EP2094441A1

EP2094441A1 - Process and apparatus for treating exhausted abrasive slurries from the lapping process for the recovery of their reusable abrasive component

Info

Publication number: EP2094441A1
Application number: EP07866781A
Authority: EP
Inventors: Guido Fragiacomo
Original assignee: GARBO Srl
Current assignee: GARBO Srl
Priority date: 2006-12-22
Filing date: 2007-12-04
Publication date: 2009-09-02
Anticipated expiration: 2027-12-04
Also published as: EP2094441B1; WO2008078349A1; ITRM20060692A1; TW200841991A; ATE546256T1

Abstract

Process and relative apparatus for recovering the reusable abrasive component contained in an abrasive slurry for the lapping of silicon wafers, or of quartz or ceramic components, when it becomes exhausted and enriched with undesired waste matter. The process, which uses wet size-sorting treatment carried out in a battery of hydrocyclones connected in series and operating in counter-current, possibly followed by chemical etching treatment in order to eliminate the fine iron particulate and, preferably, by a further wet size-sorting treatment carried out in a second battery of hydrcyclones connected in series and operating in counter-current, enables the complete recovery of the reusable abrasive grains contained in the exhausted slurry from the lapping process. These abrasive grains are the only recoverable component of this type of slurry because the suspending liquid consists of water and the suspension-stabilising additive is present in such low quantities that its recovery is not economically feasible.

Description

PROCESS AND APPARATUS FOR TREATING EXHAUSTED ABRASIVE

SLURRIES FROM THE LAPPING PROCESS FOR THE RECOVERY OF

THEIR REUSABLE ABRASIVE COMPONENT

The present invention concerns a process and relative apparatus for treating exhausted abrasive slurries from the lapping process for the recovery of their reusable abrasive component. More specifically, the invention concerns a process, with the necessary equipment for implementing it, for recovering the reusable abrasive component contained in an abrasive slurry for lapping silicon, quartz or ceramic wafers when it is exhausted and enriched with undesired waste matter. The process enables the total recovery of the still reusable abrasive grains contained in the exhausted slurry coming from the lapping process that are the sole reusable component in this type of slurries, because the suspension liquid consists of water and the suspension additive is present in insufficient amount to make its recovery economically feasible.

As is known, in the production of components for applications in the electronic and photovoltaic energy thin silicon discs ("wafers") are used, which are obtained from either polycrystalline or monocrystalline silicon ingots by first cutting the ingot perpendicularly to its length. Typically, this slicing operation is carried out by means of wire saws or cutters in which a metal wire of considerable length and suitable mechanical resistance, wound in a system of rollers and spools, is contacted while moving with the ingot, perpendicularly to the ingot length, at the points where the cut is to be made. At the same time, a slurry containing abrasive grains or particles (abrasive slurry) is fed to the contact area between the cutter wire and the ingot.

The conventional abrasive slurries used for cutting ingots of silicon, quartz or other ceramic material with a wire saw consist of a suspending, lubricant or cooling fluid such as a mineral oil or water-soluble organic liquids of high molecular weight (in particular, polyethylene glycol, PEG) in which abrasive particles of suitable hardness, generally of silicon carbide (SiC)₁ are suspended. The silicon wafers then undergo a lapping process, which is necessary in order to guarantee the strict mechanical tolerances required by the microelectronics industry and to eliminate any damage to the silicon crystalline lattice introduced by the previous cutting operation as well as to prepare the surfaces for the subsequent polishing operation. The lapping operation is carried out with lapping machines essentially consisting of two large perfectly flat and round overlapping horizontal cast-iron plates that rotate in opposite directions to one another. The lower plate is uniformly covered with an abrasive slurry and the silicon wafers coming from the cutting operation are placed on it within special satellites that are thinner than the silicon wafers to be lapped. The satellites are made to rotate by the movement of the lower round plate by means of a cogged transmission system.

The wafers are then uniformly covered with the abrasive slurry and the upper plate is lowered onto them. The whole system is thus made to rotate and the abrasive slurry is continuously fed into the space between the two plates throughout the lapping process. The operation continues until the wafer thickness required by the microelectronics technology is reached.

