How Refractory Bricks Are Fired

1. Kiln Loading Process

The firing process of refractory bricks includes three main stages: kiln loading, firing, and unloading. The first stage, kiln loading, refers to the process of arranging semi-finished refractory brick bodies in the kiln, taking into account the kiln’s structural features and the firing requirements of the product’s thermal regime. In tunnel kilns, this is also known as loading (or car loading).

• Determine the appropriate kiln loading height and method based on different brick types. For example, magnesia bricks and high-alumina bricks typically have a loading height of 1–1.1m, while silica bricks range from 1 to 1.7m, and clay bricks fall in between.
• Most bricks are loaded flat, while silica bricks are typically loaded vertically and clay bricks horizontally.
• For different brick shapes, establish the proper ratio between standard and non-standard bricks within the same kiln car. The usual ratio is around 4:6 for non-standard to standard bricks.
• To improve kiln loading efficiency, increase the density (brick load per unit car) to enhance output and reduce fuel consumption.

Ensuring the quality of the loading process is essential to maintain the correct appearance of the fired bricks. The key requirements are to keep the brick stacks level, stable, and straight, avoiding sticking together due to high-temperature firing and minimizing distortion. A layer of filler sand with a particle size of 0.5–3mm is typically applied between each layer of bricks to prevent these issues.

2. Firing Process

The firing process involves a series of physical and chemical reactions that make the brick body denser, increase its strength, stabilize its volume, and ensure precise dimensional accuracy. The firing process can be divided into three stages:

Heating Stage

This stage begins when the product enters the kiln or is ignited and continues until the highest firing temperature is reached. As the temperature rises, the product undergoes diffusion, flow, and dissolution processes. The particles at the liquid phase surface draw closer due to surface tension, compacting the body, which results in increased strength, reduced volume, and a lower porosity rate. The brick undergoes sintering during this stage.

Soaking Stage at Maximum Temperature

At the highest firing temperature, reactions in the body complete, and crystallization occurs as the liquid phase grows. This stage ensures that both the exterior and interior of the brick reach the necessary firing temperature. The heat transfer process must be evenly distributed, which takes time depending on the brick size and kiln density.

Cooling Stage

The cooling stage begins after the maximum firing temperature is reached and continues until the brick is cool enough to be unloaded. During this stage, structural and chemical changes that occurred at high temperatures are solidified. Early in this stage, further crystallization, phase transitions, and glass solidification may still occur. The cooling rate during this stage significantly affects the brick’s strength and thermal shock resistance.

To properly fire refractory products, each product type must have a specified firing schedule that includes the maximum firing temperature, the heating rate at each stage, soaking time at the maximum temperature, cooling rate, and kiln atmosphere. Firing schedules may vary based on the product’s shape and the type of firing equipment, such as large down-draft kilns, which require slower heating rates and longer soaking times for uniform temperature distribution.

3. Kiln Unloading Process

The unloading process refers to removing the fired products from the kiln after they have cooled. Proper unloading is critical to maintaining the shape and quality of the final product. Key considerations during the unloading process include:

• Handle the bricks gently to avoid shape defects caused by improper unloading.
• Keep different brick types separated to prevent confusion. If similar-shaped bricks of different types are mixed on the same kiln board, it can lead to errors during quality inspection, potentially causing usable products to be discarded.
• Properly organize and stack any tool bricks (e.g., support bricks) for future use.
• Ensure safety measures are followed during unloading and transportation to prevent accidents.