Refractories are ceramic materials capable of withstanding high temperatures. The bulk of refractory materials consist of single or mixed high melting point oxides such as silicon, aluminum, magnesium, calcium and zirconium.
Non-oxide refractories also exist and include materials such as carbides, nitrides, borides and graphite. The actual composition of a refractory material is dependent on operating factors such as temperature, atmosphere and the materials it will be in contact with.
The refractoriness of a material is a measure of its ability to withstand exposure to elevated temperatures without undergoing appreciable deformation. It is generally measured using “Seger Cones”, which are conical-shaped ceramic objects of differing compositions.
When heated to various temperatures, these cones will slump as they soften in response to the temperature, with the degree of slumping being dependent on the composition. A similar cone-shaped object is made from the material to be measured and heated along with standard Seger Cones.
After the conclusion of the heating cycle, the sample material is compared to the Seger Cones to gain a comparative measure of its refractoriness. It is important to note that if a material is subjected to a mechanical load during the test, it may well soften at well below the temperature indicated by the Seger Cone test.
The atmosphere in which a refractory is to operate can dictate what materials can be used in that application. For example, graphite refractories can operate at temperatures of up to several thousand degrees Celsius under reducing conditions or oxygen-free conditions such as a vacuum. However, they may begin to sublime at approximately 1000 °C under oxidizing conditions.
The type of materials that a refractory comes into contact with can also dictate the materials suitable to use. For instance, in steel making, basic refractories are used because the refractories often encounter slags containing magnesium and calcium oxides. If the refractory lining was made from acidic refractories, it would be eroded quickly by the chemical interaction of the slag and the acidic lining (e.g. silica) forming low melting point compounds.
Refractories also come in a range of different densities and porosities. Generally, low porosity refractories display higher thermal conductivity compared to high porosity materials. The latter are usually strong insulators due to the high volume of air they envelop since air is a very poor thermal conductor.
However, high porosity materials do not cope as well with higher temperatures and direct flame impingement and tend to shrink under these conditions. Thus, the low porosity materials are used in the hotter zones, while more porous materials are used as thermal backup materials.
Refractories come in several different forms. These can be broken up into three main forms:
- Refractory Shapes – This grouping consists of preformed refractory shapes such as bricks that are delivered from the manufacturer, ready to install.
- Monolithics – Also known as refractory castables, these materials are essentially an analog of concrete, consisting of refractory aggregates, refractory fines and refractory cements (generally calcium aluminates). They are obtained dry, mixed with water before being formed and fired in situ.
- Ceramic fiber – Generally a very lightweight refractory material, which comes in a number of different forms to cope with different applications and temperatures. Their composition is usually made up of fibers of alumino-silicate materials, alumina and zirconia, depending on the actual application.
- The ability to withstand high temperatures and confine heat within a limited area such as a furnace.
- To maintain sufficient dimensional stability at elevated temperatures and after/during repeated thermal cycling.
- To maintain sufficient mechanical properties (e.g. compressive strength) at elevated temperatures.
Refractories are used to line furnaces and keep heat within the furnace hearth. By using refractories as insulation, the furnace efficiency is increased, the formwork or frame is protected, and workers who need to be in close contact to the furnace are also protected.
Furnaces may use several layers of different refractories in their construction. For instance, a dense hot face material may be used on the inner surfaces which are exposed to the highest temperatures. On the outside, a low density highly insulating refractory layer may be employed to keep in as much of the heat as possible. Between the hot face refractory and the low-density insulation, a number of different layers of intermediate materials may also be employed. Each successive layer (moving away from the hot face) would have an increasingly lower density and more than likely lower refractoriness compared to the previous layer.
Applications of furnaces include anything from primary metal smelting, through to heat treatment, glass production or processing, ceramic component manufacture and many forms of chemical processing and testing.
This article was updated on the 26th July, 2019.