INTRODUCTION TO AIR CLASSIFIER (AIR CLASSIFYING MILL):
Air Classifying Mill or ACM is one of the most popular size reduction equipment used in Chemical, Pharmaceutical, Cosmetics, Food, Polymer coatings and Metal power industries. As discussed in my earlier blogpost on Air Jet Mill, when the product is heat sensitive, as well as, needs fine grinding (say less than 5 micon to nano), Air Classifying Mill (ACM) is considered as one of the best options alongside with the Air Jet Mill (Micronizer). However, ACM is considered to be more versatile and stronger as compared to Air Jet Mill because of its ability to handle higher volumes of the material, can achieve more uniform micron size and can handle variety of materials including metal particles and other abrasive particles.
OPERATING PRINCIPLE:
In simple words, the Air Classifying Mill Or ACM works on a dual Or a combination principle viz. Impact Milling and Particle Classifying.
A high speed rotary disk with pins Or hammers mounted on it's surface (on its circumference) achieves the size reduction along with the surrounding stationary corrugated enclosure (just like the stator of a pulverizer). The particles upon impact and collision between the rotating hammers and stationary enclosure get reduced in size and gets pulled into the classifying mill by the drag force created by the turbulent air.
In the next stage, the particles that crossover the baffle wall covering the classifier are either classified to pass through Or are rejected back to the hammer section again based on the size and density of the particles.
HOW IT OPERATES (IN DETAIL DISCUSSION):
As discussed above, the Air Classifier mill (ACM) works on the principle of size reduction plus classification. As shown in the figure above, the milling disk and the classifier are two different parts which though are concentric, however, rotates as different speeds (rpm) and are operated by two different motors.
While eighty percent of the air enters the milling chamber through a tangential entry nozzle, the twenty percent air along with the powder mass (to be milled) enters through another tangential liner popularly known as 'product inlet liner'. The high rpm of the milling disk forces the powder particles to colloid with the corrugated wall of the milling chamber and with the hammers Or pins mounted on the milling disk. The hammers Or pins are rotating, whereas the corrugated wall of the milling chamber is stationary.
The particles thus reduced in size cross over the immediate next barrier wall and enters the classifier area. The classifier wheel has vertical cage like plates arranged such that the adequate size powder particles are able to enter the classifier wheel and bigger size particles are rejected back to the product baffle wall and further to milling disk hammer section. When the required size is too fine, the plates in the classifier are arranged at an angle with the center of the axis, thus making it difficult for the power particles to enter the classifier area. Those particles which qualify for the entry gets pushed out of the milling chamber from the central exhaust pipe towards the cyclone separator and finally gets collected into the product collection bin.
MAJOR COMPONENTS:
1. Milling Disk, its motor, pulley, bearing housing: The milling disk rotates at a very high speed. Hence. the horizontal disk with vertical hammers on it has a central shaft which is either directly connected with the high rpm AC motor Or is connected through a belt pulley. Bearings and seal plays an important role so as to avoid powder leakages.
2. Classifier Wheel, its motor, pulley, bearing housing: The classifier wheel too rotates at a very high speed, independent of the milling disk. The motor and central shaft of the classifier wheel can be either top mounted and protruding Or can be bottom mounted with 'shaft in shaft' arrangement. A variable frequency drive helps in varying the speed of the classifier as per the need. The vertical plates as well as the flanges etc are precisely machined for accurate results.
3. The milling chamber housing the milling disk, baffle wall and classifier: The milling chamber is made-up of a precisely machined modular metal basket. The outermost inner wall of the basket is threaded Or corrugated and appears like the corrugated wall of the pulverizer (stator part of the stator rotor assembly). The assembly is such that the milling disk rotates freely in the closed basket, however the gap between the two is extremely small and uniform.
The classifier wheel is many a times mounted into the top disk of the basket and a provision is made to open the top part of the basket along with its motor, seal and bearing housing from the hinged joint given between the top and bottom part of the milling chamber (basket). However that may not be possible in all cases.
The provision for high pressure inlet air, as well as, for air stream for getting the product mass in is made through two different liners.
4. Powder Feeder: A gravimetric feeder assembly is usually provided consisting of a feeder tank (usually vertical cylindrical with narrow bottom) along with a rotary valve that pushes out a metered quantity of the product into the pipe that carries the powder into the milling chamber under with the help of the moderately pressurized air stream.
5. Cyclone separator and Filter assembly unit: The product particles qualified by the classifier travels upwards with the high velocity air stream and enters the cyclone separator followed by the filtration chamber with fingerbags inside. At times, only a single cyclone separator is used with the finger bags hanging from the top which arrests the powder particles and allow the air to escape.
6. Product Collection Bag: The product collection bag Or basket is kept at the bottom of the cyclone separator and the filtration chamber (carrying finger bags).
7. External Air Filtration Unit (Optional): A series of bag filters including coarse filter (say upto 10 micron) followed by fine filter (say upto 3 micron) followed by a HEPA filter (say 0.3 micron) followed by a suction blower and its motor can be provided to treat the exhaust air before releasing it to the atmosphere.
8. Operating Panel: Operating panel can be either push button type Or can be with PLC and touch screen HMI.
APPLICATIONWISE ADVANTAGES OVER AIR JET MILL AND OTHER MILLING OPTIONS:
Some of the unique advantages that makes the ACM as the preferred choice over an Air Jet Mill, as well as other milling equipments are as under:
1. Output Volumes: ACM are far more faster than AJM, as milling and classification happens simultaneously on a larger scale.
2. Output Sizes: ACM is observed to be more effective incase of certain products over AJM and other milling equipments such as metal powders, abrasive products, polymerized products, highly fluppy products etc. Output size can be as low as nano, whereas, the feed material can be as big as a few millimeters. Such wide range of size reduction is not possible with Air Jet Milling in a single pass.
3. Gentle Handling and Milling of Heat Sensitive Products: ACM too yields size reduction without generation of much heat (just like AJM). This is one of the major reasons for choosing the ACM over other mechanical grinding equipments.
4. Easy Maintenance: The equipment isn't very complex, hence preventive maintenance is easy.
5. Economic and Long Lasting: ACMs are not very costly, at the same time do lasts for very long.
MOC OR MATERIAL OF CONSTRCTION:
Incase of GMP requirement, the product contact parts are chosen to be SS316 Or SS316L and non-contact parts are chosen to be of SS304. The hammers and other components of the milling chamber Or basket can be hardened also (depending on the material to be milled). PTFE Or PU liners can also be used incase of polymerized materials.
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