What Are The Specific Performance Requirements For Refractory Materials in Different Parts Of A Cement Rotary Kiln?

 The cold end of the refractory bricks used in the cement rotary kiln is subjected to the extrusion pressure \( P \) from the kiln shell, and the two side faces are subjected to the balancing pressure \( f \) of the brick ring. The external force balance on the kiln shell and refractory bricks is shown in the figure.

 According to the equilibrium conditions, the extrusion pressure \( P \) exerted by the kiln shell on the refractory bricks, the balanced reaction force \( f \) in the refractory brick ring, and the central angle \( \alpha \) subtended by the refractory bricks satisfy the following relationship (neglecting the weight of the bricks):

 According to the geometric conditions shown in the figure, the following approximate relationship is satisfied among the rotary kiln diameter \( D \), the large-head (cold end) dimension \( A \) of the refractory brick shown in the figure, the small-head (hot end) dimension \( B \) of the refractory brick, the height \( h \) of the refractory brick, and the central angle \( \alpha \) subtended by the refractory brick:

 The diameter of the kiln cylinder in new dry process cement rotary kilns has been continuously increasing with the expansion of production capacity, increasing from 3300mm to the current 6400mm, nearly doubling. At present, there are mainly two types of brick shapes used inside cement rotary kilns. Alkaline bricks generally adopt the VDZ series brick shape (an international common series with a constant middle width of 71.5mm), while sintered silico-aluminous refractory bricks generally use the ISO series brick shape (an international common series with a constant large-head width of 103mm). The height and thickness of refractory bricks are relatively fixed, leaving little room for adjustment. If the height of the refractory bricks is too low, their thermal insulation performance will be too poor. Once the hot end of the bricks is slightly damaged, the kiln body temperature will rise above the upper limit, leading to kiln shutdown for brick replacement. If the height of the refractory bricks is too high, the weight of the bricks will increase, causing excessive deformation of the kiln body and damage to the refractory bricks.

 With the increase in kiln diameter and rotational speed, especially for the technologically advanced two - support kilns which have a higher rotational speed of up to 5r/min, it is likely to increase the ovality of the kiln shell. Correspondingly, there are higher requirements for the compressive stress on refractory bricks. Therefore, refractory bricks with high compressive strength are required. In particular, for the kiln shell in the tyre area, it is required to use refractory bricks with the same characteristics as much as possible in the vicinity of the tyre.

 As the kiln diameter expands, the central angle α corresponding to the refractory brick becomes smaller, and the stress borne by the refractory brick increases. At the same time, more stringent requirements are imposed on the dimensional tolerance of the refractory brick. The diameter of the kiln shell of a 10,000 - ton - class cement kiln reaches 6.4m, which sets extremely high requirements for the external dimensions of the refractory bricks inside the kiln. Refractory bricks with large deviations in external dimensions are prone to accidents such as brick slippage and falling during the laying and use processes, seriously endangering the normal operation of the cement kiln.

 There are many factors affecting the service life of refractory linings in kilns, which are not solely issues with refractory materials products. These include thermal stress, chemical stress, and mechanical stress generated during cement production operations, the quality of calcined raw materials, the design and selection of refractory materials, the quality, storage, construction, and masonry of refractory materials, among other factors.

 Mechanical stress refers to the internal forces that act between various parts of an object when it deforms due to external causes. These internal forces resist the external causes and attempt to restore the object to its original position before deformation. When the mechanical stress endured by refractory bricks in the kiln exceeds their own strength, the bricks will suffer partial or complete damage under the action of stress. The main factors causing mechanical stress are as follows:

 (1) Elliptical deformation. The comprehensive factors such as the refractory lining bricks in the rotary kiln, the kiln material, and the self-weight of the kiln shell cause the kiln shell to deform. Under the action of gravity and thermal load, the circular cross-section of the shell becomes elliptical. When the kiln is in operation, the ellipse imposes mechanical stress on the refractory bricks, and the greater the ovality, the greater the mechanical stress generated. The shear stress caused by the change in ovality acts in the tangential direction of each ring of bricks, resulting in annular spalling of the bricks. Generally, the spalled pieces are uniform in thickness and hard in texture.

 (2) Axial offset of the rotary kiln. The rotary kiln is supported by tyres, supporting rollers, and idlers, and its axis should be a straight line connecting the center points of the circular sections of the kiln. However, after the installation of the kiln shell and partial shell cutting and replacement, or after the kiln has been in operation for a period of time with unstable thermal working conditions, the axis of the kiln shell will shift under the action of thermal load and weight. After long-term operation, the wear of tyres and supporting rollers, the outward and inward deflection of supporting rollers, and the change in load conditions at each support point-especially when the load at the support point is too large-can easily cause burning of the supporting roller bearings, abnormal peeling, or cracking on the surfaces of tyres and supporting rollers. This will further exacerbate the axial offset of the kiln shell, resulting in extrusion deformation, damage, or shedding of the refractory bricks. The damaged refractory bricks show varying depths of shape.

