- Time:Feb 27, 2023
The rotary kiln is the core equipment for the production and operation of the cement plant. Whether the continuous operation of production can be guaranteed depends to a large extent on the use of the refractory lining of the rotary kiln. However, there are many factors that affect the use of refractories in rotary kilns, but they are mainly three factors: chemical erosion, thermal load, and mechanical load.
1. The role of refractory lining in the rotary kiln
The functions of the refractory lining of the rotary kiln are as follows: (1) Prevent direct damage to the cylinder body by high-temperature materials, flames and airflow, protect the kiln cylinder body, and avoid high-temperature deformation of the kiln cylinder body. (2) During the tissue firing process, the liquid phase clinker and harmful substances such as CO, Cl2, SO2 directly erode the cylinder body. (3) During the calcination process, the firing temperature of the clinker is about 1450°C, but the temperature of the burning gas in the kiln can reach up to 1700°C, and the refractory material in the kiln plays a role of heat insulation and reduces the temperature of the cylinder , prevent heat loss and save energy. (4) It has the function of heat storage and heat preservation. The refractory kiln lining can act as a heat transfer medium, absorb heat from the high-temperature gas in the kiln, and transfer heat to materials by conduction and radiation. (5) The refractory material kiln lining can improve the performance of hanging the kiln skin, prevent the erosion of the shell by high temperature and harmful substances, and better protect the kiln shell.
2. Factors affecting the service life of rotary kiln refractories
The rotary kiln can be divided into front kiln mouth, lower transition zone, firing zone, upper transition zone, pre-decomposition zone and back kiln mouth. According to the temperature of different parts, refractory linings of different materials need to be configured. During the production process, the refractory material will be subjected to the mechanical load from the kiln shell and the forward movement of the material, the thermal load of the pulverized coal fuel of the coal injection pipe and the high-temperature material, the chemical erosion of the clinker and harmful gas, etc.; the design of the refractory material in the kiln Factors such as quality, masonry, oven and wear from materials will cause premature damage to the refractory lining.
After theoretical analysis, combined with actual production experience, it is concluded that the main factors affecting the service life of refractory bricks in the kiln are chemical erosion, thermal load and mechanical load.
2.1 The influence of mechanical load on the service life of refractories
The mechanical load of the rotary kiln is mainly due to the relative movement and heat transfer of the cylinder body, refractory materials, materials, and dusty gas during the calcination of cement clinker, the mechanical load generated by the relative movement of the kiln lining, and the gravity and abrasive effects of the materials. . When the mechanical load on the refractory in the kiln exceeds its own strength, it will cause damage to the refractory. There are three main factors that generate mechanical loads:
(1) The cylinder is twisted and deformed. Due to the excessive cumulative weight of the refractory materials in the rotary kiln, the materials in the kiln and the cylinder itself, under the action of gravity and heat load, or due to local high temperature, the cylinder will locally undergo concave-convex deformation. The circular section of the cylinder will become elliptical. When the kiln is running, the deformation of the cylinder will generate mechanical load on the refractory material, and the greater the ellipticity, the greater the mechanical load. The shear stress generated by the change of ellipticity acts on each ring of bricks, which causes the bricks to peel off. Generally, the thickness of the strips is uniform and hard.
(2) The axis of the rotary kiln is offset. The rotary kiln is supported by tire belts, supporting wheels, and supporting rollers, and the line connecting the center points of each section circle should be on a straight line. However, after the kiln shell is installed and part of the shell is cut and replaced, or after a period of operation of the kiln body and the thermal system in the kiln is unstable, the axis of the kiln body will deviate under the action of thermal load and load. After a long period of operation, the wear of the tires and idlers, and the external and internal deflection of the idlers will cause different changes in the load of each support point, especially when the load of the support point is too large, the tires and support Abnormal peeling or cracks on the surface of the roller will further aggravate the axis deviation of the kiln body; as a result of the axis deviation, the refractory material will be squeezed and deformed to cause damage or fall off, and the shape of the refractory brick after damage is different.
