Comminution Flashcards
List some milling methods
- Cutter mill 0.1-100mm
- Runner mill 0.05-10mm
- Roller mill - 0.6-200mm
- Hammer mill - 0.03-10mm
- vibration mill - 0.001-1mm (1um-10um)
- ball mill -0.001mm-0.2mm (1um-200um)
Example of mill(s) that use compression methods and the particle size reduction range
Runner Mill - 0.05-10mm
End-Runner Mill - 0.02-100mm
Roller Mill - 0.6-200mm
Edge-Runner Mill
Sieve diameter
Sieve diameter (dA) as equivalent diameter for various shaped particles
Hammer mill
Impact method
Hammers swing out radially from the rotating central shaft, produced strains are high enough to reduce particle size through brittle fractures mechanism;
Size reduction driven by particle inertia:
As the size (mass) decrease, force of particles hitting the hammers is reduced; hence fracture is less probable;
Produced powders have narrow size distributions.
A screen allows only adequately small particles to exit the mill
NOTE: Particles passing through a given mesh can be much finer than the mesh apertures.
Fine powder size
50-100um
Very fine powder
10-50um
Particle size distribution
Ideal monodispersed particle population consists of spheres (or equivalent spheres) of the same diameter
The size of each particle can be described by a single diameter or equivalent sphere diameter
Most powders contain particles with a range of different equivalent diameters i.e. polydispersed or heterodispersed.
Pharmaceutical powder particle size
polydispersed (from nano- to milli- meters)
Size separation methods
Sieving NOTE: wet sieving methods can be used and are generally more efficient than the dry methods. Sedimentation Elutriation Cyclone
Particle size range for cutting methods
0.1mm-100mm
Particle size for combination of impact and attritution methods
0.001-10mm
Example of mill(s) that use impact methods and the particle size reduction range
Hammer mill 0.03-10mm
Vibration ball mill 0.001-1mm
Considerations about material properties
Influencing particle size
- surface hardness
- crack propagation
- energy requirements
Vibration mill
Impact method
Filled to approximately 80% total volume with porcelain or stainless steel balls;
Vibrating body of the mill: size reduction occurs by repeated impact;
A screen at the base of the mill allows particles to exit;
The efficiency of vibratory milling is greater than that for conventional ball milling
Ball mill - most important factor?
The factor of greatest importance in the operation of the ball mill is the speed of rotation. Low velocity (A): gravity exceeds frictional forces, minimal size reduction High velocity (B): balls roll on body wall due to centrifugal forces Ideal velocity (C): balls roll on body wall and then fall for gravity
Size separation efficiency
Separation efficiency is determined as a function of the effectiveness of a given process in separating particles into oversize (fO) and undersize (fU) fractions.
Hardness is measured by..
Mohn’s scale (qualitive)
where 1 is talc and 10 is diamond
Brinell or Vickers (quantitative)
particle size influences what steps?
- manufacturing
- bioavailability
Analysing particle sizes via sieving
Sieve method:
Common method to analyse particle size;
It sorts a large quantity of particles into different size ranges and determine the particle size distribution based on the mass collected in each range;
Simpler and cost effective method than microscopy, but does not provide any particle shape information (classified as particle size analysis process);
Offline analytical tool, and tends to be less accurate with non-spherical particles (due to their orientation during vibration).
Types of particles for
- Inhalers
- Solid dosage forms
Solid nanoparticles for inhalers
Micrometer particles for solid dosage forms
Importance of particle size
Particle size of the drug (API) and other excipients (powder form) in the formulation influences:
Physical performances of the medicine e.g. dissolution rate
Pharmacological effects of the drug
Accumulation of micro-particulates after parenteral administration
NOTE: plasma proteins and their interaction with particles might increase the final size in the blood stream
Particle size analysis methods
Sieve method
Microscope method
Sedimentation method
Laser diffraction
Machines that use the cutting method
Cutter mill