Insulation Refractory Bricks
Insulation Refractory Bricks
Insulating refractory brick (IRB) is the term used for heat insulating bricks and covers those heat insulating materials which are applied up to 1000 deg C. IRBs are often mistakenly referred to as rear insulation materials. These bricks are assigned to the group of lightweight refractory bricks and are manufactured on the basis of naturally occurring lightweight raw materials.
IRB is a class of brick, which consists of highly porous fireclay or kaolin. IRBs are 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. IRB 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 bricks.
IRBs were developed in the 1930s, and they were the predominant form of insulation until the development of insulating castable and fiber refractories. There are two types of bricks namely (i) bricks based on clay and gypsum using the burnout of sawdust to create high porosity (and thereby provide better insulating value), and (ii) bricks based on lightweight aggregate and clays. Like all alumina-silica brick, IRBs have a duty rating (service limit).
Over the years, IRBs have been made in a variety of ways, such as mixing of organic matter with clay and later burning it out to form pores; or a bubble structure incorporated in the clay-water mixture which is later preserved in the fired brick.
IRBs are characterized by the presence of large amount of porosity (45 % -90 %) in it. The pores are mostly closed pores. These bricks due to a highly porous structure exhibit low thermal conductivity values. The thermal conductivity not only depends on the total porosity, but also on the size and shape of the pores as well as chemical and mineralogical composition. The bricks also have low heat capacity. Due to the presence of high porosity in these bricks, the bulk density and the strength of the bricks are low. The application temperature of these bricks depends on the constituents. For example, kyanite based insulation bricks can be used at a temperature greater than 1250 deg C. Whereas fireclay based insulation refractories are usually used at low temperatures.
IRBs are shaped light weight refractory products with a total porosity of greater than 45 % and an application temperature of 800 deg C minimum. ASTM C 155-70 and DIN- EN-1094, part 2 define the temperatures at which shrinkage of the material should not exceed 2 %. The maximum bulk density is also defined. Both norms specify typical grades of IRBs. The classification of IRBs as per ASTM C155 and DIN-EN-1094, part 2 is given in Tab 1.
|Tab 1 Classification of shaped heat insulation refractory products|
|ASTM C155||DIN-EN-1094, part 2|
|Group*||Test temp.**||Max. bulk density||Group||Test temp.**||Bulk density#|
|Deg F||Deg C||kg/cum||Deg C||kg/cum|
|* abbreviated deg F, example 16 = 1600 deg F = 875 deg C|
|** test temperature at which no more than 2 % permanent linear change may occur after 24 hours.|
|# upper limit of median bulk density of group L. In each group of the L class the bulk density is a property used only for differentiation and is indicated with two digits after the decimal point.|
Important properties of insulation refractories are given in Fig 1.
Fig 1 Important properties of insulation refractories
Different types of IRBs are mainly manufactured by using the raw materials such as diatomite, perlite, expanded vermiculite, calcium silicate, fireclay, kaolin, quartz, alumina and light weight refractory aggregates by conventional method. Different types of pore formers such as sawdust, foam polystyrene, fine coke, binders and organic foams or granular materials such as hollow microspheres and bubble alumina are commonly used to obtain decreased density or to produce porous bodies in the IRBs.
The raw material of diatomaceous earth products are microscopically small shells derived from diatoms. The heat insulation results from the large number of small pores within the shells of various shapes with sizes from 5 to 500 micro meters. Diatomaceous bricks are produced by extrusion presses. Bonding clay, burning out materials and fibres can be added. The fine pore structure of these bricks has a better heat insulation than vermiculite products with the same bulk density.
Vermiculite is a 3-layer mineral which expands like an accordion when heated quickly above 700 deg C. Through the pressure of the evaporating water in the intermediate layer, the original volume enlarges by 20 to 30 times. This so-called exfoliated vermiculite (density 60 to 200 kg/cum) serves as raw material for the production of vermiculite bricks, boards and shaped parts. Concrete, water glass and phosphates are used as binder .