The abrasive slurry used for the lapping process is composed of a mixture of de-ionised water (suspending liquid), the actual abrasive, usually alumina, AI₂O₃, with a target grain size between 4 and 20 μm, depending on the required characteristics, and a commercially available suspension additive which has the task of facilitating and stabilising the suspension of abrasive particles in water. This additive is present in the suspension in an amount ranging between 3 and 4% weight, and is normally a product of such low eco- nomic value that its recovery from exhausted abrasive slurry is not considered worthwhile. A typical composition of abrasive slurry for silicon wafer lapping is as follows:

- de-ionised water 76.5% by weight

- commercial suspension additive 3.5% by weight - abrasive: pure alumina (with typical grain size

PW9 or PW15) 20.0% by weight or mixed with other oxides 200-300 g/l - suspension viscosity 450 centipoise

During the lapping process, part of the abrasive grains lose their useful characteristics for the optimal functioning of the process itself because the alumina particles break down into smaller particles and are thus no longer suitable for the lapping operation: this is shown by the fact that the grain size distribution of the abrasive particles decreases to lower average values. At the same time, the abrasive slurry becomes enriched with fine particulate coming from the silicon wafers when their thickness is reduced and from the cast iron plates (largely iron particulate) of the lapping machine that are abraded along with the silicon, as well as from the metal pipes of the machine itself. The percentage of fine particles (particles of a size < 4 μm of abrasive, iron and silicon) in an exhausted slurry is about 30% of the solid fraction.

At the same time, agglomerates of abrasive grains are created and accumulated during the lapping operation, especially owing to the deposition of fine particulate on the abrasive grains themselves, that are undesirable because they can potentially scratch and damage the surfaces undergoing the lapping process.

Therefore, the abrasive slurry coming out of the lower plate of the lapping machine no longer has the mechanical and chemical properties of fresh abrasive slurry: should it be reused as it is for another lapping cycle, it would negatively affect lapping process efficiency both in terms of outflow speed and the mechanical parameters of the silicon wafers obtained as well as in terms of metallic contamination.

The exhausted abrasive slurry that is discarded may be disposed of via municipal and industrial waste treatment plants. However, the resulting sludge must then go to a landfill.

In any case, apart from the environmental problem of having to dispose of the waste products, there is the considerable economic inconvenience of losing a considerable amount of still reusable abrasive grains contained in the exhausted slurry. These abrasive grains can be reused because they are still of the right size for further use in an abrasive slurry.

Since the need to separate and recover the reusable abrasive com- - A -

ponent for use in the lapping process is an ever more widely felt problem in the field, several specific techniques have been proposed in order to recover the exhausted abrasive slurries of the lapping process, and they all varyingly combine the following basic operations: a) separating the abrasive grains by wet size-sorting, in particular by centrifugation or by passing the slurry through a hydrocyclone (a static separator of solid particles entrained by a liquid, which exploits the action of centrifugal force); b) breaking down the undesired agglomerates of abrasive grains by means of tumbler screening.

The hitherto known technologies, as will be made clearer below with reference to some examples of the prior art, present various inconveniences, such as an insufficient quality of reusable abrasive grains in terms of size or in terms of the presence of fine silicon and/or iron particulate.

The Japanese patent application JP 2003266309 (Komatsu Denshi Kinzoku KK) describes a system for automating the operations for recovering, recycling and feeding the abrasive slurry to the lapping machines, and for improving the lapping yield by breaking down the abrasive grain agglomerates and eliminating the foreign contaminating particles. Decomposition of the abrasive grain agglomerates is carried out by means of a battery of tumbler screeners fed with the exhausted slurry coming from the lapping process; the abrasive grain agglomerates are thus broken down and the grains are then separated with the help of a recycling liquid. This liquid enables obtaining a suspension containing abrasive grains of suitable specific weight to be reused, which grains are recycled in the lapping machines.

The recycling liquid itself enables obtaining a second suspension containing the non-reusable abrasive grains (that is, grains of low specific weight) and most of the contaminants, like iron and silicon, and this suspension can then be sent to a landfill for disposal.

The sorting system described allows eliminating the abrasive agglomerates but does not yield a high quality of recovered abrasive grains, because the recovered abrasive fraction still contains a certain amount of fine contaminating abrasive particles and fine powders (iron and silicon) which build up in the exhausted abrasive slurry as residues of the lapping process. Another procedure for recovering the exhausted slurries of the lapping process is described in patent application JP 2002 239906 (Ishikawajima Hanyoki Service KK), which concerns a method and relative apparatus for reusing an abrasive slurry for the lapping process that can provide precise size-sorting of the abrasive grains and eliminate the abrasive grain agglomerates which can cause scratching and damage during the lapping operation.

