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 fluid energy mill (also known as a jet mill) operates on the principle of impact and attrition for size reduction. This type of mill uses high-velocity air or gas jets to cause particles to collide with one another at high speeds, resulting in the fragmentation of the particles. The primary mechanism involves both impact (particles striking each other at high velocity) and attrition (particles grinding against each other), making it effective for fine size reduction, especially for materials that require micronization.
The other mills listed have different operating mechanisms:
 - Roller mills typically rely on compression and shear forces to break down materials. 
- Hammer mills use impact forces primarily but often work with larger particle sizes and less fine grinding. 
- Colloid mills primarily use shearing and grinding forces for producing emulsions and suspensions, but not specifically impact and attrition. 
Thus, the correct answer is B, as the fluid energy mill is based on the mechanism of impact and attrition for size reduction.

Answer 2

The question is about which size-reduction mill works mainly through impact combined with attrition. Let's go through each mill's working principle.

1. Roller mill: A roller mill pulls material between two or more rotating rollers turning at different speeds, so particles are crushed mainly by compression and shear, not by impact or attrition.
2. Fluid energy mill: Also called a jet mill, this device injects compressed air or gas at high velocity so that particles are hurled against each other and against the chamber wall at speed. Particles break apart both from the direct collisions, which is impact, and from particles rubbing and grinding against one another as they swirl, which is attrition. This combination of impact and attrition is exactly what the fluid energy mill relies on, and it is well suited to producing very fine, even micronised, powders.
3. Hammer mill: A hammer mill uses swinging metal hammers striking the feed material, so its size reduction is essentially pure impact; there is little of the rubbing action that defines attrition.
4. Colloid mill: A colloid mill forces material through a narrow, adjustable gap between a high-speed rotor and a stator, so it works through intense shear, useful for making emulsions and suspensions rather than through impact or attrition.

Of the four, only the fluid energy mill combines particle-to-particle impact with grinding attrition as its core mechanism.

Therefore, the correct answer is Fluid energy mill.