PART 1 OF 3: MICRONIZER OR AJM (AIR JET MILL) OR A FLUID ENERGY MILL:

 INTRODUCTION TO AN AIR JET MILL OR A FLUID JET MILL OR A MICRONIZER: 

Many a times a chemical reaction Or a unit operation may yield better results when it involves fine particles over coarse ones. This is because particles as fine as Micro Or Nano offers more surface area for reaction, better absorption ability, uniformity, as well as, better solubility and mixing properties in various fluids.

To reduce the particle size of powder Or solid material, the most commonly used operations include Milling, Grinding and Micronizing.

Mechanical size reduction operations such as Milling and Grinding generates heat (due to the direct impact Or collision with the powder) and are not suitable for the heat sensitive materials. However, the Air Jet Mill Or the Micronizer uses fluid energy (mostly Air) to generate turbulence inside the milling chamber carrying the powder particles, leading to size reduction, as low as a Nanometer.

(Pic Ref: www dot sturtevantinc dot com) 

(Pic Ref: www dot promasengineers dot com) 

PRINCIPLE OF WORKING OF FLUID JET MILL OR AIR JET MILL OR MICRONIZER:

Fluid (Air) Jet Mill Or Micronizer works on the principle of 'particle to particle collision' taking place under  the influence of  induced fluid Or air turbulence. A high pressure fluid (mostly Air) is introduced tangentially inside a closed metal basket called Milling Chamber. A feeding nozzle connecting the feeder chamber, milling chamber and ventury nozzle at an angle, under the influence of another stream of fluid (air) pressure pushes the feed material inside the milling chamber. Under the high pressurized air flow inside a closed space, the particles are forced to colloid with each other and on the walls of the milling chamber, thus, reducing in size. The centrifugal and centripetal forces acting on each particle travelling in a rotational motion pushes the large size particle to stay closer to the inner calls of the chamber and the lighter or smaller particles to gather at the center of the milling chamber. The exhaust pipe at the center carries the stream of ligher particles to the cyclone separator Or series of cylone separators, which also contains a series of bag filters for arresting the fine powder particles and letting off the low pressure air.

(Pic Ref: www dot sturtevantinc dot com) 

INDUSTRIAL APPLICATIONS:

Almost all manufacturing industries, where size reduction to micrometer Or nanometer is required, makes use of Fluid Energy Mill Or Air Jet Mill Or Micronizer. Few examples are Pharmaceutical Industries, Nutraceuticals, Herbal and Ayurvedic, Cosmetics, Food, Agro-chemicals and Chemicals, Explosives  manufacturing, Metal Powders manufacturing, Polyomer industry etc.

MAJOR COMPONENTS OF THE FLUID ENERGY MILL OR AN AIR JET MILL:

1. Feeding Chamber: Material feeding can be either by a Vibratory Feeder Or by a Screw Feeder (Augar). Vibratory feeder is used for the products which are very light and may not get an appropriate push with the screw. The Screw Feeder is more prominently used in the industry.

Usually, a feeding chamber is a vertical cylindrical vessel with an open top lid for material charging and with a horizontal agitator (motorized externally) for material shuffling (which avoids ratholing). At the end of the feeder, usually a horizontal screw Or augar is fitted which is mostly powered by the same motor and gearbox which powers the agitator.

The agitator shuffles the powder mass to push it down on to the screw, whereas, the screw pushes the powder mass towards the exit ie. the ventury feeder nozzle.

2. Milling Chamber: Milling chamber is a closed basket made up of metal plates consisting of a Top Plate, Bottom Plate and the Circular Ring. Both the Top and Bottom plates are precisely machined so as to fit on the top and bottom of the Circular Ring. The assembly is closed by the use of eye bolts Or clamps such that it can easily withstand a high fluid pressure of 10 to 12 bar. The Top Plate contains a Feeder Nozzle with Ventury Tube and the Central Exhaust Pipe.

Milling Chamber can be either a Single Jet Or a Multi Jet one. The number of holes of the circular ring which allows the stream of high pressure fluid (air) enter inside the chamber tangentially decides if a milling chamber is called a Single Jet Or a Multijet milling chamber.

3. Ventury Nozzle and the Feeding Line: The Top Plate contains a feeding nozzle, which protrudes inside the top plate at an angle with the top flat surface of the plate. This nozzle has a ventury tube inline (fixed between the outlet of the screw feeder and the entry point of the milling chamber). 

4. Primary Air Pressure: This is the fluid pressure Or air pressure provided to the feeding line and the objective of which is to push the feed material down inside the milling chamber. The high pressure air stream (in other words, the Primary Air) when passed through the nozzle with ventury undergoes sudden expansion due to ventury effect, This creates a small vacuum good enough to pull the powder material from the feeder line into the milling chamber.

