PROCESS

Abstract: A process is described for the production of silicon carbide particles of a size suitable for manufacture of components by powder compaction which comprises forming granules of an intimate mixture comprising finely divided silica and carbon particles, said carbon particles being substantially spherical and substantially all having a diameter lying in the range of from about 0.1 mu m up to about 10.0 mu m, heating said granules to a temperature in the range of from about 1950 DEG K to about 2350 DEG K, and recovering the resultant particulate product.

Description: PROCESS This invention relates to the production of silicon carbide powder for use in powder compaction fabrication processes. Ceramic powders are finding increasing usage in the manufacture of engineering components as a replacement for metal. For example, it has been reported that an internal combustion engine has been produced in which such items as the cylinder block and pistons have been produced from ceramic materials, thereby obviating the need for lubrication. To produce an intricately shaped component it is necessary, according to conventional techniques, to compact a ceramic powder of defined morphology and purity. An appreciable proportion of components produced in this way have to be rejected because of the difficulties inherent in their manufacture. In some cases failure of a component to attain the desired quality control standard can be ascribed to use of powders which do not meet the required morphology specificatio . A typical specification for a silicon carbide powder for this purpose requires that it should be in the form of β-silicon carbide, that it should have a low level, e.g. less than about 2% by weight, of by-product impurities, such as silicon dioxide, carbon and silicon, and that it should have a small range of size distribution. Usually it is specified that 90% or more of the particles should lie in the range 0.1 to lOμm, whilst it is preferred that over 95% of particles should lie in the range 0.1 to 5pm. Particles of regular shape, e.g. spherical particles, are preferred. Silicon carbide can be produced in lump form by reactions of quartz and coke in an electric furnace. The resulting lumps of silicon carbide can then be milled and the appropriate fraction of the required particle size can be selected for use as the powder for production of ceramic components. However, the resulting powder is relatively impure and its particles are of a very irregular shape. Moreover, as the milling operation gives rise to a wide particle size range, an appreciable proportion of the powder has to be rejected as being of unsuitable size for compaction. In addition milling is a process requiring high cost equipment and consumes much energy. An alternative method of producing silicon carbide powder involves production of metallurgical grade silicon as a first step. This is then reacted, in a second step, with chlorine to produce silicon tetrachloride, which is'then purified in a third step. Next the purified silicon tetrachloride is reacted with a gaseous source of carbon, e.g. methane, to produce silicon carbide. In a fifth and final step the chlorine released is recovered and recycled for production of further silicon tetrachloride. Because of the difficulties inherent in production of silicon carbide powder of suitable particle size and morphology, it is a relatively expensive raw material. This factor, plus the relatively high rejection rate of compacted components, currently makes the cost of producing ceramic engineering components very high. There is a need to provide a simpler process for producing silicon carbide powders suitable for use in powder compaction techniques. The present invention accordingly seeks to provide a process whereby silicon carbide powder of appropriate morphology and purity for powder compaction can be produced in a simple manner and in relatively high yield without the need for production of silicon or silicon tetrachloride and without the need to include a milling step. It further seeks to provide a process whereby silicon dioxide can be converted directly by reaction with carbon to silicon carbide in particulate form, a high proportion of which is suitable for production of ceramic components. According to the present invention there is provided a process for the production of silicon carbide particles of a size suitable for manufacture of components by powder compaction which comprises forming granules of an intimate mixture comprising finely divided silica and carbon particles, said carbon particles being substantially spherical and substantially all having a diameter lying in the