Technology for Processing of Gibbsite-Boehmite Mixed Bauxite

Hi Friends,

In earlier posts, we have mostly discussed the process, design, engineering and technological options for processing of gibbsitic bauxite. 

In present post, we will present the different techniques for processing of gibbsite-boemite mixed Bauxite prevalent in various existing Alumina refineries in the World and placing our views  taking into account all techno-financial aspects. 

Presence of boehmite in the range of 4 to 6% of boehmite mixed with gibbsite in Bauxite is considered negligible as it is not considered economical to recover. Thus minimum boehmite content of 8% with remaining gibbsite compensating for module more than 8 is considered viable to process using Bayer process. Various technological options for digestion of boehmite-gibbsite mixed bauxite in Alumina refinery are briefly described below-

1. High temperature digestion: In this process, mixed bauxite is digested at temperature between 240 to 250 degree centigrade at about 35 kg/cm2 pressure with 5 to 7 stages of flashing for heat recovery system. Total thermal energy requirement in this process is very high to the tune of 18 to 21 GJ per tonne of alumina. Though many existing plants with this technology are operative even today but not being adopted in new Alumina refineries now because of high operating cost.

2. Double digestion: This technology has advantages over high temperature digestion technology as energy consumption is comparatively lower. In this process, the Bauxite is first digested at low pressure at digestion temperature varying from 140 to 145 degree centigrade and the settler underflow residue is again digested at high temperature at process conditions same as high temperature digestion. Thus overall energy consumption is substantially lower compared to high temperature digestion. This technology reduces the overall energy consumption of about 1.5 GJ per tonne of alumina compared to high temperature digestion technology.

3. Two step digestion: This digestion is almost same as Double digestion technology wherein Bauxite at 1st stage is digested at atmospheric pressure at around 105 to 110 degree centigrade keeping 2nd stage exactly the same as high temperature digestion. In this technique, the bauxite is digested at atmospheric pressure and settler underflow residue is again digested in second stage of digestion circuit. This technology consumes less thermal energy and lower caustic soda consumption due to partial formation of sodalite complex. Alumina extraction is comparable other available digestion technology discussed above. The total thermal energy  requirement adopting two step digestion is estimated at around 14.0 GJ per tonne of Alumina.

4. Tube Digestion : Tube digestion technology has been developed on pilot plant scale and in the process of commercialization. The details of Tube digestion technique will be covered in forthcoming posts. In this technique, bauxite slurry with desired amount of caustic soda is passed at high velocity through bundle of tubes. The residence time requirement for digestion is very low. Material and energy balance calculations with optimized process parameters and heat recovery system reveal that total thermal energy requirement adopting tube digestion technology should be around 12.0 GJ/t alumina which accounts for about 9 GJ/t for Bayer process (Hydrate) area and 3.0 GJ/t for calcination area.

Out of above four techniques, it is evident that Tube digestion technology will require less capex and opex. Pilot plant scale trials with this technology have shown encouraging results in all respect. A new Alumina refinery in Saudi Arabia is planning to adopt Tube digestion technology for their 1.8 Mtpa Alumina refinery. In our opinion, the presence of lime at high digestion temperature of 260 to 270 degree centigrade will cause high scaling rate on inner surface of tubes. Hence it may be essential to take care of the tube cleaning while adopting this technology.


We advise all concerned to carry out careful evaluation of all pros and cons before taking final decision in implementing any of the above mentioned process technology.

Please put your views / suggestions / remarks / comments, if any.

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Thanks and regards.

Kunwar Rajendra

Hydrates and Calcined Alumina

Hi Friends,

Aluminas and Hydrates are wonderful materials because of their wide range of industrial and general applications in life as well.

All of us use hydrates and aluminas in some form or other. But the process design, engineering  and processing methods including process controls are complicated which I would like to express in simpler form with clarity in expression for easy understanding. 

Regards.

Kunwar Rajendra