(Damage of refractory bricks caused by the offset of the rotary kiln axis)
(3) Masonry quality is poor. Due to construction factors, the refractory bricks are too loose and distorted. During the operation of the rotary kiln, the kiln shell and the cold surface of the refractory bricks will move relative to each other, and the thrust of the brick lining will be too large. Cracks, dropped corners, and even bricks.
(Damage of refractory bricks caused by poor masonry quality)
2.2 Effect of heat load on service life of refractories
Thermal load refers to the thermal expansion of refractory materials due to the high temperature during the temperature rise of the kiln lining, which causes mutual extrusion between adjacent refractory materials. When the extrusion stress generated by making it unable to expand completely freely is greater than the strength of the refractory brick, it will cause the hot surface of the refractory brick to peel off. Thermal stress is one of the important reasons for the peeling and damage of the refractory brick in the kiln. Taking magnesia-iron bricks or magnesia-aluminum spinel bricks as an example, the expansion rate at 1400°C is calculated as 1.6%, and the expansion of a refractory brick with a length of 198mm can reach 3.16mm. For such a large expansion, it is necessary to properly reserve a good ring. Joints (expansion joints), too large a reservation may cause refractory bricks to draw lots and bricks to fall off, and the thermal stress generated by a too small reservation will have no place to release, which will cause refractory bricks to peel off, thus seriously shortening the service life of refractory bricks .
2.3 Effect of chemical attack on service life of refractories
Chemical erosion to refractory material erosion exists in any kiln, and it is one of the important factors affecting the service life of the kiln, and the erosion mechanism is many and complex.
(1) The calcination temperature is too high or fluctuates
In the rotary kiln, the material is gradually converted into cement clinker. The liquid phase of the clinker in the firing zone is about 25%~30%. The converted clinker phase mainly includes C3S、C2S、C3A and C4AF. The phase clinker will chemically react with the surface of the refractory brick, and the refractory material will also be penetrated and eroded by harmful substances such as alkali salts and some harmful gases. If the calcination temperature is too high, the liquid phase of the clinker will increase, and the chemical reaction between the clinker and the refractory material will intensify, resulting in the deepening of the reaction layer at the hot end of the clinker and the refractory material. Due to the difference between the reaction layer and the internal structure of the brick, a Qualitative changes, such as sudden large fluctuations in the temperature in the kiln or when the kiln is stopped for cooling, will cause spalling and cracking of the metamorphic layer of the refractory material. Therefore, in order to ensure the service life of refractory bricks, the production operation should be stabilized, and a reasonable heating and cooling kiln system should be formulated.
(Unreasonable cold kiln system causes the entire layer of refractory material to peel off)
The rotational speed of the new dry-process rotary kiln is usually 3.5-4r/min, or even as high as 4r/min, and the linear velocity of the cylinder can reach more than 1m/s. If the kiln speed is increased, under the conditions of high speed, large diameter and high temperature, the contact times of clinker and refractory materials will be more frequent, so that the comprehensive damage effect of thermal load, mechanical load and chemical erosion on refractory kiln lining will be different from that of low speed Rotary kilns or static kilns are much larger. This requires that the refractory lining of large and medium-sized rotary kilns must have sufficient strength and stability, and a reasonable rotary kiln system should be formulated.
3. Conclusion
Nowadays, under the policy background of green development planning in the cement industry, the refractory lining of rotary kiln is developing in the direction of "energy saving, environmental protection, long life, lightweight, and chrome-free". Construction, kiln drying, production operation and other factors, if not handled properly, can affect the service life of rotary kiln refractories. Through the analysis of the above factors, a series of measures should be formulated to maintain the refractory material, so as to prolong the service life of the refractory material and help achieve the best use effect.
ZHENJIN REFRACTORIES is committed to one-stop general contracting services such as research and development, technical solution design, construction and supporting product provision of refractory materials in the cement, lime, non-ferrous, steel, glass and chemical industries. Kiln overall solution service provider. The service chain runs through the entire life cycle of the project, including preliminary consultation, scheme design, R&D and production, furnace construction and safety production. Through the optimized combination of various brand products, it can meet different process requirements and create value for customers.
