Recycling machine tool coolants
Is recycling metalworking coolants only for the big boys, the machine shops that buy large volumes of coolant for many machine tools? Certainly not. Virtually all shops can benefit, regardless of size and type of application.
The reason is the cost of purchasing new coolant is usually dwarfed by the cost of owning it — the costs associated with use and disposal .
The machining process and life in the sump take their toll on metalworking fluids. Over time, they collect metal fines and tramp oils and may develop bacterial activity. Machined part quality and productivity begin to suffer; cutting tools wear faster, machine tool maintenance increases, the work environment declines, and health issues surface.
Sumpside coolant maintenance slows the decline. But the machine tool operator must still adapt to a metalworking fluid that goes from excellent to nearly useless before the machine tool is recharged with new coolant and the old coolant discarded.
In contrast, frequent recycling makes it feasible to continuously operate with a coolant near its peak performance (Fig. 1). For the machinist, the fluid is no longer an operating variable. And, in situations where preventive coolant recycling along with normal top-off requirements replaces the coolant before it begins to spoil, disposal may never be required.
Timing is essential
Recycling coolant for repeated use in machine tools is about the removal of contaminants, especially tramp oil and metal particulate from the machining process. Tramp oil is essentially a brew of machine tool oils and hydraulic fluids draining into the sump and mixing with any coolant base oil that has separated from the emulsion.
Before the coolant is reused, the concentration of the emulsion should be adjusted to the requirements of the machining process. The fluid should be checked for pH and bacterial activity, and treated as necessary. It is practical to have these activities performed in a temporary storage facility, such as an aerated holding tank, from which machine tools can be refilled on demand.
The objective is to restore the fluid’s performance capability. To be effective, the process must start well in advance of significant degradation. If the fluid is appreciably degraded, especially with regard to tramp oil saturation and biological activity, there is usually little point in attempting recycling.
For successful recycling, the coolant’s pH level should be above 8.0. Around 8.5 is normal for most metalworking fluid emulsions, with its concentration above the manufacturer’s minimum recommended level. Emulsions are usually diluted with water to around 5%_25% concentration, depending on the application. Indications, through analysis or observation, of significant biological degradation (the presence of a foul smell, for example) are not good.
A dark appearance is indicative of the presence of emulsified tramp oil, which cannot be effectively removed with any recycling technique. High-pressure coolant systems, because of their aggressive agitation, require close attention. Good emulsions are either semitransparent or milky white with a sharp, rather than diffused, refractometer reading.
A maintenance routine that ensures the sumps of individual machines are serviced in a timely fashion, while the fluid is in good condition, eliminates these concerns.
To some extent, the outcome is also contingent on a coolant’s inherent properties — attributes that determine how the fluid will respond to repeated cleaning cycles. The stability of the emulsion and tolerance of biological activity are key factors. The nature of a formulation’s emulsifying agent and achieving submicron-size oil droplets during mixing are important determinants of a homogenous and stable emulsion.
The maintenance process is not demanding. Servicing individual machines involves draining the sump (a mobile sump sucker is excellent for this purpose), cleaning it out (removing chips and sludge from the bottom of the tank), and rinsing the entire fluid system. The sump then can be immediately refilled with recycled coolant drawn from a reservoir, and the machine tool is up and running again.
Dirty coolant is transferred to a recycling facility for cleaning, and then held in a reservoir. Sludge is disposed of and recovered tramp oil and metal cuttings can sometimes be turned over to dealers at low or no cost.
Two fluid cleaning techniques for the removal of both tramp oil and particulate are decanting and centrifuging. They are frequently used in conjunction with filtration and other forms of particle removal, and, in the case of decanting, with skimming techniques or coalescing techniques to remove tramp oil.
The most practical configuration depends on the individual circumstances of the shop, machining operations and parts materials, degree of automation and usage, type and volume of coolant used, and the nature and extent of fluid contamination.
Decanting. Fill a clear glass container with coolant from a machine tool coolant sump and let it sit for a few days. The effect of batch decanting becomes readily apparent: metal debris settles on the bottom, tramp oils float to the top, and the increasingly purified coolant emulsion appears in between. Time does the work, making decanting the most economical method of recycling, and it is effective (Fig. 2).
If the layer of tramp oil appears to exceed 2% of the overall volume in the container, or a lot of debris has settled on the bottom, it is time to start recycling. The decanting facility might simply be one settling tank, but two make it possible to alternate, ensuring there is always room for dirty coolant. The tanks should be sized to accommodate one or several machine tool sumps, and to provide for a dwell time of three to five days.
Ideally, cleaned coolant should be transferred to an aerated holding tank where the concentration of emulsion can be adjusted, and the fluid checked for pH and biological activity before reuse to ensure it meets manufacturer’s specifications.
