Balancing Sodium Impurities in Alumina for Improved Properties
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TRODUCTION
THE residual impurities within Bayer gibbsite are an important consideration during the refining of smelter grade alumina (SGA) from bauxite for aluminum smelting. This is despite the fact that SGA can be considered a refined, relatively high-purity commodity, typically exhibiting an aluminum oxide content in excess of 95.5 wt pct. The impact of impurities on the calcined product and the smelting process is the main motivation for maintaining a relatively high purity. Typical levels of residual impurities expected within SGA are reported in Table I with general impacts of these summarized. Sodium is the most abundant impurity in Bayer gibbsite and consequently in the calcined SGA product. Sodium content needs to be limited due to its influence on bath chemistry and costs related to surplus bath disposal from reduction cells.[1] Despite the industry trend of minimizing impurities to keep up with HASINI WIJAYARATNE, MARGARET HYLAND, GRANT MCINTOSH, and JAMES METSON are with the Light Metals Research Centre, Newmarket Campus, University of Auckland, 314390 Khyber Pass Rd, Newmarket, Auckland 1142, New Zealand. Contact e-mail [email protected] LINUS PERANDER is with the Outotec GmbH, Oberursel 61440, Germany. Manuscript submitted March 4, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
increasingly stringent alumina property requirements, there may be advantages of tolerating some impurities within the product. One example of this is in product strength. There is a loose correlation between total sodium content within calcined alumina (product) and its strength, typically reported as attrition index[5,6] as observed in Figure 1. However, it is unknown whether this is co-incidental or causative. Attrition index gives an indication of the tendency for alumina particles to be eroded/ fractured and in industry this is measured by the Forsythe–Hertwig technique.[5,7–12] The attrition index is determined by the change in the 45 lm fraction of an alumina sample after it has been subjected to the attrition test. A significant increase in fines fraction due to attrition gives a relatively large attrition index indicative of a weaker material. Although there are known limitations with this method,[7] the Forsythe–Hertwig technique is the current industry measure of SGA strength. For example, as this test is conducted on the alumina subsequent to calcination, the amount of attrition taking place in the calciner is expected to influence the attrition index result. Therefore, an alumina batch that has undergone a relatively low degree of attrition in the calciner may have an apparently higher attrition index post-calcination, giving the impression of a weak alumina. Conversely, an alumina batch that has undergone significant breakage in the calciner may produce a very low
Table I. Constituents Al2O3 Na2O SiO2 CaO Fe2O3 TiO2 ZnO P2O5 Ga2O3 V2O5
Residual Impurities Within SGA Adapted from Ref. [1–4]
Wt Pct 99.3 to 99.7 0.3 to 0.5 0.005 to 0.025 < 0.005 to 0.040 0.005 to 0.020 0.001 to 0.008 < 0.001 to 0.010 < 0.0
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