Definition:
Size Separation is a pharmaceutical process by which the particles of different sizes are separated from a mixture.
Also called as,
Sifting.
Sieving.
Classifying.
Screening.
Applications:
As a method to determine particle size and size distribution, which are useful in the production of tablets and capsules.
As a quality control tool for the analysis of raw materials such as griseofulvin and aspirin.
To test the efficiency of a size reduction equipment or process.
To optimize the process conditions such as method of agitation, time of screening, feed rate etc.
Official Standards of Sieves:
Indian Pharmacopoeia has prescribed standards for powders for pharmaceutical purposes.
Accordingly, degree of coarseness or fineness is expressed with reference to the nominal aperture size of sieve through which powder is able to pass.
The IP 1996 specifies five grades of powders.
A) Coarse Powder:
A powder, all the particles of which pass through a sieve with nominal mesh aperture of 1.70 mm (Sieve No. 10 ) and not more than 40% pass through nominal mesh aperture of 355 µm (Sieve No. 44) is called a Coarse Powder.
B) Moderately Coarse Powder:
A powder, all the particles of which pass through a sieve with nominal mesh aperture of 710 µm (Sieve No. 22) and not more than 40% pass through nominal mesh aperture of 250 µm (Sieve No. 60) is called a Moderately Coarse Powder.
C) Moderately Fine Powder:
A powder, all the particles of which pass through a sieve with nominal mesh aperture of 355 µm (Sieve No. 44) and not more than 40% pass through nominal mesh aperture of 180 µm (Sieve No. 85) is called a Moderately Fine Powder.
D) Fine Powder:
A powder, all the particles of which pass through a sieve with nominal mesh aperture of 180 µm (Sieve No. 85) is called a Fine Powder.
E) Very Fine Powder:
A powder, all the particles of which pass through a sieve with nominal mesh aperture of 125 µm (Sieve No. 120) is called a Fine Powder.
SIEVES
Sieves are the simplest and sieving is the most frequently used method for size separation.
Construction:
Sieves for pharmaceutical testing are constructed from wire cloth with square meshes, woven from wire ,of brass, bronze, 'stainless'' steel or any suitable materials.
Sieves should not be coated or plated.
There must be no reaction between the material of the sieve and the substance to be sieved.
Types of Sieves:
Woven wire sieves.
Plain weave
Twilled weave
Bolting cloth sieves.
Bar screens.
Punched plates (Perforated Screens)
Standards of Sieves:
Common standards used for sieves are:
(a) Tyler standard sieve series (in USA)
(b) US standard. sieve series (in USA)
(c) British standard sieve series (in UK)
(d) German DIN (Deutsche lndustrienormein) (in Germany"and Europe)
(e) IP standard sieve series (in India)
(f) International test sieve series (ISO) (World wide).
It is required that wire-mesh sieves will be made from wire of uniform, circular cross-section and for each sieve the following particulars are stated.
Sieve Number:
This is the number of meshes in a length of 25.4mm (i.e. 1 inch), in each direction.
Nominal size aperture:
This is the distance between the wires, so that it represents the length of the side of the square aperture.
Nominal diameter of the wire:
The wire diameter is selected to give a suitable aperture size.
It is also required to give the necessary strength to avoid distortion.
The diameter of the wire is represented by Standard Wire Gauge.
Approximate screen area:
This standard expresses the area of the meshes as a percentage of the total area of the sieve.
It is governed by the diameter of the wire.
It is generally kept within 35 to 45% of the total area of the sieve.
This represents the useful area of a sieve. Greater screen area is preferred.
Aperture tolerance average:
Some variation in the aperture size is unavoidable and this variation is expressed as a percentage, known as aperture tolerance average.
It is the maximum limit within which the dimension of meshes can be allowed to vary and still be acceptable for sieving.
Finer wires are likely to be subject to a greater proportional variation in diameter than coarse mesh.
Principles of Mechanical Size Separation:
Size Separation is done by Above methods.
Agitation:
Sieves may be agitated in a number of different ways.
Oscillation (move back and forth)
The sieve is mounted in a frame that oscillates.
Advantages: Simple method
Disadvantages: The material may roll on the surface of the sieve and fibrous materials tend to “ball”.
Vibration:
The mesh is vibrated at high speed, often by an electrical device.
Advantages The rapid vibration is imparted to the particles on the sieve and the particles are less likely to “blind” the mesh.
Gyration:
Sieve is rubber mounted and is connected to an eccentric flywheel which gives gyrating movement which helps particles passing the sieve.
Brushing:
A brush can be used to move the particles on the surface of the sieve and to keep the meshes clear.
A single brush across the diameter of an ordinary circular sieve, rotating about the midpoint, is effective;
In large-scale production a horizontal cylindrical sieve is employed, with a spiral brush rotating on the longitudinal axis of the sieve.
Useful for separating sticky powder particles.
Centrifugal Force:
Use a vertical cylindrical sieve with a high speed rotor inside the cylinder, so that particles are thrown outwards by centrifugal force.
The current of air created by the movement helps sieving.
Is especially useful with very fine powders.
Sieve Shaker
Principle:
Agitation by Oscillation.
