Refractories are used at elevated temperatures for structural purposes and they are used in many cases to contain a high temperature corrosive environment. In any process environment, refractories are potentially continuously under attack from a number of corrosive processes.
Chemical Corrosion
The corrosion envirenment usually contains materials and byproducts of the process in chemical reactions with the refractory at elevated temperatures resulting in refractory consumption or wear, potentially causing glassing or softening of the refractories. It is usually not immediately obvious, but the oxidation and reduction state of the environment can participate in and influence the chemical reactions that take place. Along with chemical reaction during corrosion, physical changes occur that may be accelerated by the corrosion process.
Erosion
Erosion is another prevalent refractory wear mechanism. Refractories can be worn away over time from the washing action of moving liquids, such as molten metals or slags. Erosion further exposes refractory to destruction by corrosive or abrasive elements.
Mechanical Abrasion
Abrasive media, including fuel, ash, and other particles, can wear away refractories over time, much like sandblasting. Refractory resistance against abrasion is a key issue for many industrial furnace applications.
Mechanical Wear
Moving parts and equipment within a process can wear against the refractory lining, jeopardizing the structural integrity of the refractory lining.
Thermal Cycling
As refractories undergo the heating and cooling cycles of a process, the refractories expands and contracts, eventually weakening and wearing down the lining. If refractories experiences a rapid change in temperature, a.k.a Thermal Shock, the refractories can experience immediate damage.
As a result of the high temperature corrosive environment, refractories will wear down over time, requiring periodic maintenance and eventual replacement. Refractory wear can be mitigated or minimized by selecting the right refractories to withstand the corrosive environments.
Sunrise Refractory offers a wide range of refractories with good wear resistance for high temperature corrosive environments in glass industry.
Sunday, May 24, 2015
Monday, May 18, 2015
Features Of High Alumina Refractories
Aluminium Oxide (Al2O3 ) or alumina is one of the most versatile of refractory ceramic oxides and finds use in a wide range of applications. High alumina refractories are alumina refractories containing more than 45% alumina.
High alumina bricks is made of high-quality bauxite chamotte as raw materials by shaping at high pressure and sintering at high temperature. Alumina is one of the most chemically stable oxides known, which offers excellent hardness, strength and spalling resistance, which imparts high alumina refractories great features.
The alumina content ranges from 45 to 100%. The refractoriness of high alumina refractories increases with increase of alumina content. So high alumina refractories have high refractoriness and excellent high temperature performance.
High alumina refractories are a kind of acid refractories. Therefore, they have good resistance to acid slags. High alumina refractories are also featured with high cold crushing strength. As the alumina content increases, the cold crushing strength also increases. This makes them to bear more load during use. The high hardness of alumina imparts high wear and abrasion resistance.
Manufacturing cost and price of these brick increase more rapidly with % alumina content, so it is essential to determine experimentally or by test installations the most economical alumina content for each service. In some areas such as the regenerators, fire clay bricks can be used as a replacement for high alumina refractories.
Due to severe service conditions prevailing in modern reheating furnaces, there is an increasing tendency to use more and more of high alumina bricks in place of conventional fireclay bricks. The applications of high alumina refractories includes the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.
Sunrise Refractory is refractory supplier from China, providing high quality high alumina refractories in various shapes at competitive prices.
High alumina bricks is made of high-quality bauxite chamotte as raw materials by shaping at high pressure and sintering at high temperature. Alumina is one of the most chemically stable oxides known, which offers excellent hardness, strength and spalling resistance, which imparts high alumina refractories great features.
The alumina content ranges from 45 to 100%. The refractoriness of high alumina refractories increases with increase of alumina content. So high alumina refractories have high refractoriness and excellent high temperature performance.
High alumina refractories are a kind of acid refractories. Therefore, they have good resistance to acid slags. High alumina refractories are also featured with high cold crushing strength. As the alumina content increases, the cold crushing strength also increases. This makes them to bear more load during use. The high hardness of alumina imparts high wear and abrasion resistance.
Manufacturing cost and price of these brick increase more rapidly with % alumina content, so it is essential to determine experimentally or by test installations the most economical alumina content for each service. In some areas such as the regenerators, fire clay bricks can be used as a replacement for high alumina refractories.
Due to severe service conditions prevailing in modern reheating furnaces, there is an increasing tendency to use more and more of high alumina bricks in place of conventional fireclay bricks. The applications of high alumina refractories includes the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.
