Design of Liquor Preparation Tanks in Alumina Refinery

Hi Friends,

Liquor preparation tanks in Alumina refinery are popularly known as Test tanks. Test tanks are the storage vessels mixed liquor used for digestion, Evaporator product liquor, Evaporator wash liquor and Fresh caustic soda. These vessels are cylindrical vessels with top cover in order to combat rain water entry to the tanks. In general, agitators are not required for such duties as solid content in liquor is negligible.

In present post, the design calculations for Test tanks have been developed and presented in simplified forms as outlined below-

The above calculations have been developed for a typical capacity, however, design of test tanks of any capacity can be done using this methodology by simply inserting the basic data and variables as applicable.

We seek your views / suggestions / remarks / comments for further improvement.

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Kunwar Rajendra

Sizing Calculations for Design of Digesters in Alumina Refinery

Hi Friends,

We are well aware that Digesters are the reaction vessels used for dissolution of alumina present in Bauxite in Caustic soda forming Sodium aluminate. Digesters are also known as Autoclaves in Alumina refinery. In present post, we will discuss the basic criteria and methodology used for design of digesters.

Basic Design Criteria: Since digesters are the pressure vessels fabricated with boiler quality steel in workshop by certified welders. The thickness of steel plate used for fabrication is 20 mm minimum. Thus the dimensions of digesters are decided based on the condition of connecting road for its transportation from workshop to project site. Number of digesters is decided based on the volumetric flow rate of digestion liquor and residence time in digestion circuit for designated plant capacity.

Design Calculations: The net volume of each digester has been taken as 200 m³ in arriving at the straight height of cylindrical vessels. Here, we have assumed the diameter of each digester as 4000 mm for sample design calculations. The cone angle of 60^o is kept at the bottom for easy discharge of digested slurry. The height of bottom discharge pot has been considered as 1115 mm. Cone end of the vessel has been added with a pipe having nominal diameter of DN900. The gross volume of each digester works out to 226 m³. Detailed sizing calculations are presented below-

The digesters of any volumetric capacity can be designed easily by following above stated procedures.

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Kunwar Rajendra

Systematic Approach for Design of Slurry Pumps for Alumina Refinery

Hi Friends,

In present post, 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.

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Kunwar Rajendra

Tube Digestion Technology for Processing of Boehmitic Bauxite

Hi Friends,

In earlier posts, we have covered variety of technological, design, engineering and operational issues of Alumina refinery. it's a truth that there is no end to advancements and thus tremendous scope for enriching our knowledge base. In continuation to our approach and efforts, we will discuss in brief about Tube digestion digestion technology as an alternative to Double digestion process for extraction of alumina. 
The digestion of predominantly Boehmitic Bauxite is carried out at an elevated temperature ranging from 220 deg C to 280 deg C, thus total thermal energy requirement for processing Boehmitic Bauxite is very high. As such Double digestion technology coupled with Pressure decantation system is a proven and established technology and many Alumina refineries are operative with this technology. The estimated overall energy consumption in efficient Alumina refinery with high temperature digestion technology is about 15 GJ per tonne of calcined alumina.
Another efficient digestion technology which was developed sometime in 1960s is popularly known as Tube Digestion Technology. In this process, wet ground Bauxite in caustic liquor followed by desilication is passed through long jacketed tubes / pipes at reasonably high velocity. The particle size of the Bauxite is controlled at around 100% minus 1000 microns. The tube jackets  contain high pressure saturated steam for raising the temperature of reaction mixture indirectly to ensure dissolution of alumina. The higher velocity of slurry in the pipelines (tubes) enhances the heat transfer coefficient and thereby heat transfer rate resulting in lower digestion time requirement. The retention time during digestion is generally not dictated by high dissolution rate of alumina rather dependent on chemical kinetics of quartz with caustic liquor. It has been practically observed in laboratory as well as plants that higher retention time across tubes during digestion at higher temperature above 250 deg C causes Quartz attack due to dissolution of Quartz with caustic soda which increases the formation of DSP and Cancrinite and thus additional consumption of caustic soda for alumina production. As per my personal experience with Boehmitic Bauxite, 15 minutes of digestion time in tube digesters will be adequate. The outcome of thermal energy calculations reveal that energy requirement with Tube digestion technology is comparatively less than that for other high temperature digestion technology. 
A photograph of Tube digestion system of an operative Alumina refinery is given below-

Facilities Installed for Tube Digestion System in an Alumina Refinery

Advantages of Tube digestion technology: There is a growing trend in adoption of Tube digestion technology in a few Alumina refineries in planning and execution stages in the World. Ma'aden Alumina refinery of 1.8 Mtpa production capacity at Ras az Zawr in Saudi Arabia is in advance stage of engineering and execution adopting this technology with the Basic engineering developed jointly by two leading engineering consultancy companies. This will be the biggest Alumina refinery in the World adopting Tube digestion technology. The plant is expected to be commissioned sometime in 2014. Tube digestion technology has following major advantages over other high temperature digestion technologies available in the World-

• Lower thermal energy consumption for digestion,
• Easy maintenance or replacement of equipment,
• Lower retention time for digestion,
• Flexibility in processing wide range of Bauxite,
• Lower evaporation requirement,
• Lower capital investment and
• Marginally lower operating cost. 

