Not a tumbling (revolving) mill:\nWhich of the following is NOT classified as a tumbling/revolving mill for size reduction?

Introduction / Context:
Tumbling (revolving) mills reduce size by cascading/cataracting media and particles inside a rotating cylindrical shell. Examples include ball, rod, pebble, and compartment mills. It is useful to distinguish these from impactor-style mills, which achieve reduction through high-speed particle impacts without a rotating, media-filled drum.

Given Data / Assumptions:

• Tumbling mills possess a rotating cylindrical shell.
• Charge motion (cascading/cataracting) is the dominant mechanism.

Concept / Approach:
Cage mills are impact mills; they use one or more high-speed cages that fling material into counter-rotating impactors and breaker plates. There is no media-filled rotating cylindrical drum as in ball/rod/pebble/compartment mills. Therefore, the cage mill is not a tumbling mill, whereas the others listed are classic tumbling types.

Step-by-Step Solution:

Verification / Alternative check:
Equipment taxonomies in unit-ops texts list cage and hammer mills under “impact mills,” separate from tumbling mills.

Why Other Options Are Wrong:

• Compartment/pebble/rod/ball mills all have rotating cylindrical shells and are tumbling mills.

Common Pitfalls:
Assuming any rotating device is a tumbling mill; internal mechanics matter.

Final Answer:
Cage mill