Mixing part 3

Industrial pharmacy I
Lec 3:
mixing
Pharmacist
Hussein kadhim alkufi
MSc in pharmaceutics
Collage of thi-qar
University of pharmacy

Solid mixing
Is more commonly used in pharmacy in comparisons
to liquid mixing.
1_ solid mixing for small scale
2_ solid mixing for large scale

solid mixing for small scale
Hand mixing: simple method for solid mixing
(manually)
1_ spatulation: we put amount of powder on paper
to be mixing by spatula
2_ traturation: by using mortal and pestle
3_ tumbling : by use of a wide mouth container in
which we put the solid particles to be mixed, so we
can get also minor reduction in solid particle size.
Usually used in final step of mixing.

solid mixing for large scale
Represent mixing of dissimilar solid particle at
particular level.
HW: the solid mixing is more difficult than the liquid
mixing.

Particulate solid variable
1_ particle size and particle size distribution:
Are important since they largely determine the
magnitude of force, gravitational, inertial, that can
cause interparticulate movement relative to surface
forces, which resist such motion.
2_ particle density, elasticity, surface roughness and
shape.
These variable can effect the bulk properties of
powder more than effect the force between particle

Particles shape: may known shape factor with index
number on which the mixing mechanism or mixer
selection depend.
Shape factor: are scalar quantities may represent
constant value between the mean particle diameter,
particle SA and volume.

Also the shape factor can give idea about the shape
particle either irregular, spherical, cubic or rods, so
the more irregular particles give more value of shape
factor.
Also shape factor are important for mixing rate, flow
rate, segregation rate and angle of repose value.
Note: segregation increase in solid than liquid

Forces acting in multiparticlate solid systems
1_ Forces that tend to result in movement of two
adjacent particles or group of particles relative to
each other.
2_ forces that tend to hold neighboring particles in a
fixed relative position
First force: are forces of acceleration result either
from the contact with the mixer or from the contact
of other particle
Other forces: that resist the particular movement and
are inter particulate interaction associate with size,
shape, the surface properties.

Powder that have high cohesive forces due to
interaction of their surfaces can be expected to be
more resistant to intimate mixing than those whose
surfaces do not interact strongly.
Factors that influence this type of interaction are
surface polarity, surface charge, adsorbed substances
such as moisture H2O

Mixing mechanism
Solid mixing proceeds by the combination of
one or more mechanism:
1.Convective mixing
2.Shear mixing
3.Diffusive mixing
MIXING MECHANISMS

1. Convective mixing
Mechanism analogous to bulk transport in fluid
mixing
Convective (bulk) mixing occurred by: Inversion of powder
bed
A- the mixer like v- shape or cone shape in which we have
inversion of power powder
B- Blades or paddles
C- Revolving screw
D- Any method of moving large mass of material from one
part of powder bed to another.
This type of mixing mechanism mainly depend on geometry or the compsotion of
mixer used EX: cone mixer, v-shape mixer
By the aid of:

1. Convective mixing
Note: the mixing resulting by this mechanism may be called macromixing

2. Shear mixing
As a result of forces within particle mass slip planes
Depending on the flow characteristic of powder, that can occur in
such a way to give singly or may be converted to [laminar
flow]
When shear occurs between regions of different composition and
parallel to their interface.
Reduce the scale of segregation by thinning the dissimilar layers

May be called micromixing can give mixing at particular level.
This type of mixing mechanism mainly depend on particles
mobility are affected by their size, density and shape
3- Diffusive mixing
Random motion of particles within a powder bed
Change position by single particles relative to one another
Reduction intensity of segregation
Occur at: 1. interface of dissimilar regions undergoing shear
shear mixing
2. Produced by any form of agitation interparticulate
motion.

Factors affecting mixing of solids
1. Particle size
The particle sizes must be nearly similar in all
particles of the mixture.
Increasing the difference in particle size will lead
to segregation (size separation), since small particles
can fall through the voids between the larger
particles.
Constant movement of the mixer may lead to
suspension of fine particles in air. So stop the
mixer immediately or remove the air.

2- Density
The difference in densities among mixed
particles will lead to segregation. The heavy
particles settle down while light ones will rise
up.
This is aggravated if the heavy particles are
coarse and the light particles are finer.
So mixed particles would be equal in density to
attain good mixing.

3- Electrostatic charges:
These charges are formed due to constant friction
among the mixed particles.
Similar charges repel particles from each others leading
to segregation.
This can be overcome by:
1. Stopping the mixing equipment. (no increase in time
of mixing)
2. Adding wetting agent or surface active agent which
neutralize similar developed charges on the particles
3. Adding some water and evaporate it after the mixing
operation is completed (if water do not affect
stability of components)
HW: Conditions to for good mixing ( numerate)

Type of mixers for powder
I- Tumbling mixers:
Here, the movement of particles occurs by tilting the material
beyond angle of repose using gravity to impel flow

1- Tumbling barrel or drum mixer :
• Consist of cylindrical vessel rotating on its horizontal axis. Gravity
impels flow.
• To increase the efficiency, put the mixer inclined.
• It gives light movement so it is suitable for friable particles when.
Disadvantageous in low shear force

2- Cubical - shaped blender
Mixing occurs by sliding of powder on its wall and gives mixing in three dimensions if hanged
from the corner.
Disadvantages:
1. Difficult in cleaning due to presence of different corners.
2. Sliding action causes abrasion of particles

3- Double cone blender: Composed of two cones joined to short
cylindrical section. It is easily cleaned. It is charged by 50 % of its capacity
by powder to ensure complete transfer of powder in the two cones.
Advantages:
1. No dead spots in mixing
2. It is easily cleaned,
3. It contains no corners.

