Sorption and Desorption of Black Liquor Xylan onto Cellulose Fibers – a New Separation Technique

The potential of xylan as a polymer in different new materials is large and not yet utilized. The availability in kraft pulp mills is large as the polymeric xylan dissolved in process liquors corresponds to up to 160 kg/adt and is today burnt to recover energy. A pre-requisite for using black liquor xylan in high-value products is to purify it from inorganic material and lignin in a technically effective and economical feasible way. This study combines two known properties of xylan: its affinity to cellulose and its solubility in alkali. The aim is to design an efficient separation process to produce black liquor xylan of high purity. A very pure xylan was reached with as low lignin contents as 1%. The hydroxide ion concentration was varied in the sorption step and it was found that an increase in hydroxide ion concentration resulted in a decreased amount of isolated xylan, decreased degree of substitution of uronic acids, but no effect on the lignin content and ash content was seen.