This document provides information about mixing in pharmaceutical processes. It defines mixing as a process that combines two or more components so that each particle is in contact with particles of the other ingredients. Ideal mixing occurs when the quantity of materials is the same in all parts of the system. The objectives, types, mechanisms, equipment, and flow patterns involved in liquid and powder mixing are described in detail. Different types of impellers like propellers, turbines, and paddles used for mixing are also explained.

1. MIXING
BY: AMJAD ANWAR
C.NO: 18
SEMESTER: 7TH (2014-19)
DEPARTMENT OF PHARMACY
UNIVERSITY OF MALAKAND
1

2. MIXING
 An operation in which two or more components (in a
separate or roughly mixed condition) are treated so
that each particle lies as nearly as possible in contact
with a particle of each of the other ingredients.
OR
 The process that tends to result in a randomization of
dissimilar particles within a system
2

3. Ideal Or Perfect Mixing
 When each particle lay adjacent to a particle of the
other component (i.e. each particle lies as closely as
possible in contact with a particle of the other
component)
OR
 When quantity of materials in all part of a system is
same (E.g. ABABAB) it is perfect mixing.
3

4. Random mixing
 A mix where the probability of selecting a particular
type of particle is the same at all positions in the mix,
and is equal to the proportion of such particles in the
total mix
OR
 Proportion is different in all parts a system. E.g. AB AA
BA AB
4

5. SEGREGATION (DE-MIXING)
 Segregation is the opposite effect to mixing, i.e.
components tend to separate out
 Segregation arises because powder mixes
encountered practically are not composed of mono-
sized spherical particles, but contain particles that
differ in size, shape and density. These variations
mean that particles will tend to behave differently
when forced to move and hence, tend to separate.
5

6. Percolation segregation:
 Smaller particles tend to fall through the voids
between larger ones and so move to the bottom of
the mass
 It may occur in static powder beds, but occurs to a
greater extent as the bed 'dilates' on being
disturbed
6

7. Trajectory segregation:
 Larger particles will tend to have greater kinetic
energy imparted to them (owing to their larger mass)
and therefore move greater distances than smaller
particles before they come to rest. This may result in
the separation of particles of different size
7

8. Elutriation segregation/dusting out:
 When a material is discharged from a container,
very small particles ('dust') in a mix may tend to be
'blown' upwards by turbulent air currents as the
mass tumbles, and remain suspended in the air.
When the mixer is stopped or material discharge is
complete, these particles will sediment and
subsequently form a layer on top of the coarser
particles
8

9. OBJECTIVES OF MIXING:
 To achieve a physical mixture.
 To bring a physical change.
 To achieve a dispersion.
 To promote a chemical reaction.
1) Simple physical mixture:
 Producing a blend of two or more miscible liquids
or two or more uniformly divided solids
9

10. 2) Physical change:
 Producing a change that is physical as distinct from
chemical
 E.g. mixing a solid with a solvent to produce a solution
3) Dispersion:
 Dispersion of two immiscible liquids (Emulsion)
 Dispersion of a solid in liquid (Suspension or Paste)
4) Promotion of reaction:
 Activate, promote and control a chemical reaction so
ensuring a uniform product.
 E.g. where accurate adjustment to PH is requires and
the degree of mixing depend on the process
10

11. NEED OF MIXING
 It is a unit operation for tablet, capsule b/c
many ingredients are used, so mixing is
required.
 Single material (potent e.g. 0.2ϻg) which is
difficult to handle so we add ingredients which
Is mixed
11

12. TYPES OF MIXTURES
1) POSITIVE MIXTURE:
 Spontaneous and irreversible mixing (i.e. requiring
no energy input for mixing)
 Mixing……..Without energy
 Separation…..Requires energy
 E.g. when two or more gases or miscible liquids are
mixed together by means of diffusion process (water
and milk)
12

13. 2) NEGATIVE MIXTURE
 Mixtures that require a higher degree of mixing along
with the expenditure of energy for mixing
 Mixing…….Requires energy
 Separation….. Without energy
 E.g. suspensions of solids in liquids and Emulsions of
two immiscible liquids
13

14. 3) NEUTRAL MIXTURE
 Static in behavior (No tendency to mix
spontaneously, nor segregate when mixed)
 Both mixing and segregation occurs with expenditure
of energy.
 E.g. physical mixing (Pastes, Ointments and mixed
powders)
14

