The Degree of Refining

4.8.1 The Proportion of Wood Resin in the Pulp Water Phase

In principle, the degree of refining may affect the proportion of wood resin in the water phase in two possible ways. Firstly, pulp with a lower freeness value has received more intensive treatment during refining, which may enhance the release of wood resin from the

parenchyma cells. Secondly, the adsorption of wood resin onto the pulp may depend on the specific surface area of the pulp. In pulp with a low freeness value, the specific surface area is higher and, thus, the amount of wood resin adsorbed onto the pulp can also be larger.

Wood resin is located in the wood tissue mainly in resin channels or inside parenchyma cells. Canal resin can be already removed from the chips by pressing and during refining the resin channels brake up completely. Parenchyma resin cannot be removed from the chips by pressing [15]. According to studies related to the breaking of parenchyma cells during refining of TMP [15, 35], it is possible that the liberation of wood resin from the parenchyma cells already takes place almost completely in pulp which has been defibrated to a higher freeness value.

The wood resin in the parenchyma cells is comprised mainly of triglycerides and steryl esters, while that located in the resin channels is composed mainly of resin acids [12]. In the study carried out on TMP, which was refined to a freeness value of 120 ml, the composition of the wood resin in the pulp water phase was the same as that of the resin in the pulp [21]. This result indicates that both the parenchyma and channel resin were equally accessible for release to the water phase and, hence, there should not exist any wood resin captured inside unbroken parenchyma cells.

If the liberation of wood resin from the parenchyma cells is influenced by the freeness value of the pulp, a decrease in the level of freeness of the pulp should cause the proportion of triglycerides and steryl esters to increase and that of resin acids to decrease in the pulp water phase. Figure 53 and Figure 54 show that the proportions of different wood resin groups remain quite constant when the freeness value of the pulp is increased from 100 to 500 ml CSF. For the steryl esters and triglycerides, only a very slight increase can be observed. These results confirm that the degree of refining does not have significant effect on the accessibility of wood resin for liberation from the parenchyma cells. These results also indicate that the parenchyma cells in pulp, that has a high freeness value, are already damaged in such of way that all the wood resin located in these cells is accessible for liberation to the pulp water phase.

0%

20%

40%

60%

80%

100%

135 175 210 260 310

Freness, ml CSF

TG SE ST FFA RA

Figure 53. The effect of freeness on the proportions of different wood resin groups in the water phase. The freeness values used in here were measured from the pulp samples before dewatering. The freeness values measured after hot

disintegration were slightly lower, namely, between 100 and 250 ml CSF, Pilot-TMP.

0 20 40 60 80 100 120 140 160

II-stage CSF 160 ml I-stage CSF 470 ml

Wood resin, mg/l

TG SE ST FFA RA

Figure 54. The effect of the freeness value on the amount of different wood resin

components in the pulp water phase. Pulp samples were taken from the same pulp production line after I-stage and II-stage refiner. The freeness values were measured from the pulp after hot disintegration, TMP II.

It has been observed that the wood resin content of fines is higher than that of fibres and it was also assumed that this difference is caused by the larger surface area of fines in comparison to that of fibres [30]. When the degree of refining is increased, the proportion of fines and, hence, the surface area of the pulp increases. This could have a increasing effect on the adsorption of wood resin onto the pulp. In Figure 54 and Figure 55, it can be seen that the amount of wood resin in the pulp water phase is independent of the freeness

value. Therefore, these results also indicate that the degree of refining does not have a significant influence on the amount of wood resin adsorbed onto the pulp.

According to these results it seems that the degree of refining does not influence the liberation of wood resin from the pulp.

4.8.2 The Effect of the Freeness Value on the Retention of Water-released Wood Resin in Dewatering

When the freeness value of the pulp decreases, the retention of wood resin has to increase, and the main question is how much the freeness value can affect retention in this case?

From Figure 54, it can be seen that when the freeness value of the pulp falls from 310 to 135 ml, the retention of wood resin is almost doubled.

For lower freeness values, the increase in retention is much sharper, Figure 56. Similar results were also obtained earlier in the measurements carried out in the mill-scale dewatering devices, Figure 12. Figure 56 also shows that the retention of substances, which cause turbidity, is higher compared to the retention of wood resin. This similar phenomenon was also observed and discussed in the mill measurements, shown earlier in the Chapter 3.2.3.

0 10 20 30 40 50 60 70 80 90 100

100 150 200 250 300 350

Freeness, ml CSF Wood resin, mg/l Retention, %

Wood resin Retention

Figure 55. The effect of freeness on both the amount of water-released wood resin in the pulp and retention during dewatering, Pilot-TMP. The determination of retention is described in the Chapter 2.3.

0 10 20 30 40 50 60 70 80

0 50 100 150 200 250 300 Freeness, ml CSF

Retention, %

Turbidity Wood resin

Figure 56. The retention of wood resin and turbidity-causing substances for pulp samples refined to different levels of freeness with Valley-beater, Pilot-TMP.