Powder mixing

Abstract

Mixing operations in the powder processing industries used to be carried out with relatively simple equipment. At present, the trend is the development of more complex mixing requirements. In this report, an overview is given of powder mixers, and the theory of mixing powders. The background of some of the latest developments in powder mixing equipment is also discussed.

Full text

. '
powder
hlu
ll
ql.
Vol.
11
·
No.
1 ·
January/March
1999
..........
1. Introduction
Mixing
of
powders
is
one
of
the oldest
unit operations in the powder processing
industries.
Thi
s fact is still reflected
in
the
design
of
most
of
the ltidustrial mixers
nowadays. Their basic design has some-
times been developed 50
or
more years
ago.
AQd
although most mixing opera-
tions still can be performed with relative
simple equipment a trend
is
developing
towards more complex mixing require-
ments.
Besides blending
of
components modern
mixers also have
to
coat
or
granulate and
also more stringent mixing quality
re
-
quirements are demanded by the market.
This paper gives a condensed overview
of,
powder mixers, the theory
of
mixing
of
powders and
it
describes the back-
ground
of
some
of
the latest develop-
ments
in
powder mixing equipment.
2.
Pow
der
Fl
ow
Behavi
our
Based
on
a thorough observation
of
pow-
der mixing processes, different mecha-
nisms have been determined: firstly, the
overall
rea
rrangement
of
parts of the mix-
ture and secondly a mechanism acting on
·
ting
le
particles
or
agglomerates. These
different mixing mechanisms can be
re
-
lated
to
the different types
of
powder flow
beha
vi
our which powders can exhibit.
In
this respect powders can
be
subdivided
in
free-flowing
po
wders and cohesive
powders. Depending on the relative
strength of the inter-particle forces
in
rela-
tion to the size and density of the single
particles powders tend
to
demonstrate ei-
P G.J van der
We
i. Hosokawa Micron B
V.
.
G1l-
denstraat 26.
NL
-
7000
AB
Doet
1n
chem.
Th
e
Netherlands
Tel.:
+3
1 31 43 73 333; Fax- +31 31 43 73
456
e-mai
l.
info@hmbv.hosokawa.
com
Deta
il
s about the author
on
page 1
38
Powder
Mixing
Peter
van
der
Wei,
The
Netherlands
ther a cohesive character
or
show a loose
and free-flowing character.
2.
1
Mi
xing of
Fr
ee-Flowing
Po
wders
The macroscopic transport process of
parts of the mixture is often referred
to
as
convective mixing. This process causes
an
overall mixing of the ingredients. After
filling a mixer with different components a
layered order exists: component A at the
bottom and component B
on
top
of
A etc.
By using some kind of ribbon
or
screw
type
of
mixing tool material
is
continuously
transported throughout the mixer, result-
ing
in
a less separated structure. Depend-
ing on the duration of the mixing process,
the transport rate and the transport
effi
-
ciency a mixture with a certain homo-
geneity
is
obtained.
Fig
. 1 shows
an
ex-
ample
of
a convective powder mixer.
Usually a convective mixing mechanism
satisfies for mixing free-flowing powders.
The loose structure of such powders al-
lows particles to mingle with each other
on
a single particle scale. However, prob-
lems can arise when one or more of the
components tends
to
segregate.
2.
1.1
Segregation or De-Mix
ing
of
Free-Flowing Powde
rs
The
main cause for segregation of com-
ponents
in
a mixture is due
to
differences
in
size of the particles, and
to
a minor ex-
tend differences
in
particle density of
shape. Usually cohesive powder with par-
ticles below about 75 micron exhibit a suf-
ficiently strong cohesive character thus
preventing segregation. However, when
particles become larger
an
_d no longer
stick together it is possible for smaller par-
ticles
to
move into voids between the
larger particles. Besides particle
size,
the
segregative movements between parti-
cles are also strongly influenced by parti-
cle shape,
i.e.
the contact area between
two particles. The dynamic behaviour of
the smaller particle moving into the voids
of
the bigger ones
is
dependent on the
density
of
the materia
l,
heavy small parti-
cles will move faster than l
ig
ht ones.