15. TYPES OF MIXING
 Liquid mixing or fluid mixing
 Powder mixing or solid mixing
 Semi-solid mixing
15

16. A) FLUID/LIQUID MIXING
 Mixing process may be easy for some fluid and
difficult for others. Following three parameters
gives necessary knowledge about basic
requirement of fluid for mixing.
 Flow characteristics.
 Mixing mechanisms.
 Mixing equipment.
16

17. FLOW CHARACTERISTICS:
 The fluid may flow freely or flow with resistance. They
show different flow characteristics, they may be
classified as,
 Newtonian fluid
 Non –Newtonian fluid
NEWTONIAN FLUIDS
 Such fluid flow like water and for them the shear
stress (force applied) is directly proportional to the
shear rate. Newtonian fluids are those whose
viscosity does not change by shear stress.
17

18. NON-NEWTONIAN FLUIDS
 Fluids whose viscosity changes with the increase
or decrease of shear stress. Non-Newtonian
fluids are of three types.
1) PLASTIC/BINGHAM FLOW
 A certain shearing stress must be exerted before the
flow begins. This stress is termed is Yield value (Flow
will not occur by applying stress below this value)
 The system behaves like a solid when small stresses
(lower than yield value) are applied i.e. the system
exhibits elastic deformations that are reversible when
these small stresses are removed
18

19. 2) PSEUDO-PLASTIC FLOW
 A type of flow where no yield value exist and the
flow begins immediately on the application of a
shearing stress
 Viscosity decreases as the shear rate is increased
until a constant value is reached
3) DILATANT FLOW
 It is opposite to that of pseudo-plastic flow in that
the viscosity increases with increase in shear rate.
As such material increase in volume during
shearing, they are referred to as dilatants and
exhibit shear thickening
19

20. MECHANISM OF MIXING
 Four types of mechanism are involved in
mixing of fluids.
 Bulk transport mixing.
 Turbulent mixing.
 Laminar mixing.
 Molecular diffusion.
20

21. Bulk Transport
 Movement of relatively large portion of a material
being mixed from one location to another in a
system
 Does not result in efficient mixing
 It is made effective by means of paddle, blade or
shuffling of system in three dimensions
21

22. Turbulent Mixing
 Turbulent mixing is the result of turbulent fluid flow
(Characterized by random fluctuation of the fluid
velocity at any given point in the system)
 The churning flow characteristics of turbulence
results in constantly changing velocities, so the fluid
has different instantaneous velocities at different
locations at the same instant in time
 Such temporal and spatial velocity differences
produces randomization of fluid particles that’s why
turbulent mixing is highly effective mixing mechanism
 Turbulent flow can be conveniently visualized as a
composite of eddies
22

23. Laminar Mixing
 Flow dominated by viscosity forces is called laminar
flow and is characterized by smooth and parallel line
motion of the fluid
 Applicable for viscous liquid or laminar liquids
 When two dissimilar liquids are mixed through laminar
flow, the shear that is generated stretches the
interface between them
 In this mechanism, layers fold back upon themselves.
Thus the number of layers increases. So, the mixing
involves reduction of fluid layer thickness by
producing folding effect. The applied shear stresses
between the interfaces of the 2 dissimilar liquids to be
mixed.
23

24. Molecular Diffusion
 The mixing result from the diffusion of molecules
caused by thermal motion is referred to as
molecular diffusion. This mechanism occurs at
molecular level.
 This type of mixing occurs whenever there is a
concentration gradient (According to Fick’s law).
24

25. MIXING EQUIPMENTS
Batch mixing
 Mixing a specific and limited quantity of material
 Impellers
 Air jet
 Fluid jet
Continuous mixing
 Equipment used for both are different. The two
components are common in the equipment used for
batch and continuous mixing processes which are;
 Tank of suitable size to hold material.
 Means of supply of energy to the system so as to enhance
the speed of mixing.
25

26. MIXING VESSELS
 The mixing apparatus consists of a container
(tank) and a mixing device.
The general construction of the mixing tank:
 Impeller, a mixing device, mounted with the help
of a shaft, which is driven by a motor.
 The tank is made up of stainless steel. The top
of the tank may be open or closed.
 The size of the tank depends on the nature of
the agitation method.
 The tank bottom is round (not flat) to eliminate
sharp corners into which the fluid can
accumulate.
 It also carries an outlet, coils, jacket,
temperature measuring device etc. wherever
necessary.
26

