Decomposition of Bayer process organics: Low-molecular-weight carboxylates

Abstract

The degradation of twenty-one low-molecular-weight organic carboxylates has been studied at 90 and 180 °C, in a synthetic Bayer liquor consisting of 6 mol kg − 1 aqueous NaOH solution, for periods of up to 36 days. The reactions were monitored and the major degradation products identified by HPLC and NMR spectroscopy. The carboxylates chosen for the study were either possible intermediates or known products arising from the decomposition of organic matter in the Bayer process. Aliphatic carboxylates without hydroxyl substituents were stable at 90 °C but decomposed at 180 °C, except for formate, acetate, oxalate and succinate. The corresponding aromatic carboxylates were stable even at 180 °C. Both aliphatic and aromatic carboxylates with hydroxyl substituents were unstable at 90 °C except for lactate and 4-hydroxy-benzoate. The most frequently detected decomposition products for both aliphatic and aromatic carboxylates were formate, acetate, oxalate, succinate and lactate. Phenolate was also observed for some aromatic carboxylates. These products are briefly discussed with reference to possible mechanisms for the degradation reactions.