METHOD FOR UTILISING A WASTE SLURRY FROM SILICON WAFER PRODUCTION

Abstract: This invention relates to method of utilising a waste slurry from silicon wafer production. More specific, this invention relates to a method for utilising the solid fraction of the slurry, which stems from the sawing or slicing of silicon wafers as a raw material in other industrial processes such as production of certain ceramics, such as silicon nitride bonded silicon carbide and/or silicon bonded silicon carbide.

Description: Method for utilising a waste slurry from silicon wafer production This invention relates to a method of utilising a waste slurry from silicon wafer production. More specific, this invention relates to a method for utilising the solid fraction of the slurry, which stems from the sawing or slicing of silicon wafers from silicon ingots as a raw material in industrial processes such as for instance production of certain ceramics. Background The world production of highly pure silicon is presently in the order of 20 000 metric tonnes per year (MT/year). This silicon is mainly consumed by the photo-voltaic industry for production of solar cells and by the semiconductor industry for production of integrated circuits. Integrated circuits have become an immensely important and very common resource in the last decades. Virtually every person in the developed countries have access to and/or own electronic equipment containing integrated circuits. There are no signs indicating that this importance will decrease in the foreseeable future. On the contrary, it is generally believed that electronic devices will be an even more widespread and integrated part of our lives. The solar cell panels are not yet common in daily life, but recent development of solar cell panels has seen panels with impressing conversion rates of solar energy to electric energy. Thus it is generally anticipated that solar cell panels will become a widely used source for electric energy. This view finds an ever increasing support from the growing focus on man-made climate change due to the developed countries dependency upon burning fossil fuels for producing our energy. Both integrated circuits and solar cell panels are thus believed to contribute to a strongly increased demand for highly pure silicon in the near future. Also, both the integrated circuits and solar cells are made from ingots of pure silicon which is sliced into thin discs or wafers. The thickness of these wafers may vary from a few hundred microns to a few millimetres. There are basically two main techniques in use today for slicing high purity silicon ingots into wafers; the wire saw or the diamond saw. However, the wire saw is gaining an increased importance and market share, and we shall therefore focus on this process. In the case of the wire saw, the ingots are sliced by forcing them through a web of rapidly running steel wires coated with a brass layer. During the slicing process, the wire is constantly rinsed with either glycol or mineral oil containing SiC as abrasive particles. This produces an oil or glycol based slurry containing SiC-particles and kerf remains (small particles of highly pure Si). Due to the small thickness of the wafers and solar cells, the kerf losses amounts up to 40wt% of the total consumption of highly pure silicon. The kerf remains are typically present in an amount ranging from 10-40wt% in the slurry. Thus the present annual slurry production amounts to 10-15000 MT/year, and is expected to increase strongly in the coming years. The waste slurry is presently deposited in landfills or burned in incinerators, and constitutes thus a large waste problem. The depositing of the slurry in landfills is an environmentally hazardous solution due to the risk of oil or glycol leaching into the soil. This has led to costly environmental safety demands for such landfills. The incineration solves the problem with soil contamination by oil or glycol, but the silicon kerf remains in the slurry are converted to contaminated (often with heavy elements) SiO 2 which also must be deposited together with the third constituent of the slurry, SiC. This solution of the waste slurry problem is unfortunate, both from an economical and environmental point of view. Especially when considering that these kerf remains and SiC-particles are leftovers from some of the commercially available purest raw materials. They thus constitutes valuable materials which are well suited to be applied as raw materials in a number of industrial applications. Object of the invention The first object of this invention is to eliminate the waste disposal by utilising the waste slurry as raw material for other industrial applications. The second object of this invention is to provide a method for reusing this solid fraction