By A.M. Garbers-Craig
Without refractory materials most of the scientific and technological inventions and developments we know today would not have been possible. The existence of virtually everything we see around us, or use in everyday life, is in some way dependent on refractory materials. Refractories are therefore facilitating or enabling materials, and are essential to the successful operation of any industry in which high temperatures are used .
The word ‘refractory’ is derived from the Latin word, ‘refractarius’, which means stubborn . These materials resist high temperatures, have high-quality mechanical and thermomechanical properties, have high corrosion resistance, act as a heat buffer between the walls of the containing vessel and the hot charge, and conserve process heat.
Reliability and long service life are required from materials when put into operation. Refractory cost is therefore expressed in terms of tons of product produced, i.e. the cost of the refractory material is weighed against useful life and replacement cost. A refractory material is a type of engineering ceramic called an ‘industrial ceramic’ . Refractory materials, however, have
coarser grain sizes and higher porosities than engineering ceramics, and consist of aggregate particles, held together by a bonding (matrix) phase, where both the aggregate and the bond can be multiphased .
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The particle size distributions are carefully controlled in order to control the microstructure, which directly influences porosity and density, strength, load-bearing capacity, corrosion resistance and thermal shock resistance . A huge range of types of refractory materials, with a variety of intricate microstructures and phase assemblages, is commercially available.
Refractory materials are mostly oxide based materials, but they are becoming increasingly composite materials, which also contain non-oxide components such as graphite, SiC, resin and metallic particles .
When refractories are classified on the basis of composition, a distinction can be made (according to the ISO [International Standards Organization Committee] classification)
between basic, non-basic (or acidic), oxidecarbon and specialized materials (Figure 1) . Refractory materials are fabricated in two forms: shaped and unshaped (monolithic) refractories (Figure 2). Shaped refractories include fired and unfired materials with predetermined shapes, precast shapes and fusion cast refractories. Monolithic materials include plastic mixes, castables, ramming materials, dry vibratables, gunning materials, fettling materials, coatings and mortars .
Monolithic materials differ from refractory bricks in that they are not shaped and fired before use. They therefore do not have high energy requirements, are more readily available, take shorter times to install, can be repaired locally and require less manpower .