Appropriately increasing the mill speed has a non‑linear effect on output and power consumption — there exists an “optimal speed range.” Before reaching this critical point, increasing speed yields positive results in both output and energy efficiency. Once the critical point is exceeded, the effect reverses. The specific impacts can be analysed from the following three aspects:

I. Positive Impact of Increasing Speed on Output (Output‑Increasing Effect)

1. Increases the impact crushing energy of grinding media

• As speed increases, the grinding media are lifted to a greater height, resulting in significantly higher impact velocity and impact kinetic energy (E∝v2).
• For materials with larger feed particle sizes or higher hardness, the increased impact force effectively enhances “primary breakage,” reducing the residence time of material in the coarse‑grinding compartment and thereby increasing throughput.

2. Increases the “number of impacts” per unit time

• As mill speed n increases, the time for one revolution of the shell decreases. With the same media charge, the total number of impacts per unit time increases, and the grinding efficiency in the fine‑grinding compartment also improves.

3. Accelerates material flow velocity and reduces “over‑grinding”

• Higher speed intensifies the centrifugal force field inside the mill, promoting faster discharge of fine particles through the diaphragm and discharge grate plates.
• Especially for closed‑circuit mills, increasing speed can appropriately raise the circulating load, allowing the air classifier to extract qualified fines more efficiently, thereby increasing finished product output.

Conclusion: In the range of 65%–80% of critical speed, appropriate speed increase can generally achieve an output increase of 5%–15% (depending on mill size and material characteristics).

II. Impact of Increasing Speed on Grinding Power Consumption (Trade‑off Between Reduction and Increase)

1. Specific power consumption may first decrease and then increase (U‑shaped curve)

• Positive phase: When output increases with speed, if the increase in motor input power is less than the increase in output, then specific power consumption (kWh/t) decreases. This is the ideal state.
• Critical point: When speed exceeds the optimum value, output growth plateaus, but motor power consumption (especially the ineffective work of overcoming centrifugal force, air resistance, and friction) continues to rise rapidly, and specific power consumption begins to increase rather than decrease.

2. Change in the proportion of “no‑load” power of the mill

• The no‑load power of the mill (used to overcome bearing friction, shell inertia, etc.) is largely independent of output. The higher the output, the lower the share of this fixed power per tonne of product, so specific power consumption naturally decreases.
• However, at excessively high speeds, centrifugal force on the grinding media increases sharply, the relative motion between the outer layer of media and the liners decreases, and the “slipping layer” thickens. A large amount of energy is converted into friction heat rather than crushing work, and the power consumption per unit of effective crushing work deteriorates sharply.

III. Negative Consequences of Excessive Speed Increase (When Exceeding the “Optimum Speed”)

1. Grinding efficiency drops sharply (centrifugal state)

• When the speed approaches the critical speed (ψ→100%), the outermost layer of grinding media begins to “stick” to the shell and rotate with it, losing its ability to fall. Only compression and attrition remain, and crushing efficiency plummets. At this point, the mill hardly crushes material, and output decreases significantly.

2. Increased wear on liners and grinding media

• The higher the speed, the greater the relative velocity and impact frequency between the grinding media and liners, leading to exponentially higher liner and media consumption. This not only increases maintenance costs but also indirectly raises overall operating power consumption (due to production losses from maintenance shutdowns and parts replacement).

3. Increased mill vibration and mechanical load

• At high speeds, unbalanced loads can cause severe vibration of the shell and transmission system, leading to premature fatigue damage of key components such as main bearings and reducer gears, shortening equipment life.

IV. How to Define “Appropriate” Speed (Reference Threshold)

The optimal speed varies for different mills, and the rotation rate (ψ) is generally used as the indicator:

Practical guidance:

• Most large cement mills operate in the range of ψ = 70% ~ 78%.
• If a mill has been operating at ψ<68% for a long time, try a small speed increase (2%–5%) and observe actual changes in output and power consumption. If the output increase exceeds the power increase, the speed increase is effective.
• If output does not change significantly after increasing speed but motor current increases significantly, the mill has likely approached or exceeded the optimal point, and the speed should be restored.

V. Summary: Concise Conclusion

Increasing mill speed appropriately within the range of 65%–76% of critical speed can simultaneously achieve “higher output” and “lower consumption” (reduced specific power consumption). Once exceeding the critical boundary of 76%–80%, the output‑increasing effect saturates, while power consumption and wear rebound significantly, ultimately making the change counterproductive.

For actual production, the best approach is: based on the current operating speed, increase by 2%–3% each time, operate stably for 24 hours, then compare output and power consumption data. Through this “step‑by‑step fine‑tuning” method, find the mill’s personalised optimal speed rather than blindly copying theoretical values.