v* eo sfy
Environmental Protection Agency
Technology Transfer Program
UPGRADING METAL FINISHING FACILITIES
TO REDUCE POLLUTION
METAL FINISHING WASTE TREATMENT
SEMINAR SERIES FOR
POLLUTION CONTROL
NEW YORK, NEW YORK
DECEMBER 12, 13, 1972
LANCY LABORATORIES DIVISION
DART INDUSTRIES, INC., CHEMICAL GROUP
CONSULTING ENGINEERS
ZELIENOPLE, PENNSYLVANIA
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UPGRADING METAL FINISHING FACILITIES
TO REDUCE POLLUTION
METAL FINISHING WASTE TREATMENT
TECHNOLOGY TRANSFER SEMINAR
NEW YORK, NEW YORK
DECEMBER 12 and 13, 1972
PREPARED FOR THE
ENVIRONMENTAL PROTECTION AGENCY
by
DR. L. E. LANCY, PRESIDENT
and
MR. R. L. RICE, P. E., EXEC. VICE PRESIDENT
LANCY LABORATORIES
DIVISION OF DART INDUSTRIES, INC.
ZELIENOPLE, PENNSYLVANIA, 16063
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TABLE OF CONTENTS
Page No.
THE NEED FOR WASTE TREATMENT 1
ENGINEERING CONSIDERATIONS 3
SOURCES OF WASTE . 5
TYPICAL METAL FINISHING PROCESSES, REQUIRING
IN-PLANT OR WASTE TREATMENT CONTROL 12
COMMONLY USED WASTE TREATMENT SYSTEMS 18
Batch Treatment 18
Continuous Treatment 27
Integrated Treatment 31
Ion Exchange Treatment 34
PROCESS SOLUTION REGENERATION AND RECOVERY
METAL RECOVERY 42
Ion Exchange 42
Evaporative Recovery 43
Reverse Osmosis 46
METAL RECOVERY WITH THE "INTEGRATED WASTE
TREATMENT" SYSTEM 47
ACID PICKLING AND ETCH SOLUTION REGENERATION BY
CRYSTALLIZATION OR ELECTROLYTIC CONTINUOUS
MAINTENANCE PROGRAMS 48
ECONOMIC CONSIDERATIONS 50
REFERENCES 53
APPENDICES
TABLE I - COMMON WASTE DISCHARGES DUE
TO ACCIDENTS IN METAL FINISHING
PLANTS 54
TABLE II - COMPARISON TREATMENT TOTAL COST 55
LIST OF FIGURES 56
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METAL FINISHING WASTE TREATMENT ENGINEERING
The Need for Waste Treatment
The processing of metals in manufacturing includes a number of finishing steps
improving and conditioning the surface for further processing for the intended
final purpose of an article. Most of these finishing steps employ wet processes
and require rinsing steps. Water pollution is caused by the deliberate or acci-
dental discharge of the processing solutions and the contaminated rinse water.
The process may be directed towards:
1. Cleaning, which is the removal of surface oils,
greases, buffing compounds, etc.
2. Removal of oxides, rust, scale, etc.
3. Electrochemical or chemical processing to
provide the basis metal with a surface coat-
ing consisting of a plated metal or a chemi-
cally deposited, so-called conversion coating,
such as phosphate, oxide film as in blackening,
etc.
In general the aim is to change the surface of a product in such a manner that
the corrosion resistance is improved, or the appearance is changed to a more
pleasing appearance, improved hardness is imparted; wear resistance is in-
creased; surface conductivity changed; finishes to either improve the appear-
ance or suit specific engineering applications. It is evident that the first
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steps in the process - that is, the cleaning and oxide removal — are maiply pre-
paratory steps for good adhesion and receptivity for the subsequent finishes to
be employed, and while these preparatory steps may be similar to the procedures
used in the primary metal manufacturing industries, or the manufacturers of
certain finished products, such as automobiles and appliances, these preparatory
steps are a major part of the activity in an electroplating plant, while they are
of relatively minor importance for the manufacturer.
An electroplating plant may also be engaged in mechanical finishing activities,
such as polishing and buffing, sandblasting, or wire brushing, or it may also
have cleaning or painting processes employing solvents, the main activity being
with various chemical solutions, using water as a solvent for the chemicals and
water as a rinsing medium between the various process solutions, through which
the work progresses.
With an activity centered around the use of various processes employing water
as a solvent material, it is evident that water pollution problems will be en-
countered whenever an effluent is discharged. The severity of the pollution
naturally will depend on the source of waste, the type of process employed,
the size of the installation, the relative concentration of the effluent, etc.
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Engineering Considerations
Waste treatment processes aim to eliminate from the effluent stream the pollution
causing ingredients. An engineering function selects and coordinates the various
treatment steps, decides the required size of equipment to be utilized, selects
the best site to meet plant requirements, and provides the necessary plans, speci'
fications, operating information, aiming for a treatment plant, which, when prop-
erly operated, will insure that the effluent will meet the design criteria.
The engineer designing a metal finishing waste treatment plant has to have mfor-
mation from many sources to be able to meet these objectives.
Each plant uses a finishing program, unique to this one plant.
The processes utilized and make up of processing solutions is highly variable.
The processing equipment employed will have its own individual features, drain-
age rates, foundation, floor contours, age of the equipment, these are all factors
to be considered. The production volume, type and shape of the articles will
determine the anticipated surface area to be processed and the anticipated drag-
out rates per unit of surface area.
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The various finishing processes contain a variety of contaminants.
These will have to be known to judge the type of material that will have to be re-
moved from the waste stream. The applicable chemical processes have to be selected
to insure the desired reactions.
As the economy of the manufacturing process is affected, the
treatment plant and its anticipated operating costs are of paramount importance.
Process solution regeneration, chemical and metals recovery, water reuse and water
savings, operating and supervisory labor costs are all factors to be considered.
The flow rate in the receiving stream, the capacity of the
municipal sewage treatment plant, the applicable federal, state, and local require-
ments will have great bearing on the design criteria to be formulated for the
effluent to be discharged.
Plant safety, OSHA regulations, and such specific needs as
meeting, as an example, the Pennsylvania "Pollution Incident Prevention Plan"
have to be all considered.
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Sources of Waste
Dumping of waste process solutions- The cleaning and descaling process solutions
are so formulated that they will have the ability to remove soil or scale, surface
metal film, and will hold the removed material without depositing it back on the work
that is being processed. Naturally, slowly the capacity for additional soil or metal
removal will be reduced in view of the soil or metal content acquired by the cleaning
solution, and the time will be reached when the particular cleaning solution is con-
sidered spent. At this time the process solution is dumped. The dumping can occur
as a batch waste discharge or perhaps a continuous, slow wastage to maintain a
certain uniform concentration of active cleaning compounds or acids and maintain
a uniform contaminant loading or metal concentration to avoid the necessity of
batch dumping.
These batch discharges occur periodically; the relative volume of waste is usually
not large, but as the chemical concentration is relatively high, the pollution effect
may be considered also relatively serious.
The cleaners employed in metal finishing are usually compounded with various
alkali phosphates and relatively high concentrations of wetting agents to provide
fast and complete oil, grease, and soil removal. The various acid solvents for
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metal removal may contain higher acid concentrations than normal in the primary
manufacturing industry in view of the greater demands regarding utmost cleanliness
of the metal film for subsequent processing and also because the usual demands
for bright finishes require high concentrations of acid solutions and frequent dump-
ing to maintain a low metal contaminant level in the acid cleaners in use.
Most of the electrochemical process solutions in use, such as electroplating and
anodic treatment processes, can be maintained in working order by periodic or
continuous filtration, purification, or additions of various chemicals for replenish-
ment, but many process solutions employed in finishing either cannot be completely
purified, or the purification is uneconomical, in which case the process solution
itself will reach a point where dumping is necessary and a new process solution
has to be made up. Under this category would come, as an example, chromium
plating solutions contaminated by iron, copper, nickel, etc., anodizing solutions
for aluminum processing, some of the cyanide-type plating solutions, and chromat-
mg or phosphating type conversion coating processes.
