-------
TABLE 18.
TYPICAL ZINC CYANIDE PLATING BATHS
Constituent
or
Condition
Plating Method
Still Tank or
Barrel Automatic
Conduit Tubing
or Strip
Zinc cyanide
Sodium cyanide
Sodium hydroxide
Sodium polysulfide
Temperature, C
Current density,
amp/sq cm
Composition Ranges, grams/liter
60-83 60-83
39-66 32-66
75-90 75-90
1.5 1.5
Operating Conditions
20-35 20-35
0.003-0.011 0.016-0.054
60-90
16-49
75-90
1.5
32-49
0.043-0.108
(a) Organic or metallic brighteners are generally employed in zinc
plating baths; most brighteners used in industry are proprietary
formulations.
46
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sodium salt of ethylene diamine tetraacetic acid, has increased to avoid
dealing with the cyanide waste problem.
Typical pretreatment of the basis metals (mostly ferrous mater-
ials) prior to zinc cyanide plating generally consists of (a) electrolytic
cleaning, (b) rinsing, (c) acid pickling, (d) rinsing, and (e) cyanide or
caustic dipping. Where no posttreatment is employed, the zinc plated
parts are first rinsed in cold water, then rinsed in hot water and air
dried. In some instances, the zinc plated parts are given a bright dip
(dilute nitric acid solution) or chromated. Chromating is done to improve
the corrosion-resistant properties of the zinc plate or to enhance the
adhesion of paint to the zinc plated parts. Table 6 illustrates the diff-
erent plating operations performed for the different basis metals. Typical
processing sequences for zinc plating-chromating steel parts in automatic-
rack and automatic-barrel operations are shown in Appendix B, Table B-2,
Lines 3 and 4, respectively.
The major source of zinc wastes results from the drag out of
plating solution into the rinse waters. Other sources of zinc wastes are
generated during the continuous or batch filtration of the bath, solution
leakage from filters, pumps, and pipes, and floor spills. Zinc plating
baths are rarely dumped.
(1-4)
Copper Platingv '
Copper coatings are extensively used as undercoatings in multi-
ple-plate coating systems. For example, the plating of zinc and steel
parts with copper before nickel and chromium plates is common practice in
the industry. Copper plating is also employed in electroforming and in
the production of heavy coatings on wire for electrical applications. For
some applications, bright electroplated copper, protected against tarnish-
ing by an overcoat of clear laquer is used as a decorative finish. Copper
plating is also widely employed in the production of printed circuit boards.
Copper can be electrodeposited from numerous electrolytes; how-
ever, four main types (i.e., alkaline cyanide, alkaline pyrophosphate,
acid sulfate, and acid fluoborate) account for most of the commercial
plating. Typical compositions and operating conditions for the copper cyan-
ide baths are shown in Table 19. Similar data for the acid sulfate and
acid fluoborate baths are presented in Table 20.
Copper can be electroplated in still tanks, barrels, and in auto-
matic equipment. Steel tanks, lined with rubber or plastic, are generally
employed for copper plating from cyanide and acid copper electrolytes.
External heating of cyanide baths is suggested using a steel heat exchanger
(brass or bronze fittings should not be employed). Special grades of carbon
or graphite pipe and tubing make efficient heat exchange or cooling coils
for acid copper baths. Soluble copper anodes in a variety of shapes are
generally employed in plating copper from the several baths, although in-
soluble anodes of steel or lead alloys are sometimes employed for special
jobs or applications.
47
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TABLE 19. TYPICAL COMPOSITIONS AND OPERATING CONDITIONS
FOR CYANIDE COPPER PLATING BATHS
Constituent
or
Condition
Plain
Cyanide
Rochelle
Cyanide
High
Efficiency
Copper cyanide
Sodium cyanide
Sodium carbonate
Sodium hydroxide
Rochelle salt
Free sodium cyanide
Composition, grams/liter
15
23
15
26
35
30
4
45
6
75
93
30
11
Operating Conditions
Temperature, C
Cathode current density,
amp/sq cm
Anode current density,
amp/sq cm
55
0.022
0.008
60
0.043
0.016
70
0.065
0.027
48
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TABLE 20. TYPICAL COMPOSITIONS AND OPERATING CONDITIONS
FOR ACID COPPER PLATING BATHS^
Constituent
or
Condition
Copper
Sulfate
Copper Fluoborate
Low Copper High Copper
Composition grams/liter
Copper sulfate, CuSO,'5H20 195 to 250
Sulfuric acid, H2S04 30 to 75
Copper fluoborate, Cu(BF,)2 - '60 120
Fluoboric acid, HBF, - To pH To pH
Operating Conditions
Temperature, C 21 to 49 27-77 27-77
Current density,
amp/sq cm 0.022 to 0.108 0.081 to 0.135 0.135 to 0.378
pH - 0.8 to 1.7 <0.6
49
-------
Typical pre treatment steps in applying copper coatings to steel
or brass parts generally include cleaning, rinsing, acid dipping, and
rinsing. A cyanide copper strike or nickel strike plate is usually em-
ployed before steel or zinc alloy parts are plated in an acid copper bath.
A typical sequence of operations used to apply a triple Cu-Ni-Cr coating
on steel or brass parts is shown in Appendix B, Table B-2, Line 1.
The primary source of copper plating chemicals in the waste
results from the solution drag out into the rinse waters. A secondary
source of wastes is from spills or leaks from filters, pumps, and piping
fixtures, etc. Copper plating baths are rarely dumped.
Tin Plating
Tin coatings are generally applied for their desirable character-
istics such as its resistance to corrosion and tarnish, its nontoxic nature,
its solderability, and its softness and ductility. The largest single use
of electrodeposited tin coatings is in the tin plate industry. Electro-
plated tin is also extensively employed as a coating on refrigerator parts,
dairy and other food handling equipment, washing machine parts, kitchen
ware, automotive pistons and piston rings, radio and electronic components,
electrical lugs and connectors, and copper wire.
Tin is deposited from both alkaline and acid solutions. Repre-
sentative compositions and operating conditions for stannate tin plating
baths are given in Table 21. Similar data for acid tin plating baths are
presented in Table 22.
Typical pretreatment steps prior to stannate tin plating are:
cleaning, rinsing, acid dipping, rinsing, cyanide or caustic dipping, and
rinsing. For acid tin plating, the cyanide or caustic dipping step is
omitted.
Tin can be electroplated in still tanks, barrels, and in auto-
matic equipment. Mild steel tanks, unlined and equipped with steel heating
coils are suitable for stannate tin plating. Rubber-lined or synthetic-
lined tanks or equipment are required for the acid tin baths. Soluble tin
and insoluble steel anodes can be used with the stannate baths, while
soluble tin anodes are generally employed in the acid baths.
