Fireclay Refractory




Fireclays are clays that are immune to deformation and melting at high temperatures in the ceramics industry. Unfired refractory bond clay and fireclays (chamotte), fired refractory clay, or similar grog ingredients are used to make fireclay refractory bricks. The basic components of fireclay refractory bricks are 18 percent to 44 percent alumina (Al2O3) and 50 percent to 80 percent silica (SiO2).


Because of the variety of clays and manufacturing techniques available, a wide range of brick varieties suitable for specific uses can be produced. The occurrence of mineral mullite, which formed during burning and is characterized by high refractoriness and low thermal expansion, contributes significantly to the utility of fireclay refractory bricks.


Raw materials for fireclay refractory bricks


The main component of refractory fireclay is hydrated aluminum silicates, with tiny amounts of other minerals. Al2O3.2SiO2.2H2O is the standard formula for these aluminum silicates, which equals 39.5 percent alumina, 46.5 percent silica, and 14 percent water (H2O). The most prevalent member of this category is kaolinite. The mixed water is driven off at high temperatures, leaving a residue that is 45.9% alumina and 54.1 percent silica in theory. Even the cleanest clays, however, include trace amounts of other components such as iron, calcium, magnesium, titanium, sodium, potassium, lithium, and free silica. The total amount of these melting point-lowering fluxing agents should be no more than 5% to 6% of the overall amount. TiO2 is not considered a fluxing agent, however it was previously counted alongside alumina.


A category of refractory clays known as fireclay can generally resist temperatures over the pyrometric cone equivalent (PCE) value of 19. The two primary qualities required in fireclay for use in the construction of refractory bricks are refractoriness and plasticity. A good fireclay should have a high fusing point (more than 1580 degrees Celsius) and be malleable. For the construction of fireclay refractory bricks, fireclays with high alumina and low iron oxide, lime, magnesia, and alkalis are commonly selected. Because of its hardness, density, and lack of iron, aluminous (kaolinitic) fireclay is more refractory, giving it a white burning color. It has an extremely high fusion temperature because to the lack of alkalis.


In India, fireclays are classified as (i) low duty, (ii) intermediate duty, (iii) high duty, and (iv) super duty, based on their ability to endure high temperatures without melting. Low-temperature fireclay (PCE 19 to 28), intermediate-temperature fireclay (PCE 30), high-temperature fireclay (PCE 32), and super-temperature fireclay (PCE 33) can all tolerate temperatures between 1515 and 1615 degrees Celsius (PCE 35).


Refractory minerals such as flint and semi-flint clays, plastic and semi-plastic clays, and kaolins are extremely important. The extreme hardness of flint clay, often known as ‘hard clay,’ gives it its name. Most super duty and high-duty fire clay bricks have it as the main ingredient. The majority of flint clays have a conchoidal, shell-like fracture. After being pulverized and combined with water, their plasticities and drying shrinkages are quite low. Their shrinking after fire is moderate. The finest clays of this class have a PCE score of 33 to 35 and are minimal in impurities.


Kaolins are mostly made up of kaolinite. They’re often made of medium-density polymers with extremely high drying and firing shrinkages. Siliceous kaolins shrink less, while bauxitic kaolins shrink more than kaolins that are virtually entirely composed of kaolinite. PCE levels of 33 to 35 are prevalent in refractory kaolins, while PCE values of 29-32 are common in less pure types.


The refractoriness, plasticity, and bonding strength of plastic and semi plastic refractory clays, sometimes known as “soft clays” or “bond clays,” vary greatly. Shrinkage during drying and fire is often rather high. Most refractory clay variants have PCE values of 29 to 33, and many clays with good plasticity and great bonding power have PCE values of 26 to 29.


The way clay reacts to water is one of its distinguishing characteristics. Clay becomes pliable when mixed with water and may thus be sculpted. The layer structure clay minerals are covered with a thin liquid film, which lowers cohesive forces between the particles, resulting in plasticity. The clay mix can be created under pressure and preserves its shape when the strength of the link between the layers is reduced by enough water layers.


When kaolin minerals are heated to temperatures between 500 and 600 degrees Celsius, they lose their crystallization water and form metakaolin, an intermediate phase. This phase, nevertheless possesses a low crystalline order. The kaolin lattice does not totally collapse until around 925 degrees Celsius. There is no reaction between the silica and alumina of the decomposed clay at first, but when the temperature rises over 950 degrees Celsius, mullite starts to form. Only mullite, cristobalite, and/or glassy phases are present above 1100 degrees Celsius. The glassy phase contains an approximate composition of 80 percent minimum silica, 10% alumina, and more than/equal to 5% alkalis and earth alkalis.


Manufacture of fireclay refractories


Fireclay refractory bricks are often made from a mixture of two or more clays. Some bricks, particularly low-duty fireclay bricks, are constructed entirely of a single clay. Flint clay and high-grade kaolin have a high refractoriness, calcined clays limit drying and fire shrinkages, and flexible clays help to form and bond. The quantity of clays used in the blend is determined by the nature and quality of the refractory brick to be created.


The clay is mixed together. Raw flint and bond clays, with or without calcined clay, are commonly used in super duty and high duty fireclay bricks. A considerable amount of the mix is precalcined while creating kaolin and other clay bricks to reduce kiln shrinkage and maintain the volume and mineral composition of the bricks.


The particles of powdered clay used to make the fireclay bricks must be of a variety of sizes, each in suitable proportion. The clays are usually pulverized in a dry pan, which is a revolving pan-shaped grinding mill with a slotted bottom aperture. The batches are filtered to the proper sizes and carefully blended with a little amount of water that is carefully controlled. The wet batch is then fed through a press that is either mechanically or hydraulically controlled, where the brick is created under pressure.


