Metalworking fluids are costly to replace and difficult to treat. They are considered as hazardous waste due to their content and concentrated nature. Therefore, they are intended to be recycled in-house, or offsite. In either case, their effective segregation is required. Spills and contamination to wastewater should be minimized. There are several technologies used for recycling and treatment of metalworking fluids. Gravity and vacuum filtration are used to separate solids. Generally, disposable filters are used .
Hydrocyclones are used to separate solids from the fluid. They work by the principle of cyclones. Fluid is directed to the top of the cyclone under pressure. Hydrocyclones cannot efficiently remove oil and particles below 5 pm. Centrifugation, employing forces of several thousand g, separates free oils as well as emulsified oil. Pasteurization is used, generally, in combination with centrifuging to clean up bacterial contamination.
However, pasteurization is an energy-intensive process and may not be effective in total destruction of specific coolant bacteria .
Oil removal is an important step. Along with conventional means, use of ozone and salts has been reported to help demulsification of cutting oil . Skimming is used to separate oil from the fluids. It is a gravity separation conducted in simple tanks with extended durations. Skimming can be carried out manually or using automatic devices with oil-attracting belts, floating ropes, or mechanical separation devices .
Coalescing is the use of special surfaces having oleophilic properties. Oil in emulsion form clings to the surface and is skimmed off by special equipment into weirs as it coalesces. Coalescing uses mediums with high surface area such as tubes or corrugated plates . One of the most effective means of oil separation is DAF. Ultrafiltration is commonly and effectively used for onsite recovery of metalworking fluids. The process is used after separation of free oil and suspended matters. Ultrafiltration is generally combined with ion exchange, microfiltration, or chemical treatment such as ozonation [114,115].
Ultrafiltration uses a minimum of chemicals. Equipment sizes are small. It is a reliable and easy-to-use technology.
The combinations of UF/RO and UF/membrane distillation have also been suggested for oily waste treatment [116,117]. High efficiencies have been obtained from UF treatment of drawing oils. Treatment of permeate with ion exchange has yielded high-quality effluent . A review and evaluation of commercial ultrafiltration systems applied to automotive oily wastewaters can be found in the literature .
Biological treatment alternatives have also been tested for metalworking fluids. Pilotplant studies were conducted on permeates of ultrafiltration of boron- and ethanolamine-containing coolants, for use as a carbon source for denitrification of electrochemical wastewater . Anaerobic removal of COD in metal-cutting fluid wastewater was studied, and the biodegradable part of COD, which was determined to be 65% of the total, was removed efficiently .
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