From a pollution hazard standpoint, these wasted process solutions may be con-
sidered primary subjects for waste treatment. In view of the periodic or infrequent
discharges, the relatively small volume, and ample time available for proper
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treatment, the waste treatment effort is relatively small. As the pollution effect, in
view of the high concentrations of chemicals to be discharged, can be. most severe,
it will be evident that the chemical consumption for treatment may also be signifi-
cant. The considerable time available between batch discharges and the usually
small total volume to be treated, on the other hand, may allow small-sized equip-
ment.
Accidental discharges of process solutions - The second most severe pollution
hazard in connection with metal finishing operations is accidental discharges of
key process solutions. The concern shown for the treatment of the periodically
dumped process waste should be multiplied with regard to the accidental loss of
process solutions, because this hazard not only is on hand for the few process
solutions that are assumed to have finite life, but may affect the contents of pro-
cessing vats which under normal conditions could be maintained by the usual
purification maintenance practiced in the particular plant.
Nearly every process solution in the plant is prone to be discharged through an
accident, mainly because in the past an engineering effort was never directed
toward the avoidance of these accidents. The usual plant is so laid out that the
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entire processing area drains on the floor and the floor is only an extension of the
sewer system leaving the area.
While it is not common that a plating tank would spring a leak of such magnitude
that the entire plating solution could leak away undetected, many plants are operated
on a haphazard basis so that a slow leak amounting to a solution loss of 1 to 2 in./day
could go undetected for months. Also it is common practice to make up evaporation
losses by adding water with a hose to a process solution or turning on a spigot and
returning to the process tank only after it is overflowing the rim of the tank for a
while.
Filter hoses, heat exchanger connections, and pumped process lines are all prone
to leak, hoses deteriorate, etc. Waste treatment engineering would anticipate a
certain frequency of accidental spillage, depending on the general maintenance
in the particular plant.
Steam coils or heat exchangers undergo slow corrosion reactions, and it may be
anticipated that pinpoint corrosion or a corrosion cracking perforated the barrier
between the process solution and heat exchange medium, that is, steam condensate
or water. As the steam condenses, vacuum forms in the heating coil or in the jacket
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of the heat exchanger, drawing in the process solution through the voids created by
the corrosion action. Proper waste treatment engineering would therefore concern
itself with the accidental contamination of either the steam condensate or the cool-
ing water utilized in the particular process.
Some state codes require containment of the most toxic process solutions, such as
cyanide and chromic acid plating vats, by requiring a surrounding outer container
capable of holding the entire volume of the process solution in case of accident.
No doubt these state codes reflect on experience accumulated with various plating
operations. The falacy of these regulations is that, as enumerated above, there are
many additional ways for serious accidents to develop, causing severe pollution con-
ditions for which the containment for the process vat would have been no insurance.
The fact that there is protection for the least common occurrence, a serious leak in
the process tank itself, has only added to the cost of the installation of a plant but
did not provide the insurance required. On the other hand, the awareness of the
regulatory agencies of some of the potential hazards with the so-called "accidents"
in a metal finishing installation should help in a waste treatment engineering effort
aiming for the utmost safety considering the particular plant and processes under
scrutiny. Pennsylvania regulations require that the engineer submit detailed plans
for each installation storing, using, or processing toxic or potentially harmful
materials to satisfy the Pollution Incident Prevention Program.
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This naturally requires the incorporation of facilities and value judgment regarding
not only the processing area, but lagoons, containment sumps, chemical storage
tanks, storage areas, etc. In California, Japan, and Mexico, earthquake hazards
should also be considered.
Table I lists some of the more common accidental discharges together with methods
for detection and prevention or correction.
Contaminated rinse water effluent - When generally discussing waste treatment in
connection with metal finishing processes, one normally assumes that the topic
will be the elimination of the toxic constituents from the rinse water effluent. As
discussed above, the most severe hazards are not with the discharge of an un-
treated rinse water effluent.
Metal finishing requires copious quantities of water to wash away the remaining
chemical film on the work surface dragged out from one process solution before it
enters the next process. The reason for this is that water is the common solvent
for the remaining chemical films carried by the workpieces. Secondly, the removal
of this tenaciously adhering chemical film can be more easily accomplished with
fast-flowing water, providing agitation around the work surface. A chemical film
dragged out from one process and remaining on the surface may react with the next
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process solution, with which it may precipitate on the metal surface insoluble salts
as barriers for good adhesion, causing subsequent roughness, etc. The chemicals
that may be dragged from one process into the other could cause contamination of
the subsequent process solution due to the slow accumulation of dragged-in im-
purities, chemical constituents of the previous process. The need for good rinsing
is also due to the fact that any chemicals remaining in the pores of finished work
may lead to later discoloration, tarnishing, or corrosion, destroying the desired
finish.
The rinse water effluent from a metal finishing plant will carry the various dis-
solved solids originally contained in the film carried out on the work surface leav-
ing the manyfold chemical processes employed in the finishing sequence. The
drag-out from alkali cleaners by now is mixed with the drag-out from acids, pickling
solutions, and the various plating processes. While the total dissolved salt concen-
tration in the water may not have increased appreciably, the effluent carries, as
either dissolved or precipitated suspended solids, the various metal salts, cleaning
compounds, and anions of the acids utilized and possibly a small quantity of the
oils and greases originally removed by the cleaners from the work surface.
The relatively large volume of the effluent discharged makes the treatment of the
rinse water effluent the major problem. An additional complication is the fact that
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after all the various process rinses are mixed, the proper chemical treatment becomes
much more complicated or maybe even impossible. In these cases, it may be necessary
to segregate the rinse waters into various chemical groupings to be able to provide
proper treatment. Mixing of the total effluent occurs only after the segregated effluent
waste streams have previously received specific chemical treatment.
Typical Metal Finishing Processes, Requiring In-Plant or Waste Treatment Control:
a. Effluents containing only solid, mechanically-produced impurities,
such as rolling or annealing scale, sand, or sludge.
b. Effluents containing liquid impurities which are immiscible with
water, such as greases, oils and their solvents including kerosene, benzene, trichlor-
ethylene, and similar solvents.
c. Effluents from acid pickling solutions for ferrous and non-ferrous
metals. Effluents from chemical or electropolishing solutions formulated with mineral
acids such as phosphoric, sulfuric, chromic, nitric, and organic acids such as acetic,
citric, gluconic, etc., with high concentration of dissolved metals. Effluents from
phosphating solutions which contain phosphoric acid and metals such as iron, zinc,
or manganese.
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d. Effluents from alkaline pickling solutions for aluminum and zinc,
including the alkaline cleaners.
e. Effluents from various types of electroplating solutions, including
acid, alkaline, and cyanide solutions from which various metals may be deposited.
f. Effluents from chromic acid and chromate solutions in the form
of electroplating, etching and anodizing solutions, including chromating solutions
for magnesium, zinc, and aluminum, and electropolishing solutions.
g. Effluents from metal heat treatment such as cyanide hardening
operations, quench waters after brazing, metal and paint stripping, water-wash paint
spray booths, metal etching, typographical and rotogravure operations and electroless
plating systems used for metal deposition on non-conductors.
Almost all of the chemical processing solution must be discarded, due to the build-
up of foreign metal impurities. Electroplating solutions, as a general rule, are seldom,
if ever, discarded. When it is necessary to discard spent processing solutions, these
are generally batch treated prior to disposal. The rinse waters following these proces-
sing operations usually contain impurities in dilute form. Except in the case of purely
mechanical contamination, most of the contamination constituents are highly toxic and
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these toxic effects usually persist even at low concentrations. Even with large
volumes of water it is impossible to sufficiently dilute the contaminate below the
toxic levels. As an example, the normal commercial chromic acid plating solutions
which may, on the average, contain 300 g/1 chromic acid, would require a dilution
by a factor of about one million to meet United States Public Health Service drinking
water standards which, for chromium, is less than 0.05 ppm.
a. Effluents Containing Solid Impurities from
Mechanical Operation.
