Odour reduction of kraft lignin by wet oxidation

This study assesses wet oxidation of partially concentrated Kraft black liquor (40% dry matter) as a method for odour reduction of the lignin obtained from it by acid precipitation. To this end, and for the first time ever, the effects of wet oxidation temperature (120-150 C-o) and pressure (10-50 bar) on the odour profile of both the black liquor and the acid precipitated lignin were analysed. 2-methoxyphenol, dimethyl disulphide and dimethyl trisulphide were the main responsible compounds of the black liquor odour and, in the case of the precipitated lignin, aldehydes and organic acids also contributed. The most odourless lignin was the one precipitated from Kraft black liquor oxidised at the most severe conditions tested (150 C-degrees and 50 bar for 2 h). However, these also caused a reduction in the lignin yield, as well as significant compositional and appearance changes, thus limiting its use in certain fields.