Size separation

1. Size separation

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

3. 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

4. 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)

5. 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

6. Different Sieves Fig : 28.1

7. Different meshes Fig : 28.2

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

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

10. Classification • Sieving method. • Sedimentation method. • Cyclone separation method.

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

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

14. Agitation method • Oscillation • Vibration • Gyration

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

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

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

18. Mechanical Sieve Shaker Fig: 29.1

19. 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

20. 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

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

22. 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

23. 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.

24. 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.

25. 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 base

26. Fig: 31.2 Cyclone separator

27. 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

28. 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

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

31. 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

33. 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

34. 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