In the rotary kiln production processes of cement, metallurgy, and other industries, the lifespan of refractory bricks directly impacts the kiln's operating cycle and production costs. As a core technology for extending the lifespan of refractory bricks, "hanging kiln skin" can reduce high-temperature material scouring, chemical erosion, and thermal stress damage by forming a dense clinker protection layer on the surface of the bricks. This can improve the lifespan of refractory bricks by 2 to 3 times.
This paper analyzes the technical logic of fast kiln skin formation from three aspects: process conditions, material properties, and operational considerations.
01. Core Conditions for Fast Kiln Skin Formation
Precise Matching of Temperature Window
The temperature in the rotary kiln firing zone should be stabilized at 1450°C ± 20°C, which is the critical zone for the formation of the material's liquid phase and the micro-melting of refractory bricks. When the temperature is below 1430°C, the amount of liquid phase is insufficient to form an effective bond. Conversely, when the temperature exceeds 1470°C, the refractory brick surface will over-melt, causing the kiln skin to loosen and peel off. Data from a cement plant shows that at 1450°C with the formation of kiln skin, the kiln skin thickness can reach up to 180mm and have a dense structure. However, when temperature fluctuations exceed ±30°C, the kiln skin thickness is reduced to less than 100mm, making it more prone to peeling.
Liquid Phase Volume and Viscosity Dynamic Equilibrium
The volume of the liquid phase should be controlled at 22%–25%, with viscosity between 100 and 300 poise. If the liquid phase volume is too low, the bonding force is insufficient; if it is too high, it can easily cause excessive hanging. The characteristics of the liquid phase can be optimized by adjusting the chemical composition of the raw meal (e.g., reducing the silicate ratio and increasing the alumina-to-oxygen ratio). For example, in an aluminum plant, after reducing the silicate ratio of the raw material from 2.8 to 2.5, the time required for kiln skin formation was reduced by 12 hours, and the density of the kiln skin increased by 15%.
Thermal Stress Gradient Control
During kiln skin formation, the temperature gradient between the surface and the interior of the refractory bricks should be maintained at ≤ 50°C/cm. If the gradient is too large, cracks are likely to form in the bricks, resulting in kiln skin peeling. The use of a segmented heating strategy (≤ 30°C per hour) can effectively control thermal stress. One steel enterprise applied this method and reduced the rate of kiln skin shedding by 40%.
02. Physicochemical Principles of Kiln Skin Formation
Liquid Phase Bonding Mechanism
When the surface temperature of the refractory brick reaches 1450°C, Al₂O₃ in the brick reacts with CaO and SiO₂ in the material to form low-melting-point calcium aluminum silicate (2CaO-Al₂O₃-SiO₂), creating a viscous liquid phase. At this point, clinker particles with a certain degree of adhesion are pressed onto the surface of the brick due to the rotation of the kiln, and preliminary bonding is achieved through liquid-phase bridging.
Solid-Phase Reaction Intensification
As the thickness of the kiln skin increases and the surface temperature rises to 1480°C, the viscosity of the liquid phase decreases, causing some of the material to fall off. At this point, solid-phase reactions occur inside the kiln skin:
Stage 1 (0-24 hours): Calcium-aluminum silicate and unreacted Al₂O₃ generate acicular calcium feldspar (CaO-Al₂O₃-2SiO₂), forming a skeletal structure.
Stage 2 (24-72 hours): Calcium feldspar reacts with MgO to form magnesium rosenbergite (3CaO-MgO-2SiO₂), which enhances the kiln skin's resistance to erosion.
A laboratory simulation test shows that the compressive strength of the kiln skin after 72 hours of solid-phase reaction can reach 15 MPa, which is three times the initial bond strength.
3. Dynamic Equilibrium Model
When the thickness of the kiln skin reaches 150–200mm, it enters the dynamic equilibrium stage:
Adhesion Rate: This depends on the amount of liquid phase, represented by the equationQ₁=k₁·C·T,
Where k₁ is the adhesion coefficient, C is the liquid phase concentration, and T is the temperature.
