Centrifugal washing and recovery as an improved method for obtaining lignin precipitated from South African kraft mill black liquor

Abstract

This study describes centrifugal recovery as an improved method for collection of lignin isolated from black liquor obtained from a South African kraft mill. Precipitation of lignin was achieved by utilising 6 M sulphuric acid. Recovery of the polymer was evaluated by comparing filtration straight after precipitation with centrifugal recovery and washing. Higher lignin yields were obtained with straightforward filtration recovery. However, filtration of the sample took a very long time, due to the colloidal nature of the lignin precipitate. Lignin recovery and washing utilising centrifugation reduced the recovery time at the final filtration step quite significantly. The lignin obtained was characterised by FTIR, (sup1)H- and (sup13)C-NMR spectroscopies to elucidate the heterogeneity of the sample, as well as to provide information on the syringyl and guaiacyl content of the lignin. Size Exclusion Chromatography revealed that the dispersity of the polymer was uniform and pyrolysis-GC/MS analysis provided insights and identification of typical degradation compounds that could be obtained from the lignin sample.