Cement Clinker Grinding Plant Vertical Roller Mill

Cement grinding process: Grinding systems can be divided into open flow systems and circle flow systems according to different grinding methods. In the grinding process, the material is called open flow after passing through the mill once. When the material is sorted after leaving the mill, the fine particles are used as finished products, and the coarse particles are returned to the mill for re-grinding, which is called circle flow. Advantages of the open flow system: simple process, less equipment, low investment, and easy operation. Its disadvantages are low crushing efficiency and high unit power consumption. Advantages of the circle flow system: it can greatly reduce over-grinding, increase mill output, reduce power consumption, and at the same time, the product particle size is uniform. The fineness of the finished product can be changed by adjusting the parameters of the grading equipment. Its disadvantages are complicated processes and large investments.

Vertical mills are widely used in raw material preparation processes due to their high system output and low power consumption. There have been reports of vertical mills grinding cement (final grinding) internationally. The grinding mechanism of the vertical mill is similar to that of the roller press, both of which are bed crushing. The difference is that the contact mode between the roller of the vertical mill and the material is cylindrical and flat, while the contact mode between the roller of the roller press and the material is cylindrical and cylindrical. In addition, the vertical mill itself does not need to set up a separate powder selection and classification system, while the roller press must be set up separately, and the system is more complicated than the vertical mill. At present, the world's largest vertical mill output is 600t/h, which is incomparable to the tube mill and roller press grinding system. At the same time, the power consumption of the vertical mill grinding system is significantly lower than that of the roller press system.

1. Excellent power saving performance:
The comprehensive power consumption of the vertical mill is stably controlled at around 25kwh/t, which saves more than 30% of power compared with the traditional ball mill system. Take a medium-sized cement factory with a daily output of 5,000 tons as an example. The daily power consumption of traditional ball mills is usually around 150,000 kWh. After adopting vertical mills, the daily power consumption is sharply reduced to about 105,000 kWh. The cumulative electricity savings easily exceeded 10 million kilowatt-hours. Such a significant power-saving effect not only significantly reduces production costs for enterprises, but also conforms to the current trend of energy conservation and emission reduction, and provides strong support for sustainable development.

2. High-quality finished product characteristics:
Vertical grinding mills excel in terms of the quality of finished cement products. In terms of particle shape, it is similar to the cement particle shape produced by the traditional ball milling process. Through the detection of professional particle size analysis instruments, there is almost no difference between the two in key indicators such as particle sphericity and aspect ratio, which ensures that the concrete is well prepared. During the process, cement can be evenly dispersed, providing good rheological properties and filling density. In terms of strength performance, vertical mill cement is even more remarkable. According to a large amount of laboratory data and engineering practice feedback, its 3d strength can reach an average of 28MPa, 7d strength is about 40MPa, and 28d strength is stable at 55MPa and above, which is much better than traditional ball mill cement. Not only are they comparable, but under certain working conditions they can also achieve a 3% - 5% increase in strength. Moreover, the vertical mill, with its advanced airflow classification and variable frequency speed regulation technology, can accurately control the distribution of cement particles, so that the water demand is stably controlled in the ideal range of 26-28%, effectively avoiding the poor hardening and durability of cement caused by water fluctuations. Reduce and other problems, and fully meet the stringent requirements of various construction projects on cement performance.

3. Extremely low wear performance: Wear has always been a thorny problem faced by grinding equipment, but vertical mills have successfully broken through with their exquisite design and material selection. Its metal consumption is as low as 4-5g/t, in stark contrast to the 20-30g/t metal loss in traditional ball milling. Taking a vertical mill that runs 300 days a year as an example, the metal consumption of parts replaced due to wear and tear is reduced by about 5-6 tons per year compared with a ball mill. This not only greatly reduces the cost of purchasing spare parts, but also reduces downtime and replacement time, and increases additional annual production costs. Effective production time is more than 100 hours. The vertical mill uses special wear-resistant alloy materials to make grinding parts and optimizes the contact mechanical structure between parts, so that the wear area is mainly concentrated between the grinding bodies, which protects the main structure of the equipment to the maximum extent and effectively ensures the continuity and stability of production. sex.

4. Ultra-high operating stability: The vertical mill system has excellent operating stability, and its operating rate can usually reach a high range of 93% - 97%. Behind this is an advanced intelligent monitoring and fault warning system to provide protection. By deploying high-precision sensors at key locations of the equipment, such as bearings, transmission gears, and motors, operating parameters are collected in real time, including key data such as temperature, vibration, and current. Once these data deviate from the normal threshold, the system immediately activates the early warning mechanism and With the help of built-in fault diagnosis algorithms, the root cause of the problem can be quickly located and accurate maintenance suggestions can be given. At the same time, the mechanical structure of the vertical mill adopts a modular design concept. Each functional module has a high degree of independence and interchangeability. Even if a sudden failure occurs, the maintenance team can complete the module replacement in a short time and quickly resume production to ensure the company's The production plan is not disturbed, laying a solid foundation for long-term stable supply.

5. Streamlined and efficient process: The technological innovation brought by the vertical mill has brought a qualitative leap in factory operations. Compared with the complicated and lengthy grinding process of traditional ball mills, vertical mills integrate core processes such as grinding, drying, and powder selection, achieving one-stop operation. This highly integrated design makes the equipment layout more compact, and the floor space is reduced by an average of 30% - 40% compared to traditional ball mills. Taking the construction of a new cement production line with an annual output of 1 million tons as an example, the use of vertical mills can save about 2,000 to 3,000 square meters of land, greatly reducing land acquisition and infrastructure costs. Moreover, the number of electromechanical equipment in the entire system has been reduced by 40%, and the operating interface is simple and intuitive. Workers who have undergone short-term training can operate it proficiently. The maintenance workload has been reduced by more than 50%, and the daily operation and maintenance costs have been effectively controlled, which has comprehensively improved the company's production management efficiency.

6. Super strong material adaptability:
In terms of material adaptability, the vertical roller mill demonstrates extraordinary tolerance. When facing wet materials with a moisture content of up to 15% - 20%, the unique hot air flow field design inside the vertical mill plays a key role. During the grinding process, the hot air quickly penetrates into the material and evaporates the moisture quickly, ensuring that the material is ground. Good fluidity and grindability are always maintained during the process, avoiding the negative impact of wet materials on grinding efficiency. For soft materials with poor abrasiveness, such as some clay minerals, the vertical mill cleverly balances the relationship between crushing efficiency and equipment protection by adjusting the grinding pressure and the gap between the rollers, which can achieve efficient crushing and effectively prevent Equipment sticking and clogging problems caused by excessive extrusion. This broad acceptance of materials has greatly broadened the raw material procurement radius of enterprises. Enterprises can make full use of local cheap, low-quality but abundant resources, reduce raw material costs by 10% - 20%, and significantly enhance the market competitiveness of enterprises.

7. Rapid cement product formula conversion:
In today's rapidly changing construction market environment, the vertical mill's rapid product conversion capability has become a highlight. In just 20 minutes, the vertical mill can switch between different types of cement. Behind this is a collaborative operation system consisting of a precise ingredient metering system, automated process parameter control software, and an efficient material cleaning device. For example, when an enterprise needs to switch from producing ordinary Portland cement to low-heat Portland cement, the batching metering system quickly adjusts the proportion of raw materials such as limestone, clay, and iron powder, and the automated process parameter control software simultaneously optimizes the grinding temperature and powder selection. At the same time, the efficient material cleaning device uses strong internal airflow to powerfully purge the residual materials, ensuring the stability of the quality of the new cement varieties and that all performance indicators meet national standards, allowing enterprises to respond quickly to market changes and seize market share. Market opportunities.

8. Excellent noise reduction and low wear design:
vertical mill also performs well in environmental protection and equipment life extension. During operation, the noise level is about 10-15 decibels lower than that of traditional ball mills. The noise at the factory boundary can be controlled below 80 decibels, which greatly improves the acoustic environment around the factory and reduces the risk of noise pollution complaints. This is due to its closed negative pressure operation structure. The grinding, drying and powder selection processes are all completed under the negative pressure environment inside the mill, effectively blocking the noise from being transmitted outward. At the same time, the wear area is mainly concentrated between the grinding bodies. Compared with the large-area friction loss between the ball and the liner, and between the material and the cylinder in the ball mill system, the vertical mill protects the main structure of the equipment to the maximum extent by optimizing the grinding trajectory and force distribution. , further reduce maintenance costs, extend the overall service life of the equipment by 3-5 years, and provide solid guarantee for the long-term and stable operation of the enterprise.

Renovation of large tube mill (diameter over 4m) In today's cement industry production, tube mills still dominate the grinding equipment. As mentioned above, the tube mill has low power utilization rate and the power consumption of grinding is higher than that of roller press, vertical mill and drum roller mill system. In order to reduce the power consumption of grinding, most companies add material pretreatment process before tube mill, and reduce the particle size of the grinding through pretreatment equipment, which greatly increases the output of the mill (30%~50%) and significantly reduces the power consumption of the powder. Reduce grinding system power consumption (20%~30%) and production costs, and improve the physical quality of cement.

Taking the roller press + sintering and grading + tube mill pretreatment grinding system (closed circuit) as an example, the power consumption of grinding new dry kiln clinker is 28kWh/t~32kWh/t, which is higher than that of using tube mill alone. The power consumption is 8kWh/t~12kwh/t lower when no pretreatment process is set. It can be seen that strengthening the pretreatment of the materials entering the mill can enable the grinding system to maintain a high and stable grinding efficiency and low Grinding power consumption. At the same time, since the particle size of the material entering the mill is reduced, the grinding body gradation in the mill can be optimized and the average size of the grinding body can be reduced, which is more conducive to significantly improving the grinding degree (specific surface area) of the cement and the strength of the mortar.

Large tube ball mills should be equipped with lifting and grading liners, screening devices, activation devices, and grinding body anti-cross-contamination devices. Due to the large amount of grinding media in large tube mills, in order to ensure that the system can maintain stable and high production for a long time, it is required to use high-quality cemented carbide grinding media, such as high- and medium-chromium alloy materials (wear <50g/t, breakage rate ＜1.0%). At the same time, other vulnerable parts in the mill, such as lining plates, partition plates, etc., should also be made of the same material as the grinding body to obtain the best anti-wear effect and good surface finish, in order to stabilize the system production, Quality creates conditions.

