China kiln refractory bricks

Research on the Erosion of Corundum Bricks and Mullite Bricks by Rotary Kiln Slag

Abstract

This study focuses on the erosion issue of corundum refractory bricks and mullite refractory bricks in the combustion zone of hazardous waste incineration rotary kilns. By sampling the used bricks from a certain company and employing XRD, SEM combined with EDS to analyze the microstructure and composition of the residual bricks, we explore the erosion mechanism of corundum bricks and mullite bricks under the attack of slag containing halogens and high - sodium salts. The results show that sodium salts, potassium salts, and elements like Br and Cl in the slag have strong erosion and penetration capabilities. Low - melting - point and low - density substances such as NaCl, KBr, and sodalite generated in the matrix not only reduce the high - temperature performance of the bricks in the combustion zone, accelerating the erosion and wear by materials, but also cause irreversible volume expansion of the bricks, leading to spalling during the operation of the rotary kiln.

1. Introduction

With the rapid development of industry, the generation of hazardous waste has been increasing day by day. Hazardous waste incineration rotary kilns, as the core equipment for handling hazardous waste, have been widely used due to their high - efficiency and stable treatment capabilities. Corundum refractory bricks, known for their high refractoriness and hardness, and mullite bricks, valued for their good thermal stability and corrosion resistance, are often applied to the key parts of rotary kilns, especially the combustion zone. However, the complex composition of hazardous waste results in the production of slag containing halogens (such as Cl and Br) and high - content sodium salts during the incineration process. These slags severely erode corundum bricks and mullite bricks, shortening the lifespan of the bricks, causing frequent maintenance of the rotary kiln, and increasing the cost and safety risks of hazardous waste treatment. Therefore, exploring the erosion mechanism of such slags on corundum bricks and mullite bricks is of great significance for improving the stability and economic efficiency of hazardous waste incineration rotary kilns.

2. Experimental Section

2.1 Sample Collection

Used corundum bricks and mullite bricks from the combustion zone of a hazardous waste treatment company's incineration rotary kiln were selected. Samples were taken from multiple points at different positions (such as the middle of the kiln wall, near the feed inlet, etc.) to ensure the representativeness of the samples. The samples were cut into appropriate sizes: some were used for macroscopic morphology observation, some were ground into powder for composition analysis, and the remaining samples were kept in their original structure for microstructure research.

2.2 Analysis Methods

1. X - ray Diffraction Analysis (XRD): An X - ray diffractometer was used to test the ground brick powder. By analyzing the diffraction patterns, the phase composition of the bricks and the newly formed phases after slag erosion were determined, and the phase change rules were explored.

2. Scanning Electron Microscopy (SEM): After sputtering the samples with gold, a scanning electron microscope was used to observe the microstructure of the bricks, including the morphology of the matrix and aggregate, the distribution of cracks, and the characteristics of the erosion interface, to obtain microscopic structural information.

3. Energy Dispersive Spectroscopy (EDS): Combined with SEM, energy dispersive spectroscopy was performed on different areas of the bricks to determine the elemental composition and content distribution of each part, clarifying the penetration path and enrichment areas of the slag components in the bricks.

3. Results and Discussion

3.1 Macroscopic Morphology Analysis

The used corundum bricks and mullite refractory bricks showed obvious erosion characteristics on the surface. In some areas, the surface of corundum refractory bricks was rough with numerous spalling pits of varying depths, while mullite refractory bricks presented more cracks and edge - wear. The edges and corners of some bricks were severely worn, and their dimensions were significantly reduced. In areas with higher slag concentration, such as near the feed inlet, the erosion of both types of bricks was more severe, indicating that the contact degree and residence time of the slag with the bricks have a significant impact on the erosion effect.

3.2 XRD Analysis Results

The main phase of the original corundum refractory bricks was corundum ($Al_2O_3$), and mullite bricks mainly consisted of mullite ($3Al_2O_3 \cdot 2SiO_2$). After slag erosion, new phase peaks appeared in the XRD patterns, including sodalite ($Na_8[AlSiO_4]_6Cl_2$), sodium chloride ($NaCl$), potassium bromide ($KBr$), etc. Sodalite was generated by the chemical reaction between the sodium salts in the slag and the components in the bricks. Its formation consumed the main phases in the bricks, destroying the original structure of the bricks. The presence of low - melting - point salts such as $NaCl$ and $KBr$ reduced the melting point of the brick surface, making both corundum bricks and mullite bricks more easily eroded by materials at high temperatures.

3.3 SEM - EDS Analysis Results

1. Microstructural Characteristics: SEM images showed that the matrix part of both corundum bricks and mullite refractory bricks was severely eroded. At the erosion interface, a large number of fine cracks and pores were visible, and the slag penetrated into the interior of the bricks through these channels. In corundum bricks, the corundum crystals were gradually surrounded and dissolved by the slag; in mullite refractory bricks, the mullite framework was damaged. As the erosion deepened, the bonding force between particles in both types of bricks weakened.

2. Element Distribution Law: EDS analysis indicated that elements such as $Na$, $K$, $Br$, and $Cl$ in the slag were distributed on the surface and inside of the bricks. On the surface of the bricks, the content of these elements was relatively high. As it extended into the interior of the bricks, the element content gradually decreased, but there was still obvious enrichment around the cracks and pores. Among them, the $Na$ element reacted with the components in the bricks to form sodalite, creating a continuous low - melting - point phase network in the matrix. The $Cl$ and $Br$ elements mainly existed in the form of halides. These low - melting - point halides were liquid at high temperatures, accelerating the penetration of the slag in the bricks.

3.4 Discussion on the Erosion Mechanism

1. Chemical Erosion: Sodium salts (such as $Na_2O$) in the slag react chemically with the components in corundum bricks and mullite refractory bricks to form low - melting - point phases such as sodalite, reducing the refractoriness and high - temperature strength of the bricks. At the same time, halogen elements such as $Cl$ and $Br$ react with the metal - containing components in the bricks to form volatile halides, further damaging the brick structure. Chemical erosion changes the phase composition and chemical composition of the bricks, deteriorating their performance.

2. Physical Effects: Low - melting - point and low - density substances such as $NaCl$ and $KBr$ melt at high temperatures and fill the pores and cracks of the bricks, causing the volume expansion of the bricks. Due to the temperature fluctuations during the operation of the rotary kiln, this irreversible volume expansion generates stress concentration inside the bricks. When the stress exceeds the strength limit of the bricks, spalling occurs. In addition, the presence of low - melting - point slag reduces the viscosity of the brick surface, making it easier for materials to erode and wear the bricks, accelerating the erosion of corundum bricks and mullite bricks.

4. Conclusion

1. Slag containing halogens and high - sodium salts in hazardous waste incineration rotary kilns has a strong erosion effect on corundum bricks and mullite refractory bricks, mainly through chemical erosion and physical effects.

2. Chemical erosion leads to a change in the phase of the bricks, generating low - melting - point phases such as sodalite. Physical effects cause volume expansion and spalling of the bricks due to the formation of low - melting - point substances, and at the same time, intensify the erosion and wear of the bricks by materials.

3. This study reveals the erosion mechanism, providing a theoretical basis for the development of new refractory materials with excellent corrosion resistance and the optimization of the design and use of corundum bricks and mullite refractory bricks for hazardous waste incineration rotary kilns. Subsequent research can focus on suppressing the formation of low - melting - point phases and enhancing the anti - expansion performance of bricks to improve the service life of refractory materials.