Total thermal energy consumption for alumina production adopting Tube Digestion Technology works out to about 12 GJ/t alumina which accounts for 9.0 GJ/t alumina for Bayer process (Hydrate) area and 3.0 GJ/t for Calcination area of Alumina refinery.

Disadvantages of Tube digestion technology: The scaling rate in inner surface of tubes will be high because of Boehmite reversion in tube digestion stage because of high alumina loading in liquor in presence of solid Boehmite. As such, the hardness of scale will be very high due to presence of DSP and Cancrinite. Also, caustic soda consumption will be higher due to dissolution of quartz and formation of Cancrinite as well. Thus following are the disadvantages of Tube digestion technology-

• High scaling rate in tube digesters and
• High caustic soda consumption.

At times, certain chemicals, like MgO, are added to bauxite slurry before entry to tube digesters for making porous scales for easy dislodging.

Since high temperature digestion beyond 250 deg C causes conversion of quartz to reactive silica, thus it is always recommended to maintain the digestion temperature at around 240-245 deg C and should avoid crossing 250 degree C to combat high caustic soda consumption for alumina production.


The statements presented here are purely based on personal experience and technical studies of the author on the subject. Hence, it is advised to carry out detailed evaluation of process by thorough scrutiny of process control data and related efficiency figures before implementation in any plant for commercial production.

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

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Kunwar Rajendra

Monetary Benefits by Reduction in Extraneous Dilution to Alumina Refinery

Hi Friends,

In present post, we will discuss the monetary impact of extraneous dilution to Bayer process in production of alumina and possible steps to control and minimize the extra water intake to process for converting the losses into monetary benefits. To analyse the dilution problems in Alumina refineries, let us run through the various process areas for identifying the major sources of extraneous dilution to process. The major sources are listed below-

1. High moisture in Bauxite fed to grinding unit,
2. Direct injection of steam to desilication tanks, digesters and causticizers,
3. Ingress of rain water to process areas,
4. Equipment flushing water to process,
5. Entry of gland seal water of rotating equipment,
6. Occasional hosing of floors of process areas,
7. Extra water intake for residue washing,
8. Extra water draw for hydrate washing,
9. Leakages in valves of fresh water line provided for intermittent washing of sight glasses and demisters of evaporators,

Extra dilution to process means additional evaporation load and thus high fuel consumption for meeting additional steam requirement. In earlier posts, we have already discussed in detail break up of water required for various process areas of Alumina refineries. 

Now let us assume a hypothetical and small reduction in fresh water to plant by 0.25 m3 per tonne of alumina which can easily be achieved, if we so desire, in any operative Alumina refineries of the World. Other assumptions for evaluating the monetary benefits are listed below-

(i) Fuel for steam raising is coal,

(ii) Calorific value of coal is around 5500 k.cal per kg,

(iii) Steam coal ratio in Boilers = 5.0,

(iv) Coal for bank firing = 4%,

(v) Issue price of coal = US$ 80 per tonne,

(vi) Steam economy in Evaporators = 4.0 .

With these assumptions, direct monetary benefits for a 1.0 Mtpa Alumina refinery works out to around US$ 1.04 Million per year. In addition to this indicated monetary benefits, there are many indirect advantages like higher liquor productivity across precipitators because of higher concentration profile which will result in reduction in specific electrical energy consumption per tonne of alumina production. Thus, it's worth taking steps to minimize extraneous dilution to process for substantial savings.

It is evident from above discussions that awareness of plant operating personnel and constant vigil of managerial staffs irrespective of their area of activities can contribute a lot in dilution control to process resulting in improving the profitability of the plant. 