In order to improve the roundness of the cement out of the mill, some companies use micro balls with a diameter of 8mm~12mm in the fine grinding bin, and the effect is good. Large tube mills have multiple bins, and the grinding media used in each bin have different specifications. The general rule is that the specifications of the grinding media in each bin gradually decrease from the feed end to the discharge end to enhance the grinding function of the grinding media on the material. The filling rate of the grinding body is generally less than 32%, and is mostly selected between 26% and 30%.

Modification of small and medium-sized cement grinding process: For small and medium-sized tube mills with a diameter of less than 4 meters, whether it is an open circuit or closed circuit grinding system, a pre-grinding material pretreatment process must be set up. The optional pretreatment methods are pre-crushing, pre-crushing and pre-grinding. Among the three pretreatment processes, pre-grinding (i.e., using short and thick rod mills or drum roller mills) has the best technical effect, low power consumption, and long-term reliable operation. The maximum particle size of the treated materials is stable below 2mm, and about 30% of them still contain finished products. The setting of the pretreatment process partially or completely replaces the function of the coarse grinding bin of the mill, which is equivalent to extending the fine grinding bin of the mill, and is more conducive to increasing the system output (30%~50%) of medium-long or short mills with a small aspect ratio (L/D≈3) and reducing the power consumption of grinding (10%~30%). Now we will discuss the transformation of several grinding processes after pretreatment.

Pretreatment open circuit high fine grinding system It is well known that the proportion of particles below 30μm in the finished cement product determines the strength of the mortar, and the more the content of characteristic particle size 16μm~24μm, the better. Small and medium-sized mills generally have shorter grinding bodies, and the time that materials stay in the mill to be ground is also short. The materials are completely crushed and ground by the grinding bodies inside the mill. The materials are often not easy to be ground finely, resulting in a high content of coarse particles in the finished product. Too much will seriously restrict the hydration activity of cement.

The setting of pretreatment process is of great significance to increasing the production, saving electricity and improving the grinding degree of cement in the open circuit grinding system. The materials entering the mill are pretreated, and the function of the first chamber of the mill is completed by the pretreatment equipment. The average size of the grinding bodies in the mill is reduced, the ability to finely grind the materials is enhanced, and the proportion of particles below 30μm in the finished cement product is significantly increased. After the pretreatment open circuit high fine grinding process is formed, it is advisable to carry out corresponding transformation inside the mill, install screening and grading partition plates, and activate the fine grinding bin lining to fully activate the grinding energy of the micro-grinding body. It improves the grinding degree and gelling activity of cement. The content of a certain particle size in the cement particle grading after open circuit grinding is relatively concentrated, which is commonly known as "narrow graded cement". The grate gaps of the partition plates and discharging grate plates in the mill are generally ≤6mm. The open circuit high fine grinding system must strengthen ventilation and dust collection measures. The wind speed in the mill should be maintained at 0.5m/s~0.8m/s, and bag dust collection technology should be selected.

If the grinding efficiency is affected by the electrostatic adsorption of fine materials on the grinding body surface, the introduction of grinding aids can be considered to solve the problem. The power consumption of this process is generally 30kWh/t~33kWh/t. 4. Pretreatment closed-circuit grinding process closed-circuit The grinding process is modified by adding high-efficiency powder selection equipment on the basis of open circuit grinding. The most important technical link in the closed-circuit grinding process is that the selected powder classifier must have high classification accuracy (such as a powder selection efficiency of more than 85%), stable performance, and reliable long-term operation, otherwise it will be difficult to achieve the best technical effect.

The best configuration of this process is: pre-grinding vertical mill + screening inside the mill + efficient powder selection outside the mill, which can avoid the phenomenon of cement particle grading becoming wider in closed-circuit grinding, and strive to make the powder content of characteristic particle size higher. These are beneficial to further enhance the hydration activity and mechanical strength of cement. The power consumption of the closed-circuit grinding system is lower than that of the open-circuit system, generally ≤28kwh/t. Tongli once used pretreatment technology to transform the domestic 2.2×6.5m diameter closed-circuit cement mill system, and the average particle size of the material entering the mill was reduced from 9.7mm to 5.3mm. At the same time, optimize the design of the grinding body grading in the mill, adjust the filling rate of the two bins, improve the internal structure of the powder classifier, and appropriately reduce the system circulation load rate.

After the transformation, the specific surface area of ​​the finished cement increased by 70% and the 3-day compressive strength increased by 65%. 5. Material Separate Grinding Process The material separate grinding process can maximize the gelling activity of the finished cement, providing a basis for the large-scale use of high Active industrial waste residues create good conditions for purifying the ecological environment. Cement that has been ground separately and then "blended" has more admixtures. At the same time, due to the reduction in clinker content, the produced cement not only has a low alkali content, but also a low hydration heat, which can significantly improve the durability of concrete products. The cement particles prepared by the separate grinding process have a more reasonable gradation, high strength gain rate, low manufacturing cost, and moderate grinding power consumption, generally 40~50, which is one of the directions of development and transformation of grinding technology.

In the complex process system of cement production, the design quality of the raw meal final grinding system is directly related to the overall efficiency and economic benefits of the production line. Tongli Cement's 6000t/d cement production line is equipped with a 12MW waste heat power generation system. Since it was put into production in 2022, its raw meal final grinding system has demonstrated unique design charm and excellent performance in many aspects.

Careful consideration of roller press process selection and configuration
The raw meal final grinding system integrates many complex processes such as extrusion, powder selection, dust collection, iron removal, and comprehensive utilization of kiln tail exhaust gas. Its design parameters are numerous and the matching connection of each link is extremely complex. In the selection of roller press, based on the maximum production capacity of the burning system for the matching capacity of raw meal finished products (about 534t/h), given that the current maximum single-unit production capacity of the roller press (520t/h) is slightly insufficient, two independent grinding systems are innovatively adopted. Each system uses HFCG180 - 160 roller press, and is cleverly combined with V-type classifier, high-efficiency powder selector and cyclone. The single set design capacity reaches 380t/h. Such a total matching capacity can fully meet the supply demand of the kiln system and is slightly more abundant, laying a solid foundation for the stable operation of production.

This selection decision is the result of comprehensive consideration of various actual working conditions. First of all, the raw material characteristics are suitable. When limestone, shale, iron slag and other main raw materials are used for raw material batching, the maximum moisture content is 4.6%, the minimum is 0.15%, and the comprehensive moisture content is less than 4%. The moisture content is low and there is no sticky material. This characteristic is highly compatible with the roller press final grinding system, providing favorable conditions for efficient grinding. Secondly, the raw material has good bond grindability. When the raw material ratio fluctuates within the allowable range, the grindability fluctuation value is only 12.5-13.2kWh/t. This stable grindability effectively ensures the balance of kiln and mill capacity, allowing the production process to proceed smoothly and avoiding the capacity imbalance caused by the difference in grindability.

Furthermore, from the perspective of market and policy adaptability, the two roller press systems are extremely flexible in responding to market factors and current policies such as capacity regulation and peak-valley electricity prices. For example, during valley electricity and peak sales seasons, two mills can be turned on to meet high production needs; during peak electricity and off-season sales, only one mill can be turned on or all can be turned off according to actual conditions. If a single mill is turned on for 14 hours a day, and two mills are turned on for 10 hours, this flexible operation and control mode can not only effectively reduce production costs, but also greatly facilitate the inspection and maintenance of the system.

In addition, according to the changes in performance indicators such as moisture and sulfur content of raw fuels, the synchronous operation of kilns and mills is easy to control. When the sulfur content is high, CaCO₃ in the raw meal can react with SO₂ in the kiln tail exhaust gas to generate solidified CaSO₄, thereby achieving self-desulfurization during the grinding process, significantly reducing the adverse effects on the firing system. At the same time, the SP boiler is the key channel for the kiln tail exhaust gas to enter the raw meal grinding system. Its start and stop status is convenient for synchronization with the production system. The overall system has strong coping capabilities, and the kiln and mill are closely connected, ensuring the smoothness of the production process. Finally, the specifications and models of the two sets of grinding system equipment are exactly the same, which makes the spare parts universal, reduces the complexity of the procurement link, and is more convenient and efficient when the equipment is maintained and replaced, further improving the efficiency and economy of production operations.

Innovation and optimization of exhaust gas channel design and configuration
The 6-stage preheater used in the firing system has specific exhaust gas parameters at the C1 outlet. The standard flue gas volume is 430,000 m³/h, the maximum temperature is 260℃, and the pressure is -5200Pa. Based on this, the treatment process of the kiln tail exhaust gas was innovatively designed, abandoning the conventional pipeline humidification method, and instead passing through the SP boiler, SCR denitrification and then being sent to the grinding system by a high-temperature fan. This design change brings many advantages. Compared with the traditional pipeline humidification system, the number of pipeline elbows is significantly reduced, thereby effectively reducing the operating resistance and improving the efficiency and stability of exhaust gas transportation. At the same time, the support platform where the SP boiler is located is raised to 45.600m, which greatly shortens the length of the air duct between the C1 outlet and the SP boiler, effectively reducing the heat loss of the exhaust gas during transportation, and creating favorable conditions for increasing the power generation of the SP boiler.

The SP boiler was built and put into operation simultaneously with the entire production system. Under normal operating conditions, it can effectively reduce the exhaust gas temperature at the C1 outlet to a reasonable range of 170-260℃. Even when only one of the two roller press systems is working, the exhaust gas temperature entering the bag dust collector at the kiln tail can still be reduced to below the safe temperature, ensuring the safe and stable operation of the dust collector. Even in extreme cases, such as when the SP boiler fails to work, the exhaust gas temperature can be quickly reduced to a safe range by opening the cold air valve on the exhaust gas air path, thereby fully ensuring the operational safety of the dust collector and the continuous stability of the grinding system, and avoiding the risk of equipment failure and production interruption due to excessive exhaust gas temperature.

