size - 2 Flashcards
fracture vs abrasion
fracture break into 2
abrasion - chipping smaller bits from big piece
4 mechanisms for breaking
shear - scissors
impact - hammer - single force
compression - two rigid forces
attrition - particle scraping against another or against a rigid surface
+ - of wet grinding
+
- no dust
- easier to handle material
- less energy
- inc mill capacity
- inc wear of medium
- cannot use for soluble material
- need to dry products after
roll mill uses
grind pastes eg creams w particulates - best for soft materials
- peripheral velocity of rolls and clearance between the rolls can be varied
3 types of hammer mill
- rotor type/ rasping screen
- low energy
knife edge rotor
- breaks down cellulose better
impact edge
- pulverisation powder grinding
hmmer mill good and bad
good
- many models , vert and horizontal designs
- medium to high shear
- interchangeable blades and screens
- can use for v hard materials
bad
- noisy and dusty
- must control the feed, cannot plug feed
- belt slip is common due to the load
- high volume air generated, need ventilation
- not scalable
- need to sift after milling
- temp rise due to friction
how does cone mill work and whta is it use for
- infeed falls to conical screen chamber
rotating impeller imparts vortex flow pattern
centrifugal acceleration forces particles to screen surface , delivered to action zone where they get crushed
sie reduced and discharged thru sceen openings
more for coarser particles
ball milling / vibratory ball milling
- used for fine gridning eg pigment efficient w vibration best wet can do dry also ball size and density is impt cascasing - takes long vibratory - fast but temp will rise
air jet mill and fluid energy mill how they work
difference
good and bad
how they work
- direct air to one spot
particles knock each other out grinding by particle particle impact
air jet uses classifier wheel but fluid energy mill uses centrifugal seperation
they are widely used for fine grinding
v efficient to micron size
can be used for v hard materials like diamond
but needs good process control
size reduction for
- ensure more efficient packing
- improve dissolubility
milling required fo r
improving performance and quality of powders for downstream processes
what can u co mill with
additives like solubilisers and lubricants
perimeter diameter vs projected area diameter
perimeter made into sphere
vs
area made into sphere
what is microscopy for and requirement
visualising particle size
- needs 10k images , not particles ! for validity
sample prep is laborious and slow
what are sieves for and
requirement
to seperate particle by size low resolution method usually only 4-8 size fractions good for qc used for > 200 microns
what is air jet sieve and what for , what result to use
and calc
for sieveing < 200 microns
( if > 200 use conventional sieve )
- particle smaller than aperture pass thru, weigh remaining material then repeat w another aperture size
plot the graph of percent weight oversize
span = d 90- d 10 / d 50
what is electrone sensing and used for what/ requirement
- used for sizing rbc in electrolyte medium
determine reisstance between 2 electrodes
resistance inc when particle enters so
extent of inc in resistance = particle size
lower limit 2-5 microns
need calibration standard - expensive and can change in distilled water and electrolyte
particle must be non porous , non conductive and insoluble
what is laser diffraction / low angle light scattering
method of choice today
- flexible, wide dynamic range can be used for dry powders, spray or particles in air/liquid
non destructive
non intrusive
rapid, fast, repeatble and high resolution
no need to calibrate
light pass thru and particle scatter based on curvature - big have lesser curve
larger particles will have low angle scatter
what is light scattering / dynmaic light scattering and quasi elastic light scattering
- based on brownian motion
particles 2-3 micrometer diameter and smaller
nalyse light intensity fluctuations from particles when they are illuminated
small particles higher kinetic energy speed of scatter faster
