Friday, March 22, 2019

Grand Success of Our Book on "Alumina Technology - Process Design & Engineering"

Hi Friends,

We have pleasure to put on record the grand success of our unique book on Alumina Technology. Content of the book has been appreciated by Consultants, Process Experts, Engineering Experts, Equipment Suppliers and Plant Operating Professionals across the book. We express our sincere thanks to all concerned.

This is a unique book as it contains insights of Alumina Technology and gives detailed coverage on following major aspects:

  • Technical data, information & calculations useful for process design engineers,
  • Characterization and detailed specifications of basic input materials,
  • Major parameters 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,
  • Required statutory clearances for setting up new 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 of units, 
  • Working out volume of work / bill of materials for new project,
  • Methodology for estimation of capital cost, production cost and pay back period,
  • Arriving at consumption factors for all input materials,
  • Procedure for working out requirements of water, steam and other utilities,
  • Sizing calculations for tailor made equipment and facilities,
  • Systematic approach for sizing of pipelines and valves,
  • Techniques for selection of critical equipment & appropriate material of construction,
  • Reference guide for Entrepreneurs and strategic planners of respective fields,
  • Planning & scheduling for execution of new 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 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 plants in the World.The 1st edition of the book was published 2018. The book is available at and
Best regards.

Rajendra Kunwar


Critical Process Steps to Improve Bauxite Residue Settling Rate in Alumina Refinery

Hi Friends,

In continuation to earlier technical papers, I would like to present this paper of utmost importance covering critical problems and their remedial actions pertaining to bauxite residue settling in Alumina refinery. The paper is purely based on personal experience of the author in operative Alumina Plants in India and abroad. Trust, you will like the content of this paper.

Bauxite residue is the waste generated in Alumina refinery. The residue generation rate is dependent of Bauxite quality and alumina extraction efficiency. In Bayer process, Bauxite with around 40% Al2O3 and less than 4% total silica (Module > 10) is considered as metallurgical grade bauxite for production of calcined alumina. The alumina digestion process results in dissolution of bauxite in caustic soda and causes generation of about 1.15 to 1.35 tonnes of dry residue as waste per tonne of alumina.

It is a known fact that storage and disposal of Bauxite residue is a big problem for all Alumina producers due to environmental problems in the vicinity because of dust generation and percolation of caustic to natural water streams. Thus stress and emphasis of concerned Governments is always given to store it in well protected area so as to combat probable environmental problems and health hazard for the community around because of the passage of alkaline water to natural streams and acute dusting in the area during summer. Though attempts are being made by researchers and scientists to find commercial utilization of residue but very limited utilization has been made possible so far because of economic disadvantages. The laboratory scale development of various processes were done till 1980s for its utilization in manufacture of red mud bricks, cement, red mud PVC plastic, pigment and many other products but only a few has been commercialized till date. 

In present post, we will discuss the key contributing parameters to improve the settling characteristics of Bauxite residue in Alumina refinery. As such, there are unlimited variables which affect the settling rate of residue but we will like to highlight the major parameters to improve the settling rate of residue particles in settlers and washers of Alumina refinery thereby improving the production rate due to steady operation, improved product quality due to less carry over of suspended solids in filtrate during polishing filtration and lower soda loss with residue due to increased residue washing efficiency in residue washers. The major parameters are briefly described in subsequent paragraphs-

1.       Mineralogical composition of Bauxite: 

The mineralogical composition of Bauxite clearly reflects that oxide of iron is present in the form of hematite, goethite and siderite whereas Oxide of titanium in the form of anatase and rutile in addition to oxides of other minor metals. The presence of goethite badly affects the settling rate of residue. Thus the ratio of hematite to goethite is preferably maintained more than 3 by adopting suitable blending techniques before processing in the Bayer circuit. 

2.       Granulometry of Bauxite in ground slurry: 

The specific gravity of Bauxite ranges from 2.50 to 2.75 depending on the chemical and mineralogical composition of Bauxite. As such, the bauxite particle of minus 60 mesh has been found to be okay for satisfactory extraction as well as recovery of alumina but to ensure the same the granulometry in ground slurry is controlled over 85% of minus 60 mesh fraction. The slurry contains about 1 to 2% of plus 10 mesh fractions. With this particle size distribution, the specific surface area of dry bauxite is observed between 12 to 15 m2 per gram. This figure is also used as a guideline for required granulometry in ground slurry. Over grinding of bauxite is avoided as it badly affects the settling rate of residue because of further disintegration of particles during digestion and slurry flashing circuit of Alumina refinery. 

3.       Selection of suitable flocculents: 

It has been seen that mixed natural and synthetic flocculent gives better result than any individual flocculent for desired level of overflow liquor clarity as well as compactness of underflow residue. However, the ratio of mixture varies widely depending on mineralogical composition as well as settling characteristics of Bauxite. Thus, the ratio of mixing is finalized by carrying out settling rate test in Laboratory with digested blow-off slurry under the plant operating conditions. The dosing rate of natural flocculent generally varies from 0.25% to 1% of dry bauxite residue entering the settlers and washers.

