INORGANIC MEMBRANES FOR CLEANER BATH RECYCLING AND OILY WASTEWATER

Transcription

1 INORGANIC MEMBRANES FOR CLEANER BATH RECYCLING AND OILY WASTEWATER Allan P. Fischer Sales Group Leader, Environmental Technology Ei senmann Corporation Crystal Lake, Illinois Presented at: 'Finishing '93 Conference and Exposition" October 25-28, 1993 Dr. Albert B. Sabin Convention Center Cincinnati, Ohio Copyrighted by SME; Technical Paper Number to be Assigned

2 Introduction: In almost every manufactured item, oil is used in its handling, machining, stamping, or for corrosion protection. These oils, while necessary, create environmental problems when they have contaminated the process waters and have to be disposed of. In finishing operations, these same oils with other soils create additional problems by preventing adhesion and have to be removed prior to the finish being applied whether it is by plating, conversion coating, painting, or porcelain enameling. While various types of cleaners have been employed, the most common in high production applications andbeing usedmore even in small operations with the phase out of chlorofluorocarbon solvents is aqueous alkaline cleaners. The cost of these cleaners as well as their disposal cost and the growing environmental regulations including the emphasis on waste minimization, chemical recovery, and resource conservation created a need for a system to extend the useful working life of these cleaners and to remove the oil contamination when these cleaners and other oily wastes are to be discarded. The heart of the systems that can respond to both these diverse applications is based on cross flow membranes in both microfiltration and ultrafiltration porosities.

3 The use of membrane filtration is not new to finishing. The most common and successful application is ultrafiltration of electro-deposited paint (E-coat). Table 1 highlights the various applications and which membranes have been utilized. In this paper we will limit our discussion to the use of membranes for removing oils from cleaners and wastes. To date, the most frequently used type of membrane is based on various types of synthetic organic polymers and is commonly referred to as polymeric with some of the more common materials listed on Table 2. The selection of the proper polymeric material is very critical since most have limited temperature ranges, chemical resistance, and suffer attack by solvents. During the sixties and seventies, both microfiltration and ultrafiltration membranes were developed and applied to both the disposal of oily wastes and recycling of aqueous cleaners. These were made from various polymeric materials andmet with varying degrees of success. The above stated limits resulted in major system problems of low flux rates, rapid fouling, limited cleanability, inability to handle high bath temperatures, low tolerance for high oil concentrations, and finally, short life. While the constant development of new polymeric membrane materials have improved on almost all of the earlier limits, there are still short falls regarding high temperatures, broad chemical resistance, maximum oil concentrations, and overall life. During this same time, mineral or inorganic membranes were also being developed based on silica (glass) or metal oxides such as aluminum (Al,O,), titanium (TiO,), and zirconium (ZrO,). The early oxide membranes were of the lldynamicll type where a thin slurry layer of ZrO, was deposited on a porous stainless steel or carbon tube, While these worked the membrane required frequent replacement which limited its use. Further fixation of the ZrO, by sintering minimized this problem and the current, third generation of this product noweprovides an almost indestructible membrane element in the range of 1400 A (0.14 micron) down to a molecular cut-off of lo3 Daltons. Ceramic membranes of A1,0, also known as alumina, on alumina support was originally developed as a single tubular microfiltration membrane for gaseous uranium enrichment. They have evolved into a broad spectrum of applications due to multiconfigurations, wide range of porosities, and the matching of the optimum ceramic material to the actual process. The ceramic membranes, like the ZrO,/carbon, can resist solvents, temperatures over 3OO0C, strong chemicals, and full ph range with the exception that the alumina membranes and support structures do not stand up to hydrofluoric acid and hot, highly concentrated sodium hydroxide solutions. The limiting factors in most applications are not the membranes, but rather the supporting component materials such as elastomeric seals, housings, pumps, and piping. I. Oily Waste Treatments: The treatment of oily wastes has always been accomplished in various steps in the past using a combination of mechanical and chemical treatments depending on whether the oil is floatable (free), emulsified or dissolved and the desired water quality required after treatment.