In the SCR denitrification process, the medium-temperature medium-dust process is adopted, the flue gas temperature is controlled at 180℃, and the TiO₂-V₂O₅ series catalyst is selected, with a quantity of 205m³. Through the synergistic effect of this process and the catalyst, the NOₓ concentration (under standard conditions) can be efficiently reduced from 200mg/m³ to 50mg/m³, and the ammonia escape amount can be accurately controlled within 3mg/m³, while effectively reducing pollutant emissions, ensuring the stable operation of the system and the reliability of the denitrification effect. In addition, as the main channel for conveying exhaust gas at the tail of the kiln, SP boilers have formulated detailed and comprehensive start/stop response measures for the 17 different working conditions they may face. For example, when the moisture content is low and more power generation is required, the SP boiler is turned on and the outlet flue gas temperature is 170℃. At this time, the mill can be fully stopped or fully opened; when the moisture content is normal, the SP boiler works normally and the outlet flue gas temperature is 180℃. The operation status of the mill can be flexibly adjusted to fully stopped, fully opened or 1 open and 1 stop according to actual production needs; when the moisture content fluctuates or is in a high moisture state, the SP boiler continues to operate, and the outlet flue gas temperature rises accordingly, but the mill can still remain fully opened according to the specific situation; and in special circumstances, such as when the moisture content is extremely high (abnormal) or the SP boiler is abnormal, there are also corresponding response strategies, such as opening the cold air valve or adjusting the mill operation status. Such a perfect response mechanism ensures that the mill can flexibly respond and accurately control according to changes in working conditions at any time to maintain stable and orderly production.

Unique advantages and innovations of powder selection system configuration
As a key component of the raw material final grinding system, the powder selection system includes V-type powder selector, ZJTL8000 dynamic powder selector, φ6m double series cyclone, bag dust collector and its supporting fan, drive motor, etc. In the design and selection process, not only the requirements of process performance parameters are strictly met, but also key breakthroughs are made in terms of structure utility model and simple and reliable operation.
ZJTL8000 dynamic powder selector is an advanced achievement of innovative development and application in recent years. Under the action of airflow, its unique three-separation mechanism plays a key role. The first rough separation can rely on the preliminary screening effect of the airflow to settle and discharge most of the coarse particles, effectively reducing the burden of the subsequent powder selection process; the second dynamic separation further utilizes the sorting principle of dynamic airflow to select most of the materials with qualified particle sizes, improving the efficiency and accuracy of powder selection; the third selection can accurately control the 0.2mm particle size screen residue through carefully designed structure and airflow parameters, which has an extremely important practical effect on improving the burnability of raw materials and helps to improve the quality and production efficiency of cement clinker. At the same time, in order to reduce the energy consumption of the system, the roller press, V selection and air selection circulation elevator are all equipped with small low-pressure dust collectors separately, and the dust-containing gases in various places are processed by independent dust collectors without interfering with each other. Compared with the conventional method of introducing dust collection air into the powder selection machine, this design can significantly reduce the load and power consumption of the high-pressure circulation fan, realizing the organic combination of energy saving and efficient powder selection.
The cyclone plays a decisive role in the material and gas separation efficiency of the flue gas at the powder selection machine outlet in the powder selection system, directly affecting the output and power consumption of the system. Since the dust concentration of the flue gas at the outlet of the dynamic powder selector is as high as 600-800g/m³, improving the separation efficiency of the cyclone has become one of the key factors to improve the performance of the system. This design uses a new type of high-efficiency and low-resistance cyclone for matching. Compared with the traditional cyclone, it has made bold innovations in structure. The number of cylinders of the new cyclone is reduced to 2, making the pipeline layout smoother and the structure more compact. The unique spiral structure in its large volute can guide the airflow to be smoothly introduced into the cylinder, effectively reducing the eddy resistance in the inlet area and avoiding the collision of airflow and backflow, so that the dust-containing gas can achieve rapid separation of gas and material under the joint action of inertia and centrifugal force. According to actual tests, its separation efficiency can reach more than 92%, and the resistance is as low as <1200Pa, which greatly improves the overall operating efficiency of the system, reduces energy consumption, and provides strong support for the efficient and stable production.

As a key equipment shared by the two roller press systems, the kiln tail bag dust collector uses glass fiber coating as its filter bag material, which has good filtering performance. The processing air volume is as high as 1000000m³/h, which can ensure that the dust emission concentration is strictly controlled at ≤5mg/m³, meeting environmental protection requirements. During the production process, even if one of the roller presses stops running, the kiln tail exhaust gas entering the dust collector can be kept below the safe temperature through reasonable design and regulation of the system, effectively avoiding problems such as filter bag damage caused by excessively high exhaust temperature. Similarly, when the SP boiler is suspended for maintenance, opening the cold air valve on the exhaust air path can quickly reduce the exhaust gas temperature to a safe range, ensuring that the dust collector is always in a safe and stable operating state, and no bag burning accidents will occur.

In addition, the design mode of two roller presses sharing one dust collector saves a lot of cost in terms of frame civil engineering investment, while reducing the longitudinal distance and air duct length, further reducing engineering costs, and improving the economy and space utilization of the production system. In terms of duct layout, the dust concentration of the three air paths, such as the high-temperature fan, the circulating fan and the bag dust collector at the end of the kiln, is relatively high. In order to reduce the system resistance, these three ducts are designed to be arranged longitudinally on the same plane. By reducing the number of elbows and shortening the length of the duct, the system operation resistance is effectively reduced, and the appearance is simple and beautiful, which reflects the scientificity and rationality of the design.

The metallic iron in the raw material mainly comes from the foreign matter mixed in the raw material yard, crushing, homogenization and other links, as well as the small amount of iron filings and their inclusions contained in the iron slag of the batching. These metallic irons are extremely harmful to the roller surface of the roller press, seriously affecting the service life and operation stability of the equipment. Under normal circumstances, most of the iron removal operations are carried out by installing a metal separator on the head of the conveyor belt. However, when the material is sticky or the conveying volume of the belt conveyor is large, the iron removal effect of this traditional method is not ideal, and it is obviously unable to meet the high conveying volume requirement of 380t/h of this production line. Therefore, the raw material iron removal process of this design adopts a combination of a four-stage self-unloading permanent magnetic iron remover and a two-stage pipeline electromagnetic iron remover to achieve the most thorough iron removal effect and reduce equipment wear. The self-unloading permanent magnetic iron remover has a bandwidth of 800mm per stage and is set in the horizontal section after the trough belt feeding. The material can be evenly stirred by turning the belt over and reversing, making the iron removal process more thorough and effective; the pipeline electromagnetic iron remover has a specification of 900mm×1100mm, and uses the magnetic adsorption principle to remove foreign matter. Due to the large flow of materials in the pipeline, a large amount of materials are often mixed with iron removal and discharged at the same time. To solve this problem, a secondary iron removal is configured for separation, which ensures the iron removal effect while controlling the material loss within a very small range. After actual production operation verification, the roller surface of the roller press has not been significantly affected since its operation, which fully proves the good effect of the iron removal scheme and effectively guarantees the normal operation of the equipment and the continuity of production.

IV. System operation effect and comprehensive evaluation
After more than half a year of continuous production assessment, the tongli raw material final grinding system has shown extremely excellent operation performance. The system runs smoothly, and the key process indicators such as output and power consumption are stable and reliable. The raw meal self-desulfurization rate has reached an impressive 50% - 80%, and the synchronous operation rate of the SP boiler, kiln and grinding system exceeds 90%. During the entire production process, there was no shutdown due to severe fluctuations in production conditions. This series of excellent operating data fully demonstrates the good design effect of the system, which not only effectively guarantees the efficient and stable production, but also has achieved remarkable results in energy conservation, emission reduction and comprehensive resource utilization. With its excellent performance, the entire production line won the "Best Innovative Design Achievement (Factory Category) of the China Building Materials Federation", among which the raw meal grinding system as the core link has made an indelible contribution, providing a valuable reference example for the process design of the raw meal final grinding system of the same type of cement production line, which is worthy of wide promotion and reference in the industry.
In summary, the raw material final grinding system of Tongli 6000t/d cement production line has achieved efficient and stable operation of the system and good economic and environmental benefits through careful design and innovative optimization of key links such as roller press selection and configuration, exhaust gas channel design, and powder selection system configuration in process design, making positive contributions to the technological progress and sustainable development of the cement production industry.

The vertical roller mill (vertical mill) integrates fine crushing, drying, grinding, sorting and conveying. It has the advantages of high grinding efficiency, strong drying capacity, simple process system and small footprint. Since the 21st century, a new generation of material bed grinding technology equipment represented by vertical mills has been widely developed and applied. Cement vertical mill final grinding technology is favored by domestic and foreign cement companies due to its simple process system flow, low unit power consumption, flexible cement product switching and easy operation.

In the 1970s and 1980s, companies such as Loesche, Pfeiffer and Polysius conducted cement vertical mill tests and applied them to production. In 1994, Loesche of Germany launched the first cement vertical mill final grinding equipment in Taiwan happiness Cement. At present, more than 70% of international companies use vertical mill final grinding to produce cement. In China, cement production capacity is in excess and competition is intensifying. Energy saving and consumption reduction have become the key to the survival of enterprises. In addition, large-scale cement production equipment has the characteristics of stable finished product quality and low energy consumption. Low-energy large-scale cement vertical mill final grinding is the development direction. Tongli Heavy Machinery Co., Ltd. launched the first domestic cement final grinding roller mill equipment in 2008, breaking the international monopoly, and continued to optimize and upgrade it, ranking at the forefront internationally. It has completed the development of the ZJTL6560 vertical mill with an annual output of 3 million tons and the application of the vertical mill with an annual output of 2.6 million tons.

1 Development History
Based on the extensive application of ZJTL type raw material vertical mill and slag vertical mill, Tongli Company started the research and development of cement vertical mill final grinding technology equipment in 2006, carried out experimental research in 2007, and in 2009, the company's two ZJTL3230 vertical mills achieved stable grinding of cement, and the mill output and main machine power consumption reached the expected indicators, completing industrial trials. In 2010, the first independently developed cement vertical mill ZJTL4540 was put into production in a cement plant in Vietnam. After a short period of commissioning, all indicators met the design requirements. The particle distribution and standard consistency water requirement of the cement finished product are equivalent to those of the closed-circuit ball mill system products. The concrete working performance is excellent, and the power consumption of the cement preparation system is 2kWh/t lower than that of the Loesche mill LM46.2 + 2CS of the same plant.

In 2011, ZJTL5640 was optimized and developed based on ZJTL4540. In 2014, ZJTLKZM5641 vertical mill was developed, which can grind slag and cement at the same time. It is convenient for material switching and has advanced operating indicators.

In 2020, the large-scale cement vertical mill ZJTL6250 was put into production. The grinding plate diameter is φ6.1m and the installed power is 6300kW. Under the conditions of 90% clinker and 370m²/kg of finished product specific surface area, the system output can reach 280t/h and the power consumption is 27-28kWh/t, which saves more than 10% of electricity compared with the "roller press + ball mill" combined grinding system of similar cement plants.