The major source of tin waste is the drag out from the plating
baths into the rinse waters. Settled sludges from the stannate bath, leak-
age from filtration equipment and piping in the acid baths, and accidental
spills are secondary sources of wastes from the tin plating baths. Tin
plating baths are rarely dumped.
(1-4*)
Iron Platingv '
Iron is used for building up heavy deposits on worn parts, for
printing plates, for electroforming and, occasionally as a substitute for
50
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TABLE 21. REPRESENTATIVE COMPOSITIONS AND OPERATING
CONDITIONS FOR STANNATE TIN PLATING BATHS
(2-4)
Constituent
or
Condition
Bath 1
Bath 2
Bath 3
Bath 4
Potassium stannate
Potassium hydroxide
Sodium stannate
Sodium hydroxide
Composition, grams/liter
100
15
210
22
420
22
100
10
Temperature, C
Current density,
amp/s q cm
Operating Conditions
65-90 70-90
0.03 to 0.11 0.17 max
70-90 60-85
0.43 max 0.005 to 0.032
TABLE 22. COMPOSITIONS AND OPERATING CONDITIONS
FOR ACID TIN PLATING BATHS (2~4)
Sulfate Bath
Fluoborate Bath
Stannous sulfate
Sulfuric acid
Cresol sulfonic acid
B-naphthol
Gelatin
Composition, grams/liter
53
98
98
1.0
2.0
Stannous fluoborate
Fluoboric acid (free)
Boric acid (free)
B-napthol
Gelatin
200
80
25
1.0
6.0
Operating Conditions
Temperature , C
Current density >
amp/sq cm
21 to 38 C
0.011 to 0.432
Temperature 3 C
Current densitys
amp/sq cm
21 to 43
0.08 to 0.14
51
-------
nickel. Iron plating solutions are slightly acidic, containing sulfates,
chlorites, fluoborates, or sulfamates. The basis metal is prepared for
iron plating by cleaning, rinsing, acid dipping, and rinsing. The parts
are rinsed and dried following plating. Soluble AKMCO iron anodes are
used for the process.
A most useful bath is the one containing 127 g/1 (17 oz/gal)
and 111 g/1 (15 oz/gal) CaCl2- The sulfate/chloride bath contains up to
20 g/1 (3 oz/gal) of ammonium chloride in addition to 250 g/1 (33 oz/gal)
ferrous sulfate and 30 g/1 (4 oz/gal) of ferrous chloride. The fluoborate
bath is sold as a concentrate and upon dilution with water contains typically
225 g/1 (30 oz/gal) ferrous fluoborate, 10 g/1 (1.3 oz/gal) sodium chloride
and 22.5 g/1 (3 oz/gal) of boric acid.
Drag-in to rinse water after plating is the major source of waste;
floor spills and leakage from filter systems are a secondary source. Fluo-
borate ions will have to be treated where such a bath is used. Small
amounts of ferric ion in the sulfate and chloride baths precipitate as
hydroxide or oxides and are removed by filtration.
Gold Plating
Gold and gold-alloy coatings are applied for decorative purposes,
as on jewelry, and for other applications requiring oxidation and tarnish
resistance. Gold plating is also used in applications of electrical con-
tacts and to achieve high solderability.
The majority of gold plating operations are performed using
cyanide solutions. Typical compositions and operating conditions for gold
cyanide plating baths are listed in Table 23. Additions can be made to the
various plating baths to produce various gold-alloy plates; for example,
7 to 15 mg/1 of antimony tartrate added to the cyanide gold-plating bath
produces a gold-antimony alloy deposit, indium cyanide additions produces a
gold-indium deposit, and gold-gallium deposits have been produced by the
addition of gallium salts. Some gold plating baths contain no free cyanide
and may contain gold and potassium phosphate or gold and ammonium citrate.
Gold coatings are predominantly produced from baths in which the
gold is supplied in the form of salts such as KAu(CN)2 or NaAu(CN)2«
Anodes used may be made of stainless steel, carbon, platinum-clad tantalum,
and platinum-plated titanium. (Gold anodes are used to supply gold to the
solution, but are used less often than are insoluble anodes in conjunction
with gold solutions.)
Gold solutions and the drag out to rinse tanks are largely re-
covered with the gold extracted either in the plating plant or by a separate
reclaiming service. The gold may be recovered by evaporation, precipitation
by zinc, electrolysis, or by ion-exchange methods.
52
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TABLE 23. RANGES OF COMPOSITIONS AND OPERATING
CONDITIONS FOR CYANIDE GOLD-PLATING
SOLUTIONS
Constituent
or
Parameter
Gold
Free KCN
K2HP04
K2C°3
PH
Temperature
Cathode Current Density
Units
g/liter
g/liter
g/liter
g/liter
PH
C
mA/cm
Range of Variables
1-8
0.1-30
15-30
0-30
10-11.5
55-70
1-10
53
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(1-4)
Silver Plating
Silver is used for decorative, protective, and engineering coat-
ings. Thin deposits of 2.5 jum (0.0001 inch) are applied over jewelry and
such. Coatings from 25 to 50 urn thickness (0.001 to 0.002 inch) are applied
to tableware and holloware. Thicker deposits of up to 1500 jum (0.060 inch)
are applied for bearings and electroforms. The bath concentrations nor-
mally increase as the desired coating thickness is increased.
The silver in plating baths is present as a cyanide complex
together with free cyanide and carbonate. Silver concentrations vary
from 25 to 75 g/1 (3.5 to 10 troy oz/gal). Additives are used for grain
refinement and brightening.
Strike solutions are generally employed in the basis metal pre-
paration. After cleaning, rinsing, and dipping in cyanide or acid solution,
and rinsing, the basis metal is activated by anodic or cathodic treatments.
For example, stainless steels may be anodically treated in sulfuric acids
or cathodically activated by striking in a Wood's nickel bath which is com-
posed of 250 g/1 (3.2 oz/gal) nickel chloride and 120 ml/1 (16 oz/gal)
hydrochloric acid. Silver strike solutions, with a low metal content (of
about 6 g/1 (0.2 troy oz/gal) of silver and high cyanide content of 75
to 90 g/1 (10 to 12 oz/gal), are used for nonferrous basis metals.
Ferrous materials are prepared first by striking in copper cyanide solutions
of similar composition prior to silver striking. Consequently, the number
of process steps is greater for silver plating than most other plating
operations.
Steel tanks, stoneware and lined tanks are normally employed for
silver plating, and the operations can be carried out manually or auto-
matically. Both racks and barrels are used for silver plating. Soluble
anodes are used for normal silver plating, whereas strike solutions would
use insoluble anodes.
The metal is generally recovered for refining and extra precau-
tions are taken to avoid spills and leaks, because of the high cost of the
metal. Dumping of solutions is not practiced. However, the cyanide por-
tion of the waste must be treated by destruction.
