Vertical Roller Mill Grinding Station

Vertical mills are mainly used to grind various raw materials, such as cement, raw materials, slag, coal, etc. It crushes materials through the interaction of rotating grinding discs and grinding rollers.

Vertical mills are widely used in the cement industry, especially for large-scale production. Taking grinding cement as an example, the grinding capacity of vertical mills is usually 80-120 tons per hour, and the energy consumption is 20-30% lower than that of traditional ball mills.

In addition, vertical mills are also used for grinding operations in industries such as slag micropowder and non-metallic minerals. Due to its high efficiency, energy saving and low maintenance cost, vertical mills have been widely used in industry.

Vertical mill grinding stations are known for their high efficiency and energy-saving performance. Compared with traditional ball mills, vertical mills usually reduce energy consumption by 30% to 50%. This is because vertical mills use vertical pressure and the rolling action of grinding rollers to make the grinding action of materials more concentrated and effective.

The pressure grinding process used by the vertical mill reduces unnecessary energy waste in the grinding process. At the same time, the compact structure of the vertical mill reduces the material transportation and circulation time, thereby further saving energy consumption.

In addition, the vertical mill also has a higher heat exchange efficiency and can better utilize the heat generated in the process. This not only reduces the operating cost of the equipment, but also reduces the heat emission to the environment. Overall, the vertical mill grinding station has significant advantages in energy saving and is an important choice for pursuing energy efficiency optimization in industrial production.

The requirements of the vertical mill grinding station for raw materials are mainly concentrated in terms of particle size, hardness and moisture. First, the particle size of the raw materials should be controlled within a certain range to ensure grinding efficiency. Too large particles may cause increased mill load and increased wear.

Secondly, the hardness of the raw materials directly affects the selection and wear of the mill. Generally speaking, the vertical mill is suitable for materials with medium and low hardness, and special design or selection of suitable wear materials are required for high hardness materials.

The moisture content of the raw materials is also very critical. Excessive moisture may cause agglomeration inside the mill, affecting material fluidity and mill efficiency. Usually, the moisture content of the material should be controlled between 1% and 2%. By controlling these characteristics of the raw materials, the efficient operation and product quality of the vertical mill grinding station can be guaranteed.

Vertical mill grinding stations are used for grinding a variety of materials due to their wide adaptability. The materials it can process include cement raw materials, cement clinker, slag, coal powder, limestone, etc.

The design of the vertical mill enables it to handle materials of different hardness and humidity. For example, in the cement industry, the vertical mill can effectively grind raw materials and clinker to improve the fineness and quality of cement.

In the steel industry, vertical mills are widely used for slag grinding to enhance its activity by increasing the fineness of slag.

In the power industry, vertical mills are used for coal powder preparation to improve combustion efficiency and reduce emissions. The chemical industry also often uses vertical mills to process various chemical raw materials and optimize the particle size distribution of products. Therefore, the vertical mill grinding station has become an ideal solution for grinding processes in various industries due to its versatility and efficiency.

Preventing wear is the key to ensure the long-term operation of vertical mill grinding station. First, choosing wear-resistant materials is an effective method, such as using high-chromium alloy steel to make grinding rollers and grinding discs.

Secondly, regular wear detection and maintenance, and timely replacement of worn parts to prevent wear expansion and equipment performance degradation. Adjust the pressure between the grinding roller and the grinding disc to maintain appropriate grinding force and reduce excessive wear on the equipment.

The improvement of the lubrication system can also effectively reduce wear. Regular inspection and replenishment of lubricating oil to ensure the smooth operation of all moving parts.

In addition, optimizing operating parameters and avoiding overload operation of equipment are also important means to reduce wear. Through these measures, wear can be significantly reduced and the service life of equipment can be increased.

Coping with raw material changes is one of the challenges in the operation of a vertical mill grinding station. First, the operating parameters of the mill, such as grinding pressure and grinding disc speed, are flexibly adjusted to adapt to the characteristics of different raw materials. The optimal process conditions for different raw materials are determined through experiments to ensure grinding efficiency and product quality.

The efficient grading system can be adjusted in time to ensure that the product particle size meets the requirements. The multifunctional design of the equipment can also handle raw materials of different types and specifications. Through these measures, the vertical mill grinding station can flexibly cope with raw material changes and maintain stable production capacity and product quality.

A Vertical Roller Mill (VRM) grinding station with a classifier is definitely a closed circuit system. The core structure of the vertical mill includes a grinding unit of "grinding disc + grinding roller" and a dynamic classifier (or static classifier) ​​on the top. After the material is crushed by the grinding roller, it is carried up to the classifier by the hot air at the bottom. The qualified fine powder is transported to the dust collector by the airflow for collection (product), and the unqualified coarse powder falls back to the grinding disc for re-grinding under the centrifugal force of the classifier blades.

This "grinding - classification - re-grinding" internal circulation mechanism itself is a typical feature of a closed-circuit system, which is essentially different from the open-circuit system of a ball mill (without built-in classification, and a separate external powder selector is required).

The closed-circuit system can make the vertical mill unaffected by natural factors such as weather, and can be arranged outdoors to reduce investment costs. In addition, in the mineral dry grinding and grading system, the vertical mill mineral dry grinding and grading system is also mostly composed of a closed-circuit hot air system, integrating drying and powder conveying functions.

The process flow are as follows:
1. Material pretreatment and feeding system: The particle size of the material entering the mill is usually ≤50mm (cement raw material) or ≤30mm (slag), and the moisture content is ≤8% (up to 15% when drying is required). The feeding adopts a quantitative belt scale or plate feeder, and the feeding amount is adjusted according to the diameter of the grinding disc. For example, the rated feeding amount of the Φ4.6m vertical mill is 200-250t/h (cement raw material grinding scene).

2. Grinding and drying: grinding disc and grinding roller assembly, grinding disc speed: 15-30rpm (the larger the diameter, the lower the speed), such as the Φ5.6m vertical mill speed is about 18rpm, and the material is thrown to the edge of the grinding disc by centrifugal force.
Grinding roller pressure: The hydraulic system provides a grinding pressure of 10-30MPa, and the pressure of a single grinding roller can reach 500-800kN (such as a total pressure of 3500kN for a 5-roller vertical mill), and the material is crushed between the grinding roller and the grinding disc liner.

3. Hot air system: The hot air temperature is 250-350℃ (cement raw material) when drying raw materials, and can reach 350-400℃ when drying slag. The air volume is matched according to the material processing capacity, such as 1t of material requires 1500-2000m³ hot air. Wind ring wind speed: 25-35m/s, to ensure that the material is evenly blown up by the air flow and forms a "material curtain", and the drying is completed at the same time (water evaporation rate 0.5-1.0kg water/m³ hot air).

