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
Runner mill
Compression method
Acts as mortar and pestle
Basic equipment consists of:
Two heavy wheels (stone or metal) connected by a shaft;
The material to be milled is fed into the centre of the pan and is worked outwards by the action of the wheels;
Scrapers constantly move fine materials from the bottom to get a continuous crushing to powders
0.05-10mm
Milling
Transformation of approximate normal particle size distribution into finer bimodal population.
Y
Different particle size range can be obtained:
Cutting methods: 100um - 100mm
Compression methods: 10um – 100mm
Impact methods: 1um – 10 mm
Attrition methods: 1um – 100um
Combination of impact and attrition: 1 um – 10mm
How to determine equivalent sphere
Several methods can be used to determine the equivalent sphere diameter, based on: Volume Surface Sieve aperture Sedimentation characteristics Etc.
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.
If the separation process is 100% efficient, then all oversize material (fo) will end up in the oversize product stream and all undersize material (fu) will end up in the undersize product stream.
Ball mill - amount of material in body does what?
The amount of material in the body is an important variable
Large feed volume produces a cushioning effect, small one causes loss of efficiency and abrasive wear of the mill parts.
Edge-Runner mill
Compression method
The Edge-runner mill has the pestle(s) mounted horizontally and rotating against a bed of powders.
Size reduction done by compression (heavy weight of mortar)
Attrition processes might promote size reduction as well
It is mostly used for all types of the drugs.
Very fine particle size is produced
Particle size range for impact methods
0.001-10mm
Ideal case
spherical, mono-sized
equivalent sphere
Equivalent sphere: reduce the three-dimensionality to a single number (simplify QC)
Equivalent sphere diameter (or equivalent diameter)
NOTE: more than one sphere can represent an irregular particle!
E.g. considering the projected area diameter
Particle size range when attrition methods are used
0.001-0.1mm
energy requirements and loss
Only 2% of the total energy is effectively involved in size reduction process Energy is lost through: Elastic deformation Plastic deformation (without fracture) Deformations initiating cracks (with fracture) Interactions with mechanical parts Heat Vibration
Crack propagation
Localised stresses produce strains in the particles to cause rupture and crack propagation
Not all the materials possess brittle behaviour e.g. plastic materials resist better to stresses
Materials properties should be considered when designing a communition process e.g. brittleness, plasticity
Ductile fracture vs brittle
Brittle - little plasticity deformation
Ductile - plastic deformation
Example of a mill that uses impact and attritution methods and the particle size reduction range
Ball Mill
0.001-0.2mm
Medium/fine powder size
100-350um
Mean particle size
A single number cannot represent (or fully describe) the size distribution of a powder
To keep things simple, it is necessary to use one value to describe the powder sample
Median or Mode can represent the tendency of the sample
The mean diameter is more informative, determined only when the size distribution is complete:
Upper and lower size limits are determined
Several ways can be used to calculate the mean diameter
Shape factor
Shape factors can be used to express particle circularity: there are several methods to measure circularity, which consider length (a) or circles (b) – respectively w/l and di/dc
Roller mill
Compression method
The material is compressed by frictional forces as it pass between rollers
One roll is mechanically driven, the other rotated by force transmission
Series of rolls can be included to obtain smaller particles
Coarse powder size
> 350um
Particle size analysis using lasers
Laser diffraction
Fully automated and in-line method of measuring the particle size distribution;
Size distribution of particles is calculated using angle of diffraction principle (light is inversely proportional to the particle size);
Theoretically an extremely accurate method, however results are poorly reproducible between different detection systems;
Only particle size can be measured (as for sieve analysis);
Another limit resides to the models used to calculated particle size, which are based on the hypothesis that measured particles are perfect spheres.
What is Comminution?
Comminution is the reduction of solid materials from one mean particle size to a smaller mean particle size
During a size reduction process the particles of feed material will be broken down and particles in different size ranges undergo different amounts of breakage.
micronised powder
<10um
End-Runner mill
Compression method
Mortar revolves and the weighted pestle rotates under frictional forces (due to powder movement)
High rotational speed is used
Mortar fixed to a flanged plate
Dumb-bell shaped pestle
Pestle can me raised from the mortar to facilitate emptying and cleaning procedures
Prolonged milling
Transformation of a fine bimodal particle population into a finer unimodal distribution.
Smaller particle sizes can impact on
Mixing of different solid powders
Production of suspensions
Sedimentation methods for separating particles
A suspension of solid particles in a liquid (usually water) is filled in a chamber. After predetermined times, particles less than a given diameter can be recovered from a fixed distance below the surface of the liquid.
Alternatively, a single separation can be carried out simply by removing the upper layer of suspension fluid after the desired time.
Disadvantages of sedimentation methods rely on being batch processes and that discrete particle fractions cannot be collected
Size fractions can be collected continuously using a pump mechanism.
Microscope method for particle size analysis
Microscope method:
Perhaps the most obvious and accurate method for determining the particle size and shape characteristics of a small sample;
Operator time required to analyse a sufficient number of particles to be representative is prohibitive;
Offline method of particle characterisation with very limited throughput which may make it unsuitable for a number of applications.
Sieving types
Agitation methods (A): oscillation, mechanical vibration, gyration (most efficient) Brushing methods (B): a brush orientate particles and prevent blocking of the sieve surface. It is fundamental that the brush does not force particles through the sieve. Centrifugal methods (C): a high speed rotor generate are flow that pushes bigger particles outwards, while fine powders are processed
Cutter mill method
Cutting method
Series on knives/blades on a rotor
Fracture of particles occurs during milling between knives/blades
A screen acts to retain material larger than a specified size
Self-classifying method
Coarse degree of size reduction in dried granules due to high shear rates
0.1-100mm
What does particle size influence?
Influences manufacturing steps
Dose/quantity of ingredients is controlled by volumetric filling e.g. capsules
Flow and packaging properties
Quality control at the manufacturing end
Smaller particles dissolve faster, hence the bioavailability is higher
NOTE: very small particles might result in a rapid absorption (not be beneficial)
Particle size influences formulation efficacy after administration
What happens when particle size is reduced to about 1-5 μm:
Particle-to-particle forces predominates comminution stresses
Particle agglomeration might happen with an increase of the average particle size
Particle size range for compression methods
0.01-100mm
Ball mill
Impact and attritution methods
A hollow cylinder body that can be rotated on its horizontal longitudinal axis contains balls that occupy 30–50% of the total volume
Ball size depends on feed (initial size) and mill (final size) particles.
Balls with different diameters help to improve the process: large balls tend to break down the coarse feed materials, the smaller ones help to form the product by reducing void spaces between balls.
Sedimentation types
Differences in settling velocities is used to separate particles with different diameters
REMEMBER Stokes’ equations
Gravitational (A) or centrifugal (B) sedimentation can be used to separate particles
