SIZE SEPARATION

SIZE SEPARATION

Content 

1.Size separation

2.Objectives of size separation

3.Official standard of powder

4.Sieve

5.Mechanism of size separation

6.Sieve shaker machine

7.Cyclone separator

8.Air separator

SIZE SEPARATION

Definition of size separation :- 

Size separation is a unit operation that used to separation of a mixture of various size particles into two or more portions by means of screening surfaces. Size separation is also known as sieving, sifting, screening. This technique is based on physical differences b/w the particles such as size, shape and density. 

OBJECTIVES / APPLICATION OF SIZE 

SEPERATION

1.To determine particle size for the production of tablets and capsules.

2.To improve mixing of powders.

3.To improve the solubility and stability of particles during production.

4.To optimize feed rate, agitation, screening during production.

5.Quality control of raw materials.

6.Useful in production of tablets, capsules, suspension, emulsion, ophthalmic preparations, ointments, creams, etc. 

OFFICIAL STANDARDS FOR POWDERS 

The Indian Pharmacopoeia has laid down the standards for powders for pharmaceutical purposes. The I.P. specifies five grades of powder which are as under :

1.COARSE POWDER- A powder of which al! the particles pass through a sieve with nominal mesh aperture of 1.7 mm (No. 10 sieve) and not more than 40.0 per cent through a sieve with nominal mesh aperture of 355 µm (No. 44 sieve) is called coarse powder.

2.MODERATELY COARSE POWDER- A powder of which all the particles pass through a sieve with nominal mesh aperture of 710 µm (No. 22 sieve) and not more than 40.0 per cent through a sieve with nominal mesh aperture of 250 µm (No. 60 sieve) is called moderately coarse powder.

3.MODERATELY FINE POWDER- If all the particles of a powder pass through a sieve with nominal mesh aperture of 355 µm (No. 44 sieve) and not more than 40.0 per cent through a sieve with nominal mesh aperture of 180 µm(No. 85 sieve), it falls in this group.

4.FINE POWDER- In case all the particles pass through a sieve with a nominal mesh aperture of 180 µm (No. 85 sieve), it is called fine powder,

5.VERY FINE POWDER - If all the particles of the powder pass through a sieve with a nominal mesh aperture of 125 µm (No. 120sieve), it is said to be very fine powder.

SIEVES :-

Sieves are constructed from wire cloth with square meshes, woven from wires of brass,bronze, stainless steel or any other suitable material. The wires should be of uniform circular cross-section and should not be coated or plated. There should not be any reaction between the material of the sieve and the substance which is being sifted from it. Standards for sieves used to testing must specify the following:

1.Number of sieve : Sieve number indicates the number of meshes in a length of 2.54 cm in each transverse direction parallel to the wires.

2.Nominal size of aperture : Nominal size of aperture indicates the distance between the wires. It represents the length of the side of the square aperture. The I.P. has given the nominal mesh aperture size for majority of sieves in mm or in cm.

3.Nominal diameter of the wire : Wire mesh sieves are made from the wire having the specified diameter in order to give a suitable aperture size and sufficient strength to avoid distortion of the sieve.

4.Approximate percentage sieving area : This standard expresses the area of the meshes a percentage of the total area of the sieve. It depends on the size of the wire used for any particular sieve number. Generally the sieving area is kept within the range of 35 to 40 percent in order to give suitable strength to the sieve.

5.Tolerance average aperture size : Some variation in the aperture size is unavoidable and when this variation is expressed as a percentage, it is known as the 'aperture tolerance average'.

MECHANISM OF SIZE SEPERATION

The working of mechanical sieving devices are based on any of the following methods.

1.Agitation

2.Brushing

3.Centrifugal

1.Agitation methods- Sieves may be agitated in a number of different ways, such as:

1.Oscillation : This sieve is mounted in a frame that oscillates back and forth. It is a simple method but the material may roll on the surface of the sieve. 

