4.5. Role of fibre coarseness in fractionation

Coarseness was studied by means of five series of screening tests performed at two TMP mills and in the laboratory. The materials, methods and experimental arrangements used in these were as follows:

TMP pulps from the first screening stage of mills A and B were used in these experiments. In addition, the accept pulp from the first stage at mill A was screened further with the laboratory screen device. The mill screening devices were axially fed Valmet TAP340 and TAP450 pressure screens (see Fig. 1) with slotted screen plates. The screen plate in mill A had a slot width of 0.18 mm and high contouring and that in mill B a slot width of 0.15 mm and low contouring. The laboratory experiments were performed with a modified axially fed Valmet TAP03 screen and a medium-contoured screen plate with a slot width of 0.09 mm. A picture of the device is presented in Appendix 2.

Pulp samples were taken from the accept and reject flows and their consistency and freeness were analysed. Fibre properties were analysed with the Kajaani FiberLab system. Coarseness values were determined by the three methods outlined in Eqs. 21, 22 and 23. Fibres longer than 0.4 mm were measured by image analysis.

Some additional FiberLab analyses (average values, not distributions) obtained in the Valmet MuST202E screening experiments at mill A and analysed at the Valmet Technology Centre were also available. These experiments used a slot width of 0.15 mm and high contouring in both screening stages. Fibres longer than 0.2 mm were subjected to image analysis.

One term used in the data analysis was — the relative coarseness index difference — , which is related either to the total difference or to the difference independent of fibre length. The former relation is defined by the following equation:

Equation 25.

and the latter by the equation:

Equation 26.

where

ΔCI_tot is the total relative coarseness index difference (%)

CI_R is the average coarseness index in the reject pulp,

CI_A is the average coarseness index in the accept pulp,

ΔCI is the relative coarseness index difference independent of length (%),

CI_R,i is the coarseness index of the reject in length class i,

CI_A,i is the coarseness index of the accept in length class i, n is the number of length classes.

The devices, screen plate designs and screening conditions used in the test series are presented in Table 8. The notations for the screen plates are similar to those in Table 6.