range of from about 0.1%pm up to about lO.Oum, heating said granules by means of a plasma arc to a temperature in the range of from about 1950°K to about 2350°K, and recovering the resultant particulate product. One method of heating said granules is by means of a plasma arc. In the process of the invention there is preferably used a particulate source of carbon in which the carbon particles substantially all have a diameter lying within a narrow range, for example in the range of from about 0.25pm up to about 10 urn, preferably in the range of from about 0.25pm up to about 5.0pm. Preferably the finely divided silica comprises particles having a grain size in the range of from about lOum up to about 250um, typically lOOum to about 200um. For example, the finely divided silica may comprise particles having a grain size in the range of from about llOum to about 150um. To remove any traces of unreacted silicon dioxide it is preferred to wash the resultant particulate product with an aqueous solution containing hydrofluoric acid. This aqueous solution may further contain nitric acid. If desired, a binding agent may be included in the mixture used to form the granules, such as water, silica gel or agents derived from hydrocarbons. Such hydrocarbon- derived binding agents may be in liquid or wax form. In a particularly preferred process the granules x> have a particle size in the range of from about 0.5 mm to about 2.5 mm. Heating can be effected in a d.c. transferred plasma arc furnace. Upon heating by means of the plasma arc the following reactions take place: C + Si0 2 > SiO + CO (A) SiO + 2C > SiC + CO (B) Reaction (A) begins to take place at about 1675°K whilst reaction (B) commences to become significant at about 2100°K. The use of thermal plasma technology for heating gas and particulate materials to temperatures which may be in excess of 3000°K has been proposed on several occasions. Publications on this topic include, for example, papers entitled "The Plasma Bed : Performance and Capabilities" by w.M. Goldberger, Che . Eng. Progr. Symp. Ser. No. 62, 62, 42-46 (1966) and "A multiple arc discharge reactor for materials processing" by J.E. Harry and R. Knight, Symposium Proceedings Seventh International Symposium on Plasma Chemistry, Eindhoven, July 1985, Paper No. B-8-5, pages 1192 to 1195. The size and shape of the silicon carbide particles produced by the process of the invention are determined largely by the size and shape of the carbon particles. Hence by appropriate choice of the particle size and shape of the particulate carbon it is possible to influence the size and shape of the resultant silicon carbide particles. Usually it will be preferred to select a source of carbon particles which are substantially spherical. Channel black and thermal black are suitable sources of particulate carbon to use in the process of the invention. A typical thermal black has substantially spherical particles with an average particle diameter of not more than about 0.35um, e.g. about 0.27um (about 270 nm) . In order to obtain a silicon carbide of appropriate purity it is necessary to utilise a suitably pure form of silicon dioxide. It is accordingly preferred to utilise a silicon dioxide with an Siθ2 content of at least 99% by weight and with as low a content of Fe and/or Al as possible. A typical silicon dioxide suitable for use in the process of the invention has an average particle size of- not more than about 250um, e.g. about 125um, and is conveniently of substantially uniform particle size. It will usually be preferred to form the granules from an intimate mixture of silica and carbon in a stoichiometric or near stoichiometric molar ratio calculated on the basis of equations (A) and (B) above taken together, i.e. a 1:3 Siθ2 C molar ratio or a ratio close thereto, for example a 1:3.2 Siθ2:C molar ratio. Usually it will be preferred to use a mixture containing a slight stoichiometric excess of carbon in order to ensure that as much Siθ2 as possible is reacted. If desired the particles may comprise a core containing a 1:1 Siθ2 C molar ratio or a ratio close thereto, corresponding to the stoichiometric ratio for equation (A) above, with a carbon rich outer layer. In such a two layer particle the quantity of carbon in the outer layer is preferably at least sufficient to provide a stoichiometric amount of carbon for equation (B) above. Hence the ratio of the amount of carbon in the outer layer to the amount of Siθ2 in the core should desirably be at least about 2:1 on a molar basis. It is also possible to utilise in the process of the invention a mixture of carbon particles and of granules of the intimate mixture