Abstract:
The present invention relates to a method and a system for treating spent abrasive slurry obtained from a process for cutting a body of a substrate material into wafer-like slices, said slurry comprising a lubricant fluid, unspent abrasive particles and fines. 
     The method comprises separating the spent slurry in a first sedimentation step into a solids concentrate comprising unspent abrasive particles and a solids depleted slurry; and subsequently separating the solids depleted slurry by cross-flow filtration into a fines containing fraction and a solids and fines depleted regenerated lubricant fluid. 
     The corresponding system comprises a first sedimentation device having an inlet for spent abrasive slurry, a sedimentation unit, a first outlet for discharging a solids concentrate and a second outlet for discharging a solids depleted slurry from said sedimentation unit; a cross-flow filtration device having an inlet for said solids depleted slurry in fluid communication with said second outlet of said sedimentation device, a first outlet for discharging a fines containing fraction and a second outlet for discharging a solids and fines depleted regenerated lubricant fluid from said cross-flow filtration device; and a conduit means providing fluid communication between said second outlet of said first sedimentation device and said inlet of said cross-flow filtration device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for treating spent abrasive slurry obtained from a process for cutting a body of a substrate material into wafer-like slices. Typically, abrasive slurries are applied in the cutting of semi-conductor materials, e.g., ingots from single crystals or polycrystalline silicon, GAs and Ge by means of wire saws into semiconductor wafers and comprise a lubricant or cooling fluid of high viscosity such as a mineral oil or water-soluble liquids (e.g., polyethylene glycol) and a particulate matter from abrasives such as silicon carbide. 
         [0002]    While the wires used for cutting the ingots into slices have a smooth surface, the cutting effect is obtained by use of the highly viscose abrasive slurry which is fed to the contact area of the cutting wire and the substrate material to be cut into slices. 
         [0003]    During the cutting operation, the substrate material is ground at a so-called saw kerf into powdery material. The slurries also serve to remove such powder substrate material from the saw kerf. 
         [0004]    During the cutting operation, the abrasive slurries are contaminated in three ways: The substrate material (e.g. silicon or other semiconductor material) is disintegrated into particles that are taken up by the slurry. The metal of the cutting wires (primarily iron) is another source for particulate contaminations due to surface wear of the wire. Finally, the grains of the abrasive material itself are partially disintegrated into smaller particles which are, of course, also incorporated in the abrasive slurry. 
         [0005]    As the concentrations of these three contaminants (in the following called fines) increase over time, the efficiency of the cutting operation decreases. When the slurry finally becomes ineffective, i.e., spent or exhausted, it must be discarded. 
         [0006]    The spent slurry is either been disposed of (by incineration or other means) or regenerated. 
         [0007]    The methods for regenerating spent abrasive slurries which were heretofore proposed rely on two different principles. The first one starts with the separation of the exhausted slurry into a first liquid fraction and a first solids fraction. Afterwards, the two fractions are regenerated separately, involving various operations such as diluting, washing, classification, filtration, etching, evaporation and others. The regenerated liquid and the regenerated solids fraction, the latter having no or a reduced content of fines, can be used for the preparation of fresh slurry. Examples of such processes are disclosed in WO 2002/096611 and in EP 1 561 557 A1. 
         [0008]    There are numerous disadvantages to this method, and the most prominent one is its complexity. The large number of operations needed and in addition the high degree of interactions between these operations would make a small system which can be located and operated near the point of use to regenerate the spent slurry from a local user rather expensive. Therefore, this technology has been used in large centralized units which make it necessary that the spent slurry is transported from the facilities of the users to the regeneration site. The transport costs involved in the regeneration of the abrasive slurry are therefore substantial. 
         [0009]    A certain improvement might be possible by lowering the viscosity of the spent slurry by heating the same as is proposed in U.S. Pat. No. 6,231,628 B1. However, the heating of rather highly concentrated slurry creates the risk of scaling, encrustation, fowling and abrasion of heat transfer surfaces resulting in lower heat transfer efficiency, limited life time of the heater and increased energy costs. 
