Recovery of organic acids from pre-treated Kraft black liquor using ultrafiltration and liquid-liquid extraction

Abstract

Kraft black liquor (BL) is a toxic residue from paper manufacture that can be revalorized to obtain high value-added products. In this study, different integrated processes, consisting of a pre-treatment (lignin recovery [LR] or partial wet oxidation [PWO]) followed by ultrafiltration (UF) and liquid–liquid extraction (LLE), have been evaluated in order to recover short-chain organic acids (OA) from BL.After the UF step, a partial purification of OA was observed. Thus, the COD fraction attributable to OA increase 26.0%, 36.6% and 11.3% for untreated BL, BL after lignin recovery (LRBL) and partially oxidised BL (WOBL), respectively.The final LLE step was optimised analysing different combinations of extractants (trioctylamine, trioctylphosphine oxide and tributylphosphate), diluents (n-hexane and heptane) and temperatures (25, 40 and 55 °C), concluding that the most suitable solvent was 30% trioctylamine in heptane (v/v) at 40 °C, in which 76% of the OA mass was recovered in LRBL, 72% in BL and 68% in WOBL.The pre-treatment of BL improved the final recovery yield of OAs, increasing from 24.7% to 28.1% and 29.4% through PWO and LR, respectively. Therefore, LR was considered the most suitable pre-treatment due to its lower operating cost, widespread use, and greater recovery of OA.