In previous post, we have developed the Design and motor rating calculations for Ball mill. In present post, we will discuss the systematic procedure for starting a Ball mill for taking into operation as well as stopping the Ball mill for taking out of circuit in Alumina refinery. Ball mill is one of the very critical equipment installed in Alumina refinery. Since the mass of the rotating cylinder with liners and balls is very high, thus the starting torque for the mill is enormously high. The drive motor for the Ball mill ranges fro 0.5 MW to 2.5 MW depending on the hardness of Bauxite, feed size and operating capacity of the mill for meeting the hourly alumina production requirement as described in earlier post. Generally squirrel cage induction motors are installed as drive motor for the Ball mill.  The mill is also provided with hydraulic pumps for lifting the Ball mill to provide a jerk to swing so that inertia of rest of the mill is broken for its rotation. The hydraulic pumps of Ball mills in Alumina refinery are popularly known as inching device for the mill. Since the drive motor rating of the mill is the highest among all the high tension (HT) motors of Alumina refinery, so it is connected with high voltage electrical supply line at about 6.60 kV. Hence all precautionary measures are taken to start the Ball mill for normal operation. Sudden start of the mill by direct power supply to its drive motor may cause serious damage to the equipment and power supply system as well.

Ball mill start up procedure: 

The systematic start up procedures for Ball mill are briefly outlined here under-

1.     Check the readiness of upstream and downstream equipment in the process circuit,

2.     Inform the turbine house of Co-generation plant to ensure the stable operation of Turbo-Generators (TGs) and connected boilers. During steam pressure fluctuations in boilers, the ball mill should not be started.

3.     Check hydraulic oil level in oil tank,

4.     Start hydraulic pumps for lifting the mill to swing,

5.     When hydraulic oil pressure reached above 2 kg/cm2 and mill swings, start the drive motor of the mill. The temperature of hydraulic oil should remain less than 40 degree centigrade,

6.     Keep close watch on voltage, amperage, temperature and sound of drive motor,

7.     Open aluminate liquor to the mill at controlled rate,

8.     Start the ground slurry pump,

9.     Start bauxite weigh feeder with low feeding rate to Ball mill,

10.   If everything is normal, put the operation of grinding circuit in auto mode with normal grinding rate.

Ball mill stoppage procedure: 

The sequence of operation for stopping the Ball mill from normal running condition are given below-

1.     Stop the bauxite feeder for stopping the feed to Ball mill first,

2.     Continue flow of digestion liquor to mill for a few minutes to ensure flushing of slurry from the Ball mill,

3.     Stop the Ball mill,

4.     Stop the slurry pump,

5.     Drain the slurry from pump including its suction and delivery lines.

Broadly, the above sequence of operations are followed for starting and stoppage of ball mills. Grinding section operators must be provided with written down procedures in order to avoid any confusion and thereby mishaps. Also, while starting the mill, presence of Area supervisor is considered essential as any small lapse during start up may cause serious damage to equipment as well as unsafe to the working personnel. Sudden start up may also cause damage to mill foundation and its alignment creating acute crisis for Alumina refinery. 

Hope, all major points have been covered under the topic. Please put your views / suggestions / remarks / comments, if any. We will always welcome your suggestions and recommendations.

Is Low Silica in Bauxite a Problem?

 Hi Friends,

After a long gap, I have appeared before you to interact on a small but very critical process issue of Bayer Alumina Plant.

In the past, we have always discussed about the negative impact of high silica in bauxite specifically with regard to operational problems, process control issues, higher consumption of input materials and deterioration in quality of product hydrate or alumina. In previous articles, this issue was elaborated at length with measurable / quantifiable approach so as to focus on its importance in Bayer Alumina Refineries.

In present post, we will briefly discuss about the advantages and disadvantages of low silica in processing bauxite for production of calcined alumina. The question mark (?) on the subject itself forces us to think and re-think about the positive and negative aspects of low silica in bauxite.

It’s a well-known fact that all Alumina producers wish to have low silica bauxite for obvious reasons discussed earlier. Now, before coming to the subject directly, following questions arise-

(i)                What type of silica is harmful for Bayer’s process?

(ii)              What should be the cut-off silica content in bauxite for acceptable quality of alumina in International market?

