Wet oxidation of precipitation yield inhibitors in sodium aluminate solutions: Effects and proposed degradation mechanisms

Organic compounds with adjacent hydroxyl groups inhibit gibbsite (“hydrate”) precipitation in the Bayer process for the production of alumina. These hydrate yield inhibitors can be degradation products from the breakdown of larger organic molecules extracted from bauxite ore in digestion. Wet oxidation can be used to remove organics from Bayer liquor, but the nature and amounts of products are difficult to predict, due to a lack of understanding of the complex mechanisms of degradation.This fundamental study was conducted to determine the susceptibility of a set of known yield inhibitors to destruction under aerobic wet oxidation conditions and compare the degradation of the compounds under anaerobic conditions typical of normal Bayer digestion. It is shown that of the yield inhibitors investigated, aliphatic compounds with more than 5 carbons in a chain, and aromatic compounds, are readily destroyed by wet oxidation at 165 °C in the presence of oxygen. Members of the C4 family of inhibitors are more stable and require higher temperatures for full destruction. Sodium oxalate was found to be the main organic degradation product from wet oxidation of the hydrate yield inhibitors studied.A general reaction mechanism involving base-catalysed oxidation by water is proposed. This mechanism explains the formation of the degradation products observed. It also explains how organics can be oxidised anaerobically, and how hydrogen is produced under Bayer process conditions.The implications of the findings are discussed in relation to the application of wet oxidation to Bayer process refineries.

Research highlights
► Hydrate yield inhibitors are unstable under wet oxidation conditions at 280 °C
► Inhibitors with < 4 carbon atoms are stable under wet oxidation conditions at 165 °C
► Proposed mechanisms explain oxidation of organics in the absence of oxygen as oxidant
► Proposed mechanisms explain hydrogen production under Bayer digestion conditions