Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/90—Carbides
  • C01B32/914—Carbides of single elements
  • C01B32/956—Silicon carbide
  • C01B32/963—Preparation from compounds containing silicon
  • C01B32/97—Preparation from SiO or SiO2

Definitions

• the present invention refers to a process for producing silicon carbide.
• the invention refers to a method for a two-step process for producing silicon carbide.
• the process for industrial production of silicon carbide commonly used ar the present comprises reacting a silicon source with a carbon source in an electrically heated furnace.
• a silicon source for industrial production the so-called "Ach- eson"-process is used, patented in 1892 and named after the inventor of said process.
• GB patent application No. 2.220.409 A refers to a method for preparing ⁇ -silicon carbide powder suitable as a sintering material by mixing a silicon powder of high purity and not larger than 50 ⁇ m in particle size with a carbon powder not larger than 1 ⁇ m in particle size and heating the mixture in a non-oxidizing atmosphere at a temperature of 1700-2000 °C for a period not exceeding 2 hours. Particular care should be taken not to use a temperature above 2000 ⁇ C, in which case there may be a risk of phase transition from ⁇ -silicon carbide to ⁇ - silicon carbide.
• GB patent No. 1.208.001 refers to a method for producing crystalline silicon carbide by heating a system compris ⁇ ing silicon and carbon and/or compounds of said elements together with lanthanum or a lanthanum compound to a tem ⁇ perature exceeding 1000°C, whereby silicon carbide crys ⁇ tals are deposited from a gaseous or vapour phase.
• ⁇ -silicon carbide is still produced in accordance with the Ache- son's process with comprises premixing of the starting material consisting of a carbon source and a silicon source, the latter in the form of quarts sand, and as a carbon source petrol coke is commonly used.
• the premixed starting materials are introduced into large rectangular electrical resistant furnaces, the end wall of which are fitted with electrodes for electrical power supply and the side walls consisting of movable concrete sections.
• the oven is half filled with the mixed starting materials, and subsequently a graphite core, forming and electrical heating element is supplied, whereafter the remaining charge of the starting material is added.
• the reaction between the starting materials takes place at a temperature above 1500"C and silicon, carbide is formed in accordance with the following overall equation Si0 2 + 3C ⁇ SiC + 2C0
• This general reaction equation is simplified in that the end product, i.e. ⁇ -SiC, is formed via a series of inter ⁇ mediates, thus complicating the reactions.
• ⁇ -silicon carbide As a result of the above mentioned only 10-15 percent by weight of the charged starting material is converted into the desired end product, ⁇ -silicon carbide.
• the partially reacted and not reacted starting material, as well as ⁇ - silicon carbide are recirculated and used together with possible optionally new starting material for a new charge in the furnace.
• the use of recirculated, non-reacted and party reacted and ⁇ -silicon carbide will result in that the end product is not "green", that is ⁇ - silicon carbide of high purity.
• the prize for green ⁇ - silicon carbide is substantially higher than for "black", non-high purity ⁇ -silicon carbide. This will, of course, make production of green silicon carbide non-profitable if not combined with the production of black ⁇ -SiC, utilizing the non-reacted starting material resulting from the production of green ⁇ -silicon carbide.
• This process for producing silicon carbide has several drawbacks: it is difficult to collect the gaseous eff ⁇ luents from the furnace, mainly carbon monoxide, which either burn on the surface of the charged starting material or are expelled to the atmosphere together with other gasses formed, such as sulphur dioxide and thus represent an environment problem.
• the carbon monoxide represents a source of energy which advantageously could be recovered if practically possible.
• Non-reacted starting materials and partly reacted starting materials can be included as new starting material for a charge, but said material can not be used for production of high purity silicon carbide. Impurities which have vaporized from the inner hot zone of the charge will condense and deposit in the outer, cooler layer, hence said non-reacted or partly reacted starting material can only be used in the manu ⁇ facture of less pure, black silicon carbide.
• the object of the present invention is to provide a new and improved method for preparing a more homogenous end product and by which method effluence to the atmos ⁇ phere for the most part can be avoided.
• the improved process for commercial production of silicon carbide comprises production in two steps; in the first step ⁇ -sili ⁇ on carbide is produced, preferably continuously, whereafter the second step the ⁇ -silicon carbide in is heat treated at a temperature at which ⁇ -silicon carbide is converted into ⁇ -silicon carbide.