Though we have discussed these issues in several articles, still it is prudent to freshen up with salient features as outlined below-

There are two types of silica in bauxite designated with regard to Bayer process of alumina – (a) Non-reactive silica and (b) Reactive silica. ROM bauxite mined adopting either open cast or underground mining methodology contains a number of metallic oxides including silica. This mined silica as percentage of total dry bauxite is termed as total silica. The part of total silica which reacts with caustic soda and forms sodalite complex through sodium silicate route is termed as Reactive silica. Majority of Reactive silica is found in form of Kaolinite. And thus, the non-reacting component of total silica with caustic soda is popularly known as Non-reactive silica. Quartz is non-reactive silica. The silica being abrasive in nature causes erosion in components of various process equipment like heat exchangers, pipelines, valves, pump casing and impellers etc. Thus higher silica content affects the capital investment of Alumina refinery because robust material of construction for equipment component in contact with bauxite slurry. Hence higher silica in bauxite causes higher initial investment for the plant and higher repair maintenance cost for routine replacement of critical components of equipment.

Generally it has been seen from bauxite analysis that Reactive silica in bauxite ranges from 50% to 80% out of Total silica in bauxite. Plant operation is affected because of high silica in bauxite as it has high settling velocity and abnormally high abrasive nature.

Now coming to the topic with raised question mark, it’s true that low reactive silica in bauxite is considered as a serious process problem as it directly affects the quality of product alumina because of higher silica content in alumina. The SiO₂ content in calcined alumina produced in all modern alumina a refinery is controlled at around 0.015%. The alumina having higher silica content is considered as inferior quality. For achieving 0.015% SiO₂ in alumina, the SiO₂ in aluminate liquor feeding to crystallizers / precipitators should be around 0.45 gpl.  The saturation level of silica in aluminate liquor is about 3.80 gpl SuO2 which is achieved within an hour in desilication circuit. The delay in achieving super-saturation of silica across desilication tanks results in inefficient desilication thereby increasing gpl SiO₂ in liquor increases to much higher level which ultimately deteriorates the product alumina quality.

The above elaboration concludes that moderate level of Reactive silica in bauxite at around 1.5% in bauxite is considered to be the best with regard to plant efficiencies and product quality as well.

Trust, the subject has been covered here to your desired level of satisfaction in understanding the criticality of the subject issue. We would welcome your  valued comments / remarks to further substantiate the subject.

Regards.

Rajendra Kunwar

Founder & CEO

CETI Enterprises, Delhi NCR, India

Cell: +91 8380043065

Efficient Bauxite Grinding System for Alumina Refinery

Hi Friends,

Size reduction of mined ROM bauxite is essential for improving its reaction with caustic soda for dissolution of alumina (Al₂O₃) forming sodium aluminate (NaAlO₂). Coarser bauxite particles affect the alumina extraction efficiency however fine bauxite creates acute problems in settling of residue in decanters and washers. Thus optimum particle size is the basic requirement for efficient Alumina refinery. Generally, particle size in ground product slurry of bauxite is minus 10 mesh  with over 80% of minus 65 mesh fraction is considered as optimum operating parameter.

Selection and sizing of bauxite grinding equipment is carried out on the basis of following major input parameters-

• Bond index of bauxite,
• Grinding rate,
• Feed size (F₈₀),
• Product size (P₈₀)

In most of the existing Alumina refineries, ball mills are operative. Few plants in the World have rod mills and a few have two stage grinding system with combination of rod mill in first stage and  ball mill in the second stage. It has been practically seen that ball mill generates more fines compared to rod mills.The cross section of a typical Ball mill has been shown below just to have the physical feel of the main components of Ball mill and movement of grinding media inside the mill in running condition. I acknowledge with thanks the efforts of my unknown friend who has developed this animation for schematic cross section of the ball mill presented here-

Schematic cross section of Ball mill in running condition

Considering the uniformity of desired particle size in ground slurry and energy consumption, single drum grinding mill having two compartments with rods and balls as grinding media in 1st and 2nd compartment has been found to be the best option as per recent studies. Thus, this system is the preferred choice for bauxite grinding for upcoming Alumina refineries in the World because it has the advantages in uniformity of product fineness as well as substantial reduction in electrical energy requirement by about 10 to 15% less compared to total electrical energy requirement in conventional single stage grinding system. The specific electrical power consumption for wet grinding in efficient system ranges from 4.6 to 4.8 kwh per tonne of bauxite.