4. Classification and product collection system: Dynamic classifier speed: 600-1200rpm, the product fineness is controlled by adjusting the blade angle (usually 15°-45°) and speed. For example: when producing P.O 42.5 cement, the classifier speed is 900rpm, and the product specific surface area reaches 350±10m²/kg. Classification efficiency: ≥85% (cutting particle size d50=15-25μm), the circulation amount of coarse powder falling back to the grinding disc is 150%-300% of the feeding amount (circulation load rate).

5. Dust collection and transportation: High-efficiency bag dust collector: filtration wind speed 0.8-1.2m/min, dust emission ≤30mg/m³, the collected fine powder is sent to the finished product warehouse through an air chute or screw conveyor.

1. Efficient grinding: In terms of grinding efficiency, it uses advanced material layer grinding principle, and applies pressure to the material layer through the grinding roller to achieve efficient crushing of materials. Compared with traditional ball mills, HRM series equipment has significant energy-saving effect, which can save 20-30% of energy consumption, and this energy-saving advantage becomes more obvious as the moisture content of raw materials increases. This is because material layer grinding can make more full use of energy and reduce ineffective losses. ​

2. Capable of handling high-humidity materials: HRM series equipment cleverly uses kiln waste heat and exhaust gas to build an efficient drying system. It can easily handle kiln raw materials with a moisture content of more than 15%, and complete the grinding process while drying.
Space saving: In terms of space utilization and process flow, the equipment integrates fine crushing, drying, grinding, powder selection, and transportation, and does not require additional drying, powder selection, and lifting equipment. According to professional calculations, its plant area is only 70% of the ball mill system, and the floor area is only 50-60% of the ball mill system. This not only saves land resources, but also simplifies the production process.

3. Quiet: In terms of environmental performance, the HRM series equipment also performs well. During operation, the grinding roller and the grinding disc are not in direct contact, avoiding the noise generated by metal collision. Its noise value is 20-25 decibels lower than that of the ball mill system, which effectively improves the working environment. At the same time, the entire system adopts a fully enclosed negative pressure operation mode to prevent dust spillage from the source, ensure a clean production environment, and meet strict environmental protection standards. ​

4. Low loss: From the perspective of product quality and cost control, the HRM series equipment has extremely low wear, with metal wear of only 5-10 grams per ton of product, which greatly reduces the metal pollution of the product and ensures the high quality of the product. In addition, the roller sleeve can be flipped for use. This innovative design effectively extends its service life, reduces the replacement frequency and maintenance cost of the equipment, and makes operation and maintenance more convenient. ​

5. Good product quality: The final grinding product has stable chemical composition and uniform particle size. This feature enables the product to fully react during the subsequent calcination process, improving the calcination quality and yield rate. With the above advantages, HRM series equipment has won the trust and choice of more than 3,000 customers and has become a highly respected high-quality equipment in the industry.

In addition to the vertical mill and its own powder selector, other equipment in the grinding station includes:

1. Feeding system
This system is responsible for evenly and stably conveying raw materials to the mill, and is usually composed of a quantitative feeder, a belt conveyor, and a bucket elevator. The VRM feeding system of a 3000t/d cement production line can feed 400~500t/h, and needs to be equipped with a material level sensor to monitor the hopper inventory in real time to avoid mill vibration or reduced grinding efficiency due to uneven feeding.

2. Air ring and hot air furnace system
The air ring is located around the grinding disc, and the ground material is brought into the powder selector through hot air. The source of hot air is mostly kiln tail flue gas or hot air furnace, and the temperature needs to be controlled at 200~350℃. The air volume is calculated according to the mill processing capacity. The hot air volume of a 5000t/d cement VRM is about 800,000~1 million m³/h. The wind speed of the wind ring is generally maintained at 60~80m/s to ensure that fine powder can be effectively carried and to avoid coarse particles settling and causing the wind ring to be blocked. The hot air furnace uses a boiling furnace, and the heat source can be coal powder or natural gas.

3. Hydraulic and lubrication system
The hydraulic system is used to drive the roller loading and rolling action. The working pressure is generally 12~16MPa, and the accumulator pressure needs to be maintained at 80%~90% of the system pressure. The lubrication system is divided into high-pressure oil lubrication and low-pressure oil lubrication. For example, the roller bearing adopts forced circulation lubrication, the oil temperature needs to be controlled at 40~60℃, the oil pressure is 0.1~0.3MPa, and the lubricating oil flow rate is determined according to the bearing specifications. For example, the flow rate of a bearing with a diameter of 500mm is about 10~15L/min to ensure that the bearing life exceeds 50,000 hours.

4. Dust removal and waste gas treatment system
The waste gas generated during VRM operation needs to be treated by a bag dust collector, and the dust removal efficiency must reach more than 99.9%, and the emission concentration must be ≤30mg/Nm³. For example, a dust collector with a processing air volume of 1 million m³/h has a filtration area of ​​about 15,000~20,000m². The filter bag material is mostly PTFE, with a temperature resistance of 180~220℃, and a pulse cleaning device is required. The cleaning pressure is 0.4~0.6MPa to ensure that the filter bag resistance is maintained at 1200~1500Pa.

5. Electric control and automation system
The system realizes full process monitoring through PLC, collects parameters such as mill vibration value ≤5mm/s, bearing temperature ≤80℃, and main motor current is 80%~90% of the rated current in real time, and adjusts the feed rate, hot air temperature and powder selector speed through the DCS system. The response time of the automation system should be ≤0.5 seconds to ensure that the mill operates under optimal conditions and reduce energy consumption. For example, the power consumption of cement grinding can be controlled at 28~35kWh/t.

To Answer this question we need to consider 3 things which is the capacity, model selection, factory size. the details are as follows:

First, the core parameters need to be calculated, and the production capacity is converted into hourly output. 1,000 tons/day corresponds to about 41.7 tons/hour, and 5,000 tons/day corresponds to about 208.3 tons/hour. At the same time, the grindability of the raw materials (such as the Bond work index of cement raw materials is usually 12~18 kWh/t) and the fineness of the finished product (such as the specific surface area of ​​cement 320~450 m²/kg) must be combined. When the grindability is poor, the hourly output needs to be multiplied by a correction factor of 0.8~0.9.

In the selection of vertical roller mill (VRM), a 1,000 t/d grinding station (taking cement raw meal as an example) usually uses a mill with a diameter of Φ3.0~3.5 m. The equipment with an effective grinding disc diameter of 3.0 m can process raw meal with an hourly output of about 50~60 tons (raw material moisture ≤15%), which can meet the demand based on 20 hours of operation. The number of grinding rollers is 3~4, the single loading force is 80~120 kN, the main motor power is 800~1200 kW, and the power consumption is controlled at 25~28 kWh/t. A 5,000 t/d grinding station (taking cement clinker grinding as an example) needs to use a large VRM of Φ4.5~5.0 m. For example, a 5.0 m diameter mill can produce 220~250 t/h (specific surface area 350 m²/kg) of clinker per unit. If it runs for 22 hours, it can meet the production capacity requirements. It has 4 grinding rollers, each with a loading force of 200~250 kN, a main motor power of 4000~5000 kW, and a power consumption of 32~35 kWh/t.

