Ultrafiltration of kraft black liquor with a ceramic membrane

Abstract

Emissions from the combustion of fossil fuels contribute to the greenhouse effect around the globe. It is therefore of great interest to reduce the use of fossil fuels and revert to some form of biofuel. Lignin, extracted from kraft black liquor, has the potential to become a significant contributor to the biofuel market. A modern pulp mill has an energy surplus that can amount to 7 GJ per tonne of pulp. One way to reduce this surplus is to extract lignin, which is the main source of energy in a kraft pulp mill, and convert it to a usable biofuel, which can be sold on the market. If membrane technology is used to extract lignin from kraft black liquor additional benefits in the overall operation of the mill can be achieved, such as, an improved pulp quality. In this investigation a tubular ceramic membrane manufactured by Orelis, France, with a cut-off of 15,000 Da was used to fractionate kraft black liquor. The experiments were performed at 60, 75 and 90°C at various pressures. The flux and retention during concentration to a volume reduction of 0.9 were studied. The fluxes were 90, 110 and 130 l/m 2h at 60, 75 and 90°C and TMP 100 kPa. The lignin retention was about 30%. The retention of monovalent ions was virtually zero, whereas multivalent ions were retained to a high degree, probably due to their association with the organic material. During concentration to a volume reduction of 0.9 at 100 kPa and 90°C, the dry substance increased from 16 wt% to 22 wt% and the lignin concentration from 55 g/1 to 158 g/I. The average flux during the concentration was 90 1/m 2h. The flux was 20 l/m 2h at the end of the concentration.