Ball mill practice: actual operating speed typically ranges from 65% to 80% of critical. In which duty is the mill usually run closest to the maximum percentage of critical speed?

Introduction / Context:
Ball mill speed affects the charge motion (cascading, cataracting) and therefore the breakage environment. Operators choose a fraction of critical speed to balance energy efficiency, liner/media wear, and required impact intensity.

Given Data / Assumptions:

• Typical operating band: 65–80% of critical speed.
• Larger feed needs more impact/cataracting to break efficiently.
• Unbaffled (smooth) mills provide less lift; higher speed is often needed to promote cataracting.

Concept / Approach:
Cataracting impacts dominate breakage of coarse, hard particles in dry grinding. In unbaffled mills, lifter action is limited; therefore, a higher fraction of critical speed helps raise media, increase impact frequency and energy, and avoid sluggish cascading that insufficiently breaks coarse lumps.

Step-by-Step Solution:
Identify duty needing strongest impacts → coarse, dry, unbaffled.Higher % of critical promotes cataracting → better coarse breakage.Therefore select: dry grinding of large particles in unbaffled mills.

Verification / Alternative check:
Plant practice often uses lower speeds for viscous wet slurries to limit power spikes and excessive heat, while coarse dry grinding benefits from faster rotation to generate impact.

Why Other Options Are Wrong:
Low-viscosity wet milling: typically mid-band speeds suffice.High-viscosity wet milling: speeds are often reduced to avoid overload and poor media mobility.Dry with large feed (baffled): lifters allow adequate cataracting at slightly lower speeds than a smooth shell.

Common Pitfalls:
Confusing critical speed (theoretical centrifuging point) with optimal operating speed; running too fast can still be inefficient if it causes centrifuging instead of controlled cataracting.

Final Answer:
Dry grinding of large particles in unbaffled mills.