Abstract
Lignin has shown a great potential to produce fuels, value-added chemicals, and functional materials due to its high-energy density and intrinsic aromatic-based structure. Yet, the lignin precipitation of different biomasses needs investigation because most of the work has been performed on softwood and much less is known about hardwoods. In fact, the lignin from these two wooden biomasses vary in composition and pulping performance, which can reflect on lignin precipitation. Therefore, the present study investigated the precipitation and composition of 40 distinct kraft lignins obtained from pine, acacia, sweetgum, and eucalyptus black liquors. Two lignin fractions were precipitated at different pHs, according to known industrial lignin separation practices (pH = 9.5 and 2.5) from black liquors taken at different levels of pulping. Overall, lignin recovery increased with increasing lignin concentration in the black liquor, i.e., higher amounts of lignin were obtained at higher levels of delignification. In addition, pine lignins showed superior yields than the hardwoods and were around five times purer. Among the hardwoods, lignin recovery increased with the S–G ratio of the native lignin, and eucalyptus showed the best performance by achieving the highest yields and purities. Finally, the present work compared the lignin recovery yield and the purity of softwood and different hardwood lignins in a systematic way, which will increase awareness of this underutilized green material and could potentially increase the interest in establishing new lignin plants across the globe.
Highlights
Lignin precipitation from kraft black liquor (BL) has been discussed for almost a century; the complexity of this wood-based polymer and its variation among species, growth site, and modifications upon processing hinder its understanding and, its utilization
Because most of lignin precipitation studies have been performed on pine black liquor samples, this species was treated as the control sample
This method was developed using a calibration line established by the S–V ratio that was obtained via nitrobenzene oxidation and the S–G
Summary
Lignin precipitation from kraft black liquor (BL) has been discussed for almost a century; the complexity of this wood-based polymer and its variation among species, growth site, and modifications upon processing hinder its understanding and, its utilization. It enables the industry to become a biorefinery by allowing for the manufacture of multiple products from biomass In line with this concept, it has already been shown that 10–15% of the lignin dissolved in BL can be isolated without adversely affecting the chemical recovery [3]. The change in the pH level and ionic strength, and any change in lignin solubility, take place in the slurry but not in the filter cake or in the filter medium during washing [7] Another motivation for redispersion is lignin recovery because this step has been shown to improve lignin yield in the range of 6 to 7.5% [4]. The final step in the precipitation process is an additional filtration to obtain solid lignin. Because most of lignin precipitation studies have been performed on pine black liquor samples, this species was treated as the control sample
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