Recovery of aliphatic low-molecular-mass carboxylic acids from hardwood kraft black liquor

•A novel separation process for the recovery of aliphatic carboxylic acids from black liquor was developed.•Formic and acetic acids were separated by direct distillation.•Low-molecular-mass hydroxy acids were separated as methyl esters by distillation.

During alkaline pulping of wood, substantial amounts of carbohydrates-derived aliphatic carboxylic acids (formic and acetic acids together with various hydroxy acids) are formed and dissolved into the cooking liquor (“black liquor”). These acids with low heating values are considered to be potential feedstocks for the chemical industry but, due to many factors, their straightforward recovery as free acids from black liquor (pH 13–14) is difficult. In this paper, we outlined the partial recovery of volatile formic and acetic acids from the acidified birch (Betula pendula) kraft black liquor (pH approximately 2.5) by distillation, and the partial recovery of significant low-molecular-mass α-hydroxy carboxylic acids (glycolic, lactic, and 2-hydroxybutanoic acids) by means of their volatile methyl esters. We noted that the maximum ester yields vary between 70% and 86% of the theoretical yield during the direct esterification (at 70 °C for 16 h) in the presence of a solid catalyst (a strong cation exchanger). Using this approach, we were able to separate the fractions of volatile and low-molecular-mass hydroxy acids from black liquor, while the crude faction of high-molecular-mass hydroxy acids (3,4-dideoxy-pentonic, 3-deoxy-pentonic, xyloisosaccharinic, and glucoisosaccharinic acids) remained as a residue.