Fluid energy mill is also known as pulverizers. 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 introduction of the air or inert gas through the nozzles. 1,2
Fluid energy mills
In 1900s the Company developed designs for crushing and grinding, blending, mixing and related materials-handling equipment. T.J. Sturtevant designed an automatic transmission IN 1904.
Further engineering skill led to the development of the Air Separator. In 1930 a contest between Sturtevant and another important manufacturer for the favor of a major cement producer proved the Sturtevant unit superior.
Sturtevant introduced an ultrafine grinder, the Micronizer in 1950.
Sturtevant innovated in 1990s, its air swept Pulver Mill and Air Separator technologies into a single unit known as the Powderizer, an air swept and air classifying mill. The Micronizer is the world's first USDA accepted jet mill, the new in 2002 "Qualification Micronizer" and newer developments yet to be announced. The Powderizer line has recently been awarded another patent for a Sturtevant innovation. 1,2,3
The fluid energy mill mainly used to grind the sensitive materials to the fine powder by the mechanism of impact and attrition forces applied by the air or inert gas from the nozzles presents in the chamber.
The main basic parts present in the fluidized energy mill are as follows
The inlet by which the solid material is introduced into the chamber which is mad of stainless steel.
The nozzles by which the air and the inert gas is introduced into the chamber at high pressure.
The classifier from which the fine reduced particles are collected.
It consists of a loop of pipe which has a diameter of 20 to 200 mm, depending on the overall height of the loop, which may be up to about 2 m.
There is an inlet for the feed and a series of nozzles for the inlet of air or an inert gas.
It also has an outlet with a classifier which allows the air to escape but prevents to pass until they become sufficiently fine. 1,2
It works mainly on the principle of attrition and impact.
In the operation of a fluid energy mill, gas of high energy content is introduced into a pulverizing chamber. The air or inert gas is introduced with a very high pressure through the nozzle. Solids are introduced into air stream through the inlet. Due to the high degree of turbulence, impact and attritional forces occurs between the particles.
The fine particles are collected through a classifier. Fluid energy mill reduce the particles to 1 to 20 micron. To get a very fine powder even up to 5 micron the material is pretreated to reduce the particle size to the order of 100 mesh and then passed through fluid energy mill.
A size-reduction unit depending for its action on collisions between the particles, the energy being supplied by a compressed fluid, (e.g., air or steam) that enters the grinding chamber at high speed. Such mills will give a product of 5 micron or less. 1-3
Working of fluid energy mill
There are two main classes of pulverizers
- Air swept pulverizer
- Air impact pulverizer
Air swept pulverizers use air to transport particles to the pulverizing section of the apparatus. Air impact pulverizers use high speed air to pulverization of the particles.
The products from both air swept and air impact pulverizers produces oversize particles and no need of further sieving or classifying.
Air Swept Pulverizer
The particles along with air is fed into the mill inlet.
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 (revolutions per minute) to reduce the size of the incoming particles.
The classifier plate separates the fine product and exit through the discharge outlet. The material is back feed to the mill inlet through the recycle housing.
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. 3
- Pulverizers are commonly used for chemicals, pigments and food processing. The micro scale air impact pulverizer is used in laboratories, where small samples are needed.
- Fluid energy mills are used because of their advantages in fine grinding.
- The mill is used to grind heat sensitive material to fine powder.
- They have been used for the fine grinding of frits, kaolin, zircon, titanium and calcium, alumina, but the energy consumed per ton of milled product is high.
- It is also an object to provide grinding of dry solids.
- The mill is used to grind those drugs in which high degree of purity is required.
- Contamination can be avoided and no excess heat is produced, pulverizers are suitable for materials that must remain ultra-pure and those that are heat sensitive. Even cryogenic applications can be reduced. 1-3
- This invention is applicable to the size reduction of a wide variety of materials capable of generating a static charge in a fluid energy mill.
- Examples of these materials are aluminum oxide, ceramic frit, powder insecticides such as DDT, diatomaceous earth, feldspar, fluorspar, graphite, gypsum, iron ore, iron oxide, iron powder, limestone, mica, paint pigments, polymers, rare earth ores carbon, talc, and the like.
- The process involves no moving parts or screens and is suitable for virtually any friable or crystalline materials, even materials that are very abrasive. 1-3
- Air needed is free.
- Large range of sizes available.
- The mill is used to grind the materials to fine powder.
- The particle size of the powder can be controlled due to the use of a classifier.
- There is no wear of the mill and hence there is no contamination of the product.
- It is useful for grinding heat sensitive substances such as sulphonamides, vitamins and antibodies.
- Homogeneous blend.
- Energy consuming.
- High head space.
- Avoid coarse materials into the chamber.
- The fed device may be clogged with the clump materials.
- Special feeding devices should be provided for the feeding of the materials.
- One of the problems encountered with the use of compressed air is the generation of static electricity.
- Material recovered in the collection bags is difficult or impossible to remove by the normal blow back procedures.
- Lieberman, Herbert A, Lachman, Leon (2009), Industrial pharmacy: Pharmaceutical dosage form-milling. Banglore, Pune. ISBN: 978-81-239-1679-8.
- Remington, Alfonso and Gennoro, Science And Practise Of Pharmacy- powders. 20th edidtion.
- Mehta RM, Pharmaceutics-I: Size reduction (1996), Page no: 117-118.