27.  The energy to the fluid can be given
by mean of;
 Impeller
 Air jets
 Liquid jets
27

28. FLOW PATTERN DURING MIXING
 Mixing occurs due to the resultant effect of 3
components acting on liquid:
 Tangential / Circular component
 Radial component
 Axial / Longitudinal component
PITCH: distance covered by liquid during axial flow
28

29. Tangential component
 Direction: Acts in the direction
tangent to the circle of rotation
around the impeller shaft.
 Effect: If shaft is placed
vertically & centrally, tangential
flow follows a circular path
around the shaft & creates a
vortex in the liquid.
29

30. Radial component
 Direction: Acts in the direction
perpendicular to the impeller
shaft.
 Effect: Excessive radial flow
takes the material to the
container wall, and then the
material falls to the bottom and
rotates as a mass beneath the
impeller.
30

31. Axial component
 Direction: Acts in the direction
parallel to the impeller shaft.
 Effect: Inadequate longitudinal
component causes the liquid
and solid to rotate in layers
without mixing. Adequate
longitudinal pattern is best used
to generate strong vertical
currents particularly while
suspending solids are present.
31

32. a) IMPELLER
 Impellers are mixing devices that provide a
definite flow pattern in liquid during mixing,
moving at various speeds.
 Liquids are mixed usually by impellers, which
produce shear forces for inducing the
necessary flow pattern in the mixing container.
CLASSIFICATION
 Impeller exists in different forms.
1. Propeller 2. Turbine 3. Paddles
32

33. 1) PROPELLER MIXER
CONSTRUCTION
 Consist of angle blades attached at the
end of the shaft, rotated by means of
motor.
 Any number of blades may be used but
three blades design is most common.
 Propeller is quiet small as compare to
size of the vessel (Ratio of diameter
between propeller and container is
1:20) but its operational speed (usually
8000rpm) compensate for the size and
produce efficient mixing in case of low
viscosity fluids
33

34. WORKING
 The material to be mixed is taken in a vessel and
the propeller bearing shaft is inserted.
 The angle blades of the propeller cause circulation
of the liquid in both axial and radial direction
ensuring good bulk transport but low shearing
force.
 The propeller may be installed in a number of ways.
 The centrally mounted vertical propeller is however
not considered good as it produces vertex.
34

35. ADVANTAGES
 Used when high mixing capacity is required.
 Effective for liquids which have maximum viscosity
of 2.0 pascals.sec or slurries up to 10% solids of
fine mesh size.
 Effective gas-liquid dispersion is possible at
laboratory scale.
Example
 Multivitamin elixirs, Disinfectant solutions
are prepared using propellers
35

36. DISADVANTAGES OF PROPELLERS
 Propellers are not normally effective with
liquids of viscosity greater than 5 pascal-
second, such as glycerin castor oil, etc
 The centrally mounted vertical propeller
produces vertex.
36

37. VORTEX
 Vertex is a powerful circular moving mass of water
or wind that can draw object into its hollow which
may result in air entrapped and bubbles formation.
 If a low viscosity liquid is stirred in an un-baffled
tank by an axially mounted agitator, tangential flow
follows a circular path around the shaft & a
swirling flow pattern is developed. This is vortex
37

38. HOW IS IT FORMED?
 In an un-baffled tank, a vortex is produced due to
the centrifugal force on the rotating liquid. This
creates a swirling motion in the liquid & the surface
tends to go upward near the vessel rim & downward
near the shaft. So a V-shaped surface is formed
which is the vortex.
38

39. REASONS
 If the shaft is placed symmetrically in the
tank.
 If the blades of the turbines are arranged
perpendicular to the central shaft.
 At high impeller speeds
 Unbaffled tank
39

40. PREVENTION OF VORTEX FORMATION
1) Impeller should be in any one of the
following positions that can avoid symmetry
such as;
 off central
 inclined
 side entering, etc.,
 and should be deep in the liquid
40

41. 2) Baffled containers should be used. In
such case impeller can be mounted
vertically at the center
41

42. 3) PULL PUSH PROPELLER
 Two or more propeller of opposite
angles or pitch are mounted on the
same shaft so that the rotary effects
are in opposite direction, cancel
each other effect (so will not
produce circulatory flow and no
vertex will be there).
 The bottom impeller is placed about
one impeller diameter above the
bottom of the tank. It creates zone
of high turbulence.
42

