Wilson, M.A., Ellis, A.V., Kannangara, K., Whelan, T.J.

Carbohydrate and lignin dissolution has been studied under Bayer conditions. In the Bayer process for preparing alumina from bauxite small molecular weight compounds such as C2-C6 mono-, di- and tri- aliphatic carboxylic acids, which have hydroxy substituents, are formed from humic material. While involatile as anions under refinery conditions, upon acidification they become volatile and can be lost either in processing or in work up of process liquors in the laboratory. The origin of these organics is unknown, however this paper shows that they are directly derived from the carbohydrate fraction in the vegetation that is dissolved with the bauxite in preparing process liquors. Glucitol is shown to be a particularly stable carbohydrate to digestion and may therefore play an important role in Bayer poisoning processes. Using 13C labelled compounds, rearrangements in alkaline solution have been demonstrated under Bayer conditions. They show that in the formation of lactate from glucose the carboxylate (COO-) carbon is formed preferentially from C1 carbons but methyl (CH3) carbon is formed preferentially from C6 carbons. From C1 labelled glucose, labelled carbon ends up as carboxylate (COO-) carbon in glycolate, but from C6 labelled glucose the labelled carbon ends up as alcoholic (CH2OH) carbon. The production of acetate and formate also discriminates between the C1 and C6 label. For the lignins the dissolution rates are 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 angiosperm lignins where reactions of aromatic carbons are much slower.