Particle Size Reduction
Particle Size Reduction
Size reduction is a process of reducing large solid unit masses into small unit masses, coarse particle or fine particles.
Size reduction process is also referred to as commination and grinding.
When the particle size of solid reduced by mechanical means it is known as milling.
Size reduction operation divided into two category –
Grinding & Cutting – If material is solid
Emulsification & Atomization – if material is liquid
Objectives of Size Reduction
Dissolution and therapeutic efficacy
Extraction
Flow ability
Mixing
Formulation
Mechanism of size reduction
Impact – this involve hammer or bar at high speed (hammer mill). Particle Size 50-8,000 µm.
Compression- particle crushed b/w rollers by the application of force (roller mill). Particle Size 50-10,000 µm.
Cutting – the material cut by a sharp blade (cutter mill), Particle Size 100-80,000 µm.
Attrition – arising from particles scraping against one another or rubbing action (colloidal mill, roller mill). Particle Size 1-50 µm
Impact and Attrition – ball mill, fluid energy mill, Particle Size 1-2,000 µm.
Advantage of size reduction
Content uniformity
Uniform flow
Effective extraction of drug
Effective drying
Improve physical stability
Improve dissolution rate
Improve rate of absorption
Increase surface area and viscosity
Facilitates bioavailability, uniform mixing and drying
Disadvantage of size reduction
Drug degradation
Poor mixing
Contamination
Factors affecting size reduction
– Hardness, toughness, abrasiveness, stickiness, material structure, bulk density, physiological effect Particle shape, Polymorphism, Temperature, and Feeding rate.
Hardness – It is easier to break soft material than hard materials. Ex: For iodine hammer mill is used.
Fibrous – These are tough in nature. A soft, tough material has more difficulty than a hard, brittle substance. Ex: Raowulfia, Ginger. Here cutters can be used.
Elastic / Sticky – Become soft during milling. Ex: synthetic gums, waxes, resins. Low melting substances should be chilled before milling. These are milled using hammer, colloid or fluid energy mill.
Melting point – Waxy substances, fats and oils are softened during size reduction due to heat generated. This is avoided by cooling the mill and the substance.
Hygroscopic – Certain substances absorb moisture content rapidly. This wet mass hampers the milling process. Ex: Potassium carbonate. Closed system such as porcelain ball mill is used.
Moisture content – presence of more than 5% moisture influences hardness, toughness, stickiness of substance, in general material having moisture content below 5% are suitable for dry grinding and above 50% for wet grinding.
Purity required – the size reduction of such hard substance leads to the abrasive wear of milling parts, causing contamination.
Laws governing size reduction
• Griffith Equation – The amount of force to be applied depends on the crack length.
• Kicks Law – Work required to reduce the size of a given quantity of material is constant for the same reduction ratio regardless of the original size.
• Rittinger ’s Law – Worked use for particulate size reduction is directly proportional to the new surface produced.
• Bond’s Law – Worked used to reduce particle size is proportional to the square root of the diameter of the particle produced.
Classification of mills
Hammer mill
Ball mill
Fluid energy mill or ultra-fine grinder or micronizer
EDGE Runner Mill
END Runner Mill
Quadro co mill
Multi Mill
Roller Mill
Rotary Mill
Disintegrator
Hammer mill
Principle –
It operates on the principle of impact between rapidly moving hammers mounted on rotor and the stationary powder material.
Parts –
Consists of a metal casing, enclosing a central shaft, to which 4 or more swinging hammers are attached.
Lower part of casing consists of a screen, through which material can pass and collected in a suitable receiver.
Construction & Working –
A hammer mill is essentially a steel drum containing a vertical or horizontal rotating shaft or drum on which hammers are
mounted.
The hammers swings on the ends or fixed to the central rotor.
The rotor is rotates at a high speed inside the drum while material is fed into a feed hopper.
The material is put into the hopper which is connected with the drum.
The material is powdered to the desired size due to fast rotation of hammers and is collected under the screen
Impact mill using high speed rotor up to 10000 rpm with a peripheral speed up to 7600 meter per minute
Dry materials, wet filter press cake, wet granules, ointments and slurries
Size reduction up to 20 to 40µ
Circular hole design for fibers and herringbone design for crystalline materials and continuous operation jump- gap screen for
abrasive and clogging materials.
