SIZE SEPARATION
A unit process used to separate or categorise the particles on the basis of differences between the physical properties of the particles, like density, shape, and size is termed as size separation (or screening or sifting or classifying). Once the size of the material is reduced, it is subjected to size separation techniques in order to obtain the powder of desired particle size range. Particle size distribution is defined as the number of particles in each size range present in a given powder of specified weight.
OBJECTIVES OF SIZE SEPARATION
- To determine the average particle size and particle size distribution which is considered as the first step in the manufacturing of tablets and capsules.
- To obtain drugs with the particle size of 10µ so that they can get absorbed from the gastrointestinal tract rapidly and effectively, e.g., griseofulvin (antifungal) and aspirin (analgesic, antipyretic).
- To evaluate the efficiency of different equipments used for the process of size reduction.
APPLICATIONS OF SIZE SEPARATION
- Release and Dissolution: Clinically, the particle size of a drug can affect its release from dosage forms administered orally, parentally, rectally, and topically. Finely powdered drug has a high surface area, hence gets easily dissolved in in vivo dissolution fluids, thereby enhancing the drug solubility and dissolution.
- Absorption and Drug Action: Drug absorption and its pharmacological response are affected by the size of particles. Higher the dissolution rate, faster the absorption rate, hence quicker and greater the drug action.
- Physical Stability: Depending upon the size of particle, the physical stability of the drug also changes; smaller the size of particles, greater will be the physical stability of the drug owing to the Brownian movement of the particles in the dispersion.
- Dose Uniformity: Good flow properties of granules and powders are required for formulating tablets and capsules. In terms of weight and number, the particles should be uniformly distributed throughout the drug.
MECHANISM OF SIZE SEPARATION
1. Agitation
- Oscillatory Motion: In this type of motion, the movement of sieves takes place with the help of rotating shaft. In this, the sieves placed on the frame are moved in to and fro motion corresponding to the plane of sieve.
- Vibrating Motion: In this type of motion, the material is placed on the sieve which is then subjected to high speed vibration by employing either an electrical or a mechanical device.
- Gyratory Motion: In this type of motion, an eccentric flywheel is used for rotating the sieve (placed on the rubber mounting) at a fixed axis in a circular motion.
2. Brushing
In this method, a brush is used for carrying the process of size separation. The brush attached with the help of a handle to the middle portion of the circular sieve, rotates along the surface of the circular sieve. This method is suitable for greasy and sticky powder materials (waxes, soaps, etc.).
3. Centrifugation
This involves vertical cylindrical sieve with a high speed rotor fixed at its centre. As the rotor rotates at a very high speed, the particles present inside the vertical cylindrical sieve are thrown in the outward direction under the influence of centrifugal force. Air currents are generated inside the equipment, and due to these air currents, the particles separate out.
CLASSIFICATION OF POWDERS ACCORDING TO IP
| Grade of Powder | Sieve Through Which All Particles Must Pass (No.) | Nominal Aperture Size (µm) | Sieve Through Which 40% Particles Pass (No.) | Nominal Aperture Size (µm) |
|---|---|---|---|---|
| Coarse powder | 10 | 1700 | 44 | 355 |
| Moderately coarse powder | 22 | 710 | 60 | 250 |
| Moderately fine powder | 44 | 355 | 85 | 180 |
| Fine powder | 85 | 180 | 120 | 125 |
| Very fine powder | 120 | 125 | 350 | 45 |
Detailed Description of Powder Grades
- Coarse Powder (10/44): All the particles of this powder pass through a sieve no. 10, having the nominal mesh aperture of 1700µm but not more than 40% of the powder by weight must pass through the sieve no. 44, having the nominal mesh aperture of 355µm.
- Moderately Coarse Powder (22/60): All the particles of this powder pass through a sieve no. 22, having the nominal mesh aperture of 710µm but not more than 40% of the powder by weight must pass through sieve no. 60, having the nominal mesh aperture of 250µm.
- Moderately Fine Powder (44/85): All the particles of this powder pass through a sieve no. 44, having the nominal mesh aperture of 355µm but not more than 40% of the powder by weight must pass through sieve no. 85, having the nominal mesh aperture of 180µm.
- Fine Powder (85/120): All the particles of this powder pass through a sieve no. 85, having the nominal mesh aperture of 180µm but not more than 40% of the powder by weight must pass through sieve no. 120, having the nominal mesh aperture of 125µm.
- Very Fine Powder (120/350): All the particles of this powder pass through a sieve no. 120, having the nominal mesh aperture of 125µm but not more than 40% of the powder by weight must pass through sieve no. 350 with the nominal mesh aperture of 45µm.
- Microfine Powder (350): The powder whose particles are not less than 90% by weight must pass through sieve no. 350, having the nominal mesh aperture of 45µm.
- Superfine Powder: The powder in which the number of particles is not less than 90% and is less than 10µm in size.
