Application Of Low Thermal Conductivity Refractory Bricks And Nano Insulation Materials in Cement Production

Refractory bricks for the rotary kiln in the cement clinker burning system, as well as thermal insulation materials for the pre-calcining system, have a significant impact on the operating rate of the rotary kiln. The use of low-thermal-conductivity refractory bricks can effectively reduce heat dissipation from the surface of the rotary kiln shell, while the application of nano-insulation materials can effectively reduce the load on the rotary kiln. Based on three rotary kilns of different capacities, this paper compares the refractory brick configuration schemes using low-thermal-conductivity bricks, analyzes their physical and chemical performance indicators, and introduces the application of nano thermal insulation materials in pre-calcining systems.

Selection and Comparison of Rotary Kiln Refractory Bricks

Table 1 shows the heat expenditure of the cement clinker burning system for a certain project. Taking this project as an example, the surface heat loss of the burning system accounts for 10% of the total heat expenditure, while the surface heat loss of the rotary kiln shell accounts for 50% of the total surface heat loss of the burning system. It can be seen that reducing the surface heat loss of the rotary kiln shell is key to lowering the overall surface heat loss of the burning system, and that the rational configuration of kiln refractory bricks is critical for achieving this goal. To minimize the surface heat loss of the burning system, a company has improved the refractory brick configurations for rotary kilns on several production lines and has achieved certain results. A comparative analysis of the refractory brick configuration schemes for rotary kilns in three projects is presented below.

1.1 Comparison of Refractory Brick Configuration Schemes for a ϕ4.3 m × 60 m Rotary Kiln (3000 t/d) Before and After Retrofit

The original refractory brick configuration of this kiln was relatively common in the domestic cement industry, adopting a design scheme that used silica-mullite bricks in the kiln's decomposition zone and upper transition zone. To achieve better energy conservation and reduced consumption, and to meet the owner's requirement that the refractory bricks be compatible with various fuels, a retrofit plan was developed after analyzing domestic refractory brick products and investigating the application of rotary kiln lining configurations in numerous cement enterprises.

It was ultimately decided to carry out the retrofit by applying DDR series low-thermal-conductivity refractory bricks, which have demonstrated satisfactory performance in the low-temperature sections of rotary kilns. Magnesium-iron spinel bricks were still used in the burning zone and lower transition zone. The thickness of the kiln refractory bricks is 200 mm. A comparison of the refractory brick configuration schemes before and after the retrofit is shown in Table 2.

1.2 Comparison of Refractory Brick Configuration Schemes for ϕ4.8m×74m Rotary Kiln (5000 t/d) Before and After Retrofit

A comparison of the refractory brick configuration schemes before and after the retrofit is shown in Table 3. The thickness of the kiln refractory bricks is 220 mm, and the magnesium-iron spinel bricks used are of a foreign brand but manufactured domestically.

1.3 Comparison of Refractory Brick Configuration Schemes for a ϕ5.0 m × 72 m Rotary Kiln (5800 t/d) Before and After Retrofit

The refractory brick configuration scheme of this kiln is similar to that of the ϕ4.8 m × 74 m rotary kiln (5000 t/d). Based on the owner's requirements, low-thermal-conductivity refractory bricks were not adopted for this kiln, and all kiln refractory bricks are from foreign brands.

The non-basic refractory bricks used in the low-temperature zone are foreign-branded products manufactured domestically, while the magnesia-iron spinel bricks and magnesia-alumina spinel bricks are foreign-branded products produced in their countries of origin. The thickness of the kiln refractory bricks is 220 mm. A comparison of the refractory brick configuration schemes before and after the retrofit is shown in Table 4.

The basic bricks include REFRAMAG CF magnesia-iron spinel bricks and ALMAG AF magnesia-alumina spinel bricks, with their refractoriness under load increased by 50 °C.

Development of Rotary Kiln Refractory Bricks and Analysis and Comparison of the Physical and Chemical Properties of Low-Thermal-Conductivity Refractory Bricks

The application of refractory bricks in rotary kilns should minimize heat loss from the kiln shell, ensure the stable operation of the rotary kiln, direct more heat to the chemical reactions of materials inside the kiln, enable more efficient and rapid clinker calcination, and provide a long service life. To meet these operational requirements, refractory brick products must have the lowest possible thermal conductivity, as well as excellent resistance to thermal stress, mechanical stress, and chemical attack.

2.1 Development and Current Status of Rotary Kiln Refractory Bricks

The first generation of refractory bricks in the cement industry mainly consisted of fireclay brick series, which were widely used in early kilns but had a short service life. High-alumina refractory bricks and magnesia refractory bricks emerged as the second generation. The third generation includes dolomite brick series, magnesia-chrome brick series, magnesia-calcia-zirconia brick series, and silica-mullite brick series.

