Grains can only be screened to the extent that screens with the suitable mesh width are available. In addition, the screened material must not be so fine that it will no longer fall through the mesh “on its own”. Below approx. 50 µ (0.050 mm), screening is no longer possible.
The “micrograin range” refers to a grain size finer than 58 µ. This can no longer be determined by vibrating screens, but by methods such as sedimentation or laser measurement (laboratory testing). In the manufacturing process, the grains are fractioned by air separation. Grain distribution is specified by the 50% value (D50), the 3% values (D3) and 94% values (D94).
Fractioning is possible down to approx. 3 µm (F 1200; FEPA classification).
| FEPA | D50 (µm) |
| F 230 | 53,0 ±3 |
| F 240 | 44,5 ±2 |
| F 280 | 36,5 ±1,5 |
| F 320 | 29,2 ±1,5 |
| F 360 | 22,8 ±1,5 |
| F 400 | 17,3 ±1 |
| F 500 | 12,8 ±1 |
| F 600 | 9,3 ±1 |
| F 800 | 6,5 ±1 |
| F 1000 | 4,5 ±0,8 |
| F 1200 | 3,0 ±0,5 |
