Is Low Silica in Bauxite a Problem in Bayer Alumina Plant ?

Hi Friends,

After a long gap, I have appeared before you to interact on a small but very critical process issue of Bayer Alumina Plant.

In the past, we have always discussed about the negative impact of high silica in bauxite specifically with regard to operational problems, process control issues, higher consumption of input materials and deterioration in quality of product hydrate or alumina. In previous articles, this issue was elaborated at length with measureable / quantifiable approach so as to focus on its importance in Bayer Alumina Refineries.

In present post, we will briefly discuss about the advantages and disadvantages of low silica in processing bauxite for production of calcined alumina.  The question mark (?) on the subject itself forces us to think and re-think about the positive and negative aspects of low silica in bauxite.

It’s a well-known fact that all Alumina producers wish to have low silica bauxite for obvious reasons discussed earlier. Now, before coming to the subject directly, following questions arise-

(i)  What type of silica is harmful for Bayer’s process?

(ii) What should be the cut-off silica content in bauxite for acceptable quality of alumina?

Though we have discussed   these issues in several articles, still it is prudent to freshen up with salient features as outlined below-

We are well aware that predominantly there are two types of silica in bauxite designated with regard to Bayer process of alumina (a) Non-reactive silica and (b) Reactive silica. ROM bauxite mined adopting either open cast or underground mining methodology contains a number of metallic oxides including silica. This mined silica as percentage of total dry bauxite is termed as total silica. The part of total silica which reacts with caustic soda and forms sodalite complex through sodium silicate route is termed as Reactive silica. Majority of Reactive silica is found in form of Kaolinite. And thus, the non-reacting component of total silica with caustic soda is popularly known as Non-reactive silica. Quartz is non-reactive silica. The silica being abrasive in nature causes erosion in components of various process equipment like heat exchangers, pipelines, valves, pump casing and impellers etc. Thus higher silica content affects the capital investment of Alumina refinery because robust material of construction for equipment component in contact with bauxite slurry. Hence higher silica in bauxite causes higher initial investment for the plant and higher repair maintenance cost for routine replacement of critical components of equipment.

Generally, it has been seen from bauxite analysis that Reactive silica in bauxite ranges from 50% to 80% out of Total silica in bauxite. Plant operation is affected because of high silica in bauxite as it has high settling velocity and abnormally high abrasive nature.

Now coming to the topic with raised question mark, it’s true that low reactive silica in bauxite is considered as a serious process problem as it directly affects the quality of product alumina because of higher silica content in alumina. The SiO₂ content in calcined alumina produced in all modern alumina a refinery is controlled at around 0.015%. The alumina having higher silica content is considered as inferior quality. For achieving 0.015% SiO₂ in alumina, the SiO₂ in aluminate liquor feeding to crystallizers / precipitators should be around 0.45 gpl.  The ineffective desilication because of delay in achieving super-saturation of silica across desilicators, gpl SiO₂ in liquor increases to much higher level which ultimately deteriorates the product alumina quality.

The above elaboration concludes that moderate level of Reactive silica in bauxite at around 1.5% in bauxite is considered to be the best with regard to plant efficiencies and product quality as well.

Trust, the subject has been covered here to your desired level of satisfaction in understanding the criticality of the subject issue. We would welcome your  valued comments / remarks to further substantiate the subject.

Regards.

Kunwar Rajendra

Bauxite : A Major Concern for Chinese Alumina Refineries

Hi Friends,

All of us are well aware that China has added the Alumina refinery capacities at faster pace during last decade though they have the limitation of quality bauxite for processing at economic level. Chinese bauxite is poor in quality as it has diasporic alumina coupled with very high reactive silica.

It will be prudent to mention here that diasporic alumina extraction is highly energy intensive and high reactive silica in bauxite causes higher Caustic soda consumption for alumina production. Also, the mining cost of bauxite in China is comparatively higher compared to other countries. Thus, imported bauxite from neighboring countries is mixed with locally mined bauxite before processing in plants. Following details will make the elaboration easy for understanding. 

