UNDERSTANDING THE ORIGIN AND MECHANISM OF HYDROGEN PRODUCTION IN THE BAYER PROCESS

Costine, A., Schibeci, M., Loh, J.S.C. and Power, G.

Hydrogen production in Bayer process digestion is known to result from the reactions with sodium hydroxide of organic compounds from the bauxite and the liquor, even in the presence of added oxygen. A fundamental understanding of the degradation reactions of organic compounds that produce hydrogen is important in the development of improved wet oxidation processes in the Bayer industry and for refinery safety in general.

The hydrogen production potential of a bauxite ore was assessed over a range of temperatures (150–275 °C) and sodium hydroxide concentrations (0–6 M). Measurements on a range of lateritic bauxites showed that the hydrogen production potential is not simply related to the total organic carbon content of the parent bauxite, indicating that the types of organic compounds present are important. Therefore, the degradation of a range of organic compounds which represented various structural classes that could be present in Bayer process digestion was studied under similar conditions. The compounds investigated include aliphatic and aromatic carboxylates, aliphatic and aromatic hydroxycarboxylates, phenols and unsaturated carboxylates.

These results demonstrate a strong dependence of hydrogen production on the structure of the organic compounds involved, and provide further evidence in support of the ionic degradation mechanism involving base-catalysed oxidation by water which we have proposed previously.