Refractories and Classification of Refractories

Refractories and Classification of Refractories

Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. The principal raw materials used in the production of refractories are normally the oxides of silicon, aluminum, magnesium, calcium and zirconium. There are some non-oxide refractories like carbides, nitrides, borides, silicates and graphite.

Refractories are chosen according to the conditions they face during their use. Some applications require special refractory materials. Zirconia is used when the material is required to withstand extremely high temperatures. Silicon carbide and carbon are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen, since they oxidize and burn in atmospheres containing oxygen.

Refractories are the materials which are resistant to heat and exposure 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. In simplified language, they are considered to be materials of construction which are able to withstand high temperatures.

Refractories are usually inorganic non-metallic materials with refractoriness greater than 1500 deg C. They belong to coarse-grained ceramics having microstructure which is composed of large grains. The basis of body is coarse-grained grog joined by fine materials.

Refractory products are a specific sort of ceramics that differs from any ‘normal’ ceramics mainly with their coarse-grained structure being formed by larger grog particles joined by finer intermediate materials (bonding).

ASTM C71 defines refractories as ‘non-metallic materials having those chemical and physical properties that make them applicable for structures or as components of systems that are exposed to environments above 538 deg C’.

Refractories are to be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion.

Refractories are used in high temperature processes because of their heat resistant properties and stability at high temperatures. They are normally used as linings for high-temperature furnaces and other processing units such as kilns, incinerators and reactors since they are able to withstand physical wear, high temperatures, and corrosion by chemical agents. They are also used to make crucibles. They find extensive uses in iron and steel industry.

The qualities of refractories are dependent on their chemical, physical, mineralogical and thermal properties. Refractory materials have a range of properties to meet the requirements imposed by different processes.

The general requirements form the refractories are (i) ability to withstand high temperatures and trap heat within a limited area like a furnace, (ii) ability to withstand sudden changes of temperature, (iii) ability to withstand load at service conditions, (iv) ability to withstand chemical and abrasive action of the materials such as liquid metal, liquid slag, and hot gases etc. coming in contact with the refractories, (v) ability to resist contamination of the material with which it comes into contact, (vi) ability to maintain sufficient dimensional stability at high temperatures and after/during repeated thermal cycling, (vii) ability to conserve heat, (viii) ability to withstand load and abrasive forces, and (ix) low coefficient of thermal expansion.

Refractories are normally tailor-made on the basis of (i) process parameters such as temperature profile, mode of operation, and operating atmosphere etc.,  (ii) expected quality characteristics, and (iii) best techniques for engineering and application, so that the final physical, chemical, and thermal properties are compatible to the application.

Refractories perform four basic functions namely (i) acting 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) insuring a strong physical protection, preventing the erosion of walls by the circulating hot medium, (iii) representing a chemical protective barrier against corrosion, and (iv) acting as thermal insulation, insuring heat retention.

Refractories are expensive, and any failure in the refractories results in a great loss of production time, equipment, and sometimes the product itself. The type of refractories also influences energy consumption and product quality. Hence, the issue of getting refractories best suited to each application is of highest importance. Economics greatly influence these issues, and the refractory best suited for an application is not necessarily the one that lasts the longest, but rather the one which provides the best balance between initial installed cost and service performance. This balance is never fixed, but is constantly shifting as a result of the introduction of new processes or new types of refractories

Classification of refractories

Refractories are classified into a number of ways on the basis of (i) chemical composition, (ii) chemical properties of their constituent substances, (iii) by the place of use, (iv) the refractoriness, (v) method of manufacture, (vi) physical form, (vii) according to the applications, (viii) based on thermal conductivity, (ix) according to the principal base features, or (x) based on compactness.

Classification based on chemical composition

Based on the chemical composition the refractories can be classified as follows.