Current production cost of calcined alumina in China is ranging from US$320 to US$380 per tonne of calcined alumina. The broad break up of cost components are outlined below-

• Bauxite : US$90 to US$170 per tonne of alumina
• Caustic : US$50 to US$60 per tonne of alumina
• Energy  : US$140 to US$180 per tonne of alumina
• Others   : Us$50 to US$60 per tonne of alumina.

Among Chinese Alumina refineries, Guangxi Alumina Plant has the lowest production cost at around US$320 per tonne of calcined alumina. However, production cost at Shandong Alumina Plant in China works out to around US$380 per tonne of calcined alumina.

The data published data clearly reveal that Indonesia had been the largest exporter of bauxite to China. In the year 2012, Indonesia exported over 36 million tonnes of bauxite to China where as the balance bauxite of about 9 million tonnes was exported by Australia, India and other countries.

Now, Indonesian Government has increased the export tax from 20% in 2012 to 50% in 2013-14. Also, the government has announced complete ban on export of bauxite from 2014 onward as they have decided to preserve their natural resources for the existing and upcoming Alumina plants in their country. The article on "Promising Alumina refinery projects in Indonesia" may be of interest to you.

Looking at the above scenario, it has become a very serious issue for China to keep their plants operative in years to come. Presently, China is consuming its total domestic alumina in their own Aluminium smelters for production of aluminium metal. 

In present crisis of bauxite availability from Indonesia, China is exploring the possibility to import bauxite from Guinea. But higher transportation cost of bauxite from Guinea will make the production cost further higher for production of calcined alumina in China.

Further, to enhance your data base, you may be interested to read Benchmark figure of Energy Consumption in Alumina Refinery.

We have assimilated the above information based on the published literature available on public domain in the interest of our friends of bauxite-alumina-aluminium fraternity across the globe.

We solicit your comments / remarks, if any.

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

Capex, Opex and Techno-economic Viability for Green-Field Alumina Refinery in India

Hi Friends,

Today is the great day for all of us who are deeply associated with this knowledge sharing technical blog on Alumina processing technology. It is needless to mention here that our blog has earned a prestigious position among the people of Bauxite-alumina-aluminium fraternity in short span of time. Congratulations to all concerned on the eve of crossing the total viewer mark of 130,000 today. This glorious moment is remarkable and memorable as we are proceeding ahead in setting the new height of success. We are continuously receiving appreciations from our learned friends for high level of quality of content published so far and feedback for further improvements. 

In present post, we are bringing out some special facts which will be useful for the top notch officials in taking strategic decisions with regard to capital investments, Operating cost and techno-economic viability aspects for setting up new Alumina refineries in India. 

As such, only a few Alumina refineries were constructed and commissioned across the globe during last decade. Broad details and present status of new refineries executed over last 15 years / under execution presently  in India are outlined in subsequent paragraphs-