To obtain a good quality final finish in a metal processing operation, whether it be
an organic, metallic, or a chemical coating, it is imperative that the surface of the
workpiece be completely free from oils, greases, and rust or other oxide films. Thus,
this requirement makes it obligatory that certain cleaning operations be performed
prior to subsequent chemical processing. Often, it is necessary to initially subject
the workpieces to a mechanical operation, such as tumbling, blast cleaning, polish-
ing, or buffing, prior to the chemical processing steps. These mechanical operations
can produce an effluent which contains solid impurities.
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Water discharged from the dust collectors, tumbling, vibratory finishing after the
solids separation, may contain impurities requiring chemical or biological treatment.
b. Effluents Containing Liquid Organic Impurities.
The work pieces when received in a metal finishing plant are often covered with
oils and greases, due to machining operations or to protect their surface during
storage and shipping. Oils and greases are usually removed by subjecting the work-
pieces to the action of organic solvents and/or inorganic alkaline cleaning solutions*
Effluents are generally contaminated with these water-immiscible materials, due to
dragover or batch dumping of the processing media.
The solvents used in vapor or the immersion types of degreasing such as the non-
flammable chlorinated hydrocarbons or the flammable solvents such as kerosene
used in emulsion cleaners, can form emulsions in water or a floating film which
not only detracts from the appearance of the water but also presents danger to all
living organisms. In addition, these organic contaminants in effluents may be in-
flammable or liberate toxic gases, which would also prohibit their discharge to a
storm or sanitary sewer system. The B.O.D. content may be sufficiently high to
require biological treatment.
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c. Effluents from Acid Pickling, Polishing, or
Phosphating Solutions.
The effluents from pickling operations, due to rinsing following processing or the
dumping of spent processing solutions, yield large quantities whenever any type
of metal is processed. The pickling solutions are usually strong acids. The acids
are consumed by the dissolution of oxides and metals processed. The acid must
be replenished and metal ion content controlled for an effective operation. In most
pickling operations the solutions cannot be controlled and must be discarded. The
effluent due to batch dumping or the rinsing following processing is toxic because
of the acidity or the metal ion content. Exposure to acid water will cause damage
to masonry and iron structures. The metal ion content of the effluent from pickling
areas may be high in copper, zinc, nickel, cadmium, iron, and other heavy metals,
which are dangerous poisons to all living organisms and may be fatal at low con-
centrations.
d. Effluents from Alkaline Pickling Solutions.
Alkaline pickling solutions are used primarily for etching aluminum and zinc. These
solutions are generally highly caustic and must be neutralized with acid or spent acid
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pickling solutions to a slightly alkaline pH for the precipitation of the metal ions.
e. Effluents from Electroplating Solutions.
Effluents from many of the alkaline plating solutions contain complex metal cyanides
which must be treated to destroy or remove the cyanide radical, and the heavy metals
must be removed before the effluents can be safely discharged. The primary toxic
constituent of the non-cyanide containing processing solutions, whether they be acid
or alkaline, is the heavy metal ion content. The heavy metal ions must be removed by
alkaline precipitation, electrolysis or ion exchange. The processing solution formula-
tion may contain complexing agents which will not allow the complete precipitation of
the metal salts upon neutralization.
f. Effluents Containing Chromium Ions.
Chromium-containing chemicals are used in many processing solutions for plating,
etching, anodi2ing, electropolishing, and chromating. The chromium ion content of
many of these processing solutions are quite high and, consequently, the rinse
effluents following processing are high in chromium ion, which is toxic even in the
most dilute of concentrations and, thus, must be reduced to a safe level. Some of the
more acceptable methods are reduction and alkaline precipitation, ion exchange, and
evaporation.
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g. Quench Water Following Cyanide Metal Hardening
These waters usually contain ferrocyanides in addition to the simple cyanides, requiring
the treatment of both of these compounds. Paint and metal stripping solutions and electro-
less plating systems may contain a wide variety of organic acids and salts, complexing
agents and cyanides. Both chemical and biological treatment may be required and the
complexity of the problem may require consultation with the supplier of the process or
waste treatment specialist.
Commonly Used Waste Treatment Systems:
Batch Treatment
For dumped processing solutions containing high concentrations of chemicals, batch
treatment may be the best system. Usually dumps are infrequent and sufficient time
is available for the slow addition of the needed chemicals. Rapid treatment of con-
centrates may generate considerable heat and this again may cause the release of
toxic fumes. With collected floor spill, the time needed to analyze for the contents
and to provide the necessary chemicals for treatment, indicates the suitability of
batch handling.
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Concentrated process solutions containing cyanides will be most economically handled
.1, 2, 3
through a batch electrolytic destruction treatment and subsequent chemical pro-
cessing. High cyanide content waste solutions should be otherwise diluted to provide
a waste containing less than 1 oz./gal. CN, to avoid overheating and the release of the
highly toxic CNC1 gas. Also some of the metal cyanide complexes cannot be treated
rapidly with the usual chlorination. Nickel and silver cyanides, as an example, require
a long time for treatment and as the solubilizing free cyanide is destroyed in the
chlorination, there is danger that the metal cyanides will precipitate as the insoluble
salts and are not available for further chemical destruction. Sludges containing slowly
soluble metal cyanides would then ensue, making the solid waste discharged by the
plant unsuitable for land disposal.
For rinse water treatment, in view of economical considerations, and because the in-
soluble metal precipitates cannot be easily separated from the treated waste water,
batch treatment should be considered only for small-sized plants. Filtration is com-
plex in view of the gelatinous nature of the metal hydroxides and decantation easily
disturbs the precipitated flocculent particles separated from the water in quiet
condition.
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Batch treatment is suitable for the neutralization of acidic and alkaline effluents and
also for the treatment of cyanide — or chromium-containing wastes.
A. Cyanide-Containing Effluents
The dilute cyanide wastes from the rinse waters following plating tanks, cyanide dips
or other cyanide-containing processing solutions are drained to a treatment tank of
large enough capacity to hold the waste accumulated in one shift plus the volume of
treatment chemicals. With two tanks, waste can be collected in one while treatment
is taking place in the other. Each tank is baffled to provide adequate mixing and to
prevent short circuiting during treatment, and each can be equipped with high - and
low-level alarms.
During all stages of the treatment, the contents are continously circulated and/or
vigorously stirred to provide rapid and intimate mixing of the reaction mass. When
the level alarm signals that a treatment tank is full, the waste flow is diverted to
the other tank. If plating or concentrated processing solutions are dumped very
frequently, a separate holding tank should be provided. The concentrated waste
from the holding tanks would be bled into the dilute waste treatment tanks by means
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of a metering pump. The size of the holding tank is determined by the frequency
of dumping.
Treatment consists of first, elevation of the pH by the addition of caustic, and
4, 5
second, simultaneous addition of chlorine and caustic or sodium hypochlorite.
After treatment, the contents of the tank are discharged.
1. Manual Batch Cyanide Waste:
A typical manual batch system for cyanide waste is shown in Fig. 1. In operation,
the circulating pump is started and the pH of the waste is adjusted by adding caustic
to the system through the caustic feed pump. The pH is checked at approximately 10-
minute intervals using either universal or narrow range pH test paper. When the pH
reaches 11.5, the caustic feed pump is stopped. A sample of the waste must then be
collected for a determination of the cyanide content in order to establish the ap-
proximate additions of chlorine or sodium hypochlorite required. The treatment
chemicals are fed continuously for a predetermined time. At the end of this time,
the chemical feeds are stopped, but circulation of the batch is continued. At the
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end of forty-five minutes, a sample of the batch is tested for residual chlorine or
cyanide. If cyanide is still present, the chemical treatment is continued by start-
ing the chemical feed pump. The batch is then tested at approximately 15-minute
intervals until it has been determined that all cyanide is destroyed. When the
absence of cyanide has been confirmed, the pumps are stopped. The treated batch
may then be dumped to a clarifier for settling the precipitated metal salts, prior
to the discharge of the treated effluent.
When the second tank has become full, the waste flow is diverted to the now empty
first tank and the batch in the second is then treated in the same manner.