In 2022, the largest cement final grinding vertical mill ZJTL6641 in China was put into operation in Fangchenggang, Guangxi, with an annual cement production capacity of 2.6 million tons, a grinding disc diameter of φ6.6m, and an installed power of 7800kW. At present, the ZJTL series cement vertical mill has completed the development of more than ten specifications of products, and completed the development of the ZJTL7000 vertical mill supporting the annual production of 3 million tons of cement grinding production line on the road to large-scale development. The company has sold more than 70 cement vertical mills in total, with more than 30 units sold overseas, accounting for nearly 50%. The projects are spread across three continents in the world, and the power consumption of the cement final grinding system is as low as 23 kWh/t.

2. Technological innovation
New grinding zone technology: Tongli cement vertical mill finished products have the characteristics of high specific surface area, poor clinker grindability, inability to spray water, and easy vibration. The company studied the relationship between the matching form of grinding parts (grinding roller and grinding disc) and grinding efficiency, and derived the correlation curve, which showed that reducing the material flow rate on the grinding disc and optimizing the grinding zone width can improve the grinding efficiency. After laboratory and industrial tests, it was determined that the original horizontal grinding disc should be changed to a slightly angled inclined disc structure. This structure changes the material movement trajectory and speed, increases the effective grinding times, improves the grinding efficiency, and also widens the grinding zone width, improving the stability and adaptability of the mill. After being applied to multiple cement vertical mills and slag vertical mills, the mill stability is greatly improved when grinding cement, and the unit power consumption of medium-grindability materials is reduced by an average of 1-2 kWh/t.

Cement product performance improvement technology: When preparing cement using the cement final grinding vertical mill system, customers are concerned about the particle distribution of the cement finished product and the water requirement for standard consistency. In the past, the final grinding of cement vertical mills was based on the design experience of vertical mills for other materials, resulting in narrow particle size distribution and high water demand. The company's R&D team conducted in-depth research on the grinding mechanism of vertical mills and developed a new cement vertical mill, which uses high grinding pressure and low cycle times to improve the performance of finished products. New parameters are adopted in the grinding area width of the grinding parts, the wind speed of the vertical mill body, the structure of the powder selector blades and the sealing form, and the design scheme is dynamically adjusted according to the cement ratio, so that the particle size distribution uniformity coefficient n value is reduced from 1.05 to below 0.92.
Practice has proved that the cement products prepared by the cement vertical mill developed by Tongli Company are comparable to the double-circle flow combined grinding system of roller presses and ball mills in terms of standard consistency water demand, setting time and concrete working performance.

Reliability improvement technology for large-scale cement vertical mills: Large-scale is the development trend of international cement vertical mills, but it also brings challenges in structural reliability and overhaul and maintenance. The company optimized the rocker bearing seat structure from split welding to integral casting, and optimized the connecting bridge to a closed box design, improving the rigidity and impact load resistance of the frame device. Large casting parts such as the disc body, rocker arm and grinding roller hub are made of ductile iron to ensure the stable operation of large vertical mills. The company adheres to the modular and standardized design concept, standardizes the specifications of grinding rollers, adapts to grinding discs of different diameters by changing the number of grinding rollers, and integrates the relevant parts of grinding rollers into the grinding roller unit, so as to realize the sharing of grinding roller units of cement vertical mills, slag vertical mills and raw material vertical mills of different specifications, improve the reliability of new specifications of vertical mills, and shorten the development, design and manufacturing supply cycle.

High-efficiency and low-resistance powder separator technology: The separation efficiency and resistance of the vertical mill powder separator affect the production capacity, energy consumption, stability and finished product performance of the vertical mill system. Tongli Company combines CFD flow field simulation data and project practical experience to launch a high-efficiency and low-resistance vertical mill powder separator. The gradient flow field design is adopted to increase the lifting wind speed step by step, preventing the dust from reciprocating in the shell and increasing the resistance; the static blades are made into a "pocket" shape to collect coarse powder to avoid secondary interference, and the dynamic blades are made into a U shape to reduce the radial wind speed entering the rotor, thereby reducing the speed and resistance of the powder classifier. Actual use shows that compared with the traditional cement vertical mill powder classifier, the resistance of the high-efficiency and low-resistance powder classifier is reduced by 500 Pa, and the average powder classifier power consumption is reduced by 0.2 kWh/t, which improves the powder selection efficiency, reduces the number of invalid internal circulation of materials, reduces the resistance of the mill, improves the product particle grading, and optimizes the performance of cement products.

Optimization and innovation of the sealing structure of the grinding roller bearing: The key to the sealing of the grinding roller bearing is to prevent dust from entering the bearing cavity and damaging the bearing. Based on many years of project practice experience, the company has developed a structure of positive pressure gas seal plus dynamic seal. Positive pressure gas is blown into the sealing cavity from the rocker arm hole to block external dust. A V-shaped seal and two skeleton oil seals in the sealing cavity prevent dust from entering and lubricating oil leakage respectively to ensure the normal operation of the bearing.

3. Tongli Typical low-energy cement vertical roller mill case
Case 1: In 2021, Dubai Cement Plant built a new 10,000t/d clinker production line with two cement grinding and preparation systems. Line 1 uses a roller press plus ball mill combined grinding system, and Line 2 uses a ZJTL4640 cement vertical mill. Under the same cement ratio and specific surface area of ​​350m²/kg, the average power consumption of the No. 2 mill system is 2 kWh/t lower than that of the No. 1 mill, which is 23 kWh/t, and the cement strength and performance are equivalent.

Case 2: The Egyptian cement plant has two different cement grinding and preparation systems. In 2020, the vertical mill system uses a ZJTL6250 cement roller mill, and the other system is a roller press plus ball mill combined grinding system. When the clinker ratio is 90% and the cement specific surface area is greater than 370m²/kg, the ZJTL6250 vertical mill runs stably, the system power consumption is 27 kWh/t, which is 25.1 kWh/t when converted to a specific surface area of ​​350m²/kg, saving more than 10% of electricity compared with the combined grinding system.

Case 3: In 2015, a domestic cement grinding station built a new 1 million t/year production line equipped with a ZJTL4640 cement vertical mill. After production, the system runs stably, the output is higher than the design value, the specific surface area of ​​the finished cement is 390m²/kg, and the vertical mill system power consumption is 27 kWh/t, which is 23.5 kWh/t when converted to a specific surface area of ​​350m²/kg.

4. Energy-saving and consumption-reducing transformation case
Case 1: Two cement vertical mills in a Vietnamese cement plant have poor stability, low operating rate, and low hourly output since they were put into production. In 2020, the company transformed its grinding area, changed the material flow rate and movement trajectory, and widened the effective grinding area. After the transformation, under the same cement ratio and finished product fineness, the No. 1 mill increased production by 38%, and the system power consumption decreased by 3 kWh/t; the No. 2 mill increased production by 22%, and the system power consumption decreased by 2.6 kWh/t. The online surfacing welding modified grinding disc liner and grinding roller sleeve were adopted, with a short transformation cycle and high return on investment.

Case 2: A cement vertical mill in a cement plant in Xinjiang required the specific surface area of ​​the cement finished product to be greater than 400m²/kg due to poor clinker strength, but the stability of the vertical mill was poor. After the transformation by Tongli Company in 2022, the grinding parts were replaced with a new grinding area structure, and the air ring and circulating air duct were transformed. After the transformation, the stability, grinding efficiency and system resistance of the vertical mill were improved, the output increased by 20t/h, and the system power consumption was reduced by 4 kWh/t, exceeding customer expectations.

There are various technical paths for carbon emission reduction in the cement industry, among which reducing the clinker coefficient is of great significance. The key lies in optimizing the hydration process and bulk density of each component of cement, which requires in-depth research on the existing cement grinding process system and product characteristics. The current cement grinding processes mainly include "roller press + tube mill" combined grinding, semi-final grinding, vertical mill final grinding and ball mill final grinding. This article focuses on the performance research and technical prospects of cement products in the vertical mill final grinding process system.

1. The vertical mill final grinding process has become the first choice for new construction and renovation projects abroad due to its small footprint, high hourly output, low energy consumption and simple operation and maintenance. However, the traditional view is that compared with ball milling, its grinding method has poor sphericity, poor hydration performance and large water requirement for standard consistency. From the perspective of cement particle shape research, characterized by sphericity (Roundness) and roughness (Convexity), there is no essential difference between vertical mill and ball mill cement in the 2-28d particle shape characteristics that play a key role in strength. For example, the sphericity range and distribution ratio of 0-12μm particles are similar. Although there are differences in the sphericity of 12-24μm, 24-36μm and larger than 36μm particles, the hydration performance of larger than 36μm particles is poor and the contribution to strength is low; in terms of roughness, 0-12μm is a smooth particle concentration area, and 12-24μm is a rough particle distribution area, and the distribution patterns of the two are similar.

2. In terms of particle distribution, the vertical mill cement has less particles smaller than 1μm than the ball mill, and the high hydration heat of this part is not conducive to the slurry structure; the content of 3-32μm particles exceeds 70%, which is higher than the 63% of the ball mill, and this part of particles has a great effect on improving strength; the content of particles larger than 32μm is also less than the ball mill and has poor hydration performance. Using the RRB equation to describe the particle distribution, the characteristic particle size of the vertical mill cement is similar to that of the ball mill, but the uniformity coefficient of about 1.1 is greater than that of the ball mill 0.78, indicating that the vertical mill cement particle distribution is narrow, which is beneficial to strength but will increase the water consumption of the standard consistency. The vertical mill cement grinding and spraying water to stabilize the material layer will cause the cement particles to prehydrate. Studies have shown that the strength and hydration degree of vertical mill cement at each age are lower than those of ball mill cement. Through specific test steps and calculations, it can be seen that the vertical mill cement has obvious prehydration phenomenon, and the increase in water spraying will aggravate the prehydration, while the gypsum has basically no dehydration phenomenon. Further research found that the increase in the vertical mill water spraying will increase the prehydration degree, resulting in a longer setting time and a decrease in 2d strength; conversely, a decrease in water spraying will increase the 2d strength and shorten the setting time.