• Ref. (
3]
shows three main mechanisms
for segregation of powders:
• Percolation
In a packed bed of powder gravity
causes small particles
to
move into the
voids between larger particles, this
is
lik
ely
to
happen when the difference in
particle size
is
relatively large.
Fig. 1: Example
of
a
con
vective mixer with a high
speed chopper: a V
ri
eco-Nauta® mixer
equi
pped
with an Intensifier. This m
ac
hine
com
bines t
he
convective mixing
by
the
orb
it-
ing screw wi
th
inten
si
ve mixing by the Intensi-
fier. In this case an optional liquid injection
through the Intensifi
er
rotor is shown.
83

Mixing
of
Powders
• Vibration
Upon vibrating
of
a bed of powder,
smaller particles will gradually move
under the bigger ones and thus lead
to
a
separation
of
the differently sized par-
ticles. It
is
this phenomenon that causes
heavy objects (e.g. stones or aircraft
bombs) to
rise
slowly
in
beds of sand or
soil upon repeated cultivation
of
the land.
• Transportation
When transporting powders the particles
wi
ll
be
constantly accelerated and decel-
erated, e.g.
in
bends
in
pneumatic con-
veyors or when charging a hopper.
Due
to
differences
in
trajectories
of
particles
with different masses and/or sizes these
particles will be separated during trans-
portation. Similar effects happen when
such powders are poured on a heap. The
heavier particles will
roll
to t
he
outside of
the heap while the smaller concentrate in
the centre
of
the powder heap. The
shape
of
the particles also plays an im-
portant role during this type of segrega-
tion process.
Measures against segregation of free-
flowing powders can be found
in
different
directions: alteration
of
the powder parti-
cle characteristics which means to give
them a more similar size or make them
more cohesive. Also special precautions
can be taken during handling of these
powders. By reducing the transportation
velocity or the falling height segregation
is
minimised.
2.2 Mixi
ng
of Cohesive Powders
More complex becomes the situation
when the powder is
no
longer a free-flow-
i
ng
powder but exhibits a cohesive char-
acter. This means that inter-particle
fo
rces are strong enough to keep parti-
cles together
in
a structured manner dur-
ing handling of the product. These inter-
particle forces can either be electrostatic
forces, VanderWaals forces or can be
forces caused
by
liquid bridges
in
humid
products. During convect
ive
mixing these
structures
are
not broken up
by
the rela-
tive mild forces used for transportation
of
the material through the mixer. This
means that after completion of the con-
vective mixing process the material
is
only mixed at a macroscopic l
evel.
When
looking at a microscopic level,
i.e.
at the
single particle scal
e,
the cohesive struc-
tures remain unchanged.
In
order to
br
eak up these cohesive struc-
tures additional mixing forces are neces-
sary. The application
of
such extra forces
during mixing will also lead to an addi-
tional energy input, the mixing becomes
more intense. For this reason these type
of mixing mechanism are often called in-
tensive mixing.
The energy required for the break-up
of
these structures can either be supplied
84
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l.
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Mar
ch
1999
powder
h11N1Na1
l
....-..
..
by
high
speed impact forces like chop-
pers of knives or by shear forces
in
rela-
tively slow moving equipment like for ex-
ample an edge runner
mill.
The impact
forces simply cut the structures
in
the
powder into pieces, the shear forces de-
form the structures enough to break
them up.
Every
impaction will break-up a
limited number
of
structures: only the
structu
res
which are hit
by
the impact el-
ement. Shear forces are much more ef-
fective:
all
structures
in
the
en
t
ire
shear-
ing zone are subjected to breakage.
2.2.1 Impact Mixers
In
practice most industrially used inten-
sive mixers apply impact forces for en-
ergy input during mixing
of
cohesive ma-
terial. This can mean that conventional
convective mixers
are
combined with
an
additional chopper, thus combining con-
vective movement
of
the material with lo-
cally
high
energy input for intensive mix-
ing,
such a mixer is shown in
Fig
.
1.
In
other cases special machi
nes
have been
developed for the intensive mixing
of
powders. Here the overall convective
mixing process which
is
always neces-
sary for producing homogeneous prod-
ucts
is
also performed by the intensi
ve
mixing element. Fig. 2 shows a typical
high speed impact mixer developed for
the intensive m
ix
ing
of
powders.