2. Instrumented Batch Cyanide Waste:
In the instrumented batch system, semi-automatic control is provided by the
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addition of pH and Oxidation-Reduction Potential (ORP) controllers. Its operation
is as follows* With the pH controller switched to manual control, caustic soda is
injected by the caustic feed pump into the untreated waste as it is pumped into
the circulation system. The pH is raised to 11.5 initially. When the pH has been
adjusted, the booster pump is started and sodium hypochlorite or chlorine is
added at the desired flow rate.
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At the same time, the pH instrument is switched to automatic control and, under
the proportional throttling control of the pH instrument, the caustic feed pump
continuously adds enough reagent to maintain a pH of 11.5 during the reaction.
The pH controller is equipped with a low pH limit switch for alarm and is inter-
locked with the booster pump to stop chlorine or sodium hypochlorite feed in the
event of a low pH condition.
The progress of the chlormation is continuously indicated by the ORP recorder-
controller. When this instrument senses that the endpoint of the reaction has been
reached, after about an hour, it shuts down the booster pump through a limit switch.
It also signals the operator.
The batch continues to circulate and if, after thirty minutes, the ORP recorder
still shows the reaction to be complete, the treated waste can be dumped. Should
the reaction be incomplete after thirty-minute mixing period, the chlorine booster
pump is started and continues to feed chlorine or sodium hypochlorite until the
limit switch on the ORP controller is again tripped, indicating that the reaction
is complete. The pH recorder-controller will automatically maintain the proper
pH during chlormation.
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7 8
3. Kastone Process '
A process developed by the DuPont Company and recommended for the treatment of
sodium, potassium, zinc, and cadmium cyanide only. The solution pH is adjusted to
10.0-11.5, the solution heated to 120-130° F, and hydrogen peroxide and formalin are
added according to recommendations of the supplier, depending on a previous analysis.
In approximately sixty minutes, the treatment is completed and the waste may be
checked for unreacted cyanide. If it is established that all the cyanide content is
treated, the waste may be decanted or filtered and discharged. The filterability of
the metal solids is improved in this process, but there is greater danger for entrap-
ment of the insoluble metal cyanides in the sludge. The chemical costs are some-
what higher with this process, but on the other hand, there is no danger of generating
cyanogen chloride gas during treatment. The effluent requires biological treatment,
so it has to be discharged to a sanitary treatment facility.
B. Chromium-Containing Effluents
The batch treatment of chromium is handled in a similar manner to cyanide batch
treatment. The chromic acid wastes from the rinses following plating solutions.
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bright dips, and conversion coatings are collected in two duplicate receiving tanks,
alternately used for collection and treatment. As in the case of cyanide, it is de-
sirable to provide a third tank for holding dumped spent processing solutions and
a metering pump to bleed a pre-determined portion of the solution from this tank into
the treatment tanks containing dilute wastes.
Treatment consists of first, lowering the pH by addition of acid, second, addition
of sulfur dioxide or sodium metabisulfite to reduce the hexavalent chromium, and
third, elevation of pH by addition of caustic to precipitate the trivalent chromium.
1. Manual Batch Chromium Waste-
In operation, the circulation pump is started and concentrated sulfuric or hydrochloric
acid added to the batch manually or by means of the acid feed pump. The pH is checked
at frequent intervals and, when it reaches 3.0, acid addition is stopped. A sample of
the waste must then be collected for determination of the hexavalent chromium content
of the waste in order to establish the required feed rate for sulfur dioxide or sodium
metabisulfite. A procedure can be set up for this purpose, using a color comparator.
For each installation, curves can be plotted for the quantity of chemical required
versus hexavalent chromium.
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When the required chemical feed rate has been determined, the pump is started and
the previously dissolved sodium metabisulfite is added or the sulfur dioxide feed
rate set on the sulfonator. The batch is then continuously circulated for about 3
hours. At the end of this time the sulfonator booster pump is stopped, but the
batch is circulated for another 15 minutes.
At the end of the 30-minute period, a sample of the treated batch is tested for hexa-
valent chromium. If the test indicates that hexavalent chromium is still present, the
treatment is continued, adding more chemicals from the sodium metabisulfite stock
solution tank, or by starting the sulfonator booster pump again. The batch is then
tested at 15-minute intervals until all the hexavalent chromium has been reduced,
at which time the sulfonator booster pump is stopped. Caustic is then added
manually or by means of a caustic feed pump until the batch attains a pH of 8.0,
at which point the trivalent chromium will precipitate out of solution. The treated
batch can then be discharged to a clarifier, or decanted to the sewer.
When the second tank has become full, the waste flow is diverted to the now empty
first tank and the batch in the second tank is then treated in the same manner.
(Fig. 2)
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2. Instrumented Batch Chromium Waste-
A semi-automatic batch treatment system can be provided by the addition of pH and
9
ORP controllers. In operation, the pH controller is switched to acid control and acid
is injected by the acid feed pump into the untreated waste as it is pumped into the
recirculation system. The pH is lowered to 3.0, at which point the acid feed pump is
cut off. The chromium booster pump is then started and sulfur dioxide fed at a pre-
set flowrate, maintained constant by the manually-set sulfonator. Using sodium
bisulfite or ferrous sulfate as a reducer, requires controlled additions of acid to
maintain the pre-set pH.
The progress of the reduction reaction is continuously indicated by the ORP
recorder-controller. When this instrument senses that the endpoint of the reaction
has been reached, it automatically shuts down the booster pump. The pH recorder-
controller is then switched to aklali control and the caustic feed pump is started.
When a pH of 8 is reached, the pH controller cuts off the caustic feed pump. The
trivalent chromium will precipitate and the batch can be discharged to a clanfier to
settle the precipitated chromium salts.
Continuous Waste Treatment
In contrast to batch treatment, continuous treatment for rinse water effluents allows
27
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closer instrumental control, better mixing of the reacting chemicals and a uniform
rate of flow pre-requisite for the successful performance of a clanfier, usually
following the chemical treatment as the first step in the liquid-solids separation
to remove the precipitated metal salts.
The rinse water streams are separated according to the various chemical treat-
ments needed, each segregated stream passing through a reaction tank of suitable
size with mixing and reaction chambers and instrumentation to allow the required
chemical feed, and retention time to provide the optimimum conditions for the com-
pletion of the intended reaction. Subsequently the treated individual rinse water
streams are mixed for self neutralization, final pH control, possible polyelectrolyte
dosing for best flocculation, and discharge to a clanfier or lagoon to settle the
precipitates and skim potential floating solids.
A. Continuous Cyanide Treatment
The dilute wastes are drained directly to the first reaction tank. A separate holding
tank is provided for concentrated wastes, which are slowly bled into the first reaction
tank by means of a metering pump. The reaction tank is baffled to assure positive
mixing and prevent short circuiting of the waste through the tank. In the first reaction
tank, the cyanides are converted by the chlorine addition into cyanogen chloride,
which will hydrolyze to cyanate in 10-15 minutes, at a pH of 11-11.5. The pH is
maintained constant by the injection of caustic through the metering pump which
is proportionally controlled by the pH recorder-controller. Chlorine is fed to the
28
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system by a chlorinator, 01 added from a sodium hypochlorite stock solution tank
through a control valve. As mentioned, the first reaction, cyanide to cyanate, takes
place at a pH of 11.5. If required, the cyanate may be completely broken down to
nitrogen and carbon dioxide gases in a second reaction which occurs more rapidly
at pH 7-5-8.0. The pH reduction is accomplished by the addition of concentrated
mineral acid. The acid is fed to the reaction tank by a control valve.
From the pH adjustment tank the waste flows to a larger, baffled tank where the
complete destruction of the cyanate is accomplished. Either chlorine and caustic
are added to this tank, controlled by pH and ORP instruments, or sodium hypochlorite
solution is fed by the ORP controller. The effluent from this final reaction tank is
discharged from the plant directly to a settling tank or lagoon.
B. Continuous Chromium Treatment
As in the case of cyanides, continuous treatment of chromate wastes is more
practical than batch treatment for plants handling large amounts of dilute wastes.
The continuous system for the treatment of chromates is basically similar to that
for cyanide treatment. In operation, the dilute wastes are drained directly to the
reaction tank. Concentrated wastes from the holding tank are slowly bled into the
tank by means of a metering pump. The hexavalent chromium is converted to the
tnvalent state in the reaction tank by the addition of a sodium metabisulfite solution
29
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or sulfur dioxide. The reaction is virtually instantaneous at a pH of 8.0 or less.