43. 2) TURBINE MIXER
CONSTRUCTION
 A turbine consists of a circular disc
impeller to which a number of
short vertical blades are attached.
Blades may be straight or curved.
 The blades are surrounded by
perforated inner and outer
diffusing rings
 The diameter of the turbine ranges
from 30-50% of the diameter of the
vessel
43

44. WORKING
 Used in similar manner as that of impeller, however it
is rotated at somewhat small speed than impeller (50-
200 rpm).
 Flat blade turbines produce radial and tangential
flow but as the speed increases radial flow
dominates. Pitched blade turbine produces axial flow
 Near the impeller zone of rapid currents, high
turbulence and intense shear is observed
 Shear produced by turbines can be further enhanced
using a diffuser ring (stationary perforated ring which
surrounds the turbine).
 Diffuser ring increase the shear forces and liquid
passes through the perforations reducing rotational
swirling and vortexing.
44

45. ADVANTAGES
 Turbines give greater shearing forces than propellers
though the pumping rate is less. Therefore suitable
for emulsification.
 Effective for high viscous solutions with a wide range
of viscosities up to 7.0 Pascal-Second.
 In low viscous materials of large volumes turbine
create a strong currents which spread throughout the
tank destroying stagnant pockets.
 They can handle slurries with 60% solids.
 Turbines are suitable for liquids of large volume and
high viscosity, if the tank is baffled.
45

46. 3) PADDLE MIXERS
 Paddles are agitator consisting of usually flat blades
attached to a vertical shaft and normally operated at low
speed (100-rpm).
 The blades have a larger surface area In relation to the
tank in which they rotate, so they can be used
effectively.
 Primarily paddle mixer produce tangential flow and
somewhat radial flow but no axial action unless blades
are pitched.
 Paddles for more viscous fluids have a number of
blades which are shaped in such a way to fit closely to
the surface of vessel (Avoiding dead spots and
deposited solids)
 At very low speeds it gives mild agitation in un-baffled
tank but as for high speeds baffles are necessary to
avoid swirling and vortexing
46

47. TYPES OF PADDLE MIXERS
 Simple paddle mixer
 Planetary motion mixer: (Small paddle rotating
on its own axis but travel also in a circular Path
round the mixing vessel. It is used for more viscos
fluids)
 Gate mixer: (It is a simple paddle, but is very
large in diameter)
 Stationary paddle mixer: (It is used for mixing of
solid with viscos liquid. Sigma plate mixer is its
improved form)
47

48. USES OF PADDLES
 Paddles are used in the manufacture of;
 Antacid suspensions
 Agar and pectin related purgatives
 Antidiarrheal mixtures such as bismuth-kaolin.
48

49. Advantages of paddles
 Vortex formation is not possible with paddle
impellers because of low speed mixing.
Disadvantages of paddles
 Mixing of the suspension is poor therefore baffled
tanks are required.
49

50. b) AIR JETS
 Air jets or other inert gas jets are effectively
used for mixing purpose with fluid of the
following characteristics;
○ Having low viscosity
○ Non foaming
○ Non-reactive with gas employed
○ Non-volatile in nature
50

51. PRINCIPLE
 When compressed air jet or suitable
gas is allows to pass at high pressure
from the inlet provided at the bottom of
the tank, air bubbles are formed in the
liquid phase.
 This causes buoyancy of the bubbles
which lifts the liquid (confined to the
central portion due to the presence of
draft tubes) from bottom to the top of
the vessel.
 The liquids flow down from the
periphery of the vessel and enter from
the bottom due to suction effect.
 The intense turbulence generated by
the jet produces intimate mixing
51

52. c) FLUID JETS
 In this device, the pumping operation is used to
transfer the liquid into the mixing tank.
MECHANISM
 In this case the fluids are pumped through nozzle
which permits good circulation of material through
the tank.
 The fluid jets in this operation behaves like propeller
in that they produce turbulent flow in the direction of
their axis, while differ from propeller b/c they don't
produce tangential flow themselves.
 They may also operate simply by pumping the liquid
from the tank and back into the tank through the jet.
52

53. CONTINUOUS MIXING
 Continuous mixing may be
accomplished in two ways.
 By using tubes or pipes
 By using mixing chamber
53

