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

Licentiate thesis, 2014

The Kraft pulping technology has been thoughtfully investigated and developed over the past hundred years and more; it is currently the most commonly-used method of pulping in the world. A new and promising opportunity for Kraft pulp mills would be 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 chem icals. The LignoB oost process is a new technique designed to extract lignin during the Kraft process with a high degree of purity, making it suitable for the manufacturing of e.g. carbon fibres. Filtration, performed by dead-end fi ltration, is one of the key step s of the LignoBoost process. The process has been recently implemented on a large scal e; in order to optimize the efficiency of the filtration stage further, obtaining information pert aining to both the physical properties of the extracted lignin and the filtration prop erties of the cake formed is of great interest. Furthermore, it is necessary to deepen knowledge regarding the impact of inter-particle interactions, such as electrostatic forces, which occur during filtration as well as to determine the local filtration properties, for the development of better models of the filtration process. The work presented here is based on two main studies. The first investigated the local and average filtration properties of the cake formed from a softwood lignin extracted using the LignoBoost process; the material was also characterized using a number of different methods. The second studied the influence of the electrostatic interactions on the local filtration properties of titanium dioxide, the model material that was chosen, by altering the ionic streng th of the initial filtration slurry. The results showed that the LignoBoost lignin inves tigated was an easy-to-f ilter material, forming weakly compressible filter cakes over the filtration pressure range studied of 2 to 28 bar. Furthermore, it was shown that the initial concentr ation of the lignin slurry did not influence the filtration properties over the rang e investigated, i.e. 6.7 to 17 vol %. From the titanium dioxide filtration experiments, it was shown that the increase in the ionic strength of the suspension decreased both the solidosity and the filtration resistance of the cake formed whilst increasing its compressibility. These results could be explained by an extensive agglomeration, with the formation of bigger and looser agglomerates, which occurs when the electrostatic repulsive interactions between the particles are decreased. Several constitutive relationships for the modelling of the filtration process could also be f itted successfully to the local data obtained during the filtration of LignoBoost lignin and titanium dioxide.