Fluid Energy Mill

 

Fluid energy mill, also known as jet mill or micronizing, is a size reduction method that operates through particle impaction and attrition. Figure 1 illustrates a form of a fluid energy mill. Both circular and oval-path designs are available for this type of milling.

Construction and working of Fluid Energy Mill

In the fluid-energy mill or micronizer, the material is suspended and conveyed at high velocity by air or steam. This air or steam is passed through nozzles at pressures of 100 to 150 pounds per square inch (psi). The high velocity of the air gives rise to zones of turbulence into which solid particles are fed. The high kinetic energy of the air causes the particles to impact with other particles and with the sides of the mill. These impacts occur with sufficient momentum for fracture to take place. Additionally, turbulence ensures that the level of particle-particle collisions remains high enough. As a result, substantial size reduction is achieved through impact. Moreover, some attrition also occurs due to these collisions.

Air is usually used because most pharmaceuticals have a low melting point or are thermolabile. As the compressed air expands at the orifice, the cooling effect counteracts the heat generated by milling.

Fluid energy mill
Fig.1: Fluid energy mill.

As shown in Fig. 1, the material is fed near the bottom of the mill through a venturi injector (A). As compressed air passes through the nozzles (B), the material is propelled outward against the wall of the grinding chamber, causing impact (C), and collides with other particles, resulting in attrition. The air moves at high speed in an elliptical path, carrying the fine particles out of the discharge outlet (D) into a cyclone separator and a bag collector. Larger particles are driven by centrifugal force to the periphery, where they are further subjected to attrition. The design of the fluid-energy mill incorporates internal classification, allowing finer and lighter particles to be discharged while heavier oversized particles are retained until they are reduced to the desired size.

Key Considerations for Fluid-Energy Mill Operation and Selection

Fluid-energy mills reduce particle sizes to 1 to 20 μm. To facilitate milling, the feed should be premilled to approximately a 20- to 100-mesh size. A 2-inch laboratory model, using 20 to 25 cubic feet per minute of air at 100 psi, can mill 5 to 10 grams of feed per minute. For a given machine, size reduction depends on the feed size, its introduction rate to the grinding chamber, and the pressure of the grinding fluid. The most important machine-related factors are the grinding chamber geometry and the number and angle of the nozzles. When selecting fluid-energy mills for production, consider the cost of the fluid-energy source and dust collection equipment, in addition to the cost of the mill itself.

Advantages and disadvantages

Powders with all particles below a few micrometers may be quickly produced by this method. The disadvantage of high capital and running costs may not be so serious in the pharmaceutical industry. This is because of the high value of the materials that are often processed. However, one drawback of this type of mill is the potential for the build-up of compressed product in the mill or on the classifier. This accumulation can affect milled particle size by changing the open volume in the mill or the open area in the classifier. This issue becomes particularly problematic if classifier vanes or gas nozzles become plugged or blocked.

Reference:

  • Aulton, M. (2018). Aulton’s pharmaceutics, the design and manufacture of medicines. Edinburgh. : Elsevier.
  • Khar, R.,Vyas, S., Ahmad, F., & Jain, G. (2016). Lachman/Lieberman’s The Theory and Practice of Industrial Industrial Pharmacy. New Delhi, ND: CBS Publishers & Distributors Pvt Ltd.