Separation of hemicelluloses and lignins from synthetic hydrolyzate and thermomechanical pulp mill process water via liquid-liquid extraction

Abstract

The separation of hemicelluloses from thermomechanical pulping (TMP) process water is of particular interest because it yields a biopolymer suitable for various value-added biomaterials production and reduces the organic loading on the water treatment facility. However, during the TMP process, hemicellulose is released in the process water at relatively low concentrations that are difficult to recover by many methods efficiently. Liquid-liquid extraction (LLE) has been deemed as a potentially viable separation technique in modern industrial processes for valuable materials recovery and undesirable impurities removal. In this work, the extraction of hemicellulose from the process water and synthetic hydrolyzate using LLE was investigated. In particular, the effects of the major experimental variables (the type of solvent, hydrolyzate to solvent volume ratio, and pH) on extraction performance were explored. The tested solvents have shown varying affinity and selectivity for the recovery of hemicellulose. It was found that the hemicellulose extraction efficiency of n-hexane (71.03%) and tributyl phosphate (TBP) (72.34%) was higher than that of 1-butanol (62.36%), and toluene (67.03%) at a solvent: hydrolyzate volume ratio of 1:3. Although TBP showed a high degree of hemicellulose extraction (72.34), it was characterized by a low selectivity coefficient, while n-hexane achieved the highest selectivity. The average selectivity coefficients of n-hexane were 7.3, 5.1, and 8.7 followed by toluene 2.7, 2.7, and 2.9 for pH values of 9.5, 7, and 4.3, respectively. Changing the pH of the hydrolyzate has resulted in varying effects depending on the type of the solvent used. The optimum extraction pH, phase ratio, and extraction time were at 4.3, 1:3 mL/mL and 30 min, respectively.