Commonly Used Mills in Pharmaceutical Manufacturing

 

Cutter Mill

• The rotary knife cutter has a horizontal rotor with 2 to 12 knives spaced uniformly on its periphery turning from 200 to 900 rpm and a cylindrical casing having several stationary knives.
• The bottom of the casing holds a screen that controls the size of the material discharged from the milling zone.
• Cutter mills are useful in producing a coarse particles.

Principle of operation: During milling, size reduction occurs by fracture of particles between. A screen is fitted in the base of the mill casing and acts to retain material in the mill until a sufficient degree of size reduction has been effected.

Roller Mill

• It consists of two to five smooth roller operating at different speed.
• Mechanism by combination of compression and shearing action.
• Two cylindrical rolls mounted horizontally and rotated about their long axes. one of the rolls is driven directly while the second is rotated by friction as material is drawn through the gap between the rolls. 
• A form of roller mill used for milling ointments, pastes and suspension where both rolls are driven but at different speeds, so that size reduction occurs by attrition.

Hammer Mill

• The hammer mill is an impact mill using a high speed rotor (up to 10,000 rpm) to which a number of swinging hammers are fixed.
• The material is fed at the top or center, thrown out centrifugally, and ground by impact of the hammers or against the plates around the periphery of the casing.
• The clearance between the housing and the hammers contributes to size reduction.
• The material is retained until it is small enough to fall through the screen that forms the lower portion of the casing. 
• Hammer mills are compact with a high capacity and size reduction of 20 to 40 μm may be achieved.

Ball Mill

• The ball mill consists of a horizontally rotating hollow vessel of cylindrical shape with the length slightly greater than its diameter.
• The mill is partially filled with balls of steel or pebbles which act as the grinding medium.
• As the speed is increased, the balls are carried up the sides of the mill and fall freely onto the material with an impact action, which is responsible for most size reduction.
• The factor of greatest importance in the operation of the ball mill is the speed of rotation.

Fluid-energy Mill

• In the fluid-energy mill or micronizer, the material is suspended and conveyed at high velocity by air or steam, which is passed through nozzles at pressures of 100 to 150 pounds per square inch (psi).
• The violent turbulence of the air and steam reduces the particle size chiefly by inter particular attrition. 
• Fluid energy milling act by impaction and attrition. 

Principle of Operation: The high kinetic energy of the air causes the particles to impact -with other particles with sufficient momentum so that the turbulence ensures that the level of particle-particle collisions is high enough to produce size reduction by impact and some attrition. 

Any mill consists of three basic parts:

(1) feed chute, which delivers the material,

(2) grinding mechanism, usually consisting of a rotor and stator, and

(3) a discharge chute.

Read also:

• Particle Size Reduction in Pharmaceutical Industry
  
• Critical Material Attributes in Pharmaceutical Manufacturing