Development trend and key points of transformation of cement grinding process

Cement is a kind of micron-sized powder with hydraulics, which is made of cement clinker, admixture, gypsum and other materials (such as grinding aids) together or separately. The new viewpoint of modern cement grinding technology is that good cement is "grind". Nowadays, cement grinding technology in the world has shown a diversified trend, and grinding equipment is also developing towards large-scale, low consumption, high efficiency and automation. With the continuous progress of science and technology, the mechanism of cement grinding is no longer confined to the traditional low-efficiency ball, calcination grinding mode, but gradually to high-efficiency and energy-saving roller grinding transition.
As far as the current cement grinding process is concerned, there are several kinds: tube mill (open or closed circuit) grinding system, vertical mill grinding system, roller mill grinding system and roller press final grinding system. The power consumption of grinding process accounts for more than 70% of the total power consumption of cement. The selection and application of grinding process directly affects the output, quality and production cost of cement, and plays an important role in cement preparation.
The present situation and development trend of cement grinding technology survey the modern cement grinding technology, most of the process still uses pipe mill as grinding equipment. At present, the design diameter of domestic cement mill has reached about 5m, and its output is above 150t/h. A large-scale pipe mill with a spine of more than 5.8m has been designed internationally for grinding cement. The output per hour is over 200t/h. The grinding mechanism of the tube mill is that the energy is transferred to the lining plate during the rotating process of the cylinder body, and the material in the mill is crushed and grinded by the lining plate to complete the grinding operation. Most of the abrasive bodies used in tube mill are spherical and cylindrical in shape. The spherical abrasive bodies impact and break the abrasive materials by point contact, so the grinding efficiency is low. Especially when the grinding material size is large and grindability is poor, the contradiction between low efficiency and high power consumption is more prominent.
In order to improve grinding operation conditions, increase mill system output and reduce grinding power consumption, cement engineers and technicians start with reducing the size of grinding materials, optimize the shape of working surface of liner plate, change the elevation of grinding bodies in grinding bin, drop trajectory and adopt grinding aids to a certain extent. The production efficiency of the mill has been greatly improved.
In addition to tube mill process, roller press, which was introduced in Germany in the mid-1980s, is mainly used for pre-grinding of raw cement and clinker, i.e. semi-final grinding. The grinding mechanism of roller press is bed grinding. At present, it has been extended from semi-final grinding to final grinding for cement preparation. The material extruded by two high pressure rollers produces a large number of cracks and fine powder, which significantly improves the grindability of the material. The crude particulate material is selected and crushed by roller press after the crushed cake is broken and classified into closed-circuit circulation. Although the power consumption of roller mill system is lower than that of tube mill grinding system, the morphology of cement finished by roller mill final grinding is polygonal, and the water requirement of standard consistency increases. The performance of concrete prepared by roller mill is not as good as that of cement grinded by tube mill.
Vertical mill is widely used in raw meal preparation process because of its high system output and low power consumption. At present, there are reports on vertical grinding of cement (final grinding) in the world. The grinding mechanism of vertical mill is similar to that of roller press. The difference is that the contact mode between vertical mill roller and material is cylinder and plane, while the contact mode between roller and material is cylinder and cylinder. In addition, the vertical mill itself does not need to set up a separate powder selection and classification system, and the roller press must be set separately, the system is more complex than the vertical mill. At present, the largest vertical mill output in the world is 600 t / h, which is incomparable with the grinding system of tube mill and roller press. At the same time, the power consumption of vertical mill grinding system is obviously lower than that of roller press system.
Key points of cement grinding process transformation
1. modification of large tube mill (above 4m)
In today's cement industry, tube mill still occupies the leading position of grinding equipment. As mentioned earlier, the power utilization of tube mill is low, and the grinding power consumption is higher than that of roller press, vertical mill and roller mill system. In order to reduce grinding power consumption, most enterprises add material pretreatment process before tube mill, reduce grinding particle size by pretreatment equipment, greatly increase mill output (30% ~ 50%), significantly reduce grinding system power consumption (20% ~ 30%) and production costs, and improve cement physical quality. Taking the roller press + breaking classification + tube mill pretreatment grinding system (closed circuit) as an example, the grinding power consumption of the new dry process kiln clinker is 28 kWh / T ~ 32 kWh / t, which is 8 kWh / t lower than that of the single tube mill without pretreatment process. It has higher and stable grinding efficiency and lower grinding power consumption. At the same time, because the particle size of the grinding material is reduced, the gradation of the grinding body can be optimized, the average size of the grinding body can be reduced, and the grinding fineness (specific surface area) and the strength of the mortar can be improved significantly.
Lifting, grading lining plate, screening device, activation device and grinding body anti-string device should be adopted in the internal of large-scale pipe mill. In order to keep the system stable and high yield for a long time, high quality cemented carbide grinding bodies, such as high and medium chromium alloy materials (wear < 50g/t, breakage rate < 1.0%), should be adopted because of the large-scale tube mill with a large amount of grinding body. At the same time, other parts of the mill vulnerable parts, such as lining board, partition board, etc., should also be selected with the same material as the grinding body and its matching, in order to obtain the best anti-wear.