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Machining coolant recovery using 3-phase disc centrifuges is a widely used method. Coolant pasteurizing in addition to centrifugation is worth considering due to its benefits. This article evaluates the benefits of coolant pasteurizing together with a disc centrifuge.
Industrial machining operations involve metal cutting. These include turning, milling, grinding, honing, gear-cutting, etc. All these operations generate heat due to the friction between the cutting tool and machined the base metal.
Machining coolants extend the working life of cutting tools by reducing the friction between the tool and the part. This reduced friction enables faster machining and improved finish of machined components.
Besides, coolants remove the metal debris from the part as it is being machined. Additives in coolants also prevent oxidation (corrosion) of the machined parts.
Machining coolants come in different forms. The common types of machining coolants are briefly described below.
The most common of all water-based coolants, soluble oils are 50 percent oil before dilution. They form a milky emulsion with water that is an excellent choice for general-purpose machining.
Synthetics do not contain any oil. They are a blend of chemicals and polymers which provide natural oil’s lubricity.
They do not produce tramp oil but their lubricity is not the same as that of oils.
Semi-synthetic coolants contain lesser oil than conventional soluble oil coolants. So they are not as prone to bacterial contamination. But they offer similar lubricity as that of soluble oil coolants.
There are 2 types of contaminants in coolants. These can be broadly classified as solid and (tramp) oil. In this section, we discuss the sources of these contaminants and the need for their removal.
Metal chips and fine metal particles are the main solid contaminants in machine coolants. The machining process generates these solids which get carried away with the coolant. These metal particles accumulate in the coolant sump tanks.
Machining coolant needs to be free of metal particles to avoid damage to the circulation system. These particles also cause a bad surface finish of the machined parts and dull machine tools.
Metal particles also cause wear on the coolant processing system. This wear can cause premature failure of components such as pumps, valves, coolant nozzles, etc.
Service oil from the machine tools leaks into the coolant during the machining operation. Machined components have protective oil coatings. This oil also gets washed into the coolant during the machining process.
The above-mentioned oils are not soluble in the machining coolant and float to the top of the coolant in the coolant sump tanks. Tramp oil is this top layer of oil floating on the coolant.
Tramp oil is not desirable in metalworking coolants. This is because it coats the machined components and prevents the coolant from performing its function.
The layer of tramp oil in the sump tank facilitates the growth of anaerobic bacteria in the coolant in the absence of oxygen. These bacteria produce hydrogen sulfide. This gas is the cause of the rotten organic odor often associated with machining coolants with tramp oil.
A disc centrifuge is a high-speed centrifugal separating machine designed for 3-phase separation. The 3 phases in contaminated machining coolant are:
Tramp Oil – Liquid Phase
Water-based Coolant (emulsion) – Liquid Phase (Desired)
Machined Metal Particles – Solid Phase.
Machining coolant recovery using 3-phase centrifuges is a proven method across industries. This application uses a disc centrifuge setup as a concentrator. A concentrating centrifuge separates a large proportion of water (or heavy phase) from a small proportion of oil (light phase).
High internal centrifugal forces (> 7,000 Gs) can separate the oil (tramp oil) from the water phase. The centrifuge bowl continuously discharges the separated oil through a light phase outlet.
The heavy phase (coolant), free of the oil phase, exits the centrifuge bowl through the heavy phase (water) outlet.
The high centrifugal force forces all metal particles (down to 1-micron level) to accumulate at the internal bowl wall. The automatic ‘self-ejection’ mechanism periodically opens sludge ejection ports along the bowl wall. The bowl instantaneously ejects the solids during the sludge ejection process.
The heating of the water-based coolant before centrifugation has the following advantages.
Any increase in temperature reduces the viscosity of the oil. In the case of tramp oil, heat causes the viscosity of the tramp oil to reduce which facilitates easier separation.
Hotter coolant helps coalescence of suspended tramp oil microdroplets. This further improves the separation of tramp oil.
Passing of hot coolant through the disc stack flushes out any sticky oils or varnish which is stuck to the discs. This improves the separation efficiency of the centrifuge.
Higher temperatures allow the odor-causing dissolved gases to escape the coolant. This helps reduce odor related issues during the machining process.
Pasteurization is the process of killing pathogens using heat. This technique is commonly used to kill pathogens in milk to increase shelf-life.
Anaerobic bacteria are the cause of foul odor in used machining coolant as discussed above. This odor issue is not fully handled by the treatment of coolants through the disc-centrifuge.
Biocides and fungicides are sometimes added to coolants as a remedy to kill the odor-causing bacteria. But the addition of these chemicals has health-related side-effects on the machinery operators.
Dermatitis is one of the common skin ailments affecting metalworking workers. These added chemicals are the cause of this skin disease.
Pasteurization is a safer and more effective way to deal with the bacteria. However, the hot coolant needs to be cooled to prevent the re-growth of the pathogens. This is the complete pasteurization process.
As explained above, pasteurizing coolants involves heating the coolant and then cooling it before returning to the sump tank.
A complete setup using a machining coolant pasteurizing centrifuge for coolant recovery with an added electric pre-heater and heat-exchanger is shown below.
The feed pump transfers the contaminated coolant to the centrifuge through the heat-exchanger. This is the ‘cold-side’ of the heat exchanger. The coolant then passes through the electric heater.
The heater raises the coolant temperature above 165 F before entering the centrifuge. Heating of water-based coolants to 165 F kills the odor-causing bacteria. This process is also known as pasteurization of coolants.
The centrifuge separates the tramp oil and fine metal particles from the heated coolant. Cooling of the clean coolant exiting the centrifuge is essential for complete pasteurization.
The clean coolant passes through the ‘hot-side’ of the heat exchanger. The incoming cold coolant cools the hot coolant. The cooled coolant is then returned to the coolant sump tank.
This setup has the added advantage of transferring the heat from the hot processed coolant to the incoming cold coolant. This saves on energy costs.
Metal wear particles or tramp oil frequently contaminate industrial coolants. However, the combination of pasteurization and disc centrifuges can efficiently recover contaminated coolant. The benefits of using such a system are evident from the above.
Though this pasteurizing process involves additional equipment, the long-term benefits of a safe and odor-free working environment often outweigh the associated costs.