1. Paste Mixing
Ashis Kumar Podder

2. Mixing of immiscible Liquids
Carried mainly in the manufacture of emulsions, and the equipment used for the preparation of an emulsion is known as emulsifier. Also known as homogenizer as it results in fine emulsion.
Fine emulsion is prepared in 2 stages.
In 1st stage coarse emulsion is prepared by using one of the following process:-
Wedge wood
Mechanical blender
Hand homogenizer
Porcelain mortar and pestle
Milk shake mixer
Propeller in a baffled tank
Some times the above equipment directly gives fine emulsion.
Otherwise coarse emulsion is subjected to homogenizer in the 2nd stage to get fine emulsion by using following process:-
Silverson emulsifier
Colloidal mill
Rapisonic homogenizer

3. Factors influencing selection of an emulsifier
Quantity of emulsion to be prepared: batch wise or continuous operation
Flow properties of liquids: Newtonian, plastic, pseudo plastic or dilatant.
Temperature maintenance: mixing will be effective at high temperatures provided the material is stable.
Desired rate of cooling: if elevated temperatures are applied

4. Equipment
Silverson emulsifier
Colloidal mill
Rapisonic homogenizer

5. Silverson mixer -Emulsifier
Principle:
It produces intense shearing forces and turbulence by use of high speed rotors.
Circulation of material takes place through the head by the suction produced in the inlet at the bottom of the head.
Circulation of the material ensures rapid breakdown of the dispersed liquid into smaller globules.
It consists of long supporting columns and a central portion. Central portion consists of a shaft which is connected to motor at one end and other to the head.
Head carries turbine blades.
Blades are surrounded by a mesh, which is further enclosed by a cover having openings.

6. Uses:
Used for the preparation of emulsions and creams of fine particle size.
Advantages:
Silver son mixer is available in different sizes to handle the liquids ranging from a few milli liters to several thousand liters.
Can be used for batch operations as well as for continuous operations by incorporating into a pipeline, through which the immiscible liquids flow.
Disadvantages:
Occasionally, there is a chance is clogging of pores of the mesh.

7. Colloid mill

8. Ultrasonic Emulsifiers – Rapisonic homogenizer
Principle:
When a liquid is subjected to ultrasonic vibrations alternate regions of compression and rarefaction are produced in the liquid.
Cavities are formed in the regions of rarefaction which subsequently collapse in the regions of compression. Which results great forces for emulsification.
Construction:
It consists of a pump driven by a motor. It is connected to inlet tube and an out let tube.
Head consists of a flat jet for liquid inlet. Facing the jet, a thin blade is present which vibrates at its natural frequency.

9. Mixing: Theory and Equipment
Advantages of rapisonic emulsifier:
Can be used for batch process (by placing it in a tank) or for continuous process (by placing it in a pipeline).
It has the capacity to produce dispersed globules of one micron size.
As this method is highly efficient to decrease globule size, reduced concentration of emulgents is sufficient.
Its capacity of mixing liquids ranges from liters per minute.
It is suitable for thermolabile substances since heat is not generated during mixing.
Disadvantages of rapisonic emulsifier:
It is useful only for low viscous liquids.
Mixing: Theory and Equipment

10. Factors affecting Paste Mixing
Particle size: should be in a reasonable range Viscosity: high enough to make paste stable but not too high so as to make mixing difficult Density: must not differ too much. Temperature: can be applied to lower the viscosity, but not fluctuate Evaporation: container must be air tight to prevent evaporation so that the mix do not dry up. Time: sufficient time must be allowed for uniform mixing.

11. Mechanism of paste mixing -stretching, folding, compaction -shear force is applied near the blade -small clearance between blades and tank wall.
Mixer design should be heavily constructed to handle materials of great consistency. Semisolids do not flow easily and the impellers provide only radial & tangential flow but axial flow is totally absent.
Therefore, mixers must consider the following points:
Mixer design must be such that agitator causes the entire volume to move and no corner remains unstirred.
Impellers must ensure high shear application
Blades must provide pulling and kneading action
Blades must move through the entire working zone, ensuring close clearance to container walls.

12. Triple Roll Mill
Principle: Has three rolls, arranged close to one another, each roll moves at various speeds. Materials, moving over the rolls, are crushed depending upon the gap & rate of movement of the two surfaces. Construction: Three parallel rolls mounted on a rigid framework, moves at different speed. The receiving roll is the slowest whereas the discharge roll is the fastest. The gap between the rolls can be adjusted.

13. Triple Roll Mill( cont.)
Operation:
Feed is introduced in the machine by a hopper
Rolls rotate and materials are abraded by the rubbing action of the rolls turning at different speed.
Between the 1st & 2nd roll, due to large space, the materials produce a thick film & aggregates are crushed here.
Between the 2nd & 3rd roll, the space is smaller. Thickness of the materials passing gets thinner and fine crushing takes place.
Finally, scraper knife is used to discharge the mixed material.

14. Triple Roll Mill( cont.)
Advantage -Continuous operation is possible -High shear applied -Size reduction possible -Mixing efficiency is high
Disadvantage -washing is difficult -high power consumption -high operating cost -there is risk of contamination

15. Change Can Mixer Construction:
Batch mixer, container is small separate unit placed on a platform, can be removed from the frame of the machine. There is a stainless steel agitator, with 3 angularly fixed blades.
Intimate mixing is accomplished in change can mixer in two ways:
Change can mixer with planetary motion
Change can mixer with rotating turn table

16. Change Can Mixer with planetary motion:
Mechanism: The rotating blade is set off centre & is carried on a rotating arm. It therefore travels around the mixing can while simultaneously rotating around its arm axis. Close clearance between the vessel and the blade is maintained. Scraping may be sometimes necessary as some of the materials are forced to the top of the can. Adv: # Good mixing efficiency # Possible to charge materials into the can without moving it from the operating table. Disadv: # batch type # open system, so there is risk of contamination or material loss.

17. Change Can Mixer with rotating turntable
Mechanism: The platform on which the can is placed can rotate. The rest of the set up & operation is same as the other change can mixer. Adv. Of change can mixer: # batch can be carefully measured or weighed before being brought to the mixer. # mixer can serve as transport media to the next operation. # mixer can serve as storage media after completion # relatively inexpensive. # easy to clean.

18. Disadv. of change can mixer: # batch type # size reduction is not possible # there is risk of contamination.