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Wednesday, 9 October 2019

Refractory Materials Examples

Refractory Materials :

The purpose of refractory material is to confine the heat within the walls of a furnace preventing excessive loss to the atmosphere. Fire clay, porcelain, silica, mica magnesite, silicon carbide, dolomite and zircon are some of the refractory materials used. The ability of a material to resist the action of heat without considerable deformation or softening under particular service conditions is termed as refractoriness. Refractory materials should be unaffected by high temperatures, thermal shocks, abrasion and rough usage and should have low thermal conductivity. It should resist corrosive action of gases and must be chemically inert. Gases and liquids should not be able to penetrate through them and should have long life without cracking. Its coefficient of expansion should be as low as possible. Machining moulding, surfacing and polishing should be practicable in the material. Those refractory materials which serve upto a temperature of 1200°C are called normal duty refractory and those which serve above 1200°C are called heavy duty refractory. Refractory Materials Examples are explained as follows.

When classified according to chemical behaviour, they are

(a) Acidic refractories
(b) Basic refractories
(c) Neutral refractories

1. Acidic Refractories :

Silica is the chief constitution of this type of refractories. They ready combine with bases and therefore called acidic refractories. Quartz, sand ganister, silica, bricks, etc., form the example of acidic refractory. The temperature of fusion of silica in the pure form is 1710°C and when used as bricks are called silica bricks which is a water resistant. The manufacturing process mainly consists of by mixing crushed ganister with two percent lime, is moulded, dried in hot air and finally baked at controlled temperatures. Silica bricks are free from air pockets, possess excellent bearing capacity and have good thermal expansion. They are mostly used for high temperature furnaces as in the case of electric furnace.

2. Basic Refractories :

Dolomite is a double carbonate of calcium and magnesium. Dolomite bricks have low conductivity and refractoriness. Though it is cheaper but has poor performance. Dolomite bricks are used in basic open hearth furnaces. The other important basic refractory material is magnesite which is made by burning magnesium carbonate. Magnesite bricks loose strength in the neighbourhood of 1500°C and cannot withstand any sudden change in temperature. It is costlier and is used in Arc furnace, Basic open hearth furnace, gold platinum and silver refining furnaces etc. The normal composition of magnesite bricks is about 6% Fe203, 4% Si02 2% Al203, 0.5% Ca0 and the rest MgO. It is prepared by dead burning at temperatures of about 1600 to I700°C, crushing it by adding some water, within iron oxide, acting as a binding material, pressing it into bricks under heavy pressure and then drying at temperature of about 1400 to 1500°C. Before use they are soaked for atleast eight hours at temperature of about 1600°C.

3. Neutral Refractories :

These refractories consist of substances which are neither acidic nor basic. Chromite and fire clay are the examples. Chromite is made of FeO and Al203. It is crushed and moulded with a small quantity fire clay, dried and baked at a temperature of about 1600°C. These refractories have high thermal conductivity and low porosity.

Fire clay bricks consist of Aluminium Silicate, Al203, 2SiO, 2H20. They are free from lime, oxides of iron or such other substances which tend to melt even the clay at low temperatures and are made in the same way as ordinary bricks and burnt in special kiln at high temperatures. Usually surfaces of bricks are glazed to protect them from the action of chemicals and other destroying agents.

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