Characterisation of efficient ECF bleaching parameters of eucalyptus pulp

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Helsinki University of Technology | Diplomityö
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Puunjalostuksen kemia
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iii + 81 s. + liitt. 7
At present, bleached eucalyptus kraft pulp is globally the most widely used short fiber raw material in paper making. One of the most extended technologies to bleach eucalyptus pulp is the so called ECF-technology which uses chlorine dioxide rather than elemental chlorine to bleach the pulp. Different bleaching arrangements exist to achieve this goal. Among them, classical sequences include conventional chlorine dioxide stages and hot stages. In the latter technologies high temperature and long retention time are the key operational parameters. The way the bleaching process is operated has a direct impact on pulp and effluent properties. One of the main environmental concerns is the load of organic chlorine in the effluent. Another concern is the formation of chlorate, both from an environmental point of view and a matter of efficiency in the process. The former is due to studies that set chlorate formation as a potential threat for some aquatic organisms present at the mill effluent discharge and the latter has to do with the inactivity of chlorate during bleaching reactions that translates into a loss of the bleaching efficiency. In this research eucalyptus pulp was bleached at 90% ISO brightness with four different bleaching sequences that included a Do-Ep-DInDZ, AID-Ep-DInDZ, DHT-Ep-DInDZ and a novel A-Ep-DInDZ. An A/D-8p sequence was also run to study the effect of acidifying the pulp with hydrochloric acid instead of sulphuric acid. Different pulp and effluent properties including chlorate formation at all bleaching stages were analysed and compared. Additionally, both the effect that filtrate circulation and chlorate dissolved in the filtrate pose on different pulp and effluent properties after a DI stage were investigated. Hot sequences demonstrated to be more efficient than conventional Do-Ep-DInDZ to fully bleach the pulp with lower chemical costs of production and lower AOX generation. Comparing A/D and DHT sequences the pulp was bleached with similar chemical costs of production, generating a lower AOX in the effluent for A/D but a higher chlorate and OX generation. However, the presence of chloride ions in A/D stage improved delignification, brightness and reduced chlorate though a higher organic chlorine generation at lower end pH conferring a higher operational flexibility to the A/D stage over DHT. On the other hand, the novel sequence proved to be the most efficient of all with remarkably lower chemical consumption requirements, lower overall AOX formation, lower chlorate formation and lower kappa number of bleached pulp. Chlorate presence in the filtrate did not influence the final chlorate formation. However, other properties were affected by chlorate presence. Results led to the conclusion that higher concentrations of chlorate present in the filtrate may adversely affect delignification.
Vuorinen, Tapani
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Vehmaa, Janne
Lehtimaa, Tuula
eucalyptus, eukalyptus, bleaching, valkaisu, chlorate, kloraatti, environmental parameters, ympäristöparametrit
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