In terms of factory building and land planning, the main factory building of 1,000 tons/day has a size of approximately 40 meters × 30 meters × 45 meters, covers an area of ​​1,200 square meters, and the raw material shed covers an area of ​​2,000 square meters (pile height 10 meters, storage capacity 3 days); the main factory building of 5,000 tons/day has a size of approximately 60 meters × 40 meters × 60 meters, covers an area of ​​2,400 square meters, and the raw material shed covers an area of ​​5,000 square meters (pile height 15 meters, storage capacity 5 days). In terms of energy consumption and investment, the total installed power of 1,000 tons/day is about 1,800 to 2,200 kilowatts, the annual power consumption is 9 to 10 million kWh, and the investment cost is about 3.5 to 4.2 million US dollars; the total installed power of 5,000 tons/day is 8,000 to 10,000 kilowatts, the annual power consumption is 50 to 55 million kWh, and the investment cost is about 17 to 21 million US dollars.

1. Low grinding efficiency

Solution: When the grinding pressure is insufficient, check whether the hydraulic system pressure reaches the design value. Usually, the main roller pressure needs to be maintained at 8-12 MPa, and confirm whether the accumulator nitrogen pressure is attenuated, which is generally 60%-70% of the working pressure; if the gap between the grinding roller and the grinding disc is too large, exceeding 8-15 mm, it is necessary to restore the gap by adjusting the hydraulic cylinder stroke or replacing the worn liner. In addition, the mismatch between the powder selector speed and the system air volume will also affect the efficiency. It is necessary to ensure that the powder selector speed is within the range of 600-1200 r/min, and monitor the system air volume through an air volume meter. Usually, each ton of raw materials corresponds to 800-1200 m³/h air volume. If the air volume is insufficient, check the pipeline leakage or the wear of the fan impeller.

2. Equipment vibration

Solution: It may be caused by abnormal particle size distribution of materials in the mill. When the proportion of coarse particles larger than 50 mm in the feed exceeds 10%, it is easy to cause uneven force on the grinding roller. At this time, the crushing ratio of the crusher needs to be adjusted to control the particle size of the material entering the mill to less than 30 mm. If the grinding disc liner or the grinding roller sleeve wears unevenly, it will cause the center of gravity to shift. The worn parts need to be replaced in time and dynamic balance calibration needs to be performed. In addition, loose foundation bolts may also cause vibration. The bolt preload needs to be checked every quarter and anti-loosening glue should be used.

3. System air leakage

Solution: It will significantly affect the grinding efficiency and energy consumption. When the system negative pressure is detected to be lower than the design value of 4000-5000 Pa, it is necessary to focus on checking the status of the sealing device: the labyrinth seal gap between the grinding roller and the grinding disc should be maintained at 2-5 mm. If the wear exceeds the standard, the seal needs to be replaced; whether the flexible seal at the connection between the feed chute and the shell is aged and cracked, and at the same time, the air tightness test of the pipeline flange connection should be carried out. The leakage rate should be ≤0.5%. An ultrasonic leak detector can be used to locate the leak point and perform repair welding or replace the sealing gasket.

3. Finished product fineness does not meet the standard

Solution: It is related to the operating parameters of the powder selector. When the product specific surface area deviates from the set value, such as the target specific surface area of ​​the cement mill is 350-380 m²/kg, and the fluctuation exceeds ±15 m²/kg, it is necessary to first verify that the accuracy error of the powder selector speed sensor should be ≤±1%, and check the blade wear. If the blade edge wear exceeds 10 mm, it needs to be replaced; in addition, the system air volume fluctuation will affect the classification effect. It is necessary to maintain a stable air volume by adjusting the opening of the fan inlet damper, and at the same time ensure that the bag filter bag is clean and the pressure difference is controlled at 1200-1500 Pa to avoid air volume fluctuations due to bag blockage.

4. Equipment bearing overheating

Bearing failures are mostly related to abnormal lubrication systems. When the bearing temperature exceeds the alarm value, the allowable temperature of rolling bearings is ≤80℃, and that of sliding bearings is ≤70℃. It is necessary to check whether the lubricating oil brand meets the requirements. For example, the kinematic viscosity of N320 medium-load gear oil should be 288 - 352 mm²/s at 40℃, and whether the oil level is at 1/2 - 2/3 of the sight glass. If the oil is contaminated or aged, the acid value is ＞0.5 mgKOH/g, and the oil needs to be replaced. At the same time, check the cooling system flow rate. The cooling water pressure should be ≥0.2 MPa and the flow rate should be ≥5 m³/h.

5. Hydraulic system mal-function

Solution: Failure of the hydraulic system will lead to unstable grinding pressure. When the pressure fluctuation exceeds ±0.5 MPa, it is necessary to check the oil pump outlet pressure (rated pressure must reach 14-16 MPa). If the pressure is slowly built up, it may be that the oil level in the oil tank is too low (it should be kept above 2/3 of the level gauge) or the suction filter is clogged (need to be cleaned when the pressure difference is ≥0.15 MPa); leakage in the hydraulic cylinder will cause the pressure to be unable to be maintained. The pressure decay rate can be observed by closing the stop valve after boosting the pressure (the pressure drop within 10 minutes should be ≤0.3 MPa). If it exceeds the standard, the piston seal needs to be replaced (the hardness of the lip seal should be between 75-85 A on Shore A); if the accumulator pressure is insufficient, it needs to be refilled with nitrogen (the pressure value is 60% of the working pressure, and the error is ≤±0.2 MPa).

6. Feed chute blockage

Solution: It often occurs in the working condition of materials with high moisture content. When the feed volume suddenly drops and the pressure difference in the mill increases (exceeding the normal range by more than 500 Pa), it is necessary to check whether the moisture content of the material exceeds the design value (usually the moisture content is allowed to be ≤15%, and the sticky materials such as clay are ≤8%). If the moisture content exceeds the standard, pre-drying measures need to be added; at the same time, the crust on the inner wall of the chute should be cleaned (it is recommended to use wear-resistant ceramic lining to reduce crusting), and the amplitude of the vibrating feeder should be checked (the normal amplitude should be 3-5 mm). If the amplitude is insufficient, the angle of the eccentric block of the vibrator needs to be adjusted (usually the angle is 45°-60°). In addition, the chute inclination design needs to be ≥65°. If the actual inclination is insufficient, the chute structure needs to be modified to improve the fluidity of the material.

