Introduction to Refractories
Introduction to Refractories
Refractories are inorganic, non-metallic, porous, and heterogeneous materials which are composed of thermally stable mineral aggregates, a binder phase, and additives. The principal raw materials used in the production of refractories are the oxides of silicon, aluminum, magnesium, calcium, and zirconium. There are some non-oxide refractories like carbides, nitrides, borides, silicates and graphite. Refractories are heat resistant materials used in almost all processes involving high temperatures and / or corrosive environment. Refractories insulate and protect industrial furnaces and vessels due to their excellent resistance to heat, chemical attack, and mechanical damage.
Refractories are defined in 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 538 deg C.
Refractories are essential lining materials for working interfaces and backup zone of the furnaces throughout the manufacturing of iron and steel, in specific sequential and consecutive operation of forming, holding, mixing, transporting hot metal, liquid steel, and slag. Despite refractory-metal direct interactions, refractories have to successively experience high temperature and corrosive environment through flues, shaft, ducts, and stack.
Refractories have the ability to retain its physical shape and chemical identity when subjected to high temperatures and are used in applications which need extreme resistance to heat. Refractories are mechanically strong and heat resistant to withstand rapid temperature change and corrosion and erosion by liquid metal, glass, slag, and hot gas.
Refractories perform four basic functions namely (i) act as a thermal barrier between a hot medium (e.g. flue gases, liquid metal, liquid slags, and molten salts) and the wall of the containing vessel, (ii) represent a chemical protective barrier against corrosion, (iii) ensure a strong physical protection, preventing the erosion of the walls by the circulating hot medium, and (iv) act as thermal insulation for ensuring heat retention.
A failure of refractory can result in a loss of production time, equipment, and sometimes the product itself. The various types of refractories also influence the safe operation, energy consumption, and product quality and hence use of refractories best suited to each application is of supreme importance.
Refractories are used by the metallurgical industry in the internal linings of the furnaces, kilns, ovens, furnaces, reactors, and other vessels for holding and transporting of liquid metal and liquid slag. In non-metallurgical industries, the refractories are mostly installed on fired heaters, hydrogen reformers, ammonia primary and secondary reformers, cracking furnaces, utility boilers, catalytic cracking units, coke calciner, sulphur furnaces, air heaters, ducting, stacks, and incinerators etc. Majority of these listed equipment operate under high pressure, and operating temperature can vary from very low to very high (around 500 deg C to 1,650 deg C). The refractory materials are hence required to withstand temperatures above these temperatures.
Refractories are a category of technical ceramics. Industrial refractories are almost all complex combinations of high-melting crystalline oxides, plus a few carbides, carbon, and graphite. Poly-crystalline ceramics are characteristically brittle, and far less strong in tension than in compression. They are subject to considerable variability in strength, resulting from local variations in their micro-structure and their lack of ductility. They display high temperature creep or plastic deformation, but almost always by a mechanism different from that operating in metals. Their mechanisms of conduction of heat and electricity are also different from those of metals. Their elastic moduli are normally quite high which along with their brittleness make them somewhat prone to failure under thermal stress and shock. Compositions can be chosen having selectively high chemical resistance (but rarely immunity) to oxidizing or reducing agents, acids or bases, metals or salts, whether liquid or gaseous.
Shaped refractory objects are typically made by particulate forming processes. Their thermal maturing or sintering produces a wide variety of phase compositions and micro-structural features. Most fired industrial refractories have experienced ‘liquid-phase’ sintering and because of this and other reasons they contain porosity. Both the lower-melting inter-crystalline phases and the porosity influence the mechanical and thermal properties, and both have much to do with the penetration by corrosive media. Only a few refractories are made essentially pore-free and without selective phase melting. The variety of phase compositions and structures needed in refractory production is hence greater than in metals and in some ways different in kind.
The manufacturing methods for refractories are more varied, and mostly different in kind. The fundamental connections made between structure and properties are in some ways unique to non-metallic materials. Further, there is an intense concern with the preparation and characterization of particulate starting materials, of pivotal importance in ceramic synthesis but infrequently of any interest at all in metal manufacture. These starting materials are frequently minerals, as opposed to high-purity synthetic chemicals.
