Most Portland cement is made in a rotary kiln. Basically, this is a long cylinder rotating about its axis once every minute or two. The axis is inclined at a slight angle, the end with the burner being lower.The rotation causes the raw meal to gradually pass along from where it enters at the cool end, to the hot end where it eventually drops out and cools. They were introduced in the 1890s and became widespread in the early part of the 20th century and were a great improvement on the earlier shaft kilns, giving continuous production and a more uniform product in larger quantities.For information on reactions in the kiln see the clinker pages.
The original rotary cement kilns were called 'wet process' kilns. In their basic form they were relatively simple compared with modern developments. The raw meal was supplied at ambient temperature in the form of a slurry.A wet process kiln may be up to 200m long and 6m in diameter. It has to be long because a lot of water has to be evaporated and the process of heat transfer is not very efficient.The slurry may contain about 40% water. This takes a lot of energy to evaporate and various developments of the wet process were aimed at reducing the water content of the raw meal. An example of this is the 'filter press' (imagine a musical accordion 10-20 metres long and several metres across) - such adaptions were described as 'semi-wet' processes.The wet process has survived for over a century because many raw materials are suited to blending as a slurry. Also, for many years, it was technically difficult to get dry powders to blend adequately.Quite a few wet process kilns are still in operation, usually now with higher-tech bits bolted on. However, new cement kilns are of the 'dry process' type.
In a modern works, the blended raw material enters the kiln via the pre-heater tower. Here, hot gases from the kiln, and probably the cooled clinker at the far end of the kiln, are used to heat the raw meal. As a result, the raw meal is already hot before it enters the kiln.
Secondly, and less obviously, the process of transferring heat is much more efficient in a dry process kiln. An integral part of the process is a heat exchanger called a 'suspension preheater'. This is a tower with a series of cyclones in which fast-moving hot gases keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas. The basic dry process system consists of the kiln and a suspension preheater. The raw materials, limestone and shale for example, are ground finely and blended to produce the raw meal. The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal. The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in temperature between the hot gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering the kiln. A development of this process is the 'precalciner' kiln. Most new cement plant is of this type. The principle is similar to that of the dry process preheater system but with the major addition of another burner, or precalciner. With the additional heat, about 85%-95% of the meal is decarbonated before it enters the kiln
An integral part of the process is a heat exchanger called a 'suspension preheater'. This is a tower with a series of cyclones in which fast-moving hot gases keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas.
The basic dry process system consists of the kiln and a suspension preheater. The raw materials, limestone and shale for example, are ground finely and blended to produce the raw meal. The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal.
The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in temperature between the hot gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering the kiln.
A development of this process is the 'precalciner' kiln. Most new cement plant is of this type. The principle is similar to that of the dry process preheater system but with the major addition of another burner, or precalciner. With the additional heat, about 85%-95% of the meal is decarbonated before it enters the kiln
Since meal enters the kiln at about 900 C, (compared with about 20 C in the wet process), the kiln can be shorter and of smaller diameter for the same output. This reduces the capital costs of a new cement plant. A dry process kiln might be only 70m long and 6m wide but produce a similar quantity of clinker (usually measured in tonnes per day) as a wet process kiln of the same diameter but 200m in length. For the same output, a dry process kiln without a precalciner would be shorter than a wet process kiln but longer than a dry process kiln with a precalciner.
In the diagram above of a precalciner kiln, raw meal passes down the preheater tower while hot gases rise up, heating the raw meal. At 'A,' the raw meal largely decarbonates; at 'B,' the temperature is 1000 C - 1200 C and intermediate compounds are forming and at 'C,' the burning zone, clinker nodules and the final clinker minerals form. A preheater tower is likely to have 4-6 stages, not the three shown here. Many designs are more complex but this diagram illustrates the principle. See the 'Clinker' pages for more information on reactions in the kiln.
The kiln is made of a steel casing lined with refractory bricks. There are many different types of refractory brick and they have to withstand not only the high temperatures in the kiln but reactions with the meal and gases in the kiln, abrasion and mechanical stresses induced by deformation of the kiln shell as it rotates.
Bricks in the burning zone are in a more aggressive environment compared with those at the cooler end of the kiln (the 'back end'), so different parts of the kiln are lined with different types of brick.
Periodically, the brick lining, or part of it, has to be replaced. Refractory life is reduced by severe changes in temperature, such as occur if the kiln has to be stopped. As the cost of refractories is a major expense in operating a cement plant, kiln stoppages are avoided as far as possible.
