Thermally stable mineral aggregates, a binder phase, and additives make up refractories, which are inorganic, nonmetallic, porous, and heterogeneous materials. The main raw materials used in the manufacture of refractories are silicon, aluminum, magnesium, calcium, and zirconium oxides. Carbides, nitrides, borides, silicates, and graphite are examples of non-oxide refractories.
Refractories are chosen based on the conditions they will be exposed to during their use. Some applications necessitate the use of specialized refractory materials. When a material must endure extremely high temperatures, zirconia is utilized. Silicon carbide and carbon are two other refractory materials that can withstand extremely high temperatures, but they cannot be employed in oxygen-rich environments because they oxidize and burn.
Refractories are heat-resistant materials that can withstand varying degrees of mechanical and thermal stress and strain, as well as corrosion/erosion from solids, liquids, and gases, gas diffusion, and mechanical abrasion at different temperatures. In layman’s terms, these are construction materials that can tolerate extreme temperatures.
Refractories are usually non-metallic inorganic materials with a refractoriness of more than 1500 degrees Celsius. They are coarse-grained ceramics with a microstructure consisting of big grains. The body is made up of coarse-grained grog and fine components. Refractory ceramics are distinguished from other types of ceramics by their coarse-grained structure, which is created of bigger grog particles connected by finer intermediate elements (bonding).
Refractories are non-metallic materials with chemical and physical qualities that allow them to be used in buildings or as components of systems that are subjected to temperatures beyond 538 ° C, according to ASTM C71.
At high temperatures, refractories must be chemically and physically stable. They must be resistant to thermal shock, chemically inert, and/or have specific ranges of thermal conductivity and coefficient of thermal expansion depending on the operating environment.
Because of their heat resistance and stability at high temperatures, refractories are utilized in high-temperature operations. Because they can endure physical wear, high temperatures, and chemical corrosion, they are commonly employed as linings for high-temperature furnaces and other processing units such as kilns, incinerators, and reactors. Crucibles are also made from them. They are widely used in the iron and steel industry.
The chemical, physical, mineralogical, and thermal features of refractories determine their quality. Refractory materials come in a variety of characteristics to satisfy the demands of various processes.
The general requirements for refractories are (i) the ability to withstand high temperatures and trap heat within a small area, such as a furnace; (ii) the ability to withstand sudden temperature changes; (iii) the ability to withstand load at service conditions; and (iv) the able to handle chemical and abrasive action of materials such as liquid metal, liquid slag, and hot gases, among others, direct contact with the refractories (v) ability to maintain sufficient dimensional stability at high temperatures and after/during repeated thermal cycling, (vi) ability to conserve heat, (vii) ability to sustain load and abrasive pressures, and (viii) low coefficient of thermal expansion.
Refractories are typically tailored to the application based on I process parameters such as temperature profile, mode of operation, and operating atmosphere, (ii) expected quality characteristics, and (iii) best engineering and application techniques, so that the final physical, chemical, and thermal properties are compatible.
Refractories have four basic functions: (i) acting as a thermal barrier between a hot medium (e.g., flue gases, liquid metal, liquid slags, and molten salts) and the containing vessel’s wall, (ii) providing strong physical protection by preventing wall erosion by the circulating hot medium, (iii) acting as a chemical protective barrier against corrosion, and (iv) acting as a thermal barrier between a hot medium.
Refractories are costly, and any failure causes a significant loss of production time, equipment, and, in certain cases, the product itself. Energy consumption and product quality are also affected by the type of refractories used.
Classification of refractories
Refractories are classified in a variety of ways based on (i) chemical composition, (ii) chemical properties of their constituent substances, (iii) place of use, (iv) refractoriness, (v) method of manufacture, (vi) physical form, (vii) applications, (viii) thermal conductivity, (ix) principal base features, or (x) compactness.
Physical properties testing
Physical properties are widely used to classify refractory materials.
1. Bulk density
It’s a measure of a product’s overall quality. The weight-to-volume ratio, expressed in g/cm3.