of the silicon sawing slurry sludge as a raw material in the production of ceramics. Brief description of the invention The objects of this invention can be achieved by what is described in the appended claims and/or in the following description. The objects of the invention can be obtained by exploiting the fact that the solid part of the slurry, being a mixture of very fine and highly pure Si- and SiC-particles, is suitable raw-material for producing certain nonoxide ceramics, including such ceramics as Si 3 N 4 -bonded SiC and Si-bonded SiC. These ceramic materials are known to be suitable for high temperature applications under conditions experienced in the metallurgical industries (aluminium, ferroalloy etc.), and also as kiln furniture materials (setter plates, beams etc.) in the ceramic industries (firing of domestic ware, sanitary ware, tiles, electrical insulators, etc.), and in many other industrial applications to resist wear and corrosion at ambient as well as high temperatures. The two main steps in the conventional process for manufacturing such ceramic materials are a powder compacting step, normally called forming, and a high temperature step, normally called sintering or firing. Forming implies shaping or moulding by applying pressure. The raw material particles are consolidated into a shape more or less similar to that of the final product. Powder pressing (uniaxial or isostatic), slip casting, extrusion, and injection moulding are examples of common techniques being applied. In the sintering step, the ceramic material comes forth as a result of chemo-physical processes (pure sintering) or a combination of chemo- physical processes and chemical reactions (reaction sintering). Infiltration with liquid metals is another possibility. A ceramic raw material must facilitate compacting and forming, and must give products of high integrity as far as material properties and consistency are concerned. It is foreseen that the Si-SiC powder mixture can be modified as a ceramic raw material to facilitate a specific forming technique or to obtain special material properties in the final ceramic product. This will include techniques such as spray drying to facilitate dry pressing, and other methods of granulation to facilitate certain semidry or wet forming techniques as such. Addition of more of one of the original ingredients, Si or SiC, to adjust the ratio for special applications is also foreseen. Additions of small amounts of chemical elements such as dopants, and of other ceramic raw materials in the form of particles, whiskers or fibres to produce ceramic composites, are foreseen as means to obtain special properties of the final products. In order to apply the waste slurry as a raw material for the production of ceramics, the sohd fraction consisting of Si- and SiC-particles must be separated from the liquid fraction, glycol or oil. This can in both cases be done by conventional processes such as settling and/or filtration. After separation, the hquid fraction, consisting of oil or glycol, can find new applications in the process industry or be recycled back to the photo-voltaic and/or semiconductor industry. However, there will normally be a liquid residue in the solid fraction after the initial separation which must be removed before the solid fraction can be used as a ceramic raw material. In the case of separation by settling or filtration, the residue is typically in the range of 5-10 wt% for both the glycol- and oil-based slurries. In the case of glycol-based slurries, the final separation of the sohd and liquid fraction can easily be performed by heating of the precipitate and/or filter cake, up to temperatures in the range of 200-400°C. The resulting solid fraction consists of a very pure mixture of Si- and SiC-particles which is well suited to be reaction sintered into a Si 3 N 4 -bonded SiC. The mixture of pure Si- and SiC-particles is first subject to a pressing and forming into the wanted shape, and thereafter a heating in a nitrogen atmosphere up to a temperature within the range of 1050-1450°C. At these temperatures the Si-particles will react according to the following reaction: 3Si(s) + 2N (g) → Si 3 N s) and the Si 3 N will bond the SiC-particles together in a sintering process, and thus forms the ceramic material. Typical compositions of the resulting precipitate and/or filter cake after the initial separation of the solid fraction of the glycol-based slurry is approximately 65-75 wt% SiC, 15-25 wt% Si and 5-10 wt% glycol. In the case of oil-based slurries, the precipitate/filter cake the figures are approximately 40-60 wt% SiC, 30-50 wt% Si and 5-15 wt% oil. As mentioned, this is an excellent ratio of Si- to SiC-particles for forming Si 3 N 4 -bonded SiC. The method according to this invention