54. TUBE TYPE/PIPE TYPE MIXER
 In this method, the material flows through tube or
pipe and there is very little back flow.
 Baffles, rods or combination of these may be
placed in tubes or pipes to enhance mixing
efficiency.
 Mixing in such systems mainly occurs through
mass transport in direction normal to that of
primary flow
54

55. CHAMBER MIXING
 Mixing chamber is used as continuous
mixing equipment, when there is difficulty in
controlling the input rate and fluctuation.
 Fluctuation in the composition final mixture
is greatly reduced by dilation effects of the
material contained in the chamber.
 It may consist of a baffled pipe or an empty
chamber.
 Liquids to be mixed are passed through the
pipe/ chamber.
 Mixing takes place through bulk transport in
the direction of flow.
 The power supplied to pump the liquid itself
accomplishes mixing.
 For effective mixing controlling the feed rate
is essential.
55

56. B) POWDER MIXING
 Powder mixing may be regarded as an operation
in which two or more than two solid substances in
particulate form intermingled in mixer by
continuous movement of particles.
 It is an example of neutral mixture and is one the
most common operation employed during
preparation of different formulation like powder,
tablets, capsules etc
56

57. MECHANISM OF POWDER MIXING
 Mixing of powder is generally carried out by one of
the following mechanism or their combination.
 Convective mixing
 Shear mixing
 Diffusive mixing
CONVECTIVE MIXING:
 It is also called bulk transport. It takes place by
transferring the part of material from one location
to another location of the system by means of
blades or paddles of the equipment.
57

58. SHEAR MIXING:
 This type of mixing occurs when a layer of
material flows over another layer resulting in
the layers moving at different speeds and
therefore mixing at the layer interface.
 Occurs when;
 The removal of mass by convective mixing
creates an unstable shear/slip plane which
causes the powder bed to collapse
 The action of the mixer induces velocity
gradients within the powder bed
58

59. DIFFUSIVE MIXING
 Diffusive mechanism occurs by random movement of
particle within a powder bed and causes them to
change their relative position in relation to one another
 When a powder bed is forced to move, it will dilate
(The volume occupied by the bed will increase). This
occurs because the powder particles become less
tightly packed and there is an increase in the air
spaces or voids between them. So there is the
potential for the powder particles to pass through the
void spaces created under gravitational forces (in
tumbling mixer) or by forced movement (in fluidized
bed). Mixing of individual particles is referred to as
diffusive mixing
59

60. EQUIPMENTS FOR SOLID MIXING
 Tumbling mixer
 High shear mixture or granulator
 Agitator mixer
 Ribbon mixer
 Planetary mixer
 Nauta mixer
60

61. 1) TUMBLING MIXER/SHEAR MIXER
 In this type of mixer movement of whole mixer is
responsible for mixing action of solid.
CONSTRUCTION:
 Tumbling mixer usually consist of metallic vessels
which are rotated on their horizental axis at optimum
speed by means of motor
 The mixing vessel is usually made up of stainless
steel and have door have loading/unloading of
material.
 The door is usually lined with rubber which provides
a perfect seal after closure.
61

62.  Tumbling mixture are available in variety of shapes and
sizes these include;
 Y-cone blender
 Cubical blender
 Double cone mixer
 Twin shell / V-shaped mixer
62

63. WORKING:
 The material to be mixed is loaded into mixing
vessel which is rotated at low speed by electric
motor.
 Due to slow speed of rotation the powder is raised
along the sides of the vessel until the angle of
repose is exceeded.
 The powder then tumbles down and mixing of
compound occurs.
 The rotation of vessel is very important in this case
 If the speed of rotation is too slow, it will cause
sliding only so proper mixing will not occurs.
 If it is rotated at high speed so the material will
adhere to the walls due to centrifugal force
63

64.  So optimum speed is required (30-100 rpm)
 Mixing mostly occurs by convective mechanism
 Shear mixing will occur as a velocity gradient is
produced, (the top layer moving with high velocity
and the velocity decreasing as the distance from
the surface increases)
 When the bed tumbles it dilates, allowing the
particles to move downward under gravitational
force, and so diffusive mixing occurs
 Addition of ‘prongs’, baffles or rotating bars will
also cause convective mixing
64

