**2. Experimental**

Recycling process of kraft pulp fibres was observed on white kraft pulp sample prepared from hardwood mixture.

Pulp sample was processed into sheets of surface weight 800 – 900 g.m2 and brightness 82.7 % MgO.

Original sample after first processing by defibering, milling and drying represents zero recycling. Following pulp fibres processing simulated recycling. Pulp fibres undergo recycling for 8 times. Simulation could be considered as sufficient because usually pulp fibres are re-utilized for 4 to 5 times in practice.

Pulp was returned back into defibering, milling and drying process. The beating value was chosen as 29 °SR because of achieving sufficient strength of paper sheets without redundant fibre weakening and ensuring the possibility of next recycling.

Recycling of the Hardwood Kraft Pulp 271

Dimensional characteristics were observed by microscope until recently. Nowadays modern apparatus which can measure faster and are able to evaluate fibre dimensions are used.

Dimensional characteristics were monitored by Fiber Tester, which allows the evaluation of app. 20 000 fibres in pulp suspension containing 0.1 g of fibres in 100 ml of suspension in one measure (Karlsson, 2006). Double-dimensional view allows fibre length measuring separately and measuring of pulp fibre deformation during the running between two-glass

Following pulp fibre characteristics in recycling process were monitored during

• Local fibre deformation – angle of fibre deviation and fibre length between particular

Average values of chosen dimensional characteristics - length, width and final fibre shape

Dimensional characteristics of pulp fibres

sample 0,824 20,8 90,1

0th recycling 0,801 0,792 0,792 20,3 21,0 20,3 89,7 89,9 89,9 1st recycling 0,774 0,792 0,782 20,2 20,9 20,2 89,9 89,5 90,1 2nd recycling 0,760 0,784 0,789 20,0 20,6 19,8 90,0 89,2 91,2 3rd recycling 0,766 0,777 0,771 19,8 20,5 20,0 89,5 89,5 89,6 4st recycling 0,769 0,770 0,772 20,4 20,4 20,2 87,2 89,6 90,2 5st recycling 0,767 0,769 0,761 19,8 20,3 19,9 86,7 89,7 89,3 6st recycling 0,757 0,768 0,761 19,5 20,3 19,9 90,3 89,5 89,4 7st recycling 0,769 0,762 0,754 19,9 20,2 19,8 89,4 89,5 89,4 8st recycling 0,771 0,762 0,744 19,8 20,2 19,8 89,8 89,7 89,3

Average fibre width [μm]

Drying temperature [°C] 80 100 120 80 100 120 80 100 120

Fibre shape factor [%]

• Fibre length distribution – in 75 classed divided by 0.1 mm upto 7.5 mm • Fibre width distribution – in 50 classes divided by 2 µm upto 100 µm

• Percentual fibre distribution within configured length and width interval

factor from multiple measures of fibre suspension are stated in tab. 2.

Average fibre length [mm]

Table 2. Dimensional characteristics of pulp fibres.

plates.

dimensional analysis:

Pulp sample

Original

• Average fibre length • Average fibre width • Fibre shape factor

• Fine ratio – fibres up to length 0.2 mm

angles of fibre deviation.

The drying temperature influence on chosen pulp fibre properties was observed at 80 °C, 100 °C a 120 °C. These chosen temperatures cover the action of usually used temperature (100 °C) and extreme temperature values (80 °C and 120 °C) which were chosen for comparison.

Pulp samples taken from each recycling level were used for sheet preparation. Sheets surface weight was determined as relative value needed for calculation of chosen characteristics.

Standardly used procedures were applied for recycled pulp treatment (dry substance determination, wet laboratory defibering, laboratory beating in PFI mill, laboratory sheet preparation for physical tests, determination of dewatering ability according to Schopper-Rieglera, determination of surface weight).

Chosen properties were observed on the sheets prepared from original pulp, pulp after first processing (zero recycling) and pulp from particular recycling degrees:

