Spectra of Lignins from Pulp and Spent Liquor

Abstract : Lignin samples isolated from pulp and spent liquor of bisulphite and sulphate cooks on spruce in successive stages of delignification have been characterized with respect to ultraviolet and visible light absorption. In the visible part of the spectrum lignin from sulphate spent liquor shows the strongest light-absorption followed by lignin from sulphate pulp, bisulphite pulp and bisulphite spent liquor. Measured at 457 nm the absorptivity of lignin isolated from bisulphite pulp decreases in the beginning of the cook passes a minimum and then increases. These changes are in qualitative accordance with those obtained earlier by direct measurements on pulp. The differences observed in the properties of lignin isolated from pulp and spent liquor strongly suggest that there is a fundamental difference between residual lignin in pulp and lignin dissolved during the cook. The implications for pulping technology of the results obtained are discussed. (Author)