4 Physical/Ch&cal: The first step in any system would be the removal of the free oil by gravity using skimmers, parallel plate separators, coalescers, or centrifuges. To remove mechanical and chemically emulsified oil requires chemical treatment to break the emulsion and bind up the residual oil. Any free oil released is collected and added to the other free oil for recovery or disposal. The oily sludge then removed by a filter press or centrifuge has to be disposed of in a regulated landfill. Figure 1 shows a typical chemical/physical treatment system employed for years. Figure 2 shows an alternate approach that binds up all of the oil in the sludge. These systems have the following disadvantages: 1. Ongoing chemical cost 2. High maintenance requirements 3. Sludge disposal costs 4. High BOD and COD levels in discharge Polymeric Ultrafiltration: The ultrafiltration treatment schematic for treating the same type of oily waste emulsion (Figure 3) is much simpler than the chemical/ physical approaches. The equipment would be similar whether polymeric or inorganic membranes are used. If the waste emulsions are hot, such as from degreasing operations, a heat exchanger may be required before the membranes on a polymeric system. Oil concentrations above 50% by weight support combustion permitting reuse as a fuel if local air permits can be met or the recycling of this oil may be economically viable. While some polymeric membrane systems permit final concentrations in the 40-60% range, the flues may be well below 17 l/m2/hr (10 GFD). High oil concentrations also increase the risk of the oil wetting the membrane so it readily passes through. High cross-flow velocities minimize this wetting action of hydrophobic membranes. Since tubular membranes can handle higher velocities, they can produce higher concentrations than spiral wound, hollow fiber or plate and frame membranes. Once the membrane has become wetted by the oil, the cleaning operations normally have to be more aggressive including the use of certain solvents. Original flux rates cannot normally be regained with the polymeric membrane and they may have suffered shortened life or irreversible fouling. Inorganic Ultrafiltration: The use of inorganic UF membranes and in particular, the ZrO,/carbon, retain all of the benefits of polymeric membranes over physical/chemical treatment, while eliminating or minimizing all of the disadvantages. These membranes average 2 to 2.5 times and higher flux rates than most polymeric membranes and can produce 50 to 60% oil concentrations while maintaining over 30 l/m2/hr (18 GFD) flux rate. The membranes can then be cleaned with standard cleaning solutions, hot caustic solutions, acid cleaners, solvents, or combinations of the above as required without damage. Ultrafiltration systems with these ZrO,/carbon membranes have been in operation in Europe on oily wastes since the mid-eighties with the original membranes.

5 11. Auu~OU~ Alkaline Cleaner Recyclincr: The forces that created the need for membrane treatment of oily wastes is even stronger in the case of recycling the cleaners used in finishing operations. These forces include the cost of the actual cleaners, the cost of chemicals and equipment to treat the frequent disposal of spent cleaners, the cost of disposing of the sludge that will be generated from chemical treatment, government regulations requiring a reduction in the amount of wastes generated and the recovery/recycling of our resources. Production can also improve by having cleaner parts/less rejects since lower levels of contaminants are maintained in the cleaner bath. The loading is also reduced to waste treatment as there is less contaminants in the rinse waters, and if counter flow rinses are employed, rinses to treatment may even be eliminated. Figures 4 and 5. The application of membranes for cleaner recycling is much more complex than utilization of UF membranes for waste disposal. In waste disposal, the object is to remove as much of the organics (oils) as possible to comply with environmental concerns. In cleaner recycling, the goal is to selectively pass some organics (detergents) while retaining other organics (oils) to satisfy production, economic and environmental goals. Cleaner Formulations: Early formulations were basically strong alkaline (caustic) soap solutions that relied on high temperatures and frequent changes to keep them and the part clean. Changes in the types of oils, more stringent cleanliness requirements and energy requirements were matched by the increased use of additives such as emulsifiers, surfactants and wetting agents. Sometimes different basic inorganic components may have to be used to prevent attack on the substrata such as in the case of caustic on aluminum. Recycling Difficulties: The many different additives found in modern cleaner formulations make recycling difficult. Even the most basic bath life extension technique, gravity separation, removes the various components that may be soluble in the oil. When cleaning solutions pass through a membrane, the oil phase is retained along with additives that are bound with the oil. The balance of the additives pass through at different percentages due to their own molecular size, shape, or membrane boundary layer effects. The difficulty in continuously determining this reject ratio precludes replacing only the rejected additives accurately but they can be fed in to maintain approximate bath concentrations. New Developments : The cost of energy today is a concern, but it is not the crisis it was in the seventies, so higher temperatures can be considered which increases the effectiveness of the basic alkaline and reduces the need for many of the additives. The environmental concerns have also started to focus on toxicity which surfactants contribute to and hence, could cause you to be out of compliance if they get into the plant's water effluent. In Germany, the government has mandated that the manufacturers formulate their products to be amenable to recycling. A similar

