size - 2

Question 1

fracture vs abrasion

Answer 1

fracture break into 2

abrasion - chipping smaller bits from big piece

Question 2

4 mechanisms for breaking

Answer 2

shear - scissors
impact - hammer - single force
compression - two rigid forces
attrition - particle scraping against another or against a rigid surface

Question 3

+ - of wet grinding

Answer 3

+

• 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

Question 4

roll mill uses

Answer 4

grind pastes eg creams w particulates - best for soft materials
- peripheral velocity of rolls and clearance between the rolls can be varied

Question 5

3 types of hammer mill

Answer 5

1. rotor type/ rasping screen
   - low energy

knife edge rotor
- breaks down cellulose better

impact edge
- pulverisation powder grinding

Question 6

hmmer mill good and bad

Answer 6

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

Question 7

how does cone mill work and whta is it use for

Answer 7

• 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

Question 8

ball milling / vibratory ball milling

Answer 8

- 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

Question 9

air jet mill and fluid energy mill how they work
difference
good and bad

Answer 9

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

Question 10

size reduction for

Answer 10

• ensure more efficient packing

- improve dissolubility

Question 11

milling required fo r

Answer 11

improving performance and quality of powders for downstream processes

Question 12

what can u co mill with

Answer 12

additives like solubilisers and lubricants

Question 13

perimeter diameter vs projected area diameter

Answer 13

perimeter made into sphere
vs
area made into sphere

Question 14

what is microscopy for and requirement

Answer 14

visualising particle size
- needs 10k images , not particles ! for validity

sample prep is laborious and slow

Question 15

what are sieves for and

requirement

Answer 15

to seperate particle by size 
low resolution method 
usually only 4-8 size fractions 
good for qc 
used for > 200 microns

Question 16

what is air jet sieve and what for , what result to use

and calc

Answer 16

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

Question 17

what is electrone sensing and used for what/ requirement

Answer 17

• 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

Question 18

what is laser diffraction / low angle light scattering

Answer 18

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

Question 19

what is light scattering / dynmaic light scattering and quasi elastic light scattering

Answer 19

• 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