J. A. Improved Rinse Design in Electro-
plating and Other Industries. Professor, Chemical
36
-------
Engineering, Drexel Institute of Technology. (Pre-
sented at the Second Mid-Atlantic Industrial Waste
Conference. Philadelphia, Pennsylvania. November
18-20, 1968.) 17 p.
13. Tallmadge, J. A., and S. U. Li. A Drop Dispersal
Model for Rinsing. A.I.Ch.E. Journal. 15 (4):521-
526, July 1969.
14. Kushner, J. B. Rinsing Techniques. In: Metal Fin-
ishing Guidebook and Directory, Westwood, New Jersey,
Metals and Plastics Publications, Inc., 1971. p. 499-
504.
15. Kramer, A. E., and H. Nierstrasz. Design of a Treat-
ment Plant for Metal Finishing Wastes. Plating,
January 1967.
16. Pinkerton, H. L. Chapter J54, Rinsing. In: Electro-
plating Engineering Handbook, Graham, A. K. (ed.).
New York, New York, Reinhold Publishing Company, 1962.
p. 705-715.
17. Editor. Self-help in Pollution Control. Products
Finishing, p. 90-94, June 1970.
18. Lancy, L. E. An Economic Study of Metal Finishing
Waste Treatment. Plating, February 1967.
19. Ceresa, M. , and L. E. Lancy. Waste Water Treatment.
In: Metal Finishing Guidebook and Directory, West-
wood, New Jersey, Metals and Plastics Publications,
Inc., 1971. p. 766-784.
20. Ceresa, M., and L. E. Lancy. Metal Finishing Waste
Disposal. Metal Finishing, April 1968, p. 56-62.
21. Spring, S. Water Supply, Rinsing, and Drying. Metal
Finishing, April 1965. p. 46-53.
-------
22. Tweed, R. E. Rinsing Practices. Metal Finishing,
July 1968. p. 48-52.
23. Delos, J. S. Administration of a Water Pollution Con-
trol Program. Plating, March 1970. p. 258-261.
24. Barnes, G. E. Disposal and Recovery of Electroplating
Wastes. Plating, September 1966. p. 1115-1121.
25- Graham, A. K., and H. L. Pinkerton. Dollars and Sense
about Plating Waste Treatment and Water Conservation.
Plating, October 1966. p. 1222-1229.
26. Olson, A. E. Upgrading Metal - Finishing Facilities to
Reduce Pollution: In-Process Pollution Abatement.
Environmental Protection Agency Technology Transfer
Program. U.S. Government Printing Office, Washington,
B.C. Publication Number 0-503-729. 1973. p. 13-18.
38
-------
SECTION VI
WASTE TREATMENT FACILITY
GENERAL
The waste treatment facility installed at New England Plating
Company provides for segregation of waste by both type and
concentration. The various subsystems are based upon the
electrolytic reduction of hexavalent chromium and oxidation
of cyanide, the precipitation of heavy metal hydroxides by
pH adjustment and the removal of suspended solids with con-
centration of sludges. In addition to the electrolytic
treatment processes, chemical means for oxidation/reduction
reactions have been installed to obtain operating cost data
for comparison purposes and to provide back-up treatment pro-
cesses for the electrolytic treatment processes.
WASTE ISOLATION TECHNIQUES
Three separate drain systems were installed for flowing rinse
waters. These drains segregate the wastes into those that
are chromium bearing rinses, cyanide bearing rinses and rin-
ses free of cyanides and chromium compounds. Spent process
solutions that are dumped periodically are segregated into
39
-------
those which are cyanide bearing, chromium and acid bearing
and non-cyanide alkali bearing. These waste streams drain by
gravity to sumps located in the lowest section of the plant
from where the waste water is pumped to appropriate subsys-
tems . Accidental discharges, process equipment wash water
and any other floor spills find their way into an old floor
trench system and are collected with non-cyanide alkali batch
dumps. All drains are piped to the appropriate sump to pre-
clude accidental contamination that is possible when using
trenches for sewers. Each sump is equipped with a high level
alarm to alert operators when more water is entering than can
be removed by the sump pumps. This safety feature was in-
corporated to prevent sumps from overflowing unnoticed, thus
permitting isolated wastes to intermix and overtax the floor
spill collection system.
DESIGN BASIS
The chromium rinse water treatment process was designed for a
hydraulic loading of 150 1/min (40 GPM). Considering the
concentrating effect to be experienced when chromium waste
water was isolated, it was expected that these combined rin-
ses would contain hexavalent chromium in the order of magni-
tude of 120 mg/1. Electrochemical treatment cells were se-
lected that were believed to treat concentrations as high as
200 mg/1.
The cyanide rinse water treatment process was designed for a
hydraulic loading of 115 1/min (50 GPM). Considering the
40
-------
concentration effect produced by isolation, it was expected
that the cyanide bearing rinse waters would contain cyanide
in the order of magnitude of 30 mg/1. Electrochemical treat-
ment cells were selected that were believed to treat concen-
trations as high as 100 mg/1 (although experience was slight
in this potential application.)
The volume of those rinses known to be free of cyanide and
chromium compounds and whose treatment requires pH adjustment
was estimated to be 225 1/min (60 GPM). Since the pH adjust-
ment system would also service treated cyanide rinse water to
lower its pH, the pH system was sized for a hydraulic loading
of 380 1/min (100 GPM). Normally this combined waste would
be slightly acidic. When no treated cyanide rinse water was
entering the system, the pH would be quite acidic. Converse-
ly, when only treated cyanide rinse water entered the system,
the waste would be alkaline.
The batch treatment processes were designed to handle chro-
mium bearing waste at 11,400 1 (3000 gal.) per week, cyanide
bearing waste at 850 1 (225 gal.) per week and non-CN/Cr
bearing waste at 22,800 1 (6000 gal.) per week.
The equipment for removing suspended solids from flowing
rinse water was initially sized for a hydraulic loading of
570 1/min (150 GPM) at a normal suspended solids content of
100-200 mg/1. In order to maintain this relatively low
solids concentration, the design took into consideration that
solids generated by batch neutralization processes would not
pass through the suspended solids removal equipment. The
41
-------
concentration of solids in the sludge from settling of sus-
pended solids was expected to be in the range of 0.5-1 per-
cent. Suspended solids coming from batch neutralization
were expected to be in the range of 0.5 percent to 6 percent
(with occasions where 15 percent would be reached).
Contingency factors that were also a part of the design basis
considered the newness of the electrochemical treatment pro-
cess and the lack of significant production experience. The
chromium treatment process included the capability of con-
verting to acidic-bisulfite reduction of hexavalent chromium.
The electrochemical treatment process for cyanide oxidation
was followed by a chemical oxidation system using sodium hy-
pochlorite.
The combined effluent from the treatment facility considered
significant re-use of treated rinse water to reduce the hy-
draulic flow to less than 380 1/min (100 GPM). It was antic-
ipated that the discharge to the stream would have the char-
acteristics as shown in Table 3 and be in the pH range of
6.5 - 8.5.
Table 3- ANTICIPATED EFFLUENT
(milligrams/liter)
Cyanide, amenable . . . .0.1
Chromium, hexavalent. . . 0.2
Chromium, trivalent . . . 0.1
Copper 0.5
Zinc 1.2
Iron 0.1
42
-------
These concentrations pertain to dissolved contaminants. It
was realized that some portion of the 20 mg/1 of suspended
solids anticipated would contain metallic hydroxides.
RINSE WATER TREATMENT
Details of the subsystems installed at New England Plating
follow. Schematic flow diagrams are provided as an aid to
the explanation. Operating conditions are stated. The re-
.sults of the various studies conducted are to be found in
-Section VII, Demonstration of Treatment.
Chromium Rinse Water Treatment
Figure 6 should be referenced to assist in the description
of the treatment system for chromium bearing rinse waters.
The combined chromium bearing rinse waters that have been
isolated from other process water drain by gravity into a
collection sump. The sump is equipped with a level control,
a high level alarm and a pump for transporting the water into
the chromium rinse surge tank.
The chromium bearing rinse waters entering the surge tank
have a pH range of 2 - 6.5. The electrolytic treatment pro-
cess for reduction of chromium from the hexavalent to the
trivalent state is most efficient at low pH. Therefore,
these raw rinse waters flow through a pH control chamber up-
on entering the surge tank. This chamber is equipped with an
air agitation header and a pH probe and has as auxiliary
equipment a pH controller which actuates a proportional con-
trol feed pump delivering dilute sulfuric acid from a storage
43
-------
•1
o
COMBINED
CHROMIUM
RINSES
V
A
-------
tank. The pH is adjusted to a range of 1.5 to 2.0 for the
electrochemical reduction step to follow. The rinse water
thus collected in the surge tank is pumped through a filter
to four electrochemical treatment units connected in paral-
lel. As the water passes through the semi-conductive carbon
bed in the cells, the electrical power supplied by the recti-
fier reduces the hexavalent chromium to the trivalent chro-
mium state at the many cathode sites within the bed. At a pH
of 1.5 to 2, the resulting trivalent chromium remains in so-
lution and discharges as pretreated water from the electro-
chemical treatment units, draining by gravity into a sump for
pH adjustment.
Final treatment of the chromium rinse water is accomplished
in the chromium pH adjustment sump which is equipped with a
mixer and a pH controller to add dilute caustic soda solu-
tion from a storage tank. The controller permits addition of
caustic soda to maintain a pH of 8.2 in the sump. The mixer
assures thorough intermixing of the chemical solutions and
thorough solution washing of the pH electrode. To insure
complete reduction, a small amount of reducing agent (such as
sodium hydrosulfite) is mixed in with the caustic soda solu-
tion.
The treated water now free of hexavalent chromium and dis-
solved trivalent chromium intermixes with other treated rinse
water for removal of suspended solids.
Figure 7 is a picture of the installation at New England
Plating wherein the electrochemical treatment process is
45
-------
1. Surge Tank
2. Filter
3. Rectifier
4. Electrochemical Treatment Cells
Figure 7. Electrochemical Reduction Installation
at New England Plating Co.
46
-------
being used for chromium reduction.
For chemical reduction of hexavalent chromium (refer to
Figure 8), the hydraulic surge tank is converted to a reduc-
tion tank and the pump is used for solution mixing. The re-
duced waste water drains by gravity from an overflow weir to
the pH adjustment sump, by-passing the filter and the elec-
trochemical treatment cells. An ORP controller regulates the
addition of a solution of sodium bisulfite by activating a
proportional control feed pump.
CHROMIUM RINSES
AIR
H2S04
STORAGE
'*
d
-
=rl_
:
3H ADJUST REDUCTION
r^LJAkJDC'D TAK1IS
TO pH
ADJUST
SUMP
NaHSOj
STORAGE
Figure 8. Chemical reduction of chromium schematic
47
-------
In normal operations, the following conditions prevail when
the system is operating by electrochemical reduction of hexa-
valent chromium:
pH 1.8 - 2.1
Flow 58-150 1/min (10-40 GPM)
Cell potential 7.7 volts
Current 175 amps (at 75 1/min)
When operations are being conducted under chemical reduction
of hexavalent chromium, the following conditions prevail:
pH 2.0-2.5
Flow 58-150 1/min (10-40 GPM)
Solution potential +500 mv
The acid neutralization step used for final pH adjustment re-
mains the same for either reduction process. The pH is main-
tained at 8.0 - 8.5 for minimum solubility of trivalent chro-
mium hydroxide.
Cyanide Rinse Water Treatment
Figure 9 will assist in the description of the treatment sys-
tem for cyanide bearing rinse waters.
The combined cyanide bearing rinse waters that have been iso-
lated from other process water drain by gravity into a collec-
tion sump. The sump is equipped with a level control, a high
level alarm and a pump for transporting the water into the
cyanide rinse surge tank.
48
-------
COMBINED 1 U
CYANIDE HYDRAULIC
RINSES SURGE -
5 TANK ' '
CIJ
* » 1 ., *• •
T«; L — 1 ':l RECTIF
^
COLLECTION
SUMP
HYPOCHLORITE ORP
STORAGE 1 |~ CO NT.
U-i:
, CN/Cr FREE RINSES ~\
>
PH ....
r "i r CONT.
i r
• i
vJ V-x^^p 1* 0
T » . ..._.^J>fi 1 I 1 1 i
oOH H2S04 ,7^ni^ ^f
STORAGE :-. -=-1-=^ ^^
-------
The cyanide bearing rinse waters have a pH that is always
above 10.5» and it is not necessary to adjust the pH as in
the case of the chromium rinses. The rinse water thus col-
lected is pumped through a filter to three electrochemical
treatment units connected in parallel. As the water passes
through the semi-conductive carbon bed in the cells, the
electrical power supplied by the rectifier oxidizes the cya>
nide compounds at the many anode sites within the bed. TMe
discharge from the electrochemical treatment units drains by
gravity into a chemical oxidation tank which is equipped with
a mixer and an ORP controller to regulate the addition of so-
dium hypochlorite from a storage tank. The controller per-
mits the addition of the hypochlorite so as to always main-
tain a slight excess of oxidizing agent (measured as free
chlorine). The mixer assures thorough intermixing of the
chemical solutions and thorough solution washing of the ORP
electrode.
The. overflow from the chlorination step intermixes with rinse
water that is normally free of cyanides and chromium come
pounds for final pH adjustment and precipitation of metal hy-
droxides. This step is performed in a sump which is equipped
with a mixer and pH controller to add either dilute sulfuric
acid or dilute caustic soda from storage tanks. The control-
ler permits acid or alkali addition to maintain a pH range of
6.5 - 8.5. The. mixer assures thorough intermixing of the
chemical solutions, and thorough washing of the pH electrode.
The treated water now free of cyanide and metal ions inter-
50
-------
mixes with other treated rinse water for removal of suspended
solids. The dashed line shown in Figure 9 shows how the flow
can be diverted around the electrochemical treatment process
when straight chemical oxidation of cyanide is desired.
Figure 10 is a picture of the installation at New England
Plating wherein the electrochemical treatment process is be-
ing used for oxidation of cyanides.