In
case of
in
tensive mixing by impact
forces the structures which are destruc-
t
ed
have
no
preference for enrobing other
particles, they can easily form simi
lar
structures again or adhere to other parti-
cles
in
a random manner. The latter
means that the high capacities
of
high im-
pact mixers are obtained by using a less
intensive mixing process.
2.2.2 Shear Mixers
The principle
of
applying shear forces for
breaking-up structures
in
cohesive mix-
tures
is
best illustrated with one
of
the
Fig.
2:
Example of an
Intensive mixer
mixing
by
high
speed impaction
most intensi
ve
mixers
av
ailab
le
at t
he
market t
he
Mechanofusion. Here the
materi
~I
to be mixed is cent
ri
fuged
in
a
rotat
in
g chamber. Due to the high radial
velocities the
ma
teri
al
is compacted to
the wall and in this compacted material
extremely high shear forces are produced
by a stationary shear element. Fig. 3
shows a schematic drawing
of
this mech-
•
Flg.
3: Working principle of the Mechanofusion,
showi
ng
the material which is compacted at
the wa
ll
due
to
the rotation of the vesse
l.
A
stationary shearing device introduces shear
into the product while a scraper transports
the material in the machine.
..

powder
llalHl
ll
q
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Vol.
11
·
No
. 1 ·
January/March
1999
,,...,
..
Mixing
of
Powders
anism. This figure also shows the scraper
which removes the compacted
and
sheared material from the wall, thus pro-
ducing the necessary convection or
transportation
of
material.
Although the enormous shear forces pro-
duced
in
the Mechanofusion are almost
ideal for the intensive mixing of material,
applications
of
this technique are
limi
ted
to
smaller batches and special mixing
processes, whereas components
are
re-
ally fused onto other materials
by
the ex-
treme mechanical forces. Due to con-
structive restrictions the high shear forces
are
only produced
in
a small zone in the
product, making the process somethi
ng
less effective and strongly reducing the
overall mixing capacity.
By
shearing material a rolling motion
of
the particles
is
obtained and when struc-
tures
of
cohesive components are bro-
ken-up
in
the shearing zone the single
particles encapsulate directly the larger
rolling particles. This process results
in
a
nearly ideal state
of
mixing
.
Fig
. 4 sum-
marises the differences between the vari-
ous mechanisms available for mixing
solid materials.
2.3
New
Developments
in
Intensive Mixing
In
a search for the ideal intensive mixer
recently successful attempts have been
made in combining the principle of
in
-
tensive mixi
ng
by
shear forces and the
high capacities
of
the conventional im-
pact mixers. A stationary, vertical conical
vessel has been equipped with a fast ro-
tating shaft with paddles which move
along the wall, Fi
g.
5 shows a drawing
of
this mixer geometry. Due to the high ro-
F
ig
.
5:
Flow pattern
of
product inside the conical
high speed shear mixer, the Cyclomix.
Due
to
the conical shape the material a convective
transport pattern
is
obtained
in
the mixer.
High shear
is
produced between the fast
moving paddles and the stationary wall.
•
<-
tation speed
of
the paddles the material
is
centrifuged towards the wall. At the
wall a fast rotating ring
of
material
is
ob-
tained. Due to the conical shape
of
the
vessel the material
in
this
ring
is
gradually
transported upwards. The increased ra-
dius of this ring also introduces a con-
stant acceleration
of
the material.
In
the
upper part
of
the mixer no paddles are
present and the material
is
decelerated.
The shape
of
the cover
of
the vessel
guides the material into the centre of the
mixer where it falls down to the bottom
where it
is
accelerated again.
The up and down transport of the mater-
ial
ensures a rapid overall mixing of the
components to be mixed. The constant
acceleration and deceleration
in
combi-
nation with high friction with wall provide
the necessary intensive mixing capacity.
The mixing mechanism
in
this mixer can
be best described
as
high speed shear
mixing. The centrifugal
fo
rces produce
the necessary compression
of
the mater-
ial
while the high speed, constantly accel-
erating ring of material experiences high
shear forces with the wall. Some impact
on the material
is
produced when the ma-
ter
ial
hits the bottom scraper of
th
e mix
er,
although this effect
is
of
minor
im
por'-
tance for the mixing process.
