The production process of Portland cement is representative in cement production. It uses limestone and clay as the main raw materials, which are crushed, batched, and ground to make raw materials, and then they are fed into the cement kiln to calcinate mature materials, and then cooked The material is finely ground with appropriate amount of gypsum (sometimes mixed with mixed materials or additives).
Cement production varies with raw meal preparation methods, and can be divided into dry method (including semi-dry method) and wet method (including semi-wet method).
① Dry production. Grinding and simultaneously drying the raw material, before or after grinding and drying as feed raw material powder is dry kiln clinker calcining the method. However, there is also a method of adding raw meal powder to an appropriate amount of water to make raw meal balls , and then sending them into the Liboer kiln to calcinate the mature material. This method is called semi-dry method, which is still a dry method.
New dry process cement
The new dry process cement production line refers to the cement produced by the new process of decomposition outside the kiln. Its production is centered on suspension preheater and decomposition technology outside the kiln . It adopts new raw materials, fuel homogenization and energy-saving grinding technology and equipment. The entire line adopts computer distributed control to realize the automation and high efficiency, high quality, low consumption and environmental protection of the cement production process . .
New dry-process cement production technology was developed in the 1950s, Japan, Germany and other developed countries. The rate of new dry-process cement clinker production equipment with suspension preheating and pre-decomposition as the core accounted for 95%, and China’s first set of suspension preheating And the precalcining kiln was put into operation in 1976. The technical advantages: rapid heat transfer, high thermal efficiency, larger output per unit volume than wet cement, and lower heat consumption.
② Wet production. The raw material is ground into raw material slurry by adding water powder, and then fed into the wet kiln to calcinate the mature material. There is also a method of dewatering the raw material slurry prepared by the wet method, and then making the raw material block into the kiln to calcinate the mature material, which is called the semi-wet method, which is still a kind of wet method production.
The main advantage of dry production is low heat consumption (for example, the heat consumption of dry kiln clinker with preheater is 3140～3768 J/kg). The disadvantage is that the raw meal composition is not easy to be uniform, the workshop dust is large, and the power consumption is high. . Wet production has the advantages of simple operation, easy control of raw meal composition, good product quality, convenient slurry transportation, and less dust in the workshop. The disadvantage is high heat consumption (the heat consumption of clinker is usually 5234~6490 J/kg).
Cement production can generally be divided into three processes: raw meal preparation, clinker calcination, and cement production. The entire production process can be summarized as " two grinding and one burning ".
Raw meal grinding
Cement process
There are two types: dry method and wet method. The dry method generally uses a closed-circuit operating system, that is, the raw materials are ground by the mill, and then enter the separator for sorting, and the coarse powder flows back into the mill and then re-grinds. Most of the materials are dried and ground in the mill at the same time. The equipment used includes tube mill, intermediate discharge mill and roller mill. The wet method usually uses an open circuit system such as a tube mill and a baseball mill that passes through the mill once and does not return back. However, there are also closed circuit systems with a classifier or an arc screen.
Clinker calcination
There are two main types of equipment for calcining clinker: vertical kiln and rotary kiln . Vertical kiln is suitable for small-scale factories. Large and medium-sized factories should adopt rotary kiln.
①Shaft kiln:
The kiln shell is called vertical kiln without rotating. It is divided into ordinary shaft kiln and mechanized shaft kiln. Ordinary shaft kilns are manual feeding and unloading or mechanical feeding and unloading; mechanical shaft kilns are mechanical feeding and unloading. The mechanical shaft kiln is continuously operated, and its output, quality and labor productivity are higher than ordinary shaft kilns. Most shaft kilns abroad have been replaced by rotary kilns, but in the current Chinese cement industry, shaft kilns still occupy an important position. According to the technical policy requirements of building materials, small cement plants use mechanized shaft kilns to gradually replace ordinary shaft kilns.
