Understanding the Question:
The question asks which mill reduces particle size through the combined mechanism of impact and attrition.
Key Concept:
Each type of size reduction mill is built around a dominant force: compression, single-direction impact, shear, or a mix of impact plus attrition, and the equipment design decides which mechanism is at play.
Checking Each Option:
A roller mill compresses material between rotating rollers, a compression mechanism rather than impact and attrition.
A hammer mill relies mostly on direct impact from swinging hammers, without a real attrition component.
A colloid mill works by shear, forcing material through a narrow rotor-stator gap, mainly for wet suspensions and emulsions.
A fluid energy (jet) mill uses high-velocity air jets to make particles collide with each other and the chamber wall while also rubbing against one another, combining impact and attrition in one process.
Final Answer:
The fluid energy mill works by the combined mechanism of impact and attrition.