The refractory lining of lime kiln is affected by high temperature thermal shock, mechanical wear, structural stress and chemical erosion.
Lime rotary kiln is a kind of equipment used to calcine limestone at high temperature to prepare quicklime. Due to the high temperature, chemical corrosion and mechanical stress in the internal environment of lime rotary kiln, the application of refractory materials in the kiln is crucial. The following are the specific requirements and commonly used materials for refractory materials in lime rotary kiln.
1. High refractoriness and thermal stability
The calcination temperature of lime rotary kiln is usually between 1000°C and 1400°C. The refractory materials must maintain structural stability at high temperature without softening, deformation or melting.
Refractory: The material is required to remain solid under conditions higher than the temperature range in the kiln. For example, high alumina bricks and magnesia alumina spinel bricks have high refractoriness and are suitable for use in high temperature areas.
2. Excellent wear resistance and mechanical strength
There are mechanical impacts and frictions of limestone and lime in the kiln, especially in the preheating zone and the burning zone. The refractory materials need to have good wear resistance to reduce the wear and replacement frequency of the materials.
Mechanical strength: such as high compressive strength and flexural strength to cope with the impact of stones on the kiln wall. Materials with good wear resistance such as high alumina bricks, silicon-molybdenum bricks, etc. can be used.
3. Good thermal shock resistance
The lime rotary kiln will experience temperature fluctuations during operation. The refractory materials need to have good thermal shock resistance to withstand rapid temperature changes without cracking.
Thermal shock resistance: Materials such as magnesia-alumina spinel bricks and mullite bricks have good thermal shock resistance and are suitable for areas with large temperature changes in the kiln.
4. High resistance to alkaline corrosion
The quicklime and its dust in the lime rotary kiln are alkaline corrosive, especially in the preheating zone and the cooling zone. The refractory materials need to have good corrosion resistance to alkaline substances.
Alkali corrosion resistance: such as magnesia chrome bricks and magnesia alumina spinel bricks, which have strong resistance to erosion by alkaline compounds.
5. Low porosity and high density
Low porosity helps reduce the penetration of corrosive media and extend the service life of the material. High-density materials are more resistant to erosion and wear.
Porosity: Choosing dense refractory materials such as sintered magnesia bricks can reduce the penetration of alkaline substances.
6. Thermal conductivity and insulation performance
Refractory materials need to strike a balance between thermal conductivity and insulation performance. In the firing zone of the kiln, high thermal conductivity helps to evenly distribute heat, while in the cooling zone, lower thermal conductivity helps to keep warm.
Thermal conductivity: The firing zone uses materials with better thermal conductivity such as high-alumina bricks, while the cooling zone can use lightweight refractory bricks with better insulation.
7. Machinability and assembly
Refractory materials need to be able to be processed into suitable shapes and sizes to adapt to the complex structure and installation requirements in the kiln. Good machinability and dimensional accuracy can ensure that the materials are tightly installed in the kiln, reducing gaps and misalignment.
Assembly: such as prefabricated parts and castables, which can meet the lining requirements of complex areas.
The use conditions of refractory materials for lime kilns are very harsh. In view of the harsh use conditions, refractory materials are required to have good chemical corrosion resistance, good high-temperature volume stability and masonry integrity, good thermal shock resistance, good mechanical erosion resistance and wear resistance. The specific requirements for each section of the lime rotary kiln are as follows:
1. Preheating section
The preheating section is preceded by a preheater. The limestone is first heated in the preheater. The heat source is the waste flue gas from the cooling section. The raw material particle size is 10~30mm. The temperature entering the preheating section is 1000~1050℃. The requirements for the material are mainly high strength, good wear resistance, and certain thermal shock resistance. Dense clay bricks or high-grade clay bricks are generally used at this stage. Generally, no layers are used during masonry. The furnace shell temperature is controlled at no more than 350℃. Use fire mud for wet masonry.
2. Transition zone (lifting belt)
The temperature of this section is 1200~1300℃, which is the transition stage of limestone to the burning zone. The temperature changes greatly in this stage, and some limestone has begun to react. The thermal shock stability and material strength of the material at this stage are required to be good. Generally, B-HM55 mullite bricks are selected. At the same time, in order to make the material distribution and heating uniform at this stage, about 3 raised lifting belts are generally added in the length direction to drive the raw materials to turn over during masonry. The raised materials are generally prefabricated blocks of the same material. Mullite bricks and fire clay are used for direct masonry in this section, and no layers are required.
3. High temperature zone
The high temperature zone is adjacent to the burning zone, with a use temperature of 1300~1400℃ and frequent temperature changes. This section requires the material to have a higher load softening starting point and higher volume stability (reburning is micro-expansion), and better thermal shock resistance. Generally, special high-alumina bricks are selected to be directly masonry with fire clay without layers. The representative material of this section is KE-70 high-alumina brick.
4. Burning zone
The burning zone is the final reaction stage of lime. This section uses a large burner for heating at a temperature of 1400~1500℃. The higher the activity requirement of lime, the higher its burning temperature. For this reason, this section uses an insulation layer, and the insulation material is required to have considerable strength to prevent it from being worn and powdered by the working layer. This section uses magnesium-rich spinel bricks, which are dry-laid without fire mud. Steel plates are added between bricks. At high temperatures, the steel plates and materials generate magnesium-iron spinel and are firmly bonded together.
5. Cooling zone
The cooling zone is very short, with only a few rings of bricks or castables, and the temperature is 1300~1400℃. It is generally built with KE-70 high-alumina bricks.
6. Cooler
This equipment is the cooling equipment for finished lime. It is cooled by a blower. This section requires the temperature of the cooled lime to be lower than 100℃. The cooled lime enters the silo through the belt. The temperature of the hot air after heat exchange is about 600℃, and it enters the rotary kiln as combustion-supporting air to mix with coal gas for combustion. This section adopts wet masonry construction with clay bricks and fire clay.
7. Refractory clay
Due to the masonry types in various stages of the lime rotary kiln, most of them need to be masonry with fire clay, and there are special requirements for fire clay. Generally, suitable refractory clay should be selected according to the temperature and requirements of each part to prevent the pursuit of high quality of fire clay and the inability to match the temperature and materials of this stage, which affects the overall operation of the kiln.
The selection and arrangement of refractory materials for lime rotary kiln is a process that comprehensively considers refractoriness, wear resistance, thermal shock resistance, alkaline corrosion resistance and structural characteristics. Reasonable selection and application of refractory materials can significantly improve the operating efficiency and service life of the kiln, and reduce maintenance frequency and cost.
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