Thus the technology of refractory materials and their properties alone is itself a multi-disciplinary matter. Refractory materials are best comprehended on a foundation of ceramic science and engineering or of materials science and engineering. Specifically, it relates mostly to the matters associated with (i) the inorganic chemistry, kinetics, and thermodynamics, (ii) solutions and phase equilibrium, (iii) the rudiments of non-metallic crystalline and vitreous structure, (iv) the thermo-mechanical properties of solids including fundamentals of elasticity, plastic flow, and fracture, and (v) concepts of heat, heat transport, and electrical transport, and (vi) ceramic synthesis.
Refractory materials, by definition, are supposed to be resistant to heat and are exposed to different degrees of mechanical stress and strain, thermal stress and strain, corrosion / erosion from solids, liquids, and gases, gas diffusion, and mechanical abrasion at various temperatures. Different refractories are designed and produced so that their properties are appropriate for the intended applications. In most cases, refractory properties can be predicted from the results of appropriate tests while for some of the refractory characteristics where direct tests correlating the properties are not available, knowledge and experience predict the behaviour of refractories during their use. The testing of refractory properties can, in most cases, indicate the performance of a refractory in actual application. Hence, appropriate testing of refractories is of great importance for predicting properties, closely simulative to their applications.
Refractories are classified in the different ways. The different types of classification of refractories are described below. Fig 1 shows classification of refractories.
Fig 1 Classification of refractories
Classification based on chemical composition
Refractories are classified on the basis of their chemical behaviour into following three classes namely acid refractories, basic refractories, and neutral refractories.
Acid refractories are those refractories which are attacked by alkalis or basic slags. These are used in acidic environment or where slags are acidic. Example of these refractories are silica and zirconia.
Basic refractories are attacked by acid slags but stable to alkaline slag, dust, and fumes at the elevated temperatures. These refractories are used in alkaline environments. Example of these refractories are magnesia, dolomite, and chromite.
Neutral refractories are chemically stable to both acids and bases and used in the areas where slag and environment are either acidic or basic. Examples are carbon graphite, chromites and alumina. Graphite is the least reactive and is extensively used in the furnaces where the process of oxidation can be controlled.
Classification based on physical form
Refractories are classified according to their physical form in the two types namely (i) shaped refractories, and (ii) unshaped refractories.
Shaped refractories are normally known as refractory bricks. These refractories have fixed shape. The refractories bricks can be of standard shape or of special shape. Standard shaped bricks have dimensions which are conformed to by most of the refractory manufacturers and are normally applicable to kilns and furnaces of the same type. On the other hand, special shaped refractories are specifically made for particular furnace. Special refractory shapes of a furnace are normally not applicable to another furnace of the same type. Standard shaped refractories are always machine pressed and hence have uniformity of properties. Special shapes are generally hand moulded and are normally associated with slight variation in properties. For shaped refractories, attaining of the maximum density after the shapes are formed is the main objective of the process.
Unshaped refractories are without definite form and are only given shape during their installation and heating. They form joint-less lining and are also known as monolithic refractories. These refractories are further categorized as plastic refractories, ramming mixes, castable and pumpable refractories, gunning mixes, fettling mixes, and mortars.
The physical characteristics of plastics and ramming mixes are their ease of ramming to consolidate to proper density. For mortars and coatings, they are required to have appropriate consistency to be used as desired for the specific applications. For gunning mixes, the material is required to have good adhesion to the gunned surface, low rebound, and appropriate properties as designed. For castables and pumpable refractories, the primary characteristics are particle size distribution, which in effect controls the flowability. The proper addition of minor ingredients in the castable and pumpable refractories controls the workability, setting, and strength development upon curing and heating, and the bonding system, which dictates the high temperature properties.
With the development of vibratable and pumpable castables, methods have been standardized for measuring the flowability of a castable, which can predict the flow characteristics. The test consists of filling up a cone with the castable refractory and then letting it flow under vibration for a specified time. The pre-determined flowability characteristics define the use of the castable refractory in actual applications.
Classification based on method of manufacture
Refractories can be classified by the method of their manufacture. The refractories based on the method of manufacture are (i) dry pressed refractories, (ii) fused cast refractories, (iii) hand moulded refractories, (iv) formed (normal, fired, or chemical bonded) refractories, and (v) unformed (monolithic, plastics, ramming masses, gunning, castables, and spray masses) refractories.