         [0010]    The other principle used for slurry regeneration is based on the use of a combination of two centrifuges. In the first centrifuge, a classification of particles is achieved, i.e., the slurry is divided into two fractions that contain both a part of the lubricant and a part of the solids. The two fractions differ in their solids concentration and in particle size distribution of the solids. The overflow of the first centrifuge mainly contains the majority of the lubricant and the smaller particles (fines or debris from semiconductor material, wire and abrasive material, typically below about 10 μm). The sludge resulting from the first centrifugation step (flowable concentrate) contains preferably the abrasive grains with sizes near to those of new slurry (essentially above about 10 μm). The volume of the sludge containing the re-usable abrasive grains is much smaller than the volume of the overflow. The classification effect results from the differences in settling velocities of the particles in the centrifugal field. 
         [0011]    Larger particles sink faster, and they are favored to leave the centrifuge with the sludge. Smaller particles that settle with lower speed are easier carried and swept along with the liquid overflow of the centrifuge. The overflow is clarified by means of a second centrifuge resulting in a sludge to be discarded (small waste particles or fines in a small amount of lubricant fluid) and a more or less clarified stream of lubricant. Although this principle necessitates less operations than the first one described above and although it might be easier installed as a point-of-use process, two major disadvantages prevent the method from being widely used. 
         [0012]    First of all, the second centrifuge is not capable of clarifying the overflow fraction of the first centrifuge satisfactorily to provide a readily re-usable lubricant. A substantial part of the fines remains in the lubricant and pollutes, when applied to prepare fresh slurry, this slurry from the beginning. 
         [0013]    Secondly, the classification effect in the first centrifuge is far from being ideal: Substantial amounts of fines leave the centrifuge with the unspent abrasive particles contaminating fresh slurries prepared with the recovered abrasives in addition and limit the life time of the fresh slurries. 
         [0014]    The reasons for this are some limitations of the centrifugal process:
   i) High solids concentration of a common saw sludge favor particle-particle interactions; large particles can entrain fines along with them, forcing them to leave the centrifuge the wrong way out. The effect is strong especially near the inner wall of the centrifuge bowl where most of the solids are concentrated.   ii) The high viscosity of the lubricant amplifies the momentum transfer between the particles; small particles are much influenced by coarse ones.   
 
         [0017]    The object of the present invention is to provide a method for treating spent abrasive slurries in a more economical way. Especially, the present invention provides a method for treating the spent abrasive slurry which may be carried out at the point of use obviating long transportation of the spent slurry for regeneration. 
       SUMMARY OF THE INVENTION 
       [0018]    The present invention provides a solution to this object by the method defined in claim  1 . 
         [0019]    The first sedimentation steps may be accomplished by simple gravity sedimentation. Advantageously, the preferred embodiments of the present invention make use of a centrifugal field in this step which allows for more compact equipment. 
         [0020]    The present invention uses a cross-flow filtration device (e.g., ultrafiltration or cross-flow microfiltration) which allows removing practically all fines from the lubricant. The cross-flow filtration device may be incorporated in the treatment system downstream of the second centrifuge mentioned for the second method above or it can fully replace such a second centrifuge. The regenerated lubricant fluid, i.e., the filtrate or permeate equals an unused lubricant as far as its content of fines, i.e., particulates and colloidal components are concerned. 
         [0021]    Its use for preparing a fresh slurry greatly reduces fines contaminations and increases slurry life time even if combined with a solids concentrate obtained in a conventional way by the centrifugation of spent slurries, which is due to the excellent quality of this liquid. 
         [0022]    In a preferred embodiment the filtrate is split and one portion thereof is mixed with the spent slurry in the feed of the first sedimentation step. This lubricant recycling dilutes the spent slurry increasing the distance between the particles and, as a consequence, lowers the particle-particle interactions and improves classification efficiency of the first sedimentation step. Thereby, the amount of fines polluting the coarse abrasive particles obtained in the sludge of the first sedimentation step is diminished. The combination of the regenerated lubricant fluid and the solids concentrate obtained according to this embodiment provides fresh slurry of excellent quality and further increased slurry life time. 
         [0023]    Preferably, the regenerated lubricant fluid and said spent slurry are admixed in a volume ratio of from about 0.5:1 to about 3:1, more preferably of from about 1:1 to about 3:1. 
         [0024]    Mixing of the lubricant and the spent slurry may be preferably accomplished in a static mixer. 
         [0025]    This allows low-cost equipment, and the efficiency provided by a static mixer is sufficient for the purpose. 
