On the Local Filtration Properties of LignoBoost lignin: Studies of the influence of xylan and ionic strength

Abstract: Kraft pulping technology is currently the most commonly-used method for producing paper pulp from wood. A new and promising opportunity for Kraft pulp mills is to take a step towards becoming biorefineries by implementing technologies able to extract and convert the organic by-products, such as lignin, into a wide range of value-added products and chemicals. The LignoBoost process, which is a new technique that has recently been implemented on an industrial scale, is designed to extract lignin from the Kraft process with a high degree of purity, making it potentially suitable for the manufacture of e.g. carbon fibres. Following a lignin precipitation stage, filtration, performed by dead-end filtration, is one of the key steps of the LignoBoost process. The aim of this work is to improve the efficiency of the filtration stage of the LignoBoost process further. The local and average filtration properties of the cake formed from softwood lignins extracted using the LignoBoost process were investigated through the use of model liquors and by varying the condition parameters and the preparation procedures. The influence of the hemicellulose xylan on the filtration and precipitation of LignoBoost lignin was studied. LignoBoost lignin was (i) suspended in acid water with xylan added and (ii) dissolved together with xylan and then re-precipitated. The effects of ionic strength, applied pressure, slurry concentration, pH, precipitation temperature and rate of acidification on the resulting material and its filtration properties were all investigated. Moreover, the evolution of the size of the particles agglomerates during the course of precipitation was monitored in situ using the Focus Beam Reflectance Measurement (FBRM) technique.  The lignin-xylan mixtures were more difficult to filter than the original LignoBoost lignin. The latter was found to be a material that was relatively easy to filter (2 to 6 ·1011 m/kg in filtration resistance), forming weakly compressible filter cakes over the filtration pressure range studied (2-28 bar). The slurry concentration (8.8-21.6 wt%) was not found to affect the filtration behaviour. Chemical analysis of different layers of the filter cakes formed showed that xylan was distributed evenly on the solid lignin when both solids were precipitated together. It is thus likely that xylan is sorbed onto the surface of the lignin particles-agglomerates, opening their structure and increasing the contact area between solid and liquid during filtration: the flow resistance is increased. Furthermore, it was found that increasing the ionic strength of the slurries made the solid/liquid separation process easier. A plausible explanation for this is a decrease in the electrostatic repulsive interactions between the solids and the subsequent formation of a denser solid structure. Similarly, lowering the pH below the pKa values of the carboxylic acid groups of xylan made the lignin-xylan mixtures significantly easier to filter. Finally, the onset of precipitation (particle sizes ≥ 1µm) was not found to be affected by either the precipitation temperature (45-77 °C) or the addition of xylan; a broader particle size distribution was obtained when acidification was rapid compared to that performed slowly and stepwise.