Features of Draft Tube and Open Impeller Agitators for Precipitators in Alumina Refinery

Hi Friends,

Precipitators are most critical equipment considered in Alumina refinery because of its effect on process efficiency and product quality as well. Relevant process efficiency refers to liquor productivity during crystallization process in precipitators which has direct bearing on total thermal and electrical energy consumption for alumina production in Bayer process. On product quality front, product coarseness along with granulometry of hydrated alumina is controlled to the desired level in precipitators.

As discussed in earlier posts, in precipitators, freshly prepared seed hydrate is added in the saturated and filtered aluminate liquor at controlled temperature and the whole content is kept under agitation / circulation resulting in crystallization of hydrated alumina. The crystallization process requires about 40 hours of slurry holding time. There are three types of agitation / slurry circulation systems prevalent in Alumina refineries-

• Circulation  by air lift,
• Draft tube agitators and
• Open impeller agitators.

Slurry circulation by air lift requires huge flow of compressed air, thus it has lower slurry agitation efficiency. Till 1960s, many Alumina plants had compressed air based slurry agitation system in precipitators but now most of them have modified the system adopting either draft tube agitators or open impeller agitators. In addition to inefficiency, air lift system has the capacity limitation for bigger volume precipitators. Earlier days, the crystallization circuit had smaller capacity precipitation tanks with volume of each precipitation tank ranging from 1000 m3 to 3000 m3. Now a days, all modern Alumina refineries installed in 1980s and afterwards has higher volumetric capacity varying from 4000 m3 to 4500 m3 per precipitation tank. Installation of higher capacity precipitation tanks has resulted in lower space requirement for installation, lower man power requirement for operation and substantial reduction in operating and maintenance expenses. Agitators of precipitation tanks requires following key features-

1. Higher pumping capacity for keeping solids in suspension and homogeneity of solids,

2. Low shearing rate of impeller to minimize particle attrition and

3. Lower electrical energy requirement.

As per my personal experience and interactions with professionals of Alumina refineries in India and abroad, both draft tube agitators as well as open impeller agitators are equally efficient for solid consistency in precipitation tanks ranging from 400 to 500 gpl hydrate as alumina. This level of solids in precipitation tanks is adequate for achieving the liquor productivity upto 75 gpl Al2O3 and most of the plant in the World has liquor productivity in the same level. But, all new Alumina refineries under execution and planning stage have precipitation tanks of further higher capacity of 5500 m3 to 6000 m3 with solids in slurry over 600 gpl Al₂O₃ with the prime objective to have liquor productivity to the level of plus 92 gpl Al₂O₃.

For higher solid consistency, open impeller agitators will not have any problem, however, certain design modifications may be required in draft tube agitators for meeting the process requirement for its successful uninterrupted operation with high solids in precipitation tanks of Alumina refinery.

The views expressed here are purely my personal experience without any bias / prejudice for any particular individual / organization involved in designing of such a complex equipment. I have freely shared my views and request you to put your views / remarks / comments, if any, if you have different experience on the subject.

Rajendra Kunwar

www.ceti.co.in

Systematic Approach for Design of Slurry Pumps for Alumina Refinery

In this paper, we will discuss the detailed design calculations for slurry pumps of Alumina refinery. The entire design sample calculations have been covered in three major steps for easy understanding as outlined below-

Step-I:

This step contains the basic input data for specified duty conditions including the details about the suction lines and fittings as given below-

Step-II:

This step contains the description of delivery lines and assumed efficiency figures for pumps and motors as outlined below-

Step-III:

This step may be considered as the final step which presents the calculated output in terms of pressure drop in suction and delivery lines, NPSHA and required electrical power of motor to be connected for successful operation of the pump.

The derivation has been presented in very simplified fashion and may not require further explanation at any step. It is evident from this sample calculation that any process pump of Alumina refinery can be designed just by changing the input conditions and basic assumptions, if so required.

