Lignin extraction from kraft black liquor by ultrafiltration

Extn. of lignin from kraft black liquor is one method of lowering the large energy surplus in pulp mills. The lignin can be used as a bio-fuel to replace fossil fuels in heat or power generation, and the lignin-depleted black liquor can be reused in the cooking operation. As kraft black liquor lignin has a fairly high mol. wt., ultrafiltration can be used to selectively fractionate lignin from cooking chems. However, ultrafiltration of kraft black liquor calls for membranes that can withstand high temps. at high pH, i.e. ceramic membranes. In this investigation the performance of a ceramic membrane manufd. by Orelis, France, with a cut-off of 15 kDa was studied. The temps. employed during the expts. were 75 and 90°C and no adjustment of the pH was made. The influence of transmembrane pressure, temp., cross-flow velocity and vol. redn. on flux and retention of lignin and inorg. compds. (cooking chems. and multivalent, non-process elements) was studied. A correlation was found between the av. flux and retention during concn. The highest av. flux attained during concn. to 90% vol. redn. was 160 L/m2 h with a lignin retention of 35%. The retention of cooking chems. was found to be insignificant, while multivalent inorg. compds. were found to be retained to a high degree.