Various Processing Techniques
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Powder Preparation:
-- physical method: grinding/
granulation/ mixing
-- chemical method: precipitation/ solgel/ …etc.
Forming: pressing/ casting/ .. etc.
Sintering
 Get powder
you want by
simple
separation
methods of
minerals
 Purity is
usually not an
issue
Che5700 陶瓷粉末處理
Beneficiation
To improve both microscopic characteristics
and macroscopic uniformity of the system, and
also to modify the liquid content and rheology
for forming
 Include processing operations such as:
comminution, dispersion and mixing, particle
separation and concentration, granulation;
Che5700 陶瓷粉末處理
Crushing and Grinding
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Crushing, Grinding, Comminution, Milling, Blunger,
Pulverizing: several different terminology

Purpose: to reduce particle size, to release impurity
(from large raw material particles, e.g. minerals), to
modify particle shape, size distribution, to increase fine
particles, to destroy agglomerates
 good for: purity improvement, increase surface
area(for reaction or sintering), improve flowability, to
change microstructure etc.
Often require screening after grinding, take out the
qualified portion, remove the un-qualified particles; may
be contaminated due to wear
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Taken from TA
Ring, 1996;
Comminution and
classification often
go together.
Dry or wet
operation
Batch or
continuous
operation
Equipment
 Forces acting
on particles:
impact,
compression &
shear
Jaw
Crusher;
Rotary
Crusher;
Crushing
Rollers;
Hammermill
取自JS
Reed, 1995;
Che5700 陶瓷粉末處理
Ball Mill
media:
porcelain 2.3
g/cm3;
alumina 3.6;
zircon 3.7;
zirconia 5.5;
steel 7.8;
WC 15.6;
* size/quantity of ball (ball volume often 50% of total
volume), rotation speed, quantity of material, property of
material, ball material, container material; if wet milling
then viscosity also important factor
Laboratory Ball Mills
Critical rotation
speed: the mill is
centrifuging;
nc = (1/2)[g/(R-r)] 0.5
R = radius of mill; r =
radius of grinding
element;
Common practice:
65 – 80% of critical
value
A special product from a company: (as compared to a
conventional ball mill)，claim to save quite some energy。
Can reduce size down to 5m (not attainable by ball
mills)。 (energy efficiency: problem in grinding)
Che5700 陶瓷粉末處理
Energy Requirement

Crushing particles: mainly by shearing & impacting
forces, coming from velocity difference between
particles and balls;

