Lignin isolation from spruce wood with low concentration aqueous alkali at high temperature and pressure: influence of hot-water pre-extraction

The aim of this study was to isolate lignin from spruce wood with an accelerated solvent extractor (Dionex ASE 350) using low concentration alkali solutions. Two concentrations of alkali were studied - 1% and 2% aqueous NaOH. Extraction was performed for 4 hours total time, but different extraction sequences were studied: 1 cycle per 240 minutes static time (1 x 240 min), 2 cycles per 120 minutes static time (2 x 120 min), 4 cycles per 60 minutes static time (4 x 60 min), 6 cycles per 40 minutes static time (6 x 40 min), and 12 cycles per 20 minutes static time (12 x 20 min). The effect of preliminary hot-water extraction on lignin isolation with alkali was also investigated. It was found that higher alkali concentration allows extracting more lignin from spruce wood. The extraction sequence has a great influence on the process - shorter extraction cycles extracted up to 10 times more lignin compared to longer ones. It was possible to extract up to 22.5% of the wood as lignin, which corresponds to 75% of the total lignin content in the wood. Extraction kinetics, with regard to the alkali concentration, was also studied. The rate constant of extraction was shown to be almost linearly proportional to the concentration of alkali. The reaction order was established to be 1.25.