Milling Flashcards
Forms of energy requirements in breaking mechanisms
- Elastic & PLastic deformation
- Slip Systems
- Fracture Mechanims
- Brittle Fracture
- Crack initiation
Mechanisms of particle breakage
- Impact (single rigid force)
- Compaction (two rigid forces)
- Shear (fluid or particle-particle interactions)
- Attrition (Particle scraping against eachother or rigid surface)
Key factors to milling
- physicochemical & mechanical properties
- Initial size
- Requirements
- Temp Control
- Cleaning
- Safety
- Energy Requirement
Name the 3 Crushing Laws
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)
Which crushing law is appropriate when:
Rittinger:
Formation of new surfaces (fine grinding)
Kick: Elastic deformation before fracture occurs (coarse grinding)
Bond: Intermediate
Energy Losses in Milling processes
- Elastic deformation
- Inelastic deformation
- Elastic distortion in equipment
- Friction between particles/wall
- Noise, heat & vibrations
Effects of milling
- Increased rate of reaction
- Greater flow impeding
- Greater leaching
- Increased drying rate
Benefits of agglomeration
- Ease of particle handling
- Fine particles tend to adhere
- reduce environmental/ health issues
- Flowability
- Reduced electrostatic charges
3 stages of wet granulation:
- Wetting & Nucleation (Formation of initail agglomerates)
- Consolidation & Coalescence
- Attrition & Breakage
Critical Stokes Number
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
Stokes number
St = 8mu/(3pimu*d^2)
Measure of the relative kinetic energy absorbed plastically by viscous binder
Three types of granule growth
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)
Iveson Model
Gives Maximum granule pore saturation
S = wro_s(1-eps)/(ro_l*eps)
Physcial properties causing segregation
Particle shape
Size
Density
Size distribution
Size most important and density the least
Consequences of segregation
Variations in:
- size distribution
- Bulk density
- Chemical composition
- Effect functionality of mixing equipment
Mechanisms of segregation
- Trajectory (side to side)
- Sieving/ Vibration (Top to bottom)
- Elutriation (Top to Bottom)
- Agglomeration
Trajectory Segregation
- Horizontal movement of particles
- Drag governed by stokes law
D = Urox^2 /(18*mu)
D = limiting horizontal distance it can travel x = diamter
Percolation of fine particles (Sieving) Segregation
- gaps created allow small particles to fall from above
- occurs whenever mixture is disturbed
(Larger particles move upwards)
Elutriation Segregation
Vessel with air flowing upward and air velocity exceeds terminal freefall velocity
Agglomration Segregation
- one components forms agglomerates easily whilst others do not
Methods of reducing segregation
- Decrease particle size (stronger interparticle forces)
- Reduce particle mobility (addition of liquid)
- Low free fall height
- remove vibrations
- decrease heap size
Mechanisms of mixing
- 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)
Mixing Index
Ratio of mixing achieved to mixing possible
If zero - completely segregated
If 1 - completely random mixture
Define Bin or Silo
Container for bulk solids
Define bulk solid
Material consisting of discrete solid particles
Define Discharger
Device to enhance material flow from a bin (not capable of controlling rate)
Feeder
Device for controlling the rate of withdrawal of bulk solid from a bin
Flwo channel
Space in a bin through which bulk solid is flowing during withdrawaL
Hopper
Converging part of a bin
Silo vs Bunker
Both types of bin:
Silo = H > 1.5D
Bunker = H < 1.5D
3 flow patterns in Silos
- Mass flow
- Funnel flow
- Expanded flow
Mass Flow in Silos
- All contents in motion whenever any is withdrawn
- Discharge bulk density independent of head of material
- Stable/predicatble flow channel
Design considerations for mass flow in silos
- Outlet should be large enough to prevent arching
- Wall of the hopper should be smooth and steep
Funnel flow
- 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
Expanded Flow
- Mass flow section below a funnel flow section
5 Flow Problems in bulk solids
- No flow
- Erratic Flow
- Flooding
- limited discharge rate
- Segregation
Cause of No Flow in silo
Arch/bridge forms over outlet
Cause of erratic flow
Formation of stable rat hole which collapses due to vibrations which may result in arch formation
Cause of flooding
- Material from above falls into channel/rat hole and becomes entrained in the air so the channel becomes fluidised
Cause of low discharge rate in silos
Material may have low air permeability
Factors affecting flowability of powder
- Pressure
- Moisture content
- Particle size/shape
- Temperature
- Storage time
- surface roughness
