Equilibrium of lignin precipitation: The effects of pH, temperature, ion strength and wood origins

The kraft process is the predominant method for producing pulp in Scandinavia. However, the material efficiency in modern kraft pulp mills for pulp production is only 40−55% and the final product consists mainly of cellulose. A novel process called “LignoBoost” has been introduced recently for extracting lignin from alkaline black liquor. The separated lignin can either be utilized as solid fuel or raw material of other value-added products such as carbon fibre or phenols, which is one step towards a combined pulp and biorefinery process. The LignoBoost process consists of four major steps: precipitation, filtration, re-dispersing and final washing. Whilst filtration and washing have already been investigated extensively, more knowledge needs to be gained of the precipitation step. In this work, lignin precipitation was performed by the acidification of black liquor at various process conditions, namely pH, temperature and ion strength, and the influences exerted by these parameters on the equilibrium of lignin precipitation were investigated. Carbohydrate analysis was performed on the precipitated solid and lignin-lean liquor, and the average molecular weight of the precipitated lignin was determined. The content of the functional groups (i.e. phenolic and methoxyl) in the precipitated lignin was analyzed and, finally, new parameters in a given model from a previous study for estimating the precipitation yield of lignin were provided and evaluated.The results show that the precipitation yield of lignin increases with decreasing pH and temperature and/or with increasing ion strength of the black liquor used. The concentration of carbohydrates in lignin decreases with decreasing pH or with increasing temperature, and that an increasing amount of lower molecular weight lignin is precipitated at a higher precipitation yield. Furthermore, it was found, for the experimental conditions chosen, that the precipitation pH has a much stronger influence than the temperature and the ion strength on the precipitation yield and the average molecular weight of the precipitated lignin. According to 1H and 13C NMR spectra analysis, the content of phenolic and methoxyl groups in the precipitated lignin increases with increasing precipitation yield