Smelting-reduction of bauxite for sustainable alumina production
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Bauxite ore is the main raw material for aluminum production, it is converted into alumina (Al2O3) to obtain a proper charge material for aluminum production in Hall-Heroult process. Bayer process is the main commercial alumina production method in which bauxite ore is first digested by NaOH at moderate hydrothermal conditions, followed by aluminum hydroxide (Al(OH)3) precipitation from the obtained sodium aluminate solution. Calcination of the hydroxide to alumina yields a high purity alumina feedstock for aluminum production. One of the main by-products of the Bayer process is red mud, which is separated from the bauxite solution after digestion and contains a significant amount of iron oxide. Red mud is one of the most important disposal materials in the world with no commercial use. Alternative sustainable process for bauxite treatment to alumina is Pedersen process, which was patented in the 1920s. This process was run for 40 years on a commercial scale before it was closed down due to the high cost of the alumina produced. The Pedersen process is more sustainable compared to Bayer process regarding lower CO2 emission and energy consumption. In this process, iron (the main impurity in bauxite) is removed from the ore through a smelting reduction process, which yields pig iron and high alumina containing slag. The slag is further hydrothermally treated to alumina through an economical process. In the present study, the carbothermic reduction of bauxite is studied at elevated temperatures through experimental work with lime addition as the flux. The effect of process temperature, slag chemistry, and type of carbon material on iron removal is studied. Moreover, the distribution of the elements such as Si, Ti, Al, Ca, Mn, and so forth is investigated under different process conditions. The chemical, physical, and microstructural properties of the produced slags are also studied.