Wednesday, February 6, 2019

Technological options for Design of New Alumina Refinery

Hi Friends,

I would like to share the glimpse of one of my technical papers presented in an International seminar wherein I expressed my feelings for utilizing the experience and expertise of individuals in execution of new Alumina refineries by adoption of Public domain technology. The paper was well appreciated by one and all in the seminar. Trust, you will also find it informative and interesting as well.

The production of alumina is a hydrometallurgical process comprising of several unit operations and unit processes prevalent in chemical industries. Basically it is a chemical enrichment process wherein the undesired components are removed from bauxite. The extent of the presence of these components and their behavior play a vital role in economic exploitation and selection of an appropriate process route. The selection of an appropriate process route for production of alumina depends on various factors like chemical and mineralogical composition of bauxite, extraction efficiency, the module, and sodalite factor and energy consumption. Mineralogically, alumina occurs in bauxite in the mono-hydrated form (Al2O3.H2O) known as Boehmite & Diaspore and the tri-hydrated form (Al2O3.3H2O) referred as Gibbsite.


Based on available geological exploration data and chemical analysis of bauxite average compositions of bauxite is considered for design of Alumina refinery.

With available mineralogical composition of bauxite, various technological options can be worked out to arrive at the best-suited process technology to be adopted for new Alumina Refinery. Various technological options considered for associated major unit operations and unit processes are outlined below-

Bauxite Crushing:

As per prevailing practices, secondary crushing of bauxite from around 120 mm to 20 mm is done using following types of crushers:

(a) Hammer crushers
(b) Impactors
(c) Double Roll crushers

Availability and operational reliability of such equipment are the basic consideration for their selection. Comparatively the availability and reliability of double roll crusher is much more than hammer crushers and impactors for the required duty.

Bauxite Grinding:

Crushed bauxite is ground to increase the surface area of bauxite resulting in improvement in its reactivity with caustic soda. Following are the various ways for grinding of bauxite:

(a) Dry grinding
(b) Closed circuit wet grinding
(c) Open circuit wet grinding

Earlier till 1960’s, Alumina Refinery had dry grinding system using Ball mills as the main equipment. Dry grinding of bauxite causes dust generation affecting the working environment and hence the dry grinding system was modified to wet grinding system. Under wet grinding, following types / combinations are used-

(a) Single stage closed circuit wet grinding
(b)Two stage closed circuit wet grinding
(c) Single stage open circuit wet grinding

Generally Rod mills / & Ball mills are used as grinding equipment for bauxite in Alumina refineries in above-mentioned types of combinations.

Out of the above-mentioned grinding system, single stage open circuit grinding is considered to be an appropriate grinding technique for green-field Alumina Refinery.


Under mentioned digestion processes have been adopted in various Alumina refineries:

(a) Atmospheric digestion
(b) Medium pressure digestion
(c) High Pressure digestion
(d) Double digestion
(e) Tube digestion
(f) Two-Step digestion

Atmospheric and medium pressure digestion methods are considered for gibbsitic alumina however high pressure and two-step digestion techniques are adopted in case of boehmitic bauxite. Atmospheric digestion technology has following advantages over medium pressure digestion technology:

· Lower capital investment as it does not require pressure vessels and slurry flash tanks.
· Ease in operation
· Lower process steam requirement

In spite of above advantages of atmospheric digestion, lower alumina recovery is its main disadvantage which causes higher consumption of bauxite per tonne of alumina and hence under-utilization of natural resources. In addition, lower alumina recovery requires high flow of plant liquor. Hence medium pressure digestion is considered to be most appropriate technology for gibbsitic bauxite.

For Boehmite-Gibbsite mixed bauxite, Double digestion, Tube digestion and Two step digestion technology are adopted. In my opinion Two-Step Digestion (Atmospheric followed by high pressure) Digestion shall be most efficient and economical for processing Gibbsite-Boehmite mixed Bauxite.

Residue thickening, Washing & Disposal:

The undesired oxide of iron, titanium, silicon and other minor metals is termed as residue. The separation of residue from saturated aluminate liquor is carried out using gravity settlers / decanters. Earlier the concept was to use the large diameter settlers for the purpose but over the period of time lot of technological advancement has taken place in this area and smaller diameter deep thickeners have been developed as efficient equipment for residue settling and washing.

