Characterization of Hardwood Soda-AQ Lignins Precipitated from Black Liquor through Selective Acidification

Abstract

In the development of integrated biorefinery process alternatives to produce value-added by-products, various black liquors from sulfur-free pulping processes offer potential feedstocks for recovering their main chemical constituents, lignin and aliphatic carboxylic acids. In this study, lignin fractions were obtained from silver birch (Betula pendula) sodaanthraquinone black liquor by carbonation (pH to about 8.5) or by acidification (pH to about 2) with H2SO4 after carbonation or directly. These fractions were characterized by Fourier transform infrared (FTIR), ultraviolet (UV), energy dispersive X-ray fluorescence (ED XRF), and 13 C nuclear magnetic resonance ( 13 C NMR) spectroscopy. In addition, the molecular weight distributions of these lignin fractions were determined. All the experimental data clearly suggested that only small differences between the precipitated lignins existed, and thus, their equal chemical utilization seems possible.

A quantitative comparison of the precipitation behavior of lignin from sweetgum and pine kraft black liquors

“…In general, at higher pH, the fraction of lignin that precipitates has a higher molecular weight, whereas, at lower pH, the lignin that precipitates has a lower molecular weight (García et al 2009;dos Santos et al 2014;Kumar et al 2016). A decrease in the amount of lignin precipitated at pH 9.5 between 120 to 150 min (487 and 800 H-factor, respectively) was observed, followed by an increase in the fraction obtained at pH 2.5.…”

“…It was initially thought that the lignin precipitation methods developed and proven to work for the majority of softwood species would be acceptable for hardwood species. However, investigations into various hardwood lignin compositions have used the softwood precipitation techniques and obtained unexpectedly low lignin yields (Santos et al 2011(Santos et al , 2012Kumar et al 2016;Gomes et al 2018;Giummarella et al 2019). Lack of understanding of hardwood lignin precipitation is a significant hurdle toward fully characterizing and utilizing this raw material, which prompted this study.…”

Lignin recovery from spent alkaline pulping liquors using acidification, membrane separation, and related processing steps: A review

“…Primarily due to different determination methods, there are only limited reliable data available on the molecular mass distribution of various lignins in black liquors during alkaline delignification. In practice, these data would be of great importance when considering the mass transfer aspects, but also with respect to the full-scale separation of lignin from different black liquors; for example, when predicting the lignin portion that can be separated by acid precipitation at different pHs (Pakkanen and Alén 2012;Kumar et al 2016). As general trends, it is known that milled wood lignin preparations have weightaverage molecular masses of between 15,000 and 20,000 g/mole, which is four to five times that of kraft lignin (Glasser et al 1983) and, on the other hand, that soluble hardwood lignins typically have slightly lower molecular masses than softwood lignins (Goring 1971).…”

“…Studies have shown that CO2 can be effective for reducing the pH of black liquor to values in the approximate range of 7 to 10.5 (Tomlinson and Tomlinson 1946;Alén et al 1979Alén et al , 1985bWeinhaus et al 1990;Howell and Thring 2000;Theliander 2001, 2007;Wallmo et al 2007Wallmo et al , 2009aNagy et al 2010;Theliander 2011, 2015; Velez and Thies 2013; Lake and Blackburn 2016; Kouisni et al 2016;Kumar et al 2016). Stocklosa et al (2013) found that although CO2 readily reduced the pH to 9.5, the majority of softwood kraft lignin already became precipitated when the pH was lowered to a level between 10 and 11.1, which suggests that their specimen was mainly stabilized by phenolate functional groups having a pKa of about 10 or possibly higher ( Table 2).…”

“…Lignin also can be utilized as a portable fuel (Uloth and Wearing 1989b;Tomani 2010) or as feedstock for preparation of biodiesel, which in some cases may have higher value than combustion of the material as part of a chemical recovery process in a pulp mill. Finally, there may be opportunities to rethink the entire pulping process as a biorefinery in which multiple chemical compounds, including breakdown products of polysaccharides, are fractionated, making them available as replacements for petroleum-based chemicals (Alén et al 1989;Moshkelani et al 2013;Alén 2015;Kumar et al 2016;Teguia et al 2017).…”