can be employed for any mixture of Si- and SiC-particles where the concentration of Si-particles ranges from 2-100 wt%, more preferably from 10-50 wt%, and most preferably from 15-30 wt%. It is preferred to employ mixtures with 15-30 wt% Si-particles since this gives the strongest and most dense ceramic after sintering. From the typical composition of the slurry after the initial removal of the liquid phase, one sees that the most typically resulting composition of the slurry after completion of the removal of the liquid phase normally lies within this range. This is one of the reasons why these slurries are well suited for production of these ceramic materials. In the case of oil-based slurries, one has that some fractions of the oil residue have too high vaporisation temperatures to be completely removed by heating the precipitate/filter cake. Since the introduction of oxygen into the ceramic material is undesirable, the heating should be performed in an inert atmosphere. This leads to a decomposition of some or all of the hydrocarbons in the oil to form elementary C and H 2 in a pyrolysis process. And as a result, the sohd fraction consisting of the Si- and SiC-particles becomes mixed with elementary C, typically in the range of 1-10 wt% of the mixture. However, elementary C is detrimental for many refractory and/or metallurgical applications, and the presence of elementary C should therefore be eliminated. Fortunately, this occurs all by itself during the sintering process, since elementary C will react with Si and form SiC before reaching the sintering temperature. All one has to ensure is that the initial powder mixture contains sufficient Si-particles such that there remains sufficiently amounts of Si to form the ceramic bonds after all C has been consumed. Thus, when producing Si 3 N -bonded SiC from oil-based slurries, a typical process includes a heat pre-treatment in a nitrogen atmosphere up to a temperature in the order of 700-800°C until the pyrolysis is completed, and then increase the temperature to the range of 1050-1450°C in order to convert all elementary C to SiC and thereafter converting the remaining Si to Si 3 N to form the ceramic. In addition to making Si 3 N 4 -bonded SiC, one can also make Si-bonded SiC (SiSiC) from the oil-based slurries. In this case one substitutes the nitrogen atmosphere with an inert atmosphere, typically an argon atmosphere. In this case one must first decompose the hydrocarbons in a pyrolysis step by heating the slurry to approximately 700-800°C and then increase the temperature to initiate the sintering process. The temperature ranges for the sintering process are essentially the same as in the case of forming Si 3 N -bonded SiC from oil-based slurries. The sintering temperature must be adjusted according to the Si-particle content in the mixture, and can be as high as above 2000°C. Such adjustments are known to a skilled person. As mentioned, it is preferred to apply slurries where the resulting Si- and SiC-particle mixture contains from 5-25% Si-particles. Further, there are also known several forming processes for these types ceramics, including isostatic pressing, dry pressing, slip-casting, sintering, extrusion etc. The main idea of this invention is to exploit a waste slurry to produce these types of ceramic materials. Thus these additives, production methods and their resulting ceramics should be considered as incorporated in this invention. In this way, the problem with disposal is completely solved by reusing substantially all of the liquid phase (glycol or oil) in an industrial application, and by fully reusing the solid components (Si- and SiC-particles) of the waste slurry to form a new set of ceramic products, nonoxide ceramics. Detailed description of the invention The invention will now be described in more detail by way of examples of preferred embodiments. These should not be construed as a limitation of the invention. Example 1. formation of a Si ? N -bonded SiC from a glycol-based waste slurry After the initial separation of the solid fraction of the slurry by settling, one achieved a precipitate with the following composition; 70 wt% SiC, 20 wt% Si and 10 wt% glycol. The precipitate was then uniaxially pressed at a force of 100 MPa to form test specimens with dimension lO x lO x 100 mm. The specimens were placed in a gas tight, water cooled, vertical mullite lined tube furnace. The final removal of the glycol was performed by evacuating the furnace and heating the specimens to 400°C for 24 hours. During the nitridation, the furnace was filled with pure nitrogen at a pressure monitored between 1025 and 1050 mbar. As the nitrogen was consumed, the pressure dropped until the lower set-point where nitrogen was supplied until reaching the upper set-point. The