Lignin solubilisation

Lignin is water-insoluble and it contains only a few hydrophilic functionalities (primarily the phenolic groups). In order to increase its solubility, aqueous pulping processes therefore seek either to introduce water solubilising groups, or to reduce the degree of polymerisation or both. 

Recovery of inorganic chemicals is crucial to the cost effectiveness of the Kraft process. The black liquor which is obtained from delignification is rich in solubilised lignin and carbohydrate degradation products and, after concentration, is combusted in a recovery furnace. The Carbon dioxide which is produced during combustion converts unused sodium hydroxide into sodium carbonate. In addition, the sodium sulfate is converted, under the reducing atmosphere of the furnace, to sodium sulfide. 

Pressure filtration of the liquid is very difficult due to the complex interaction of the char and pyrolytic lignin, which appears to form a gel-like phase that rapidly blocks the filter. Modification of the liquid micro-structure by addition of solvents such as methanol or ethanol that solubilise the less soluble constituents will improve this problem and also contribute to improvements in liquid stability as described below. 

A patent on biomass fractionation using ionic liquids describes extraction of cellulose from lignocellulose by dissolution in [C4mim CI, followed by addition of a caustic solution to precipitate cellulose while solubilising lignin and hemi-cellulose [191]. One of the main advantages of the process was the partial phase separation between the ionic liquid and the aqueous caustic phase, which enabled easier recycling of the ionic liquid. 

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