The described process is based on the separation of abrasive grains by centrifugation: the exhausted slurry coming from the lapping machines is fed into a first centrifuge which enables separating the lapping process liquid from the abrasive grains and from the impurities. The abrasive grains of the first centrifuge are mixed with a liquid for cleaning and separating the fine particles and are then fed into two consecutive centrifuges which separate the reusable abrasive grains from the cleaning liquid, which will thus contain the fine abrasive grains and impurities. The abrasive grains obtained from the two consecutive centrifuges are mixed with the lapping liquid obtained from the first centrifuge and the recovery suspension thus obtained is fed to the lapping machines.

In the aforesaid process, the separation of the abrasive component and of the impurities from the lapping process liquid is carried out by a single centrifugation operation (first centrifuge). Hence, the lapping liquid, and thus the recovered suspension, is expected to contain part of the abrasive fines and also a part of the impurities.

On the basis of this prior art, an object of the present invention is thus to provide a method for treating exhausted abrasive slurries of the kind used in the lapping of silicon wafers by means of lapping machines, and enabling the virtually complete recovery of the abrasive grains that are still of a suitable size to be reused in the lapping process, with an economically feasible process that is easy to run. The method must also be able to guarantee the complete elimination, from the recovered and reusable component, of the waste matter coming from the lapping process (such as silicon and iron from the cast-iron plates of the lapping machines) and the broken-down grains of abrasive material that is no longer active. Starting from the specific need of recovering the abrasive slurry used in the lapping of silicon wafers, which typically have the following characteristics:

^■ abrasive: pure alumina or in a mixture with other oxides, with grain size between 4 and 20 μm, suspension liquid: deionised water, additive: of a commercially available type with a high suspension capacity, the present invention aims at applying a technology similar to the one already used for treating exhausted abrasive slurries coming from the cutting of ingots of silicon, quartz or ceramics by means of wire saw cutters, which is the object of the patent application RM2005 A000329 by the same inventor. This technology, which for the specific case of recovering all the useful components coming from the cutting of silicon wafers is certainly more complex, includes a section that can be adapted and advantageously used for recovering the abrasive component of the lapping process, wherein the exhausted abrasive slurry is subjected to separation by removing the reusable abrasive grains from the rest of the suspension by means of wet size-sorting in a hydrocyc- lone, wherein the said operation is optimally carried out not inside a single hydrocyclone but in a battery of hydrocyclones suitably connected in series and fed in counter-current.

As is known, in a hydrocyclone the feed to be treated is fed from the top at high speed and tangentially into the apparatus, so that the centrifugal force pushes the heavier particles towards the sides of the container. Moving in a spiral manner, the heavier particles are then collected in the container's conical bottom (underflow), while the clarified liquid comes out at the top from a central duct (overflow). In the hydrocyclone battery proposed, the abrasive- grain enriched suspension obtained as an underflow (UF) from the bottom of the hydrocyclone goes to feed the next hydrocyclone in the series, and the bottom of the latter hydrocyclone yields a suspension of abrasive grains of suitable size for their reuse.

According to some preferred solutions of the procedure envisaged in the present invention, other technological characteristics of the process described in patent application RM2005 A000329 are also used. In particular, in the acid leaching treatment carried out on the grains obtained as underflow from the hydrocyclone battery, which is intended to achieve a chemical etch- ing of the fine iron particles still contained in the abrasive grain stream recovered from the passage in the hydrocyclone battery. Also, in some cases, in the operation of filtering the suspension of recovered abrasive grains after the chemical etching.