Skimming. Skimming equipment is available to remove tramp oil accumulating on the coolant surface during decanting. Three designs, all mounted on top of the tank, rely on a disk, continuous belt, or band made of material that attracts oil. As the partially submerged disk, belt, or band slowly rotates, oil sticking to the surface is pulled from the decanting tank, scraped off, and collected (Fig. 3).
Removal is improved by using coolants formulated to reject tramp oil, way lube oils, and hydraulic fluids designed to separate quickly from a coolant emulsion. Installation of coalescing equipment in the decanter will also help.
The construction material and design of coalescers promotes suspended tramp oil droplets to combine into larger agglomerations that float to the surface.
Centrifuging. Centrifuges are widely used in machine shops with large, central fluid systems, particularly if they are in continuous operation. The equipment is also practical for intermittent use in batch operations, often in conjunction with a decanter, for example, to clean out smaller-size contaminants not readily removed through settling.
Centrifuges are potent tools and they can damage coolants. It is critical not to exceed the permissible holding times at different levels of g-force, and to be alert to unintended side effects. At the higher levels of g-force, where emulsified tramp oil is removed, emulsion ingredients can separate, emulsions split, and coolant spin out.
Depending on contaminants to be removed, centrifuges for coolant emulsions are operated within a range from 1000 g to 7000 g. Low-speed units are designed to remove solids, while high-speed units also remove tramp oil. Emulsified tramp oil is separated out in the range of 4000 g to 6000 g.
Filtration. Particle filters are valuable adjuncts to settling and centrifuging, both for the removal of particles and quality control. Operating principles vary. Some filters for coolant recycling rely on gravity to drain the fluid through a filter medium. In other designs, such as bag filters, the fluid is forced through the medium, or vacuum pulls the fluid through the medium. The media include wire mesh, cloth, or paper, with nominal ratings that indicate the size of particles that will remain in the fluid (Fig. 4).
A high-quality recycled coolant should not contain particulate larger than 50-microns. For some applications, keeping particulate around 10-microns is appropriate to ensure parts quality and reach desired tool life. Less than a 10-micron particle size may be required to ensure good surface finish when machine tools are equipped with high-pressure, through-the-tool fluid systems.
Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, email@example.com .
Coolant recycling lowers cost, increases production
USAeroteam, of Dayton, OH, produces parts and subassemblies for aerospace, automotive and defense-related industries. They began recycling some of the metalworking coolant used in its plant in April 1997. By the end of the year, the direct costs of consumption and disposal of spent coolant had been reduced by $23,000.
Since then, the shop has doubled in size with the addition of 11 CNC machine tools served by a decanting-type recycling system. Yet no coolant has been discarded, and consumption is 60% below what it would have been without recycling; 60 drums of coolant concentrate per year compared to an estimated 140 drums.
In the aerospace products shop, the coolant for 21 CNC machines, with a combined sump capacity of 2500 gal, is recycled. Every couple of days, a few machines at a time are recycled. The coolant is vacuumed out of the sumps, chips and sludge removed from the bottom, and the machine tool operators refill the fluid systems from hose connections in the ceiling. The process takes about 2 hr per machine.
The maintenance staff designed the decanting system around a 1000-gal tank. It was positioned in the basement, immediately below the machine shop, and divided into three sections. The first two sections are used for initial and secondary decanting; the third, as a holding tank for on-demand refills of the machine tools.
Spent coolant, drained into a connection in the shop floor, is allowed to settle for two to three days in the first tank to separate larger particulate. Rapidly separating tramp oil is removed with a belt-type skimmer. Decanted coolant is pumped through a 5-micron bag filter into the second tank, with a disk skimmer and coalescer, for an additional three to four days.
At the aerated holding tank, equipped with a coolant mixer, the concentration of the emulsion is adjusted, and the fluid checked for pH and biological activity. Recovered tramp oil is collected by an outside vendor at no cost.
A new facility was designed to initially serve 13 machines with long production runs of mostly ductile iron automotive parts. Rather than expand the existing coolant treatment facility, a separate system was installed, principally to avoid cross contamination from mixing the fluid streams.
Machining ductile iron produces exceptionally small fines, difficult to remove with decanting. Adding particle removal capability might not have prevented some of the ductile iron fines from recirculating back to the machines, and the impact on parts quality and tool life in the aerospace operation would be unacceptable.
Also, the two groups of machines (aerospace and automotive products) use different coolant formulations. They are both mineral oil-based, but the automotive version is fortified to provide a higher level of lubricity for heavy-duty machining. Both are tolerant of decanting as well as centrifuge recycling, formulated to be resilient to biological activity without the use of biocides, remain chemically stable, and quickly release tramp oils.