Treatability of kraft spent liquor by microfiltration and andultrafiltration

The application of membrane technology on black liquor from pulp mills has been greatly limited due to the high temperature, high pH value and high suspended solid (SS) concentration of the black liquor, although many researchers have made a great effort. Attention has been paid to the inorganic membrane in this paper because it has the advantages of resistance to acid and alkali, a long life span, etc. The aim of this work was to study the feasibility of black liquor treatment by various organic OF and inorganic MF membranes. Based on the batch experiments with 200 kPa transmembrane and a temperature of 30C, the results showed that approximately 80% lignin retention was achieved with MF membranes and 90% lignin retention with OF membranes. The COD reduction in the permeation mainly resulted from lignin rejection. Both MF and OF membranes could reject silica and their efficiencies were all over 80%. The observed total filtration resistance was evaluated for the intrinsic resistance (R_pp), adsorption (R_a), pore plugging (R_pp), and concentration polarization (R_cp), using a series resistance model based on the pure water flux. The concentration polarization dominated the process for the inorganic membranes. However, the distributions of the filtration resistance for the organic membrane depended on the materials characteristics and the pore sizes. By a 40-day continuous experiment with a crossflow velocity of 2.3 m/s and transmembrane pressure of 200 kPa, the flux ofthe 0.2 μ inorganic MF membrane was 2001/m²h flux at 32°C after operation for 374 h, and 4001/m²h at 63°C for 625 h. During the operation process, no cleaning was carried out and the smooth operation period was more than 40 days. The experimental results highlighted the treatment of black liquor by membrane separation.