Silica Brick

Firing Expansion of Silica Bricks

Silica bricks undergo phase changes during the firing process and have large volume changes. In addition, the amount of solution formed by bricks at the firing temperature is small (about 10%), making it much more difficult to fire than other refractory products. The firing system of silica bricks is related to a series of physical and chemical changes that occur during the firing process of the bricks, the quantity and nature of the added materials, the shape and size of the bricks, and the characteristics of the firing kiln.

 

Physical and chemical changes of silica brick during the firing process. Silica bricks undergo the following changes in temperature sequence during the firing process. Drain residual moisture from bricks below 150°C. At 450°C, Ca(OH)2 begins to decompose. At 450~500℃, the dehydration of Ca(OH)2 is completed, the combination of silica particles and lime is destroyed, and the strength of the green body is greatly reduced. In the range of 550~650℃, β-quartz transforms into α-quartz. Since the transformation process is accompanied by a volume expansion of 0.82%, the quartz crystal will have microscopic cracks with varying densities. Between 600 and 700°C, the solid phase reaction between CaO and SiO2 begins, and the strength of the brick increases. Starting from 1100°C, the transformation speed of quartz increases greatly and the specific gravity of bricks also decreases significantly. At this time, the brick volume is greatly increased due to the transformation of quartz into a low specific gravity variant. Although the liquid phase is also increasing at this time, cracks can still occur in the range of 1100~1200.

 

The volume of siliceous bricks changes during the heating process, which is different at different temperature stages. According to the changing characteristics of various types of silica green bodies, they can be divided into stages below 600℃, 600~1100℃, 1100~1250℃, and above 1250℃. There is a significant expansion in the β→α quartz transition between 500 and 600°C. At 700~800℃, there is a solid phase reaction between CaO and SiO2, which causes microscopic shrinkage. At 1000~1100℃, the shrinkage is more significant due to the formation of liquid phase. Starting from about 1200°C, the green body begins to expand violently due to the acceleration of quartz transformation.

Process of Silica Brick

Raw Material Preparation
The first step in the manufacturing process of silica bricks is the preparation of raw materials. This involves selecting high-purity quartzite or silica sand and grinding it into a fine powder. The ground silica is then mixed with a small amount of lime (typically around 2-3%) and water to form a homogenous mixture.

The goal of this stage is to ensure that the silica particles are uniformly distributed and that the mixture has the proper consistency for shaping. In some cases, other materials such as binders or additives may be introduced to improve the workability of the mixture or enhance specific properties of the finished bricks, such as thermal shock resistance or chemical durability.

The grinding process is crucial because the particle size of the silica affects the brick’s density and mechanical strength. Finer particles lead to denser and stronger bricks, while coarser particles may result in bricks with lower strength and durability.

Shaping (forming The Bricks)
Once the raw materials are prepared, the next step is shaping the mixture into bricks. There are several methods for shaping silica bricks, depending on the desired properties and application:

Dry pressing: Dry pressing is the most common method for shaping silica bricks. In this process, the raw material mixture is placed into a mold, and a hydraulic or mechanical press applies high pressure to compact the material into a brick shape. Dry pressing allows for the production of bricks with uniform dimensions and high density, making it suitable for large-scale production.

Extrusion: In the extrusion method, the raw material mixture is forced through a die to form a continuous column, which is then cut into individual bricks of the desired size. Extrusion is often used when producing bricks with complex shapes or when a higher degree of flexibility in design is required.

Hand molding: Hand molding is a labor-intensive process in which the raw material mixture is manually placed into molds and shaped by hand. This method is typically used for small-scale production or for creating custom-shaped bricks that cannot be produced by dry pressing or extrusion.

Drying
After shaping, the formed silica bricks must undergo a drying process to remove excess moisture from the material. Drying is essential because any remaining water in the bricks could cause cracking or warping during the firing process. The drying process usually takes place in drying chambers or kilns, where the bricks are gradually heated to temperatures between 110°C and 200°C to ensure even moisture removal.

The drying stage must be carefully controlled to prevent the bricks from drying too quickly, which could lead to cracking or deformation. Typically, the drying process takes several hours to several days, depending on the size and thickness of the bricks, as well as the drying method used.

Firing
Firing is the most critical stage in the manufacturing process of silica bricks, as it is during this stage that the silica grains are sintered together to form a strong, durable structure. Firing takes place in high-temperature kilns, where the bricks are heated to temperatures ranging from 1,400°C to 1,450°C (2,552°F to 2,642°F).

During firing, the lime present in the mixture acts as a flux, promoting the formation of a bond between the silica grains. This bonding process is essential for the brick’s mechanical strength and thermal stability. The duration of the firing process varies depending on the type of kiln used and the size of the bricks, but it typically lasts for several hours to ensure that the bricks are fully sintered and achieve the desired properties.

Quality Control And Testing
Once the firing process is complete, the finished silica bricks undergo a series of quality control tests to ensure that they meet the required specifications. These tests include:

Dimensional accuracy: Ensuring that the bricks meet the specified size and shape tolerances.

Bulk density: Measuring the density of the bricks to ensure that they are sufficiently compacted and durable.

Cold crushing strength: Testing the mechanical strength of the bricks by applying pressure until they break.

Refractoriness: Verifying the brick’s ability to withstand high temperatures without melting or deforming.

Thermal shock resistance: Testing the brick’s ability to resist cracking or spalling when exposed to rapid temperature changes.

Only bricks that pass these rigorous quality control tests are approved for use in high-temperature industrial applications.