The world wouldn’t be where it is now without machine shops. Manufacturing operations such as tool and die plants, aerospace parts manufacturers, surgical fitting fabricators, firearms manufacturing, and other metalworking industries (particularly precision work) have had a long history in Massachusetts, from the founding of the Springfield Armory in 1777 through the present day.
As any good machinist knows, though, if you want to work with metal you have to know a fair amount about oil, which is used in many forms in metalworking operations.
The two kinds of oil most commonly used are:
- Way oil, also known as lube oil, slide oil, or brown oil, is a high-grade hydraulic oil formulated with a tackifier, an additive that improves the oil’s adhesion to metal surfaces such as the hydraulic pistons or sliding surfaces found in CNC machines, lathes and other heavy machine tools, to prolong the useful life of the oil and prevent it from oozing into the working process.
- Modern cutting fluids, sometimes called metalworking fluids, cutting lube, or cutting oils, are sprayed, misted or flowed onto machining surfaces in manufacturing for several purposes—they lubricate the cutting process and allow machines to go faster, they cool the process and prevent tip welding, where the drill bit or other machine tool overheats to the point where it welds itself onto the workpiece. These products are typically an emulsified mixture of oils and water (either water outside oil or oil outside water), by means of an oil engineered to be water soluble, or a surfactant or detergent additive. Most cutting fluids range from 1% to 5% oil by volume, and the emulsions can remain stable for weeks. Cutting oils used in these mixtures may be petroleum based or derived from plant or animal materials (lard and fish oil are surprisingly common, especially for manufacturing food grade equipment components), or based on synthetic oils (often used in milling and grinding). High-flash kerosene is sometimes used for working with aluminum.
Close-up view of a CNC machine and cutting fluid
In addition, some facilities use quantities of other kinds of oil, such as lubricants, rust preventatives (especially in firearms manufacturing), quench oils, and other products.
It makes economic sense to reuse cutting fluids as much as possible, but cutting fluids deteriorate and almost inevitably become contaminated with way oil and other oils, such as oil films used to protect bar stock, etc., which form separate phase liquids called “tramp oils” that float in blobs on top of a container of cutting fluid, and that would foul the process if allowed to recirculate through the system. If you let a drum of well-used waste cutting fluid sit for a few hours, more often than not a layer of tramp oil will partition out on top. Many modern CNC machines have onboard sumps in which the cutting fluid accumulates before it is recirculated, fitted with skimmers or other devices to remove separated tramp oils. Some larger facilities have central cutting fluid management systems with sophisticated tramp oil separators.
Eventually any machine shop or metalworking facility generates waste oil. In the late 1980s, EPA developed a regulatory framework for waste oils that didn’t meet the RCRA criteria for hazardous wastes (40 CFR 279), and which has been implemented by most states, in many cases along with the states’ identification of waste oil (variously defined) as a state-listed waste.
These regulations ultimately had two goals. The first was regulatory, in order to prevent the inappropriate disposal of hazardous wastes. The second reason was to provide for the beneficial reuse of oils that would otherwise have to be disposed.
Cutting fluids can also accumulate concentrations of RCRA metals (chromium, cadmium, lead, etc.) or chlorinated solvents such as perchloroethylene (PCE), trichloroethylene (TCE), or 1,1,1-trichloroethane (TCA), which were historically widely used for degreasing and cleaning metal parts before and after working on them. The older generation of consultants and manufacturing veterans remember the ‘old days’ when jet engines or other machines were dipped whole into vats of solvents for degreasing, like deep-frying a Thanksgiving turkey, and “waste oil” was historically a sort of catch-all waste stream that could contain many other things, including solvents, PCBs from transformer and hydraulic oils, pesticides and caustics. The use of waste oil containing highly toxic dioxins for oiling dirt roads is what turned Times Beach, Missouri into a ghost town. The use of solvents like PCE, TCE and TCA has decreased greatly over the last couple decades (down by about 90% since 1991, based on data provided by the Massachusetts Toxics Use Reduction Program) but they are still used in reduced quantities and remain of concern.
Uncontaminated way oils are readily recyclable, useful for fuel blending or lube base production, and can typically be recycled as heating fuel in standard waste oil burners if no other option is economical. It’s therefore a good idea to keep spent hydraulic and way oils and tramp oils separate from cutting fluids.
Cutting fluids, by contrast, can pose a number of problems:
- Although the percentage of oil in a cutting fluid is small, environmental regulations in many states require that the whole volume of the material be managed as a waste oil or hazardous waste, because of the RCRA “mixture rule” requirement that goes with being a listed waste. This can result in relatively small facilities generating enough oil/water mixtures to trigger Large Quantity Generator status, which comes with higher annual regulatory fees, planning and training requirements, etc.
- On their own, dewatered non-petroleum cutting oils typically have little fuel value, limiting their reuse options, although they can be blended with other oils with higher fuel values to produce a marketable fuel product.