Energy required to reduce particle from size Lo to L,
can be estimated by the following equations: energy
used in (1) rotating the equipment; (2)crushing the
particles; (3)wear of balls
L
C
E    n dL
L
Lo
n=1 Kick’s Law
n=2 Ritenger’s law
n=1.5 Bond’s law
Che5700 陶瓷粉末處理
Comminution Efficiency
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
The following two equations: for n = 1 and n = 2 (from
previous page)
Other empirical model: UT = Ac [1/am – 1/aom]; UT =
energy input in producing a unit of product; Ac:
efficiency constant; m = fracture constant; a = mean
size
Lo
E  k ln( )
L
1 1
E  k (  )
L Lo
Energy Consumption
Taken from
McCabe & Smith,
6e, p. 981
One of the most
inefficient
operations; probably
99% go to operating
the equipment,
producing noise,
heat, 1% creating
new surface;
(cooling is
necessary)
Che5700 陶瓷粉末處理
Taken from TA Ring, 1996; size on the right - feed, on
the left - product
Che5700 陶瓷粉末處理
Fracture of Particles
* Forces on a particle during grinding: compressive and
shearing forces; leading to crushing of particle into
several pieces, or gradually worn out due to attrition,
frictional stress, or rubbing (to produce very fine particles)
Taken from Chem. Eng. Sci.,
Aug, 56-62, 1989.
Terminology different here
Che5700 陶瓷粉末處理
Fracture Strength
•Attrition: produce many very
fine particles, yet the original
size distribution roughly remains;
•Some particles may undergo
anelastic deformation to reduce
stress, difficult to crush (tough
particles)
•Fracture strength of particle:
proportional to size and defect
of particle: Sf ~ KIc/ (y c 1/2) ;
c= depth of flaw; y=constant,
depending on defect geometry;
Small particles, size of defect
K material fracture toughness
also small  greater strength Ic
(influence by microstructure)
Che5700 陶瓷粉末處理
Milling Performance
•Action of grinding  formation of internal lattice defects
of particles, e.g. dislocation, may change properties of
particles, such as refractive index or phase change, etc.
•Chemical effect: additives (e.g. surfactants,
deflocculants) can wet the new surfaces  good for the
milling process.  CMP chemical mechanical polishing
(used in IC processing)
•Milling performance by:
• [particles generated/time] = (media
collision/time) x (particle impact/collision) x
(particles generated/impact)
Che5700 陶瓷粉末處理
Milling Performance (2)
•Media collision frequency  increase for small ball size and
rotation speed;
•Particle impact frequency  cylinder ball greater than
spherical balls, better when particle stick to ball surface, wet
grinding improve frequency (at the cost of more energy
input)
•Large balls for large particles, small balls for small particles.
Particle Fracture
Original microstructure: very important factor,
defects like vacancy, dislocation will grow due to
external force, if crack branching during fracture 
then produce finer fragments
If external force: frictional stress type  more
attrition effect
Che5700 陶瓷粉末處理
Milling Practices
•In general: ball volume ~ 50% of container volume,
particle volume – fill in the space between balls.
•Media size/feed size: ~ 25/1
• Wear-resistant property of ball: important factor.
• size distribution often log-normal distribution after
grinding;
• Some estimate: only 1% of energy for producing new
surface.
•Mohr scale of hardness: 1 – talc; 2 – rock salt or gypsum;
3 – calcite; 4 – fluorspar; 5 – apatite; 6 – feldspar; 7 –
quartz; 8 – topaz; 9 – carborundum; 10 – diamond;
Taken from JS
Reed, 1995;
ball mill vs
vibratory mill:
more cheap
 Vibratory mill:
better results
Particle Size Distribution
Mostly follow lognormal distributions,
but maybe skewed to
large sizes
Different
revolutions = different
grinding time
Example 17-1
Q: compare ball mill, vibratory mill, planetary mill: particle
volume fraction and their grinding efficiency
 ball media = mill 50% volume; material = interstices of
the media (media packing fraction = 55%), particle in slurry
= 40%  then media charge = (1-0.55) x 0.4 = 0.18 
effective volume fraction in mill = 0.18 x 0.5 = 0.09
 For vibratory mill, planetary mill: media up to 80%
volume then effective volume fraction = 0.8 x 0.18 = 0.14;
(greater than 0.09), collision efficiency increase, grinding
rate increase
Che5700 陶瓷粉末處理
Ultrafine Grinding
Difficulty in getting very fine particles:
 small particles more tough, less internal defect
 re-agglomerate (key: how to prevent this)
 energy utilization: low
 semifluid nature of fines
 protection of fines by large particles (cushion effect)
 Ways to improve:
 use closed-loop, continuous operation to remove fine
products
 use grinding aids: surfactant, deflocculants, etc.
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Effect of Ball Size
Sample: BaTiO3; ball: zirconia; size: 0.35 and 0.2 mm; sample
conc. 1 vol% (7 wt%) 陳奕安提供
Che5700 陶瓷粉末處理
Fluid Energy Mill
Taken from McCabe & Smith,
6e, p.978
One of ultrafine grinder
Carried by high speed fluid,
cause particle crushing on
impact, + classification, to
remove fines continuously
Particles can be reduced to
0.5 – 10 μm
Jet-O-Mill: high
pressure air to carry
particles, particleparticle collision;
particle-wall collision,
breaking particles;
small particles carried
away, large ones
return to system;
relative velocity 100
m/sec
10- pulverizing zone;
2 –classifier
6 – venturi nozzle;
7 –compressed air