METALWORK1 NG FLU IDS 0 Society of Manufacturing Engineers fjf MARCH 12-14,1991 DEARBORN INN HOTEL DEARBORN (DETROIT), MICHIGAN VOLUME 2
COOLANTS RECOVERY USING A CENTRIFUGE AND HEAT/PASTEURIZATION Steven R. Friedman Vice President/General Manager Sanborn. Inc. Wrentham, PA PRESENTED AT: SPONSORED BY: "Metalworking Fluids" March 12-14. 1991 Dearborn. MI The Society of Manufacturing Engineers Courses and C1 inics Department One SME Drive. P.O. Box 930 Dearborn. MI 48121-0930 Phone: (313) 271-1500 FAX: (313) 271-2861
Coolant Recovery Using an Integrated Disc Centrifuge System and Heatedpasteurizer lntroduct ion Coolants are used in metalworking operations to remove heat, lubricate, clear the workface of chips and swarf and to control corrosion. Clean coolant is an important part of manufacturing cost containment, especially with -the ever increasing environmental pressures to reduce waste liability through waste minimization. When operating with clean coolant the manufacturer achieves higher production rates, longer tool life, better finishes on parts, and reduced machine maintenance. Frequent disposal of used coolants generates unacceptable long-term waste discharge liabilities and excess costs for replacement coolants. The goal of coolant recovery is to provide a maximum of recovered "like new" fluid to the machine tool with a minimum of waste. To achieve this it is necessary to remove virtually all of the tramp oil that has been introduced to the coolant, remove solids to the lowest level practical, and to return to the manufacturing floor a product that has both low biological activity and a biological preventative for long sump life. An integrated fluid recovery system employing high speed disc centrifugation, a heatedpasteurizer, a heat exchanger, a hydrocylone preseparator, a continuous centrifuge feed pump, and a make-up fluid renewal system is required.
Discuss ion Coolant recovery is a relatively new concept. It has been thought by some to be interchangeable with fluid management, but that is not correct. Fluid management encompasses a much broader scope including fluid selection, machine maintenance, and other issues along with recovery and waste minimization. The focus of this paper is to discuss state-of-the-art methods and effectiveness of coolant recovery for the manufacturing plant in order to optimize fluid reuse and minimize waste disposal. The discussion also focuses on deriving these benefas with minimal maintenance. The application of high speed disc centrifuges (HSDC) on coolants was pioneered in the late 1970's. Before that time there was little interest in reusing water based coolants. This was primarily because the fluids were inexpensive to purchase and used fluids were generally acceptable as non-regulated wastes with virtually no associated disposal costs. The advent of new environmental laws and the ever increasing regulation on manufacturing waste discharges has brought the need for maximum recovery to every facility. HSDC's are liquid/liquid/solids separating devices that separate solids and two liquids of different specific gravities. The separation requires a minimum of 7800 gravities (GIs). This implied force can be directed to split the coolant stream from the mechanically and loosely chemical emulsified 'tramp oil. The use of heat to 160 degrees F has proven to enhance this effective separation. Designed originally for creamlmilk separation, HSDC's are relatively intolerant of solids. To deal with the fine and very abrasive solids that are significant in the metalworking industry, additional solids separating devices must be integrated into a recovery system. There are primarily two types of HSDC's: 1) manual solids discharge and 2) automatic solids discharge. While all HSDC's automatically separate the tramp oil from the coolant and direct the two streams to different locations, the solids handling and concern over maintenance intensity is what differentiates the type of HSDC chosen. As its name implies a manual HSDC entrains solids. On a periodic basis the operator must open the centrifuge and manually remove the solids. While manual machines offer initial lower capital costs they can be maintenance intensive for most shops. The extra maintenance is necessary to avoid solids backing-up into the centrifuge's internal disc stack. Manual HSDC's are also not able to recover all types of fluids a shop will be using, such as grinding fluids, because of the volume of solids present in such fluids. z
