AOS Stade is one of the most energy effi cient alumina refineries in the world. The low energy consumption is due to the application of highly efficient technologies, such as the tube digestion units and the Circulating Fluidized Bed (CFB) calciners, both installed from the beginning of production in the early 1970s.
However, since the AOS CFB calciners were started, some significant advances in the technology have been made and several calciners with more modern layouts have been built. With every new installation the energy efficiency has been improved. Prior to the most recent upgrade, the AOS calciners’ energy consumption of 3.15 GJ/t was at the upper end of the range.
To improve the fuel consumption and hence also the operating costs at AOS Stade, Outotec has in a joint effort with the refinery, changed one of the existing calciners according to the most modern CFB calcination flowsheet, and have thus been able to decrease the specific energy to the 2.7 GJ/t mark.
The modifications enabling this unprecedented energy efficiency included implementation of a second cooling stage and a hydrate bypass, which made the configuration comparable to the latest Outotec CFB calciners. Furthermore, the addition of a hydrate dryer and a third preheating stage advanced the configuration to Outotec’s latest developed flowsheet CircoCal™ and reduced the energy efficiency to a world record low.
In this article the modifications, achievements and experiences are discussed, with implementation potential for other plants also outlined.