Q. What are refractory's? How are they classified? Give the essential requirements of good refractories.

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Discuss classes of refractory.

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Classify the refractory with suitable example. Explain in detail refractory.

Ans. Refractory: Refractory are those specially designed materials, capable to withstand at extremely high temperature without being destroyed or softened by virtue of their high melting point. Refractories retain their chemical and physical identity at the elevated temperature.

Classification: Refractories are of three types

(a)          Acid refractory's

(b)          Basic refractory

(c)           Natural refractory

1.      Acid refractory: These are acidic in nature, thus they are resistant to acidic materials but are attacked by basic materials. They include: (1) silica refractory (2) Fireclay refractory (3) high alumina bricks.

2.      Silica refractory: Silica (Quartz or ganister) is mainly used for making refractories. The content of basic oxide should be minimum as they decrease the m. p. Of silica. Silica bricks tend to undergo slight permanent expansion upon prolonged exposed to high temperature. Tendency of silica bricks to expend is useful in producing a good furnace arched. Silica refractories are used as the lining material for acid open hearth and acid Bessemer convertors and for roofs of furnaces converts with refractory clay.

3.      Siliceous or semi-silica refractories: They are clay bonded silica bricks. They may be made from silica naturally occurring sand clay mixtures or from batches prepared from ground ganister bonded with refractory clay. These are used for constructing steel furnace plants.

4.      Fireclay refractory: Main ingredient in Kaolinite, hydrated aluminium silicate containing 18-45% Al₂O₃. During firing, the product shrinks due to expulsion of water and fluxing effect of impurities. They are uses as a casting pit refractories and high for the bricks used for the blast furnace hearths and the roofs of open hearths checkers.

5.      Basic refractory's: They are composed of basis materials such as magnesia (MgO), dolomite etc. Examples of such refractories are magnetite, dolomite and chrome-magnetite. These are attached by acidic slugs but are resistant towards basic slag.

1. Magnetite: It is basic refractory. Basic slag has little or no corrosive effect. It has good chemical resistance. Mechanical strength is much more than ordinary bricks. It has low thermal spelling. The resistance to abrasion is low. They can be used up to 2000⁰C without load and under load (3.5 kg/cm²)            

2. Dolomite refractory's: Dolomite refractory's (Cao.MgO) is highly basic in nature and has proved a good and cheap substitute for magnetite bricks. They can withstand high temperature load also a very good mechanical load.

3. Chrome -magnetic refractory's: Chrome-magnetite bricks containing about 70%, chrome and 30% magnetite are much superior to the magnetite or chromate bricks because these refractory's is swelling and bursting in the presence of slag containing iron oxide.

4. Neutral refractory's: This group includes a wide range of materials such as chromites, silicon carbide, zirconium, carbon etc.

Chrome refractories: The chromites bricks are neutral in character. Chromites bricks are resistant to basic and acids slag. They are not resistant to thermal shock. These refractory's can be used up to 1800⁰C and their refractory's under load is 3.5 kg/cm² at 1430⁰C. They have moderate thermal conductivity and good mechanical strength.

Silicon carbide: Silicon carbide decomposes in the temperature range of 2200: 2700⁰C before to basic and acid slag. They are not resistant before its melting and do not react with hydrogen nitrogen or carbon monoxide. A small proportion of clay or betonies are added as binder, and the bricks are fired to a high temperature say 1500⁰C.