Liquid purification or separation – Processes – Using magnetic force
Reexamination Certificate
2002-07-18
2004-11-23
Drodge, Joseph (Department: 1723)
Liquid purification or separation
Processes
Using magnetic force
C209S039000, C210S634000, C210S770000, C210S773000, C210S774000, C210S787000, C210S806000
Reexamination Certificate
active
06821437
ABSTRACT:
FIELD OF APPLICATION
The present invention relates to a method of separating a machining suspension into fractions, particularly of the type accumulating when silicon, quartz or ceramic materials are machined. A used machining suspension consists, as a rule, of a cutting fluid in which abrasive grains or attrition material of a cutting tool or a machining equipment, respectively, or of the machined material are dispersed.
Such machining suspensions are used, for example, in the manufacture of thin wafers of silicon or quartz for the semiconductor or solar technology, wherein the thin wafers are separated from mono-crystalline rods or cast blocks by application of the separating lapping technique. In one embodiment of this technique, a wire having a length of several kilometers is passed over a coil system in such a way that a wire netting will be created, which includes up to several hundreds of wires in side-by-side relationship. With the addition of the machining suspension, it is hence possible to saw several hundreds of thin wafers from a block or rod of appropriate length at the same time, in a single operation. The used machining suspensions are composed of a cutting fluid—preferably polyvalent alcohols or highly refined mineral oils with additives—with abrasive grains, preferably of silicon carbide (SiC) with different particle sizes, suspended therein. In the course of the mechanical machining of the silicon or quartz, the abrasive grains carried in the machining fluid is conveyed to and compressed at the site where it is to produce its machining effects, by means of the machining tools such as band saws, sawing wires or lapping disks.
The suspension used for the machining process, which carries the accumulating attrition material, is collected and recycled into the machining process again. As a matter of fact, however, the efficiency of the machining suspension reduces as the application period prolongs so that it must be replaced in the end because the given aim of the machining operation can no longer be achieved. The reason for this resides not only in the wear of the SiC material but mainly also in the increasing percentage of the fine particles of the silicon or quartz attrition material and in the iron attrition material from the machining tools. The range of a typical composition of such used suspensions is illustrated in Table 1.
TABLE 1
Typical composition of machining suspensions from the process of
silicon wafer sawing
Component
Percentage (mass fraction)
Silicon carbide
20-70
Cutting fluid
20-70
Silicon
0-20
Metal attrition material
0-10
PRIOR ART
For processing a used machining suspension, only some few methods are known at present so that in practical operation the machining suspensions must frequently be disposed of as hazardous waste for incineration.
As a matter of fact, there are various possibilities available to increase the period of service, i.e. the number of the possible cuts to be performed per kilogram of suspension. To this end, for example, one can replace a partial volume of the used machining suspension after each cutting operation by a new machining suspension, or separate one part of the attrition material from the suspension by decanting. In all these methods, however, the used machining suspension must be disposed of after a more or less high number of cutting operations. As a rule, this is performed by way of disposal as hazardous waste by incineration with subsequent dumping of the residues.
As the costs of providing the machining suspension account for a substantial part of the total costs incurred by the mechanical machining of silicon, quartz or ceramic material, this widespread practical approach in disposal is not economic.
A method is known from the Patent Abstracts of Japan, in relation to JP 1-316170, wherein used machining suspensions are centrifuged and the abrasive or grinding grains, respectively, so separated are re-dispersed in fresh cutting fluid. It is a disadvantage of this method, however, that a high fraction of attrition material and of used cutting fluid remains still in the separated abrasive grains. This method does therefore not permit a distinct separation between the abrasive grains and the attrition material so that the possibilities of recycling the constituents of the machining suspension are very strongly restricted, too.
The European Patent EP 0916463A1 discloses a further method for separation of a machining suspension into fractions. In that method, the suspension is initially disintegrated by a distilling drying step into a solids component and a liquid component. Then a fractionating step, particularly a screening or air sizing technique, is applied for separating the dry solids component into a fine attrition fraction and a coarse abrasive grain fraction. Subsequently to this dry sizing step, the fraction containing the abrasive grains may be used again in a machining suspension. What is a disadvantage in the drying step, however, is the fact that the additives difficult to volatilize in cutting fluid remain in the solids component. Moreover, even the preferred vacuum operation for the successful drying of the solid material requires temperatures that may be detrimental to the cutting fluid in terms of disintegration. The cutting fluid material is then precluded from re-use. As a result of the subsequent dry air sizing technique, moreover undesirable agglomerates consisting of extremely fine particles may be transferred into the re-used coarse fraction. The reason for this are residues of the cutting fluids that fuse the particles together.
A strict separation between the fractions contained in the machining suspensions can therefore not be achieved either with this method. Successful separation into fractions is, however, the prerequisite for a high-quality salvage and for re-use of the components. Particularly in the case of an intended salvage of used SiC material in the field of ceramics or abrasive grain manufacture, a strict isolation of silicon attrition material from the metal attrition material is required. With the known methods according to prior art, such a separation could so far not be achieved.
In another known method according to the U.S. Pat. No. 3,9978,359 for the separation of individual components from a cutting fluid, the separation of the attrition material from the abrasive grains is carried out only after a separation of the cutting fluid in several washing and rinsing operations with a subsequent drying step so that the aforedescribed problems may occur.
The European Patent EP 0 786 317 A2 discloses a method of processing used machining fluids of the claimed general type, such as cutting fluids; this method serves to separate the abrasive grains from the machining liquid. In that method, initially water is added to the oily machining liquid in order to reduce the viscosity of the machining liquid by the formation of an oil/water emulsion. Then, in a wet sizing step, the abrasive grains are separated from the liquid phase consisting of oil, water and attrition material. With such a technique, however, the intended strict separation of the abrasive grains from the attrition material and the cutting fluid is not achieved either.
The present invention is now based on the problem of providing a method of separating a machining suspension into fractions, by means of which the abrasive grains can be reliably and precisely separated from the attrition material and the cutting fluid so as to permit the successful re-use of the individual components of the machining suspension.
SUMMARY OF THE INVENTION
The problem is solved with the method according to claim
1
. Expedient embodiments and improvements of the method are the subject matters of the dependent claims.
In the inventive method, initially a liquid processing aid is selected for the reduction of the viscosity as a function of the respective cutting fluid used and of the attrition material, which processing aid can be mixed with the cutting fluid (i.e. is miscible therewith), which mixture in its turn forms a sus
Borcherding Axel
Eisner Peter
Malberg Andreas
Menner Michael
Breiner & Breiner L.L.C.
Drodge Joseph
Fraunhofer-Gesellschaft zur Forderung der ange-wandten Forschung
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