Micro pulverizer for nonabrasive materials
Determination of particle size
Rotor speed
Feed rate
Clearance between hammers and grinding plates
Size of discharge opening
Advantages –
It is rapid in action, and is capable of grinding many different types of materials.
They are easy to install and operate, the operation is continuous.
There is little contamination of the product with metal abraded from the mill as surface move against each other.
The particle size of the material to be reduced can be easily controlled by changing the speed of the rotor, hammer type, shape and size of the screen.
Disadvantages –
Heat buildup during milling is more, therefore, product degradation is possible.
Hammer mills cannot be employed to mill sticky, fibrous and hard materials.
The screens may get clogged.
Uses
Brittle material is best fractured by impact from blunt hammers.
Fibrous material is best reduced by cutting edges.
Variants
Fitzatrick comminuting machine – fitz mill
Strokes tornado mill
Ball mill
Principle –
It operates on the principle of impact and attrition between the rapidly moving balls and the powder material, both enclosed in a hollow cylinder.
Both wet and dry grinding, up to 10µ
Pebble, rod mill, tube mill and Hardinge mill
Sliding, cascading and centrifuging
The critical speed is n=76.6/D2=KD (feed particle)
Optimum diameter of the ball= D ball
Parts
Consists of a hollow cylinder mounted on a metallic frame such that it can be rotated along its longitudinal axis.
The cylinder contains balls occupying 30–50 % of mill volume.
The weight of ball is constant; Size depends on the feed quantity and diameter of mill.
Determination of particle size and Shape
Size of ball
Feed rate
Speed of rotation of cylinder
Advantages:
It can produce very fine powder.
Ball mill is used for both wet and dry grinding processes.
Toxic substances can be ground, as the cylinder is closed system.
Installation, operation and labour costs are low.
Disadvantages:
The ball mill is a very noisy machine.
Ball mill is a slow process.
Soft, fibrous material cannot be milled by ball mill.
Uses
Produced fine powder, can grind large variety of materials.
As it is a close system toxic substance can be ground.
Fluid energy mill or ultra-fine grinder or micronizer
Principle
It operates on the principle of impact and attrition.
• Air or steam at pressure of 100- 150 psi
• Particle size to 1-20µ
• Feed should be pre milled to 20 to 100 mesh
• Nozzle design and direction of air jets
• Efficiency of air compressors
• Efficiency of filters and separators
Parts
Consists of a loop of pipe with diameter 20-200 mm. The overall height of the pipe is 2 m.
Inlet for feed and a series of nozzles for air, inert gas. Outlet with classifier which prevents the particles to pass until they become
sufficiently fine.
Construction
It consists of a loop of pipe, which has a diameter of 20 to 200 mm , depending on the overall height of the loop ,which may be upto about 2m.
There is an inlet for the feed and a series of nozzles for the inlet of air or an inert gas.
It also has an outlet with a classifier which allow the air to escape but prevents the particles to pass until they become sufficiently
fine.
Working
The air or inert gas is introduced with a very high pressure through the nozzles.
Solids are introduced into air steam through inlet due to high degree of turbulence, impact and attritional forces occurs between the particles.
The fine particles are collected through a classifier.
Fluid energy mill reduces the particles to 1 to 20 micron.
To get a very fine powder, even up to five micron, the material is pretreated to reduce the particle size to the order of 100 mesh and then passed through fluid energy mill.
Determination of particle size and Shape
To speed of air/inert gas
The impact between the feed and air.
Uses
The mill is used to grind heat sensitive material to fine powder.
The mill is used to grind those drugs in which high degree of purity is required.
Advantages
The mill is used to grind the material to fine powder.
The particle size of powder can be controlled due to the use of a classifier.
There is no wear of the mill and hence there is no contamination of the product.
It is useful for grinding heat sensitive substances such as sulphonamides, vitamins and antibiotics.
Disadvantages
Tendency of forming aggregates or agglomerates after milling.
Generation of amorphous content due to high energy impact.
Formation of ultra-fine particles
High energy consumption.
EDGE Runner Mill
Principle
The size reduction is done by crushing due to heavy weight of stone.
Construction
It consist of two heavy rollers and may weigh several tons.
The roller move on a bed which is made up of granite or stone.
Each roller has a central shaft and revolve on its axis.