Sunrise Refractory is refractory supplier from China, providing high quality high alumina refractories in various shapes at competitive prices.
Sunday, May 10, 2015
The Pros and Cons of Refractory Bricks and Monolithic Refractories
Refractory materials can be generally divided into two kinds: refractories bricks (shaped refractories) and monolithic refractories (unshaped refractories). Both have its pros and cons and applications in the refractory lines.
Refractories bricks are shaped refractories which have fixed shapes. The shapes of refractories bricks maybe divided into two types: standard shapes and special shapes. Standard shapes have dimensions that are conformed to by most refractory manufacturers and are generally applicable to kilns and furnaces of the same type. Special shapes are specifically made for particular kilns and furnaces. This may not be applicable to another furnaces or kiln of the same type.
Shaped Refractory bricks are almost always machine-pressed, thus, high uniformity in properties are expected. Special shapes are most often hand-molded and are expected to exhibit slight variations in properties.
Refractory bricks possess high thermal strength at elevated temperatures. Combined with chemical resistance to alkali attack, the performance is improved for areas with mechanical and chemical impact. These performance characteristics are important in where archways and belly band areas are exposed to mechanical wear and chemical attack. One advantage of refractory bricks over monolithic refractories is that the structural properties are defined during the production process.
Unshaped refractories are without definite form and are only given shape when used. It forms joint less lining and are better known as monolithic refractories. These are categorized as plastic refractories, ramming mixes, castables, gunning mixes, fettling mixes and mortars.
Monolithic refractories exhibits properties that outperform traditional refractory bricks. They have better volume stability and better mechanical resistance to vibration and impact. Another advantage over fired bricks is that shrinkage and expansion of monolithic linings can be matched to the application. In some cases shrinkage can offset the thermal expansion resulting in a significantly different thermo-mechanical behavior compared to refractory bricks.
Use of Monolithic refractories eliminates difficult brick laying tasks and joints which may be accompanied with weakness in construction. Under certain conditions, monolithic linings of the same composition as firebrick provide better insulation, lower permeability and improved resistance to the spalling effects of thermal shock. With little or no preparation, monolithic refractories can be applied to form monolithic or joint free furnace linings in new constructions or to repair existing refractory lining.
Monolithic refractories are widely used in the construction of new kilns and furnaces and in the maintenance of older ones because substantial repairs can be made with a minimum loss of time and, in some cases, even during operations, and in a variety of other applications.
Refractories bricks are shaped refractories which have fixed shapes. The shapes of refractories bricks maybe divided into two types: standard shapes and special shapes. Standard shapes have dimensions that are conformed to by most refractory manufacturers and are generally applicable to kilns and furnaces of the same type. Special shapes are specifically made for particular kilns and furnaces. This may not be applicable to another furnaces or kiln of the same type.
Shaped Refractory bricks are almost always machine-pressed, thus, high uniformity in properties are expected. Special shapes are most often hand-molded and are expected to exhibit slight variations in properties.
Refractory bricks possess high thermal strength at elevated temperatures. Combined with chemical resistance to alkali attack, the performance is improved for areas with mechanical and chemical impact. These performance characteristics are important in where archways and belly band areas are exposed to mechanical wear and chemical attack. One advantage of refractory bricks over monolithic refractories is that the structural properties are defined during the production process.
Unshaped refractories are without definite form and are only given shape when used. It forms joint less lining and are better known as monolithic refractories. These are categorized as plastic refractories, ramming mixes, castables, gunning mixes, fettling mixes and mortars.
Monolithic refractories exhibits properties that outperform traditional refractory bricks. They have better volume stability and better mechanical resistance to vibration and impact. Another advantage over fired bricks is that shrinkage and expansion of monolithic linings can be matched to the application. In some cases shrinkage can offset the thermal expansion resulting in a significantly different thermo-mechanical behavior compared to refractory bricks.
Use of Monolithic refractories eliminates difficult brick laying tasks and joints which may be accompanied with weakness in construction. Under certain conditions, monolithic linings of the same composition as firebrick provide better insulation, lower permeability and improved resistance to the spalling effects of thermal shock. With little or no preparation, monolithic refractories can be applied to form monolithic or joint free furnace linings in new constructions or to repair existing refractory lining.
Monolithic refractories are widely used in the construction of new kilns and furnaces and in the maintenance of older ones because substantial repairs can be made with a minimum loss of time and, in some cases, even during operations, and in a variety of other applications.
