FINE GRINDING OF FOOD AND FIBROUS MATERIALSBY
Satoshi Akiyama
Senior Research Scientist
Nisshin Seifun Group Inc.

Carl K. Ishito
General Manager
Nisshin Engineering Inc.

ABSTRACT
A more efficient approach based on numerical simulations for determining optimum blade
geometry of the mechanical mill is described. The performance of the mechanical mill with theblades optimized by means of the approach is compared with that of a conventional mechanicalmill and a jet mill. In addition, effects of grinding conditions on changes in particle size, colorand flavor of the ground materials, which are important properties for food materials, have been
investigated.

1. INTRODUCTION
Mechanical impact mills with rotating blades are widely used in the food processing industry
for the processing of dry materials because of their efficiency. The demand for fine powder has
recently increased in the food processing industry as well as other industries.
In the case of mechanical impact mills, particles are mainly ground by mechanical stresses,
including impact and friction. These stresses are caused by the collision of particles with therotating blades, other particles and stator walls [1]. Therefore, in order to improve theperformance of mechanical impact mills, it becomes necessary to understand the flow fields andparticle trajectories in such mills because they play an important role in promoting the collisionof particles.
Mostly empirical approaches based on experiments are being used for design andoptimization of mechanical impact mills. For example, Leschonski and Drogemeier [2] havebeen carried out experimental studies on the impact speed and the design of an impact machine.
However, our understanding of the particle motions in mechanical impact mills is limited.

Recently, computational fluid dynamics (CFD) techniques have been developed andapplications of CFD to powder processes have been reported. By use of a three-dimensional
numerical simulation technique, Yoshida et al. [3] have been carried out a three-dimensional
numerical simulation of cyclones. In our previous report [4], the effects of the number of blades
and blade angle on the grinding performance of a mechanical impact mill for calcium carbonate
particles were studied numerically and experimentally.
In this report, we describe a more efficient approach based on numerical simulations fordetermining optimum blade geometry of the mechanical mill for fine grinding. The performanceof the mechanical mill with the blades optimized by means of the approach is compared withthat of a conventional mechanical mill and a jet mill. In addition, we show the effect of grinding
conditions on changes in particle size, color and flavor of the ground materials, which areimportant properties for food materials.

2. EXPERIMENTAL
2.1 Experimental apparatus
Figure 1 shows a schematic diagram of the experimental apparatus used in the studies. Raw
materials to be ground were continuously fed into the mill by means of a screw feeder. The raw
materials were introduced at the bottom of the mill with surrounding air through the inlet. The
materials were then ground by means of swirling shear flow, eddies generated by the blades andimpact with the walls. After grinding, the materials were discharged to a bag filter.
Figure 2 shows a cross-sectional diagram of the mechanical impact mill. It is possible toexamine a wide range of blade designs by using various fittings on a drive shaft. A cylindricalstator contains a multi-blade rotor, which is driven by a variable-speed motor. The inner wall of
the stator has many small grooves. Figure 2 also shows a summary of its geometric dimensions
and the definition of the blade angle θB.
Bag filter
When the blades inclined at a positive value,
the inside air is discharged in a downward
Orifice
direction. The number of blades NB andblade angle θB were variables.
Screw feeder
Blower

Raw materials used for the grinding
experiments were wheat bran, dried carrots
and green tea leaves. The airflow rate, Qa,
through the mill was measured using an
orifice meter and was maintained at 1.5
m3/min. The powder feed rate, Qp, was also
constant at 1 kg/h.

Manometer

Tachometer
Mechanical mill

Figure 1

Experimental apparatus

Outlet
D1
Blade

D2
z
r

T

Rotor
C

Figure 2

H1
W1

Dimensions
C =2mm
D1=150mm
D2=136mm
H =40mm
T = 6mm
W =20mm

Stator

Inlet

θΒ>0
Blade
Air flowωDefinition of blade angle

Cross-sectional diagram of mechanical impact mill

2.2 Characterization
Particle size distributions of ground powder were measured by means of a Microtrac
particle-size analyzer FSA (Nikkiso, Japan). A tristimulus colorimeter CR-310 (Minolta, Japan)
was used to measure color of green tea powder in the CIE L* a* b* color space, defined by CIE(Commission Internationale de I'Eclairage) in 1976. A GC-MS system GC-17A&QP-5000(Shimazu, Japan) was used to determine flavor of green tea powder ground in the mill.

