Size separation 1 Objectives Size separation Importance of

Size separation 1

Objectives • Size separation • Importance of size separation • Sieve • Grades of powders • Classification of size separation 2

Size separation: • Size separation is a unit operation that involves the separation of a mixture of various sizes of particles into two or more portions by means of screening surfaces. • Its also known as sieving, shifting, classifying or screening. • This is based on the size, shape, and density of the particles 3

Importance: • To formulate a uniform dosage form • To prepare granules of required size • As a quality control tool for analysis • To test the efficiency of a size reduction equipment or process • • To separate undesirable particles To optimize the process condition (screening feed rate) 4

Sieve • It is simplest and sieving is the most frequently used • It is made up of wood or metal fitted with a wire cloth which is made up of copper alloys, stainless steel, silk, nylon or terylene , having a specific diameter of the wire. • Each sieve is given a definite number which indicate a number of the meshes present in a length of 2. 54 cm or one inch. • Sieve numbers: 4, 6, 10, 22, 44, 60, 85, 120 5

Different Sieves Fig : 28. 1 6

Different meshes Fig : 28. 2 7

Sieves commonly used in pharmaceutical processing includes: • Woven wire sieves • Bolting cloth sieves • Closely spaced bars • Punched plates 8

Grading of powders Coarse powder : 10/44 Moderately coarse powder : 22/60 Moderately fine powder : 44/85 Fine powder : 85 Very fine powder : 120 9

Classification 1. Sieving method. 2. Sedimentation method. 3. Cyclone separation method. 10

Objectives • Size separation by sieving or • sifting by different methods. 11

Sieving • Separation of coarse particles from fine particles by using set of sieves. Sieving Methods: • Agitation • Brushing • Centrifugation 12

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Agitation method • Oscillation • Vibration • Gyration 14

Oscillation method • Moves back and forth • Material roll on the surface of the sieve 15

Vibration method • Sieve move with High speed • Rapid vibration imparted to the particles on the sieve • Particles are less likely to “blind” the mesh 16

Gyration method • Rotatary movement small amplitude. • Considerable intensity. • Spinning motion to the particles. • Particles suitably oriented to pass through the sieve. 17

Mechanical Sieve Shaker Fig: 29. 1 18

Sieve details • Set of 12 sieves • Arranged in ascending order • From 6 to 200 No • Large size sieve at the top • Small size at the bottom • Receiver at the bottom of the small size sieve 19

Sieving procedure • Powder kept in top sieve • Shake the sieve • Sieve gyrates and vibrates • Weight of powder on each sieve • Find the percentage of residue in each sieve • Percentage = Residue in each sieve X 100 Total wt of powder 20

Advantages • Size analysis data under controlled condition • Inexpensive • Simple & Rapid process • Little variation between operation 21

Disadvantages • Over loading sieve result error • Insufficient time leads to wrong results • Electrostatic attractions leads to aggregation • Humidity, Hygroscopic powders leads to aggregation • Pale like or long fibrous will not pass 22

Brushing method • Brush is fitted about midpoint of sieve. • Reorient the particles on the surface of sieve. • Prevent blocking of sieve. • Spiral brush is used for large scale production. • Fitted on to a horizontal cylindrical sieve. • Rotates about the longitudinal axis. 23

Centrifugal method • Centrifugal force separates particles. • A vertical cylindrical sieve. • Rotates with high speed. • Current of air created by rotor movement. • Particles are thrown outwards. • Assist in sieving. 24

Cyclone Separator Principle: • It is used to separate solid from fluids • It depends not only on the particle size, but also on density of particles • It is also possible to allow fine particles to be carried by the fluid Working: • Feed is introduced tangentially at very high velocity of air • Rotatory movement takes place within the vessel • Centrifugal force through the solids to the walls • As the speed of air diminishes the particles fall to the conical 25 base

Fig: 31. 2 Cyclone separator 26

Uses • used to separate the solids from gases • used to separate the solids from liquids • used to separate the heavy or coarse fraction from fine dust 27

Air Separator Principle: • Centrifugal force is used to separate solids • The air environment is obtained by means of rotating disc and blades • To improve separation the stationary blades are used 28

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Uses • its often attached to ball mill / hammer mill 30

Bag Filter Principle: • In a bag filter, size separation of fines. • The fines is achieved in two steps. • In the first step, the milled powder is passed through a bag filter(cloth) by applying suction on the opposite side of feed entry. • In the next step, pressure applied in order to shake the bags so that powder adhering to the bag falls off, which is collected from the conical balance 31

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Construction • It consist of a number of bags made of cotton or wool fabric. • 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 the exhaust. • Adjacent to this, a bell crank level arrangement is made to bring the filters to normal atmospheric conditions 33

Working • The working of bag filter consist of two steps • In the first step, the feed is separated from air by passing it through the cloth bags • In the subsequent step, the bags are shaken to collect the fines that are adhere to the bags. Uses: • Its used along with the cyclone separator. • It is connected to the discharge end of the fluidized energy mill 34