- Time:Oct 27, 2023
Kiln coating refers to the cement clinker that adheres to the surface of refractory materials in the high-temperature zone inside the kiln. It plays a crucial role in protecting the kiln lining and extending the lifespan of refractory materials.
In large-scale dry-process cement kilns, the flame temperature can exceed 1700°C. Without the protection of kiln coating, the refractory materials would quickly deteriorate due to the effects of high temperature and chemical erosion.
I. Aspects tobe considered when forming kiln coating in a cement kiln
1. Coordination of feed rate and time
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, depending on the specific condition of the kiln coating. Within 60 to 72 hours, the feed rate can be increased to the normal level.
If a thick layer of kiln coating is formed in a short period of time, it will collapse. Conversely, if the kiln coating is hung for too long, it will have a significant impact on production. The first layer of kiln coating is crucial for the long-term operation of a rotary kiln, but it is also important to protect the kiln coating under normal conditions.
Therefore, reducing the feed rate appropriately can stabilize the thermal system of the kiln, control the particle size of the materials in the burning zone, and ensure tighter bonding of the kiln coating.
2. Raw material composition during kiln coatingforming
Using conventional raw material composition is suitable for smaller diameter kilns or kilns with lower saturation ratio (KH) during regular operation because it is easier to adhere.
For larger kilns or kilns with higher saturation ratio (KH) during regular operation, the fixed value of calcium carbonate in the raw material composition for kiln coating forming can be slightly lower than normal kiln operation. This decision should be based on the specific situation of each plant while ensuring the quality of clinker.
3. Control of firing temperature
The clinker should have fine and uniform grain size, and the bulk density of clinker should be slightly lower than the normal level.
During kiln coating forming, it is necessary to maintain a stable firing temperature with minimal variation, and the distribution of the high-temperature zone should be uniform. Only in this way can the kiln coating be firmly bonded, compact, and smooth. It is strictly prohibited to have large flames or run raw materials during the entire kiln coating forming period.
Currently, there are two different methods for kiln coating forming:
(1）Strictly control the firing temperature when the materials reach the burning zone, maintain fine and uniform particles to prevent the formation of large clumps and running raw materials (the bulk density of clinker at this time is only used as a reference). This approach is beneficial for achieving dense and uniform kiln coating.
(2) Increase the burning zone temperature slightly higher than the temperature for regular kiln coatingforming when the materials reach the burning zone. After forming the first layer of kiln coating tightly to the refractory bricks, reduce the burning zone temperature to the usual level. Then continue operations at normal temperature, gradually forming kiln coating until reaching the specified time and thickness.
4. The coal powder composition should be appropriate
Using coal powder with volatile content controlled between 5% and 17%. It is important to strictly control the moisture content of raw coal entering the plant, which should not be less than 8‰. Such fuel can stabilize the kiln condition, maintain a stable flame position for the burner, form an appropriate and stable melt, and facilitate the adhesion of kiln coating.
II. Methods of protecting and patching kiln coating
1. Protecting the kiln coating
After forming the kiln coating, it is important to protect it throughout the operation of the rotary kiln. The quality of kiln coating protection often depends on the operational practices. To protect the kiln coating, the following points should be considered:
- Maintain the correct flame shape, avoid concentrating high temperatures, and prevent the flame from sweeping across the kiln coating.
- Prevent localized high temperatures and strictly control large flames, ensuring that clinker particles are fine and uniform without forming large clumps.
- Promptly burn off any coal powder rings (coke rings) at the kiln inlet, and remove any large clumps that cannot roll out.
- Regularly inspect the kiln coating and promptly replace or repair any damaged areas.
- Make efforts to stabilize the thermal system inside the kiln.
- During shift changes, thoroughly inspect the kiln coating and inform the incoming shift about any changes in the previous shift's kiln coating to ensure timely awareness of the kiln coating condition.
2. Repairing kiln coating
When the kiln coating thickness is insufficient or local detachment occurs due to unevenness, it should be repaired promptly to prevent further deterioration and disruption of normal kiln operation. If the entire kiln coating is thin, adjusting the raw material composition or reducing the feed rate and kiln speed can be considered before reforming the kiln coating.
If there are specific areas of poor kiln coating condition, adjust the firing position accordingly, but limit the movement to around 150-200 cm. If this does not yield satisfactory results and the kiln coating condition is still severe, a slow cooling "pressure patch" can be performed by slightly raising the burning temperature when there is an ample amount of material.
Typically, for kilns with cooling water, a pressure patch takes around fifteen minutes, while for kilns without cooling water, it may take around half an hour. It is important to avoid damaging the kiln structure during the patching process. If the kiln becomes dry or red, immediate stoppage and refractory replacement are necessary, and pressure patching without bricks is strictly prohibited.
III. Significant performanceof Periclase-hercynite refractory bricks
The burning zone of a cement rotary kiln is a critical region where the transformation of silicate phases in cement occurs. Within this zone, the flame temperature can reach as high as 1800°C to 2000°C, emitting a significant amount of radiation energy to its surroundings. Prolonged exposure of the flame's tongue to any refractory brick leads to rapid decomposition and damage.
To withstand high temperatures, it is necessary to form a protective layer called "kiln coating" on the lining bricks. Magnesia iron spinel refractory bricks are used for the burning zone of rotary kilns due to their excellent kiln coating adherence, ensuring long-term normal operation of the kiln system and effectively protecting the kiln shell.
1. Why are Periclase-Hercynite refractory bricks prone to kiln coating adherence?
Periclase-Hercynite refractory bricks contain iron and aluminum oxides that readily react with calcium oxide in cement clinker, forming low-melting-point mineral phases such as dicalcium ferrite and calcium ferrite. These mineral phases, which have a certain viscosity, adhere to the working surface of the refractory brick, gradually forming a uniform and stable kiln coating with a thickness of around 200mm. The long-term presence of the kiln coating reduces the high-temperature melting and wear of the refractory brick caused by the raw materials, resulting in good performance. It has gained widespread market recognition in the cement industry.
2. Applications of Periclase-Hercynite refractory bricks in cement industry
Periclase-Hercynite refractory bricks are commonly used in various sizes of cement rotary kilns with operating periods of over 12 months. In particular, these bricks have achieved outstanding results in the continuous operation of a 6500T/D kiln at Huaxin Cement (Yangxin) for 24 months. In Huangshi Company, known as the cradle of China's cement industry, a 10,000T/D cement kiln completed its first operating period using Periclase-Hercynite refractory bricks and ran safely for 410 days. The second operating period successfully lasted 414 days, and the third period has been running safely since January 28, 2023.
In conclusion, choosing high-quality refractory bricks that are easy to form a uniform and stable kiln coating is crucial for achieving long-lasting adherence. Proper selection of refractory materials reduces production costs, improves product quality, and brings about better economic and social benefits.
Additionally, reducing the frequency of kiln start-ups and shutdowns, controlling the composition of raw materials and coal powder, ensuring fine and uniform clinker granules, and maintaining thermal stability are factors that contribute to the development of a good kiln coating.
