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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.3 MICROFILTRATON
4.3.4 Technology/Equipment Description
4.3.4.1 General
4.3.4.2 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).
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 for use.
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.