Bauxite Digestion Technology for Processing of Gibbsite-Boehmite Mixed Bauxite

Hi Friends,

We appreciate your remarks on "Typical schedule for commissioning of Alumina Refinery" as realistic and systematic approach. All our papers are based on our experience in operative Alumina plants in various parts of the globe.

In this technical paper, we would like to present different techniques for processing of Gibbsite-Boehmite mixed Bauxite prevalent in various existing Alumina refineries in the World and placing our views taking into account all techno-financial aspects. 

Presence of boehmite in the range of 4 to 6% of boehmite mixed with gibbsite in Bauxite is considered negligible as it is not considered economical to recover such small component of boehmite. Thus minimum boehmite content of 8% with remaining gibbsite compensating for module more than 8 is considered viable to process using Modified Bayer process. Various technological options for digestion of boehmite-gibbsite mixed bauxite in Alumina refinery are briefly described below-

1.       High temperature digestion technology: 

In this process, mixed bauxite is digested at temperature between 240 to 250 degree centigrade at about 35 kg/cm2 pressure with 5 to 7 stages of flashing for heat recovery system. Total thermal energy requirement in this process is very high to the tune of 18 to 21 GJ per tonne of alumina. Though many existing plants with this technology are operative even today but not being adopted in new Alumina refineries now because of high operating cost.

2.       Double digestion technology:

This technology has advantages over high temperature digestion technology as energy consumption is comparatively lower. In this process, the Bauxite is first digested at low pressure at digestion temperature varying from 140 to 145 degree centigrade and the settler underflow residue is again digested at high temperature at process conditions same as high temperature digestion. Thus overall energy consumption is substantially lower compared to high temperature digestion. This technology reduces the overall energy consumption of about 1.5 GJ per tonne of alumina compared to high temperature digestion technology.

3.       Two step digestion technology:

This digestion is almost same as Double digestion technology wherein Bauxite at 1st stage is digested at atmospheric pressure at around 105 to 110 degree centigrade keeping 2nd stage exactly the same as high temperature digestion. In this technique, the bauxite is digested at atmospheric pressure and settler underflow residue is again digested in second stage of digestion circuit. This technology consumes less thermal energy and lower caustic soda consumption due to partial formation of sodalite complex. Alumina extraction is comparable other available digestion technology discussed above. The total thermal energy requirement adopting two step digestion is estimated at around 14.0 GJ per tonne of Alumina.

4.       Tube Digestion technology:

Tube digestion technology has been developed on pilot plant scale and in the process of commercialization. The details of Tube digestion technique will be covered in forthcoming posts. In this technique, bauxite slurry with desired amount of caustic soda is passed at high velocity through bundle of tubes. The residence time requirement for digestion is very low. Material and energy balance calculations with optimized process parameters and heat recovery system reveal that total thermal energy requirement adopting tube digestion technology should be around 12.0 GJ/t alumina which accounts for about 9 GJ/t for Bayer process (Hydrate) area and 3.0 GJ/t for calcination area.

Out of above four techniques, it is evident that Tube digestion technology will require less capex and opex. Pilot plant scale trials with this technology have shown encouraging results in all respect. A newly constructed Alumina refinery has adopted Tube digestion technology for their 1.8 Mtpa Alumina refinery in Saudi Arabia. We strongly feel that the presence of lime at high digestion temperature of 260 to 270^oC will cause high scaling rate on inner surface of tubes. Hence it may be essential to take care of the tube cleaning while adopting this technology in any Alumina refinery.

Our technical paper on "International benchmark of energy consumption in Alumina refinery" may of interest to you.

It is advisable to carry out careful evaluation of all pros and cons of considered digestion technology before taking final decision in implementing any of the above-mentioned digestion technology in the plant. The content of this paper is purely our own experience and views in the interest of global bauxite-alumina-aluminium fraternity.

Your comments / remarks on above will be highly appreciated.

Best regards.

Rajendra Kunwar

CETI Enterprises.

www.ceti.co.in

Systematic Approach for Design of Slurry Mixing Agitators

         Hi Friends,

In various technical forums, process experts as well as equipment manufacturers have opined that the design of agitators for mixing process slurry handled in Chemical industries is complicated and tricky issue. In this paper, we would like to discuss the subject in very simplified manner with systematic approach starting with brief description of involved terminology, associated design parameters and methodology with sample motor rating calculations for the slurry mixing agitator.

Basic Formula Drive Rating Calculations for Agitator:

Power Required for Agitator Drive Motor = N_p X ρ x N³ x D⁵ watt for 1 Impeller.