3. NUMERICAL SIMULATION
3.1 Flow field and particle trajectory
A numerical simulation was conducted to examine how geometry of blades affects particle
behavior in the mill. The CFD software Fluent (Ver. 5.5) was used to calculate the flow fields
and particle trajectories. Figure 3 shows the calculation region and boundary conditions. The
domain enclosed by the two blades, the stator wall and the rotor disks above and below theblades was calculated. The stator was treated as a fixed wall and the rotor was treated as amoving wall. In the simulations, the small grooves on the inner surface of the stator wall were
not taken into consideration.
When the Fluent software is used, many models are available for describing turbulent fluid
flow. Examples include the standard k-ε model, the renormalization group (RNG) k-ε model, therealizable k-ε model and the Reynolds stress model (RSM). The realizable k-ε model [5],modified for swirling flows, was used in the calculations because of its stability and reasonableaccuracy. The particle trajectories were calculated using a Lagrangian dispersed phase model.
Collisions between particles and each wall were assumed to be perfectly elastic, with acoefficient of restitution equal to 1. It was also assumed that particle-particle interactions and theeffects of the particle volume fraction on the properties of the surrounding air were negligible.
The 100 mono-sized particles were uniformly released at the inlet with a velocity of 0 m/s. The
particle density in the calculations was assumed to be the same as the wheat bran (ρp = 1,400kg/m3) in the experiments.
Rotor disc
OutletBlade

Blade (uθ= r ω )Rotation
Rear side

Front side
z
Blade

r
u
Inlet

θ

Periodic
Stator wall(uz =0,ur=0,uθ=0)Figure 3 Calculation region

3.2 Impact energy
Using the particle trajectory calculations in the mill, information concerning the residence
time of the particles, the number of impacts between the particles and the walls, and the impact
velocity were obtained. Finally the energy applied to the particles thorough the grinding zone
was estimated.
The impact energy per unit mass for an i-th particle for a j-th impact are given by:

en i , j =1 2vn
2 i, j(1)where vn i,j is the normal components of the relative velocity of an i-th particle.
The total impact energy per unit mass, En, is defined as the amount of impact energy applied
to a particle in the calculations and the definition is shown as [6]:En =1 N t ni∑ ∑ enN t i =1 j =1 i , j(2)where Nt is number of particles used in the calculations and n i is total number of impacts for thei-th particle.

4.1 Effect of number of blades
4.1.1 Experimental results

Figure 4 shows the mass median particle
diameter, Dp50, of wheat bran powder groundunder the rotational speed ω = 14,000min-1
and the blade angle θB = 0o as a function of
number of blades NB. The median particle
diameter of ground powder decreases up to
NB = 16 and then increases.

Median particle diameter Dp50 [µm]4. RESULTS AND DISCUSSIONS

1000
Exp.
θB=0deg.

100

10
0

10

20

30

40

Number of blades NB [-]Figure 4 Relationship between median particle
diameter and number of blades
4.1.2 Comparison with calculated results

Figure 5 shows the calculated result of theflow field in the mill under the rotational
speed ω = 14,000 min-1, the number of
blades NB = 16 and the blade angle θB = 0o.
Radial and tangential velocity vectors areindicated in the figure. A strong swirling
flow is observed in the mill. Contours ofconstant pressure on the blade surfaces arealso plotted. Pressure is highest near the
lower corner of the blade on the front side.
Figure 6 shows the radial and tangential
fluid velocity vectors in the r-θ plane at aheight of 20 mm for the number of blades
NB = 8, 16 and 32. For NB = 8, there are alarge swirling flow near the blade of thefront side and a secondary flow near that of
the rear side. For NB = 16, a large swirling
flow between blades appears. Increasing thenumber of blades NB to 32, two small
vortices near the root of blades are induced
by a swirling flow near the tips of blades.

