Types of Mixers for Cohesive Solids

Disperser

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Extruder

Roller Mill

Banbury Mixer

Change Can Mixer

Masticator

Kneaders Mixers

Muller Mixer

Kneader

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Kneader Mixer

Working Principle:

Some kneaders achieve their mixing action by squashing the mass flat, folding it, and squashing it again. Others tear the mass apart and shear it between a moving blade and a stationary surface.

Operation:

A two-arm kneader handles suspensions, pastes, and light plastic masses. Typical applications are in the compounding of lacquer bases from pigments and carriers and in shredding cotton linters into acetic acid and acetic anhydride to form cellulose acetate.

Advantages and Disadvantages:

ADVANTAGES DISADVANTAGES

Rugged construction allows considerable power per volume to be applied.

Intensive agitation results in short mixing time.

Close blade clearance helps eliminate stagnant material areas.

Easy to clean after mixing sticky material.

Double arm kneaders available in wide range of sizes.

Too much friction produced in the confined area may result in heating, which would limit mixing of heat-sensitive materials.

Discharge of finished mixture sometimes difficult.

If blades are not overlapping, material may simply rotate about the axis in a cylinder-like glob without getting mixed (rubbery materials have a tendency to "ride" the blades like this).

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Extruder

Working Principle:

Extruders are used to mix liquid-solid systems and to extrude molten or plasticized polymers to semifinal and final product forms.

Operation:

During processing in the extruder, the polymer is melted and the additives are mixed. The feed enters a channel where it is mixed by one or two rotating screws. The extradite product is delivered at high pressure and at a controlled rate from the extruder for shaping by a die or mold.

In single screw extruders, the mixing quality is established by the total shear deformation for a given material volume. There are two types of flow in the channel: longitudinal, along its helical axis, and transverse, a type of circulatory motion. Single screw extruders are effective for highly viscous materials, because they allow for the attachment of larger gears.

Advantages and Disadvantages:

ADVANTAGES DISADVANTAGES

Capital cost and power requirement lower for single screws.

Mixing performance can be improved by using additional mixing heads or in-line static mixers.

Single screw shearing is not intense.

Single screw mixers not as effective as co-rotating and counter-rotating twin screw mixers.

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Change Can Mixer

Working Principle

Change can mixers work by the relative motion of the blades and the can.

Operation:

Change can mixers blend viscous liquids or light pastes, as in food processing or in paint manufacture, in a removable agitated can or vessel 5 to 100 gal in size. They have two types:

a) Pony Mixers.b) Beater Mixers.

In the pony mixer, the rotating agitator carries several vertical blades positioned near the vessel wall. The can is driven by a turntable in a direction opposite to that of the agitator. While in the beater mixer, the can is stationary and the agitator has a planetary motion, so that as it rotates it precesses, repeatedly all parts of the vessel.

a) Pony Mixer b) Beater Mixer

Muller Mixer

Working Principle:

Mullers are liquid-solid system mixers consisting of a circular pan and a turret, with large rollers.

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Operation:

There are 3 types of batch mullers: Stationary pan with rotating turret, shown below. Rotating pan with stationary turret. Counter-current pan and turret.

The material to be mixed is moved from the outside to the inside of the pan and is crushed and rolled over by the mullers. The rollers sometimes skid, resulting in shearing and coarse-scale mixing, which is aided by the plows and scrapers.

A continuous muller consists of two batch pan/turret systems joined to make a figure eight design. In a continuous muller, material builds up in one pan until the feed rate into that pan equals the discharge rate from the second pan.

Advantages and Disadvantages:

ADVANTAGES DISADVANTAGES Effectively breaks up lumps and

aggregates. Versatile:

o material to be mixed may be wet or dry

o used over a broad range of densities and viscosities

o can handle large capacities Useful for mixing problems requiring

aggregate breakdown, frictional and storage of particles to one another, and densification of the final mixture.