Perlite is water containing volcanic rock, solidified to a glass shape. Water was absorbed by the magma under high pressure during litho genesis and can evaporate when heated quickly. Expanded perlite is formed with a settled density of 35-150 kg/cum. The production process of bricks, boards and shaped parts is almost the same as that for vermiculite. The application limit of this product range is from 750 deg C to 1000 deg C due to intensive shrinkage at higher temperatures.
Rice husk/ sawdust straw or low cost biomaterial are used as combustibles or pore former in manufacturing of insulation brick. However naphthalene, starch are also used for pore former in high duty insulation brick. These combustibles either evaporate or burnout during initial stage of firing and creates pores in the brick. The size and shape of pore former controls the pore morphology in insulation brick. Polystyrene foam each particle, which is dissipated during firing process and leaves behind a cavity that can improve thermal insulation properties of the brick. Polystyrene foam is, therefore, used as a pore forming material in the brick body .To that correspondence should be addressed for reducing thermal conductivity. Sawdust is combustible material, which produce channel and porosity at high temperature.
Normally plastic clay based binders are used in manufacturing of insulation brick. Other binders are ethyl cellulose, starch and molasses. Calcium oxide based binder lime or sometime gypsum is used during manufacturing of insulation brick via casting and setting process.
IRBs are produced predominantly in rectangular dimensions or arch shapes. End arches and side arches for round shapes can be cut from the rectangular sizes and adapted to the radius.
The maximum size of IRBs bricks is limited by the manufacturing process, because uniform properties can only be realized up to certain sizes. Larger dimensions (e.g. hanging blocks and roller bushings) are produced by gluing smaller sizes with high temperature refractory mortars. In general, the strength of the bonding joint is higher compared to the strength of the IRB itself. It is important to pay attention to the usage instructions of the mortar adhesive, as mortars are sensitive to extreme weather and climate conditions.
If the chemical structure is considered, IRBs are classified in aluminum silicate lightweight refractory bricks, silica lightweight refractory bricks, zircon lightweight refractory bricks and corundum lightweight bricks. The group of aluminum silicate IRBs (fireclay and mullite bricks) is the most important and common group. Raw materials based on Al2O3, SiO2 and in some cases CaO are used for the production of these bricks. Raw materials such as clay, kaolin, fireclay, sillimanite, andalusite, kyanite, mullite, alumina, alumina hydrate and corundum are used as alumina carriers. In addition to the fine grained raw materials, coarse grained and porous raw materials are also applied. These include lightweight fireclay and hollow spheres (balls) consisting of corundum or mullite.
The thermal decomposition process is best known and most applied technique to produce pores in the insulating firebricks. Fine saw dust, petroleum coke, lignite abrasion, styropor balls, fine waste products of the cellulose and paperboard (carton) industry are used as burning out materials. Decomposition materials with low ash content are required in order to prevent negative effects on the hot properties (e.g. alkalines). The so called foaming process is another further method for the production of pores in insulating fire bricks. Special soaps, saponins and sulfonates are used to produce stable foams. The slurry for the ceramic body is often made separately from a foam emulsion. Foam and slurry are homogenized in an intensive mixer. The required bulk density is adjusted by controlled mixing of foam and slurry.
In practice the gas impellent process is used less frequently. Gas producing substances are mixed into the compound.
Lightweight refractory bricks which are produced by mixing in evaporating substances (naphthalene) have distinctive differences in their properties compared to other brick qualities. It is possible to produce bricks with low density and high strength. Very fine pores guarantee low thermal conductivity values.
Shaping of the lightweight refractory bricks is done by casting, slinger method or pressing. During casting, the perforated metal moulds (forms) are lined with filter paper before being filled. Sulphite liquor, gypsum or concrete can be added to improve the mixing of the raw material and to speed the setting.
The slinger method is very efficient due to the continuous shaping of large blocks.
Plastic, semi-dry and dry mixes are shaped with the corresponding presses (extrusion presses, hydraulic presses or mechanical presses).
