Acid precipitation of kraft lignin from aqueous solutions: the influence of anionic specificity and concentration level of the salt

Tor Sewring,Hans Theliander

doi:10.1515/hf-2018-0302

Abstract

Abstract In this study, the formation of particles and evolution of the particle size distribution in the micron range were monitored in situ during acid precipitation of kraft lignin. The objective of this work was to study the influence of anionic specificity and the ion concentration level. The concentrations of ions in the solution were altered both in terms of the concentration of Na+ and the type of anion in the salt added ( SO 4 2 − ${\rm{SO}}_4^{2 - }$ and Cl−). The results indicate that a salting-out phenomenon occurred as NaCl was added (Na+ ≥2 mol kg−1 water) to the kraft lignin solution at high pH, but not when Na2SO4 was added. However, the onset pH of the formation of particles (≥1 μm), triggered by acidification, showed to be virtually non-specific to the anion but strongly dependent on the Na+ concentration. As the pH decreased further to below the onset pH of the formation of particles ≥1 μm, the chord length distributions (particle-size related) indicated that most of the volume of the precipitated kraft lignin (and thus possibly also the mass) may be found among the micron-sized particles, despite the fact that a relatively large number of submicron particles may also be present. The volume-based distributions tended to be wide at relatively low pH and high Na+ concentrations (e.g. pH 9.4 and 2.0 mol kg−1 water).

Highlights

The extraction of lignin from black liquor is an efficient way of increasing the material yield of the kraft pulp mill
The results indicate that a salting-out phenomenon occurred as NaCl was added (Na+ ≥2 mol kg−1 water) to the kraft lignin solution at high pH, but not when Na2SO4 was added
This study aims at exploring the ion-specific effects of SO24− and Cl− on the precipitation process of kraft lignin in a rather wide range of Na+ concentration levels, i.e. 1.0–5.8 mol Na+ kg−1 water, with either SO24− or Cl− as the anion in the added salt

Summary

Introduction

The extraction of lignin from black liquor is an efficient way of increasing the material yield of the kraft pulp mill. Most of the studies found in the literature (Norgren et al 2001a, 2002; Fritz et al 2017) focusing on the phase behavior of kraft lignin have been conducted in dilute solutions at concentrations of salt and lignin of up to about 1 mol dm−3 and 3 g dm−3, respectively. At these conditions, it has been suggested that electrostatics has a significant impact on the phase behavior of kraft lignin (Norgren et al 2001a). Results reported in the literature have shown that the kinetics of the precipitation is influenced by the composition of the ionic species in the solution, i.e. display ion-specific effects (Norgren et al 2002; Norgren and Mackin 2009)

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Methods

Conclusion