Sätt vid framställning av kemisk massa

摘要

The pulp is treated between two successive oxygen gas stages with acetic acid. The pulp for treatment with acetic acid has a low content of harmful heavy metals, partic. a low content of manganese which is less than 50 mg Mn/Kg of pulp and pref. less than 10 mg Mn/kg of pulp. The pulp is treated with a complex builder in a stage before treatment with acetic acid in order to reduce the content of harmful heavy metals. This treatment occurs in a stage after the first oxygen gas stage, or before that stage. It can also occur in connection with the boiling process, during or before it. Treatment with acetic acid takes place over a period between 1 min and 1 hour, pref. less than 60 mins. The acetic acid is added in an amt. of at most 50 kg/ton of dry pulp. the treatment with it occurs at a pH of pref. between 8 and 7. The treatments with acetic acid and oxygen gas are carried out without intermediate washing. The pulp after the last oxygen stage is bleached to the required lightness.