Rotary Kiln Bricks: Selection Criteria & Key Functions - Complete Guide

Kiln bricks are a layer of refractory material lining the inside of the kiln cylinder, and their main functions are as follows:

• To protect the kiln shell by reducing chemical corrosion and mechanical abrasion caused by high-temperature flue gas and materials.

• To act as a heat transfer medium: kiln bricks can absorb part of the heat from the flue gas and transfer it to the materials through conduction and radiation.

• To provide thermal insulation and reduce heat loss from the kiln body.

According to the different application positions of refractory materials inside the kiln, the main requirements for kiln bricks of a rotary kiln are as follows:

1.Excellent high-temperature resistance

The temperature inside the kiln remains above 1000 ℃ regardless of the sintering conditions in the kiln tube. This requires refractory bricks to be non-melting at high temperatures and to maintain a certain level of strength below their melting point. Meanwhile, they must also be capable of maintaining a stable volume even when exposed to high temperatures for a long period.

2.Superior thermal shock resistance

This refers to a strong ability to resist damage caused by drastic temperature fluctuations inside the kiln. The kiln temperature changes significantly during shutdown, start-up, and unstable operation of the rotary kiln. Therefore, kiln bricks must not crack or spall under severe temperature variations, which also requires the kiln temperature to be kept as stable as possible during operation.

3.High chemical corrosion resistance

The ash, slag, and vapor formed during sintering in the rotary kiln can cause severe corrosion to kiln bricks.

4.Good wear resistance and mechanical strength

The sliding of feed materials and friction from dust in the airflow inside the kiln cause heavy wear to kiln bricks, especially during the initial stage of kiln start-up when no kiln coating is present for protection. Kiln bricks must also withstand expansion stress at high temperatures and stress caused by the elliptical deformation of the kiln shell; therefore, they are required to have a certain level of mechanical strength.

5.Excellent kiln coating adherence

Kiln coating adhering to kiln bricks provides significant protection. If kiln bricks have good kiln coating adherence and the coating can be maintained for a long time, the bricks can be effectively protected from corrosion and wear.

6.Low porosity

High porosity allows corrosive kiln gases to penetrate and condense inside the kiln bricks, causing damage-this is especially true for alkaline gases.

7.Stable thermal expansion performance

Although the thermal expansion coefficient of the kiln shell is higher than that of kiln bricks, the temperature of the kiln shell is generally around 280–450 ℃, while that of kiln bricks is usually above 800 ℃ (and can exceed 1300 ℃ in the burning zone). As a result, the thermal expansion of kiln bricks is still greater than that of the kiln shell, making the bricks prone to spalling under compressive stress.

The service life of kiln bricks is affected by the quality and refractory properties of the bricks, the appropriateness of their selection, and the quality of the bricklaying work. In turn, the service life of kiln bricks directly affects the maintenance costs and production output of rotary kilns, underscoring the critical importance of refractory bricks.

When selecting kiln bricks, the general practice is to make choices based on the different temperatures and loads in various kiln zones, with particular attention paid to ensuring that the thermal expansion coefficient of the kiln bricks closely matches that of the kiln shell. Increasing the thickness of kiln bricks improves thermal insulation for the kiln shell; however, it reduces the effective cross-sectional area inside the kiln, thereby lowering the kiln's production output. Conversely, thinner kiln bricks increase the effective internal volume of the kiln but result in greater heat dissipation from the kiln shell, leading to more severe high-temperature damage to equipment and significant energy loss.

At present, two brick systems are most widely used internationally: the ISO system (International Standard) and the VDZ system (German Standard). In fact, the raw materials used in both systems are identical, with the only differences being their specifications and dimensions. ISO system bricks are larger and thicker, with fewer brick joints, but they are inconvenient to transport and handle. By contrast, VDZ system bricks are smaller and thinner, making them easier to handle and install; however, they have lower strength and a greater number of brick joints. For the ISO system, it is advisable to standardize specifications as much as possible-adopting a height of 220 mm and a width of 200 mm-as this reduces the number of spare bricks required and lowers investment costs.

Refractory bricks can be classified into the following types according to their constituent materials:

Fire clay bricks (insulating bricks):

Used in the preheating and calcining zones, with a refractoriness grade below SK35.

High alumina bricks:

Their main component is Al₂O₃. The higher the Al₂O₃ content, the higher the refractoriness of the brick, along with improved spalling resistance, thermal conductivity, mechanical strength, and chemical corrosion resistance. However, high alumina bricks exhibit significant reversible expansion and have poorer slag resistance than basic refractory bricks. They are generally used in the calcining (transition) zone, with SK37 being the most commonly used refractoriness grade. SK37 or SK36 is mostly adopted in the cooling zone.