In normal operations, the following conditions prevail when
the system is operated in the electrochemical oxidation mode:
pH 10.5
Flow 50-115 1/min (7-30 GPM)
Cell potential 40 volts
Current 560 amps (at 115 1/min)
The chlorination operation that is applied whether or not the
electrochemical treatment process is in use is conducted un-
der the following conditions:
pH 10.5
Flow 30-115 1/min (7-30 GPM)
Solution potential +550 mv
The pH adjustment step following the chlorination remains the
same for either oxidation process. The pH is maintained at
6.5 - 8.5.
Electrochemical Treatment Cell Configuration
The electrochemical treatment process employed in the demon-
stration is based upon the concept of obtaining oxidation/
-------
1. Electrochemical treatment cells (3) for rinse water
2. Electrolysis cell for concentrated cyanide
3. Rectifier for concentrate cell
4. Not shown, but to left: surge tank, pump, filter,
and rectifier for electrochemical treatment cells
Figure 10. Electrochemical oxidation installation
at New England Plating Co.
52
-------
reduction reactions in a bed of inert conductive particles
that are loosely packed to impart a semi-conductive nature to
the bed. Potentials are maintained between particles and/or
agglomerates of particles. This produces a bipolar effect of
the particles (or agglomerates) and a miniature cell is de-
veloped between the particles. This multiplicity of minia-
ture cells produces many anodic and cathodic sites within the
major cell (or unit). As the waste water passes through the
bed, the ionic forms in solution are subjected to very close
contact with the miniature electrodes and undergo oxidation
and reduction reactions. A secondary effect is believed to
occur through electrolysis of the water and other chemicals
present. These form oxidizing and reducing agents that are
liberated into the waste water and undergo chemical reactions
with the contaminants. Figure 11 depicts the semi-conductive
bed concept. The bed particles are shown in Figure 12.
Figure 15 is a photograph of the patented27 cells employed in
this demonstration project. Other cell configurations have
come into being since the start of this project and are men-
tioned in Section IX.
The flow of waste water through the cells is adjusted for a
maximum effective contact time within the bed. The major
electrodes act as baffles and are over- and under-flowed to
prevent solution from bypassing the beds. These cells have
an effective bed length of 2.5 m (100 in.), and at normal
flow conditions maintain a solution contact time of 20 min.
If greater bed length were used to provide longer contact
-------
SOLUTION
TRAVEL
Figure 11. Semi-conductive bed electrolysis
Figure 12. Bed particles
54
-------
.
^
'
1. Major Anodes
2. Major Cathodes
3. Bus Bars
4. Air Manifold
5. Solution Inlet
6. Solution Outlet
Figure 13. Electrochemical treatment cell
55
-------
times, it is expected that higher levels of contaminants
could be removed. The hydraulic capacity is determined by the
width of the cells when anode to cathode distances remain
constant at 15 cm (6 in.), as in the present case.
The cells are equipped with air manifolds at the bottom of the
the individual bays so that the beds can be agitated periodi-
cally in order to flush out precipitated solids that would
tend to plug the bed and restrict flow. This agitation is
also used to maintain bed fluidity, as packing tends to re-
duce the semi-conductive nature of the cells and reduces ef-
ficiency.
Rinse Waters Free of Cyanides and Chromium Compounds
The combined rinse waters that are known to be free of cya-
nides and chromium compounds drain by gravity into a collec-
tion sump. The sump is equipped with a level control, a high
level alarm and a pump for transporting the water into the
final pH adjustment sump.
This waste water is normally acidic and is intermixed with
the discharge from the cyanide rinse water treatment system
in the sump. The sump is equipped with a mixer and pH con-
troller to add either dilute sulfuric acid or dilute caustic
soda from storage tanks. The controller permits acid or al-
kali addition to maintain a pH range of 6.5 - 8.5. The mixer
assures thorough intermixing of chemical solutions and wash-
ing of the electrode.
56
-------
The treated waste now intermixes with chromium treated waste
water for removal of suspended solids.
BATCH WASTE TREATMENT
Details of the subsystems used for batch treatment of peri-
odic batch dumps follow. A schematic flow diagram is pro-
vided as an aid to the explanation. The results of the stud-
ies conducted with these systems are to be found in Section
VII, Demonstration of Treatment.
Acid and Chromium Batches
Figure 14 will assist in the description of the treatment
system for acid and chromium bearing batch discharges.
Acids and spent chromate dips drain or are pumped into a col-
lection sump for transfer to a holding and neutralization
tank located in the waste treatment area. This tank is
equipped with a high level alarm to alert the operator when
the tank is about two-thirds full. Auxiliary equipment in-
cludes mixers and storage tanks for dilute acid, dilute caus-
tic soda and sodium bisulfite solution.
When it is desired to treat a batch of waste, the mixer in
the tank is turned on to thoroughly intermix the various
dumps that have found their way into the tank. At the same
time, the batch treatment pump is turned on to assist in so-
lution intermixing. A sample is withdrawn for pH measure-
ment and hexavalent chromium analysis. If chromium is pres-
ent, reduction is required. If the pH is below 4.5, no acid
57
-------
ACIDS AND
CHROMIUM
WASTE
I
STORAGE AND
NEUTRALIZATION
TANK
ALKALI AND
FLOOR SPILL
WASTE
i* I
—cxi-
STORAGE AND
NEUTRALIZATION
TANK
CYANIDE
BATCH
WASTE
STORAGE TANK
RECTIFIER
EXHAUST
-txv
^ TO SLUDGE
STORAGE
H2S04
STORAGE
NoOH
STORAGE
NoHS03
STORAGE
NoOCI
STORAGE
STEAM
TO FLOOR
SPILL
ELECTROLYSIS TANK
Figure 14. Batch treatment schematic
58
-------
addition is required. If the pH is above 4.5, sulfuric acid
is used to lower the pH. Sodium bisulfite is slowly added
until tests show the absence of hexavalent chromium. Follow-
ing chromium reduction, the pH is increased by the addition
of caustic soda (or other waste alkali) to precipitate the
heavy metal hydroxides. The pH is normally raised to 7-5 to
8.5.
After chromium is reduced and the metal hydroxide precipi-
tated, the treated batch is sent to the solids handling
facilities for storage and sludge dewatering.
If no hexavalent chromium is present, only pH adjustment
takes place.
Alkali and Floor Spill Batches
Figure 14 is a schematic presentation of the method employed
for treating batch discharges from alkalis and floor spills.
The floor spill collection sump also receives all spent al-
kalis that are normally free of heavy concentrations of cya-
nides. The waste is transferred from this sump to a holding
and neutralization tank located in the waste treatment area.
This tank is equipped with a high level alarm to alert the
operator when the tank is about two-thirds full. Auxiliary
equipment provides for mixing and the addition of dilute
acid, dilute caustic soda and sodium hypochlorite from stor-
age tanks.
When it is desired to treat a batch of waste, the mixer and
batch treatment pump are used to intermix the various dumps
59
-------
in the tank. A sample is withdrawn for analysis and pH
measurement. If cyanide is found to be present, oxidation is
required. If the pH is below 10.5, caustic soda is added to
raise the pH for rapid oxidation. When the pH is above 10.5,
chlorination of the waste is practical. Normally this pro-
cess does not see high concentration of cyanide waste. Small
additions of hypochlorite (either sodium or calcium) are made
to the agitated waste water until a test for free chlorine is
positive for at least a half hour, when all cyanide is con-
sidered to have been oxidized. After oxidation, the pH is
reduced to 7-5 - 8-5 to precipitate any heavy metal hydrox-
ides .
After cyanide oxidation and pH reduction, trace quantities of
hexavalent chromium that might be present are reduced by the
manual addition of sodium hydrosulfite.
If no cyanide or hexavalent chromium is present, only pH ad-
justment takes place.
The treated batch is finally sent to the solids handling
facilities for storage and sludge dewatering.
Concentrated Cyanide Batches
Batches of concentrated cyanide waste are transferred by
various means to a holding tank located in the treatment
area. Conventional electrolysis at elevated temperatures is
used for oxidation of free cyanides and some of the more
tightly held metal cyanide complexes. Details of this elec-
28-30
trolytic process can be found in the literature.
60
-------
The electrolytic process is used to reduce high concentra-
tions to low levels so that final treatment can be accom-
plished by chemical means in the aforementioned process with
a minimum amount of hypochlorite consumption.
This process operates at a temperature near the boiling
point. The voltage applied depends upon the current demand
of the particular batch being treated, but normally 6 volts
is used. Exhaust is required.
After a given batch has been electrolyzed to the point of
diminishing returns, the batch is discharged to the floor
spill collection system for further treatment.
SOLIDS HANDLING
The waste treatment facility provides for the removal of sus-
pended solids by gravity separation from flowing waste water
in a tube settler. The solids resulting from settling are
further concentrated by centrifugal force. Solids from batch
neutralization practices are further concentrated by centrif-
ugal force. The details of the solids handling installation
are to be found in Section VIII, Solids Handling Program.
REFERENCE
2?. U. S. Patent Number 3,692,66!, Apparatus for Removing
Pollutants and Ions from Liquids.
28. Dobson, J. Disposal of Cyanide Wastes. Metal Finish-
ing, February 194?. p. 78-81.
29. Gray, A. G. Practical Methods for Treatment of Plating
Room Wastes. Products Finishing, August 1950. p.68-84.
61
-------
30. Oylar, R. W. Disposal of Waste Cyanides by Electrolytic
Oxidation. Plating, April 1949. p.
62
-------
SECTION VII
DEMONSTRATION OF TREATMENT
GENERAL
The various studies being reported were designed to demon-
strate the economics and reliability of the various subsys-
tems employed. Tests were conducted to characterize each
separate waste stream to provide more data on the amount of
cyanide and chromium compounds being treated, as it was an-
ticipated that changes in plating practices would result in
quantities that varied from the design basis reported in
Section VI. Further tests were conducted to widen the appli-
cability of data to others electing to use this electrochemi-
cal treatment process.
The systems described earlier (Section VI) provide means for
operating the treatment facility under either chemical or
electrochemical mode on the identical waste being generated.
Exhaustive chemical processing was not conducted, as these
techniques are amply described in the technical literature.
The studies of the chemical treatment steps were of adequate
depth to insure process reliability and of sufficient length
to obtain reasonably accurate operating cost data for com-
parison purposes.
63
-------
Further data was obtained to explain the roles of several
electrochemical process variables and their impact upon the
specific process under study.
The effluent character and quality with regard to the primary
contaminant are being reported for the subsystems and some
combination of subsystems, and, of course, for the total
treatment plant.
The results of the tests and studies are found under the fol-
lowing divisions of this section of the report: chromium
treatment, cyanide treatment, wastes free of cyanide and
chromium compounds. Similar information relative to solids
handling is reported in Section VIII of this report.
CHROMIUM TREATMENT
Chromium bearing rinses were processed with the semi-conduc-
tive bed electrolytic process for reduction of hexavalent
chromium to the trivalent state. Subsequent hydroxide pre-
cipitation was achieved using sodium hydroxide for pH adjust-
ment. Also, the same waste was reduced with bisulfite and
the same techniques employed for subsequent pH adjustment.
Batch treatment was employed for reduction of spent solutions
containing hexavalent chromium. Electrolysis was also used
on some spent chromium bearing batch wastes.
Characterization of Waste Waters
The original design basis for treatment of chromium bearing
rinses considered a hydraulic loading of 150 1/min (40 GPM)
64
-------
and hexavalent chromium concentrations in the order of magni-
tude of 120 mg/1. Upon completion of water conservation ef-
forts and deletion of some low volume metal finishing proces-
ses, flowing rinses totaled approximately 75 1/min (20 GPM).
During the twelve months of study on this system, daily grab
samples of the raw waste were monitored for CCr + O. The con-
centration values do not fit a normal frequency distribution
curve. However, the log [Cr+e} does fit the curve and is
shown in Figure 15. Data from this histogram and factors
from standard probability tables were used to construct the
curve in Figure 16 which depicts the probability of seeing
any given concentration in the rinse water. Analysis of this
data shows that during 90 percent of the time, the concentra-
tion was less than one-half of the design criteria. During
one-third of the time, the concentration was less than 10
percent of the design criteria.
In view of the concentration variations that could occur
within a given day and the potential error in drawing conclu-
sions solely from analysis of one daily grab sample, hourly
grab samples were taken on two separate days. The results
are shown in Table 4. It was felt that no severe anomalies
existed that would prohibit use of the statistical data from
grab samples in subsequent calculations.
The pH of the raw waste was monitored over a three-week peri-
od. While the rinse water was always acidic, the pH varied
from 1.7 to 6.3. With effective pH control of the raw waste
prior to electrolysis, the pH range was narrowed to 1.7 - 2.3,
65
-------
00
c
t-s
03
<
03
I—1
fD
p
n-
n
o
3
i-1-
3
t-t
0)
CO
3)
m
m
I
5 6 7 8 9 10 20 30 40 50 60 80 100
Cr+6 , mg/liter
2C» 247
-------
% OVER
.3.99 99.5 95 90 80 60 40 20 10 |
|00°|—iiiii—n i i i i i i i—r~i—TTT
aoi
100-
a.
ui
u.
UJ
UJ
on
0.01 O.I I
10 20 40 60 80 90 95
% UNDER
995 99.99
Figure 16. Concentration of hexavalent chromium
in raw waste - probability curve
67
-------
Table 4. HEXAVALENT CHROMIUM IN ISOLATED
RAW WASTE DURING PRODUCTION DAY
railligrams/liter
Time
0800
0900
1000
1100
1200
1500
1400
1500
1600
1700
1800
1900
2000
2100
2200
4/11/73
11.0
5.5
5-5
3.3
4.4
4.4
5.5
6.6
4.4
6.6
13-2
12.1
--
18.6
17.6
2/12/73
9.9
6.6
9-9
13.2
11.0
11.0
13.2
22.0
24.2
20.9
24.2
20.9
18.9
17.6
17.6
Batch waste containing hexavalent chromium can be character-
ized as amounting to 9000 1 (2350 gal.) per week with an
average concentration of 430 mg/1 of hexavalent chromium.
