Silica Brick

Silica brick is a refractory product with silicon oxide (SiO2) as the main component. Silica brick for open furnace and silica brick for coke oven.

Performance Silica brick has a high refractoriness under load, which is only 30~50℃ different from the refractoriness, a characteristic that other fire-resistant materials do not have. The volume expansion caused by the crystal transformation during heating is concentrated before 600℃, so the thermal shock resistance is poor at low temperature stage, but very good at high temperature stage. The low density of the product can reduce the load of the furnace. Silica brick raw materials are widely distributed and easy to mine. Bricks can be made without ore dressing, so the price is relatively low.

 

Silica brick manufacturing process: Clean silica or quartzite is crushed to less than 3mm, and then ground into fine powder less than 0.088mm by ball mill or other fine grinding equipment. Raw materials of various particle sizes are reasonably graded according to the product and quality requirements. In order to reduce the expansion during firing, waste silica bricks accounting for 5% to 20% of the total amount can be added.

Lime (usually added in the form of lime milk), iron oxide and other mineralizers that can react with SiO2 to form a certain liquid phase at high temperature without seriously affecting the refractoriness are added to the ingredients. The liquid phase can promote the transformation of quartz to tridymite and can alleviate the stress caused by expansion during firing, thereby reducing cracks in the product. The above raw materials and mineralizers are mixed in a wet mill and a binder sulfite pulp waste liquid is added. The mixed mud is placed in a brick mold and made into a brick blank by machine or hand. The brick blank is dried in a tunnel dryer or a chamber dryer, and the initial temperature is not more than 70℃. Brick blanks larger than 40kg should be dried in air for more than 48 hours. The moisture content of the dried bricks is not more than 1%, and the moisture content of the large bricks is not more than 0.5%. After the appearance and cross-section inspection, they are fired in tunnel kilns, downdraft kilns or shuttle kilns. The maximum firing temperature is 1380~1450℃, which varies depending on the quartz conversion rate in the raw materials and the quality requirements of the products. Firing is a key process and the heating curve must be strictly followed. The cooled products should be inspected piece by piece. The appearance should be regular, and the dimensional deviation and surface defects such as melting holes and cracks should not exceed the regulations.

The manufacturing process determines the mineral composition of silica bricks. Under the same firing system, products with more mineralizers and finer particles have more tridymite content, which is suitable for silica bricks for coke ovens and silica bricks for hot blast furnaces. Silica bricks for glass kilns require high SiO2 content and less mineralizer addition, so the bricks contain more quartz.

The main crystal mineral composition of silica bricks at room temperature is a mixture of γ-tridymite, β-cristobalite and β-quartz. The proportions vary according to different processes, ranging from β-cristobalite 15%~60% γ-tridymite 35%~80% β-quartz 0%~10%

Silicon bricks are mainly used in coke ovens, blast furnace hot blast furnaces, glass kilns, and some countries still use silica bricks for steelmaking electric furnace covers.

Silicon bricks should be stored in waterproof warehouses and can still be used after being slowly dried below 100°C after being damp. The strength of water-containing silica bricks is affected by freezing and needs to be re-inspected. When building a furnace, expansion joints should be left according to the linear expansion rate provided by the manufacturer. The temperature-time heating curve for baking and periodic use should be reasonably formulated, especially in the stage of more intense expansion before 600°C, which should be strictly controlled.

Microstructure of Siliceous Refractory Materials

Silica bricks are composed of a multi-phase heterogeneous structure of tridymite, cristobalite, residual quartz, silicate, glass phase, etc. Tridymite, cristobalite, silicate and glass phase constitute the matrix structural unit. Orthorhombic low-temperature γ-tridymite often exists in the form of a pseudo-body after the transformation of hexagonal high-temperature α-tridymite, or forms a "wedge-shaped" twin crystal with a (10T6) twin bonding surface. The volume fraction and direct bonding rate of γ-tridymite are quantitative parameters for evaluating the degree of tridymite and the continuity of the network structure.

Pseudowollastonite, calcium iron pyroxene, calcium iron olivine and dicalcium silicate constitute the silicate phase, which is dispersed or enriched in the voids of the tridymite network. Unevenly distributed iron oxides can also appear in the matrix in the form of α-Fe2O3 phase. Metastable or stable cristobalite, residual quartz, etc. constitute the particle structural unit. Metastable cristobalite is a metastable phase of α-cristobalite between 870 and 1470℃. It shows a homogeneous effect under a single polarization system and is difficult to distinguish from stable cristobalite in the X-ray diffraction spectrum. In the early stage of the transformation of α-quartz to metastable cristobalite, metastable cristobalite forms a continuous network, separating the residual quartz into isolated islands. As the thermal process continues, the volume fraction of residual quartz decreases or disappears from the particle structure unit. The brown reaction layer at the edge of the particle is composed of glass phase, tridymite, and metastable cristobalite, and is the product of the interaction between the mineralizer in the matrix and the particles.

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