The service life of kiln mouth refractory castables is relatively short-typically only 4 to 6 months-making it a critical limiting factor for the operational cycle of rotary kilns. By implementing a series of technical measures and prioritizing process management during subsequent production, significant improvements have been achieved.
1. Selection of Novel Castables
Following a comparative analysis of the performance characteristics of various castables suitable for kiln mouth applications, the previously used high-strength, explosion-proof, and erosion-resistant castable was replaced with a novel tabular alumina castable. Tabular alumina-also known as tabular aluminum oxide-is a type of sintered alumina produced through rapid sintering at an extremely high temperature of 1925°C (slightly below the melting point of α-Al₂O₃, which is 2050°C). This process results in fully recrystallized α-Al₂O₃ grains. Microscopically, it exhibits a distinctive structure characterized by two-dimensional, plate-like grains containing numerous closed, spherical pores located both within and between the grains.
Performance Characteristics of the Novel Tabular Alumina Castable: (1) Utilizing high-density, high-melting-point tabular alumina as its primary raw material, it demonstrates excellent performance under high-temperature operating conditions. (2) It possesses superior resistance to alkali corrosion at high temperatures; specifically, its alkali resistance rating at 1350°C meets or exceeds Level 2, thereby effectively resolving the issue of poor alkali corrosion resistance typically associated with conventional alumina-based castables. (3) It exhibits excellent thermal shock stability, effectively preventing issues such as spalling and block detachment during operation.
2. Optimization of Anchors and Welding Electrodes
Anchors fabricated from Cr25Ni20 alloy were selected for use. Each incoming batch of anchors undergoes rigorous sampling and inspection to ensure full compliance with specified physicochemical parameters and performance standards. Furthermore, the anchor configuration was optimized: the Y-shaped anchors previously used on the inner curved surface of the kiln shell were replaced with U-shaped, split-type anchors featuring a stainless steel base plate. This new design increases the weld volume, thereby establishing a more robust connection with the kiln's base metal and significantly reducing the risk of anchor detachment. Additionally, the split-type structure is more effective in mitigating thermal stress damage caused by the differential thermal expansion rates between the anchors and the castable lining. Circular anchors have been added to connect the refractory castable on the end face with that on the curved surface; this enhances the castable's strength and resolves the issue of poor durability previously observed in the end-face castable.
3 Selection of Optimal Construction and Curing Methods
An "open-style" construction method was adopted, utilizing iron molds solely for the end faces while dispensing with molds for the curved surfaces. The characteristics of this method are as follows: the castable possesses high viscosity, ensuring it remains in place without sagging after pouring; furthermore, it features a short initial setting time, thereby meeting the operational requirements of the rotary kiln. During construction, the work is segmented into sections of 0.8 m to 1.0 m in length. Upon the completion of pouring, leveling, and troweling for each section, an electric heater is applied for drying for one hour before the kiln is rotated to commence work on the subsequent section; a 10 mm-thick expansion joint is inserted between adjacent sections. The iron molds on the end faces are removed 24 hours after the entire pouring process is completed; following demolding, the castable undergoes a natural curing period of 24 hours before the kiln is ignited and heated up. Although this open-style method extends the overall construction duration by 12 hours compared to traditional methods, it offers the following distinct advantages: (1) Low water addition rate, low porosity, high mechanical strength, and excellent resistance to erosion. (2) Attenuation of structural and thermal stresses, thereby inhibiting crack propagation. (3) Enhanced resistance to explosive spalling.
4 Emphasis on Daily Process Management
(1) Rationally control the silica modulus and SO3 content to mitigate the generation of flying grit (airborne particulate matter). An excessive volume of flying grit exacerbates damage to the kiln hood, coal injection pipe, and the refractory castable at the kiln mouth; in severe cases, it can even lead to the structural collapse of the kiln hood. Through continuous experimentation and refinement, it has been determined that the optimal parameters involve maintaining the clinker's silica modulus at 2.3 ± 0.05 and limiting the SO3 content in the raw meal to ≤ 0.30%.
(2) Rationally control the negative pressure within the kiln hood to maintain a proper balance between the air supplied to the kiln system and the air entering the kiln hood. Insufficient negative pressure in the kiln hood can easily result in damage to peripheral equipment, obstruct visual monitoring of the flame within the kiln, and pose a safety risk to personnel due to the splashing of high-temperature materials. During production, strictly maintaining a slight negative pressure within the kiln hood helps to alleviate the abrasive wear inflicted upon the refractory materials by flying grit under high-temperature conditions.
(3) Promptly remove coke deposits from the tip of the kiln head burner to ensure a smooth, lively, and vigorous flame, thereby preventing flame bifurcation or deformation that could lead to direct high-temperature erosion of the refractory castables at the kiln mouth. (4) Whether the kiln is started or shut down-whether on a planned or unplanned basis-the cooling and drying procedures must be strictly carried out in accordance with process requirements; this serves to minimize damage to the refractory castables resulting from improper operation during kiln startups, shutdowns, or abnormal operating conditions.
