Energy Consumption for Processing of Gibbsite-Boehmite Mixed Bauxite

Hi Friends,

In earlier posts, we have already discussed the Technology for Processing of Gibbsite-Boehmite Mixed Bauxite, Energy Consumption in Gibbsitic Bauxite based Alumina refinery and Technological options for setting up new Alumina refinery.  In present post, we will elaborate the total thermal energy requirement for processing Gibbsite-Boehmite mixed Bauxite adopting efficient high temperature digestion technology.

As such, Double digestion technology and Tube digestion technology have been rated as energy efficient high temperature digestion technology for processing of mixed Bauxite. Between these two techniques, Double digestion followed by Pressure decantation system has been successfully commercialized and operative in many plants in the World but Tube digestion technology has been adopted only in Comalco Alumina refinery, Australia. 


Comalco Alumina Refinery (CAR-1), Australia has adopted Tube digestion technology in their 1.4 Mtpa Alumina refinery. The system contains 3 slurry tubes each having 150 mm diameter fitted in a jacketed shell of 500 mm diameter pipe in 'U' shape arrangement for maintaining continuity. In this system keeping the straight length of tubes about 60 meters. In CAR, the digestion temperature is 265 degree centigrade which is attained by using high pressure steam at about 100 bar and temperature of 320 degree centigrade as heating media for Bauxite slurry in digestion. Initially, CAR Australia had all process units accept Desilication but Pre-Desilication unit was added afterwards to ensure product quality of International standard.


A new Alumina refinery owned by Ma'aden-Alcoa joint venture is planned to be executed in Saudi Arabia adopting Tube digestion technology. This project having 1.8 Mtpa Alumina production capacity is in advance stage of engineering with adoption of Tube digestion technology and planned to be commissioned by the end of 2014. This will be the 4th Alumina refinery adopting Tube digestion technology in the World. Rio Tinto Alcan owned Yarwun Alumina refinery in Australia is the 3rd operative Alumina refinery having Tube digestion technology in the World. The present capacity of Yarwun Alumina refinery is 1.4 Mtpa alumina which is under expansion to 3.4 Mtpa alumina. This brown-field expansion is planned to be completed by end of 2012 with capital investment of about US$ 1.90 billion.


The broad break up of thermal energy consumption for processing of Gibbsite-Boehmite mixed Bauxite incorporating efficient high temperature digestion technology is given below-

The above details clearly reveal that total thermal energy requirement for efficient High temperature digestion technology works out to 15.12 GJ / tonne of calcined alumina. The above figures have been arrived by carrying out detailed material balance and thermal energy balance calculations for high temperature digestion system. It is evident from above tabular data that only the digestion area accounts for over 10 GJ of thermal energy per tonne of calcined alumina which is equivalent to total energy consumption for alumina production with Medium pressure digestion technique. However, with adoption of efficient energy recovery system, it is possible to reduce the energy consumption figure by about 10% to a level of 9 GJ for digestion area which will ultimately result in overall energy consumption to the level of 14.12 GJ per tonne of calcined alumina. As per the published literature available on public domain, all existing efficient Alumina refineries in the World adopting Double digestion technology have their thermal energy consumption ranging from 15 GJ to 16 GJ per tonne of calcined alumina. This proves that calculations carried out by us  for energy consumption is in order. It also reflects that there is a scope for further reduction in thermal energy consumption in operative Alumina refineries by 10 to 15% from the present level.

It's a fact that the heat transfer co-efficient increases with increase in slurry velocity resulting in lower overall energy consumption at around 12 GJ/t alumina (9.0 GJ/t for hydrate area and 3.0 for calcination area) which makes the Tube digestion as most energy efficient process technology for processing of gibbsite-boehmite mixed bauxite. But digestion of Bauxite at high temperature at around 260 to 270 degree C in presence of lime will be pron to hard scaling on inner surface of tubes which must be kept in mind while designing the Tube digestion system for any plant for production on commercial scale. It is essential to have unique high pressure jet system for descaling the Tube digesters in order to ensure minimum downtime of the plant.

Trust, the above data / information gives clear understanding of energy requirement figures for Alumina refinery. In case you have difference in opinion on any of the above figures, please feel free to put your views / suggestions / remarks / comments, if any.

Regards.

Kunwar Rajendra