3. In summary, the cement products of the vertical mill final grinding process system have specific properties. In the key strength contribution range of 3-10μm and 10-32μm, the cement particle shape, sphericity and surface roughness are not fundamentally different from those of ball milling; the characteristic particle size is almost the same as that of ball milling, but the particle distribution is narrower, which is beneficial to strength but increases the water consumption of standard consistency; water spraying in the mill will cause cement particles to prehydrate, affecting strength and setting time. Based on this, although it is feasible to use vertical mills for precise grinding process to produce low clinker cement, it is necessary to develop new controlled material bed stabilization technology to avoid water spraying prehydration, develop step powder selection devices to accurately sort materials, and increase fine mixing processes to reduce power consumption.

4. The shape of cement particles can be characterized by two parameters: roundness and roughness. The calculation formula for roundness is Roundness = Length/Width, and its value range is between 0 and 1. The larger the value, the higher the sphericity. The calculation formula for roughness is Con - 1 - A/(A + B), and the smaller the value, the smoother the particles. By analyzing the distribution of sphericity and roughness of representative samples of cement product particles from vertical mills and ball mills, it can be seen that there is no qualitative difference between vertical mills and ball mills in terms of particle shape, which plays a key role in cement strength. From the perspective of sphericity distribution, for particles of 0-12μm, the sphericity range of vertical mill and ball mill cement is relatively wide, ranging from 0.2 to 1.0, and the distribution ratio is very close, and the particle size of particles with a sphericity from 0.8 to 1.0 is all between 0-12μm. For particles of 12-24μm, although most particles have a sphericity between 0.2-0.6, there are relatively few particles of 12-24μm in vertical mill cement with a sphericity between 0.6-0.8 compared to ball milling. For particles of 24-36μm, the sphericity of vertical mill is between 0.2-0.4, and that of ball mill is between 0.2-0.6. For particles larger than 36μm, the sphericity of ball mill particles is slightly higher than that of vertical mill cement, but these particles have poor hydration performance and contribute very little to the 28d strength. From the perspective of roughness distribution, smooth surface particles are concentrated in 0-12μm, while rough surface particles are distributed in 12-24μm. The distribution patterns of vertical mill and ball mill cement are similar.

5. In terms of cement particle distribution, the particle size distribution curves of the vertical mill cement product sample and the experimental ball mill cement sample (specific surface area is controlled at 3600cm²/g) show certain differences. The particles smaller than 1μm in the vertical mill cement are significantly less than those in the ball mill. These particles will quickly hydrate after contacting water, but they do not contribute much to the strength. Instead, they may cause microcracks in the cement paste due to the large hydration heat, which has an adverse effect on the paste structure. The content of 3-32μm particles in the vertical mill cement is more than 70%, while that of the ball mill is only about 63%. The more these particles are, the higher the 2d and 28d strengths are.

In addition, the content of particles larger than 32μm in the vertical mill cement is also less than that in the ball mill. These particles have poor hydration performance and only have a certain impact on the later strength of the cement. Therefore, from the perspective of the impact of particle size distribution on strength, the particle distribution of vertical mill cement is relatively more reasonable. The distribution of cement particles can be described by the Rosin Rammer Benet equation (RRB equation), that is, R = 100 Exp {-(D/Dₑ)ⁿ}, in which R represents the sieve residue (%) of particle size D (μm), D is the characteristic particle size, and its physical meaning is R = 36.8% of the particle size (μm), and n is the uniformity coefficient, which indicates the width of the particle distribution. The larger the value, the narrower the particle distribution range. By analyzing and calculating the RRB curves of the vertical mill cement product sample and the ball mill cement sample, the characteristic particle size and uniformity coefficient of different samples can be obtained. The results show that the characteristic particle size of the vertical mill cement is almost the same as that of the ball mill, but the uniformity coefficient of the vertical mill cement is about 1.1, and that of the ball mill sample is only 0.78, which indicates that the particle distribution of the vertical mill is narrower. Although this particle size distribution form is more beneficial to strength, it will also lead to an increase in the water consumption of the standard consistency.

In order to stabilize the material layer during the vertical mill cement grinding process, it is necessary to spray water for a long time. However, this operation may cause the cement particles, especially the fine powder with large specific surface area and high activity, to be prehydrated. During the prehydration process, a layer of hydration products will grow on the surface of the cement particles. This results in the difficulty of water molecules to penetrate the hydration product layer during the coagulation and hardening of cement when it is used in cement paste, mortar, and concrete, and it is impossible to continue to react with the internal clinker minerals, thereby reducing the hydration rate and degree of hydration, making it impossible to generate C-S-H with a large number of mesh structures, overlapping each other, and mainly contributing to strength, which ultimately results in a long coagulation time and a decrease in strength. In order to further study the effect of vertical mill water spraying on cement performance, the researchers took samples, prepared samples, and formed slurry from the vertical mill final grinding system and the laboratory small mill, and determined the hydration degree and strength of cement slurry at different ages by measuring the non-evaporable water content of cement slurry at different ages. The test results show that the strength and hydration degree of vertical mill cement at different ages are lower than those of ball mill cement samples.

8. In order to further verify whether cement is prehydrated during the vertical mill grinding process, the researchers conducted a series of detailed tests on the cement out of the mill. First, place the crucible in a high-temperature furnace and calcine it at 950℃ for a certain period of time, then quickly transfer it to a dryer and cool it to room temperature, and weigh it until it reaches a constant weight a; then place about 10g of cement in the crucible and weigh the total weight b; then place the crucible with cement in a drying oven, heat it up and keep it at 105℃ for 0.5h, quickly transfer it to a dryer and cool it to room temperature, and quickly weigh it c; then place the crucible in a drying oven, heat it up and keep it at 180℃ for 1h, quickly transfer it to a dryer and cool it to room temperature, and quickly weigh it d; then place the crucible in a high-temperature furnace, heat it up and keep it at 340℃ for 1h, quickly transfer it to a dryer and cool it to room temperature, and quickly weigh it e; then place the crucible in a high-temperature furnace, heat it up and keep it at 450℃ for 1h, quickly transfer it to a dryer and cool it to room temperature, and quickly weigh it; finally place the crucible in a high-temperature furnace, heat it up and keep it at 180℃ for 1h, quickly transfer it to a dryer and cool it to room temperature, and quickly weigh it d. 950℃, quickly transferred to the dryer to cool to room temperature, and quickly weighed g. In the whole process, each step needs to prevent water absorption as much as possible to avoid large errors, and all weighings are accurate to 4 decimal places. Through the corresponding calculation formulas: humidity W=(b - c)/(b - a)×100%, gypsum dehydration degree W₈ = LOI₁₈₀ - LOI₁₀₅=(c - d)/(b - a)×100%, prehydration degree Wₖ = LOI₃₄₀ - LOI₁₈₀=(d - e)/(b - a)×100%, etc., relevant data can be obtained.

The test results show that the prehydration degree Wₖ is around 0.2%, which fully indicates that there is obvious prehydration phenomenon in the water spraying in the mill, and the prehydration degree will be higher with the increase of water spraying amount. At the same time, it can be concluded that the gypsum dehydration degree W₈ is about 0.7%, which means that there is basically no dehydration of gypsum. It is speculated that this may be due to the short residence time of gypsum in the mill. In addition, by studying the effect of different vertical mill water spraying amounts on cement strength and setting time, it is found that when the amount of water added gradually increases, the prehydration degree Wₖ gradually increases, the setting time of cement also increases significantly, and the 2d strength decreases significantly; conversely, when the vertical mill water spraying amount decreases, the 2d strength increases significantly and the setting time also decreases. Through a comprehensive and in-depth study of the particle shape, particle distribution, and prehydration in the mill of the vertical mill final grinding cement product, it can be concluded that the cement products of the vertical mill final grinding process system have the following significant characteristics:

First, in the two cement particle distribution ranges of 3-10μm and 10-32μm, which play a major role in cement strength, there is no fundamental difference in the particle shape, sphericity, and surface roughness of the cement final grinding of the vertical mill compared with the ball mill. Secondly, the characteristic particle size of cement particles in the final grinding of the vertical mill is almost the same as that of the ball mill, but the uniformity coefficient of cement in the final grinding of the vertical mill is about 1.1, while that of the ball mill sample is only 0.78. The particle distribution of cement in the vertical mill is narrower. This particle size distribution is more favorable for strength, but it will also lead to an increase in the water consumption of the standard consistency. Finally, the operation of spraying water in the vertical mill to stabilize the material layer will cause the cement particles in the mill to prehydrate, and as the amount of water sprayed increases, the degree of prehydration increases, which in turn leads to a decrease in strength and a decrease in setting time.

In summary, the above research conclusions show that it is completely feasible from a technical point of view to use the vertical mill grinding process system for the precise separation grinding process system to produce low clinker cement. However, in order to better achieve this goal, further in-depth research is needed in the following aspects: First, develop new controlled material bed stabilization technology to avoid directly stabilizing the material layer by spraying water inside the mill, thereby completely eliminating the pre-hydration phenomenon inside the mill; second, develop a stepped powder selection device, which can accurately select cement and auxiliary cementitious materials in different particle size ranges under the guidance of the low clinker dosage cement particle distribution model; third, increase the fine mixing process, so as to eliminate the process of shaping the cement particles through the tube mill, thereby effectively reducing the system power consumption.

The above research conclusions show that in order to better achieve the goal of producing low-clinker cement, further in-depth research is needed in the following aspects: First, develop new controlled material bed stabilization technology to avoid directly stabilizing the material layer by spraying water in the mill, thereby completely eliminating the pre-hydration phenomenon in the mill; second, develop a stepped powder selection device, which can accurately select cement and auxiliary cementitious materials in different particle size ranges under the guidance of the low-clinker cement particle distribution model; third, increase the fine mixing process, so as to eliminate the process of shaping cement particles through the tube mill, thereby effectively reducing the system power consumption.

In the vertical roller mill cement grinding process, the fluctuation of raw material moisture easily causes the vertical mill to vibrate. Generally, water spraying is used to stabilize the material bed, but water spraying easily causes hydration of clinker and reduces cement strength. In order to achieve grinding without water spraying, Tongli Heavy Machinery Cement Research Institute has transformed the grinding roller, powder selector, air ring, etc. The results show that the technical transformation reduced the clinker ratio by 2.9%, increased the output by 19%, and reduced the unit power consumption by more than 3.3 kWh/t, achieving the goal of increasing production, reducing consumption, saving energy and reducing carbon emissions.

