API 936: Refractory Installation, Inspection and Testing
PART -II PROPERTIES AND TYPES OF REFRACTORIES
PROPERTIES AND TYPES OF REFRACTORIES What are Refractories Any material can be described as a ‘refractory,’ if it can withstand the action of abrasive or corrosive solids, liquids or gases at high temperatures. The various combinations of operating conditions in which refractories are used, make it necessary to manufacture a range of refractory materials with different properties. Refractory materials are made in varying combinations and shapes depending on their applications. General requirements of a refractory material are: Withstand high temperatures
Withstand sudden changes of temperatures
Withstand action of molten metal slag, glass, hot gases, etc
Withstand load at service conditions
Withstand load and abrasive forces
Conserve heat
Have low coefficient of thermal expansion
Should not contaminate the material with which it comes into contact
Table 1 compares the thermal properties of typical high density and low density refractory materials. Table 1. Typical Refractory Properties Property
High Thermal
Low Thermal Mass
Mass(Highdensity refractories) (Ceramic fiber) Thermal conductivity (W/m K) Specific heat (J/kg K) Density (kg/m3)
1.2 1000 2300
0.3 1000 130
Depending on the area of application such as boilers, furnaces, kilns, ovens etc, temperatures and atmospheres encountered different types of refractories are used. Some of the important properties of refractories are: Melting point: Pure substances melt instantly at a specific temperature. Most refractory materials consist of particles bonded together that have high melting temperatures. At high temperatures, these particles melt and form slag. The melting point of the refractory
is the temperature at which a test pyramid (cone) fails to support its own weight. Size: The size and shape of the refractories is a part of the design of the furnace, since it affects the stability of the furnace structure. Accurate size is extremely important to properly fit the refractory shape inside the furnace and to minimize space between construction joints. Bulk density: The bulk density is useful property of refractories, which is the amount of refractory material within a volume (kg/m3). An increase in bulk density of a given refractory increases its volume stability, heat capacity and resistance to slag penetration. Porosity: The apparent porosity is the volume of the open pores, into which a liquid can penetrate, as a percentage of the total volume of the refractory. This property is important when the refractory is in contact with molten charge and slag. A low apparent porosity prevents molten material from penetrating into the refractory. A large number of small pores is generally preferred to a small number of large pores. Cold crushing strength: The cold crushing strength is the resistance of the refractory to crushing, which mostly happens during transport. It only has an indirect relevance to refractory performance, and is used as one of the indicators of abrasion resistance. Other indicators used are bulk density and porosity. Pyrometric cones and Pyrometric cones equivalent (PCE): The ‘refractoriness’ of (refractory) bricks is the temperature at which the refractory bends because it can no longer support its own weight. Pyrometric cones are used in ceramic industries to test the refractoriness of the (refractory) bricks. They consist of a mixture of oxides that 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 oC above which the refractory cannot be used. This is known as Pyrometric Cone Equivalent temperatures. Refer Figure below.
Figure 3: Pyrometric Cones (Bureau of Energy Efficiency, 2004)
Creep at high temperature: Creep is a time dependent property, which determines the deformation in a given time and at a given temperature by a refractory material under stress. Volume stability, expansion, and shrinkage at high temperatures: The contraction or expansion of the refractories can take place during service life. Such permanent changes in dimensions may be due to: The changes in the allotropic forms, which cause a change in specific gravity
A chemical reaction, which produces a new material of altered specific gravity
The formation of liquid phase Sintering reactions Fusion dust and slag or by the action of alkalies on fireclay refractories, to form alkalialumina silicates. This is generally observed in blast furnaces. Reversible thermal expansion: Any material expands when heated, and contracts when cooled. The reversible thermal expansion is a reflection on the phase transformations that occur during heating and cooling.