Anodizing
Anodizing is an electrolytic oxidation process by which the sur-
face of the metal is converted to an insoluble oxide having desirable
chemical and physical properties. Considerable aluminum is treated, some
magnesium and limited amounts of zinc and titanium. Aluminum is anodized
in 12 to 15 percent sulfuric acid to produce an oxide coating for corrosion
protection and as a basis for decorative color finishes with dyes, or a
hard coat for extra wear resistance. A 5-10 percent chromic acid bath is
used where solution may become entrapped without current in recesses. In
such a case sulfuric acid would attack the aluminum. Aluminum may also be
54
-------
anodized in oxalic acid or boric acid. The characteristics of anodic coat-
ings on magnesium can be varied from thin coatings to give good paint
adhesion to heavy coatings for abrasion and corrosion resistance in solu-
tions containing fluorides, phosphates and dichromates. Some zinc parts
are anodized to improve corrosion resistance.
Pretreatment operations for anodizing involve one or more of the
following steps: Alkaline cleaning, caustic etching, deoxidizing, desmut-
ting and bright dipping depending on the alloy used and the desired finish.
Posttreatment includes color eying and sealing with hot water or nickel
acetate solution.
Steel, rubber-lined, plastic, and lead-lined tanks are used for
anodizing. The latter may be used as the cathode in sulfuric acid ano-
dizing. Lead cathodes must be used elsewhere except for chromic acid
anodizing where a steel tank could serve as a cathode. Wastes are generated
by drag in of process solutions into the rinse waters. Solutions are
occasionally dumped. Spills are a secondary source of waste from anodizing
solutions.
Materials Reclamation
In conventional electroplating operations, a sludge is produced
from the dump streams and the treatment of rinse waters by the so-called
"destruct" systems in plants from a wastewater treatment system. Examples
of these systems are the oxidation of cyanide wastewater with sodium hypo-
chlorite, NaCIO, and the reduction of hexavalent chromium to trivalent
chromium with waste pickle liquor before neutralization and precipitation
of both with either sodium hydroxide or a lime slurry. The resulting
sludge is separated and sometimes dewatered. The common method for hand-
ling this sludge is disposal in landfill sites.
Several recent developments have forced changes in this procedure.
Among these are new regulations on the discharge of toxic materials, the
small number of certified landfill sites, and the rising costs of plating
metals and chemicals. The alternative methods are aimed at reclaiming the
dissolved plating metals in the wastewater during the plating operation in
closed or partially closed systems, thus circumventing sludge formation al-
together. However, in the present state of the art there is still a need
for a small destruct system to neutralize spills, fumes, dumped plating
baths, unrecovered plating acid drag out, etc. Recovery units used to
recover chemicals from plating bath rinse water are fairly wideily used;
however, they have not been installed on cleaner or acid dip lines because
the cost of chemicals is not sufficient to make recovery worthwhile. Also,
buildup of contaminants such as oil and grease makes the use of such closed
systems difficult.
Treatment of the effluent to recover plating chemicals falls into
two categories: (1) treatment of the entire volume of effluent by various
recovery methods, and (2) partial recovery. The simple schematics shown
in Figure 2 illustrates the difference. The total recovery unit illustrated
-------
A. Total Reclamation
Drag In
Evapor-
ation
Losses
Plating
Tank
Reclaimed
I Plating
I Solution
Drag
Drag
Out
Drag
Out
Drag
Out
Rinse
Tank
No. 1
f
*
'
Rinse
Tank
No. 2
Rinse
Tank
No. 3
Rinse
*" "Water ""
Recovery
Unit
Purified
Water
Makeup
Water
1
B. Partial Reclamation
Evapor-
T ation Drag
Drag In T n <-
« — a — — Losses Out
? .
Drag
I ^
Plating
Tank
Reclaimed
Uj'latijig. _ „.____ „ _
Solution
Rinse
Rank
No. 1
1
.mf
^
k
Recovery
Unit
-I
PL
Iwa
?
Rinse
Tank
No. 2
i
rif ied '
ter i
^
Drag
Out
N-«l ^x ~"^ *'
Rinse
Tank
No. 2
>
Rinse
Rank
No. 3
Drag
Out .
— *
. ^ ^Rinse^
^"
To Waste
I Makeup Water
_ _ Treatment
FIGURE 2. TOTAL AND PARTIAL RECLAMATION OF_PLATING
CHEMICALS LOST THROUGH DRAG
56
-------
in Figure 2A handles three times as much water as the partial recovery unit,
Figure 2B, and has a capital cost of about 2.5 times that of Figure 2B.
With careful water conservation practices, 90 percent recovery of the chro-
mium and nickel plating chemicals can be achieved with the partial recovery
unit shown in 2B. Where the main purpose is to recover plating chemicals
and return them to the process baths, partial recovery units such as 2B
are used. Further improvement in the recovery of plating chemicals (up to
99 percent) is possible if rinse tank No. 2 is added to the recovery loop
shown in 2B. It is not often economically feasible (or even physically
possible) to treat the entire volume of effluent. Usually selective
and integrated treatment is combined with conventional treatment.
Ion-Exchange Recovery Units '
One method for recovering metals from the plating bath is by ion
exchange. In using this technique, the wastewater passes through an ion-
exchange system to regenerate the captured plating solution and selectively
capture the contaminants that build up in the plating bath. The treated
water is recovered and reused as rinse water. Figure 3 is a schematic of
an ion-exchange system used for the recovery of chromic acid. Note that the
principle is to return as much drag out as possible to the plating baths
through countercurrent rinse baths. Rinse water from the last rinse stage
is then recirculated over cationic and anionic filters arranged in series.
The eluate from the anionic exchanger contains sodium chromate, Na2Cr04.
It is returned to the chromium bath through a cationic exchanger saturated
with hydrated hydrogen ions.
Ion exchange is an economically attractive method of concentrating
plating chemicals. It also permits the removal of metallic impurities from
rinse water and its reuse. It has not worked well on cyanide rinse water;
however, a 3-bed system consisting of a strongly acidic, weakly basic, and
strongly basic ion exchanger has been used in Europe for removing cyanides.
Small evaporative recovery units may be needed to augment ion-exchange
recovery; in some cases small evaporators may be needed to raise the con-
centration of the output from the ion-exchange unit.
(5-7}
Evaporative Recovery Units
Evaporation is a firmly established procedure for recovering
plating chemicals and rinse water from plating waste effluents. Over 100
evaporative units (i.e., single-, double-, multiple-effect, and vapor-
recompression evaporator units) have been installed in the U.S. These
units are used in partial recovery loops such as that illustrated in
Figure 2B, and also in combination with other recovery methods such as ion
exchange and reverse osmosis. Vapor-recompression evaporators are used
only where steam evaporators are not available, and thus the high cost of
the expensive and complex compressor can be economically justified.