heating of the samples was performed according to the following time/temperature scheme: After completion of the removal of the glycol, the nitrogen was supplied and the temperature was raised with a ratio of 400°/hour until reaching 1100°C where it was kept for 72 hours. Then the temperature was further raised to 1150°C for 24 hours, 1200°C for 48 hours, 1300°C for 6 hours and finally 1350°C for 48 hours. After nitridation, the resulting test specimens were characterised as follows: - measurement of bulk density and apparent porosity according to ISO 5017, - measurement of cold flexural strength according to ASTM C651-91, and - XRD powder diffraction. The bulk density was found to be 2.53+0.00 g/cm 3 , apparent porosity 17.64+0.07%, and flexural strength 57.4+6.36 MPa. The XRD Powder diffraction analysis was performed on samples of the specimens that was milled to particle diameter < 45 μm. The analysis gave that the Si 3 N mainly consisted of α- Si 3 N 4 with minor amounts of β- Si 3 N 4 , and that the SiC consisted of mainly α-SiC. In addition there was found minor amounts of ferro- silicon (Fe 3 Si). Commercial silicon nitride bonded silicon carbide materials have typically a density of 2.6-2,7 g/cm 3 , an apparent porosity of 14.6-18%, and a flexural strength about 60 MPa. Thus, the material according to this invention has a somewhat lower density and the porosity is within normal values. The flexural strength is however below the commercial materials. The test specimens showed clear signs of delamination, which reduces the flexural strength considerably. The reasons for delamination is presently not clear, but is probably connected to the pressing of the specimens with the relatively high glycol content. The material can easily be made stronger by adjusting the pressing operation, and the slurry is thus well suited as a raw material for producing silicon nitride bonded silicon carbide. Example 2. formation of a Si-bonded SiC from an oil-based waste slurry After the initial separation of the solid fraction of the slurry by settling, one achieved a precipitate with the following composition; 50 wt% SiC, 40 wt% Si and 10 wt% oil. The precipitate was then uniaxially pressed to form cylindrical test specimens with dimension 20 x 20 mm under a force of 160 MPa. The specimens were placed in a gas tight, water cooled, vertical mulhte lined tube furnace in two batches. The first half of the specimens were heated in an argon atmosphere to about 700°C, while the other half were heated in argon atmosphere up to 1300°C. The first half of the specimens was heated at 150°C/hour up to 700°C, and was kept at that temperature for 2 hours before the furnace was shut down. The second half of the specimens was heated at 150°C/hour up to 1300°C, and was kept at that temperature for 5 hours before the temperature was lowered at a rate of 150°C back to room temperature. Also these test specimens were characterised by the following investigations: - measurement of bulk density and apparent porosity according to ISO 5017, - measurement of cold crushing strength according to ASTM C133-91, and - XRD powder diffraction. The resulting bulk density and apparent porosity for both batches are given in Table 1. The table shows that the apparent porosity decreased by 11%) during sintering. No liquid phases are expected at these temperatures with the possible exception of minor amounts of copper, zinc and ferrosilicon compounds stemming from wear of the steel wire of the wire saw. The cold crushing strength of the specimens in batch I, heating temperature 700°C, were , less than 3.5 MPa. The specimens in batch II, heating temperature 1300°C showed a cold crushing strength of 69.1+14.4 MPa. Samples of the specimens from both batches were milled to particle diameter < 45 μm and analysed in a Phillips PW 1710 XRD powder diffractometer. The analysis of the pyrolysed sludge (batch I) showed that it mainly consisted of α-SiC with some sihcon and ferro silicon. There were also traces of cristobalite. The sintered sludge (batch II) consisted mainly of α-SiC with some amounts of cristobalite and ferro sihcon, and possibly some traces of quartz. Table 1 Bulk density and apparent porosity of heat treated oil-based slurry precipitate