Plastically compressed fireclay bricks no longer keep up the specifications for numerous applications. By pressing the fireclay bricks semi-dry and dry, the qualities of the fireclay bricks are enhanced. Because the clay mix for super duty and high duty bricks contains a lower amount of bond clay, these bricks are typically pressed by semi dry and dry pressing.



The use of a high vacuum during the forming of the brick improves certain physical features in a modification of the power press method. Vacuum-formed bricks have a more uniform texture and are harder, stronger, less porous, and more dense than non-vacuum-formed bricks. As a result, these bricks are more immune to slag impregnation and corrosion, as well as gas penetration.


Extrusion is a method that is occasionally used to create unique shapes. Clays are ground in a dry pan, blended wet or dry in a mixer, brought to the right consistency in a pug mill, and extruded in the shape of a stiff column through the die of an augur machine. A deairing system within the auger machine chamber removes the air from the clay before to extrusion. Wires are used to cut the column into brick. Typically, the bricks are re-pressed to give them sharper corners and edges, as well as fine surface. Vertical piercing and establishing presses, which totally reconfigure blanks from the extrusion machine, produce a variety of complicated and unique designs.


The bricks are dried in tunnels or humidity driers, depending on how they were made. Significant shrinkage is caused by a high moisture content and a high clay content. The dimensions of burnt bricks vary greatly, and if the bricks are not dried properly and gently, further anomalies such as warping or bloating might occur.


The temperature at which the bricks are fired is determined by the maturation temperatures of the clays, as well as the planned use of the bricks. Several important goals are met when the brick is fired. These include the evaporation of free and mixed water, the oxidation of iron and Sulphur compounds, as well as organic matter, the alteration of minerals, and volume changes. After that, the clay particles are ceramically fused together to create mechanically robust bricks.


Mullite, cristobalite, residual quartz, and glass make up the fireclay bricks after they’ve been fired. Mineral components are not present in a balanced state in burnt bricks. The bricks only approach equilibrium at the brick hot phase after the fireclay bricks have been inserted in the furnace. The content of mullite changes little with rising temperature and a longer holding period at high temperature, but the content of cristobalite and quartz falls and disappears completely at 1400 deg C to 1500 deg C. The fireclay bricks are thus made entirely of mullite and a viscous glass that may involve alkalis and other fluxing agents in addition to silica and alumina.


Fireclay refractory bricks have a number of important properties:-


The number and nature of the glassy phase impact the softening behavior of fireclay refractory bricks. This phase begins to soften about 1000 degrees C due to the alkali content and the presence of other impurities, and it gives a high softening intervals to the fireclay bricks because of its viscosity. The refractoriness under load (RUL), thermal expansion under load (creep), and hot crushing strength of fireclay bricks are used to determine their softening behavior.


Fireclay refractory bricks come in a variety of shapes and sizes.


There are five standard classifications of fireclay bricks, according to the American Society of Testing Materials (ASTM). (i) super duty, (ii) high duty, (iii) medium duty, (iv) low duty, and (v) semi silica


Types of fireclay refractory bricks

Super duty fireclay bricks – Super duty fireclay bricks have a high refractoriness and strength. They contain a 40 percent to 44 percent alumina concentration and good volume stability at high temperatures. When exposed to fast temperature changes, super duty bricks are more resistant to cracking or spalling. In terms of PCE values, their refractoriness cannot be less than 33. The super duty fireclay bricks can be modified in a variety of ways, including being burned at temperatures hundreds of degrees higher than the standard product. The high temperature fire improves the brick’s high temperature strength, stabilizes its volume and mineral compositions, boosts its resistance to fluxing, and leaves it practically immune to carbon deposition breakdown in carbon monoxide gas atmospheres.

High-density fireclay bricks — These bricks are used in large quantities and for a variety of purposes. High-duty fireclay bricks are frequently more cost-effective than medium-duty bricks for lining furnaces that are operated at moderate temperatures for significant periods of time but are prone to periodic shutdowns due to their superior resistance to thermal shock. The PCE value of the high-duty brick must be at least 31.5, and it often ranges from 31.5 to 33.

Fireclay bricks with a medium tensile strength – Medium-duty fireclay bricks are suitable for applications that are subjected to moderately severe circumstances. Within their usable temperature ranges, these bricks can tolerate abrasion better than many high-duty bricks. PCE ratings for medium-duty fireclay bricks range from 29 to 31.

Low-refractoriness fireclay bricks — These bricks are used as back-ups for higher-refractoriness fireclay bricks. They are employed in services when the temperature is reasonably reasonable. Low-duty fireclay bricks have PCE values ranging from 15 to 27-29.

Semi silica fireclay bricks – These bricks have an alumina percentage of 18 to 25% and a silica content of 72 to 80%, with a low content of alkalis and other contaminants. These refractories have exceptional load bearing strength and volume stability at relatively high temperatures, as well as a high resistance to shrinking.


The applications of fireclay bricks


In addition to refractoriness, many other qualities affect the usage of fireclay bricks. Dimensional precision, crushing strength, porosity, and refractoriness under strain are some of these qualities. Many applications call for machine-pressed, fired fireclay refractories. The materials are subjected to a wide range of stresses. It is normal to create bricks that are designed to satisfy specific needs for unique uses. In the steel industry, fireclay refractory bricks are used in coke oven batteries, blast furnaces, hot blast stoves, and a variety of other furnaces.