HiEnergy Requirement for Efficient Alumina Refinery

In earlier posts, we have discussed various critical issues pertaining to design, engineering and operational aspects wherein we have observed that consumption of thermal energy and caustic soda are the major cash components in production cost of calcined alumina. Thus process experts, technologists and equipment manufacturers are continuously striving to develop energy efficient Alumina refinery so as to improve profitability of the plant. In present post, we will discuss in detail with break up of thermal energy consumption for various unit operation and unit processes of gibbsitic bauxite based Alumina refinery to arrive at a realistic figure as a bench mark for comparison purposes.

                                               

Here, we would like to discuss the energy consumption in Alumina refineries processing gibbsitic bauxite which may be taken as guiding tool to develop similar figures for other types of bauxitic and and non-bauxitic ores, if so desired. Thus our purview of discussion will remain within the limits of energy requirement for Alumina refineries adopting Medium pressure digestion with two stage continuous precipitation followed by stationary calciners for production of smelter grade calcined alumina. We are aware that thermal energy in Alumina refinery is used in two forms as outlined below-

1.       Saturated steam for heating the process streams in Pre-desilication, Digestion and Evaporation areas. Minor steam is also required for miscellaneous purposes in the plant.

2.       Natural gas or Fuel oil (HFO / LSHS) in calciners for calcination of hydrate for converting to desired product.

Our calculation is based on pressure of saturated steam at 12 bar to be used in medium pressure digestion at 145 degree Centigrade as well as evaporation and HFO as the fuel oil for calcination. The break up of steam consumption and fuel oil consumption along with respective heat value have been tabulated below-

 

The above presented data clearly reveal that total thermal energy consumption  for alumina production works out to around 8.14 GJ per tonne of calcined alumina. If we take 5% additional energy to compensate for unforeseen inefficiencies and losses, even then total thermal energy consumption in efficient Alumina refinery processing gibbsitic bauxite should not exceed 8.50 GJ/t of calcined alumina. Hence, specific energy consumption figure of 8.5 GJ per tonne of calcined alumina is the International benchmark for efficient Alumina refineries in the World.

On the process and energy audit of efficient Alumina refineries in various part of the globe, it has been observed that if one refinery is energy efficient in digestion area because of efficient heat recovery system then the same plant is lagging in operating the evaporators efficiently and reverse order as well. All the operative plants can become energy efficient provided they implement modifications in equipment and operating practices by exchanging the merits and shortcomings with one another. Such things are rarely happening today and everybody has the feeling that he has the best plant. But, with comparison of any two Alumina refineries, you will find that many changes can be incorporated immediately without any investment which may attribute in reducing the production cost substantially. 

The indicated figures are realistic figures which can be achieved with a little efforts and  constant vigil on design, equipment, process control and operational aspects of Alumina refinery. Hope, all experienced people of bauxite-alumina-aluminium fraternity will agree with my above indicated observations and analysis in reducing the thermal energy consumption drastically for alumina production. Request to kindly put your views / suggestions / remarks / comments, if any. Friends,