65. ADVANTAGES:
 Good for free flowing powders/granules
 Mix from approximately 50 g (Laboratory scale) to
over 100 kg (Large scale)
 Can be used to produce ordered mixes
 Used in the blending of lubricants, glidants or
external disintegrants with granules prior to tableting
DISADVANTAGES:
 Less effective for cohesive/poorly flowing powders
 Segregation is likely to occur if there are significant
differences in particle size
65

66. 2) HIGH SHEAR MIXER-GRANULATOR
 It is so called because mixing mainly occurs
by shear mixing mechanism and at same time
granulation is carried out
CONSTRUCTION:
 It consist of a vessel having propeller
with long blades
 The clearance (distance b/w propeller
blades and walls of vessel) is low.
 There is a closing lid that closes the
vessel after material to be mixed is
added.
 For introduction of material/granulating
agent funnel is used.
 For the purposes of granulation a
chopper is present on side wall
66

67. WORKING:
 The material is to mixed id introduced to the mixer.
 The centrally mounted propeller blade at the bottom
of the mixer rotates at high speed, throwing material
towards the mixture bowl wall by centrifugal force.
 The material is then forced upward before dropping
back down towards the centre of the mixer.
 The particulate movement within the bowl tends to
mix the components quickly owing to high shear
forces (arising from the high velocity) and expansion
in the bed volume that allows diffusive mixing
67

68.  After mixing the granulating agent (water or
alcohol) is then added through funnel.
 It will produce wet mass that will go to the side
wall of mixer because of propeller.
 On sides, chopper with vertical, short and sharp
blades, is present that is rotating at speed higher
than that of the propeller and will broke the wet
mass so as to produce granules.
68

69. ADVANTAGES:
 Can be used for both wet and dry mixing
 For granulation purposes
DISADVANTAGE:
 Materials being mixed can fracture easily due to
high speed movement
 Cannot be used for blending lubricants
69

70. 3) AGITATOR MIXER:
 These are the mixers in which the container to
hold the material is fixed. Mixing is done by means
of mixing screws, paddles or blades.
 Well known mixers of this type include the
following:
 The ribbon blender/Mixer
 Planetary mixer:
 Nauta mixer
70

71. RIBBON MIXER
Construction
 Consists of horizontal cylindrical
trough with semicircular bottom
usually open at the top. It is fitted
with two helical blades, which are
mounted on the same shaft through
the long axis of the trough.
 Blades have both right and left hand
twists.
 Blades are connected to fixed speed
drive.
 It can be loaded by top and emptying
is done through bottom port.
71

72. Principle:
 Mechanism of mixing is shear. Shear is transferred
by moving blades. High shear rates are effective in
breaking lumps and aggregates.
 Convective mixing also occurs as the powder bed is
lifted and allowed to cascade to the bottom of the
container. An equilibrium state of mixing can be
achieved.
Uses:
 Used for mixing of finely divided solids, wet solid
mass and plastic solids.
 Uniform size and density materials can be easily
mixed.
 Used for solid – solid and liquid – solid mixing.
72

73. Advantages:
 High shear can be applied by using perforated
baffles, which bring about a rubbing and breakdown
of aggregates.
 Headroom requires less space.
Disadvantages:
 It is a poor mixer, because movement of
particles is two dimensional.
 Shearing action is less than in planetary mixer.
 Dead spots are observed in the mixer, though
they are minimum
 It has fixed speed drive.
 Not suitable for fragile crystals and sensitive
materials.
73

74. PLANETARY MIXER
 The name “planetary mixer” comes from the system
used in the equipment that mixes the dough in the
planets rotation direction.
Construction:
 Consists of vertical cylinder shell for ingredients
placement which can be removed.
 Mixing element (whisk, hook, flat beater, scrapper or
other system)
 It consists of a rod that rotates in its own axis and also
moves forward (around the bowl axis). As the movement
is just like planet so it is called planetary mixer
 The blade is mounted from the top of the bowl.
 Mixing shaft is driven by planetary gear and it is normally
built with variable speed drive
74

75. Principle
 Mechanism of mixing is shear. Shear is applied
between moving blade and stationary wall.
 Mixing arm moves around its own axis and around
the central axis so that it reaches every spot of the
vessel.
 The plates in the blades are sloped so that powder
makes an upward movement to achieve tumbling
action also.
Uses
 Break down agglomerates rapidly.
 Low speeds are used for dry blending and fast for
wet granulation.
75