The automatic discharging centrifuge, sometimes called a split bowl centrifuge, are far more versatile. Properly designed into an integrated system they offer the ability to process every type of metalworking fluid in a shop with minimal operator intervention. These devices are designed for a productive environment and can operate completely unattended without regard to solids loading. This is achieved in an automatic centrifuge by holding solids in a chamber that can open and evacuate the solids to a solids chute. These solids are ejected when the split bowl opens, and a small amount of hot water flushes the centrifuge and drives the solids out of the chamber with the imparted G Force of typically >7800. Two devices are necessary in the system to improve the solids removal efficiency of the automatic machine. 1 ) Hydrocyclone preseparator: Solids will be automatically ejected from a chamber, but too many solids in the feed stream will cause excess ejection and wear on the automatic self-discharge mechanism. This would create an unacceptable maintenance situation. To avoid this,, presolids separation must be designed into an integrated system. One successful design has been to employ a multipass hydrocyclone system before the centrifuge. These devices are convenient, use no consumable media, and can prevent 70% of the solids over 30 micron from entering the centrifuge. When the Automatic HSDC is employed on a central coolant system with adequate solids prefiltration, typically 25 micron, additional solids preseparation is not required. 2) Heater/Pasteurizer: The addition of heat to the system allows the flush water to become hot through contact with the centrifuge. This flushing cycle removes any residual solids and sticky varnishes adhering to the separating discs. Both the heater and hydrocyclone allow the recovery system to be flexible enough to handle all the used metalworking fluids in a plant with a minimal amount of maintenance. 3
Through the use of heat, the viscosity of the liquids is lowered. The separation characteristics by the HSDC is improved. Results achieved from field systems1 indicate that solids removal will improve to 1 micron clarity and that tramp oil remaining in the cleaned coolant will be less than 0.25% by volume. This is 1/2 of the volume of tramp oil that would remain in an unheated separation with a HSDC, and solids can only be separated to 5 micron when no heat is employed. Not only do the benefits of heat in the recovery systems allow for improved separations and internal cleanliness of the HSDC but also with a properly designed system the heat allows recovery of straight oils. Heat allows the dissolved gases that cause the coolants to smell (H2S) to be removed from the recovered product. Finally, to kill biological activity the heat can beneficially be used to pasteurize the coolant. The most common cause of coolant disposal is biological degradation. Usually identified as a rotten egg smell, the situation formerly required changing out the old coolant and replacing it with new fluid. Since the cost of disposal has increased so dramatically over the last few years, it is a significant liability to dispose of used fluids. For this reason, operating managers have tried to extend fluid usage in the machine sumps beyond their usefulness. That situation creates low moral among employees and more importantly produces a host of aualitv Droblems associated with bioloaical dearadation. Those problems include:' 1) staining of parts 2) coolant destabilization 3) increased maintenance at coolant pumps and supply lines. Additionally, when biological problems are treated with chemicals such as biocides or fungicides the machine tool operator may develop a skin disorder such as dermatitis. It has been studied by Bennett2 that worker sensitivity to biocidal chemicals is the second most frequent cause of worker dermatitis in the metalworking industry. The use of pasteurization on a recovery system has been proven to reduce biological activity to that of freshly mixed coolant3. This technique in concert with the HSDC produces a recovered coolant that is free of the contaminants of solids, tramp oil, biologicals, and odors but because of the characteristics of most new coolant biocides there is no residual biological control after long term ifl-sump usage and pasteurization. For this reason proportioned additions of new coolants are mixed with recovered coolants to provide the residual protection fluids needs for reintroduction to the machine sumps. Several years 4
