Process for cleaning metal working fluids for re-use

In a continuous recycling process for removing suspended foreign matter including tramp oil from contaminated metal working fluid in an effective manner with little or no foaming and without removing or otherwise adversely affecting any oil or synthetic emulsions present in the fluid, said process comprising the steps of:

• (a) providing a tank through which the metal working fluid is able to continuously flow, said tank having a mixing zone within the lower region thereof in which the contaminated metal working fluid and a saturated dissolved air solution are able to mix quiescently prior to formation of air bubbles within the solution,
• (b) locating the tank in the process at a location that enables the tramp oil to enter the tank prior to the emulsification thereof by any emulsifying agents present in the fluid,
• (c) introducing the metal working fluid into the mixing zone of the tank of step (a),
• (d) dissolving air into a portion of the metal working fluid contained in a pressure vessel under sufficient pressure to provide a saturated dissolved air solution,
• (e) introducing the pressurized saturated dissolved air solution of step (d) into said mixing zone of the tank of step (c) in such a manner that the solution is able to mix quiescently with the contaminated metal working fluid before the formation of air bubbles within the solution begins to occur,
• (f) permitting the pressure on the saturated dissolved air solution to lessen within the tank so as to enable the air to spontaneously nucleate and expand into a multitude of bubbles that adhere to and carry the foreign matter and tramp oil but not the emulsion upwardly through the metal working fluid to form a concentrated layer of the foreign matter and tramp oil in the upper level of the metal working fluid, and
• (g) removing the concentrate of step (f) from the metal working fluid.