Mixing of Solids

By

Sidra Jabeen

Department of Chemical Engineering,

University of Engineering & Technology Lahore

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Intermingling of two or more separate

components to form more or less uniform

product.

Some other terms are

Agitation

Blending

Mixing

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MixingAgitation Blending

Mixing is the random distribution, into and through one another, of two or more initially separate phases.

Agitation refers to the induced motion of the material in a circulatory pattern in some type of container.

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 – well-mixed

product

(homogeneous)

Solid – no such

currents so mixing is

done by other means

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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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

Mixers for cohesive solids

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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

Mixers for Cohesive solids

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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

Change Can Mixer (Pony Mixer)

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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

Change Can Mixer (Beater)

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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

Banbury Mixer

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 and develop high shearing forces

Disperse powders or liquids into rubbery masses.

Kneader (Sigma Blades)

Kneader (Sigma Blades)

Double Naben Blades Z-type Disperser blade

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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.

Mixer Extruder

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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 has two rolls set in horizontal

plane and require long mixing time and attention

Used for dispersing solids in rubber and plastic

materials

Mixing Rolls