Minimering av ditionitdoseringen i linje 12 : Optimering av kemikalieanvändning för pappersblekning på Hallsta pappersbruk

Sammanfattning: Manufacturing of paper is a large industry in Sweden. At Hallsta paper mill dithionite is used to bleach the thermo mechanical pulp (TMP) at their production line 12. The purpose of this work was to minimize the addition of dithionite while maintaining the brightness of the produced paper. In order to achieve this goal three different dosages, high, medium and low dosage, were used at the first dosage points. Furthermore, the working conditions were examined by measuring SO2 along the production line to verify that the levels were according to the national guidelines. To evaluate the brightness of the pulp along line 12, pulp was collected at eight different sites of sample withdrawal and made into test paper. The brightness of the test paper was measured with a spectrophotometer. The levels of SO2 were measured at various points of interest using a multi gas meter. Results show that the total amount of dithionite used was lower when a lower dosage at the first dosage points was used. The brightness gain was approximately the same for all dosages. According to this study, the strategy of dosage should be chosen in relation to the wood received. The high dosage at the first dosage points was an unnecessarily high dosage for the recieved wood. The dosage at the later dosage points should be chosen with respect to the brightness goal of the produced paper. The measurements of SO2 show no relation with dosage of dithionite at the first dosage points, because of a large uncertainty. Further improvements on the working environment must be applied at the critical stages in the production line.