material is one that retains its strength at high temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
ASTM International, known until 2001 as the American Society for Testing and Materials , is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services...
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 1000 °F (810.9 K; 537.8 °C)".
Refractory materials are used in linings for furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
s, incinerators and reactors. They are also used to make crucible
A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...
Refractory materials must be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...
, be chemically inert, and/or have specific ranges of thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
and of the coefficient of thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s of aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
(silica) and magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
) are the most important materials used in the manufacturing of refractories. Another oxide usually found in refractories is the oxide of calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
). Fire clay
Fire clay is a term applied to a range of refractory clays used in the manufacture of ceramics, especially fire brick.High grade fire clays can withstand temperatures of 1775°C , but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1515°C...
s 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
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
) are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, as they will oxidize and burn.
Binary compounds such as tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
or boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...
can be very refractory. Hafnium carbide
Hafnium carbide is a chemical compound of hafnium and carbon. With a melting point of about 3900 °C it is one of the most refractory binary compounds known. However, it has a low oxidation resistance, with the oxidation starting at temperatures as low as 430 °C.Hafnium carbide is usually carbon...
is the most refractory binary compound known, with a melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
of 3890 °C. The ternary compound
In chemistry, a ternary compound is a compound containing three different elements. An example of this is sodium phosphate, Na3PO4. The sodium ion has a charge of 1+ and the phosphate ion has a charge of 3-. Therefore, three sodium ions are needed to balance the charge of one phosphate ion. Another...
tantalum hafnium carbide
Tantalum hafnium carbide is a refractory chemical compound with a general formula TaxHf1-xCy, which can be considered as a solid solution of tantalum carbide and hafnium carbide...
has one of the highest melting points of all known compounds (4215 °C).
Classification of refractory materials
Refractories can be classified on the basis of chemical composition, method of manufacture, physical form or according to their applications.
These are used in areas where slag and atmosphere are acidic. They are stable to acids but attacked by alkalis. The main raw materials belongs to the RO2
group, ex- silica (SiO2
), zirconia (ZrO2
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, R2
group. The common examples of these materials are alumina (Al2
), chromia (Cr2
) and carbon.
These are used on areas where slags and atmosphere are basic, stable to alkaline materials but reacts with acids. The main raw materials belong to the RO group to which magnesia (MgO) is a very common example. Other examples includes 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)
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.
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.
All refractory require anchorage systems such as wire formed anchors, formed metal (for example, hexmetal) or ceramic tiles to support the refractory linings. The anchorage used for refractory on roofs and vertical walls are more critical as they must remain able to support the weight of refractory even at the elevated temperatures and operating conditions.
The commonly used anchorages have circular or rectangular cross-section. Circular cross-section are used for low thickness refractory and they support less weight per unit area; whereas the rectangular cross-section is used for high thickness refractory and can support higher weight of refractory per unit area. The number of anchors to be used depend on the operating conditions and the refractory materials. The choice of anchors material, shape, numbers and size has significant impact on the useful life of the refractory.
Usually, refractories require special heat-up techniques
After building of a new refractory-lined industrial furnace or equipment, or refractory maintenance or relining of existing equipment, a necessary step is the start-up of the operation, which usually involves heating-up the unit in a controllable way, in order to prevent spalling or shortening of...
to ensure that their performance will be attained as designed, and to avoid thermal shock and drying stresses until the operational status is achieved.