1. Process configuration and operating parameters before transformation
The mill of the cement vertical mill system of Tongli Company has 4 feeding points, 1 for feeding clinker, 1 for feeding gypsum and mixed materials, and 2 for feeding dry fly ash. In order to avoid the agglomeration of dry and wet materials and the agglomeration of the air lock valve, clinker and mixed materials are fed into the grinding disc from their respective feeding points. Due to its high specific surface area, dry fly ash enters the vertical mill through the air lock valve, and is first selected by the powder selector. The coarse material returns to the grinding disc for grinding, which is conducive to increasing production and reducing the pressure difference of the mill. The finished product obtained by grinding and selecting powder in the vertical mill is collected by the dust collector and transported to the storage elevator. The heat source of the vertical mill drying comes from the exhaust gas (about 120℃) at the outlet of the cooling head exhaust fan to ensure that the moisture content of the cement is controlled to < 0.5%.

Output: P・042.5 grade cement 205 t/h; Mill model: LM56.3 + 3c/s, grinding disc diameter 5600mm; grinding disc speed 22.09r/min; number of grinding rollers 3; grinding roller middle diameter 2360mm; grinding roller width 785mm; hydraulic cylinder diameter 480mm; piston rod diameter 145mm; power 5300kW; powder concentrator: diameter 5150mm, height 3445mm, maximum speed 125r/min, power 400kW; fan: air volume 300 000m/h, air pressure 6000Pa, power 2200kW; bag dust collector: maximum processing air volume 650000m/h, filtration air speed 0.8m/min, resistance 1200Pa; return material elevator: lifting capacity 150t/h, power 30kW

The relevant parameters of this project are as follows: output is 210t/h, mill power is about 3900kW, water spray volume is 2-3t/h, mill pressure difference is 4800Pa, material retaining ring height is 385mm, grinding roller linear speed is 41r/min, material layer thickness is between 50-70mm, powder concentrator power is 130-180kW, powder concentrator speed is 80r/min, wind ring effective area is 3.98m², windmill baffle opening is 100%, horizontal vibration value is 3-5mm/s, rod chamber pressure is 6.5-7.5MPa, rodless chamber pressure is 2.0-2.4MPa, windmill power is 1450-1650kW, hot gas temperature is 90°C.

2. Analysis of existing problems
Current VRM uses mill water spraying 2-3 t/h to stabilize the material bed. The P・042.5 grade cement ratio is 1%-2% more clinker than the roller press + ball mill semi-final grinding system. Not only is the cement production cost high, but the power consumption is also high, reaching 31 kWh/t.
The structure of the vertical mill grinding area plays a vital role in the grinding capacity and stability of the mill. According to the poor stability of the water spraying operation required on site, it is inferred that the grinding area may be narrow and the width of the grinding area needs to be widened to give full play to the grinding capacity of the mill.
The mill needs to spray water to stabilize the material layer, but the vibration may also be related to the speed of the grinding roller and the structure of the air ring.
The internal circulation volume of the powder selector is large, and the rotor speed of the powder selector is fast, resulting in low classification efficiency and affecting the performance of the mill capacity.

3. Tongli Renovation plan
Wide the grinding area. Update 3 roller sleeves (see Figure 2), increase the roller sleeve diameter from 2360 mm to 2570 mm, increase the roller projection area, improve the mill output and grinding efficiency, and reduce power consumption.
The grinding disc speed remains unchanged. Since the grinding disc speed remains unchanged at 22.09 r/min, the linear speed is reduced from 41 r/min to 37 r/min after the roller diameter is increased. After the linear speed is reduced, the grinding pressure can be increased, the grinding efficiency can be improved, and the mill vibration can be reduced to achieve non-water spraying grinding.
Optimize the roller taper angle. After the roller diameter is increased, the roller assembly can be reused, but the roller taper angle needs to be optimized. The grinding disc size is adjusted by surfacing, and the roller taper angle is adjusted from horizontal 0° to 1.6° (see Figure 3) to ensure that it matches the working surface of the new roller.
Optimize the height of the retaining ring. The height of the retaining ring has a corresponding relationship with the material layer thickness, which has a great influence on the grinding efficiency of the mill. Under the condition of no vibration, the lower the height of the retaining ring, the higher the grinding efficiency of the mill. The increase in the middle diameter of the grinding roller and the reduction in the linear speed can reduce the vibration of the mill. Therefore, the height of the retaining ring of the mill is reduced from 385 mm to 265 mm, and the grinding efficiency is improved by reducing the thickness of the material layer.
Switch to a low-resistance and high-efficiency powder classifier. The particles ground by the mill need to be sorted by the powder classifier, and the semi-finished products are returned to the mill for grinding. The existing powder classifier has large resistance and low powder selection efficiency. The powder classifier and the upper shell of the mill are replaced as a whole with a low-resistance and high-efficiency powder classifier to improve the powder selection efficiency and reduce the system resistance.
Optimize the angle of the wind ring blades and the air guide cone. The mill wind ring structure has a certain influence on the mill resistance. By optimizing the angle of the wind ring blades and the air guide cone, the resistance of the wind ring itself is reduced, and at the same time, the flow field in the mill is optimized, the overall resistance of the mill is reduced, and the powder selection effect of the wind ring is improved.

4. Optimize operating parameters
After the transformation, the operating parameters such as the height of the retaining ring, grinding pressure, and system air volume were optimized, and cement grinding without water spraying was successfully achieved, so that the output of P・042.5 grade cement increased from 210 tons per hour before the transformation to 250 tons per hour, and the power consumption was greatly reduced. The operating parameters after the transformation showed that the mill pressure difference dropped to 4300 Pa, the height of the retaining ring was reduced to 265 mm, the linear speed of the grinding roller was adjusted to 37 revolutions per minute, the speed of the powder selector was reduced to 68 revolutions per minute, the power of the powder selector was controlled at 110 to 130 kilowatts, and the effective area of ​​the wind ring was 3.85 square meters. In addition, the windmill damper opening was maintained at 100%, the rod chamber pressure was stable at 7.0 to 7.3 MPa, the rodless chamber pressure was 2.0 to 2.2 MPa, the hot gas temperature was 90 degrees Celsius, and the horizontal vibration value was 3 to 5 mm per second.

Reduced clinker ratio
Due to the cessation of water jet grinding after the transformation, the clinker ratio of P・042.5 grade cement decreased by 2.9%. Before the transformation, the clinker ratio was 78.5%, and the proportions of gypsum, limestone, coal slag, volcanic stone and fly ash were 5.0%, 6.0%, 4.0%, 2.0% and 4.5% respectively. After the transformation, the clinker ratio dropped to 75.6%, the proportion of coal slag increased to 5.1%, the proportion of volcanic stone increased to 5.5%, and the proportion of fly ash decreased to 2.8%. The proportions of gypsum and limestone remained unchanged at 5.0% and 6.0% respectively.

Increased output and reduced power consumption
The output and power consumption performance of P・042.5 grade cement also changed significantly before and after the transformation. Before the transformation, its output was 210 tons per hour, the specific surface area was 3500 to 3600 square centimeters per gram, the unit power consumption was 31.1 kWh per ton, the 3-day compressive strength was 27.2 MPa, and the 28-day compressive strength was 51.5 MPa. After the transformation, the output increased to 250 tons per hour, the unit power consumption dropped to 27.8 kWh per ton, the 3-day compressive strength increased to 29.1 MPa, and the 28-day compressive strength increased to 52.3 MPa. The specific surface area remained unchanged at 3500 to 3600 square centimeters per gram.

Transformation benefits
According to the scale of 1.2 million tons of cement per year, combined with the reduction of clinker ratio by 2.9%, this transformation has brought significant economic and environmental benefits. Among them, 3.96 million kWh of electricity was saved annually, the use of low clinker was reduced by 34,800 tons, the direct economic benefits were close to 8 million yuan, and the project investment recovery period was only more than one year. In addition, by saving electricity and reducing the amount of clinker, carbon dioxide emissions are reduced by about 30,000 tons per year, further reflecting the value of green environmental protection.

Comprehensive evaluation
Tongli Company combined the characteristics of cement vertical mill grinding and carried out structural transformation of key equipment such as grinding rollers, powder selectors, and air rings. After the transformation, not only did it achieve grinding without water spraying, but it also reduced the clinker ratio by 2.9%, increased the output by 19%, and reduced the unit power consumption by more than 3.3 kWh per ton. The transformation effectively achieved the goals of increasing production and reducing consumption, saving energy and reducing carbon emissions, and achieved significant economic and social benefits.

Energy conservation and consumption reduction in cement grinding preparation process is one of the main ways to achieve carbon emission reduction and process carbon reduction in cement industry. A combined grinding system or a single independent grinding final grinding system consisting of a vertical mill or roller press and a ball mill with higher grinding efficiency and lower energy consumption has gradually become the mainstream equipment. This article introduces a ZJTL-Y type external circulation cement vertical mill final grinding technology and industrial application based on the material bed grinding mechanism. This technology has the characteristics of low power consumption and excellent cement performance. It can be used as a practical technical solution to achieve energy conservation and consumption reduction in cement grinding preparation and carbon emission reduction in cement industry.

In the cement production process, cement grinding power consumption accounts for 60% - 70% of the total power consumption in the cement production process. In order to reduce more cement industry emissions and achieve process carbon reduction, one of the main ways is to take necessary measures to achieve energy conservation and consumption reduction in cement grinding preparation. In order to maximize energy saving and manufacture and develop more products that meet the requirements, it is necessary to study the mechanism of grinding energy saving, study the particle morphology of materials, and develop more effective grinding technology.

At present, cement grinding main machine is equipped with ball mill based on single particle crushing mechanism and roller mill, roller press and drum roller mill based on material bed grinding mechanism, among which ball mill, vertical roller mill (abbreviated as "vertical mill") and roller press are the most common. Since the energy utilization rate of traditional ball mill is only below 3%, the final grinding system of ball mill has been basically eliminated or modified in the cement grinding preparation process. With the development of material bed grinding technology, the combined grinding system composed of vertical mill or roller press and ball mill with higher grinding efficiency and lower energy consumption or a separate vertical mill final grinding system has gradually become the mainstream. This article introduces a ZJTL-Y type external circulation cement vertical mill final grinding technology based on material bed grinding mechanism.

