The new dry-process cement production line technology was developed in the 1950s. So far, in Japan, Germany and other developed countries, the production rate of new dry-process cement clinker with suspension preheating and pre-decomposition as the core accounted for 95%. A suspension preheating and precalcining kiln was commissioned in 1976. The advantages of this technology: rapid heat transfer, high thermal efficiency, large volume per unit volume of wet cement, and low heat consumption. Development stage: The first stage, from the 1950s to the early 1970s, was the birth and development stage of suspension preheating technology. The second stage, in the early 1970s, was the birth and development stage of pre-decomposition technology.
Process steps of the new dry process cement production line: crushing and pre-homogenization → raw material preparation → raw material homogenization → preheating decomposition → cement clinker burning → cement grinding → cement packaging.
First, Cement production of raw fuel and ingredients
The main raw materials for the production of Portland cement are lime raw materials and clay raw materials. Sometimes, according to fuel quality and cement varieties, correcting raw materials are added to supplement the deficiency of certain components, and industrial waste residue can also be used as raw materials or mixed materials of cement. in production.
1. Limestone raw materials
The calcareous raw material refers to limestone, marl, chalk and shells containing calcium carbonate as a main component. Limestone is the main raw material for cement production. Each production – ton of clinker requires about 1.3 tons of limestone, and more than 80% of the raw meal is limestone.
2. Clay raw materials
Clay raw materials mainly provide a small amount of cement clinker. Natural clay materials include loess, clay, shale, siltstone and river mud. Among them, loess and clay are used the most. In addition, there are industrial wastes such as fly ash and coal gangue. Clay is a finely dispersed sedimentary rock composed of different minerals such as kaolin, montmorillonite, hydromica and other hydrated aluminosilicates.
3. Correcting raw materials
When the raw material composition of the calcareous raw material and the clay raw material can not meet the requirements of the batching scheme (some content is insufficient, some and the content is insufficient), the corresponding calibration raw materials must be added according to the missing components: (1) siliceous The calibration raw material contains more than 80%; (2) the aluminum-corrected raw material contains more than 30%; and (3) the iron-corrected raw material contains more than 50%.
Second, the mineral composition of Portland cement clinker
The mineral of the Portland cement clinker is mainly composed of tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite.
Third, the process
1. Fragmentation and pre-homogenization
(1) In the process of crushing cement production, most of the raw materials are crushed, such as limestone, clay, iron ore and coal. Limestone is the raw material for producing the largest amount of cement. After mining, the particle size is larger and the hardness is higher. Therefore, limestone is the raw material for producing the largest amount of cement. After mining, the particle size is larger and the hardness is higher. Therefore, the limestone is crushed in the cement plant. The fragmentation plays a more important role. The crushing process is economical and convenient than the grinding process. It is very important to use crushing equipment and grinding equipment reasonably. Before the material enters the grinding equipment, the bulk material is broken into small and uniform particle size as much as possible to reduce the load of the grinding equipment and increase the output of the corrosion machine. After the material is crushed, the separation of materials of different particle sizes during transportation and storage can be reduced, and the raw materials with uniform composition can be obtained, and the accuracy of the ingredients can be improved.
(2) Raw material pre-homogenization pre-homogenization technology is to use the scientific stacking and reclaiming technology to realize the initial homogenization of raw materials in the process of depositing and taking raw materials, so that the raw material storage yard has the functions of storage and homogenization. The basic principle of raw material pre-homogenization is that when the materials are stacked, the incoming raw materials are continuously stacked in a certain manner into as many parallel layers, upper and lower overlaps and the same thickness. When taking the material, in the direction perpendicular to the layer of the material, all the layers are cut as much as possible, and cut in order, until the end is taken, that is, “tiling straight”.
A. Homogenize raw material components, reduce quality fluctuations, facilitate the production of higher quality clinker, and stabilize the production of the firing system.
B. Expand the utilization of mine resources, improve mining efficiency, maximize the coverage of the mine and the interlayer, and produce no or less waste rock during the mining process.
C. It is possible to relax the quality and control requirements of mining and reduce the mining cost of the mine.
D. It has strong adaptability to sticky materials.
E. Provide long-term stable raw materials for the factory. It is also possible to mix the raw materials of different components in the yard to make it a pre-mixing yard, creating conditions for stable production and improved equipment operation rate.
F. High degree of automation.
2. Raw material preparation
In the cement production process, at least 3 tons of materials (including various raw materials, fuels, clinker, mixture, gypsum) should be ground for every ton of Portland cement produced. According to statistics, the dry cement production line needs to be consumed. The power accounts for more than 60% of the power of the whole plant, of which raw material grinding accounts for more than 30%, coal grinding accounts for about 3%, and cement grinding accounts for about 40%. Therefore, rational selection of grinding equipment and process, optimization of process parameters, correct operation, and control of operating systems are of great significance for ensuring product quality and reducing energy consumption.
