PARTICLE TECHNOLOGY

Topic: Mixing of Solids

 Mixing

Random distribution, into and through one another, of two or

more initially separated phases (components)

Segregation

One or more key components congregates or it condensed locally

in microscopic sense.

The mechanism by which mixing & segregation occur are identical

in principle

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 Segregation

 Desired - Separation

 Undesired - avoided when homogeneity is required

 In chemical, pharmaceutical, agricultural & smelting industries

segregation is undesirable as it cause serious problems such as

 Uneven quality of products

 Fluctuation in weight

 Low mechanical strength

 Poor refractory material

 Low rate of contact & reaction

 In screening or other separations, segregation is favourable.

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 Mixing of solids

 Liquids blending depends on the creation of flow currents

which transport the unmixed material to the mixing zone.

 Solid – no such currents, so mixing is done by other means.

that's why large power requirement for pastes & dry solids
(sand, cements, gravels etc.)

 Liquid – well-mixed product (homogeneous)

 Solids – identifiable phases

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 Types of mixture

 Perfect mixture

 Random mixture

 Segregated mixture

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 Perfect mixture

 In which a group of particles taken from any position in the

mixture will contain the same proportion of each particle as

the proportion present in whole mixture

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 Random mixture

 A mixture in which the probability of finding a particle of any

component is the same at all locations and equal to the proportion

of that component in the mixture as a whole.

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 Segregated mixture

 Particles of one component have a greater probability of being

found in one part of the mixture and so a random mixture

cannot be achieved

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 Mixing Mechanism

Shear mixing Force of attraction are broken down so that each

particle moves on its own between region of different component
and parallel to the surface.
Diffusive mixing (micro mixing) Involve the random motion of
particle within powder bed, thereby particle change their position
relative to one another.
Convective mixing(macro mixing) Inversion of powder bed using
blade/screw in which large mass of material moves from one place to
another.

 Shear and convective mixing can produce the rough mix but
groups of particles remain unseparated unless subject to diffusive
mixing.

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 Properties affecting solid mixing

 PSD

 Density

 Particle shape

 Surface characteristics

 Flow characteristics

 Friability

 State of agglomeration

 Moisture or liquid content of solid

 Temperature limitation of ingredients

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 Types of mixers

For dry solids

 Mixing is by slow speed agitation of the mass with an impeller

 By tumbling

 By centrifugal smearing and impact

 Mixer for dry solids are

 Light in construction

 Moderate power consumption

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For paste, rubber & heavy plastic masses

 Appropriate contact

 Mixing is by combination of low speed

 Shear
 Smearing
 Wiping
 Folding
 Stretching
 compressing

 Mixer are

 high power consumption

 Cooling is required

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 Mixers for Cohesive solids

 Cohesive solids – most difficult of all mixing problems

 High viscosity – Different and powerful equipments

 Mixing elements cannot generate flow currents

 Shear, fold, stretch and compress the material to be mixed

 Change-Can mixer
 Kneaders
 Dispersers
 Masticators
 Continuous kneaders
 Mixer extruders
 Mixing rolls
 Muller and pan mixers
 Pug mills
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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 – cleaning of blades are performed

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 moving surface.

 For deformable & plastic solids

 Large energy requirements

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a) Two-arm kneader

 Suspension, pastes and light plastics

 Applications:

 Compounding of lacquer bases from pigments and

carriers

 Shredding cotton linters into acetic acid & acetic

anhydride to form cellulose acetate

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 b) Dispersers

 Heavier in construction

 Draws more power than a kneader

 Application: additives and colouring agents into stiff materials

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 c) Masticators

 Still heavier

 More power consumption

 Application: disintegrate rubber and toughest plastic masses

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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

 Circles of rotation of blades are tangential

 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

 Sometimes heating but more commonly cooling is provided

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 Open trough

 Closed trough during operation (internal mixers)

 Used for dissolving rubber and dispersion of rubber in

liquids

 Internal mixer – Banbury mixer

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 Banbury mixer

 Heavy duty two-arm mixer

 30 to 40 rpm

 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
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 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

 Sometime large in diameter

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 Kneader
(Sigma Blades)
 Kneader
(Sigma Blades)
 Z Baldes

Masticator
Double Naben Baldes Balde
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 blades in axial or longitudinal direction

as well as radial shear

 Several tons per hour 32

 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, doughs,
clays, and other hard-to-mix materials.
 Heating jacket and vapour discharge is provided.

