Sintered Silica Refractories

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Introduction of Sintered Silica Refractories

Sintered Silica Refractories

Silica refractories were first produced in the United Kingdom in 1822 from Ganister (carboniferous sandstone) or the so-called Dinas sand. Silica occurs in a variety of crystalline modifications, e.g. quartz, tridymite, and cristobalite, and also as an under-cooled melt called quartz glass.

The crystalline modifications each have a high and low-temperature form that can transform reversibly. The crystal structure of the individual SiO2 modifications can differ widely so that distinct density changes occur during transformation. This is of great importance during heating and cooling because of the change in the volume.

Production of the Silica Refractory Bricks

sintered silica refractory bricks

The silica refractories are manufactured as multiple asymmetric shapes, which are normally keyed or interlocked with each other by means of tongues and grooves.

It is the objective of the manufacturer of silica refractory bricks to select the raw materials and the firing process in such a manner that the degree of quartz transformation is suitable for the intended application of the brick. The raw material for silica brick is naturally occurring quartzite which must meet certain requirements to achieve optimum brick properties. If refractoriness or thermal expansion under load (creep) are the main requirements, a quartzite of high chemical purity must be selected. Raw materials for volume-stable products should have good transformation properties.

The chemical composition of quartzite is important in its evaluation as a raw material, in particular the content of alumina and alkalis, as these lower the melting point and considerably reduce the possibilities of application. In addition, the firing behavior of the quartzite must be taken into account.

After the washed raw materials have been crushed, ground, and screened to the various grain fractions, the individual fractions are combined in predetermined proportions according to the required application properties. In most cases, muller mixers are used for mixing and special bonding agents. Generally around 2 % slaked lime in liquid form (lime water) and some sulfite solution as a temporary binder, are added at the same time. The friable mix is then processed on friction presses or hydraulic presses. Complicated shapes or those where short runs are required, are still rammed by hand. Drying takes only a short time, one to two days, as lime-bonded silica is not sensitive when drying.

Sintered Silica Refractories
Sintered Silica Refractories

Silica refractory bricks are fired at a temperature of about 1450-1500 deg C with longer holding times being required at the highest temperatures. For this reason, firing in annular kilns or bogie hearth furnaces is preferred. Because the transformation of the silica modifications takes place suddenly, cooling must be carried out slowly or the bricks will crack. It is necessary to maintain a carefully planned time-temperature cycle during firing because there are critical temperature ranges through which the silica brick must pass so that strong, well-bonded bricks are obtained.

During firing, the linear growth of silica brick is about 4 %. The growth is less than the increase in the molar volume would suggest, because there is a contraction of the pores.

Applications of Silica Refractory Bricks

  • Metallurgy: Silica refractory bricks are used in blast furnaces due to their high-temperature resistance.
  • Glass Manufacturing: They’re used in glass melting kilns for their resistance to thermal shock and acids.
  • Ceramics: Silica refractory bricks are chosen for ceramic kilns because they can withstand high temperatures and thermal shock.
  • Chemical Industry: Due to their acid resistance, silica refractory bricks are used as building materials for corrosion-resistant equipment.
Sintered Silica Refractories

Physical Performance of Silica Refractory Brick

  • Acid-base Resistance
    Silica refractory brick belongs to acid refractory material and has strong resistance to acid slag erosion and oxides of CaO, FeO, and Fe2O3. But silica fire brick is easily damaged by Al2O3 oxide when eroded strongly by alkali slag.
  • Expansibility
    Silica refractory bricks’ thermal conductivity and bulk increase with the service temperature rising. There is no linear after-contraction during the baking process. During the baking process, the maximum expansion is about 100~300℃, expansion amount is about 70~75% accounting for the total expansion amount before 300℃. As the SiO2 happens four crystalline form inversion points of 117, 163, 180~270, and 573℃ during the baking process. Of which 180~270℃ is mainly affected by cristobalite.
  • Refractoriness Under Load
    Silica refractory bricks have high refractoriness under load. The fusion point is about 1640~4680 ℃ which is close to tridymite and cristobalites.
  • Thermal Stability
    Silica refractory brick has low thermal shock resistance and its refractoriness is about 1690~1730℃, which results in the limit for its application range. True density is the most important fact for silica fire bricks’ heat conductivity. The lower the true density and its lime convert more completely, there will be smaller after-expansion during the baking process.

Properties of Silica Refractory Bricks

The fired silica refractory brick contains the crystalline SiO2 modifications cristobalite, tridymite, and some residual quartz. During the firing process, the lime reacts with the finest quartzite components to form wollastonite (CaO.SiO2). The matrix also contains very small quantities of calcium ferrite, hematite, magnetite, calcium olivine, and hedenbergite [calcium ferrous silicate, CaFe(SiO3)2], which are formed from impurities. These crystalline phases are the reason for the discoloration and spot formation on the fired bricks.

The transformed coarse grain generally consists of cristobalite, a proportion of residual quartz corresponding to the degree of transformation, and very little tridymite, whereas the fine-grained matrix is enriched with tridymite, glass, and wollastonite. Silica refractory bricks with identical chemical composition can have differing mineralogical compositions and this can cause quite different behavior during use. Therefore, it is not always sufficient to evaluate silica bricks solely by their chemical composition. It is essential to also consider the degree of transformation (residual quartz content) and the thermal expansion behavior of the bricks.

The degree of transformation of the bricks can be determined easily and accurately by the density of the residual quartzite content. The density of a fired silica brick is lowest when the degree of transformation is farthest advanced and attains the value of 2.33 g/cu cm with a complete transformation. The density allows conclusions to be drawn with respect to the irreversible after-expansion which must be expected during service. The degree of transformation can be evaluated even more accurately with the help of the residual quartz content, which is determined by the radiographic phase analysis or X-ray diffraction analysis.

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