The power consumption of a complete VRM (vertical roller mill) grinding station depends on the nature of the grinding material, the product fineness requirements, the equipment specifications and the system configuration. It is worth noting that advanced process configuration can reduce power consumption. For example, the use of a closed-circuit system with an efficient powder selector, or a combined grinding process with pre-grinding equipment (such as a roller press) can reduce power consumption by 10% - 15%.

1. Taking cement raw material grinding as an example, when the processing capacity is 100 tons/hour (2400 tons/day), the comprehensive power consumption of the entire grinding station is usually 18-25 kWh/ton, of which the main mill accounts for about 60% - 70%, and the rest comes from auxiliary equipment such as fans, feeding equipment and dust collection systems. If calculated at 20 kWh/ton, the daily power consumption of a grinding station of this scale is about 48,000-60,000 kWh, and the annual power consumption (based on 300-day operation) can reach 14.4 million-18 million kWh.

2. For difficult-to-grind materials such as slag powder, the power consumption of grinding will increase significantly. For example, when processing 50 tons/hour or 1,200 tons/day of slag, the system's comprehensive power consumption may reach 40-50 kWh/ton. The main mill power is usually configured to be 3,000-4,000 kW, plus the supporting equipment such as 800 kW of powder selector and 1,200 kW of fan, the total installed power can reach 5,000-6,000 kW, and the daily power consumption is about 48,000-60,000 kWh, and the annual power consumption is about 14.4 million-18 million kWh, which is about twice as high as raw material grinding.

3. In the cement clinker final grinding scenario, the power consumption index is even higher. Taking a large-scale VRM system of 200 tons/hour (4,800 tons/day) as an example, if the product specific surface area requirement reaches 350-400 square meters/kg, the comprehensive power consumption is usually 32-40 kWh/ton, the main mill power is mostly 8,000-10,000 kW, the total installed power of the supporting system can reach 12,000-15,000 kW, the daily power consumption is about 153,600-192,000 kWh, and the annual power consumption (300 days) reaches 46.08 million-57.6 million kWh. At this time, the proportion of fan power consumption will increase to 30%-40%, mainly used to maintain the gas-solid two-phase flow transportation and classification efficiency in the system.

Conclusion: For large cement production lines with an annual output of more than 600,000 tons, VRM has significant advantages in energy consumption, land occupation and long-term operating costs, while the flexibility and low initial investment of ball mills are still attractive for small and medium-sized production lines or special material scenarios. However, ball mills have a slight advantage in raw material adaptability, especially for highly abrasive materials. When processing high-siliceous clinker (SiO₂ content > 25%), the wear rate of VRM grinding rollers will increase to 0.15-0.2mm/kt, while the wear of ball mill steel balls will only increase by 0.05-0.1mm/kt. However, by adopting high-chromium cast iron liners and ceramic-coated grinding rollers, modern VRMs can adapt to most clinker conditions. If we discuss it in detail then we can separate it in 4 dimensions:

1. Grinding efficiency

From the perspective of grinding efficiency, VRM has significantly higher energy utilization than ball mill due to its material bed grinding principle. Taking the production of cement with a specific surface area of ​​350-400㎡/kg as an example, the comprehensive power consumption of the VRM system is usually 32-40 kWh/ton, while the power consumption of the traditional ball mill open circuit system can reach 50-60 kWh/ton, and the closed circuit system also requires 45-55 kWh/ton. The power consumption advantage of VRM is 20%-30%. This is because the material is subjected to the dual effects of rolling and shearing during the VRM grinding process, and the energy is directly used for particle crushing, while the ball mill relies on steel ball impact and grinding, and a large amount of energy is consumed in mechanical friction and cylinder rotation.

2. Energy consumption

In addition to energy consumption, VRM has more advantages in system integration. It integrates grinding, drying, and powder selection, and can use the waste heat at the end of the kiln to dry materials with a moisture content of ≤8%, reducing the investment and energy consumption of independent drying equipment. Taking a 5,000 t/d production line as an example, the VRM system can save about $2.74 million in drying equipment investment (converted at $1 ≈ 7.3 RMB, 20 million RMB ≈ $2.74 million), and the waste heat utilization reduces the heat consumption by 150,000-200,000 kcal/hour. The ball mill needs to be equipped with an independent powder selector and drying equipment, and the system occupies 30%-50% more area than the VRM. For example, a 4.2×13 m ball mill grinding station occupies about 2,500 square meters, while the VRM with the same capacity only requires 1,800 square meters.

3. Price:

In terms of investment cost, the initial investment of VRM is higher. The investment for a VRM grinding station with an annual output of 1 million tons of cement clinker is about 16.44 million to 20.55 million US dollars, while the ball mill system is about 10.96 million to 13.7 million US dollars. The investment for VRM is 20% to 30% higher. However, in the long run, the low energy consumption and low maintenance cost of VRM can shorten the payback period. The life of its grinding roller and grinding disc lining is usually 8000-10000 hours, and the replacement cost is about 2.05-2.74 US dollars/ton, while the ball consumption of the ball mill is 0.8-1.2 kg/ton. Based on the unit price of steel balls of 822 US dollars/ton (6000 yuan ≈ 822 US dollars), the cost per ton is 0.66-0.99 US dollars (4.8-7.2 yuan ≈ 0.66-0.99 US dollars). Together with the cost of lining replacement, the maintenance cost of the ball mill is 30%-40% higher than that of VRM.

4. Product quality:

In terms of product quality control, VRM's dynamic powder selection system can accurately adjust the product fineness, and the specific surface area fluctuation range is ≤±10㎡/kg, while the ball mill open circuit system fluctuates up to ±20㎡/kg. Although the closed circuit system is improved through cyclic load adjustment, there is still the problem of uneven particle grading. In addition, the cement particles ground by VRM have higher sphericity and reduce water demand by 5% - 8%, which is more beneficial to the working performance of concrete. A project case shows that when using VRM-ground P.O 42.5 cement to prepare C30 concrete, the water consumption can be reduced by 10 - 15kg/m³ while maintaining a slump of 180mm.

Taking the VRM grinding station with an annual output of 1 million tons of cement clinker as an example, the overall installation takes about 6-12 months, and the specific stages are as follows:

1. Preliminary preparation stage (1-2 months)

Complete the site leveling and foundation pouring drawing design. According to the load requirements of the VRM host (such as the 4.5-meter diameter grinding disc) and supporting equipment (hot air furnace, powder selector, fan), pour a reinforced concrete foundation with a thickness of ≥2 meters, and the maintenance period is not less than 28 days.
Equipment order and arrival coordination, core components such as grinding rollers and grinding disc liners need to be customized 3 months in advance, and the transportation cycle is about 2-4 weeks.

