Configuration and application progress of amorphous refractory materials in new dry process cement kilns

Different parts of cement kiln systems of different specifications have different requirements for refractory materials; when designing and matching, refractory materials of the type that match their requirements must be selected; in addition, the differences in raw materials, fuel quality, equipment conditions, operating habits, etc. used by various cement manufacturers objectively have slightly different performance requirements for refractory materials. The following focuses on the main problems existing in the use of amorphous refractory materials (mainly castables) in new dry process cement kilns and their current reasonable configuration in new dry process cement kilns.

Application and existing problems of amorphous refractory materials in cement systems

1.1 Rotary kiln front and rear kiln mouths

Most of the refractory materials used in the front and rear kiln mouths of rotary kilns are currently designed as castables, especially the refractory materials of the front kiln mouth. In addition to withstanding high temperatures above 1400℃, they are also mainly damaged by mechanical stress, thermal stress and chemical erosion of sulfur, chlorine and alkali substances during use4. At present, the more common problems are: the castables are not wear-resistant, not resistant to erosion, easy to peel off, and have a short service life. The service life of the refractory materials in the front and rear kiln mouths is mostly 4 to 8 months.

1.2 Coal injection pipe (cement kiln burner)

The castables for coal injection pipes, in addition to bearing the high temperature of 1400℃ in the working environment, must also bear the thermal cycle stress and the scouring of dust airflow S. The current problem is that the use temperature changes frequently and the temperature difference is large. The refractory lining 1m in front of the coal injection pipe is prone to coking and peeling, especially the lower part of the front end is prone to coking and sulfur formation, forming fish scale-like shedding, and its service life is short, generally 3 to 7 months.

1.3 Kiln head cover

The castables for the kiln head cover include the top and lower side walls of the kiln head cover; the top of the kiln head cover is mainly subjected to the scouring of high-temperature dusty airflow; the side walls of the kiln head cover not only have to withstand the scouring of high-temperature dust, but also have to bear the wear of cement clinker. The current problem is that the construction is difficult, and it is easy to burst if the baking is not in place after construction, and the service life is short, generally 6 to 12 months.

1.4 Grate cooler and low wall

The castables for the grate cooler include the top and side walls of the first stage of the grate cooler and the top and side walls of the second stage of the grate cooler.

The top of the first stage of the grate cooler is subject to high heat radiation from cement clinker and heat convection from air, and the working temperature changes frequently, resulting in severe thermal shock; the low walls on both sides of the first stage of the grate cooler are subject to long-term wear and tear from high-temperature clinker, and also suffer from damage caused by thermal stress and mechanical stress; the temperature of the second stage of the grate cooler is relatively lower than that of the first stage. The current problem is that the construction is difficult, and it is easy to burst if the baking is not in place after construction, and the wear is severe, and the service life is short, generally 6 to 12 months.

1.5 Decomposition system outside the kiln

The decomposition system outside the kiln mainly includes 1 to 5-level cyclones, feed pipes, decomposition furnace cones and smoke chambers. The cyclone is mainly affected by the continuous volatilization and condensation of harmful components such as alkali, chlorine, and sulfur in the cement raw materials, which are repeatedly circulated and continuously enriched, causing strong chemical erosion on the refractory castables and causing "alkali cracking" damage. The discharge pipe, decomposition furnace cone and smoke chamber are mainly affected by the calcination of low-grade raw materials, garbage waste, and inferior fuel coal powder in the cement production process, resulting in the enrichment of harmful substances such as chlorine, sulfur, and alkali in the entire kiln system. The 4th and 5th level cyclone cones, 4th and 5th level discharge pipes, decomposition furnace cones and smoke chambers are prone to crusting, resulting in frequent use of air cannons and water guns, which is not only risky, but also affects the normal production of the process and reduces the operation rate of the cement kiln system.

1.6 Three-way air duct gate and elbow

The temperature of the tertiary air containing dust in the tertiary air duct is 800~1000℃, and the wind speed is about 25m/s. The refractory lining of the tertiary air duct elbows, tertiary air duct gates and other parts of the tertiary air duct with clinker is easy to fall off or even break. The shortest service life is only 3 to 4 months, and the longest is only about 6 months. Once the refractory lining of the tertiary air duct gate or elbow is worn out, the tertiary air duct gate and elbow will be worn through, and the kiln has to be stopped for replacement.

In view of the problems existing in the refractory materials in the new dry process cement system production line mentioned above, this article recommends the following configuration of amorphous refractory materials used in different parts of large and medium-sized dry process cement kiln production lines.

2.1 Castables for kiln mouth

According to the use characteristics of the front kiln mouth, it is recommended to use JD-80AS thermal shock resistant low expansion andalusite kiln mouth castables for 5000t/d and below; JD-85AS high performance andalusite wear-resistant castables for 5000t/d and above; JD-75M corundum-mullite wear-resistant castables for the rear kiln mouth; this type of product has excellent wear resistance, high temperature alkali resistance and corrosion resistance, as well as good thermal shock resistance and spalling resistance.

2.2 Coal injection pipe castable

Since the temperature of coal injection pipe changes frequently and the temperature difference is large, the front end is prone to cracking and peeling, especially the lower part of the front end is prone to coking and sulfur deposition. It is recommended to use JD-80AS thermal shock resistant low expansion andalusite coal injection pipe castable for 5000t/d and below; JD-85AS high performance andalusite wear-resistant castable for 5000t/d and above. This type of product has excellent thermal shock stability and good erosion resistance.

