Ultrafine grinders: identify the dominant size-reduction mechanism typically exploited to reach sub-100 micrometre and micron-scale product sizes.

Introduction / Context:
Ultrafine grinding targets very small particle sizes with narrow distributions for pigments, pharmaceuticals, and specialty minerals. Different machines (jet mills, stirred media mills) employ different physics than coarse crushers. Recognizing the dominant mechanism aids in choosing the correct mill and media.

Given Data / Assumptions:

• Target sizes: tens of micrometres down to a few micrometres or below.
• Equipment examples: fluid energy (jet) mills, attritors/stirred media mills, pin mills.
• Feed material is friable enough for fine comminution.

Concept / Approach:
At ultrafine scales, attrition (rubbing/shearing along surfaces) dominates because it can continuously erode particle asperities and break weak points, especially in high-shear zones between media or in opposed jets. Impact alone becomes less efficient for very small targets due to decreasing fracture probability at low mass and the need for high collision frequency.

Step-by-Step Solution:
Identify typical ultrafine equipment and their internal motion (e.g., stirred beads).Relate micro-scale breakage to shear/attrition prevalence.Select “Attrition” as the dominant mechanism for ultrafine grinders.

Verification / Alternative check:
Process descriptions for jet mills and stirred media mills emphasize particle–particle and particle–media shear interactions, with operating metrics (specific energy, tip speed) tuned for attrition-driven breakage.

Why Other Options Are Wrong:
Slow compression: characteristic of coarse crushers.Impact: important in some mills but not the primary driver for consistent sub-100 μm production across machines like attritors.Cutting action: relevant for fibrous materials, not typical for mineral ultrafines.

Common Pitfalls:
Expecting ball mills alone to achieve ultrafine sizes efficiently; specialized attrition-based mills are more suitable.

Final Answer:
Attrition