The effects of chip pretreatment on ctmp pulp properties

In this investigation, the chip pretreatment stage in the mechanical pulpin g process has been studied and the effects of screw press pretreatment on t he initial fiber separation and strength property development in refining a re discussed. The study was carried out in a CTMP pilot plant equipped with a plug screw press. The pretreatment was carried out by conventional PREX- impregnation or by means of so-called SOAK-impregnation consisting of steam ing of the chips followed by immersion in cold impregnation liquor and preh eating to allow for sulfonation. The chips were then passed through the sam e screw press as in the PREX-impregnation. CTMP pulps were then produced at different pretreatment conditions such as varied sulfonate content and dif ferent temperatures in the screw press. In order to study the initial fiber separation, scanning electron microscopy was used and fiber characteristic s were studied with a fiber analyzer PQM1000 as well as specific surface me asurements. It was found that the two pretreatment methods resulted in different types of fiber separation, which gave different mechanical pulp properties. The c hips were less deformed after the screw press when they hard been SOAK-impr egnated. Extensive fiber separation could be found after the PREX-impregnat ion and the resulting first stage pulp had a surface that was well fibrilla ted. This was also shown by the specific surface study where the long fiber fraction of the PREX-pulp had higher specific surface than the SOAK-pulp. The higher specific surface for the PREX-pulp resulted in both higher tensi le strength and, higher sheet density when compared at constant refining en ergy consumption. The scattering coefficient was also slightly higher far t he PREX-pulp but the SOAK-pulp, on the other hand, had higher long fiber co ntent which resulted in higher tear strength, both at constant freeness and constant refining energy. The refining energy consumption, compared at con stant pulp freeness and constant sulfonate content, was about 10% lower for the PREX-pulp.