Lignin Recovery from Black Liquor Using Integrated UF/NF Processes and Economic Analysis

Lignin is a polyphenolic biopolymer present in large amounts in black liquor (BL). This work investigated the recovery of lignin from BL (pre-filtered by ultrafiltration (UF)) by nanofiltration (NF). For the NF tests, laboratory-made mixed matrix membrane (MMM) prepared with 0.1% activated carbon (AC) nanoparticles were used in crossflow filtration mode. The effect of pressure (6–15 bar) and volume reduction (VR) (~65%) were analyzed, and the filtration performance was evaluated in terms of permeate flux, lignin rejection rate, and flux reduction. The lignin rejection rate varied in the range of 67–80% with the pressure, however, the highest increases in flux and rejection were observed at 12 bar, which was found to be the optimum pressure. At a VR of ~65%, the permeate flux decreased by ~55% and the lignin rejection rate increased from 78% to 86%. In addition, an economic evaluation was performed for the preparation of UF and NF MMM. The minimum-to-maximum price range was estimated considering the costs of the laboratory and commercial grade regents. It showed a difference of ~10-fold and ~14-fold for UF and NF membranes, respectively. The results of the laboratory-scale study were used to evaluate the economic feasibility of the process for recovering lignin- and hemicellulose-rich retentate streams.