Pressure×10 4 Pa
1.0
0.5
0.0
-0.5

z

r

Figure 5θInletCalculated Velocity vectors
and pressure contours for
NB=16, θB=0

ω

ω

ω
W1

S
(a) NB=8(b) NB=16(c) NB=32

Figure 6 Calculated fluid velocity distributions
in r-θ plane at z=20mm
100
Impact energy En [kJ/kg]

Impact frequency F i [1/s]

4

10

Calc.
Dp=100µm
θB=0deg.
3

10

2

10

10
Calc.
Dp=100µm
θB=0deg.

1
0

10

20

30

40

Number of blades NB [-]Figure 7 Relationship between impact
frequency and number of blades

0

10

20

30

40

Number of blades NB [-]Figure 8 Relationship between impact
energy and number of blades

The calculated impact frequency, Fi, which means the total number of impacts with blades
and stator per second, is shown in Figure 7. Figure 8 shows the impact energy, En, as a functionof number of blades. For NB = 16, particles collide more frequently with the blades and statorwall because of the large swirling flow between blades, as shown in Figure 6 (b). As a result, theimpact energy has a maximum value at NB = 16. The trend for size reduction, as shown in Figure
4, is similar to that for the impact energy, as shown in Figure 8. It can be seen that the grinding
performance was improved because the impact energy of particles with blades and stator has amaximum value at NB = 16.

4.2 Effect of blade angle
4.2.1 Experimental results

Figure 9 shows the median diameter, Dp50, of wheat bran powder ground under the rotational
speed ω = 14,000min-1 and number of blades NB = 16 as a function of blade angle. The mediandiameter decreases with increasing blade angle up to 30o and then increases.

4.2.2 Comparison with calculated results

Figure 10 shows typical particle trajectories for a single particle with a diameter of 100 µm inthe mechanical impact mill using various blade angles. The positions of impact on the statorwall are also shown in Figure 10. For the blade angle θB = 0o, the particle collides with theblades and the stator wall while moving upward through the mill.

For a blade angle of 15o, the particle
collides with the stator wall several times
after the rebound of the particles from theleading edge of the blades, because a blade
inclined at a lower angle leads to a lower
rebound velocity in the z-direction. As aresult, the particle collides more frequently
with the stator wall. On the other hand, for a
blade angle of 30o, the particle collides a few
times with the stator wall while moving
downwards after the rebound from theMedian particle diameter Dp50 [µm]When particles collide with the inclined blades, they are accelerated downwards and movetowards the inlet boundary against the airflow. If the particles reached the inlet boundary, they
were injected again at the same positions and with a velocity of 0 m/s until they pass through therotor. It can be seen that the angle of incline of the blade clearly influences the residence time ofthe particles. In addition, it should be notedthat the blade angle has an influence on the
1000
number of impacts with the stator wall afterExp.
the rebound of the particles with the blades,
NB=16
as shown in Figure 10.
100

10
0

10

20

Figure 10(b) θΒ=15o

40

50

Blade angle θB [deg]Figure 9 Relationship between medianparticle diameter and blade angle

Impact positions
with stator(a) θΒ=0o30(c) θΒ=30o

Typical particle trajectories of a particle withdiameter of 100 µm for various blade angles

leading edge of the blade walls because of its higher rebound velocity in the z-direction.
Figure 11 shows the mean residence time of the particles with a diameter of 100 µm as afunction of blade angle. As can be seen from Figure 11, the mean residence time increases
rapidly with an increase in blade angles from 0 to 15o, and then gradually increases. This is dueto the fact that the inclined blades prevent the particles from passing through the rotor directly
because the particles are accelerated downwards against the airflow after colliding with theblades.

Figure 13 shows the calculated results of the
impact energy En. It can be seen that the impact
energy reaches a maximum value at a blade angleof 30o. The reason for this is due to the balance
between the effect of impact velocity and that ofresidence time. The impact energy depends, notonly on the impact velocity of the particle, but on
the residence time corresponding to the numberof impacts as well. As shown in Figure 12, thenormal impact velocity decreases with increasing
blade angle. On the contrary, the residence timeof particles increases with an increase in blade

1
Mean residence time [s]Figure 12 shows the normal component of
impact velocity of particles, vn, with a diameterof 100 µm as a function of blade angle. The
impact velocity decreases with increasing blade
angle.

Calc.
Dp=100µm

0.1

0.01
0

10

20

30

40

50

Blade angle θB [deg.]

