Miling- Mill used in pharmaceutical Industry

 The main objective in milling is to reduce particle size to within a specific range while minimizing the generation of fines​ (dusty particles).  Particle size distribution (PSD) is essential to flow, compression, ejection, friability, disintegration, dissolution, machine downtime, changeovers, yields, product quality attributes, and, most importantly, tablet weights. Size reduction mills are used widely throughout the pharmaceutical industry for the reduction of active ingredients (API’s), as well as bulk pharmaceuticals (BPI’s) and excipients such as lactose, HPMC (hydroxypropylmethyl cellulose) and others. Size reduction is used to increase surface area and improve formulation dissolution properties. It is also used to maintain a consistent average particle size distribution (PSD) for the formulation, thus allowing for a better quality mixture when creating solid dosage forms such as tablets and capsules.

Laws governing size reduction:

 • Griffith Equation - The amount of force to be applied depends on the crack length.

• Kicks Law - Work required to reduce the size of a given quantity of material is constant for the same reduction ratio regardless of the original size.

• Rittinger ’s Law - Worked use for particulate size reduction is directly proportional to the new surface produced.

• Bond’s Law - Worked used to reduce particle size is proportional to the square root of the diameter of the particle produced.

 

Mechanism of size reduction:-

 

·         Impact – this involve hammer or bar at high speed (hammer mill). Particle Size 50-8,000 µm.·        

·         Compression- particle crushed b/w rollers by the application of force (roller mill). Particle Size 50-10,000 µm.·        

·          Cutting – the material cut by a sharp blade (cutter mill), Particle Size 100-80,000 µm.

·         Attrition - arising from particles scraping against one another or rubbing action (colloidal mill, roller mill). Particle Size 1-50 µm·     

 

Impact and Attrition – ball mill, fluid energy mill, Particle Size 1-2,000 µm.  

 

 

Factors affecting size reduction– Hardness, toughness, abrasiveness, stickiness, material structure, bulk density, physiological effect Particle shape, Polymorphism, Temperature, and Feeding rate.

 

·         Hardness – It is easier to break soft material than hard materials. Ex: For iodine hammer mill is used.

·         Fibrous – These are tough in nature. A soft, tough material has more difficulty than a hard, brittle substance. Ex: Raowulfia, Ginger. Here cutters can be used.

·         Elastic / Sticky – Become soft during milling. Ex: synthetic gums, waxes, resins. Low melting substances should be chilled before milling. These are milled using hammer, colloid or fluid energy mill.

·         Melting point – Waxy substances, fats and oils are softened during size reduction due to heat generated. This is avoided by cooling the mill and the substance.

·         Hygroscopic – Certain substances absorb moisture content rapidly. This wet mass hampers the milling process. Ex: Potassium carbonate. Closed system such as porcelain ball mill is used.

·         Moisture content – presence of more than 5% moisture influences hardness, toughness, stickiness of substance, in general material having moisture content below 5% are suitable for dry grinding and above 50% for wet grinding.

·         Purity required – the size reduction of such hard substance leads to the abrasive wear of milling parts, causing contamination.

 

Size Reduction Equipment

A. Crushers (Coarse and Fine)

 1.Jaw Crusher

 2.Gyratory Crusher

 3.Crushing Rolls

·         A primary crusher operates on run of mine material accepting anything that comes from the mine face and breaking into150-250mm(6-10in).

   Ø A Secondary crusher reduces these lumps into 6 mm (1/4in).

 

B.Grinders (Intermediate and Fine)

 1.Hammer Mills

 2.Rolling Compression Mills

      A) Bowl Mills

        B) Roller Mills

 3.Attrition Mills

 4.Tumbling Mills

         A) Rod Mills

        B) Ball Mill; Pebble Mill

        C) Tube Mills; Compartment Mills

 

·         Grinders reduce crushed feed to powders

  Ø    The product from a intermediate grinder might pass a 40-mesh screen ØMost of the product from a fine grinder would pass a 200 mesh screen with 74 micron opening.

 

 C. Ultrafine Grinders

 1. Hammer Mills with internal classification

 2. Agitated Mills

 3. Fluid Energy Mills

·          Feed Product is Size size

 

D. Cutting Machines

 1. Knife Cutters; dicers; millers.

Ø Definite Product in length size size .

 

 

 

 

 

 

MECHANISM OF ACTION	EXAMPLE	PRODUCT SIZE	TYPE OF MATERIAL
Impact	Hammer mill	Moderate to fine	Brittle and dry material
Shear	Extruder and hand screen	coarse	Deagglomeration, wet granulation
attrition	Oscillating granulator	Coarse to moderate	Dried granulation
Shear compression	Conical screening mill	Moderate to coarse	Wet, dry granulation

 

  1.Cone  Mill

 A conical mill (or conical screen mill) is a machine used to reduce the size of material in a uniform manner. It is an alternative to the hammermill or other forms of grinding mills. As the name implies, the conical mill varies in diameter from where the feed enters to where the product exits. Conical mills come in a variety of sizes from tabletop lab models to full-size high-capacity machines for use in processing large quantities of material, and the impeller and screen can be customized for each individual use. The machines can be used not only to reduce the size of particles, but also for deagglomeration, sieving, dispersion, and mixing.

Cone Mill is used for high speed SHREDDING, SIZING, GRINDING of powder Granules, Tablets and a wide range of wet and dry materials without special attachments. As compared to the four common principles of size reduction i.e. grinding, compression, impact and shearing, which often do not produce controlled size reduction, this machine utilizes the principle of Centrifugal Shearing force.

 2.Multimill

The principle involves variable force rotate blades having both knife and sharp edges with validated screen size to reduce particles in a controlled manner. The mechanism involves the pulverization process (Pulverization is grinding and crushing of materials into small particles) knife is used to cut the large particles into small size particles. These big particles are made from granules during the drying process in FBD. The hummer process is used to break the large particles into small sizes.