Therefore, the present invention specifically provides a process for treating exhausted abrasive slurries from the lapping process, comprising a suspending liquid essentially composed of water and an additive for stabilising the suspension, as well as reusable abrasive grains, fine abrasive grains, fine silicon particles and fine metallic particles, which process comprises the following steps: a) separating an exhausted slurry, by wet size-sorting treatment, into: i) a liquid suspension containing reusable abrasive grains and H) a liquid suspension containing the fine abrasive grains as well as the fine silicon particles and fine metal particles; b) eliminating the said liquid suspension containing the fine abrasive grains and the fine silicon particles and fine metal particles, and collecting the said liquid suspension containing the reusable abrasive grains in order to feed it back into the lapping process; the process being characterised by the fact that the said wet size-sorting treatment of operation a) is carried out by treating the mixture in a first battery of at least two hydrocyclones connected in series, fed in counter-current with water, thereby obtaining, from the bottom of the last hydrocyclone of the battery, a liquid suspension containing the reusable abrasive grains and a negligible quantity of fine abrasive grains. Preferably, the said first battery consists of between two and six hydrocyclones, but preferably four. The aforesaid described process represents the simplest version of the procedure according to the present invention, to be used when the final concentration of fine iron particulate in the recovered abrasive component is not critical. According to a first preferred variant of the process proposed according to the present invention, the liquid suspension containing the reusable abrasive grains^" obtained from the bottom of the last hydrocyclone of the said first battery undergoes further treatment in order to obtain abrasive grains purified of traces of fine iron particles by chemical etching, by adding a solution of acid agent to the liquid suspension containing the reusable abrasive grains. The acid agent may be selected from the group consisting of nitric acid, hydrochloric acid, oxalic acid, sulphuric acid, citric acid and tartaric acid or their mixtures, and is used in excess so that the outflow liquid obtained from the treatment, which is discarded, contains the iron salts deriving from the acid leaching operation, along with the excess acid and any fine iron residues.

According to the preferred version of the process, after adding the said acid agent solution the liquid suspension containing the reusable abrasive grains undergoes further wet size-sorting treatment, also carried out by treating the mixture in a battery of at least two hydrocyclones connected in series and fed in counter-current with water. A liquid suspension containing the reusable abrasive grains and essentially free of fine unusable abrasive grains is obtained from the bottom of the last hydrocyclone of the second battery. The top of the first hydrocyclone of the said second battery yields a liquid suspension of fine iron particulates, excess acid and iron salts, which is discarded.

Also with the second hydrocyclone battery, the number of hydrocyclones connected in series is normally between two and six, and is preferably four. According to a different variant of the process, equivalent to the first one in terms of results, after adding the acid agent solution the liquid suspension containing the reusable abrasive grains is subjected to a filtering operation by adding water, thereby obtaining - in the filtration cake - a solid phase containing the reusable abrasive grains and essentially devoid of fine abrasive grains. The filtrate of the said filtration operation yields a liquid suspension of excess acid and iron salts, and is discarded.

The abrasive grains resulting from the process proposed according to the present invention are suitable to be reused in the lapping process, and to this end they are preventively placed in a suspension maturation tank with a shaker mechanism, after adding the necessary amount of water and fresh stabilising additive. The recovered abrasive suspension thus obtained may be mixed, before use, with a suitable quantity of fresh abrasive slurry and the whole lot is fed to the lapping process.

According to a further aspect thereof, the present invention concerns an apparatus for treating exhausted abrasive slurries coming from the lapping process according to the aforesaid described procedure, consisting of the following interconnected elements in sequence:

I. a first battery of at least two hydrocyclones connected in series, fed in counter-current with water, wherein the first hydrocyclone is fed with the exhausted abrasive slurries and the underflow of each hydrocyclone feeds the next hydrocyclone, along with the recycling of the overflow of the next but one hydrocyclone;

II. a tank equipped with a shaker mechanism for the preparation of lapping slurry to be reused in the lapping process;

III. a tank equipped with a shaker mechanism for the maturation of the recovered suspension. Preferably, upstream of the said tank with shaker mechanism for the preparation of the lapping suspension liquid to be reused in the lapping process, there is also a tank with a shaker mechanism for the chemical etching of fine iron particles, and downstream of the said tank for the chemical etching of fine iron particles there is a second battery of at least two hydrocyclones con- nected in series, fed in counter-current with water, wherein the first hydrocyclone is fed with the exhausted abrasive slurries and the underflow of each hydrocyclone feeds the next hydrocyclone along with the recycling of the overflow of the next but one hydrocyclone. The suspension from the bottom of the last hydrocyclone feeds the said tank with shaker mechanism for the prepara- tion of the lapping suspension to be reused.

