At The Present Stage Of The Cement Rotary Kiln Commonly Used in Seven Kinds Of Refractory Bricks

With the rapid development of the cement industry, refractory bricks for cement kilns have made significant progress in both variety and quality. In different parts of the cement rotary kiln, the atmosphere and operating temperature vary, so the refractory bricks used in different zones of the rotary kiln-such as the decomposition zone, transition zone, and cooling zone-also differ. At the present stage, the refractory bricks used in rotary kilns are mainly of the following types:

In the firing zone of cement rotary kilns, magnesium-chromium bricks made from directly bonded magnesia and chromite are commonly used. These bricks primarily consist of MgO and Cr₂O₃. They offer numerous advantages, such as high refractoriness, excellent high-temperature strength, strong resistance to slag erosion, and good thermal shock resistance. They also exhibit good kiln skin adherence and low thermal conductivity.

Due to their excellent high-temperature performance, magnesium-chromium bricks are widely used in the firing zone and can easily interact with kiln materials to form a stable kiln skin. However, during rotary kiln maintenance or shutdowns, the temperature fluctuations can cause structural loosening, reduced strength, spalling, and damage to the bricks.

Additionally, under high-temperature conditions and in the presence of alkali metal oxides, the Cr₂O₃ in magnesium-chromium bricks can form toxic hexavalent chromium (Cr⁶⁺) compounds such as R₂CrO₄. If not properly managed, these water-soluble Cr⁶⁺ compounds can cause severe water pollution. As a result, the use of magnesium-chromium bricks in cement kilns is subject to strict environmental regulations.

Dolomite bricks are relatively low in cost, and despite their affordability, their kiln skin adherence properties are well recognized within the industry. In the firing zone of rotary kilns, the free f-CaO in dolomite bricks reacts with C₂S in the cement clinker to form C₃S, which has a high melting point. This reaction helps create a stable kiln skin that adheres firmly to the hot surface of the brick.

Because CaO is evenly distributed throughout the brick, the entire hot surface is uniformly covered with the adhered kiln skin. This results in a strong and stable bond between the kiln skin and the brick, effectively making them function as a single unit. As a result, dolomite bricks can offer a longer service life.

However, if the kiln skin is thin, the expansion of the brick body can lead to structural spalling. Therefore, dolomite bricks are mainly used in areas where the kiln skin remains relatively stable, such as the middle section of the firing zone. Additionally, the free calcium in dolomite bricks is highly prone to hydration, which further limits their application.

Currently, research into bonding agents and waterproofing methods for dolomite bricks is ongoing, with continuous improvements being made.

Magnesia-zirconia bricks have a melting point of 2715°C, which is significantly higher than the temperature at which clinker begins to cause erosion (above 1660°C). This gives them a major advantage in terms of refractoriness, as it is higher than that of many other bricks.

Zirconia possesses high fracture strength under both hot and cold conditions due to the presence of microcracks around its particles, which help absorb external stresses. In addition, magnesia-zirconia bricks offer strong resistance to erosion from harmful substances such as SO₃, CO₂, alkalis, chlorine vapor, and the liquid phase of clinker. They also exhibit high compressive strength and excellent resistance to oxidation and reducing atmospheres.

However, magnesia-zirconia bricks have a high thermal expansion coefficient and relatively poor structural toughness. During use, they are prone to cracking and subsequent spalling damage.

Magnesite, which is highly resistant to the erosion caused by silicate cement clinker, shows improved kiln skin adherence and erosion resistance when CaO and ZrO₂ are introduced, leading to the formation of calcium zirconate. Additionally, the thermal shock resistance of the refractory is enhanced due to the volume expansion effect that promotes the growth of magnesite crystals and the martensitic phase transformation of ZrO₂.

With the application of advanced manufacturing techniques and appropriate formulations, MgO-CaO-ZrO₂ refractories can be produced with magnesite as the primary crystalline phase and CaO-ZrO₂ and C₃S as secondary phases. The resulting materials are suitable for use in the central sintering zone of the sintering belt, where operating conditions are particularly demanding. These bricks address the shortcomings of magnesia-dolomite and magnesia-spinel products-for example, the susceptibility of magnesia-dolomite to hydration and the performance instability of magnesia-spinel under kiln skin conditions, as well as its high thermal conductivity.

However, the high production cost of calcium zirconium magnesite bricks significantly limits their widespread application.

Magnesium-iron spinel brick is an environmentally friendly refractory material with no chromium content, resulting in very low pollution. It has excellent thermal shock stability, good kiln skin adhesion, strong erosion resistance, low thermal conductivity, low thermal expansion, and superior structural flexibility.

