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Recovery of Lignin

Abstract Recovery of lignin from different processes—kraft process, steam explosion process, organosolv process—is discussed in this chapter. Commercial suppliers of lignin are also listed.











Keywords Lignin Lignin recovery Kraft process Steam explosion process Organosolv process Kraft lignin Steam explosion lignin Organosolv lignin










Several studies have investigated the possibility of using various technical lignins as raw material for carbon fibre production, e.g. lignosulfonates, organosolv lignins, steam explosion lignins and kraft lignins (Otani 1981; Kadla and Kubo 2002; Uraki et al. 1993; Sudo and Shimizu 1992; Sudo et al. 1993; Kubo and Kadla 2005; Attwenger 2014; Wang and Lu 2010; Stelte 2013). Organosolv lignin is generally much purer than the commercial kraft lignin (Huang 2009). Steam explosion and organosolv processes have been known in the pulping industry for decades and have a better environmental impact than kraft or sulphite processes. By comparison with cellulose which is commercially used for pulp and paper production, lignin has very limited applications as a chemical and is not intentionally produced in industry. It generally originates in huge amounts as a side product when cellulose is isolated from the lignocellulosic material during the pulping process. There are various isolation technologies based on chemical or mechanical treatment for the separation of cellulose and hemicellulose from lignin (Smook 1992; Biermann 1996). Whereas mechanical treatments are used to mechanically separate the constituents from each other, the predominant chemical processes usually use harsh process conditions and pulping chemicals that uniquely alter the structure of lignin (Zakzeski et al. 2010). They either aim at the removal of cellulose and hemicellulose by solubilization leaving an insoluble lignin fraction behind or vice versa. Lignin from potential biorefineries that produce ethanol by fermentation of cellulosic sugars is derived from processes of the first category. The pulp and paper industry on the other hand uses chemical pulping processes resulting in an insoluble fibrous pulp and lignin-rich black liquor. Kraft pulping and to a little extent sulfite pulping are the predominant processes for wood in industry. © The Author(s) 2017 P. Bajpai, Carbon Fibre from Lignin, SpringerBriefs in Materials, DOI 10.1007/978-981-10-4229-4_6

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6 Recovery of Lignin

Other pulping processes are so far mainly used in pilot-scale plants or in research (Lora 2008; Sixta 2006; Gosselink et al. 2004). The structure and properties of lignins vary based on different plant source they are separated from, such as hardwood, softwood, wheat straw or bamboo.

6.1

Kraft Lignin

The kraft process is also known as the sulphate process. It is the dominating chemical pulping technology worldwide with over 22 million tonnes produced in Europe as of 2011 (FAOSTAT 2012). The process is based on an alkaline solution of sodium hydroxide and sodium sulphide, which degrades the carbohydrates by