As the meal passes through the burning zone, it reaches clinkering temperatures of about 1400 C - 1500 C. Nodules form as the burning zone is approached. When the clinker has passed the burning zone, it starts to cool, slowly at first, then much more quickly as it passes over the 'nose ring' at the end of the kiln and drops out into the cooler.
The cooled clinker is then conveyed either to the clinker store or directly to the clinker mill. The clinker store is usually capable of holding several weeks' supply of clinker, so that deliveries to customers can be maintained when the kiln is not operating.
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Cement rotary kilns refer to cement rotary calcining kilns (sometimes called rotary furnaces). It is a pyroprocessing device used to heat materials to high temperatures in a continuous process. The kiln body is a cylinder vessel with a certain degree of tilt to the horizontal level. Raw materials are fed into the vessel from the upper end and moved to the lower end, being stirred and mixed relying on the inclination and rotation of the kiln. The kiln burner produces a lot of heat by burning fuel. This kind of heat is usually transferred to materials through flame radiation, hot gas convection, kiln brick conduction, etc., which causes the chemical reaction between raw materials and finally forms clinker.
Rotary kilns can be divided into cement kiln, metallurgical and chemical rotary kiln, lime rotary kiln and so on. Cement rotary kilns are used for calcining cement clinker in the cement plant, which can be divided into dry cement kiln and wet cement kiln. Metallurgical and chemical rotary kilns are mainly applied in the metallurgical industry. As for the lime rotary kiln, it is the main equipment for calcining active lime and light burned dolomite used in iron and steel plants, ferroalloy plants, calcium carbide plants, and magnesium metal plants.
The cement rotary kiln is mainly composed of cylinder, supporting device, drive gear, refractory lining, catch-wheel device, kiln head sealing device, kiln tail sealing device, kiln hood, and other components. On the cylinder, there is a large gear ring fixed with a spring plate near the kiln tail, some pinions below are engaged with it, jointly forming the drive gear. In normal operation, the main drive motor will transfer power to this gear device through reducer to run rotary kiln. The raw material usually enters the rotary kiln from the upper end and move slowly to another end along with the chamber as it rotates. In this process, raw materials will be heated by high temperature and then decompose and produce chemical reactions so that their state finally changed. Under normal conditions, the heat source of indirect fired rotary kilns is supplied from the kiln burner outside the kiln. This kind of way protects the integrity of raw materials, while the heat source of the direct-fired rotary kiln is inside the kiln. Besides, the rotation speed and temperature of the cylinder are tightly controlled and changed according to different desire processes and material applications. After the calcination is completed, the clinker will be pre-cooled in the chamber and then be sent into the cooler for further cooling.
AGICO Group is an integrative enterprise group. It is a Chinese company that specialized in manufacturing and exporting cement plants and cement equipment, providing the turnkey project from project design, equipment installation and equipment commissioning to equipment maintenance.
Cement Rotary kiln, is one kind of lime kiln, belongs to building material equipment. The rotary kiln has wide applications, such as metallurgy, chemical industry, cement, refractory materials, lime, environmental protection and other industries.
Cement rotary kiln, also called clinker kiln, can be divided into dry-process cement kiln and wet-process cement kiln according to the cement production technology. Dry-process cement rotary kiln is mainly used to calcine cement clinker. As the core equipment of NSP cement production line, cement rotary kiln is consist of shell, supporting device, transmission device, lubrication device, moving kiln head, sealing device for kiln tail, burning device and etc. dry-process cement rotary kiln has advantages of simple structure, reliable operation and easy to automatic control in the cement production line.
AGICO Cement is one of leading cement plant manufacturers offers cement kiln and other cement equipment, our cement rotary kiln has been widely applied to many cement plant. Based on 60+ years experience and advanced production equipment, we can provide EPC project and custom-designed solution to cement manufacturing.
Cement rotary kiln is a piece of cement equipment with strong durability, also plays a vital role in the cement plant. If you find any problem in the operation, solve it immediately. In general, the maintenance of rotary kiln is a complex process, here are some tips will be helpful.
First of all, check the fire hole of the rotary kiln is closed or not, ensure the close state to avoid too much cold air. Then, check the windshield of the burner at the kiln door is close to the kiln door. If the air leakage is not closed, it should be pushed to the position of close contact with the kiln door. Third, check the door cover of rotary kiln, if there is the phenomenon of positive pressure ash, should inform the central control to adjust properly. Finally, remember to check air leakage or wear out the phenomenon of resistant material, when you find the above problems, promptly inform the relevant leadership to deal with them.
