Silica bricks are acid-based refractory materials primarily composed of tridymite, cristobalite, and small amounts of residual quartz and glass. They offer strong resistance to acid-based slag, but are susceptible to corrosion by alkaline slag and are vulnerable to corrosion by oxides such as Al₂O₃, K₂O, and Na₂O. Their refractoriness under load is high, ranging from 1640°C to 1680°C, close to the melting points of tridymite and cristobalite (1670°C and 1713°C, respectively). Their greatest drawback is their low thermal shock resistance, but their refractoriness is similar to their refractoriness under load. They withstand long-term use at high temperatures without deformation, helping to ensure the structural strength of masonry structures during operation.
Silica bricks are primarily used in partition walls of the carbonization and combustion chambers of coke ovens, as well as in the roofs or vaults of soaking pits, hot blast furnaces, acid open-hearth furnaces, and glass kilns. In ironmaking technology, new technologies such as direct reduction and molten reduction are gradually being transformed into productive forces. In the coking industry, a "formed coke" produced without the use of a coke oven has been developed, which can partially replace traditional coke.
Silica refractory bricks, like most sintered refractory bricks, are produced using a semi-dry process and fired in tunnel kilns. Cracks that occur during the production process are one of the main reasons for the high scrap rate.
Types of Cracks in Silica Bricks
Cracks in silicon bricks can be categorized as surface cracks and internal cracks, the latter also known as layer cracks. Surface cracks are further categorized as transverse cracks, longitudinal cracks, and network cracks. Silica bricks are produced using a semi-dry press-forming method to create dense green bodies. Cracks occurring along the direction of pressure applied to the green body are transverse cracks, while cracks occurring perpendicular to the direction of pressure are vertical cracks. Network cracks are composed of several cracks distributed in a spiderweb pattern on the surface of a silica brick.
Typically, for a standard silica brick, the green body is pressed through its thickness. The forming process of silica fire bricks is essentially a process of compacting the particles within the blank and removing air, thereby forming a dense blank. After being machine-pressed, the bricks exhibit advantages such as high density, strength, minimal drying and firing shrinkage, and easily controlled product dimensions. However, if the machine-pressing process is improperly controlled, lamellar cracks perpendicular to the direction of pressure may form in the blank during the pressurization process. Therefore, lamellar cracks, or simply laminations, within silica firebricks are also longitudinal cracks.
Large laminations can be detected immediately after the bricks are formed or dried. However, minor laminations within the bricks only become noticeable after firing, as they continue to expand due to thermal stresses during firing. Bricks containing cracks, especially laminations, are prone to breakage, making them unusable and reducing the yield of silica brick products.
Key Measures for the Formation and Prevention of Cracks in Silica Bricks
1. Machine Pressing
Laminations in silica bricks are primarily caused by improper control of the machine-pressing process and are sometimes referred to as machine-pressed cracks. The raw materials and blanks of silica refractory bricks are composed of three phases of matter: solid, water or other binders and air. During the entire process of mechanical compression molding or die pressing, the amount of solid and liquid phases does not change, while the amount of air in the blank is compressed and reduced due to the action of pressure, and the volume of the compressed blank is also reduced accordingly. The die pressing process can be roughly divided into the following three stages: (1) In the first stage, under the action of pressure, the particles in the blank begin to move and reconfigure into a denser stack. The characteristic of this process is obvious compression. When the pressure increases to a certain value, it enters the second stage. (2) In the second stage, the particles undergo brittle and elastic deformation. After the blank is compressed to a certain extent, further compression is hindered. When the pressure increases and reaches the external force that causes the particles to deform again, the blank is re-compressed, and the density of the blank increases accordingly. This stage is a stage where compression and pressurization become short and frequent. (3) In the third stage, under the limit pressure, the relative density of the blank is basically stable and difficult to increase. The die pressing of the brick blank is completed. During the compression molding process, the delayed expansion of the green body due to elastic aftereffects must be controlled to less than 2%. Failure to do so will often result in product rejection during the pressing process. If the green body forms "layered density" along the direction of pressure applied, with a density difference exceeding 2%, layered cracking is likely to occur within the green body. This leads to uneven thermal expansion during firing, resulting in significant thermal stress and the formation of longitudinal cracks parallel to the density layers, resulting in product rejection.
During compression molding, pressure is used to overcome internal friction between particles, external friction between particles and the mold wall, and deformation of the pressed green body. As the distance from the pressing head increases, the internal pressure of the green body decreases.
Therefore, when pressing silica bricks, it is best to use short molds with a small aspect ratio, rather than tall molds with a large aspect ratio, to ensure uniform pressure distribution within the green body. At the same time, certain plasticizers and surfactants are introduced into the blank to reduce internal friction and pressure transmission losses; mold finishes are improved or lubricated to reduce external friction; double-sided pressing is used to reduce the L/D ratio of the blank; and multiple pressurizations, starting with light and then heavy, are employed to prevent excessive pressure accumulation within the blank and eliminate elastic aftereffects. These measures improve the uniformity of pressure and density within the blank. This prevents high density near the pressure surface and low density far from the pressure surface within the silica brick blank, thereby reducing the formation of layer density and cracks.
