Size Reduction Techniques in Particle Engineering

Size Reduction Influence on Size Distribution

• Size reduction can change the size distribution of particles, shifting towards smaller diameters.
• Initial normal size distribution can transform into a bimodal population due to different fracture behaviors of coarse and fine particles.
• Continued milling can lead to a unimodal population again.
• The lower particle size limit during milling depends on energy input and material properties.

Cutting Methods

• Size reduction range: Refer to figure in source material.
• Principle of operation: Stationary and rotating knives with a screen.
• High shear rates are used to produce coarse powders and fibrous crude drugs.

Compression Methods

• Size reduction range: Refer to figure in source material.
• Principle of operation:
  • Small scale: mortar and pestle.
  • Larger scale: End-runner and edge-runner mills.
  • Size reduction occurs through attrition and compression.
• Alternatives: Roller mill with two horizontally mounted cylindrical rolls, one directly rotated, the other by friction.

Impact Methods

• Size reduction range: Refer to figure in source material.
• Principle of operation:
  • Hammer mill: Contains multiple hammers hinged on a central shaft within a rigid case.
  • High strain rate from angular velocity causes brittle fracture.
  • Can produce a narrow size range.
  • Particles are retained within the mill for repeated processing.
  • Vibration milling:
    • Filled with steel or porcelain balls.
    • The entire mill body vibrates, resulting in impact-based size reduction.
    • Comminuted particles fall through a screen at the base.
    • More efficient than ball mills.

Attrition Methods

• Size reduction range: Refer to figure in source material.
• Principle of operation:
  • Roller mills: Two or three horizontally mounted metal or porcelain rolls with adjustable gap.
  • Rolls rotate at different speeds, causing shearing and attrition of the material.

Combined Methods

• Size reduction range: Refer to figure in source material.
• Principle of operation:
  • Ball mills: Consists of a hollow cylinder rotating on its horizontal axis.
  • Different sized balls (larger for breaking coarse materials, smaller for fine grinding) are used.
  • Amount of material in the mill and the speed of rotation significantly impact efficiency.
  • At low speeds, balls move with the drum, resulting in minimal size reduction.
  • High speed results in balls being thrown against the mill wall, also reducing efficiency.
  • Optimal size reduction occurs at around 2/3 of the maximum angular velocity, resulting in cascading action.

Fluid Energy (Jet) Mill (micronizer)

• Consists of a hollow toroid with a diameter of 20-200 mm.
• High pressure jet of air is injected from the bottom nozzles.
• Air velocity creates turbulent zones, imparting kinetic energy to particles.
• Particles impact each other and fracture.
• A particle size classifier removes fine particles, retaining larger ones for further milling.

Selecting the Milling Method

• Factors:
  • Intended use of the product.
  • Cost (increases with more size reduction).
  • Material properties (hardness, toughness, friability, moisture content, melting point, flammability).
  • Batch size.
  • Safety (toxicity, explosivity).

Importance of Particle Size (PS) and Lifetime of Drug

• Particle size determination is crucial.
• Crystalline form, in vitro dissolution, powder flow, compressibility, in vivo dissolution, absorption, drug distribution, and protein binding are all affected by particle size.
• Drugs in nano or micro sizes can be absorbed as particles.

Influence of Material Properties on Size Reduction

• Toughness:
  • Size reduction occurs through crack propagation.
  • Cracks are initiated in regions with flaws or discontinuities.
  • The stress multiplier at the crack tip is calculated using Inglis’ equation.
  • The stress concentration diminishes with distance from the crack tip.
  • When a crack is initiated, its tip propagates at a speed of 40% of the sound speed in the solid material.

Ball Mill Operation

• Cylinder diameters vary based on scale (3 meters for large scale, 0.2 meters for small scale).
• Ball size is determined by mill size (e.g., 75 mm balls for a 1 meter diameter mill).
• Mills generally contain balls with varying diameters for improved efficiency.
• Material amount in the mill influences efficiency: Too much material cushions impact and reduces effectiveness; too little material causes loss of efficiency and abrasive wear on mill parts.

This quiz explores the principles and methods of size reduction in particle engineering, detailing how various milling and compression techniques affect particle size distribution. Understand how initial size distributions can transform through processes like milling, and the factors influencing particle size limits. Perfect for students studying material science or chemical engineering.