5. Secondary Air Pressure: The fluid Or air pressure required to the create turbulence inside the milling chamber is also known as Secondary Air Pressure. This pressure along with the flow rate of the fluid media (Air in today's discussion) decides the output micron size, as well as, yield Or output in measurable quantity viz kilograms.

6. Central Exhaust Pipe: This is the pipe that connects the center of the Milling Chamber and the Cyclone Separator or Dust Collector. The powder particles when broken into smaller particles inside the milling chamber due to particle to particle collisions, get pushed towards the center due to density difference and eventually enters the central pipe which is directly connected to the cyclone separator.

7. Cyclone Separator And the Filter Bag Housing:  This vertical chamber is divided into two parts viz. Filter Bag Housing and Cyclone Separator. The filter bag housing is nothing but the vertical cylindrical topmost cylinder which has a set of PP (polypropylene) filter bags fixed inside of approx. 5 to 10 micron size. The bottom conical part which acts as a cyclone separator is connected to the filter housing cylinder at its bottom. The butterfly valve is provided at the bottom of the conical cylinder to facilitate the discharge of the the fine powder into the receiver tank.

8. Powder Collection Vessel Or Receiver along with the Mechanical Lifting Trolley:  A vertical cylindrical powder collection vessel Or receiver is attached to the bottom of the cyclone separator. The flange of the butterfly valve and the neck flange of the receiver must match each other so that both can be sandwiched in a common triclover clamp Or a Quick release coupling Or Eye Bolts. The neck to body part of this receiver is slanted. This is because even if the diameter of the cylindrical part of the receiver is large, the top neck flange can be of much smaller Or standard size viz 8 inch Or 10 inch or 12 Inch etc and hence can easily connect. The role of the mechanical trolley is to unload the receiver back on the trolley Or lift the receiver to a height such that the receiver flange and butterfly valve flange can match each other.

9. External Filtration Assembly Or  Air Handling Unit: The exhaust air passing though the primary filter Or bag filters sometimes needs additional set of filtration. This optional feature can provide a set of three different filters fixed inside the housing filtration  viz. Primary Filter (usually 5 or 10 micron), Secondary Or Fine Filter (usually 3 micron) and HEPA filter (0.3 micron)

10. Rupture Disc (Optional): In order to mitigate the possibility of any kind of an explosion, a rupture disc is provided on the cylindrical hood carrying the finger bags.

11. Air Manifold for branching the Incoming Air Flow:  Air Jet Mill Or Micronizer essentially need an incoming Air Flow to be split into two halves viz. Primary Or Feeding Pressure and Secondary Or Grinding Pressure. This manifold separates the incoming Air Flow into the Primary and Secondary line with the help of the throttling valves and provision in the operating panel.

HOW TO ACHIEVE DESIRED PARTICLE SIZE IN MICRON 

Please follow my next blog focusing on Desired Particle Size, Feed Pressure, Grinding Pressure and Air Flow Rate

MATERIAL OF CONSTRUCTION:

For pharmaceutical Or food applications (where GMP validation is applicable) the Contact Parts are of SS316 Or Higher Grade, whereas, the Non Contact Parts are of SS304. 

Incase of Highly Abrasive Materials, the Milling Chamber is usually made up of Hardened Steel Or Alloy Steel i.e Lined with Silicon Carbide.

Incase of Non-GMP applications, both the contact and non contact parts can be SS304 Or MS based on the process requirement. 

SURFACE FINISH: 

In case of GMP setup, Product Contact surface is usually required to be of minimum 240 grit Mirror Finish and Non Contact surface is 180 grit matt finish. 

SCOPE OF AUTOMATION IN MILLING OPERATIONS: 

There are a few OEMS and Integrators who claims to have achieved an Automation in Air Jet Milling Or Micronization operation such that the inline sampler and tester randomly picks a scoop of powder from the output material and instantly determines its average micron size based on the pre-filled parameters. Based on the outcome of the computer based analysis, instantly the powder is either sent for further packaging (if passes through) Or sent back as a reflux to the feeding chamber (if it doesnt pass through). This kind of automation is extremely complex, as, the parameters to be chosen that can help us to infer that a particular set of particles can be of a particular micron size are different for different users. Also, there isn't much clarity if a set of parameters that can deicide the particle size by the help of a software can be applied and give effective results for all types of material. Therefore, this kind of an automation will always be a complex one and may not be accurate always.

However, many OEMS and manufacturing companies are able to successfully automate the Feeder Line and Output Line. Which means, the feed to the Feeder of Micronizer can be automated with the hep of a Screw feeder or a Vacuumized Powder Transfer. Same can be applied to the output material. We will talk in details about it in my upcoming blog on Material Handling Systems.

Note: Please do not forget to read through Part 2 and Part 3 of this series, as it is the continuation of the discussion.