Particle Size Reduction

Question 1

what is comminution?

Answer 1

particle size reduction

Question 2

what is milling?

Answer 2

fracturing particles by applying mechanical stress

Question 3

what are the pros of comminution?

Answer 3

1. higher dissolution rate for same amount of drug
2. increases bioavailability if drug absorption is dissolution rate-limited
3. improves content uniformity in tablets and capsules
4. increases bulk density for more efficient transportation

Question 4

how do you calculate the specific surface area of the particles?

Answer 4

total surface area divided by the mass of the particles

Question 5

what happens during particle size reduction?

Answer 5

1. partiles produce a bi-modal size distribution
2. hence there’s 2 size sub-populations
3. these sub-populations resolve upon milling

Question 6

what does the smallest possible particle size depend on?

Answer 6

the smallest particle size achievable depends on the milling method and the material’s properties

Question 7

what should be considered when choosing what milling method to use?

Answer 7

1. the target particle size desired
2. the material’s properties
3. mechanisms of comminution

Question 8

What is the ease of fracture and deformation for brittle, strong, tough and hard materials?

Answer 8

Brittle: easily fractured, minimal deformation
Strong: not easily fractured
Tough: prone to plastic deformation
Hard: resist deformation

Question 9

what is Griffith’s theory of fracture?

Answer 9

• materials have microscopic flaws so energy applied during comminution is concentrated at the tip of the microscopic flaws
• the energy overcomes bond strength causing rupture
• crack propagates along the weakest regions so elastic energy dissipates through the material and concentrates at other flaws (cracks)
• this has a cascade effect causing brittle fracture.
• tough materials undergo ductile fracture with slower crack propagation

Question 10

what is elastic deformation?

Answer 10

reversible deformation (so object returns to the original shape when stress is removed. All materials deform elastically to some extent)

Question 11

what is plastic deformation?

Answer 11

irreversible deformation (so there’s a permanent change in the shape of the object)

Question 12

what is hardness?

Answer 12

the ability of a material to resist plastic deformation. This makes them hard to comminute and may wear out equipment’s.

Question 13

During comminution, only a small amount of energy is used to reduce the particle size. What happens to the rest of the energy?

Answer 13

lost via deformation, friction, heat, sound and vibration

Question 14

what is Rittinger’s hypothesis?

Answer 14

energy used is proportional to increase in particle surface area.

Question 15

what is Kick’s theory?

Answer 15

energy used is proportional to particle size reduction ratio

Question 16

what is bond’s theory?

Answer 16

Energy used in crack propagation is

proportional to new crack length formed

Question 17

how does a cutter mill work?

Answer 17

• particles cut by rotating and stationary knives

- particles are retained until the desired particle size is reached

Question 18

how does a runner mill work?

Answer 18

two types: end and edge

end: grinding under flat base of rotating wheel
edge: grinding on edge of rotating wheel

Question 19

how does a roller mill work?

Answer 19

• two rollers rotate against each other or one roller rotates against a stationary roller and crushes particles in the gap between the rollers
• if there’s a difference in the speeds between the rollers, this affects shearing

Question 20

how does a hammer mill work?

Answer 20

-hammers radiate from central rotating shaft and hit the particles at high speed causing brittle fracture in most materials

Question 21

how does the vibration mill work?

Answer 21

• particles are poured into the equipment alongside milling balls
• Vibration causes impact between particles and mill balls.

Question 22

how does ball mill work?

Answer 22

• a hollow ball filled with mill balls and particles that you want to reduce the size of.
• spinning speed and feed volume affects the efficacy of the ball mill.
• there’s an avalanche effect.

Question 23

what is the avalanche effect in the ball mill?

Answer 23

the balls are lifted and cascade back down when dynamic angle of repose is exceeded

Question 24

how does the fluid energy mill work?

Answer 24

• high velocity fluid injected into the toroid milling chamber
• particles collide with each other and the mill wall as they’re carried by the air

Question 25

how does the pin mill work?

Answer 25

2 discs with closely positioned pins rotate and fracture the particles by impact on collision with pins or by attrition on shearing between pins.