10. Production of Smelter Grade Alumina (SGA) by Calcination

This chapter covers the changes in calcination technology, energy consumption, air pollution control and quality of SGA developed over time since the end of the 2nd World War and up until today. The demand for SGA with high specific surface area ranging from 50 to 80 m²/g (BET method) originated from the alumina smelter customers driven by the increasing need to capture HF gas emitted from the smelter cell’s by so-called Dry-Scrubbing with virgin alumina in Gas Treatment Centers (GTC) located at the alumina smelter. The physio-chemisorbed HF on the virgin alumina collected in the GTC as secondary alumina, was then fed to the alumina smelter cells, or pots, via pot feeding systems. This development starting around 1970 made the Floury type SGA, with very low specific surface area around 5 m²/g, obsolete, and Floury SGA was gradually replaced with so-called Sandy type SGA. The oil crisis around 1972 accelerated the development, scale-up and commercialization of stationary calciners for production of SGA initiated by Alcoa in the mid 1950 ties. The 25–30% reduction in specific thermal energy consumption in Rotary Kilns, when compared to Stationary Calciners was a very strong driver. The retention time in the Rotary Kiln was reduced from hours to minutes in Fluidized Bed calciners and from minutes to seconds in Gas Suspension or Flash calciners. Simultaneously with the above development several Rotary Kilns were retrofitted with pre-heater cyclones, and in one case also with a calciner furnaces, to reduce the specific thermal energy consumption and increase the production capacity of SGA. The largest Rotary Kilns for production of SGA had a design capacity of 1400 tpd SGA. This capacity was exceeded several times at Queensland Alumina, Australia, where the new Gas Suspension Calciners installed in 2002–2004 was designed for 4500 tpd SGA, driven by economy of scale. Today, the preferred design capacity of stationary calciners is around 3500 tpd SGA to match the single train production capacity of the Bayer Process circuit in modern alumina refineries. However, the above technology shifts have not come without new challenges to the Bayer process design. One such major challenge is to produce a hydrate quality, which upon calcination to SGA in stationary calciners, results in a particle size distribution and strength as SGA, that meets the specification of the smelters. Driven by environmental requirements of reduced dust emission Queensland Alumina, Australia, as the first in the alumina industry, decided in 2002, to install Baghouses or Fabric filters on their new Gas Suspension Calciners instead of Electrostatic Precipitators (ESPs), in order to avoid excessive dust emission during a power failure. Up until today heavy fuel oil, natural gas and coal gas (CO + H₂) is used as fuel in stationary calciners. But the challenges laying ahead with a predicted increase of the global warming of mother earth, makes hydrogen produced by electrolysis powered with renewable energy the preferred fuel for the not so distant future with potential to make the production of SGA CO₂ free.