Fireclay Refractory Bricks
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).
The variety of clays and manufacturing techniques allows the production of numerous brick types appropriate to particular applications. The usefulness of fireclay refractory bricks are largely due to the presence of mineral mullite, which forms during firing and is characterized by high refractoriness and low thermal expansion.
Raw materials for fireclay refractory bricks
Refractory fireclay essentially consists of hydrated aluminum silicates with minor proportion of other minerals. The general formula for these aluminum silicates is Al2O3.2SiO2.2H2O, corresponding to 39.5 % alumina, 46.5 % silica, and 14 % water (H2O). Kaolinite is the most common member of this group. At high temperature, the combined water is driven off, and the residue theoretically consists of 45.9 % alumina and 54.1 % silica. However even the purest clays contain small amounts of other constituents , such as compounds of iron, calcium, magnesium, titanium, sodium, potassium, lithium, and usually some free silica. The total quantity of these fluxing agents, which lower the melting point, should be at a level of 5 % to 6 % maximum. TiO2 is not regarded as fluxing agent and was previously counted together with alumina.
The name fireclay is given to a group of refractory clays which can generally withstand temperatures above pyrometric cone equivalent (PCE) value of 19. Refractoriness and plasticity are the two main properties needed in fireclay for its suitability in the manufacture of refractory bricks. A good fireclay should have a high fusion point (greater than 1580 deg C) and good plasticity. Fireclays containing high alumina and low iron oxide, lime, magnesia and alkalis are usually preferred for the production of fireclay refractory bricks. The aluminous (kaolinitic) variety of fireclay is more refractory because of its hardness and density and absence of iron, giving it a white burning colour. The absence of alkalis gives it a very high fusion temperature.
The fireclays are graded in India into (i) low duty (ii) intermediate duty (iii) high duty and (iv) super duty, depending upon their capacity to withstand high temperature before melting. The low duty fireclay can withstand temperatures between 1515 and 1615 deg C (PCE 19 to28), intermediate duty fireclay up to 1650 deg C (PCE 30), high duty fireclay up to 1700 deg C (PCE 32) and super duty beyond 1775 deg C (PCE 35).
Flint and semi flint clays, plastic and semi plastic clays, and kaolins are of great importance as refractory materials. Flint clay, also known as ‘hard clay’ derives its name from its extreme hardness. It is the principal component of most super duty and high duty fire clay bricks. Most flint clays break with a conchoidal, and shell like fracture. Their plasticities and drying shrinkages, after they have been ground and mixed with water, are very low. Their firing shrinkages are moderate. The best clays of this type are low in impurities and have a PCE value of 33 to 35.
Kaolins consist essentially of kaolinite. They usually are moderately plastics and have extremely high drying and firing shrinkages. Siliceous kaolins shrink less and bauxitic kaolins shrink more than kaolins which consist almost wholly of kaolinite. Refractory kaolins generally have a PCE value of 33 to 35, less pure varieties with PCE values of 29-32 are common.
Plastic and semi plastic refractory clays often called ‘soft clays’ or ‘bond clays’, varies considerably in refractoriness, plasticity and bonding strength. Drying and firing shrinkages are generally fairly high. The PCE values of these clays ranges from 29 to 33 for most of refractory clays varieties, and from 26 to 29 for many clays of high plasticity and excellent bonding power.
A characteristic property of clay is its behaviour to water. When mixed with water, clay become plastic, and thus can be shaped. This plasticity occurs because the layer structure clay minerals are surrounded by a thin liquid film, which reduces cohesion forces between the particles. When the strength of the bond between the layers is reduced by sufficient water layers , the clay mix can be formed under pressure and retains its subsequent shape.
During heating up to 500 deg C to 600 deg C, kaolin minerals lose their crystallization water and an intermediate phase, known as metakaolin is formed. However, this phase still exhibits a low crystalline order. The kaolin lattice does not disintegrate completely until about 925 deg C. Initially, there is no reaction between the silica and alumina of the decomposed clay, but with further heating to slightly above 950 deg C, mullite begins to form. Above 1100 deg C only mullite, cristobalite and/or glassy phase are present. The approximate composition of the glassy phase is 80 % minimum silica, 10 % alumina, and more than/equal to 5 % of alkalis and earth alkalis.
Manufacture of fireclay refractories
A blend of two or more clays is normally used for the manufacture of fireclay refractory bricks. Some bricks, especially those of the low duty fireclay bricks, are made of a single clay. Flint clay and high grade kaolin impart high refractoriness, calcined clays control the drying and firing shrinkages, plastic clays facilitate forming and impart bonding strength. The character and the quality of the refractory brick to be made determines the relative proportion of clays used in the blend.
The clay mixes for super duty and high duty fireclay bricks usually contain raw flint and bond clays, with or without calcined clay. In making bricks of kaolin and various other clays, a large proportion of the mix is precalcined to control firing shrinkage and to stabilize the volume and mineral composition of the bricks.