  • Silica refractories – Silica refractories comprise of silicon oxide (SiO2) also known as silica. These refractories are produced either from quartz or fused silica. Silica refractories contain at least 93 % SiO2. The raw material is quality rocks. Various grades of silica brick have found extensive use in the iron and steel industry furnaces. In addition to high fusion point multi-type refractories, the other important properties of silica refractories are their high resistance to thermal shock (spalling) and their high refractoriness. The outstanding property of silica brick is that it does not begin to soften under high loads until its fusion point is approached. Other advantages are flux and slag resistance, volume stability and high spalling resistance.
  • Fireclay refractories – Fireclay refractories comprise around 75 % of the production of refractories on a volume basis and are essentially hydrated aluminum silicates with minor proportions of other minerals. Typical composition consists of SiO2 less than 78 % and Al2O3 less than 44 %. As a type, they are extremely versatile and least costly of all refractory bricks and are extensively used in the iron and steel industry. Fireclay refractories are produced by firing of certain types of clays. The principal mineral for these refractories is kaolinite. Fireclay bricks are divided into five different classes namely (i) super-duty, (ii) high-duty, (iii) semi-silica, (iv) medium-duty, and (v) low-duty. The super- duty and high-duty classes are divided further into three types under each class.
  • Alumina refractories – Alumina refractories are also having basic constituents of Al2O3 and SiO2 but these refractories have a minimum of 50 % Al2O3. Alumina refractories are divided into seven different classes by percent alumina. These classes are (i) 50 % Al2O3, (ii) 60 % Al2O3, (iii) 70 % Al2O3, (iv) 80 % Al2O3, (v) 85 % Al2O3, (vi) 90 % Al2O3, and (vii) 99 % Al2O3.
  • Magnesia refractories – The main constituent of these refractories is magnesia (MgO). The main sources of magnesia are brines (often deep well type) and seawater as well as sintered and fused magnesia. Magnesia- carbon refractories are important refractories of this type.
  • Dolomite refractories – The main constituents of these refractories are MgO and calcium oxide (CaO). These refractories are produced from sintered dolomite.
  • Magnesia-chrome or chrome-magnesite refractories – The main constituents of these refractories are MgO and chromium oxide (Cr2O3). Depending upon the percentage of MgO and Cr2O3 in the refractories these refractories are called either magnesia-chrome refractories or chrome-magnesite refractories. Here, a distinction is required to be made between chrome-magnesite refractories and magnesite-chrome-refractories. Chrome-magnesite material usually contains 15 % to 35 % Cr2O3 and 42 % to 50 % MgO whereas magnesite-chromite refractories contain at least 60 % MgO and 8 % to 18 % Cr2O3. Chrome- magnesite refractories are used for building the critical parts of high temperature furnaces. These materials can withstand corrosive slags and gases and have high refractoriness. The magnesite-chrome refractories are suitable for service at the highest temperatures and in contact with the most basic slags used in steelmaking. Magnesite-chrome refractories usually have a better spalling resistance than chrome-magnesite refractories.
  • Silicon carbide refractories – Silicon carbide refractories are produced from silicon carbide (SiC), a raw material synthesized in a resistance-type electric furnace at temperature greater than 2500 deg C, through the reaction of silica with carbon.
  • Zirconia refractories – In zirconia refractories, main constituent of the refractory is zirconium oxide (ZrO2). Zirconia refractories are useful as high temperature construction materials. They tend to be used in applications where temperatures are above 1900 deg C such as casting nozzles and gates, crucibles, furnace liners and kilns. The thermal conductivity of zirconium dioxide is found to be much lower than that of most other refractories and the material is therefore used as a high temperature insulating refractory. Since Zirconia shows very low thermal losses and does not react readily with liquid metals, it is mainly useful for making refractory crucibles and other vessels for metallurgical purposes.
  • Carbon refractories – The main constituent for these refractories is carbon. Carbon, formed carbon, manufactured carbon, amorphous carbon and baked carbon are the terms which are used for these refractories. Carbon can also be in graphitized or semi-graphitized form. The carbon refractories are primarily used in highly reducing environments. Carbon refractories have a high refractoriness and high temperature of softening under load. They resist well the action of slags and have high thermal stability.

Classification based on chemical properties of their constituent substances

Refractories are typically classified on the based on the chemical behaviour of their constituent substances, i.e. their reaction to the type of slags (Fig 1).  Accordingly to this classification, refractories are of three types namely (i) acid refractories, (ii) basic refractories, and (iii) neutral refractories. Also there are some special types of refractories.

