MECHANISMS OF THE WET OXIDATION OF MALONATES AND COOXIDATION OF BENZOATE IN HIGHLY ALKALINE SOLUTION

Dong, J., Power, G., Loh, J., Tardio, J., Vernon, C. and Bhargava, S.

Organic compounds in Bayer liquor cause significant reductions in alumina production and hence the development of processes for removing them from the liquor is of great interest to alumina refineries. Of the various organics removal processes known, wet oxidation has received significant interest. Further development of this technology requires an improved understanding of the underlying chemistry of the wet oxidation of Bayer organics. A significant step has been taken through the study of the oxidation mechanism of sodium malonate (a common Bayer organic). We have previously confirmed that the first reaction step in the wet oxidation of sodium malonate and malonate derivatives is the formation of a carbanion, as a consequence of the slight acidity of the α-hydrogen(s) in these compounds. In this study the subsequent reaction step, which has been proposed to involve the formation of alkyl and peroxyl radicals via the reaction between the malonate carbanions and oxygen, is investigated. Consistent with this, the results show a linear dependence of the oxidation rate constant of the malonates on the oxygen partial pressure. The proposed mechanism is further supported by the reaction products identified, and by the effect of adding benzoate, a known free-radical scavenger. Malonate appears to catalyse the reaction of benzoate in the presence of oxygen, causing a co-oxidation effect, which is consistent with the presence of free radical intermediates in the overall mechanism.