During the process of size reduction, it is not possible to obtain the particle of uniform size. Hence special technique is used to separate particles of a specified size, which is known as the process of size separation. 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
When the fineness of the powder is described by means of a number, it is intended that all the particles of the powder shall pass through a sieve of which the nominal mesh aperture, in µm , is equal to that number.
Official Standard for Powder: IP (Indian Pharmacopoeia)
Coarse or fineness of powder is expressed by Reference to the nominal mesh aperture size of the sieve through which powder is able to pass.
- Indian pharmacopoeia- 5 grade of powder
| S. No. | Grade of powder | all particles must pass through Sieve No. /Nominal mesh aperture | Sieve through which 40% of the particles pass/ Nominal mesh aperture |
|---|---|---|---|
| 1 | Coarse powder | 10 /(1700 µm) | 44 /(355 µm) |
| 2 | Moderately coarse powder | 22 /(710 µm) | 60/ (250 µm) |
| 3 | Moderately fine powder | 44/(355 µm) | 85/(180 µm) |
| 4 | Fine powder | 85/(180 µm) | Not specified |
| 5 | Very fine powder | 120/(125 µm) | Not specified |
| S. No. | Grade of powder | Comparison of powder size |
|---|---|---|
| 1 | Coarse powder | 1 |
| 2 | Moderately coarse powder | 1/6 |
| 3 | Moderately fine powder | 1/24 |
| 4 | Fine powder | 1/90 |
| 5 | Very fine powder | 1/2000 |
Note: 1.70 mm=1700 µm
A) Coarse Powder:
- All the particles of which pass through a sieve with nominal mesh aperture of 1.70 mm/1700 µm (Sieve No. 10 ) and not more than 40% pass through nominal mesh aperture of 355 µm (Sieve No. 44) is called a Coarse Powder.
B) Moderately Coarse Powder:
- All the particles of which pass through a sieve with nominal mesh aperture of 710 µm (Sieve No. 22) and not more than 40% pass through nominal mesh aperture of 250 µm (Sieve No. 60) is called a Moderately Coarse Powder.
C) Moderately Fine Powder:
- All the particles of which pass through a sieve with nominal mesh aperture of 355 µm (Sieve No. 44) and not more than 40% pass through nominal mesh aperture of 180 µm (Sieve No. 85) is called a Moderately Fine Powder.
D) Fine Powder:
- All the particles of which pass through a sieve with nominal mesh aperture of 180 µm (Sieve No. 85) is called a Fine Powder.
E) Very Fine Powder:
- All the particles of which pass through a sieve with nominal mesh aperture of 125 µm (Sieve No. 120) is called a Fine Powder.
- Note:
- For Fine & Very fine grade of powder: IP has prescribed only the upper limit (Fine & Very fine)
- Fineness of powder is described by means of Number: it indicates that all particles of powder shall pass through that sieve number (respective nominal mesh aperture in µm).
f) Microfine powder
A powder of which not less than 90 percent by weight of the particles pass through a sieve with a nominal mesh aperture of45 µm.
G) Superfine powder
A powder of which not less than 90 percent by a number of the particles are less than 10 µm in size.