Figure 11 Relationship between mean
residence time and blade angle

Impact energy En [kJ/kg]

Impact velocity v n [m/s]

60
Calc.
Dp=100µm

50
40
30
20
10
0

10

20

30

40

50

Blade angle θB [deg.]

Figure 12 Relationship between impact
velocity and blade angle

Calc.
Dp=100µm
NB=16

100

10

0

10

20

30

40

50

Blade angle θB [deg]Figure 13 Relationship between impact
energy and blade angle

angle, as shown in Figure 11. A blade angle of 30o results in a longer residence time of particles
than that of 15o and a higher impact velocity than that of 45o. As a result, the impact energy has
a maximum value at a blade angle of 30o.

5. PERFORMANCE OF THE OPTIMIZED MILL
We have developed a mechanical impact mill with the blades optimized using the numericalsimulation technique mentioned above. In this section, the performance of the optimized
mechanical mill (Blade Mill BM-15, Nisshin Engineering Inc.) is compared with that of aconventional mechanical mill (SRC-25, Nisshin Engineering Inc.) and a jet mill (Current Jet
CJ-10, Nisshin Engineering Inc.).
Figure 14 shows particle size distribution curves for the wheat bran powder ground in the
optimized mechanical mill and the conventional mechanical mill under the same conditions. The
feed material used here has a mass median diameter of about 500 µm. The median particle
diameters for the optimized mechanical mill and the conventional mechanical mill are 9.6 and35.2 µm, respectively. The maximum particle diameters of powder ground in the optimized
mechanical mill and the conventional mechanical mill are 124 and 352 µm, respectively.

Frequency [wt%]10
8Wheat bran

Optimized

6

Conventional

4
2
0
0.1

1

10

100

1000

Particle diameter [µm]Figure 14 Particle size distributions ofwheat bran powder groundin optimized mill andconventional mechanical millSpecific energy consumption w [kWh/kg]

Figure 15 shows the relationship between the median particle diameter of product and thespecific energy consumption for the optimized mechanical mill, the conventional mechanicalmill and the jet mill. The optimized mechanical mill produces powder with a finer and narrower
100
Optimized
Conventional
Jet mill

101Wheat bran

0.1
10
100
Median particle diameter Dp50 [µm]Figure 15 Specific energy consumption
as a function of median particle
diameter of product for optimized
mechanical mill, conventional
mechanical mill and jet millparticle distribution and consumes less energy than the conventional mechanical mill. When
powder with a median diameter of 10 µm is produced, the energy consumption for the optimized
mechanical mill is 80 % less than that for the jet mill.

6. EFFECT OF GRINDING CONDITIONS ON POWDER PROPERTIES
6.1 Particle size distribution

Particle size distributions of powder ground in the mechanical mill depend on operation
conditions including rotational speed, airflow rate and powder feed rate. In particular, therotational speed is the most important parameter to control them. In order to study the effect of
the rotational speed on particle size distributions, dried carrot was ground in the optimized
mechanical mill at rotational speeds between 8,000 and 14,000 min-1.

Cumulative undersize [wt%]The particle size distributions of dried carrot powder ground at different rotational speeds areshown in Figure 16. The particle size distribution of product shifts to finer sizes as rotational
speed increases. The median particle diameters for the rotational speed at 8,000, 12,000 and
14,000 min-1 are 18.2, 13.7 and 6.4 µm, respectively. The shapes of the particle size distribution
curves are similar although the median particle diameter varies. The maximum particle
diameters of carrot powder ground at 8,000, 12,000 and 14,000 min-1 are 124.5, 74.0 and 18.5
µm, respectively.

100
80
60

Rotational speed-18,000 min
10,000 min-1

14,000 min

-1

40
20
0
0.1

Carot

1

10

100

1000

Particle diameter [µm]Figure 16 Particle size distributions of dried carrot powder
ground in optimized mechanical mill at different
rotational speeds

6.2 Color and flavor

Color and flavor of powder are important properties for food materials as well as particle size
distributions because they have a great influence on the quality of final products. Effects ofgrinding temperatures and particle sizes of product on color and flavor of green tea powder havebeen investigated.
The CIE L* a* b* color coordinates were used to determine the color of green tea powder
ground in the optimized mechanical mill under various conditions. The L* value shows lightnessof a color and ranges from black at 0 to white at 100. The a* value shows red when positive andgreen when negative, and the b* value shows yellow when positive and blue when negative.
Chroma C* can be determined from values of the a* and b*, which is given by:

C * = (a * ) 2 + (b * ) 2(3)

Figures 17 and 18 show effect of air temperature at the outlet of the mill on lightness andchroma of the green tea powders with different median diameters. The air temperature in themill has no significant effect on the lightness and chroma of the green tea powder. However,particle size has a significant effect on the lightness and chroma. As can be seen from Figures 17
and 18, the lightness L* and chroma C* increase with decreasing median particle diameter.
Figure 19 shows intensity of flavor for green tea powder ground under various conditions.
Flavor intensity was determined using the GC-MS analysis. It can be seen that the flavor
intensity decreases with increasing air temperature at the outlet in the mill. In order to preventfrom reducing flavor, it is important to control temperature in the mill.

D50
8µm
12µm
15µm

*

72
68
64
60
-20

0

20

D50
8µm
12µm
15µm

32
Chroma C

Lightness L

*

76

40

60

80

Outlet temperature T o [oC]

Figure 17 Effect of grinding temperatureon lightness of green tea powderfor various particle sizes

28
24
20
-20

0

20

40

60

80

Outlet temperature T o [oC]

Figure 18 Effect of grinding temperatureon chroma of green tea powderfor various particle sizes

Flavor intensity

1.0
0.8
0.6
D50
8µm
12µm
15µm

0.4
0.2
0.0
-20

0

20

40

60

80

Outlet temperature To [oC]

Figure 19 Effect of grinding temperature on flavor ofgreen tea powder for various particle sizes

7. CONCLISIONS
Experimental and numerical studies have been conducted on the effect of blade geometry andoperating conditions for a mechanical impact mill. The following conclusions can be made:(1) The simulated impact energy is one of the most important parameters for determining
optimum blade geometry of the mechanical mill.
(2) The mechanical mill with the optimum blade geometry produces powder with a fine andnarrow particle distribution compared with the conventional mechanical mill.
(3) Energy consumption of the optimized mechanical mill is 80 % less than that of the jet mill
for wheat bran powder with a median diameter of 10 µm.
(4) For green tea powder ground in the mechanical mill, the particle size has a significant
effect on the color and the grinding temperature influences the flavor.

NOMENCLATURE=
parameters in the CIE L* a* b* color space
a*, b*=
chromaC*=
diameter of particles
Dp=
median diameter of particles
Dp50=
total impact energy per particle massEn
en=impact energy=
frequency of impact
Fi=lightnessL*=number of blades
NB=
average number of impacts between particles and wallsNa=
total number of injected particles
Ntn=total number of impacts

[-]
[-]
[µm]
[µm]
[kJ/kg]
[J]
[1/s]
[-]
[-]
[-]
[-]
[-]

r
S
t
To
u
vn
W1
w
z

=
=
=
=
=
=
=
=
=

radial coordinate
blade pitchtime
air temperature at outlet
fluid velocity
normal component of particle impact velocity
blade width
power consumption per powder feed rate
axial coordinate

[mm]
[mm]
[s]
[oC]
[m/s]
[m/s]
[mm]
[kWh/kg]
[mm]

=
=
=
=

circumferential coordinate
blade angle
particle density
rotational speed of rotor blade

[rad]
[°]
[kg/m3][1/min]

Greek

θ
θB
ρp
ω

REFERENCES[1] R. Drogemeier and K. Leschonski, Int. J. Mineral Process, 44-45 (1996) 485[2] K. Leschonski and R. Drogemeier, XVIII Int. Mineral Proc.Congress,Sydney (1993) 227[3] H. Yoshida, T.Saeki, K.Hashimoto and T.Fuyuki, J. of Chem.Eng. of Japan, 24 (1991) 640[4] S. Akiyama, K. Kozawa and H. Yoshida, WCPT4, Sydney, Australia, No.697 (2002)[5] T. H. Shih et al., Comp. Fluids, 24 (1995) 227[6] S. Akiyama, K. Kozawa and H. Yoshida, Kagaku Kogaku Ronbunshu, 30 (2004) 108