In addition, silica brick blanks are prepared by mixing aggregate, clinker, ball mill powder, mineralizer, sulfite pulp waste liquor, and plasticizer. Improving the blank kneading process can also help increase the density of the blank. In terms of physical mixing technology, the movement of materials in the same phase is called mixing, the movement of materials in different phases is called stirring, and the mixing of high-viscosity liquids and solids is called kneading (kneading and mixing). Through proper kneading, fine powder can be coated around larger particles, effectively removing gases and increasing the densification of the brick, thereby reducing the porosity of the brick.
2. Firing Process
The sintering of silica bricks is essentially a polycrystalline transformation of SiO2. Under the action of mineralizers, the silica raw material is slowly sintered, essentially transforming into tridymite and cristobalite, with only a small amount of residual quartz. During use, silica firebricks experience a total volume expansion of 1.5% to 2.2% when heated to 1450°C. This residual expansion seals the mortar joints, ensuring good tightness and structural strength in silica brick masonry. Furthermore, this polycrystalline transformation of SiO2 dictates that silica fire bricks be the focus of refractory material monitoring during the initial kiln firing phase, with a slow and uniform heating rate being the characteristic. Because the crystal transformation of β- and α-cristobalite in silica firebricks is accompanied by a significant volume effect within the temperature range of 150-300°C, special care should be taken to slowly increase the temperature within this range during kiln firing.
The physical and chemical changes that occur during the firing of silica bricks can be summarized as follows:
① Residual moisture in the bricks is removed below 150°C.
② Ca(OH)2 begins to decompose between 450-550°C and is complete by 550°C. At this point, the bonds between the silica brick particles are broken by the action of CaO and other substances, resulting in a decrease in strength and a brittle brick.
③ At 550-650°C, β-quartz bricks transform into monoquartz, causing volume expansion.
④ At 600-700°C, a solid-phase reaction occurs between CaO and SiO2, increasing brick strength.
⑤ At 800-1100°C, a liquid-phase reaction occurs in the bricks, rapidly increasing brick strength. Starting at 1100°C, the rate of quartz conversion increases significantly, and low-density quartz forms, causing significant volume expansion.
⑥ At 1300-1350℃, due to the increase in the amount of tridymite and cristobalite, the true specific gravity of the green body decreases, and the volume expansion increases, which may lead to cracking.
⑦ At 1350-1470℃, the degree of quartz conversion and the resulting expansion are very large. Only monoquartz, metastable cristobalite, mineralizers and impurities interact to form a liquid phase, and invade the quartz particles to form cracks when metastable cristobalite is formed, which promotes the continuous dissolution of monoquartz and metastable cristobalite in the formed liquid phase, making it a supersaturated melt of silicon and oxygen, and then continuously crystallizes from the melt in the form of stable tridymite. At this time, the greater the viscosity of the liquid phase, the faster the silica brick conversion speed, and the greater the possibility of cracks in the brick green body. Therefore, in order to prevent the silica brick from undergoing crystal form changes during the firing process, accompanied by large volume changes leading to the formation of cracks, the following process measures must be taken:
(1) Control the heating rate of different firing temperature ranges. The heating rate should be slowed down when the temperature is less than 600℃. The heating rate can be accelerated when the temperature is between 600℃ and 1000℃. The heating rate should be slow when the temperature is between 1100℃ and 1300℃. When the temperature is between 1300℃ and the firing temperature (1430℃ to 1450℃), the heating rate should be the slowest during the firing process. When the fired silica bricks are cooled below 600℃, especially at 300℃, they should be cooled slowly. This can effectively buffer the volume change of the crystal transformation, making the content of tridymite and cristobalite higher and avoiding the formation of cracks.
(2) A reducing atmosphere should be used during the high-temperature firing stage, which is conducive to the mineralization of low-valent iron oxide and promotes the large-scale production of tridymite. Otherwise, in an oxidizing atmosphere, especially when the mineralizer is insufficient, most of the α-quartz is converted into α-cristobalite. This conversion is called "dry conversion". During dry conversion, due to the uneven volume expansion of the brick body and the lack of liquid phase buffering stress, the product structure will become loose and cracked. At the same time, proper insulation should be carried out at different temperature stages of silica brick firing to ensure that the silica bricks have a reasonable phase composition and meet the requirements of use.
(3) Improve the semi-finished product loading system to reduce the probability of cracks. Transverse cracks in silica refractory bricks, that is, cracks parallel to the pressure direction of the product, are usually caused by uneven heating of the various parts of the product during firing. They mostly appear on the fire-exposed surface outside the brick stack, especially the surface of the top product. The mesh cracks on the surface of silica fire bricks, in addition to the microscopic unevenness of the green body itself due to uneven kneading or changes in raw materials, are usually caused by the product being heated at too high a temperature with large fluctuations. When loading, special silicon bricks need to be placed inside the kiln car, and standard ordinary bricks need to be placed outside the kiln car; the protruding parts of special-shaped bricks or parts prone to cracks should be placed inward; the top of the kiln car should be covered with some thin bricks to avoid direct impact from the flame, etc. Otherwise, more cracks will occur.
Cracks are a major factor affecting silicon brick yield and performance. Mastering the press molding and firing processes is key to preventing cracks in silica bricks. Theoretical and actual conversions of silica raw materials vary, and the firing schedule must be adjusted in real time based on changes in raw materials, brick type, and other factors. The preparation and quality of silica brick blanks are important, even critical factors. Only by strictly controlling each process step can high-performance silica bricks be produced efficiently and with low energy consumption.