In practical production, magnesium-iron spinel bricks typically have a service life exceeding 12 months. In the firing zone, the kiln skin forms steadily and uniformly on these bricks, without widespread damage or spalling. Even during kiln shutdowns, these bricks do not suffer from destructive falling-off phenomena, which helps maintain a low shell temperature and reduces thermal energy loss.

During firing, some microcracks develop slowly in the magnesium-iron spinel bricks. These microcracks play a positive role by improving the bricks' thermal shock stability. The presence of an appropriate amount of FeO in the bricks enhances their kiln skin adhesion properties. FeO, incorporated as part of the magnesium-iron spinel phase, not only improves the kiln skin adherence but also contributes to excellent thermal shock resistance.

The monolayer structure is the basic form of the new dry-process cement kiln lining. High thermal conductivity in lining bricks can lead to excessive energy loss and cause shell overheating and deformation. Compared with other bricks, magnesium-iron spinel bricks feature a low coefficient of thermal expansion and low thermal conductivity. This low thermal expansion protects the lining and reduces thermal stress on the kiln shell, thereby minimizing refractory damage and kiln body degradation. Additionally, low thermal conductivity lowers the cylinder surface temperature and reduces kiln skin thickness, which improves kiln operation by decreasing the kiln's load and extending its service life.

Magnesium-iron spinel bricks exhibit outstanding structural flexibility, attributable to their mineral composition, microstructure, and the formation of fine microcracks during activation. This flexibility helps reduce damage and material loss caused by mechanical stresses from kiln shell deformation, further extending the bricks' service life.

Compared to ordinary magnesium-chromium bricks, magnesium-iron spinel bricks perform far better in these aspects and significantly outperform magnesium-chromium bricks overall.

Magnesium-aluminum spinel bricks are used as basic refractory materials to replace magnesium-chromium bricks in the firing zone of cement rotary kilns. Although the production of MgO–Al₂O₃ spinel bricks using combinations of MgO and Al₂O₃ began around the 1930s, this solution was not widely adopted or utilized.

While magnesium-aluminum spinel bricks have performed well in the transition zone, their application in the firing zone still faces the following shortcomings:

a. Poor kiln skin adherence:

During production, the burner zone reaches very high temperatures. If the kiln skin in the firing zone falls off, the bricks are directly exposed to cement raw materials at high temperatures, leading to unfavorable reactions.

b. High thermal conductivity:

High thermal conductivity reduces the rigidity of the kiln shell, causing deformation and increasing mechanical stress on the refractory materials.

c. Weaker corrosion resistance compared to magnesium-chromium bricks:

Magnesium-aluminum spinel bricks have inferior resistance to corrosion from clinker components when compared to magnesium-chromium bricks.

At present, there are still many issues that need to be resolved when using magnesium-aluminum spinel bricks in the firing zone of cement rotary kilns. As a result, it is currently difficult for magnesium-aluminum spinel bricks to fully replace magnesium-chromium bricks in this application.

Magnesia-ferroaluminum spinel bricks were invented and developed in the 1990s. They are produced by sintering MgO and FeAl₂O₄ at high temperatures. Today, these bricks are widely used in the lining of cement rotary kilns, introducing a new concept of kiln lining that is both environmentally friendly and more cost-effective.

The specific properties of magnesia-ferroaluminum spinel bricks are as follows: the spinel group contains relatively stable divalent iron ions, which enhances the elasticity of FeAl₂O₄ and increases the flexibility of the lining, significantly reducing brittleness. To ensure that chromium-free magnesia materials form a better kiln skin, it is important to retain the iron in the material and control its valence state to remain divalent (Fe²⁺). This prevents the transformation between divalent and trivalent iron states (Fe²⁺/Fe³⁺), which can affect performance. Iron-aluminum spinel maintains Fe²⁺ stability at lower temperatures.

Iron-aluminum spinel serves as a key source of iron and aluminum in these bricks. When Fe-Al spinel comes into contact with cement clinker, it generates calcium plagioclase silicate and alumina plagioclase silicate phases, forming a protective layer. This contributes to strong adaptability and improved resistance to alkali and salt erosion.

The uniformity of the brick structure is attributed to the ability of ferroaluminum spinel to dissolve and integrate with both natural and synthetic magnesite. Compared to conventional magnesium-aluminum spinel bricks, magnesia-ferroaluminum spinel bricks have a denser structure and higher high-temperature flexural strength. Their strength and density are further enhanced by optimizing the grain size distribution and using high-quality synthetic magnesium sand.

These bricks also demonstrate excellent kiln skin adherence, very low thermal conductivity and thermal expansion, and enhanced resistance to wear and erosion at high temperatures. As a result, they help lower kiln shell temperatures and significantly extend the service life of the kiln lining.

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