According to the executive variant of the apparatus arrangement which adopts a filtering operation in place of the second battery of hydrocyc- lones, downstream of the said tank for the chemical etching of the fine iron particles there is a filtering apparatus from which the outflowing clarified liquid, consisting of a liquid suspension of excess acid and iron salts, is discarded. The filtration cake obtained, containing the reusable abrasive grains, feeds the said tank with the shaker mechanism for the preparation of the lapping suspension to be reused.

The specific features of the present invention, as well as its advantages and relative operational modalities, will be more evident with reference to the detailed description below, presented merely for exemplification pur- poses and concerning some of its preferred embodiments. The same are illustrated in the attached drawings, wherein:

Figure 1 shows an overall block diagram of a process for treating exhausted abrasive slurries according to a first embodiment of the present invention; Figure 2 shows an overall block diagram of a process for treating exhausted abrasive slurries according to a second embodiment of the present invention;

Figure 3 shows an overall block diagram of a process for treating exhausted abrasive slurries according to a third embodiment of the present invention;

Figure 4 shows a simplified apparatus arrangement of the first battery of hydrocyclones connected in series that represents the main section of the arrangement; and

Figure 5 shows a simplified apparatus arrangement of the second battery of hydrocyclones connected in series that represents a preferred added section of the arrangement.

As shown in the block diagram of Figure 1 , which represents the main arrangement of reference, the process according to the preferred variant of the present invention is essentially composed of three sections: 1^st hydrocyc- lone battery - chemical etching - 2^nd hydrocyclone battery.

The illustrated arrangement is used when the final concentration of iron in the recovered abrasive component is critical, and is essentially com- posed of the following operations: a) collecting the exhausted abrasive slurry from the lapping machines inside a container called Tank 1 , which is necessary because the process may be carried out either in continuous or batch mode, depending on the desired process arrangement; in Figure 1 the process is continuous; b) separating the exhausted slurry, carried out by a wet size-sorting treatment in a first battery of hydrocyclones fed in counter-current, into two components: - an underflow (U. F.) suspension containing the reusable abrasive grains (> 4 μm) in the suspending liquid; the fine iron particles being still present up to a level of 1000-1500 ppm;

- an overflow (O. F.) suspension containing the non-reusable abrasive grains (< 4 μm), the fine particles of silicon and iron as well as the additive contained in the exhausted slurry; c) collecting the U. F. suspension coming from the battery of hydrocyclones of operation b) into a container called Tank 2, which serves as a storage tank for the suspension containing the reusable abrasive component (> 4 μm); d) adding an acid agent solution (oxalic, sulphuric, hydrochloric or other acid) in Tank 2 of operation c) in order to further reduce the concentration of fine iron particles<, e) separating the reusable abrasive slurry contained in Tank 2, by wet size-sorting treatment carried out in a second hydrocyclone battery, into two components:

■ an underflow (U. F.) suspension containing the reusable abrasive grains (> 4 μm) in the suspending liquid, wherein the fine iron particles is reduced to 100-500 ppm;

- an overflow (O. F.) suspension of fine iron particles, containing the ex- cess acid and iron salts, that is discarded; f) collecting the abrasive slurry containing the reusable abrasive grains (>4 μm) in a container for preparing the regenerated suspension and then adding water and the additive; g) conveying the regenerated abrasive slurry to a container called Tank 3, wherein the suspension is kept under shaking and can stabilise; h) mixing the suspension contained in Tank 3 with the suspension pre- pared with fresh abrasive material and then conveying the resulting abrasive slurry to the lapping machines for use. Alternatively, the same qualitative features of the final product can be obtained with a process carried out according to the arrangement illustrated in Figure 2 and essentially consisting of three sections: 1^st hydrocyclone battery - chemical etching - filtering.

