What are refractories? Give the preparation, properties and uses Carborundum bricks.
Refactory is any material that can withstand high temperature, without softening or suffering a deformation in shape.
Carborundom (SiC) bricks:
Carborundom bricks belong to the class of neutral refactory.
- SiC is made in an electric furnace at a temperature of 15000C, from sand (60%) and coke(40%) with the addition of some saw dust and little salt..
- Saw dust is added to increase porosity when burned. Saw dust evolves gases which by circulation increases the porosity of the charge.
- Salt is added because it reacts with iron and similar impurities forming volatile chlorides.
- SiC in form of interlocked crystals are formed.
- These are mixed with bonding agents (<10%) like plastic, fire clay, graphite, etc.
- After missing with bonding agent, final firing is done in a reducing atmosphere, at about 15000C.
- Self-bonding type SiC refractory bricks results when SiC particles are mixed with glue and fired at 20000C
- ‘Clay-bonded’ SiC can be used upto 17500C as this product softens near this temperature.
- ‘Silicon-nitride bonded’ bricks have a high strength and superior thermal shock resistance than clay-bonded products.
- ‘Self-bonded’ products have high refractoriness, superior strength, high density, high chemical resistance and high abrasion resistance.
- SiC are mainly used in muffle furnaces because of their good thermal conductivity.
- It is also an ideal choice for recuperators because of good resistance to spalling under repeated temperature fluctuations and ability to absorb and release heat rapidly.
- Furnace heating elements (Globars) are also made by SiC refractory.
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