2. Main installation stage (3-5 months)
Construct in the order of "from bottom to top, first the main machine and then the auxiliary machine": first hoist the grinding disc base and calibrate the horizontality (error ≤0.5mm/m), then install the grinding roller bracket and hydraulic loading system to ensure that the parallelism deviation between the grinding roller and the grinding disc is ≤1mm. The installation of hot air ducts and powder selectors requires the flatness of flange sealing surfaces to avoid air leakage (designed air leakage rate ≤5%), and the thickness of the pipe insulation layer ≥100mm to reduce heat loss.

3. Installation of electrical and automation systems (2-3 months)
Laying high-voltage cables (such as 10kV incoming lines) and control cabinets, the configuration programming of the DCS system (distributed control system) must match the equipment action logic, for example, the interlocking time between the lifting of the grinding roller and the start of the main motor is set to ≤3 seconds. The installation of sensors (such as temperature, pressure, and vibration sensors) requires calibration accuracy, the temperature sensor (PT100) error ≤±1℃, and the vibration sensor (acceleration type) range must cover 0-50mm/s.

4. Installation of auxiliary systems (1-2 months)
When installing the lubricating oil station, the cleanliness of the pipeline must be ensured (NAS 8-level standard), and the filter element accuracy must be ≤25μm; the oil contamination of the hydraulic system must reach ISO 18/15 level. The filter bags of the dust collection system (bag dust collector) must be installed to prevent wrinkles, the filter bag material (PTFE) must be temperature resistant ≥ 200°C, and the filtration wind speed must be controlled at 0.8-1.2m/min.

Then the debugging process is divided into three stages: single machine test run, linkage test run, and material test run. The total time is about 1-2 months. The core steps are as follows:

1. Single machine test run (10-15 days)
Main motor idling test: continuous operation for 8 hours, monitoring bearing temperature (≤70℃), vibration value (≤4.5mm/s), current fluctuation range ≤±5% rated current. Grinding roller lifting system test: The hydraulic station pressure must be stable at 12-14MPa, the time for the grinding roller to rise from the lowest position to the highest position is controlled at 45-60 seconds, and the action is repeated 5 times without stagnation. Powder selector rotor dynamic balance: no-load speed increases from 0 to 1200rpm (design speed), vibration amplitude ≤0.05mm, noise ≤85dB (A).

2. Linkage test (15-20 days)
Hot air system linkage: start the hot air furnace (fuel is coal powder or natural gas), raise the hot air temperature to 350-400℃, control the pipeline wind speed to 18-22m/s, and ensure that the moisture content of the material in the drying section is reduced from 8% to below 1%. Feeding and grinding system interlocking: start in the order of "first start the fan and then start the feeding", and gradually increase the feeding amount from 30% of the rated load (such as 150 tons/hour), and monitor the linear relationship between the main motor power growth and the feeding amount (deviation ≤10%). Dust collection system test: When the system is running at full load, the outlet dust concentration must be ≤30mg/Nm³, the induced draft fan current fluctuation is ≤±8%, and the bag chamber pressure difference is stable at 1.2-1.5kPa.

3. Test run with materials and performance assessment (20-30 days)
Full-load continuous operation: Continuously run for 72 hours according to the designed capacity (such as 200 tons/hour), test key indicators: Grinding power consumption ≤40kWh/ton (specific surface area 350㎡/kg); Product fineness qualification rate ≥95% (screen residue ≤1.5%, 0.08mm square hole screen); Equipment operation rate ≥95%, wear of main components (grinding roller, lining plate) ≤0.1mm/thousand tons of material. Heat balance test: When using the waste heat at the kiln tail, the system thermal efficiency must be ≥75%, the amount of fuel added to the hot air furnace ≤30% of the design value, and the tail gas temperature is controlled at 80-100℃ to reduce heat loss.

1. Bottom control layer (PLC):
The hardware configuration uses redundant PLC controllers such as Siemens S7-1500 and ABB AC 800M, which are deployed in the field control cabinet and directly connected to sensors and actuators. Typical control objects include roller lifting hydraulic system, main motor drive device, feeding belt scale, powder selection machine inverter, etc. Core functions: real-time collection of field data (such as mill vibration value, bearing temperature, material flow), execution of logical control (such as equipment start and stop sequence, fault chain protection), and communication with the upper system through industrial buses such as PROFINET and EtherCAT.

2. Middle monitoring layer (SCADA)
Software platform: WinCC, Wonderware InTouch and other SCADA systems are selected to build a human-machine interaction interface (HMI) to achieve full-process visual monitoring. Key functions: Dynamically display process flow charts (such as material flow direction, equipment operation status), support real-time curve drawing of parameters such as roller pressure, grinding temperature, and powder selection efficiency; integrate alarm management system, automatically trigger sound and light alarms for abnormal working conditions (such as mill overload, lubrication system failure), and record historical data for fault tracing; provide data reports and trend analysis, support automatic statistics of production indicators such as shift/day/month output and power consumption.

3. Upper management layer (DCS)
System integration: DCS, as the central control hub, integrates SCADA data through the OPC UA protocol to achieve production scheduling for the entire plant. Optimization control: Based on the model predictive control (MPC) algorithm, the feed rate, grinding pressure and powder concentrator speed are dynamically adjusted to maintain a stable mill load (such as controlling the material bed thickness at 50-80mm) to avoid "empty grinding" or "full grinding"; the kiln tail waste heat system is linked to automatically adjust the drying hot air temperature (usually controlled at 250-350℃) according to the moisture content of the material to optimize energy consumption matching; the enterprise MES system is connected to upload production data (such as hourly output and product fineness), receive order scheduling instructions, and realize "one-click production change".

Functions of the central control system:

1. Material bed stability control
The pressure sensor monitors the roller loading force (usually 50-100kN) in real time, and the PLC performs PID adjustment of the feeding amount in combination with the weighing bin material level signal to ensure uniform material bed thickness. When the mill vibration value exceeds the threshold (such as 5mm/s), the system automatically reduces the loading force and increases water spray (cooling the material) to prevent vibration from tripping.

2. Grinding efficiency optimization
The SCADA system collects the dust concentration of the powder selector outlet gas and the product specific surface area data, and adjusts the powder selector speed (typical range 150-300rpm) through the fuzzy control algorithm to stabilize the specific surface area at 350-400㎡/kg, with a fluctuation of ≤±10㎡/kg.

3. Intelligent energy consumption management
DCS integrates the power data of each motor (such as the main motor and fan power consumption), and automatically switches the operating mode in combination with the production load: start the "economic mode" at low load, reduce the fan frequency (such as from 50Hz to 40Hz), and maintain grinding efficiency at the same time. A case study shows that the automation system can reduce the overall power consumption by 5-8 kWh/ton.

4. Safety interlock design
Establish a three-level protection mechanism: PLC performs equipment-level interlocking (such as automatic grinding stop when the lubrication system fails), SCADA implements process-level early warning (such as starting inert gas protection when CO concentration exceeds the standard), and DCS coordinates plant-level emergency response (such as interlocking the fire protection system in case of fire).