It is the ratio of the volume of its pores to the overall volume of the body. Porosity is an important refractory quality since it influences several other characteristics such as abrasion resistance and heat conductivity. A good refractory must have a low porosity in general. Here are some of the benefits and drawbacks of high porosity.
High porosity has a number of advantages.
(i) Thermal conductivity is poorer in highly porous refractories. This is because there are more air gaps in the material, which act as insulation and can thus be utilized to line furnaces.
(ii) High thermal shock and heat spalling resistance.
(iii) Pores operate as a crack inhibitor.
High porosity has a number of drawbacks.
(i) It weakens the body.
(ii) Reduces abrasion and corrosion resistance.
(iii)Lower load bearing ability
3. Compressive strength in the cold
This reveals how well a refractory material can sustain installation loads or resist the rigors of transportation, whereas the hot compressive strength indicates how well it will operate at high temperatures. A compression test is used to determine it. It is used to ensure that refractory brick does not crush when subjected to high pressures.
4. Flexural strength
Also referred to as “bend strength.” A specimen sitting on two edges is subjected to a concentrated centre load in this test. Flexural strength is determined by bending the specimen until it raptures.
5. Resistance to wear
The shock effect of gases and high-velocity abrasive raw solids flow over the refractory walls of furnaces and kilns. As a result, the refractory must be resistant to wear.
Testing of Thermochemical Properties
1. Melting point of the substance
Its melting point should be quite high.
2. Expansion of heat
It is the measurement of a material’s linear stability when subjected to high temperatures. Under the effect of temperature, all bodies undergo a reversible change in dimension. The surface of refractory material will flake if it expands more than it should.
3. Dimentional consistency
It is a refractory’s resistance to changing volume when exposed to high temperatures for an extended period of time.
If exposed to a large number of thermal cycles, temperature fluctuations can drastically degrade the strength of refractory surfaces, producing layer breakage or peeling. It’s crucial because most procedures use alternate heating and cooling methods. Thermal shock resistance is an important feature of a good refractory.
5. Thermal conductivity
When exposed to a high temperature, the ability to transmit heat from a hot face to a cool face. This is an important measurement for thermal insulation.
6. Equivalent of a pyro metric cone (PCE)
It is the measurement of heat work (the effect of temperature and time combined).
7. Corrosion resistance
Because they are in direct contact with hot gases and slag, the refractory material should be corrosion resistant.
Testing of Thermal Properties
Cold crushing strength – The capacity to endure the rigors of transportation and handling prior to the installation of refractories in the furnace is determined by the cold crushing strength. In conjunction with other parameters such as bulk density and porosity, it can be used as a good indicator of the adequacy of firing and abrasion resistance. The procedures provided in several standards are used to determine the cold crushing strength. In addition to the cold crushing strength, the hot crushing strength is sometimes computed to evaluate the behavior at service temperatures.
Abrasion resistance – Refractory mechanical stress is created not only by pressure, but also by the abrasive action of solid charge materials as they pass over the brickwork inside the furnace. The impingement of fast moving vapors filled with small solid dust particles can also cause mechanical stress. The abrasive stress is well approximated by Bohme’s grinding machine, but the results are rarely applicable to the conditions found in furnaces operating at high temperatures, especially when the refractory brick strength is changing owing to chemical factors. There is currently no authorized method for assessing abrasion resistance, and the abrasion factor established by Bohme is still used as a reference value.
Deformation modulus and cold modulus of rupture – Thermal stress causes strain conditions in the refractory brickwork, which can lead to rupture and crack formation due to changing physical-chemical conditions caused by infiltrations. The resistance to deformation under bending stress, or rupture strength, is measured to determine the size of the rupture stress. Refractory bricks are purely elastic within a restricted deformation area due to their heterogeneous coarse ceramic structure. The modulus of elasticity is defined as the ratio of stress of deformation inside this purely elastic initial region, where the deformation is still reversible. The frequency of ultrasonic or resonance waves is commonly used to measure this dynamically. With the deformations on the less resistant fines content, progressive deformation up to rupture is outside the completely elastic area, and as a result, the stress grows more slowly relative to the deformation. The modulus of elasticity is no longer adequate for calculating the stress that happens. As a result, the modulus of deformation has been defined as the ratio of rupture stress to rupture deformation. In the case of refractories, this modulus is less than or equal to the elasticity modulus. On a test bar resting on two bearing edges and applying a force to the centre of the bar, the modulus of deformation in cold state is determined, along with the modulus of rupture.
