2
C.V.G. Esteves: Oxygen deligni fi ed fi bers as a greener alternative for tissue paper
maximize the increase in the total fi ber charge of the fi bers subjected to oxygen deligni fi cation assessing kraft cooked pulp with di ff erent starting kappa numbers and investigating the potential of using those pulps as a replacement for fully bleached fi bers.
250 g (o. d.) wood chips. With a vacuum pump, the air inside the vessels was removed for 30 min and after that time, the cooking liquor was sucked into the autoclaves. The chips were fi rst subjected to 0.5 MPa nitrogen injection for about 30 min, and then released before starting the cook. For the impregnation step, the autoclaves were placed in a steam- heated glycol bath at 100 ° C for 30 min, and at 160 ° C for the cooking step. Rotation and slight inclination of the auto- claves ensured good mixing inside. After the cooking step, the autoclaves were cooled down in a water bath for 10 min, and then the spent liquor was drained o ff the chips and collected for analysis. The trials were performed with an e ff ective alkali of 22%, a sul fi dity of 30 % and with an ionic strength of 0.1 M. The trials were stopped at di ff erent H factors (cooking times) to achieve distinct kappa numbers. The deligni fi ed chips were washed with deionized water for 10 h in self-emptying metal cylinders and subsequently de fi brated and screened in an NAF water jet de fi brator (Nordiska Armaturfabriken). The shives were collected, dried at 105 ° C, and weighed.
2 Materials and methods 2.1 Materials
2.2.2 Oxygen deligni fi cation
A mixture of softwood chips from an industrial mill (80 % Spruce and 20 % Pine) was used in this study for kraft cooking and oxygen deligni fi cation. A never dried commercial fully bleached pulp (reference pulp) from the same mill was used for comparison. For future comparisons, it is important to highlight that the fully bleached pulp was produced indus- trially and, therefore subjected to much harsher treatments than the pulps produced in the laboratory.
Oxygen deligni fi cation was carried out with 60 g of oven- dried (o. d.) pulp, placed in polyethylene bags with the required amount of NaOH, MgSO 4 , and water at a consis- tency of 12 %. The bags were closed by heat-welding, knea- ded initially by hand and then in a vibrational shaker. The pulp was then removed from the bags and transferred to pressurized te fl on-coated stainless-steel autoclaves. The autoclaves were closed, pressurized with oxygen and placed in an electrically heated glycol bath, with rotation at a slight inclination. The trials were performed with single or double oxygen stages with no washing in between. The single oxy- gen stages were pressurized with 0.7 MPa of oxygen and the reaction was carried at 100 ° C. For the double stages of ox- ygen deligni fi cation the pressure used in the fi rst stage was 1 MPa and the temperature was 85 ° C, while for the second stage the pressure was 0.5 MPa and 110 ° C. After the oxygen deligni fi cation, the pulps were washed with deionized water and fi ltrated. The conditions for each trial are illustrated in Figure 1. Pulps were denominated KX_OY, where X is the kappa number of the cooked pulp and Y is the kappa number after the oxygen deligni fi cation stage.
2.2 Methods
2.2.1 Kraft cooking
Three series of kraft cooks were done in this study to obtain pulps with di ff erent kappa numbers, two were produced in one recirculatory digester and one was produced in autoclave. Recirculator digester: The wood chips were kraft cooked in a recirculated digester with a dry chips capacity of 2 kg, controlled temperature and a forced liquor fl ow. The wood chips were impregnated in water under 0.5 MPa nitrogen pressure the day before the cooking. The water was then removed and cooking liquor was added to give a liquor/wood ratio of 4.5 l/kg. The impregnation step was performed for 30 min at 100 ° C and the cooking step was performed at 160 ° C. After the cooking, the steam fl ow was stopped, and the spent liquor was drained o ff and collected for analysis. Autoclave: The trials were performed in steel auto- claves with a volume of 2.5 dm 3 , which were loaded with
2.2.3 Re fi ning, paper making
The pulp samples were re fi ned at two di ff erent levels (2000 and 4,000 rpm) in a PFI-mill according to ISO 5264 – 2 and
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