Thermal conductivity: Thermal conductivity depends on the chemical and mineralogical composition and silica content of the refractory and on the application temperature. The conductivity usually changes with rising temperature. 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. Additional insulation conserves heat but at the same time increases the hot face temperature and hence a better quality refractory is required. Because of this, the outside roofs of open-hearth furnaces are normally not insulated, as this could cause the roof to collapse. Lightweight refractories of low thermal conductivity find wider applications in low temperature heat treatment furnaces, for example in batch type furnaces where the low heat capacity of the refractory structure minimizes the heat stored during the intermittent heating and cooling cycles. Insulating refractories have very low thermal conductivity. This is usually achieved by trapping a higher proportion of air into the structure. Some examples are: Naturally occurring materials like asbestos are good insulators but are not particularly good refractories Mineral wools are available which combine good insulating properties with good resistance to heat but these are not rigid Porous bricks are rigid at high temperatures and have a reasonably low thermal conductivity.
Various Types of Refractories Fireclay refractories consist essentially of hydrated aluminum silicates (generally Al2O3 · 2SiO2 · 2H2O) along with smaller quantities of other minerals. Hard clays are the flint and semiflint clays, which form the principal component of the super and high-duty fire bricks having PCE of 33–35. Plastic and semiplastic refractory clays, also called soft clays, vary considerably in refractoriness, bonding strength, and plasticity having PCE values ranging from 29 to 33. High-alumina refractories. Several minerals can be used in the making of alumina refractories. Most high-alumina refractories are made from bauxite (Al2O3 · H2O + Al2O3 · 3H2O) or diaspore (Al2O3 · H2O) or a mixture of the two blended with flint and plastic clay. Alumina types are more refractory than fireclay, approximately in proportion to their content of alumina. They are highly resistant to chemical attack by various slags and fumes and, in
general, have greater resistance to pressure at high temperature than fireclay refractory. Silica refractories are made from high-purity crystalline mineral quartz. Their thermal expansion is high at low temperatures and negligible beyond 550°C. They possess high refractoriness, strength, and rigidity at temperatures close to their melting point. They are susceptible to thermal spalling (cracking) at 650؛C, but at higher temperatures, they are free from spalling. Basic refractories. The raw materials used include magnesite, dolomite, and chrome ores. Insulating refractories are lighter and porous, as they trap a lot of air and hence possess much lower conductivity and heat storage capacity. They are normally used as the backing to the dense refractory facing. If the furnace conditions are clean, they can also be used as facing materials.
1.Refractory Bricks: Bricks are preformed shapes obtained by pressing the green mass to the required shape and size and firing at the specified temperature until the refractory bond is formed by chemical action under heat. The word brick is used in the acronym BRIL to cover both bricks and the tiles. Standard refractory bricks are made in two thicknesses, namely, 227 mm × 116 mm × 76 mm and 63 mm (9 in. × 4 1/2 in. × 3 in./2½ in.). Tiles are flattened bricks usually made in the range of 50–76 mm thickness, with overlapping edges and an arrangement to hold the tile. Tiles can also be made in different shapes and in more advanced composition than bricks. Today, refractory brick construction is confined mainly to the following areas: ● Around the fire in pile burning, in horse shoe or ward furnaces in firing bagasse or similar fuels. This type of burning is nearly extinct. ● Shaped refractory arches in chain grate stokers for radiating the heat onto the bed. ● Boiler enclosure in brick set boilers. ● Brick lining for underground brick flues.
● Stack lining. ● Brick lining in cyclones and external HXs in CFBC boilers. The use of refractory bricks in modern boilers is negligible in comparison to the former times. Since the bricks are pressed in hydraulic presses, they are strong, dense, and heavy, which makes them ideally suited to face the fire and dust-laden gases. Several types of refractory bricks are manufactured to suit various types of furnaces: 1. Fireclay 2. High alumina 3. Insulating bricks ● Firebricks (FBs) are made of a blended mixture of flint and plastic clays. Some or all of the flint may be replaced by highly burnt or calcined clay called grog.The dried bricks are burnt at 1200–1500°C. Firebricks can withstand spalling and many slag conditions but are not suitable for lime or ash slags. ● High-alumina bricks are graded by alumina content as 50, 60, 70, 80, and 90% and are used for unusually severe temperature and load conditions. They are more expensive than firebricks. ● Insulating FBs are made from porous fireclay or kaolin. They are light, one-half to one-sixth of the weight of equivalent refractory brick, low in thermal conductivity, and capable of withstanding high temperatures. They are graded by thermal withstand temperatures as 870, 1100, 1260, 1430, and 1540°C (1600, 2000, 2300, 2600, and 2800°F). They are not slagresistant. ● High-burned kaolin (alumina–silica china) refractories can withstand high temperatures and heavy loads or severe spalling conditions as in oil-fired boilers.