Where Np = Power number (Dimensionless number dependent on type of impeller),

ρ = Density of Fluid in kg/m3,

N = Revolution per second and

D = Diameter of Impeller in m.

Indicative Power Numbers for Various Types of Impellers:

S.No.	Type of Agitator Impeller	Power Number (Np)
1.	Four bladed 45 degree Pitch Turbine	2.1
2.	Rushtom Turbine	5.5
3.	Propeller Type Impeller	0.70
4.	Hydro-Foil Type Impeller	0.30 to 0.51

Sample Calculations to Arrive at Drive Motor Rating for Agitator: 

Simplified sample calculations have been presented to arrive at the drive motor rating for the agitator of a Process tank having around 3000 m³ gross capacity with key input figures as well as realistic assumptions-

Fluid height in the tank, H = 16 m and Diameter of tank, D_t = 14 m

Slurry volume in tank   = π *D_t²*H/4 =  π * (14)²*16 /4 = 2463 m³   

Solid consistency in Slurry = 50 % (w/w),   Density of slurry, ρ = 1602 kg/m³,

Viscosity of slurry, μ = 550 cp

Agitator Impeller Diameter, D_i= 33 % of tank diameter = 14 * 33% m = 4.62 m

Tip speed of Impeller = π *D_i *N = 3.1416 * 4.62 *20 = 290 m/minute,      

 Drive motor RPM = 1500 rpm, Gear Box Reduction Ratio = 75

∴ Agitator RPM = Drive Motor RPM/Gear Box Reduction Ratio = 1500/75 = 20 rpm,

Thus revolution per second of impeller, N = 20/60 = 0.333 Revolution per second.

Flow Number N_q   = 0.56 and Power Number,     N_p  = 0.51 (assumed figures for Impeller)

∴ Pumping Capacity  =  N_q * N * D_i³  m³/ second

= 0.56 * 0.333 * (4.62)³ = 18.41 m³/sec.

Area of Tank               = π  * D_t²   = π  *(14)² / 4 = 153.94 m²

Bulk fluid Velocity      = pumping capacity/area of tank

= 1104.44 / 153.94= 7.18 m/min. = 23.55 ft./min.

Degree of Agitation   = bulk fluid velocity / 6     

(For 6 ft/min., degree of agitation =1 and Degree of agitation varies from 0 to 10)

= 23.55 / 6 = 3.93 ~ 4

Annular Area = π  * (D_t²- D_i² ) /4  

Where D_t = Diameter of tank      and        D_i = Diameter of impeller in meters.

= 3.14 * (14² – 4.62²) / 4 = 137.18 m²

Rising velocity of particles = pumping capacity / annular area

= 1104.44 / 137.18 = 8.051 m/min.  = 0.1342 m/sec. 

Tank Turnover rate   = Pumping capacity / tank capacity

= 1104.44 / 2463 = 0.45 times / min.

Power Number N_p    = 0.51

Shaft Power,      P      = N_p* ρ *(D_i)⁵ * N³ /1000 kW

Where N_p = Power number of impeller (Dimensionless number),

D_i = Diameter of impeller in meters, Shaft RPS,             N = revolutions per second,

∴ Shaft Power, P   = 0.51 * 1602 * (4.402)⁵ * (0.333)³ /1000 kW = 49.86 kW

Taking Gear Box Efficiency   = 80% and Drive Motor Efficiency = 95%,

Design margin           = 1.15

∴ Drive Motor Rating = 1.15 * 49.86/(0.80 * 0.95) =73.95 kW.

Thus the drive motor of about 75 kW shall be adequate for successful operation of agitator of 3000 m³ Process tank containing fluid of considered solid content. 

Conclusions:

The developed methodology clearly reveals that motor rating calculations for any slurry mixing agitator can be carried out easily by simply replacing the associated input process conditions, operating parameters, dimensions of tanks / vessel and appropriate power number for impeller in above simplified derivation. 

Trust, you will like the methodology adopted in this technical paper. We would like to have your comments.

In case, your company needs assistance of our team in resolving any technological, process, engineering or operational problems of your plant; we may be contacted. We are operating out of Gurugram, India. You may like to have a look at our website.

Regards.

Rajendra Kunwar

E.Mail: rajendra@ceti.co.in

Our Website: www.ceti.co.in

Footsteps for Enhancing Data Bank on Bauxite, Hydrate, Alumina & Alumina Chemicals

Hi Friends,

In present post, we have put forward our aggressive steps to enhance the inventory of technical data bank pertaining to bauxite, hydrate, alumina and alumina chemicals covering all spheres of activities for all friends of our professional fraternity.