         [0026]    In an even more preferred embodiment, the one portion of the regenerated lubricant fluid used to dilute the spent slurry is heated to an elevated temperature prior to admixing the same with the spent slurry. The resulting mixture of spent slurry and regenerated lubricant fluid is at an elevated temperature, preferably at about 35° C. or more, more preferably at a temperature of 50° C. or more. The elevated temperature lowers the viscosity of the slurry/lubricant mixture. The lower viscosity accelerates the sedimentation of the particles within the fluid and such overcompensates the increased feed rate into the centrifuge and to the cross-flow separation device. It furthermore improves the classification efficiency and thus provides a higher quality first solids concentrate. 
         [0027]    Heating the regenerated lubricant fluid prior to admixing the same with the slurry is of advantage over a direct heating of the spent slurry itself. Heating the regenerated lubricant fluid obtained in the present invention prior to admixing the same with spent slurry avoids the risk of scaling, encrustation, fouling and abrasion of heat transfer surfaces, since the liquid passing the heater practically no particles and almost no colloidal components. 
         [0028]    The lubricant fluid obtained in the cross-flow filtration step which is re-used to produce fresh slurry may be cooled down in a cooling device prior to admixing the same with the solids concentrate obtained from the first sedimentation step and optional supplemental fresh slurry. 
         [0029]    The present invention also resides in a system for treating spent abrasive slurries according to a method as described above. Such a system is set out in claim  15 . 
         [0030]    Preferred embodiments of such systems are evident from the above discussion of the inventive process where the various devices advantageously used in such an inventive system have already been discussed. They are furthermore subject matter of the claims dependant on claim  15 . 
         [0031]    The present invention is explained by way of the following examples and embodiments in more detail. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  shows a schematic representation of a cutting wire in a saw kerf; 
           [0033]      FIG. 2  shows a schematic enlarged representation of abrasive particles; 
           [0034]      FIGS. 3  a and b show two basic systems for implementing the inventive method; 
           [0035]      FIG. 4  shows a preferred embodiment of a system according to the inventive system; 
           [0036]      FIG. 5  shows an even further preferred embodiment of a system according to the present invention; and 
           [0037]      FIG. 6  shows another preferred embodiment of a system according to the preferred invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]      FIG. 1  shows a cut-out of a cross-sectional representation of an ingot body  10  of, e.g., silicon which has been cut already to a certain depth starting from an outer circumference  12  to produce a kerf  14 . Within kerf  14 , a cutting wire  16  is positioned which has an approximately circular cross-section and a smooth outer surface  18 . In the portion of kerf  14  where the cutting wire  16  contacts the ingot body  10 , an abrasive slurry  20  is present to cool and lubricate the cutting tool and the substrate material of the ingot while at the same time supporting abrasion of the substrate material of ingot  10 . To that effect, the abrasive slurry  20  includes besides a viscous lubricant fluid abrasive particles or grains  22  which are shown in  FIG. 2  in more detail. The average size of the abrasive grains  22  shown in  FIG. 2  is about 20 μm and these abrasive grains  22  work on the substrate material of ingot  10  driven in a longitudinal motion of cutting wire  16 , a motion which would be perpendicular to the surface of the drawing. The lubricating liquid which forms part of the abrasive slurry is a highly viscous fluid, for example a mineral oil or a water-soluble organic liquid like polyethylene glycol. 
         [0039]    The high viscosity is needed in order to provide for sufficient abrading action of the abrasive grains  22  in kerf  14 . The high viscosity is further needed in order to provide for sufficient lubrication. 
         [0040]    In addition to the above functions of the abrasive slurry  20 , the slurry also provides for removal of the deteriorated substrate material (fines) produced during the cutting operation. 
         [0041]    Therefore, the slurry  20  which is recycled in the cutting device until it is exhausted comprises besides the lubricating fluid, the abrasive grains  22  and the powdered substrate material which usually has an average particle size very well below about 10 μm, deteriorated material from the abrasive grains as well as metal particles created from wear of the cutting wire (herein in summary called fines). 
         [0042]    The present invention provides for a cost-effective method and economical means for treating the spent slurry to an extent that at least large parts of it may be re-used for the cutting operation. 
         [0043]    The present invention in its simplest, but nevertheless already very efficient configuration is shown in  FIG. 3   a . The device according to  FIG. 3   a  comprises a first sedimentation unit  32  which comprises a centrifuge and which receives the spent or exhausted slurry directly from the cutting process described above. The centrifuge of the first sedimentation unit  32  separates the spent abrasive slurry into a solids concentrate and a solids depleted slurry. 
