Influence of the membrane cut-off during ultrafiltration of kraft black liquor with ceramic membranes

Fractionation of kraft black liquor by ultrafiltration is receiving a great deal of interest at the moment, mainly because the extracted lignin can be used as a biofuel, but also because there are indications that the pulp quality is improved when permeate is used instead of black liquor as the diluent in the cooking process. Fractionation of kraft black liquor calls for membranes that can withstand high temperatures at high pH. In this investigation the performance of two ceramic membranes manufactured by Orelis, France, with cut-offs of 5 and 15 kDa was studied. The temperature during the experiments was 90degreesC and no adjustment of the pH was made. The retention of cooking chemicals was almost zero for both membranes. The volume of black liquor was reduced by 80% with the 5 kDa membrane and by 90% with the 15 kDa membrane. The average flux during concentration with the 5 kDa membrane was 451m(-2) h(-1) (transmembrane pressure 400 kPa and circulation velocity 3.6 m s(-1)) while the flux of the 15kDa membrane was 951m(-2) h(-1) (transmembrane pressure 100kPa and circulation velocity 4.5 m s(-1)). The lignin recovery was 66% and 28% for the 5 and 15 kDa membranes, respectively.