• the ⁇ -silicon carbide in a preferably con ⁇ tinuous process one can obtain a very even quality of the ⁇ -silicon carbide with approximately 100% conversion, that is, in essence all of the charge starting material will be converted to ⁇ -silicon carbide which subsequently can be submitted to crystal growth to the desired end product, ⁇ -silicon carbide.
• the effluent generated in said step can be recovered and utilized, for instance gene ⁇ rated car ⁇ - -i monoxide can be utilized as a heat source.
• the starting materials, silicon dioxide and carbon are reacted at a temperature in the range of 1600-1800°C.
• the necessary heat energy can for example be applied by means of an arc or by means of a plasma torch or any other suitable heating arrangement.
• a plasma torch mounted in connection with a rotating furnace, the plasma torch providing heat to a reaction chamber.
• a preheating chamber can be mounted before the reaction chamber in order to pre-heat the supplied starting materials consisting of silicon dioxide and a carbon source.
• furnaces which can be used by practising the instant process.
• Fig. 1 is shown a rotating furnace without a pre ⁇ heating chamber.
• a graphite electrode or a plasma torch for heating indicates a reaction chamber in which the reactants are present.
• 3 shows feeding of the reactants, preferably in the in the form of pellets, and 3 also serves as an outlet for carbon monoxide.
• 4 shows the exit for the ⁇ -silicon carbide produced
• 5 indicates a roller bearing and 6 a sealing.
• Fig. 2 shows a rotating furnace provided with a pre ⁇ heating chamber.
• the same reference numbers as in Fig. 1 are also used in the corresponding parts in Fig. 2.
• 1' indicates a plasma torch and 2' indicates a pre-heating chamber.
• 7 is a magnetic coil.
• the produced ⁇ -silicon carbide with a very small particle size can be transferred for instance to a high tempera ⁇ ture furnace, for heating to a temperature in the range above 1800°C, preferably in the range of 1800-2200°C, for a desired period of time, in order to give the desired product, ⁇ -silicon carbide with desired particle size.
• the present process is a 2-step process in which ⁇ - silicon carbide is produced in the first step and in a subsequent, separate step the obtained ⁇ -silicon carbide is converted into ⁇ -silicon carbide with the desired particle size.
• the ⁇ -silicon carbide obtained can subsequently be converted to desired end product in a conventional manner, which will be well known for the person within the state of art.
• the chemical conversion of the starting material to ⁇ -silicon carbide the formed carbon monoxide can be separated and collected in a controlled manner and optionally purified and used as an energy source.
• the present invention is advantageous in that the ⁇ - silicon carbide is produced in a pure form and can be used as such for the preparation of SiC-products of low density, grinding grit and the like, or be submitted to crystal growth and conversion into ⁇ -silicon carbide.
• Prior to the conversion of the starting material to ⁇ - silicon carbide doping agents can be added to the raw material and subsequently preferably pelletized in a conventional manner and dried before being fed to the first conversion step.
• the produced ⁇ -silicon carbide can then be heated to 1800-2200°C, either in a suitable high temperature furnace or by using an already existing "Acheson" furnace without causing pollution problems, because the chemical reactions taking place in the con ⁇ version of the starting material to silicon carbide already have taken place and hence discharge of CO and S0 2 will be substantially reduced compared with the conventional process.
• the instant process for production of silicon carbide represents an advantageous feature in respect to the known process, the chemical reactions taking place in a closed chamber so that the effluent gases can be taken care of, the obtained silicon carbide can be worked up in a conven- tional high temperature furnace, for instance an "Ache- son"-furnace or in a suitable shaft furnace, without the danger of a discharge of gaseous reactants to the envi ⁇ ronment.
• the process is less work intensive and will give a improved control over the production process and lead to a hight yield of ⁇ -silicon carbide with a suitable crystal size, and also offering the possibility of pro ⁇ ducing a cleaner end product, and hence it is more profitable than the known process.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Carbon And Carbon Compounds (AREA)

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• 1994
  • 1994-06-06 NO NO942090A patent/NO306815B1/no not_active IP Right Cessation
• 1995
  • 1995-06-02 AU AU26844/95A patent/AU2684495A/en not_active Abandoned
  • 1995-06-02 EP EP95922009A patent/EP0766645B1/en not_active Expired - Lifetime
  • 1995-06-02 JP JP8500700A patent/JPH10500933A/ja active Pending
  • 1995-06-02 US US08/750,393 patent/US6022515A/en not_active Expired - Lifetime
  • 1995-06-02 DE DE69504196T patent/DE69504196T2/de not_active Expired - Lifetime
  • 1995-06-02 WO PCT/NO1995/000091 patent/WO1995033683A1/en active IP Right Grant

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