SILICON Carbide

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• SILICON Carbide

Silicon Carbide
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Discovery

• In 1891 Edward G Acheson produced a small amount
  of Silicon Carbide
• while conducting experiments with the aim of
  obtaining a hard material
• from the reaction of clay and carbon.
• He passed a strong electric current from a carbon
  electrode through a mixture
• of clay and coke contained in an iron bowl, which
  served as the second electrode.
• Acheson recognized the abrasive value of the
  crystals obtained, had them
• analyzed, found the formula to be SiC,
  incorporated The Carborundum Company
• in September 1891, and filed application for a
  patent on May 10, 1892.

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SiC Properties

• Silicon Carbide is also called carborundum,
  including black and green silicon carbide both
  with a
• shape of hex crystal. The black silicon carbide
  is classified into coke-made and coal-made black
• silicon carbide depending on different raw
  materials. The material is extremely hard and
  sharp,
• with excellent chemical properties. The hardness
  is between diamond and fused alumina, but
• the mechanism hardness is higher than fused
  alumina. The micro hardness is in the range of
• 2840-3320kg/mm².
• Silicon Carbide is sharp but fragile with good
  heat-resistance, heat-conductibility, can be
  antacid
• and antalkali, lower dilatability and can be
  aseismatic.
• SiC has
• high hardness
• high thermal consistency
• very good resistance at high temperatures
• low thermal expansion
• electrical conductivity
• is a semiconductor
• non linear electrical resistance
• Si and C as alloying additive

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SiC Properties cont.
Crystal Structure of Silicon Carbide

• Appearance
• When removed from the furnace, the Silicon
  Carbide is a mass of interlocking iridescent
  crystals,
• the crystals themselves being largely twinned and
  often coalescent. Their iridescence is due to a
• thin surface layer of silica resulting from
  superficial oxidation of the carbide. Washing in
• hydrochloric acid will remove this layer. The
  crystals vary in colour from very pale green to
  black
• depending on the amount of included impurities.
• Hardness
• Silicon Carbide was the first material entering
  the range of hardness between corundum and
• diamond. It is given the relative position of 9.5
  on Mohs scale and diamond at 10.
• Properties
• SiC is quite stable chemically. It is stable to
  acids, not reacting with fuming nitric acid, nor
  with
• boiling sulphuric hydrochloric or hydrofluoric
  acid. Sodium silicate attacks it above 1300ºC,
• calcium and magnesium oxides attack above 1000ºC.
  Copper oxide reacts at 800ºC to form the
• metal silicide. It oxidizes slowly in air above
  1000ºC.
• Silicon Carbide dissociates in molten iron and
  the silicon reacts with the metal oxides in the
  melt.
• This reaction is of use in the metallurgy of iron
  and steel.

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Applications for SiC

• Silicon carbide forms natural crystals, which are
  very hard, very abrasive and dissociate or
  sublimate
• at high temperatures. It is for these reasons
  that silicon carbide is used in the following
  applications
• Abrasive Industry
• With a good hardness, silicon carbide is the
  first choice raw material for manufacturing
  abrasive pipe,
• impeller, pumping chamber etc. Its abrasiveness
  is 5-20 times than that of cast iron and rubber.
• Macrogrits are used to make items like sandpaper,
  grinding wheels, disks, wire saws and a number of
• other abrasive products.

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Applications for SiC cont.

• Refractory Industry
• Because of its high temperature and abrasive
  resistance it is used to make
• refractories for furnaces and other high
  temperature components. The ceramic
• industry is one of the largest users of SiC
  refractory products.
• Metallurgical Industry
  
• Silicon carbide is used for the deoxidation and
  recarburation of cast iron
• and steel in foundries. Metallurgical grade
  Silicon Carbide grain is a unique material for
  use in the
• production of iron and steel. It is used in the
  foundry industry for the electric furnace
  production of
• gray, ductile, and malleable iron. It is an
  excellent source of carbon and silicon, promotes
  nucleation and
• renders the iron more responsive to inoculation,
  and deoxidizes the iron, which enhances furnace
  lining
• life.
• Silicon carbide can also be used to enhance
  efficiency in ferroalloy production using the
  patented
• Maxred process developed by Sublime Technologies.
  
• Other Industries
• Silicon carbide is used in several specific
  electro-technical applications such as autovalves
  
• andresistance. It is also used in traditional
  mechanical fields such as non-slip floors.

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Typical Silicon Carbide uses

• Fixed and moving turbine components
• Suction box covers
• Seals, bearings
• Ball valve parts
• Hot gas flow liners
• Heat exchangers
• Semiconductor process equipment

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Sublime Technologies (Pty) Ltd.
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History

• Sublime Technologies is South Africas first
  silicon carbide producer.
• Sublime originated within Pyromet Technologies
  (now Tenova Pyromet), an organisation
  specialising in
• smelting technology.
• Sublime was established in 2001 and produces
  Silicon Carbide making
• use of state of the art Acheson Electric furnaces
  (diagram).
• The company exports most of it production to
  consumers in Europe
• with only a small percentage being used in South
  and Southern Africa.
• Sales into America have now also started and will
  increase as production
• increases. Products are crushed and screened
  into various size fractions
• where after it is sold in bulk and bagged format.
  
• Scope exists for producing more value added
  variations of the current
• products in future which will enhance margins.
• Our marketing team in North America and Europe
  have extensive
• experience in all areas of silicon carbide
  production and sales.

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Production Process

• Silicon carbide is made today in much the same
  way as it was when invented in 1891
• High purity quartz is mixed with a high quality
  coke or anthracite in large electric resistance
• furnaces at temperatures of over 2 000C
  according to the following reaction  
• SiO2 3CSiC 2CO 

The process is an endothermic reaction requiring
between 8 000 10 000kWh per tonne of
product. The product is removed from the furnace
when cool and separated into different grades.
Sorting is aimed at separating high-grade
crystalline silicon carbide from metallurgical
grade silicon carbide. Lower grades of silicon
carbide are recycled in the production system.
The sorted silicon carbide is then crushed and
screened to a saleable product.
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Products we produce

• Crystalline Grade (98)
• Our premium quality product has a variety of
  applications ranging from abrasives,
• (bonded, coated and granular), refractories and
  metallurgical applications. Products
• are uncompromisingly precision graded packaged
  to customer specifications.
• Metallurgical Grade (82 - 92)
• Our production process inherently produces grades
  of silicon carbide with lower
• SiC content, material ideally suited to re
  carburetion of cast iron and steel in
• foundries. Metallurgical grade Silicon carbide
  can also be used to enhance
• efficiency in ferroalloy production using the
  patented Maxred process developed
• by Sublime Technologies.
• Maxred Process
• In terms of Maxred there is currently no rival to
  this process from other technologies. The
  alternative
• for ferrochrome users would be to revert to the
  conventional carbon reduction process and hence
• have no additional benefits that the Maxred
  process allows. Testing of the material has
  shown strong
• potential, but pricing and limitations of
  Sublimes ability to supply have retarded market
  penetration.
• The increased volumes from the expanded plant as
  well as a much lower unit production costs can be

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New Developments

• Pelletising
• Silicon Carbide can be used in the production of
  Ferrochrome.
• Currently Silicon Carbide is added directly into
  Ferrochrome furnaces but new research by the
  company is
• suggesting that by adding the product into the
  Chrome ore pellets which are used to charge
  Ferrochrome
• furnaces, significant improvements in Ferrochrome
  production recoveries and efficiencies may be
  possible.
• To this end improved furnace recovery techniques
  are being trialed in conjunction with local major
  steel
• producers, which are showing encouraging results.