Experience Sharing on the Safe Operation of Magnesium-Iron-Aluminium Spinel Refractory Bricks for Over Two Years
Refractory Bricks for Cement Kilns
With heightened environmental awareness and stricter standards, magnesia-ferrite spinel refractory bricks have replaced directly bonded magnesia-chrome bricks in cement kilns to mitigate the environmental impact of Cr⁶⁺. However, compared to direct-bonded magnesia-chrome bricks, magnesia-iron spinel bricks exhibit certain disadvantages, such as high thermal conductivity and poor adhesion to kiln linings. These drawbacks result in a shortened service life, elevated cylinder temperatures, and kiln overheating issues. Through continuous adjustment and optimization, magnesia-iron-aluminium spinel bricks have achieved two years of safe operation within the firing zone. The optimization experience is summarized below for industry reference.
Part 1: Overview of Refractory Brick Usage
The 4000 t/d clinker production line commenced operations in December 2008. The Φ4.6m × 68m rotary kiln is equipped with three types of cement kiln refractory bricks: magnesia-alumina spinel, magnesia-ferrite spinel, and silicon-mullite bricks. The kiln's mouth and tail utilize unshaped refractory materials. The refractory brick configuration within the kiln, as designed by the design institute, is shown in Figure 1.
Magnesium-aluminium spinel and magnesium-iron spinel are used in the firing zone to replace directly bonded magnesium-chromium bricks. Compared to the latter, magnesium-aluminium spinel and magnesium-iron spinel exhibit poorer kiln skin adhesion and higher thermal conductivity. During production, high-temperature spots frequently develop in the firing zone refractory bricks after 6 to 8 months of use. These spots are concentrated at both ends of the firing zone, specifically 0.8 to 3 meters from the kiln mouth and 18 to 23 meters from the kiln mouth. Annual patching is required to maintain operation until the annual major overhaul, which adversely affects the kiln's continuous operation.
Part 2: Analysis of Existing Issues
1.Analysis of Damage Causes at 0.8–3m from Kiln Mouth
Whenever high temperatures occur at the kiln mouth, or the kiln shuts down due to red kiln conditions, the refractory lining at this location becomes extremely thin. The refractory bricks suffer severe overall spalling, typically around 5 cm in depth, with some areas completely devoid of bricks, exposing the cylinder body. No displacement is observed. Analysis indicates that this location lies at the junction of the firing zone and cooling zone, where significant temperature fluctuations cause instability in the kiln lining, leading to frequent spalling. Combined with abrasion from exiting clinker, this results in refractory brick spalling and damage, necessitating adjustments to the brick layout.
2.Analysis of Damage Causes at 18–23m
The limestone from the quarry exhibits a high magnesium oxide content. The average magnesium oxide content in the clinker is approximately 5.0%, with the liquid phase content reaching 29.29% and the crusting value at 40.82%. Both values exceed the upper limits of the optimal control range (with upper limits being 26% and 37%, respectively). Furthermore, thick kiln skin and crusting phenomena are present at the 16–17m section. This section lies within the rear calcination zone, where temperatures are comparatively lower, and the kiln skin is less stable. Compounded by the presence of a crust ahead, clinker abrasion intensifies at this point, leading to damage to the refractory bricks.
Part 3: Measures Implemented
Given the current situation where the high magnesium oxide content in quarry limestone cannot be improved, efforts should focus on optimizing brick selection and performance metrics for magnesia-alumina spinel and magnesia-ferrite spinel refractory bricks. The measures are as follows:
1.Tailor designs to individual kilns, optimize brick selection, and fully leverage the properties of each refractory material type
Based on the actual brick layouts employed by long-term customers of refractory manufacturers, adjustments have been made to the kiln's brick configuration. At the kiln mouth, magnesium-aluminium spinel bricks have been replaced with 1680 silicon-magnesium bricks and magnesium-iron spinel bricks. Given the high magnesium oxide content in the clinker, unstable kiln lining, and the fact that magnesium-iron spinel bricks do not adhere to the kiln lining as effectively as directly bonded magnesium-chromium bricks, the firing zone brick length has been reduced by 1 meter. For energy efficiency, low-thermal-conductivity, anti-spalling refractory bricks were employed in the rear transition zone. The specific brick layout is shown in Figure 2. Concurrently, partial adjustments were made to certain physical and chemical properties of the magnesia-ferrite spinel bricks. The use of magnesia-ferrite-alumina spinel refractory bricks is recommended, combining the advantages of both magnesia-ferrite spinel and magnesia-alumina spinel. This ensures proper kiln lining adhesion while enhancing brick strength and wear resistance.
2. Optimize the Batch Formulation to Promote Uniform Kiln Skin Formation and Enhance Its Adhesion Properties
The quality of the kiln skin directly determines the service life of refractory bricks in the firing zone. Under normal protective kiln skin thickness, the surface temperature of alkaline bricks can be reduced from 1400–1450°C to 600–700°C, placing the lining bricks in an extremely safe state. Even a kiln skin thickness of just 23 mm can lower the brick surface temperature from 1450°C to approximately 1230°C. Under kiln skin protection, the temperature gradient within alkaline bricks becomes markedly more uniform, the infiltrate layer significantly thins, and brick deterioration is minimized.