The arrangement illustrated in Figure 2 is used instead of the one illustrated in Figure 1 when the final concentration of iron in the reusable abrasive component is critical, and essentially involves the following operations: a) collecting the exhausted abrasive slurry from the lapping machines in- side a container called Tank 1 , which is necessary because the process may be carried out either in continuous or batch mode, depending on the desired process arrangement; in Figure 2 the process is in batch; b) separating the exhausted slurry, carried out by wet size-sorting treatment in a first battery of hydrocyclones fed in counter-current, into two components:

■ an underflow (U. F.) suspension containing the reusable abrasive grains (> 4 μm) in the suspending liquid; the fine iron particles are still present up to a level of 1000-1500 ppm;

■ an overflow (O. F.) suspension containing the non-reusable abrasive grains (< 4 μm), the fine particles of silicon and iron as well as the additive contained in the exhausted slurry; c) collecting the U. F. suspension coming from the battery of hydrocyclones of operation b) into a container called Tank 2, which serves as a storage tank for the suspension containing the reusable abrasive com- ponent (> 4 μm); d) adding an acid agent solution (oxalic, sulphuric, hydrochloric or other acid) in Tank 2 of operation c) in order to further reduce the concentra- tion of fine iron particles; e) separating the reusable abrasive slurry contained in Tank 2, by wet size-sorting treatment carried out in a second hydrocyclone battery, into two components: ■ a liquid phase, consisting of water, the excess acid added and the iron salts of the said acid, that is discarded;

■ a solid phase consisting of alumina grains of size > 4 μm and residual fine iron particles which are within the limits of 100-500 ppm; f) mixing the solid obtained by filtering in operation e) with water and an additive in order to obtain the suspension of reusable alumina particles

(> 4 μm) in water; g) conveying the regenerated abrasive slurry to a container called Tank 3, wherein the suspension is kept under shaking and can stabilise; h) mixing the suspension contained in Tank 3 with the suspension pre- pared with fresh abrasive material and then conveying the resulting abrasive slurry to the lapping machines for use. The process carried out according to the arrangement illustrated in Figure 3 is a simplification of the two aforesaid arrangements and consists of only the first hydrocyclone battery; this process is used when the initial con- centration of iron in the abrasive component is not critical. In this case, the process consists of the following operations: a) collecting the exhausted abrasive slurry from the lapping machines inside a container called Tank 1 , which is necessary because the process may be carried out both in continuous mode or in batch, depending on the desired process arrangement; in Figure 3 the process is of the continuous type; b) separating the exhausted slurry, carried out by wet size-sorting treatment in a first battery of hydrocyclones fed in counter-current, into two components: ■ an underflow (U. F.) suspension containing the reusable abrasive grains

(> 4 μm) in the suspending liquid; the fine iron particles are still present up to a level of 1000-1500 ppm; ■ an overflow (O. F.) suspension containing the non-reusable abrasive grains (< 4 μm), the fine particles of silicon and iron as well as the additive contained in the exhausted slurry. c) collecting the U. F. suspension coming from the battery of hydrocyc- lones of operation b) into a container called Tank 2, which serves as a storage tank for the suspension containing the reusable abrasive component (> 4 μm); d) conveying the regenerated abrasive slurry thus obtained to a container called Tank 3, wherein the suspension is kept under shaking and can stabilise; e) mixing the suspension contained in Tank 3 with the suspension prepared with fresh abrasive material and then conveying the resulting abrasive slurry to the lapping machines for use.

The hydrocyclone battery arrangement illustrated in Figure 4 can be used in order to separate the reusable abrasive grains (size > 4 μm) from the used and no-longer reusable abrasive grains (size < 4 μm) and from the fine particles of iron and silicon. The first hydrocyclone of this battery is fed with the exhausted abrasive slurry mixed with the overflow of the second hydrocyclone. The first hydrocyclone generates an underflow suspension, which goes to feed the second hydrocyclone, and an overflow suspension containing the abrasive grains of size < 4 μm, the fine particles of iron and silicon and the additive present in the exhausted slurry, which is discarded and sent to the waste treatment plant.

The second hydrocyclone is fed with the underflow suspension of the first hydrocyclone mixed with the overflow suspension of the third hydrocyclone; the overflow suspension generated by the second hydrocyclone goes to feed the first hydrocyclone while the underflow suspension feeds the third hydrocyclone.

The third hydrocyclone is fed with the underflow suspension of the second hydrocyclone mixed with the overflow suspension of the fourth hydrocyclone; the overflow suspension generated by the third hydrocyclone goes to feed the second hydrocyclone while the underflow suspension feeds the fourth hydrocyclone.

The fourth hydrocyclone is fed with the underflow suspension of the third hydrocyclone preventively mixed with water; the overflow suspension generated by the fourth hydrocyclone goes to feed the third hydrocyclone while the underflow suspension containing the reusable abrasive grains of size > 4 μm, with an iron content within the limits of 1000-1500 ppm, is conveyed to collection Tank 2.

