Silica brick is the most common silica refractory material. Because of its high high temperature strength and load softening temperature, good high temperature creep resistance and strong resistance to acid slag erosion, it is widely used in industrial production. application. There are generally three crystal phases in silicon bricks, namely tridymite, cristobalite and a small amount of residual quartz, and the true density increases sequentially. Generally speaking, true density, thermal expansion coefficient, tridymite and residual quartz content are the most critical performance indicators for characterizing silica bricks. During the firing process, the greater the degree of conversion of quartz into tridymite with stable volume and cristobalite with excellent high temperature performance, the less residual quartz content and the lower the true density of the silica brick, the better the high temperature volume stability. During use The further expansion is also smaller.
1.Selection of raw materials for silica bricks
The silica suitable for refractory materials is mainly quartzite, which can be divided into crystalline silica and cemented silica according to the type of its structure. Generally speaking, the purity of crystalline silica is high, the raw material density is large, the quartz crystal particles are larger, and the transformation speed is slow when heated; cemented silica often contains a small amount of impurities, the purity is relatively low, and the quartz particles in the cemented silica are small in crystals , The cement content is more, the conversion speed is faster when heating. Therefore, a reasonable production process should be formulated according to the characteristics of silica raw materials to produce silica bricks suitable for different purposes.
Crystalline silica and cemented silica have their own advantages and disadvantages. It is also a good choice to compound the two to give full play to their respective advantages.
2. The choice of mineralizer
In the production process of silica bricks, a certain amount of mineralizer is often introduced. Its function is mainly to use the mineralizer and SiO2 or other impurities to form a low-melting high-temperature liquid phase, which promotes the conversion of quartz into tridymite and square quartz during the firing process. Quartz can also buffer the rapid expansion of the volume caused by the rapid phase change during the firing process, which leads to the loosening and cracking of the product.
At present, the widely used mineralizers are lime and iron scale. Lime is usually added in the form of lime milk. It can not only increase the strength of the brick after forming, but also can react with SiO2 in the low-temperature firing stage (600~700℃) to increase the strength of the brick. Wollastonite can form a liquid phase with other mineralizers to convert quartz to tridymite. Iron scale is often added as a mineralizer at the same time as lime, which can significantly reduce the temperature and viscosity of the liquid phase and reduce product cracks. In order to make the scales evenly distributed in the ingredients to achieve a good mineralization effect, the mass fraction of particle size ≤0.088mm is required to be >80%. In addition to lime and iron scales, fluorite and feldspar composite, MnO2, and C3S have also been shown to have a positive effect in promoting the formation of tridymite.
In addition to the type of mineralizer, the particle size of the mineralizer is also more important. The finer the particle size of the mineralizer, the more evenly distributed it is in the siliceous raw material, and the better its effect. Nano-scale mineralizers have good dispersibility and higher mineralization efficiency, which makes the internal particles of siliceous products and the volume expansion and contraction between the particles in the process of crystal transformation better synchronization, reducing the volume stress caused by Crack pores, while improving the physical and mechanical properties of silica bricks, reducing the true density of siliceous products, and reducing the content of residual quartz in the products.
3.Introduction of additives.
For different purposes, certain properties of silica bricks, such as thermal conductivity, abrasion resistance and thermal shock resistance, need to be further strengthened. At this time, in addition to the rational selection of silica raw materials and the addition of appropriate mineralizers, a certain amount of additives need to be introduced to achieve the desired effect.
Adding SiC to silica bricks can promote the formation of tridymite, reduce its thermal expansion rate and creep rate, increase thermal conductivity and high temperature flexural strength; adding Si3N4 can improve the thermal shock stability of silica bricks, and the addition amount is 5 %, it has a higher content of tridymite and a dense microstructure; metal and its oxides as additives such as TiO2 added to siliceous refractories can reduce the material’s apparent porosity, increase bulk density, reduce residual quartz content, and increase tridymite Content to optimize material strength and fire resistance.