The disposal of washed residue from Alumina Refinery to residue pond is achieved by two ways:

(a) Wet residue ponding
(b) Dry residue stacking

As per latest guidelines of Environmental Control Authority, wet residue ponding method is not permitted for green-field Alumina Refineries. Hence disposal of residue at higher solids at around 55-65% is done which is popularly known as dry residue stacking technique.

Polishing Filtration: 

Supernatant overflow liquor collected from Decanters are polished in security filtration unit of Alumina refinery. Till 1980s, Horizontal Leaf Filters having manual removal of residue sludge were operative in most of the Alumina refineries of the World. But, now almost all new Alumina refineries are preferring the installation of Auto-dumping vertical security filters.

Heat Interchange:

There are two methods for recovery of thermal energy from filtered aluminate liquor going to crystallizers:

(a) Liquor flashing system
(b) Plate heat exchangers

The heat recovery system employs a battery of flash tanks and shell & tube heat exchangers. 
Liquor flashing system requires more space however; PHEs need small space and ease in operation. Both techniques are used in Alumina Refineries but most of the existing Alumina Refineries have Plate Heat Exchanger System.


Three types of precipitation technologies are normally used in most of the Alumina Refineries of the World.

(a) Batch precipitation
(b) Single stage continuous precipitation
(c) Two stage continuous precipitation

Because of higher liquor productivity and consistency in coarseness of product alumina, two-stage continuous precipitation technology is preferred over batch precipitation technology. Two-stage continuous technique is based on controlled precipitation of alumina hydrate adopting agglomeration and crystal growth principles.


Calcination is the last unit step of Alumina Refinery. Two types of equipment are used for producing metallurgical grade calcined alumina:
(a) Horizontal rotary kiln
(b) Stationary calciner

Stationary calciner is thermally efficient and hence requires less fuel per tonne of alumina production. Hence stationary calciner is a preferred choice over rotary kilns.


Following efficiency figures are achievable with selected process route and equipment:

- Overall alumina recovery: 94 %
- Liquor Productivity: 90 gpl Al2O3
- HFO Requirement: 73 kg/t Al2O3
- Process Steam requirement: 1.85 t/t Al2O3
- Total Energy requirement: 8.5 GJ/t Al2O3


The recommended process route of modified process route has under mentioned tangible and intangible advantages:
  • Consistency in quality of alumina
  • Optimal consumption of thermal energy
  • Lower consumption of caustic soda
  • Lower scaling rate in heat exchangers
  • Reduction in raw material consumption factors
  • Improved working environment
The above stated technological options have been elaborated as Public domain technology available for adoption and execution of an efficient Alumina refinery in any part of the globe. Request to put your views / valued suggestion / remarks / comments, if any, for further improvement in considered options.

Rajendra Kunwar
CETI Enterprises,

Friday, January 18, 2019

Major Considerations in Development of General Layout of Alumina Refinery

Hi Friends,

In earlier papers, we have discussed many design, engineering, operational and quality control issues pertaining to Alumina refinery. In present paper, we would like to discuss the major considerations in development of overall general layout of a new green-field Alumina refinery. Basic considerations are outlined below-

  1. Location of Project Site:  
Site selection of project site plays a vital role in capex and opex of Alumina refinery. If the site is located near the sea coast then minimum elevation level of plant should be 5 meters above the high tide level observed in last 100 years.

  1. Natural Site Conditions: 
a.       Following parameters are of utmost importance which have direct bearing in the long run on successful operation of Alumina refinery-
b.      Topography of land,
c.       High flood level,
d.      Maximum wind velocity,
e.      Predominant direction of wind,
f.        Maximum rain fall,
g.       Safe bearing capacity (SBC) of soil,
h.      Total natural evaporation and
i.         Elevation of site from MSL.
j.        Natural topography of land should be utilized for gravity flow of process streams so as to minimize the expenses towards cutting and filling during site grading and expenses towards pumping energy during normal operation of the plant.

  1. Process Technology: 
Each and every Alumina refinery is the combination of tailor-made equipment and facilities depending on the plant capacity and technology considered.  Depending on technological requirement and material flow sequence of process streams, substantial reduction in length of piping network and their structural supports can be achieved. Thus, Plant layout must be frozen after finalization of process technology with due consideration of Bauxite quality and process efficiency figures.

