A Technical Guide To The Grinding Pressure Control In Vertical Mills

A Technical Guide To The Grinding Pressure Control In Vertical Mills

Vertical mills have become the industry standard for raw material, coal, and cement grinding in modern processing plants. Their energy efficiency, compact design, and ability to handle moist materials make them superior to traditional ball mills in many applications. Among the critical operational parameters that determine the performance, product quality, and energy consumption of a vertical mill, grinding pressure stands out as one of the most significant. This guide provides a comprehensive technical overview of grinding pressure control, its impact on the milling process, and best practices for optimization.

1. The Fundamental Role of Grinding Pressure

In a vertical mill, the material bed between the grinding table and the rollers is subjected to a specific force, known as the grinding pressure. This pressure is generated by a hydraulic system that applies force to the rollers, pressing them onto the material bed. The primary functions of this pressure are:

• Comminution: To exert sufficient compressive and shear forces on the particle bed to cause particle breakage.
• Bed Stability: To maintain a stable and consistent material bed, which is crucial for efficient grinding and smooth operation.
• Power Transmission: To translate the rotational energy of the table into grinding work.

An optimal grinding pressure ensures that the energy consumed is used effectively for particle size reduction rather than being wasted on excessive wear or vibration.

2. Key Factors Influencing Optimal Grinding Pressure

Determining the correct grinding pressure is not a one-size-fits-all calculation. It depends on a complex interplay of several factors:

• Material Grindability (Hardgrove Index): Harder materials generally require higher grinding pressures to achieve the same degree of comminution as softer materials.
• Feed Material Size and Moisture: Coarser or wetter feed may require adjustments in pressure to maintain a stable bed and prevent roller slippage.
• Required Product Fineness: A finer product often necessitates a higher specific grinding force to increase the number of particle fractures.
• Mill Throughput (Capacity): As the feed rate increases, the bed depth increases, which may require an adjustment in pressure to maintain grinding efficiency.
• Roller and Table Wear Condition: Worn grinding parts can lead to an unstable grinding bed, often requiring operational compromises on pressure settings.

3. The Consequences of Improper Pressure Control

Deviating from the optimal pressure range has direct and often severe consequences for the milling process.

3.1. Excessively High Grinding Pressure

• Increased Wear: Accelerated wear of grinding rollers and the table liner, leading to higher maintenance costs and downtime.
• High Vibration: Can cause the mill to trip on high vibration limits, disrupting production.
• Overloaded Main Motor: Increases specific energy consumption (kWh/t) and risks motor damage.
• Unnecessarily Fine Product: While sometimes desirable, producing a product finer than required wastes energy.

3.2. Excessively Low Grinding Pressure

• Unstable Grinding Bed & Vibration: The rollers may bounce or slip on the material bed, causing severe vibration and potential damage to the mill.
• Poor Grinding Efficiency: Inadequate force results in insufficient particle breakage, leading to a coarse product and high recirculation rates.
• Reduced Mill Capacity: The mill cannot reach its designed throughput as the classifier rejects a large amount of coarse material back to the bed.
• Spillage: Material may be pushed out from under the rollers without being ground, leading to buildup and operational issues.

4. Advanced Control Strategies and System Integration

Modern vertical mills are equipped with sophisticated control systems like Siemens PCS7 or similar DCS/PLC platforms. Grinding pressure is not controlled in isolation but is part of a multi-variable control loop. Key strategies include:

• Constant Grinding Force Control: The hydraulic pressure is maintained at a setpoint, providing stable grinding conditions.
• Mill Vibration Limiting: The control system can automatically reduce the grinding pressure setpoint if vibration levels approach a dangerous threshold.
• Integration with Mill Motor Power: The pressure setpoint can be adjusted to keep the main motor load within a safe and efficient range, especially during start-up or feed interruptions.
• Feed-forward Control: Anticipating changes in feed material characteristics and adjusting the pressure preemptively.

5. Zenith’s Advanced Solutions for Precision Grinding

Selecting the right equipment is the foundation for effective pressure control. Shanghai Zenith Machinery Co., Ltd., a leading manufacturer in the field of ultra-fine powder grinding, offers vertical mills engineered for stability, efficiency, and precise operational control. Our machines integrate crushing, grinding, powder selection, drying, and material conveying into a single, robust unit.

For operations requiring high capacity and robust performance in processing minerals, the LM Vertical Grinding Mill series is an exemplary choice. Its design ensures a stable material bed, which is paramount for consistent grinding pressure application and overall mill stability.

For applications demanding the highest levels of fineness, the LUM Ultrafine Vertical Mill represents the pinnacle of Zenith’s engineering. It features advanced material bed control and an intelligent control system that seamlessly manages grinding pressure in relation to other parameters, ensuring optimal performance for superfine powder production.

6. Operational Best Practices and Conclusion

Effective grinding pressure management is an ongoing process. Operators should:

1. Establish a baseline pressure for each material type based on manufacturer guidelines and operational data.
2. Continuously monitor trends in pressure, vibration, power, and product fineness.
3. Perform regular inspections and maintenance of the hydraulic system and grinding elements.
4. Utilize the advanced control capabilities of modern mills like those from Zenith to automate responses to process disturbances.

In conclusion, grinding pressure is the linchpin of vertical mill performance. A deep understanding of its role, coupled with precise control and high-quality equipment such as Zenith’s LM and LUM series vertical mills, is essential for achieving optimal productivity, minimizing wear, and ensuring the long-term, economical operation of any grinding circuit.