thumb|Refractory bricks in a [[torpedo wagon|torpedo car used for hauling molten iron]]
A fire brick, firebrick, fireclay brick, or refractory brick is a block of ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. Made of primarily oxide materials like silica and alumina in varying ratios, these insulating materials are able to withstand extremely high temperatures, and have a low thermal conductivity for greater energy efficiency. Refractory bricks generally range from 25-45% alumina, and ~60% silica, with additional magnesium, calcium, potassium oxides. They are weaker, but they are much lighter and easier to form and insulate far better than dense bricks. In any case, firebricks should not spall, and their strength should hold up well during rapid temperature changes.
Manufacturing
There are three main manufacturing methods for firebrick: firing, cementing and geopolymerization. In the first method, clay is fired in the kiln until it is partly vitrified. For special purposes, the brick may also be glazed. Firing is the most conventional process, with easy execution using traditional equipment. However, it is an energy intensive process, consuming a significant amount of energy - on the scale of 2.0 kWh per brick. Firing also releases around 0.41 kg of CO<sub>2</sub> High-temperature refractory cements include furnace cements or high-heat mortar. These mixtures are applied to the brick surfaces and pressed together, and allowed to cure with time - taking anywhere from days to weeks.
A technique with the potential to replace firing and cementing is geopolymerization. Firing and cementing both require huge amounts of energy and release large amounts of greenhouse gases, thus motivating research into additional methods of production. Efforts to incorporate up to 30 wt% of waste materials into clay ceramics have been shown. Clay ceramics with 17 wt% additives displayed the highest bending strength of 30 MPa. Also available are firebrick "splits" which are half the thickness and are often used to line wood stoves and fireplace inserts. The dimensions of a split are usually . The most common basic refractory bricks used in smelting non-ferrous metal concentrates are "chrome-magnesite" or "magnesite-chrome" bricks (depending on the relative ratios of magnesite and chromite ores used in their manufacture).
Lower temperature applications
A range of other materials find use as firebricks for lower temperature applications. Magnesium oxide is often used as a lining for furnaces. Silica bricks are the most common type of bricks used for the inner lining of furnaces and incinerators. As the inner lining is usually of sacrificial nature, fire bricks of higher alumina content may be employed to lengthen the duration between re-linings. Very often cracks can be seen in this sacrificial inner lining shortly after being put into operation. They revealed more expansion joints should have been put in the first place, but these now become expansion joints themselves and are of no concern as long as structural integrity is not affected. Silicon carbide, with high abrasive strength, is a popular material for hearths of incinerators and cremators. Common red clay brick may be used for chimneys and wood-fired ovens.
Material properties
The linear shrinkage of firebrick under compression was found to be around 12.8%. This number can be decreased to below 8% with the addition of 25wt% coal ash. This is due to the high porosity of pure refractory bricks, around 31%, which compromise some of its structural integrity. When even 5wt% of coal ash is added, the porosity decreases to 24%,
See also
- Harbison-Walker Refractories Company
- Equivalent VIII
- Niles Firebrick