Tuesday, February 6, 2018

Bauxite Digestion Technology for Processing of Gibbsite-Boehmite Mixed Bauxite

Hi Friends,

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 270oC 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.

Monday, February 5, 2018

Impact of Design Defect on Steady Operation of Alumina Refinery

         Hi Friends,

We are well aware that alumina production is a continuous hydro-metallurgical process. The steady operation of plant and equipment results in remarkable plant performance. At times, minor design defects causes severe operational problems in Alumina refinery. Unsteady operation of Alumina refinery directly impacts the efficiency of the plant. Thus, all out efforts are made to minimize the design defects during implementation / modification stages of plant / equipment / facilities. 

In earlier posts, we have highlighted a few design defects and the methodology to overcome those limitations as well. In present post, we will discuss in brief a few additional design defects which have been experienced by the author in the operative plants in India and abroad. A few design defects are briefly outlined in subsequent paragraphs-

1.       Bauxite slurry heating system: 
Generally, heating of ground bauxite slurry at around 100 degree centigrade is carried out either during pre-desilication or before feeding to pre-desilication tanks. The heating of slurry is carried out to attain the desilication temperature by following four methods-
·       Direct injection of steam to pre-desilication tanks,
·       Indirect heating using Double pipe heat exchangers,
·       Indirect heating using spiral heat exchangers and
·       Indirect heating using Shell & tube heat exchangers.
Direct injection to bauxite slurry increases the dilution to process and hence never considered. Double pipe heat exchanger for bauxite slurry heating is not recommended because of acute scaling problems for the reasons already explained in earlier post. Last two options stated above are recommended among which the last one i.e. Shell & tube heat exchanger is the preferred choice because of lower capital investment as well as ease in maintenance.

2.       Schedule of pipes for slurry handling: 
Slurry generated in red area of Alumina refinery has high abrasiveness because of presence of silica which causes erosion of pipelines. Thus, pipelines of adequate thickness and schedule 80 should be installed.  Schedule-40 pipes should always be avoided as it will require frequent replacement.

3.       Selection of MoC for pumps, valves and fittings: 
As such carbon steel is the best suited material of construction (MoC) for Alumina refinery having alkaline environment. But presence of silica causes fast erosion in stationary as well as rotating equipment. Thus, for handling abrasive slurry in red area, equipment / components made of carbon steel as well as Nicast are never used. For such critical service conditions, Nihard / high nickel steel / high chrome steel is the right choice. Never compromise on MoC of impeller and casing of slurry pumps, connected valves and inlet Tees of slurry flash tanks particularly for handling Bauxite slurry. Ensure the right MoC for these critical components for steady operation of Alumina refinery.

4.       Minimum shell thickness of Digesters:
Digesters are the pressure vessels installed for dissolution of alumina present in bauxite in caustic liquor. The digestion vessels are designed on the basis of its recommended operating conditions. The shell thickness is always based on the pressure but shell thickness is always kept more than 20 mm to take care of any eventualities because of instrument failure and operational lapses as well.

5.       Provision of non-return valve in steam lines to process vessels: 
Alkalinity in boiler feed water is controlled below 0.01 gpl Na2CO3. Though direct injection of steam to process are always avoided but a few designers keep provisions. In such situation, non-return valve in steam line near process area is a must as sudden drop in boiler pressure for any reason may case backing of alkaline liquid to steam lines which may develop stress corrosion ultimately causing sudden failure of steam line within short duration. It is essential for safeguarding the equipment as well as safety of operating personnel in the plant.

6.       Provision of flanges in pipelines: 
Sometimes, flanges are not provided in slurry pipelines to minimize capital investment in project stage. But, it cause severe operational problems because of difficulties in cleaning of welded lines. Thus flanges must be provided at the interval of 6m to 10m depending of practical experience operating personnel.

7.       Adequate capacity of Evaporator Hot well: 
Tail pipe of Barometric condenser is kept submerged to a depth of about 0.50 meter in water of Hot well of evaporation unit. The Hot well is designed to maintain adequate seal water for circuit. The volume of Hot well is always kept more than five times the submerged volume of leg of Barometric condenser. If water volume in Hot well is inadequate, the vacuum seal will be broken and air drawn through tail pipe will drastically affect the performance of evaporation unit.

8.       Minimum plate thickness of tail pipe of Barometric condenser: 
Generally, the vacuum requirement for efficient evaporation system is 685 mm of mercury. Thus, the minimum acceptable plate thickness for the tail pipe is 6 mm. Otherwise, sudden increase in vacuum during start up with negligible load of non-condensable may cause rupture of tail pipe of Barometric condenser. This problem was faced in a newly constructed Alumina refinery which could be rectified by carrying out modification for re-startup.

9.       RPM of impellers of slurry pumps: 
RPM of impeller is the prime criteria for selecting the pumps for Alumina refinery. In Alumina refinery, impeller rpm for handling bauxite residue as well as hydrate is relatively kept at lowest possible level so as to avoid breaking of the flocs and particles. This issue has already been discussed at length in earlier posts.

These are just a few examples of design defects of Alumina refinery. There are many such design defects observed in Alumina refinery which badly affects the continuous operation of Alumina refinery and thereby by huge monetary losses for the company. Hence, role of Owner’s consultant becomes very important to avoid such design defects during engineering state of execution of the plant. We would like to have your comments / remarks for further improvement.

You may like the article on "Systematic approach for design of Slurry mixing agitator" published earlier.
Best regards.

Rajendra Kunwar
Principal Consultant-Engineering
CETI Gurugram, Haryana, India.


Saturday, February 3, 2018

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 5th 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

Website: www.ceti.co.in

Thursday, January 25, 2018


Hi Friends,
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