Question

Which of the following mills is based on the mechanism of impact and attrition for size reduction:

• A. Roller mill
• B. Fluid energy mill
• C. Hammer mill
• D. Colloid mill

Answer 1

The Correct Option is B

The correct option is (B): Fluid energy mill.

Answer 2

Different size reduction mills rely on different combinations of mechanical forces: compression, impact, attrition, cutting or shear. The question asks which mill works specifically through the combined action of impact and attrition.

1. Roller mill: A roller mill crushes material by squeezing it between two or more rotating rollers, which is a compression-based mechanism, not impact and attrition.
2. Fluid energy mill: Also called a jet mill, this equipment injects particles into a chamber using high-velocity compressed air or gas jets. Inside the chamber, particles collide violently with each other and with the chamber wall (impact) while also rubbing against each other as they circulate (attrition), and it is this dual mechanism that grinds the material down to very fine, often micron-sized, particles.
3. Hammer mill: A hammer mill reduces particle size mainly through direct impact, as swinging hammers strike the material against a screen, without a meaningful attrition (particle-on-particle rubbing) component built into its mechanism.
4. Colloid mill: A colloid mill works by shear, forcing material through a narrow, adjustable gap between a high-speed rotor and a stationary stator, mainly used for wet-milling suspensions and emulsions rather than dry impact-and-attrition size reduction.

Since only the fluid energy mill combines high-speed particle collisions with particle-on-particle rubbing inside its air stream, it is the mill defined by both impact and attrition together.

The correct answer is Fluid energy mill.