Greco 2 Flashcards
List 5 methods of size reduction
- Combined Impact and Attrition methods
- Attrition methods
- Impact methods
- Compression methods
- Cutting methods
List 2 mills used for combined impact and attrition methods
- Pin mills
2. Ball mills
List 2 mills used for impact methods
- Vibration mills
2. Hammer mills
List 2 mill used for compression methods
- End runners mills
2. Roller mills
Which method produces the smallest particles? (Diameter, um)
Compression methods (end runner and roller mills) Cutting methods close second
Which methods produce the largest particles? (Diameter, um)
Ball mills (combined), attrition methods and vibration mills (impact)
How do impact methods reduce the size of particles?
Particles are hit by a moving surface or moving particles hit a surface
How do compression methods reduce the size of particles?
By applying pressure
e.g. pestle and mortar
How do cutting methods reduce the size of particles?
Material cut by blades
How does a roller mill (compression) work?
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
How do attrition methods reduce the size of particles?
By pressure and friction
e.g roller mills rotating at different speeds
List the 3 factors which are used to decide the appropriate type of mill
- Particle size to obtain
- Characteristics of the material
- Other factors (cost, time, stability of ingredient)
Which type of mill is best for elastic, fibrous materials?
Cutter mill
Which type of method is best for ointments, solid in suspension and pastes?
Attrition methods
Which type of method is best for brittle materials?
Impact methods
What 2 factors affect the efficiency of the ball mill?
- Amount of material
2. Speed of the rotating mill
How does the amount of material affect the efficiency of a ball mill?
Too much material = reduced chance of hitting balls = cushioning effect
Too little material = balls hit themselves = wear of equipment
Which 2 forces are the balls in a ball mill subject to?
- Gravity = balls downwards
2. Centrifugal forces = balls upwards
What is the critical centrifugal velocity?
The speed at which centrifugal forces overcome gravity
At what point is the best milling obtained? (Ball mill)
At 2/3 of the critical centrifugal velocity
How does a fluid energy mill work?
- Material is let in to the bottom on the mill
- It is pushed round by high pressure fluid
- Material particles knock together to reduce size
- Centrifuging action throws the coarser (larger) particles outwards so they cannot leave the mill via the classifier
What is the approximation of size analysis of particles?
That the particles are circles/spheres
What is the projected area diameter? (da)
It is assumed that the spheres have the same projected area as that of the particle
What is the projected perimeter diameter? (dp)
Circle drawn around the particle
It is assumed the sphere has the same perimeter as that of the particle
List the 2 statistical diameters used in size analysis
- Feret’s diameter (dF)
2. Martin’s diameter (dM)
What are statistical diameters?
Diameters that change depending on the orientation of the particle
How is Martin’s diameter used?
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
How is Feret’s diameter used?
Measures the length of the particle as the particle is rotated
Average of results is taken = particle size
What are the direct methods of measuring particle size?
Physically measure particle size
1) Sieving
2) Microscopy
What are the indirect methods of measuring particle size?
Physical particle size not measured, properties that are connected to the particle size are measured
1) Sedimentation rate
2) Permeability
What is the aperture diameter of a sieve?
The distance between its wires (pores?)
Measured in um
How are sieves used to measure particle size?
- 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
Name the 2 types of microscopy which can be used as a direct method of measuring particle size
1) Light microscopy (1-1000um)
2) Electron microscopy (as low as 0.001um)
How does light microscopy work to calculate particle size?
- A drop of very diluted suspension (so not dissolved) is placed on a microscope slide
- Particles are measured in relation to the equivalent diameter chosen (dp, da, dF, dM)
- The images are 2D
Why is it important that a suspension of particles is used instead of a solution for light microscopy?
So the particles are not dissolved (and so visible)
Why is it important that the suspension is very dilute in light microscopy?
So the particles to not aggregate = cluster