Single-effect evaporators are less efficient than double effect
or vapor-recompression units; however, they require less initial capital,
57
-------
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and are easier to operate with inexperienced personnel. There are a number
of single-effect evaporator units available. They are as follows: (1)
evaporation based on the cooling tower principle, (2) submerged-tube
evaporation, (3) flash evaporation, and (4) "climbing film" evaporation.
The latter, a new concept introduced by the Corning Glass Company, is
illustrated in Figure 4. The illustration shows an arrangement for
partial recovery such as that shown in Figure 2B. Only minor changes
would be required to achieve total recovery such as that diagrammed in
Figure 2A.
The recovery unit shown at the right in Figure 4 consists of
a glass shell and a tube heat exchanger mounted vertically. The solution
is fed through the bottom of this evaporative unit. Boiling of the solution
causes the liquid to surge and this produces a "climbing film" effect which
improves the heat transfer. Vapor and liquid overflow from the top of the
tube are separated in the cyclone. Recycled rinse water containing less
than 0.05 ppm of chromic acid can be produced from chromium plating rinse
water using this method.
In concentrating the rinse water by evaporation to recover chem-
icals in such operations as chromium and nickel plating, relatively high
plating bath temperatures aid recovery. Nickel baths operate at around
60 C (140 F). Accordingly, considerable evaporation at this temperature
permits the recovery of most of the drag out by countercurrent rinsing
alone. However, to reduce the number of rinsing stages and provide for a
good rinse efficiency, evaporators (or alternative recovery mechanisms) are
used. Chromium baths operate at lower temperatures than nickel baths, 40-
50 C (104-122 F), thus it is more difficult to recycle chemicals by counter-
current baths alone, and, therefore, evaporators (or alternative recovery
mechanisms) are necessary.
Closed-loop evaporative recovery systems require close control of
the bath for good operation. For example, nickel baths are treated with
activated carbon to prevent contamination by organic substances. Metallic
impurities such as iron, copper, and zinc are precipitated selectively by
electrolysis.
In addition to its use in nickel and chromium plating, evapora-
tive recovery units are being used on gold plating baths. Systems for zinc,
copper, brass, and cadmium cyanide baths, and for lead-tin-copper fluoborate
baths have also been installed.
Zinc cyanide baths operate most efficiently at 25-30 C (67-86 F)
and require cooling to maintain this temperature. Accordingly, cooling
towers have been used to cool the baths and concentrate them at the same
time. Carbon dioxide is absorbed in the process which in turn leads to
the rapid formation of zinc carbonate. This must be removed either by
freezing or precipitation with calcium in a separate treatment. This and
problems arising from small amounts of impurities in the zinc cyanide bath
make this method of recovery marginal.
59
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Reverse Osmosis Recovery Units '
Concentration of the rinse waters from nickel plating baths is
performed in a number of installations utilizing reverse osmosis units.
Commercial reverse osmosis units for zinc and copper acid plating rinse
water have also been installed.
In reverse osmosis, a pressure differential across a membrane
forces the water through the membrane, leaving behind most of the dissolved
salts. Using this method, salts in the rinse water from nickel and copper
sulfate plating baths can be concentrated to solutions containing up to
15 percent salts, by weight.
Several membrane support systems are in commercial use; these
include plate and frame, tubular, spiral wound, and hollow fine fiber
designs. The cellulose acid membrane used in most of the development work
to date has a limit on the pH of solutions that can be handled. The pH
range of 3 to 8 precludes treating either strongly acid or alkaline solu-
tions. Spiral wound and hollow fiber polyamide membranes have been tried
on copper, zinc, and cadmium cyanide baths on an experimental basis. Pilot
plant trials have also been run on copper cyanide rinse water; however,
there have been problems with this system with fouling of the membrane.
One of the disadvantages of reverse osmosis recovery units is the tendency
toward fouling of the membrane by slightly soluble components in solution
and by suspended solids in feeds (feeds must be amenable to solids separa-
tion before treatment by reverse osmosis).
Other Recovery Methods;
Current State of the Art
There are a number of other recovery methods which are being tried
out but have not yet reached commercial scale. These include freezing,
electrodialysis, ion-flotation techniques, electrolytic stripping, carbon
absorption, and liquid-liquid extraction. These are scattered operations
using chemical precipitation and crystallization from solution. Commercial
electrolytic recovery of tin and silver from plating bath rinses is also
being done.
It is evident from the discussions on materials reclamation that
there are still many types of metal plating processes which are not yet
amenable to problem-free, closed-loop reclamation. This is particularly
true in the case of zinc, copper, and cadmium cyanide plating baths. Con-
tinuous closed-loop reclamation of plating chemicals from nickel and
chromium plating baths has proven to be commercially feasible; however,
there are still problems with these recovery systems. Closed-loop svsterns
are subject to impurity builiup which requires hleedoff fi uui lii.e o lime.
While the main process loops may be closed, secondary purification loops
are much more difficult. Thus, nickel impurities can be removed from
chromium plating baths by ion exchange and returned to the ndckol bath in
a closed-loop system, but sodium sulfate and sodium chloride at 6 excess
-------
which cannot be completely returned to the process. There is also the
problem of sludge from acid dip and cleaner line rinses for which recycling
usually is not economical. Research is continuing in this area of recla-
mation; the Metal Finishers Foundation has put priority on the concentration
and recycling of cyanide compounds in plating baths.
WASTE STREAM GENERATION
In performing the study of the electroplating and metal finishing
industry, four waste types were identified as being destined for land dis-
posal. These are as follows:
Water-pollution control sludges
Process wastes
Degreaser sludges
Salt precipitates from electroless nickel bath regeneration.
Water-pollution control sludges have been identified as a class
because they constitute the largest single waste category destined for
land disposal. Included in this category are rinse waters from each plating
step which contain potentially hazardous chemicals dragged out from concen-
trated plating solutions. Also included are process solutions—such as
alkaline cleaners, acid dips and pickles, and conversion coating solutions—
some of which are dumped at regular intervals. Water pollution control,
when practiced at an electroplating and metal finishing facility, will
precipitate the dissolved potentially hazardous facility materials, thus
generating a sludge for land disposal.
Process wastes include grinding, polishing, and buffing dusts,
filter aids, anode sludges and anode bags, plating racks, and rack mater-
ials. Most of these materials bear occluded or absorbed process chemicals
and are generally disposed of by landfill means, either separately dis-
posed or mixed with the normal refuse.
Degreaser sludges are in most cases chlorinated hydrocarbons which
are used to remove greases and oils from mechanically finished parts.
These sludges then contain dissolved greases and oils, buffing compounds,
abrasives, cloths, and metals. They may be discarded through the routes
of direct disposal to the land or may be routed to a reclamation operation
(e.g., distillation).