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Kunwar Rajendra

Impact of Design Defects on Steady Operation of Alumina Refinery

Hi Friends,

We are well aware that alumina production is a continuous hydro-metallurgical process. The steady operation of plant and equipment results in remarkable plant performance. At times, minor design defects causes severe operational problems in Alumina refinery. Unsteady operation of Alumina refinery directly impacts the efficiency of the plant.Thus, all out efforts are made to minimize the design defects during implementation / modification stages of plant / equipment / facilities. 
In earlier posts, we have highlighted a few design defects and the methodology to overcome those limitations as well. In present post, we will discuss in brief a few additional  design defects which have been experienced by the author in the operative plants in India and abroad. A few design defects are briefly outlined in subsequent paragraphs-


1. Bauxite slurry heating system: Generally, heating of ground bauxite slurry at around 100 degree centigrade is carried out either during pre-desilication or before feeding to pre-desilication tanks. The heating of slurry is carried out to attain the desilication temperature by following four methods-

• Direct injection of steam to pre-desilication tanks,
• Indirect heating using Double pipe heat exchangers,
• Indirect heating using spiral heat exchangers and
• Indirect heating using Shell & tube heat exchangers.

Direct injection to bauxite slurry increases the dilution to process and hence never considered. Double pipe heat exchanger for bauxite slurry heating is not recommended because of acute scaling problems for the reasons already explained in earlier post. Last two options stated above are recommended among which the last one i.e. Shell & tube heat exchanger is the preferred choice because of lower capital investment as well as ease in maintenance.


2. Schedule of pipes for slurry handling: Slurry generated in red area of Alumina refinery has high abrasiveness because of presence of silica which causes erosion of pipelines. Thus pipelines of adequate thickness > schedule 80 should be installed.  Schedule-40 pipes should always be avoided as it will require frequent replacement.


3. Selection of MoC for pumps, valves and fittings: As such carbon steel is the best suited material of construction for Alumina refinery having alkaline environment. But presence of silica causes fast erosion in stationary as well as rotating equipment. Thus, for handling abrasive slurry in red area, equipment / components made of carbon steel as well as Nicast are never used. For such critical service conditions, Nihard / high nickel steel / high chrome steel is the right choice. Never compromise on MoC of impeller and casing of slurry pumps, connected valves and inlet Tees of slurry flash tanks particularly for handling Bauxite slurry. Ensure the right MoC for these critical components for steady operation of Alumina refinery.


4. Minimum shell thickness of Digesters: Digesters are the pressure vessels installed for dissolution of alumina present in bauxite in caustic liquor. The digestion vessels are designed on the basis of its recommended operating conditions. The shell thickness is always based on the pressure but shell thickness is always kept more than 20 mm to take care of any eventualities because of instrument failure and operational lapses as well.


5. Provision of non-return valve in steam lines to process vessels: Alkalinity in boiler feed water is controlled below 0.01 gpl Na2CO3. Though direct injection of steam to process are always avoided but a few designers keep provisions. In such situation, non-return valve in steam line near process area is a must as sudden drop in boiler pressure for any reason may case backing of alkaline liquid to steam lines which may develop stress corrosion  ultimately causing sudden failure of steam line within short duration. It is essential for safeguarding the equipment as well as safety of operating personnel in the plant.


6. Provision of flanges in pipelines: Sometimes, flanges are not provided in slurry pipelines to minimize capital investment in project stage. But, it cause severe operational problems because of difficulties in cleaning of welded lines. Thus flanges must be provided at the interval of 6m to 10m depending of practical experience operating personnel.

7. Adequate capacity of Evaporator Hot well: Tail pipe of Barometric condenser is kept submerged to a depth of about 0.50 meter in water of Hot well of evaporation unit. The Hot well is designed to maintain adequate seal water for circuit. The volume of Hot well is always kept more than five times the submerged volume of leg of Barometric condenser. If water volume in Hot well is inadequate, the vacuum seal will be broken and air drawn through tail pipe will drastically affect the performance of evaporation unit.


8. Minimum plate thickness of tail pipe of Barometric condenser: Generally, the vacuum requirement for efficient evaporation system is > 685 mm of Hg. Thus, the minimum acceptable plate thickness for the tail pipe is 6 mm. Otherwise, sudden increase in vacuum during start up with negligible load of non-condensables may cause rupture of tail pipe of Barometric condenser.


9. RPM of impellers of slurry pumps: RPM of impeller is the prime criteria for selecting the pumps for Alumina refinery. In Alumina refinery, impeller rpm for handling residue as well as hydrate is relatively kept at lowest possible level so as to avoid breaking of the flocs and particles. This issue has already been discussed at length in earlier posts.


These are just major examples of design defects of Alumina refinery. Please put your views / suggestions / remarks / comments, if any.



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Kunwar Rajendra

Basic Technical Data Required for Process Design Calculations

Hi Friends,

In present post, we will discuss a few important data which are essentially required for carrying out process design calculations including material balance, thermal energy balance and sizing calculations for various pipelines of Alumina refinery.