With the widespread application of alternative fuels and raw materials in the cement industry, the variety of fuels has increased and mixed fuels have been adopted. Meanwhile, clinker output has continuously increased, and kiln dimensions have become larger, resulting in higher requirements for refractory bricks. In response, magnesia spinel series refractory bricks with improved heat resistance, erosion resistance, and stability have been developed, including magnesia-iron spinel and magnesia-alumina spinel series, whose service life generally meets production demands.

At present, refractory brick manufacturers are conducting research on low-thermal-conductivity refractory bricks in two main directions. One approach is to produce composite refractory bricks using low-thermal-conductivity refractory raw materials or composite low-thermal-conductivity materials, such as DDR30 and DDR50 refractory bricks. The other approach is to retain the original high-performance materials in the working layer of the refractory bricks during production, apply low-thermal-conductivity materials in the non-working layer, and co-sinter the two materials into an integral structure to reduce the overall thermal conductivity of the brick product, such as DDR80 refractory bricks. Practice has shown that the research, development, and production of both types of composite products have been quite successful.

2.2 Comparison of Physical and Chemical Properties of Rotary Kiln Refractory Bricks

The performance indicators of refractory bricks include bulk density and apparent porosity, which reflect their structural properties, as well as cold crushing strength, flexural strength, modulus of elasticity, refractoriness under load, and thermal shock resistance, among other indicators that characterize the service performance of refractory bricks under specific conditions.

Tables 5 and 6 present comparisons of the main physical and chemical properties of several commonly used non-basic refractory bricks and basic refractory bricks, respectively.

It can be seen from Table 5 that the bulk density and apparent porosity of the low-thermal-conductivity bricks DDR30 and DDR50 are significantly lower than those of high-alumina bricks and silica-mullite bricks, and their thermal conductivity is also lower than that of high-alumina bricks and silica-mullite bricks. DDR30 and DDR50 low-thermal-conductivity bricks adopt an interlocked structure of crystalline and amorphous phases, which reduces the thermal conductivity of the refractory bricks. Their main crystalline phase is mullite, with the chemical formula 3Al₂O₃·2SiO₂. Their Al₂O₃ content is lower than that of high-alumina bricks and silica-mullite bricks, and they exhibit excellent alkali resistance and good service performance.

It can be seen from Table 6 that the thermal conductivity of the insulation layer of the composite magnesia–alumina spinel brick DDR80 is lower than that of the working layer. The insulation layer is made of magnesia–iron composite forsterite, with forsterite as its main crystalline phase. It has a melting point of 1890℃, a true density of 3.22 g/cm³, a thermal conductivity of 1.88 W/(m·K) at 1000℃, and a cold crushing strength greater than 35 MPa.

The two refractory materials are formed integrally in a single step and co-fired at the same temperature, ensuring essentially synchronized high-temperature strength. When tested at 1100℃, the flexural strength at the bonding interface can reach 4.12 MPa.

ALMAGAF basic refractory bricks are selected for the upper and lower transition zones of the ϕ5.0 m × 72 m rotary kiln (5800 t/d), mainly to accommodate high fuel substitution rates. They feature high structural strength, good resistance to gas permeability, and excellent alkali resistance, but have the disadvantage of relatively high thermal conductivity.

ALMAGES bricks are recommended for the upper and lower transition zones. This grade of refractory brick not only retains the advantages of ALMAGAF but also overcomes its drawback of high thermal conductivity. Its thermal conductivity, tested at 300℃, 700℃, and 1000℃, is approximately 30% lower than that of ALMAGAF, reaching only 2.1 W/(m·K) at 1000℃. Without coating formation, the kiln shell surface temperature can be reduced by 20℃ to 40℃, demonstrating a favorable energy-saving effect.

Application of Low-Thermal-Conductivity Refractory Bricks

The temperature data for the three kilns presented in this paper were all measured using kiln shell scanners during commissioning. Energy-saving calculations were performed under the assumption that the temperatures inside and outside the kiln shell remained unchanged before and after the retrofit.

3.1 ϕ4.3 m × 60 m Rotary Kiln (3000 t/d)

Figure 1 shows the temperature measurement image obtained from the shell scanner of the ϕ4.3 m × 60 m rotary kiln (3000 t/d).

In the cold zone of the rotary kiln where DDR30 low-thermal-conductivity bricks are applied, the temperature is below 220℃. In the section where DDR50 low-thermal-conductivity bricks are used, the temperature is around 260℃. In the area lined with DDR80 composite magnesia–alumina spinel bricks, the temperature ranges from 300℃ to 320℃. Overall, the temperature distribution of the kiln shell and refractory lining is relatively uniform and ideal.

Magnesia–alumina spinel bricks have thermal conductivity similar to that of AZM1680 silica-mullite bricks and exhibit relatively good performance. At present, magnesia–alumina spinel bricks are widely used in the transition zone of rotary kiln refractory linings.