Salt precipitates from electroless nickel plating operations are
produced during the regeneration of the bath composed of calcium ortho-
phosphite and calcium sulfate. The plating bath chemicals contained in the
precipitate may be returned to the bath by washing the filtrate. This parti-
cular type of waste may not always be present since electroless nickel
plating operations are not as common as electroplating operations or small
volume solutions are not regenerated.
A simplified representation of the relationships between the
sources a ad the types of wastes is shown in Figure 5. Details and varia-
tions from the concept are discussed in the following paragraphs.
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Water Pollution Control Sludges
The source of these sludges is the process solutions drag out
from the workpieces as they are moved from tank to tank in any plating
operation. The concentrated process solutions carried out as drag out are
rinsed in continuously flowing water at a rate which ranges from 1 to 10
gallons per minute. The water flow rate depends on the number of rinses
and whether or not they are connected to each other in a countercurrent
or select mode.
The rinse waters contain alkalies, acids with dissolved metals,
and possibly cyanides from the preplating steps. The metal or metals
being plated onto the workpieces appear as dissolved salts in the rinse
waters following metal deposition steps. Also present are conductivity
salts and additives which were introduced to enhance electrodeposition or
the properties of the deposits. Some plating processes incorporate a post-
plating step intended to alter the metal surface by conversion or filming
to improve on the corrosion properties of the deposits. Drag outs from
these solutions also contain metals and chemicals. The rinse waters are
then collected throughout the plant in three distinct streams. One stream
carries all cyanide-bearing wastes, another all chromium-bearing wastes,
and a third stream contains all alkalies and acids and metal salt solu-
tions other than chromium and those metals which are chemically bound to
cyanide. The hazardous chemicals are then destroyed or reduced by passing
the streams through a water pollution control system, metals are precipi-
tated and separated, and the effluent is discharged to a stream or sewer.
Plating solutions contain valuable metals in concentrations as
high as 200 g/1 (27 oz/gal), as well as chemical salts and additives. For
these reasons, plating solutions are maintained by purification and filter-
ing and are rarely ever disposed of. In contrast, pre- and postplating
solutions are dumped at regular production intervals as spent baths, and
metered into the water pollution control equipment for treatment along with
the rinse waters. Such solutions are alkaline soak and electrolytic
cleaners, acid and cyanide dips, pickles, descaling baths, chemical and
electrochemical polishing baths, oxidizing, phosphating, coloring, and
conversion coating solutions.
Spills and leaks from process tanks may also occasionally occur.
With proper plant equipment maintenance and good housekeeping such hazard-
ous waste generation can be kept to a minimum. When it does occur, the
wastewater is handled by the water pollution control equipment adding
additional but relatively minute quantities of sludges for disposal.
Water Pollution Control Technology
The chemical treatment of wastewater from electroplating and
metal finishing operations involves simple chemical reactions carried out
on a batch or continuous type basis. This type of operation should pro-
vide sufficient holding time to complete these reactions, continuous
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monitoring of oxidation-reduction potentials and pH, and controls for regu-
lating reagent additions. The amount of metals precipitated depends on the
insolubility of their hydroxides. When solubilizing complexing agents are
present, precipitation is not as complete, so additional or different chem-
ical steps may be required. For example, cyanide ions must be destroyed,
not only because they are toxic but also because they prevent the effective
precipitation of metals as hydroxides. Similarly, chelating agents, such
as EDTA (ethylene-diamine-tetraacetic acid), when used in some process
solutions, make complete metal hydroxide precipitation more difficult.
But since these compounds are used only in very small quantities at concen-
trations of a few mg/1, the amount of water pollution control sludge is
reduced by only an indescribably small fraction.
The first reaction to be considered is the destruction of cyanide
to nitrogen and carbon dioxide using chlorine or hypochlorides. Another
process to destroy cyanide is to use ferrous sulfate which forms ferro-
cyanide and does produce a solid waste. This process is used very infre-
quently and then only for the destruction of very concentrated cyanide
wastes if for no other than economic considerations.
Other anions found commonly in electroplating and metal finishing
operations with the exception of fluorides, fluoborates, and phosphates
cannot be removed by precipitation and therefore escape in the effluent and
do not produce a solid waste. Fluorides and fluoborates can be used with
practically all metals for the electrodeposition and at times for etching
of certain basis metals. The most common use is in the electrodeposition
of lead and tin coatings and their alloys for bearing materials and solder
applications. Fluoborates hydrolize to HF and BF3 in dilute solutions and
the fluoride can be precipitated with lime for disposal as a sludge. Fluo-
ride removal is not as complete as desired, with more than 10 ppm escaping
with the effluent stream. Phosphates are present in alkaline cleaning
solutions; however, there is a trend to reduce their concentration or elim-
inate their use because they are difficult to remove from plant effluents
by precipitation as calcium diphosphate (CaHPO^-Zt^O), which has a solu-
bility of 0.2 g/1 in cold water. The sodium diphosphate which would result
from the use of caustic is much more soluble. Heavy concentrations of
phosphates aie found in rinses following electrdpolishing and chemical
polishing operations where concentrated phosphoric acid is employed. In
plants which process aluminum, aluminum phosphate is precipitated (AlPC^)
when these two ions are present in the same neutral solution. Thirty to
thirty-five percent of the concentrated phosphoric acid solutions are not
expected to produce solid waste since they can be used as a raw material
for the manufacture of fertilizer. Zinc or iron phosphates are used in
the processing of steel as a corrosion-protective undercoating and/or as
a bonding agent in electropainting processes. Their concentrations in
this application are about one-tenth of those used in electroplating oper-
ations, ane.1 the phosphates as well as the metals are removed from solution by
dry lime precipitation for disposal with the water pollution control wastes.
The next chemical reaction to be considered is the reduction of
hexavalent chromium to trivalent chromium using sulfite-containing compounds
or ferrous sulfate.
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In acid solutions these reactions are as follows:
or
2Cr03 + 6FeS04 + 6H2S
and
Cr2(S04)3 + eNaOH^^F 2Cr(OH)3
In using ferrous sulfate as a reducing agent, approximately twice the vol-
ume of sludge is produced since three moles of ferric sulfate must also be
precipitated and removed as the hydroxide.
In alkaline solutions hexavalent chromium can be reduced and pre-
cipitated with hydrazine.
4H0CrO, + 3N0H. fZ^4Cr (OH) , + 3N + 4H_0.
24 24 J /
Chromium wastes are generated from chromium plating solutions,
chromates and dichromates from bright dipping, electropolishing, and the
conversion coatings on zinc and cadmium. Large installations for bright
chromium plating, where the plating time is only from 5 to 15 minutes
contribute large quantities of chromium to the waste stream, whereas fewer
installations for hard chromium plating with a plating time of several
hours contribute much less to the waste. Conversion coating solutions,
although having a much smaller bath concentration of chromium (~15 g/1),
are periodically dumped and consequently contribute significant quantities
of chromium to the waste stream.