The liquid phase content of clinker is fundamental to kiln skin formation. C₂S melting enhances skin strength, while Fe₂O₃ significantly reduces sintering temperature and liquid phase viscosity-all three factors promote skin development. When the skin formation value is <30%, skin formation is often difficult; at 33–37%, skin forms relatively easily; when the crusting value exceeds 40%, large lumps and crust rings form. The crusting value is calculated as follows: Crusting Value = Liquid Phase Content + 0.2 × C₂S + 2 × Fe₂O₃. This demonstrates Fe₂O₃'s dual influence on clinker: reducing its content simultaneously lowers both liquid phase content and crusting value, while being relatively straightforward to adjust in the raw material mix and highly practical. Therefore, a low-iron raw material formulation was adopted, with the crusting value controlled at the midpoint of the range (35.43%). The optimized composition is presented in Table 1.
3. Establishing Cooling Curves
While heating curves are routinely established in the cement industry, cooling processes receive less attention. The skin formation formula indicates that C₂S is pivotal for enhancing kiln skin strength. However, C₂S minerals undergo crystalline transformations; during slow cooling between 400–600°C, they readily shift to the γ phase, compromising kiln skin integrity and causing refractory brick spalling. Therefore, during temporary shutdowns, careful attention must be paid to the cooling rate within this temperature range. A cooling curve should be established (see Figure 3) to protect the kiln skin and extend the service life of the bricks.
The purpose of the cooling curve is to control the cooling rate, preventing damage such as spalling of the silicon-magnesium bricks in the transition zone and detachment of the kiln skin in the firing zone, which can lead to spalling of the magnesia-ferro-aluminate spinel bricks. This, in turn, extends the service life.
4. Maintenance of the Kiln Mouth Castable
The kiln mouth castable comes into direct contact with the first row of silicon-magnesium bricks. Operational observations indicate that when the castable is damaged, the brick surface adjacent to the castable rapidly exhibits spalling, and in severe cases, fractures or chipped corners, leading to kiln shutdowns. Damage to the kiln mouth castable frequently manifests as complete radial detachment, with no residual anchor pins observed at the failure points. Analysis suggests that this occurs when some rake pins are inadequately welded, or when welding rods stored on-site for extended periods dry out and subsequently reabsorb moisture. This causes localized refractory material detachment, which then pulls away surrounding material. Consequently, a new rake pin structure and an open-type construction process for the refractory material have been adopted. See Figure 4 for details.
The rake tines comprise a square base plate and U-shaped steel bars, both fabricated from heat-resistant steel. The square base plate is fully welded, increasing the welded surface area compared to the original tines, thereby enhancing structural integrity. The U-shaped steel bar is inserted through the central circular hole in the square base and spot-welded to eliminate stress. The kiln's radial circumference is divided into 24 equal sections at 600 mm intervals. Steel plates secure both sides for pouring the kiln mouth refractory material. After initial setting, vent holes are arranged on the working surface, followed by thermal insulation curing. Subsequent blocks are constructed sequentially following this procedure. While the open-construction method is slower and yields uneven surfaces, practical production experience over one year has demonstrated no instances of entire rings detaching, indicating satisfactory quality.
5. Maintenance of Coal Injection Pipes
Coal injection pipes significantly impact kiln mouth bricks. Two incidents occurred: one involved dimensional deviations in the manufacturer's cyclone head spare parts, which caused excessive internal air velocity and flame dispersion, resulting in deep grooves around the kiln mouth silicon-mullite bricks; the second involved three large, deep pits in the kiln mouth silicon-mullite bricks. Analysis suggested that this resulted from an improper ratio of internal to external airflow, causing an unstable flame with insufficient force and drifting behavior. Improper positioning may also have been a factor. It is recommended that coal pipe positioning and flame shape be adjusted conservatively, prioritizing the safe operation of refractory bricks. Attention to detail and meticulous maintenance will extend the lifespan of the refractory bricks.
Prior to kiln start-up, maintain clean coal nozzle end faces and verify their dimensions. Inspect the flatness of the end-face castable material, ensuring precise positioning data. Document and mark the correspondence between the kiln's internal position and the external coal nozzle markings (along the X and Y axes). During operation, the coal nozzle end face must be cleaned of coking deposits once per shift to stabilize flame shape. Accumulated material at the top and whisker-like deposits at the bottom of the coal nozzle should be removed to prevent prolonged bending deformation of the nozzle head, which could alter the flame direction.
Part 4: Conclusion
The use of magnesia-iron-aluminium spinel refractory bricks combines the advantages of both magnesia-iron spinel and magnesia-aluminium spinel. While maintaining consistent kiln lining adhesion properties, these bricks enhance strength and wear resistance. Optimizing the physical quality of the bricks and their arrangement within the kiln forms the foundation, with kiln lining maintenance being the critical factor. During routine production, monitor the crusting index to ensure a smooth kiln lining, preventing process issues such as crusting rings or egging. Simultaneously, pay attention to the heating and cooling processes, as well as the operational status of the castable refractories and coal pipes. By focusing on details and meticulous maintenance, the service life of refractory bricks in the firing zone can be extended.