Monday, May 4, 2015
How To Improve The Thermal Shock Resistance Of Refractories
Thermal shock is the direct result of exposing the surface of refractory installations to rapid heating and cooling conditions which cause temperature gradients within the refractory blocks. Such gradients, in the case of uneven cooling or heating, may cause cracking. Thermal shock is one of the most important potential failure modes of refractory installations.
In many service conditions, refractories can undergo rapid temperature changes. These temperature fluctuations develop unequal thermal stresses, within the refractory, by causing either rapid expansion or contraction of a section of material. The failure occurs when the thermal stress exceeds the strength of the material in that mode of stressing. It is one of the common reasons of refractory lining damages, more dangerous, than chemical and mechanical tear and wear of the lining.
Thermal shock is a key property in refractory selection process. The most undesirable consequence of thermal shock is obviously spalling. Spalling is the loss of fragments or “spalls” from the face of a refractory brick or structure through cracking and rupture, which exposes inner portions of the refractory.
In many instances, a material properties or/and heat transfer conditions is taken to characterize thermal shock behavior of the refractories.
One of the most important parameters for thermal shock resistance is the coefficient of thermal expansion. Generally, the refractory with the lowest rate of thermal expansion (lowest coefficient of expansion) has the best thermal shock resistance. Inversely, the material with a high expansion has a low thermal shock resistance.
Aside from the thermal conductivity, another parameter which must be considered in thermal shock resistance, is the surface heat transfer coefficient.
Recommendations to improve the thermal shock resistance of refractories:
1) Use materials with a low thermal expansion coefficient, or a combination of a raw materials that would result in a low permanent linear contraction to reduce the thermally induced stresses.
2) Heat up or cool down the refractory line as slowly as possible. From an operational point of view, however, the faster heat-up or cool-down that can be achieved the less time is wasted in a furnace or vessel being not in operation and hopefully making money. These two considerations must be balanced: reducing downtime; against avoiding damaging the lining.
3) Use insulating refractories which play an insulation role and avoid too much weight and thickness.
4) Select refractories with high heat transfer coefficient.
Thermal shock resistance dictates refractory performance in many applications. The thermal shock occurs, then object temperature changes much in a short time. It can become a reason of a sudden failure of the lining at the very beginning. So it is important to take the thermal shock resistance into account when selecting and installing refractory linings.
In many service conditions, refractories can undergo rapid temperature changes. These temperature fluctuations develop unequal thermal stresses, within the refractory, by causing either rapid expansion or contraction of a section of material. The failure occurs when the thermal stress exceeds the strength of the material in that mode of stressing. It is one of the common reasons of refractory lining damages, more dangerous, than chemical and mechanical tear and wear of the lining.
Thermal shock is a key property in refractory selection process. The most undesirable consequence of thermal shock is obviously spalling. Spalling is the loss of fragments or “spalls” from the face of a refractory brick or structure through cracking and rupture, which exposes inner portions of the refractory.
In many instances, a material properties or/and heat transfer conditions is taken to characterize thermal shock behavior of the refractories.
One of the most important parameters for thermal shock resistance is the coefficient of thermal expansion. Generally, the refractory with the lowest rate of thermal expansion (lowest coefficient of expansion) has the best thermal shock resistance. Inversely, the material with a high expansion has a low thermal shock resistance.
Aside from the thermal conductivity, another parameter which must be considered in thermal shock resistance, is the surface heat transfer coefficient.
Recommendations to improve the thermal shock resistance of refractories:
1) Use materials with a low thermal expansion coefficient, or a combination of a raw materials that would result in a low permanent linear contraction to reduce the thermally induced stresses.
2) Heat up or cool down the refractory line as slowly as possible. From an operational point of view, however, the faster heat-up or cool-down that can be achieved the less time is wasted in a furnace or vessel being not in operation and hopefully making money. These two considerations must be balanced: reducing downtime; against avoiding damaging the lining.
3) Use insulating refractories which play an insulation role and avoid too much weight and thickness.
4) Select refractories with high heat transfer coefficient.
Thermal shock resistance dictates refractory performance in many applications. The thermal shock occurs, then object temperature changes much in a short time. It can become a reason of a sudden failure of the lining at the very beginning. So it is important to take the thermal shock resistance into account when selecting and installing refractory linings.