Through calculation, comparison is made on the heat dissipation loss from the kiln shell surface before and after the retrofit using low‑thermal‑conductivity bricks.

Table 7 shows a comparison of heat dissipation loss using low-thermal-conductivity bricks for the ϕ4.3 m × 60 m rotary kiln (3000 t/d) before and after the retrofit.

It can be seen from Table 7 that the application of low-thermal-conductivity bricks in the low-temperature zone achieves an energy saving of 39%, indicating a highly significant energy-saving effect.

In terms of weight, the total mass of 7.2 m of high-alumina bricks plus 19.2 m of AZM1650 silica-mullite bricks is 219.2 t, whereas the total mass of the two types of low-thermal-conductivity bricks (13.4 m + 13 m) is 192 t, resulting in a weight reduction of 27.2 t.

Table 7 does not include a comparison between DDR80 composite magnesia–alumina spinel bricks and AZM1680 silica-mullite bricks. Although silica-mullite bricks have low thermal conductivity, their service performance is inferior to that of magnesia–alumina spinel bricks, and their service life is relatively short, making them no longer suitable for the operating conditions of rotary kilns.

3.2 ϕ4.8 m × 74 m Rotary Kiln (5000 t/d)

Figure 2 shows the temperature measurement image obtained from the shell scanner of the ϕ4.8 m × 74 m rotary kiln (5000 t/d).

In the cold zone of the rotary kiln where DDR30 low-thermal-conductivity bricks are applied, the temperature is below 220℃. In the section lined with DDR50 low-thermal-conductivity bricks, the temperature is around 240℃. In the area lined with DDR80 composite magnesia–alumina spinel bricks, the temperature ranges from 300℃ to 330℃. Overall, the temperature distribution of the kiln shell and refractory lining is relatively uniform and ideal.

Table 8 presents a comparison of heat dissipation loss using low-thermal-conductivity bricks for the ϕ4.8 m × 74 m rotary kiln (5000 t/d) before and after the retrofit.

As shown in Table 8, the combined use of DDR30 and DDR50 low-thermal-conductivity bricks in the low-temperature zone, along with DDR80 composite magnesia–alumina spinel bricks in the transition zone, achieves an energy saving of 36%, indicating a remarkable energy-saving effect.

In terms of weight, the total weight of refractory bricks before the retrofit was 350 t, and the total weight after the retrofit was 323 t, representing a reduction of 27 t.

3.3 ϕ5.0 m × 72 m Rotary Kiln (5800 t/d)

Figure 3 shows the temperature measurement image obtained from the shell scanner of the ϕ5.0 m × 72 m rotary kiln (5800 t/d).

As shown in Figure 3, the temperatures in the low-temperature zone and upper transition zone of the rotary kiln are generally higher than those achieved when using low-thermal-conductivity refractory bricks.

Specific heat dissipation loss calculations are not presented here. If ALMAGES magnesia–alumina spinel refractory bricks were selected, an energy saving of approximately 30% could also be achieved, and the total weight of the kiln lining bricks would also be reduced.

Application of Nano Thermal Insulation Materials in Pre-Calcining Systems

In conventional designs, ordinary calcium silicate boards with a thickness of 114 mm are used as the thermal insulation layer in the pre-calcining system. Improvements in this aspect mainly follow two approaches:

The first approach maintains the total thickness of the thermal insulation layer while dividing the 114 mm insulation layer into two sections. One section continues to use ordinary calcium silicate boards, while the other section adopts nano thermal insulation materials with thicknesses of 35 mm or 50 mm, depending on the installation location. This configuration can reduce the outer wall temperature by 15℃ to 20℃.

The second approach involves the use of nano thermal insulation materials while reducing the total thickness of the insulation layer to 65–75 mm, thereby increasing the effective cross-sectional area of the cyclone or upper air duct. Under this configuration, the outer wall temperature remains essentially the same as that achieved when using a 114 mm thick ordinary calcium silicate board as the insulation layer.

Tables 9 and 10 present comparisons of service performance and laboratory-measured temperatures between ordinary calcium silicate boards and different types of nano insulation materials, respectively.

As shown in Tables 9 and 10, nano insulation materials exhibit significantly better service performance than ordinary calcium silicate boards, with much lower thermal conductivity and more favorable measured cold-side temperatures.

It should be noted that the installation of thermal insulation materials must be carried out strictly in accordance with design and construction requirements, using appropriate binders and ensuring that all gaps are fully filled; otherwise, heat dissipation losses will increase significantly.

Conclusion

The application of low-thermal-conductivity refractory bricks in rotary kilns, together with nano thermal insulation materials in pre-calcining systems, can significantly reduce heat dissipation losses from the rotary kiln shell, lower the operating load of the kiln, and achieve energy conservation and consumption reduction. This approach is therefore worthy of further research and wider application.