Refractory

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Refractory bricks in a torpedo car used for hauling molten iron Torpedo car refractory bricks.JPG
Refractory bricks in a torpedo car used for hauling molten iron

In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. [1] They are inorganic, non-metallic compounds that may be porous or non-porous, and their crystallinity varies widely: they may be crystalline, polycrystalline, amorphous, or composite. They are typically composed of oxides, carbides or nitrides of the following elements: silicon, aluminium, magnesium, calcium, boron, chromium and zirconium. [2] Many refractories are ceramics, but some such as graphite are not, and some ceramics such as clay pottery are not considered refractory. Refractories are distinguished from the refractory metals , which are elemental metals and their alloys that have high melting temperatures.

Contents

Refractories are defined by ASTM C71 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)". [3] Refractory materials are used in furnaces, kilns, incinerators, and reactors. Refractories are also used to make crucibles and molds for casting glass and metals. The iron and steel industry and metal casting sectors use approximately 70% of all refractories produced. [4]

Refractory materials

Refractory materials must be chemically and physically stable at high temperatures. Depending on the operating environment, they must 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). [5] Fire clays are also widely used in the manufacture of refractories.

Refractories must be chosen according to the conditions they face. Some applications require special refractory materials. [6] Zirconia is used when the material must withstand extremely high temperatures. [7] 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 would 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. [8] [9] The ternary compound tantalum hafnium carbide has one of the highest melting points of all known compounds (4215 °C). [10] [11]

Molybdenum disilicide has a high melting point of 2030 °C and is often used as a heating element.

Uses

Refractory materials are useful for the following functions: [12] [2]

  1. Serving as a thermal barrier between a hot medium and the wall of a containing vessel
  2. Withstanding physical stresses and preventing erosion of vessel walls due to the hot medium
  3. Protecting against corrosion
  4. Providing thermal insulation

Refractories have multiple useful applications. In the metallurgy industry, refractories are used for lining furnaces, kilns, reactors, and other vessels which hold and transport hot media such as metal and slag. Refractories have other high temperature applications such as fired heaters, hydrogen reformers, ammonia primary and secondary reformers, cracking furnaces, utility boilers, catalytic cracking units, air heaters, and sulfur furnaces. [12] They are used for surfacing flame deflectors in rocket launch structures. [13]

Classification of refractory materials

Refractories are classified in multiple ways, based on:

  1. Chemical composition
  2. Method of manufacture
  3. Size and shape
  4. Fusion temperature
  5. Refractoriness
  6. Thermal conductivity

Chemical composition

Acidic refractories

Acidic refractories are generally impervious to acidic materials but easily attacked by basic materials, and are thus used with acidic slag in acidic environments. They include substances such as silica, alumina, and fire clay brick refractories. Notable reagents that can attack both alumina and silica are hydrofluoric acid, phosphoric acid, and fluorinated gases (e.g. HF, F2). [14] At high temperatures, acidic refractories may also react with limes and basic oxides.

  • Silica refractories are refractories containing more than 93% silicon oxide (SiO2). They are acidic, have high resistance to thermal shock, flux and slag resistance, and high spalling resistance. Silica bricks are often used in the iron and steel industry as furnace materials. An important property of silica brick is its ability to maintain hardness under high loads until its fusion point. [2] Silica refractories are usually cheaper hence easily disposable. New technologies that provide higher strength and more casting duration with less silicon oxide (90%) when mixed with organic resins have been developed.
  • Zirconia refractories are refractories primarily composed of zirconium oxide (ZrO2). They are often used for glass furnaces because they have low thermal conductivity, are not easily wetted by molten glass and have low reactivity with molten glass. These refractories are also useful for applications in high temperature construction materials.
  • Aluminosilicate refractories mainly consist of alumina (Al2O3) and silica (SiO2). Aluminosilicate refractories can be semiacidic, fireclay composite, or high alumina content composite.[ clarification needed ] [15]

Basic refractories

Basic refractories are used in areas where slags and atmosphere are basic. They are stable to alkaline materials but can react to acids, which is important e. g. when removing phosphorus from pig iron (see Gilchrist–Thomas process). The main raw materials belong to the RO group, of which magnesia (MgO) is a common example. Other examples include dolomite and chrome-magnesia. For the first half of the twentieth century, the steel making process used artificial periclase (roasted magnesite) as a furnace lining material.

  • Magnesite refractories are composed of ≥ 85% magnesium oxide (MgO). They have high slag resistance to lime and iron-rich slags, strong abrasion and corrosion resistance, and high refractoriness under load, and are typically used in metallurgical furnaces. [16]
  • Dolomite refractories mainly consist of calcium magnesium carbonate. Typically, dolomite refractories are used in converter and refining furnaces. [17]
  • Magnesia-chrome refractories mainly consist of magnesium oxide (MgO) and chromium oxide (Cr2O3). These refractories have high refractoriness and have a high tolerance for corrosive environments.