6 direction is foreshadowed here by RCRA (Resource Conservation and Recovery Act). Inorganic membranes in both ultrafiltration and microfiltration have been developed that permit the matching of the correct porosity and high temperature resistance to the type of cleaners that will be required to meet the environmental limitations. The use of polymeric membranes are limited by the need to cool the cleaners prior to separation and then reheating the solution upon returning to the process. Selection: To determine the proper membrane for each application requires good cooperation and communication between the chemical supplier, membrane system supplier, and the end user. They have to review jointly the cleaning application, cleaner composition, operating parameters, economic and environmental considerations to make a preliminary membrane selection. A pilot system should then be initiated on a working system to determine membrane performance regarding oil retention, cleaner permeability and cleaning ability of the recycled solution. From this pilot work the optimum membrane can be selected; changes in cleaner composition can be made; process conditions can be set; and the total system can be sized and supplied. Prior to pilot testing, a bench top laboratory test should be performed for preliminary screening of compatibility of membrane to the application Case Histories: Waste Treatment: A German automotive manufacturer's transmission plant is directly discharging into a small river. The plant's existing biological treatment plant could not meet new stricter C.O.D. limits. It was determined that the plant could comply if the loadings were reduced from the oily water sources. There were three distinct sources that were identified: 1. Cleaning emulsions with a weekly volume of 1,040 m3 (275,000 gal.) containing approximately % oil and a C.O.D. value ranging between 2,000-3,500 mg/l. 2. Coolant emulsions with a weekly volume of 240 m3 (63,500 gal.) containing between % oil and a C.O.D. value of 100, ,000 mg/l. 3. Skimmed oil from various locations in the plant that amounted to 50 m3 (13,200 gal.) per week that contains approximately 80% oil.

7 The pretreatment system was to reduce oil content in the discharge to the biological system to 10 mg/l. The recovered oil was to contain less than 5% water so it could be sold. The solids to be removed in a dry form to facilitate local land disposal. It had to be reliable and economical. Initial investigation indicated that after preliminary separation, ultrafiltration and evaporation were the only two processes that were viable. These were then investigated in detail including piloting with both polymeric and ZrOJcarbon UF membranes. The the best The results of the pilot study concluded that ultrafiltration was available technology over the evaporation approach due to: The dissolved metals would scale out in the evaporation system and also result in a high metal content in the concentrated oily emulsion. If the metals were removed from the starting emulsion, the resulting sludge would require special treatment or disposal due to the combination of oils and heavy metals. The evaporator's concentrate contained all of the detergent chemicals resulting in a strong water in oil emulsion. The evaporator system was not flexible enough for changes in process flows or future recycling. The capital and operating costs for evaporation would be very high. UF presented the following advantages: The dissolved metals stayed in the permeate so no special treatment was required. The detergents and emulsifiers split between the concentrate and permeate so final oil concentration (after desorbtion) was 98%, yet the permeate's residuals were readily biodegradable. The cleaning solutions used in the UF systems did not require separate treatment but could be recycled to the incoming waste collection. The UF installation was supplied in modules that provides for more flexibility allowing it to match production. The modular approach also permitted separation of coolant and cleaner emulsions that meet current limits and provides for expected future recycling requirements.