The rollers are mounted on horizontal shaft and move around the bed
Working:
The material to be ground is placed on the bed with the help of the scrapper in such a way that it comes in the path of the stone wheel.
These stones revolve on its own axis and at the same time travel around the shallow stone bed.
The material is ground for definite period.
The powder is collected and passed through a sieve to get powder of required size.
Advantages
Does not require attention during operation.
Disadvantages
More space than other mill, Contamination, Time consuming, not use for sticky materials.
Uses
Grinding tough material to fine powder.
END Runner Mill
Principle
Size reduction is done by crushing due to heavy weight of steel pestle.
Shearing stress is also involved during movement of mortar and pestle.
Construction
It is considered as mechanical mortar pestle.
It consist of a steel mortar which is fixed to a plate.
The construction of mortar is connected to horizontal shaft bearing a pulley so the plate with mortar can be rotated at high speed.
The pestle is dumb-bell shaped and bottom of pestle is flat.
Construction of pestle is done in such a way that it can be raised from mortar for cleaning and emptying.
Working
The material to be ground is placed in the mortar.
The mortar revolves at a high speed.
The revolving mortar causes the pestle to revolve during this process, size reduction is achieved.
Advantages
End runner mill provides moderately fine powder and operates successfully with fibrous materials, bark, woods fruits, leaves, etc.
Wet grinding with very viscous material such as ointments and paste is also possible.
Disadvantages
Not suitable for unbroken or slightly broken condition of drug.
Uses
Use for fine grinding.
Quadro co mill
• Control feed product into upper conical screen chamber
• A rotating impeller calibrates incoming material
• Calibrated product then passes through to the lower chamber
• A second intensifying impeller accelerates the particles.
• Particles are continuously delivered to “action zone” between screen and impeller
• Particles are size reduced (as fine as 150 micron) and instantly discharged through screen openings
Multi mill
• Multi mill machines are widely used for wet and dry granulation, pulverization etc.
• SS beaters having knife and impact edges rotate within a selected screen
• The material fed in the hopper enters into the processing chamber where it moves to the pentery and passes through screen
• Output & quality of final products depend on (I) shapes of beaters (Knife/ impact edge) (ii) speed (iii) size of screen
Roller mill
Principle
Material is compressed by application of stress and attrition.
Stress is applied by rotating heavy wheels, Muller or rollers.
Parts
2 cylindrical rollers of stone or metal mounted horizontally, having diameter ranging from few mm to a meter.
They rotate in longitudinal axis, one roller is run by motor and other freely.
Determination of Particle size
Gap between rollers controlled to obtain desired particle size.
Uses
For crushing of seeds before extraction of fixed oils.
Variants
Multiple/ Corrugated rollers
Ribbed/ Saw toothed rollers
Rotary cutter mill
Principle
Size reduction involves successive cutting/shearing the feed material with help of sharp knife.
Parts
Hopper
Milling chamber – horizontally mounted rotor disk consisting 2-12 rotating knives spaced uniformly and casing has stationary knives.
Screen
Discharge chute
Determination of particle size and shape
Rotor size
Gap between the 2 sets of knives
Sieve
Uses
Size reduction of tough fibrous materials
Medicinal plants, animal tissue are converted to small parts
Variants
Double runner disc mill
Single runner disc mill
Disintegrator
Principle
The size reduction is done by impact
Parts
Consists of a steel drum enclosing a central shaft, which has a disc to which 4 beaters are fixed.
The side are upper inner surface of drum is rough.
Lower part of casing consists of a detachable screen.
Use
To powder all type of drugs including very hard drugs.
Other important technique in size reduction
Sono-crystallization –
Utilizes ultra sound of frequency range 20-100 kHz for inducing crystallization.
It is an effective means of size reduction and controls size distribution of API.
Spray Drying –
It is a common method of drying a liquid feed through a hot gas.
This hot gas is air, but sensitive material such as ethanol required oxygen free drying and nitrogen gas.
Supercritical fluid process –
It is a dance non-condensable fluid whose temperature and pressure are grater then its critical temperature and critical pressure.
Drug particle are solubilized within super critical fluids and re-crystallized to get greatly reduced particle size.
Size Reduction – Objective, Methods, and Factor Affecting in Pharmaceutics PDF
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