         [0044]    The solids concentrate is drained from the sedimentation unit  32  via line  34  and contains a smaller amount of the lubricant fluid of the abrasive slurry. The composition of the solids concentrate drained via line  34  is such that it may be directly used for the preparation of fresh abrasive slurry by combining same with regenerated lubricant fluid. 
         [0045]    The overflow generated in the sedimentation unit  32  is a solids depleted slurry which is withdrawn from the sedimentation unit  32  via line  36  and fed into a cross-flow filtration unit  38 . In the cross-flow filtration unit  38 , which preferably is a membrane separation device, essentially all of the fines including colloidal dispersed particulate matter in the lubricating fluid may be removed such that the permeate which may be withdrawn from the cross-flow filtration unit  38  via line  40  is ready for re-use and preparation of a new or fresh abrasive slurry. This solids and fines depleted permeate is called herein regenerated lubricant fluid. The fines-containing fraction withdrawn from the cross-flow filtration unit  38  is removed by line  42  and comprises essentially all of the fines previously contained in the spent abrasive slurry. This fraction may be discarded in a conventional manner via line  43 . In order to increase the efficiency of the cross-flow filtration unit, i.e., to increase the proportion of regenerated lubricant fluid which may be withdrawn via line  40 , part of the fines-containing fraction may be recycled via line  45  and mixed with the solids depleted slurry which is fed via line  36  into the inlet of the cross-flow filtration unit  38 . 
         [0046]      FIG. 3   b  shows a preferred variant of the device of  FIG. 3   a  comprising in addition to the first sedimentation unit  32  a second sedimentation unit  44 . The second sedimentation unit  44  comprises preferably a centrifuge and receives as its feed the solids depleted slurry from the first sedimentation unit  32  via line  36 . The second sedimentation unit  44  serves to remove part of the fines included in the solids depleted slurry prior to transfer the same to the cross-flow filtration unit  38 . In this embodiment, a solids and partly fines depleted slurry is fed via line  46  into the cross-flow filtration unit  38 . A fines concentrate withdrawn from the second sedimentation unit  44  via line  48  mainly contains fines and is conventionally discarded. 
         [0047]    The particle size in the fines concentrate is much lower than that of the particles contained in the first solids concentrate and also may comprise a substantial portion of abrasive grain debris, powdered substrate material and abraded metal particles from the cutting wire of the cutting system. 
         [0048]    As described in connection with  FIG. 3   a  already, the retentate (fines containing fraction) from the cross-flow filtration unit  38  may be partly recycled via line  45  to the solids and partly fines depleted slurry to improve the separation efficiency of the cross-flow separation unit  38  while the remainder is discarded via line  43 . 
         [0049]      FIG. 4  shows a preferred embodiment of a system  50  of the present invention for treating spent abrasive slurries, said system  50  receiving spent abrasive slurries via line  52 . The spent abrasive slurry is fed into a centrifuge or first sedimentation unit  54  which provides an overflow in the form of a solids depleted slurry which is drained from centrifuge  54  via line  56 . The sludge separated in centrifuge  54  from the spent slurry as a solids concentrate comprises the main portion of abrasive particles which still may be used in the cutting process. The sludge is withdrawn from centrifuge  54  via line  58 . 
         [0050]    The solids depleted slurry still including fines may be fed via line  56  into a second sedimentation unit in the form of centrifuge  60  to withdraw a fines concentrate as sludge via line  62 . This fines concentrate comprises a substantial portion of fines. The solids and partly fines depleted slurry is withdrawn from centrifuge  60  as overflow via line  64  and sent to a cross-filtration unit  66 . 
         [0051]    In a variant of system  50 , the second centrifuge  60  may be omitted and line  56  may be directly connected to line  64  so that the solids depleted slurry created in centrifuge  54  is directly fed into the cross-flow separation unit  66 . 
         [0052]    In the cross-flow separation unit  66 , a retentate is withdrawn via line  68  as a fines containing fraction whereas the permeate is withdrawn via line  70  in the form of a (solids and fines depleted) regenerated lubricant fluid. One part of the retentate may be recycled via line  69  and the remainder discarded via line  71 . 
         [0053]    The quality of the so obtained regenerated lubricant fluid is such that it may be used without any further treatment for preparing fresh slurry, e.g., by combining the solids concentrate received from centrifuge  54  via line  58  and the regenerated lubricant fluid withdrawn from the cross-flow filtration unit  66  via line  70 . 