The pH is maintained constant by the addition of mineral acid through a control
valve regulated by the pH recorder-controller. Sulfur dioxide may be fed to the
system by an S02 feeder.
From the first reaction tank the flow of the waste is directed to the small pH ad-
justment tank. Here, the pH is increased to 8-0 by alkali addition. The pH recorder-
controller automatically maintains the pH constant by the addition of the caustic
soda solution. From the pH adjustment tank, the waste can be discharged to a
settling tank for separation of the precipitated solids. Figure 2 shows the schematic
arrangement for continuous treatment of a cyanide and chromium containing effluent.
The advantages of the continuous system are:
a. Simplicity of design. Assuming the presence of only conven-
tional metal finishing processes with well-known chemical
treatment requirements, engineering problems are minimized.
b. With proper instrumentation and incorporation of safety features,
the supervisory attention is minimal; therefore, no separate waste
treatment operator is needed.
The disadvantages are:
a. Minimal opportunities for water saving or reuse.
b. The precipitated metal salts are mixed, eliminating the
opportunity for economical metal recovery from the sludges.
c. Chemical costs are high because with the metal precipitation,
also water hardness, has to be precipitated and the chemicals
30
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for pH adjustment for the various treatments can become
significant.
d. Unless care is exercised and complexing agents are rigor-
ously kept out of the rinse water, meeting of effluent limits
may be marginal or not possible.
Integrated Waste Treatment10'11
This system was devised primarily to meet the need of the plating industry for im-
proved rinsing, and to create savings to offset the costs of waste treatment. The
basic concept of the integrated system is the segregation and treatment of the waste
at the source. To accomplish this, the liquid film of plating solution which adheres
to the part as it is removed from the bath is simultaneously treated and removed from
the plated or processed part. The waste treatment is integrated into the processing
sequence and no separate treatment plant is required. The system can be employed
following any processing step which would result in toxic waste carryover, regard-
less of its position in the processing line. Its simplicity and reduced space re-
quirements make Lt easily adaptable to existing processing lines.
In operation, a treatment wash tank is substituted for the first rinse tank following
the plating operation. A treatment wash solution is continuously recirculated through
this tank and physically removes the dragout, at the same time reacting chemically
with it. The part is then rinsed with fresh water in the subsequent rinse tank. The
31
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effluent from this rinse tank is now uncontaminated with toxic dragout or precipitated
metal hydroxides and requires no additional treatment.
The treatment solution is continuously recirculated between the treatment wash tank
and a larger reservoir. The reservoit tank serves three major functions:
1. It is the all-important buffering component in the system
which neutralizes the shock load caused by sudden and
irregular changes in the quantity of plating or processing
solution dragout treated.
2- It serves as a clarifier, settling out the insoluble metal
oxides and hydroxides formed in the first stage of the
reaction.
3. It serves as a retention tank, providing adequate time for
the desired chemical reaction, such as oxidation of cyanates
to carbon dioxide and nitrogen, treatment of nickel cyanide,
silver cyanide complexes, etc., which take hours for completion.
Only one reservoir tank is required and several treatment wash tanks can be served by
a common reservoir tank. Various metal wastes should not be mixed if metal recovery
from the sludges is the aim.
The integrated system can be completely automated, but in smaller plants operates
without the need for close control since high excesses of treatment chemicals are
used in the closed loop. Relatively simple paper and spot tests are sufficient for
control of the treatment process. Treatment chemicals are added either continuously
32
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or batch-wise as they are consumed by the toxic materials. A typical integrated
system is shown in Figure 3.
The system has many advantages, such as:
a. Equipment costs are low, the equipment is integrated into
the finishing line, occuping a minimum of floor space, and
requiring no separate waste treatment plant.
b. Ease of supervision and control, because control is restricted
to simple checks to ascertain the availability of excess treat-
ment chemical in the system.
c. Better rinsing, elimination of staining, and reduction in rinsing
rejects. Reduced quantities of water are used in view of the
pre-nnse with chemicals, allowing 80-90% reuse of the waste
waters, since the contaminating chemicals are kept out of the
rinse water flow and no treatment chemicals are added to in-
crease the salt content of the waste water.
d. Minimum waste treatment chemical cost, since one of the
major chemical consumption factors, the addition of caustic
or acid to bring the waste rinse water into the correct pH
range for treatment, is eliminated.
e. Sludge handling is simplified. The chemical system is so
formulated that the precipitates are densely settled. The
metal sludges are segregated, allowing simple and economical
recovery of the metal values.
f. The individual chemical rinse stations operated with high
content of excess reacting chemicals provide a fast and more
complete treatment. Only the dragout from the treatment rinse
reaches the rinse water, therefore it is easier to meet effluent
quality requirements.
33
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The disadvantages of the system are:
a. Additional rinse tanks may be required where such are
presently not used, unless one of the rinse tanks in a
double rinse system can be converted to chemical
rinsing.
b. While the integrated rinse, if properly maintained, leads
to a quality improvement in the finish, the improperly
operated plant may affect the desired finish quality and
may be the cause for re-works.
c. The metal finisher is made responsible for waste treat-
ment and has to attend to the maintenance of additional
process solutions, while it would be more convenient to
leave these problems to a remote waste treatment plant
operator.
Ion Exchange Treatment
Ion exchange is one method of concentrating the chemical contaminants in rinse
waters so that they can be treated more easily. It also makes possible the recovery
of valuable materials. As a by-product, ion exchange produces deiomzed water
which is useful in rinse tanks and in preparing new plating solutions.
Basically, ion exchange is a system for removing one ion from solution and sub-
stituting it with another ion to produce a solution that has a more desirable compo-
sition than the one being treated. The basic material involved is a granular solid
known as an ion-exchange resin, which has the property of exchanging one of its
34
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ions for one of those in the solution being treated. The process itself is cyclic.
The solution being treated passes through the exchanger until it exhausts the resin.
In essence, the resin itself can be used indefinitely. Ion exchange can be used to
concentrate rinse water wastes that must be neutralized prior to discharge or, when
economics dictate, it can be used to recover metals and regenerate process solutions.
Theoretically, it is possible to continuously remove the chemicals contained in the
drag-out from the process solution from the rinse water effluent and to recover these
chemicals in a useful form when backwashing the ion-exchange column. The back-
wash waters, since they are usually more dilute than the original process solution,
would have to be only reconcentrated by evaporation before returning them to the
process. At the same time, the rinse waters that have been passing through the
ion-exchange column would be returned for repeated rinsing. Waste treatment op-
erated in this manner becomes a closed cycle, greatly reducing the quantity of
water that has to be purchased, but at the cost of chemicals that are required to
backwash the ion-exchange column—that is, to free the ion-exchange resin of the
process chemicals that were picked up while the waste rinse water was purified.
When an ion-exchanger installation is used for the purification of rinse water
effluent from various processes, the reclamation of the process chemicals con-
tained in the backwash water cannot be utilized any more for return to the
original process from which they originated. In an installation of this type, the
-------
mam function of the ion-exchange installation is to avoid waste treatment of large
volumes of rinse water effluent and, by backwashing the exchanger, all the chemicals
that would have required treatment become available in a far more concentrated form
for waste treatment. The functions of the ion-exchanger in an application of this type
would aim to return nearly all the rinse water to the process for repeated usage and
allow a simplified waste treatment with regard to the volume of the total waste to be
treated. The chemical and maintenance cost of the ion-exchange installation would
have to be balanced against the water savings.
Consideration has to be given in a system of this nature to the fact that the ion-
exchange resin bed is capable of repeated regeneration without deterioration if it is
loaded with chemicals which the resin can release on simple backwashing with the
chemicals utilized for this purpose. With a mixed rinse water waste stream, careful
engineering is required to insure the success of the installation. Oil that may be
carried by the rinse water after cleaning should be removed. Some wetting agents
and organic brighteners may also foul the resin bed. For this reason, it is best
to insert a carbon filter into the recirculation system to remove all deleterious
organic materials before passing the waste rinse water through the ion-exchanger.
