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Particles > Milling > Flashcards

Milling Flashcards

1
Q

Forms of energy requirements in breaking mechanisms

A
  • Elastic & PLastic deformation
  • Slip Systems
  • Fracture Mechanims
  • Brittle Fracture
  • Crack initiation
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2
Q

Mechanisms of particle breakage

A
  • Impact (single rigid force)
  • Compaction (two rigid forces)
  • Shear (fluid or particle-particle interactions)
  • Attrition (Particle scraping against eachother or rigid surface)
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3
Q

Key factors to milling

A
  • physicochemical & mechanical properties
  • Initial size
  • Requirements
  • Temp Control
  • Cleaning
  • Safety
  • Energy Requirement
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4
Q

Name the 3 Crushing Laws

A

dE/dx = -k*x^m

Rittinger: Energy/mass is proportional to new surface area (m = -2)

Kick: Energy is proportional to size reduction ratio (m = -1)

Bond: Intermediate (m = -3/2)

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5
Q

Which crushing law is appropriate when:

A

Rittinger:
Formation of new surfaces (fine grinding)

Kick: Elastic deformation before fracture occurs (coarse grinding)

Bond: Intermediate

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6
Q

Energy Losses in Milling processes

A
  • Elastic deformation
  • Inelastic deformation
  • Elastic distortion in equipment
  • Friction between particles/wall
  • Noise, heat & vibrations
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7
Q

Effects of milling

A
  • Increased rate of reaction
  • Greater flow impeding
  • Greater leaching
  • Increased drying rate
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8
Q

Benefits of agglomeration

A
  • Ease of particle handling
  • Fine particles tend to adhere
  • reduce environmental/ health issues
  • Flowability
  • Reduced electrostatic charges
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9
Q

3 stages of wet granulation:

A
  • Wetting & Nucleation (Formation of initail agglomerates)
  • Consolidation & Coalescence
  • Attrition & Breakage
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10
Q

Critical Stokes Number

A

St* = (1 + 1/e)*ln(h/h_a)

e = coefficient of restitution
h = thickness of liquid surface layer
h_a = characteristic height of surface asperities
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11
Q

Stokes number

A

St = 8mu/(3pimu*d^2)

Measure of the relative kinetic energy absorbed plastically by viscous binder

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12
Q

Three types of granule growth

A

Non-inertial growth (St < St*)
- Collisions lead to coalescence

Inertial growth (St = St*)

Coating (St > St*)
- Kinetic energy too high to be absorbed by liquid layer (no coalescence)

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13
Q

Iveson Model

A

Gives Maximum granule pore saturation

S = wro_s(1-eps)/(ro_l*eps)

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14
Q

Physcial properties causing segregation

A

Particle shape
Size
Density
Size distribution

Size most important and density the least

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15
Q

Consequences of segregation

A

Variations in:

  • size distribution
  • Bulk density
  • Chemical composition
  • Effect functionality of mixing equipment
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16
Q

Mechanisms of segregation

A
  • Trajectory (side to side)
  • Sieving/ Vibration (Top to bottom)
  • Elutriation (Top to Bottom)
  • Agglomeration
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17
Q

Trajectory Segregation

A
  • Horizontal movement of particles
  • Drag governed by stokes law

D = Urox^2 /(18*mu)

D = limiting horizontal distance it can travel
x = diamter
18
Q

Percolation of fine particles (Sieving) Segregation

A
  • gaps created allow small particles to fall from above
  • occurs whenever mixture is disturbed
    (Larger particles move upwards)
19
Q

Elutriation Segregation

A

Vessel with air flowing upward and air velocity exceeds terminal freefall velocity

20
Q

Agglomration Segregation

A
  • one components forms agglomerates easily whilst others do not
21
Q

Methods of reducing segregation

A
  • Decrease particle size (stronger interparticle forces)
  • Reduce particle mobility (addition of liquid)
  • Low free fall height
  • remove vibrations
  • decrease heap size
22
Q

Mechanisms of mixing

A
  • Convective mixing (transfer of larger particle groups from one location to another)
  • Shear mixing (particles of different velocities leads to velocity distribution)
  • Diffusive mixing (random motion of particles)
23
Q

Mixing Index

A

Ratio of mixing achieved to mixing possible

If zero - completely segregated
If 1 - completely random mixture

24
Q

Define Bin or Silo

A

Container for bulk solids

25
Q

Define bulk solid

A

Material consisting of discrete solid particles

26
Q

Define Discharger

A

Device to enhance material flow from a bin (not capable of controlling rate)

27
Q

Feeder

A

Device for controlling the rate of withdrawal of bulk solid from a bin

28
Q

Flwo channel

A

Space in a bin through which bulk solid is flowing during withdrawaL

29
Q

Hopper

A

Converging part of a bin

30
Q

Silo vs Bunker

A

Both types of bin:
Silo = H > 1.5D
Bunker = H < 1.5D

31
Q

3 flow patterns in Silos

A
  • Mass flow
  • Funnel flow
  • Expanded flow
32
Q

Mass Flow in Silos

A
  • All contents in motion whenever any is withdrawn
  • Discharge bulk density independent of head of material
  • Stable/predicatble flow channel
33
Q

Design considerations for mass flow in silos

A
  • Outlet should be large enough to prevent arching

- Wall of the hopper should be smooth and steep

34
Q

Funnel flow

A
  • some material moves towards outlet whilst rest is stationary
  • Friction between hopper and material is great enough to inhibit flow at interface
  • First in Last Out
  • Not suitable for cohesive materials (outlet bulk density can be affected)
  • Suitable for free flowing coarse materials
35
Q

Expanded Flow

A
  • Mass flow section below a funnel flow section
36
Q

5 Flow Problems in bulk solids

A
  • No flow
  • Erratic Flow
  • Flooding
  • limited discharge rate
  • Segregation
37
Q

Cause of No Flow in silo

A

Arch/bridge forms over outlet

38
Q

Cause of erratic flow

A

Formation of stable rat hole which collapses due to vibrations which may result in arch formation

39
Q

Cause of flooding

A
  • Material from above falls into channel/rat hole and becomes entrained in the air so the channel becomes fluidised
40
Q

Cause of low discharge rate in silos

A

Material may have low air permeability

41
Q

Factors affecting flowability of powder

A
  • Pressure
  • Moisture content
  • Particle size/shape
  • Temperature
  • Storage time
  • surface roughness

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