         [0054]    It has, however, turned out to be more efficient not to recycle all of the regenerated lubricant fluid to the process for preparing fresh abrasive slurry, but to recycle one part of it via a line  72  and combine it with the spent slurry received via line  52  from the cutting process. Surprisingly, recycling of part of the regenerated lubricant fluid and combining the same with the spent abrasive slurry received from the cutting process enhances the separation efficiency of centrifuge  54  and provides for a better quality of the solids concentrate withdrawn from centrifuge  54  via line  58 . 
         [0055]    The proportion of regenerated lubricant fluid combined with the spent slurry is preferably such that a ratio of from about 0.5:1 to about 3:1 results. More preferably, the ratio of regenerated lubricant fluid and spent abrasive slurry is in the range of from about 1:1 to about 3:1. 
         [0056]    The proportion of the solids depleted regenerated lubricant fluid received in line  70  from the cross-flow separation unit  66  will be recycled to be combined with spent slurry independent of whether a second sedimentation unit is provided in the system  50  or not. 
         [0057]    An even more preferred embodiment of the present invention is shown in the form of system  80  in  FIG. 5 . The treatment system  80  receives spent abrasive slurry via a line  82  from a cutting device. 
         [0058]    The spent abrasive slurry received from line  82  is first of all passed through a mixer unit  84  and then proceeds via line  86  to a first sedimentation unit in the form of a centrifuge  88 . The solids depleted slurry is withdrawn from centrifuge  88  as an overflow via line  90  whereas a solids-concentrate is withdrawn as sludge from centrifuge  88  via line  92 . As discussed in connection with the afore-described systems already, the quality of the solids-concentrate is such that it may be used without any further treatment as additive to lubricating fluid for producing fresh abrasive slurry. 
         [0059]    The solids depleted slurry is directed via line  90  into a second sedimentation unit  94  which provides via line  96  a solids and partly fines depleted slurry whereas a fines concentrate comprising a substantial portion of fines is withdrawn from centrifuge  94  via line  98 . The fines concentrate is usually discarded. 
         [0060]    The solids and partly fines depleted slurry withdrawn via line  96  from the second sedimentation unit, i.e., centrifuge  94 , is directed to a cross-flow separation unit  100 . The cross-flow separation unit  100  yields a retentate which is withdrawn from the unit  100  via line  102  and discarded at least in part via line  103 . Another portion of the retentate is recycled via line  105  to improve the separation efficiency of unit  100 . The permeate in the form of a (solids and fines depleted) regenerated lubricant fluid is withdrawn via line  104 . 
         [0061]    As described already in connection with the system of  FIG. 4 , in the present, even more preferred embodiment of the inventive system also a part of the regenerated lubricant fluid is recycled via a line  106  and used to dilute the spent slurry received in the system via line  82 . Here, line  106  communicates with the mixer unit  84 , a static mixer which allows homogeneously distributing the spent abrasive slurry and the recycled portion of the regenerated lubricant fluid. The proportions of regenerated lubricant fluid and spent abrasive slurry admixed in mixer  84  correspond to the recommendations given already above in connection with the description of system  50  of  FIG. 4 . 
         [0062]    In the presently described inventive system  80 , optionally line  106  passes through a heating unit  108  which is used to heat the recycled regenerated lubricant fluid to a temperature of, e.g., 80° C. The heated fluid from heater  108  is fed into mixer  84  and provides for a substantial increase of the temperature of the admixed spent abrasive slurry and regenerated lubricant fluid which serves to decrease the viscosity of the fluid sent via line  86  to centrifuge  88 . This improves the separation process in centrifuge  88  and provides for a better quality first solids concentrate which is withdrawn via line  92 . 
         [0063]    Also, the second sedimentation step performed in centrifuge  94  is improved by the increased temperature of the fluid received via line  90 . 
         [0064]    Still, the fluid sent via line  96  to the cross-flow separation unit  100  will be at an elevated temperature, such that the permeate (regenerated lubricant fluid) withdrawn from line  104  is at a temperature higher than ambient temperature. 
         [0065]    Therefore, the portion of regenerated lubricant fluid passing through line  110  for re-use in a fresh abrasive slurry preferably is cooled down to about 20° C. via a cooler  112  prior to combining the same with the solids concentrate received from centrifuge  88  via line  92 . 
