Primary Fines and their Positive Influence on Energy Consumption during Refining with Regard to Strength Development for Softwood Kraft Pulp

Besides the fiber fraction the so called fines fraction affects both production process and product properties and has
received increasing interest in recent years and several research groups have experimented with either primary and/or
secondary fines to assess their corresponding properties with not always consistent results.
Our work focuses on the primary fines fraction of an unbleached softwood kraft pulp. Primary fines are separated
from the pulp using a laboratory pressure screen to be added again in controlled amounts afterwards. Thereby three
pulp blends, showing a primary fines content of around 5%, 9% and 12% where prepared. These pulp blends were
refined in a PFI mill at 1000, 4500 and 6000 revolutions.
The results show that the primary fines enhance tensile strength in the unrefined and slightly refined state while they
do not increase the maximum tensile strength of the highly refined reference. The results also show a slightly increased
dewatering resistance (Schopper-Riegler) but comparable air permeability (Gurley) for a given tensile strength. The
linear relationship between tensile index and apparent sheet density seems to be affected in the unrefined and slightly
refined state where the breaking length of the fines-enriched samples is higher for a given apparent density.