Silica refractories were first created in 1822 in the United Kingdom from Ganister (caboniferous sandstone) or Dinas sand.
Quartz, tridymite, and cristobalite are examples of crystalline modifications of silica, as well as an under-cooled melt known as quartz glass. Each of the crystalline alterations has a high and low temperature form that can transiently transition. Individual SiO2 modifications can have quite different crystal structures, resulting in discrete density variations throughout transformation. Due to the sheer volume change during heating and cooling, this is quite important.
Production of silica refractories
The silica refractories are composed of a number of asymmetrical structures that are generally linked or locked together using tongues and grooves. The goal of a silica refractory brick producer is to choose raw materials and fire them in such a way that the degree of quartz transformation is appropriate for the brick’s intended application. Natural quartzite is used as the raw material for silica brick, and it must meet particular specifications in order to create the best brick qualities. If refractoriness or thermal expansion under load (creep) are important, a quartzite with a high chemical purity should be chosen. Volume of raw materials The transformation properties of stable goods should be good.
The chemical composition of quartzite is significant in determining its suitability as a raw material, particularly the presence of alumina and alkalis, which lower the melting point and limit the range of applications. In addition, the quartzite’s firing tendency must be taken with consideration.
The distinct grain fractions are blended in predefined proportions according to the needed application qualities after the washed raw materials have been crushed, ground, and screened to the various grain fractions. Muller mixers are typically used for mixing and specific bonding agents. Around 2% slaked lime in liquid form (lime water) is added at the same time, along with some sulphite solution as a temporary binder. Friction presses or hydraulic presses are used to process the friable mix. Hand-ramming is still used for complicated forms or those that require short passes. Because lime bound silica is not sensitive to drying, it simply takes one to two days to dry.
Silica bricks are burnt at temperatures ranging from 1450 to 1500 degrees Celsius, with higher temperatures necessitating longer hold periods. As a result, circular kilns or bogie hearth furnaces are favoured for fire. As a result, circular kilns or bogie hearth furnaces are favoured for fire. Because the silica changes change so quickly, cooling must be done gently or the bricks would crack. Because there are key temperature ranges over which the silica brick must transit in order to produce strong, well-bonded bricks, it is vital to maintain a well designed time temperature cycle throughout firing.
The linear growth of silica brick during burning is roughly 4%. Because the pores are contracting, the growth is smaller than the rise in molar volume would recommend.
Silica bricks have certain properties.
Silicon dioxide is the major component of silica refractories (SiO2). Silica refractories also contain calcium compounds generated from calcium hydroxide, which is used as a bonding agent, despite the presence of contaminants in the raw materials.
The crystalline SiO2 modifications cristobalite, tridymite, and some residual quartz are present in the burnt silica brick. The lime combines with the finer quartzite components during the burning process to generate wollastonite (CaO.SiO2). Calcium ferrite, hematite, magnetite, calcium olivine, and hedenbergite [calcium ferrous silicate, café(SiO3)2] are all impurities found in the matrix. The discolouration and spot formation on the burnt bricks are caused by these crystalline phases.
Cristobalite, a fraction of residual quartz proportional to the degree of conversion, and very little tridymite make up the transformed coarse grain, but the fine-grained matrix is richer with tridymite, glass, and wollastonite. Silica bricks with the same chemical composition can have quite varied mineralogical compositions, resulting in very diverse behaviour when used. As a result, judging silica bricks merely on the basis of their chemical makeup is not always sufficient. The degree of transformation (residual quartz content) and the thermal expansion characteristic of the bricks must also be considered.
The density of the residual quartzite material may quickly and correctly identify the degree of alteration of the bricks. When the degree of transformation is farthest progressed, the density of a burnt silica brick is lowest, reaching 2.33 g/cu cm with complete transformation. Because of the density, conclusions can be formed about the irreversible after-expansion that must be expected during service. The residual quartz content, which is assessed by radiographic phase analysis or X-ray diffraction analysis, can be used to assess the degree of transformation even more precisely.
Application
The most common refractory utilized in the construction of a coke oven battery is silica refractory (COB). Silica is the preferred refractory because, at ordinary COB operating temperatures, silica refractories exhibit negligible creep. Furthermore, because practically all silica brick expansion occurs below 650 degrees Celsius during typical COB operation, the minor temperature changes of the walls have no impact on the volume integrity of the refractory that makes up the wall. In the development of a COB design, there are over 400 distinct shapes that can be used. A silica mortar is used to install these shapes.
Since the late 1950s, there has been a widespread trend in COB building to utilise high bulk density (BD) silica bricks (BD more than 1850 kg/cum), because increasing BD results in increased cold strength and heat conductivity.
Silica bricks are primarily used in glass melting furnaces, hot blast burners, and electric arc furnace roofs, in addition to COB.
Silica mortar
Silica mortar is mostly made out of quartz, a silica mineral. An air establishing, which contains a small amount of sodium silicate, and a heat setting, which is essentially the same mortar except without the sodium silicate, are the two main varieties of mortar used in common. At normal battery operating temperatures, neither type of silica mortar bonds to the silica brick. As a result, the wall does not gain any strength from it. The mortar is also not volume stable because the principal mineral element is quartz. During typical battery operation, the quartz in silica mortar put in an operating battery slowly transforms to the high temperature forms of silica—tridymite and cristobalite. There is a large increase in volume as a result of this conversion. On the hotter flue side of the wall, the conversion takes place first. This means that the mortar in the horizontal joints takes on a wedge form, with the flue side of the wall being thicker and the oven side being thinner.