Refractories and Fire Bricks

By: Adrian Rocker

Refractories are materials that have an unusually high melting point. They also maintain their structural properties at very high temperatures. They are composed of ceramics and are employed in the metallurgical, glassmaking and ceramics industries. They line the interiors of furnaces, kilns and other devices that process materials at high temperatures.

The most important aspects regarding refractories are: the essential properties of ceramic refractories and their applications, the processing techniques employed in the manufacture of refractories and the connection between the properties of refractories and their chemistry and microstructure.

Ceramics are useful as refractories due to the property of chemical inertness. In addition to temperature and corrosion resistance, refractories must possess superior physical wear or abrasion resistance and must be resistant to the thermal shock that occurs when an object is rapidly cooled from high temperature. Ceramics can be made resistant to thermal shock by adjusting their microstructure which is quite coarse when compared with white wares such as porcelain or less finely textured structural clay products such as brick.

Moreover, most refractories are quite porous and are presented with large amounts of air spaces of varying size incorporated into the material. These pores can reduce the load-bearing strength of the product but it can also reduce susceptibility to thermal shock.

The composition and processing of ceramic refractories vary according to their applications. Most refractories are either clay-based or non-clay based. Among clay-based refractories are: fireclay, high-alumina, silica and zircon materials. The category of clay-based products also includes: mortars for cementing bricks, mixes for ramming and gunning, fireboards, blankets and vacuum-cast shapes.

Fire bricks are also called refractory bricks and are in the above mentioned category of materials. They are used in lining furnaces, fireboxes and fireplaces. Fire bricks are primarily built to withstand temperature and ensure energy efficiency by means of low thermal conductivity. Dense fire bricks are also used in applications with extreme mechanical, chemical and thermal stresses.

Porous fire bricks are also useful but only under less harsh conditions. They are weaker and lighter but insulate better than dense fire bricks do. Fire bricks are primarily made in the United States and shipped to many locations around the world.

Fire bricks have an aluminum oxide content that can even insulate the tiles of space shuttles, but silica fire bricks are the most common type of fire bricks used for the inner lining (of furnaces) which is of sacrificial nature that is often presented with cracks which result from repetitive operations.

Common red clay fire bricks are used for chimney and wood-fired ovens. Similarly, silicon carbide (with high abrasive strength) is used for incinerators and cremators.

All in all, refractories are extremely useful in terms of furnace lining regardless of type of furnace that is being used. Among all refractory products, fire bricks are the most popular with furnace manufacturers.

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