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Introduction to Coolant Reconditioning

Before the effectiveness of any coolant recycling system can be assessed, it is necessary to keep in mind the three basic processes which degrade coolant:

  1. Included Contaminants
  2. Biological Attack
  3. Tramp Oil Contamination

These three processes are universal, occurring in systems composed of either central or individual sumps.  Both soluble and synthetic coolants are degraded by the same process.  They are constant, beginning as soon as coolant is added to a sump.  They result in a coolant totally degrading in between two weeks and six months, (depending on conditions).  The usual time frame for complete degradation is four to six weeks.

Each of these processes is addressed in various ways by different recycling systems.  However, it is imperative to remember that all three must be dealt with or the coolant will fail prematurely.  Treating only one or two of the three degradation processes will improve coolant life, but it will not give a true recycling system.

I. Included Contaminants: there are three forms

  1. Machining Fines and Grit
  2. Hard Water Salts
  3. Biological By-Products

Any materials which are truly dissolved in the coolant, like how sugar is dissolved in water, cannot be removed from the coolant by mechanical means.  Their removal would require complex chemical reaction which would probably destroy the coolant.  For this reason, most hard water salts and biological by-products cannot be removed from coolant.  this means that the coolant will eventually reach a point at which it must be disposed of properly.  To help postpone disposal as long as possible, the use of deionized water is strongly recommended in recycling systems.  Even then, the best any recycling system can realistically promise is an 80% to 90% reduction in waste disposal.

There are claims for coolant systems with zero disposal.  These claims are generally made for either the "super coolant" or "super recycling" system.  The claims are true, but they have little to do with either the " super coolant" or the "super recycling" system.  The important factor is "drag-off".  Drag-off is all the coolant which is carried off with the parts, thrown away with the chips, splashed on the floor, etc.  It the drag off is sufficient (usually greater than 25% per week), the hard water salts and biological by-products will not build up to levels which will cause problems.  In this case, if the other two degradation processes are adequately addressed, a system can last "indefinitely".  Drag-off of this magnitude, however, is extremely expensive and few well-run plants would tolerate the cost.

The most common method for removing undissolved contaminants is the use of either a centrifuge or a cyclone filter.  Although the mechanics are different, they both operate on similar principals.  The coolant is caused to spin at high speeds.  This spinning creates a type of artificial gravity through centrifugal force.  The "heavier" particles, such as fines, are forced to the outside.  "Lighter" particles, such as tramp oil, float on the inside.

Cyclonic Filters create the spin by pumping coolant through a spiral path in a cone-shaped chamber.  Heavier particles are forced downward and disposed of and lighter segments flow upwards and are returned to the system.  Cyclonics are mechanically simple and efficient at removing solids.  However, because of their design, cyclonics can remove only the heavier segments of the coolant.  Since tramp oil is lighter than either soluble or synthetic coolants, cyclonics do not remove it from the coolant system.  They can only be used for removing fines.

Centrifuges induce the spin by bringing the coolant into contact with either the rotating plates or chambers.  An intricate collection system is then used to separate contaminants.  Centrifuges can be designed to remove both the heavier and lighter segments from the coolant being passed through them.  This makes them more versatile than cyclonics.  When properly adjusted and maintained, centrifuges are among the best mechanical means for removing contaminants.  However, there are several problems with centrifuges.  Because the soluble oils have almost exactly the same "weight" as tramp oil, the centrifuge must be very carefully adjusted.  In actual operation, the centrifuge is usually either passing through tramp oil or removing the soluble oil concentrate from the emulsion.  Centrifuge systems are somewhat difficult to maintain and usually have significant downtime.  Lastly, they are also relatively high-energy consumers.

The second type of system for separation of contaminants is Filtration.  These types of systems are familiar to almost everyone.  They are easy to maintain and to use.  Proper filtration helps to maintain surface finishes on the parts and prevent the sump from physically filling up with fines and swarf.  Combined with oil skimming ability, these filters can be cost effective and reliable.

II. Biological Attack

In the vast majority of cases, coolant is dumped because of biological attack.  this attack can cause corrosion, tool wear, blocked lines and the major problem - ODOR.  There are only two methods used for the control of biological attack:  Heat Sterilization (sometimes incorrectly called pasteurization) and the use of biocide.

