Precipitation and valorisation of lignin obtained from South African Kraft mill black liquor

Abstract

Lignin is a phenolic, carbon-rich, heterogeneous polymer that has, in recent years, become a focus of research in the biorefinery field, as a potential source of chemicals, energy and materials. Generally, research on lignin in the biorefinery field mainly focuses on three aspects: understanding lignin and its chemistry; the forms in which it can be extracted/obtained from biomass or processing liquids, and ultimately: its valorisation into valuable compounds or materials. The pulp and paper industry kraft mill process produces a byproduct called black liquor, which contains lignin. In this form, lignin in the black liquor is processed in recovery operations where it is utilised as a source of energy for mill processes. However, it has become increasingly evident that this is an underutilisation of an important source of carbon. Additionally, kraft mills have limited capacity in their recovery operations and thus cannot process all the liquor. Hence there are opportunities to use the excess black liquor to collect lignin for use in biorefinery operations. Although biorefinery research is well-established, it has not received much attention in the South African context, which has prompted this research into valorisation of lignin in South Africa. In this thesis, the isolation and recovery of lignin from kraft mill black liquor was examined, an in-depth characterisation of the polymer was undertaken, and finally, a method for the potential valorisation of the lignin from a South African kraft mill was established. Lignin was precipitated from kraft mill black liquor by utilising sulphuric acid and three organic acids (acetic, citric, and formic acids). During recovery of the lignin, it was noticed that recovery of the precipitated lignin was a long and tedious process, as has been reported in the literature. Consequently, a novel method for overcoming this limitation was developed. The recovery of the polymer was achieved by the newly developed stepwise centrifugal recovery and washing method. The method was optimised to achieve comparable yields to utilising filtration immediately after precipitation. The precipitated lignin was characterised by a number of techniques, viz., Fourier transform Infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), size exclusion chromatography (SEC), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). SEC data revealed a narrow molecular weight distribution with a small polydispersity index in the lignin sample – all desirable features for the further valorisation of vi the lignin samples. Results from FTIR and NMR data showed the presence of both syringyl and guaiacyl functional groups thus confirming that the wood furnish used was a mixture of softwood (Pine) and hardwood (Eucalyptus). Py-GC/MS data was used to glean information on potential compounds that could be obtained via pyrolysis of the lignins: the major compounds that were identified were guaiacol compounds. Further studies on lignin precipitation were conducted on black liquors obtained from separate softwood and hardwood pulping at the mill. The studies were to evaluate and compare precipitation of lignin by acidification with sulphuric acid, versus acidification with organic acids, viz, acetic, citric, and formic acids. Precipitation of lignin with organic acids can be considered as a novel ‘green’ alternative to utilising sulphuric acid as a precipitating agent. The centrifugation recovery and washing method was optimised utilising all four acids and the results confirmed that the yields of the collected lignins were comparable with those obtained with vacuum filtration immediately after precipitation. The overall times for recovering the organic acid precipitated lignins varied between 2-4 hours depending on the acid. The lignin samples, together with commercial kraft lignin and Alcell® lignin used for comparison, were used for further investigation in valorisation studies of the lignin. All lignin samples were analysed by elemental analysis, as well as thermogravimetry (TG), under air and nitrogen environments to investigate their thermal characteristics. The morphology of the chars obtained from the TG studies under nitrogen were also studied by scanning electron microscopy (SEM). Elemental analysis showed a carbon, hydrogen, and oxygen (CHO) profile that was typical of values reported in the literature for hardwoods and softwoods. The sulphur content was higher in hardwood lignin than in softwood lignin samples, but more importantly, organic acid precipitated lignin samples exhibited lower sulphur content than lignins obtained from precipitation with sulphuric acid. The fixed carbon and char yields observed from TG studies under nitrogen for all samples indicated that the black liquor, and hence the lignin raw material obtained in this work was ‘good biomass’ for further processing into valuable materials. The SEM morphology of the char samples produced under nitrogen displayed differences in their swelling characteristics with differently formed cracks and cavities in the char particles. In this study, the pathway chosen for lignin valorisation was via production of chars and activated carbon. Lignin chars were prepared by carbonisation of the raw lignin samples vi under a nitrogen environment. Lower carbonisation yields were observed for hardwood (34 – 43%) lignin samples than their softwood (39 – 47%) counterparts, due to the higher oxygen content in hardwood lignins. The chars obtained prior to activation showed spherical particles for softwood samples and a blend of particles for hardwood samples under SEM. Activated carbon were produced. by means of physical activation through partial gasification of lignin chars with carbon dioxide. Only softwood lignin samples were utilised for the activation reactions, due to their higher char yields. After activation, there was a significant increase in the porous structure of the activated carbon compared with the preceding chars, with specific surface areas of 1000–1300 m2/g for all the softwood lignin samples studied. The micro-pores were also observed to show signs of broadening at the lower ends of the pores. Reactivity studies showed that the activation reactions were independent of temperature at higher temperatures. The gasification reactions had a high propensity toward mass-transfer limitation. The characteristics observed in the porous structure of the activated carbon obtained in this study displayed features suitable for applications such as adsorption, catalysis, energy storage, and molecular sieve uses. In summary, lignin from South African black liquor can be collected and valorised after precipitation with acid. A faster method for isolation, recovery and washing of the precipitated lignin has been developed. Acidification with organic acids produces high-quality lignins that have less sulphur contamination – unlike lignins obtained by acidification of black liquor with sulphuric acid. The collected lignins can be valorised by conversion into activated carbon that have the potential for use in various industrial applications. This work has resulted in 2 papers that have been published in peer-reviewed journals and one paper that has been submitted for publication in a peer-reviewed journal.