Another problem is the precipitated water hardness and metal hydroxides clogging
the rpsin bed. Considering a pool of mixed rinse water effluent, the pH, of the
stored water will determine to what extent the metal salts in the waste may have
been precipitated. The ion-exchange resin is capable only of removing ionized
36
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substances; therefore, solids such as precipitated metals, soaps, etc., will mechan-
ically clog the resin bed. Some compounds may not be released as easily by some of
the ion-exchange resin materials as some other resin formulations. It is necessary,
therefore, to consider all the possible organic and inorganic materials that will be
contained in the rinse water and select the resin that is least affected and easiest
to regenerate for a successful installation. Figure 4 provides a schematic presenta-
tion of an ion-exchange installation of this type.
Assuming that the precipitated metal salts are removed by previous pressure filia-
tions, and the organic content of the waste stream was purified through the use of a
carbon filter, of all the materials suspected of fouling of the ion exchange resin bed,
the rinse water as is passed through the ion-exchanger is freed of all salt content and
the water can be reused in a closed loop. The cation bed will remove all cations until
exhausted and similarly, the anion exchanger will remove the anions. As an example,
sodium, calcium, the metal ions, and trivalent chromium would be retained on the
cation resin, while the anion exchanger retains the sulfates, chlorides, etc., hexa-
valent chromium, and cyanide complexes.
Each-ion-exchange resin column has a certain capacity to combine with equivalent
weights of the various chemicals contained in either the process solution or in the
37
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rinse water. Calculations have to be conducted for each installation to establish
the volume of resin that should be utilized to provide a reasonable length of time
for usage between backwashings. Normally, parallel ion-exchange systems are
used for waste water treatment to allow the backwashing of one while the other is
on stream. The size of the installation will depend on these calculations and a
certain percentage of deterioration in time should be calculated to allow for a
20-25% efficiency drop in the utilization of the ion exchange column.
As the capacity of the exchanger is based only on the chemical equivalent weights
of the various chemicals to be removed from the recirculated rinse water, the volume
of water being recirculated is immaterial. This allows the accelerated water recircu-
lation for better rinsing without economical detriment. While the fact that the recircu-
lated water entering the rinse tank is desalted may not be of significant value because
desalted water doesn't have buffering capacity, the fact that the flow rate can be ac-
celerated is an advantage.
The performance of the ion-exchange system usually is monitored with a conductivity
controller. When the exchange capacity of the system nears exhaustion , the salt con-
tent increases the conductivity of the water. This breakthrough of dissolved salt in
38
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the recirculated effluent is an indication that the system requires regenera-
tion. Usually dilute hydrochloric acid is used for the regeneration of the cation
exchange column, while caustic soda solution is used for the regeneration of the
anion exchanger. Cyanide and wetting agents may not be completely retained and
may be present in the recirculated rinse water, while the complete release of
chromic acid from the anion bed can be another difficulty to guard against.
The regenerant and backwash waters carry all the salts earlier retained on the
resin. Usually a batch waste treatment system is used to treat this mixed waste.
Since the acid and alkali values are near balance, the regenerant and backwash
waste is mixed. After cyanide treatment the chromium is reduced and precipitated
in the alkaline range and the metal salts are precipitated, settled, and the clear
effluent decanted to allow the separation of the solids from the water.
The chemical treatment of these batches have to be tailored to the particular
installation, and the anticipated constituents of the mixed waste. Process
changes and variable processing in the finishing plant may require variations
in the treatment process. Complex salt formation is always a danger if the po-
tential of iron cyanide and nickel cyanide formation is on hand; the cation and
39
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anion regenerant waters may have to be segregated. Complexing agents should be
kept out of the effluent in a similar manner as indicated for continuous or batch
treatment. Sometimes it is necessary to pass the treated effluent through a secon-
dary cationexchanger to remove the metal salts that could be only incompletely
precipitated in the treatment.
Dumped processing solutions and floor spill are batch treated, sometimes mixed with
the regenerate and backwash from the ion exchanger system. Inconsiderate mixing of
these wastes may make the treated effluent or sludges, or both, unacceptable for dis-
charge. The potential hazard of creating a sludge high in slightly soluble metal
cyanides, insoluble iron cyanides, soluble metals that cannot be precipitated or
separated from the water phase, is extremely great.
Moving bed ion exchange systems are from a theoretical concept similar to the fixed
12
bed installations. Their advantage may be obvious when large capacities are needed.
As shown on Figure 5 schematically, the resin is recirculated in pulses and only a
short column length is used for absorption, while simultaneously at other areas re-
generation and backwashing take place. Beside potentially large capacity, the
greatest advantage may lie in the fact that since resin is in the loading cycle only
40
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for a few minutes, subsequently it can be washed, then regenerated, and washed
again-the chances of clogging the bed with insoluble precipitates and fouling with
organic compounds being greatly reduced.
The advantages of the ion exchange treatment system are:
a. Significant water savings up to 90% in view of water
recirculation. At the same time, sewer rental charges
are equally reduced.
b. The volume of the effluent discharged is greatly reduced,
thereby allowing potential reduction of the polluting
substances.
The disadvantages of the ion-exchange type treatment are:
a. Design considerations have to be very carefully
weighed.
b. Batch treatment chemical costs, labor, and supervisory
expenses, also equipment maintenance, are high. Sludge
handling can be expensive since the metal separation can
be complex.
c. The investment in equipment and installation is relatively
high.
41
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Process Solution Regeneration and Recovery. Metal Recovery.
Ion Exchange
The best opportunities for ion exchange systems in metal finishing waste treatment
13
may lie in the field of valuable metal recovery or regeneration of process solutions.
As an example, when rinse waters after chromium plating are passed through a cation
exchange column, the system may serve the function of recovering the valuable chrom-
ium chemicals by removing the impurities, such as trivalent chromium, copper, zinc,
nickel, and iron, in the cation exchange column, the backwash waters from which would
go to waste treatment. An evaporation system allows the reconcentration of valuable
chemicals and reuse of the rinse waters.
An example of the maintenance of a process solution would be the removal of aluminum
from a chromic acid anodizing bath, avoiding the necessity of periodic disposal of the
bath!4 Chromic acid, as a strong oxidizer, will deteriorate the resin to some extent
and, therefore, concentrated chromic acid solutions should first be diluted with water
before regeneration through an ion-exchanger is attempted.
An ion exchanger receiving rinse waters from only one process can retain the desired
cation or anion in a sufficiently pure condition so that the regenerant could serve as
replenishment source back to the process, especially after reconcentration by evapora-
tion. Nickel sulfate, as an example, can be reclaimed in this manner.
42
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Process solution regeneration is, perhaps, the field which, from economical con-
siderations, is best suited for the use of ion-exchangers. The limited capacity of
the ion-exchange systems and the necessity for large installations to provide the
necessary salt absorbancy between backwashings has limited to some extent more
common use of this type of equipment. Moving bed ion exchange columns overcome
the common limitations assumed regarding the capacity of ion-exchange installations
(Figure 5). Successful installations indicating good economy have been accomplished
with this type of system for such process solution recovery as bright dip solutions
used for aluminum which require resins to be able to accept high-strength oxidizing
acids and have removal rates of large quantity of aluminum, maintaining the process
solution at the optimum aluminum concentration.
Evaporative Recovery
There are basically two systems used in the recovery of chemicals by the evapora-
tion technique.
One is the vacuum evaporator, concentrating the process solution waste at reduced
temperature by depressing the boiling point, maintaining a vacuum in the evaporative
vessel.15 Reducing atmospheric conditions and temperature, the oxidative breakdown
of cyanide compounds may be reduced. The atmospheric evaporator uses forced air
through a chamber into which the processing solution is sprayed to accelerate
43
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evaporation rates and remove water vapor with the air that is discharged through a
stack. The water removed by the vacuum evaporator can be recondensed and reused
for rinsing.
Two modes and their variants are usually employed, depending on the number of
countercurrent rinse stations available and the water flow rate required for good
16
rinsing.