Experiments pro
ve
that wi
th
this type of
intensive mixer approach
es
the mixing re-
sults which can be obtained wi
th
the
aforementioned Mechanofusion process,
in
combination with production capaciti
es
which equalise
th
e capacities of industr
ia
l
impact mixers. Although no real mechan-
ical fusion
of
particles has been ob-
served, the high speed shearing process
in
the new mixer leads to nicely coated
particles.
fffi
• A particle and a cohes,ve lump before
m1xmg
This specific type
of
in-
te
nsi
ve
mixing mecha-
nism makes the new
mixer
of
extreme interest
for applications
in
for ex-
ample toner or powder
pa
in
t additive blending,
whe
re
it is essent
ial
that
components a
re
evenly
distributed over the sur-
face
of
the primary parti-
cles. Of course, normal
mixing will
re
m
ain
of im-
portance for the powder
process industries, but
more and more mixing
processes require spe-
cial
effects
of
the mixer
in order to obtain special
products. Not only
in
the
toner and powder paint
industry, but also in
pharmaceutica
ls,
plas-
tics and ceramics indus-
try this trend is
ob-
served.
t
t
•
'
•
M1xmg
result after
m1xmg
with a cohe-
sive
powder
M1x1ng
result after
1ntens1Ve
mixing with
an
1mpact1on
mixer
M1xmg
result after
mtensJVe
mixing with
a high shear mixer
M1xmg
result after treatment with the
Mechanotus1on®
Fig.
4:
The m
ix
ing quality obtained with the different mixing mechanisms.
Shown
is
a b
in
ary mixture
of
a larger particle and a l
ump
of
cohesive
material.
Fig. 6: Simplified mixer selection chart
_ I
Mixer
Selection I
Free
Flowing
Powders
I
Segregation
Problems?
no
Tumbler
SlloMlxen
yes
Shear
Mixers
I
Extruders
Mills
Cyclomlx
Impact Mixers
I
Henschel
type
mixers
L6dlge type
mixers
8rtch type
mixers
85

Mixing
of
Powders
3.
Mixer
Sele
ct
ion
On the basis
of
the flow characteristics of
the
powder to be mixed, mixers can be
selected according
to
the scheme given
in
Fig
. 6 (see previous page).
This selection diagram
is
solely based on
the flow characteristics
of
the materials to
be mixed. When selecting a mixer for a
certain indust
rial
application more factors
will influence the final choice. The capac-
ity
of
the mixer, the batch size, contami-
nation risk, mixing time, dimensions, etc.
will
all
play a role
in
the definitive choice.
4.
Mi
xe
r Evaluation
Once a
mi
xer
is
selected for a certain ap-
plication it
is
necessary
to
monitor the
performance
of
the equipment. This
means that the quality of the mixture
is determined, usually by taking some
samples form the mixer. Both sampling
and the statistical phenomena around the
quality
of
mixtures give valuable informa-
tion on the performance
of
the mixer.
4
.1
Sampling
Regardless whether a batch
or
a continu-
ous
mixer
is
sampled it
is
preferred
to
sample
in
the discharge flow of the ma-
chine. In this way the entire volume of the
mixer can be observed, without missing
dead spots
in
the mixer volume which
would not be reached when taking sam-
ples from the bulk
in
the mixer. Because
of this it
is
also preferable to sample the
entire batch
in
stead
of
a limited number
of samples, although in most cases this is
impossibl
e.
Usually because
of
econom-
ics the number
of
samples is limited. Care
should also
be
taken for sampling
in
phase with some process parameter, like
e.g. the passage
of
a rotating mixing ele-
ment. Especially free-flowing powder
mixtures are very sensitive for this kind
of
regular sampling, because these
pow
-
ders tend
to
segregate quite easy.
Besides the number of samples the size
of the sample is also important for evalu-
ating the quality of the mixture. This
proper size
of
the sample is often referred
to as the scale of scrutiny. Usually the
size of the samples
is
determined by the
size of the packages, cans or bags
in
which the mixture
is
to
be filled.