Thursday, April 30, 2015
5 Common Manufacturing Methods Of Refractory Materials
Refractories are manufactured by various methods. The common manufacturing methods include dry press process, fused cast, hand molded, formed and un-formed.
1) Dry press process
This process is ideally suitable to the formation of simple solid shapes. It is particularly suited to clays of very low plasticity. Clay is mixed with a minimal amount of water, then pressed into steel molds under pressures by hydraulic or compressed air rams. Because the dry press process is so simple and involves low capital equipment costs it is the most widely uased high –volume forming process for ceramics.
The manufacturing process has six general steps: 1) mining and storage of raw materials, 2) preparing raw materials, 3) forming the brick, 4) drying, 5) firing and cooling and 6) de-hacking and storing finished products.
2) Fused cast
Fused cast involves melting refractory material in a electric furnace followed by casting and annealing are treated with oxygen while in the molten state to place the constituents in the most highly oxidized state. This method minimizes the exudation of the glassy matrix of the refractory during service. The raw materials for the refractories may be oxidized before melting by heat treating to reduce the oxygen necessary for oxidizing the molten refractory. High density, small or large shapes are obtained. When appropriate, a finish is made by grinding with diamond tools.
3) Hand molded
The mold is made by hand. Hand molded refractories do not have the smooth surface of machine made brick. This method is especially well adapted to small
4) Formed
Firebrick is a common example of formed refractory. Formed refractories are manufactured by either fired or chemically bonded method.
Fired refractories is formed by heating the refractory material to high temperatures in a kiln to form a ceramic bond. This process gives the raw materials their refractory properties.
Chemically bonded refractory brick, also referred to as unfired brick, is formed with the aid of selected additives that set up at room temperature and provide structural integrity, eliminating the need for high-temperature sintering. It offers significant energy savings by eliminating the need for high-temperature processing. In addition, the many methods for modifying the chemical bond can develop new compositions to withstand a variety of severe environments encountered in many industrial processes.
5) Un-formed
Un-formed refractories, also called monolithic, do not comes in any specific form. Unformed refractories are made and marketed in granulated or plastic forms or as spray mixes. Thus, they can be used as patching materials for maintenance. Common unformed refactories include monolithic-plastic, ramming and gunning mass, castables, mortars, and dry vibrating cements. They are manufactured in various ways.
1) Dry press process
This process is ideally suitable to the formation of simple solid shapes. It is particularly suited to clays of very low plasticity. Clay is mixed with a minimal amount of water, then pressed into steel molds under pressures by hydraulic or compressed air rams. Because the dry press process is so simple and involves low capital equipment costs it is the most widely uased high –volume forming process for ceramics.
The manufacturing process has six general steps: 1) mining and storage of raw materials, 2) preparing raw materials, 3) forming the brick, 4) drying, 5) firing and cooling and 6) de-hacking and storing finished products.
2) Fused cast
Fused cast involves melting refractory material in a electric furnace followed by casting and annealing are treated with oxygen while in the molten state to place the constituents in the most highly oxidized state. This method minimizes the exudation of the glassy matrix of the refractory during service. The raw materials for the refractories may be oxidized before melting by heat treating to reduce the oxygen necessary for oxidizing the molten refractory. High density, small or large shapes are obtained. When appropriate, a finish is made by grinding with diamond tools.
3) Hand molded
The mold is made by hand. Hand molded refractories do not have the smooth surface of machine made brick. This method is especially well adapted to small
4) Formed
Firebrick is a common example of formed refractory. Formed refractories are manufactured by either fired or chemically bonded method.
Fired refractories is formed by heating the refractory material to high temperatures in a kiln to form a ceramic bond. This process gives the raw materials their refractory properties.
Chemically bonded refractory brick, also referred to as unfired brick, is formed with the aid of selected additives that set up at room temperature and provide structural integrity, eliminating the need for high-temperature sintering. It offers significant energy savings by eliminating the need for high-temperature processing. In addition, the many methods for modifying the chemical bond can develop new compositions to withstand a variety of severe environments encountered in many industrial processes.
5) Un-formed
Un-formed refractories, also called monolithic, do not comes in any specific form. Unformed refractories are made and marketed in granulated or plastic forms or as spray mixes. Thus, they can be used as patching materials for maintenance. Common unformed refactories include monolithic-plastic, ramming and gunning mass, castables, mortars, and dry vibrating cements. They are manufactured in various ways.