During the demonstration period, two batches of chromium con-
taining waste were treated that were abnormal. With the
68
-------
deletion of several process operations, a chromic acid ano-
dizing solution (at 100 g/1 of Cr03) and a chrome plating
solution (at 240 g/1 of Cr03) required batch treatment.
These two occasions, while providing sludge for solide hand-
ling studies, contributed little to studies in this section
of the report and were not used in characterizing the waste
water.
Rinse Treatment
Rinse waters containing hexavalent chromium were processed
through the electrolytic cell and by chemical reduction for
cost determination and comparison. Tests were also conducted
to determine the maximum hexavalent chromium concentration
allowable in the raw waste for full and effective utilization
of the electrolytic process.
Preliminary pH Adjustment Step -
Due to wide pH variations in the raw waste water (1.7 to
6.J5)> problems caused by heavy metal hydroxide precipitation
within the bed were experienced initially. Trivalent chro-
mium hydroxide precipitated as well as other metals, such as
iron and zinc. These solids were retained in the bed at the
higher pH values and increased the back pressure of the sys-
tem causing the individual bays within the cell to overflow.
With the waste water overflowing bays and bypassing the bed,
significant reduction in overall cell efficiency was experi-
enced.
69
-------
Adjustment of the pH to values in the range of 1.8 to 2.1
resulted in a modest sulfuric acid consumption and a slightly
higher power consumption. However, within this range a bed
fluidity was maintained which minimized cell maintenance and
produced consistent results. In addition, the filter car-
tridge replacement was reduced to three changes every two
months from at least three changes a week. Adjustment of the
pH to a value lower than 1.8 was not considered desirable in
cells of the design used in this demonstration, as the life
of the graphite anodes drops off with lower feed pH levels.
At the established pH values, the graphite anodes have a use-
ful life of 1.5 years.
The acid consumed during electrolytic reduction for a contin-
uous 75 1/min (20 GPM) flow was 0.84 kg (1.84 Ib) per hour.
To obtain high efficiencies with bisulfite reduction, a pH
range of 2.0 - 2.5 was used when operating in the chemical
reduction mode. Sulfuric acid consumption was 0.71 kg (1.57
Ib) per hour for the same averaged flow rate.
Electrochemical Reduction Step -
Four cells were used in parallel, each having a nominal ca-
pacity of 38 1/min (10 GPM). Since the flow was one-half of
this value, the system operated at maximum capacity in a pul-
sing ON/OFF mode. Each cell was used 50 percent of the time
as the level of accumulated waste water in the surge tank
rose and fell. Samples of rinse water before and after elec-
trolysis were analyzed for hexavalent chromium and the pH was
measured. At the same time, power consumption (voltage and
70
-------
amperage) was recorded and the flow measured.
Periodically, air was introduced into the bottom of the cell
sections to re-suspend the bed so as to keep the carbon
particles from compacting during treatment. Occasionally
water was introduced in place of air to aid in flushing
occluded particles from the bed. These fines were for the
most part particles that had fallen from the graphite anodes
as they eroded. Some carbon fines were observed during
cleaning with water which were believed to have come from bed
attrition. Bed attrition, which amounted to 2.5 percent per
month, is caused by particles abrading against each other and
against the partitions.
The 2.5 m (100 in.) of travel through the bed provided ample
time (20 min.) for virtually complete reduction of hexavalent
chromium. Flows higher than 150 1/min (40 GPM) could not be
processed due to the cell design. Lower flow rates did not
produce any improvement in the quality of the cell discharge.
It was recognized that activated carbon is a good adsorber
for hexavalent chromium. The carbon for the bed was primar-
ily selected for its size, shape and hardness. When compared
for adsorption with other carbons, it was obvious that better
adsorbing properties were available if this property were the
governing performance factor. Since it was felt that the pri-
mary factor was electrolytic action, only minor attention was
paid to the adsorption phenomenon. There were periods during
the demonstration when hexavalent chromium bearing waste
water passed through the cell with D.C. power off. In the
71
-------
absence of applied voltage, the level of hexavalent chromium
in the cell discharge gradually increased. When power was
again applied, the concentration level returned to the low
values obtained prior to power interruption.
The amount of power applied to these cells using a 15 cm
(6 in.) spacing between major opposing electrodes was suf-
ficient to produce a slight amount of gassing at the major
cathodes. Higher voltages apparently produced little if any
improvement in reduction efficiency. At the normal operating
voltage, the amount of current drawn by the cells was depen-
dent upon the pH and the chromium concentration in the waste
water. In addition, as the bed became more compact when the
time between bed agitation was extended an increase in the
amount of current consumed was observed.
The amount of operating labor required by the electrochemical
treatment process during the 12-month study period was 100
hours. Approximately half of this was consumed in preparing
the dilute sulfuric acid solution, one-fifth each in bed main-
tenance and filter tube replacement. Maintenance labor during
this period amounted to 70 hours. One-third of this amount
was devoted to anode replacement, with an equal amount di-
rected towards delivery pump repairs.
During the first six months of 1975, analysis of the dis-
charge data shows:
of the time, cell discharge was 0.05 mg/1 or less
" " " » " " 0.08 " " "
72
-------
83$ of the time, cell discharge was 0.10 mg/1 or less
ti 11 ft ti n n n -xr\ " " f
.30
100$ " " tr " " " less than 1.0 mg/1
The weighted average discharge from the cell was 0.06 mg/1.
With an average feed concentration of 17 mg/1, this amounts
to an average removal efficiency of 99'7 percent. Higher
efficiencies were realized when peak loadings greater than
100 mg/1 were observed in the raw waste. Even with signifi-
cant variation in feed concentration (see waste characteriza-
tion information), there was virtually no change in cell dis-
charge concentration.
To insure that instantaneous daily observatbns were indica-
tive of results throughout the day, hourly samples throughout
a given production day were monitored. The concentrations in
the cell discharge are listed in Table 5.
Power consumption during this period averaged at 390 amperes
and 7-8 volts. A prolonged period of stability was demon-
strated with the current range being 350 - 455 amperes (with
monthly averages of 380 - 425) and the applied voltage range
being 7-5 - 8.0 volts (monthly averages 7.7 - 7-8). The
average kilowatts consumed were 3-0 (monthly averages 2.9 ~
5.5).
A graph of the power consumed per kilogram of Cr03 reduced as
a function of the concentration of feed is shown in Figure
17. With the obvious economic advantage of the process for
higher feed concentrations, additional tests were run at a
73
-------
Table 5. HEXAVALENT CHROMIUM IN ELECTROCHEMICAL
CELL DISCHARGE DURING PRODUCTION DAY
Time
0800
0900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
(milligrams /liter)
4/11/73 2/12/73
0.07
0.05
0.025
0.055
0.05
0.06
0.04
0.05
0.055
0.05
0.055
0.03
--
0.06
0.045
0.04
0.00
0.02
0.02
0.07
0.025
0.025
0.04
0.03
0.02
0.025
0.015
0.02
0.01
0.025
74
-------
(Jl
H-
00
ro
h-1
»j
O 20
m
15
o
o
01
20
40
60
80
100
120
140
160
[cr+6] IN FEED, mg/liter
-------
later date to determine the tolerance of these specific cells
for hexavalent chromium loading.
Chemical Reduction Step -
A series of production evaluations of the chromium treatment
system when operated under a mode for chemical reduction of
hexavalent chromium with acidic sodium bisulfite were con-
ducted to establish cost factors for comparison purposes.
Early evaluations were directed towards determining the opti-
mum operating technique to meet the same quality criteria
demonstrated by the electrochemical reduction process. It was
-realized that the equipment to be used was less than ideal,
with improvements possible in the intermixing of the reducing
agent and the waste water being treated. With less than per-
fect mixing, the ORP probe location took on greater signifi-
cance. It was also recognized that the turbulence produced
by the system undoubtedly caused oxygen to be entrained in
the water and would account for some bisulfite consumption.
During February of 197^> over 450,000 1 (120,000 gal.) of
waste water was processed during a two-week period following
preliminary tests to optimize the system.
The pH was controlled in a range of 2.0 to 2.5 by the addi-
tion of dilute sulfuric acid. Retention time in the reduc-
tion tank was 100 minutes. The hexavalent chromium concentra-
tion was approximately 20 percent less than average condition
when the electrochemical treatment was accomplished.
76
-------
Operating labor showed a 70 percent increase over that re-
quired for the electrochemical process. Of the 170 hour/
year, better than half was consumed in preparing the bisul-
fite solution. Maintenance labor was predicted to be 70 per-
cent of that realized by the electrochemical process.
In order to achieve the same quality of reduction, it was
necessary to add an excess of bisulfite to the waste; 2.7 kg
(5.95 Ib) of Na2S205 was consumed for each kilogram (2.2 Ib)
of Cr03 reduced. This amounts to 1.9 times the theoretical
amount.
Precipitation Step -
Sodium hydroxide was used to adjust the pH of the reduced
waste to a range of 8.0 to 8.5. The retention time in the
neutralization sump was 24 minutes under average conditions.
Operating labor, which for the most part was used to prepare
the dilute alkali, amounted to 100 hr/yr. Maintenance labor
consumed one-fourth that amount.
Sodium hydrosulfite (Na2S204) was added daily in small quan-
tities to the alkali to reduce any hexavalent chromium escap-
ing the electrolytic cells; 6.8 kg (15 Ib) per year was the
minimum feed rate established to consistently produce the de-
sired level of hexavalent chromium in the chromium system dis-
charge. When bisulfite reduction was in use, the excess so-
dium bisulfite eliminated the need for this hydrosulfite addi-
tion.
77
-------
The efficiency of this step when monitoring hexavalent chro-
mium in the discharge is exhibited by:
57$ of the time, discharge was 0.05 rag/1 or less
n " " " f1 0.05 mg/1 "
w " " " " 0.10 mg/1 "
100$ " " " " " 0.30 mg/1 " "
Chromium Tolerance Tests
It was realized that the specific cells being used for rinse
water treatment provided excess capacity for chromium reduc-
tion. It was also recognized that chemical costs for chro-
mium reduction in batch treating spent chromate solutions
were higher than desired. A series of experiments were con-
ducted to establish the maximum concentration allowable in
the raw waste being fed into the electrochemical treatment
cells that would be consistent with the quality desired in
the effluent. It was expected that levels could be reached
which would permit electrochemical treatment of spent chro-
mate dips when added to the rinse water.
Chromic acid was prepared in a small tank adjacent to the
electrochemical cells and metered into the incoming raw
waste. Various feed concentrations were selected to deter-
mine the tolerance level for these cells and to determine the
rate of removal as an aid for future cell design.
Based upon the data obtained, it is believed that these spe-
cific cells (2.5 meters of bed length) can tolerate a feed
concentration of 150 mg/1 and continue to produce an
78
-------
.acceptable discharge (less than 0.2 i^g/1). This results in
New England Plating being able to discharge 85 percent of the
spent chromate dips directly into the rinse water system,
with the level always remaining below the critical concentra-
tion. Highly concentrated chromate dips (the remaining 15
percent) can be metered into the rinse water to further maxi-
mize utilization of the electrolytic system.
At feed concentrations approximating 150 mg/1, power consump-
tion was 4 kilowatts when operating at maximum hydraulic ca-
pacity. At this cell loading, 1.47 kwh per kilogram of Cr03
was realized. This compared favorably with 14.5 kwh/kg which
represented the average conditions while processing normal
rinse water.
By selecting specific points for analysis that were along the
path of solution travel through the bed, additional data be-
came available to monitor the rate of reduction of hexavalent
chromium as a function of concentration. With feed concen-
trations in the range of 220-250 mg/1, 145 tng/1 per meter of
travel was obtained. When in the range of 160-180 mg/1, 90
mg/1 per meter of travel was obtained. At a range of 60-140
mg/1, 40 mg/1 per meter of travel was obtained.
BATCH TREATMENT
During the study period, batch treatment of collected waste
acid and chromate dips averaged one batch per week. Sodium
bisulfite was added at low pH until a slight excess of re-
ducer was present (500-750 mg/1 excess). An average of
79
-------
15.4- kg (34 Ib) of anhydrous sodium bisulfite (Na2S2Os) was
used for each batch. Labor claims for operating and analyti-
cal labor amounted to li hours per batch.
CYANIDE TREATMENT
Cyanide bearing rinses were processed with the semi-conduc-
tive bed electrolytic process for oxidation of cyanide to
less harmful species. Following the electrolysis treatment,
alkaline chlorination was employed to remove residual cya-
nides amenable to chlorination that might escape the electro-
lytic oxidation. The electrolytic process was by-passed to
obtain operating cost data with only chlorination for com-
parison purposes.
Batch treatment of spent cyanide bearing process solutions
and spill water was accomplished with electrolysis for con-
centrated wastes and alkaline chlorination for dilute wastes.
Characterization of Waste Waters
The original design basis for treatment of cyanide bearing
rinses considered a hydraulic loading of 115 1/min (30 GPM).
and cyanide concentrations in the order of magnitude of 30
mg/1. Upon completion of the water conservation efforts and
other process changes, the flowing rinses totaled approxi-
mately 75 1/min (20 GPM). From February 1973 to February
1974, daily grab samples of the raw waste were always moni-
tored for cyanides amenable to chlorination and for approxi-
mately one-fourth of the time for total cyanide. Figure 18
80
-------
oo
09
c
n
n>
00
*
n
05
ol
ft)
CO
03
n>
03
cr
h-•
n>
rt
O
n
!T
03
n-
H-
o
H«
a
03
03
0)
rr
fD
1 I 1 Mill
30
m
o
m
14
12
8
IO
0-0
O
I I 1 I I I I I
20 30 40 50 60 70 80 100
AMENABLE CN~ , mg/liter
200
4OO
-------
shows the log [CNA] as a normal frequency distribution curve.
Data from this histogram and factors from standard probabil-
ity tables were used to construct the curve in Figure 19
which depicts the probability of seeing any given concentra-
tion in the rinse water. Analysis of the data shows that fore
98 percent of the time, the concentration was above the dfe-
sign criteria. During one-third of the time, the concentra-
tion was more than 300 percent of the design criteria.