In
case
of
the production of tablets
or
capsules
it
is
ttie size
of
these units which should
be
taken as appropriate sample size.
4.2 Mixing Qual
ity
By applying elementary statistics
it
is
very
we
ll
possible
to
evaluate the qua
li
ty of
86
Vol.
11
· No. 1 ·
January
/
Ma
rc
h 1999
powder mixtures on a
quantitative basis. In
this respect four de-
grees of mixing are im-
portant:
0000000
0000000
0000000
0000
•••
0000000
oe
oooo
e
oo
e o
•••
0000
••
0
oo
•••
oo
o
ee
o
••
o
4.2.1 The Unmixed
State
This state exists directly
after charging the mixer
with the components
to
be mixed. Since no mix-
ing has taken place a
sample of a binary mix-
ture will either consist of
component A
or
com-
ponent
B.
When sam-
pling this mixture one
will find either compo-
nent A
or
component
8, the standard devia-
tion
cr
0
in
the samples is
in this case defined as a
function
of
p,
the frac-
tion
of
component A
in
the mixture.
•••••••
•••••••
••••••• •••••o•
a)
the unmixed state b) the actual state of mixing
••
oe
oo
e
oe
oo
ee
o
oo
e
ooo
e
•
ooo
eoo
eo
ee
oo
e
oe
oo
ee
o
e
oo
e
oo
e
• o• o• o• .
..
.
oe oeoeo
• o• o• o•
oeoeoe o
• o• o•o•
oeoe oeo
• o• o• o•
c)
The state
of
random mixing d) the ideal state
of
mixing
Fig. 7: Visualisation
of
the d ifferent qualities of mixing
o;
= p .
(1
-p)
(
1)
4.2.2 The Actual State of Mix
ing
During mixing when taking n samples
from the mixer the standard deviation
in
the concentration x
of
one of the ingredi-
ents
is
defined as:
1 n
<fa
= - 1 .
L,
(x-
x)2
(2
)
n-
i=1
where x
is
the average concentration of
this component
in
n samples
4.2.3 The State of Random Mixing
In
practice the ultimate goal for most mix-
ing applications
is
the random state
of
mixing. This state is defined as:
-2
p ·
(1
-p) w · p ·
(1
-p)
CJ.=
= p
(3
)
r z m
whereas z
is
the number of particles
in
a
sample, alternatively written as a function
of the single particle weight
wP
and the
total mass
of
the sample m.
4.2.4 The Ideal State of Mixing
In
theory, when particles of the different
components are ordered
in
very regular
manner it is possible to reach a state
of
mixing which is even less distributed
than the state of random mixing. This is
e.g. the case when particles of compo-
nent A are
ho
mogeneously coated with a
layer
of
particles
of
compon
en
t
B.
In
this
ideal case the standard deviation in the
co
mpo
sition of the mixture approaches
zero!
The different phases
in
the state
of
mixing
are illust
ra
ted in Fig. 7 showing a model
re
presentation
of
a binary powder mix-
ture.
4.3 Mixing Indices
Usually, the quality
of
a powder mixture is
quantified
by
comparing the stanqard
de-
vi
ation of the actual state of mixing with
the initial state
of
mixing and the best
possible state of mixing.
In
the literature
various ways of,c:omparing those mixture
numbers- are mentioned, whereas the
most frequent
ly
used mixing index M is
de
fi
ned by
LA
CEY
[1
J:
M=~
- ~
~-of
(4)
This
m1
x1
ng index wi
ll
approach unity
when the mixing process
is
completed.
Referen
ces
[1]
H
AR
NB
Y, N., M.F. E
DWARDS
,
AND
A.W.
NIE
NOW
(E
DS.): Mixing
in
the Process In-
dustries; Butterworth & Heinemann,
1992
[2]
KAYE,
B.H.: Powder Mixing; Chapman
& Hall, London, 1997
[3]
FAY
ED
, M.E.
AN
D L. O
ITEN
(
EDS)
: Hand-
book of Powder Science and Technol-
ogy; Van Nostrand Reinh
ol
d Company
In
c., New York, 1984
I
I
..
!.