4.3. Effects of operating parameters on fractionation
The materials, methods and experimental arrangements used for studying the effects of operating parameters on pulp fractionation were as follows:
Thermomechanical pulp (TMP) made from Norway spruce (Picea abies) was used in the mill-scale screening experiments. Key data on the fed pulp in pressure screening are presented in Table 3.
Table 3. Key data on the fed pulp in the pressure screening experiments.
| Mean | Stdv | ||
|---|---|---|---|
| Bauer-McNett | R16 | 27.3 w-% | 1.6 w-% |
| R30 | 20.4 w-% | 1.2 w-% | |
| R50 | 13.1 w-% | 0.4 w-% | |
| R200 | 13.1 w-% | 0.5 w-% | |
| P200 | 26.1 w-% | 2.2 w-% | |
| Pulmac shives (#0.10 mm) | 1.1 w-% | 0.1 w-% | |
| Freeness (CSF) | 135 ml | 11 ml | |
| Length-weighted fibre length (FSA) | 1.85 mm | 0.05 mm | |
A screen plate (#0.15/HP) having a slot width of 0.15 mm and a contour height of 1.1 mm (determined as in Fig. 4) was used at both screening stages. The effects of operating parameters on the fractionation result were examined with that screen plate design, the screening conditions being varied over a wide range by altering the feed consistency, volumetric reject rate, rotor frequency and volumetric accept flows. Feed freeness and temperature were uncontrollable variables. The operating ranges used in the experiments are presented in Table 4. Samples were collected under steady-state screening conditions, and on-line process information was recorded by means of a laptop computer.
The experimental screen was a two-stage, axially fed Valmet MuST202E pressure screen having a screening area of 1.2 m2 in the first stage and 0.6 m2 in the second. Photographs of the screen are shown in Appendix 1 and a schematic diagram in Fig. 13. A foil-type rotor and wedged wire screen baskets were used. The screen was installed in the circulation of the pulp from the latency tower as shown in Fig. 13. The pulp in the latency tower, of a consistency of about 2.5%, was diluted with white water from the paper machine before the screen. White water was also used for dilution between the screening stages. The inlet and outlet pipes of the screen were equipped with magnetic flow meters, pressure sensors and manual ball valves as sampling devices. In addition, microwave consistency transmitters were installed in the feed and reject lines to help monitor the screening process. An inverter was installed to facilitate adjustment of the rotor frequency.
Figure 13. Flow diagram showing the connection of the two-stage pressure screen to the mill process.
Consistency, freeness and fibre length distribution were analysed in the laboratory from non-disintegrated pulp samples. The fibre length distribution, measured optically with the Kajaani FSA, and freeness were used to model the Bauer-McNett fractions R16, R30, R50, R200 and the P200 in the pulp samples. Fractionation was characterised by means of Eq. 14, using the yields of these fractions.
Table 4. Operating ranges used in the 71 screening tests.
| Screen plate and operating parameters | Screening stage | Range |
|---|---|---|
| Temperature | 1st, 2nd | 75 – 85 °C |
| Tip speed of foil | 1st, 2nd | 13 – 21 m/s |
| Slot velocity | 1st | 0.5 – 1.9 m/s |
| 2nd | 0.3 – 1.9 m/s | |
| Volumetric reject rate | 1st | 29 – 47% |
| 2nd | 38 – 58% | |
| Mass reject rate | 1st | 45 – 67% |
| 2nd | 58 – 80% | |
| Feed consistency | 1st | 0.9 – 2.0% |
| 2nd | 1.0 – 3.1% | |
| Feed freeness | 1st | 111 – 161 ml |
| 2nd | 160 – 310 ml | |
| Accept flow per unit screening area | 1st | 90 – 330 m3/h /m 2 |
| 2nd | 45 – 330 m 3/h /m 2 | |
| Accept production per unit screening area (odt = oven-dry ton) | 1st | 20 – 122 odt/d /m 2 |
| 2nd | 8 – 95 odt/d /m 2 |