1. Introduction to external circulation cement vertical mill final grinding technology
Different from the traditional internal circulation vertical mill, ZJTL-Y vertical mill itself does not have a powder selector. During operation, the motor drives the reducer to drive the grinding disc to rotate. The material falls to the center of the grinding disc through the central discharge pipe, and gradually moves to the edge of the grinding disc under the rotation of the grinding disc. When it moves to the bottom of the grinding roller, it is ground under the high specific roller pressure generated by the tension of the hydraulic pressure cylinder. The ground material passes over the retaining ring under the action of centrifugal force and is thrown out of the grinding disc, and is scraped into the discharge port by the scraper device, and then all discharged to the circulation elevator. ZJTL-Y vertical mill final grinding system process system All materials fed into the ZJTL-Y vertical mill are ground and discharged to the circulation elevator, and all are primary sorted by the external V-type static powder separator. After the primary sorting, the coarse material enters the vertical mill for grinding again, and the fine material enters the fine powder separator for secondary sorting. After the secondary sorting, the coarse powder in the fine material returns to the vertical mill for further grinding, and the fine powder in the fine material is collected and stored as cement finished products through the bag dust collector. Because this system process does not have a ball mill, it is simpler than the pre-grinding or semi-final grinding system.


2. Technical characteristics of external circulation cement vertical mill final grinding
2.1 High grinding specific pressure: Grinding specific pressure is one of the important indicators reflecting the grinding force of vertical mill, which is generally expressed as the ratio of roller pressure to the projected area of ​​grinding roller. Generally, the grinding pressure of ZJTL-Y type external circulation pre-grinding vertical mill (is 20% higher than that of traditional internal circulation vertical mill or other types of external circulation vertical mills used for cement pre-grinding, and the grinding pressure of ZJTL-Y vertical mill developed on the basis of ZJTL-K vertical mill is 20% higher. Through the adjustment of hydraulic pressure cylinder, its grinding pressure can be freely adjusted between 1.0 - 1.5 MPa, which ensures the first-time yield and the grinding efficiency of multiple material cycles.

2.2 Optimized roller curve
ZJTL-Y vertical mill optimizes the "grooved tire roller" roller grinding structure. Under the same conditions, the material can obtain greater shear force during the grinding process, thereby obtaining better grinding effect and achieving efficient grinding. Different shear effects can also be obtained by adjusting the curvature radius of the roller curve to change the speed difference between the rollers. At the same time, this shear force can sphericalize the material produced by extrusion, thereby improving the adaptability and workability of cement. In addition, due to The rubbing effect brought by the shear force during the grinding process of the ZJTL-Y vertical mill makes the material highly dispersed after extrusion grinding and shear grinding, and it will not clump like the roller press, and it also reduces the difficulty and energy consumption of subsequent powder selection.

2.3 Low system resistance
The materials after the ZJTL-Y vertical mill final grinding system are all mechanically conveyed, while the traditional internal circulation vertical mill requires pneumatic conveying, that is, the outlet of the injection ring needs to have a suitable wind speed. The resistance here accounts for more than 40% of the system resistance. The ZJTL-Y vertical mill final grinding system only has the resistance of the pipeline, powder selector and bag dust collector. Compared with the traditional internal circulation vertical mill system, its system resistance can be reduced by more than 1500 Pa, and the air volume per unit product can be reduced by about 50% compared with the traditional internal circulation vertical mill system.

2.4 External powder selection system
The external powder selection system can be refined and personalized according to the actual situation of the project. The powder selection system in the vertical mill final grinding system consists of a V-type static powder selection machine and a fine powder selection machine. Of course, a combined powder selection machine or other powder selection types can also be used, which is easier to design and select reasonably and achieve efficient sorting.

2.5 Less system maintenance workload
Compared with the pre-grinding or semi-final grinding system, like the traditional internal circulation vertical mill grinding system, the ZJTL-Y vertical mill final grinding system is a single fan system without a ball mill, so the system has a small number of equipment, a simple process flow, and a small system maintenance workload. At the same time, through the application of unique single expansion sleeve connection between the grinding roller shaft and the rocker arm, and the use of outer ring widened high-load non-standard self-lubricating spherical bearings for hydraulic pressure cylinders, the difficulty and workload of operation and maintenance are reduced while ensuring high reliability.

2.6 Low wear inside the mill
Since all materials are circulated externally and mechanically conveyed into the powder selection device, the ZJTL-Y vertical mill cancels the mill blowing ring device, which greatly reduces the wear of high-speed dust-containing airflow on the grinding roller and the inner wall of the shell. The mill only needs to pass through the micro-negative pressure dust collection pipe to ensure that the mill is in a micro-negative pressure state during operation. At the same time, since there is no large amount of airflow passing through the mill, the gas content of the material in the mill is less than that of the traditional internal circulation vertical mill, and the material bed is more stable.

2.7 Real-time monitoring and adjustment
The grinding pressure, height of the retaining ring, height of the central feed pipe and circulation volume of the ZJTL-Y vertical mill can be adjusted according to different material conditions and the requirements of the grinding process. At the same time, with the help of monitoring methods such as material layer, vibration, current, and temperature, the overall operation status of the vertical mill can be observed remotely online in real time. Combined with the DEM-MBD simulation of the mechanism of factors affecting the stability of the material bed, the reasonable adjustment range guided by the results and the intelligent expert system can realize the stable online intelligent real-time adjustment control of the material bed. The hydraulic system of ZJTL-Y vertical mill is composed of the basic unit of "1 station controlling 2 rollers" or "1 station controlling 3 rollers", which can not only realize the function of "lifting and dropping rollers", but also realize no-load start. Especially for ZJTL-Y vertical mill with multiple hydraulic system units, when a hydraulic system unit has a problem, it will not affect the normal operation of other units, ensuring the high operation rate of the equipment.

3. Tongli ZJTL-Y vertical mill industrial application
The first ZJTL-Y external circulation cement vertical mill final grinding system developed, designed and applied by our company was officially completed and put into production at the end of 2014. The system uses TL46.4-C external circulation cement final grinding vertical mill as the core grinding equipment, and is equipped with an external dynamic-static coupling powder selection system consisting of a V-type powder selector + LAX type double separation high-efficiency dynamic powder selector, as shown in Figure 5. It has been stably and reliably produced for nearly 10 years, and no water spraying is required during the production process, avoiding the impact of water spraying on the performance of cement products. The P・O42.5 cement products produced are recognized and well received by the market.

3.1 Equipment Performance
ZJTL-Y46.4—c external circulation cement final grinding vertical mill is a four-roller vertical roller mill. The loading hydraulic system adopts a shared oil tank and dual loading drive configuration scheme of "1 station controls 2 rollers". The corresponding rod chamber and rodless chamber inlet and outlet pipelines between the systems are connected or disconnected by bypass docking and manual ball valve switch adjustment. Therefore, when one of the loading hydraulic systems fails, the other loading hydraulic system can be used to achieve two-roller operation or four-roller operation; or when the grinding roller controlled by one of the loading hydraulic systems needs to be repaired, the other loading hydraulic system can be used to achieve two-roller operation. During operation, a high-pressure operation mode is adopted, and the operating pressure is generally set to 13-14 MPa, which can achieve a high grinding pressure of 1.1-1.2 MPa. It can also be adjusted according to actual conditions. At the same time, an optimized roller disc structure is adopted to achieve efficient grinding. ZJTL-Y—c The consumption of wear-resistant parts of the external circulation cement final grinding mill is mainly the wear and tear of the roller sleeve and the grinding disc lining. According to production statistics, the comprehensive wear of the wear-resistant roller sleeve and the grinding disc lining in the form of composite wear-resistant cladding is 2.5-3.2, which is much lower than the metal wear consumption of the combined grinding system 22 and the ball mill system (circular flow) 45.

3.2 System operation performance
The powder selection efficiency of the external powder selection system of the ZJTL46.4-c external circulation cement vertical mill final grinding system and the stability of the inlet flow are crucial to the stable and reliable operation of the equipment and system. During the operation of the vertical mill final grinding, the sorting efficiency of the V-type powder selector and the LAX-type double-separation high-efficiency dynamic powder selector in the external powder selection system can reach more than 90% and more than 70% (45). At the same time, by taking measures such as intermediate bin + quantitative feeding, iron removal, and removal of large material blocks to stabilize the inlet flow, the vibration value of the vertical mill during operation is basically below 1.8 mm/s. In addition, due to the full external circulation of materials, there is no wind resistance in the vertical mill itself. When the system is running, the outlet pressure of the finished product dust collector is only -3500--4000 Pa, which is lower than the outlet pressure of the finished product dust collector of the traditional internal circulation vertical mill of -5500--6500 Pa.

The output is 183.73, and the process power consumption is 21.92, indicating the energy consumption level of the production process. The specific surface area is 374, indicating the fineness of the material. The 45-sieve residue is 7.8%, and the 80-sieve residue is 1.5%, which respectively reflect the screening of the product. The standard consistency water consumption accounts for 26.9%, which affects the workability of the product. The 3-day compressive strength is 31.7 MPa, and the 28-day compressive strength is 53.7 MPa, which shows the strength characteristics of the product under different curing times.

3.33.3 System energy consumption distribution
Under normal circumstances, the power consumption of the bed final grinding system is 22-26, which is better than the bed pre-grinding system (26-35) or semi-final grinding system (25-28), and better than the ball mill final grinding system (35-42). The energy consumption distribution of different cement bed final grinding systems is different, but the energy saving effect of the ZJTL-Y46.4-c external circulation cement vertical mill final grinding system is more obvious, especially in terms of system fan power consumption. The roller press final grinding system, which is also a full external circulation material system, has some technical application breakthroughs, but the industry is concerned about the morphology and performance of cement finished particles in the roller press final grinding system and has not been promoted on a large scale.

3.4 Particle morphology
Thanks to the high grinding pressure of the ZJTL-Y vertical mill and the high speed difference between its rollers, the final grinding cement product of the ZJTL-Y vertical mill (using the "grooved disc tire roller" roller grinding structure) is better than the final grinding cement product of the HX traditional internal circulation vertical mill (using the "flat disc cone roller" roller grinding structure) from the perspective of overall sphericity. Different from the ZJTL-Y vertical mill, the roller press mainly completes the material bed grinding through the high pressure and slow extrusion between the moving and fixed rollers, which also makes the sphericity of its finished product worse than that of the ZJTL-Y vertical mill with grinding and shearing effects. Figure 7 is an electron microscope photo of the semi-finished particles of the ZJTL-K vertical mill and roller press combined into the ball mill from the same factory. From the perspective of overall sphericity, the semi-finished particles of the ZJTL-K vertical mill are better than those of the roller press, especially the semi-finished particles of the ZJTL-K vertical mill are less long and flake-like. ZJTL-Y vertical mill is developed on the basis of ZJTL-K vertical mill. It has the same grinding mechanism as ZJTL-K vertical mill. Therefore, the particle morphology of the cement finished product of ZJTL-Y vertical mill also has similar characteristics.