Porosity and density – A refractory brick with a low porosity improves the mechanical strength and other qualities of the refractory. True porosity of a refractory brick is defined as the ratio of a body’s total pore space (which includes both open and closed pores) to its volume, expressed in volume percent. True porosity is calculated using the following formula.
True porosity is defined as (S- R)/S X 100 volume percent, with S denoting density and R denoting bulk density.
The density is calculated using finely crushed material and is defined as the quotient of mass and volume excluding pore space. The values of apparent porosity (open porosity) are frequently employed as the application property instead of genuine porosity values. Only those holes that can be infiltrated by water are included in the apparent porosity, not closed holes.
High porosity materials tend to be highly insulating as a result of high volume of air they trap, since air is a very poor thermal conductor. As a result, low porosity materials are generally used in hotter zones, while the more porous materials are usually used for thermal backup. Such materials, however, do not work with higher temperatures and direct flame impingement, and are likely to shrink when subjected to such conditions. Refractory materials with high porosity are usually not chosen when they are to be in contact with liquid slag since they can be penetrated as easily.
The bulk density is generally considered in conjunction with apparent porosity. It is a measure of the weight of a given volume of the refractory. For many refractories, the bulk density provides a general indication of the product quality. It is considered that the refractory with higher bulk density (low porosity) is better in quality. This is because an increase in bulk density increases the volume stability, the heat capacity, as well as the resistance to abrasion and slag penetration.
It is a measurement of the weight of refractory material in a specific volume. The bulk density of many refractories serves as a general indicator of product quality. Refractory with a higher bulk density (low porosity) is thought to be of superior grade. This is because an increase in bulk density improves volume stability, heat capacity, and abrasion and slag penetration resistance.
Thermal stress properties
Pyrometric cone equivalent – Refractories melt gradually over a wide temperature range due to their chemical complexity. One of the most essential aspects of refractories is their refractoriness. Because refractories are rarely made up of a single compound, they are described in terms of a softening zone rather than a melting point. This is assessed using comparison ceramic samples with established softening behavior, sometimes referred to as “Seger cones.” The refractory bricks that will be examined are chopped into pyramids. Various standards specify the testing procedure. The pyrometric cone equivalent is the equivalent standard cone that melts to the same amount as the test cone (PCE). Because refractory cone values are based on a standard time-temperature relationship, differing heating rates result in variable PCE values.
Refractoriness under load – When hot and liquid materials are enclosed while being transported and/or processed, refractoriness refers to the resistance to severe heat (temperatures more than 1000 degrees Celsius) and corrosion. Refractoriness is a measurement of a material’s capacity to resist greater temperatures without deforming significantly. The refractoriness under load tests are used to assess the softening behavior of burned refractory bricks as the temperature rises and the load remains constant. The softening behavior under load is determined by the quantity and degree of dispersion of low melting point flux agents, and is not similar to the melting range of pure raw materials.
The temperature at which the refractory bricks collapse in service settings with similar load is determined by the refractoriness under load test. However, in actual use, where the bricks are only heated on one face, the somewhat cooler rigid portion of the bricks carries the majority of the load. As a result, the refractoriness under load test only provides an index of refractory quality, not a figure that can be used in refractory design. The test data is extremely significant under service conditions, where the refractory is heated from all sides, such as checkers, partition walls, and so on.