1.     Alumina means Al₂O₃ having molar mass = 2x27+3x16=102 g per mole.

2.  Hydrate alumina means Al₂O₃.3H₂O having molar mass= 102+3x18=156 g per mole.

3.  Thus 156 tonnes of Hydrate alumina = 102 tonnes of Alumina, this implies 1 tonne of Hydrate is equal to 102/156 = 0.65385 tonne of alumina.

4.     Specific gravity of dry hydrate = 2.42

5.     Specific gravity of dry Bauxite = 2.55 to 2.80

6.     Bulk density of alumina = 0.95 to 1.05 tonne per m³.

7.     Molar mass of Na₂O = 62 gm/mole ;

8.     Bulk Density of Bauxite = 1.20 - 1.30 t/m³.

9.     Molar mass of Na₂CO₃ =106 gm/mole;

10. Static Angle of Repose of Bauxite = 30 to 32 degree

11. Dynamic angle of repose of Bauxite = 19 to 20 degree

12. Molar mass of Na₂O = 62 gm/mole

13. Bulk Density of Bauxite = 1.20 - 1.30 t/m³

14. Molar mass of Na₂CO₃ = 106 gm/mole

15. Molar mass of NaOH = 40 gm/mole

16. Hardness of Bauxite = 2.5 to 3.0 Moh

17. Bond Index = 10.5 t0 18 kW/st

18. Specific Gravity of dry redmud = 2.30

19. Velocity of slurry in pipe line = 1.5 to 2.44 m/sec.

20. Design velocity of steam in pipeline = 20 to 40 metres per sec.

21. Water requirement  = 135 litres / capita.day

22. Sewage generation rate = 96 litres/capita.day.

23. Solid waste generation = 0.90 kg/ capita.day

24. Stack Height (in metres) = 14x (Q)^0.3;where Q=Emission rate of SO₂ in kg/hr.

25. Total water for Alumina Plant = 6.5 m³/tonne alumina

26. Norms for Green belt = 25% of total plant area (min.)

27. Residue Factor is the ratio of sum total oxides in Bauxite to sum total oxides in residue (red mud) which gives tonnes of residue generated per tonne of Bauxite processed. Residue generation rate with respect to alumina production varies from 1.10 - 1.35 tonnes residue per tonne of alumina. Higher residue factor signifies either poor quality of bauxite or lower alumina recovery or both.

Hope, all major input data have been covered here. It will be our pleasure to receive your valuable remarks / suggestins / comments, if any.

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Kunwar Rajendra

Severe Damage in Steam Line of High Pressure Digestion Unit

Hi Friends,

In present, we will present a few photographs to assess the severity of damage in bypass line of high pressure steam line in high pressure digestion unit of an operative Alumina refinery adopting  Double digestion technology for processing of gibbsite-boehmite mixed bauxite. It may not be proper to mention the name of technology supplier, detailed engineering consultant, erector and plant owner as fault finding is not our objective. These photos have been presented to take a lesson as precautionary measures to prevent such incidents in future.  This incident occurred in a newly constructed Alumina refinery within a year of commissioning. It was a sudden burst and you can have the feel of severity of damage by observing the following photographs. I am thankful to my friends who have shared these photographs with us, specifically for the benefits of professionals who are involved in planning and execution stage of Alumina refinery in any part of the globe.

The reasons for damage of  bypass line in high pressure steam line are still a matter of investigation. The possible reasons may be one of the following-

• Material defects,
• Design defects,
• Welding defects,
• Stress corrosion,
• Combination of two or more of above.

The exact reason will be known as outcome of investigations in progress. Thanks to almighty God for safeguarding the operating personnel  as no body was present in the vicinity of indicated steam bypass line area at the time of incident. Thus it's a word of caution for all concerned to take all possible measures to avoid such incidents in future.

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Kunwar Rajendra

Major Facilities for Processing Gibbsite-Boehmite Mixed Bauxite

Hi Friends,

In earlier post, we have discussed about the Technology for processing of gibbsite-boehmite mixed bauxite in Alumina refinery. Here in present post, we will discuss briefly about the major technological and services facilities required for setting up new Alumina refinery for processing of gibbsite-boehmite mixed bauxite. Considering the recent developments and efficiency of operative Alumina refineries in the World operative on commercial scale have been taken in to account to highlight the major facilities for forthcoming plants are given below-

Adoption of Double digestion system, Pressure decantation and Two stage continuous precipitation are the proven process techniques which attribute in marginal reduction in the energy consumption compared to other established process technology for gibbsite-boehmite mixed bauxite in the World. A few Alumina refineries adopting this technology are operative and under execution as well. As sizing calculations for all these equipment and facilities have direct bearing on capital investment of the project, thus it requires systematic approach and inputs from experienced experts in the field while designing and execution of plant.


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Kunwar Rajendra