Table of Contents
Sieving/ SHIFTING
Sieve for Pharmaceutical testing are constructed from wire cloth with square meshes woven from wire of brass, bronze, stainless steel or any other suitable material.
The wire should be of uniform circular cross- section and should not be coated or plated these should not be any reaction between the material of the sieve and the substance which is being shifted from it.
- Generally iron wire is used as screen material because it is cheap but their disadvantage and corrosive nature and chances of contamination by iron. This disadvantage can be overcome by coating iron with galvanizing agents which increase the strength and also make it corrosive resistant.
- For separation of fine powder bolting cloth sieve are used. They are woven from twisted multi strand fibers made of silk, nylon and cotton. NyIon cloth is generally designated for their micrometer opening and also their availability in different grades.
Sieve Analysis
the International Standards Organization (ISO) fix lowest a sieve diameter 45µm. powder are generally defined as particles having a maximum diameter of 1000µm, so this is the upper limit. In practice, sieve analysis can be performed in a range of 5 to 125,000µm. sieve analysis used to monitor material quality based on particle size.
Standard for Sieve
Standards for sieves used to testing must specify the following:
Holes in the screen are called mesh. Mesh number indicate number of holes included in a length of 1 inch. Aperture of screen is the clear space between wires of screen opening and screen number denotes number of meshes in a linear length of 25.4mm.
- 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.
- Nominal size of aperture: – Nominal size of aperture indicates gap between two adjacent 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.
- 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.
- 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.
- 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’
The working of mechanical sieving devices is based on any of the following methods.
- Agitation.
- Brushing.
- Centrifugal.
- Agitation methods- Sieves may be agitated in a number of different ways, such as:
- 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.
- 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.
- Gyration: In this method, a system is made so that sieve is on rubber mounting and connected to an eccentric flywheel. 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.
- 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.
- 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
- 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
- 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 sieve
Advantages of Sieving
- It is much quicker than other conventional filtration methods.
- Sieving is a traditional method of filtration and does not require a unique skill-set to perform.
- It does not require any high-end equipment for working, thus making it highly cost-effective.
Disadvantages of Sieving
- Sieving cannot be used to separate mixtures with identical sizes of particles.
- Applying force during sieving can also cause a breakup of mixture particles, thus rendering it inefficient for filtering brittle particles.
- Sieving can be used for filtering only dry particles as a sieve cannot separate wet particles.