Fundamentals of wet oxidation of bayer liquor organics: Effect of oxygen on malonates

Organic impurities in Bayer liquor adversely affect a number of the processes used to refine alumina. Development of processes for removing these impurities from Bayer liquor is therefore of great interest to the alumina refining industry. Removal processes based on oxidation that do not involve the introduction of foreign species have received the most interest for achieving this goal. Of such processes, wet oxidation has received particular attention. Better understanding of the reaction mechanisms involved in wet oxidation is a key aspect in the development of this technology. Sodium malonate, which is present in most Bayer liquors, has been shown to be unusually reactive under typical wet oxidation conditions. In a previous study the formation of a carbanion has been confirmed as the first step in the oxidation mechanism of malonate, as a consequence of the slight acidity of the alpha-hydrogen atoms. The observation of a linear dependence of the rate constant of malonate on the oxygen partial pressure is reported here. The current work demonstrates that the second step is a 1:1 reaction of the carbanion with oxygen, which most likely results in the formation of alkyl and oxygen free radical intermediates. Substituting an electron-donating group at the alpha-carbon of malonate reduces the effect of the oxygen partial pressure on the oxidation rates of the substituted malonates, consistent with the decreased acidity of the alpha-hydrogen. Analysis of the final reaction products indicates that there are two possible pathways for the subsequent reactions of the carbanions, the dominant of which results in the formation of formate and oxalate in all cases, while the minor pathway produces formate, acetate or propionate, depending on the starting material.