Claims: CLAIMS 1. A method for producing non-oxide ceramics, characterised in - that the method employs a waste slurry containing a mixture of Si- and SiC- particles and a liquid comprising oil or glycol, - that the waste slurry is subject to a separation process for removing all or most of the liquid fraction of the waste slurry, and - that the resulting powder mixture is subject to conventional processes to form the nonoxide ceramic material. 2. A method according to claim 1, characterised in that the main part of the liquid fraction of the waste slurry is separated from the solid fraction by settling and subsequent decanting. 3. A method according to claim 2, characterised in that when the method is applied for producing silicon nitride bonded silicon carbide from a glycol based waste slurry, the production process comprises - removing the glycol residue in the precipitate by gentle heating in vacuum, - forming the resulting mixture of Si- and SiC-particles into a shaped object, and - sintering the ceramic material by heating the shaped object in a nitrogen atmosphere at a temperature where the Si-particles react with N 2 to form the silicon nitride bonding the SiC-particles together. 4. A method according to claim 3, characterised in that - the gentle heating for vaporisation of the glycol residue is performed at a temperature in the order of 200-400°C, - the compaction is made by a force in the order of 100 MPa, and - the sintering is performed at a temperature in the range of 1050-1350°C at a nitrogen pressure in the order of 10 5 Pa. 5. A method according to claim 3 and 4, in the case when the waste slurry is oil based and contains a mixture of Si- and SiC-particles and mineral oil, characterised in that - the separation of the main part of the hquid phase comprises settling and subsequent decanting, and in that - the removal of the oil residue comprises heating the precipitate in an inert atmosphere until the hydrocarbons in the oil decomposes by pyrolysis to form elementary carbon, - the resulting powder mixture is formed into a shaped object, and - the ceramic material is formed by sintering the shaped object in a nitrogen atmosphere at a temperature where the elementary C react with Si to form SiC, and subsequently where the remaining Si-particles react with N 2 to form the silicon nitride which sinters the SiC-particles together. 6. A method according to claim 5, characterised in that - the pyrolysis temperature is in the range of 700-800°C, and which is kept for 2 hours, - the compaction is made by a force in the order of 100 MPa, and - the sintering is performed at a temperature in the order of 1050-1350°C and at a nitrogen pressure in the order of 10 5 Pa. 7. A method according to claim 2, characterised in that when the method is applied for producing silicon bonded silicon carbide, the production process comprises - removing the oil residue in the precipitate by pyrolysis to form elementary C, - forming the resulting mixture of Si- and SiC-particles and elementary C into a shaped object, and - sintering the ceramic material by heating the compacted object in an inert atmosphere at a temperature where the Si-particles first react with C to form SiC and then at a temperature where the remaining Si-particles bonds the SiC- particles together. 8. A method according to claim 7, characterised in that - the pyrolysis temperature is in the range of 700-800°C, and which is kept for 2 hours, - the compacting is made by a force in the order of 160 MPa, and - the sintering is performed at a temperature in the range of 1050-1350°C at an inert gas pressure in the order of 10 5 Pa. 9. A method according to any of claims 1 - 6, characterised in that the composition of the purified Si- and SiC-particle mixture should preferably contain from 2-100 wt% Si-particles, more preferably from 10- 50 wt% Si-particles, and most preferably from 15-30% Si-particles. 10. A method according to claim 9, characterised in that when the waste slurry is oil based, the resultmg composition of the Si- and SiC-particle mixture after the elementary C which formed during pyrolysis of the oil has been converted to SiC, should preferably contain from 2- 100 wt% Si-particles, more preferably from 10-50 wt% Si-particles, and most preferably from 15-30%) Si-particles. 11. A method according to claim 1, characterised in that the conventional production process comprises isostatic pressing, dry pressing, shp-casting, sintering and/or extrusion. 12. Use of a waste slurry containing a mixture of Si- and SiC-particles, and a liquid phase comprising glycol or mineral oil to produce a non-oxide ceramic material, Si 3 N -bonded SiC. 13. Use of waste slurry from the production of highly pure silicon wafers in the photo-voltaic and/or semiconductor industry to make a non-oxide ceramic material, Si 3 N 4 -bonded SiC according to the method as given in claims 1 - 6 and/or 9. 14. Use of waste slurry from the production of highly pure sihcon wafers in the photo-voltaic and/or semiconductor industry to make a non-oxide ceramic material, Si-bonded SiC according to the method as given in claims 7 - 8 and/

Inventors: HENRIKSEN KNUT, HENRIKSEN, KNUT
Applicants: HENRIKSEN KNUT, METALLKRAFT AS, HENRIKSEN, KNUT
Application year: 2001
Main classification: C02F1
IPC codes: C02F1, C02F11, C04B33, C04B35, C21B11