Mechanical and chemical chip pre-treatment in mechanical pulp production

Abstract: The mechanical pulping industry has been developing throughout the years, due to competitive prices in the electricity market and good accessibility of wood. This has made it possible for such and “expensive” process to further develop. Today, with increasing electricity prizes, it is of great interest to reduce electrical consumption in mechanical pulping industry, since the process consumes large amounts of electricity. Braviken paper mill is starting up a new thermomechanical pulping line, scheduled for start-up in August 2008, which aims to reduce electrical consumption. The new line will include chip pre-treatment equipment such as an impregnator, an Andrtiz Impressafiner (Screw press), a high intensity primary stage refines double disc (DD), and a new low consistency refiner (LC), significantly bigger than those earlier available on the market. This master´s thesis is one out of three Master´s thesis made at Braviken paper mill during spring 2008. They all are connected, and are investigating the possibility to reduce electric energy consumption within TMP production. Master´s thesis concerning high consistency refining was done by Dino Muhic, “High consistency refining of mechanical pulps during varying refining conditions”, and low consistency refining written by Fredrik Johansson “Increased energy efficiency in low consistency refining”. Chip pre-treatment is to be used to reduce electrical consumption. Mechanical pre-treatment, such as using an Andrtiz Impressafiner demolishes the chips while also making it possible to impregnate the chips with chemicals, the later giving additional possibilities to reduce electricity consumption. Chemical chip pre-treatment decreases the lignin softening temperature, which benefits the refining process, yielding longer and less damaged fibers that will create a fibrous pulp with reduced electrical energy input.The goal for this study was to investigate the effect of alkaline-peroxide on chip pre-treatment by using a design of experiment method, in terms of electric energy consumption for the process, strength properties, opacity and ISO-brightness within the pulp/sheets. The trials were built up as a factorial experiment, with two factors, alkaline and peroxide, with two levels each (high and low). The high level for alkaline was 15 kg/t and 10 kg/t for the low level, and the high level for peroxide was 10 kg/t and 5 kg/t for the low level. This resulted in four trials with two zero-points, and two reference pulps, one normal TMP, thermomechanical pulp, and the other TMP with pressafiner and water. The trials showed that adding alkaline-peroxide clearly had an impact on pulp properties, such as increased strength properties, fiber length improvements and less shives could be found in the alkaline-peroxide treated pulps. The yield was highest for the normal TMP, about 99% and it decreased with increasing alkaline addition, the lowest value was achieved for the pulps containing the highest dose of alkaline, about 95%. The optical properties were more or less as expected. Opacity had the highest value for the pulps that had been made from chips with the highest total alkaline level. The ISO brightness was highest for pulps containing low level of alkaline. It could not be decided if the electricity demand had been reduced for the chemically treated pulps; it actually had the opposite effect as expected. The chemically treated pulps demanded a higher SEC, specific energy consumption, compared to the reference pulps. This result could have depended on the small pilot plant high consistency refiners at CTP, Centre technique du papier, Grenoble, France, due to the plate size and what kind of plats that were used. To do trials like this and to be able to draw correct conclusions relevant for a full scale plant, bigger refiners might give a more comparable result. It was clear that the fiber properties had improved, which could be the key to reduce electricity when LE- (low-energy) plates are used in the HC-refiner. A higher intensity could be used and electricity energy could be saved.