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 belong to, but are not confined to, the R2O3 group. Common examples of these materials are alumina (Al2O3), chromia (Cr2O3) and carbon. [2]

  • Carbon graphite refractories mainly consist of carbon. These refractories are often used in highly reducing environments, and their properties of high refractoriness allow them excellent thermal stability and resistance to slags.
  • Chromite refractories are composed of sintered magnesia and chromia. They have constant volume at high temperatures, high refractoriness, and high resistance to slags. [18]
  • Alumina refractories are composed of ≥ 50% alumina (Al2O3).

Method of manufacture

  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.

Size and shape

Refractory objects are manufactured in standard shapes and special shapes. Standard shapes have dimensions that conform to conventions used by refractory manufacturers and are generally applicable to kilns or furnaces of the same types. Standard shapes are usually bricks that have a standard dimension of 9 in × 4.5 in × 2.5 in (229 mm × 114 mm × 64 mm) and this dimension is called a "one brick equivalent". "Brick equivalents" are used in estimating how many refractory bricks it takes to make an installation into an industrial furnace. There are ranges of standard shapes of different sizes manufactured to produce walls, roofs, arches, tubes and circular apertures etc. Special shapes are specifically made for specific locations within furnaces and for particular kilns or furnaces. Special shapes are usually less dense and therefore less hard wearing than standard shapes.

Unshaped (monolithic)

These are without prescribed form and are only given shape upon application. These types are known as monolithic refractories. Common examples include plastic masses, ramming masses, castables, gunning masses, fettling mix, and mortars.

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.

Fusion temperature

Refractory materials are classified into three types based on fusion temperature (melting point).

Refractoriness

Refractoriness is the property of a refractory's multiphase to reach a specific softening degree at high temperature without load, and is measured with a pyrometric cone equivalent (PCE) test. Refractories are classified as: [2]

Thermal conductivity

Refractories may be classified by thermal conductivity as either conducting, nonconducting, or insulating. Examples of conducting refractories are silicon carbide (SiC) and zirconium carbide (ZrC), whereas examples of nonconducting refractories are silica and alumina. Insulating refractories include calcium silicate materials, kaolin, and zirconia.

Insulating refractories are used to reduce the rate of heat loss through furnace walls. These refractories have low thermal conductivity due to a high degree of porosity, with a desired porous structure of small, uniform pores evenly distributed throughout the refractory brick in order to minimize thermal conductivity. Insulating refractories can be further classified into four types: [2]

  1. Heat-resistant insulating materials with application temperatures ≤ 1100 °C
  2. Refractory insulating materials with application temperatures ≤ 1400 °C
  3. High refractory insulating materials with application temperatures ≤ 1700 °C
  4. Ultra-high refractory insulating materials with application temperatures ≤ 2000 °C

See also

Related Research Articles

A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, and brick.

<span class="mw-page-title-main">Graphite</span> Allotrope of carbon, mineral, substance

Graphite is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on a large scale for uses in pencils, lubricants, and electrodes. Under high pressures and temperatures it converts to diamond. It is a good conductor of both heat and electricity.

<span class="mw-page-title-main">Aluminium oxide</span> Chemical compound with formula Al2O3

Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly called alumina and may also be called aloxide, aloxite, or alundum in various forms and applications. It occurs naturally in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. Al2O3 is used to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point.

<span class="mw-page-title-main">Sintering</span> Process of forming and bonding material by heat or pressure

Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing process used with metals, ceramics, plastics, and other materials. The nanoparticles in the sintered material diffuse across the boundaries of the particles, fusing the particles together and creating a solid piece.

<span class="mw-page-title-main">Zirconium dioxide</span> Chemical compound

Zirconium dioxide, sometimes known as zirconia, is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabilized cubic structured zirconia, cubic zirconia, is synthesized in various colours for use as a gemstone and a diamond simulant.

<span class="mw-page-title-main">Mineral wool</span> Fiber made from spun molten minerals

Mineral wool is any fibrous material formed by spinning or drawing molten mineral or rock materials such as slag and ceramics.

<span class="mw-page-title-main">Silicon carbide</span> Extremely hard semiconductor

Silicon carbide (SiC), also known as carborundum, is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal since 1893 for use as an abrasive. Grains of silicon carbide can be bonded together by sintering to form very hard ceramics that are widely used in applications requiring high endurance, such as car brakes, car clutches and ceramic plates in bulletproof vests. Large single crystals of silicon carbide can be grown by the Lely method and they can be cut into gems known as synthetic moissanite.