Cannot be adapted for vacuum or different pressures.

Excessively fluid or sticky materials cannot be mixed.

Not used for basic blending because of high cost and high power requirements.

Treats material roughly.

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Disperser

Working Principle:

A Disperser is a single shaft mixer used to break apart or dissolve solid particles in a liquid.

Operation:

This is typically accomplished using a "saw tooth" blade rotating at high speed (approximately 5200 feet per minute peripheral speed). The blade imparts high shear forces to the ingredients being dispersed, breaking them apart. Through proper blade selection and sizing, a flow can be established that will repeatedly turn over the batch and yield a homogeneous product. Dispersers are most often used to mix low viscosity products such as paints.

Advantages and Disadvantages:

Dispersers are available with variable speed mixing shafts. Some are directly mounted atop a tank and are fixed to operate with the blade in only the original mounting position. Other tank mounted dispersers can raise and lower the blade by several feet (to better control the vortex) without exiting the tank. Another design, perhaps the most popular, places the disperser on top of a hydraulic lift that is mounted to the floor. The lift enables the operator to raise the blade completely out of the mixing vessel and change to another vessel. This technique uses small portable tanks (up to 500 gallons) that can be rolled away on wheels or picked up with a fork truck. Larger stationary tanks are often centered within the arc of rotation from the center of the hoist to the center of the mixing shaft. The bridge containing the mixing shaft at one end and the motor at the other is then rotated from one tank to the next. Choosing the best configuration of available designs is a combination of functional need and economic justification.

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Roller Mill

Working Principle:

Roller mills are mills that use cylindrical rollers, either in opposing pairs or against flat plates, to crush or grind various materials, such as grain, ore, gravel, plastic, and others.

Operation:

To produce refined (white) wheat flour, grain is usually tempered, i.e. moisture added to the grain, before milling, to optimize milling efficiency. This softens the starchy "endosperm" portion of the wheat kernel, which will be separated out in the milling process to produce what is known to consumers as white flour. The addition of moisture also stiffens the bran and ultimately reduces the energy input required to shatter the kernel, while at the same time avoiding the shattering of bran and germ particles to be separated out in this milling process by sieving or sifting.

With the invention of the roller milling system in the late 19 th century, the bran and the germ were able to be removed, dramatically improving the appeal of baked products to the public. The moistened grain is first passed through the series of break rollers, and then sieved to separate out the fine particles that make up white flour. The balance is intermediate particles of endosperm (otherwise known as product middling or farina) and coarse particles of bran and germ. The middling then makes multiple passes through the reduction rolls, and is again sieved after each pass to maximize extraction of white flour from the endosperm, while removing coarser bran and germ particles. To produce whole wheat flour, 100% of the bran and germ must be reintroduced to the white flour that the roller milling system was originally designed to separate it from. Therefore, these elements are first ground on another mill (usually a pin mill). These finer bran and germ fractions are then reintroduced to the endosperm (white flour) to produce whole wheat flour made of 100% of the kernel of wheat.

Masticator

Working Principle:

A masticator is heavier and draws more power than a disperser.

Operation:

A masticator can disintegrate scrap rubber and compound the toughest plastic masses that can be worked at all. Masticators are often called intensive mixers. Masticator

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blades are heavier and little larger than the shafts that drive them. Spiral, flattened and elliptical designs of masticator blades are used.

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Banbury Mixer

Working Principle:

Banbury mixers compound rubber and plastic solids, masticate crude rubber, devulcanize rubber scrap, and make water dispersions and rubber solutions.

Operation:

Banbury mixer is a type of internal screw mixer. It is a heavy duty two-arm mixer in which the agitators are in the form of interrupted spirals. The shafts turn at 30-40 rpm. Solids are charged in from above and held in the trough during mixing by an air-operated piston under a pressure of 1-10 atm. Mixed material is discharged through a sliding door in the bottom of the trough.