What is cement raw meal?

Raw meal preparation includes the process from raw material crushing to batching and then to qualified raw meal. There are two methods to prepare raw materials: dry method and wet method. In the process of dry preparation, large hard raw materials such as limestone are crushed into blocks with the size of about 100mm by traditional technology, or into blocks with the size less than 25mm by secondary technology (block technology with the size less than 25mm has been developed in recent years). Water-bearing raw materials such as clay should be dried, sent to a mill in proportion with limestone and iron ore, ground into fine raw meal powder, then sent to a mixing bin, stirred with compressed air, and the ingredients adjusted to qualified raw meal powder. The main difference between wet method and dry method is that clay is washed into slurry with water first, and then ground with limestone and iron ore until the water content is about 35%. The main advantage of preparing raw meal by dry method is that the heat consumption when calcining cement clinker is lower than that by wet method. The heat consumption per kilogram of clinker is only 3.6 ~ 4.6 MJ, while the wet process needs 5.2 ~ 6.3 MJ. However, the ingredients of raw meal prepared by wet method are easy to be uniform. In recent years, some advanced dry cement plants have adopted measures such as pre-homogenization of raw meal and automatic control of raw meal composition to ensure the uniformity of raw meal powder composition. The grinding of raw materials is carried out in different types of mills, mainly including ball mill, tube mill, vertical mill and intermediate discharge mill, while drying and grinding. In order to save electricity in the grinding process and improve the efficiency of the mill, closed-circuit (circulating) grinding is often used in production, that is, the materials coming out of the mill first pass through a particle classifier, and the fine particles are selected as products, and the coarse particles return to the mill to continue grinding. Compared with open-circuit grinding (classification without classifier), the output of closed-circuit grinding system can be increased by about 15% ~ 25%, and the over-grinding phenomenon is reduced. Disadvantages are large equipment investment and complicated operation and management. In recent years, a new type of vertical roller mill with classifier has been adopted, which can crush, grind, dry and classify in the same equipment. At present, the maximum vertical mill output can reach 400 tons per hour.

Clinker calcination The prepared raw meal is calcined into cement clinker in different types of kilns. Usually, raw meal powder or slurry is calcined in a rotary kiln. Most small cement plants in China use vertical kiln for calcination. When the shaft kiln is used for calcination, the raw meal powder is mixed with the required pulverized coal, and an appropriate amount of water is added to make raw meal balls with a diameter of10 ~ 30 mm. The quality of cement clinker calcined in the shaft kiln is slightly poor, but the calcination temperature is low and the coal consumption is low. In order to save energy and improve the production capacity of rotary kiln, the out-of-kiln decomposition technology with preheater and calciner has been developed since 1970s. A series of physical and chemical changes have taken place in the heating process of cement raw meal in kiln, such as evaporation of free water, removal of crystal water in clay, decomposition of calcium carbonate into calcium oxide and so on. The latter reacts with silica, alumina and iron ore in clay to form compounds, which mainly exist in four forms, namely tricalcium silicate (3CaO SiO2, abbreviated as C3S), dicalcium silicate (2CaO SiO2, abbreviated as C2S), tricalcium aluminate (3 Cao al2o 3, abbreviated as C2A) and tetracalcium aluminate (4cao Al2O3).

Cement is manufactured and packaged. After cooling, appropriate amount of gypsum is added to the cement clinker discharged from the kiln (SO3 in cement is controlled to be less than or equal to 3.5%), and it is ground in a mill to make portland cement. The fineness of cement grinding has a great influence on cement quality. Increasing fineness can improve the strength of cement, but the corresponding power consumption also increases. Generally, the fineness is controlled on a 0.08mm square hole screen, and the screen allowance is not more than 10%, or the specific surface area is about 3000 cm2/g ... The dust in the cement grinding process is relatively large, so dust removal equipment, such as settling chamber, cyclone and bag filter, should be installed at the equipment import and export, transportation and packaging. Some advanced factories are equipped with electrostatic precipitator. In China, clay or potash feldspar with high K2O content is also used to replace clay raw materials, so that oxides are volatilized into dust during calcination, and the dust with high K2O content is collected and can be used as potash fertilizer. Cement powder is usually packed in paper bags, but in recent years it has been transported by bulk carriers and bulk trucks, which improves the transportation efficiency and reduces the cost.

Extended data:

Cement, powdery hydraulic inorganic cementing material. Mud can be hardened in air after being mixed with water, and the hardening effect is better in water, and materials such as sand and stone can be firmly bonded together. The word "cement" is developed from the Latin caementum, which means crushed stone and flaky. The history of cement can be traced back to Dadiwan people in Qin 'an, China, 5000 years ago. They paved the ground similar to modern cement. Later, the ancient Romans used a mixture of lime and pozzolan in their buildings, which was very similar to modern lime-pozzolan cement. The concrete made of its [1] cemented macadam not only has high strength after hardening, but also can resist the erosion of fresh water or salt water. For a long time, as an important cementing material, it has been widely used in civil engineering, water conservancy, national defense and other projects.

In the cement industry, ball milling technology is applied to two production links. One link, like the thermal power industry, is used to grind pulverized coal and provide coal for production. The other link is used to grind the sintered cement clinker into powder, which plays a vital role in the production efficiency and product quality of cement enterprises. In recent years, due to the increase of investment in fixed assets, the rapid development of infrastructure construction and real estate industry has played an obvious role in pulling cement output. Driven by huge demand, cement output will still maintain a relatively stable growth. According to relevant statistics, the output of cement industry in 20 12 years has reached 265,438+1100 million tons.