4- Twin -shell or V -shaped mixer: It gives more efficient and
more precise mixing due to high shearing force

Factors for good mixing in tumbling mixers:
1. Capacity of the mixer must not more than 50% of its capacity.
2. optimum time of mixing.
3. Optimum speed of the mixer ,increasing the speed causes adhesion
of powder on the walls of the mixer.
. Light handling of the powders to minimize size reduction.
5. The method of charging the powders in the tumbling mixers is
important: The materials must not be placed in layers. If placed in
layers, mixing will affect the upper layer only. So, materials are pre-
mixed or put side - by - side and mixing starts immediately.

Advantages of tumbling mixers:
1. They are mild equipment (not aggressive).Thus they are suitable
for friable materials.
2. They are preferable when different particle sizes and densities
powders are to be mixed due to repeated reversal of direction of
flow.
3. If some mixers trough contains an arm which rotates to transmit
shearing action to parti-cles representing an advantage.

II. Agitators mixers:
In these mixers, shear is applied by means of agitating
ribbon force.
Example : Ribbon blender

Ribbon blender:
There is an outer spiral ribbon to move the materials in one direction
and an inner spiral ribbon to move materials in the opposite direction.
The outer ribbon must have a fairly close clearance with the wall so no
material remains in the bottom sur-face.
It is used for blending of materials (or powders) tend to aggregate or
don't flow freely.
Dry Mixer

Disadvantages:
1. It is not precise .
2. Consumption of powder is more than that in tumbling.
3. Grinding of materials may occur.
4. It is not used for fragile particles

3- Special Mixers for Powder
1.Impact wheel mixer.
2. Pneumatic mixer.
4. Entolator impact wheel mixer:

• This is based on rotating a disc or a wheel 20-68 cm in diameter,
rotate 1750 - 3500 revolution / min.
• The materials are feed from above in the center of the disc. The shear
force developed cause continuous powder mixing.
• The particles of powder are then ejected by the centrifugal force
from the disc periphery onto the walls of a conical tank which gives
the mix-ture a spiral movement towards the bottom

4- Pneumatic mixer: (Airmix mixer)
Here, the driving force is the com-pressed air
which is introduced through nozzles present at
the lower part. These nozzles are arranged in a
man-ner that escaped air stream in a vertical
mo-tion gives a chance for powder to settle.

Evaluation of a blend of powder after mixing :
1.The sample mixture is taken and evaluated by:
2. Microscopic Counting if particles are different in shape.
3. Analytical techniques for the drug and other components in the
mixture
4. Screen analysis of the mixture, if the powder mixture is different in
size.

V- Type mixer:
It has two motors :One for rotating the shell and
shaft in opposite direction.
Advantages of the V-type mixer:
1. It is efficient in mixing
2. It has no dead spot
3. It has variable speed rotation
4. Easily cleaned
the other for rotating the baffled

III. Mixers of pastes and plastic masses (semi -
solids)
The equipment are similar to liquid mixers but heavy built so shearing action is
distributed to all parts of apparatus.
Types of flow in semisolids:
Materials are classified according to the type of flow into:
1- Newtonian flow:
2- Non-Newtonian Flow:
A_ Plastic
B_ pseudo-plastic
C_ Dilatant
D_ Thixotropic systems

Types of mixers for semisolids and pasts: There are four classes of
equipment:
(1) Beaters (2) Kneaders
(3) Mixer extruders (4) Mixing rolls.

1. Beaters:
These are similar in principle to the agitator mixers for powders and
liquids, but are more heavily built to handle materials of greater
consistency. The agitator arms are designed to give a pulling and
kneading action, and the shape and the movement of these arms are such
that there are no "dead spots" in the mixing vessel. An example of such
mixers is the Hobart's Mixer shown in the following Figure
Hobart's Mixer: (a) The mixer (b)
Planetary motion.

2. Kneaders:
Kneading involves squashing the material flat, folding
it over on itself, and squashing it once more. Most
kneading machines also tear the mass of material apart
and shear it be between a moving arm and a stationary
surface.
Double-Arm Mixer

Blades of various designs as those shown in the following Figure are
available for several kneading purposes:
Blades of Various Designs: (a) Sigma blade (b) double-naben blade,
(c) Dispersion Blade.

3- Mixer-Extruders:
The operation of a mixer-extruder depends essentially upon cutting and
folding the material in a mixing chamber by means of special blades, and
extruding It through a die, thereby subjecting the material to additional shear.
Mixer-extruders continuously mix materials that are difficult to mix such as
clays and thermoplastics.
An example of such mixers is the "Roto-feed Mixer" shown in the following
figure.
Roto-feed mixer

4. The Mixing Rolls:
In mixing rolls, the materials subjected to intense shear by passing
between smooth metal rolls turning at different speeds. By repeated
passage between such rolls, solid additives can be thoroughly distributed
into pasty or plastic materials.
An example of such mixing rolls is the "Triple-Roll Mill".

Mixing part 3

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