Partial Recovery of Aliphatic Carboxylic Acids and Sodium Hydroxide from Hardwood Black Liquor by Electrodialysis

The partial recovery of sodium as NaOH and sodium-free aliphatic carboxylic acids (formic and acetic acids together with various hydroxy carboxylic acids) from alkaline hardwood black liquor (BL, pH about 14) was studied by using electrodialysis (ED). In the first phase, the lignin was partly (about 59% of the initial lignin) precipitated from BL by carbonation (pH to 8.5). Furthermore, the carbonated liquor, mainly containing NaHCO3/Na2CO3 and Na salts of aliphatic carboxylic acids and lignin, was subjected to ED for recovering NaOH and sodium-free aliphatic acids. In the second phase, the carbonated BL was further acidified with H2SO4 (pH to about 2.5) for precipitating more lignin (about 90% of the initial lignin) with the simultaneous liberation of aliphatic acids. The Na2SO4 that was formed was then separated from the acidified liquor by precipitation with methanol and treated by ED to produce NaOH and H2SO4. For ED experiments, a batch laboratory-scale apparatus containing cation selective and bipolar membranes was used at 20 V. Within an ED treatment time of 5 h the recovery rate of sodium from carbonated BL, model Na2SO4, and precipitated Na2SO4 was 0.044, 0.110, and 0.104 mol Na/h, corresponding to an energy consumption of 1.76, 0.38, and 0.43 kWh/mol Na, respectively. In each case, the total amount of sodium recovered was about 90%.