PARTICLE SIZE REDUCTION

Question 1

Factors affecting particle size reduction

Answer 1

Brittleness, Hardness, Elasticity and plasticity, melting point and glass transition

Question 2

Size reduction via…

Answer 2

Milling equipment technique used

Question 3

Milling equipment methods (5)

Answer 3

Cutting method
Compression method
Impaction method
Attrition method
Combined impact and attrition method.

Question 4

Cutting method

Answer 4

1) Cutter mill (Series of stationary and moving knives a screen is fitted until a sufficient size of size reduction is reached)

Question 5

Compression methods (3)

Answer 5

1) Mortar and pestar
2) End runner mill (pestle is mounted horizontally and rotated against the powder bed)
3) Edge runner mill (pestle is turned by the friction of material underneath it as a mortar rotates under powder

Question 6

Impaction method (2)

Answer 6

1) Hammer (hammer spin our radially from rotating shaft, produces powders with narrow size distribution)
2) Vibration (80% filled with steel/porcelain balls) the body of the mill is vibrated and size reduction occurs via repeated impaction.

Question 7

Attrition method

Answer 7

1) Roller mill (used for size reduction of particles in suspensions, pastes and ointments, rules are mounted horizontally with adjustable gap the material is sheered as it goes through the gap and removed through means of a scrapper.

Question 8

Both impactions and attrition

Answer 8

1) Ball mill
attrition - through friction of rolling balls
impact- through impact of falling balls
- large balls - coarse grinding
- small balls - fine particles
- too slow or two fast efficiency decreases.

2) Pin mill
Cylinder rotates at high speed against the upper fixed cylinder
Particle size reduction occurs by impaction and attrition with pins as particles flow outwards

3) Fluid energy and jet energy
High pressure air jet is injected, the increase air velocity gives rise to zones of turbulence causing particles to impact and therefore particle size reduction occurs.