Preparation:

• 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 in order 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 behaviour 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 sulphite 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 are not sensitive when drying.
• Silica bricks are fired at 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 are 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 a 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.

Properties:

• Silica refractories consist mainly of silicon dioxide (SiO2). Not taking into account raw materials impurities, silica refractories also contain calcium compounds formed from calcium hydroxide which is used as a bonding agent.
• The fired silica 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 bricks with identical chemical composition can have differing mineralogical compositions and this can cause quite different behaviour 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 behaviour 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 attain the value of 2.33 g/cu cm with 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.
• A characteristic of silica bricks is the difference of only 10 K between the beginning and the completion of softening during the refractoriness under load test. Because of their low glass content, the bricks form very small amounts of liquid at high temperatures. The amount of liquid phase at 1600 deg C is between 10 % and 20 % and only increases strongly above 1650 deg C. For this reason, silica bricks can be used practically up to the melting point. During use of too high temperatures, combined with chemical attack, the bricks drip or run.

Uses:

• Silica refractory is the most abundant refractory used in the construction of a coke oven battery (COB). Silica is the refractory of choice primarily because, at normal COB operating temperatures, silica refractories are subject to minimal creep. Also, since nearly all of the expansion of silica brick takes place below 650 deg C, during normal operation of a COB, the moderate temperature fluctuations of the walls have no effect on the volume stability of the refractory comprising the wall. A COB design can have well over 400 different shapes used in its construction. These shapes are installed with a silica mortar.
• Since the later part of 1950s there had been a general trend to use high bulk density (BD) silica bricks (BD greater than 1850 kg/cum) in COB construction, since increasing BD is accompanied by corresponding increases in cold strength and thermal conductivity.
• Other than COB, silica bricks are used mainly in glass melting furnaces, hot blast stoves, and electric arc furnace roofs.