It's my natural temperament to exchange ideas particularly on technical topics. It is also the truth that blessings of my superiors and best wishes of my friends have attributed in enriching my knowledge base which further encouraged me to move forward with accelerated pace in exchanging ideas and opinions. I do not believe in diary culture in hiding information unnecessarily in my desk. I know, I have very limited data / information with me compared to many others in the World. My this approach has given me more than 2500 trust-worthy friends who are directly or indirectly associated with bauxite, hydrate, alumina and aluminium including specialty chemicals. As such, I have faced many road blocks in my career path so is impossible for me to forget the blessings of society and almighty God. Majority of my life, I have enjoyed with smile. What ever work I do, I get immense pleasure in contributing to my fullest satisfaction.  

During early days of my professional careers, I faced acute problems in learning technical things as limited people had the desire to exchange their fresh or outdated perceived ideas as they used to keep those data as very confidential just like keeping the personal property in the Bank lockers. Those days i.e in early eighties, internet in India was available to defense people only. You can imagine the extent of difficulties faced by me in learning the basics and design aspects of process, calculations, equipment, capex, opex and profitability aspects. Truly, it was hard time for me when I recall. I struggled a lot for finding even tiny technical information. You will be astonished to know that people were reluctant to tell even the basic process and operating parameters of Alumina refinery. Can you think of doing process design engineering calculations using scientific calculators and preparation of drawings using Mini Drafters on the Drawing boards. For the first time in 1984, I got opportunity to work on computer that to with 640 kb RAM. Those days, Word star and Lotus-123 were the basic software for  word processing and calculations respectively. In 1990, I got the computer with MS-Excel on which I initiated to take up the assignment for developing a mathematical model for Design of Alumina Refinery during extra hours. I was fortunate to be awarded for development of that mathematical model sometime in 1995. I kept on updating the mathematical model considering it as my person achievement. If I see the model today, I am surprised on myself for completing this unique and wonderful developmental work alone. The outcome of this model has been well tested with the results provided by other leading technology suppliers in the World. This mathematical model has been developed on MS-Excel platform and requires only 150  basic input data and it gives detailed process calculations like material balance, thermal energy balance, water balance, caustic soda balance, alumina balance etc and sizing calculations for all major technological equipment, tailor made equipment and requirement of utilities and services within no time. Utmost accuracy in outcome is the essence of this mathematical model. Since this Mathematical model has been awarded by Internationally reputed Engineering & Consultancy Organization and they are using the same even today for design of Alumina refinery. Thus, I consider it as their asset but as an originator, I keep on updating the Mathematical model incorporating advancements in process technology. 

To summarize the facts of my professional life, I had to face a lot of difficulties which can not be expressed in words. All of us know the fact of our life that we are not going to remain alive on this earth for more than 200 years under any circumstance. Thus, it would be a wiser decision to dissipate the knowledge and expertise acquired by all of us. Though our first forefathers were monkeys, but we can not behave like monkeys today in keeping intact the assimilated information with us as the World has become very small because of advancements in communication technology. I feel proud of myself as I initiated this move of exchanging the ideas of public domain nature among my friends about a decade back which was appreciated by almost all across the globe. Now, I have observed that my friends are contributing much more than what I could and what I am. I have always advised one and all to share the information of public domain nature and never disclose any data, information and drawings of proprietary and confidential nature. My interest  has remained at the same level among the global team but I am contributing little less because of paucity of time due to other pressing assignments.Till date, I have published 283 technical articles on this specialized subject and could present few papers in national and International seminars covering all aspects starting from concept to commissioning of Alumina Refinery.

No matter, I am always (24x7) available for serving the community around and specifically to bauxite, hydrate, alumina, aluminium and alumina chemical fraternity on techno-commercial issues across the globe even during odd hours.

If any individual particularly the young professionals / Entrepreneurs has any question with regard design, engineering, execution, commissioning, process calculations and costing (such as bauxite mining cost, alumina production cost, specific capital cost etc.) related to Bauxite, mining, Hydrate, Alumina and Specialty grades of hydrates and aluminas including Alumina chemicals, please feel free to ask any information / clarification clearly indicating your e.mail ID for receiving your reply. Our team of Experts will definitely put efforts for reply to you. Sometimes, you may be required to wait a while for the reply. Simplicity in our language and authenticity in information are major strength. 

We wish to have your opinion / comments / remarks on our new move on the very specialized subject like alumina. We will welcome your suggestions.