Shedding Rate: This is related to the kiln skin surface temperature (T8) and the material impact force (F), represented by the equationQ₂=k₂·Tₛ²·F
When Q₁=Q₂, the kiln skin thickness is stable. One company controls the equilibrium thickness error within ±5mm by monitoring the kiln skin surface temperature (using infrared thermography) and material flow rate (using laser velocimetry) in real-time.
03. Operational Points for Fast Kiln Skin Formation
Matching of Feed Quantity and Kiln Speed
Initial Stage (0-24 hours): The feed quantity should be 60%-70% of the designed capacity, and the kiln speed should be controlled at 0.8-1.0 rpm to ensure that the material stays in the firing zone for ≥15 minutes.
Incremental Phase (24-72 hours): Increase the feed quantity by 5%-10% every 8 hours, and simultaneously increase the kiln speed to 1.2-1.5 rpm to avoid local overheating.
After adopting this strategy in a cement plant, the time required for kiln skin formation was reduced from 96 hours to 72 hours, and the uniformity of kiln skin thickness improved by 20%.
Flame Pattern Control
Length: The flame length should be controlled to 0.8-1.0 times the kiln diameter to prevent a long flame, which can cause thin kiln skin at the front and thick kiln skin at the back.
Shape: A "short and thick" flame should be used (with the black flame head length ≤ 1.5 m) so that the high-temperature zone is concentrated in the middle of the firing zone.
Position: Initially, the coal injection pipe should be positioned 100-150 mm outside the kiln. As the kiln skin thickens, the pipe should gradually move inward to prevent kiln skin formation from reaching the transition zone.
Raw Material Composition Optimization
Saturation Ratio (KH): Should be controlled at 0.88-0.92 to ensure a moderate amount of liquid phase.
Silicic Acid Rate (n): Should be adjusted to 2.5-2.8 to optimize the liquid phase viscosity.
Alumina-Oxygen Ratio (p): Should be increased to 1.6-1.8 to enhance kiln skin erosion resistance.
At an aluminum plant, the success rate of kiln skin formation increased from 75% to 92% by reducing the KH of the raw meal from 0.95 to 0.90.
04. Case Study: Rapid Kiln Skin Formation in a Cement Plant
Background
A 5000 t/d cement production line rotary kiln, with refractory brick thickness of 300mm. The original kiln skin formation cycle was up to 120 hours, with kiln skin thickness fluctuations of ±30mm.
Improvement Measures
Temperature Control: Install an infrared thermometer to monitor the firing zone temperature in real time and reduce the fluctuation range to ±15°C.
Feeding Strategy: Adopt the "low speed and high material" mode, with the initial feed quantity at 70% and kiln speed at 0.9 rpm.
Flame Adjustment: Shorten the flame length from 28m to 22m, and control the length of the black flame head to 1.2m.
Raw Material Optimization: Reduce the KH to 0.90, adjust the n to 2.6, and increase the p to 1.7.
Effect
The kiln skin formation time was shortened to 78 hours, with an average kiln skin thickness of 185mm and fluctuation of ±8mm.
The refractory brick life was extended from 8 months to 14 months, saving 1.2 million yuan in refractory material costs per year.
The surface temperature of the kiln cylinder was reduced from 320°C to 260°C, resulting in an 18% reduction in heat loss.
05. Conclusion
Rapid skinning technology is key to the efficient operation of rotary kilns. By precisely controlling temperature, liquid phase, and heat balance, combined with optimized operational strategies, the efficiency and quality of kiln skin formation can be significantly improved. In the future, with the continued application of intelligent technology, the kiln skinning process will progress toward higher levels of automation and precision, providing stronger support for energy savings, consumption reduction, and long-life operation of industrial kilns.