  • Acid refractories – These refractories are attacked by alkalis (basic slags). These are used in areas where both slag and atmosphere are acidic. Examples of acid refractories are silica refractories, zirconia refractories and alumino-silicate refractories.
  • Basic refractoriesBasic refractories are those which are attacked by acid slags but stable to alkaline slags, dusts and fumes at high temperatures. Since they do not react with alkaline slags, these refractories are of considerable importance for furnace linings where the environment is alkaline such as steelmaking operations. The most important basic refractories are magnesite refractories, dolomite refractories, and magnesia-chrome refractories.
  • Neutral refractories – Neutral refractories are chemically stable to both acids and bases and are used in areas where slag and atmosphere are either acidic or basic. The common examples of these refractories materials are carbon graphite (most inert), chromite refractories, and alumina refractories. Out of these graphite is the least reactive and is extensively used in metallurgical furnaces where the process of oxidation can be controlled.

Special refractories are expensive refractory materials which have been manufactured using synthetic (fused / sintered) grains free from impurities, under highly controlled production parameters. They are used for special purposes like – construction of crucible, some parts of furnaces and, research and development purposes etc. where the cost of the refractory is of no consideration. They include materials like pure alumina, sialons (Si – Al – O – N), thoria (ThO2), beryllia (BeO), zirconia, boron nitride, and spinel etc.

Classification of refractories based on chemical behaviour of constituents

Fig 1 Classification of refractories based on chemical behaviour of constituents

Classification based on the place of use

Iron and steel industry is the major consumer of refractories with around 70 % of refractories being consumed by the industry. The different areas of the manufacturing processes in iron and steel industry are exposed to different temperatures, slag and sulphur gases. The diversity in the operating conditions of different equipment’s demands different grades for different areas of application.  Also, each production shop has requirements of refractories which need special shapes as well as technical specifications necessary for meeting the process requirements for the shop. Hence, the refractories are often named after the shop names. Under this classification, refractories are classified as (i) coke oven refractories, (ii) blast furnace refractories, (iii) steelmaking refractories, (iv) ladle refractories, (v) tundish refractories, (vi) calcining plant refractories, and (vii) reheating furnace refractories etc.

Classification based on refractoriness

Based on the property of refractoriness, the refractories are usually classified usually in four classes. These are namely (i) super-duty, (ii) high-duty, (iii) intermediate duty, and (v) low duty. Super duty refractories have PCE (pyrometric cone equivalent) value ranging from 33-38. High duty refractories have PCE (pyrometric cone equivalent) value ranging from 30-33. Intermediate duty refractories have PCE (pyrometric cone equivalent) value ranging from 28-30, while low duty refractories have PCE (pyrometric cone equivalent) value ranging from 19-28.

Classification based on method of manufacture

The refractories can be manufactured by several methods consisting mainly of (i) dry press process, (ii) fused cast process, (iii) hand molding process, (iv) forming process consisting of normal, fired or chemical bonded, and (v) unformed refractories such as monolithic, plastics, ramming masses, gunning materials, and castable etc. and are classified accordingly.

Classification based on physical form

Refractories are classified according to their physical form. These are the shaped and unshaped refractories. The shaped is commonly known as refractory bricks and the unshaped as monolithic refractories.

Shaped refractories are those which have fixed shapes when delivered to the user. The shaped refractories are normally known as refractory bricks. Brick shapes are usually divided into two types (i) standard shapes and (ii) special shapes.  Standards shapes have dimensions which 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 refractories 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.

Unshaped refractories are without definite form and are only given shape during their application. They form 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.

Ramming refractory materials are in loose dry form with graded particle size. They are mixed with water for use. Wet ramming masses are used immediately on opening. Ramming masses are used mostly in cold condition so that desired shapes can be obtained with accuracy.

Castables refractory materials contain binder such as aluminate cement which imparts hydraulic setting properties when mixed with water. These materials are installed by casting and are also known as refractory concretes.

Mortars are finely ground refractory materials, which become plastic when mixed with water. These are used to fill the gap created by a deformed shell, and to make wall gas tight to prevent slag penetration. Bricks are joined with mortars to provide a structure.

Plastic refractories are packed in moisture proof packings and the packings are opened at the time of use. Plastic refractories have high resistance to corrosion.