②Rotary kiln:
The kiln shell is placed horizontally (with a slight slope, about 3%) and can make a rotary movement is called a rotary kiln. It is divided into dry kiln for calcining raw meal powder and wet kiln for calcining slurry (usually about 35% water content).
a. Dry kiln
Dry kiln can be divided into hollow kiln, waste heat boiler kiln, suspension preheater kiln and suspension calciner kiln. Around the 1970s, a calcining process that can greatly increase the output of the rotary kiln was developed—the decomposition technology outside the kiln. It is characterized by the use of a precalcining kiln, which is based on a suspension preheater kiln, and a decomposing furnace is added between the preheater and the kiln. Add 50% to 60% of the total fuel consumption in the calciner, so that the fuel combustion process, the preheating of raw meal and the carbonate decomposition process are moved from the area with low heat transfer efficiency in the kiln to the calciner. , The raw meal exchanges heat with the hot air flow in the suspended state or boiling state, thereby improving the heat transfer efficiency, so that the calcium carbonate decomposition rate of the raw meal before entering the kiln reaches more than 80%, reducing the heat load of the kiln and extending the kiln lining The service life and the operation cycle of the kiln, while maintaining the heating capacity of the kiln, are the purpose of greatly increasing the output.
b. Wet kiln
Cement kilns used in wet production are called wet kilns. In wet production, raw materials are made into slurry with a water content of 32% to 40%. Since it is prepared into a fluid slurry, the raw materials are mixed well, the raw meal composition is uniform, and the quality of the fired clinker is high, which is the main advantage of wet production.
Wet kilns can be divided into wet long kilns and wet short kilns with slurry evaporators. Long kilns are widely used, but short kilns are rarely used. In order to reduce the heat consumption of the long wet kiln, various types of heat exchangers are installed in the kiln, such as chains , slurry filter preheaters, metal or ceramic heat exchangers .
Cement grinding
The fine grinding of cement clinker usually adopts the loop grinding process (ie, closed-circuit operation system). In order to prevent dust during production from flying, cement plants are equipped with dust collection equipment. Electric dust collectors , bag dust collectors and cyclone dust collectors are commonly used dust collection equipment in cement plants.
Due to the adoption of new technologies and new equipment in the pre-homogenization of raw materials, the homogenization and transportation of raw meal powder and dust collection, especially the emergence of decomposition technology outside the kiln, a new dry production process has emerged. The use of this new technology makes the quality of dry clinker no less than that of wet production, and the power consumption is also reduced, which has become a trend in the development of the cement industry in various countries.
Example (vertical kiln)
After the raw materials and fuels enter the plant, they are sampled and analyzed by the laboratory, and at the same time, they are matched and homogenized according to their quality and stored in the raw material storage shed. Clay, coal, and pyrite powder are dried by the dryer to the process index value, and then lifted to the corresponding raw material storage by the hoist. After being crushed in two stages, limestone, fluorite and gypsum are sent to their respective storage warehouses by elevators.
The laboratory calculates the process formula according to the quality of limestone, clay, anthracite, fluorite, and pyrite powder. The raw material microcomputer batching system is used to batch all black raw materials. The raw material mill is used for grinding and sampling every hour. Test the percentage content of calcium oxide, iron trioxide and fineness of the raw meal once , and make adjustments in time to make the data meet the requirements of the process formula. The ground black raw material is carried into the raw material storage by the bucket elevator . The laboratory uses the multi-storage collocation and mechanical dumping method to homogenize the raw material according to the quality of the raw raw material. Raw meal homogenization silo. The raw meal is matched through the two homogenization silos, and the raw meal is lifted to the ball storage bin. The pre-water ball control device installed on the vertical kiln performs the ratio of material and water. The balling plate is used for balling of the raw meal. The ball from the kiln into the hopper the raw material balls distributed in different positions kiln firing, the fired clinker was crushed through a delivery tube, to the slat clinker crusher unit, is sampled by laboratory hour Perform chemical and physical analysis of clinker at one time .
According to the quality of the clinker, the hoist is put into the corresponding clinker warehouse. At the same time, according to the requirements of production and operation and the building materials market , the laboratory will mix the clinker, gypsum, and slag through the clinker microcomputer batching system for cement proportioning, and the cement mill 425 and 525 ordinary Portland cement were ground respectively, and samples were taken every hour for analysis and inspection. The ground cement is carried into 3 cement storages by a bucket elevator, and the laboratory will homogenize the cement through multiple storage collocation and mechanical dumping methods according to the quality of the ground cement. It is sent to 2 cement homogenization warehouses by hoist, and then the two cement homogenization warehouses are matched, and the packaging machine is controlled by the computer to package the cement. The packaged cement is stored in the finished product warehouse, and then after the laboratory sampling inspection is qualified Issuing the cement factory notice.
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