For the production of the fireclay bricks, the particles of ground clay must include a range of graded sizes, each in proper proportion. The clays are typically ground in a dry pan, which is a rotating pan shaped grinding mill, having slotted opening in the bottom. The batches are screened to the desired sizes and thoroughly mixed with a small but closely controlled amount of water. The moistened batch is then fed to a mechanically or hydraulically operated press in which the brick is formed under pressure.
For many applications , the plastically pressed fireclay bricks no longer meet the technical requirements. Improved properties requirement of the fireclay bricks is achieved through semi dry and dry pressing. Generally super duty and high duty bricks are pressed by semi dry and dry pressing, since clay mix for these bricks contain lower percentage of bond clay.
In a modification of the power press process, certain physical properties are enhanced by the application of a high vacuum during the forming of the brick. Bricks made with the application of vacuum, typically have a more homogeneous textures and are harder, stronger, less porous and more dense than the bricks which are made without vacuum. Hence these bricks are more resistant to impregnation and corrosion by slags and to penetration by gases.
The extrusion process is sometimes used for making special shapes. In making extruded bricks, clays are ground in a dry pan, mixed wet or dry in a mixer, and brought to proper consistency in a pug mill, and extruded through the die of an augur machine in the form of a stiff column. The air is removed from the clay before extrusion by a deairing system within the auger machine chamber. The column is cut into brick by means of wires. The bricks are then typically re-pressed to give them sharp corners and edges and smooth surfaces. Many intricate and special shapes are formed in vertical piercing and forming presses, in which blanks from the extrusion machine are completely reshaped.
Bricks formed by any process are dried in tunnel or humidity driers. A high moisture content and a high clay amount lead to considerable shrinkage. The dimensions of the fired bricks vary considerably and other irregularities such as warping or bloating can happen if the bricks are not dried carefully and very slowly.
The temperature of firing depends upon the maturing temperatures of the clays, and often upon the service for which the bricks are intended. In firing the brick, several necessary ends are accomplished. These include, driving off the free and combined water, oxidizing of iron and sulphur compounds and organic matters, transformation of the minerals, and changes in volume. Finally the particles of clay are ceramically bonded together into mechanically strong bricks.
After firing, the fireclay bricks consist of mullite, cristobalite, residual quartz and glass. In fired bricks, the mineral components are not present in equilibrium condition. Only after the fireclay bricks have been installed in the furnace, then only the bricks approach equilibrium at the brick hot phase. With rising temperature and a longer holding period at high temperature, the mullite content changes little, where as the content of cristobalite and quartz declines and disappears totally at 1400 deg C to 1500 deg C. The fireclay bricks then consist only of mullite and a viscous glass which, in addition to silica and some alumina, may contain alkalis and other fluxing agents.
Important characteristics of fireclay refractory bricks
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 1000 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.
Types of fireclay refractory bricks
As defined by the American Society of Testing Materials (ASTM), there are five standard classes of fireclay bricks (Fig 1). These classes are (i) super duty, (ii) high duty, (iii) medium duty, (iv) low duty, and (v) semi silica.
Fig 1 Types of fireclay refractory bricks
- Super duty fireclay bricks – Super duty fireclay bricks have good strength and refractoriness. They have good volume stability at high temperatures and an alumina content of 40 % to 44 %. Super duty bricks have superior resistance to cracking or spalling when subjected to rapid changes of temperature. Their refractoriness, in terms of their PCE values, may not be less than 33. There are several possible modifications in the super duty fireclay bricks, including bricks fired at temperatures several hundred degree higher than the usual product. The high temperature firing enhances the high temperature strength of the brick, stabilizes their volume and mineral composition, increases their resistance to fluxing, and renders them practically inert to disintegration by carbon deposition in atmospheres containing carbon monoxide gas.
- High duty fireclay bricks – High duty fireclay bricks are used in large quantities and for a wide range of applications. Because of their greater resistance to thermal shock, high duty fireclay bricks can often be used with better economy than medium duty bricks for lining of furnaces operated at moderate temperatures over long periods of time but subject to frequent shutdowns. The PCE value of the high duty brick may not be less than 31.5, and usually varies from 31.5 to 33.
- Medium duty fireclay bricks – Medium duty fireclay bricks are appropriate in applications where they are exposed to conditions of moderate severity. These bricks, within their serviceable temperature ranges, can withstand abrasion better than many bricks of the high duty class. Medium duty fireclay bricks have PCE values in the range of 29 to 31.
- Low duty fireclay bricks – These bricks find application as back up bricks for bricks with higher refractoriness. They are used for services where relatively moderate temperature prevails. The PCE values of low duty fireclay bricks cover the range 15 to 27-29.
- Semi silica fireclay bricks – These bricks contain 18 % to 25 % alumina and 72 % to 80 % silica, with a low content of alkalis and other impurities. With notable resistance to shrinkage, these refractories also have excellent load bearing strength and volume stability at relatively high temperatures.
Application of fireclay bricks
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 steel industry in coke oven batteries, blast furnace, hot blast stoves, and various other furnaces used in the steel industry.