What are refractories? Give the preparation, properties and uses of Carborundum bricks

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What are refractories?

Refractory is any material that can withstand high temperature, without softening or suffering a deformation in shape.

The term refractories embrace all materials used in the arts for the construction of heat-resisting containers, using the word in its broadest sense, whether it be to afford space for the evolution of gases in combustion processes or the holding of molten charges or of solids undergoing heat-treatment.

Carborundum (SiC) bricks: Carborundum bricks belong to the class of neutral refractory.

Preparation of refractories:
  1. SiC is made in an electric furnace at a temperature of 15000C, from sand (60%) and coke(40%) with the addition of some sawdust and little salt..
  2. Saw dust is added to increase porosity when burned. Saw dust evolves gases which by circulation increase the porosity of the charge.
  3. Salt is added because it reacts with iron and similar impurities forming volatile chlorides.
  4. SiC in form of interlocked crystals are formed.
  5. These are mixed with bonding agents (<10%) like plastic, fire clay, graphite, etc.
  6. After missing with a bonding agent, the final firing is done in a reducing atmosphere, at about 15000C.
  7. Self-bonding type SiC refractory bricks result when SiC particles are mixed with glue and fired at 20000C
Properties of refractories:
  1. ‘Clay-bonded’ SiC can be used up to 17500C as this product softens near this temperature.
  2. ‘Silicon-nitride bonded’ bricks have high strength and superior thermal shock resistance than clay-bonded products.
  3. ‘Self-bonded’ products have high refractoriness, superior strength, high density, high chemical resistance, and high abrasion resistance.
Uses of refractories:
  1. SiC is mainly used in muffle furnaces because of its good thermal conductivity.
  2. It is also an ideal choice for recuperators because of its good resistance to spalling under repeated temperature fluctuations and its ability to absorb and release heat rapidly.
  3. Furnace heating elements (Globars) are also made by SiC refractory.
Types of refractories:

1. Acid refractories.

2. Basic refractories.

3. Neutral refractories.

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