Evolution of nano- and microstructure during the calcination of Bayer gibbsite to produce alumina

Abstract

The microstructure of Metallurgical Grade Alumina (MGA) has extensive consequences on how it performs as a raw material for aluminium production. MGA is produced primarily in the Bayer process in which both the precipitation and the calcination steps play important roles for the development of alumina microstructure. Calciner technology is progressively moving from rotary kilns to more energy efficient gas suspension or circulating fluidised bed processes, which influences the intricate interplay of calcination parameters and therefore effects the development of structure and properties in the alumina. The evolution of structural features during the calcination of Bayer gibbsite is a complex process that involves both a dehydroxylation reaction and a rearrangement of the crystal lattice. Due to the highly agglomerated starting material, the pseudomorphic nature of the reactions and the incomplete transformation to the thermodynamically stable alpha alumina, a mixed phase meso- to macro porous material results. As a consequence of the wide range of particle sizes, rapid heating rates, short residence times and other inhomogenities in the modern calcination process a diversity and mixture of aluminium oxide products are formed. Deviations from the average transition alumina structures are also observed. In this thesis an integrated approach to evaluate alumina micro- and nanostructure is presented. Using a suite of modern analytical techniques, in conjunction with more traditional approaches, the structures that make up the MGAs are probed and insights into the development of structural features and the transformation reactions during the calcination process are obtained. It is shown that the alumina phase composition is influenced by the calcination conditions, which for the MGAs are dictated by the calciner technology, and that compositional variations are observed even within individual alumina grains. The results indicate that these compositional variations arise both due to the distribution of sodium and other impurities which catalyse the transition to alpha alumina, and as a result of the rapid heating rates. It is proposed that the rapid heating rates in modern calcination processes result in the formation of a boehmite-like structure within the particles. The internal boehmite formation may be promoted by desorption resistance of the product water, from surfaces and pore openings, due to the rapid decomposition rates. Electron microscopy investigations reveal that a lamellar and sponge-like network of interconnected pores and channels is formed with relatively large alpha alumina crystallites growing in the transition alumina matrix. Furthermore, it is shown that alpha alumina may be directly observed in cross sectioned alumina grains through Charge Contrast Imaging using an Environmental SEM. It is also demonstrated that multiple field Solid-state Magic Angle Spinning NMR can be used to quantitatively assess the phase composition of MGAs. The availability of high magnetic fields significantly aids this quantification. The NMR results indicate that the MGAs are relatively ordered on the short range, and that only small amounts of pentahedral aluminium are present. Furthermore, using X-ray Absorption Near-edge Structure (XANES) spectroscopy it was observed that the short range order is influenced by the precursor material and the calcination conditions. This offers a possible explanation for some of the discrepancy in the literature regarding the structural relationship between the gibbsite and boehmite derived transition aluminas. The results also indicate that changes in the intermediate range ordering occur with increasing calcination temperature. Even in the complex mixed phase MGAs some long range order is observed, which seems to be closely tied to the formation of alpha alumina in the samples. XANES is very sensitive to these long range structures. However quantitative or structural analysis is difficult due to the complex and overlapping resonances arising from multiple scattering interactions in the samples. The complex mixture of phases and the high level of disorder in the MGAs present significant challenges in understanding their structure, particularly with relation to the performance as a raw material for aluminium metal production. The structures that make up the MGAs transcend short-range ordering, on the nano-scale, and into longer range domains. Therefore, to examine MGA microstructure and better understand its impact in smelter operations, it is essential to evaluate structural ordering on several different scales. This requires advanced and often multiple analytical techniques.