Monolithic refractories are replacing conventional brick refractories in steelmaking and other metal extraction industries. These refractories are loose materials which can be used to form joint free lining. Various means are employed in the placement of monolithic refractories like ramming, casting, gunniting, spraying, and sand slinging, etc. The main advantages of monolithic linings are (i) they eliminate joints which is an inherent weakness with brick lining, (ii) hey have greater volume stability, (iii) they have better spalling tendency, (iv) they can be installed in hot standby mode, (v) they have easier transportation and handling, (vi) the method of application is faster and skilled measures in large number are not required, (vii) they offer better scope to reduce downtime for repairs, (viii) they offer considerable scope to reduce inventory and eliminate special shapes, (ix) they are heat saver.

Classification according to applications

Chemical characteristics of the furnace process usually determine the type of refractory required. Theoretically, acid refractories should not be used in contact with basic slags, gases and fumes whereas basic refractories can be best used in alkaline environment. Actually, for various reasons, these rules are often violated.

Classification based on thermal conductivity

Based on thermal conductivity, refractories can be (i) conducting such as SiC, ZrC, (ii) non conducting such as silica, alumina, or (iii) insulating refractories.

The function of insulating refractory is to reduce the rate of heat flow (heat loss) through the walls of furnaces. The desirable feature of insulating refractories is the low thermal conductivity, which usually results from a high degree of porosity. Structure of air insulating material consists of minute pores filled with air which have in them very low thermal conductivity. The air spaces inside the brick prevent the heat from being conducted but the solid particles of which the brick is made conduct the heat. So, in order to have required insulation property in a brick a balance has to be struck between the proportion of its solid particles and air spaces. The thermal conductivity is lower if the volume of air space is larger. Importantly, the thermal conductivity of a brick does not so much depend on the size of pores as on the uniformity of size and even distribution of these pores. Hence, uniformly small sized pores distributed evenly in the whole body of the insulating brick are preferred.

The high porosity of the brick is created during manufacturing by adding a fine organic material to the mix, such as sawdust. During firing, the organic addition burns out, creating internal pores. Other ways to accomplish high porosity involves (i) by using materials which expand and open up on heating, (ii) by using volatile compounds like naphthalene, (iii) by using aluminum (Al) powder in combination with NaOH solution (called chemical bloating), (iv) by using substances which by themselves have open texture e.g. insulating brick grog, vermiculite, ex-foliated mica, raw diatomite etc. (v) by using foaming agents to slip, and (vi) by aeration etc.

Because of their high porosity, insulating bricks inherently have lower thermal conductivity and lower heat capacity than other refractory materials.

Insulating materials can be classified into four types with respect to application temperature. These four types are (i) heat resistant insulating materials for application temperatures up to 1100 deg C (examples are calcium silicate materials, products from siliceous earth, perlite or vermiculite, silica based micro porous heat insulators, and alumino-silicate fibers), (ii) refractory insulating materials for application temperatures up to 1400 deg C (examples are lightweight chamotte and kaolin bricks, lightweight castables, and mixed  and aluminum oxide fibers), (iii) high refractory insulating materials for application temperatures up to 1700 deg C (examples are lightweight mullite and alumina bricks, lightweight hollow sphere corundum castables and bricks, and special high refractory fibers) and (iv) ultra-high refractory insulating materials for application temperatures up to 2000 deg C (examples are  zirconia lightweight bricks and fibers, and non-oxide compounds). Several other types of insulating refractories include castables, granular insulation, and ceramic fiber insulation, which is light weight. Extremely lightweight materials have a porosity of 75 % to 85 % and ultra-lightweight, high-temperature insulating materials have a total porosity greater than 85 %.

Classification according the principal base features

Based on principal base features refractories are of two types. The first type is oxide-containing refractories. They are namely based on oxides and their compounds, the most important oxides are Al2O3, CaO, MgO, SiO2, Cr2O3, and ZrO2. The second type is non-oxide refractories. Example of non-oxide refractories are carbon-based refractory materials, and carbides, nitrides, borides, and silicides. This group included also sialons which are the silicon nitride sinterable derivatives.

Classification based on compactness

Based on this classification refractories are two types. The first type consists of dense refractories with their true porosity less than 45 %. The second type consists of insulating refractories with their true porosity more than 45 %.