Other finishing operations performed in electroplating and metal
finishing facilities will also generate a waste. Chemical milling of
aluminum is one of these operations and is carried out in an alkaline
solution. The resulting wastes can be used for phosphate precipitation
where the phosphates are part of the plant operations, which is generally
the case in the metal finishing of aluminum. Milling of other metals is
carried out in acid solutions and hydroxide sludges are formed by neutrali-
zation.
Neutralization and chemical precipitation are also used to remove
heavy metals from the rinses of etching operations. The etching solutions
themselves are generated by electrolyzing in a closed-loop system. Where
the etchants contain chromates, acetates, or ammonia, as is the case for
some printed-circuit manufacturing, and as a pretreatment for plating on
polystyrene, polyethylene, polyvinylchloride, polycarbonate, polysulfone,
polypropylene, and ABS, they are shipped to recovery plants. With the
exception of the last two materials, all require solvent treatment to make
the surface hydrophilic. The contaminated solvents are reclaimed by dis-
tillation after being shipped in drums to a reclaimer or the supplier and
are, therefore, not destined for land disposal by the electroplating industry.
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The plating of plastics is also performed and requires some addi-
tional process steps to those commonly employed for plating on metallic
surfaces. Some of these steps are: (1) sensitizing the plastic, which
involves the use of stannous chloride solution; (2) nucleation, which
deposits a film of palladium metal; and (3) electroless plating. As an
alternative to steps (1) and (2), palladium-chloride may be applied dir-
ectly onto the plastic and then reduced to the metallic form by the use
of formaldehyde, hypophosphite, hydrazine, or borane compounds. Of all
the process steps, only the use of stannous chloride, when precipitated
as the hydroxide in chemical waste treatment practices, adds to the sludge.
Spent palladium solutions are returned to the supplier for recovery, while
the palladium concentration is in the rinse on the order of t>pb and is lost
with the effluent.
The electroless plating solutions, which are also used to produce
deposits on metallic surfaces, contain either nickel or copper,, Nickel and
copper wastes from both rinse waters and spent plating solutions are
treated at pH 13 with caustic soda to destroy the complex and precipitate
the metal hydroxides. Copper may also be precipitated as a metallic sludge
by heating to ~60 C (150-160 F) for more than 4 hours. The resulting
metal or metal hydroxide wastes go to land disposal, although it is not
uncommon for the plater to return spent solutions to a supplier or
scavenger.
The choice of the treatment chemical for metal hydroxide preci-
pitation is between slurried lime, Ca(OH) , or caustic soda, NaOH,
purchased dry or in liquid form« The stoichiometric quantities of hydroxide
sludge produced per kg of metal are as follows:
A1(OH)3 2.89 Fe(OH)2 1.91
Cd(OH)2 1.3 Ni(OH)2 1.52
Cr(OH)3 Io98 Zn(OH)2 1.52
Cu(OH)2 1.54
Insoluble calcium carbonate is formed with the use of lime thus
adding to the sludge volume; the remaining calcium or all of the sodium
part of the reagents appears in the plant water effluent. Precipitation
with lime is generally preferred since it produces a sludge wherein the
gelatinous hydroxide colloids are less difficult to settle and easier to
separate than with caustic. Reaction rates and efficiency are affected
by the presence of other cations (Mg) or anions (CO-j), the pH of the
solution, the time allowed before separating the solids, the precipitation
agent (lime or caustic) used, and the degree of agitation. Reagent chem-
icals are added in excess from 5 to 10 percent.
Coagulant aids are usually required for sedimentation and separa-
tion. Iron salts as well as alum may serve this purpose; however, this
will increase the quantity of sludge generated as the hydroxide. Poly-
electrolytes are most commonly used in concentrations of 1 to 40 mg/1
(0.01 to 0.03 lb/1,000 gal). For an average-size job shop with a water
use rate of 230,000 I/day (60,000 gal/day), as much as 9.2 kg/day (20 lb/
day) of flocculant is added. Solids adhering to the workpieces are removed
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during the alkaline or acid cleaning steps prior to electroplating are
added to the total solid wastes for disposal. Alkaline wastes, such as
soaps and detergents from burnishing or tumbling operations, are simply
neutralized. Where ion-exchange systems are used for purifying tap or
process water, any solutions used to regenerate these systems are also
passed through the treatment plant.
Toxic fumes are also generated in many plating operations.
Electrolytic cleaning solutions, cyanide, and chromium plating solutions
all work at elevated temperatures and at current efficiencies ranging from
12 percent to less than 100 percent. That portion of the current which
is not used for metal deposition or anode dissolution produces H2 or C>2,
respectively. The escaping gases carry with them various amounts of solu-
tion in the form of a mist which must be exhausted. Similar conditions
exist for pickling, descaling, bright dipping, and chemical or electro-
chemical polishing. Dispersion of such fumes into the atmosphere is
generally not permitted so that fume scrubbers or other cleaning devices
may be required. Wet scrubbers or spray chambers are most often used.
Depending on the waste treatment effort exercised by a plant, the result-
ing liquid wastes may either be handled with the plating wastes in the
treatment system or periodically collected from the scrubber and returned
to a scavenger. When processed through the treatment system, the acids
or alkali are simply neutralized; but cyanides, chromic acid, and solutions
containing metal ions add to the solid waste load of the plant.
Quantities of Sludge. The quantity of sludge produced in an
electroplating and metal finishing plant is not uniform for any given
process because the drag out and spillage varies from plant to plant.
The reasons are: (1) processes plating the same finishes on the same
basis metal vary from plant to plant, depending on the state of the art
practiced in each plant; (2) process lines may be hand operated, semi-
automated, or automated; (3) variations in the size and shape of the parts
causes variations in the volume of solution drag out; (4) tank sizes
affect rack design; (5) rinsing practices differ from high water-flow
single rinse tanks to save rinses, or mists which collect and return drag
out chemicals to the plating baths; (6) deposit thickness directly affects
the production rate, i.e., the thinner the coating applied the more drag
out results for a given size installation; (7) simple housekeeping prac-
tices by maintaining all equipment and containing spills reduces the waste
load; and (8) water conservation through control technology with the use
of ion exchange, evaporative recovery, reverse osmosis, freezing, electro-
dialysis, electrolytic stripping, carbon adsprtion, ion flotation, and
liquid-liquid extraction.