The influence of material characteristics on grinding efficiency can be expanded from the following 7 aspects:

1. Material particle size
The influence of material particle size on the performance of VRM grinding station is reflected in crushing efficiency and equipment load. When the proportion of particles larger than 50 mm in the feed exceeds 10%, the grinding roller needs to apply greater pressure (such as from the normal 80 kN to 120 kN) to complete the crushing, which will cause the main motor current to increase by 15%-20% and the power consumption to increase by about 5-8 kWh/ton. At the same time, the uneven distribution of large pieces of material on the grinding disc can easily cause eccentric grinding of the grinding roller. In one case, when the feed particle size exceeds the standard, the wear rate of the grinding roller liner increases from 0.1 mm/kt to 0.15 mm/kt, and the equipment maintenance cost increases by 30%. Therefore, by controlling the feed particle size below 30 mm (90% pass rate) through pre-crushing, the VRM processing capacity can be increased by 10%-15% and the risk of vibration failure can be reduced.

2. Grindability
The grindability of materials directly determines the energy consumption and output of grinding. Taking the Bond work index as an example, when the material work index increases from 12 kWh/ton to 18 kWh/ton (such as high-silica clinker), the VRM's grinding power consumption will increase from 32 kWh/ton to more than 40 kWh/ton, and the output per unit will decrease by about 20%. This is because difficult-to-grind materials require higher grinding pressure (such as increasing the roller loading force from 200 kN to 250 kN), and the powder selector needs to increase the speed (from 200 rpm to 250 rpm) to ensure product fineness, resulting in an increase in the overall energy consumption of the system. In actual production, by mixing easy-to-grind materials (such as adding 3%-5% gypsum), the work index can be reduced by 10%-15%, effectively improving the grinding efficiency.

3. Moisture content
The influence of material moisture on VRM is dual: 3%-5% moisture content helps to form a stable material bed. At this time, hot air only needs to reduce the material moisture content from 8% to below 1%, with a heat consumption of about 150,000 kcal/hour; when the moisture content exceeds 8%, the material will stick to the edge of the grinding disc, forming a mud cake with a thickness of 100 mm, causing the wind ring wind speed to drop from 30 m/s to below 20 m/s, and the fine powder cannot be effectively carried. The system cycle load rate increases from 200% to 350%, and the hourly output decreases by 30%. In a slag grinding case, after the feed moisture content was reduced from 10% to 5%, the VRM processing capacity increased from 150 tons/hour to 200 tons/hour, and the power consumption decreased by 6 kWh/ton.

4. Particle size
The particle size distribution of the material affects the porosity and compactness of the material bed. When the proportion of 5-30 mm particles reaches 70%, the stacking density of the material bed on the grinding disc is about 1.4 tons/cubic meter, and the energy transfer efficiency is the highest when the grinding roller is rolling; if the particle size is concentrated in 20-30 mm (accounting for more than 80%), the porosity of the material bed is too large, the pressure of the grinding roller is difficult to be effectively transmitted, and the grinding energy consumption increases by about 12%. Optimizing particle size distribution can be achieved through multi-stage crushing. For example, after a cement production line adopts the "jaw crusher + cone crusher" process, the proportion of 5-30 mm particles in the feed increases from 55% to 75%, and the grinding power consumption of VRM decreases by 3.5 kWh/ton.

5. Composition
The chemical composition and mineral composition of the material affect the equipment life through hardness and abrasiveness. The microhardness of high-calcium clinker (C3S content > 60%) is about 5-6 GPa, while the hardness of high-magnesium materials (MgO>4%) can reach 7-8 GPa, which will increase the wear rate of the grinding roller lining from 0.1 mm/kt to 0.2 mm/kt and shorten the replacement cycle from 10,000 hours to 5,000 hours. In addition, low-melting-point minerals (such as alkali metal oxide content > 1.5%) are easily melted by hot air above 300°C and adhere to the powder concentrator blades, reducing the classification efficiency from 85% to 70%, resulting in a fluctuation of the finished product fineness of more than ±20 m2/kg.

6. Bulk density
The bulk density of the material affects the stability of the grinding disc layer. Materials with a bulk density lower than 1.2 tons/m3 (such as fly ash content exceeding 30%) are easily blown up by the airflow on the grinding disc, causing the material layer thickness to be reduced from the normal 50 mm to less than 30 mm, and the grinding efficiency to drop by 15%; when the bulk density exceeds 1.8 tons/m3 (such as metal tailings), the grinding disc drive torque needs to be increased by 20%, the main motor current exceeds 90% of the rated value, and long-term operation will cause the bearing temperature to rise to above 80°C. Therefore, it is necessary to control the bulk density within the range of 1.3-1.6 tons/m3 through ingredient adjustment.

7. Impurities
Impurities and foreign matter have a sudden impact on VRM. Metal blocks with a diameter of more than 50 mm entering the mill will directly hit the surface of the roller, causing partial peeling of the lining (the area can reach more than 100 square centimeters), triggering violent vibration of the equipment (acceleration exceeds 50 mm/s²); fibrous impurities entangled in the roller seal will increase the sealing gap from the normal 2-5 mm to more than 10 mm, causing the system air leakage rate to increase from 5% to 15%, the hot air temperature to drop by 50℃, and the drying capacity to decrease by 20%. Therefore, it is necessary to set a metal detector (sensitivity ≥Φ10 mm) and an iron remover (magnetic field strength ≥15000 Gauss) at the feed end, and regularly clean the fiber impurities in the chute.

There are different approachs to achieve this goal, and here we are going to list 14 suggestions for your referenece:

1. Optimization of raw material particle size and moisture content: Ensure that the particle size of the material entering the mill is ≤50mm (ideal value ≤30mm) to avoid excessively large particles increasing the grinding load; control the moisture content to ≤1.5% (if the moisture content is too high, it is necessary to strengthen the utilization of waste heat at the end of the kiln or adjust the drying system) to prevent the material from adhering to the grinding disc and causing a decrease in grinding efficiency.

2. Raw material homogenization management: Through the pre-homogenization yard or the matching feeding device, ensure that the composition (such as limestone, clay, etc.) and particle size distribution of the raw materials entering the mill are uniform, reduce the sudden change of the mill load caused by the fluctuation of the material properties, and reduce the ineffective power consumption.

3. Matching of grinding pressure and speed: Dynamically adjust the roller pressure (usually 10-15MPa) according to the grindability of the material (such as the Bond work index), and avoid excessive pressure to increase the mechanical friction energy consumption while ensuring the grinding efficiency; at the same time, optimize the grinding disc speed (usually 20-30rpm) to achieve the best balance between the residence time of the material on the grinding disc and the grinding efficiency.