 Thermal stress refers to the stress generated in an object when temperature changes, as it cannot expand or contract completely freely due to external constraints and mutual constraints between internal parts. High-temperature thermal expansion easily causes axial expansion and extrusion stress in refractory bricks, which is one of the important reasons for spalling and damage of refractory bricks in the kiln. Taking magnesia-chrome or spinel bricks as examples, the expansion rate at 1400°C can be calculated as 1.6%, and the expansion amount of a refractory brick with a length of 198mm can reach 3.17mm. With such significant expansion, if the circumferential joints are not properly reserved, being too large or too small will cause brick slippage, falling, and spalling, seriously shortening the service life of refractory bricks.

 The influence mechanism of production operations on the service life of refractory bricks is complex and involves many factors. The analysis is mainly carried out from the following two aspects:

 (1) Brick damage caused by excessively high calcination temperature.
The flame temperature inside the new dry process pre-decomposition kiln can exceed 1700°C, and the working temperatures in the transition zone, burning zone, cooling zone, kiln hood, throat and high-temperature zone of the cooler, and the outer nozzle area are also much higher than those of traditional kilns in corresponding positions. Even with high-quality refractory materials, the service life of kiln linings in the transition zone, burning zone, and cooling zone of large rotary kilns is generally 0.5 to 1 year, and in some cases, as short as 3 to 5 months; the service life of kiln mouth and nozzle linings is typically only 0.5 to 1 year or even shorter; and the service life of kiln hood and cooler throat linings is approximately 2 years. During the trial production stage, the operation rate of rotary kilns is usually only 70% to 75% or even lower, with very few reaching 85% to 90%. If the operation of the preheater and decomposition furnace is poor, and the decomposition degree of the material entering the kiln is unstable, the positions of various process zones in the kiln will frequently change, leading to unstable kiln operation and faster damage to the kiln lining. For example, excessively high calcination temperature can cause damage and molten pits in the refractory bricks inside the kiln, as circled in Figure 5.

 (2) Brick damage caused by high kiln rotation speed.
The rotation speed of new dry process pre-decomposition kilns often reaches 3 to 3.7 r/min, and can even exceed 4 r/min, with the linear speed of the rotary kiln shell exceeding 1 m/s. In new dry process kilns with high rotation speed, large diameter, and high temperature, the comprehensive damage effects of thermal stress, mechanical stress, and chemical erosion on the kiln lining are much greater than those in traditional kilns. This requires that the kiln lining must have sufficient strength and stability in both cold and hot states.

 The main function of refractory bricks in a rotary kiln is to protect the kiln shell from damage caused by high-temperature gases and materials, ensuring the normal operation of production. In industrial production, the service life of refractory bricks in the burning zone is very short, often leading to unplanned kiln shutdowns for maintenance. This has become a key factor affecting the high-quality, high-yield, low-consumption operation and annual operation rate of cement kilns.

 Whether in wet-process kilns or new dry-process rotary kilns, during the clinker calcination process, the gas temperature inside the kiln is much higher than the material temperature. Each time the kiln rotates, the surface of the kiln lining undergoes periodic thermal shock, with temperature variations ranging from 150 to 250°C, generating thermal stress within the 10–20 mm surface layer of the kiln lining. The kiln lining also withstands alternating radial and axial mechanical stresses from the brickwork due to kiln rotation, as well as erosion and wear from calcined materials.

 Silicate melt is simultaneously generated, which easily interacts with the surface of the kiln lining refractory bricks in high-temperature environments to form an initial layer. It also penetrates into the interior of the refractory bricks along their pores and bonds with them, altering the chemical composition and phase composition within the 10–20 mm surface layer of the refractory bricks and degrading their technical performance. When the material has a narrow sintering range or local high temperatures occur due to short-flame rapid firing, the minimum temperature on the kiln skin surface may exceed the solidification temperature of the material liquid phase. The surface layer of the kiln skin then transitions from solid to liquid and peels off, penetrating from the surface to the interior of the kiln skin before forming a new initial layer. When this cycle repeats, the kiln lining in the burning zone gradually thins and may even completely detach, exposing the kiln shell locally and causing "red kiln" (overheating). In reality, this is how the burning zone kiln lining deteriorates: in high-temperature areas, the residual brick thickness generally distributes along a curve with a large radius of curvature, sometimes with the curve base falling on the inner surface of the kiln shell.

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