Separation methods for closed-loop manufacture of bleached kraft pulp — overview of an EU project

Abstract

A summary of an EU project concentrating on separation processes for the closed-loop manufacture of bleached kraft pulp is presented. The project includes four separate tasks in which the need and function of specific “kidneys” are studied. In the first task, the reuse of condensates from black liquor evaporation in pulp bleaching has been studied, including a detailed characterization of the condensates. The discharge of accumulating non-process elements (NPEs) from acidic or neutral bleach filtrates was the focus of the second task of the project. Precipitation of NPEs occurs during a separate pre-evaporation of the filtrates to about 20% DS content. Further precipitation may be obtained by pH adjustment to a level where the precipitation and separation properties can be optimized to favour the discharge of some specific NPEs (e.g., silicon). The third task concentrates on the discharge of accumulating compounds from alkaline bleach filtrates by the use of ceramic nanofiltration membranes developed within the project. In the fourth task an overall system analysis of the closed cycle pulp mill is made.