Industrial machining processes depend upon cutting oils for lubrication and cooling. Liberal use of a fluid having needed lubricity and heat transfer properties is a key requirement for efficient, high-quality production. Unfortunately, because they are organic, machines coolants can lose desirable properties. This is particularly true of the commonly used water emulsion type coolants. The fluids can lose lubricating capabilities, become corrosive to the metal parts being manufactured and give off smell gas that is obnoxious and sometimes harmful to machine operators.
Progressive machine shops have learned that coolant deterioration can be controlled. Disposal of "rotten" batches need not be an every day practice. Coolant life can be extended with care and attention, with simple procedures such as following maintenance schedule and practicing good housekeeping. With proper treatment and periodic cleaning, coolants can be used for extended periods. The result can be major savings in coolant purchasing costs and reduced disposal costs.
Coolant deterioration results from bacterial and fungal growths. The hydrocarbon content of the coolant provides plenty of nourishments for such organisms. Metal chips, fines and globules of machine lubricants that have fallen into the coolant, termed "tramp oil", also seem to stimulate growth. The bacteria reduce coolant lubricity, stain many metals and are responsible for the generation of hydrogen sulfide gas, which smells like "rotten eggs" and is most detectable after the coolant system has been idle for two or three days. The bacteria are also though to be a major cause for many skin disorders and viral infections that workers experience around metalworking machines.
Four procedures will help inhibit the growth of bacteria and fungus in coolant systems and thereby prolong coolant life.
Ask your coolant supplier what additives you can use to inhibit bacterial growth. There are a variety of possibilities, depending upon the composition of the coolant.
The chemical inhibitor should be added to the coolant bath on a periodic basis in accordance with the supplier's recommendations.
The metalworking system should be equipped to collect and remove metal chips and tramp oil routinely.
Metal chips can be collected on screens positioned over the entrances to the coolant sump. The chips may be removed from the screens manually or with a vacuum.
Skimmers can be used to collect tramp oil from the coolant surface. Skimmers should be located near the inlet to the pump that can be used to collect tramp oil from the coolant surface. Skimmers should be located near the inlet to the pump that circulates the coolant to "catch" the oil globules as they are drawn to the pump.
Periodically, you should shut metalworking machines down, drain the coolants completely and clean all machine and coolant system surfaces. The frequency of cleaning will be dependent upon several factors, most importantly coolant composition, whether the coolant is circulated continuously when the metal working equipment is idle and the presence of contaminants in the coolant system. Usually, as bacterial count increases, emulsion-type coolants turn color from milkish white to gray and the color the odor of hydrogen sulfide gas becomes more pronounced.
Thorough cleaning can effectively reduce bacterial and fungal growth, as shown below:
| Maintenance Procedure | Bacterial Growth Rate Comparison |
|---|---|
| Drain and Recharge | 20 |
| Drain, Clean and Recharge | 2* |
| Drain, Clean, Rinse and Recharge | 1 |
| * Assumes that chip residues are removed. | |
The data was obtained in a study of bacterial counts in a coolant system after the listed maintenance procedures. The data indicate that the bacterial growth rate after cleaning is only 10 percent of the rate before cleaning. Further, rinsing after cleaning reduced the growth rate another 5 percent. With thorough cleaning, including rinsing, the bacterial growth rate was reduced a total of 95 percent.
Thorough cleaning involves the following steps:
Used coolant can usually be reused after a micro-cleaning process that removes all metals fines, bacteria and fungus and other contaminants from the workplace. Several ultrafiltration processes can be used. In addition, to filtration, the pH of the coolant should be adjusted. Micro-cleaning can be done on-site or off-site. On-site cleaning will be less costly usually.
Because of the need for frequent cleaning, all surfaces of the coolant circulating system should be easily accessible. Your coolant system should be modified to eliminate all potential havens for bacterial and fungal growths, as follows:
Modern coolants are carefully formulated to have desirable characteristics. When preparing emulsion-type coolants, it is important to use deionized water. Do not use tap water, whether from wells or surface waters, as a general rule. Minerals in natural water can destroy desirable coolant properties. Additionally, organic contaminants in such water can accelerate bacterial growth.
Eventually, after repeated usage, coolants become unusable because the inhibitors, stabilizers, wetting agents and other functional additives lose effectiveness. Remember that used coolant may not be dumped down sewers, poured into surface waters or dumped on land.
Many suppliers will take back "spent" coolants. When negotiating purchasing contracts for coolants, try to stipulate that the supplier will accept the return of "spent" materials.
When suppliers will not accept "spent" coolants, try to find a way to reuse or recycle the material. Coolants that are essentially oils, such as mineral oil, can be re-refined or blended with other oils. Oil refiners are available to provide such services.
Emulsion type coolants, however, must be "broken down" into oil and water components. The separation is most often done chemically by adding an acid or salt. Normally, the water can be drawn off with sufficient purity to be capable of discharge into municipal sewage treatment facilities. The oil fraction, however, must be handled as a waste oil.
The Office of Waste Reduction Services thanks the Minnesota Technical Assistance Program (MnTAP) for the use of portions of their materials in this fact sheet.
Last Updated: November 10, 1995