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

A to Z on Bauxite, Hydrate, Alumina, Alumina Chemicals and Aluminium

Hi Friends,

We are planning to publish a wonderful and unique technical book on "A to Z on Bauxite, Hydrate, Alumina, Alumina Chemicals and Aluminium" covering various technical aspects with emphasis on process, design, engineering, project management, commissioning, process control and successful operation including trouble-shooting of metallurgical and non-metallurgical (Specialty grades) Alumina Plants including byproducts, chemicals and glimpse of major critical issues of aluminium smelters as well. We have already structured a table of content with variety of technical inputs for inclusion in the book. Previously, we had a plan to publish this book by early this year but could not be done due to many major changes in the content of the book on request of our friends and well wishers. Now, our team members are planning to publish this book at the earliest latest by the mid of 2016.

The proposed new book will be unique with regard to following key features:

• Technical data, information & detailed calculations useful for process design engineers,
• Detailed specifications of basic input materials,
• Treatise for process control, process development & product development,
• Production, consumption and forecast data for products across the globe for taking strategic decisions,
• Method to optimize techno-economically viable plant with provision for expansion,
• Basic parameters for selection of suitable project site for new plant,
• List of required statutory clearances for setting up new Alumina plant,
• Specifications of different grades of bauxite, hydrate, calcined alumina & chemicals,
• Applications and production process routes for standard and specialty grades of bauxite, hydrate, calcined alumina and value added chemicals,
• Methodology to develop PFD, P&ID, general layout of plant & shop layout / equipment arrangement drawings of units, 
• Clear understanding about minimum feasible capacity for Alumina refinery,
• Working out volume of work / bill of materials / Material take-off for civil, structural, electrical, co-generation and instrumentation and control units for new Alumina project,
• Methodology for estimation of capital cost, production cost, IRR and pay back period,
• Arriving at consumption factors for all input materials,
• Procedure for working out requirements of fresh water, cooling water, steam and other utilities,
• Sizing calculations for tailor made process equipment and facilities,
• Systematic approach for sizing of pipelines and valves,
• Techniques for selection of critical equipment & appropriate material of construction,
• Finalization of process technology based on characterization of bauxite,
• Basic inputs and methodology for developing Basic engineering documents,
• Methodology with sample drawings, data sheets and documents to be developed during Detailed engineering for execution,
• Reference guide for Entrepreneurs  and strategic planners of respective fields particularly with regard to specific capital investment, profitability and other related aspects,
• Planning & scheduling for execution of new Alumina plant,
• Detailed systematic planning for successful commissioning,
• Wide coverage on safety, occupational health and environmental protection,
• Technical input data for research and development and
• Many more topics of interest which have not been published in any document so far.

This book will be the outcome of life time experience of a group of professionals located in different parts of the globe who have designed, executed, commissioned and successfully operated many such Alumina plants in the World.

We are thankful to all individuals and organizations who helped us directly or indirectly in making this book with rich data, information and calculations. We would also like to put on record our sincere thanks to those individuals who approached us for early publication of this unique technical book. All friends of bauxite-alumina-aluminium fraternity will appreciate the wide coverage of technical information with addition of many topics / contents compared to earlier envisaged. Editing of technical information and calculations are in advance stage and concerted efforts are being made by our team members to expedite the finalization of manuscript with utmost accuracy.

As such all out efforts will be made to cover all technical issues, however if you wish to include detailed coverage on any topic, please let us know by writing your opinion under comment column which will be passed on to the editorial board for inclusion in appropriate place in the proposed book.

We will welcome your comments, remarks and suggestions, if any.

Best regards.

Kunwar Rajendra
www.bauxite2aluminium.blogspot.com

Scope Matrix for Execution of Alumina Refinery Projects on EPC Contract Methodology

Hi Friends,

All of us are aware that each and every Alumina refinery is tailor made because of changes in processes, equipment and facilities because of variation in quality of basic input raw materials. Thus, generally Basic engineering & Process technology supplier, Detailed engineering consultants, equipment suppliers and of course the Project owners are involved in execution of Alumina refinery projects. In the recent past, some changes in execution methodology for Alumina refinery projects have been distinctly observed. 

These days, project owners across the globe are approaching various design, engineering and construction agencies to carry out execution of Alumina refinery projects on EPC basis. This methodology has its own advantages like single point responsibility, less involvement of project owner and remote chances of schedule & cost over run for the project. But capex may be little high because of loading on prices by EPC Contractor for their efforts in execution of project such as procurement of equipment & facilities from different sources and appointment of various agencies for inspection, quality control, safety, security and other associated activities within as well as outside the premises of project site. 

