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 Testing & Commissioning of Newly Constructed Alumina Refinery

Hi Friends,

We are well aware that Alumina refinery is a continuous hydro-metallurgical process involving complicated unit operations and unit processes. Testing & commissioning is the 5^th stage in execution of new Alumina refinery which comes after Basic engineering, Detail engineering, Procurement and Erection ensuring mechanical completion. Safe and smooth startup of Alumina refinery with due care of human safety, equipment, process input materials and environment are the basic objectives of successful testing and commissioning of Alumina refinery. Thus, successful commissioning of Alumina refinery has five basic objectives as listed below-

·        No loss time accident,

·        No equipment damage,

·        No risk to environment,

·        No wastage of input materials and

·        Stabilization of plant within shortest period.

Sequence of testing and commissioning of Alumina refinery include following major steps-

Field Checking and Punch Listing: 

Field checking of construction completion status of all equipment, instruments, electrical and associated facilities of Alumina refinery, Co-generation plant, Water supply system, Power distribution system, Compressed air system, cooling tower and Distributed control system is carried out before testing and commissioning of technological plant and equipment. Based on field checking, a list of required additions / deletions / modifications and design changes are recorded with specific identification number for rectification on planned manner as per priority decided by a team of senior technical professionals of the plant.

System Configuration Checking:

You must have gone through the paper on Execution of Alumina refinery on EPC Methodology. Soon after completion of erection activities and before commencement of testing and commissioning trials, system configuration checking is considered to be one of the important steps. This step is the visual inspection of technological and services equipment as per PFDs, P&IDs, ISOs, specifications and design drawings to ensure their readiness for cold water trials. System configuration checking includes the verification of following-

·        Proper installation of equipment,

·        Fixing of valves and pipelines,

·        Alignment and lubrication of rotating equipment,

·        Direction of rotation of motors,

·        Sealing arrangement of pumps,

·        Proper connections of electrical power supply system.

Checking of Instrumentation and Control System: 

This activity is carried out to ensure the readiness of Instrumentation and control system with respect to following critical aspects-

·        Installation of all instruments and controls as per P&IDs,

·        Setting of alarms and tripping system,

·        Process and Safety interlock system,

·        Microprocessor signals,

·        Functioning of local and remote sensing equipment,

·        Correct relay of data / information from field instruments to local as well as Central control rooms,

Checking Cleanliness of Process Vessels and Pipelines: 

This task is carried out by flushing out the process vessels and pipelines to ensure removal of obstruction and blockages in the process circuit just before cold water test run.

Checking of Auxiliary System: 

Performance of auxiliary system like Air conditioners, exhaust fans and connected ducts are checked to ensure desired working environment for instruments, controls, equipment and operating personnel.

Training of Plant Operating Supervisors and Operators: 

Objective of training of supervisory staffs and plant operators is to guide them to commission the Alumina refinery safely. It involves following steps-

·        Formation of team comprising of Managers of various disciplines / departments headed by an Expert.

·        Train the Supervisors aiming to ‘Train the Trainer’ who will train the operators,

·        Review the startup and commissioning procedures,

·        Educating the team with operating and process control parameters including benefits and consequences,

·        Provide technical leadership to facilitate safe, reliable and steady operation of Alumina refinery.

Process and Operating Manual for Systematic Start up: 

This document includes the estimated requirement of input materials and stepwise process sequence for meeting the following basic requirements-

·        Planning for required quantity of input materials,

·        Readiness of Area wise log sheets

·        Fixing the responsibilities of individuals in commissioning team,

·        Normal plant start up procedure,

·        Normal shut down procedure,

·        Emergency plant shut down procedure,

·        Plant Inventory procedure,

·        Procedure for increasing the plant volume steadily,

·        Method for increasing temperature profile of process stream,

·        Guidelines for ensuring adequate supply of condensate to cogeneration plant,

·        Sequence for raising concentration profile of process streams,

·        Planning for aluminate liquor generation and seed charge to initiate generation of seed,

·        Recycling of seed to regular precipitation circuit,

·        Production of hydrate and product alumina.

·        Stabilization of operation of Alumina refinery.

Start up and commissioning of Alumina Refinery:

Bauxite charge to process is considered as the zero date for startup and commissioning of Alumina refinery which is done after caustic concentration build up in selected process vessels to start with for generation of spent liquor and aluminate liquor subsequently. Seed charge enhances the precipitation efficiency thereby generation of seed and product hydrate. Steady control on process and operating parameters stabilizes the plant resulting in target production rate. Calciner is generally commissioned after creating adequate stock of hydrate in the storage shed.

Performance Tests for Plant and Equipment: 
Test runs are conducted with standard agreed procedure for individual plant, equipment and facilities in order to assess the guaranteed performance. However, performance tests for entire Alumina refinery are carried out after stabilization of plant operation to check the assigned production rate and efficiency parameters as agreed by technology supplier, equipment supplier and engineering consultant. 

The above stated guidelines are generally followed for systematic testing, start up and commissioning including establishing performance guarantee parameters of newly constructed Alumina refinery.

Hope, you will like to have a look at the article on "Engineering and execution stages of Alumina refinery."

Best regards.

Rajendra Kunwar

E.Mail: rajendra@ceti.co.in

Website: www.ceti.co.in

MAJOR ENGINEERING & EXECUTION ACTIVITIES FOR SETTING UP NEW CHEMICAL INDUSTRIES

Hi Friends,

Before going through the current article, you will like to have a look at "Caustic embrittlement in Alumina Refinery". This will help you to know the impact of caustic at high temperature.

As such it is difficult to list out major engineering and execution activities, however all out efforts have been made to assimilate the information in systematic manner for guiding the young professionals engaged in engineering of Mineral processing / Specialized Chemical Industries 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  input materials

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.

The above list clearly indicates the volume of engineering and execution activities involved in putting up any new chemical industries in any geographical location across the globe. 

Also, you may be interested to read our paper on "Systematic approach on testing and commissioning of Newly Constructed Alumina Refinery" .

We  will welcome your feedback and suggestions for further improvement. 

For any assignment related to design and engineering activities, we may be contacted at rajendra@ceti.co.in. Details of services are available at our website.
Best regards.

Rajendra Kunwar
India
www.ceti.co.in