Bauxite Digestion Technology for Processing of Gibbsite-Boehmite Mixed Bauxite

Hi Friends,

We appreciate your remarks on "Typical schedule for commissioning of Alumina Refinery" as realistic and systematic approach. All our papers are based on our experience in operative Alumina plants in various parts of the globe.

In this technical paper, we would like to present different techniques for processing of Gibbsite-Boehmite 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 such small component of boehmite. Thus minimum boehmite content of 8% with remaining gibbsite compensating for module more than 8 is considered viable to process using Modified Bayer process. Various technological options for digestion of boehmite-gibbsite mixed bauxite in Alumina refinery are briefly described below-

1.       High temperature digestion technology: 

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 technology:

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 technology:

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 technology:

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 newly constructed Alumina refinery has adopted Tube digestion technology for their 1.8 Mtpa Alumina refinery in Saudi Arabia. We strongly feel that the presence of lime at high digestion temperature of 260 to 270^oC 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 in any Alumina refinery.

Our technical paper on "International benchmark of energy consumption in Alumina refinery" may of interest to you.

It is advisable to carry out careful evaluation of all pros and cons of considered digestion technology before taking final decision in implementing any of the above-mentioned digestion technology in the plant. The content of this paper is purely our own experience and views in the interest of global bauxite-alumina-aluminium fraternity.

Your comments / remarks on above will be highly appreciated.

Best regards.

Rajendra Kunwar

CETI Enterprises.

www.ceti.co.in

International Benchmark of Energy Consumption for Alumina Production in Bayer Alumina Refinery

Hi Friends,

In earlier posts, we have covered various issues pertaining to design, engineering, execution and cost estimation of plant, equipment and facilities of Alumina refinery. In present post, we will discuss the International benchmark of energy consumption for production of Metallurgical grade calcined alumina.

As per the literature published by United Nations Industrial Development Organization (UNIDO) available on public domain, typical energy consumption data presented below clearly reveal that specific thermal energy consumption per tonne of alumina produced in various Alumina refineries across the globe ranges from 10.2 GJ/t to 24.5 GJ/t alumina. The thermal energy consumption figures for different regions are presented in following graph-

Data Source : International Aluminium Institute

It is evident from the above graph that specific energy consumption for production of calcined alumina in South America is the lowest at 10.2 GJ/t whereas it is highest in China at about 24.5 GJ/t of alumina. Reasons for wide range of variation in energy consumption figures have already been described in earlier posts. On comparative analysis of these figures, it can be concluded that Asian and American Alumina refineries have the potential to improve the performance of plants for reduction in specific energy consumption by 23 to 25% from the present level. However, Chines Alumina refineries have tremendous scope for further reduction in energy consumption by about 50% through modifications and modernisation in the existing plants. As such, many Alumina refineries under execution in the World have set their total thermal energy consumption target at 8.5 GJ/t alumina.

On critical analysis of plant efficiency figures, process control parameters of operative Alumina refineries and technological advancements in the World, the global benchmark for specific thermal energy consumption in new Alumina refinery has been set by the International Expert Committee as 7.80 GJ/t of calcined alumina.

It may be a little difficult to achieve the set benchmark figure in existing Alumina plants because of inefficient heat recovery system. However, it is challenging and achievable target for modern Alumina refineries with installation of efficient heat recovery system, high degree of automation and continuous monitoring of thermal energy consumed in various units of Alumina refinery. Let us join hands to save unwanted thermal energy and get additional advantage on production cost of alumina. 

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

Regards.

Rajendra Kunwar
rajendra@ceti.co.in
www.ceti.co.in