Pollution Prevention and Control Technologies for Plating
Section 4 - Chemical Solution Maintenance
4.3.4 Technology/Equipment Description
126.96.36.199 Commercial Equipment
Equipment selected for this application should have a simple mechanical
configuration and be physically sturdy and compact. The materials
of construction, including gaskets and seals, must be resistant
to high alkalinity, high temperature and sharp temperature fluctuations.
Soils and fine metal shavings must not impede the proper functioning
of the system (ref. 311).
The available systems typically have two sections within their
equipment package. In the first section, free oil and settleable
solids are separated from the remaining feedstream. The second
section houses or is connected to the microfiltration unit. A
key element of the design is the membrane cleaning function. This
can be achieved by a pulsing valve fitted into the permeate line.
Compressed air is periodically cycled into the permeate discharge
line to generate a series of back pulses, creating a momentary
(e.g., 0.5 seconds) reversal in the direction of the permeate
flow (ref. 311). This prevents solids from fouling the membrane.
The performance and sizing of microfiltration systems is dependent
primarily upon the specified flux. The flux is the amount of flow
per unit time that will permeate a unit of area of filter space.
U.S. industry usually expresses flux as gallons per square foot
of filter space per day (gfd). The selection of the membrane and
designation of the pressure, retenate flow rate, and concentration
of oil in the influent are the most critical factors for proper
system operation (ref. 477).
188.8.131.52 Commercial Equipment
This subsection contains a description of commercially available
microfiltration equipment used for degreasing/cleaning bath maintenance.
This is intended to provide the reader with information and data
on a cross section of available equipment. Mention of trade names
or commercial products is not intended to constitute endorsement
U.S. Filter markets a bath maintenance system, Membralox®
3000, which is applicable to aqueous and non-aqueous degreasing/cleaning
baths. It is a packaged unit requiring a 24 in. by 40 in. floor
space. It utilizes ceramic membranes with pore sizes ranging from
500 angstroms to 0.8 microns, depending upon the chemical make-up
of the bath. Systems using membranes with pore sizes of 0.2 microns
and larger contain an integral backpulsing system to help prevent
plugging of the filter membrane. In operation, the contaminated
cleaner enters a two-compartment holding tank through a bag filter
which initially removes large particulate material from the feed
stream. The level in the tank is maintained by a level switch,
which controls the tank inlet valve and also acts as a low-level
cutoff for the system pump. The oils accumulate in the initial
compartment and can be drained on a periodic basis. The liquid
then moves to a second tank compartment through a sub-surface
passage, thereby leaving the floating oils in the first compartment.
The liquid in the second compartment is pumped through the ceramic
filter. The water and cleaner chemicals are forced through the
ceramic membrane, while the oil and other soils are retained and
recycled back to the initial tank compartment. A range of system
capacities is available for feed stream flow rates of 300 gpd
to 1,500 gpd. The flux rate of the membrane is approximately 50
to 100 gfd. The expected membrane life is 10 or more years.
Kinetic Recovery Corporation markets a cleaner purification system
(CPS). A diagram of their equipment is shown in Exhibit 4-5. The
contaminated cleaner enters the system through a tank compartment
(1) that provides laminar flow conditions due to the presence
of baffles. Oil accumulates in the tank and is periodically drained
(2). The liquid then moves into the main tank compartment (3)
(working tank) from where it is pumped by the immersed pump (4)
through the ceramic membrane (5). The water with the dissolved
cleaner chemicals are forced through the membrane while the oil
and solids are retained and moved to the laminar flow compartment
(1). The dirt and solids (sludge) which settle to the bottom of
the tank can be periodically drained (6). The continuous operation
of the system provides a constant stream of purified cleaner back
to the cleaner bath.
The CPS utilizes microfiltration membranes (pore sizes of 0.2
µ or higher) and contains an integral back pulsing system
to prevent plugging and fouling (impurity layer on the surface
of the membrane). The integral back pulse is an air purge that
forces permeate from the reservoir back through the membrane without
pushing air into the membrane. This purge takes only a fraction
of a second. Dirt and solid impurities end up as slurry at the
bottom of the tank and are drained off.
Prosys Corporation manufactures microfiltration equipment for
various applications, including: recycle of caustic cleaner, end-of-pipe
treatment, polishing of treated effluent, vibratory media filtration,
zyglo removal, and coolant recovery. They have been marketing
equipment to the plating industry since 1987 to which they have
sold over 70 microfiltration units.
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