Pollution Prevention and Control Technologies for Plating
Section 4 - Chemical Solution Maintenance
4.2 COMMONLY APPLIED PREVENTATIVE AND CORRECTIVE BATH MAINTENANCE
4.2.2 Common Corrective Measures
126.96.36.199 Carbon Treatment
188.8.131.52 Electrolysis (Dummying)
184.108.40.206 Carbonate Freezing
Filtration is the most commonly applied method of corrective bath
maintenance. It is used to remove suspended solids from plating
and other metal finishing solutions. Suspended solids in plating
solutions may cause roughness and burning of deposits. Various
equipment are used for filtration, with the most common being
cartridge filters and precoat (diatomaceous earth) filters. Sand
or multimedia filters are also employed. Cartridge filters are
available with either in-tank or external configurations, with
the former used mostly for small tanks and the latter for larger
tanks. Most cartridges are disposable, however, washable and reusable
filters have been recently commercialized. Several respondents
reported use of the reusable filters (PS 180, PS 265, PS 229).
Precoat filters are used mostly for large tank applications. Filter
media are selected based on the chemical composition of the bath.
Filtration systems are sized based on solids loading and the required
flow rate (turnovers per hour). Typical flow rates for plating
solution applications are 2 to 3 bath turnovers per hour. Other
operating criteria for filtration processes can be found in the
literature (ref. 421).
A total of 1,020 applications of bath filtration were reported
by the 318 respondents of the Users Survey or an average of 3.2
per shop. Approximately two thirds of these filtration applications
involved the use of the less expensive in-tank devices and one-third
the external devices. The distribution of filtration applications
among the different types of plating baths was approximately the
same as the usage of the baths themselves (see Exhibit 4-1 which
indicates for each process the total number of plating processes
and the number of filtration applications identified). One notable
exception was chromium plating, where filtration was not frequently
Cost data for common bath maintenance techniques were not specifically
requested during the Users Survey. However, four shops provided
the following data for filtration.
PS 160 reported a capital cost of $14,000 (1991 costs, includes
$1,000 of installation costs), annual non-labor costs of $2,000
and an annual labor requirement of 50 hours for an external sand
filtration system (Techmatic) that serves three zinc chloride
tanks containing 7,600 gal of solution. They reported that the
sand bed requires manual stirring once per month and that the
sand must be replaced once each year. They indicated that the
downtime of the unit was 2% and reported a high satisfaction level
for the filter.
PS 191 reported that they have purchased multiple filter units
(Serfilco) that had a total capital cost (purchased during various
years dating back to 1965) of $40,940 (includes $3,000 of installation
costs) to filter two nickel plating tanks totaling 7,200 gal.
They employ standard filter cartridges, but rather then discard
them after a single use, the spent cartridges are washed in a
standard washing machining (inexpensive Sears unit). They have
found that by washing the cartridges (after first soaking them
in a tank), they can reuse them 3 to 4 times. They expend approximately
30 man-hrs per year washing cartridges. They first implemented
this procedure in 1989 and had to replace the washer in 1992.
The discharge from the soak tanks and the washer is sent to their
conventional treatment system. They originally implemented this
procedure in order to dispose of the filters as non-hazardous
waste, but after seeing the results of the cleaning process, decided
to reuse them. Used anode bags are processed in the same manner.
PS 114 reported the purchase of used filtration equipment for
$500 with installation costs of $350 (1992) for filtering an electrocleaner
and acid pickle (105 gal each). They indicated that annual non-labor
costs are $100 and the labor requirement is only 10 hrs/yr. They
estimated that by using filtration, they extend the life of the
electrocleaner by 10 times and the life of the acid pickle by
PS 253 reported cost data for a carbon treatment/filtration system
(see Section 220.127.116.11).
18.104.22.168 Carbon Treatment
Carbon treatment of plating baths is a common method of removing
organic contaminants. The carbon adsorbs organic impurities that
are present as a result of oil introduction or the breakdown of
bath constituents. It is used on both a continuous and batch basis.
Various application methods are available, including carbon filtration
cartridges (contain up to 8 oz of carbon and are restricted to
use on small applications), carbon canisters (up to 10 lbs of
carbon), precoat filters, and bulk application/agitation/filtration
(ref. 421). Typical dosages are 1 to 4 pounds of carbon per 100
gallons of solution (ref. 341, 421).
A total of 505 applications of carbon treatment were reported
by respondents to the Users Survey. The most frequently cited
applications were nickel electroplating (mostly Watts nickel and
nickel sulfamate), which accounted for 50% of all carbon treatment
applications. The other most common applications were copper electroplating
(18% of all applications) (mostly copper cyanide and copper sulfate),
zinc plating (10% of all applications) and cadmium cyanide plating
(4% of all applications). Survey respondents used continuous (46%)
and batch (54%) carbon treatment methods on an almost equal basis.
Cost data were not requested in the Users Survey for common bath
maintenance techniques. However, two shops provided the following
data for carbon treatment. PS 253 reported a capital cost of $14,200
(installed cost for external Mefiag 6500 SS filter/carbon treatment
system, 1986), non-labor annual costs of $616 and 39 hours/yr
of labor for maintaining a 3,600 gal Watts nickel bath. The bath
is continuously pumped through a carbon filter at a rate of 55
gpm. This operation generates 125 lbs/mth of spent filter pads
which are sent to off-site disposal. PS 253 indicated that they
are very satisfied with the unit and that it has a downtime of
0%. PS 160 reported a capital cost of $120 (1992) for an in-tank
unit (Flo-King) and annual operating costs of $240 (includes both
labor and non-labor costs). They indicated that they are satisfied
with the unit and that it has a downtime of only 2%.
