Global Refractory Gateway

Selection of Refractories

The selection of a suitable refractory depends on a large number of factors classified into the following groups

I	Factors related to refractories

The properties of the refractories available need to be studied in detail before starting selection of refractories for any particular application. The main characteristics of refractories are displayed in Table 1 below

Table I

Type of bricks	Characteristics
High-Alumina	High refractoriness, which increases with increasing alumina content. High mechanical strength at high temperatures. Excellent to fair resistance to spalling. Greater resistance (than fireclay brick) to corrosion by most basic slags & fluxes
Fire-clay	The chemical and physical properties of fireclays vary between a wide range; hence, fireclay bricks with widely varying combination of properties are available. This fact accounts for their suitability for service under widely different operation conditions. Most fireclay bricks have relatively good spalling resistance and thermal insulation value. They offer fair resistance to acid slags and fluxes, lower resistance to basic slags and fluxes.
Silica	High refractoriness and resistance to abrasion. High mechanical strength at high temperatures. Thermal conductivity at high temperatures is about 25% greater than that of fireclay brick High resistance to corrosion by acid slags. Fair resistance to attack by lime, magnesia and iron oxide. Readily attacked by basic slags and fluorine. Not subject to thermal spalling at temperature above 650oC Poor resistance to thermal spalling at low temperature.
Magnesite	Extremely high refractoriness and high thermal conductivity Great resistance to corrosion by basic slags Poor resistance to slags containing high percentages of silica Chemically bonded and metal encased bricks have marked resistance to spalling.
Chrome	High resistance to corrosion by basic slags and moderately acid slags and fluxes. In general, basic slags do not adhere to chrome bricks. Under certain unusual conditions, iron oxide is absorbed and causes a damaging expansion Thermal conductivity is lower than that of magnesite brick but higher than that of fireclay brick
Magnesite Chrome	Mechanical strength and stability of volume at high temperatures. Excellent resistance to spalling. High resistance to corrosion by basic slags.
Chrome Magnesite	Mechanical strength and stability of volume at high temperatures. Excellent to good resistance to spalling. High resistance to corrosion by basic slags
Forsterite 2MgO.SiO2	High refractoriness Excellent strength at high temperatures Marked resistance to corrosion by alkali compounds. Fair resistance to most basic slags; attacked by acid slags
MgO-C	Excellent corrosion to slag resistance & spalling resistance
Al2O3-MgO-C	Excellent thermal spalling & corrosion to molten steel

II	Factors related to furnace design

When the refractories are to be used in a certain furnace, the selection of refractory will depend upon the type of the furnace, conditions of heating (e.g. a coke oven is kept continuously at a high temperature for years but a cupola is cooled daily) and loading, degree of insulation, provision for thermal expansion made in the furnace, whether there are moving parts or not (e.g. as are in rotary kilns and tunnel kiln cars) etc.

III	Factors related operation

The most important factor in this group is the chemical nature of the materials (like the ores, fluxes, fuels etc.) in contact with the refractory materials, temperature in different parts of the furnace, temperature fluctuations, fluidity of the metals & slags, velocity of gases in contact with the furnace lining and the abrasion that takes place due to movement of charge, gases and molten metals.

IV	Cost factors

While considering the initial cost of different refractories, their life during use should not be overlooked. As can be seen, the selection of a suitable refractory material is quite complicated. A significant amount of data is available from the usage of refractories in similar furnaces elsewhere. These can be helpful in the actual selection process. Out of the large number of factors only a few are so essential that by comparison other factors are of minor importance. For example, in some cases exposure to high temperature is the main problem; in some other cases high load has to be supported; in only a few cases the refractory has to withstand both high temperature and high load; in some cases high insulating property is needed; in others, high thermal conductivity is of prime importance. Greatest weight, in the selection should be given to the more essential properties for the particular use.

The cost of handling the refractory, transportation, brick laying cost, are the same whether the bricks used are of superior or inferior quality. If an inferior refractory is used and it has a shorter life, it will have to be replaced more often. This will involve additional handling costs, in addition to loss in production. Therefore the true cost of refractories should be considered to get the overall cost of production per unit output.

Conclusion

Refractories must be selected after careful study of all operational conditions of the furnaces and properties of the bricks by looking into most severe operating conditions which are responsible for high wear and tear of refractories. In case of high wear, zones of furnaces can be lined with zoning by quality or thickness to get balanced wear out with most economical results.