1. 1.4 Mtpa Vedanta Alumina Refinery, India: This plant was designed for 1.4 Mtpa alumina with two streams each stream having 0.70 Mtpa production capacity. One of the stream of this plant was commissioned  in 2006 with non-designated source of bauxite as Niyamgiri deposits could not be cleared  by the Government of India for carrying out bauxite mining activities.  The specific capital investment for this plant was very low as the company took advantage of volatile market of equipment and facilities during execution period. But processing of low alumina content and high reactive silica content in bauxite of unidentified areas during commissioning itself made the plant uneconomical. Finally, the shutdown of the plant was forced on 9th December 2012 because of non-availability of bauxite. Plant remains shutdown as yet. However, the Vedanta management is striving to restart the operation of this plant shortly even with imported bauxite.
2. 1.4 Mtpa Anrak Alumina Refinery, India: This plant is located at about 70 km from Visakhapatnam in the state of Andhra Pradesh in India. This Alumina refinery has two streams of 0.70 Mtpa each and the execution is expected to be completed for commissioning by end of the current year - 2013. Most of the equipment and facilities of this plant have been installed and one of the process streams out of two are in readiness for trial but no adequate bauxite available as the earmarked Jarrella bauxite deposits are yet to be cleared by the Government. The trial run for Stationary calciner supplied by Outotec Germany was done in early first week of April 2013 with purchased hydrate and a few thousand tonnes of calcined alumina could be produced. HSD was used as the fuel for firing followed by HFO for calcination during commissioning trial. The estimated specific capital investment of this plant was much higher than that of Vedanta. But this plant could save substantially on technology, engineering and site activities during implementation.
3. 1.5 Mtpa Utkal Alumina Refinery, India: This plant is owned by Aditya Birla group of companies. The execution of this plant was conceived sometime in 1996. The construction of this plant is in advance stage and mechanical completion of this plant is expected by the end of 2013. This plant has been designed by Rio Tinto Alcan Australia who are considered leader in process technology across the globe. But because of inordinate delay in implementation because of various socio-political reasons, the capex of this project has increased substantially. Thus the specific capital investment of Utkal alumina plant was much higher than targeted figure. 
4. 1.4 Mtpa JSW Alumina Refinery, India: Under diversification plan, JSW group company decided to set up alumina refinery in Vijaya Nagaram district of Andhra Pradesh with state of the art and most modern plant associating Rio Tinto Alcan Australia as technology supplier in the year 2005. The company has acquired adequate land but execution of project is under hold awaiting clearance from Government of India for bauxite mining from Araku group of deposits.
5. 1.0 Mtpa NALCO Alumina Refinery, India: National Aluminium Company (NALCO) has initiated studies for setting up 1.0 Mtpa Alumina refinery near Mandavi in Gujarat. As per our estimates, the specific capital investment for this project will be high at around US$ 1000 per   tonne of Calcined alumina mainly because of Lignite based Co-generation plant, required Desalination plant, marginally bigger size equipment requirement because of lower alumina recovery from available bauxite in Gujarat and additional expenditure towards development of infrastructure facilities like port etc. for the project.

The capital expenditure data analysis clearly reveal that specific capital investment in setting up Alumina refineries in India varies from US$ 650 to US$ 1000 per tonne of calcined alumina depending on geographic location of the plant in the country. The capex will be lowest for the plant in Orissa, Gujarat and Jharkhand where as the same will highest in Gujarat.  The reasons are well known to all concern as elaborated in above paragraph. Also, the opex will be higher mainly because of poor plant efficiency figures due to wide variation in mineralogical and chemical composition of basic input material, Bauxite. Also, the volume effect of plant capacity because of low reserve of Bauxite in Gujarat is the other reason. It will be prudent to mention that comparatively lower grade of bauxite, non-availability of desalinated water, availability of low calorific value solid fuel source (Lignite) for co-generation plant are the main reasons for highest capex. Again, on opex part, the same will be abnormally high because of poor plant efficiency figures coupled with low grade of bauxite in Gujarat. At current price level of Caustic soda, Heavy furnace oil and Lignite, even the economic viability for Alumina refinery in Gujarat is a question mark because of high specific capital investment of around US$1000 per tonne and estimated operating cost excluding financing charges (interest & depreciation) at around US$ 200 per tonne of Calcined alumina as per preliminary estimates.However, the actual figures of Capex and Opex can be ascertained by Project Owner with detailed studies before taking financial decision on the matter. In spite of all these facts stated above, Green-field Alumina refineries will be installed mostly in India and Vietnam in coming decade because of huge bauxite reserves of comparatively improved quality than other countries across the globe. In other words, India and Vietnam are the future destinations for green-field Alumina refineries in years to come.

Trust, the above deliberation will give a glimpse of required capex, opex and economic viability aspects for setting up green-field Alumina refinery to one and all across the globe. The author of this technical article will welcome your comments on the subject, if any. Please feel free in sharing your views.

Regards.

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

BENEFICIATION OF BAUXITE FOR ENRICHMENT IN QUALITY

Hi Friends,

In continuation to my earlier posts, I thought of sharing my views on beneficiation of Bauxite with the objective to optimum utilization of natural resources available with us. I am presenting here my views on the subject as outlined here under in subsequent paragraphs.

