Study On Alkali Erosion Resistance Of Aluminosilicate Refractory Bricks For Cement Kilns

 In recent years, with the rapid economic development and accelerated urbanization in China, the quantity of industrial and domestic waste has continued to increase, becoming a social problem that plagues urban development and affects the quality of life of residents. The co-processing technology in cement kilns has achieved "reduction, harmlessness, and resource utilization" of waste, and has achieved good social, environmental, and economic benefits. However, due to the presence of large amounts of harmful components such as sulfur, alkalis, and chlorine in waste, which are cyclically enriched in the preheater system of cement kilns, refractory bricks are prone to erosion, spalling, and damage. At present, aluminosilicate products used in the low-temperature zone of cement rotary kilns, such as series silicon-mullite bricks, spalling-resistant high-alumina bricks, and new low-alumina mullite bricks, differ significantly in raw material selection and physical and chemical properties. In this work, the alkali erosion resistance of the above different products was compared, and combined with the analysis of microstructure and phase composition, their alkali erosion resistance was judged, so as to optimize the material selection and configuration of refractory materials for cement kilns.

 1.1 Refractory Brick Selection

Commercially available silicon-mullite bricks 1680, silicon-mullite bricks 1550, spalling-resistant high-alumina bricks JA, and low-alumina mullite bricks M55 were selected. The physical and chemical properties of the sample bricks are shown in Table 1.

Table 1 Physical and Chemical Indexes of Commercially Available Aluminosilicate Refractory Bricks

 1.2 Alkali Erosion Resistance Test and Performance Evaluation

The static crucible method was used for the alkali erosion resistance test. Cut sample blocks with a size of 80 mm × 80 mm × 80 mm from each sample brick, drill a cylindrical groove of 36 mm × 40 mm at the center position to make a crucible, and then cut a thin plate of 60 mm × 60 mm × 30 mm to make a crucible cover. Dry the crucible and the crucible cover. Put 20 g of commercially available chemical - pure K₂CO₃ into each crucible, and use fire clay to seal the gap between the crucible cover and the crucible. Dry the entire crucible in an oven at 110 °C for 12 h, then place it in an electric furnace, keep it warm at 1100 °C for 5 h, and let it cool naturally. Evaluate the alkali erosion resistance of the samples by observing the appearance of the samples. Take the eroded area at the bottom of the crucible for microstructure and phase composition analysis. Divide the area from the bottom of the crucible to the bottom of the brick sample at intervals of 5 mm (see Figure 1), and use EDS to analyze the K element content in each area. Use an X - ray diffractometer to detect the phase composition in the 0 - 10 mm area of the eroded and metamorphosed layer at the bottom of the crucible.

Figure 1 Schematic Diagram of Sampling from the Crucible After Alkali Erosion

 2.1 Appearance Analysis of Crucibles after Alkali Erosion Test

From the appearance photos of the four crucibles after alkali erosion, it was found that silicon-mullite brick 1680, silicon-mullite brick 1550, and low-alumina mullite brick M55 showed no cracks, indicating excellent alkali erosion resistance; while the spalling-resistant high-alumina brick JA had through-going large cracks, showing relatively poor alkali resistance.

 2.2 Distribution of K Element at the Bottom of the Eroded Crucible

EDS surface scanning was performed on the bottom of the crucible close to each partition line, and the measured K element content is shown in Figure 2. As can be seen from Figure 2, the distribution of K element in different sample bricks varies significantly. Among them, the change curves of K element content in silicon-mullite brick 1680 and silicon-mullite brick 1550 are consistent, with the maximum K element content at 0 mm. As the erosion depth increases, the K element content decreases sharply. After the depth reaches 20 mm, the K element content approaches 1% (w) and no longer changes. The distribution curves of K element content in spalling-resistant high-alumina brick JA and low-alumina mullite brick M55 are similar. The spalling-resistant high-alumina brick JA has the highest K element content at 0, 5, and 10 mm; in the low-alumina mullite brick M55, the highest K element content is at 0 and 5 mm. After reaching the highest value in these two sample bricks, the K element content decreases sharply with the distance from the bottom of the crucible hole. In the spalling-resistant high-alumina brick JA, the K element content approaches 1% (w) after 30 mm and no longer changes; in the low-alumina mullite brick M55, the K element content approaches 1% (w) after 20 mm and no longer changes.

The alkali erosion resistance of silicon-mullite bricks is related to the introduction of silicon carbide and the apparent porosity of silicon-mullite bricks. Silicon dioxide formed by the oxidation of silicon carbide at high temperatures reacts with potassium carbonate on the surface of refractory bricks to form a glassy phase and a dense layer, which can effectively inhibit the erosion and penetration of K elements, making K elements concentrate in the surface area of the bricks. Spalling-resistant high-alumina bricks and low-alumina mullite bricks have high apparent porosity, and the pores provide a fast penetration channel for molten potassium carbonate. Potassium carbonate enters the interior of the bricks along the pores and reacts with the bricks at high temperatures to form minerals such as nepheline or leucite. The formation of nepheline or leucite produces large volume expansion, causing the bricks to crack. The spalling-resistant high-alumina brick with high apparent porosity has a higher K element content at the same depth than the low-alumina mullite brick with low apparent porosity, and its K element enrichment in the surface area is greater.