of silica and carbon particles for heating by means of the plasma arc. In this case it will usually be preferred to utilise granules containing an approximately 1:1 C:Siθ2 molar mixture, i.e. a mixture that is approximately stoichiometric for equation (A) above, to generate gaseous SiO which can then react with the free carbon particles according to equation (B) above. Again it is preferred to use overall an Siθ2:C molar ratio for the mixture of carbon particles and granules that is near stoichiometric for equations (A) and (B) taken together, i.e. a 1:3 Siθ2:C molar ratio, for example a 1:3.2 Siθ2:C molar ratio overall. Again it is preferred to utilise a slight excess of carbon in this variant of the process. The granules may be formed by pelletising. Typically the granules are substantially spherical and range in diameter from about 1 mm up to about 4 mm, e.g. about 1.5 mm to 2.0 mm. Although the use of non-spherical granules (e.g. cylindrical pellets) is not ruled out, it is advantageous to use spherical granules since these flow better and are less likely to cause blockages in the feed passages of the plasma reactor. The invention is further illustrated in the following Examples. Example 1 The apparatus is illustrated in the accompanying drawing. Plasma arc furnace 1 is lined with MgO and contains a graphite crucible 2 and a graphite upper sleeve 3, the upper end of which is provided with an annular top 4, also made of graphite, through which projects cathode 5. Reference numeral 6 represents the anode. Crucible 2 is supported on graphite rods 7 and is surmounted by a graphite liner 8. Furnace 1 is itself fitted with a graphite liner 9. The furnace roof 10 is also made of graphite and supports a graphite lower sleeve 11. A plasma forming gas, such as argon, can be introduced into the furnace at a pressure slightly above atmospheric pressure through the annular gap 12 between annular top 4 and cathode 5. Pellets of a mixture of silica and thermal black are fed to furnace 1 through feed ports 13 and are blown into furnace 1 by means of a non-oxidising gas, such as carbon monoxide or argon. Reference numeral 14 indicates diagrammatically the d.c. transferred are discharge that is formed when a suitably high d.c. voltage is struck between cathode 5 and anode 6. Gas exits furnace 1 through gas port 15 and enters primary scrubber 16 in which it encounters a spray of finely dispersed droplets of water. Make up water is introduced via line 17. This spray is indicated diagrammatically at 18 and is produced by forcing the water under pressure through nozzles 19. The water and silicon carbide particles collect in primary tank 20. Water is recycled to the top of the primary scrubber in line 21 by means of pump 22. The scrubbed gas passes on via duct 23 to secondary scrubber 24. Scrubbing water is pumped via line 25 to nozzles 26 to form spray 27. Water and further silicon carbide collects in secondary tank 28. The scrubbed gas is vented via line 29. Reference numeral 30 indicates a gas analyser sampling point. Crushed quartz (99.9% purity) with a particle size of 125um and an amorphous carbon black (thermal black) with substantially spherical particles having a particle size of 270 nm (0.27um) were intimately mixed in a Siθ2 C mole ratio of 1:3.2. (The stoichiometric ratio for reactions (A) and (B) together is li3 so that the Siθ2:C mole ratio used was approximately stoichiometric but with carbon present in slight excess with a view to ensuring that, so far as is possible, all Siθ2 was reacted) . This mixture was compacted in a roller compactor to give granules of approximately 1.5 mm diameter. The fraction passing through a sieve with a mesh aperture of 1.56 mm was selected for use. Using argon as plasma forming gas at a feed rate of 100 Nl/minute, a d.c. arc 400 mm long was established resulting in the sleeve reactor 3 reaching a temperature of 2243°K. The current was then reduced to 1000A and the sleeve temperature dropped to 2190°K. Feed of the granules to the sleeve reactor 3 then commenced via feed port 13 at a rate of 0.6 kg/minute. As soon as feed injection commenced the voltage and therefore the power input to the arc increased to 213 kw as the feed material and CO generated by reactions (A) and (B) were entrained in the arc. Argon was used as feed gas, the supply rate being 80 Nl/minute. Analysis of the exit gas indicated flow rates of CO of 300 Nl/minute and H2 of 110 Nl/minute. The hydrogen was believed to result from reaction of unreaσted carbon with the spray water: C + H 2 0 > CO + H 2 (C) After 10 minutes the power input to the arc was reduced to 188 kW. The run was continued for a further 24 