The mineral, Bauxite, is a rich source of alumina for making aluminum. The aluminium metal and its alloys have strong growth potential in international market for wide range of industrial applications. Bauxite is also used in refractory, cement, absorbents, steel, abrasives, rubber, plastic, cosmetics, paints, paper, polishes, glass, enamel and ceramics.

Since bauxite is a mineral occurring in earth’s crust, it has a number of impurities, like iron, silica, titania, calcium, and small quantities of phosphorous, sulfur, zinc, magnesium and various carbonate and silicate minerals. These impurities create quality problems during processing thereby increase production costs. Therefore, the removal of impurities to the extent possible economically is essential before processing further for any application.

Most mines abroad subject their run of mine ore (ROM) to the mineral dressing operation most suitable for their material. Crushing the ROM, usually in hammer mills, is an operation practiced worldwide. Depending on the bauxite, and the quality/grade required, the next stages are screening, scrubbing and washing, magnetic separation and drying. Magnetic separation is done to remove iron, however, is normally practiced in a limited way to produce high value, special grade bauxites. At present, the production of bauxite in most operative mines in India is not sufficiently high as to warrant a capital-intensive beneficiation plant, which usually requires a large capacity to be really economical. Hence, ore dressing efforts have been mainly confined to removal of silica by manual and/or mechanized breaking, crushing, manual sorting and dry screening.

Efforts towards research are being emphasized in most bauxite producing countries to develop cheap and innovative bauxite beneficiation processes. Many new methods of magnetic separation are also being tried out. These include rare earth roll separators, super conducting high gradient magnetic separators and open gradient, non-cryogenic high gradient magnetic separators. Other methods being researched include fluidized bed acid leaching, hydrogen assisted beneficiation and bio leaching. However, despite decades of intensive studies, economical bauxite beneficiation technology for removal of impurities satisfactorily is not available as on date.

Research in India is along international lines but still confined to the laboratory. The Indian Bureau of Mines, Nagpur (IBM), the Jawaharlal Nehru Research Development and Design Center, Nagpur (JNARDDC), and the Regional Research Laboratory, Bhubaneswar (RRL), all claim to have laboratory scale processes that are ready to be up scaled to pilot plant levels. The IBM has done numerous beneficiation tests, using different methods, on various bauxites found in India. The RRL claims to have developed a beneficiation process to produce non-metallurgical grade bauxite, whereas the JNARDDC says it has processes for both metallurgical and non-metallurgical grades.

As per the available published literatures and analysis, the savings on raw ore cost brought by JNARDDC's process through beneficiating metallurgical grade bauxite appears to be over 17% and that of non-metallurgical grade at least 19%. Bauxite users in India have indicated that they are willing to pay between 10-50% higher for a reduction in impurities by a similar amount. This would make the production and marketing of beneficiated bauxite more profitable.

We will welcome your comments / remarks on the elaborated topic.

Regards.

                        Kunwar Rajendra

It's Right Time to Write Our Success Story

Hi Friends,

It gives me immense pleasure to share this moment of achieving a record number of viewers of over 50,000 of our popular technical blog in such a short span of time. With this achievement, we have reached to a new height of success. All of us have contributed in this remarkable  success thus it is the right time to celebrate.

Let us recall in brief the moments of this glorious event as Success Story. Initially, I had no idea about sharing the knowledge through known and unknown friends as I have limited knowledge about the computers and internet as well. I would like to express my sincere thanks to my elder son, Vivek, for educating me about the blog. Vivek being a Computer engineering professional inspired me to write something about my areas of expertise. 

Soon after, I took the lead and published a few lines assuring my friends to share the technical knowledge of public domain nature pertaining to bauxite, alumina and aluminium. Till date, we have followed this strict ethical principle in publishing each and every technical article on this blog. We have never copied any information from any published literature. We have always used our simple language in expressing our own personal thoughts. 

Of course, right in the beginning itself, I had a dream to achieve a number of 100,000 viewers in  a time span of five (5) years and today on auditing of data, we find that we made a very conservative target.

