Liquid purification or separation – Processes – Separating
Patent
1996-03-15
1998-06-30
Popovics, Robert
Liquid purification or separation
Processes
Separating
210168, 210171, 210 87, 210 961, 210101, 210143, 2101951, 210252, 2102571, 210418, 451 88, B01D 1712
Patent
active
057729000
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Art
The present invention relates to a method and an apparatus for reclaiming a used working fluid produced after the working fluid which contains abrasive grains dispersed in a medium liquid is used in abrasive machining on a workpiece.
2. Discussion of the Prior Art
In various kinds of working operation on a workpiece such as a grinding operation on a silicon wafer or a cutting operation using a wire saw, an oily or aqueous working fluid is used, which is prepared by dispersing abrasive grains such as GC or WA in a medium liquid such as kerosene or water. In abrasive machining using the thus prepared working fluid, a workpiece is processed while the working fluid is applied to the surface of the workpiece. The working fluid which has been applied to the workpiece is recovered and applied again to the workpiece. Thus, the working fluid is circulated and repeatedly used in the abrasive machining. The working fluid is applied to the workpiece so that the abrasive grains in the working fluid are supplied to the surface of the workpiece for assuring an intended machining operation. In addition, the working fluid functions to wash away cutting chips or swarf produced during the machining operation on the workpiece.
In the abrasive machining as described above, since the working fluid is circulated and repeatedly used, the working fluid is inevitably contaminated with the cutting chips. As the working operation proceeds, the concentration of the cutting chips in the working fluid is gradually increased. When the concentration of the cutting chips exceeds a certain threshold (about 0.5 kg/l, for instance, where the working fluid containing GC abrasive grains of #1000 size and having a concentration of 1.6 kg/l is used for lapping operation on a silicon wafer) determined depending upon the working conditions and other factors, the working accuracy is deteriorated since the temperature of the working fluid is raised due to an increase in the friction. Further, the cutting chips included in the working fluid hinder contact of the abrasive grains in the working fluid with the workpiece, undesirably lowering the working efficiency. In view of this, the concentration of the cutting chips in the working fluid should be maintained lower than the above-described threshold. Therefore, the working fluid whose cutting chip concentration exceeds the threshold is reclaimed by removing the cutting chips from the used working fluid, or the used working fluid is replaced with a fresh working fluid which does not contain the cutting chips.
The cutting chips produced during the working operation are extremely small generally having an average grain size which is not larger than about one tenth of that of the abrasive grains. Thus, it is difficult to remove the cutting chips from the used working fluid. When the cutting chips are removed by a membrane filtration method using a membrane filter having pores which are sufficiently smaller than the average grain size of the abrasive grains, the cutting chips can be separated from the medium liquid containing the abrasive grains. In this method, however, the abrasive grains are deposited on the surface of the membrane filter and form a cake layer thereon. This cake layer prevents the cutting chips from being passed through the membrane filter and only the medium liquid is transmitted therethrough. In this case, the membrane filter needs to be frequently replaced with new ones for attaining a sufficient removal of the cutting chips. Further, when the cake of the abrasive grains is reduced, it is difficult to disperse the abrasive grains of the cake in the medium liquid. The cutting chips of silicon, for instance, have a smaller specific gravity and a smaller grain size than the abrasive grains such as GC grains. Accordingly, it is possible to remove the cutting chips from the working fluid by precipitating the abrasive grains, and throwing away a supernatant liquid and thereby separating the cutting chips. In this case, too, it is extremely di
Yorita Hiroshi
Yoshikawa Takashi
Noritake Co., Limited
Popovics Robert
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