It is, therefore, essential to evaluate all pros and cons of techno-economic features for all possible alternatives before taking final call to go ahead with the best suited option of grinding system meeting the process requirement with optimum energy consumption.
Kindly put your views / suggestions / remarks / comments, if any on the subject.

If you like this article, then please press your rating as  +1  .
Thanks and regards.

Kunwar Rajendra

System Design Calculations for Setting up Refractory Grade Alumina Refinery

Hi Friends,

So far, we have covered the operation, technological options, broad design basis, process engineering and process calculations for metallurgical grade Alumina refineries with glimpse of specifications and production processes for non-metallurgical grades of bauxites, hydrates and calcined aluminas. The non-metallurgical grades of hydrate and alumina are also known as specialty grades of products as they have special characteristics particularly with respect to their quality specifications and wide range of industrial applications. Specialty products are value added products mostly manufactured using chemical grade hydrate as the feed stock under strictly controlled process conditions with or without addition of suitable mineralizer. 

Refractory grade alumina (RGA) has high degree of calcination and lower impurities.   In RGA,  Na₂O content is controlled at lower level as Na₂O causes formation of voids in refractory at high temperature due to melting and creates cracks which is not desirable.

In present post, we will discuss about the product specifications, production process route and system design including list of major equipment for a typical RGA plant of 3000 tpa production capacity in subsequent paragraphs-

Specifications of Refractory grade alumina:

Al₂O₃ : 99.0% min.

SiO₂ : 0.020% max.

Fe₂O₃: 0.020% min.

Na₂O (total) : 0.35% max.

Na₂O (soluble): 0.15% max.

LOI (at 1100^o C) : 0.30 % max.

α Al2O3 : 90% min.

Typical physical properties of Refractory grade alumina:-

Appearance: White crystalline,

Melting point: 2040^oC,

Specific surface area: 0.50 to 1.0g/m²,

Refractive index: 1.76

Hardness: 9 on Moh scale.

Typical sieve analysis:-

+ 100 Mesh: 5 to 20%,

+ 200 Mesh: 40 to 90%,

-325 Mesh: 5 to 10%.

Production Process Route:-

Chemical grade hydrate, the intermediate product of Metallurgical grade Alumina plant, is used as the feed stock for production of Refractory grade calcined alumina. As such, the calcination system with associated material handling and alumina cooling system are the basic facilities of RGA plant but it is preferred to install hydrate washing and filtration equipment at the 1^st operating stage of the plant to ensure desired control on leachable Na₂O content in the feed hydrate to Kiln. The washed and filtered hydrate is calcined in Rotary kiln at about 1400^oC and passed through Air cooler and water cooler for recovery of thermal energy so as to minimise specific HFO consumption for alumina production.

Operating parameters for Rotary vacuum filters:-

Solids in feed hydrate slurry: 45% (w/w),

Cake thickness: 40 mm max.,

Heel thickness: 5 mm,

Vacuum requirement: 700 mm Hg min.

Moisture in filtered cake: 10% max.,

Hot water requirement: 1 m³ per tonne of hydrate,

Specific filtration rate: 1.5 t/m².hr.

Specifications of Fuel oil:-

Type of fuel oil: Heavy furnace oil (HFO),

Calorific value of oil: 9600 k.cals./kg,

Specific heat of oil: 0.56 k.cal./kg,

Flash point of oil: 250^oC.

Typical analysis of Fuel oil:-

C = 87%,          H=10.5%,          S =1.0%,

O=0.10%,         N=0.20%           Moisture=1.2%.

Sizing of Required Horizontal Rotary Vacuum Filter:-

It is assumed that 0.5% of alumina will be lost with flue gas through stack.

Alumina production rate = 1.0 tph

                                                         1.0 *156

Thus dry hydrate to Rotary kiln = ------------- = 1.54 tph.

                                                      0.995*102

No. of washing stages on filter =2

Number of filtration stage = 1,

Total no. of washing & filtration stages =3.

Specific filtration rate = 1.5 t Hydrate/m².hr.

Total filtration area required = 3*1.5/1.54 = 3.1 m².

Nearest available filter = 4.2 sq.m.

Sizing of Required Rotary Kiln:-

Let effective inner diameter of Kiln = 1.80 m,

Thickness of refractory bricks = 150 mm.

Thus inner shell diameter of Kiln=1.80 m + 2*0.15 m =2.10 m.

                                                                                K*L*D²

Throughput rate of Kiln, tonnes per day = ------------

                                                                                    3

Where,

L = Length of Kiln in meters,

D = Diameter of Kiln in meters and

K = Constant.