         [0066]    It is understood that the abrasive slurry needs to have a relatively high viscosity and therefore a low temperature, e.g., ambient temperature, in order to maintain abrasion efficiency of the slurry. 
         [0067]    In the following example, the operation of a system  120  similar to the one explained in  FIG. 5  will be explained in some more detail by way of  FIG. 6 . 
         [0068]    The spent slurry received via line  122  contains two populations of solid particles. The coarser one consists of abrasive silicon carbide (e.g., SiC) particles ranging mainly from about 6 to about 50 μm in size with a pique at about 18 μm. The finer one is mainly a mixture of (SiC) fines and ground substrate material, e.g., silicon, mainly in the range from about 0.2 to about 5 μm in size with about 1 μm pique value. 
         [0069]    The spent slurry flows into a heat exchanger  124  at ambient temperatures, e.g., at about 20° C. A fraction of regenerated lubricant fluid (e.g., PEG) enters the heat exchanger  124  at higher temperature, e.g., at about 60° C. A part of its enthalpy is transferred to the exhausted slurry, raising its temperature from about 20° C. to about 41° C. The regenerated PEG is simultaneously cooled down in heat exchanger  124  from about 60° C. to about 30° C. 
         [0070]    The pre-heated spent slurry exits heat exchanger  124  via line  126  and is mixed and diluted with another fraction of the regenerated PEG in a static mixer  128 . This fraction of regenerated PEG is preferably heated to a temperature of, e.g., about 80° C. which will allow to increase the temperature of the diluted spent slurry exiting the mixer  128  via line  130  to about 60° C. This mixture enters the centrifuge  132 , e.g., a cylindrical-conical helical-conveyor solid bowl centrifuge. Here, the suspended solids and fines are classified into the two particle size fractions mentioned above. The coarser fraction of the grains (the “good grains”) moves preferably to the inner wall of the rotating bowl. It is discharged as a solids concentrate or sludge from the centrifuge bowl at about 80° C. by means of the helical conveyor and is transferred via line  136  to a mixing tank  156 . The finer fraction (“fines”) preferably remains suspended in the solids depleted slurry and leaves the centrifuge  132  through a liquid overflow port and line  134 . 
         [0071]    The solids depleted slurry comprising the fines is conveyed via line  134  to a cross-flow membrane separation unit  138 . The major volume of the slurry leaves this filtration unit  138  clarified as regenerated lubricant fluid. The fines are concentrated in a smaller volume of PEG (fines containing fraction). 
         [0072]    One part of the fines containing fraction drained from filtration unit  138  via line  140  may be fed back into line  134  via line  141  and combined with the solids depleted slurry received from centrifuge  132 . In order to obtain a thorough mixture of the one part of fines containing fraction drained from filtration unit  138  and the solids depleted slurry received from centrifuge  132  a holding tank (not shown) may be provided into which these fluids are fed. An outlet of the holding tank serves to feed the mixture to the filtration unit  138 . 
         [0073]    Another part of the fines containing fraction leaves the system via line  142  and is conventionally discarded. 
         [0074]    The clarified regenerated lubricant fluid exits the filtration unit  138  as permeate via line  144  and is split into two fractions. One fraction is transferred via line  146  and heated up to about 80° C. by means of the heat exchanger  148  as mentioned before and is mixed with the spent slurry in the static mixer  128  in order to dilute the spent slurry and to heat it up to about 60° C. before flowing into the centrifuge  132 . 
         [0075]    The other fraction of the regenerated lubricant fluid first flows via line  150  to the heat exchanger  124  where it is cooled down to about 30° C., heating up the spent slurry from ambient temperature to about 41° C. From heat exchanger  124  the cooled-down regenerated lubricant fluid is withdrawn via line  152  and fed into a second heat exchanger  154  where it is further cooled down to about 9° C. in order to obtain ambient temperature of the mixture prepared in the mixing tank  156 . Here an agitator  158  mixes the regenerated lubricant fluid with the sludge coming from the centrifuge  132  via line  136  and with some fresh PEG and with fresh abrasive particles received via line  160  in order to compensate the loss of materials due to the fines fraction discharged from the membrane filtration unit  138 . The resulting mixture obtained in tank  156  is a regenerated abrasive slurry ready for re-use and sent back via line  162  to the cutting device. It is well understood that fresh PEG and fresh abrasive particles alternatively may be fed into mixing tank  156  via separate lines (not shown).