  1. Finalization of Plant and Equipment: 
Finalization of capacity and dimensions of equipment are essential to work out the preliminary shop layouts for developing the overall plant layout.

  1. Space for Operational and Maintenance Convenience: 
Adequate space must be kept in every area of the plant for safe movement of operating personnel and mechanized maintenance equipment.

  1. Storage for Input Raw Materials and Products: 
The storage sheds and vessels should be adequately considered for the plant. The inclination of belt conveyor for movement of bulk solid materials should not exceed the dynamic angle of repose of the material. This parameter is important for deciding the height of storage and space requirement.

  1. Adherence to Safety and Environmental Protection Norms: 
Statutory requirements with regard to safety and environment protection guidelines are strictly followed while developing the layout of Alumina refinery.

  1. Capex and Opex of Alumina refinery:
Lower Capex and Opex are considered as prime objectives for development of final layout of Alumina refinery.

These are a few major parameters essentially considered in developing the overall general layout of Alumina refinery.

We solicit your valued comments / remarks.

Rajendra Kunwar
Principal Consultant-Engineering,
CETI Enterprises.

Tuesday, June 19, 2018

Grand Success of My Book on " A Complete Guide to Alumina Technology"

Hi Friends,

I am highly obliged to my seniors for their guidance in completing the publication of my book on "A Complete Guide to Alumina Technology" as planned. The book is available at and 

I am sincerely thankful to all my friends for their positive feed back about the book. The book has earned good reputation among the friends of bauxite-alumina fraternity within such a short span of time. Your encouraging words will always remain memorable for me.
Best regards.

Rajendra Kunwar

Friday, June 15, 2018

Tube Digestion Technology for Processing of Boehmitic Bauxite

Hi Friends,

In previous papers, we have covered variety of technological, design, engineering and operational issues of Alumina refinery. It’s true 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 present here in brief about Tube 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 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. 

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. An Alumina refinery of 1.8 Mtpa production capacity has been newly constructed and commissioned a few years back in Saudi Arabia adopting tube digestion technology. The plant has adopted this technology with Basic engineering developed jointly by two leading engineering consultancy companies. This will be the biggest Alumina refinery in the World adopting Tube digestion technology.

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 250oC causes conversion of quartz to reactive silica, thus it is always recommended to maintain the digestion temperature at around 240-245oC and should avoid crossing 250oC 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.

We solicit your valued comments / remarks for further improvement from our end.
Rajendra Kunwar

Wednesday, May 30, 2018

The Book on "A Complete Guide to Alumina Technology"

Hi Friends,

As informed through previous article, the book on "A Complete Guide to Alumina Technology" is ready now in final stage of publication. Given below, please find the actual physical appearance of the book.

Assimilation of technical data, information and calculation related to design & engineering took long time as utmost care was taken on accuracy aspects of the book.

As communicated, the book will be available on sometime in the first week of June 2018 onward.

Best regards.

Rajendra Kunwar

Saturday, May 26, 2018

A Complete Guide to Alumina Technology - Process Design and Engineering

Hi Friends, 

We have pleasure to inform you that approval from concerned authority of Government of India was received yesterday for publication of my book "A Complete Guide to Alumina Technology." Soon after approval, the process of publication has been initiated by the publishing house.

Given below please find the pictorial view of the book which will be available at from 8th June 2018 onward.

This book is the exposition for engineers involved in preparing basic engineering, carrying out detailed engineering and developing various process design calculations for Alumina refinery. It provides systematic approach, required technical data and methodology for process design & engineering in execution of Alumina refinery projects. It provides systematic approach, methodology and calculations pertaining to material & energy balance, sizing of critical technological equipment & pipelines, estimation of utility requirements, arriving consumption factors of input materials, determining mining cost of bauxite, working out manufacture cost of calcined alumina and estimation of capital cost of Alumina projects. 

This book will be proved as a reference guide for strategic planners and entrepreneurs as it elaborates techniques for pre-investment studies with focus on specific capital investment, profitability and other associated techno-economic aspects of the project. This book covers key technical data and information with regard to composition of major input materials and products including data sheets and specifications of equipment. It gives the glimpse of quality specifications, applications and production process routes for specialty grades of hydrate, alumina, by-products and value-added alumina chemicals. This book also provides, tips, tricks and techniques in carrying out engineering activities efficiently thereby optimising the capex and opex of Alumina refinery. 
Best regards.

Rajendra Kunwar