Improved Breaking Length Development of Unbleached Softwood Kraft Pulp in PFI Refining by Addition of Primary Fines

Nowadays the so called fines fraction is experiencing increasing interest for the papermaking society, as an essential component of any papermaking pulp. It shows distinctive properties affecting both production process and product properties to a large extent. Several research groups have experimented with either primary and/or secondary fines to assess their corresponding properties in the recent years. When it comes to the influence of primary fines on paper and process, these studies do not always show consistent results, attributed to different raw materials considered, to retention issues during sheet forming or maybe to formation issues arising at higher dosages. This work focuses on the clarification, how primary fines of unbleached softwood kraft pulp (Kappa number ~27) influence the product and process parameters, especially if the total fines amount is risen compared to the original stock. 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 and compared with the unbeaten reference. The refining treatment mainly resulted in fiber flexibilisation and internal fibrillation while barely any secondary fines were produced. Because retention of the fines material might be an issue, a Rapid-Köthen sheet former with white water recirculation was used. The results of paper testing show that the tensile index develops at lower specific refining energy when adding primary fines prior to refining due to increased densification of the sheets. The results also show increased dewatering resistance (Schopper-Riegler) at a given tensile index, while densification and air permeability (Gurley) are comparable. Considering the linear relationship between tensile index and sheet density – independent of the fines content – it can be concluded that fibre flexibilisation and primary fines both enhance fibre-fibre bonding and that both strategies result in the same increase in mechanical strength with the downside of slightly reduced dewatering in case of the introduction of primary fines. Introduction When talking about pulp, it is necessary to differentiate between the coarse fiber material and the fines material. Fines again can be separated in primary fines, which are created during the pulping process, and secondary fines which are a product of refining and mechanical stresses in stock preparation. The morphological character of the produced fines is different depending on their origin. Primary fines mainly consists of flake like fines, which are mostly ray cells and fragmental parts from middle lamella and primary fiber wall. Secondary fines are mainly fibrillar fines torn out from the secondary wall. [1][2] Several studies have been done by different groups focusing on the technological properties of these fines and the effects of their removal or addition. Because of the diversity of experimental designs, these studies did not always show consistent results. Chauhan et.al. for example show that the addition of primary fines, separated from bleached hardwood pulp by means of a Bauer McNett classifier, up to 10 and 20% total fines content prior to refining, leads to a reduced strength potential after PFI refining compared to the reference without additional primary fines [3]. Feirreira et.al. on the other hand show that removal of primary fines from unbleached hardwood kraft pulp, leads to a decrease in tensile and burst index, but an increase in tear index [4]. Similar results are presented by Bäckström et al. in an experimental design where fines were added instead of removed. It is shown that the addition of either primary or secondary fines to refined fibers allows an improvement of tensile index, burst index and TEA [2]. This work focuses on the primary fines of a softwood kraft pulp and the effect of different primary fines ratios in the pulp to shed more light on their role when it comes to pulp properties in the unrefined state and especially the development of said properties during refining and the accompanied energy saving for beating. 16th Minisymposium Verfahrenstechnik & 7th Partikelforum, TU Wien, Sept. 21/22, 2020 MoV4-(02) page 2/5 Materials and Methods