- Time:Nov 02, 2023
Kiln coating is cement clinker adhering to the surface of refractory material in the high temperature zone of the kiln, which plays a very important role in protecting the kiln lining and prolonging the life of the refractory material.
The flame temperature of new dry-process cement kiln is up to 1700℃ or more. Without the protection of kiln skin, the refractory material will soon be destroyed due to high temperature and erosion.
1、Aspects to consider when forming kiln coating
(1) Coordination of feed rate and timing
Generally, during the first 24 hours of kiln operation, the kiln should maintain 60% to 70% of the normal feed rate. After 24 hours, the feed rate can be increased by 5% to 10% every 8 to 16 hours, based on the specific condition of the kiln coating. Within 60 to 72 hours, the feed rate can gradually reach the normal level.
If a thick layer of kiln coating forms too quickly, it may collapse. Conversely, if the kiln coating hangs for too long, it will have a significant impact on production. It is important to establish the initial kiln coating for the long-term operation of the rotary kiln, but it is even more crucial to protect the kiln coating under normal conditions.
Therefore, reducing the feed rate appropriately can help stabilize the thermal system of the kiln, control the particle size of materials in the burning zone, and achieve better adhesion of the kiln coating.
(2) Raw material composition during kiln coating:
Conventional raw material composition can be used for smaller diameter kilns or those with lower saturation ratios (KH).
For larger diameter kilns or those with higher saturation ratios (KH), the calcium carbonate content of the raw material composition during kiln coating can be slightly lower than normal, depending on the specific circumstances of each plant, while ensuring the quality of clinker.
(3) Control of firing temperature:
The standard is to achieve fine and uniform clinker granulation, paying attention to the bulk density of the clinker and ensuring it is slightly lower than the normal bulk density. During kiln coating, it is necessary to maintain a stable burning temperature with minimal fluctuations. The distribution in the high-temperature zone should be uniform to ensure a solid, dense, and smooth kiln coating. Throughout the kiln coating process, large flames and material runout should be strictly prohibited.
Currently, there are two different methods for kiln coating:
- Strictly control the burning temperature when the materials reach the burning zone, focusing on achieving fine and uniform particle size distribution to prevent the formation of large clumps and material runout (the clinker bulk density at this stage is only used as a reference). This approach greatly benefits the compactness and uniformity of the kiln coating.
- Increase the temperature of the burning zone slightly higher than the usual kiln coating temperature when the materials reach the burning zone. After applying the first layer of kiln coating tightly adhered to the bricks, reduce the temperature back to the normal kiln coating temperature. Subsequent operations can proceed at the normal temperature, gradually applying kiln coating according to the specified time and thickness.
(4) Suitable coal powder composition:
The composition of pulverised coal needs to be appropriate, the choice of volatile matter control in 5%-17% of pulverised coal, strict control of incoming raw coal moisture is not less than 8 ‰, such fuel can stabilise the kiln conditions, so that the position of the burner flames to remain stable, in order to form the number and viscosity of the appropriate, stable melt, more conducive to the kiln skin of the sticking hanging.
2、Methods for protecting and repairing the kiln coating:
(1) Protecting the kiln coating:
- Maintain correct flame shape, prevent concentrated high temperature, and avoid direct flame impingement on the kiln coating.
- Prevent localized high temperatures and large clinker formation. Timely remove any accumulated material rings at the kiln inlet.
- Regularly inspect the kiln coating, promptly repair any damaged areas, and ensure stable thermal conditions.
- Thoroughly communicate the condition of the kiln coating during shift changes.
(2) Repairing the kiln coating:
- When the kiln coating thickness is insufficient or local detachment occurs, timely repair is necessary to prevent further deterioration and maintain normal kiln operation.
- If the entire kiln coating is thin, adjust the raw material composition or reduce feed rate and kiln speed before reapplying the coating.
- For localized kiln coating issues, adjust the flame position conservatively (150-200 cm limit).
- If the problem persists, consider a slow kiln "compression repair" by slightly increasing the firing temperature during periods of higher material load. Cooling water-equipped kilns require around 15 minutes for the repair, while non-water-cooled kilns may need approximately 30 minutes.
- Caution should be exercised to avoid damaging the kiln structure, and brick replacement is mandatory if the kiln becomes dry or red during the repair process. No repair without bricks should be allowed.
3. The significant impacts of magnesia-spinel brick on cement kiln coating are as follows:
(1) Why is magnesia-spinel brick prone to forming kiln coating?
Magnesia-spinel brick, among magnesia bricks, exhibits a strong affinity with the calcium oxide in cement clinker. This results in the formation of low-melting minerals such as dicalcium iron silicate and calcium ferrite when they react. These mineral phases, with a certain viscosity, adhere to the working surface of the refractory brick, gradually forming a uniform and stable kiln coating with a thickness of approximately 200mm. The long-term presence of this "kiln coating" reduces the high-temperature melting and erosion of the refractory brick by the materials, leading to improved performance. It has gained widespread recognition in the cement industry.
(2) Industrial applications of magnesia-spinel brick:
Magnesia-spinel brick is commonly used in various sizes of cement rotary kilns, with operational cycles over 12 months. Notably, this product achieved excellent results in continuous operation for 24 months in the 6,500 T/D kiln of Huaxin Cement Group Yangxin Company. The first kiln zone in the 10,000 T/D kiln of Huangshi Company, the cradle of China's cement industry, exclusively employed magnesia-spinel brick and operated safely for 410 days. The second cycle operated successfully for 414 days, and the third cycle has been running safely since January 28, 2023.
To consistently form a well-distributed and stable kiln coating, selecting high-quality refractory bricks that are prone to kiln coating is of paramount importance. Proper refractory material selection not only reduces production costs but also improves product quality, yielding significant economic and social benefits.
Additionally, minimizing kiln start-ups and shutdowns, controlling the composition of raw materials and coal powder, achieving fine and uniform clinker formation, and maintaining thermal stability are crucial factors in facilitating the development of a good kiln coating.