The second battery of hydrocyclones, schematically shown in Figure 5, is used in order to further reduce the iron content in the reusable abrasive slurry by combining the separating centrifugal action of the hydrocyclones with the chemical etching activity of an acid on the iron-based particles.

The first hydrocyclone of the second battery is fed with the reusable abrasive suspension coming from Tank 2 mixed with the overflow of the second hydrocyclone. The first hydrocyclone generates an underflow suspension, which goes to feed the second hydrocyclone, and an overflow suspension containing the eliminated fine iron particles and the excess acid, that is discarded.

The second hydrocyclone of the second battery is fed with the underflow suspension of the first hydrocyclone mixed with the overflow suspension of the third hydrocyclone; the overflow suspension generated by the second hydrocyclone goes to feed the first hydrocyclone while the underflow suspension feeds the third hydrocyclone.

The third hydrocyclone is fed with the underflow suspension of the second hydrocyclone mixed with the overflow suspension of the fourth hydro- cyclone; the overflow suspension generated by the third hydrocyclone goes to feed the second hydrocyclone while the underflow suspension feeds the fourth hydrocyclone. The fourth hydrocyclone is fed with the underflow suspension of the third hydrocyclone preventively mixed with water; the overflow suspension generated by the fourth hydrocyclone goes to feed the third hydrocyclone while the underflow suspension containing the reusable abrasive grains of size > 4 μm, with an iron content within the limits of 100-500 ppm, is conveyed to collection Tank 2, where the regenerated suspension is prepared by adding water and an additive.

From the foregoing, it may be seen that the process according to the present invention aims to achieve the result of advantageously applying to the field of recovering abrasive grains from the lapping process a technology similar to the one described in the co-pending patent application RM2005

A000329. The main differences between the two processes are as follows: a) the present invention is applied to exhausted slurries coming from a different process (lapping instead of cutting), which uses a different abrasive component (alumina or a mixture of oxides instead of silicon carbide), a different liquid support (water instead of polyethylene glycol or oil) and different additives; b) the process phases and relative arrangements have the following main differences: a. the underflow of the hydrocyclone battery of the previous process feeds a particular filtration system which, within the same apparatus, also carries out caustic etching, washing, chemical etching, washing and drying of the filtrate, while in the present process the underflow of the first hydrocyclone battery feeds one of the following process flows, depending on the level of iron contamination acceptable in the recovered abrasive component:

I. chemical etching followed by treatment in a second hydrocyclone battery (Fig. 1).

II. chemical etching followed by a filtration operation (Fig. 2).

III. or even the direct preparation of the suspension with the re- covered abrasive component without any chemical etching operation (Fig. 3). b. the overflow of the hydrocyclone battery of the previous patent application is subjected to filtration, microfiltration, distillation and final filtration in order to recover the suspension fluid (polyethylene glycol or oil) and to completely eliminate the fine particles, while in the present invention the overflow of the first and second battery of hydrocyc- lones is directly discarded (the suspending fluid is water and is thus not recovered); c. the process described in the present invention can be run either in continuous or in batch mode, depending on the flow arrangement used, while the process in the previous patent application was only in continuous mode.

The present invention has been disclosed with particular reference to some specific embodiments thereof, but it should be understood that modifications and changes may be made by the persons skilled in the art without departing from the scope of the invention as defined in the appended claims

Claims

1. A process for treating exhausted abrasive slurries from the lapping process, comprising a suspending liquid essentially composed of water and an additive for stabilising the suspension, as well as reusable abrasive grains, fine abrasive grains, fine silicon particles and fine metallic particles, which process comprises the following steps: a) separating an exhausted slurry, by wet size-sorting treatment, into: i) a liquid suspension containing reusable abrasive grains and H) a liquid suspension containing the fine abrasive grains as well as the fine silicon particles and fine metal particles; b) eliminating the said liquid suspension containing the fine abrasive grains and the fine silicon particles and fine metal particles, and collecting the said liquid suspension containing the reusable abrasive grains in order to feed it back into the lapping process; the process being characterised by the fact that the said wet size-sorting treatment of operation a) is carried out by treating the mixture in a first battery of at least two hydrocyclones connected in series, fed in counter-current with water, thereby obtaining, from the bottom of the last hydrocyclone of the battery, a liquid suspension containing the reusable abrasive grains and a negligible quantity of fine abrasive grains.

