Mixing of Solids
By
Sidra Jabeen
Department of Chemical Engineering,
University of Engineering & Technology Lahore
Mixing
 Intermingling of two or more separate components to form
more or less uniform product.
 Some other terms are
 Agitation
 Blending
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Agitation
Agitation refers
to the induced
motion of the
material in a
circulatory
pattern in some
type
of
container.
Mixing
Mixing is the
random
distribution, into
and through one
another, of two or
more
initially
separate phases.
Blending
Blending
is
the
mixing
of
two
components to form a
homogenous mixture.
Sometimes, blending
is the term used for
mixing of liquids.
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Mixing of solids
Liquids
–
currents
are
generated
Liquid
Solid – no such currents
so mixing is done by
other means
–
well-mixed
product (homogeneous)
Solids
–
identifiable
phases
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Types of mixture
 Perfect mixture / Solution
 Segregated mixture / Suspension
 Random mixture / Colloidal Dispersion
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Perfect mixture / Solution
 Solutions are mixtures made by mixing a solute and a solvent.
The solute is the substance that dissolves. The solvent is the
substance that does the dissolving. Solutions are homogeneous
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Suspension / Segregated mixture
 Suspensions are heterogeneous mixtures of a solid and a
liquid in which the solid does not dissolve. Particles of one
component have a greater probability of being found in greater
amount in one part of the mixture than the other.
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Random mixture / Colloidal Dispersion
 Colloidal Dispersions are mixtures with characteristics part
way between a solution and a suspension. Colloidal
dispersions may appear homogeneous but are actually
heterogeneous.
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Types of mixers
 Mixing is by slow speed agitation of the mass with an impeller
 By tumbling
 By Centrifuging smearing and impact
 For cohesive solids – powerful and heavier machines
 For free flowing solids – light in construction
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Mixers for non cohesive solids
 Mixer for dry and free flowing solids are
 Light in construction
 Moderate power consumption
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Mixers for cohesive solids
 Appropriate contact
 Mixing elements cannot generate flow currents
 High viscosity
 Mixing is by combination of low speed
 Shear
 Smearing
 Wiping
 Folding
 Stretching
 compressing
 Mixer have
 high power consumption
 Cooling is required
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Mixers for Cohesive solids
 Different types of mixers are:
1.
Change-Can mixer
2.
Kneaders
3.
Dispersers
4.
Masticators
5.
Continuous kneaders
6.
Mixer extruders
7.
Mixing rolls
8.
Muller and pan mixers
9.
Pug mills
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1. Change-Can Mixers
 For viscous liquids and light pastes as:
 Food processing
 Paint manufacturing
 5 to 100 gal. in size
 Change-Can mixer are:
1.
Pony mixer
2.
Beater mixer
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1. Pony mixers
 Agitator consist of vertical blades held on rotating head
positioned near the wall of the can
 Blades are slightly twisted
 Agitator is mounted eccentrically
 Can rest on a turn table driven in direction opposite to that of
agitator
 All the material in the can is brought to the blade to be mixed
 When mixing is completed –blades are cleaned and can is
replaced
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Change Can Mixer (Pony Mixer)
2. Beater mixers
 Can or vessel is stationary
 Agitator has a planetary motion
 Visits all parts of vessel
 Beaters are shaped to pass with close clearance over the side &
bottom of mixing vessel
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Change Can Mixer (Beater)
Kneaders, Dispersers and Masticators
 Kneading – “squashing the mass flat, folding it over on itself
and squashing it once more”
 Also tear the mass apart and shear it between a moving blade
& a stationary surface.
 For deformable & plastic solids
 Large energy requirements
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Kneaders, Dispersers and Masticators
 Mixing is done by two heavy blades on horizontal shaft in a
short trough with a saddle shaped bottom
 Blades
turn towards each other at top, drawing mass
downward over the point of the saddle, then shearing it
between the blades and wall
 They turn at different speeds usually 1.5 : 1 or 2 : 1
 Mixing time – 5 to 20 min or longer then trough is tilted for
discharging or can be unloaded through an opening
 Cooling is provided commonly
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Banbury mixer
 Heavy duty two-arm mixer
 30 to 40 rpm
 Solids enter from top and discharge from bottom
 Applications:
 Compound rubber and plastic solids
 Masticate crude rubber
 Devulcanize rubber scrap
 Make water dispersion and rubber solution
 Shorter time and small batches
 Cooling is done by:
 Water spraying on walls
 Circulating through hollow agitator shaft
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Banbury Mixer
Banbury Mixer
Design of Blades
 Sigma blade:
 Used for general purpose kneading
 Edges are serrated to give a shredding action
 S-type Double-naben (fish-tail blade):
 Effective with heavy plastic materials
 develop high shear force
 Z-type Disperser blade:
 Heavier and develop high shearing forces
 Disperse powders or liquids into rubbery masses.
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Kneader
(Sigma Blades)
Kneader
(Sigma Blades)
Double Naben Blades
Z-type Disperser blade
Continuous Kneaders
 Continuous operation
 deals light to fairly heavy materials (heavy, stiff, or gummy
materials)
 consist of
 Single horizontal shaft
 Slow in motion
 Rows of teeth on shaft arranged in spiral pattern
 Stationary teeth on the wall of casing
 Close clearance between dynamic and stationary teeth
 Shaft turns and also reciprocates in the axial direction
 Smearing action b/w teeth in axial or longitudinal direction as
well as radial shear
 Several tons per hour
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Mixer Extruders
 The discharge of continuous kneader is restricted by covering
it with an extrusion die
 Pitched blades of the rotor build up considerable pressure in
the material
 Material is cut and folded and subjected to additional shear
 Contain one or two horizontal shafts, rotating but not
reciprocating , carrying blades set in a helical pattern.
 Pressure is built by reducing the pitch of helix or by reducing
the diameter of chamber or both
 Continuously mix, compound, and work thermoplastics, clays,
and other hard-to-mix materials.
 Heating jacket is provided.
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Mixer Extruder
Mixing Rolls
 Smooth metal rolls at variable speeds
 For pastes & deformable solids to intense shear
 3 to 5 horizontal rolls in vertical stack
 Material moves from slower to faster ones
 Batch roll mills has two rolls set in horizontal plane and
require long mixing time and attention
 Used for dispersing solids in rubber and plastic materials
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Mixing Rolls