Obtaining a specific particle range is often the objective of the size reduction operation. The FitzMill Comminutor is ideal for this application because by reconfiguring the mill a wide variety of particle ranges can be obtained.

In other cases obtaining the smallest possible product is the objective. Various techniques can be used to meet this goal. Generally 40 Micron can be considered as the lower limit of size reduction in a FitzMill. For some products which exhibit a crystalline structure however, a finer grind can be obtained.

Fitzpatrick provides units ranging from laboratory scale to continuous-process units capable of operating at 10 to 15 metric tons/hours.

Benefits of Controlled Particle Reduction:
Particle size affects any number of characteristics in the manufacturing process. Controlling the particle size helps assure that the milled material will be consistent and repeatable with respect to:

• Color - uniform particles assure batch-to-batch color consistency
• Flowability - critical to packaging, tableting, weighing
• Uniformity - consistent bulk density
• Density - helps control shipping costs and minimize dust
• Reconstitution - assures the desired dissolution rate
• Chemical reaction - vital for uniform, controlled chemical change
• Taste - allows precise portion control for consistent taste

The FitzMill Comminutor operates by feeding material uniformly into a chamber in which a rotating blade assembly reduces the particles of the material by cutting or impacting them. The material discharges through a screen which regulates final particle size at the outlet of the milling chamber. The blade and screen act in conjunction to determine final product sizing.

The Comminutors rotor speed directly affects the particle size range. As a general rule, and with all other variables remaining constant, the faster the rotor speed, the finer the grind:

Rotor speeds of 3000 to 7200 rpm are typically used with flat blades in fine grinding applications, while speeds of 1000 to 3000 rpm are typically used with sharp blades in coarse grinding applications. For general purpose applications, variable speed rotors are often specified to accommodate the physical characteristics of different materials.

Screen
The Comminutor sizing screen, located directly beneath the blades rotation arc, prevents particles from leaving the grinding chamber until they are at least as small as the screen holes. These holes can have round or square perforations, diagonal or straight slots, or wire-mesh openings.

The screen thickness and the total open surface area of the screen also affect the comminuting operation.

The screen size (diameter of the screen holes) doesn't necessarily designate the particle size of the finished product. The reason is that the impacted particles follow a tangential trajectory from the blades and approach the screen at a shallow angle. Therefore, particles effectively see an opening as an ellipse. The higher the rotor speed, the smaller the angle under which the particle approaches the screen and the smaller the screen openings appear to the particle. This is illustrated in the figure below:

Above figure clearly shows that at higher rotor speeds only smaller particles will pass through the screen. Also, the thickness of the screen determines the size of final product. The thicker the screen, the smaller the particles must be to pass through the screen. This is illustrated in the figure below.



The maximum screen thickness is limited mechanically by the distance between the screen and blades.

The total open surface area affects the amount of fines generated in the milling process. When a particle with the right size to pass through the screen, hits a screen with a bigger total open surface area, the probability that it finds an opening is greater and therefore the probability that it bounces off on the screen back into the chamber is smaller.

Screens with rectangular openings or wire mesh screens have generally larger total open areas than the round opening screen. However, increasing the total open area reduces the mechanical strength of the screen.