minutes. The material collected in the primary and secondary tanks 20 and 28 was analysed. The material from the primary tank 20 consisted mainly of β-silicon carbide and unreacted carbon plus some silicon and forsterite (Mg2Siθ4). No e*»-phase silicon carbide was detected. (The presence of forsterite in the material collected in the primary tank was ascribed to reaction of siliceous materials with magnesium from to furnace refractory) . The major component of the minor amount of material collected in the secondary tank 28, was elemental silicon, together with some β-silicon carbide, forsterite and silica. There was no carbon in this material. Besides the material collected in the primary and secondary tanks 20 and 28 a quantity of unreacted material collected in crucible 2. The results are set out in Tables 1 and 2 below. A sample of the powder product recovered from the primary tank was heated to 700°C in air for 15 hours in order to remove residual carbon. The resulting heated treated powder was then immersed in concentrated aqueous hydrofluoric acid for 12 hours in order to dissolve residual Si, Si0 2 and other impurities. The acid treated powder was then washed eight times with toluene and dried at 150°C for 12 hours in a fan-assisted oven. The dried powder was then characterised by measuring its surface area using nitrogen absorption by the well known BET method (i.e. the method developed by Brunnaurer, Emet and Teller) and by scanning electron microscopy (SEM) . The BET surface area measurement was carried out under the following conditions: Outgassing conditions 120°C for 17 hours Ambient conditions 745.4 mm Hg at 23°C Sample weight 1.3959 g The specific surface area of the sample was found to be 36.39 m /g, corresponding to an average particle diameter of 51 n (0.051um) . The SEM studies were carried out using an S700 Hitachi scanning electron-microscope. The photomicrographs obtained indicated that the SiC powder is highly agglomerated but the agglomerated particles appear spherical and free from sharp edges and corners. Comparative Examples A and B. Using a similar procedure but replacing the granulated material by the powdered mixture of Siθ2 and C, product was collected in primary and secondary tanks 20 and 28. The results are also set out in Tables 1 and 2 below. In a modification of the apparatus of the drawing the gas exiting furnace 1 through gas port 15 is passed to a bag plant for collection of the powder product. liable 1 ) π Table 2

Claims: CLAIMS 1. A process for the production of silicon carbide particles of a size suitable for manufacture of components by powder compaction which comprises forming granules of an intimate mixture comprising finely divided silica and carbon particles, said carbon particles being substantially spherical and substantially all having a diameter lying in the range of from about 0.1pm up to about 10.0pm, heating said granules to a temperature in the range of from about 1950 β K to about 2350°K, and recovering the resultant particulate product. 2. A process according to claim 1, in which heating of said granules is effected by means of a plasma arc. 3. A process according to claim 1 or claim 2, in which the carbon particles substantially all have a diameter lying in the range of from about 0.25pm up to about 10.0pm. 4. A process according to claim 3, in which the carbon particles substantially all have a diameter lying in the range of from about 0.25pm up to about 5.0pm. 5. A process according to any one of claims 1 to 3, in which the finely divided silica comprises particles having a grain size in the range of from about lOum to about 250pm. 6. A process according to any one of claims 1 to 5, in which the finely divided silica comprises particles having a grain size in the range of from about 110pm to about 150pm. 7. A process according to any one of claims 1 to 6, in which the resultant particulate product is washed with an aqueous solution containing hydrofluoric acid. 8. A process according to claim 7, in which the aqueous solution further contains nitric acid. 9.- A process according to any one of claims 1 to 8, in which the mixture used to form the granules also includes a binding agent. 10. A process according to any one of claims 1 to 8, in which the granules have a particle size in the range of from about 0.5 mm to about 2.5 mm. 11. A process according to any one of claims 1 to 9, in which heating is effected in a d.c. transferred plasma arc furnace.

Inventors: BAGHA SHAHIN, NADEN DAVID, BAGHA, SHAHIN, NADEN, DAVID
Applicants: DAVY RES & DEV LTD, DAVY RESEARCH AND DEVELOPMENT LIMITED
Application year: 1990
Main classification: C01B31
IPC codes: C01B31