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M.Sc. Zheni Radeva, Dr.-Ing. Peter Müller , Prof. Dr.-Ing. habil. Jürgen Tomas
INFLUENCE OF THE PELLETIZING PROCESS PARAMETERS
ON THE MECHANICAL PROPERTIES OF RECEIVED ALUMINA
OXIDE PELLETS
Otto von Guericke University, Magdeburg, Germany, Institute of Process Engineering, Mechanical Process Engineering, [email protected]
I. Introduction
II. Materials : Primary particles and binder
Pelletizing is a common process to optimize the mechanical properties of various powder
materials. Industrially produced alumina oxide (γ-Al2O3) granules are used here as primary
model particles for the pelletizing process, carried out in a laboratory rotating pan-pelletizer
(spheronizer) at constant processing time of 20 minutes. Solution of viscoelastic polymer -
hydroxypropyl methylcellulose (HPMC) is used as binder. Its concentration was increased
until finding of the most suitable binder content for the model pellets. The rotational
velocity of the pan-pelletizer was varied in order to find the optimal speed, which provides
pellets with improved properties. The influence of the process parameter on the received
pellet product properties like density and porosity, size distribution, breakage characteristics
and breakage probability was analysed.
III. Experimental setup
IV. Results and discussion
It was clearly shown that the change in the rotational velocity of the bottom rotating disk and the amount of
added binder exert an enormous influence on the mechanical properties of the received agglomerated product.
It was proven that larger pellets can be obtained by increasing the binder amount.
Strong and compact pellets can be produced at large rotational velocities.
The binder content in the pellet increases its breakage resistance, but changes the breakage behaviour because of
the binder's plasticity and proves to be more problematic in case of binder addition to the batch of primary
particles.
Future outlook: V. Conclusions
2. Compression Test
1. Pelletizing
1. Light and scanning electron microscopy of the produced pellets
3. Force-displacement behaviour and breakage probability
As future outlook, an investigation of the influence of the primary particle diameter on the mechanical
properties of the pellets is suggested.
The influence of different binders will be proved in future research.
In order to understand better the process of breakage of the pellet by compression, DEM simulations
should be accomplished in future.
Specific mass-related Breakage energy
Em =EBMP= (ρPVP)
−1 F s ds
sB
s0
γ-Al2O3 properties
diameter (mm) 1.0
crush strength (MPa) 45
bulk density (g/m3) 740-820
porosity (%) 75
Hypromellose
(HPMC) Solution
- non Newtonian fluid -pseudoplastic behavior
- prepared by directly dissolving HPMC
powder into distilled water
AMW
(g/mol) pH chemical reactivity
748.807 5.0-7.5
combustible and react
vigorously with oxidizing
agents
Critical velocity : ωcrit
=μ
R∗gr
, rad/s ; Vcrit
=ra∗ω
crit, m/s
Force balance
Fn=0=F
c−F
R ; F
c−μ
R∗ F
G+F
A
O
=0; Fc=m
P∗r∗ω2;F
R=μ
R∗(F
G+F
A
O
)
The breakage test was carried out using special equipment provided by Ewete, Germany.
The loading velocity ν was fixed to 0.05 mm/s.
In order to perform a reliable compression test the pellet has to take an initial stable
position. During the stressing a deformed contact area will be developed between the
pellet and the piston.
The roughness of the irregularly shaped pellet is considered as a distribution of
hemispherical asperities with the curve radius (diameter) of the primary particles d <
dpellet that forms the surface of the pellet3. Thus for the approach, one may assume
characteristic, microscopically smooth, hemispherical asperities, whose curves are
equivalent to the primary particles so that the force-displacement models of Hertz and
Tomas can be applied2.