Figure 2 Variation of K Element Content at Different Positions in Different Regions at the Bottom of Crucibles of Different Sample Bricks After Alkali Erosion

 2.3 Analysis of Microstructure and Phase Composition in the 0 - 10 mm Region of the Eroded Layer

 2.3.1 Silicon - Mullite Brick 1680

Figure 3 shows the SEM photos at different magnifications of the 0 - 10 mm region of the eroded layer of silicon - mullite brick 1680 after alkali erosion. It is observed from Figure 3(a) that: the surface layer structure at the bottom of the crucible in contact with K₂CO₃ is dense, and cracks appear in some areas at the edges of large particles. It can be seen from Figure 3(b) that: the edges of bauxite particles in the matrix are obviously eroded, while silicon carbide has no obvious change.

Figure 3 Microstructure Photos of the 0 - 10 mm Region of the Eroded Layer of Silicon - Mullite Brick 1680 at Different Magnifications

Figure 4 shows the XRD pattern of the 0 - 10 mm region of the eroded layer of silicon-mullite brick 1680. It can be seen that its phase composition mainly consists of corundum, mullite, and silicon carbide, and the K element exists in the glass phase.

Figure 4 XRD Pattern of the 0 - 10 mm Region of the Eroded Layer of Silicon - Mullite Brick 1680

In the matrix of the mullite brick shown in Figure 4, silicon dioxide generated from the oxidation of bauxite and silicon carbide in the matrix reacts with potassium element to form a liquid phase. The liquid phase fills the pores of the silicon - mullite brick, seals the pores and cracks in the area where the refractory brick contacts with alkali, forms a dense layer, and hinders the further penetration of K element.

2.3.2 Spalling - Resistant High - Alumina Brick JA

The microstructure photos of the 0 - 10 mm region at the bottom of the crucible of the spalling - resistant high - alumina brick JA after alkali erosion are shown in Figure 5.

Figure 5 Microstructure Photos of the 0 - 10 mm Region of the Eroded Layer of Spalling - Resistant High - Alumina Brick JA

From the observation of Figure 5(a), it is found that: the surface layer structure at the bottom of the crucible becomes dense, the bauxite particles are obviously eroded, and the particle structure in the matrix becomes indistinct. From the observation of Figure 5(b), it can be seen that: the edges of the matrix particles become very blurred, and a large number of light - colored new phases fill the pores or grain boundaries. The spalling - resistant high - alumina brick JA has a high apparent porosity, and potassium carbonate enters the interior of the brick along the pores or grain boundaries to erode the inside of the sample, resulting in a large erosion depth. Phase analysis of the eroded layer shows that kalsilite is generated in the eroded layer (see Figure 6). The generation of a large amount of kalsilite causes volume expansion, leading to the formation of large through - cracks in the spalling - resistant high - alumina brick.

Figure 6 XRD Pattern of the 0 - 10 mm Region of the Eroded Layer of Spalling - Resistant High - Alumina Brick JA

The microstructure photos of the 0 - 10 mm region at the bottom of the crucible of low - alumina mullite brick M55 after alkali erosion are shown in Figure 7. It can be seen from Figure 7(a) that: a small number of cracks are also generated in the surface layer structure at the bottom of the crucible of low - alumina mullite brick M55, and the structure also becomes dense, but it is quite different from that of the spalling - resistant high - alumina brick. Observed at a high magnification (see Figure 7(b)), obvious erosion also occurs at the edges of dense mullite particles. It can be seen that potassium carbonate also forms a certain erosion and penetration effect on the low - alumina mullite brick, but it is significantly improved compared with the spalling - resistant high - alumina brick. It can be known from the diffraction pattern (see Figure 8) that: the main phases of the eroded layer are mullite and a small amount of andalusite that has not been transformed, and a small amount of kalsilite also exists. The raw materials used in low - alumina mullite brick M55 are mullite homogeneous material and andalusite. The mullite homogeneous material has a dense structure and contains a certain amount of high - silicon amorphous phase. At high temperature, potassium melts into the amorphous phase to form a high - viscosity glass phase; at the same time, the amorphous SiO₂ formed by the decomposition of andalusite absorbs part of the potassium carbonate and also generates a high - viscosity glass phase, and the eroded layer of the refractory brick is sealed by it, hindering the penetration of K element. A small amount of potassium carbonate reacts with mullite in the 0 - 10 mm region of the eroded layer of low - alumina mullite brick M55 to generate kalsilite. The generation amount of kalsilite is low, and the generated expansion is not enough to damage the brick structure. Therefore, low - alumina mullite brick M55 shows good alkali erosion resistance.

Figure 7 Microstructure Photos of the 0 - 10 mm Region of the Eroded Layer of Low - Alumina Mullite Brick M55

(1) The addition of silicon carbide and partial oxidation into silicon dioxide to fill pores in silicon-mullite brick 1680 and silicon-mullite brick 1550 can significantly reduce alkali erosion, showing excellent alkali erosion resistance. After the alkali erosion test, K elements are mainly concentrated in the erosion surface area, and the K element content decreases sharply with the increase of distance.

(2) The commercially available spalling-resistant high-alumina brick JA has high apparent porosity and poor alkali erosion resistance. The alkali erosion depth reaches 10 mm, and the K element content (w) in the eroded layer reaches 20%−25%. The formation of a large amount of kalsilite leads to cracking of the brick body.

(3) Low-alumina mullite brick M55 has low apparent porosity. The mullite homogeneous material and andalusite raw materials used have a dense structure. A certain amount of high-silicon amorphous phase in the matrix can absorb alkali to form a high-viscosity glass phase to further seal pores. The alkali erosion depth is only 5 mm, and the amount of kalsilite generated by alkali erosion is relatively small, which is insufficient to damage the brick structure, so it has relatively good alkali erosion resistance.