8 The ZrO, membranes were selected over the polymeric membranes even through they had a higher capital cost due to: 1. Over double the flux rates l/m2/hr (47-59 GFD) for inorganic membranes to l/m2/hr (12-24 GFD) for the organic membranes. 2. Long service life. 3. Chemical resistance, 0-14 ph, and cleaner chemistry only limited by the Viton seals. 4. Insensitivity to pressure and temperature. 5. Consistently high final oil concentration (50-60% without irreversible fouling. The treatment scheme is shown on the block diagram (Figure 6) and the final simplified flow schematic (Figure 7) shows the as-built system with dual emulsion trains and oil concentration treatment. Cleaner Recycling: An aluminum coil line had to meet very critical cleaning conditions for a special high quality sheet. In addition to removing all of the soils and oils, the cleaner had to be non-etching to the aluminum. Through diligent work by a chemical company, a special commercially available cleaner was proposed that was built on a non-caustic inorganic cleaner with various surfactants and builders to remove the soils at the design temperature range of OF. Lab and pilot plant work confirmed that the cleaner could perform to specifications, but the oil content in the bath must remain less than 1,000 mg/l or the sheet would not always pass a water break test after rinsing. The ultrafilter was part of the overall coil line and was based on similar applications in Europe now has to be evaluated based on this very low allowable oil concentration and non-etching formulation that has a critical surfactant balance. A program was put together through the joint efforts and cooperation of the chemical supplier, the coil line owner and UF system supplier. This program consisted of a chemical supplier-developed analytical method for determining the passage of the surfactant package. The owner provides a pilot line where the cleaner could be used under actual conditiom to produce the actual soiled cleaner for initial tests and materials to make up additional synthetically soiled cleaner for followup and confirming tests plus room for the UF pilot system plus a technician. The owner also had the laboratory to perform the surfactant and oil concentration tests. A rental pilot unit was provided by the system supplier with a selection of high temperature ZrO, ultrafiltration and microfiltration membranes and a start-up engineer. The first test runs were done on the same 300,000 Dalton cut-off membrane that proved successful in Europe The lab results showed that this membrane was very effective in removing oil, but it also removed approximately 70 percent of the surfactant package. The pilot runs were

9 repeated with 1,000 A (Angstrom) to 2,000 A (0.1 to 0.2emicron) range microfiltration membranes. The membranes in the 2,000 A range passed better than 90 percent of the surfactants, but the oil level exceeded the desired control level. A membrane with a pore size of 1,400 A (0.14 micron) proved to pass a similar level of surfactants as the 2,000 A membranes, but the oil concentrations were below the desired control level. The 1,400 A and 2,000 A membranes passed 100% of the surfactants when no oil was present indicating 10% of the surfactants were bound with the oil. Upon review of the pilot test data and the cleaning line process, it was concluded by the project committee that normal bath additions to replenish drag-out losses will maintain adequate surfactant levels and balance when the 1,400 A membranes are used. The production line is currently under construction with a MF/UF system based on design data developed from the pilot study work and cooperation of the three principles. The final system Figure 8 combines multi-stage rinses with MF/UF degreasing recycling to provide the ultra clean finished product, recover the valuable cleaner while minimizing the generation of waste. The chemical supplier and system supplier has a happy client that will provide more business in the future on this line and others. Conclusion: The use of membranes has come of age and the use of inorganic membranes are proving their ability to solve the inherent shortcoming of polymeric membranes in these difficult applications. The correct matching of the inorganic membrane to waste treatment for oil removal is fairly straight forward based on the years of experience in Europe and the installations in North America. The utilization of either ultrafiltration or microfiltration for degreasing solution recycling will continue to grow based on increasing environmental and economic pressures and can best be serviced by inorganic membrane systems. To ensure that this will happen to the benefit of all interested parties will depend on the commitment and cooperation of the end user, the chemical supplier and the system designer.