Classification according to refractoriness
Refractories are classified into four types according to their refractoriness. The first type is the low heat duty refractories. These refractories have refractoriness in the range of 1,520 deg C to 1,630 deg C and have pyrometric cone equivalent (PCE) value in the range of 19 to 28. Example of these refractories is silica bricks. The second type is the intermediate heat duty refractories. These refractories have refractoriness in the range of 1,630 deg C to 1,670 deg C and have PCE value in the range of 28 to 30. Example of these refractories is fire clay bricks.
The third type is high heat duty refractories. These refractories have refractoriness in the range of 1,670 deg C to 1,730 deg C and have PCE value in the range of 30 to 33. Example of these refractories is chromite bricks. The fourth type is super heat duty refractories. These refractories have refractoriness higher than 1,730 deg C and have PCE value greater than 33. Example of these refractories is magnesite bricks.
Classification based on oxide content
As per this classification, refractories are classified as (i) single oxide refractories such as alumina, magnesia, and zirconia, (ii) mixed oxide refractories such as spinel and mullite and (iii) non oxide refractories such as borides, carbides and silicates.
Classification based on density of refractory
Under this classification, refractories are classified either as dense refractories or insulating refractories. The majority of the high temperature refractories have high density and these high density bricks have resistance to slags of different chemical compositions, fumes, dust, and gases. On the other hand insulating refractories are of low densities and provide insulating properties besides offering resistance to corrosion and chemical reactions with the operating environment.
Important refractories used in iron and steel industry
Some of the important refractories which are used in the iron and steel industry are given in Fig 2 and described below.
Fig 2 Important refractories used in the iron and steel industry
Fireclay refractories – Fireclay refractories are essentially hydrated aluminum silicates with minor proportion of other minerals. These refractories have maximum share of the production of refractories on a volume basis.
Fireclay refractory bricks are manufactured from unfired refractory bond clay and fireclays (chamotte), fired refractory clay, or similar grog materials. Fireclay refractory bricks have two main components namely 18 % to 44 % of alumina (Al2O3) and 50 % to 80 % of silica (SiO2). Fireclay refractories are least costly and are used extensively.
The variety of clays and manufacturing techniques allows the production of numerous brick types appropriate to particular applications. The usefulness of fireclay refractory bricks is largely because of the presence of mineral mullite, which forms during firing and is characterized by high refractoriness and low thermal expansion.
ASTM subdivides fireclay refractories into four major classifications depending primarily upon fusion temperature (PCE). Four standard classes of fireclay refractories are super duty, high duty, medium duty and low duty. These classes cover the range from around 18 % alumina to 44 % alumina and from about 50 % silica to 80 % silica.
The softening behaviour of fireclay refractory bricks is determined by the amount and the composition of the glassy phase. Due to the alkali content and the presence of other impurities, this phase starts to soften at 1,000 deg C and it imparts a high softening interval to the fireclay bricks because of its high viscosity. The softening behaviour of fireclay bricks are determined by testing the bricks for refractoriness under load (RUL), thermal expansion under load (creep), and hot crushing strength.
Characteristically, fireclay bricks begin to soften far below their fusion temperature and under load actual deformation take place. The amount of deformation depends upon the load and once started this deformation is slow but continuous process unless the load or the temperature is reduced. Due to this reason fire clay bricks are not being used in wide sprung arches in furnaces operating continuously at high temperatures.
The application of fireclay bricks is influenced by several other properties in addition to the refractoriness. These properties are dimensional accuracy, crushing strength, porosity, and refractoriness under load. Machine pressed, fired fireclay refractories are used for many applications. The stress on the materials differs widely. For special applications, it is customary to manufacture bricks which are tailored to meet specific requirements. Fireclay refractory bricks are used in iron and steel industry in coke oven batteries, blast furnace, hot blast stoves, and various other furnaces used in the industry.
Alumina refractories – Alumina refractories contain alumina (Al2O3) which is one of the most chemically stable oxides. These refractories are the part of alumina- silica group of refractories and belong to the SiO2 -Al2O3 phase equilibrium system. These refractories differ from fireclay refractories in term of Al2O3 content and normally have Al2O3 content of more than 45 %. The raw material base for these refractories is different than the fireclay bricks.
Alumina refractories are specified by the amount of alumina in it. The 50 %, 60 %, 70 % and 80 % classes contain their respective alumina content with an allowable range of +/- 2.5 %. Alumina bricks with 72 % alumina and 28 % silica are known as mullite bricks. These bricks have excellent volume stability and strength at high temperatures. Alumina refractories with 99 % alumina are called corundum refractories. These refractories contain single phase poly-crystalline and alpha alumina.