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Mixer Extruder
Mixer Extruder
(Side View)
Extrusion Die

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 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 require long mixing time and attention

 Additives, rubber and plastic materials

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Mixing Rolls
 Muller mixers

 Mulling is smearing or rubbing action similar to that in mortar

and pestle

 Wide, heavy wheels of the mixer did the same job

 Pan is stationary & central vertical shaft is driven – causing the

Muller wheels to roll in circular path on solid

 Rubbing action
 Plows – solid under wheels or to discharge opening

 Axis of the wheels is stationary & pan is rotated

 Pan & wheels both are driven – wheel are offset

 Good mixer for batches of heavy solids and pastes

 Effective in coating the granular particles

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Muller Mixer
 Pug Mills

 Mixing is done by blades or knives set in helical pattern on a

horizontal shaft.
 Open trough or closed cylinder

 Continuous system

 Cut, mixed and moved forward

 closed mixing chamber - Single shaft

 Open trough – double shaft for more rapid & thorough mixing

 Mostly cylindrical but sometimes polygonal in cross section

for sticky solids
 Heating or cooling jackets

 Can operate under vacuum

 Blend and homogenize clays, break up agglomerates, thick

heavy slurries
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Pugmills
Pugmills
 Mixers for free flowing solids

 Lighter machines are there for dry powders and thin pastes

 Ribbon blender - mechanical shuffling

 Tumbling mixer – repeatedly lifting and dropping the

material and rolling it over

 Vertical screws – same as above

 Impact wheel / rotating disc – smearing it out in a thin

layer

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 Ribbon Blenders

 Horizontal trough – central shaft and a helical ribbon agitator

 Two counteracting ribbon mounted on same shaft

 One moving slowly in one direction

 Second quickly in other direction

 Ribbon – continuous or interrupted

 Mixing – turbulence by counteracting agitators

 Mode of operation – batch or continuous

 Trough – open or closed

 Moderate power consumption

 Effective for thin pastes and solids that do not flow readily
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Ribbon Blender
Ribbon Blender
 Internal screw mixers

 Vertical tank containing a helical conveyor that elevates and

circulates the material

 For free flowing grains and light solids

 Mixing is slower than ribbon blenders but power requirement

is less

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Internal Screw
Mixer
Internal Screw Mixer
 Tumbling mixer

 Partly filled container rotating about horizontal axis

 Mostly no grinding element

 Effectively mix – suspension of dry solid in liquid, heavy dry

powders

 Wide size range and material of construction

 Double cone mixer

 Batch – charged from above – 50 to 60 %full
 Free flowing dry powders
 Close end of vessel – operated 5 to 20 min

 Twin shell blender

 Two cylinder joined to form a V
 rotated about horizontal axis
 Internal sprays or mechanical devices
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 More effective than double cone mixer
Tumbling Mixer
Double Cone Mixer
Twin Shell Blender
 Impact wheels

 Operating continuously by spreading them out in a thin layer

under centrifugal action

 Several dry ingredients are fed continuously near the high

speed spinning disk 10 to 27 in. in diameter throwing it in a

stationary casing.

 Intense shear cause mixing

 Disk – vertical or horizontal

 1750 to 3500 rpm

 Several passes through same or in series

 1 to 25 tons/hr

 Fine light powders like insecticides

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Impact Wheels
END

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