2.Refractory Tiles: Refractory tiles are used in the following areas in modern boilers: ● On top of the floor tubes to protect the tubes from overheating in package boilers ● In burner quarls to give the right shape to the flame ● Between tubes to form gas baffles ● In the lining of cyclones ● In the lining of hot and dust-laden gas ducts and hoppers
3.Refractory Castables: These refractory materials are not preformed but are cast in situ to any desired shape. Because of this flexibility and the advances made in the materials and binders, the castable refractories have become greatly popular, displacing the shaped and formed types in many applications. They are available in special mixes or blends of dry granular or stiffly plastic refractory materials with which (a) practically joint-free linings (monolithic) can be made or (b) repair of masonry can be carried out. These are packed in a way that makes transportation and handling easy. The application is also made very easy with little or no preparation. There are four types of castables or monolithics: 1. Plastic refractories 2. Ramming mixes 3. Gun mixes 4. Castables The monolithics develop their strength by either air or hydraulic setting. The entire thickness becomes hard and strong at room temperatures. At higher temperatures, it becomes even stronger due to the development of the ceramic bond. Heat setting monolithic refractories have very low strength at low temperatures and develop their full strength only on attaining the full temperature. Linings on furnace wall tubes require that the water walls be fully drained before the application of a refractory layer, lest the water-cooled walls hinderthe lining
from attaining the necessary temperature. Usually castable linings need some anchor material to hold. ● Plastic refractories are mixtures of refractory materials prepared in stiff plastic conditions of proper consistency, for ramming into place with pneumatic hammers or mallets. Plastics are similar to castables in formulation, as both use calcined aggregates and a binder. However, the plastics that are premixed at the factory use phosphates or other heat-setting agents to develop a bond when fired. Castables use hydraulic cements that form the permanent bond when mixed with water. ● Plastic chrome ore (PCO) linings are proven lining materials for black liquor (BL) recovery boilers. The air setting plastic compound is rammed into position on the studded walls to develop a dense monolithic layer, which has high resistance to spalling, erosion, hot gases, and smelt. High-alumina phosphate-bonded plastics are used in hot cyclones of circulating fluidized bed (CFB) ● Ramming mixes are ground refractory materials with minor amounts of other materials added to make the mixes workable. Most ramming mixes are supplied dry. Ramming mixes are required to be mixed with water and rammed into place, followed by drying and heating when they form a dense and strong monolithic refractory structure by self-bonding. ● Gun mixes are granular refractory materials prepared for spraying at high velocity and pressure by guns. The resulting lining is homogeneous and dense and free from lamination cracks. The spray can be either by dry mix or by wet mix. The gun has a water nozzle to moisten the mixture. The gun mixes can be either air or heat setting. Refractory lining of steel stacks is often done by gunning. ● Castables are granular refractory materials combined with suitable hydraulic setting bonding agent. They are supplied in dry form to be mixed, at site, with water and poured or cast in place to develop a strong hydraulic set. They are rammed or troweled or tamped into position and occasionally applied with air guns. These castables have negligible shrinkage in service and low coeff cients of thermal expansion. They are resistant to spalling. Some are capable of withstanding severe erosion. Some are good insulators and others are good conductors. High-alumina dense castables with varying alumina contents in excess of 90% are used up to an operating temperature of 1800ºC to withstand high erosion .Burner quarls and furnace linings are some examples of the usage of this castable.
Low-cement, ultra low–cement, and no-cement castables are relatively recent developments where refractory cement used as binder is progressively brought down to withstand high temperatures without weakening the lining. Less cement means reduced lime content and reduced water requirement for setting. The porosity is reduced by more than 50%, resulting in a very dense structure capable of standing up to very high erosion as seen in cyclones of CFB boilers. Insulating castables serve as hot face lining in clean applications and as backup lining for dusty applications. They are light, strong, and low in conductivity, making the whole liner thin and cheap. They are made in a range of densities from ~ 400 to 1600 kg/m3. Silicon carbide lining of FBC furnaces is done to withstand extreme levels of erosion and have good heat transfer to the water walls.