Check the fish scales sealed in the kiln head, and then check the fitting condition of the fish scales in the rotary kiln and friction ring. If there is any bad condition such as non-fitting, the weight should be adjusted by tightening the device to make it fit. If there is too much aggregate in the reliable rotary kiln, the discharging pipe should be adjusted timely. After that, check the wear condition of the friction ring of rotary kiln. If the friction ring is found to be badly worn or worn out, replace it.
Check the cooling air at the kiln head, and then check the friction between kiln head and the kiln door cover. In order to ensure safety, check the oil film contact of the supporting wheel shaft and thrust plate one by one. Check oil level, oil scoop and oil distribution of rotary kiln. Next, check whether there are impurities, water droplets, oil leakage, collision, loosening and other abnormal conditions, if necessary, the temperature can be measured and control.
The service life of rotary kiln is affected by many factors, such as quality, regular maintenance, etc. AGICO CEMENT ensures the quality of the rotary kiln by strict quality control and advanced technology. Besides, regular maintenance can prolong the useful life of your rotary kiln.
Yes, in order to prevent cold air from entering the rotary kiln and smoke dust from spilling out of the cylinder, the rotary kiln is equipped with a reliable sealing device for the inlet end (tail) and an outlet end (head) of the cylinder, ensuring the sealing performance of the rotary kiln.
The cement rotary kiln produced by AGICO has been optimized and added with e dust filter products for environment protection, which is a new type of environmental protection equipment to reduce the environmental pollution caused by industrial production.
(I) LIME: It is main constituent of cement. Calculated amount of lime is added in the manufacturing of Portland cement, If it is excess in amount, it reduces strength of cement, because it makes the cement to expand and disintegrate.
(III) ALUMINA: Tricalcium aluminate is required for setting of cement. If it is an excess quantity, then cement undergoes setting very fastly, because with the reaction of water, there is evolution of large amount of heat.
Raw material is crushed into small pieces -----> Small pieces grind in fine ball mills and stored separately -------> Mixed in proper proportions ------->pulverized in tube mills and homogenized in a mixing mill.
The grinding process can be done in ball or tube mill or even both. Then the slurry is led into collecting basin where composition can be adjusted. The slurry contains around 38-40% water that is stored in storage tanks and kept ready for the rotary kiln.
The burning process is carried out in the rotary kiln while the raw materials are rotated at 1-2rpm at its longitudinal axis. The rotary kiln is made up of steel tubes having the diameter of 2.5-3.0 meter and the length differs from 90-120meter. The inner side of the kiln is lined with refractory bricks.
The kiln is supported on the columns of masonry or concrete and rested on roller bearing in slightly inclined position at the gradient of 1 in 25 to 1 in 30. The raw mix of dry process of corrected slurry of wet process is injected into the kiln from the upper end. The kiln is heated with the help of powdered coal or oil or hot gases from the lower end of the kiln so that the long hot flames is produced.
As the kiln position is inclined and it rotates slowly, the material charged from upper end moves towards lower end at the speed of 15m/hr. In the upper part, water or moisture in the material is evaporated at 400oC temp, so this process is known as Drying Zone.
The central part i.e. calcination zone, the temperature is around 10000C, where decomposition of lime stone takes place. The remaining material is in the form of small lumps known as nodules after the CO2 is released.
The lower part (clinkering zone) have temperature in between 1500-17000C where lime and clay are reacts to yielding calcium aluminates and calcium silicates. This aluminates and silicates of calcium fuse to gather to form small and hard stones are known as clinkers. The size of the clinker is varies from 5-10mm.
The lower part i.e. clinkering zone has the temperature around 1500-1700C. In the region lime and clay reacts to yield calcium aluminates and calcium silicates. This product of aluminates and silicates of calcium fuses together to form hard and small stones known as clinkers. The size of the small and hard clinkers varies from 5 to 10mm.
The clinkers coming from the burning zone are very hot. To bring down the temperature of clinkers, air is admitted in counter current direction at the base of the rotary kiln. The cooled clinkers are collected in small trolleys.
The cooled clinkers are received from the cooling pans and sent into mills. The clinkers are grinded finely into powder in ball mill or tube mill. Powdered gypsum is added around 2-3% as retarding agent during final grinding. The final obtained product is cement that does not settle quickly when comes in contact with water.
After the initial setting time of the cement, the cement becomes stiff and the gypsum retards the dissolution of tri-calcium aluminates by forming tricalcium sulfoaluminate which is insoluble and prevents too early further reactions of setting and hardening.