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PR3117 > Size reduction & measurement > Flashcards

Size reduction & measurement Flashcards

1
Q

What is size reduction

A

Mechanical process of reducing a solid into a smaller state of sub-division

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2
Q

List methods for size reduction

A
  • Wet grinding
  • Roll mill
  • Hammer mill
  • Cone mill
  • Micronizing mill
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3
Q

Reasons for size reduction

A
  • 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
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4
Q

How does energy loss occur during size reduction?

A
  • 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
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5
Q

Considerations in size reduction

A

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

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6
Q

Milling process

A
  • 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)

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7
Q

What is wet grinding?

A

Size reduction in liquid media, usually water

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8
Q

Advantages of wet grinding

A

1) Eliminate dust
2) Easier to handle material
3) Use less energy
4) Increase mill capacity

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9
Q

Disadvantages of wet grinding

A

1) Increase wear of grinding machine
2) Not applicable to soluble materials
3) May need to dry products

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10
Q

What is roll mill used for?

A

Best for soft materials, popular for creams!

  • Grind pastes; coarse crushing
  • Peripheral velocity of rolls & clearance between rolls can be varied
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11
Q

Hammer Mill

What, Advantages, Disadvantages

A

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)

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12
Q

How does the Cone mill work?

A

Infeed falls into conical screen chamber -> Rotating impeller imparts a vortex flow pattern to the infeed material
-> Centrifugal acceleration forces particulates to screen surface

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13
Q

Micronizing Mill (What)

A

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
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14
Q

Air Jet Mill

What, Advantages, Disadvantages

A

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
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15
Q

Advantages of Micronizing Mills

A

1) Widely used for fine grinding
2) Extremely efficient
3) Can mill very hard materials

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16
Q

Disadvantages of Micronizing Mills

A

Need good process control

17
Q

What’s the purpose of Milling?

A

To facilitate down stream processing such as blending & tableting

18
Q

What should we consider when choosing a size measurement method

A

1) Reliable
2) Reproducible
3) Sensitive

19
Q

Basic problem of particle size analysis

A

To describe a 3D object with just 1 number

20
Q

What shape can be described by a single number?

A

Sphere

- Perimeter diameter/Projected area diameter

21
Q

What is the equivalent sphere theory?

A

Convert weight of __ to weight of sphere

22
Q

List methods for sizing measurement

A
  • Scales
  • Microscopy
  • Sieves
  • Laser diffraction
  • Light scattering
23
Q

Microscopy

Use, Advantages, Disadvantages

A
  • 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
24
Q

Sieves

Use, Advantages, Disadvantages

A
  • 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
25
Q

What sieve can be used for particles <200um?

A

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
26
Q

How does Electrozone Sensing work?

A

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
27
Q

Disadvantages of Electrozone Sensing

A

1) Requires calibration standards that are expensive

2) Porous materials & dense materials pose additional problems

28
Q

Mechanism of Laser Diffraction?

A

Curvature of particles causes light to scatter

  • > Small: Bigger scatter b/c sharper curvature
  • -> From scatter determine size
29
Q

Advantages of of Laser Diffraction

A

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

30
Q

Mechanism of Light Scattering?

A

Analyzes the time dependency of light intensity fluctuations scattered from particles when they are illuminated
–> Smaller particles (Higher KE) -> Faster light scattering

PR3117 (13 decks)

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