Fundamentals of Pharmaceutical Processes Milling

Main ideas, concepts, and lessons

• Pharmaceutical processes are the stages involved in formulating pharmaceutical dosage forms, converting raw materials into final products.
• Because industrial production is complex, processes are broken into smaller, repeatable components called unit operations.
  • This supports:
    • Optimization
    • Reproducibility of product characteristics
    • Efficiency and yield
• Different dosage forms use different subsets of unit operations.
  • Examples given:
    • General example: mixing → heating → filtration
    • Tableting process (sequence): milling → blending/co-milling → blending → compaction → granulation
    • Liquid dosage forms (sequence): mixing → filtration → (if needed) evaporation → (if required) centrifugation
• Unit operations are categorized (in this lecture) as:
  • Milling, mixing, filtration, drying, compression
• Milling is the focus.

Milling: definition and related terms

• Definition: size reduction—reducing particle size to suit the dosage form type.
• Other names/related terms: crushing, dispersion, grinding, pulverization, disintegration.

Why mill? Fundamental requirements

• Increase surface area
  • Smaller particles → more surface area → more interaction/solubilization/absorption/adsorption (as needed by the process goal).
• Particle size distribution (PSD)
  • Milling produces a distribution of particle sizes.
  • PSD describes:
    • Size ranges
    • Number/frequency of particles in each range
  • PSD can be shown as:
    • Table
    • Bar graph
    • Histogram
    • Cumulative frequency graph
    • Normal (Gaussian) distribution curve
  • Interpretation:
    • If the distribution is symmetrical (two halves mirror each other), it is normal/ideal.
    • If not symmetrical, it is skewed/asymmetric, indicating deviation in process outcome.

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How particle size and distribution can be determined

Sieving (one of the oldest techniques)

• Uses circular/rectangular sieves with mesh openings.
• Sieves are characterized by number of openings per linear inch (often related to sieve mesh number).
• Key procedure:
  • Pass a sample through a series of successively smaller apertures.
  • Weigh the material retained on each sieve.
  • Determine what size range the material corresponds to.
• Typical arrangement:
  • Sieves arranged from largest openings/highest size at the top to smallest openings at the bottom.

Microscopy

• Used especially for micron- or nano-sized particles.
• Instruments mentioned:
  • Scanning Electron Microscope (SEM)
  • Transmission Electron Microscope (TEM)

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Factors to consider during size reduction / milling (sieving-related discussion)

• Hardness
  • Harder surfaces require more strength to reduce size (contrasted with toughness).
• Toughness
  • Measures difficulty to break.
  • Examples:
    • Chalk powder: hard but not tough → breaks more easily.
    • Rubber: soft but hard to break → higher toughness than chalk.
    • Fibrous root: described as hard and tough.
• Abrasiveness
  • Brittleness/susceptibility to shedding under friction/rubbing.
• Stickiness
  • Sticky/gummy/resinous materials are harder to break (tend to agglomerate/stick during milling).
• Thermosensitivity
  • If heat/pressure softens material, size reduction becomes harder.
  • Techniques/mechanisms can help prevent temperature rise.
• Material structure
  • Organized lattice/internal structure → easier to break.
  • Unorganized/complex internal structure (e.g., vegetable-origin fibrous/resinous products) → harder to break.
• Moisture content
  • Hygroscopic materials absorb moisture and become soft → harder to mill dry.
  • Guidance:
    • < 5% moisture: easier to use dry grinding
    • > 50% moisture: avoid dry grinding; use wet grinding

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Method/mechanisms used for size reduction (detailed list)

• Cutting
  • Sharp blade/knife cutting.
• Compression
  • Crushing under applied pressure.
• Impact
  • Material hits an object, or an object hits the material (reducing size by collisions).
• Attrition
  • Material placed between two relatively moving surfaces; repeated rubbing/shearing reduces size.

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Types of mills and their main parts

• Mills are instruments for reducing size.
• Three basic parts described:
  1. Feeder (where material is loaded)
  2. Grinding component (the grinder; changes depending on mechanism)
  3. Discharge chute (where ground product is collected; typically via a screen/mesh)

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Specific milling equipment described (operating principle + use notes)

Cutter mill (cutting mechanism)

• Rotor with knives.
• Material fed through a feeding hopper.
• Rotor moves knives → material is cut.
• A sieve/screen at discharge determines when particles are small enough to pass and be collected.

Compression mill

• Two rotating rolls/cylinders:
  • One rotates; the other may be stationary (with rotational control often fine-tunable for the rotating one).
• Material fed from top; passes between cylinders → crushed.
• Used generally for suspensions, pastes, and ointments.
• Modification mentioned:
  • Both cylinders can be tuned to required speed, or only one can be tuned.

Hammer mill (impact mechanism)

• Hammers on a rotating shaft.
• Material fed from top; hammers strike material → reduces size.
• Discharge includes a changeable screen/mesh to achieve required size range.

Ball mill

• Hollow rotating container (horizontal rotation).
• Contains balls (examples: metal, porcelain, pebbles).
• Rotating motion causes balls to strike and break material.
• Final particle size depends on:
  • Speed of rotation
  • Number and size of balls
  • Type of material

Fluid energy mill / Jet mill

• Uses compressed air (or steam) to create high-velocity jets.
• Material is fed and strikes jets; turbulence creates attrition and impaction.
• Prerequisite: material must be pre-treated to ~100 mesh so it is fluidizable (suspended in air flow).
• Advantages mentioned:
  • Suitable for heat-sensitive materials
  • Temperature can be controlled by regulating generated heat/air usage
  • Can connect to vacuum pumps to help regulate temperature.

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Speakers / sources featured

• No specific speakers, instructors, or external sources are named in the provided subtitles.