Size reduction & measurement Flashcards
What is size reduction
Mechanical process of reducing a solid into a smaller state of sub-division
List methods for size reduction
- Wet grinding
- Roll mill
- Hammer mill
- Cone mill
- Micronizing mill
Reasons for size reduction
- Increase SA for reaction
- Improve extraction of active principles
- Improve dispersibility in solution
- Allow for better mixing/blending
- Enable rapid dissolution
- Preliminary process in preparation of products
How does energy loss occur during size reduction?
- Elastic deformation
- Plastic deformation
- Deformation to initial cracks/of mechanical parts
- Inter-particle friction
- Heat
- Sound
- Vibration
- Important to select correct mill type & scale for size reduction
Considerations in size reduction
Material property:
- Thermolability, MP, flammability, deformation characteristics
Fracture mechanics of particles:
- Hardness, tensile strength
Mechanical, type of equipment:
- material in contact w product (SS 304/316), temp control, impact, shear or pressure
Milling process
- Particles fracture/abrade to give smaller particles
Modes of cracking:
1) Crack OPENING by tensile spreading at crack tip
2) Crack SLIDING by shear deformation parallel to crack direction
3) Crack TEARING by shear deformation perpendicular to crack direction
Force applied (F=kX): - For successful milling, supplied energy has to exert stress beyond the material's break or fracture point
Particle size distribution:
Unimodal –> Bimodal –> Unimodal (In smaller size range)
What is wet grinding?
Size reduction in liquid media, usually water
Advantages of wet grinding
1) Eliminate dust
2) Easier to handle material
3) Use less energy
4) Increase mill capacity
Disadvantages of wet grinding
1) Increase wear of grinding machine
2) Not applicable to soluble materials
3) May need to dry products
What is roll mill used for?
Best for soft materials, popular for creams!
- Grind pastes; coarse crushing
- Peripheral velocity of rolls & clearance between rolls can be varied
Hammer Mill
What, Advantages, Disadvantages
What: Spin hammer at high speed, particles smaller than the screen will pass through
- screen determines size of particles
- rotor speed affects particle size (High speed, might get thrown out at angle)
Advantages:
- Several models avail
- Cheap & efficient
- Blades & screens interchangeable
- Medium-High shear applications
- Suitable for very hard materials
Disadvantages:
- Less precise
- Noisy & dusty
- Cannot plug feed
- Not scalable (screen selection & installation complex)
- Sifting required after milling
- Temp rise due to friction
Type of blades:
Knife edge - Good for TCM, break cellulose materials easier
Blunt edge
(Blade rotor consists of knife & blunt edge)
How does the Cone mill work?
Infeed falls into conical screen chamber -> Rotating impeller imparts a vortex flow pattern to the infeed material
-> Centrifugal acceleration forces particulates to screen surface
Micronizing Mill (What)
Eg. Vibratory Jet Mill, Fluid Energy Mill, Air Jet Mill
Use: Fine grinding
- Involves tumbling ball that’s rotating
- Ball size & density important
- Cascading/vibratory (shorten time but generates heat)
- Extremely efficient with vibration; best wet BUT need to be mindful of heat
- Cascading: Long process time
Air Jet Mill
What, Advantages, Disadvantages
Use: Hard materials
Air jet with classifier wheel:
- Directs air to a particular slot in the chamber & spun at high speed
- Particles knock against each other
- Only small particles can escape
Advantages of Micronizing Mills
1) Widely used for fine grinding
2) Extremely efficient
3) Can mill very hard materials