As in the work on the chromium system, periodic grab samples
were taken on a day considered typical for cyanide proces-
sing. The results are listed in Table 6. Again, it was felt
the statistical data from grab samples could safely be used
in subsequent calculations.
Monitoring of the pH during the twelve-month period showed a
consistent range of 11.0 to 12.0 with only six excursions to
the range of 10-5 to 11.0 and two above 12.0.
By multiple regression analysis, a relationship was estab-
lished; between cyanides amenable to chlorination and the cor-
responding total cyanide value. This curve, Figure 20, is
shown to be linear except at high values.
Batch cyanide wastes collected for treatment can be charac-
terized in two general groups. Highly concentrated waste
from dumped process solutions averaged 570 1 (150 gal.) per
week at concentrations in the range of 30-50 g/1 (4-6.7 oz/
gal.). More dilute waste generated from floor spills, main-
tenance cleaning,, etc., had volumes in the range of 15,000 1
82
-------
%OVER
oo
00
1
n>
o n o
C ET O
H I-1 P
< o n
H- P
P rt
03 t-t
rt 03
(-•• rt
O h"
P O
P
M-
P O
l-h
o> n
0)
QJ H.
co a.
rt fl>
fD M
I 0)
B
"O rt
O 03
cr a*
05 I—1
o" n>
H-
f • o
rt
99.99
1000
0.01
i
CD
i
100-
0.01
O.I 02 05
ZQ 4O
% UNDER
995
99.99
-------
I80h-^ I 1 I I "/
145
z~no
o
ti
75
40— O
t;
/
/
40 75 MO 145 180 2 5
FEED CNA
Figure 20. Total cyanide vs cyanides amenable
to chlorination
(CNT: total CN; CNA: amenable CN)
84
-------
Table 6. CYANIDE IN ISOLATED RAW WASTE
DURING PRODUCTION DAY
(milligrams /liter)
Time
0815
0915
1015
1115
1215
1315
1415
1515
CNA
--
--
Sj.l
65-3
52.1
59-9
47.2
86.1
CNx
85.3
61.1
111.8
90.0
71.8
81.6
75.8
103-9
(CN.: amenable; CNT: total)
(4000 gal.) per week with concentrations normally in the
neighborhood of 100 mg/1. Occasionally this more dilute
waste contained 2-3 g/1 (0.25 - 0.40 oz/gal.).
Rinse Treatment
Rinses containing cyanide were processed under varying cell
potentials to optimize the electrolytic process. Prolonged
evaluations at what were considered to be optimum conditions
were conducted to establish quality of treatment and cost
data. Further analysis was conducted in an attempt to deter-
mine the reaction products of the electrolytic process.
Role of pH -
The pH of the combined cyanide rinses was at all times above
10.5, the minimum considered safe for chlorination, and
85
-------
normally in the range of 11.0 to 12.0. Previous experience
had shown no improvement in electrolytic oxidation by using
higher (or lower) pH values.
It was noted that a drop in pH of about 0.5 was realized as
the waste passed through the electrolytic cells.
Electrochemical Oxidation Step -
Three cells were used in parallel, each having a nominal
capacity of 38 1/min (10 GPM). Since the flow was two-thirds
of this value, the system operated at maximum capacity in a
pulsing ON/OFF mode. Each cell was used 6? percent of the
time as the level of accumulated waste water in the surge
tank rose and fell. Samples of the rinse water before and
after electrolysis were analyzed for total cyanide and cya-
nides amenable to chlorination, and of course the pH was
measured. At the same time, power consumption was recorded
and flow measurements made.
As was found during operation of chromium reduction units,
oxidation cells periodically required air to re-suspend the
bed to prevent compaction and a corresponding reduction in
treatment. Water was used to augment the air and to aid
in flushing solids from the bed. At the higher pH,
characteristic of the cyanide rinse water, less graphite
and bed attrition were noted than with the low pH waste.
A four-year life was estimated for the anodes in this
service. Bed consumption was calculated to be 1.0 - 1.5
percent per month.
86
-------
The flow rate used in the demonstration provided a 20 minute
retention time. Longer dwell times did not appreciably im-
prove the oxidation. In fact, too slow a flow resulted in the
liquid developing localized points of high flow and in reality
a lower contact time in the bed is realized.
Carbon is known to be an effective adsorber of cyanide. Ini-
tial results with the electrochemical cells containing a new
carbon bed can be misleading as the dominant mechanism of cy-
anide removal is adsorption. According to theory, the amount
of a component adsorbed depends upon the concentration in the
mass of the solution. Following periods of high concentra-
tion of waste being processed (and greater adsorption taking
place), if more dilute waste is processed, desorption activ-
ity takes place. It was demonstrated under this condition
that more cyanide was being discharged than was entering the
system. Obviously, the adsorption phenomenon was dominating
the results. To insure complete adsorption, waste was
passed through the cell for a sufficient period (longer than
200 hr) so that the discharge concentration was equal to the
inlet concentration. At this point, current was applied. A
steady decrease in cyanide concentration in the discharge was
observed. When a steady state condition existed, data was
taken for cell efficiency.
A definite relationship exists between applied voltage (and
increase in current) and cell efficiency as shown in Figure
21. The voltage generally used in this demonstration was
40 v., a value believed to optimize economies. At this
87
-------
Figure 21
Cyanide oxidation efficiency as a function
of applied voltage
88
-------
applied voltage, 60-70 percent of the cyanide entering the
cell was oxidized. This removal rate was consistent for
many months, and was maintained over a fairly wide range of
feed concentration. It was concluded that lower discharge
levels could be reached with a large cell having a length of
travel greater than the 2.5 m (100 in.) used in this demon-
stration.
Electrical power expended during the demonstration period
averaged 22.4 kw (5^0 amperes at 40 volts). Based upon the
average amount of cyanide removed, power consumption has been
calculated at 45 kw/kg CN (20.4 kw/lb CN).
Little or no reduction occurred in metal concentrations, in-
dicating that complexed metals remained in solution through
the electrolytic process. This is probably influenced by the
fact that "free" cyanide was always present in the discharge,
presenting a ready source for complexing any metals that
might otherwise have been removed from solution. It should
also be noted that with the metals present (Fe, Zn, Ni and a
minor amount of other heavy metals), no obvious effect on cy-
anide oxidation could be observed as a function of the con-
centration of these metals.
Analysis of the raw waste water shows that cyanate and ammo-
nia are present. Cyanate (CNO ) is formed by anode oxidation
of CN" in the plating bath, while low concentrations of NH3
results from the slow hydrolysis of CNO . When comparing CNO
concentrations in cell effluents with corresponding feed
values, evidence is provided that the electrolytic process
89
-------
oxidizes CN~ all the way to C02 and N2. With 65 percent oxi-
dation of CN" within the cell, for example, it would be ex-
pected that the CNO" concentration would show a marked in-
crease if this were the end product. Results show a CNO" in-
crease that is less than that calculated from CN loss. In
some cases the discharge CNO" concentration is less than in
the raw waste. If the apparent loss of CNO" is attributed to
hydrolysis to NH3, an appropriate increase in NHa would be
noted. However, in 82 percent of the tests monitored (18/22)
the NH3 concentration in the discharge from the electrolytic
cell was equal to or less than that entering the cell.
The drop in pH observed as the waste passes through the cell
can be attributed to the formation of carbon dioxide (C02)
which consumes alkali.
On several occasions during the demonstration period, oil
entered the cyanide rinse system. It is believed that the
oil came from cleaning baths and from the surface of the zinc
plating solution. Excessive oil adsorbed on the carbon bed
renders the latter ineffective for cyanide destruction and
consequently leads to frequent bed replacement. If the
degree of oil contamination is slight and infrequent, the bed
will normally recover as the impurity is desorbed when oil-
free rinse water is processed through the cells.
Chlorination Step -
Periodically the electrolytic process was by-passed to obtain
operating data on chlorination of the raw waste for cost
90
-------
comparison purposes. The system installed provides for only
high pH oxidation with hypochlorite. It must be realized
that while the dominant reaction is one producing cyanate
from the cyanide, some "chlorine" is consumed in oxidizing
other species as well as a slight amount of cyanate, leading
to slightly higher consumption rates.
The results obtained were good to excellent for cyanides
amenable to chlorination and poor for the more refractory
cyanide complexes. When the hypochlorite consumption was
only slightly above the theoretical amount of 2.86 kg NaOCl/
kg CN, marginal quality was obtained. At 50 percent excess,
excellent quality was obtained. Higher hypochlorite feed
rates did not improve oxidation of the more resistant cyanide
complexes of nickel and iron.
In both the chemical and combined electrochemical/chemical
treatment mode of operation, it was felt the additional hypo-
chlorite consumption required to promote more consistently
high removal rates was a proper manner to reduce pollution.
With this in mind, the data that showed 4.5 - 5.0 kg NaOCl/kg
CN are used for subsequent calculations.
Operating labor for the combined electrochemical-chlorination
system amounts to 100 hr/yr and includes bed maintenance, hy-
pochlorite handling and filter replacement. General mainte-
nance labor requires 60 hr/yr, 40 percent of which was at-
tributed to pump service. Operating labor for chemical
treatment alone remained the same as with the combined elec-
trochemical/chemical treatment. Additional labor in
91
-------
hypochlorite handling replaced bed maintenance labor.
Maintenance labor decreased to 35 hr/yr with the elimination
of the pump service requirements.
Batch Treatment
Spent process solutions containing high concentrations of cy-
anide (strip solutions, plating solutions, etc.) were iso-
lated, stored and periodically oxidized by electrolysis in a
batch process tank using conventional cell design. Dilute
cyanide solutions (floor spills for the most part) were
chlorinated in a batch mode using spent alkaline cleaners as
a readily available source of alkali to maintain the desired
high pH. Prolonged and detailed evaluations were not con-
ducted as these processes are well described in the litera-
ture.
Electrolysis -
During the demonstration period several cell configurations
were evaluated. Each varied slightly in anode to cathode
spacing, electrode areas and anode material of construction.
Variations in voltages required and current consumption were
noted that could not be related to the amount of cyanide re-
moved when comparing one series of tests to another. It is
believed that these variations are more dependent upon cell
configuration than upon any other fact. It is recognized
that the resistivity to oxidation varies from one metal com-
plex to the next and different results would be expected when
comparing the treatment of nickel strip with a copper plating
solution.
92
-------
Removal rates varied from 68 g (0.15 lb) of CN per 1000 am-
pere-hours to 360 g (0.80 lb) per 1000 amp-hr. The curves
shown in Figure 22 are typical in showing the change in con-
centration of total cyanide and amenable cyanides as a func-
tion of time. These tests were on 1150 1 (300 gal.) batches
at 96 - 102°C (205 - 215°F). The amenable cyanides were con-
sistently oxidized (or converted to iron complexes) within
three days. After that time, the rate of oxidation decreases
drastically.
Chlorination -
Data consistent with information published on chemical con-
sumption was obtained. With slight demands for hypochlorite
the tendency to overtreat to save time was repeatedly demon-
strated. It was felt that any attempt to use electrolysis in
the semi-conductive bed technique for these collected dilute
waste batches would prove futile due to the presence of oils
from spent cleaners. Expenses known to occur with bed con-
tamination exceeded any economic gain by electrolysis treat-
ment.
NON-CYANIDE/CHROMIUM WASTE TREATMENT
A minor amount of studies were conducted on the waste waters
normally considered free of cyanides and chromium compounds in
order to obtain cost data to complete the total treatment sys-
tem analysis.
93
-------
6 10
It ME .days
\2
14 16 18 20
Figure 22. CN concentration during batch electrolysis
94
-------
Characterization of Waste Water
Rinses known to be initially free of cyanides and chromium
compounds were monitored. The original design criteria used
a hydraulic loading of 225 1/min (60 GPM) and anticipated an
acidic nature. Following water conservation-oriented changes
and some process deletions, the flow averaged at 125 1/min
(52.5 GPM) with occasional peaks of 325 1/min (85 GPM) as
well as occasional periods of no flow. The pH monitoring of
this raw waste showed a rather consistent range of 1.8 to 2.5.
Batch waste originally free of cyanide and chromium become
intermixed with dilute cyanide batch waste water or dilute
chromium batch waste water by the collection techniques em-
ployed. As a consequence, it is not possible to characterize
the batches themselves. Since these waters are basically
cleaners and pickles, their neutralization is simple and re-
quires little if any discussion in this report.
Rinse Treatment
The neutralization sump receiving this raw waste water also
receives the rinse water from the cyanide chlorination pro-
cess. The collection system and the cyanide treatment system
both operate in a pulsing mode. Consequently, there are peri-
ods when one or the other is not delivering waste to the sump.
The cyanide waste is always alkaline and the non-cyanide
rinse is always acidic. Therefore,this sump sees a wide range
of pH values (1.8 to 12.0) in the raw waste.
95
-------
The non-cyanide rinse water averages 125 1/min (32.5 GFM)
and the cyanide treatment system contributes 75 1/min
(20 GPM) for a total average flow of 200 1/min (52.5 GPM) .
Operating labor amounts to 60 hr/yr for preparation of the
dilute acid and alkali solutions used. Maintenance labor
amounts to 20 hr/yr.
The pH is controlled at 7.0 to 8.5 with sulfuric acid and
sodium hydroxide. Chemical consumption amounts to 8.2 kg
(18 Ib) per day of suIfuric acid and ?4.5 kg (76 Ib) per day
of caustic soda.
SYSTEM ANALYSIS
There are three important areas in studying the results of
the various systems. The combination of treated cyanide
rinse water and neutralized rinses originally free of cyanide
and chromium is of prime interest as this water is used for
less demanding rinsing in the various metal finishing opera-
tions. Analysis of the total combined rinses leaving the
treatment facility before solids removal indicates the qual-
ity of the treatment systems. The final discharge is of
interest as it addresses the quality of the total plant.
