COST AND ENERGY EFFICIENCY IMPROVEMENT IN ALUMINA CALCINATION

Perander, L.M., Gasafi, E., Scarsella, A.

Alumina refining and aluminium smelting are two energy intense industries, the former requiring some 8- 12 GJ per ton alumina of energy, mainly supplied in the form of fossil fuel and the latter some 12-15 kWh per ton aluminium of electrical energy. For this reason, aluminium smelters are often located in regions where electrical energy is available at competitive prices, which is often neither close to the consumers of the produced aluminium nor the suppliers of alumina – the refineries. Many large refineries processing domestic ore bodies are located close to the bauxite deposits.

Calcination, the final step in the Bayer process, consumes a considerable part of the overall energy used in the alumina refinery. This energy is supplied as fuel oil or gas and is used to heat up the precursor aluminium hydroxide to sufficient temperatures to produce a material meeting Smelter Grade Alumina specifications. Reducing the specific fuel energy consumption has been a key driver for the continuous development of the Circulating Fluidised Bed calcination technology, culminating in the recently achieved consumption of 2.7 GJ/t alumina.

Although the calcination stage is often considered a part of the Bayer process, this processing step is in fact mostly independent of the so called Bayer circuit. It is perhaps more out of tradition and convenience that the calciners are located at the refinery rather than close to the smelter. In today’s industry, where many producers are focusing on the whole value chain, it may in fact be challenged whether it is more economic to locate the calciner at the smelter site.

This paper explores on a technical and economic basis several trade-off scenarios:

  • comparing the traditional approach where calcination is carried out at the refinery location and the alumina shipped to the smelter, with processing of the Bauxite into hydroxide at the source and thereafter shipping dry or moist hydroxide to the smelter site where the calcination step is then carried out,
  • reviewing heat exchange optimisation, including additional heating and cooling stages, recovery of heat from waste gas and cooling water, hydrate by-pass.