So far, we have published over 265 technical articles and from last few months we have not added new article because of paucity of time. However, my friends all over the World are enjoying the published articles every moment.

This blog has given me thousands of global friends in every corner of the World as this blog has the in-built translation facility to read in any international language.

At this moment, I would like to thank to Google Management to provide such a lovely platform free of cost for sharing our knowledge.

Let us join hands to assimilate technical data related to design, operation, engineering, technology, execution, commissioning and other related issues on bauxite, alumina and aluminium and enrich our knowledge by exchanging those data with one other. This technique will make all of us more knowledgeable in the field. Trust you will agree with my views.

I take the opportunity to wish you a very happy, healthy and prosperous new year.
Thanks and regards.

Kunwar Rajendra
www.bauxite2aluminium.blogspot.com

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

Major Requirements for Successful Commissioning of New Alumina Refinery

Hi Friends,

In continuation to our earlier post, we would like to discuss the requirement of critical facilities, quantity of input materials and documents in readiness for successful commissioning of new Alumina refinery as described here under.

Readiness of plant and facilities:

It is essential to keep the keep following plant, equipment and facilities before starting the commissioning of Alumina refinery-

• Bauxite receipt, unloading, stacking and reclaiming system.
• Readiness of Bauxite crushing, Grinding, Predesilication, Digestion, Residue settling & washing, Security filtration, Heat interchange unit, Precipitation, Classification, Evaporation, Hydrate storage, Calcination  and alumina handling system.
• Major infrastructure facilities like co-generation plant, raw water supply system, cooling towers, compressed air station, emergency power supply system, railway and road network.
• Readiness of Process water storage and Drinking water distribution network.
• Storage vessels for Caustic lye, Fuel oil and other consumables.
• Availability of filter cloth, lubricants and other consumables.
• Adequate stock of input materials like Bauxite, Caustic soda, Coal, Fuel oil, Diesel, Burnt lime, Acids, Flocculent and Seed hydrate.
• Availability of experienced technical executives and operating workforce.

Materials Requirement:

Adequate quantity of major input raw material must be ensured in the stock for smooth commissioning of Alumina refinery as stoppage of plant after commissioning may cause losses for the company. Major raw materials are Bauxite, Caustic soda, water, lime, seed hydrate, flocculent and furnace oil / LSHS.

Documentation Requirement:

Following documents are required for systematic commissioning of Alumina refinery-

• Detailed commissioning procedures.
• Step by step process commissioning sequence.
• Safety and risk management Plan.
• Broad schedule for commissioning.
• Organization chart of Commissioning team.
• Training of commissioning team.
• Formats for recording various parameters and activities.

The above information will suffice the commissioning requirement of new Alumina refinery. Please put your views / suggestions / remarks / comments, if any, further improvements in planning the required activities.
Regards.


Kunwar Rajendra
www.bauxite2aluminium.blogspot.com

Technology Driven and Equipment Oriented Units of Alumina Refinery

Hi Friends,

We have studied so far in detail the design and operating features of Alumina refinery. We have also made ourselves well acquainted with the technological advancements as well as modifications and improvements in major equipment considered for efficient operation of Alumina refinery.

In present post, we will discuss the highlights of our previous posts with more specific to technology and equipment oriented unit operations and unit processes of Alumina refinery. The process sequence of various units of Alumina refinery has been shown below-

As such, unit operations are complicated and unit processes of Alumina refinery are complex in nature because of inter dependence on the major operating and process parameters of upstream and downstream units as even minor deviations / fluctuations in process control parameters abruptly affects the plant efficiency figures. Over a period of time, lots of developments in process technology as well as improvements and modifications in equipment have occurred which has resulted in drastic reduction in overall energy consumption and raw material consumption as well ultimately improving the profitability of the plant. 

Based on our studies and experience in the field, we have clearly depicted in above diagram, the technology driven and equipment oriented process and operating areas of Alumina refinery. Selection of right technology and right equipment matching with optimized process and operational parameters will result in improved efficiency figures thereby lower capex and opex of Alumina refinery. 

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

Regards.

Kunwar Rajendra