76. Advantages:
 Speed of rotation can be varied at will.
 Avoid dead zones and vortex formation
 More useful for wet granulation process.
Disadvantages:
 Mechanical heat is buildup within the powder mix.
 It requires high power.
 It has limited size and is useful for batch work only
76

77. NAUTA MIXER
 It is a vertical screw mixer Originally designed as a powder and
semi-solid mixer but now-a-days also used as a mixer-
granulator
Construction
 It consists of conical vessel fitted at the base with a
rotating screw, which is fastened to the end of
rotating arm.
 Accessory equipments include;
 Lump breaker, attached at the bottom of the conical
chamber
 Temperature monitor
 A nuclear non-contact density gauge
 Infrared moisture analyzer
 Sampling system
77

78. Working
 Material is added (powder to be mixed, liquid
granulating agent)
 The screw is moving in a planetary motion and
also lifting the material to be blended from bottom
to the near the top, where it cascades backs into
the mass, thus imparts 3 dimensional mixing.
 The mixer thus combine
 Convective mixing.
 Shear.
 Diffusive mixing
78

79. C) SEMI-SOLID MIXING
MECHANISM:
 It has been found that, in mixing an
insoluble powder with, a liquid, a
number of stages can be observed as
the liquid content is increased
79

80. a) Pellet & powder state
 Addition of small amount of liquid to a bulk of
powder causes the solids to ball up and form small
pellets.
 The pellets are embedded in a matrix of dry
powder which has a cushioning effect to make the
pellet difficult to break up.
80

81. b) Pellet state
 Further addition of liquid results in the conversion
of more dry powder to pellets until all the material
is in this state.
 The mass has coarse granular appearance but
pellets do not cohesive and agitation will cause
aggregates to break down into small granules.
 The rate of homogenization is even lower than in
pellet and powder state and is the stage aimed it
in moistening powder for tablet
81

82. c) Plastic state
 As the liquid content is increased further the
character of the mixture changes markedly.
 Aggregates of material adhere, Granular appearance
is lost and mixture become more or less homogenous
and of clay like consistency
 Plastic properties are shown the mixture being
difficult to shear, flowing at low stresses but break
under high stress.
 Homogenization is achieved much rapidly
82

83. d) Sticky state
 Continual incorporation of liquid causes the
mixture to attain the sticky state; the appearance
become paste like, the surface is shiny and the
mass adheres to solid surface.
 The mass flows easily, even under low stresses
but homogeneity is attained only slowly
83

84. e) Liquid state (semisolid state)
 Eventually the addition of liquids result in a
decrease of consistency until a fluid state is
reached
 In this state, the liquid flows under its own weight
and will drain off vertical surface.
 The Homogenization is rapid
84

85. SIGMA-BLADE/ARM MIXER
 Used for semi-solid of plastic consistency
Principle:
 Shear (Inter meshing of sigma blades creates high
shear and kneading action
85

86. Construction and working
 It consists of double tough shaped stationary bowl.
 Two sigma shaped blades are fitted horizontally in
each trough of the bowl.
 These blades have very low clearance and are
connected to a fixed speed drive.
 Mixer is loaded from top and unloaded by tilting the
entire bowl.
 The blades rotate tangentially at different speeds,
one about twice than the other (2:1), which allows
movement of powder from sides to centers.
86

87.  The material also moves top to downwards and
gets sheared between the blades and the wall of
the tough resulting cascading action.
 Perforated blades can be used to break lumps and
aggregates which create high shear forces.
 The final stage of mix represents an equilibrium
state
87

88. Uses:
 Used in the wet granulation process in the manufacture
of tablets, pill masses and ointments,
 It is primarily used for liquid-solid mixing, although it can
be used for solid-solid mixing.
 This mixer is well suited to high viscosity materials like
grease, putty, toffee and bubble gum.
 With its strong construction and high power, the sigma
blade mixer can handle the heaviest plastic materials
and products like tablet granules, and ointments that
are mixed readily
 Sigma blade mixer is used in chemical and
pharmaceutical industries, to make food products,
adhesives, rubber
88

89. Advantages:
 Sigma blade mixer creates a minimum dead space
during mixing.
 It has close tolerances between the blades and
the sidewalls as well as bottom of the mixer shell.
Disadvantages:
 Sigma blade mixer works at a fixed speed.
 Problems of entrainment of the air and therefore
lead to decomposition of oxidisable materials
89

90. 90