Size-exclusion chromatographic separation of hydroxy acids and sodium hydroxide in spent pulping liquor

• Hydroxy acids and NaOH of black liquor were separated using SEC.
• Co-operative sorption of NaOH enables using large injection volumes.
• Separation mechanism was explained considering size-exclusion of individual ions.

In this work, chromatographic recovery of hydroxy acids and cooking alkali (NaOH) from spent pulping liquor using size-exclusion chromatography (SEC) is investigated. Ultrafiltered black liquors from soda cooking of hardwood and softwood were used as feed and Sephadex G-10 as the stationary phase. Hydroxy acids were successfully separated from sodium hydroxide and the lignin content of the product fraction was reduced significantly. Fouling did not reduce the separation capability of the separation medium in extended runs with more than 40 consecutive injections. High column loadings, up to 25% of the bed volume, were found applicable for separation of authentic solutions without compromising the resolution. This was attributed to size-exclusion of individual ions which leads to co-operative sorption of NaOH in the presence of the sodium salts of hydroxy acids. The proposed mechanism was verified using data from experiments with model solutions.