Sunday, April 26, 2015
What Are The Causes Of Refractory Failures
Refractories are heat-resistant materials that constitute the linings for high-temperature furnaces and reactors and other processing units. In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractories must also withstand physical wear and corrosion by chemical agents. Any failure of refractory could result in a great loss of production time.
The refractory material failure may caused by many different factors, such as chemical reaction and corrosion, spalling, material selection, plant operations, material storage, mixing, installation, curing, and drying. Only by understanding all aspects pertaining to the design and installation of the refractory material can one find the cause of the failure and help eliminate future failures.
The most common cause for failure of refractory is chemical reaction with the environment in which it is operating and chemical corrosion from molten slag and hot gas/molten salt. Chemical corrosion of a refractory is caused by slag attack at the refractory surface. The material selected must match the chemical environment that exists. For example, an acidic refractory should not be used in furnaces using basic fluxes, slag, etc. and vice-versa.
The porosity of refractory plays an important role in the chemical reaction. The more porous it is, the greater will be the depth to which the slag will penetrate and destroy the refractory. As the temperature increases, the rate of chemical reaction gradually increases. Sometimes, rise in temperature beyond the safe limit quickly brings about the destruction of the refractory. These chemical aspects are complementary to the engineering plant aspects and must be taken into consideration for a successful realization of the process.
Another important cause is spalling. It may be thermal, mechanical or structural. Thermal spalling may be due to unequal expansion or contraction caused by the difference in temperature at different parts. Mechanical spalling is mostly due to carelessness in loading the furnace or in the removal of materials from furnace, thereby damaging the refractory. Structural spalling takes place due to change in composition of the refractory because of reaction with slags, flux, etc. as a result its coefficient of expansion changes. Thus, different parts expand and contract to a different extent.
Improper material storage, mixing, installation, curing and drying will also cause refractory failure. Refractory material should always be stored in dry, well-ventilated conditions. Use fresh refractory materials and follow proper storage procedures to ensure that the refractory will not lose strength. Use potable water (suitable for drinking) for mixing. The use of the wrong type of water will hinder the ability of the refractory material to reach its proper strength. Using the right type of mixer, following proper mixing procedures, and staying within recommended pot life are other important installation factors. Using the wrong mixer or pneumatic gun could also affect the strength of the refractory material. Almost all refractory materials (except those that are phosphate bonded) must be cured prior to the drying process. Failure to properly cure a cement-bonded refractory material is the number one contributor to refractory failure and lack of longevity.
Problems with the quality of the refractory material itself is also an important reason for the failure of refracoties. A selection of the right refractories for a specific application is important.
The refractory material failure may caused by many different factors, such as chemical reaction and corrosion, spalling, material selection, plant operations, material storage, mixing, installation, curing, and drying. Only by understanding all aspects pertaining to the design and installation of the refractory material can one find the cause of the failure and help eliminate future failures.
The most common cause for failure of refractory is chemical reaction with the environment in which it is operating and chemical corrosion from molten slag and hot gas/molten salt. Chemical corrosion of a refractory is caused by slag attack at the refractory surface. The material selected must match the chemical environment that exists. For example, an acidic refractory should not be used in furnaces using basic fluxes, slag, etc. and vice-versa.
The porosity of refractory plays an important role in the chemical reaction. The more porous it is, the greater will be the depth to which the slag will penetrate and destroy the refractory. As the temperature increases, the rate of chemical reaction gradually increases. Sometimes, rise in temperature beyond the safe limit quickly brings about the destruction of the refractory. These chemical aspects are complementary to the engineering plant aspects and must be taken into consideration for a successful realization of the process.
Another important cause is spalling. It may be thermal, mechanical or structural. Thermal spalling may be due to unequal expansion or contraction caused by the difference in temperature at different parts. Mechanical spalling is mostly due to carelessness in loading the furnace or in the removal of materials from furnace, thereby damaging the refractory. Structural spalling takes place due to change in composition of the refractory because of reaction with slags, flux, etc. as a result its coefficient of expansion changes. Thus, different parts expand and contract to a different extent.
