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Abstract

The present invention relates to a method of membrane-filtrating a lignin containing composition in order to obtain a lignin fraction more suitable for further treatments. In particular it relates to treating a liquid lignin composition to obtain a lignin fraction having a desired molecular weight distribution by a membrane filtration. It comprises subjecting the liquid lignin composition to a first membrane filtration with a first filter cut-off adapted to remove species having a first molecular weight thereby providing a permeate with a molecular weight distribution defined by said cut-off. Then, the permeate from the first membrane filtration is; subjected to at least one further filtration step with a second filter cut-off, different from said first filter cut-off to provide a retentate (concentrate) with a molecular weight distribution defined by both the cut-off in the first filter the cut-off in said second filter. A dilution is performed on a desired lignin containing fraction at some point downstream of the first filtration unit. A lignin containing retentate (concentrate) is collected from the further ultrafiltration for further processing.

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First Claim