2.Vibration : The sieve is vibrated at high speed by means of an electric device. The rapid vibration is imparted to the particles on the sieve which helps to pass the powdered material through it. 

3.Gyration : In this method, a system is made so that sieve is on rubber mounting and connected to an eccentric fly wheel. This gives a rotary movement of small amplitude to sieve which turn gives spinning motion to the particles that helps to pass them through a sieve. 

Agitation methods are not continuous methods' but can be made so by inclination of the sieve and the provision of separate outlets for undersize and oversize particles.

2.Brushing methods- In this case, a brush is used to move the particles on the surface of the sieve and to keep the meshes clear. The brush is rotated in the middle in the case of a circular sieve but spiral brush is rotated on the longitudinal axis in case Of a horizontal cylindrical sieve. 

3.Centrifugal methods- In this method, a high speed rotor is fixed inside the vertical cylindrical sieve, so that on rotation of rotor the particles are thrown outwards by centrifugal force. The current of air which is produced due to high speed of rotor helps in sieving the powder

SIEVING METHOD FOR SIZE SEPRATION 

 >In this method, the fine powder is separated from the coarse powder by using sieves of desired number. The degree of fineness of a powder is known with the help of sieve through which the powdered material is passed. Sieves are numbered in order to distinguish from each other. 

Working & construction of sieving method

>Size separation of powder is done by passing the powdered material through a set of sieves. Sieves are arranged in descending order i.e. sieve of larger size is at the top and the smallest one at the bottom. The bottom sieve is attached to the receiving pan. The material is placed in the uppermost sieve. The sieves are shaken with the help of mechanical sieve shaker or electromagnetic devices. It helps the particles to pass through the sieves

Advantages-

1.inexpensive

2.Easy to use 

Disadvantages –

1.Chance of clogging of sieve if powder is not properly dry

2.During shaking, attrition may occur.

CYCLONE SEPARATOR 

Principle -

In cyclone separator, the centrifugal force is used to separate solids from fluids. The separation depends not only on the particle size but also on density of particles. Hence depending on the fluid velocity, the cyclone separator can be used to separate all types of particles or to remove only coarse particles and allow fine particles to be carried through with the fluid.

Construction -

It consists of a cylindrical vessel with a conical base. In the upper part of the vessel is fitted with a tangential inlet and a fluid outlet and at the base it is fitted with solid outlet.

Working of cyclone separator

The suspension of a solid in gas (usually air) is introduced tangentially at a very high velocity, so that rotary movement takes place within the vessel. The fluid is removed from a central outlet at the top. The rotatory flow within the cyclone separator causes the particles to be acted on by centrifugal force. The solids are thrown out to the walls, thereafter it falls to the conical base and discharged out through solids outlet.

Uses 

1.Cyclone separators are used to separate the suspension of a solid in a gas (air).

2.It can be used with liquid suspensions of solids

AIR SEPARATOR 

Principle 

It works on the same principle as that of cyclone separator. But in this case the air movement is obtained by means of rotating disc and blades. To improve the separation, the stationary blades are used. By controlling these blades and the speed of rotation, it is possible to vary the size at which separation occurs.

 

Construction

It consists of a cylindrical vessel with a conical base. In the upper part of the separator the vessel is fitted with feed inlet, and at the base there are two outlets, one for light particles and other for heavy particles. The rotating disc and rotating blades are attached to the central shaft, to produce air movement. 

Working

the sample powder is passed through the feed inlet, which falls on the rotating disc. The rotating blades are attached to the same shaft. These produce a current of air as shown by the arrows. The fine particles are picked up and are carried into space, where air velocity is sufficiently reduced. The fine particles are dropped and ultimately collected at an outlet meant for fine particles. The heavy particles which fall downward are removed at an outlet meant for heavy particles. 

Uses 

Air separator is often attached to the ball mill or hammer mill to separate and return oversized particles for further size reduction

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