2.3 Refractory materials for kiln head hood

Since the top of the kiln head hood is mainly subjected to the erosion of high-temperature dusty airflow, the side wall of the kiln head hood not only has to withstand the erosion of high-temperature dust, but also has to bear the wear of cement clinker. If it is not baked in place after construction, it is easy to burst. According to the characteristics of this part during operation, it is recommended to use JD-75M corundum-mullite wear-resistant castable with good high-temperature volume stability, good anti-stripping performance, and relatively wear-resistant.

2.4 Refractory materials for grate coolers and low walls

According to the temperature and usage characteristics of the top and side walls of the first stage of the grate cooler and the top and side walls of the second stage of the grate cooler, JD-M20-65 mullite high-strength wear-resistant castable can be used in the high-temperature zone of the first stage, and JD-16F high-aluminum high-strength wear-resistant castable can be used in the second stage of the grate cooler. The low wall of the grate cooler can use JD-M20-70 grate cooler low wall castable or JDY-M70 low wall special wear-resistant prefabricated bricks. In particular, the special wear-resistant prefabricated bricks for low walls have the characteristics of easy masonry, long life, and quick replacement, and should be vigorously promoted.

2.5 Refractory materials for decomposition system outside the kiln

In view of the high concentration of harmful components such as alkali, sulfur, and chlorine in the system outside the kiln, and the easy crusting of the cone of the 4th and 5th cyclone, the 4th and 5th feed pipes, the cone of the decomposition furnace, and the smoke chamber, it is recommended to use JD-13NL high-strength alkali-resistant castables for the 1st, 2nd, and 3rd levels of the preheater; JD-16 high-aluminum high-strength castables for the 4th and 5th levels of the preheater, the decomposition furnace, and the straight section of the tertiary air duct; silicon carbide anti-scaling castables for the smoke chamber and the feed pipe; in addition, the smoke chamber can also be built with special wear-resistant prefabricated bricks for the smoke chamber.

2.6 Refractory materials for tertiary air duct elbows and tertiary air valves

In view of the characteristics of tertiary air duct elbows and high-speed dust-containing fluids in tertiary air ducts, JD-17MF high wear-resistant and corrosion-resistant castables can be used for tertiary air duct elbows; prefabricated tertiary air duct gates can be used for tertiary air ducts. This product is directly cast and fired before leaving the factory, and can be directly installed and used after leaving the factory. It has the advantages of convenient hoisting and use and long service life.

In addition, when emergency repairs are carried out in cement plants, refractory spray coatings can be used, which have the advantages of moldless construction, high spray construction efficiency, and good explosion-proof performance. JD-PT-M45 mullite refractory spray coatings can be used for preheaters, decomposition furnaces, kiln head hoods, tertiary air ducts, grate coolers and other parts; JD-PT-50S anti-scaling refractory spray coatings can be used for smoke chambers and decomposition furnaces.

Precautions for the construction of amorphous refractory materials

Except for a small number of prefabricated bricks, such as wear-resistant quick-change prefabricated bricks for low walls and special wear-resistant prefabricated bricks for smoke chambers, which can be directly assembled and laid on site, and spray coating materials that can be directly sprayed on site, the rest of the amorphous refractory materials used in cement kilns are mainly refractory castables, which are bulk materials that need to be cast and formed at the production site. Moreover, the amount of water added and the construction time of each type of material are different. The pouring and vibration methods of different construction sites are also different. The quality of construction directly and seriously affects the service life of subsequent castables7. Therefore, when constructing castables, it is necessary to strictly follow the specified construction process and baking system of the castable manufacturer to ensure the service life. The basic construction requirements are as follows:

(1) The amount of water added to the castable stirring should meet the construction requirements. The less water added, the better the overall performance; the stirred castable must be used up within 30 minutes, and the refractory material that has reacted and initially set shall not be used.

(2) A forced mixer should be used for mixing the castables. It should be carefully cleaned before use to ensure that it is clean. During mixing, the aggregate, powder and binder should be mixed evenly, and then about 80% of clean water should be added. Finally, the remaining water should be added until the appropriate construction fluidity is obtained.

(3) The refractory castable poured into the formwork should be vibrated evenly. It should not be vibrated for a long time in the same position to avoid material segregation. Do not touch the anchoring metal parts and lightweight insulation layer. After the surface of the castable is observed to be slurry and the bubbles become less, the vibrating rod should be slowly pulled out to prevent segregation and holes in the castable layer.

(4) When pouring on a large scale, it should be constructed in blocks, and the area of each block should be 1.5m². Expansion joints should be reserved according to the requirements of the designer and should not be omitted.

(5) After the surface of the cast lining is hardened, the strength requirement must reach more than 70% before the mold can be removed.

(6) After the formwork is removed, the cast lining should be systematically inspected as soon as possible. If quality problems such as peeling, holes, and honeycombs are found, the same grade of castable can be used to continue repairing, vibrating, and curing. The surface of the cast lining should be smooth and neat, and no pits should appear.

Conclusion

The new dry-process cement kiln system has advantages that wet-process cement kilns and traditional dry-process cement kilns cannot match. The selection of amorphous refractory materials is also different from other types of cement kilns; each enterprise can optimize the configuration of amorphous refractory materials according to the actual characteristics of its own equipment, fuel, and raw materials, and strictly follow the construction guidance of the refractory manufacturer to ensure that the expected life is achieved, thereby reducing the number of kiln shutdowns and maintenance time, improving the system's operating rate, and achieving the goal of stable production, high production, and low energy consumption of the kiln system.