Combined Rinses from Cyanide Treatment and from Those
Initially Free of Cyanide and Chromium
Samples taken at various times throughout the day and over a.
two-week period were analyzed for the major contaminants that
might influence the use of this second class water for
96
-------
rinsing. The range of cyanides amenable to chlorination was
0.11 - 0.55, with an average value of 0.23 mg/1. Hexavalent
chromium was always below the limit of detection (0.00 mg/1).
The pH ranged from 6.8 to 9.7 with an average value of 8.5.
At an average volume of 200 1/min (52.5 GPM), this stream
provides an ample supply of water with quality sufficient for
rinsing after: cleaning and pickling in the manner described
in Section V.
Combined Rinses after Treatment
Filtered samples taken prior to solids removal were analyzed
for cyanide and chromium. Cyanide amenable to chlorination
was less than the design criteria of 0.1 mg/1. Average con-
ditions showed less than 0.06 mg/1. Hexavalent chromium con-
centrations were also below the design criteria of 0,2 mg/1
with averages less than 0.01 mg/1.
A typical analysis of a filtered sample is shown in Table 7-
In relating this discharge at 285 1/min (75 GPM) as a func-
tion of production in the same manner the initial pollution
load was presented in Table 2, Section IV, one arrives at the
data presented in Table 8.
Combined Rinses after Solids Removal
The results of the third important area, after solids removal,
were not available at the time the project was completed. It
was felt that the success in the re-use of partially treated
water would eliminate the requirements for solids removal for
water re-use. It was also realized that there will be little
97
-------
Table 7. DISCHARGE AFTER TREATMENT
Cyanide, amenable 0.02 mg/1
Cyanide, total 8.? "
Chromium, hexavalent 0.04 "
Chromium, trivalent O.j58 "
Copper 0.11 "
Zinc 0.5 "
Iron 0.8 "
Nickel 0.3 "
PH 8.4 "
98
-------
Table 8. TREATED DISCHARGE AS A FUNCTION OF PRODUCTION
(on filtered sample)
mg/m2 lb/106ft2
Cyanide, amenable 1.61 0.33
Chromium, hexavalent 3-18 0.65
Chromium, trivalent 30.3 6.2
Copper 8.8 1.8
Zinc 40.1 8.2
Nickel 24.0 4.9
Iron 64.5 15-2
Dissolved solids 113,500 23,200
Suspended solids3 12,700 2,590
Q
prior to filtering
99
-------
change in the concentration of dissolved materials as re-
ported above. With less than 20 mg/1 of suspended solids
after settling, the total discharge would not change signifi-
cantly for this plant. Consequently, the more important
evaluations reported in Section VIII, Solids Handling, were
given a higher priority.
100
-------
SECTION VIII
SOLIDS HANDLING PROGRAM
GENERAL
Properly designed and operated waste treatment facilities
produce effluents acceptable to regulatory agencies in that
the discharge is considered safe for disposal to surface
waters or to sanitary sewerage systems. In the process of
chemically treating the waste water generated by metal
finishing operations to render it non-toxic, solid waste is
produced. The proper handling of these solids is essential
to insure that any negative effect on the total environment
is held to a minimum.
When considering the total impact on the environment caused
by solids, one must address both the dissolved solids in the
waste water and those solids that are not dissolved but are
suspended and are carried along with the waste water. This
section of the report is concerned only with those solids that
are out of solution and is not concerned with the total dis-
solved solids content.
The treatment of metal finishing waste water normally con-
sists of one or more processes used for oxidation of cyanide,
101
-------
reduction of hexavalent chromium, and pH adjustment. These
processes may be either of batch operation or of continuous
nature (or both). In the treatment of cyanide, the metals
that had been in solution as the cyanide complex are removed
from solution as the hydroxide.
Reduction of hexavalent chromium followed by pH adjustment
produces insoluble trivalent chromium hydroxide. With the pH
controlled to a value close to the minimum solubility of the
various metal hydroxides, solids result that are suspended in
the neutralized waste water. The combined waste streams nor-
mally contain objectionable amounts of these suspended solids,
Aside from the aesthetic appearance of waste water high in
suspended solids, concern is expressed for the potentially
toxic nature of these solids. In metal finishing waste,
these solids contain a significantly high proportion of heavy
metals. When discharged to a sanitary sewer, their accumula-
tion in the sewage sludge presents problems. They may be
toxic to the microorganisms in the sewage treatment plant.
Their presence in the sewage sludge may prohibit use of the
sewage sludge as fertilizer because of subsequent crop
damage. If incineration is used for sewage sludge disposal,
atmospheric pollution may result from the heavy metals pres-
ent. When the metal finishing waste is discharged to surface
waters, the solids will accumulate in the bottom of the
streams or lakes. This silting action can significantly dis-
turb the ecology of the organism originally living in this
environment.
102
-------
The need for removal of the precipitated solids has been well
recognized. Low limits have been established for effluents
discharge both to surface waters and to sanitary treatment
systems.
Filtration of the treated waste water for solids removal is
seldom considered because of the expense involved. The
solids are relatively gelatinous in nature and rapidly re-
strict the flow of water through the filter. As a result,
where space permits, the most commonly used approach for
removal of suspended solids is through gravity settling.
The use of mechanical aids such as a series of parallel tubes
across the top of the settling tank will significantly reduce
the size of the equipment. An excellent discussion of this
concept is provided by Lancy.31
Solids that have precipitated to the bottom of the settler
must periodically be removed. Concentrations in the range of
0.5-2 percent solids are common to the metal finishing indus-
try. The frequency of withdrawal and concentration is pri-
marily dependent upon the detail design of the settler. This
relatively low solids concentration requires further dewater-
ing prior to ultimate disposal.
Thickening of sludge is an essential part of any solids hand-
ling program. Aside from the added cost of hauling water,
there are other considerations requiring solids contents far
higher than 0.5-2 percent. When used for land fill applica-
tions, the very "thin11 sludge is impossible to contain within
a given area, as the water will flow away, carrying solids
103
-------
with it. Compaction, common to most land fills, is impossible
with excess water. A more desirable range is 15 percent
(comparable to thick mud) to 35 percent (paste-like). Vari-
ous techniques are available for thickening the sludge. The
approach employed in this demonstration uses a centrifuge.
SOLIDS HANDLING AT NEW ENGLAND PLATING COMPANY
Characterization
The rinse waters contain suspended solids resulting from
chromium and cyanide treatment processes and from acid/alkali
neutralization. The average flow rate of the combined rinses
is 284 liter/min (75 GPM). The suspended solids content is
in the range of 100-225 mg/1.
Batch neutralization practices produce 24,000 liter/week
(6350 gal./week) of waste water. Suspended solids contents
seldom exceed 5 percent by weight. On two occasions higher
values were obtained. Spent chromic acid anodizing solution
and a spent chrome plating bath were treated, producing solids
contents of 20-25 percent.
Facilities
A test settler was used to accelerate data collection after
completion of the various tests described in the treatment
study program. This unit is one-tenth the size of the pro-
duction settler planned and has the same geometry as the full-
sized settler. At the time of report preparation, the final
production-size settler was still in the design stage. The
104
-------
test unit provided valuable information for this design. The
data being reported is believed to be representative of that
which will be experienced with the production-sized unit.
The information on sludge concentration was obtained from
full-scale production equipment.
As an aid to understanding the solids handling facility,
refer to Figure 23 for a schematic diagram of the installa-
tion. The combined rinse waters from the treatment area
drain by gravity into a sump located in the Solids Handling
Building. A pump is used to deliver the water to the tube
settler.
Upon entering the settler, the waste water passes through a
chamber to permit the formation of flocculant particles. Af-
ter flocculation the water enters the settling chamber, flow-
ing in an upward direction, through the tubes and overflowing
at the top by way of an overflow dam. The clarified water
drains by gravity into an observation sump and finally to a
sewer leaving the plant.
The suspended solids precipitate in the settling chamber,
flowing counter-current to the water itself. The solids drop
to the bottom of the settling chamber and slowly accumulate.
Periodically the accumulated solids are withdrawn from the
bottom of the settling tank.
A centrifuge is used for concentration of the sludge. The
solids withdrawn from the bottom of the settling tank are
pumped into the centrifuge. Centrate leaving the centrifuge
105
-------
SETTLER
WITHDRAWAL HEADER
BATCH
WASTE
SLUDGE
SLUDGE
STORAGE
CENTRIFUGE
-••TO
DRUMS
Loj ^
PUMP
CENTRATE
a 4.
OBSERVATION^
SUMP "!
.
FINAL
EFFLUENT
•.. « • o* r •
•» A . . * A »
Figure 23. Solids handling schematic
106
-------
drains by gravity into the sump receiving the combined rinse
from the treatment area, and passes through the settling
operation a second time.
The partially concentrated sludge is delivered to a sludge
storage tank whenever the sludge leaving the centrifuge is
below 15 percent solids.
The partially thickened sludge is periodically withdrawn from
the sludge storage tank for another pass through the centri-
fuge. With a higher feed concentration, the centrifuge pro-
duces the thick sludge desired (15-30 percent). The centrate
during this operation is again recycled through the settling
tank.
Waste water produced by batch treatment practices is normally
higher in solids content than the rinse waters. These wastes
are transferred from the batch treatment tanks directly to
the sludge storage tank and are processed along with partial-
ly thickened sludge.
The thickened sludge is placed in drums for hauling away to a
suitable land fill operation.
DEMONSTRATIONS
Suspended Solids Removal
In operating the test tube settler, a portion of the total
waste water was used under varying hydraulic loadings. Meas-
urements of suspended solids were made on both the feed water
and the settler effluent. To study various hydraulic loading
107
-------
effects (and a corresponding total solids loading), the unit
was run for several days at any given flow rate to establish
what was considered equilibrium conditions between solids
entering and solids leaving.
The range of flow rates used was 22.7 to 56.7 liter/min (6
to 15 GPM). This corresponded to a range of velocities
through the tubes of 0.05 to 0.13 cm/sec. (0.1 to 0.25 ft/
min).
The data obtained are shown in Figure 24. The results show
the optimum flow rate to be near 37.8 liter/min (10 GPM) when
considering the quality of effluent. Lower flow rates did
not improve the quality. It will be noted that when the
solids concentration into the settler at the low flow rate
rose to four to five times the average concentration at New
England Plating, the quality of discharge improved signifi-
cantly and approached that of the more ideal flow rate. This
fact may indicate that equilibrium conditions had not been
reached at the lower concentrations for this low flow rate.
As would be expected, the quality of effluent suffered when a
high flow rate was used. The shorter contact time in the
unit, together with the higher velocities in the tubes, per-
mitted excess solids to escape in the effluent.
The frequency of removal of suspended solids in this test
settler was observed under various operating conditions. If
the period between removal was too long, the quality of ef-
fluent suffered. In addition, when the solids became too
108
-------
z
UJ
D
70 —
60 —
SO-
to
Q
_J
O
(0
O
UJ
O
z
UJ
0_
(O
D
(O
SO—
20 —
10
0
V
56 liters/min.
(ISgpm)
1 I T
23 liters/min.
(6gpm)
f
38 liters/mi n.
(lOqpm)
I I I I I
I i
100 200 300 400 5OO 600 700
SUSPENDED SOLIDS IN FEED, mg/llt«r
Figure 24. Settler effluent under solids input variations
109
-------
concentrated after too long a period between withdrawal, the
sludge header in the bottom became blocked with solids.
Since the primary concern was the removal of suspended solids
from the waste water, and not with the design of the bottom
of the tube settler, little factual data can be presented.
Sludge Concentration
The centrifuge employed in this demonstration was obtained
from Leon J. Barrett Company. This unit employs a contin-
uous-intermittent cycle. Waste is introduced into the bowl
for a preset time. The unit then continues to spin for a
second preset period. Finally a scoop removes the accumu-
lated sludge from the bowl, completing the cycle. The feed
rate becomes a fourth variable. The unit used has a maximum
hydraulic loading of 37.8 liter/min (10 GPM).
At a given hydraulic loading, the time of the feed determines
the total solids loading, and is perhaps the most significant
variable. During the feed period, the amount of solids re-
moved decreases as the sludge accumulates. With this centri-
fuge design, any excess of solids that cannot be retained
within the unit escapes into the centrate.
During the demonstration period, little effect was noticed
by variations in the length of the second step during which
the accumulated sludge in the bowl is dewatered. Variations
in the time required to removed the accumulated and dewatered
sludge from the bowl also did not produce any significant re-
sults in the concentration of sludge discharged.
110
-------
During the evaluations, various concentrations of solids in
the feed to the centrifuge were processed. At a given hy-
draulic loading, the other three machine variables were ad-
justed to produce what was considered to be the optimum con-
ditions for producing the most dense sludge. Comparison of
the sludge density before and after the centrifuge showed the
concentrating effect obtained. In feed ranges of 0,7 - 1.6
percent at 57.8 liters/min (10 GPM), the solids content in-
creased an average of 19 times (i.e., a feed at 0.71 percent
resulted in a sludge at 13-5 percent solids). When the hy-
draulic loading was changed to 18.9 liters/min (5 GPM), this
same comparison showed the solids content increase to average
20 times the feed concentration.
During this same period, measurements of solids content in
the centrate were made to determine the percent of solids in
the feed that were captured by the centrifuge. The control-
ling factor was demonstrated to be the hydraulic feed. At
the lower hydraulic loading, 50 percent of the solids in the
feed were captured; at the higher value, the average amount
captured was only ten percent of the feed solids. When the
hydraulic loading was kept at the lower volume and the dura-
tion of feed into the centrifuge was reduced, the percent of
solids captured increased to 56 •* 63 percent as the feed
time per cycle was steadily decreased. At the lower feed
time, the solids content in the sludge discharge decreased.
Ill
-------
DISCUSSION
The rectangular tubes used in this demonstration were ob-
tained from Neptune Microfloc Company. Other shapes are
available and may well produce results that vary from those
demonstrated.
All data presented on the settler were obtained without the
use of coagulant or polyelectrolyte addition agents, it is
felt that the proper application of these chemicals will
further enhance solids removal, especially for the smaller
particles approaching colloidal size.