Basic refractory bricks:

These bricks exhibit excellent chemical resistance to ash and slag at high temperatures and possess good kiln coating adherence, with the coating firmly bonded to the brick surface. Because the bricks and the kiln coating fuse into an integrated whole, part of the kiln bricks may spall off together with the kiln coating when the coating detaches, resulting in poorer spalling resistance compared with high alumina bricks.

There are two main types of refractory bricks based on their material composition:

a. Dolomite bricks:

The main component of dolomite bricks is periclase (MgO), whose composition is very similar to that of the raw meal. Therefore, these bricks exhibit excellent kiln coating adherence and do not react with the raw meal. They feature high refractoriness and good volume stability; however, part of the bricks may spall off together with the kiln coating when the coating detaches. Attention must be paid to preventing carbonation during application, and strict waterproof measures must be taken during the storage and conditioning of these bricks.

b. Magnesia-chrome bricks:

The main component of magnesia-chrome bricks is chromite ore. They have good spalling resistance and mechanical strength and are generally used in the burning zone of rotary kilns. In this zone, the bricks are subjected to damage from three factors: thermal stress, mechanical stress, and chemical corrosion; therefore, the requirements for bricks used in this zone are particularly stringent. Magnesia-chrome bricks were previously the primary choice due to their high refractoriness and excellent resistance to alkaline corrosion. However, they contain a high chromium content; when discarded, chromium can leach into water and cause environmental pollution. For this reason, the use of magnesia-chrome bricks has gradually declined, and they have largely been replaced by spinel bricks. At the Hualien Plant, discarded kiln bricks are mixed with limestone at the mines and recycled as raw material for production, thereby reducing waste and mitigating environmental pollution.

Refractory mortar can be divided into the following main categories:

a. Air-setting refractory mortar (also known as castable refractory mortar):

This type is used to form a protective refractory layer on kiln head burners and as a refractory lining for certain parts of preheaters. It is generally applied by grouting.

b. Insulating refractory mortar:

This mortar is used to repair and level deformed flat surfaces of kiln shells or to provide thermal insulation on the ceilings of coolers.

c. Heat-setting refractory mortar (also referred to as regular refractory mortar):

This type is mainly used for bricklaying. When using heat-setting mortar, it must be mixed with water glass (sodium silicate solution, Na₂SiO₃·H₂O). Generally, DiDOTECT 135 (resistant to temperatures above 1350 ℃, SK34) is used at the kiln tail; DiDOTECT 150 (resistant to temperatures above 1500 ℃) and Couprit 160H (matching SK35 for high-temperature conditions) are mainly applied at kiln burner heads; Couprit 135H is primarily used in preheaters and coolers.

Selection of Kiln Bricks

a. Selection of Refractory Bricks Inside the Kiln

For Kiln #1, high alumina bricks are used from the 35-meter mark onwards and in the outlet cooling zone, while magnesia-chrome bricks or spinel bricks are used for all other areas.

For Kiln #2, high alumina bricks are used from the 43-meter mark onwards, and magnesia-chrome bricks or spinel bricks for the remaining areas.

For Kiln #3, high alumina bricks are used from the 35-meter mark onwards, with magnesia-chrome bricks or spinel bricks for other areas. Refractory mortar is applied in the outlet cooling zone of Kiln #3.

Near the riding rings, to prevent excessive heat dissipation that could cause thermal expansion of the kiln shell and subject it to high mechanical stress, the refractory bricks are arranged in a double layer of high alumina bricks and insulating bricks.

b. Selection of Refractory Bricks Inside the Cooler

The temperature of clinker in the cooler varies between the front and rear sections-higher in the front section and relatively lower in the rear section-so different refractory bricks are selected for each section. The front section mainly uses a double layer of high alumina bricks and insulating bricks, while only insulating bricks are used in the rear section.

In areas of the cooler that come into frictional contact with the clinker bed, the refractory bricks are exposed to severe high temperatures and abrasion, which imposes stricter quality requirements for the bricks used in these positions.

c. Selection of Refractory Materials Inside the Preheater

The main refractory materials used in the preheater are insulating bricks and air-setting refractory mortar of the corresponding grade. These materials are required to be lightweight while providing adequate refractoriness and fire resistance.

Inside the preheater, the inner walls of the cyclone cylinders are subjected to abrasion from powder entrained in the airflow, especially at bends and direction-changing sections of the air ducts. At the bottom of the cyclone cylinder cones, high-temperature deacidified raw meal comes into contact with the cylinder walls.

Coal is injected into the kiln gas riser pipes and tertiary air ducts to support the decomposition of carbonates in the preheater, where temperatures are also relatively high. Consequently, the refractory materials used in these parts must meet more stringent requirements.