Recovery of lignin from Kraft black liquor in the methods oxygen and carbon dioxide pretreatment from hardwood Kraft black liquor

Black liquors are produced as a byproduct of chemical pulping and About 500 million tons of black liquor are produced annually in pulp mills around the world, of which about 95% for fuel consumption and only 2% for the production of special products with higher added value are used. In order to recover more effectively, acidic precipitation of black liquor using gas injection pretreatment has been studied recently. Previous studies have shown the advantage of carbon dioxide injection as pretreatment to reduce sulfuric acid consumption, increase yield and purity of lignin. The Iranian’s pulp and paper mills have the potential to recover lignin, which is burned as fuel. Therefore, this study was performed to investigate new methods of lignin recovery on hardwood Kraft black liquor in Iran. Pretreatment was done using carbon dioxide injection in lignoboost method as well as two-stage oxygen-carbon dioxide injection in Lignoforce method under 2 bar pressure and 80 ° C on Kraft black liquor, Then, the pH was decreased to 2-3 using 6 M sulfuric acid to precipitate the lignin, which was then separated by filtration and washed with water. Finally, the recovered lignin was dried at 50º C. TGA, FTIR, SEM and AFM tests were performed to evaluate the thermal stability, chemical structure, morphology and dimensions of recovered lignin. Also, acid consumption, yield and ash content were calculated. The amount of acid consumed and the ash percentage of lignin recovered from gas injection processes was less than the non-injection method. In addition, the oxygen-carbon dioxide injection method showed higher efficiency compared to the carbon dioxide injection. FTIR results demonstrated a similar chemical structure in the recovered lignin. AFM and SEM images of lignin produced by pretreatment with carbon dioxide injection showed larger particles as well as higher thermal stability compared to oxygen-carbon dioxide injection.