Bayer Poisons: Degradation of Angiosperm and Gymnosperm Water-Soluble Extracts in Sodium Hydroxide at 145 °C Amanda V. Ellis, † Michael A. Wilson,* ,‡ and Kamali Kannangara † Department of Chemistry, Materials and Forensic Science, University of Technology Sydney, P.O. Box 123, Broadway, NSW 2007, Australia, and Office of the Dean, College of Science Technology and Environment, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia In the Bayer process of preparing alumina from bauxite small molecular weight compounds such as C 2 -C 6 , mono-, di-, and trialiphatic carboxylic acids which have hydroxy substituents are formed from a humic material. While involatile as anions under refinery conditions, upon acidification they become volatile and can be lost either in processing or in workup of process liquors in the laboratory. The origin of these organics is unknown; however, this paper shows that they are directly derived from the water-soluble (mainly carbohydrate) fraction in the vegetation that is dissolved with the bauxite in preparing process liquors. Glucitol is shown to be a particularly stable carbohydrate to digestion and may, therefore, play an important role in Bayer poisoning processes. Introduction The Bayer process prepares aluminum hydroxide trihydrate by high-temperature (145-245 °C) partial dissolution of aluminum oxides in bauxite ore in a strongly alkaline sodium aluminate solution (3.5-5 M). Iron oxides are removed by filtration, and the aluminum trihydrate product is crystallized out by evaporation. Alumina as the final product is then prepared by later calcination. Organic compounds in bauxite result in both small (<1000 Da) and large (>1000 Da) molecular weight organic compounds in the refinery circuit. Typi- cal concentrations of these accumulated organic degra- dation products can reach up to 100 g/L present as sodium salts. 1 The structure of some of the small organic compounds present such as oxalate, formate, and ace- tate are known to significantly interfere with the precipitation of this hydrate. 1-9 For example, sodium oxalate can be included in the product and hence can reduce the alumina purity. 1 Sodium oxalate has been implicated in the coprecipitation of humates of high molecular weight and low hydrophilicity. 10-12 Accurate estimates of oxalate 1,10 are possible, but many of the other compounds present are difficult to measure with precision. Isolation of extracts after acidification and evaporation often leads to compound loss and poor mass balances. 13 A logical approach to understanding the composition of organics in the Bayer process is to study the decom- position of plant remains that are present in bauxite so that the role of each plant component can be elucidated and the pathways of decomposition determined. This is the approach used here. One important component is carbohydrates. Gymnosperms contain up to 20% and angiosperms 20-30% pentosanes and hexosanes. The cellulose content for both ranges from 40 to 60%. 14 These compounds must, therefore, contribute significantly to the organic matter in the Bayer liquors. One way of studying the role of carbohydrates in Bayer refining is to study the water-soluble fraction of plant remains in the bauxite because this is readily removed from plants and is believed to be almost entirely carbohydrate. The oxidation under mild alkaline conditions of carbohydrates is not well understood. It has been shown that sodium hydroxide treatment yields a multitude of monocarboxylic acids (in particular iso-, meta-, and saccharinic acids (compounds I-III, respectively) with further degradation to give lactic acid. 15,16 The effect of the carbohydrate concentration on the formation of lactic acid with a 3-10 M sodium hydroxide solution at room temperature has been studied. 17,18 At higher temperatures (262-294 °C), xylose in a 2.5 M sodium hydroxide solution is instantaneously converted to lactic, formic, acetic, propionic, and glycolic (R-hydrox- acetic acid) acids in up to 84% total yield (g/g) depending on conditions. 19 Other products are formed, probably carbon dioxide. Xylan (IV) and cellulose (V) decomposition in 0.25-2 M sodium hydroxide at 150-180 °C has been studied 20 and the kinetics of loss of these substances described by exponential decay constants. However, yields of lactic acid or other small molecules were not reported in the above work. Little is known of the products under harsh conditions that model Bayer processes. There have been no re- ported studies on model systems or carbohydrate ex- tracts from plants under these conditions. In this study we follow the decomposition of water-soluble extracts isolated from the wood of Callitris rhomboidea (gym- nosperm) and the roots of Corymbia calophylla and Eucalyptus marginata (angiosperms), under laboratory- simulated Bayer process conditions using solid-state 13 C * To whom correspondence should be addressed. E-mail: ma.wilson@uws.edu.au. Phone: 61-2-4570-1210. † University of Technology Sydney. ‡ University of Western Sydney. 2842 Ind. Eng. Chem. Res. 2002, 41, 2842-2852 10.1021/ie010803a CCC: $22.00 © 2002 American Chemical Society Published on Web 05/10/2002