4. Improve the efficiency of the powder selector: By adjusting the speed of the dynamic powder selector (commonly 600-1000rpm) and the blade angle, the specific surface area of ​​the product (such as 350-400㎡/kg) is accurately controlled to reduce the phenomenon of over-grinding; at the same time, the accumulated material inside the powder selector is cleaned to avoid rotor blockage, which leads to increased circulation load and increased power consumption.

5. Optimization of air ring and air volume: Check the wear of the air ring, replace the worn parts in time (wear ≤10%), ensure that the wind speed is stable at 40-60m/s, so that the material forms a uniform suspension state in the mill, and improve the gas-solid heat exchange and grinding efficiency; at the same time, adjust the system air volume (usually 1000-1500m³/h・t) according to the mill load to avoid excessive air volume and increase the power consumption of the fan.

6. Residual heat deep recovery: optimize the waste heat flue gas introduction system at the kiln tail to ensure that the flue gas temperature is ≥300℃, improve the material drying efficiency, and reduce the additional energy consumption in the drying process; at the same time, a flue gas heat exchanger can be added to reheat the hot air entering the mill to reduce the heat loss of the system.

7. High-voltage motor and frequency conversion control: upgrade the main motor to a high-efficiency permanent magnet synchronous motor (efficiency ≥96%), and install frequency converters on auxiliary equipment such as fans and feeders to dynamically adjust the speed according to the real-time load (such as the fan frequency conversion range of 50%-100%) to avoid the "big horse pulling a small cart" phenomenon, which is expected to reduce power consumption by 5%-8%.

8. DCS system intelligent optimization: optimize the control system parameters through machine learning algorithms, establish a dynamic model of power consumption, raw material properties, and equipment parameters (such as training prediction models based on historical data), realize real-time coordinated adjustment of grinding pressure, air volume, and feed amount, and reduce parameter fluctuations caused by manual intervention.

9. Maintenance of grinding roller and grinding disc lining: Regularly check the wear degree of lining (replace when the wear depth is ≤30%), ensure the flatness of the grinding surface, and avoid the reduction of grinding efficiency due to uneven wear of the lining; at the same time, optimize the lining material (such as high chromium cast iron), extend the service life to more than 10,000 hours, and reduce the frequency of shutdown and replacement.

10. System leakage control: Seal the mill housing, pipeline interface and other parts (such as installing silicone sealing rings), control the system leakage rate to ≤5%, and avoid air volume loss and increased fan load due to leakage.

11. Optimization of open and closed systems: If the current system is a closed system, the material circulation energy consumption can be reduced by increasing the diameter of the powder selector return pipe (such as from DN300 to DN350) and reducing the circulation load rate to 150%-200%; if it is an open system, consider adding a high-efficiency static powder selector to improve the product fineness control accuracy and reduce over-grinding.

12. Capacity and load matching: Avoid the mill running at low load (<70% rated capacity) for a long time, and adjust the feed rate to keep the mill power at 85%-90% of the rated value to ensure maximum energy utilization.

13. Install energy recovery device: Add a hydraulic coupling or permanent magnet speed regulator at the fan outlet to recover the excess kinetic energy of the system; or install a synchronous compensator at the high-voltage motor end to increase the power factor to above 0.95 and reduce reactive power loss.

14. Introduce new grinding aids: Add 0.1%-0.3% of high-efficiency grinding aids (such as triethanolamine) to the raw materials to improve the grindability of the material and reduce the grinding energy barrier. It is expected to reduce power consumption by 3%-5%. At the same time, attention should be paid to the impact of grinding aids on cement performance.

So let us explain how the vrm grinding station connect with the upstream pyro-line:

1. The vertical mill grinding station needs to dynamically match the clinker output of the kiln line
The clinker outflow, temperature and chemical composition data (such as CaO and SiO₂ content) of the kiln line are collected in real time through the DCS (distributed control system) and transmitted to the vertical mill central control system. For example, when the clinker output of the kiln line fluctuates (such as the increase or decrease of clinker generation due to changes in kiln temperature), the vertical mill needs to adjust the feed amount synchronously to avoid excessive clinker storage or material shortage upstream. Automatic adjustment can be achieved by establishing a "kiln line - vertical mill" capacity mathematical model (such as a feed amount calculation formula based on the clinker grindability index).

2. Cooperative optimization of process parameters
The calcination state of the kiln line directly affects the physical properties of the clinker (such as particle grading and crystallinity), which in turn affects the grinding efficiency of the vertical mill. For example, when the kiln line clinker is overburned and the hardness increases, the grinding pressure of the vertical mill, the speed of the powder selector and other parameters need to be adjusted synchronously. A parameter linkage mechanism can be established through SCADA (Supervisory Control and Data Acquisition): When the free calcium oxide (f-CaO) content of the kiln line clinker exceeds 1.5%, the system automatically increases the grinding pressure of the vertical mill by 5%-8% and reduces the feed rate by 3%-5% to maintain stable grinding efficiency.

3. Buffer inventory and fault linkage
A clinker buffer bin (usually with a capacity of 2-4 hours of processing capacity of the vertical mill) is set between the kiln line and the vertical mill. When the kiln line is temporarily shut down, the buffer bin can maintain the operation of the vertical mill for a short time; at the same time, the vertical mill central control system needs to establish a fault interlock with the kiln line PLC (Programmable Logic Controller): If the kiln line is shut down for more than 30 minutes, the vertical mill automatically enters the low-speed operation mode to avoid idle grinding wear.

1. TONGLI vertical roller mill Equipment models: including HRM30/2, HRM34/3, HRM42/4, HRM44/4, HRM50/4, HRM53/4, HRM56/4, HRM60/4, HRM65/6.

2. Grinding disc diameter (mm): Different models correspond to different grinding disc diameters, HRM30/2 is 2500mm, HRM34/3 is 2800mm, HRM42/4 is 3400mm, HRM44/4 is 3700mm, HRM50/4 is 4200mm, HRM53/4 is 4500mm, HRM56/4 is 4800mm, HRM60/4 is 5100mm, HRM65/6 is 5600mm.

3. Output (t/h): The output range of each model is different, HRM30/2 is 85-100t/h, HRM34/3 is 130-160t/h, HRM42/4 is 190-240t/h, HRM44/4 is 240-300t/h, HRM50/4 is 320-400t/h, HRM53/4 is 400-500t/h, HRM56/4 is 440-550t/h, HRM60/4 is 550-670t/h, HRM65/6 is 600-730t/h.

4. Raw material moisture (%): There are two relevant data, one is <8-10%, and the other is ≤1%.
Raw material fineness: R0.08 <14% (80 micron sieve) is required.

5. Main motor power (kW): Different models correspond to different power ranges, HRM30/2 is 800/900kW, HRM34/3 is 1120/1250kW, HRM42/4 is 1800/2000kW, HRM44/4 is 2500/2800kW, HRM50/4 is 3150/3350kW, HRM53/4 is 3800/4200kW, HRM56/4 is 4200/4500kW, HRM60/4 is 5000/5400kW, HRM65/6 is 5600/6000kW.