4.       Temperature profile of Settling and Washing circuit: 

Though, there may not be wide fluctuations in temperature profile of thickeners and washers but at times even the minor deviations in temperature profile either due to ignorance of operating personnel or malfunctioning of instrumentation and control system may cause serious problem in the settling rate as well as overflow liquor clarity due to increase in viscosity of aluminate liquor at lower temperature. Thus close watch on steady temperature profile is considered as one of the major factors.

5.       Monitoring and control on P2O5 content in process liquor: 

This is one of the very important parameters which requires constant vigil for steady control within limits by taking required preventive measures in the upstream of the process. For the purpose, suitable chemical is added continuously to the process circuit.

It has been experienced that close watch on these critical parameters results in trouble-free as well as steady and efficient operation of Alumina refinery. We would appreciate your valued comments.
Best regards.

Rajendra Kunwar
Principal Consultant-Engineering

Wednesday, March 6, 2019


           Hi Friends,

As such it is difficult to list out all engineering and execution activities, however all out efforts have been made to assimilate major engineering and execution activities containing  information in systematic manner for guiding the young professionals engaged in engineering of Alumina Refinery Project as outlined here under-

A.      Pre-Project Activities

1.       Finalization of Product mix
2.       Quality specifications of products
3.       Preparation of Conceptual Report
4.       Technological testing of bauxite
5.       Site selection and land acquisition
6.       Identification of available infrastructure like roads, rails, rivers and port facilities
7.       Estimation of Raw material consumption factors
8.       Estimation of utilities requirement like steam, power, chilled water &compressed air
9.       List of major equipment with broad specifications
10.   Estimation of Capex and Opex
11.   Development of Plot plan
12.   Preparation of Feasibility Report
13.   EIA / EMP Report
14.   Statutory clearances for site
15.   Receipt of Consent to Establish (CTE)

B.      Basic Engineering

1.       Process Design Basis
2.       Fixing the process & operating parameters
3.       Material flow balance
4.       Thermal energy balance
5.       Water Balance
6.       Caustic Balance
7.       Alumina Balance
8.       Condensate balance
9.       Development of  Overall Process Flow Diagram
10.   PFDs for All Units
11.   P&IDs for All Units
12.   Broad specifications of major technological equipment
                                                  i.      Primary & secondary crusher
                                                ii.      Conveyors
                                               iii.      Vibro-pulse screens
                                              iv.      Ball mills
                                                v.      Slurry pumps
                                              vi.      Heat Exchangers
                                             vii.      Autoclaves / Digesters
                                           viii.      Flash Tanks
                                              ix.      Evaporators
                                                x.      PHEs
                                              xi.      Boilers
                                             xii.      Compressors
                                           xiii.      Cooling Towers
                                           xiv.      Rotary Disc Filters
                                            xv.      Rotary Drum Filters
                                           xvi.      Calciners
13.   List of major equipment with motor ratings
14.   Specifications of other process equipment
15.   Instrumentation and control philosophy
16.   List of cranes and hoists
17.   Sizing of heat exchangers
18.   Sizing of Desilicators
19.   Sizing of Liquor decanters & washers
20.   Sizing of Precipitators
21.   Sizing of condensate & other liquid storages tanks
22.   Specifications for liquor and water pumps
23.   Sizing of pipelines
24.   Sizing of valves
25.    Acid cleaning system for heat exchangers
26.   Chemical cleaning system for precipitators

C.      Detailed engineering

1.       Preparation of tender documents
2.       Development of General layout of the plant
3.       Preparation of shop layout
4.       Assistance in evaluation of offers
5.       Assistance in inspection of equipment
6.       Assistance in procurement of plant & equipment
7.       SLD for Power Control Centre (PCC)
8.       SLDs for Motor Control Centre (MCC)
9.       Design of Digesters
10.   Design of Flash tanks
11.   Design drawings for tanks and vessels
12.   Inspection and expediting activities
13.   Drawings for process & utility piping network
14.   Design drawings for Civil & Structural Buildings
15.   Foundation drawings for all equipment & facilities
16.   Certification of bills payable to contractors and suppliers

D.      Construction activities

1.       Preparation of schedule of activities
2.       Planning for materials, machines and manpower
3.       Planning of construction activities
4.       Appointment of Competent Contractors
5.       Supervision of construction at site
6.       Cost & quality control during erection

E.       Assistance in Testing & Commissioning

1.       Checking Mechanical completion
2.       Cold trial run
3.       Hot trial run
4.       Commissioning of equipment
5.       Plant commissioning procedure
6.       Performance guarantee test

F.       Stabilization of plant

1.       Finalization of  process and operating parameters
2.       Guiding the team for steady plant operation

G.     Quality control methodology

H.     Safety and Environmental Management & Control Techniques

I.        Preparation of As-Built Drawings.

Hope, you will realize the extent of efforts put by our team in preparing such exhaustive list of information. The above list clearly indicates the volume of work associated in engineering and execution activities involved in putting up any Alumina Refinery in any geographical location across the globe. Our team will welcome your feedback and suggestions in further improvement. Please feel free to contact us for clarifications, if any.
Best regards.

Rajendra Kunwar

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,