1.0
mm
Solid bridges formed from
consolidated binder
Primary particles (Al2O3)
Solid bridge bond
400 μm 400 μm
Solid
bridge bond
1 mm 1 mm
2. Influence of process parameters on the pellet cumulative size distribution
0
20
40
60
80
100
0 5 10 15 20 25
Pa
rtic
le s
ize
dis
trib
uti
on
Q3
in %
Pellet size d50,3 in mm
0.17 m/s
0.37 m/s
0.74 m/s
Rotational tip velocity
d50,3i
0
20
40
60
80
100
0 5 10 15 20 25
Pa
rtic
le s
ize
dis
trib
uti
on
Q3
in %
Pellet size d50,3 in mm
0.027 g/g
0.053 g/g
0.0109 g/g
Binder content
d50,3i
Elastic: Hertz law1 Fel =2
3E∗ R∗
s
2
3
=4
3
E1
1 − ν12 R1
s
2
3
=1
3
E1
1 − ν12 d1s
3
Elastic-plastic: Tomas2 Fel−pl,w−p−w = λel − pl πR1pF 1 −1
3
sFs
s
2
0
2
4
6
8
10
12
14
0.004 0.009 0.014 0.019 0.024 0.029
Forc
e in
N
Displacement in mm
Exp.valus
Hertz
Tomas
values
Force-displacement diagram for γ-Al2O3 pellets with d50,3=2.5 mm,
produced at rotational velocity 0.37 m/s, 20 min processing time and
binder content 0.053 g/g
Breakage
point
sB s0
EBi = F(s)dssB
s0
Elastic
deformation
Elastic-plastic
deformation
Yield
point 1000
1500
2000
2500
3000
0 0.05 0.1 0.15
Ela
stic
mo
du
lus
E1 i
n M
Pa
Binder content in g/g
0
200
400
600
800
0 0.05 0.1 0.15
Ela
stic
co
nta
ct s
tiff
nes
s k
n,e
l
Binder content in g/g
Elastic modulus E1 and elastic contact stiffness kn,el as function of the binder content
El-pl. contact stiffness kn,el-pl and fracture strength σB as a function of the binder content
0
200
400
600
800
0 0.05 0.1 0.15
El-
pl.
co
nta
ct s
tiff
nes
s k
n,e
l-p
l
Binder content in g/g
0
5
10
15
0 0.05 0.1 0.15
Fra
ctu
re s
tren
gth
σB i
n M
Pa
Binder content in g/g
0
0.2
0.4
0.6
0.8
1
0 500 1000 1500 2000 2500
Bre
ak
ag
e p
rob
ab
ilit
y P
Mass-related breakage energy in J/kg
0.17 exp.values
0.37 exp.values
0.74 exp.values
0.17 lognormal fit
0.37 lognormal fit
0.74 lognormal fit
Rotational tip velocity in m/s
Em,50i
0
0.2
0.4
0.6
0.8
1
0 500 1000
Bre
ak
ag
e p
rob
ab
ilit
y P
Mass-related breakage energy in J/kg
0.027 exp.values
0.053 exp. values
0.109 exp.values
0.027 lognormal fit
0.053 lognormal fit
0.109 lognormal fit
Em,50i
Binder content in g/g
1 H. Hertz „Ueber die Berühung fester elastischer Körper, J. reine u.angew. Math. 1881, 92, 156-171; 2 J.Tomas: Zur Mechanik trockener kohäsiver Schüttgüter, Schüttgut 2002, 8(6), 522-537; 3 S.Aman, J.Tomas and H. Kalman „Breakage probability of irregularly shaped particles“ Chem.Eng. Science, 2010. 65(5):p. 1503-1512.
deq =4A
π
Elastic modulus E1 and elastic contact stiffness kn,el as function of the tip velocity ν
600
800
1000
1200
1400
0 0.5 1
Ela
stic
mo
du
lus
E1 i
n M
Pa
Rotational tip velocity in m/s
0
200
400
600
0 0.5 1
Ela
stic
co
nta
ct s
tiff
nes
s k
n,e
l
Rotational tip velocity in m/s
El-pl. contact stiffness kn,el-pl and fracture strength σB as a function of the tip velocity ν
0
200
400
600
0 0.5 1
El-
pl.
co
nta
ct s
tiff
nes
s k
n,e
l-p
l
Rotational tip velocity in m/s
0
5
10
15
0 0.5 1
Fra
ctu
re s
tren
gth
σB i
n
MP
a
Rotational tip velocity in m/s
0.109
*
* Cowork with M.Sc. Reihaneh Pashmineh, Otto von Guericke University, Magdeburg Germany, Institute of Process Engineering, Chair „Thermal Process Engineering“