<span class="mw-page-title-main">Slag</span> By-product of smelting ores and used metals

Slag is a by-product of smelting (pyrometallurgical) ores and recycled metals. Slag is mainly a mixture of metal oxides and silicon dioxide. Broadly, it can be classified as ferrous, ferroalloy or non-ferrous/base metals. Within these general categories, slags can be further categorized by their precursor and processing conditions. "Slag generated from the EAF process can contain toxic metals, which can be hazardous to human and environmental health".

<span class="mw-page-title-main">Fire brick</span> Building material

A fire brick, firebrick, fireclay brick, or refractory brick is a block of ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. A refractory brick is built primarily to withstand high temperature, but will also usually have a low thermal conductivity for greater energy efficiency. Usually dense fire bricks are used in applications with extreme mechanical, chemical, or thermal stresses, such as the inside of a wood-fired kiln or a furnace, which is subject to abrasion from wood, fluxing from ash or slag, and high temperatures. In other, less harsh situations, such as in an electric or natural gas fired kiln, more porous bricks, commonly known as "kiln bricks", are a better choice. They are weaker, but they are much lighter and easier to form and insulate far better than dense bricks. In any case, firebricks should not spall, and their strength should hold up well during rapid temperature changes.

<span class="mw-page-title-main">Electric arc furnace</span> Type of furnace

An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc.

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<span class="mw-page-title-main">Zirconium carbide</span> Chemical compound

Zirconium carbide (ZrC) is an extremely hard refractory ceramic material, commercially used in tool bits for cutting tools. It is usually processed by sintering.

<span class="mw-page-title-main">Ceramic engineering</span> Science and technology of creating objects from inorganic, non-metallic materials

Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components and the study of their structure, composition and properties.

<span class="mw-page-title-main">Soda–lime glass</span> Type of glass

Soda–lime glass, also called soda–lime–silica glass, is the most prevalent type of glass, used for windowpanes and glass containers for beverages, food, and some commodity items. Some glass bakeware is made of soda-lime glass, as opposed to the more common borosilicate glass. Soda–lime glass accounts for about 90% of manufactured glass.

Ceramic membranes are a type of artificial membranes made from inorganic materials. They are used in membrane operations for liquid filtration.

<span class="mw-page-title-main">Thermal barrier coating</span> Form of exhaust heat management

Thermal barrier coatings (TBCs) are advanced materials systems usually applied to metallic surfaces on parts operating at elevated temperatures, such as gas turbine combustors and turbines, and in automotive exhaust heat management. These 100 μm to 2 mm thick coatings of thermally insulating materials serve to insulate components from large and prolonged heat loads and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface. In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue. In conjunction with active film cooling, TBCs permit working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications. Due to increasing demand for more efficient engines running at higher temperatures with better durability/lifetime and thinner coatings to reduce parasitic mass for rotating/moving components, there is significant motivation to develop new and advanced TBCs. The material requirements of TBCs are similar to those of heat shields, although in the latter application emissivity tends to be of greater importance.

<span class="mw-page-title-main">Yttria-stabilized zirconia</span> Ceramic with room temperature stable cubic crystal structure

Yttria-stabilized zirconia (YSZ) is a ceramic in which the cubic crystal structure of zirconium dioxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" (ZrO2) and "yttria" (Y2O3), hence the name.

Ultra-high-temperature ceramics (UHTCs) are a type of refractory ceramics that can withstand extremely high temperatures without degrading, often above 2,000 °C. They also often have high thermal conductivities and are highly resistant to thermal shock, meaning they can withstand sudden and extreme changes in temperature without cracking or breaking. Chemically, they are usually borides, carbides, nitrides, and oxides of early transition metals.

<span class="mw-page-title-main">Kiln furniture</span>

Kiln furniture are devices and implements inside furnaces used during the heating of manufactured individual pieces, such as pottery or other ceramic or metal components. Kiln furniture is made of refractory materials, i.e., materials that withstand high temperatures without deformation. Kiln furniture can account for up to 80% of the mass of a kiln charge.

Hafnium compounds are compounds containing the element hafnium (Hf). Due to the lanthanide contraction, the ionic radius of hafnium(IV) (0.78 ångström) is almost the same as that of zirconium(IV) (0.79 angstroms). Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties. Hafnium and zirconium tend to occur together in nature and the similarity of their ionic radii makes their chemical separation rather difficult. Hafnium tends to form inorganic compounds in the oxidation state of +4. Halogens react with it to form hafnium tetrahalides. At higher temperatures, hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Some compounds of hafnium in lower oxidation states are known.

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