The control technology which may be utilized has specific appli-
cations. In all instances, it is applied to the plating solutions, but
not to the cleaners or acid rinses. Ion exchange is used for in-process
control of raw water, processing baths, and rinse waters. If the appli-
cation is to produce deionized water or to remove impurities from the
plating bath, more heavy metals are generated and have to be disposed of
as a concentrate from the backwash operations or fed to the chemical
treatment system. On the other hand, concentrates may be returned to the
baths, eliminating the need for dispersing of a waste to the land. Examp-
les would be the recovery of chromic acid or phosphoric acid.
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Evaporative recovery systems greatly reduce the quantity of
waste by returning large portions of the drag out chemicals to the baths
and also recycling the rinse waters. These units are commercially applied
to zinc, copper, cadmium, chromium, and nickel baths. Because continuous
recycling increases the contaminant concentration in the baths, some of the
concentrate is discharged, and the system falls short of beinp able to achieve
zero discharge and, consequently, zero sludge disposal.
Reverse osmosis employs a pressure differential across a mem-
brane to separate the solution into a concentrate and a solution approach-
ing the purity of the solvent. Units having a through-flow rate of less
than 1200 1/hr (300 gal/hr) have been installed to recover plating bath
chemicals and make closed-loop operations of the plating baths possible.
The disposable waste would be worn and fouled membranes.
Electrolytic stripping has been used for the recovery of precious
metals, copper and tin. In order to strip a solution by electrodeposition
of its metal, it is necessary that the metallic ions in a dilute solution
reach the cathode surface at a sufficient rate so that essentially all
of the metal ions can be deposited in a reasonable time. This can be
accomplished by pumping the solution between closely spaced electrodes,
through beds of metal or metal-coated glass spheres, through expanded
metal or porous carbon electrodes. If cyanide is present in these solu-
tions, and is not oxidized at the anode, it must be destroyed subsequently
by chemical means.
Freezing, electrodialysis, carbon adsorption, ion flotation,
and liquid-liquid extraction have also been considered but are still in
the development stage. Pilot plant installations will not affect the
quantities of the pollution control sludge, either now or in the near-
term future.
Solids Separation and Sludge Disposal
The first step in separating the precipitated metals and undis-
solved solids from the treated wastewater is settling through clarifiers
or solid contactors. Settling is accomplished by a batch process in a
stagnant tank; after a certain period of time (normally over 2 hours) and
after the clear effluent has been drawn off the side or the top of the
tank, the sludge may be emptied through the bottom. The continuous system
uses a baffled tank allowing the stream to flow first to the bottom and
then rise with a decreasing vertical velocity until the precipitates can
settle in a practically stagnant fluid. Clarifier underflow or "sludge"
contains typically 1 to 2 percent solids and is pumped to a pond, lagoon,
or holding sump or further processed by dewatering. Flowthrough ponds,
lagoons, or sumps discharge some more effluent to a stream or sewer system
causing a thickening of the sludge to 4 to 6 percent solids. (Their
holding capacity is no less than the sludge accumulated during a 30-day
production.) This sludge is periodically removed by tank trucks to a
disposal site. In other instances, seepage-type ponds are used where the
effluent part of the sludge is allowed to percolate through the ground
causing the sludge to thicken to where it possibly could contain as much
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as 12 percent solids. The dangers associated with lagooning are seepage
of wastewater associated with the sludge into the aquifer or to adjacent
streams. Percolating ponds are found with plants located in rural areas
where there is sufficient land available. Plants in urban areas are
forced to use holding sumps. Depending on the size of the plant and the
space available the sumps may have to be emptied weekly, biweekly,
monthly, or in rare cases, less frequently.
Centrifuges will also thicken sludges to a consistency of 8 to
12 percent solids. The effluent, however, contains suspended solids in
excess of 20 mg/1 and needs to be recirculated to the clarifier. Centri-
fuges require small amounts of floor space and less than half the land area
that is required for lagoons or ponds. Lagooning may be avoided entirely
by dewatering the sludge to a condition containing as much as about 50
percent solids. Rotary vacuum filters will concentrate a sludge containing
4 to 8 percent solids to 20 to 25 percent solids. The same solids content
can be achieved with pressure filters but the filtrate contains less than
3 mg/1 of suspended solids so that a return of the effluent to the clari-
fier is not needed. Semicontinuous tank filters may further increase the
solids content to as high as 35 percent. Automated tank-type pressure
filters and plate-and-frame presses produce the highest solids-content
waste of about 50 percent. The choice of which dewatering device to use
is one of economics, available space, or the limitations imposed on the
ultimate disposal. The availability and area of privately owned land at
or near a plant site is a factor in determining the disposal mode.
Sludge may also be solidified by addition of chemical fixing
agents which insolubilize the metal hydroxides. The additions, of course,
increase the volume of sludge destined for land disposal. The sludge is
then similar in consistency to dried clay and reduces leaching to a frac-
tion of a ppb of heavy metal.
Concentrated wastes rich in metals may be considered for recovery.
This depends on the number and ratio of the constituents and the volume of
the sludge. Large volumes of sludge in one location are economically fav-
orable. For similar sludges from different plants a central processing
station may accomplish such a task. However, very little effort, if any,
is being expended at present to undertake such a task. As raw material
prices increase and their availability decreases, the economics of recovery
of sludges may become more favorable.
Ultimate sludge disposal, at the present, occurs at the follow-
ing locations:
• Company on-site lagoons
• Municipal or private dumps
• Covered landfill
• Sewers.
Other possibilities are disposal at lagoons or landfill areas, incinera-
tion, recovery and reuse, or some disposal by deep-well injection, but
none of these are practiced to any large extent.
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Process Wastes
Process wastes can be separated into two distinct types (i.e.,
pre- and postplating preparation wastes and miscellaneous wastes. The
first type of waste is generated in the electroplating and metal finishing
process in the preparation of the basis material prior to metal deposition
or the application of surface coatings. The appearance of the product is
in many ways reflected by the way the underlying surface has been finished.
Similarly, the protection given to a product against corrosion and wear,
or the physical-mechanical properties of the deposit are, among other
criteria, dependent on the preparation for plating. Depending on the size,
shape, and number of parts, and finish desired, parts may be manually,
automatically, or bulk finished. Depending on its surface condition, one
or more operations may be necessary in preplating preparation. A post-
plating mechanical operation may also be desirable. All of these operations
produce solid wastes which are composed of the materials being processed
and the materials used to achieve a desired finish. These operations
are: grinding, abrasive blasting, polishing, buffing, brushing, mass
finishing by vibratory deburring, and barrel processes.
The second type of waste results from daily plant operations
which are not precipitated metal hydroxide sludges or solids. They can
be metallics from used anodes, plating racks, etc., or nonmetallics such
as woven anode bags, coatings on plating racks, filter cakes, worn tank
linings, etc.