Time did not permit evaluations of these two factors during
the demonstration period. As a result, the 37-8 liters/min
(10 GPM) flow rate has been selected for final settler
design. It is felt that further investigations will result
in further improvements. However, the effluent discharge
meets the current regulatory requirements.
The design of the sludge accumulation section in the bottom
of the tube settler determines the degree of concentration
obtainable. Important factors to be considered in this de-
sign include the frequency of solids withdrawal, the amount
removed at any given time, and, of course, the physical tech-
nique employed for withdrawal. From preliminary observations,
it is felt that the sludge accumulation section in the pro-
duction unit will be larger than that used in the test unit.
In this manner, there will be a smaller impact of the efflu-
ent quality as the sludge accumulates.
112
-------
With the centrifuge being used for sludge concentration, the
centrate is intermixed with the treated water and passed
through the settler for another pass. This recycle of solids
increases the settler feed concentration for levels consider-
ably higher than the normal rinse waters. Values as high as
1300 mg/1 were witnessed. Except when the hydraulic loading
exceeded 38.4 1/min (10 GPM), the settler effluent quality
improved at higher feed concentrations. With the feed at a
level of 600 to 675 mg/1, effluents in the 10-12 mg/1 range
were obtained. In the test at 1^00 mg/1, the effluent was
1 mg/1. These observations appear to support the advantages
of sludge recycling. Taylor and Jester38 describe their suc-
cess in sludge recycling at a brass mill treatment plant. It
is believed future developments in this concept will enhance
tube settler performance.
It is recommended to others considering the tube settler ap-
proach to removal of suspended solids that they use a pilot
operation on the actual waste to maximize the advantages of
the tube settlers in reducing capital and space requirements.
For efficient centrifuge operation, the initial consideration
should be the hydraulic loading. As shown, lower feed rates
show significant improvement in the amount of solids captured
by the centrifuge. With higher capture, it is believed a
greater concentrating effect can also be achieved. Caution
must be observed that the feed concentration is not too high
as the mechanism employed for removing the sludge from the
centrifuge used in the demonstration becomes plugged by the
113
-------
very dense sludge. It is believed that with the variables
available in operating the machine, a wide range of feed
concentration can be properly handled. At low concentra-
tions, multiple passes will result in the dense sludge de-
sired for land disposal. Since the centrate contains sig-
nificant solids concentration (even at 80 percent capture),
it is required that this waste water be returned to the set-
tler for recycle. Fortunately, settler studies show an im-
proved effluent quality with higher feed concentrations.
REFERENCE
51. Lancy, L. E. Upgrading Metal Finishing Facilities to
Reduce Pollution, Environmental Protection Agency
Technology Transfer Seminar Publication, U.S.
Government Printing Office, Washington, D. C.
Publication Number 1974 - 546-315:240, p. 35-38.
32. Taylor, T. H., and T. L. Jester. Industrial Waste
Treatment at Scovill Manufacturing Co., Waterbury,
Connecticut. (presented at the 28th Purdue Indus-
trial Waste Conference. LaFayette, Indiana.
May 2, 1973-) 16 p.
114
-------
SECTION IX
DISCUSSION
GENERAL
This section of the report provides a summary discussion of
the in-plant changes prompted by waste treatment practices,
the effectiveness and efficiencies of the chromium and cya-
nide systems, as well as a cost analysis of each, and of the
total treatment plant. In addition, guidance for others is
provided and a brief description of parallel electrolytic
treatment work done concurrently with this demonstration pro-
ject.
IN-PLANT CHANGES
In resolving the pollution problems initially presented,
many in-plant changes were prompted that not only affected
the waste treatment facility, but also the processing of parts
being plated.
Previously there were a variety of cleaning and pickling pro-
cesses available to accommodate all the needs of many and
varied customers. In order to reduce the complexity of the
pretreatment processing, these processes were reduced in
115
-------
number and those selected would successfully prepare the
greatest amount of production. In a few cases when customer's
work would not be satisfied by the resulting processes, the
work was refused.
Processing of aluminum alloys was never considered a signifi-
cant volume in respect to total plant production. Rather
than install isolation and collection piping, the work was
discontinued and process equipment removed. Process opera-
tions deleted included sulfuric acid anodizing and dyeing,
chromic acid anodizing, hard coating of aluminum and conver-
sion coating application. For the same reason, phosphating
of steel was discontinued.
As explained earlier, the water conservation practices
started earlier were accelerated to reduce the hydraulic
loading on the treatment facility. Initially the plant was
separated into process centers for cost control purposes.
Each process center was supplied with its own water distribu-
tion system. Past practices had been to hold the machine
operator (or process center lead operator) responsible for
production and quality at his process center. The operator's
job description was expanded to include concern for water
conservation. The extended counterflow rinse systems reduced
the amount of involvement for the operator, as well as water
consumption.
In addressing the optimum rinse water flow, flow rates were
selected after a variety of tests at higher and lower flows.
A minimum flow was determined that was consistent with
116
-------
quality objectives and solution life. A slightly higher
value was used to provide a safety margin. In some cases,
reductions of better than 70 percent were realized. In the
overall plant, attention to water conservation reduced the
hydraulic load by better than 30 percent in the raw waste.
This reduction in the amount of water purchased by New Eng-
land Plating Company resulted in a savings of $2867 per year.
The program for re-use of treated water is continually being
expanded after a thorough investigation at each location. At
the time of writing this report, this second class water is
being effectively used following cleaning and pickling steps,
as well as for maintenance cleaning. It is expected that
this program when fully implemented will reduce the amount of
plant discharge by an additional 15 percent through re-use of
30-35 percent of the treated water.
Additional attention is being given to solution life. With
the added cost of batch treatment, there is a greater eco-
nomic incentive to longer use of process solutions than
before pollution control went into effect.
During the demonstration program, more automatic processing
equipment was installed with a corresponding decrease in
hand processing. While the major reasons for the change to
automation were based upon normal business considerations,
waste treatment considerations did have an effect. In hand
operations, additional counterflow rinsing increases labor
costs. The chance of operator error in using the wrong
rinse tank is eliminated with automation. When a given hand
117
-------
operated process center was replaced by automatic machines,
the high cost associated with isolation of drain lines with
the replaced hand line was avoided and influenced financial
considerations.
CHROMIUM TREATMENT
In discussing the effectiveness and efficiencies of the chro-
mium treatment system at New England Plating, it must be
recognized that the electrochemical process and the equipment
employed should be separate and distinct subjects. The equip-
ment installed was sized to treat 120 mg/1 of hexavalent
chromium with an anticipated cell discharge of less than 0.2
mg/1. During the demonstration period, the average concen-
tration of hexavalent chromium in the normal rinse waters was
17 mg/1. As a result, the length of travel through the cells
was greater than required for this concentration. It is be-
lieved that this is the single most important factor contrib-
uting to the superior treatment achieved. With only 15 per-
cent of the anticipated hexavalent loading, it is understand-
able as to why the discharge was an order of magnitude lower
than anticipated. At higher feed concentrations (100-150
mg/1) the cell discharge was closer to the predicted value.
Control of the pH of raw waste entering the electrolytic
cells is essential for maintenance of the electrochemical
reduction process. At concentrations of hexavalent chromium
less than 50 mg/1, a pH of 1.8-2.1 is adequate. As the con-
centration is increased to values as high as 150 mg/1, lower
pH values in the range of 1.5 to 1.6 are preferred.
118
-------
With the electrolytic process, the amount of chromium reduced
is dependent upon pH, concentration of hexavalent chromium
and distance of travel through the bed at a given velocity.
Most of the data was obtained in the narrow pH range of
1.8-2.1. From the limited data available, it is believed
that lower pH values will result in greater reduction rates .
If the extent of reduction (mg/1 reduced per meter of travel)
is plotted against the initial hexavalent chromium concentra-
tion, a curve as shown in Figure 25 is obtained.
The limited experiments directed towards removal rate varia-
tions with applied power variations indicated that after a
threshold voltage is reached, no significant improvement is
obtained with higher voltages. From a process standpoint,
the voltage depends upon the spacing between major elec-
trodes. The distance in these specific cells is 15 cm
(6 in.). With a greater distance, it is expected that a
higher voltage would be required. The demonstration cells
under normal operating conditions used 7.5 v. When higher
chromium concentrations were being processed, this minimum
voltage was 8.0 v.
In considering the efficiency and effectiveness of the elec-
trolytic process itself, it is believed that a very stable
process exists that should be considered for other applica-
tions. Initial results can be misleading because the adsorp-
tion effect will show extremely high removal rates. However,
after this initial period, a steady state will exist for
reduction of hexavalent chromium.
119
-------
UJ
UJ
40
80
120 160 200
IN FEED, mg/liter
240
Figure 25.
Extension of chromium reduction with initial
concentration variations
(at velocity of 0.325 m/min)
120
-------
In order to consistently maintain a low level of discharge,
it is important to use a contact time adequate for the high-
est feed concentration and not the average concentration.
Power consumption varies with the chromium concentration in
the waste being treated. For a 75 1/min (20 GPM) flow at
17 mg/1, 1.4 kw of power was consumed. At the same flow
rate but at 156 mg/1, 2.0 kw of power was used. This results
in these cells reducing nine times the amount of chromium
with only a 40 percent increase in power. Figure 2b shows
the results obtained during the demonstration for the amount
of chromium reduced per kilowatt hour as a function of hexa-
valent chromium loading.
The pH adjustment step following the electrolytic reduction
process proved effective in trivalent chromium hydroxide
formation. The performance was considered typical for com-
parable processes employed in this industry.
Equipment Costs
The equipment costs provided in Table 9 represent purchase
prices to New England Plating factored for an average flow
rate of 75 1/min (20 GPM). It should be recognized that this
equipment was purchased and installed in 1971- Price adjust-
ment undoubtedly would influence the capital picture today,
whether caused by inflation or by advances in cell construc-
tion technology.
It is estimated that equipment that would normally be se-
lected for bisulfite reduction if used in place of the elec-
trochemical process would cost the amount shown in Table 10.
121
-------
M
c
»-!
ro
ho
O D* O
o o rr
D C *-t
one
rt> 3
rr en ^
i-! 3
03 rr
rr n> i-<
i_j, v r*
i /s i ^
O &2 C-
P < C
(-• re
ro a.
re
O
h1-
I
PC
h"
i—1
O
CD
0.4
O
I 0.3
0.2
rfl
O
O
O.I
0.88
— K>.66
CT
O
— 0.44
).22
20
40
60
80
100
120
140
160
[o+6J IN FEED,mq/liter
-------
Table 9- ELECTROCHEMICAL REDUCTION SYSTEM EQUIPMENT COSTS
pH control system (raw waste) $ 1,400
Surge tank 1,825
Transfer pump 9^5
Filters (2) 510
Electrochemical cells (2) 18,900
Rectifier 450
pH control system (reduced waste) 2,100
Electrical controls 225
Total Equipment $26,175
Installation costs
(rigging, plumbing, electrical)
electrochemical system $ 3>800
precipitation system 1,075
Total installation cost & 4,875
Total installed cost $30»050
123
-------
Table 10. CHEMICAL REDUCTION SYSTEM EQUIPMENT COSTS
Reduction tank $ 1,900
pH control system (reduction step) 1,4-00
ORP control system 1,400
Agitator 1,000
pH control system (reduced waste) 2,100
Electrical controls 225
Total equipment $ 8,025
Installation costs
(rigging, plumbing, electrical)
reduction step $ 1,075
precipitation step 1,075
Total installation cost
Total installed cost
124
-------
Operating Costs
Operating costs are provided for both the electrochemical
(Table 11) and the chemical (Table 12) reduction processes.
Figures are provided at two chromium concentrations in the
raw waste. Data given are on an annual basis with 16 hr/day
of production (4000 hr/yr) for a 75 1/min (20 GPM) flow.
Operating costs shown for the chemical reduction step on
dilute waste were obtained from operating history. Figures
shown for the high chromium level are calculated by adding
the theoretical amount required to lower the chromium level
to the lower point of the operating data for the lower level.
Cost Comparison
Direct comparison between the two operating modes shows that
at the low concentrations of hexavalent chromium, there is no
significant operating cost advantage of one system over the
other. However, at higher concentrations, the economic ad-
vantage of the electrochemical process would pay for the ad-
ditional capital investment in four to five years.
At New England Plating, treatment of 450 kg (1000 Ib) of
chromic acid from spent chromate solutions in the rinse water
system will permit greater utilization of the electrochemical
system and avoid chemical costs and labor associated with
batch reduction.
Other Considerations
In drawing too significant a cost comparison, one must remem-
ber that the New England region has higher than national
125
-------
Table 11. OPERATING COSTS FOR ELECTROCHEMICAL REDUCTION & PRECIPITATION
(annual basis)
Item
Sulfuric acid
Carbon bed replacement
Sodium hydroxide
Sodium hydrosulf ite
Electricity, cell use
Electricity, other use
Replacement parts
anodes for cells
filter cartridges
miscellaneous
Labor
operating, reduction
step
operating, precipi-
tation step
maintenance , reduc-
tion step
maintenance, precip-
itation step
Total direct cost to o
[Cr-1-6] at 17 mg/1
amount cost
33^0 kg (7350 Ib)
218 kg (480 Ib)
3720 kg (8200 Ib)
7 kg (15 Ib)
5500 kwh
14,700 kwh
100 hr
100 hr
70 hr
25 hr
perate
$ 440
480
1560
15
175
468
640
297
246
350
350
280
100
$5401
[Cr*6] at 156 mg/1
amount cost
3340 kg (7350 Ib}
241 kg (530 Ib)
9530 kg (21,000 lb)a
7 kg (15 Ib)
8000 kwh
14,700 kwh
100 hr
200 hra
70 hr
25 hr
$ 440
530
4000
15
254
468
765a
297
246
350
700
280
100
$8445
NJ
At 75 1/min (20 GPM) $17.20/kg Cr (*7.82/lg) $2.94/kg Cr ($1.33/lb)
Estimates
-------
Table 12. OPERATING COSTS FOR CHEMICAL REDUCTION AND PRECIPITATION
(annual basis)
Item
Sulfuric acid
Sodium bisulfite,
anhydrous
Sodium hydroxide
Electricity
Replacement parts
Labor ,
operating, reduction
step
operating, precipi-
tation step
maintenance, reduc-
tion step
maintenance, precip-
itation step
Total direct cost to
operate
[Cr^l at 17 mg/
amount
2870 kg (6325 lb)
1850 kg (4075 lb)
3720 kg (8200 lb)
18,500 kwh
170 hr
100 hr
50 hr
25 hr
'1
cost
$ 380
895
1560
588
196
595
350
200
100
$4864
[Cr*6] at 156 mg
amount
10,200 kg (22,500 lb)
8800 kg (19,400 lb)
9530 kg (21,000 lb)
18,500 kwh
450 hr
200 hr
50 hr
25 hr
/I
cost
$ 1350
4270
4000
588
196
1575
700
200
100
$12,979
At 75 1/min (20 GPM) $15-50/kg Cr ($7.05/lb) $4.50/kg Cr ($2.05/lb)
-------
averages for power costs. It should also be considered that
the power costs shown represent prices that reflect upon the
"fuel crisis" under which the utilities had been permitted to
increase their rates to consumers. The prices shown for chem-
icals represent prices that had only recently been released
from Federal price control and do not necessarily reflect
upon the anticipated price increases expected by economists.
