Breakage mechanisms by equipment: in which of the following machines does size reduction not predominantly occur by compression of particles?

Introduction / Context:
Crushers and mills reduce size via different dominant mechanisms: compression, impact, and attrition/abrasion. Recognizing the primary mode helps predict product size distribution, fines generation, and energy use, thereby guiding equipment selection and operating strategy.

Given Data / Assumptions:

• Jaw and gyratory crushers rely on compressive forces between hard surfaces.
• Smooth roll crushers nip and compress particles to break them.
• Rod mills use tumbling action with line contact among rods producing abrasion and selective grinding.

Concept / Approach:
Compression-dominant devices trap particles between converging surfaces, causing fracture under compressive stress. Rod mills, however, predominantly reduce size by abrasion/attrition as particles are caught between moving rods and the mill liner, with less direct compressive pinching than rolls or jaws.

Step-by-Step Solution:
List each machine’s dominant breakage mode.Identify the one where compression is not the primary mechanism.Select “Rod mills,” which emphasize abrasion/attrition and selective grinding.

Verification / Alternative check:
Operating descriptions and product profiles confirm crushers yield more angular, compression-fractured product, while rod mills produce fewer fines and more uniform particles due to attrition-dominant action.

Why Other Options Are Wrong:
Gyratory and jaw crushers: classic compression machines.Smooth roll crushers: rely on nip and compressive squeezing between rolls.

Common Pitfalls:
Confusing rod mills with ball mills; both are tumbling mills, but ball mills use point contacts among balls, leading to more impact events than rod mills. Rod mills are chosen when minimizing fines is important.

Final Answer:
Rod mills