A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as non-metallic materials having those chemical and physical properties that make them applicable for structures or as components of systems that are exposed to environments above 1,000 °F (811 K; 538 °C)”.
Refractory materials must be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion.
The oxides of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacturing of refractories. Another oxide usually found in refractories is the oxide of calcium (lime). Fire clays are also widely used in the manufacture of refractories.
Refractories must be chosen according to the conditions they will face. Some applications require special refractory materials. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide and carbon (graphite) are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen, as they will oxidize and burn.
Binary compounds such as tungsten carbide or boron nitride can be very refractory. Hafnium carbide is the most refractory binary compound known, with a melting point of 3890 °C. The ternary compound tantalum hafnium carbide has one of the highest melting points of all known compounds (4215 °C).
Classification of refractory materials:
Based on chemical composition
1. Acidic refractories
They consist of mostly acidic materials like alumina (Al2O3) and silica (SiO2). They are not attacked or affected by acidic materials, but easily affected by basic materials e.g.: silica, alumina, fire clay refractories, etc.
2. Neutral refractories
These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases. The main raw materials belongs to, but not confined to, R2O3 group. The common examples of these materials are alumina (Al2O3), chromia (Cr2O3) and carbon.
3. Basic refractories
These are used on areas where slags and atmosphere are basic; they are stable to alkaline materials but react with acids. The main raw materials belong to the RO group to which magnesia (MgO) is a very common example. Other examples include dolomite and chrome-magnesia.
Based on method of manufacture
Dry press process
Fused cast
Hand molded
Formed (normal, fired or chemically bonded)
Un-formed (monolithic-plastic, ramming and gunning mass, castables)
Un-formed Dry Vibratable refractories.
Shaped
These have fixed size and shapes. These may be further divided into standard shapes and special shapes. Standard shapes have dimension that are conformed by most refractory manufacturers and are generally applicable to kilns or furnaces of the same types. Special shapes are specifically made for particular kilns or furnaces.
Unshaped (Monolithic refractories)
These are without definite form and are only given shape upon application. These types are better known as monolithic refractories. The common examples are plastic masses, Ramming masses, castables, gunning masses, fettling mix, mortars etc.
Dry vibration linings often used in Induction furnace linings are also monolithic, and sold and transported as a dry powder, usually with a magnesia/alumina composition with additions of other chemicals for altering specific properties. They are also finding more applications in blast furnace linings, although this use is still rare.
Applications
Different applications of refractory materials are given below:
Furnaces
The unit processes utilized in the metal casting industry are lined with a wide range of refractory compositions (including silica, alumino-silicate, high alumina, zircon, magnesia, spinel, chrome, and magnesia-carbon) and forms (monolithics, precast shapes, and bricks). Most metals casting industry melting and holding furnaces are lined with ceramic refractories which are selected to minimize reaction with the specific metal being processed. Major refractory lined units include reverberatory furnaces, crucible (pot) furnaces, channel induction furnaces, coreless induction furnaces, electric arc furnaces, and pouring ladles. These furnaces are lined with wide ranges of refractory compositions, including silica, alumino-silicate, high alumina, zircon, magnesia, spinel, chrome, and magnesia-carbon.
Substantial problems do still exist with molten material containment and handling. For example, cupola linings for ferrous alloy melting usually require weekly repairs because of corrosion and mechanical damage. An alternative to refractory linings is “skull melting” (in titanium melting, for example) where in a mechanism similar to the slag splashing in the steel industry or the formation of a “frozen ledge” in the aluminum industry, a protective layer of metal is solidified on the containment vessel surface, creating a corrosion-resistant layer but lowering the thermal efficiency of the system.
Iron foundries are the major energy consumer for this sector, representing about 12% of the industry, followed by steel foundries at about 10%. Within each process, approximately 55% of the total energy cost is used for the melting of the metal, with other processes (core making, mold making, heat treatment, and post-casting processes) accounting for the other 45%. Although the total energy cost (fuel + electricity) accounts for only about 10% of the total material cost, it can be as high as 20% or more for many foundries depending on efficiency and the materials processed.
Other : Nuclear engines, Missiles, Gas Turbines, Nickel-base superalloy, Jet Engines
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