A fluid energy mill is also known as a pulverizer, micronized, or jet mill. It is used for fine grinding and close particle size control. The reduction of the particles takes place by the attrition and impact mechanism by the air or inert gas introduced through the nozzles present in the chamber. This mill is mainly used to grind heat-sensitive materials to the fine powder.
Principle of Fluid Energy Mill:
It
operates on the principle of impact and attrition. The inlet and outlets are
attached with the classifier which prevents the particles to pass until they
become sufficiently fine, Fig.1(a). It helps in the determination of particle
size and shape. The speed of air/inert gas is directly related to efficiency.
Solids introduced into the stream through the inlet result in a high degree of
turbulence, impact, and attritional forces occurring between the particles.
This erratic motion between the feed and air results in the breakdown of
particles. This mill involves no heat generation and is hence used to grind
heat-sensitive materials.
Construction of Fluid Energy Mill:
The main
basic parts present in the fluidized energy mill are inlet, nozzles, classifier,
and hollow toroid (Loop). Through the inlet, the solid material is introduced
into the chamber made of stainless steel. The air or the inert gas is
introduced through nozzles into the chamber at the bottom of the loop. The
cyclone separator called classifier is attached at the top from which the fine
particles are collected. The loop of a pipe has a diameter of 20 to 200 mm
depending on the overall height of the loop which may be up to about 2 meters.
The high pressure of fluid exerts a high-velocity circulation in the loop in a
very turbulent manner. A classifier is incorporated into the system so that
particles are retained in a loop until sufficiently fine.
Fluidized
energy mills are available in other subclasses. They have no moving parts and
are primarily distinguished from one another by the configuration and/or shape
of their chambers, nozzles, and classifiers. They include tangential jet,
loop/oval, opposed jet, opposed jet with dynamic classifiers, fluidized bed,
moving target, fixed target, and high-pressure homogenizers.
Working of Fluid Energy Mill:
The feed
introduced into the fluid energy mill is pre-treated to reduce the particles
size to the order of 100 meshes. This enables the process to yield a product as
small as 5 micrometers or less. Despite this, mills capable of output up to 40
kg/h are also available. Air or inert gas is injected as a high-pressure jet
through nozzles at the bottom of the loop.
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| Fig.1: Fluid Energy Mill |
The powder
particles in the mill are accelerated to high velocity by gas pressure. The kinetic
energy of the air and the resulting turbulence due to high pressure causes
inter-particle collision and attrition due to particle-wall contact resulting
in particle size reduction up to 5 µm. Size reduction in this mill also depends
on the energy supplied by compressed air that enters the grinding chamber at
high speed. The fluidized effect carries particles to a classifier zone where
the larger particles are retained until they become sufficiently fine. Fine
particles are collected through a classifier.
Types of Fluid Energy Mills:
There are
two main classes of pulverizers (fluid energy mills)
(i) Air
Swept Pulverizer:
In this mill, the particles along with air are fed into the mill inlet. Air
swept pulverizers use air to transport particles to the pulverizing section of
the apparatus. The beater plates support the hammers and distribute the
particles around the periphery of the grinding chamber. The hammers grind the
solid against the liner of the grinding chamber. The beater plates rotate between
1600 and 7000 rpm to reduce the size of the incoming particles. The classifier
plate separates the fine product and exits through the discharge outlet. The
larger material is back feed to the mill inlet through the recycle housing.
(ii)
Air Impact Pulverizer:
In air impact pulverizers superheated steam or compressed air produces the
force that reduces the size of large particles. It results in the smashing of
the particles into smaller particles. This pulverizer uses high-speed air to
pulverize the particles. The products from both breaths of air swept and air
impact pulverizers produce particles that do not require further sieving or
classifying.
Factors determining the efficiency of fluid energy mills:
- The speed of air/inert gas.
- Feed rate and size.
- The configuration of the mill.
- Design of the classifier.
- The position of the nozzle.
- The impact between the feed and air.
Uses of Fluid Energy Mill:
- A fluid energy mill is used when fine powders are required, for example, antibiotics, sulphonamides, and vitamins.
- Suitable for laboratories where small samples are needed.
- The mill is used to grind heat-sensitive material to a fine powder.
- The major advantage is fine grinding of pigments, kaolin, zircon, titanium and calcium, alumina, ceramic frit, powder insecticides such as DDT, diatomaceous earth, feldspar, fluorspar, graphite, gypsum, iron ore, iron oxide, iron powder, limestone, polymers, rare earth ores carbon, talc, etc.
- It is the choice of the mill when a higher degree of drug purity is required.
Advantages of Fluid Energy Mill:
- The particle size of the product is smaller than that produced by any other method.
- Expansion of gases at the nozzles leads to cooling, counteracting the frictional heat thus protecting heat-sensitive materials.
- There is little or no abrasion of the mill and so no contamination of the product.
- To protect sensitive drugs from oxidative degradation this mill has the facility to use inert gases.
- The presence of a classifier permits control of particle size and particle size distribution.
- Suitable for size reduction of materials capable of generating a static charge.
- The process is suitable for friable, abrasive, or crystalline materials.
- The air needed is freely available.
- A homogeneous blend of a large range of sizes is available.
- The equipment is easily sterilized.
- At the end of milling product particle size between 2 and 10 µm is obtained.
Disadvantages of Fluid Energy Mill:
- This mill is energy consuming and the energy consumed per ton of milled product is high.
- High headspace is required.
- Coarse feed size is not suitable.
- The fed device may be clogged with the clump materials.
- Special feeding devices should be provided for the feeding of the materials.
- The use of compressed air leads to the generation of static electricity.
- Material recovered in the collection bags is difficult or impossible to remove by the normal blowback procedures.
- The tendency of forming aggregates or agglomerates after milling.
- Generation of amorphous content due to high energy impact.
- Formation of unwanted ultra-fine particles.
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