Alumina offers excellent hardness, strength, and spalling resistance. It is insoluble in water, super-heated steam, and in most inorganic acids and alkalis. Alumina refractories carry all purpose characteristics of fireclay refractories into higher temperature ranges which make these refractories suitable for lining furnaces upto 1,850 deg C. Alumina refractories have high resistance in oxidizing and reducing atmospheres. With increase in alumina content, the refractoriness of the high alumina refractories increases.
Silica refractories – Silica refractories are those refractories which contain at least 93 % SiO2. These refractories have second highest share of the production of refractories on a volume basis. Silica refractories have the outstanding property of excellent mechanical strength at temperatures approaching their actual fusion point. This property of silica refractories contrast that of many other refractories which begins to fuse and creep at temperatures considerably lower than their fusion points.
Silica refractory is the most abundant refractory used in the construction of a coke oven battery. Silica is the refractory of choice primarily because, at the normal operating temperatures of the coke oven battery, silica refractories are subject to minimal creep.
The major drawback of silica refractories is that they are susceptible to spalling at temperatures below 650 deg C. Temperature fluctuations above 650 deg C donot affect silica refractory adversely and in this range it is classed as a good spalling resistant refractory. Silica refractories need special precaution during heating and cooling since it undergoes phase changes. Silica refractories are not of practical use if the furnace is to cool down to room temperature frequently.
Magnesite refractories – Magnesite refractories are chemically basic refractories containing at least 85 % magnesium oxide. These refractories are one of the most widely used basic refractory bricks. They are manufactured either from natural occurring magnesite or sea water magnesia. Magnesite bricks are made from dead burnt magnesite. These bricks are strong and extremely durable. Their main advantage is very high slag resistance to basic slags specially lime and iron rich slags which is very important for steelmaking processes.
Magnesite refractories have the properties of bearing high temperature, high refractoriness under load, and low vulnerability to attack by iron oxide and alkalis. Magnesite refractory bricks are widely used in the basic zone of metallurgical furnaces. These refractories have spalling resistance, strong abrasion and corrosion resistance, and high cold crushing strength. Physical properties of magnesite refractories are relatively poor. Magnesite bricks cannot resist thermal stock, loose strength at high temperature, and are not resistant to abrasion.
The refractoriness of a magnesite brick is frequently determined by the amount and the type of impurity within the grain. The refractoriness of the dead burnt magnesite is improved by lowering the amount of impurities or adjusting the chemistry of the impurities or both.
Magnesite refractories are employed as sub-hearth bricks for electric arc furnaces and sometimes as back up lining for basic oxygen converters. They are also used as slide gate refractories or in pouring nozzles. They are frequently impregnated with pitch. For steelmaking furnaces especially BOF (basic oxygen furnace) normally carbon is added to magnesia to produce magnesia carbon refractories. Magnesia carbon refractories have better resistance to highly basic slags at high temperatures.
One of the more important types of magnesite bricks is those which have low boron oxide contents and di-calcium silicates bonds. These chemical features give the bricks excellent refractoriness, hot strength, and resistance to load at elevated temperatures. Magnesite bricks containing higher content of boron oxide have improved hydration resistance.
Dolomite refractories – These are basic refractories made from dead burnt dolomite. Dolomite (CaCO3+MgCO3) when dead burnt by high temperature firing produce CaO+MgO. For dolomite refractories, CaO+MgO content of greater than 97 % is desirable. This percentage of CaO+MgO is normally obtained from high purity dolomite. Dolomite refractories have very good resistance to thermal shock and alkali attack. These refractories with zirconia enrichment are used for crack arresting.
Dolomite bricks have one major disadvantage which is their tendency to disintegrate when stored for a short period of time due to reaction of free lime with moisture in the air. Dolomite bricks have a good balance between low cost and good refractoriness for certain uses. They also offer good metallurgical characteristics for certain clean steel applications.
Dolomite bricks are made both in dead burnt and carbon bonded compositions. The carbon bonded varieties include both pitch and resin bonded versions. Some of the carbon bonded dolomite bricks contain flake graphite and are analogous to magnesite carbon bricks.
Chromite refractories – In these refractories along with chromite, magnesite is present. There is difference between chrome magnesite and magnesite chrome refractories. While chrome-magnesite refractories normally contain 15 % to 35 % Cr2O3 and 42 % to 50 % MgO, magnesite-chrome refractories contain at least 60 % MgO and 8 % to 18 % Cr2O3.
