Why Can the Use of Magnesia Spinel Bricks Prevent Ring Formation in the Rotary Kiln?

For the firing zone of the cement rotary kiln, a stable and strong kiln skin is required. Because it can protect the refractory lining and reduce the heat loss through the kiln body. A few centimeters of stable kiln skin can not only extend the life of the lining material by nearly 100% but also reduce the temperature of the kiln body by several hundred degrees. This means reducing heat loss by up to 25%. When the local kiln appeared thick skin, it is known as "ring formation." It will hinder the operation of the kiln considerably. The most serious is that it has built an unfixed cross-section of the kiln, which hinders the steady flow of materials and fuel gas into the kiln. Once the kiln skin is looped, it cannot fall off by itself, and the kiln has to be stopped in order to deal with them.

Rotary Kiln in the Cement Plant

The rate of kiln shutdown due to loop formation accounted for 80%. The change cycle ranges from once a month to more than once a week. It is for this reason that people have been studying methods to prevent or deal with kiln skin loops that hinder operation. Therefore, it is considered whether it can be prevented by selecting suitable refractory materials, or at least the occurrence of ring formation can be reduced.

Classification and Location of Kiln Skins and Ring Formation

According to the material flow, the kiln skin and ring formation in the preheater, rotary kiln, and cooler is divided into different types. According to these classifications, the raw material loop occurs in the decomposition zone where the material temperature is between 800°C and 1200°C. The sintering ring occurs at the front end of the firing zone where the material temperature is 1200~1350℃. The clinker ring occurs at the end of the firing zone where the material temperature is 1200~1400℃. Another obstacle to the formation of kiln skins is the slurry ring on the wet long kiln. The skin on the suspension preheater and decomposition belt of the rotary short kiln, and the well-known "clinker mushroom tumor" on the cooler inlet chute.

Refractory Material Affects the Possibility and Limit of Ring Formation

The chemical atmosphere in the kiln and the normal operating conditions affect the desired formation of the kiln skin and cause the trouble of material ring formation. However, refractory materials can also affect the stability of the kiln skin due to their different bonding strength. Experiments have shown that dolomite bricks have the strongest bonding force, followed by magnesia-chrome bricks and direct-bonded magnesia-chrome bricks (MgO content 60~70%), and magnesia spinel bricks (Almag 85) are the weakest.

Magnesia Spinel Bricks Manufacturer

The "Mineral-Chemistry" study confirmed that the bonding force at the interface between the kiln skin and different bricks was measured by the mineral composition of the "kiln skin-brick" interface and the liquid "wetting" angle. Therefore, a layer of C3S enrichment zone with high refractoriness is formed on the interface between kiln skin and brick. This layer of C3S also contains tricalcium aluminate and ferrite phases. If the temperature drops below the stability limit of C3S (1250°C), C3S usually decomposes into C2S and secondary free CaO. But obviously, the incorporation of foreign ions, such as MgO, will stabilize C3S.

Due to the decrease of the local CaO/SiO2 ratio in the area close to the surface of the brick, a C2S-rich boundary layer is formed, which results in a lower bonding force on the magnesia-chrome brick. In the presence of Cr2O3, C2S is partially stabilized. However, if the interface temperature drops, that is, if the kiln skin becomes thicker, β-C2S will be converted to γ-C2S, with a volume expansion of 10%, resulting in "pulverization" and weakening of the binding force of the ring to the lining. So that the knot loops fall off. Magnesia spinel bricks have a particularly strong "chalking" effect. Although a C2S-rich layer is formed, the liquid phase obviously does not infiltrate the bricks to such an extent. On the other hand, there is no adequate amount of ions that can stabilize β-C2S. If the temperature drops in the contact zone, it is easier to form γ-C2S, so that the ring of materials will fall off.

Because the magnesia spinel brick has higher control of Fe2O3 content, it will not generate a C2S layer enriched with iron-free aluminate. The situation of magnesia chrome brick is the opposite because it contains higher Fe2O3. Based on the previous observations, it is clear that magnesia spinel bricks can cause the ring to fall off, or prevent the formation of a C2S-rich ring layer that hinders operation.

These loops mainly occur before and after the decomposition zone and the firing zone. Lining with magnesium spinel bricks can also prevent the formation of "clinker mushroom tumors" at the inlet chute of the cooler. As to whether the magnesia spinel brick can successfully eliminate the problem of skinning containing potassium sulfate, potassium carbonate, silica calcite, alkali metal sulfate, and alkali metal chloride, it is still worth discussing. The looping of slurry in the cyclone, short dry kiln flue chamber, long wet kiln, and skinning on the wall and top of the hot gas chamber of the grate preheater are all such problems. Obviously, the lining made of Almag85 grade magnesia spinel bricks shows that it can effectively prevent the formation of clinker rings, sintered rings, and raw material rings.

Learn more about the magnesia alumina spinel bricks from Rongsheng Refractory Manufacturer.