In traditional cement rotary kilns, the material temperature in the drying zone is 20–150 degree , and the gas temperature is 250–400 degree .

 In new dry process cement kilns, the material entering the kiln is heated to above 150 degree the moment it enters the kiln tail Stage I cyclone. Therefore, the drying zone only exists in a very short section at the raw material inlet. There is almost no drying zone in the kiln tail system of new dry process cement kilns.

 In traditional cement rotary kilns, the material temperature in the preheating zone is 150–800 degree , and the gas temperature is 450–850 degree .

 In new dry process kilns, the preheating zone spans from the first-stage cyclone to the penultimate-stage cyclone dust collector. In the preheating zone, the organic components in the raw material begin to dry distill and decompose, and clay starts to dehydrate, creating conditions for the decomposition of calcium carbonate.

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 In traditional cement rotary kilns, the material temperature in the decomposition zone is 800–1000 degree , and the gas temperature is 1000–1400 degree .

 In new dry process cement kilns, the decomposition zone extends from the decomposition furnace to a point 7D (D is the steel plate diameter of the kiln) away from the kiln head inside the kiln. For example:

For a new dry process kiln with dimensions Φ4m×60m, the decomposition zone ranges from 28–60m from the kiln head.

For a new dry process kiln with dimensions Φ4.7m×74m, the decomposition zone ranges from 33–74m from the kiln head.

 The material temperature in the exothermic reaction zone is 1000–1300 degree .

 In new dry process cement kilns, the transition zone is calculated as the area from 7D to 4D from the kiln head. Examples:

For a Φ4m×60m new dry process cement kiln, the transition zone ranges from 16–28m from the kiln head.

For a Φ4.7m×74m new dry process cement kiln, the transition zone ranges from 19–33m from the kiln head.

 The temperature of the kiln charge in the cooling zone is 1100 - 1300 degree . After the clinker leaves the burning zone, it is cooled or solidified in the cooling zone.

 The burning zone is at the location of the highest temperature in the cement kiln. The flame temperature can reach 1800 - 2000 degree , and the temperature of the kiln charge reaches 1350 - 1400 degree .

 In this part, there are a large number of alkali - sulfur compounds in the kiln charge. Cement clinker needs to form a kiln coating to protect the lining bricks. At the same time, the lining bricks also need to have the property of adhering to the kiln coating.

 During the clinker calcination process, the temperature of raw meal, kiln charge, and clinker is heated from normal temperature to approximately 1430 degree , and the fuel combustion temperature in the kiln drops gradually from 1800 - 2000 degree to below 350 degree . During the clinker cooling process, the clinker is cooled to about 100 degree by air, while the air is heated from normal temperature to over about 1000 degree . In the above - mentioned processes, the radiation, convection, and heat transfer of flue gas, air, raw meal, kiln charge, and clinker to the lining and metal components are called thermal stress.

 During the cement clinker calcination process, various compound components in the raw meal and fuel, under different technological conditions, undergo chemical reactions to produce various compounds. These compounds penetrate into the refractory linings and metal components in solid, molten, or gaseous phases, react with the compounds in the linings and metal components, and form low - melting - point compounds with volume changes, leading to the damage of the linings and metal components. This is called chemical stress.

 The heat exchange capacity of the preheating system in new dry process cement kilns is excellent. However, the heat exchange capacity of the rotary kiln section is relatively poor, which reduces the sintering capacity. Therefore, cement rotary kilns need to increase the burning temperature for compensation. The maximum temperature of the flame in the cement rotary kiln can reach 1,700 degree , and the effect of high temperature is extremely intense. Without the protection of the kiln skin, the exposed refractory materials will be damaged quickly.

 As the output of the cement kiln increases, the high-temperature load on the rotary kiln gradually increases. When the output increases from 2,000 t/d to 7,000 t/d, the cross-sectional heat load of the kiln increases from 4.2×10⁹ cal/(m²·h) (1 cal=4.18 J, the same below) to 6.2×10⁹ cal/(m²·h). The heat load on the surface of refractory materials in the high-temperature zone increases from approximately 3.0×10⁹ cal/(m²·h) to 4.8×10⁹ cal/(m²·h), and the surface output load also increases from 8 t/(d·m²) to 11 t/(d·m²).

 Large-scale pre-decomposition (PC) kilns use high-efficiency coolers with a heat recovery efficiency of over 60% and multi-channel burners with sufficient combustion and a low proportion of primary air. The kiln head is also equipped with enhanced airtightness and thermal insulation. In a 4.7m×74m pre-decomposition kiln, the secondary air temperature reaches 1,150 degree , the kiln tail gas temperature reaches 1,050–1,100 degree (maximum 1,200 degree ), and the temperature of clinker leaving the kiln cylinder reaches 1,400 degree . The operating temperatures in the transition zone, burning zone, cooling zone, kiln door hood, throat and high-temperature zone of the cooler, and the outer side of the burner are far higher than those of corresponding parts in traditional cement kilns.

 Although high temperature is beneficial for cement burning, it weakens the performance of refractory materials and accelerates their erosion, leading to damage and affecting the service life of refractories. Therefore, cement pre-decomposition kilns must use a series of new refractory materials to replace the traditional materials used in conventional kilns. For example, in the direct firing point (center) of the burning zone of a cement rotary kiln, high-grade magnesia-based refractory materials are required, including directly bonded magnesia-chrome bricks with a load softening temperature >1,650 degree or high-quality chrome-free alkaline bricks.

 If part of the refractory bricks in the cement kiln is damaged, the thickness of the remaining kiln lining will decrease, the thermal insulation effect of the lining will weaken, and the surface temperature of the kiln body will rise. This will further accelerate the damage rate of the refractory materials. For example, when the cylinder temperature in the high-temperature zone increases from the normal 250–300 degree to 350–400 degree , significant deformation of the kiln body will occur.

 The rotation speed of traditional cement kilns is 1 r/min, while that of large-scale pre-decomposition kilns can reach as high as 3–4 r/min. In pre-decomposition kilns with high temperature, high speed, and large diameter, the operating environment of the kiln shell and lining is much harsher than that of traditional rotary kilns.

 The cylinder of a cement rotary kiln is supported on supporting rollers via riding rings. Due to the significant self-weight of the cylinder, as well as the gravitational forces from refractory bricks, kiln skin, and kiln charge, large radial deformation occurs in the cross-section of the cylinder between the riding rings. Continuous measurement of the operating kiln shell using a cylinder measuring instrument shows that the dimensional difference between the horizontal and vertical diameters of the cylinder can reach 0.3%, and sometimes even 0.6%–0.7%. Each revolution of the kiln shell causes its curvature to change cyclically. Consequently, the refractory lining is inevitably subjected to periodic squeezing forces from the kiln shell and internal equilibrium stresses within the brick ring, leading to damage under fatigue loading.

Direct-Bonded Magnesia Chrome Brick