Refractoriness under load (differential) – To eliminate errors caused by the test equipment’s inherent expansion when testing refractoriness under load, and to allow tests to be conducted in an oxidizing atmosphere, a new method for determining resistance at rising temperature and constant load, known as refractoriness under load (diff), has been developed. In this method same type of samples are used as for the refractoriness under load test bur they have an internal bore to permit rods to be fitted to the upper and the lower sides. With this method, temperature values are obtained by differential measurements in an oxidizing atmosphere and these values are considerably lower than the refractoriness under load values.
Refractory materials must maintain dimensional stability under severe temperatures (including frequent thermal cycling) and continual corrosion from extremely hot liquids and gases. A long-term test termed creep under load can be used to determine the thermal expansion under load (creep) of refractory bricks that are heated equally on all sides over a lengthy period of time during service. It is a time-dependent feature that specifies how much a material under stress deforms in a given amount of time and at a specific temperature. The test is carried out according to the various standards. After reaching the requisite test temperature, the sample of 50 mm diameter and 50 mm height with an internal bore for the measurement rod is heated at a consistent speed and under a given load (usually 0.2 N/sq mm) and kept for 10 to 50 hours under constant load. At a specific test temperature, the compression of the sample after maximum expansion is presented in relation to the test time as a measure of creep.
The resistance to bending stress at high temperatures is known as the hot modulus of rupture. The deformation behavior of refractory products at high temperatures is determined by their resistance to bending stress. The test samples are bars which are heated in electric chamber kilns. For the test, the bars are placed on bearing edges of the kiln and are stressed until fracture occurs at the test temperature by applying an increasing load to the center of the bar.
Thermal expansion – All materials experience a change in volume under the influence of temperature. The contraction or expansion of the refractories can take place during service. Such permanent changes in dimensions may be due to (i) the changes in the allotropic forms which cause a change in specific gravity, (ii) a chemical reaction which produces a new material of altered specific gravity, (iii) the formation of liquid phase, (iv) sintering reactions, and (v) may happen on account of fluxing with dust and slag or by the action of alkalis on fireclay refractories, to form alkali-alumina silicates, causing expansion and disruption. The reversible linear expansion curve of most of the refractory bricks is more or less straight although the absolute amount varies considerably. Silica bricks, however, have an irregular and strong thermal expansion in the temperature range of up to 700 deg C. By changes in structure or in firing methods of refractory bricks, the expansion curve can be influenced within certain limits. Bricks with high expansion are very susceptible to thermal shock. Thermal expansion is important in service, as the effects of expansion are to be taken into account during the installation of refractory lining. If not done, then edge pressure and premature spalling of the bricks take place.
Reheat change (after shrinkage and after expansion) – After heating to a high temperature and then cooling, a permanent change in dimension (permanent linear change) is frequently observed, which is referred to as after expansion or after shrinkage. When a refractory brick shrinks significantly, the joints expand, and the brickwork loosens and no longer holds together. After expansion, on the other hand, is dangerous since it might cause the brickwork to be destroyed due to pressure. The refractories’ permanent linear change (PLC) can be altered. The burning of the raw materials and the firing of the bricks must be managed in such a way that equilibrium is achieved at the correct temperature in order to obtain a brick with a constant volume.
Thermal shock resistance – One of the most significant service features is thermal shock resistance. It describes how refractories react to abrupt temperature changes, which occur often during furnace operation. Temperature changes can significantly impair the strength of the brick construction and cause disintegration or spalling in the layers. The thermal shock resistance can be tested using one of two ways. They are (i) water quenching method, and (ii) air quenching method. In the water quenching method, the test piece is a standard cylinder which is heated to 950 deg C and then quenched in running cold water. The test is repeated until the sample is destroyed, but no more than 30 times. The thermal shock resistance is determined by the number of quenching required until the material is destroyed. For bricks that are prone to hydration, the air quenching method is utilized. The technique is the same in this manner, except that instead of running water, compressed air is used to quench the water. With increased firing levels, refractories’ thermal shock resistance often declines. The most beneficial refractories in practice are those that preserve their quenching resistance even after higher firing temperatures or service temperatures.