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B. Process-Specific Cleaner Production
Technologies This section provides a general
overview of cleaner production technologies available to
platers. These technologies are organized according to
the three general stages of a plating operation:
cleaning, plating, and rinsing.
1. CLEANING
Surface cleaning may occur at several stages in a
plating operation: when parts come in from a supplier;
after parts undergo a metal finishing process; or prior
to a coating or painting operation. To lower production
costs by reducing the quantity of chemicals purchased and
the amount of wastewater discharged, plating shops may:
- reuse spent plating bath as influent for an
alkaline cleaning system;
- replace chlorinated solvents with aqueous
solvents for surface cleaning; or
- reuse spent cleaning solvents from upstream
operations in dirtier downstream processes.
The Surface Cleaning Laboratory at the Toxics Use
Reduction Institute can assist manufacturers in planning
and testing surface cleaning alternatives. For more
information, please call 978/934-3249.
2. PLATING
A plating shop can improve its plating process through
a variety of technologies that minimize waste treatment
or disposal costs, improve plating process efficiency,
and/or optimize plating quality. A shop can extend the
life of its plating bath solution, or recover metals and
chemicals from its spent plating bath solution, by:
operating its plating bath at optimal pH and
temperature to support the lowest possible concentration
of metals and bath chemistry;
dumping its plating bath only when necessary, based on
plating quality rather than on a calendar schedule;
reducing generation of acid vapors by installing acid
mist sprays, using ventilation/exhaust systems, or
adding vapor barriers;
recovering plating bath chemicals through the use of drag-out
tanks, electrodialysis,10 or
membrane technologies such as ultrafiltration,11
reverse osmosis12, or diffusion
dialysis13 (See Appendix B for a
case study of an electroplating facility in Springfield,
Massachusetts, that adopted membrane technology);
Figure 5
- recovering plating metals (particularly copper,
nickel, and precious metals) from rinsewater
using ion exchange;14
- recovering metals from spent process bath
solution by using an electrolytic process, electrowinning15
("dummy plating");
- substituting trivalent chrome, cobalt
alloys, or nickel tungsten alloys for hexavalent
chrome (in chrome plating);
- substituting non-cyanide for cyanide plating bath
solution (in copper, zinc, and cadmium plating);
- replacing chelating compounds with
non-chelated plating bath solution chemistries16;
or
- using alternative technologies for depositing
metal ions onto the workpiece, including chemical
or physical vapor deposition,17
which transports metals in a vapor state to a
part to achieve a solid metal coating,18
or thermal spray coating.19
| 10
Electrodialysis
employs ion-permeable and selective membranes
under an applied direct current potential
difference to separate out ions (most often
nickel) from an aqueous solution. Cushnie, George
C., Jr., Pollution Prevention and Control
Technology for Plating Operations, National
Center for Manufacturing Sciences and National
Association of Metal Finishers: Ann Arbor, MI,
1994, p. 138. 11 Ultrafiltration uses
higher pressure and membranes with smaller pore
sizes than other types of membrane technologies
to recover plating bath components from
wastewater solution. NEWMOA Manual, p.
133.
12 Reverse osmosis has been
applied to a range of processes, including brass,
chromium, copper, tin, and zinc plating
solutions, but it is most frequently and most
successfully used to recover nickel. Cushnie,
George C., Jr., p. 143.
13
Diffusion dialysis is used to purify acid baths
by removing metals such as copper, chrome,
nickel, iron, and aluminum so that the acid can
be reused. NEWMOA Manual, p. 143.
14 In
ion exchange, a plating chemical ion is exchanged
for a similarly charged ion attached to an
immobile solid particle (i.e. hydrogen and
hydroxyl ions located on an ion exchange resin).
The exchange occurs when rinsewater is passed
through vessels containing the resin. Ion
exchange is used more frequently to recover metal
from non-cyanide solutions, as ion exchange is
much more complex with cyanide solutions.
Cushnie, George C., Jr., pp. 98-99.
15
Electrowinning is a technique that uses a low
current to plate out metal contaminants from
process bath solution. It is commonly used in
precious metals plating operations, which are
important in the manufacture of jewelry and
electronics products. NEWMOA Manual , p.
64.
16
Chelating compounds are often added to plating
baths to prevent metal from building up on the
plating racks. However, because they form
tightly-bound compounds with the metal ions that
are difficult to precipitate out of spent
solution, chelating compounds increase the volume
and toxicity of process wastewater that a
facility discharges. Consequently, in order to
comply with the discharge limits in its permit, a
facility will need to use additional chemicals to
adjust the pH of a chelated bath discharge.
17
Chemical or physical vapor deposition includes
the following technologies: ion plating, ion
implantation, sputtering and sputter deposition,
and laser surface alloying. These technologies
eliminate the use of toxic components such as
cyanide from the plating process, and they reduce
the amount of metal-contaminated wastewater and
sludge that is generated from plating. NEWMOA
Manual, p. 148.
18 NEWMOA
Manual, p. 158.
19
Thermal spray coating may involve the use of a
combustion torch, electric arc spraying, or
plasma spraying. NEWMOA Manual, p. 149.
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Sectors - Metal Finishing
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