Its important to note that water-diluted metalworking fluids used in metal removal operations can be recycled, as well as straight oils or other fluids that are not diluted. There are certain operations where the water-diluted metalworking fluid is used only once, including warm and hot forging, die casting, ferrous hot rolling and minimum quantity lubricant metal removal fluids. Recycling is not applicable in these operations, either.

Understanding Fluid Failure

There are five basic failure mechanisms that can degrade metalworking fluids:

Attack by positively charged contaminants, including calcium and magnesium from hard water and iron, aluminum and other reactive metals from parts being machined. This leads to loss of emulsion stability and wetting.

Effects of negatively charged contaminants such as sulfates, phosphates and chlorides, which can fuel growth of microorganisms and corrosion.

Tramp oil effects, which upset the hydrophilic-lipophilic balance of the surfactant and also leads to growth of microorganisms.

Degradation by microorganisms such as bacteria and fungus, leading to additive consumption, odors, clogged filters, corrosion and health risks.

Loss of alkalinity, either through evaporation of alkalinity components or through acids released by microbial activity.

These failure mechanisms will act upon the fluid in both series and parallel failure modes as the metalworking fluid is used in the manufacturing process. It is rare that only one failure mechanism is working against the stability of the fluid. In some cases it can be degraded by these failure mechanisms to the point of no return, and the fluids cannot be improved by any recycling process.

The Recycling Process

The recycling system must address contaminants that are insoluble (metal particles, grinding-wheel debris, bacteria, fungus), semi-soluble (tramp oil, grease, metallic soaps) and soluble (salts).

For the most effective treatment, the metalworking fluid should be transferred to a separate reservoir or series of reservoirs, where the contaminants leading to failure mechanisms can be greatly reduced or eliminated. After the contaminants are removed down to an acceptable level, the metalworking fluid can be readditizedto improve performance and resistance to microorganisms.

The first step is to filter out the micro-particles. Filtration is best done in two stages. Stage one is filtered at 100 microns and stage two at 25 microns. If the micro particles are magnetic, then a magnetic separator may be used after the 25-micron filter. If the product being recycled is a full synthetic solution, then it can safely be filtered down to 5 microns in the second stage.

Filtration can be done using pleated cartridge filters, bag filters or roll media under pressure or vacuum. In all cases, avoid the use of polypropylene media, as it can adsorb oil-like components and defoamers from the metalworking fluid.

Next comes the separation of the tramp oil, which is unwanted oil that has found its way into a metalworking solution. There are two effective strategies for tramp oil removal. One method is by simple decanting in a holding tank, and the other works by use of a centrifuge.

Decanting involves placing the filtered metalworking fluid into a holding tank and allowing it to stand for approximately 24 hours. The majority of the tramp oil will separate and float to the top, where it can be skimmed by conventional methods. Drain off approximately 90 percent of the fluid beneath the tramp oil layer. Filter this fluid again, then transfer it to a second holding tank.

When centrifuging, process the filtered metalworking fluid through a liquid/liquid high-speed disc bowl centrifuge, then transfer it to a holding tank. Allow the centrifuge to make at least three passes of the fluid for effective tramp oil removal. Filter this fluid again, then transfer the fluid to a second holding tank. Use caution, as some semi-synthetic and oil emulsion fluids can be damaged by extended centrifuging.

The last step in the recycling process is additization, where the product is tested and adjustments are made. In a second holding tank, the metalworking fluid is tested for some of the following parameters: concentration, pH, microbial activity, tramp oil, dirt levels, foam potential and perhaps specific component analysis.

It is likely that some tramp oil will continue to be released in the holding tank while the product is waiting for testing to be completed. Therefore, a method to further skim off the tramp oil in the second holding tank is necessary.

What to Look For

After testing has been completed, the product can be readditized as necessary to bring the recycled metalworking fluid back to an acceptable performance level. These additions can be, but are not limited to, acidity or alkalinity boosters, biocides, lubricity additives, product additions, defoamers, corrosion inhibitors, detergency boosters and specific compounds that are unique to each formula.

One key consideration is the use of robust equipment for this recycling strategy. For example, some plastics can be attacked by combinations of additives in metalworking fluids such as amines blended with petroleum oils. Some elastomers used in pump seals and those used in diaphragm pumps can also degrade in the presence of some types of metalworking fluids. It is important to check with the fluid manufacturer to ensure that the construction materials of the recycling system are compatible with the metalworking fluid used in the recycling process.

