Greco lecture 2: Mechanical Methods

Question 1

What is milling?

Answer 1

Mechanical method used to give you reduction in particle size

Question 2

What is the principle method that’s followed for size reduction?

Answer 2

1. Cut
2. Compression
3. Impact
4. Attrition

Question 3

Describe the cutting process?

Answer 3

1. Material is entered through a funnel which is cut up by many knives
2. The particle goes through a screen which allows it to go through if it’s small enough size

Question 4

Describe the compression method?

Answer 4

1. A pressure is applied which breaks up the material
2. Roller mill: cross section of cylinders attached to an engine that ensure it roll
3. Good for breaking up crystalline material
4. Can be traditionally done via pestle and mortar

Question 5

Describe the impact method?

Answer 5

1. Material is hit by rotating hammers

2. Particles are hit by a moving surface or the particles hit a moving surface

Question 6

Describe the attrition method?

Answer 6

1. Pressure and friction
2. Roller mills that are attached to a engine which rotate at different speeds
3. Ointment with a powder dispersed would use this milling method

Question 7

Describe how impact and attrition methods can be combined together?

Answer 7

1. Main chamber filled with balls that reduces the size of the particles- ball mill
2. Factors which affect efficiency of it is the amount of material and the speed at which the mill rotates
3. Subject to gravity as they roll upwards they are pushed down again

Question 8

What is the problem that can occur if too much material is added to the impact and attrition ball mill method?

Answer 8

Decreases chance of the material heating the balls is held to a minimum this is called the cushioning effect

Question 9

What is the problem that can occur if too little material is added to the impact and attrition ball mill method?

Answer 9

Balls will start heating among themselves which leads to a wearing of equipment

Question 10

How do you decide to choose which appropriate mill to use?

Answer 10

1. Particle size you want
2. Characteristics:
   - Cutter mills: elastic, fibrous material like roots/wood
   - Attrition: ointments, solids in suspensions and paste
   - Impact: brittle materials
3. Other factors:
   - Cost
   - time
   - stability of the ingredients
   - Specific milling conditions

Question 11

What is particle size analysis?

Answer 11

Approximating the size of the particles by comparing it to a spheres diameter

Question 12

What is the projected area diameter?

Answer 12

The diameter of the sphere who’s area is the same as the particle

Question 13

What is the projected perimeter diameter?

Answer 13

The diameter of the sphere who’s perimeter is the same as the particle

Question 14

What are the two direct methods for working out particle size?

Answer 14

1. Sieving

2. Microscopy

Question 15

How are sieve sizes expressed?

Answer 15

Based on the sieve aperture diameter measured in micro meters that are the size of squares

Question 16

Explain how a 100g powder is sieved?

Answer 16

A large column contains many sieves that each individually filter the powder

Question 17

Explain how light microscopy works?

Answer 17

1. A drop of diluted suspension is placed on a microscope slide
2. Particles are measured in relation to the equivalent diameter chosen
3. Images are 2D and range between 1 to 1000 micrometers

Question 18

Explain what electron microscopy is and the advantages/disadvantages?

Answer 18

1. Scanning electron microscopy
2. Provides a 3D image and gives information on shape
3. Advantage:
   3D images gives information on shape
4. Disadvantages:
   Expensive, high level of operator expertise

Question 19

Explain what a coulter counter is and does?

Answer 19

1. Measures the volume of particles from 0.1 to 1000 micrometers
2. Emerged in solution filled with electrolytes

Question 20

What are the indirect methods to measure particle size?

Answer 20

1. Sedimentation rate

2. Permeability

Question 21

How does the laser light scattering method work?

Answer 21

1. Laser light interacts with particles
2. Light is diffracted by particles at an angle that is inversely proportional to the volume of particles
3. Detector analyses the radiation diffracted by the particles

Question 22

How does the sedimentation method work?

Answer 22

Diameter of particles estimated based on sedimentation rate