Separation and recovery of alkali lignin and NaOH based on size exclusion methodology

Alkali lignin and NaOH are two main components in the black liquors from alkaline pulp. Adding sulfuric acid directly to precipitate lignin results in high consumption of acid as well as production of waste salts (Na₂SO₄). In this work, a benign process was proposed to separate alkali lignin and NaOH using ultra-high crosslinked non-ionic microporous resin Mn270 as an adsorbent and water as an eluent. Special attention was paid to explore the separation mechanism and establish a proper chromatographic model for this process. First, the adsorption isotherms showed the presence of alkali lignin increased the adsorption capacity of NaOH on the Mn270 resin. The separation mechanism was assumed to be size exclusion. Simultaneously, the alkali lignin promotes the adsorption of NaOH on the resin, which was verified by analysis of the interaction energy among NaOH-resin-Alkali lignin from a molecular dynamics simulation point of view. Afterwards, a linear driving force model combined with binary isotherms was used to predict the breakthrough / elution profiles of the two components on the column system fairly well. Finally, the alkaline pulp was used as a feed solution and got a good separation effect. After removal of NaOH, the amount of sulfuric acid and the corresponding salts were reduced by 37.86% in subsequent acid precipitation process.