The reaction between chrome ore and magnesite outline the fundamental chemistry of the magnesite chrome bricks. Magnesite chrome bricks can be either silicate bonded or direct bonded. Silicate bonded bricks have a thin film of silicate minerals which surrounds and bonds together the magnesite and chrome ore particles. Direct bonded bricks have the direct attachment of the magnesia to the chrome ore without intervening films of silicate. Direct bonding is obtained by combining high purity chrome ore and magnesite and firing them at extremely high temperatures.
Direct bonded bricks have high strength at elevated temperatures, better slag resistance, and better resistance to peel spalling than silicate bonded bricks. The balance of properties of the bricks is a function of the magnesite to chrome ratio.
Chrome magnesite refractories are used for building the critical paths of the high temperature furnaces. These refractories can withstand corrosive slags and gases and have high refractoriness. Magnesite refractories are suitable for service at the highest temperatures an in contact with more basic slags. These refractories has better spalling resistance than chrome magnesite refractories.
Carbon refractories – In these refractories, the principle component is carbon. These refractories are characterized by a high refractoriness, high thermal conductivity, and high chemical resistance but are highly susceptible to oxidation. Because of low interfacial tension between carbon and slag melts, there is little slag infiltration. Carbon refractories are extremely resistant to thermal shock because of high thermal conductivity and low thermal expansion. Carbon refractories are susceptible to attack by oxygen, steam, and CO2 in an oxidizing atmosphere above 400 deg C.
Carbon refractories behave differently than the typical ceramic refractories, primarily because carbon refractories are conductive rather than insulating. All carbon refractory lining systems perform as a ‘conductive cooling system’ as opposed to a classic definition of a refractory lining which is typically an ‘insulating system’. Consequently, proper cooling is always to be utilized with the carbon refractory lining system to assist in maintaining refractory temperatures which are to be below the critical chemical attack temperature for mechanisms such as oxidation, alkali, CO (carbon mono-oxide) degradation, or dissolution of the carbon by liquid metal. The words ‘carbon’ and ‘graphite’ are frequently used inter-changeably, but the two are not synonymous. Additionally, the words ‘semi-graphite’ and ‘semi-graphitic’ are also similarly misused.
Zirconia refractories – Zirconia refractories contain zirconium dioxide (ZrO2) which is a polymorphic material. It has certain difficulties in its usage and fabrication as a refractoy material and hence it is stabilized by incorporating small quantities of calcium, magnesium, and cerium oxides. The properties of zirconia refractories are dependent on the degree of stabilization and quantity of stabilizer as well as the quantity of original raw material.
Zirconia refractories have a very high strength at room temperature which is maintained upto temperatures as high as 1,480 deg C. They are used for high temperature applications. Since the thermal conductivity of zirconium dioxide is much lower than other refractory materials, it is, hence, used as a high temperature insulating refractory. Zirconia refractories have very low thermal losses and does not reacts with liquid metals and hence it is useful making refractory crucibles.
Zirconia refractory bricks are produced from natural zircon. Their main advantages are good slagging resistance, high load softening temperature, high performance to wear, and good thermal shock stability. These bricks are mainly used for metallurgical kilns, melting pools of glass furnaces, regenerative chambers of glass furnaces, and solution erosion area in chemical industry furnaces.
Zirconia refractories are useful as high temperature construction materials for furnaces and kilns because of their very high strength at room temperature. The thermal conductivity of zirconia refractories is much lower than most of other refractories, because of that these refractories are used as high temperature insulating refractories.
Zircon tundish nozzles are produced with 65 % to 96 % of zirconia content and are used for controlling the flow of steel from the tundish to the continuous casting mould. Zircon refractory nozzles have the advantages of high refractoriness, good thermal shock resistance, erosion resistance and scouring resistance, small hole diameter change, and long service life.
Monolithic refractories – Monolithic refractories are special mixes or blends of dry granular or cohesive plastic materials used to form virtually joint free linings. They are unshaped refractory products which are installed as some form of suspension that ultimately harden to form a solid mass. Most monolithic formulations consist of large refractory particulates (an aggregate), fine filler materials (which fill the inter particle voids) and a binder phase (that gels the particulates together in the green state). Types of these refractories are castable refractories, insulating castables, plastic refractories, ramming mixes, patching refractories, coating refractories, mortars, gunning and fettling mixes etc.