Improper material storage, mixing, installation, curing and drying will also cause refractory failure. Refractory material should always be stored in dry, well-ventilated conditions. Use fresh refractory materials and follow proper storage procedures to ensure that the refractory will not lose strength. Use potable water (suitable for drinking) for mixing. The use of the wrong type of water will hinder the ability of the refractory material to reach its proper strength. Using the right type of mixer, following proper mixing procedures, and staying within recommended pot life are other important installation factors. Using the wrong mixer or pneumatic gun could also affect the strength of the refractory material. Almost all refractory materials (except those that are phosphate bonded) must be cured prior to the drying process. Failure to properly cure a cement-bonded refractory material is the number one contributor to refractory failure and lack of longevity.
Problems with the quality of the refractory material itself is also an important reason for the failure of refracoties. A selection of the right refractories for a specific application is important.
Tuesday, April 14, 2015
What are Refractories?
Refractories are high temperature resistant materials. Refractory materials can retain their strength at high temperatures. They are used to make crucibles and are used in linings for high-temperature industrial furnaces, kilns and reactors and other processing units.
In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractories must also resist abrasion wears and erosion by chemical agents. Refractory materials must be strong at high temperatures, resistant to thermal shock, chemically inert, and have low thermal conductivities and coefficients of expansion. Refractories are more heat resistant than metals and are required for heating applications above1000F(538C).
Refractories are produced from natural and synthetic materials, usually nonmetallic, or combinations of compounds and minerals such as alumina, fireclays, bauxite, chromite, dolomite, magnesite, silicon carbide, zirconia, and others. Aluminium oxide (Al2O3), Magnesium oxide (MgO) and Silicon oxide (SiO2) are the most important refractory materials, though fireclay is widely used as well. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide is another refractory material. It is very strong at high temperatures, but will burn in the presence of oxygen, if the protective silica coating comes off.
Refractories must be chosen according to the temperature inside the unit and the chemical nature of the material being processed. For example, carbon cannot be used when it will be in contact with oxygen, as it will burn. Acidic refractories cannot be used in a basic environment and basic refractories cannot be used in acidic environment because they will be eroded.
Zircon, fireclay and silica are acidic. They are generally not attacked or affected by acidic materials, but easily affected by basic materials. At high temperatures, acidic refractories may also react with limes and basic oxides. Dolomite and magnesite are basic. These are used on areas where slags and atmosphere are basic; they are stable to alkaline materials but could react with acids. Alumina, chromite, silicon carbide, carbon and mullite are neutral. These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases.
Refractories are made in varying combinations and shapes depending on their applications. Refractories are widely used in the blast furnaces, hot stoves, and open-hearth furnaces, cement kilns, glass furnaces, nonferrous metallurgical furnaces, ceramic kilns, steam boilers, and paper plants. Special types of refractories are also used in rockets, jets, and nuclear power plants.
In addition to being resistant to thermal stress and other physical phenomena induced by heat, refractories must also resist abrasion wears and erosion by chemical agents. Refractory materials must be strong at high temperatures, resistant to thermal shock, chemically inert, and have low thermal conductivities and coefficients of expansion. Refractories are more heat resistant than metals and are required for heating applications above1000F(538C).
Refractories are produced from natural and synthetic materials, usually nonmetallic, or combinations of compounds and minerals such as alumina, fireclays, bauxite, chromite, dolomite, magnesite, silicon carbide, zirconia, and others. Aluminium oxide (Al2O3), Magnesium oxide (MgO) and Silicon oxide (SiO2) are the most important refractory materials, though fireclay is widely used as well. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide is another refractory material. It is very strong at high temperatures, but will burn in the presence of oxygen, if the protective silica coating comes off.
Refractories must be chosen according to the temperature inside the unit and the chemical nature of the material being processed. For example, carbon cannot be used when it will be in contact with oxygen, as it will burn. Acidic refractories cannot be used in a basic environment and basic refractories cannot be used in acidic environment because they will be eroded.
Zircon, fireclay and silica are acidic. They are generally not attacked or affected by acidic materials, but easily affected by basic materials. At high temperatures, acidic refractories may also react with limes and basic oxides. Dolomite and magnesite are basic. These are used on areas where slags and atmosphere are basic; they are stable to alkaline materials but could react with acids. Alumina, chromite, silicon carbide, carbon and mullite are neutral. These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases.
Refractories are made in varying combinations and shapes depending on their applications. Refractories are widely used in the blast furnaces, hot stoves, and open-hearth furnaces, cement kilns, glass furnaces, nonferrous metallurgical furnaces, ceramic kilns, steam boilers, and paper plants. Special types of refractories are also used in rockets, jets, and nuclear power plants.
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