In present post, we are presenting the major responsibility matrix with indicative scope of services of EPC Contractor and The Project Owner for successful execution of green-field Alumina refinery project as tabulated here under-

  RESPONSIBILITY MATRIX FOR EXECUTION OF ALUMINA PROJECT ON EPC BASIS

S.No.	Activity Description	EPC Contractor	Project Owner	Remarks
1	Feasibility Study Report		√	Owner to provide data pertaining to hydrological, meteorological, geological, estimated ore reserves, bauxite quality specifications with range including available infrastructure in the vicinity of proposed site.
2	Technological Testing Report for characterization of Bauxite		√	Owner to provide, hard groove index of bauxite, digestion conditions, residue settling characteristics and other associated figures derived from quality specifications with detailed test results and conclusions.
3	Project Clearances		√	Owner to provide land acquisition status, site details with topography, EIA / EMP reports, consent for establishment, safety assessment and fire rating.
4	Geological Survey Report		√	Preliminary exploration and Detailed exploration reports.
5	Basic Engineering	√		Design basis, process description, PFDs, P&IDs, equipment List, motor list, data Sheets, MSDS, utility requirement, plot plan, SLD, list of packages, details of DCS with control philosophy, design drawings for tailor made equipment and tankages, list of instruments & controls, specifications for critical equipment, pollution control equipment, 3D piping drawing upto pipe size of 50 mm and more including instruments and other details as per scope for Basic Engineering.
6	Detailed Engineering	√		Development of general layout of plant, area-wise equipment arrangement drawings, technical specifications for equipment & facilities including utilities & services, SLD for MRDS & MCCs, construction drawings for civil, architectural & structural buildings, pipe & cable racks, roads, drains, environment & safety equipment, all foundation drawings, cable routing diagrams, firefighting system, 3D piping drawings incorporating utilities & services, equipment for maintenance workshop, water supply & distribution network, erection tenders for civil, architectural, structural, electrical, instrumentation and control work and other LSTK packages plus other work as per scope for detailed engineering.
7	Development of General Layout of Plant.	√		Starting from raw material unloading, handling & storage to storage and loading of products including all processing, utility & services facilities and Land escaping.
8	Construction Drawings	√		Construction drawings for all civil buildings including boundary wall, Main gates, foundations for footings of Pipe racks, footings for PEBs and operating platforms.
9	Site Preparation	√		Marking location of plant units and offices depending on topography of land, geotechnical study reports.
10	Temporary Construction Facilities	√		Temporary office space for Client as well as Contractor officials, communication equipment, arrangement for supply of construction power and construction water, Labour sheds and stores.
11	Fabrication drawings	√		Fabrication drawings of tanks with nozzle orientation and Fabrication drawings for pipe racks.
12	Structural buildings	√		Fabrication and erection drawings for Structural buildings and ware houses.
13	Cogeneration Plant	√	√	Only system design drawings to be provided by Owner and detail drawings to be developed by contractor.
14	Design, Fabrication and erection of non-standard equipment	√
15	Installation of Piping network	√
16	Design for electrical power receiving and distribution system.	√
17	Erection of PCC & MCCs	√
18	Electrical and Control Cables	√
19	Distributed Control System	√
20	Installation of all instruments & controls	√
21	Laboratory Equipment	√
22	Effluent Treatment Plant		√
23	Sewage Treatment Plant		√
24	Construction of Office buildings		√
25	Fire Fighting System		√
26	Plantation and development of greeneries		√
27	Preparation of Operation manuals		√
28	Preparation of Maintenance Manuals		√
29	Supervision during testing & Commissioning		√
30	Stabilization of plant operation		√
31	Supply of Commissioning Spares		√
32	Establishing plant performance guarantee parameters		√
33	Submission of ‘As Built’ drawings		√	‘As built’ drawings for entire plant within the premises to be prepared and submitted by contractor.

The above stated scope matrix is for reference only which is also applicable for brown-field expansion of Alumina refinery and other industrial projects to be executed on EPC basis. These are just the guidelines and the same may be further elaborated on case to case basis.

Trust, our friends of Bauxite-alumina-aluminium fraternity will find this article useful more specifically while freezing & finalizing the scope of EPC contract. We will welcome your remarks and comments for further improvement by incorporating your valued suggestions, if any, at our end. Please feel free to put your remarks under "Comments" provided below each article published on this unique technical blog.

Regards.

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