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
Disadvantages of Micronizing Mills
Need good process control
What’s the purpose of Milling?
To facilitate down stream processing such as blending & tableting
What should we consider when choosing a size measurement method
1) Reliable
2) Reproducible
3) Sensitive
Basic problem of particle size analysis
To describe a 3D object with just 1 number
What shape can be described by a single number?
Sphere
- Perimeter diameter/Projected area diameter
What is the equivalent sphere theory?
Convert weight of __ to weight of sphere
List methods for sizing measurement
- Scales
- Microscopy
- Sieves
- Laser diffraction
- Light scattering
Microscopy
Use, Advantages, Disadvantages
- Direct visual examination
- Require min of 10,000 images for statistical validity
- Provides a 2D image with max particle length as diameter (implying that particle is sphere (equivalent) of this max dimension)
Advantages:
- Relatively cheap
Disadvantage:
- Operator-dependent
- Laborious & slow manual method
Sieves
Use, Advantages, Disadvantages
- Longer measurement time, smaller the size as particles orient themselves to fall through the sieve
Use: Particles >200um
Advantages:
- Long in existence
- Economical
- Robust technique for larger particles
- Good for quality control
Disadvantage:
- Low-resolution method
- Usually only 4-8 size fractions are provided
What sieve can be used for particles <200um?
Air Jet Sieve
- Can be used with microsieves
- Particles smaller than aperture size will be passed through the sieve
- Material remaining on sieve is weighed
- Process repeated with sieves of another aperture size
- Cumulative graph of percent weight oversize plotted
How does Electrozone Sensing work?
Measures resistance between electrodes
USE: Developed originally for sizing blood cells
- Based on orifice obscuration
- Particles are dispersed in an electrolyte as a suspension
- Measures resistance between electrodes
- -> Rise in resistance = Particle size
- Needs a reference size calibrator
Disadvantages of Electrozone Sensing
1) Requires calibration standards that are expensive
2) Porous materials & dense materials pose additional problems
Mechanism of Laser Diffraction?
Curvature of particles causes light to scatter
- > Small: Bigger scatter b/c sharper curvature
- -> From scatter determine size
Advantages of of Laser Diffraction
1) Wide dynamic range & very flexible
2) Rapid results (< a minute)
3) Repeatable & high resolution
4) No need to calibrate against a standard but equipment performance can be easily verified
5) Non-destructive & non-intrusive method
Mechanism of Light Scattering?
Analyzes the time dependency of light intensity fluctuations scattered from particles when they are illuminated
–> Smaller particles (Higher KE) -> Faster light scattering