The water used in the system should have a hardness at or below 20 milligrams per liter (expressed as calcium carbonate) and chlorides below 15 mg/L. Such water quality is not usually found in many places; therefore, water pre-treatment may be necessary. Reverse osmosis and deionization are excellent methods to produce water that is suitable for metalworking operations. These two types of water purification systems can be incorporated into a master control panel for the entire recycling system.

After treatment and appropriate chemical additions, the recycled metalworking fluid can be returned to the machines. It can be transferred by overhead piping or by a sump cart. If you are using overhead piping, do not use galvanized pipes, as the amines in some metalworking fluids will dissolve the galvanized coating. That chemical attack of the piping will add zinc to the recycled metalworking fluid, which can lead to harmful effects on its stability. Black steel pipe schedule 40 and CPVC pipe schedule 80 are acceptable for metalworking fluid transport in many cases. Check with the manufacturer of the metalworking fluid for acceptable materials.

Reality Check

Since failure of the fluid is primarily occurring in the machining environment, it has already lost some of its performance attributes when transferred to the recycling system. The recycling system is not likely to return the fluid back to its original condition, meaning the rate of failure is only reduced, not stopped.

Treatment and recycling will generally reduce less than 30 percent of purchases of new metalworking fluids. But likewise, there will be a reduction in the amount of waste metalworking fluid to haul. That reduction in waste haulage cost will help offset the capital cost of the recycling equipment.

It is not possible to remove all the contaminants that enter the fluid during the manufacturing process. This is especially true in the case of tramp oil in relation to certain types of oil emulsions. Most tramp oils are somewhat mutually soluble into emulsion products. Therefore, high-speed centrifuges or decanting will not effectively remove tramp oil in all cases. In some instances, a centrifuge can inadvertently remove desirable components from the metalworking fluid.

The recycled metalworking fluid will probably not provide the same tool life as a new fluid. This is due to the ingress of tramp oil and other contaminants that are not completely removed by recycling methods.

Other Considerations

Chloride levels can increase due to preferential evaporation of the water phase if the water used was not pretreated by deionization or reverse osmosis. Chlorides naturally occur in most waters throughout the world. When levels of chlorides exceed 130 mg/L, corrosion of iron-alloy components can occur.

At 700 mg/L chloride, the potential for corrosion is essentially guaranteed. There is no practical method to remove chlorides from metalworking fluids. Do not confuse the chlorine your municipality adds for disinfection of drinking water (sodium hypochlorite or chlorine gas) with soluble chloride, such as sodium chloride.

Controlling bacteria and fungus to levels below 100 microorganisms per milliliter is essential to maintain a healthy fluid that is suitable for recycling. The proper use of biocides and fungicides is necessary for extended fluid stability. There are concerns and evolving regulations in some countries with regard to the use of formaldehyde-condensate biocides, thus leaving very few chemical choices for microbial control.

Another issue is the safety data sheet and the actual hazard a metalworking fluid places on the worker in the manufacturing environment. Considering the contaminants from metals machined, tramp oil (and associated additives), bacteria, fungus, biocide and fungicide additions (as required) and endotoxins, it is not easy to truly evaluate the health effects of a recycled fluid. Individual chemicals are most likely to interact with each other as these chemicals enter into the fluid mix. The potential reactions and side reactions, further amplified by heat and pressure, can be incalculable.

Where does that leave an industry that wants to reduce product consumption and the volume of spent metalworking fluids to be hauled away?

The first step is to find a biostable fluid that can resist degradation by hard water and machined metal interactions. Second is to build a robust recycling system with a high level of automation and continuous measurement systems. Third, consider a realistic endpoint to the longevity of the recycled metalworking fluid.

Typically, pushing the fluid to 1.5 years is technically feasible. However, going beyond 1.5 years of fluid life may create additional hazards that will not be immediately apparent in a review of the combined safety data sheets of all the interacting chemicals. In the long run, the health and safety of the exposed workers comes first.

John Burke is global director of engineering at Houghton International. He has over 46 years of experience in the metalworking fluid industry, and has been an instructor for the Society of Tribologists and Lubrication Engineers metalworking fluid education course for the past 26 years. Contact him at jburke@houghtonintl.com.