As a way to reduce the increasing costs of purchasing coolant concentrate, many companies are implementing coolant management programs. Some of the reasons for reclaiming coolants regularly include longer tool life, lower reject rates, improved surface finishes, and a cleaner, odorless work area.
The primary causes for coolant discards are the build-up of solids, growth of bacteria, and excessive tramp oil in the coolant. On the other hand, if coolants are removed from the machine sumps, treated to remove the contaminants, and brought back to specification by the addition of new coolant concentrate on a regular basis, the quantity of coolants used yearly could be drastically reduced. Approximately 10 percent of the coolants is lost to dragout and evaporation. However, biocide levels usually are maintained if this 10 percent is replenished with new makeup solution, but other additives such as rust inhibitors may also be needed.
Reducing Tramp Oils in Coolants
Since tramp oils such as hydraulic and lube oil are major causes of coolant breakdown and rancidity associated with the formation of hydrogen sulfide, all hydraulic oil leaks should be repaired. A preventive maintenance program that requires periodic inspection and replacement of gaskets, wipers, and seals should be performed.
Coolant Selection
Only quality stable cutting and grinding fluids with the best recyclability should be used. At the time they are being evaluated for cutting and grinding capabilities, the coolants should be examined for resistance to bacterial attack, corrosion protection, residues left on work pieces, and hazardous additives. Also, if the number of different coolants used is reduced, recycling is easier to facilitate. Sometimes only the coolant concentration needs to be changed for different applications.
Coolant Makeup Water
Tap water with a high mineral content can adversely affect coolant performance by deteriorating emulsions, causing corrosion, and enhancing microbial growth. Purification of makeup water through dionization can help. The possibility of using dionized water for coolant bath makeup water can be explored.
Coolant Concentration and Chemistry
Refractometers or titration kits should be made available to operators (along with training on using them) to monitor and maintain proper coolant concentrations. Refractometers cost between $150 and $200. Low pH readings signify low coolant concentrations and related problems of increased metal fines and suspended solids. Also, a drop in pH can result from increased microbial growth.
Sump Maintenance
Another step that can be taken to reduce the required frequency of coolant treatment or coolant dumps is to install oil skimming devices on each machine sump. Removal of excess tramp oil from the sump will also remove part of the food source for bacteria that cause the coolant to turn rancid. A number of companies make oil skimming devices that remove floating tramp oils. Skimming wheels cost between $100 and $500. The oils will not float to the top to be skimmed unless the sump is still while the oil skimmer is operating. Thus, a good time to operate the oil skimmer is during employee breaks such as lunch hour. The oil that is skimmed from each sump could be collected and managed as waste oil.
To help extend the life of the coolant, machine sumps clean-out should be scheduled on a regular basis. The frequency will, of course, depend upon the use of the machine. It is particularly important that the shavings be removed and the sumps cleaned with an alkaline cleanser when dirty coolant is removed from the sump for cleaning or disposal. The metal shavings that have built up harbor bacteria that can recontaminate the new or rejuvenated coolant. These areas with accumulated shavings are also somewhat impenetrable to the biocides contained in the coolant.
Coolant Circulation
With the exception of the occasions when the sump is still during operation of the oil skimmer, the sump should remain agitated. Bothersome anaerobic bacteria are able to grow and reproduce much more rapidly in a stagnant sump than in one that is well agitated.
Coolant Recycling Equipment
Many different types of equipment are available for recycling coolants. This equipment ranges from simple filtration or oil skimming equipment to multiple-stage machinery that incorporates filtration, centrifugation, and heat pasteurization.
- Centrifugation: This process involves mechanical rotation of the coolants to provide several thousand G's of separating force. Centrifugation is able to remove hydraulic oils and other emulsified tramp oils as well as free floating oils and solids.
- Coalescing Separators: This technique uses an olephilic high-surface-area medium to coalesce oil to form larger droplets. These larger oil droplets float to the surface and can be skimmed off with adjustable weirs. Coalescers are not effective for removing water miscible hydraulic oils or other emulsified oils.
- Hydrocyclones: This process uses centrifugal force to remove solids from the fluid. It is not effective for removing small quantities of tramp oil, but it can remove solids down to 5 microns. This system is mechanically simple and easy to operate.
Proper Trash Disposal
Care must be taken to keep trash out of the coolant sumps and chip bins. Cleanliness fosters good work habits and can have a positive impact on coolant recycling efforts.
Conclusions
Proper coolant management and coolant recycling are effective ways for metalworking operations to save money on raw materials and waste management and to reduce waste generation.
For more information about coolant management and/or assistance with your waste management concerns, call the Office of Waste Reduction at (919) 715-6500 or e-mail nowaste@p2pays.org.
The NC Office of Waste Reduction provides free, non-regulatory technical assistance and education on methods to eliminate, reduce, or recycle wastes before they become pollutants or require disposal.
OWR. 1993.