The electric motor drives the grinding disc to rotate through the speed reducing device, and the material is dropped into the center of the grinding disc through the wind-feeding device through the wind-locking feeding device, and is crushed by the grinding roller under the action of the centrifugal force, and the crushed material is rubbed from the grinding disc. The edge overflows and is dried by the hot air flow from the nozzle at a high speed. According to the different air flow speed, part of the material is taken by the air flow to the high-efficiency classifier, and the coarse powder is separated and returned to the grinding disc for re-grinding; The powder is ground with the airflow and collected in the system dust collecting device, which is the product. The coarse-grained material that has not been taken up by the hot air is fed to the classifier by the bucket elevator that is externally circulated after overflowing the grinding disc, and the coarse particles fall back to the grinding disc and are again squeezed and ground.
3. Raw material homogenization
In the process of new dry process cement production, the stable raw material composition is the premise of stable clinker burning thermal system, and the raw material homogenization system plays the role of stabilizing the raw material composition.
Homogenization principle: air agitation and gravity are used to produce a “funnel effect”, so that when the raw meal powder is discharged downward, the multi-layered material surface is cut as much as possible and thoroughly mixed. Different fluidizing air is used to cause different sizes of fluidization expansion in the parallel material surface of the reservoir. Some areas are unloaded and some areas are fluidized, so that the inner surface of the reservoir is inclined and radially mixed and homogenized.
4. Preheat decomposition
The preheating and partial decomposition of the raw material are completed by the preheater, instead of the partial function of the rotary kiln, the length of the return kiln is shortened, and the gas heat exchange process in the kiln is carried out in a stacked state, and moved to the preheater. The suspension is carried out in a state of being suspended, so that the raw material can be thoroughly mixed with the hot gas discharged from the kiln, the contact area of the gas material is increased, the heat transfer speed is fast, and the heat exchange efficiency is high, thereby improving the production efficiency of the kiln system and reducing the heat of clinker burning. The purpose of consumption.
The main function of the preheater is to make full use of the residual heat of the exhaust gas discharged from the rotary kiln and the decomposition furnace to heat the raw material, so that the raw material is preheated and some carbonate is decomposed. In order to maximize the heat exchange efficiency between the gas and solid, to achieve high quality, high yield and low consumption of the entire calcination system, it is necessary to have three functions of uniform gas-solid dispersion, rapid heat transfer and high-efficiency separation.
(1) Material dispersion
80% of the heat transfer takes place in the inlet pipe. The raw material fed into the preheater pipe, under the impact of the high-speed updraft, the material turns and moves with the airflow while being dispersed.
(2) Gas-solid separation
When the airflow carries the powder into the cyclone, it is forced to rotate in the annular space between the cyclone cylinder and the inner cylinder (exhaust pipe), and rotates side-down, from the cylinder to the cone The body can extend to the end of the cone and then rotate upwards upwards and exit by the exhaust pipe.
The emergence of pre-decomposition technology is a technological leap in the cement calcination process. It is a decomposing furnace added between the preheater and the rotary kiln, and a flue gas is used to raise the flue by the kiln tail, and a fuel injection device is provided to make the exothermic process of burning the fuel and the endothermic carbonate decomposition process. The furnace is rapidly carried out in a suspended state or a fluidized state, so that the decomposition rate of the raw material into the kiln is increased to 90% or more. The carbonate decomposition task originally carried out in the rotary kiln is moved to the decomposition furnace; most of the fuel is added from the decomposition furnace, and a small part is added by the kiln head, which reduces the heat load of the calcination zone in the kiln and extends the lining. The material life is conducive to the large-scale production. Because the fuel and the raw material are uniformly mixed, the fuel combustion heat is transferred to the material in time, and the combustion, heat exchange and carbonate decomposition processes are optimized. Therefore, it has a series of excellent performances and features such as high quality, high efficiency and low consumption.
4. Burning of cement clinker
After the raw material is preheated and pre-decomposed in the cyclone preheater, the next step is to enter the rotary kiln for clinker firing. In the rotary kiln, the carbonate is further rapidly decomposed and a series of solid phase reactions occur to form minerals in the cement clinker. As the temperature of the material rises, the minerals become liquid, and the dissolved in the liquid phase reacts to form a large amount (clinker). After the clinker is fired, the temperature begins to decrease. Finally, the cement clinker cooling machine cools the high temperature clinker discharged from the rotary kiln to the temperature that can be withstood by the downstream conveying, storage and cement grinding, and recovers the sensible heat of the high temperature clinker to improve the thermal efficiency and clinker quality of the system.
5. Cement grinding
Cement grinding is the last step in cement manufacturing and the most energy-intensive process. Its main function is to grind cement clinker (and gelling agent, performance adjusting material, etc.) to a suitable particle size (expressed by fineness, specific surface area, etc.) to form a certain particle gradation and increase its hydration area. Accelerate the hydration speed to meet the requirements of cement slurry condensation and hardening.
6. Cement packaging
The cement is shipped in both bagged and bulk shipping modes.