Decomposition of Bayer process organics: Phenolates, polyalcohols, and additional carboxylates

The degradation of nineteen low-molecular-weight phenolates, polyalcohols and selected aliphatic and aromatic carboxylates of relevance to the Bayer process has been studied in 6 mol kg−1 NaOH(aq) at 90 °C for up to 36 days, and (for some species) at 180 °C for up to 12 days, using HPLC and 13C NMR spectroscopy. Aliphatic polyalcohols degraded readily at 90 °C to lactate, oxalate, acetate, and formate. As observed previously, aliphatic carboxylates with hydroxyl groups also degraded readily at 90 °C but there is evidence that the position of the hydroxyl group may be important. The observed degradation products for most, but not all, of these species can be explained in terms of well-known organic reaction mechanisms. Phenolate and 5-hydroxyisophthalate were stable at 180 °C but other phenolic species degraded partially at 90 °C. However, the reaction products could not be identified and no trends in reactivity were discernible. Consistent with previous studies both aliphatic and aromatic carboxylates without hydroxyl groups were generally stable in NaOH(aq) even at 180 °C.

Research highlights
► Behaviour of 19 phenolates, polyalchohols & carboxylates in hot NaOH(aq) is reported.
► Reactivity is related to presence and position of hydroxyls and of leaving group.
► Lactate was detected from the degradation of polyalcohols.
► Relative stability of carboxylates and unsubstituted aromatics was confirmed.