Process flow is different than working principle, so the mechanism of a vertical roller mill grinding station are as follows:

1. Vertical mill grinding and energy transfer mechanism
The material forms a 30-50mm thick material layer on the grinding disc, and the grinding roller rolls the material layer through "surface contact", with an energy utilization rate of 15%-20% (much higher than the 3%-5% of the ball mill). Taking the Φ4.8m vertical mill as an example, the unit power consumption when grinding cement raw materials is 18-22kWh/t, which is more than 30% lower than that of the ball mill.

2. Principle of gas-solid two-phase flow classification of finished products of vertical mill
When the hot air carries the material particles upward, the aerodynamic force (F=0.5ρv²ACd, ρ is air density, v is wind speed, A is the windward area of ​​the particles, and Cd is the resistance coefficient) is balanced with gravity, and the separation of coarse and fine particles is achieved: fine particles (d＜20μm): aerodynamic force＞gravity, are brought into the classifier; coarse particles (d＞45μm): gravity＞aerodynamic force, fall back to the grinding disc for re-grinding.
Typical case: In a slag vertical mill system, when the hot air speed is 30m/s, the fall rate of particles above 50μm reaches 90%, ensuring that the content of particles ≥45μm in the product is less than 5%.

3. Synergistic mechanism of vertical mill drying and grinding
Heat exchange efficiency: the material is suspended in the airflow, and the contact area with the hot air is 100-200m²/kg, the heat exchange coefficient is 50-80W/(m²・K), and the water evaporation rate can reach 0.8kg water/(kW・h) (better than 0.5kg water/(kW・h) of the traditional dryer). Process data: when the moisture content of the slag entering the mill is 10%, the moisture content can be reduced to ≤0.5% in the mill through 350℃ hot air (air volume 1800m³/t material), meeting the storage requirements of finished products.

The disadvantage of vertical roller mill can be summarized as price, complexity, maintenance:

1. High initial investment cost
The manufacturing process of the main equipment of the vertical mill (such as grinding discs, grinding rollers, and powder concentrators) is complex and requires the use of high-wear-resistant alloy materials (such as high-chromium cast iron). The cost of a single set of equipment is 30%-50% higher than that of a ball mill. Taking a 300t/h cement vertical mill as an example, the equipment procurement and installation costs can reach US$3 million to US$4.5 million. The investment in supporting auxiliary facilities such as the DCS control system and hot air furnace is also significantly higher than that of traditional grinding systems, making it difficult for small and medium-sized enterprises to afford it.

2. Sensitive to material properties, poor adaptability
When the proportion of coarse particles with a particle size of >50mm in the feed exceeds 5%, it is easy to cause local wear of the grinding disc lining to intensify, and even cause vibration to stop. For example, a 250t/h vertical mill was shut down for maintenance due to mixing with kiln skin blocks and the vibration value of the grinding roller exceeded 4.5mm/s); when the moisture content of the material exceeds 15%, it is easy to cause paste in the mill (such as adhesion in the gap between the grinding roller and the grinding disc), and it is necessary to increase the energy consumption of the hot air furnace to dry it. When the moisture content is >20%, it may not even be able to grind normally (traditional ball mills can process materials with a moisture content of ≤25%); for high-hardness materials (such as sandstone with a quartz content of >20%), the grinding efficiency of the vertical mill is significantly reduced, and the power consumption can rise to more than 35kWh/t (the ball mill can still maintain production capacity by extending the grinding time).

3. Equipment maintenance is difficult and downtime costs are high
Special hydraulic tools are required to replace the linings of the grinding rollers and grinding discs. A single overhaul takes 8-12 hours, and the life of wear parts is usually 8000-10000 hours (the life of the ball mill lining can reach 15000 hours), and the annual maintenance cost increases by 15%-20%;
The rotor bearings of the powder selector need to be oil-cooled regularly. If the cooling system fails, it is easy to cause speed fluctuations, affecting the qualified rate of finished product fineness (for example, in a case, the overheating of the powder selector bearing caused the cement specific surface area to fluctuate by more than ±50m²/kg);
When shutting down for maintenance, the material in the mill needs to be emptied (about 50-80 tons), and it takes 2-3 hours to restart to adjust the process parameters, resulting in production capacity loss (the ball mill can be started and stopped with material, and the restart time is ≤1 hour).

But to be fair, ball mill and roller press also have drawbacks, therefore the owner shoud carefully evaluate which grinding equipment suit you the most.

The most important disadvantge in my opinion is because the finished product is completely produced by extrusion, the particle gradation is not ideal, and the particle surface shape is not conducive to the hydration and strength of cement. But at the same time since there are so many company using vertical roller mill to grind cement so as a owner you can feel safe that these disadvantages are minor, otherwise those big cement group will not be using vertical roller mill.

1. The control range of finished product particle grading is narrow
The vertical mill adjusts the fineness through the speed of the powder selector, but the particle grading distribution is more concentrated than that of the ball mill:
When producing P.O 42.5 cement, the 3-32μm particles in the vertical mill product account for about 65%-70% (the ball mill can reach 70%-75%), and the content of coarse particles (＞65μm) is relatively high (about 8%-10% for the vertical mill and about 5%-7% for the ball mill), which may affect the early strength of the cement;
For special cement (such as oil well cement requires particles concentrated in 10-20μm), it is difficult for the vertical mill to meet the requirements through process adjustment, and secondary grinding equipment (such as roller press) is required, which increases the complexity of the system.

2. High product temperature
During the vertical mill grinding process, the material is subjected to grinding and hot air, and the finished product temperature can reach 100-120℃ (the finished product temperature of the ball mill is ≤80℃), which may cause the following problems:
Gypsum dehydration (when the temperature is greater than 95℃, dihydrate gypsum dehydrates into hemihydrate gypsum), resulting in abnormal cement setting time (in one case, the initial setting time of cement was shortened from 45 minutes to 28 minutes);
The packaging bag is damaged due to high temperature aging, especially when stored in summer, the strength of the packaging bag decreases by more than 30%, increasing the bag breakage rate (the vertical mill cement bag breakage rate is about 0.5%-1%, and the ball mill is about 0.2%-0.5%).

3. Low activity of micro powder affects concrete performance
During the grinding process of vertical mill, the material is subjected to rolling, and the particles are mostly angular and have low surface energy:
Comparative tests show that the 28-day compressive strength of C30 concrete prepared with vertical mill cement is 3-5MPa lower than that of ball mill cement (hydration degree is 5%-8% lower at the same age);
When used for premixed concrete, the water demand of vertical mill cement increases by 5%-8%, which can easily lead to an increase in the slump loss of concrete over time (loss in 1 hour > 30mm), and an additional amount of water reducer needs to be used.