An advantage of the electrochemical treatment system that may
readily be overlooked is the impact upon the total dissolved
solids in any given installation. The use of derivatives of
S02, or for that matter other reducing chemicals, introduces
additional dissolved solids to the waste water. The bisul-
fite used in the demonstration increased the sulfate concen-
tration in the final effluent. When the electrolytic process
is employed, less sulfate is discharged to the environment.
CYANIDE TREATMENT
In discussing the effectiveness and efficiencies of the cya-
nide treatment system at this installation, it is also impor-
tant (as with chromium treatment) to consider the electro-
chemical process separate from the specific equipment em-
ployed.
The equipment installed was sized to treat 50 mg/1. With an
anticipated efficiency of 90 - 95 percent, the cells would
have discharged less than 2 mg/1. During the demonstration
period the average concentration of cyanides amenable to
chlorination was 80 mg/1. The average concentration of total
128
-------
cyanides was 110 rag/1. The average removal was 52 mg/1.
While more cyanide was removed electrolytically than had
been anticipated, these cells had an overall removal effi-
ciency of 65 percent when 40 volts was applied.
These cells have an effective length of solution contact that
is 2.5 m (100 in.). Samples withdrawn at intermediate points
along this contact route show a straight line relationship
for removal of cyanide (except at very low concentrations).
At 20 percent of the distance of travel (0.5 m), 20 percent
of the cyanide being removed was removed. At 60 percent of
the distance, 60 percent was removed. Based upon these ob-
servations, it is believed that 20 mg/1 is being removed for
each meter of travel (0.5 mg/1 per in.). Had cells been
selected that had a length of travel of four meters, the
average discharge from the cells would have been closer to
the design concept of 2 mg/1. The distance between major
electrodes in these cells is 15 cm (6 in.). Previous work
with other cyanide solutions had indicated a potential be-
tween these electrodes of 12-14 volts would be adequate.
However, the work done during the demonstration shows that
with cyanide rinses that are predominately from zinc plating,
higher potentials are required.
In considering the efficiency and effectiveness of the pro-
cess itself and taking into account the cell design consider-
ations just covered, it is believed that a stable process
exists. Initial results can be misleading because the
adsorption effect will show extremely high removal rates.
129
-------
However, after this initial period, a steady state will exist
for removal of cyanide.
In order to consistently maintain a low level of discharge,
it is important to use a contact time adequate for the high-
est feed concentration. At this facility, with concentra-
tions at 140 mg/1 for 10 percent of the time, it is believed
that a bed length of 7~8 meters would be required. This
would be roughly twice the size of that required for the
average concentration in the feed. The equipment costs for
maximum loading would be higher than for average loading. If
this route is selected, then the process will exhibit an even
greater stability since the last portion of the bed will nor-
mally only receive very low concentrations and the adsorption
role would probably play a dominant role for a long period of
time.
When this demonstration project was initiated, power costs
were approximately one-third of that existing at the end of
the project. Initial considerations showed a significant
cost advantage in using the electrolytic process over the
chlorination process. During the investigation it was estab-
lished that the amount of power required was two to three
times that expected. This discovery, together with the in-
creased power costs, resulted in an economic picture signifi-
cantly different than that initially predicted. In reviewing
the removal rate variations, demonstrated to be affected by
applied voltage, one can establish an optimum trade-off
between power costs for electrochemical removal rates and
130
-------
hypochlorite costs. During the demonstration, 40 v. was
used. A slightly lower value would have been more economi-
cal. Hypochlorite was in short supply and price increases
were anticipated. For this reason, the higher voltage was
selected to establish data over a long period.
The chlorination system installed following the electrochemi-
cal treatment step proved effective when adequate excess hy-
pochlorite was maintained.
For the total cyanide system, effective and efficient treat-
ment of cyanides amenable to chlorination was achieved. Nor-
mally one would expect those cyanides represented by the dif-
ference between total cyanide and amenable cyanides to show
an inferior removal rate. In the average conditions during
the demonstration this difference is JO mg/1 (CN @ 110 mg/1
and CN @ 80 mg/1) in the raw waste. These more refractory
cl
cyanides were partially removed. While the total system,
i.e., electrochemical/chlorination, removed more of these
tightly complexed cyanides than anticipated, time did not
permit thorough studies of the treatment of iron and nickel
cyanide complexes.
Equipment Costs
The equipment costs provided in Table 15 represent purchased
prices to New England Plating factored for an average flow
rate of 75 1/min (20 GPM). It should be recognized that this
equipment was purchased and installed in 1971- Price adjust-
ments undoubtedly would influence the capital picture today,
131
-------
whether they be caused by inflation or by advances in cell
construction technology,
Table 13. ELECTROCHEMICAL & CHEMICAL TREATMENT
EQUIPMENT COSTS FOR CYANIDE REMOVAL
Surge tank $ 1,500
Transfer pump 9^5
Filters (2) 310
Electrochemical cells (2) 18,900
Rectifier 1,500
Chlorination equipment 2>200
Electrical controls 225
Total equipment $26,bOO
Installation costs
(rigging, plumbing, electrical)
electrochemical system $ 3,600
chlorination 1.175
Total installation costs $ ^,775
Total installed cost $31,11
132'
-------
Operating Costs
Operating costs are provided for both the electrochemical/
chemical operating mode and the chlorination mode alone
(Tables 14 and 15, respectively). Figures given are on an
annual basis with 16 hr/day of production (4000 hr/yr) for a
75 1/min (20 GPM) flow.
Cost Comparison
Direct comparison between the two operating modes shows a
cost reduction of $1,855 per year with electrolysis in use.
However, it must be remembered that the electrochemical sys-
tem shows evidence of complete oxidation to carbon dioxide
and nitrogen, while the chemical oxidation step employed is
known to only oxidize cyanide to cyanate. Had the two-step
chlorination process been employed to oxidize the cyanide to
carbon dioxide and nitrogen, the direct costs have been cal-
culated to increase by $16,675 for chemical oxidation and
$7,185 for the electrochemical/chemical mode. Under these
circumstances, the cost advantage of the electrolytic process
increases to approximately $11,000 per year.
Other Considerations
In drawing cost comparisons, one must remember that New Eng-
land has a higher than national average cost for electricity.
It must also be remembered that the power costs shown repre-
sent prices that reflect upon the "fuel crisis" under which
the utilities had been permitted to increase their rates to
consumers. The prices shown for chemicals represent prices
133
-------
Table 14. OPERATING COSTS
FOR ELECTROCHEMICAL/CHEMICAL OXIDATION
(at 65$ by electrolysis, 35# by chlorination
on annual basis)
Item Amount used Cost
Sodium hypochlorite 27,0001 (7200 gal.) $3600
Carbon bed replacement 218 kg (480 Ib) 480
Electricity, cell use 60,000 kwh 1910
Electricity, other use 10,000 kwh 320
Replacement parts,
anodes for cell 240
filter cartridges 400
miscellaneous 130
Labor,
operating 100 hr 350
maintenance 60 hr 240
Total direct cost to operate $7^70
At average conditions of 102 mg/1 CN removed from a
75 1/min (20 GPM) flow:
$4.l4/kg CN or $1.83/lb CN
134
-------
Table 15. OPERATING COSTS FOR CHEMICAL OXIDATION
(annual basis)
Item Amount Cost
Sodium hypochlorite 66,0001 (l?,600gal.) $8800
Electricity 5500 kwh 175
Replacement parts 60
Labor ,
operating 100 hr 350
maintenance 35 hr
Total direct cost to operate $9525
At average conditions of 102 mg/1 CN removed from a 75 1/min
(20 GPM) flow:
$5.l4/kg CN or $2.33/lb CN
135
-------
that had only recently been released from Federal price con-
trol and do not necessarily reflect upon the price increases
expected.
One advantage of the electrochemical treatment process not
covered earlier is concerned with the impact of this system
on the total dissolved solids content. By replacing chlori-
nation with electrolytic treatment, a significant reduction
in chloride can be expected in the final effluent.
TOTAL WASTE TREATMENT PLANT
In discussing the overall effectiveness of the waste treat-
ment plant prior to solids removal, comments are being
offered on analytical data coming from filtered samples so as
to remove from any issues those contaminants that might re-
main after solids removal. In this manner, the electrochemi-
cal and chemical treatment systems themselves may be properly
discussed.
A comparison of the initial raw water contamination level
prior to the installation of the waste abatement system and
the final discharge during the study period, as well as the
relationships to production, is provided in Table 16.
Analysis of this data shows the superior results of the
treatment facility. For cyanides amenable to chlorination
and hexavalent chromium effluent analysis consistently is
lower than the originally anticipated effluent. For cyanides
not amenable to chlorination, higher than desired values are
seen. However, it is believed the low quantity (8.3 mg/1)
136
-------
Table 16. MAJOR CONTAMINANTS BEFORE AND AFTER TREATMENT
GO
Cyanides,
amenable
Cyanides,
Chromium,
hexavalent
Chromium,
trivalent
Copper
Zinc
Nickel
Iron
Solids, total
dissolved
Solids, ,
suspended
Before at 757 1/min (200 GPM)
mg/1
4
otal NA
24
it
11
0.5
0.2
4
2
al 1036
rag/m2*
920
5,500
2,520
112
44
900
460
238,000
lb/10eft*a
188
1,125
516
23
9
188
94
48,600
After at 274 1/min (75 GPM)
mg/1
0.02
%:&
0.38
0.11
0.5
0.3
0.8
1422
mg/m2
1.61
660 n
3.18
30.3
8.8
40.1
24.0
64.5
113,500
Ib/106ft2a
0.33
1350.65
6.2
1.8
8.2
4.9
13-2
23,200
46
10,500
2,160
159
12,700
2,590
a
related to production volume
b prior to filtration
Note: With suspended solids removal to 20 mg/1, contamination of fully
treated effluent as related to production volume becomes 1,597
mg/m2 or 326 lb/106ft2.
-------
will have little effect upon the environment. Any attempt to
lower this material must rest with reduction of quantity at
the source, namely in the zinc and copper plating solutions
themselves. The treatment facility will do no more than is
presently being done.
In regard to the total dissolved solids, the concentration in
the discharge has increased. With the reduction in volume
that has taken place, there is better than a 50 percent re-
duction in the TDS when the total environment is considered.
It should also be recognized that of the IDS, at least 37
percent comes from the waste treatment chemicals themselves,
and not from the production facility.
The sludges generated by the waste treatment processes will
ultimately be hauled away by a State licensed contractor for
disposal in an appropriate landfill. The metal hydroxides in
the sludge will not pose a threat to the environment when
handled in this manner.
Of the products produced by the treatment facility, only the
treated water is considered recoverable. The various metal
hydroxides are intermixed and of little interest for salvage
due to the low volume and high expense of separation. Only
partial recovery of water is anticipated. The high dissolved
solids content in the treated water limits its use in the
production facility.
Capital Costs for Total Plant
Those items normally considered to be capitalized are summa-
rized in Table 1?.
138
-------
Table 17'. CAPITAL COSTS FOR TOTAL PLANT
System Equipment Installation
Isolation and collection $ 13,312 $25,900
Cyanide rinses 26,600 4,775
Chromium rinses 26,175 4,875
Non-CN/Cr rinses 2,700 1,325
Batch 11,3^8 4,300
Solids handling 29,790 7,150
Miscellaneous 1,500 300
Total $111,425 $48,625
Total equipment $111,425
Total installation 48,625
Waste treatment building 23,047
Engineering fees 18,000
Total capital cost
Operating Expense for Total Plant
The annual operating expense for the waste treatment facility
is shown in Table 18. The costs shown for the solids hand-
ling are those projected and are based upon the data obtained
from pilot runs.
Waste Treatment Costs Relative to Production
Waste treatment costs discussed earlier have been provided
as an aid to others designing and selecting specific types
139
-------
Table 18. DIRECT OPERATING EXPENSE FOR TOTAL TREATMENT PLANT
System
Chromium
rinse
Cyanide
rinse
Non-Cr/CN
rinse
Batch
Solids
handling*
chemicals
$ 2,495
4,080
389
4,935
250
(per
electricity
$ 643
2,230
162
1,700
225
year)
parts
$1,183
770
50
125
150
labor-
oper.
$ 700
350
210
1,138
1,400
labor-
main.
$ 380
240
80
225
225
total
$ 5,401
7,670
891
8,123
2,250
Total $12,149 $4,960 $2,278 $3,798 $1,150 $24,335
^estimated
-------
of treatment. It is also believed that the total expenses
incurred at this facility should be related to production
volume. In a captive shop, it is easy to relate the waste
treatment expenses to the specific items being manufactured.
In a diversified job shop, serving many different customers
with a wide variety of parts, the only suitable relationship
can be to one that is relative to added value, or dollar
volume of production. A summary of waste treatment costs on
an annual basis is given in Table 19-
Table 19. SUMMARY OF WASTE TREATMENT COSTS
(annual basis)
Labor $ 4,9^8
Purchases 19,38?