A. Closed Loop System
The closed loop system is an effective way to recover cyanide, metal cyanides,
chromium-and other metal-containing chemicals from plating operations so that
chemical treatment of rinse water is eliminated or minimized. This technique is
applied only to processing lines using countercurrent rinsing. In a typical system,
Figure 6, a single effect evaporator concentrates flow from the rinse water holding
tank. The concentrated rinse solution is returned to the plating bath and the dis-
tilled water is returned to the final rinse tank.
In the closed loop system, no external rinse water is added for makeup except that
required by atmospheric evaporation. The only chemicals added to the plating bath
are those required for replacing what is actually deposited on parts and any spillage
44
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or accidental losses. The system is designed to recover 100 per cent of the plating
chemicals normally lost in dragout, for reuse in the plating cycle.
B. Open Loop System
The open loop system is adaptable to those plating installations where there is an
insufficient number of countercurrent rinse tanks, and is a system for partial recovery
of plating chemicals. A small portion of the chemical dragout that accumulates in the
final rinse tank is not circulated to the evaporator for concentration. The circulation
loop through the evaporator is opened by creating another flow path for the chemical
dragout. This small fraction of dragout solution not returned to the evaporator can be
treated by an appropriate chemical method before disposal. A typical example of an
open loop system with only two rinse tanks can be operated economically. In this
system a small percentage of the dragout is lost, which must be treated.
45
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Reverse Osmosis
Functionally, the reverse osmosis applications in metal finishing are very similar to
the opportunities available by evaporation. Theoretically, reverse osmosis aims to
apply high pressure to a suitable thin membrane, overcoming the osmotic pressure,
passing water through the membrane which at the same time rejects the salt molecules,
and thereby separates a relatively salt-free water stream and a salt solution at a higher
concentration than the original input was.
Rinse waters from a specific process can thereby be treated, the water product re-
turned for rinsing, and the concentrates, possibly after further concentration by evapora-
tion, returned to the process. If the recovered water stream is sufficiently free of the
dragout salts, and a sufficient number of counter-current rinse stations are available,
the level of the original contaminant concentration can be sufficiently reduced so that
a last rinse doesn't require treatment. Otherwise the configuration is similar to the
17 18
"Open Loop System" as shown for evaporative recovery.
Suitable membrane materials for cyanide and chromium type rinse water reconcentration
are not yet commercially available.
46
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Care has to be exercised with reverse osmosis systems so that the rinse water doesn't
contain precipitated salts; otherwise, these may in time reduce the permeability of the
membrane.
Metal Recovery with the "Integrated Waste Treatment" System
As discussed earlier with the various Waste Treatment Systems available, the "Inte-
grated Waste Treatment" aims to follow closely the processing step and is segregated
with respect to the general metal treatment process. The design theory assumes that
only the metal is worth recovering, both from an economical standpoint, to reduce the
sludge volume and to avoid returning contaminants to the process and unduly increas-
ing the metal content of the solution.
A. Electrolytic Recovery
19
After certain processes, such as for example gold, silver, tin, a dragout solution is
maintained as a suitable electrolyte to allow the continuous recovery of the metal in
the process and maintaining the electrolytic system at such metal level that the dragout
loss of the metal into the following treatment wash system is of negligible economical
consequence.
B. Metal Recovery Values from the Sludges
The relative "Integrated" closed loop segregates the metal in a dense sludge collected
47
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in the respective reservoir tank. Since the sludge is uncontaminated, it has a good
market and can be sold to the suppliers of the plating chemicals or anodes for nearly
the full value of the metal content. Especially for smaller plants this provides a
simple and unsophisticated means to recover the dragout losses from, as an example,
20,21
nickel and copper plating installations. A significant metal loss also occurs at
times of purification of these plating solutions and from filter drains. All these
wastes can be easily recovered up to 100% with this technique.
Larger installations may avail themselves of further savings by recovering the full
metal values with highly economical automatic electrolytic recovery plant, operating
at near 100% efficiency, to reclaim all the nickel and copper lost. This is an obvious
step, for instance, for the non-ferrous metals processing plants, even though some
waste sludges are mixed, as an example copper sludge containing zinc, tin, nickel,
22
etc.
Acid Pickle and Etch Solution Regeneration by Crystallization
or Electrolytic Continuous Maintenance Programs
Processes depending on cooling or heating of the acid solutions to avoid dumping
and recovering the crystallized metal salts have been known for many years. Their
wider use may come in the future with added emphasis on the need of recycle and
increasing costs of sludge handling. We may mention here, as an example, the
48
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crystallization recovery for pickling solutions for steel, removing ferrous sulfate or
the regeneration of ammonium persulfate copper-etch solutions as used for printed
circuit boards. The removal of the copper-ammonium sulfate extends considerably
the usefulness of the processing solution.
Continuous electrolytic maintenance of a desired metal concentration has been
practiced for many years for pickling acids in non-ferrous metal manufacturing
plants. With greater emphasis on waste treatment, the processes expanded, re-
cycling the metal that was dumped earlier with significant reduction in acid
handling costs and improved processing.
49
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Economical Considerations
From the foregoing, it will be obvious that waste treatment costs will be greatly de-
23
pendent on many factors. Under plant management control are such, as an example:
drag-out control, economical water usage, good housekeeping, and the selection of
processes from the standpoint of waste treatment needs also. Investment for solu-
tion, regeneration, purification, reduction of dumping rates, closer chemical control
of processing systems are all issues that require a new look from the waste treatment
cost angle.
The engineer in charge of waste treatment design should consider this question from
many angles also.
(a) The design should consider meeting not only today's requirements, but
the best treatment in view of the mounting restrictions anticipated in the future, and
the safest possible margin for his client.
(b) Water consumption and sewer rental charges should be considered as
much part of the overall cost as the chemicals used in treatment.
(c) Savings achieved in water reuse opportunities, chemical and metal re-
covery steps built into the waste treatment scheme, may allow economies to offset
24
treatment costs, reducing the overall operating costs.
50
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(d) Increasing the supervisory and operating labor costs can be avoided
only if the waste treatment system is integral or at least close to the processing area.
(e) An extensive and expensive waste treatment plant does not necessarily
insure better treatment, more flexibility, or freedom from upsets.
(f) Design criteria should not be established solely on the basis of the needs
of the receiving stream or the leniency or restrictive attitudes of the local Regulatory
Agency.
25
A case history reported elsewhere in detail is presented as an illustration. The report
covers cost comparison between three types of general waste treatment systems as re-
lated to the cost of rinse water treatment for the same size plants with identical drag-
out rates, production volumes, and other basis costs. The report is specific regarding
a plating process consisting of cyanide copper strike, followed by acid copper, nickel
and chromium plating. The cost comparison is tabulated in Table II.
The following general conclusions may be drawn from this example:
1. (a) Water costs and sewer rental charges were the most significant
cost factor.
(b) Sludge handling was the second important cost item, followed by
(c) Amortization charges.
51
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2. Chemical costs for the treatment erf the drag-out are not a major
item. When large volumes of water are processed, the precipita-
tion of water hardness may exceed the chemical cost of treating
the chemicals from the drag-out. The study does not reflect what
the chemical costs of treatment would have been if solution dump-
ing and excessive floor spill would have to be treated. Chemical
costs could be a major item in a sloppy operation.
3. Treated rinse water due to water reuse options may cost less than
the untreated rinse water and sewer rental charges.
4. The cost comparison study did not take into consideration that all
the copper and nickel is recovered in the plant under study. The
actual savings far exceed the reported values. We may say, there-
fore, that suttable waste treatment design in many installations
may significantly reduce operating costs.
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REFERENCES
1. R. W. Oyler, Plating 36, 341 (1949).
2. R. W. Oyler, Proc. 4th Indust. Waste Conf., Purdue Univ., 250 (1949).
3. J. M. Conrad 8c G. P. Beardsley. Metal Finishing, 59, #5, 54 (1961).
4. J. G. Dobson, Sewage Works J., 19, 100 (1947). Metal Finishing, 45 #2, 78;
45, #3, 68 (1947).