2. A process according to claim 1, wherein the said first battery of hydrocyclones is composed of between 2 and 6 hydrocyclones connected in series.

3. A process according to claim 2, wherein the said first battery of hydrocyclones is composed of four hydrocyclones connected in series.

4. A process according to claim 1 , wherein the said liquid suspension containing the reusable abrasive grains obtained from the bottom of the last hydrocyclone of the said first battery is further treated in order to obtain abra- sive grains devoid of traces of fine iron particles, by chemical etching by adding an acid agent solution to the said liquid suspension containing the reusable abrasive grains.

5. A process according to claim 4, wherein the said acid agent is selected from the group consisting of nitric acid, hydrochloric acid, oxalic acid, sulphuric acid, citric acid and tartaric acid or their mixtures.

6. A process according to claims 4 or 5, wherein, after adding the said acid agent solution, the said liquid suspension containing the reusable abrasive grains is subjected to further wet size-sorting treatment through a second battery of at least two hydrocyclones connected in series, fed in counter- current with water, and obtaining, from the bottom of the last hydrocyclone of the second battery, a liquid suspension containing the reusable abrasive grains and essentially devoid of non-reusable fine abrasive grains.

7. A process according to claim 6, wherein from the top of the first hydrocyclone of the said second battery there is obtained a liquid suspension of fine iron particles, excess acid and iron salts, that is discarded.

8. A process according to claims 6 or 7, wherein the said second battery of hydrocyclones is composed of between two and six hydrocyclones connected in series.

9. A process according to claim 8, wherein the said second battery of hydrocyclones is composed of four hydrocyclones connected in series.

10. A process according to claims 4 or 5, wherein, after adding the said acid agent solution, the said liquid suspension containing the reusable abrasive grains undergoes a filtration operation by adding water thereby yielding, in the filtration cake, a solid phase containing the reusable abrasive grains and essentially devoid of non-reusable fine abrasive grains.

11. A process according to claim 10, wherein the filtrate of the said filtration operation yields a liquid suspension of excess acid and iron salts that is then discarded.

12. A process according to any one of the aforesaid claims, wherein the said reusable abrasive grains are fed back into the lapping process after remaining for a time in a shaking tank for suspension maturation.

13. A process according to claim 12, wherein the said abrasive grains are fed back into the lapping process after adding a necessary quantity of supplementary abrasive slurry prepared with fresh abrasive material, water and fresh stabilising additive.

14. An apparatus for treating exhausted abrasive slurries from the lapping process according to the process as defined in claim 1 , including the following interconnected elements in sequence: I. a first battery of at least two hydrocyclones connected in series, fed in counter-current with water, wherein the first hydrocyclone is fed with the said exhausted abrasive slurries and the underflow of each hydrocyclone feeds the next hydrocyclone, along with the recycling of the overflow of the next but one hydrocyclone; II. a tank equipped with a shaker mechanism for the preparation of lapping slurry to be reused in the lapping process.

III. a tank equipped with a shaker mechanism for the maturation of the recovered suspension.

15. An apparatus according to claim 14, wherein upstream of the said tank with shaker mechanism for the preparation of lapping slurry to be reused in the lapping process there is a tank with a shaker mechanism for the chemical etching of the said fine iron particles.

16. An apparatus according to claim 15, wherein downstream of the said tank for the chemical etching of the fine iron particles there is a second battery of at least two hydroyclones connected in series, fed in counter-current with water, and wherein the first hydrocyclone is fed with the said exhausted abrasive slurries and the underflow of each hydrocyclone goes to feed the next hydrocyclone, along with the overflow of the next but one hydrocyclone, and the underflow of the last hydrocyclone goes to feed the said tank with shaker mechanism for the preparation of the reusable lapping slurry.

17. An apparatus according to claim 16, wherein downstream of the said tank for the chemical etching of the fine iron particles there is a filtration device from which the outflowing clarified liquid, consisting of a liquid suspension of excess acid and iron salts, is discarded and the obtained filtration cake, containing the reusable abrasive grains, feeds the said tank with a shaker mechanism for the preparation of the reusable lapping slurry.