Greco 2

Question 1

List 5 methods of size reduction

Answer 1

1. Combined Impact and Attrition methods
2. Attrition methods
3. Impact methods
4. Compression methods
5. Cutting methods

Question 2

List 2 mills used for combined impact and attrition methods

Answer 2

1. Pin mills

2. Ball mills

Question 3

List 2 mills used for impact methods

Answer 3

1. Vibration mills

2. Hammer mills

Question 4

List 2 mill used for compression methods

Answer 4

1. End runners mills

2. Roller mills

Question 5

Which method produces the smallest particles? (Diameter, um)

Answer 5

Compression methods (end runner and roller mills)
Cutting methods close second

Question 6

Which methods produce the largest particles? (Diameter, um)

Answer 6

Ball mills (combined), attrition methods and vibration mills (impact)

Question 7

How do impact methods reduce the size of particles?

Answer 7

Particles are hit by a moving surface or moving particles hit a surface

Question 8

How do compression methods reduce the size of particles?

Answer 8

By applying pressure

e.g. pestle and mortar

Question 9

How do cutting methods reduce the size of particles?

Answer 9

Material cut by blades

Question 10

How does a roller mill (compression) work?

Answer 10

Material placed between 2 rolls rotating in opposite directions
The material can pass through the gap when it it small enough
The left sphere is driven directly and the right rotates as a consequence of friction

Question 11

How do attrition methods reduce the size of particles?

Answer 11

By pressure and friction

e.g roller mills rotating at different speeds

Question 12

List the 3 factors which are used to decide the appropriate type of mill

Answer 12

1. Particle size to obtain
2. Characteristics of the material
3. Other factors (cost, time, stability of ingredient)

Question 13

Which type of mill is best for elastic, fibrous materials?

Answer 13

Cutter mill

Question 14

Which type of method is best for ointments, solid in suspension and pastes?

Answer 14

Attrition methods

Question 15

Which type of method is best for brittle materials?

Answer 15

Impact methods

Question 16

What 2 factors affect the efficiency of the ball mill?

Answer 16

1. Amount of material

2. Speed of the rotating mill

Question 17

How does the amount of material affect the efficiency of a ball mill?

Answer 17

Too much material = reduced chance of hitting balls = cushioning effect
Too little material = balls hit themselves = wear of equipment

Question 18

Which 2 forces are the balls in a ball mill subject to?

Answer 18

1. Gravity = balls downwards

2. Centrifugal forces = balls upwards

Question 19

What is the critical centrifugal velocity?

Answer 19

The speed at which centrifugal forces overcome gravity

Question 20

At what point is the best milling obtained? (Ball mill)

Answer 20

At 2/3 of the critical centrifugal velocity

Question 21

How does a fluid energy mill work?

Answer 21

1. Material is let in to the bottom on the mill
2. It is pushed round by high pressure fluid
3. Material particles knock together to reduce size
4. Centrifuging action throws the coarser (larger) particles outwards so they cannot leave the mill via the classifier

Question 22

What is the approximation of size analysis of particles?

Answer 22

That the particles are circles/spheres

Question 23

What is the projected area diameter? (da)

Answer 23

It is assumed that the spheres have the same projected area as that of the particle

Question 24

What is the projected perimeter diameter? (dp)

Answer 24

Circle drawn around the particle

It is assumed the sphere has the same perimeter as that of the particle

Question 25

List the 2 statistical diameters used in size analysis

Answer 25

1. Feret’s diameter (dF)

2. Martin’s diameter (dM)

Question 26

What are statistical diameters?

Answer 26

Diameters that change depending on the orientation of the particle

Question 27

How is Martin’s diameter used?

Answer 27

A horizontal line is drawn as the particle is rotated
Line crosses the particle in half (area of top = area of bottom)
An average of the results is taken = particle size

Question 28

How is Feret’s diameter used?

Answer 28

Measures the length of the particle as the particle is rotated
Average of results is taken = particle size

Question 29

What are the direct methods of measuring particle size?

Answer 29

Physically measure particle size

1) Sieving
2) Microscopy

Question 30

What are the indirect methods of measuring particle size?

Answer 30

Physical particle size not measured, properties that are connected to the particle size are measured

1) Sedimentation rate
2) Permeability

Question 31

What is the aperture diameter of a sieve?

Answer 31

The distance between its wires (pores?)

Measured in um

Question 32

How are sieves used to measure particle size?

Answer 32

• Stack of sieves used
• At top of stack = biggest aperture, at bottom = smallest aperture
• Material put in top sieve and different fractions are collected
• Therefore particles of a material are separated by their weight

Question 33

Name the 2 types of microscopy which can be used as a direct method of measuring particle size

Answer 33

1) Light microscopy (1-1000um)

2) Electron microscopy (as low as 0.001um)

Question 34

How does light microscopy work to calculate particle size?

Answer 34

1. A drop of very diluted suspension (so not dissolved) is placed on a microscope slide
2. Particles are measured in relation to the equivalent diameter chosen (dp, da, dF, dM)
3. The images are 2D

Question 35

Why is it important that a suspension of particles is used instead of a solution for light microscopy?

Answer 35

So the particles are not dissolved (and so visible)

Question 36

Why is it important that the suspension is very dilute in light microscopy?

Answer 36

So the particles to not aggregate = cluster