- Breaking the emulsions and separating the oil from water is advantageous, but this can involve some fairly complicated chemical treatment, such as heating, acidification to a pH of roughly 2 and subsequent neutralization, or the addition of a salt or acetate. Even then the separated decant water will still likely contain some oil and may need to be evaporated, recycled into the process with new oil additives, or treated as an industrial wastewater.
- Residual cutting fluids will also often cling to metal turnings, and well-managed shops will typically clean their turnings with a centrifuge, wringer, bath, or other means to remove most of these residues before shipping them for recycling.
- Potentially most seriously, waste cutting fluids or their sludges can contain chemical impurities picked up during use, including metals and solvents. These contaminants can greatly increase the cost of managing the oil, ranging from “off-spec” costs for water, solids or halogens, to needing to manage the oil as a hazardous waste. Addressing these complications after they come up can cost time and money.
One common problem with waste oil is based in simple chemistry. Much of the waste oil generated by commerce and industry is reused for fuel, whether burned in the ubiquitous waste oil fired space heaters, or sent to a plant for batching, re-refining and resale. When oil containing chlorine-containing compounds is burned, the chlorinated compounds break down and the result is hydrochloric acid (HCL). This poses health hazards to workers and the public, and can also corrode and damage the oil-burning equipment. The more chlorine there is in the oil means the more acid there is in the off-gas.
EPA’s policy therefore centered on a “rebuttable presumption” that oils containing less than 1,000 parts per million (0.1%) total halogenated compounds were unlikely to have been mixed, intentionally or not, with a listed hazardous waste, while oil containing more than this threshold were considered to be hazardous unless shown not to be by further testing or generator knowledge. Most waste oil handlers will accept oil with high halogens, but will typically assess a surcharge on a sliding scale according to the halogen concentration.
Halogens are a family of chemicals including chlorine, fluorine, bromine and iodine, so called because they readily form salts (halides) with alkaline metals such as sodium (e.g. sodium chloride, calcium chloride, or potassium bromide). They also readily bond with hydrocarbons to form ‘organochlorine’ compounds, and many of the “better living through chemistry” era’s hazardous legacy products were based on organochlorine technology, whether old standbys such as DDT, perchloroethylene, pentachlorophenol, polychlorinated biphenyls, trichloroethylene, their lesser-known cousins such as Halowax or polybrominated fire retardants, or the increasingly notorious perfluorinated compounds such as the PFAS and PFOS families.
One of the problems with this approach, of course, is that oil technology has changed a great deal since the late 1980s, and in some respects the regulations and analytical methods haven’t kept pace. Many modern waste oils contain concentrations of chlorine greater than EPA’s 1,000 ppm threshold even though they aren’t contaminated with RCRA-listed solvents, or weren’t even generated at facilities where these old solvents are used at all (not even the old and sparingly-used-just-for-repair-emergencies bottle of old-formulation 3-in-1 oil (the kind loaded with trichloroethylene) that so many maintenance men kept in their toolboxes)!
Many modern synthetic or vegetable-based machine cutting oils, as used in machine shops, contain engineered chlorinated compounds in the form of biocides such as CMIT (to keep bacteria from degrading the oil) or as “EP” temperature and pressure additives (typically chlorinated paraffins, although there has been considerable regulatory whiplash over the now-aborted phase-out of shorter-chain hydrocarbons in favor of less toxic long and very-long-chain paraffins). Let’s just emphasize that these compounds are NOT currently listed by EPA as hazardous wastes, and for the most part didn’t even exist in trade when EPA’s waste oil policy was developed in the late 1980s. It’s also worth noting that, as we discussed in a prior blog post, cutting oils that don’t contain petroleum and that aren’t otherwise a hazardous waste often do not need to be managed as a hazardous waste or state-listed waste oil.
The sticking point is that the common ‘total halogens’ analyses (SW-846 laboratory methods 9253, 9056, 9075, 9076 and the Method 9077 field test kits such as Chlor-N-Oil) report only a total concentration of all the chlorinated, brominated or fluorinated compounds in the sample, which doesn’t tell you if your oil was formulated with a non-regulated chloroparaffin or brominated ingredient, or if it somehow became contaminated with a regulated degreaser such as trichloroethylene or a nonregulated product like a chlorinated brake cleaner. “Failing” a total halogens screening test does not automatically mean your oil is a hazardous waste. Most environmental laboratories can run chemical tests for solvents or other regulated chlorinated compounds in waste oil, and this may be necessary, but the cost can be several hundred dollars per sample to cover EPA’s entire list of potentially regulated compounds.
The first step in a solution to this conundrum is, of course, plain old good recordkeeping. Safety Data Sheets, product formulation spec sheets, and other documentation that provide information on the chemical makeup of the parent product, any additives, and most particularly, what your facility doesn’t use (e.g. solvent products containing more than the 10% chlorinated hydrocarbons threshold in EPA’s listing descriptions for solvent wastes), go a long, long way towards demonstrated that the oil doesn’t contain a listed solvent, and reducing the effort and cost of testing, handling and disposing of these materials.