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

Key Controls for Improving Settling Characteristics of Bauxite Residue

Hi Friends,

We are well aware that 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 5% total silica (Module > 8) is preferred which generally causes generation of about 1.15 to 1.35 tonnes of dry residue per tonne of alumina.

It is a known fact that storage and disposal of Bauxite residue is a big problem for all Alumina producers with stress and emphasis of concerned Governments 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 where as 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 flocculants: 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.

Thus, close watch on these critical parameters must be kept for trouble-free as well as steady and efficient operation of Alumina refinery. We would like to have your views / remarks / suggestions / comments, if any, for further improvements in quality of future articles.

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

International Benchmark of Energy Consumption for Alumina Production in Bayer Alumina Refinery

Hi Friends,

In earlier posts, we have covered various issues pertaining to design, engineering, execution and cost estimation of plant, equipment and facilities of Alumina refinery. In present post, we will discuss the International benchmark of energy consumption for production of Metallurgical grade calcined alumina.

As per the literature published by United Nations Industrial Development Organization (UNIDO) available on public domain, typical energy consumption data presented below clearly reveal that specific thermal energy consumption per tonne of alumina produced in various Alumina refineries across the globe ranges from 10.2 GJ/t to 24.5 GJ/t alumina. The thermal energy consumption figures for different regions are presented in following graph-

Data Source : International Aluminium Institute

It is evident from the above graph that specific energy consumption for production of calcined alumina in South America is the lowest at 10.2 GJ/t whereas it is highest in China at about 24.5 GJ/t of alumina. Reasons for wide range of variation in energy consumption figures have already been described in earlier posts. On comparative analysis of these figures, it can be concluded that Asian and American Alumina refineries have the potential to improve the performance of plants for reduction in specific energy consumption by 23 to 25% from the present level. However, Chines Alumina refineries have tremendous scope for further reduction in energy consumption by about 50% through modifications and modernisation in the existing plants. As such, many Alumina refineries under execution in the World have set their total thermal energy consumption target at 8.5 GJ/t alumina.

On critical analysis of plant efficiency figures, process control parameters of operative Alumina refineries and technological advancements in the World, the global benchmark for specific thermal energy consumption in new Alumina refinery has been set by the International Expert Committee as 7.80 GJ/t of calcined alumina.

It may be a little difficult to achieve the set benchmark figure in existing Alumina plants because of inefficient heat recovery system. However, it is challenging and achievable target for modern Alumina refineries with installation of efficient heat recovery system, high degree of automation and continuous monitoring of thermal energy consumed in various units of Alumina refinery. Let us join hands to save unwanted thermal energy and get additional advantage on production cost of alumina. 

Please put your views / suggestions / remarks / comments, if any.

Regards.

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