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11 Membrane Applications in Finishing Po I y m e r i c Inorganic Materials Polyacrylonitrile, Polyamides (Aliphatic & Aromatic), Polypropylene Polyethersulfone, Polysulfone, Polyvinylidene fluoride, Polytetraf luoroet hylene Aluminum Oxide (A1203) Titanium Oxide (Ti02) Zirconium Oxide (Zr 02) TABLE 2

12 Emulsion Splitting by Floatation ACID COAGULANT CAUSTIC FLOCCULANT NEUTRALIZATION - FIGURE 1 I

13 Alternate Emulsion Splitting by Flotation I I ' DH RECORD FILTER PRESS NEUTRALIZATION p :ORFINAL M0; ph SLUDGE TO DISPOSAL SLUDGE AIR SATURATION TANK SEWER

14 Emulsion Splitting by Ultrafiltration OIL FREE FILTRATE (PERMEATE) SEWER FIGURE 3

15 Basic Aqueous Degreasing Recycling Loop MAKE-UP DEGREASED PARTS WATER CLEAN PARTS TO FINISHING OPERATIONS - WORK FLOW PERMEATE CONCENTRATE A,I UF MODULE(S) FEED RECIRCULATION FINAL PUMP PUMP CONCENTRATION FIGURE 4

16 tlulti-stage Aqueous Degreasing tecycling Loop "Closed Rinse" OILY PARTS MAKE-UP DEGREASED PARTS WATER WORK FLOW - CONCENTRATION - PERMEATE COh t I UF MODULES (S) WASTE OIL HOLDING v I FEED PUMP CIRCULATION PUMP & TO DISPOSAURECYCLE r FIGURE 5

17 Emulsion Splitting with u1 t rafil tration in an Automotive Transmission Plant DEREASING EMULSIONS/COOLANT EMULSIONS DISSOLVED WATER I SALTS I EISMI*I OIL I DIRT SKIMMED OIL WATER I OIL DEWATERING ' ' CONCENTRATED OIL WATER OIL EISMI* CLEAN WATEWPURE WATER DISSOLVED I E/s/w* WATER 1 SALTS *EMULSIFIERS, SURFACTANTS AND WETTING AGENTS PROCESS DISCHARGE I i OIL V V I I USAN* OIL DISCHARGE FIGURE 6

18 Emulsion Splitting with Ultrafiltration in an hutomotive Transmission Plant DEGREASING EMULSION 274,700 GALLONS ~ % OIL UF- MODULES 317,000 GALLONS CLEAN WATER 5 mgll OIL --b COOLANT EMULSION 68JWGALLONS 3-30% OIL U F- MODULES HOLDING TANK WI SKIMMER SKIMMED OIL 13,200 GALLONS I' I I -+ 2OOo LBS DRY SOLIDS 80% OIL --.._ CONCENTRATED OIL 31,700 GALLONS OF 98% OIL FIGURE 7

19 Multi-Stage Aqueous Degreasing Recycling Loop OILY PARTS MAKE-UP DEGREASED PARTS STAGE#2 STAGE#3 PERMEATE CONCENTRATE WORK FLOW CONCENTRATION UF MODULES (S) FEED CIRCULATION TO DISPOSAURECYCLE FIGURE 8

20 Ultrafiltration of Degreaser and Rinse Baths from Metal Cleaning I I

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22 FINISHING '93 CONFERENCE AND EXPOSITION October 25-28, 1993 Dr. Albert B. Sabin Convention Center Cincinnati, Ohio 0 Sponsored by the Association for Finishing Processes of the Society of Manufacturing Engineers