3.5 Particle size distribution
Sampling and analysis were conducted from several common cement final grinding production processes. Among them, the cement finished product of ZJTL-Y external circulation vertical mill and the cement finished product of ball mill came from the same factory. It can be seen that due to the dual effects of extrusion and shearing caused by high specific pressure grinding of roller disc, the particle size of the cement finished product of ZJTL-Y external circulation vertical mill is similar to that of the ball mill in the same factory, but the particles in this range are finer, and the overall particle size distribution is wider than that of ball mill final grinding, and the water consumption of cement standard consistency is also lower. The particle distribution of cement finished products of traditional internal circulation cement mill is more concentrated, the proportion of 3-32 particles is also relatively small, and the water consumption of cement standard consistency is also higher. This may be related to its small grinding pressure and weak shearing effect, which leads to more flaky particles in cement finished products, less fine powder in cement finished products, and higher water consumption of cement finished products with standard consistency. In addition, the traditional internal circulation cement mill adopts the form of flat disc cone roller. In actual operation, the grinding disc speed is also higher than that of ZJTL-Y vertical mill, resulting in faster material flow rate on the grinding disc. Research has found that the faster the material flow rate, the fewer times the material is ground on the grinding disc in a single cycle, and the more times the material needs to be circulated, resulting in fewer coarse particles and fine particles in the particle size distribution of cement finished products, a more concentrated particle size distribution, and a slightly higher water consumption of cement finished products with standard consistency. Therefore, the slower the material flow rate, the better the cement finished products will be. Due to the difficulty of modifying the main motor and roller curve of the vertical mill, in actual production, the traditional internal circulation cement mill adopts methods such as adjusting the mineral composition of clinker and optimizing the raw material ingredients of cement to improve the particle size distribution of cement finished products.

At the same time, for ZJTL-Y vertical mill, the particle distribution of cement products can be changed by changing the radius of curvature of the roller, so as to obtain better performance of cement products. Because at the same grinding disc speed, the flow rate of the same material on the grinding disc is positively correlated with the radius of curvature of the roller. From the above analysis, it can be seen that slow material flow rate will lead to wider particle size distribution and lower water consumption of standard consistency of cement products. Therefore, the relatively small curvature radius R of the ZJTL-Y vertical mill roller will have a wider particle size distribution and lower water consumption of standard consistency of cement products. However, the roller curvature radius R is too small.

3.6 One mill for multiple uses
The ZJTL-Y external circulation cement vertical mill final grinding system can produce not only P・O42.5 cement, but also PO52.5 cement. When producing PO52.5 cement, due to the increase in the cement clinker ratio, the overall output is lower than that of P・O42.5 cement, and the power consumption is higher. Its ratio is (clinker + slag) 93, desulfurized gypsum 7, the output is 145-155, the finished product specific surface area is 390, the finished product 45 sieve residue, 3d compressive strength 30.4MPa, 28d compressive strength 54.9MPa, vertical mill main machine power consumption 18-19, system process power consumption 24-25. In addition, the system can also produce desulfurized limestone powder (raw material is 100% limestone). The shared oil tank and double loading drive configuration of the loading hydraulic system "1 station control 2 rollers" can realize two-roller or four-roller operation. The output when two rollers are running is not mentioned, the power consumption of the vertical mill host is not mentioned, and the system process power consumption is 20-20.5; when four rollers are running, the output is 190-200, the power consumption of the vertical mill host is 11.5-12, and the system process power consumption is 19-20. In addition, the overall roller pressure of two rollers is small, the vertical mill is more stable, and the energy consumption is not much different. The operation mode can be adjusted as needed.

3.7 Intelligent control
The external circulation cement vertical mill final grinding system adopts a variety of intelligent control methods. For example, APS one-button start and stop technology, integrated with multiple technologies such as composite variable automatic control, shortens the system start and stop time from 5-8min to about 3min, and reduces the workload of operators by more than 20%. By adding fault analysis methods such as monitoring and detection sensors and data acquisition kits to the main and auxiliary machines, and combining intelligent monitoring and intelligent inspection technologies, intelligent operation and maintenance of equipment and systems can be achieved. Intelligent hydraulic lubrication technology is used at the hydraulic lubrication points of key main and auxiliary machines to identify the degree of oil contamination and deterioration and the wear status of components, and to achieve precise control of timing, quantity, and fixed points. The system production control integrates APC advanced process control technology such as MPC multivariable predictive model control to achieve intelligent control of the entire system and the entire process, and can also be incorporated into the intelligent control system of the entire plant as a subsystem. In the future, it will also combine the "grinding +" automatic driving intelligent control technology to achieve weak manual or unmanned control operation.

1. Introduction:
A cement company in Dubai selected a UBE vertical roller mill system for cement preparation. The main machine specification is TRM4540. It is designed to produce P·O 42.5 cement with an output of 150 tons/hour and a certain specific surface area. In the original process design, the drying heat source was extracted from the outlet of the high-temperature fan at the end of the kiln at 210℃ hot air.

Due to the influence of environmental protection policies, the hot air duct was removed.
After the hot air duct was removed, the problem of low temperature of cement out of the mill occurred during the production process. The temperature dropped from 90℃ to 63℃, and the moisture content of cement increased from 0.2% to 0.5%. This resulted in the later strength of cement out of the factory being attenuated by 2-3 MPa after being stored for a period of time. In order to control the moisture content of cement out of the mill and increase the temperature out of the mill, measures were taken to reduce the amount of water sprayed in the mill, but this adjustment caused the grinding disc material layer to be unstable, the vibration value increased from 1.0 to 3.5, and the hourly output dropped from 220 tons to 180 tons. In order to stabilize the material layer, the height of the retaining ring was increased from 200mm to 220mm. Although the vibration value dropped to 2.0, the main motor current increased by 25A, and the power consumption of cement products increased significantly from 26kWh/t to 28.5kWh/t.

2. Vertical mill modification plan

2.1 Adjust the size of the grinding disc surfacing
On-site inspection found that the grinding disc working surface was narrow, with a force surface width of 250-300mm, accounting for only 50%-60% of the surfacing width of the grinding disc working surface (the designed working surface width is 500mm), and the force surface showed corrugated strip wear, which further aggravated the vibration value of the mill. After in-depth analysis and discussion of Tongli vertical mill, combined with its excellent design concept and engineering technology, it was decided to widen the grinding disc working surface to 600mm to increase the contact area between the grinding roller and the grinding disc. The technical team of Tongli vertical mill worked closely with the surfacing manufacturer and selected more wear-resistant welding wire, which effectively reduced the corrugated strip wear caused by the force on the working surface and reduced the vibration value of the vertical mill, reflecting the professional level of Tongli vertical mill in detail optimization.

2.2 Adding a scraper device
The vertical mill was originally designed as a four-roller vertical mill, of which two rollers were used to keep the material layer flat, and the other two rollers were working rollers. When the four rollers are running at the same time, the output is low due to the low loading pressure. During normal operation, only two working rollers are usually used, and the other two auxiliary rollers are suspended for standby. In order to ensure that the grinding disc material layer is flat when the two working rollers are working, the professional design team of Tongli vertical mill recommended and implemented a scraper device (see Figure 5). This design not only ensures the stability of the material layer, but also reduces the vibration of the system, further improving production efficiency. This innovative solution fully demonstrates Tongli's advancement and professional technology in vertical mill design.

2.3 Improve the thermal efficiency of the system
The operation of the heat source-free cement vertical mill system requires special attention to the management of the system's thermal efficiency, which is mainly achieved by increasing the utilization of the circulating air in the system, controlling the system's air leakage, and improving the system's thermal insulation performance, thereby ensuring the maximum control of the cement temperature out of the mill. Tongli vertical mill is particularly outstanding in this regard. Through advanced system design and optimized management, it has successfully improved the system's thermal efficiency.
First, during the daily startup process, the temperature of the incoming clinker and the heat generated by the vertical mill operation can gradually increase the system air temperature to 63°C, but it is still 22°C lower than the design value of the vertical mill outlet air temperature of 85°C.

In order to make up for this gap, Tongli's engineering team took the measure of fully opening the circulating air valve to make full use of the circulating air temperature of 63°C to supplement the system air volume and prevent cold air from entering the system. Secondly, the centrifugal fan air inlet of the mill tail bag dust collection chute was modified, and the inlet of the two centrifugal fans was connected to the circulating air duct, so as to transport the circulating air to the mill tail bag dust collection chute, reduce the intake of outdoor cold air, and reduce the system temperature.
In terms of air leakage management, Tongli vertical mill adopts an efficient sealing system, the mill feed inlet uses a clinker sealing plate chain scale, the slag discharge outlet uses a non-powered curtain-type air lock valve, and a new type of active soft connection is used between the grinding roller and the shell. These efficient sealing and optimized designs not only improve the thermal efficiency of the system, but also ensure the stability and production efficiency of the equipment.
In order to enhance the thermal insulation effect of the system, Tongli also insulated the shell of the vertical mill and installed a shutter valve at the outlet of the tail exhaust fan. When the mill is shut down, the inlet and outlet valves of the system are closed for thermal insulation operation. This measure further reduces heat loss and makes full preparations for the next startup.

Conclusion
By adjusting the size of the grinding disc surfacing and installing the scraper device, the stability of the material layer was successfully guaranteed, the vibration value of the vertical mill was significantly reduced, the hourly output was restored to 220 tons, and the cement power consumption was controlled within 26kWh/t. By improving the thermal efficiency of the system, the temperature of the cement out of the mill increased by 10℃ to 73℃, and the moisture content of the cement out of the mill was controlled within 0.35%, which basically achieved the expected goal. Through the above-mentioned transformation measures, the operating effect of the cement vertical mill without heat source has been close to the level of the cement vertical mill with drying heat source. Tongli vertical mill ensures the efficient operation and optimization of the entire system with its advanced technology, precise design and professional capabilities of the engineering team. Whether in terms of equipment durability, energy efficiency optimization, or technological innovation, Tongli vertical mill has demonstrated its outstanding professionalism in the field of cement production equipment, and deserves wide recognition and reference from peers and the industry.