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

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,

Operational Sequence for Startup & Commissioning of Alumina Refinery

In this technical paper, we have elaborated the simplified operational sequence for starting and commissioning of Alumina refinery. As such, there may be many permutations and combinations of operational sequence for starting and commissioning of Alumina refinery. We are presenting here one of them which may be simple and most appropriate. The schematic diagram is presented here under-

Start up and commissioning of Alumina refinery is taken up after following activities-

·         Thorough checking for Mechanical completion,

·         Readiness of raw water supply system and DM plant,

·         Synchronization of boilers and turbo-generators,

·         Cold water run and 

·         Hot water run.

·         Ensuring the availability of adequate input materials.

Building up caustic concentration and simultaneously raising its temperature to around 85 degree centigrade are pre-commissioning activities required for startup of Alumina refinery. Sequence of caustic concentration build up, temperature raising, start up and commissioning steps are taken up in following sequence-

1.       Fresh caustic lye received by road / rail tankers are stored in Caustic storage tanks (CSTs) and 6 or 7 nos. precipitators of 2nd line as 1st line precipitators shall be taken into range for commissioning.

2.       Initially, Evaporator feed tank, Liquor preparation tank, Digesters, Flash tanks, Heaters and one of the Decanters are taken into range for concentration build up and temperature raising. Accordingly closed circuit with these equipment is made by temporary connections for circulation of liquor before start up. 

3.       Water is taken in Evaporator feed tank through temporary lines and operation of evaporator is initiated at low vacuum and operation is stabilized slowly with boiling. This ensures the adequate condensate supply to boilers as well as hot water for dilution of caustic lye to around 150 to 200 gpl caustic in Liquor preparation tank. 

4.       Run the Test tank pump at about 50% flow for filling Heaters, Digesters and Decanter connected in the circuit. Keep the drain valves of Heaters in open condition in the beginning for purging out entrapped air in pipelines otherwise it may cause damage to chambers of heaters.

5.       Open steam in Live steam heater maintaining the outlet temperature below 100 degree centigrade. 

6.       Circulation of liquor in this circuit is continued for about 24 hours for checking the readiness of process circuit.

7.       Start Bauxite charging to grinding mill at about 50% digestion liquor flow and taking charging ratio at around 0.300. Bauxite charging to grinding circuit is considered as start of plant commissioning i.e. Zero hour of plant commissioning.

8.       Two desilication tanks will be filled up with slurry taking Bauxite slurry heaters into service. After attaining about 16 hours of desilication time, temporary circuit of Red area will be discontinued and permanent process circuit will be energized for generation of spent liquor.

9.       Start flocculent preparation and dosing to settler, as required.

10.   Initially Security filters will be operating as a spool but will be taken into range afterwards for ensuring required purity of plant liquor.

11.   Heat Interchange Unit will be kept bypassed and filling of precipitators shall be started taking only five (5) precipitators in range for the purpose of adequate seed generation.

12.   Continue circulation of Precipitation liquor through Evaporators and Digesters for slowly raising the temperature profile of liquor.

13.   Slowly increase the digestion liquor concentration to around 250 gpl caustic and step up charging ratio to around 0.620 level for raising the ratio profile of aluminate liquor to the level of pregnant liquor. 

14.   Take Residue washers into circuit.

15.   After attaining the aluminate liquor ratio above 0.600 in feed liquor of Precipitation tanks, start seeding with purchased solid hydrate at controlled rate for generation of seed hydrate. For the purpose, solid hydrate will be mixed in Seed mixing tank with Pregnant liquor and pumped to those five (5) Precipitation tanks taken in range.

16.   Maximize charging ratio to the desired saturation level and increase Bauxite charge at increase digestion liquor flow.

17.   Hydrate generated in Precipitation circuit will be used as seed till adequate hydrate is built up in the process.

18.   Discontinue all temporary connections and continue operation as normal by taking desired number of equipment and vessels of one line of process circuit.

19.   Start hydrate classification circuit and transfer of product hydrate to Hydrate storage shed.

20.   After making a stock of adequate stock of hydrate in storage shed, commissioning of Calciner need to be taken up for production of calcined alumina.

21.   Stabilize plant operation for maximizing alumina production level.

We have presented the methodology for commissioning of Alumina refinery in simplified manner for easy understanding.