EQUIPMENT FOR SIZE SEPARATION
The common size separation equipment used in pharmaceutical industries are: Sieves, Cyclone separator, Air separator, Sedimentation Tank, and Elutriation. Among these, the two most commonly used equipment, i.e., sieve and cyclone separator are discussed below:
Sieves
Normally, a sieve is used for separating the unwanted or useless material from that of the desired one with the help of different tools, such as net, mesh, or other distillation and filtration techniques. For the separation of liquids from the solids, a particular type of sieve is used, known as strainer. Sieving is also used as an analytical technique for the measurement of size and classifying the powders according to the size of the particles.
Types of Sieves
- Woven Wire Sieves: These sieves are of two types: Plain weave and Twilled weave. Metal wire woven sieves are preferred for fine sieving, e.g., hand sieves. These sieves are incorporated during milling within the roller mill, ball mill, etc.
- Bolting Cloth Sieves: Silk, nylon, and cotton are woven from twisted multi-strand fibres. Nylon cloths have openings of micrometre range, are available in different grades, and are used for separating fine powders. This type of sieves is used in hummer screens.
- Bar Screens: These are used while handling large and heavy pieces of materials. The bars are parallelly fixed by cross bars and spacers. Bars with tapered thickness from one end to the other avoid blinding and thus are recommended. This type of sieves is used in grizzlies.
- Punched Plates (Perforated Screens): These are used for coarse sizing. A sheet metal having round, oval, square, or rectangular perforations and of variable thickness are used for preparing the screens. This type of sieves is used in hammer mills.
Standards of Sieves
- Tyler standard sieve series (in U.S.A.)
- US standard sieve series (in U.S.A.)
- British standard sieve series (in U.K.)
- German DIN (Deutsche Industrie-Normen) (in Germany and Europe)
- IP standard sieve series (in India)
- International test sieve series (ISO) (Worldwide)
Specifications for Pharmacopoeial Testing
- Sieve Number: This is the number of meshes present per linear length of 25.4mm.
- Nominal Aperture Size: This is the distance between the two adjacent wires, representing the side of a square aperture. The nominal mesh aperture size for most of the sieves is given in the I.P. 1996 (either in mm or in µm).
- Nominal Wire Diameter: The wire used in wire mesh sieves has a specified diameter which provides a suitable aperture size and sufficient strength so that sieve distortion can be avoided.
- Approximate Percentage Sieving Area: This is the mesh area expressed as a percentage of the total sieve area. It depends on the wire size used for a sieve. The sieving area is appropriately maintained within 35-40% to provide the required strength to the sieve.
- Aperture Tolerance Average Size: Some variations which the aperture size undergoes cannot be avoided. Such variations are termed aperture tolerance average, expressed as a percentage.
Cyclone Separator
Principle
Cyclone separator is a type of sedimentation technique which works on the principle of centrifugal force rather than the gravitational force. Thus, based on the fluid velocity, cyclone enables the separation of all the particles or only the coarse particles can be removed, leaving behind the fine particles which are then carried away by the fluid.
Construction
- Upper part: Consists of a tangential inlet and a fluid outlet at the centre of the top portion and extending inwardly into the separator.
- Lower part (conical base): Fitted with the outlet for the solid particles.
Working
The suspension is introduced into the cylindrical vessel through the tangential inlet at high speed in order to provide the rotary movement inside the vessel. The fluid outlet located at the top of the vessel helps in removing the fluid. The solid particles are thrown out towards the cyclone walls due to the rotary movement occurring inside the cyclone which causes the movement of particles by the centrifugal force. The particles thrown out then fall down on the conical base which is removed ultimately from the solid outlet at the bottom of the cylinder.
Applications
- It is used for the separation of suspensions of solids in liquids with the preference for suspension of a solid in a gas (air).
- It is also used for separating the heavy or coarse fraction from fine dust.
SUMMARY OF POWDER GRADES AS PER IP
| Grade | Sieve Pass (All) | Sieve Pass (40%) | Particle Size |
|---|---|---|---|
| Coarse | #10 (1700µm) | #44 (355µm) | Coarse |
| Moderately Coarse | #22 (710µm) | #60 (250µm) | Medium-coarse |
| Moderately Fine | #44 (355µm) | #85 (180µm) | Medium-fine |
| Fine | #85 (180µm) | #120 (125µm) | Fine |
| Very Fine | #120 (125µm) | #350 (45µm) | Very fine |
| Microfine | #350 (45µm) – 90% pass | Microfine | |
| Superfine | 90% particles <10µm | Superfine | |
COMPARISON: TYPES OF SIEVES
| Type | Material | Best Suited For | Common Equipment |
|---|---|---|---|
| Woven Wire Sieves | Metal wire | Fine sieving | Hand sieves, roller mill, ball mill |
| Bolting Cloth Sieves | Silk, nylon, cotton | Fine powders | Hummer screens |
| Bar Screens | Metal bars | Large, heavy pieces | Grizzlies |
| Punched Plates | Sheet metal | Coarse sizing | Hammer mills |
