I would like to share the glimpse of one of my technical papers presented in an International seminar wherein I expressed my feelings for utilizing the experience and expertise of individuals in execution of new Alumina refineries by adoption of Public domain technology. The paper was well appreciated by one and all in the seminar. Trust, you will also find it informative and interesting as well.
The production of alumina is a hydrometallurgical process comprising of several unit operations and unit processes prevalent in chemical industries. Basically it is a chemical enrichment process wherein the undesired components are removed from bauxite. The extent of the presence of these components and their behavior play a vital role in economic exploitation and selection of an appropriate process route. The selection of an appropriate process route for production of alumina depends on various factors like chemical and mineralogical composition of bauxite, extraction efficiency, the module, and sodalite factor and energy consumption. Mineralogically, alumina occurs in bauxite in the mono-hydrated form (Al2O3.H2O) known as Boehmite & Diaspore and the tri-hydrated form (Al2O3.3H2O) referred as Gibbsite.
Based on available geological exploration data and chemical analysis of bauxite average compositions of bauxite is considered for design of Alumina refinery.
With available mineralogical composition of bauxite, various technological options can be worked out to arrive at the best-suited process technology to be adopted for new Alumina Refinery. Various technological options considered for associated major unit operations and unit processes are outlined below-
As per prevailing practices, secondary crushing of bauxite from around 120 mm to 20 mm is done using following types of crushers:
(a) Hammer crushers
(c) Double Roll crushers
Availability and operational reliability of such equipment are the basic consideration for their selection. Comparatively the availability and reliability of double roll crusher is much more than hammer crushers and impactors for the required duty.
Crushed bauxite is ground to increase the surface area of bauxite resulting in improvement in its reactivity with caustic soda. Following are the various ways for grinding of bauxite:
(a) Dry grinding
(b) Closed circuit wet grinding
(c) Open circuit wet grinding
Earlier till 1960’s, Alumina Refinery had dry grinding system using Ball mills as the main equipment. Dry grinding of bauxite causes dust generation affecting the working environment and hence the dry grinding system was modified to wet grinding system. Under wet grinding, following types / combinations are used-
(a) Single stage closed circuit wet grinding
(b)Two stage closed circuit wet grinding
(c) Single stage open circuit wet grinding
Generally Rod mills / & Ball mills are used as grinding equipment for bauxite in Alumina refineries in above-mentioned types of combinations.
Out of the above-mentioned grinding system, single stage open circuit grinding is considered to be an appropriate grinding technique for green-field Alumina Refinery.
Under mentioned digestion processes have been adopted in various Alumina refineries:
(a) Atmospheric digestion
(b) Medium pressure digestion
(c) High Pressure digestion
(d) Double digestion
(e) Tube digestion
(f) Two-Step digestion
Atmospheric and medium pressure digestion methods are considered for gibbsitic alumina however high pressure and two-step digestion techniques are adopted in case of boehmitic bauxite. Atmospheric digestion technology has following advantages over medium pressure digestion technology:
· Lower capital investment as it does not require pressure vessels and slurry flash tanks.
· Ease in operation
· Lower process steam requirement
In spite of above advantages of atmospheric digestion, lower alumina recovery is its main disadvantage which causes higher consumption of bauxite per tonne of alumina and hence under-utilization of natural resources. In addition, lower alumina recovery requires high flow of plant liquor. Hence medium pressure digestion is considered to be most appropriate technology for gibbsitic bauxite.
For Boehmite-Gibbsite mixed bauxite, Double digestion, Tube digestion and Two step digestion technology are adopted. In my opinion Two-Step Digestion (Atmospheric followed by high pressure) Digestion shall be most efficient and economical for processing Gibbsite-Boehmite mixed Bauxite.
Residue thickening, Washing & Disposal:
The undesired oxide of iron, titanium, silicon and other minor metals is termed as residue. The separation of residue from saturated aluminate liquor is carried out using gravity settlers / decanters. Earlier the concept was to use the large diameter settlers for the purpose but over the period of time lot of technological advancement has taken place in this area and smaller diameter deep thickeners have been developed as efficient equipment for residue settling and washing.
The disposal of washed residue from Alumina Refinery to residue pond is achieved by two ways:
(a) Wet residue ponding
(b) Dry residue stacking
As per latest guidelines of Environmental Control Authority, wet residue ponding method is not permitted for green-field Alumina Refineries. Hence disposal of residue at higher solids at around 55-65% is done which is popularly known as dry residue stacking technique.
Supernatant overflow liquor collected from Decanters are polished in security filtration unit of Alumina refinery. Till 1980s, Horizontal Leaf Filters having manual removal of residue sludge were operative in most of the Alumina refineries of the World. But, now almost all new Alumina refineries are preferring the installation of Auto-dumping vertical security filters.
There are two methods for recovery of thermal energy from filtered aluminate liquor going to crystallizers:
(a) Liquor flashing system
(b) Plate heat exchangers
The heat recovery system employs a battery of flash tanks and shell & tube heat exchangers. Liquor flashing system requires more space however; PHEs need small space and ease in operation. Both techniques are used in Alumina Refineries but most of the existing Alumina Refineries have Plate Heat Exchanger System.
Three types of precipitation technologies are normally used in most of the Alumina Refineries of the World.
(a) Batch precipitation
(b) Single stage continuous precipitation
(c) Two stage continuous precipitation
Because of higher liquor productivity and consistency in coarseness of product alumina, two-stage continuous precipitation technology is preferred over batch precipitation technology. Two-stage continuous technique is based on controlled precipitation of alumina hydrate adopting agglomeration and crystal growth principles.
Calcination is the last unit step of Alumina Refinery. Two types of equipment are used for producing metallurgical grade calcined alumina:
(a) Horizontal rotary kiln
(b) Stationary calciner
Stationary calciner is thermally efficient and hence requires less fuel per tonne of alumina production. Hence stationary calciner is a preferred choice over rotary kilns.
TARGET EFFICICIENCY FIGURES:-
Following efficiency figures are achievable with selected process route and equipment:
- Overall alumina recovery: 94 %
- Liquor Productivity: 90 gpl Al2O3
- HFO Requirement: 73 kg/t Al2O3
- Process Steam requirement: 1.85 t/t Al2O3
- Total Energy requirement: 8.5 GJ/t Al2O3
The recommended process route of modified process route has under mentioned tangible and intangible advantages:
- Consistency in quality of alumina
- Optimal consumption of thermal energy
- Lower consumption of caustic soda
- Lower scaling rate in heat exchangers
- Reduction in raw material consumption factors
- Improved working environment
The above stated technological options have been elaborated as Public domain technology available for adoption and execution of an efficient Alumina refinery in any part of the globe. Request to put your views / valued suggestion / remarks / comments, if any, for further improvement in considered options.