5. B. F. Dodge 8c W. Zabban, Plating, 38, 561 (1951), 39, 385 (1952).
6. L. E. Lancy, W. Zabban, ASTM - STP No. 337, 32 (1962).
7. B. C. Lawes, E. I. DuPont, l'. S. Pat. 3,617,582.
8. B. C. Lawes, L. B. Fournier, and 0. B. Mathre, "A New Peroxygen Method for
Destroying Cyanide in Zinc and Cadmium Electroplating Rinse Waters," 58th
Annual Convention, American Electroplaters' Society, Inc., Buffalo, N. Y. (1971).
9. W. N. Greer, Sewage 8c Indust. Wastes 25, 930 (1953).
10. L. E. Lancy, U. S. Pat. #2,725,314
11. L. E. Lancy - Sewage 8c Indust. Wastes 26, 1117 (1954)
12. I. R. Higgins, U. S. Pat. #3,492,092
13. R. Kunin, Products Finishing (April, 1969)
14. J. Thompson and V. J. Miller, Plating 58, 809 (1971).
15. J. M. Culotta, Plating 52, 545 (1965)
16. J. M. Culotta and W. F. Swanton, Plating 58, 783 (1971)
17. A. Golomb, Plating 57, 1001 (1970)
18. A. Golomb, Plating 59, 316 (1972)
19. L. E. Lancy, U. S. Pat. #3,431,187
20. R. Pinner, IMF, Symposium Rep. 14, 272 (1968)
21. W. Nohse 8c D. Wystrach. "Nature of the Effluent Sludge from Lancy Treatment
of Rinse Waters from Acid Copper Plating." Electroplating and Metal Finishing
19, 146 (1966)
22. Water Pollution Control Research Report #12010DPF 11/71
23. L. E. Lancy - Plating 54, 157 (1967)
24. R- Pinner, V. Crowle - Electroplating and Metal Finishing, March, 1971
25. L. E. Lancy, W. Nohse, 8c D. Wystrach - Plating, 59, 126 (1972)
53
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TABLE I - COMMON WASTE DISCHARGES DUE TO ACCIDENTS IN METAL FINISHING PLANTS
Source
Method of Detection
Correction or Prevention
1. Process tank overflow
a. Unattended water additions
b. Leak of cooling water into
solution from heat exchanger
or cooling coil
2. Process solution leakage
a. Tank rupture or leakage
b. Pump, hose, pipe rupture or leakage,
filtration, heat exchanger, etc.
c. Accidental opening of wrong valve
3. Normal drippage from work-pieces
during transfer between process tanks
4. Process solution entering cooling
water (heat exchanger leak)
5. Process solution entering steam
condensate (heat exchanger or
heating coil leak)
6. Spillage of chemicals when making
additions to process tanks or spillage
in the chemical storage area
1. High-level alarms in floor
collection system to signal
unusual discharges
2. Integrated floor spill treatment
Same as above
Inspection
1. Conductivity cell and bridge
to actuate an alarm
2. Use of the cooling water as
rinse water in a process line
where the contamination will
be immediately evident
Conductivity cell and bridge to
actuate an alarm
Make the solution maintenance
man responsible for chemical
additions
1. Provide proper floor construction
for floor spill segregation and
containment (curbs, trenches,
pits)
2. Provide treatment facilities for
collected floor spill
3. Integrated floor spill treatment
system
4 Use of spring-loaded valves for
water additions
5. Provide automatic level controls
for water additions.
Same as 1-3 above
1. Provide drainage pans between
process tanks so that drippage
returns to the tanks
2. Collect floor spillage
3. Integrated floor spill treatment
Use conductivity controller to
switch contaminated condensate
to a waste collection and treatment
system
1. Careful handling and segregation
of chemical stores
2. Segregation and collection of all
floor spillage
3. Integrated floor spill treatment
-------
TABLE II
- COMPARISON TREATMENT TOTAL. COST
ION EXCHANGER
CONTINUOUS
TREATMENT
INTEGRATED
TREATMENT
Fresh Water
187
28,894
165
Ca , Mg, Precipitation
21
4,597
-
pH Correct, for ORP Control
4,367
—
Neutralization
2,784
79
Sewer Rental Charges
157
24,143
138
Deionized Water
289
289
289
Treatm.Conc.Sol.& Sludge Hdlg.
33,589
27,981
2,588
Electric Energy
1,289
3,405
960
Chemicals
6,759
1,743
6,084
Wages
3,863
4,0 17
2008
Amortization
31,091
16,966
4,809
Maintenance
3,999
1 2,767
1,443
Cost Per Year #
81,244
131,953
18,563
-------
LIST OF FIGURES
Figure 1
Manual Batch Cyanide System
Figure 2
Continuous Treatment of Cyanide and Chromium
Figure 3
Integrated Treatment System
Figure 4
Ion Exchange System
Figure 5
Moving Bed Ion Exchange System
Figure 6
Evaporative Recovery Closed Loop
56
-------
TO CLARIFIER
RINSE WATER
CONTAMINATED WITH CYANIDE
MIXER
MIXER
SODIUM
HYPO-
CHLORITE
CAUSTIC
SODA
TREATMENT TANK NO. I
TREATMENT TANK NO. 2
Fig. I. Manual batch cyanide system.
-------
RINSE WATER RINSE WATER
CONTAMINATED
WITH CYANIDE
CONTAMINATED
WITH CHROMIUM
SODIUM
HYPO-
CHLORITE
SODIUM
BI-
SULFITE
CAUSTIC
SODA
SUL-
FURIC
ACID
~~l
ALL OTHER
RINSE WATER
MIXER
MIXER
CYANIDE TREATMENT TANK
CHROMIUM TREATMENT TANK
CMl —
FURIC CAUSTIC
ACID SODA
ySL,
MIXER
TO CLARIFIER
pH ADJUSTMENT TANK
Fig. 2. Continuous treatment of cyanide and chromium.
-------
REUSE WATER
CYANIDE SOLUTIONS
PWAO OUT
CYANIDE
WASTE
TREATMENT
WATER
RIN8E
CHROMIC
ACID
PROCESS
80DIUM
HYPOCHLORITE
PEED
PUMP
1 {
CHROMIC
ACID WASTE
TREATMENT
-txJ-
CYANIDE WASTE
TREATMENT RESERVOIR
&L&
¦0
-04-
WATER
RINSE
TO pH CONTROL
CLARIPIER
WATER REUSE PUMP
WATER BLOW DOWN
TO SEWER
G"
J
CHROMIUM WASTE
TREATMENT RESERVOIR
R
PEEO 1
PUMP
-|
*
SODIUM CARBONATE
8001UM HYDRO-
8ULFITE
¦Q
TO SLUDQE BED
Fig. 3. Integrated treatment system
-------
TO CLEAN WATER
RESERVOIR AND
PROCESS RINSE
WASTE FROM
CONTAMINATED
RINSE OVERFLOW
WASTE WATER
RESERVOIR
CARBON
FILTER
CARBON
FILTER
FILTER
FILTER
AAA A
CATION
ANION
CATION
ANION
flV VlV
Fig. 4 Ion exchange system .
Lo
CAUSTIC
HYDROCHLORIC
SODA
ACID
i—Cw
o
TO WASTE
TREATMENT
-------
RESIN RESERVOIR
RESIN
VALVE
BACKWASH WATER" IN"-^_
CONDUCTIVITY PROBE
HYDRAULIC PULSE FOR
RESIN MOVEMENT
RESIN REOENERANT IN
PULSING CHAMBER
PROCESS WASTE 'bUT
REOERATION COLUMN
REGENERATION OF
PROCESS SOLTION
PROCESS WASTE IN
RESIN REGENERANT
AND
BACKWASH WASTE "OUT
RESIN
valve
Fig, 5 "Moving bed" ion-exchanger
-------
PLATING
SOLUTION
CONCENT-
RATE
OtSTILLATE
HOLDING
TANK
PLATING TANK
RINSE
RINSE
RINSE
COOLING WATER "OUT'
WATER RINSE
HOLDING TANK
C0NPEN80R
EVAPORATOR
FLASH BOILER
•STEAM
STEAM CONDENSATE
CONCENTRATE
RETURN PUMP
n n
Fig. 6 Evaporative recovery- closed loop
------- |