Applied overhead 9^01
Capital depreciation* 36,532
Total annual cost $70,268
^depreciation of non-building capital is
5-year, straight line and for building
capital, 25-year, straight line.
This value is considered total annual expense. Since a por-
tion of the treated water is essentially reclaimed water, a
net operating expense is obtained by deducting from the total
annual expense, the value of re-used treated water. At New
England Plating, it is projected that 30-35 percent of the
treated water will be re-used, with a savings of 22.6 x 10s
141
-------
1/yr (6.0 x 106 gal./yr). At the present cost of water in
Worcester, Massachusetts, a salvage value of $1600 is antici-
pated. This will result in a net annual expense of $68,668.
Based upon current business dollar volume, the total cost of
waste treatment represents 6.4 percent of sales and the net
cost is 6.25 percent of sales.
PARALLEL WORK WITH ELECTROLYSIS
During the demonstration project, there were considerable
hardware and process developments, as well as several other
applications using the same basic semi-conductive bed electro-
chemical technology. A brief description of the more signi-
ficant aspects of this parallel work is being offered as
additional information. These details relate to cell design,
applications within the Metal Finishing Industry, and uses
in industries having similar pollution problems.
Cell Design
The cells used in this demonstration are described in detail
in Section VI. These cells use electrodes as baffles, creat-
ing an upward and downward directed flow through the bed.
Each bay (or compartment) represents 20 percent of the total
bed length.
At low levels of contamination (below 5 mg/1), the required
contact time within the bed becomes much lower. Cells have
been employed which have the same general anode to cathode
spacing and bed width as in the demonstration units, with the
142
-------
exception that the bed length is shortened to the point that
only upflow is used. These shortened cells have proved ef-
fective in reclamation of precious metals where economics
dictate low level contamination. Lower capital cost per unit
volume of hydraulic loading is realized by this alteration in
cell design.
A logical extension of the single upflow cell just described
is the circular cell. These cells use concentric electrodes
with one of the major electrodes being the outer shell (or an
electrode in close contact with a conforming shell) and the
other electrode being centered within this shell. The semi-
conductive bed is placed between the electrodes and the flow
of liquid is parallel to the electrodes.
A thorough description of this cell configuration is provided
by Franco and Balouskus.33 Their major effort was directed
towards iron reduction under an EPA demonstration grant. The
data presented shows the influence of the area relationships
of the major electrodes. When reduction is the desired reac-
tion, the outer electrode is preferred to be the cathode. It
is believed that when the polarity is reversed so that the
center electrode (with its smaller surface area) becomes the
cathode and the outer electrode (with its larger surface area)
becomes the anode, then the cell will exhibit improved oxida-
tion properties. Cells using this basic concept have been
evaluated on hexavalent chromium bearing waste water and have
received limited market acceptance.
143
-------
The cells demonstrated use a bed that is totally comprised of
particles that are conductive and which exhibit the bipolar
effect described earlier. Lancy34 presents a modified cell in
which there is intentional intent to separate the conductive
particles with inert, non-conductive particles. It is be-
lieved that this alteration in cell design produces a greater
potential between the conductive particles. Evidence to date
from pilot plant applications indicates that the more refrac-
tory cyanide complexes (iron and nickel) are oxidized in
this modified cell.
Metal Finishing Applications
The demonstration project addressed primarily the complete re-
duction of hexavalent chromium and the partial oxidation of
cyanides coming from zinc plating. The amount of cyanide from
copper and cadmium plating played a very minor role (less than
5 percent each). Other production experiences within the
Metal Finishing Industry where this technology has been em-
ployed are worthy of comment.
In one job shop, cells comparable to those demonstrated are
employed prior to a complete hexavalent chromium chemical re-
duction waste treatment installation to reduce the amount of
chemical consumed in the chemical plant. Reduction in chemi-
cals consumed in the range of 50-90 percent have been real-
ized. In another job shop where the primary cyanide bearing
waste is from copper plating, greater cell efficiencies have
been realized. In this particular application, the normal
cyanide level is less than one-fourth that experienced in
this present demonstration. With the same contact time in
the bed, all cyanide is oxidized and the copper content in the
144
-------
effluent is less than 0.5 mg/1. At a mid-western manufacturer
of builders hardware, a cell similar to that demonstrated in
this project is employed on rinse water following brass
plating. In this case, the contamination level is in the
same range as at New England Plating Company. Cell effi-
ciencies greater than 99 percent have been continuously demon-
strated. In these two applications, the applied voltage is
in the range of 12-14 volts as opposed to the 40 volts re-
quired for the demonstrated waste stream with zinc cyanide.
It should be noted that at this latter facility only cyanides
measured by the silver nitrate titration technique are moni-
tored. The results of treatment relative to the iron cyanide
complex known to exist in this waste water cannot be
reported.
Rectangular cells similar to those employed at New England
Plating and single upflow cells described earlier have been
used in production applications for gold reclamation. In
the case of the longer cell contact time, the gold is com-
pletely retained within the cell and the cyanide (less than
3 mg/1) is oxidized. In the cases employing the lower con-
tact time, attention is directed solely to gold reclamation.
In these applications, the values attributed to the gold re-
claimed from the bed provides an entirely different economic
incentive than when only waste treatment is considered.
In the application of conversion coatings to aluminum, a com-
mon additive to the hexavalent chromium containing process
solution is sodium ferrocyanide. This additive enhances the
145
-------
gold color desired by consumers. A production installation
using a cell as demonstrated is used for simultaneous reduc-
tion of hexavalent chromium and removal of the ferrocyanide.
Contamination levels are comparable to those at New England
Plating for hexavalent chromium and approximately 3-5 mg/1
for the cyanide.
The modified cell described by Lancy31* has received extensive
evaluation in pilot plant sized equipment. In one applica-
tion, a solution of sodium chloride is recirculated in a man-
ner similar to the well-known Lancy Integrated System. As
the chloride solution passes through the modified cell,
sodium hypochlorite is produced. As cyanide is introduced
into this closed loop system (simulating dragout on work
pieces), the cyanide is oxidized with the hypochlorite to
produce cyanate and chloride. The chloride is again avail-
able for re-oxidation to hypochlorite. It is expected that
operating economies will be realized and that lower chloride
contamination will be available when this application is
fully developed.
In the other application of this modified cell, attention is
being directed towards oxidation of cyanide complexes experi-
enced in the immersion stripping of nickel from steel. Pilot
plant efforts indicate that this low temperature process
effectively oxidizes this more refractory complex. It is
expected that production experience in this use will be ob-
tained in Europe during 197*1-.
146
-------
Applications Outside of Metal Finishing
Several applications of the same basic semi-conductive bed
technology have met with positive results in fields outside
of metal finishing that are similar to problems within this
industry.
In a copper rolling mill, rinse water following sulfuric acid
pickling contains approximately 35 mg/1 dissolved copper at a
pH of 2. As this waste passes through the bed, copper is
deposited at cathode sites. The cell discharge is virtually
copper free (less than 0.5 mg/1). Subsequent pH adjustment
to neutralize the acid is provided. Due to the low copper
content in the cell discharge, no copper hydroxide is pro-
duced. The need for solids separation and sludge disposal is
eliminated by extracting the copper as copper metal.
Ferrous iron common to acid mine drainage has been oxidized
to ferric iron in round cells. The sludge stability problem
attendant with ferrous hydroxide is eliminated. These are
indications that a more dense sludge is formed by the elec-
trolytic process than that obtained by air oxidation. De-
tails of this particular demonstration project are provided
by Franco and Balouskus.3*
Rectangular cells are being used to collect trace quantities
of platinum group metals in a refinery. The waste water is
low in pH (about 0.5). The value of the metals collected on
the carbon bed provides economic incentives for using the
technology. Considerations are being given to using the
147
-------
process in place of the cementation process currently being
used prior to the electrochemical process.
The same modified cell described earlier in work being done
on nickel cyanide strip is being used for oxidizing cyanide
heat treating salts. In this application in West Germany,
the primary concern is for removal of ferrocyanides.
GUIDANCE FOR OTHERS
Should others faced with resolution of pollution problems
wish to consider application of the technologies being re-
ported for this demonstration project, it is suggested they
consider the following comments.
Water Conservation
The use of counterflow rinses provides significant reduction
in water use. The extended counterflow expands upon this
concept. In using the extended counterflow technique in
cleaning cycles, one must consider the type of cleaner in
use. Heavily silicated cleaners may produce a precipitate on
the surface of the work pieces when the cleaner film is acid-
ified. This silicate film may require the addition of fluor-
ide salts to the pickle so that they will redissolve in the
pickle.
Good mixing is required. Introduce the incoming water at the
bottom of the rinse tank and overflow the top. Existing
facilities may not have sufficient hydraulic head between
rinse tanks which were not designed for counterflow rinsing
148
-------
to permit the use of small diameter piping. At New England
Plating, J.6 cm (3 in.) diameter pipe permitted the desired
flows to be obtained.
Virtually all of the evaluations of this extended counterflow
rinsing approach to the plating solution itself has been done
on zinc and cadmium plating. With these two process solu-
tions, immersion deposition of metals is not experienced.
Caution must be taken when other metals are being processed,
and a thorough evaluation is recommended by others looking
for the advantages of extended counterflow rinsing.
In the re-use of treated water (with or without suspended
solids removal) one must consider subsequent processing.
With further cleaning operations, as in this demonstration
project, preliminary cleanliness is not essential. Obviously,
treated water may be used for maintenance cleaning. It
should not be considered for applications where solids (dis-
solved or suspended) present problems as in the addition to
plating tanks and final hot water rinses.
In all efforts directed towards water conservation, a pre-
requisite is a thorough knowledge of the various metal finish-
ing operations employed.
Electrochemical Reduction of Chromium
In considering the application of electrolytic reduction of
hexavalent chromium, the maximum contamination level must be
established prior to selection of equipment size to produce
the desired discharge level.
149
-------
Purveyors of equipment for this application will be able to
answer questions as to the tolerance level of their cells.
It is believed because of the interest established in this
technology that many of the relatively high costs associated
with capital and operating expense will be resolved.
Where the process is used in cost reduction efforts prior to
a chemical treatment system, it is obvious that less demands
will be placed upon the cell design.
Control of raw waste pH is essential to the successful appli-
cation of this process. The maximum pH selected will depend
upon the equipment itself, the concentration of hexavalent
chromium and the degree of removal required.
Electrochemical Oxidation of Cyanide
Greater caution is required in the application of the electro-
lytic process to cyanide bearing waste. The role of metals
normally associated with cyanide complexes in the metal
finishing industry is not fully understood when electrolysis
is employed.
In using this technology one must consider the hardness of
rinse water. Production experience has shown that when the
rinse water is "hard,1' calcium carbonate is produced within
the cell and precipitates from solution due to its low solu-
bility. In hard water areas, water softening is an essential
prerequisite.
Oil should be kept from entering system to prolong carbon
life.
150
-------
As with using the process in reduction applications, the
equipment suppliers should be consulted in each application
to avoid misapplication. Today's negative features may well
have been overcome for many uses.
Suspended Solids Removal
The advantages of the "tube" settler in reducing capital
costs and space requirements have been demonstrated on treated
plating waste water. Several purveyors of this type of
equipment exist, each having their own unique features.
Sludge recycling appears to show a significant improvement in
the clarity of effluent.
Sludge Concentration
A centrifuge provides a small compact tool for the production
of thickened sludge. It is believed that less labor is re-
quired than with filtrations.
Multiple passes can be employed to reach the desired solids
concentration.
REFERENCES
33. Franco, N. B., and R. A. Balouskus. Electrochemical
Removal of Heavy Metals from Acid Mine Drainage. Envir-
onmental Protection Agency. Cincinnati, Ohio 45268.
Publication Number 670/2-74-023. May 1974. 87 p.
34. U.S. Patent Number 3,719,570. Modified Electrochemical
Cell.
151
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-75-028
3. RECIPIENT'S ACCESSIOI»NO.
4. TITLE ANDSUBTITLE
ELECTROLYTIC TREATMENT OF JOB SHOP METAL
FINISHING WASTEWATER
5. REPORT DATE
September 1975 (Issuing Date)
6. PE'RFOFiMMSIG ORGANIZATION CODE
T. AUTHOR(S)
Bruce E. Warner
8. PERFORMING ORGANIZATION REPORT NO,
9. PERFORMING ORGANIZATION NAME AND ADDRESS
New England Plating Company, Inc.
Worcester, Massachusetts 01605
10. PROGRAM ELEMENT NO.
1BB056;ROAP 21AZO;Task 15
11.1
12010 GUG
1 NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Full scale in-plant production studies demonstrated the reliability and
economics of electrolytic cells containing beds of conductive particles
between cathodes and anodes for reduction of hexavalent chromium and
oxidation of cyanide in plating rinse water. The heavy metals are
subsequently removed from the waste water by alkali precipitation.
Seventy-five liter/min. (20 GPM) sized cells
and cyanide rinses. Chromium concentrations
concentrations to 150 mg/liter were processed.
parallel equipment using chemical treatment for
chromium concentrations (less than 25 mg/liter)
more economical. At higher concentrations (150
reduction became the more economical process.
the electrochemical process reduced direct costs associated with chlori-
nation. Waste treatment costs, capital and operating, for the job shop
are provided with an assessment of total costs on the price of services
provided. Water conservation techniques are described. Experiences
with tube settling equipment for removal of suspended solids and
centrifuge for sludge concentration is provided.
were employed for chromium
to 250 mg/liter and cyanide
Data were obtained with
cost comparison. At low
, chemical reduction was
mg.liter), electrolytic
In cyanide oxidation,
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
*Metal finishing, Plating, Chlorination,
*Electroplating,*Electrolytic cells
Electrochemical treatment;
Chemical treatment,
Wastewater treatment,
Pollution control, *Semi-
conductive bed electroly-
sis, Chromium reduction,
Cyanide oxidation
13B
B. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19, SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
162
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
152
aUSGPO: 1975 - 657-695/5313 Region 5-11
------- |