LOGICAL AND PRACTICAL ASPECTS OF MICRONIZATION:
Having discussed about Micronizer Or Air Jet Mill Or Fluid Energy Mill, as well as its working (in Part 1 of 3), it is now the time to discuss the practical aspects of the milling operation with the logical explanations.
MATERIAL FEEDING:
As discussed earlier, a vertical cylindrical vessel with usually a hemispherical bottom is used for charging the feed material. The horizontal agitator keeps the material shuffling and the horizontal screw at the bottom pushes it towards the ventury based feeding line. Important aspects to be taken note of are as under:
# Feed Size Desired: Less than 1000 Microns (Or 1 mm). It is so because Micronizer is meant for milling already fine particles to super fine level.
# RPM of the Agitator and Screw: Usually the RPM is never more than 100 and not lesser than 5 to 6. Higher the RPM, faster is the pushing of the material into the Milling Chamber.
# Volumetric Vs Gravimetric Feeding: Micronizers usually use Volumetric feeding only, where the RPM of the Screw decides the material to be pushed Or feeded to the milling chamber. On the other hand, a gravimetric feeder means a mechanism that always keeps an eye on the pre-set value of the weight of the material and the weight variation drives the flow of the feed. For the purpose of Micronizer, feeding of known quantity (in terms of weight) is actually necessary. However, looking at the complexities of the operation, most of the organizations prefer the auger driven feeding i.e. the Volumetric Feeding.
MICRONIZATION:
As discussed in the previous post, the micronization here necessarily is achieved by particle to particle collision under the influence of the turbulence created by high pressure fluid (usually air) jet entering the milling chamber. Important aspects to be taken note of are as under:
# Primary Pressure: Also known as the Feeding Pressure. The Primary Pressure Or P1 is usually never more than 8 to 10 bar, and, can be as less as 1 to 2 bar. Lower P1 leads to slowing of the overall output i.e. the milled powder collected at the receiver.
# Secondary Pressure: Also known as the Milling Pressure. The Secondary Pressure Or P2 is usually never more than 10 to 12 bar, and, can be as less as 1 to 2 bar. Lower P2 leads to fine milling. It is because when we lower the P2, the powder particles inside the chamber gets more residence time, resulting into finer milling.
# Vacuum Balancing Line and Ventury: As discussed earlier, the feeding line has a ventury line attached at the bottom at an angle (as shown in the fig below). The primary pressure P1 pushes the air through the ventury inside the chamber. Due to sudden expansion of the air in the nozzle (after ventury opening), a good amount of negative pressure is created, which is responsible for pulling the material down inside the milling chamber.
Sometimes the negative pressure created is so high that it can choke the feeding line by the powder particles and imbalance the pressure inside the chamber resulting into back pressure (i.e. powder can be gushed out of any opening into open atmosphere). To avoid this, a bypass line is provided near the feeding nozzle, and the other end of this line is either kept open to atmosphere with a filter cloth covering it, Or is attached to the Hood of the Filter Bags Or Finger Bags.
(Pic Ref: www dot sturtevantinc dot com)
SEPARATION OF THE MILLED PARTICLES FROM AIR:
As discussed in the previous post, the fine particles travel through a central pipe emerging from the center of the milling chamber to the Cyclone Separator and a series of Filters Or Finger Bags. Important aspects to be taken note of are as under:
# Sudden Pressure Drop due to a Volumetric Expansion: While inside the milling chamber, the air and material particles are subject to a very high pressure (say in the range of 8 to 12 bar). However, post milling the high pressure air stream along with low density (milled) powder particles enters to expansion chamber Or filter bag hood, which is more than 20 times bigger in volume (as compared to the milling chamber volume). Hence the pressure drops to nearly atmospheric or lesser than one bar.
# Cyclonic Action coupled with Coarse and Fine Filtration: The air stream with sudden expansion looses its pressure, however, due to the already attained high momentum can escape the cyclonic action. To avoid this, a series of finger bags Or filter bags (usually 5 to 10 micron size and made up of Polypropylene or Satin material) are used to arrest the powder inside and allow the air to escape through. Note - calculating the required surface area of filtration is a crucial part here.
The powder particles carried by low pressure and low velocity air are then allowed to settle down under the cyclonic action in the receiver collected at the bottom of the cyclone separator.
# Further Filtration of the escaping Air: There is a possibility of powder particles reaching a Nano Or super fine size range. In such cases, the preliminary finger bag filters may not be able to arrest the powder particles completely. This can lead to both loss of material, as well as the contamination Or Air pollution.
To avoid this, a series of external filters are attached in the exhaust air stream viz. Fine Filter (say close to 3 micron aperture) and a HEPA filter (0.3 micron aperture). Incase of a HEPA filter, a differential pressure guage Or transmitter might be required, which will indicate if the filter is choked Or not (so that it can be replaced).
# Quantity, Quality and Pressure of the Air Requirement for the System: As discussed earlier, the milling media in Fluid Energy Mill or Micronizer is mostly Dry Air only.
It needs to be Dry and Oil free, as, otherwise it will help create agglomeration of the powder particles instead of fine milling.
In general a 12" Or 350mm diameter Milling Chamber needs almost 220 CFM (cubic feet per min) worth Air Flow Rate in the main header. Incase the flow rate is lower, the milling will not be consistent.
There is a possibility of powder particles reaching a Nano Or super fine size range. In such cases, the preliminary finger bag filters may not be able to arrest the powder particles completely. This can lead to both loss of material, as well as the contamination Or Air pollution.
Note: Please do not forget to read through Part 1 and Part 3 of this series, as it is the continuation of the discussion.
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