What role do calcium-free silica bricks play in kilns?

The kiln lining must withstand temperatures exceeding 1600°C and chemical erosion. Choosing the wrong material for one layer can shorten the life of the entire furnace. Because calcium-free silica bricks do not contain CaO, they are more stable against high-temperature creep and alkali corrosion. For this reason, they are increasingly being included in furnace lining configurations.

1. First, let’s look at the composition: What exactly is “free” from them?

Calcium-free silica bricks contain less than 0.1% CaO. Their primary binding phase is high-purity SiO₂ itself, rather than the traditional calcium-magnesium silicate glass phase.

As a result, after heating to 1500°C, the glass phase content within the brick is more than half that of ordinary silica bricks, resulting in a corresponding decrease in creep rate.

At the same time, the thermal expansion curve is almost linear. This reduces internal stress during thermal cycling, reducing the likelihood of cracks.

Due to the concentrated distribution and small pore size of the pores, the thermal conductivity is approximately 20% lower than that of clay bricks of the same density, which directly helps reduce heat loss from the furnace shell.

Simply put, it uses the “removal of CaO” tactic to simultaneously reduce the two problems of high-temperature plastic deformation and volume instability.

2. Typical Positions of Calcium-Free Silica Bricks in the Furnace

(1)Glass Melting Furnace

In the large arch of a glass melting furnace, alkaline volatiles are the primary source of corrosion. Calcium-free silica bricks barely react with Na₂O and K₂O, and thus do not form a low-melting-point glaze layer on their surface. Therefore, they are often placed in the middle section of the arch, where the temperature is highest.

(2)Coke Oven

Moving into the coke oven’s carbonization chamber, the intense cooling and heating cycles of coal loading and coke discharge are experienced.

Its low expansion rate allows the furnace walls to remain intact even after multiple rapid cooling cycles. Therefore, it has replaced traditional silica bricks as the preferred choice for the upper carbonization chamber.

(3)Ceramic Roller Kiln

Looking at the firing zone of a ceramic roller kiln, although the temperature is slightly lower, it is subject to severe erosion from high-velocity airflow. Calcium-free silica bricks have a high SiO₂ skeleton and high hardness, resulting in a lower wear rate than mullite bricks.

Therefore, they are also commonly used for sidewall linings. In short, it should be placed where temperatures are high, alkali corrosion is severe, and frequent hot and cold cycles occur, allowing it to act as a “high-temperature stabilizer.”

3. How to Combine with Other Bricks

Calcium-free silica bricks are acid-resistant and creep-resistant, but are susceptible to slagging caused by strong alkalis. In the same kiln, they are often paired with zirconia mullite bricks or magnesia-alumina spinel bricks in sections.

If a high-alkalinity melt appears in the lower slag line area, magnesia bricks or magnesia-chrome bricks can be used to resist corrosion. Calcium-free silica bricks can be retained in the upper, clean, high-temperature area to maintain the overall profile.

By adopting a “silicon-on-top, magnesium-bottom” or “silicon-mid, zirconium-side” transition strategy, the advantages of each material are maximized. This also prevents local defects in a single material from degrading the entire furnace life.

To further reduce energy consumption, a layer of lightweight clay insulation bricks can be added to the backing layer. The hot surface remains supported by the calcium-free silica bricks, with a sliding joint of ceramic fiber paper between the two layers to prevent them from interfering with each other during temperature rise. In general, when selecting refractory bricks, we should first check the furnace temperature curve and chemical atmosphere, then determine whether the bricks’ physical and chemical indicators match. Finally, we should also select a supplier with stable raw materials and small dimensional errors. This will ensure the longevity of the kiln and control energy consumption.