22.214.171.124 Electrolysis (Dummying)
Dummy plating is an electrolytic treatment process in which metallic
contaminants in a metal finishing solution are either plated out
(low current density electrolysis) or oxidized (high current density
electrolysis). Dummy plating has been documented to be in use
since 1916 (ref. 339).
Low current density (LCD) dummy plating is applied to a range
of plating and other metal finishing processes. The contaminant
metals that are most frequently removed by dummy plating are copper,
zinc, iron and lead. Dummy plating is usually performed using
a corrugated steel sheet cathode with an anode to cathode spacing
of approximately 4 in. The optimal current density will depend
on the metal contaminants being removed. The normal range is 2
to 8 ASF. The duration of treatment is typically 2 to 5 amp-hr/gal.
Agitation is essential for speedy removal of contaminants and
air agitation should be used if the type of bath permits (ref.
270, 339, 340, 341).
LCD dummy plating can be performed on a batch or continuous basis.
Batch treatment is usually performed in the process tank and requires
down-time. Continuous treatment is usually performed in a side-tank
and cathodes are typically sized to permit 0.05 amp/gal of solution
(ref. 339, 341). The solution is preferably returned to the process
tank through a filter (ref. 314). An example of a continuous LCD
dummy plating application used by PS 118 on a closed-loop nickel
plating line is shown in Section 2.4.
High current density (HCD) dummy plating typically refers to the
practice of oxidizing trivalent chromium to hexavalent chromium
in chromic acid baths (e.g., chromium plating and chromic acid
anodizing). It is also used to gas-off chloride as chlorine.
The HCD process requires an anode to cathode ratio of between
10:1 and 30:1. Lead or lead alloy anodes are typically used in
the process. A lead peroxide film is formed on the anode which
functions as the oxidation agent. Current densities of between
100 to 300 ASF are used. The rate of conversion is controlled
by the overall cathode and anode areas and current flow.
Respondents to the Users Survey employed dummy plating most frequently
with nickel electroplating (48% of all electrolysis applications),
chromium electroplating (13%), zinc electroplating (11%), copper
electroplating (10%) and cadmium electroplating (7%).
126.96.36.199 Carbonate Freezing
Cyanide baths are adversely affected by carbonate buildup. Carbonates
are formed by the breakdown of cyanide (especially at high temperatures),
excessive anode current densities and the adsorption of carbon
dioxide from the air. Excessive cabonates cause increased resistance
in the bath, yielding low plating current densities, which normally
accentuate the poor appearance that metallic impurities cause.
An excessive carbonate concentration can affect the smoothness
of deposits, plating efficiency and plating range. Both sodium
and potassium baths are affected by carbonates. However, the sodium
bath is affected at a much lower concentration (14 oz/gal vs 40
oz/gal) (ref. 340, 482).
Sodium cyanide baths can be treated for excessive carbonate buildup
by "carbonate freezing" or crystallization. Potassium
cyanide baths must be treated by precipitation, which is also
applicable to sodium cyanide baths (see Section 4.2.6). Carbonate
freezing takes advantage of the low solubility of carbonate salts
in the sodium cyanide bath. The method involves lowering the bath
temperature to approximately 26F (-3C) where hydrated salt/Na2CO3
10H2O crystallizes out. This treatment procedure will also remove
sodium sulfate and sodium ferrocyanide (ref. 339).
A total of 111 applications of carbonate freezing were reported
by the respondents to the Users Survey. The vast majority of these
applications were for cadmium cyanide plating, copper cyanide
plating and copper cyanide strike baths.
Various chemical treatments of plating baths are performed to
remove bath contaminants via precipitation. Precipitation is generally
a batch process that is often performed in a spare tank where
the solution is chemically treated and filtered and returned to
its original tank. Electrolytic purification is sometimes applied
following chemical treatment to remove metal contaminants less
affected by precipitation (e.g., following purification of a Watts
nickel bath to remove copper, zinc and cadmium). Precipitation
is an alternative method to carbonate freezing for cyanide baths
and is especially applicable to potassium cyanide baths. Chemicals
used for this purpose include: barium cyanide, barium hydroxide,
calcium hydroxide, calcium sulfate or calcium cyanide. The least
expensive of these chemicals, calcium sulfate, forms a bulky precipitate
that is less easily removed. Other relatively common uses of precipitation
include lime addition for the removal of carbonates from silver
cyanide baths, sodium sulfide treatment of cyanide baths for zinc
and lead removal and nickel carbonate (1 to 2 lbs/100 gal) or
nickel hydrate treatment of nickel plating baths to remove miscellaneous
metal contaminants (e.g., iron, aluminum, silicon). The latter
of these methods is termed "high-pH treatment." Peroxide
is sometimes added during high-pH treatment to enhance precipitation
and destroy organics (ref. 340). Hydrogen peroxide is also used
for oxidizing soluble ferrous iron to insoluble ferric hydroxide
in acid chloride zinc baths (ref. 340, 482). Two respondents reported
the use of potassium permanganate (oxidizing agent) for iron removal
from zinc baths (PS 076, PS 268). The precipitated iron is removed
by filtration. Also, lime is sometimes used to remove phosphorus
compounds from electroless nickel solutions; however, this process
is not widely applied (ref. 286). One respondent reported the
use of silver oxide to precipitate chloride from a decorative
chromium bath (PS 162).
Respondents to the Users Survey reported 35 applications of precipitation
and another 42 applications of high pH treatment. The use of high-pH
treatment was restricted mostly to Watts nickel and sulfamate
nickel baths. Other precipitation applications in order of decreasing
frequency include: zinc plating, nickel plating, silver plating,
copper plating, bronze plating, chromium plating and cadmium plating.
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