Impact grinding equipment:\nWhich mills are commonly referred to as “impactors” in size reduction practice?

Introduction / Context:
Size-reduction mechanisms include compression, impact, attrition, and cutting. Equipment is often grouped by its dominant breakage mode. “Impactors” rely primarily on high-velocity impacts between particles and moving elements or breaker plates to shatter the feed. Correctly identifying impactors aids in selecting machines for brittle materials and in predicting product top size and fines content.

Given Data / Assumptions:

• Standard industrial definitions are assumed.
• Multiple answers could apply; select the option that captures the commonly accepted set.

Concept / Approach:
Hammer mills use rapidly rotating hammers to impart impact energy to particles, which then collide with breaker plates/screens. Cage mills employ counter-rotating cages to throw particles at high speed for multiple impacts. Both are classic impactors. Rolling-compression devices (smooth/ toothed rolls) rely on nipping and compressive crushing rather than impact and therefore are not classed as impactors.

Step-by-Step Solution:

Verification / Alternative check:
Equipment catalogs and unit-ops texts group hammer and cage mills together under impact mills, distinct from compression and attrition devices.

Why Other Options Are Wrong:

• Choosing only one (hammer or cage) is incomplete.
• Rolling-compression: not an impactor.
• “None”: incorrect given the above.

Common Pitfalls:
Confusing toothed rolls (which do impart some local impact) with true impactor action; the dominant mechanism remains compression/cleaving in rolls.

Final Answer:
Both hammer and cage mills