Monolithic refractories represent a wide range of mineral compositions and vary greatly in their physical and chemical properties. Some are low in refractoriness while others approach high purity brick compositions in their ability to withstand severe environments. Various means are employed in the placement of monolithic refractories like ramming, casting, gunniting, spraying, and sand slinging etc
The advantages of monolithic refractories are (i) elimination of joints, (ii) faster application, (iii) heat saving, (iv) better spalling resistance, (v) volume stability, (vi) easy to transport, handle and install, and (vii) reduced downtime for repairs.
Insulating refractories – These are high porosity refractories with low thermal conductivity used to reduce the rate of heat flow and thus reduce heat losses by maximizing heat conservation within the furnace. These refractories are lighter with low densities.
Insulating refractory brick is the term used for heat insulating bricks and covers those heat insulating materials which are applied upto 1,000 deg C. These bricks are assigned to the group of light-weight refractory bricks and are manufactured on the basis of naturally occurring light-weight raw materials.
Insulating refractory brick is a class of brick, which consists of highly porous fireclay or kaolin. The brick is lightweight, low in thermal conductivity, and yet sufficiently resistant to temperature to be used successfully on the hot side of the furnace wall, thus permitting thin walls of low thermal conductivity and low heat content. The low heat content is particularly important in saving fuel and time on heating up, allows rapid changes in temperature to be made, and permits rapid cooling.
Insulating refractory brick is characterized by the presence of large amount of porosity in it. The pores are mostly closed pores. The presence of porosity decreases the thermal conductivity of the insulating brick. The three basic types of insulating refractories are (i) very thin low density fibres made from organic or inorganic materials, (ii) cellular material in closed or open cell made of organic or inorganic material, and (iii) flaked or granular inorganic materials bonded in the desired form.
Properties of refractories
Refractories are those materials which have high melting points and have properties which make them suitable to act as heat resisting barriers between high and low temperature zones.
Refractories are inorganic, nonmetallic, porous, and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. The general requirements of refractories include (i) ability to withstand high temperatures and trap heat within a limited area such as a furnace, (ii) ability to withstand action of liquid metal, hot gasses, and liquid slag by resisting erosion and corrosion etc. (iii) ability to withstand load at service environment, (iv) ability to resist contamination of the material with which it comes into contact, (v) ability to maintain necessary dimensional stability at high temperatures and after/during repeated thermal cycling, and (vi) ability to conserve heat.
Properties of the refractories can be classified to resist four types of service stresses namely (i) chemical, (ii) mechanical, (iii) thermal, and (iv) thermo-technical. A suitable selection of the refractories for the furnace lining can only be made with an accurate knowledge of the refractory properties and the stresses on the refractories during service.
Important properties of refractories are chemical composition, bulk density, apparent porosity, apparent specific gravity, and strength at atmospheric temperatures. These properties are frequently among those which are used as ‘control points’ in the manufacturing and quality control process. Some of the important properties of refractories are described below.
Melting point – Melting temperatures (melting points) specify the ability of materials to withstand high temperatures without chemical change and physical destruction. The melting point of few elements which constitute refractory composition in the pure state varies from 1,700 deg C to 3,500 deg C. The melting point serves as a sufficient basis for considering the thermal stability of refractory mixtures and is an important characteristic indicating the maximum temperature of use. The melting point of the refractory is the temperature at which a test pyramid (cone) fails to support its own weight.
Size and dimensional stability – The size and shape of the refractories is an important feature in design since it affects the stability of any structure. Dimensional accuracy and size is extremely important to enable proper fitting of the refractory shape and to minimize the thickness and joints in construction.
Porosity and density – Low porosity of the refractory brick is desirable since it improves the mechanical strength and other properties of the refractories. True porosity of a refractory brick is the ratio of the total pore space (i.e. open and closed pores) of a body to its volume and is expressed in volume percent. The formula for true porosity is ‘True porosity = (S- R)/S X 100 volume %, where S is the density and R is the bulk density’.
The density is the quotient of mass and volume excluding pore space and is determined on finely crushed material. Frequently, not the values of the true porosity but those of apparent porosity (open porosity) are used as the application property. The apparent porosity includes only those holes which can be infiltrated by water and not the closed holes.