of one manufacturer% experience with this technique indicate that only a 10% fresh coolant make-up ratio is required for residual protection through a reuse cycle. Other constituents such as rust preventatives are also maintained with this minor addition. The use of a heat exchanger along with the heater/pasteurizer has been shown to aid recovery in three ways: 1) It is necessary to immediately reduce the outgoing clean fluid temperature below the ideal biological regrowth temperature zone so biological contamination stays low. 2) When the clean coolant is directly entering a machine sump or central system, overall sump temperature stability helps to maintain tolerances. 3) Lastly, the exchanger reduces energy consumption. It exchanges the heat in the clean outgoing fluid with the ambient incoming dirty fluid. Preheating the incoming fluid allows an electric heater to cycle on 50% of the time during the recovery operation. A typical heater/pasteurizer with a heat exchanger consumes only 9 KW/Hr during operation. The recovery system with an automatic HSDC, pasteurizer/heat exchanger, and presolids separation require minimum maintenance and are highly efficient in changing used coolants into recovered fluids. The use of an HSDC system does not,however, guarantee that the lowest level possible of waste minimization will be achieved. To maximize the efficiency af a HSDC system for waste minimization it is important that an integrated system be designed to direct all of the dirty fluids, coolants and tramp oils, into the centrifuge. A HSDC also requires that a homogeneous and consistent feed rate be introduced into it for maximum separating efficiency and the least amount of coolant in the waste stream. The proven method for introducing a consistent feed rate into a HSDC is through a gear or positive displacement (PD) pump. More tolerant of solids, the PD pump should be chosen for reliability. 5
Two techniques are currently commercially employed to introduce a homogeneous feed to the centrifuge: 1) Hydraulic Agitation: This technique is continuous during operation. Hydraulic agitation of the dirty fluid to be processed allows no stratification of tramp oils and coolants before the centrifuge. With this technique, all tramp oil is separated in the HSDC. The disposed oil phase can contain less than 5% water. 2) Oil Skimming Device: An alternative technique employs the use of an oil skimming device on the dirty coolant processing tank. Although efficient at supplying the HSDC with a homogeneous feed, these devices classically drag out as much as 70% coolant and 30% oil in their discharges. This method should be avoided if possible due to the high level of recoverable fluid in the waste and the associated liability of such disposal. Through the use of programmable controllers, integrated HSDC systems can be engineered to recover different brands of coolants or oils with very high separating efficiencies. A minimum of operator intervention'will be necessary to process most fluids for maximum separation qualities and waste minimization concerns. These systems can also be engineered for both batch and central system recovery. They can default when the batch project is complete to automatically process central systems. Conclusion The advent of strict environmental regulations have forced the manufacturing community to seek sound methods for recovery and waste minimization for metalworking fluids. The use of an integrated high speed disc centrifuge system highly engineered for operational flexibility and reduced operator maintenance allows for virtually all of the recoverable fluids to be reused. An integrated HSDC system can generate the least amount of waste products requiring disposal. Recovery rates of over 95% and waste tramp oils containing only 5% water can be achieved. 6
References 1) Williams, George F., P.E., 1986 Pasteurization: Key To Quality Coolant Recycling 2) Bennett, E.O., Ph.D.. 1989 Nonmedical Viewpoint Of The Causes Of Dermatitis Among Machinists, SME Metalworking Coolants Clinic, Chapter 10 3) Sanbom, George W., 1991, A Guide To Coolant Recovery 7
References 1) Williams, George F., P.E., 1986 Pasteurization: Key To Quality Coolant Recycling 2) Bennett, E.O., Ph.D., 1989 Nonmedical Viewpoint Of The Causes Of Dermatitis Among Machinists, SME Metalworking Coolants Clinic, Chapter 10 3) Sanbom, George W., 1991, A Guide To Coolant Recovery 7
SME METALWORKING FLUIDS OUTLINE OF SLIDES FOR THE PRESENTATION COOLANT RECOVERY UTILIZING A HIGH SPEED DISC CENTRIFUGE SYSTEM WITH A HEATEWPASTEURIZER STEVEN R. FRIEDMAN SANBORN INC. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.,14. 15. Goal of coolant recovery Fluid managements 7 point objectives Environmental disposal regulations since 1971 Schematic of disc centrifuge Manual solids removal form a centrifuge Automatic solids ejection from a centrifuge Hydrocyclone schematic Flush water circuit for centrifuge Graphic depiction of Separation efficiency and temp. Problems associated with biological contamination Proportional coolant mixing Heater/Pasteurizer -Heat exchanger photo Hydraulic agitation in the dirty tank Positive displacement feed pump Centrifuge tramp oil for an integrated system