Size separation is one of the most important unit operations in pharmaceutics, used to separate particles according to their size using mechanical or air classification methods.
This process ensures uniformity, improves drug mixing, tablet compression, and dissolution rate, which directly affect the quality of pharmaceutical products.
Definition of Size Separation
Size separation (also known as sieving or screening) is the process of separating particles based on their size using a sieve, mesh, or other separating devices.
ORIGINAL NOTES:
Objectives / Applications
- To achieve uniform particle size for better blending and mixing.
- To remove oversized or undersized particles.
- To improve flow properties and compressibility of powders.
- To ensure uniform drug content and dissolution rate in tablets and capsules.
- To classify powders according to the Indian Pharmacopoeia (IP) standards.
Principle of Size Separation
The process works on the principle of sieving and air classification, where:
- Particles smaller than the mesh openings pass through.
- Particles larger than the openings remain on the sieve.
- Air or mechanical vibration helps separate particles efficiently.
1. Classification of Powders According to Indian Pharmacopoeia (IP)
The Indian Pharmacopoeia (IP) classifies powders based on particle size and the mesh number of the sieve through which they pass.
| Type of Powder | Definition (as per IP) |
|---|---|
| Coarse Powder | Passes through Sieve No. 10 and not more than 40% passes through Sieve No. 44 |
| Moderately Coarse Powder | Passes through Sieve No. 22 and not more than 40% passes through Sieve No. 60 |
| Moderately Fine Powder | Passes through Sieve No. 44 and not more than 40% passes through Sieve No. 85 |
| Fine Powder | Passes through Sieve No. 85 |
| Very Fine Powder | Passes through Sieve No. 120 |
2. Sieves and Standards of Sieves
Principle
Sieves separate particles by mechanical agitation or vibration.
Each sieve has a wire mesh with precise openings defined by standards like IP, BP, or USP.
Construction
- A sieve consists of a circular brass or stainless steel frame.
- The bottom part has a metal mesh with uniform-sized openings.
- Sieves can be stacked together in a nest (set of 6–8 sieves) to separate powders into different size ranges.
Working
- A known quantity of powder is placed on the top sieve.
- The stack is shaken manually or mechanically for a set time.
- Particles are separated according to their size—smaller particles pass through, while larger ones remain on the upper sieve.
- The weight of powder retained on each sieve is measured to determine particle size distribution.
Advantages
- Simple and easy to use.
- Accurate for dry powders.
- Standardized mesh sizes ensure repeatability.
Disadvantages
- Not suitable for very fine powders or wet materials.
- Possibility of mesh clogging.
- Requires cleaning and calibration.
Applications
- Used in tablet and capsule formulation.
- Determines particle size distribution for quality control.
- Useful in research and development of solid dosage forms.
3. Cyclone Separator
Principle
A cyclone separator works on the principle of centrifugal force.
When air or gas containing solid particles enters tangentially into the cyclone chamber, the centrifugal action forces the heavier particles to move outward and downward, while clean air moves upward.
Construction
- Cylindrical or conical metal chamber with a tangential inlet at the top.
- A vortex finder at the center allows clean air to escape.
- A dust collector or receiver is fitted at the bottom to collect separated particles.
Working
- The mixture of air and fine powder enters the cyclone tangentially at high speed.
- Due to centrifugal force, heavy particles move toward the wall of the chamber.
- These particles lose momentum and fall into the dust collector.
- The clean air exits through the central outlet at the top.
Advantages
- No moving parts – low maintenance.
- Handles large volumes of air and dust.
- Suitable for coarse and medium-sized particles.
- Low operating cost.
Disadvantages
- Inefficient for very fine particles.
- May require secondary filtration.
Applications
- Used in dust collection systems.
- Helps in classifying powders after grinding.
- Used in pharmaceutical, chemical, and food industries.
4. Standards of Sieves
According to Indian Pharmacopoeia, sieves are standardized based on their mesh size (number of openings per linear inch).
Common sieve numbers used in pharmacy are 10, 22, 44, 60, 85, 120, etc.
Each sieve must be calibrated and maintained regularly to ensure accuracy and consistency.