Heat Sterilization is a deceptively simple process.  The coolant temperature is raised to between 140 degrees F and 160 degrees F.  It is maintained there from 8 to 12 hours, then allowed to cool or sometimes run through a refrigerator unit to bring it down to room temperature.  This is a very ineffective method of biological control.  The high temperature and long dwell time make the concept very energy inefficient.  In addition, temperature high enough to sterilize the coolants frequently oxidize components of the coolant.  The worst drawback is that heat sterilization does not control bacteria where the main problem is in the sump.  Though the coolant may be relatively bacterial free when it is returned to the machine coolant system, the bacteria which still coats the inner walls and lines of the sump begin multiplying immediately.  They usually reach their original levels of contamination within 24 - 48 hours.

Biocides can either be added manually or metered in through the recycling system.  They have, when added in proper concentrations, several advantages.  They persist in the sump and continue to kill bacteria and mold for several weeks or more.  This helps extend the interval between recycling.  The amount or type of biocide can be altered to meet changing plant conditions.  Biocides can help keep the sump and coolant lines free of biological contamination, to the point at which extensive cleaning is not necessary.  The concerns with the use of biocides are relatively few.  Improperly used biocides can cause dermatitis or skin irritation.  In addition to depleting biocides can also cause disposal problems.

III. Tramp Oil Contamination:

It is a commonly held view in most shops that the primary cause of coolant degradation is the presence of free of "tramp" oils floating on the surface of the coolant.  This view is reinforced by the observance of clumps of mold or bacteria attached to the masses of oil or accumulating on sump walls in the presence of oil.

It is true that tramp oils are an organic nutrient and are one of the causes of coolant degradation.  However, coolants will support biological growth even without the presence of tramp oils.  Heavy layers of floating oils do promote the growth of anaerobic bacteria by shutting off oxygen to the coolant sump.

There are several methods of removing tramp oils.  Besides coalescers, skimming with a belt of wheel skimmer can remove large quantities of oil.  Absorption type filters work well if the quantities are low, but get expensive if volumes are excessive.  Decanting or overflow systems work but require quiet surfaces and remove some coolant along with the oil.

The real problem with tramp oils is their complete removal from the sump.  This is often difficult because of the configuration of the sump and/or its location (for example, under the machine bed).  Oil sometimes accumulates in difficult-to-reach areas such as under plumbing or baffles.  The only effective way to remove these tramp oils is to use a powerful sump vacuum.  This defeats the purpose of in-process recycling because it necessitates emptying out the sump.

A much preferred alternative is to clean out the machine prior to instituting a recycling program.  This will loosen any clumps of oil/grease and hidden colonies of mold/bacteria/fungus.  Once the machine is thoroughly cleaned out, a regular recycling program will significantly prevent build-up of hidden tramp oil.

Please note that although tramp oils contribute to microbial problems, there removal does not necessarily eliminate bacteria/fungal growth.  Some bacteria are capable of living under both aerobic and anaerobic conditions.  Proper addition of biocide is the only means of correcting this problem.  Removal of tramp oils does help but does not prevent microbial contamination.

Other Concerns:

In an overall coolant management program, other concerns which must be dealt with include chemical depletion.  As a coolant is used, whether it is a soluble oil of synthetic coolant, its components change.  Individual components can plate out on parts, combine with hard water salts, be broken down by bacteria or oxidized by heat.  As the recycling lop is closed and coolant life is extended for longer and longer time periods, the addition of fresh coolant to "bring up concentration" results in an unbalanced coolant formulation.  This can cause many of the same problems the system was designed to eliminate.

For a coolant recycling system to work properly, the coolant must be analyzed regularly.  On the basis of this analysis, the coolant should be selectively fortified.  This requires an ongoing program of coolant maintenance.  The more effective the mechanical system for recycling coolant is, the more important this maintenance program becomes.

One problem which arises in any recycling system is the delegation of responsibilities.  In the typical scenario, when a coolant recycling system is chosen, the plant already has a specific soluble of synthetic coolant in use.  Any problems which arise are then bounced back and forth between the supplier of the coolant and the supplier of the recycling system - one saying the coolant is causing the problem and the other saying the recycling system is causing he problem.  Wherever possible, it is best to establish a working relationship between the coolant supplier and the supplier of the recycling system.

In a well-managed coolant recycling system, wherever close attention is paid to the three degradation processes - contaminant inclusion, biological attack and tramp oil contamination, coolant disposal can be reduced by up to 90%  With the proper ancillary equipment, such as evaporators, this can be reduced even further.  The goal of any good recycling system is to reach the point wherever hazardous waste disposal can be changed from an nightmare to a minor irritation.


For details contact Pan Pacific Environmental Group or email us from our Literature Request Page

Copyright 2009 Pan Pacific Environmental
Last modified: 02/10/09