Comminution Flashcards
Define comminution.
Reduction of solid particles to lower average particle size
What size classification are granules?
Up to 2mm
What size classification is a coarse powder?
> 350um
What size classification is a medium/fine powder?
100-350um
What size classification is a fine powder?
50-100um
What size classification is a very fine powder?
10-50um
What size classification is a micronised powder?
<10um
What is a disadvantage of using a sieve for particle size analysis?
Limited information about shape or size range
What is a disadvantage of using microscopy for particle size analysis?
Not representative
What is a disadvantage of using laser diffraction for particle size analysis?
Must consider interaction with the solvent
Particles assumed to be spheres
Define milling.
Transformation of approximate normal particle size distribution into a finer bimodal population
What effect does prolonged milling have?
Forms a unimodal population
What happens when particle sizes are reduced to 1-5um?
Particle-particle forces predominate and agglomeration can occur
What are the features of cutting methods?
Series of knives/blades on rotor
High shear rates
Screen acts to retain particles larger than a specified size
What are the features of compression methods?
Material passes through rollers, one is mechanically driven
Series of rollers can be used to achieve smaller particles
What are the features of impact methods?
Filled to 80% with steel balls
Body of the mill vibrates
Give examples of size separation methods.
Sieving
Sedimentation
Cyclone
Elutriation
Give examples of agitation methods.
Oscillation
Mechanical vibratioon
Gyration
What is crack propagation?
Localised stress producing strains in particles that lead to ruptures/cracking
At low temperatures, what happens to materials?
Transition from plastic to brittle behaviours
How is energy lost?
Elastic/plastic deformation Deformation initiating cracks Interaction with mechanics Heat Vibration
What is the Moh’s scale?
Qualitative determination of hardness
What methods can be used to determine hardness qualitatively?
Brinell and Vickers
What is the range of attrition?
1-100um
