Operational Sequence for Start up & Commissioning of Alumina Refinery

Hi Friends,

In present post, we will discuss the 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.

Caustic concentration build up and raising its temperature to around 85 degree centigrade are pre-commissioning activities required for start up 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 about 20,000 tonnes of hydrate in storage shed, commission Calciner for production of calcined alumina.
21. Stabilize plant operation for maximizing alumina production level.

Hope, starting and commissioning procedure for Alumina refinery has been covered with clarity in understanding.

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

Regards.

Kunwar Rajendra

Production of Gallium from Bayer Liquor of Alumina Refinery

Hi Friends,

In earlier post, we have already discussed the characteristics and applications of Gallium metal. In present post, we will describe various production processes for recovery of Gallium as byproduct from Bayer liquor of Alumina refinery. Crude Gallium of 97.0 to 99.9% purity is produced as byproduct from the circulating liquor of Bayer alumina plants. 


Production process:
Over a period of time, many processes have been developed for recovery of this strategic metal as listed below-

1. Carbonation process,
2. Mercury process, 
3. Cementation process,
4. Solvent extraction process and 
5. Extraction with chelating agents.

Most of the old plants in the World are operative with Mercury process but this process is not recommended now because of associated environmental problems. Here, we are describing the Carbonation process for recovery of Gallium from Bayer liquor as it is commercially viable eco-friendly process where as other processes are yet to be commercialized. The method for purification, storage and transportation have also been briefly described here in subsequent paragraphs.

Carbonation process involves treatment of aluminate liquor of Bayer process in two controlled stages. In first stage, major portion of hydrated alumina present in Bayer liquor gets precipitated but over 90% of Gallium remains in solution because of its higher solubility in NaOH than that of hydrated alumina. The Gallium rich decanted liquor is subjected to second stage carbonation resulting in precipitation of Gallium along with remaining hydrated alumina. This precipitate of mixed hydroxides contains about 0.30% to 1.0% of Gallium. The precipitate is dissolved in aqueous solution of sodium hydroxide yielding a concentrated solution of sodium aluminate and sodium gallate. Gallium concentration in this solution is sufficiently high for carrying out electrolytic deposition of Gallium. The electrolysis of this filtered alkaline solution is performed with stainless steel as cathode and pure nickel as anode. At times, liquid Gallium cathode is also used. The cells operate at 60-70 degree C with voltage at about 4 v. The concentration of Gallium hydroxide and Sodium hydroxide is kept at around 1:1 and not less than 1:6 in order to prevent separation of Gallium hydroxide. The cathodic current density is about 100 A/dm2 and current yield of 75% with Gallium concentration more than 40 gpl. The liquid Gallium accumulated at the bottom of electrolytic cell is periodically run into a cast mold. The crude Gallium thus obtained has the purity of 97.0% to 99.9%. The depleted liquor containing about 0.30 gpl Gallium is returned back to Bayer circuit of Alumina refinery.

Purification:

The crude gallium is washed in stages with hydrochloric and nitric acids followed by its filtration through porous plates like sintered glass. Here, 99.0% to 99.9% Gallium is obtained. For technological applications, minimum six-nines i.e. 99.9999%, is the required specification which is achieved by adopting fractional crystallization technique.

Storage and Transportation:

The Gallium metal is either packed in cold gelatine capsules or plastic containers and sold in lumps of 10 grams to 20 kg. During transportation, the temperature is strictly controlled below 20 degree C.

Trust, the production, purification, storage and handling of Gallium have been described to the extent possible based on which the process flow diagram can be developed for implementation of project. Please put your views / suggestions / remarks / comments, if any.

Regards.

Kunwar Rajendra

Characteristics and Applications of Gallium

Hi Friends,

In present post, we will discuss about characteristics and applications of Gallium which is produced as one of the byproducts of Alumina refinery. We already discussed the production process of Vanadium sludge in previous post. The details about Gallium are outlined here in subsequent paragraphs-

Characteristics of Gallium:

Gallium is one out of  four strategic metals which is found in liquid  state at ordinary room temperature, the other being Mercury, Rubidium and Cesium. Gallium is a silvery white metal having melting point of 28.3 deg.C and boiling point at around 2403 deg.C. Like Antimony, Gallium expands when freezes to solids with volume expansion of about 3.2%. The solubility of Gallium in water is negligible but it dissolves in dilute mineral acids. Its solubility is very high in alkaline solution and aqua-regia.

Applications of Gallium:

High purity Gallium having purity at 99.9999% has many technological applications as listed here under-

• Light emitting diodes,
• Opto electronics,
• Industrial thermometers,
• Heat transfer medium,
• Eutectic alloys,
• Liquid seal and
• High temperature lubricants.

The characteristics and applications of Gallium have been broadly described here. The production process of Gallium from circulating Bayer liquor shall be presented in forthcoming post. Please put your views / suggestions / remarks / comments, if any.

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Kunwar Rajendra