Particles are made to pass through different sieves placed in nest which oscillates back and forth in motion.
Construction:
Consists of a metal frame on which sieves of different no. can be mounted.
The sieves are arranged in a dissenting order of mesh size.
The metal frame is fixed using screws and a capping on a vibrating frame.
The vibrating frame is operated by an electrical motor whose speed can be controlled.
The timer helps in maintaining time of the operation.
Working:
The powder to be screened is kept on the uppermost sieve and is capped and bolted.
The machine's timer is set to the desired time (Usually 20 mins) and operated at desired speed.
The vibrations cause oscillations resulting in passing of material from one sieve to another.
After a given time the machine is stopped, metal frame is loosed and screens are separated to collect the material.
Advantages:
Easy operation.
Less power consumption.
Economical.
Disadvantages:
Fines may generate due to attrition.
Clogging may happen if powder is not sufficiently dried.
Cyclone Separator.
Principle:
It uses centrifugal force to separate solid from fluids.
The separation process depends on particle size and particle density.
Construction:
It is made up of a short vertical, cylindrical vessel with a conical base.
The upper part of the vessel is fitted with a tangential inlet.
The solid outlet is at the base.
Fluid outlet is provided at the center of the top portion, which extends inwardly into the separator.
Such an arrangement prevents the air short-circuiting directly from the inlet to the outlet of the fluid.
Working:
The solids to be separated are suspended in a stream of fluid (usually air or water).
Such feed is introduced tangentially at a very high velocity, so that rotary movement takes place within the vessel.
The centrifugal force throws the particles to the wall of the vessel.
As the speed of the fluid (air) diminishes, the particles fall to the base and are collected at the solid outlet.
The fluid (air) can escape from the central outlet at the top.
Uses:
Cyclone separators are used to separate solid particles from gases.
It is also used for size separation of solids in liquids.
It is used to separate the heavy and coarse fraction from fine dust.
Variants :
Cyclone separators are also used for size separation or solids suspended in a liquid such as water.
Such separators are known as wet or liquid cyclones.
One such apparatus used for this purpose Is : dorrclone.
AIR SEPARATOR
Principle:
A current of air combined with centrifugal force is used.
The finer particles are carried away by air and the coarser particles are thrown by centrifugal force, which fall at the bottom.
Construction:
It consists of a cylindrical vessel with a conical base.
A rotating plate is fitted on a shaft placed at the center of the vessel.
A set of fan blades are also fitted with the same shaft.
At the base of the vessel two outlets are provided: one for the finer particles and the other for coarse particles.
Working:
The disc and the fan are rotated by means of a motor.
The feed (powder) enters at the center of the vessel and falls off the rotating plate.
The rotating fan blades produce a draft (flow) of air in the direction as shown in the diagram.
The fine particles are picked up by the draft of air and carried into the space of the settling chamber, where the air velocity is sufficiently reduced so that the fine particles are dropped and removed through the fine particle outlet.
Particles too heavy to be picked up by the air stream are removed at the coarse particle outlet.
Uses
Air separators are often attached to the ball mill or hammer mill to separate and return oversized particles for further size reduction.
BAG FILTER:
Principle:
Particle Size and density of particles.
In a bag filter, size separation of fines (or dust) from the milled powder is achieved in two steps.
In the first step, the milled powder is passed through a bag (made from cloth) by applying suction on the opposite side of the feed entry. This facilitates the separation.
In the next step, pressure is applied in order to shake the bags so that powder adhering to the bag falls off, which is collected from the conical base.
Construction:
It consists of a number of bags made of cotton or wool fabric.
These are suspended in a metal container.
A hopper is arranged at the bottom of the filter to receive the feed.
At the top of the metal container, a provision is made for vacuum fans and exhaust through a discharge manifold.
At the top of the vessel a bell-crank lever arrangement is made to change the action from filtering to shaking.
Working:
(a)Filtering period:
During this period the vacuum fan produces a pressure lower than the atmospheric pressure within the vessel.
Gas to be filtered enters the hopper, passes through the bags, and out of the top of the apparatus.
The particles are retained within the bags.
(b)Shaking period:
During this period the bell-crank lever first closes the discharge manifold and air enters through the top so the vacuum is broken.
At the same time it gives a violent jerking action to the bags so that they are freed from the dust.
The fine particles are collected at the conical base.
Uses:
Bag filters are used along with other size separation equipment, e.g. a cyclone separator.
They are used on the top of a fluidized bed dryer for drying to separate the dusts.
They are used to clean the air of a room.
Household vacuum cleaner is a simple version of a bag filter.
Elutriation:
Elutriation is a process for separating particles based on their size, shape and density, using a stream of gas or liquid flowing in a direction opposite to the direction of sedimentation.
The apparatus consists of a vertical column with an inlet near the bottom for suspension, an outlet at the base for coarse particles and an overflow near the top for fluid and fine articles.
One column will give single separation into two fractions. If more than one fraction is required a number of tubes of increasing area of cross section can be connected in series.
The velocity of the fluid decreases in succeeding tubes as the area of cross section increases, thus giving a number of fractions.
These fractions are separated and dried.
Continuous process.
Particles with low density which are not in path of fluid current remain unaffected.
