FIRE CLAY

Fireclay denotes a silica-rich natural clay that can withstand a high firing temperature above the pyrometric cone equivalent (PCE) of 19 without melting, cracking, deforming, disintegrating, or softening. Typically, a good fireclay should have 24 to 26 vol.% plasticity, and shrinkage after firing should be within 6 to 8 vol.% maximum. Fireclays are mostly made of kaolinite, but some Fe2 O3 and minor amounts of Na₂O, K₂O, CaO, MgO, and TiO₂ are invariably present depending on the mineralogy and geology of the deposit, making it gray in color.

As a general rule fireclays are used in both shaped refractories (i.e., bricks) and monolithic refractories (i.e., castables), while super-duty plastic fireclay is used in the preparation of castable recipes. Therefore, the major applications of fireclays are in power generation, such as in boiler furnaces, in glass-melting furnaces, in chimney linings, in pottery kilns, and finally in blast furnaces where the backup lining is done almost entirely with fireclay bricks. Pouring refractories like sleeves, nozzles, stoppers, and tuyers are also made of fireclay.

Mined clay is stacked in the factory yard and allowed to weather for about 1 year. For daily production of different types of refractories, this weathered clay is taken and mixed in different percentages with grog (i.e., spent fireclay). The mixture is sent to the grinding mill from where it is transferred to the pug mill. In the pug mill a suitable proportion of water is added so as to give it proper plasticity. The mold is supplied to different machines for making standard bricks or shapes. Intricate shapes are made by hand. The bricks thus made are then dried in hot floor driers and after drying are loaded in kilns for firing. The firing ranges are, of course, different for different grades of refractories. After firing, the kilns are allowed to cool, then the bricks are unloaded. Upon burning fireclay is converted into a stonelike material that is highly resistant to water and acids, while manufacturing high aluminous fire-bricks bauxite is added along with grog in suitable proportions.

Upon firing, fireclay yields a strong ceramic product with a composition close to the theoretical composition of metakaolin (i.e., 54.1 wt.% SiO2 and 45.9 wt.% Al₂O₃), but in practice it contains between 50 and 60 wt.% SiO₂, 24 and 32 wt.% Al₂O₃, no more than 25 wt.% Fe₂O₃ and a loss on ignition of 9 to 12 wt.%.

Fireclay is classified under acid refractories, that is, refractories that are not attacked by acid slags. In practice, refractoriness and plasticity are the two main properties required for the manufacture of refractory bricks; hence fireclays are grouped according to the maximum service temperature of the final product before melting in: low-duty fireclay (max. 870°C, PCE 18 to 28), mediumduty fireclay (max. 1315°C, PCE 30), high-duty fireclay (max. 1480°C, PCE 32), and super-duty fireclay (max. 1480°C–1619°C, PCE 35). In practice, it has been observed that the higher the alumina content in the fireclay, the higher the melting point. All fireclays are not necessarily plastic clays. In such cases, some plastic clay, like ball clay, is added to increase plasticity to a suitable degree. A good fireclay should have 24 to 26% plasticity, and shrinkage after firing should be within 6 to 8% maximum. It should also not contain more than 25% Fe₂O₃.