Bricks, unfinished cylindrical pieces or blocks are fired in chamber furnaces, bogie hearth furnaces or tunnel kilns. The firing temperature corresponds approximately to the classification temperature indicated by the producers. Cutting or grinding is necessary for most brick qualities in order to obtain the standard shapes due to high drying and firing shrinkage.
IRBs with complicated shapes are produced by hand forming, vibration or moulding processes.
For long term exposures near the rated service temperature, shrinkage may occur sufficiently to allow joint opening. For this reason, it can be important to use IRBs with a higher duty rating than seems required by the process temperature alone.
IRBs are susceptible to alkali attack. The products made using gypsum and sawdust burnout material (indicated by high CaO contents) may undergo an expansion reaction in a service environment containing alkali. In some cases, this reaction has been deleterious and reduces service life. By contrast, the IRB made with lightweight aggregate may experience surface glazing in an alkali environment.
The uses of IRBs ranges from laboratory furnaces to foundry furnaces and large tunnel kilns. The requirements on IRBs are diverse and in some cases even contradictory. On the one hand, high thermal insulating capability and low bulk density are the desirable properties of IRBs, on the other hand the bricks need to have sufficient mechanical strength but also good machining. Additionally the properties needed in these bricks are high thermal resistance under a multitude of atmospheric conditions as well as resistance to temperature shocks and changes.
The operation time of the industrial furnace is a decisive criterion for the IRBs. For furnaces operated on a continuous basis the mass of the constructed refractory lining is less important concerning energy efficiency. The thermal conductivity (l) is significant for efficient operation, therefore bricks with low l-values are preferred. The basic rule is ‘The lower the bulk density, the lower the thermal conductivity’. The bulk density needs to be adapted to the service temperature of the kiln. The reason is the existence of a minimum thermal conductivity which is shifted to higher bulk densities at higher operating temperatures.
The cold compressive strength is less important compared to the hot properties of the brick. In general the mechanical loads are not high. The strength of insulating fire bricks is fully sufficient. Approximately 0.5 N/sq mm is required as a minimum strength. That value ensures safe transport, handling and installation work. Often a compromise between strength, bulk density and thermal conductivity has to be found.
Constructions with higher mechanical loads require bricks with higher cold crushing strength. It is important that the higher cold compressive strength is not the result of a higher fluxing agent content in the brick, otherwise the creep under compression increases at high temperatures.
Thermal shock resistance is an important property for periodically operated furnaces.
Depending on the heating up and cooling down conditions of the furnace, it is important to know the stress limits of the material. The material will crack, if temperature differences between cold and hot side are higher than 130 to 250 deg K. Such temperature differences are often exceeded repeatedly in wall linings. Each temperature change affects the structure of the brick.
The critical heating up speed for lightweight fireclay bricks (depending on the shape) is 5-10 deg K/min. according to the literature.
The different IRBs vary in their thermal shock behaviour due to their specific composition and porosity. High cristobalite contents (more than 10 %) have a negative effect on thermal shock resistance. In case of less than 10 % other criteria are more essential.
Micro cracks in the structure are an advantage since they absorb stresses without further crack development
IRBs remain a choice for construction and repair when castable or fiber products may not be preferred. IRBs constitute the working lining of many furnace structures. Construction is usually convenient since most IRBs can be cut with a hacksaw. Caution should be used in selecting IRBs when abrasion resistance or impact resistance is required.
Mortar joints and adhesive joints are the weak points in the brickwork. Due to high porosity and therefore fast removal of water, there are the following two important factors that are to be considered.
- The mortar must be ‘plastic’ and must have a high capability to bond water. If not, the bricks cannot be laid precisely according to measures and the bond of the individual bricks is lost when corrected later.
- The mortar should have low alkali content. The insulation firebricks are infiltrated with the binder liquid resulting in a higher alkaline concentration in the border areas. This causes lower melting points/increased sintering of the material and increased creep at high temperatures. Also thermal shock resistance is lowered by alkaline impurities. Hence mortars without water glass are preferred for high temperature applications.