Comminution and mechanism of milling

Objectives of comminution

• Increase surface area, which can improve dissolution and extraction.
• Obtain a desired and more uniform particle-size range.
• Facilitate blending of powders.
• Improve flow, packing, drying, and downstream processing.
• Break agglomerates in powders or granules.

Theories of comminution

1. Rittinger’s law
   Rittinger proposed that the energy required for size reduction is proportional to the new surface area produced.
   [ E = K_R \left(\frac{1}{d_2} - \frac{1}{d_1}\right) ]
   Where (d_1) is initial particle size and (d_2) is final particle size.
   • Most applicable to fine grinding, where a very large new surface area is generated.
   • Fine powders require more energy because surface-area increase is high.
2. Kick’s law
   Kick stated that the energy needed is proportional to the size-reduction ratio, rather than the newly created surface area.
   [ E = K_K \ln\left(\frac{d_1}{d_2}\right) ]
   • Mainly applicable to coarse crushing.
   • It assumes that equal fractional reductions in size require equal amounts of energy.
3. Bond’s law
   Bond’s theory is an intermediate approach. It states that energy required is related to the change in reciprocal square root of particle diameter.
   [ E = K_B \left(\frac{1}{\sqrt{d_2}} - \frac{1}{\sqrt{d_1}}\right) ]
   • Often useful for practical industrial milling.
   • It applies best to intermediate-size reduction, between coarse crushing and fine grinding.

Mechanisms of milling

• Impact: A rapidly moving object strikes a particle, causing fracture. Example: hammer mill.
• Compression: Material is crushed between two surfaces. Example: roller mill.
• Attrition: Particles are broken by rubbing or friction between surfaces or other particles. Example: ball mill.
• Cutting: Sharp edges cut the material into smaller pieces. Useful for fibrous materials. Example: cutter mill.
• Shear: Oppositely directed forces cause layers of material to slide and fracture. Example: colloid mill.

Hammer Mill

Principle

Construction

1. Feed hopper: Introduces material into the grinding chamber.
2. Metallic grinding chamber or casing: Encloses the milling zone and prevents escape of powder.
3. Rotor or central shaft: Rotates at high speed.
4. Hammers: Usually four or more hardened steel rectangular pieces pivoted or hinged to the rotor. They swing outward due to centrifugal force.
5. Breaker plates or grinding plates: Stationary surfaces against which particles may strike and break.
6. Screen or sieve: Located at the bottom or around the lower portion of the chamber. It controls the maximum particle size of the product.
7. Discharge outlet: Collects the milled material passing through the screen.
8. Motor and drive mechanism: Provide rotary motion to the shaft.

Working and mechanism of milling

1. Material is fed through the hopper into the milling chamber.
2. The rotor rotates at high speed and the hammers swing radially outward due to centrifugal force.
3. The hammers strike the particles forcefully, producing primary size reduction by impact.
4. Particles are also broken by:
   • collision against the casing or breaker plates,
   • collision among particles,
   • attrition and shear between hammers, screen, and particles.
5. Fine particles pass through the screen and are collected through the outlet.
6. Coarse particles cannot pass through the screen, remain in the chamber, and undergo repeated impacts until sufficiently small.

Factors affecting product size

• Rotor speed: Higher speed generally gives greater impact and finer product, though it can also increase heat generation.
• Hammer size, number, and design: More hammers increase frequency of impact.
• Hammer-to-screen clearance: Smaller clearance can increase attrition and promote finer milling.
• Screen aperture size: Smaller openings produce finer particles but reduce throughput and may increase clogging.
• Feed rate: Excess feed causes poor milling, choking, and uneven particle size.
• Nature of material: Brittle materials mill easily; tough, elastic, sticky, plastic, or fibrous materials are less suitable.
• Moisture content: High moisture can cause sticking and block the screen.
• Feed particle size: Very large feed may require pre-crushing.

Types

• Reversible hammer mill: Rotor can rotate in both clockwise and anticlockwise directions. This permits use of both sides of the hammers and may prolong hammer life.
• Non-reversible hammer mill: Rotor runs in only one direction.

Pharmaceutical uses

• Milling of dry granules before tablet compression.
• Processing wet granules after drying.
• Deagglomeration and dispersion of powder blends.
• Milling of APIs and excipients, when suitable.
• Powdering dried herbal drugs, including roots, bark, leaves, and other crude plant material.
• Milling substances such as sugar and certain crystalline materials.

Advantages

• Simple construction and relatively low cost.
• Continuous operation with high production capacity.
• High reduction ratio.
• Easy installation, operation, cleaning, and maintenance.
• Screen provides control of maximum particle size.
• Suitable for many brittle, dry materials.
• Requires relatively little floor space.
• Can be used for primary, secondary, or tertiary size reduction.

Limitations and disadvantages

• Not suitable for very hard or abrasive materials because of excessive hammer and screen wear.
• Not ideal for very fine, narrow particle-size distributions.
• Heat generation may damage heat-sensitive materials.
• Sticky, waxy, low-melting, or plastic materials may foul the chamber and clog the screen.
• Feed rate must be controlled to prevent choking.
• Foreign metallic particles or stones can damage the machine.
• Produces dust and noise, so containment and dust-control measures may be required.
• Screen blockage can reduce output and cause non-uniform milling.