On the local filtration properties during cake filtration: Studies on LignoBoost lignin and the influence of ionic strength

The Kraft pulping technology has been thoughtfullyinvestigated and developed over the pasthundred years and more; it is currently the mostcommonly-used method ofpulping in the world. Anew and promising opportunity for Kraft pulp millswould be to take a step towards becomingbiorefineries by implementing technologies ableto extract and convert the organic by-products,such as lignin, into a wide rangeof value-added products and chemicals. The LignoBoost process isa new technique designed to extract lignin duringthe Kraft process with a high degree of purity,making it suitable for the manufacturing of e.g. carbon fibres.Filtration, performed by dead-end filtration, is one of the key steps of the LignoBoost process. Theprocess has been recently implemented on a large scale; in order to optimize the efficiency of thefiltration stage further, obtaining information pertaining to both the physical properties of theextracted lignin and the filtration properties of the cake formed is of great interest. Furthermore, it isnecessary to deepen knowledgeregarding the impact of inter-particle interactions, such aselectrostatic forces, which occur during filtration as well as to determine the local filtrationproperties, for the development of better models ofthe filtration process.The work presented hereis based on two main studies. The first investigated the local and average filtration properties of thecake formed from a softwood lignin extracted usingthe LignoBoost process;the material was alsocharacterized using a number ofdifferent methods. The secondstudied the influence of theelectrostatic interactions on the local filtrationproperties of titanium dioxide, the model materialthat was chosen, by altering the ionic strength of the initial filtration slurry.The results showed that the LignoBoost lignin investigated was an easy-to-filter material, formingweakly compressible filter cakesover the filtration pressure range studied of 2 to 28 bar.Furthermore, it was shown that the initial concentration of the lignin slurry did not influence thefiltration properties over the range investigated, i.e. 6.7 to 17 vol%. From the titanium dioxidefiltration experiments, it was shown that the increase in the ionic strength of the suspensiondecreased both the solidosity andthe filtration resistance of thecake formed whilst increasing itscompressibility. These results could be explainedby an extensive agglomeration, with the formationof bigger and looser agglomerates, which occurswhen the electrostaticrepulsive interactionsbetween the particles are decreased. Several constitutive relationships for the modelling of thefiltration process could also be fitted successfully to the local dataobtained during the filtration ofLignoBoost lignin and titanium dioxide