Particle Size Reduction

What is comminution?

The process of reducing particle size, also called particle size reduction.

Why is particle size reduction important in pharmaceutics?

Improves material handling, dissolution rate, bioavailability, and deposition of inhaled particles.

What is specific surface area?

Surface area per unit mass; increases with particle size reduction.

What does the Noyes-Whitney equation describe?

The rate of dissolution of a particle based on surface area, diffusion coefficient, concentration gradient, and boundary layer thickness.

Key mechanisms of comminution?

Cutting, compression, impact, and attrition.

Primary mechanism of Cutter Mill?

Cutting.

Primary mechanism of Hammer Mill?

Impact.

Primary mechanisms of Roller Mill?

Compression and shearing (attrition).

How does a ball mill work?

Rotating drum partially filled with balls; balls cascade to grind materials through impact and attrition.

What happens if a ball mill spins too fast?

Balls stick to the drum walls and do not grind the material effectively.

What is the principle of a vibration mill?

Vibration causes milling balls to impact each other and the particles.

How does a pin mill work?

Particles fracture on collision with pins and shear between rotating discs.

Mechanism of a fluid energy mill?

Particles collide with each other and the walls under high-velocity air turbulence.

What does Griffith’s theory of brittle fracture explain?

Comminution by stress concentration at crack tips causing rupture and cascade failure.

Difference between elastic and plastic deformation?

Elastic deformation is reversible; plastic deformation is permanent.

What is hardness?

Material's resistance to plastic deformation; affects ease of comminution.

Which type of materials are harder to comminute?

Hard materials (e.g., diamond, stainless steel) due to high resistance to deformation.

What can happen to equipment when milling hard materials?

Equipment parts may wear out faster.

How is hardness measured?

In Nm² or on Mohs' scale (e.g., diamond = 10, stainless steel = 5).

Which mills use both impact and attrition?

Pin mills, ball mills, and vibration mills.