Bulk density – In order to know the stored heat, it is necessary to know the bulk density of the refractories. The term bulk density describes the measure of mass and volume including the pore space. Bulk density is normally 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 normally considered that refractories with higher bulk density (low porosity) are better in quality. An increase in bulk density increases the volume stability, the heat capacity, as well as the resistance to abrasion, and slag penetration. Bulk density is by far one of the most important characteristics and serves, together with the density, to calculate the true porosity and, together with water absorption, to calculate the apparent porosity.
Cold crushing strength – The cold crushing strength determines the ability to withstand the rigorous of transport and handling before the installation of refractories in the furnace. It only has an indirect relevance to refractory performance. It can be seen as a useful indicator to the adequacy of firing and abrasion resistance in consonance with other properties such as bulk density and porosity. The cold crushing strength is determined by the methods described in various standards. In order to evaluate the behaviour at the service temperatures, the hot crushing strength is also sometimes determined in addition to the cold crushing strength.
Pyrometric cone equivalent – Refractories due to their chemical complexity melt progressively over a range of temperature. The refractoriness of the refractories is one of the most important properties of the refractory. The ‘refractoriness’ of refractory bricks is the temperature at which the refractory bends because it can no longer support its own weight. As the refractories hardly ever consist of a single compound, reference is made, not to a specific melting point, but to a softening region. This is determined with the help of comparative ceramic samples.
Pyrometric cones also known as ‘Seger cones’ are used in ceramic industries to test the refractoriness of the refractory bricks. They consist of a mixture of oxides which are known to melt at a specific narrow temperature range. Cones with different oxide composition are placed in sequence of their melting temperature alongside a row of refractory bricks in a furnace. The furnace is fired and the temperature rises. One cone will bends together with the refractory brick. This is the temperature range in deg C above which the refractory cannot be used. This is known as the pyrometric cone equivalent (PCE). The PCE values reported for refractories are based on a defined standard time-temperature relationship, so different heating rates result in different PCE values.
Thermal expansion under load (creep) – Refractory materials are required to maintain dimensional stability under extreme temperatures (including repeated thermal cycling) and constant corrosion from very hot liquids and gases. The thermal expansion under load (creep) of refractory bricks, which are heated evenly on all sides over a long period during service, can be tested by a long term test called creep under load. It is a time dependent property which determines the deformation in a given time and at a given temperature by a material under stress.
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 can 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) can 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 upto 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 high temperature and subsequent cooling, a permanent change in the dimension (permanent linear change) frequently occurs which is described as after expansion or after shrinkage. If a refractory brick has very strong after shrinkage then the joints get enlarged and the brickwork is loosened and no longer tight. In opposite case, after expansion is also dangerous since this can cause the destruction of the brickwork through pressure. The permanent linear change (PLC) of the refractories can be influenced. For obtaining a brick with a constant volume, the burning of the raw materials and the firing of the bricks is to be controlled in such a manner so that equilibrium is achieved at the desired temperature.
Reversible thermal expansion – Refractories like any material expand when heated, and contracts when cooled. The reversible thermal expansion is a reflection on the phase transformations which occur during heating and cooling.
Thermal conductivity – Thermal conductivity is defined as the quantity of heat which flows through a unit area in direction normal to the surface area in a defined time with a known temperature gradient under steady state conditions. It indicates general heat flow characteristics of the refractory and depends upon the chemical and mineralogical compositions as well as the application temperature.The unit of the thermal conductivity of refractories is W/K*m. In addition to the temperature, the co-efficient of thermal conductivity depends also on the composition of raw materials, the mineralogical structure of the brick mix, true porosity and pore size, firing temperature and grading. Hence the absolute values of the thermal conductivity vary widely for the different types of the refractories.
There is a negative temperature gradient of thermal conductivity in bricks which consist almost exclusively of crystalline components. Bricks with a high portion of the glassy phase normally have a positive and small gradient. With rising temperatures the vibration conditions of the crystalline, non-metallic materials become similar to those of the amorphous materials and this leads to converging values of the thermal conductivity. Porosity is a significant factor in heat flow through refractories. The thermal conductivity of a refractory decreases on increasing its porosity.
High thermal conductivity of a refractory is desirable when heat transfer though brickwork is required, for example in recuperators, regenerators, muffles, etc. Low thermal conductivity is desirable for conservation of heat, as the refractory acts as an insulator. Light-weight refractories of low thermal conductivity find wider applications in low temperature heat treatment furnaces.
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