Precipitation and filtration of lignin from black liquor of different origin

The separation of lignin from black liquor is an attractive option in the pulp mill. Lignin extraction provides a modern mill with the opportunity of removing an energy surplus in the form of a solid biofuel. In pulp mills where pulp production is limited by the capacity of the recovery boiler introducing lignin separation can de-bottleneck the process and thus allow an increase in production. A well-known method for separating lignin from black liquor is precipitation by acidification. After precipitation, the lignin is filtered and further purified. The filtration properties of lignin precipitated from one single kraft black liquor was studied in a previous investigation. The present work expands that study to embrace several other black liquors of different types. It is shown here that the filtration properties, measured by the average specific filtration resistance, differ between black liquors. Black liquors that were concentrated using a membrane prior to precipitation were more difficult to filter than those taken directly from the evaporator train. Subjecting the black liquor to heat treatment before precipitation, however, improved filtration properties. In addition, black liquors, lignin filter cakes and filtrates were chemically characterized so that possible differences in organic composition could be detected.