Relationships Between Smelter Grade Alumina Characteristics and Strength Determined by Nanoindentation and Ultrasound-Me
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UCTION
SMELTER grade alumina (SGA) is a complex material (in terms of microstructure and crystallinity) produced via the Bayer process, a method converting aluminum-bearing bauxite into a refined precursor gibbsite that is then calcined at ~1273 K (1000 C) in large-capacity industrial calciners. Over 100 million tonnes of SGA are produced each year to supply the demand of the global aluminum industry.[1] The transformation of gibbsite to SGA is pseudomorphic, where the original gibbsite morphology is largely retained during conversion, with internal shrinkage (up to 24 pct by volume[2]) leading to the formation of significant porosity and large internal surface area.[3] The evolution of internal pore structure and the different phase transformation pathways to the final thermodynamically stable a-alumina form (corundum) has been well studied but nevertheless there is still some ambiguity.[4] Figure 1 shows the phase diagram of the proposed transformation pathways of gibbsite under different conditions as they are currently understood. However, knowledge on the exact structures of some of the transition alumina is still limited.[9] Similarly there HASINI WIJAYARATNE, GRANT MCINTOSH, MARGARET HYLAND, and JAMES METSON are with the Light Metals Research Centre, Newmarket Campus, University of Auckland, 314-390 Khyber Pass Rd, Newmarket, Auckland 1142, New Zealand. Contact e-mail: [email protected] LINUS PERANDER is with the Outotec GmbH, 61440, Oberursel, Germany. Manuscript submitted December 18, 2016. Article published online March 28, 2017 3046—VOLUME 48A, JUNE 2017
is a lack of understanding about the nature of modern SGAs, agglomerates of gibbsite crystallites transformed in modern stationary calciners and exhibiting a range of phases typically distributed within a single particle.[6] Stationary calciners operate with more rapid heating rates and shorter residence times than the historical rotary kilns. Modern calciners lead to aluminas that are ‘‘kinetically frozen’’ as opposed to reaching a thermal equilibrium. Few publications[6,10] have discussed the nature of these modern SGAs, and thus more understanding is needed as there are consequences on the performance of these materials when they are used in smelters. Alumina strength is one of the most important quality parameters in a smelting context.[11] It is known that transportation and handling leads to alumina attrition creating dust, which has been shown to increase emissions of gaseous fluorides and particulates.[11] In the aluminum and alumina industries, SGA strength is determined by an attrition index using the standard Forsythe–Hertwig test[12] quantifying the increase in fines generation (typically change in 45 lm fraction) during testing under high-velocity air jets. As this aggressive test only measures the relative change in mass fraction of fines, it provides little understanding about material properties that influence the inherent strength of SGA. A strong alumina is generally indicated by a low attrition index. Plant-measured attrition indice
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