Refractory

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Refractory bricks in a torpedo car used for hauling molten iron

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)".[1]

Refractory materials are used in linings for furnaces, kilns, incinerators and reactors. They are also used to make crucibles.

Refractory materials[edit]

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.[2][3] The ternary compound tantalum hafnium carbide has one of the highest melting points of all known compounds (4215 °C).[4][5]

Classification of refractory materials[edit]

Refractories can be classified on the basis of chemical composition, method of manufacture, physical form or according to their applications, fusion temperature.

Based on chemical composition[edit]

Acidic refractories[edit]

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. They include substances such as silica, alumina, and fire clay brick refractories.

Neutral refractories[edit]

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 belong to, but are not confined to, the R2O3 group. The common examples of these materials are alumina (Al2O3), chromia (Cr2O3) and carbon.

Basic refractories[edit]

These are used on areas where slags and atmosphere are basic; they are stable to alkaline materials but could 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[edit]

  1. Dry press process
  2. Fused cast
  3. Hand molded
  4. Formed (normal, fired or chemically bonded)
  5. Un-formed (monolithic-plastic, ramming and gunning mass, castables, mortars, dry vibrating cements.)
  6. Un-formed Dry refractories.

Shaped[edit]

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. Standard shapes are usually firebrick that have a standard dimension of 9 x 4-1/2 x 2-1/2 inches and this dimension is called a "one brick equivalent". "Brick equivalents" are used in estimating how many firebrick it takes to make an installation into an industrial furnace. Special shapes are specifically made for particular kilns or furnaces.

Unshaped (Monolithic refractories)[edit]

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.

Based on fusion temperature[edit]

Based on fusion temperature, they are classified into three types:

  1. Normal refractory.
  2. High refractory.
  3. Super refractory.

Normal refractory[edit]

Fusion temperature = 1580-1780° (e.g. Fire clay)

High refractory[edit]

Fusion temperature = 1780 - 2000° (e.g. Chromite)

Super refractory[edit]

Fusion temperature > 2000° (e.g. Zirconia)

Refractory anchorage[edit]

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.

Refractory heat-up[edit]

Usually, refractories require special heat-up techniques to ensure that their performance will be attained as designed, and to avoid thermal shock and drying stresses until the operational status is achieved.

Major producers[edit]

Some of the major refractory producers in the world are OCL India Ltd. www.ocl.in Intocast, www.intocast.com, Siam Refractory, RHI AG, Vesuvius, Calderys, Magnesita, TRL Krosaki Refractories Limited, Minteq, and Refratechnik. Resco Products, Inc, Metaflux Company Pvt.Ltd., and Capital Refractories, www.capital-refractories.com

See also[edit]

References[edit]

  1. ^ ASTM Volume 15.01 Refractories; Activated Carbon, Advanced Ceramics
  2. ^ Hugh O. Pierson (1992). Handbook of chemical vapor deposition (CVD): principles, technology, and applications. William Andrew. pp. 206–. ISBN 978-0-8155-1300-1. Retrieved 22 April 2011. 
  3. ^ Hafnium, Los Alamos National Laboratory
  4. ^ McGraw-Hill encyclopedia of science and technology: an international reference work in fifteen volumes including an index. McGraw-Hill. 1977. p. 360. ISBN 978-0-07-079590-7. Retrieved 22 April 2011. 
  5. ^ "Hafnium". Encyclopædia Britannica. Encyclopædia Britannica, Inc. Retrieved 17 December 2010. 

External links[edit]