Bayer poisons : degradation of Klason Lignin in Sodium Hydroxide at 145 deg. C

During bauxite refining, roots are dissolved in sodium hydroxide with the bauxite. This material is largely responsible for the poisoning activity of organic matter in the Bayer process. Although plant components are modified during extraction, they can be useful in demonstrating the role of different types of structural bonds during dissolution under bauxite refining. In this work, three Klason lignins have been prepared from Callitris rhomboidea plants (a gymnosperm) and the roots of Corymbia calophylla and Eucalyptus marginata (both angiosperms), and the dissolution of these materials in 3.5 M sodium hydroxide at 145 °C has been studied to understand β-O-4 aromatic carbon cleavage from syringyl and guaiacyl groups. The dissolution rates fall in the order Corymbia calophylla > Eucalyptus marginata > Callitris rhomboidea, which is the same order as their syringyl contents. By means of carbon balances and solid-state NMR spectroscopy, it has been established that, for Callitris rhomboidea, aromatic carbon is rapidly hydroxylated on initial dissolution and converted to other carbon types. This differs from the behavior of angiosperm lignins, where reactions of aromatic carbons are much slower.