Defence of doctoral thesis: Marie Bäckström – The effect of environment on refining efficiency of kraft pulps

Abstract

Although the pulp and paper mill processes have been operational for long time there is still a need to understand the unit operations in paper making and how they interact with the ingoing pulp material. This is crucial in order to fully utilize the potential of the wood as well as of the unit operations. In order to do that it is vital to have an understanding about the produced pulp fibres, how they are constructed and how they respond to different conditions of their environment.

The aim of this work has been to clarify how the environment influences the refining efficiency of kraft pulps in terms of energy requirement and paper property development. The main hypothesis has been that the swelling of the fibres, due to their inherited polyelectrolytic gel nature, will not only affect the fibre as such but also the strength and properties of the fibre flocs that are mechanically treated between bars in the refiner and in this way affect the refining efficiency.

The main focus has been to study how the chemical environment, in particular the initial fibre swelling, affects the refining efficiency. Therefore, the influence of counter-ions to the charged groups, the number of charged groups, electrolyte concentration, pH and rheological behaviour was studied. Additionally, the importance of fibre flocculation for the refining efficiency was investigated by chemical means, i.e. to chemically flocculate and deflocculate a fibre suspension just before entering the refiner and evaluate the refining efficiency. An investigation to clarify the importance of refining homogeneity was also performed. The work was performed both on a laboratory scale and in pilot scale using industrial refiners to ensure the validity of the results.

The importance of the counter-ions to the charged groups on refining was demonstrated. When the counter-ions was sodium the refinability, defined as the required energy input to reach a certain WRV or tensile index, was reduced by up to 50%. The more energy-efficient refining of pulps in the Na+-form may be explained as a co-operation between a higher osmotic pressure in the fibre wall and the mechanical stress applied during refining, so called “electrostatic repulsion-assisted refining”. When mechanical forces are applied on the fibre and the fibre wall, the electrostatic repulsion forces due to the ionization act as an additional aid to increase the swelling, and this in turn helps to delaminate the fibre wall. This “electrostatic repulsion-assisted refining” also resulted in another type of external fibrillation of the fibres than that produced for the reference pulp in calcium form. The fibrils were very short and tiny. The improved refining efficiency could not be attributed to any rheological effect such as floc strength or floc size.

The number of charges correlated to refinabiliy of the pulp material, but there has to be a balance between the number of charges and ultrastructure of the pulp fibre. If too many charges were introduced, the internal ultrastructure was eventually damaged, and no property development was obtained in the refining.

The importance of refining heterogeneity on paper properties was investigated by mixing less refined or unrefined pulps and highly refined pulps in different proportions giving a wide distribution on energy input to individual fibres, as well giving swelling distribution curves. The mechanical properties of the produced paper were surprisingly alike, and the influence of the extreme inhomogeneity was rather small. In a pilot paper machine trial, clear effects due to heterogeneous refining was only observed when 50% of the stock fed to the paper machine was unrefined. This implies that the mills can have large freedom in refining strategies without any significant negative impact on the tensile strength properties at a given density.