Because of the cost, many captive and commercial heat treaters are looking at reclaiming or recycling their quench oil as a method of saving money. In many cases, reclamation or recycling of the quench oil can be done, provided that some simple guidelines are followed. However, should these guidelines not be followed, there is a risk of fire, equipment downtime, and personnel injury.
Reclamation and Recycling
There is a subtle difference between reclamation and recycling. From the Merriam-Webster Dictionary, the words are defined in the following manner  :
re·claim 1 a: to recall from wrong or improper conduct 2 a: to rescue from an undesirable state b: to make available for human use by changing natural conditions 3: to obtain from a waste product or by-product: RECOVER 4 a: to demand or obtain the return of b: to regain possession of
re·cy·cle: 1: to pass again through a series of changes or treatments: as a: to process (as liquid body waste, glass, or cans) in order to regain material for human use 2: to adapt to a new use: 3: to bring back: REUSE 4: to make ready for reuse
Based on these definitions, there are two primary methods for extending the life of quench oil. The first method is based on preventative maintenance of the oil to clean and filter the oil; while the second method reprocesses the oil to make it fit for use, where it would normally be thrown away.
Reclamation of Quench Oil
Reclamation of quench oils can be accomplished continuously or in batch. The basic principles are the same, but the steps necessary are slightly different. This means that the oil is filtered on a regular basis, using a high-quality filtration unit. In continuous reclamation, in-line filtration systems are used to remove soot, particulate, and other debris. The use of other types of filtration media, such as Fuller’s Earth (diatomaceous earth) is used to remove deleterious organic acids. Long oil life can result from quality filtering of quench oil.
This reclamation of quench oil can be done in a continuous fashion, on-site. There are two ways of doing this. First, oil can be continually cleaned to prevent the buildup of soot, scale, and oxidation by products. Using high-quality filtration systems, oil can be maintained in a nearly like-new condition.
Filtration of the oil is usually accomplished using a series of increasingly smaller filtration steps, initially 50 microns to remove the larger particles of scale. The filter size is then progressively reduced to approximately 1 micron. This will remove the largest percentage of soot, scale and oxidation precursors. A typical dispersion of particles in quench oil is shown in Figure 1.
This graph clearly illustrates the importance of very fine filtration of oil. In the case of a 20-micron filter, the filter is only removing no more than 5 percent of the particulate present in the oil.
It is only until the oil is filtered to less than 3 microns that half the particulate is removed. It is necessary to filter to below 1-2 microns to achieve a truly clean oil.
One misconception that is often cited for not filtering quench oil, is that very fine filtration will remove the additive package. This is absolutely incorrect. Fine filtration will not remove the additive package as the additive package (speed improvers and anti-oxidants) is in solution with the oil.
These types of filtration systems are often set up as a “kidney loop” where the filtration system is separate from the system .
As an alternative, oil that is resulting in staining can be collected and placed in a large tank located outside the heat-treating facility. Oil can be filtered on-site, or an outside company can be contracted to clean the oil, and return it to the tank for use. The reclaimed oil is then mixed with the new oil — typically 50/50.
Once the reclaimed oil is completed with filtering, it should be tested to verify that the oil is suitable for use. Testing should include comparative cooling curves, total acid number, viscosity, flashpoint, and water. This testing will determine if any additional additive package is needed. If the oil is mixed 50/50, often no additive package is required.
As a general rule, fine filtering can extend the life of the oil up to double the life as well as increase the service interval by up to 50 percent. Additional benefits include improved surface finish with fewer rejected parts.
Recycling Quench Oil
Recycling quench oil can reduce quench oil consumption by as much as 75 percent. This is accomplished by careful recovery of quench oil from washers and other sources. Usually, the post-quench washer is used to recover the quench oil for reuse. This washer must only be used to clean parts after the quench and must not be contaminated with other coolants or other contaminants. In this process, the oil is recovered from the post-quench wash and segregated to a separate tank. Once a sufficient quantity of oil is gathered (typically 1,500 gallons or more), then it is cleaned and recycled in a batch process. Figure 2 shows the typical process for recycling quench oil.
Oil from the washer is recovered by skimmers. This means that the oil skimmers must be in good working order, and that the cleaner used will properly split the oil for recovery. The oil must not be emulsified, as this will reduce the efficiency of the recycling process  .
The oil is segregated to a separate tank farm, typically outside of the facility. Three tanks are usually used: one for new oil, one tank for recycled oil, and finally, a tank for oil to be recycled. In the bulk tank containing oil to be recycled, the oil is allowed to sit for a period of time until full. Any residual water will split out and settle to the bottom of the tank. An initial draining of the tank to eliminate the gross water collection is performed.
An initial filtration (10-20 µm) through blotter paper to remove water remaining is done, and then the oil is passed through secondary filtration, often with multiple passes, being filtered with progressively lower pore size. Typically filtration down to 1µ can be readily accomplished.
A washing step is sometimes performed to neutralize any residual organic acids and to help remove any of the cleaner soap that has made its way into the oil. Buildup of cleaner in the oil can behave as a speed improver, and result in oil that is too fast and oil that can leave inorganic stains on the part. The buildup of the cleaner can be monitored by the use of AA or ICP . Because of this potential buildup of cleaner in the oil, the amount of recycled oil to new oil is typically limited to 50-75 percent.
After filtration, any residual water is removed by centrifugal separation and vacuum dehydration. The oil is heated to 70°C or so under a vacuum. Water, due to its much higher vapor pressure, will be distilled from the oil. This oil is progressively decanted off.
Once the water is removed and the oil has been filtered, it should be properly tested to make sure that it is suitable for continued use. Testing should include cooling curves and water testing to ensure that the water has been removed.
This batch recycling of the oil can either be done in-house or by contracting an outside firm to perform the recycling of the oil. Typically, when contracting out recycling of oil, it can cost upwards of 25-75 percent of the cost of new oil.
One common refrain when discussing filtration, reclamation, and recycling of oil is, “We don’t get paid to filter oil — only to heat treat parts.” Proper filtration can increase the life of the oil by nearly double, so that the interval between oil dumps is halved. This means that the purchase of new oil is reduced, and new purchases of oil are extended over a longer period of time.
With recycling of quench oil, the purchase of new oil can be reduced by up to 75 percent or more, with 50 percent the most common. Between proper reclamation of the oil and recycling of oil from the washers, the total oil consumption can be reduced to 25 percent of operations that do not reclaim and recycle. This is oil that is thrown away and hauled off as hazardous waste that could be used to heat-treat parts. It doesn’t cost to reclaim and recycle oil — it pays for itself in reduced consumption, reduced maintenance, and cleaner parts.
In this short article, I have illustrated the basic process of reclamation and recycling of oil, and the basic cost savings that can be achieved through proper reclamation and recycling process. Should you have any questions regarding this article, or other articles, please contact the author.
- Merriam-Webster, “Merriam-Webster,” [Online]. Available: https://www.merriam-webster.com/dictionary/recycle. [Accessed 10 August 2019].
- Merriam-Webster, “Merriam-Webster,” [Online]. Available: https://www.merriam-webster.com/dictionary/reclaim. [Accessed 10 August 2019].
- J. Pfaffenberger, “Why Fine Filtering for quench oil systems is a must!” in Proceedings of the 23rd IFHTSE Congress, Savannah, GA, 2016.
- D. S. MacKenzie and R. Johnston, “Cleaning for Heat Treating,” in Proc. 26th ASM Heat Treating Society Conf., 31 October – 02 November, Cincinnati, OH, 2011.
- D. S. MacKenzie, “Care and Maintenance of Oil Quenchants used for Quenching Automotive Components,” in European Conference on Heat Treatment and Surface Engineering, 11-13 May, Prague, Czech Republic, 2016.
- D. S. MacKenzie, M. Fretz and D. Schuster, “Cleaning for Heat Treatment – Part I,” Heat Treating Progress, no. October, pp. 30-35, 2008.