Pre- and Postplating Preparation
Process Wastes
Grinding. The most frequent application of grinding is associated
with hard chromium plating„ The uses of hard chromium are many, and it may be
found in all types of industry for improving hardness, heat-, wear-, and
corrosion-resistance, and for its low coefficient of friction. Grinding
can be carried out before or after hard chromium plating. Worn parts, such
as crankshafts, propeller shafts, cams, dies, molds, etc., can be salvaged
by grinding back or cutting beyond the old surface coating. New parts can
be processed in the same way, but it is common to manufacture these parts
to within close tolerances and then plate the metal to the thickness desired.
Sometimes the parts are overplated and then reground to finish dimensions.
Included in this group are gauges, taps, molds for plastic and rubber,
cutting tools, printing rolls, dies, gun barrels, and various shafts.
The wastes that are generated are, of course, metallic chromium and the
basis material to which the coating is applied, or an intermediate coating.
Waste is normally disposed of along with the weekly refuse collection, or
if in sufficient quantity, to a metal scrap dealer. Grinding carried out
on bulk material is discussed later.
Abrasive Blasting. Materials are frequently finished by wet or
dry abrasive blasting, for the removal of scale, rust, or other coatings;
for roughening the surface or to improve or develop the surface for finish;
for cold working (shot peening) to increase fatigue life; or to decrease
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susceptibility to stress corrosion or correct objectionable distortion.
The amount of material removed depends on the type and quantity of abra-
sive used and the amount of mechanical or air pressure applied. Angular
grit, steel shot, manufactured abrasives, such as aluminum oxide and
silicon carbide, natural abrasives, slag products, and some glass beads
are the most common recyclable materials, ranging in size from 50 to 500
mesh. Precleaning of the parts for the removal of soil is recommended
to keep these materials clean for recycling. The natural abrasives include
such items as pumice, silica, quartz, etc., and are used where metallic
impurities introduced to the part to be finished cannot be tolerated.
Sawdust, ground corncobs, crushed nut shells, glass beads, thermoplastic
materials, etc., comprise the soft abrasives which are normally discarded
with the weekly refuse. Dirt, soil, and scale are the only waste products
from dry blasting operations where recycling equipment is attached. Loss
of abrasives can occur by change of abrasive materials in the cabinet or
through dust filters.
Polishing and Buffing. In metal finishing, after grinding comes
the polishing operation which is done to achieve an intermediate surface
that can be further refined by buffing prior to plating. The polishing
operation may be carried out on metals or nonmetals; it removes some
material from the surfaces. The purpose of buffing is to smooth and
brighten the surface without much metal removal.
Polishing is carried out on hard-faced wheels varying in diam-
eter, thickness, and material depending upon the part that is being pro-
cessed and the finish and material removal rate that is desired. Wheels
are constructed of woven cotton fabrics, canvas, felt, or leather discs
glued or sewn together. Felt wheels are used where true surfaces are
required or where a contoured shape is being finished. Leather wheels
produce a finer finish, and wood wheels covered with leather are normally
used for flat surfaces. Abrasives are generally applied to these wheels
with synthetic adhesives or cements which have generally replaced the
hide-glue formerly used. The ratio of abrasive to glue used in the facing
of the wheels changes with grit size. Used wheels may be recoated after
removal of lubricants from the facing or the facing itself may be replaced.
The abrasives are fused-aluminum oxide, silicon carbide, and turkish emery.
Tallow, grease, special bar lubricants, and spray lubricants are also used.
Woven cloth belts are also used for polishing operations after having been
treated in the same way as the wheels. Greaseless polishing can be accomp-
lished with flexible polishing wheels using softer cloths. Burrs on
castings or stampings may also be removed with the use of polishing wheels.
Buffing normally follows the polishing operation as the last step
before plating. Depending on the desired finish of the product, the oper-
ations may vary. Satin finishes are obtained using fast cutting abrasives,
mostly aluminum oxide and silicon carbide. Glue or adhesive binders are
used to hold the abrasives, forming a greaseless compound buffing bar.
Grease-base buffing bars are made of animal, vegetable, mineral fats,
and waxes together with an adhesive. "Cut-down" buffing bars are composed
of various materials, depending on the metal to be finished. Once-ground
tripoli and aluminum oxide are the most widely used. An operation known as
"cut-and-color" buffing is used in place of the cut-down operation to
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achieve a small amount of metal removal while at the same time giving some
luster to the metal. Basically, the same constituents are used in different
compositions to produce cut-and-color bars. Red iron oxide powder is
frequently used for nonferrous metals. For producing a final finish having
the best color and luster, very small amounts of very fine abrasives are
used consisting of the materials listed above plus bars containing
chromium oxide for finishing stainless steel and chromium,,
Where semiautomatic or automatic machines are used, the buffing
compound is not in a bar form but is applied in a liquid form from air
pressure feed tanks or circulating drum pumping equipment. The newest
development is a system of airless spray buffing. Liquid compounds are
also available for manual buffing operations. The same abrasives that
are used in buffing bars are used in liquid buffing; however, materials
are oil solutions or water emulsions instead of grease, fat, waxes, and
oils. Liquid buffing compounds produce better finish at a lower cost than
maual operations with bar compounds, are required in smaller amounts,
cause less wear on the buffing wheel, and cause less dirt to be packed onto
the parts, which makes subsequent liquid cleaning easier. The different
production techniques are summarized in Table 24.
The wastes produced from these operations contain three basic
materials: (1) the metal removed by the abrasive media which will vary
in quantity depending on the type and size of the abrasive, the contact
time, the wheel diameter and speed, (2) the constituents of the specific
compounds used, and (3) the materials used on the wheels and buffs. Items
(1) through (3) are combined as one waste and collected from mechanical
exhaust systems. This waste is disposed of in a landfill, although other
means, such as incineration, are possible. Precious-metal wastes are
more likely to be collected and incinerated, with the metals recovered from
the ashes.
Wheels and buffs are consumed to about one-half of their diameter
and then discarded as solid waste or returned to a supplier for recycling.
In shops with small polishing and buffing operations, the former procedure
is more likely to occur.
Where plastics are being processed (thermosetting, thermoplastic,
molded or laminated, with or without fibrous filters), these materials are
also part of the waste.
Mass Finishing. The improvement of surface finishes, the removal
of burrs, edges and scales, and the forming of radii is specially obtained
by tumbling or vibratory finishing of several parts at one time. Just a
few large parts may be finished to specification or thousands of small
parts may be handled by machines with revolving (tumbling) or vibratory
motion. A medium is added to separate the workpieces and perform a finish-
ing operation similar to that in polishing and buffing. The media which
are used are listed in Table 25. Some of the media are used in conjunc-
tion with proprietary chemical compounds, or these compounds can be used
by themselves. Most operations are carried out wet. Equipment may be
made of wood or steel and may be lined with neoprene, vinyl, or polyure-
thane. Metal is removed in the process either as solid or dissolved ions
depending on the specific compound used. The proprietary chemical compounds
72