For alumina, K =1.10.

Thus 1.0*24 = 1.10*L*(2.1)²

Hence, Length of Kiln, L =14.85 meters.

Rotational Speed of Kiln:-                     

For production of specialty grade alumina,

Kiln rotates at N*D = 2.40,

Where,

N=Rotational speed of Kiln in rpm and

D = Diameter of Kiln.

Thus N*2.10 = 2.40

Hence Rotational speed Rotary Kiln = 1.14 RPM.

Trust, the system design calculations for RGA have been described systematically. The material balance and thermal energy balance for RGA plant shall be covered in forthcoming posts.

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

Thanks and regards.

Rajendra Kunwar
www.ceti.co.in

Preferred Classification Equipment for Ground Bauxite Slurry

Hi Friends,

In earlier posts, we have discussed about various techniques for grinding of bauxite. The basic objective of grinding of bauxite is to increase the surface area of material substantially so as to improve its reactivity with caustic soda to a desired level during digestion process. As such wet grinding of bauxite has  been found to be energy efficient and environment friendly as well. Ball mill or Rod mill or SAG or combination of any two is generally used in Alumina refineries for the purpose. The particle size of ground bauxite is decided based on the laboratory test work ranging from 149 microns to 1000 microns, however, minus 10 mesh size (100%) has been found to be optimum size in most of the operative Alumina refineries in the World.

In present post, we will discuss the preferred equipment for classification of bauxite slurry in Alumina refinery. Though, open circuit wet grinding may require less capital investment as well as lower energy consumption but it is difficult to ensure desired product fineness. Thus, to ensure the consistency in desired product fineness, closed circuit grinding is adopted in most of the Alumina refineries. For bauxite slurry classification, any one of the following type of equipment are installed in Alumina refineries-

• DSM Screens, 
• Banana Screens,
• Hydro-cyclones.

With regard to capital investment for installation, all three types of slurry classification systems are comparable. But observing the availability for continuous operation, hydro-cyclones are always rated at number one position. The reason being it does not require any descaling and thus downtime is minimal where as periodic descaling of DSM as well Banana screens are mandatory for removal of accumulated scales to ensure desired classification efficiency. Hence considering the availability, reliability and process efficiency, hydro-cyclone is always the preferred classification for Bauxite grinding circuit.

I personally like trouble-free operation of hydro-cyclones over others and love to operate it efficiently. This is purely my personal experience. In case, you differ to my views, please come forward in freely exchanging your own operating experience. We will welcome your views.

regards.

Rajendra Kunwar
www.ceti.co.in

Basic Approach for Testing & Commissioning of 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 start up 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 etc 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:

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,

·         Equipment interlocking system,

·         Micro processor 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 start up 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,

·         Fixing the responsibilities of individuals in commissioning team,

·         Normal plant start up procedure,

·         Normal shut down procedure,

·         Emergency plant shut down 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,

·         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 start up 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 essentially followed for systematic testing, start up and commissioning of newly constructed Alumina refinery.

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

Regards.

Rajendra Kunwar
Principal Consultant-Engineering
CETI Enterprises,
www.ceti.co.in

REASONS FOR HIGH READERS TRAFFIC ON ALUMINA TECHNOLOGY BLOG

Hi Friends,

Looking at the astonished and unimaginable interests on readers on our Alumina Technology Blog forced us to analyse the reasons of such high density of readers traffic.

Members of our team did the thorough analysis to find out the most applicable reasons in consultation with our learned followers across the globe. Basic observations were systematically analysed with statistical tools. The key results and conclusions are listed below:

1. Unique technical platform with authentic data and information,
2. Terminology nicely elaborated for easy understanding,
3. Detailed process and engineering calculations with systematic approach,
4. Process and control parameters elaborated with clarity,
5. Equipment design calculations presented with suitable examples,
6. Methodology described for project costing and production costing,
7. A to Z from bauxite mining to Alumina production in detail with glimpse of aluminium production technology,
8. Detailed specifications for variety of bauxite, alumina and alumina chemicals,
9. Good coverage on production process of each product with focus on market perspective, demand and project profitability,
10. Benchmark figures on efficiency parameters such as raw material consumption figures and energy consumption aspects,
11. Process of developing basic engineering documents for all unit operation and processes including design of Residue storage pond,
12. Simplicity in presentation highlighting important steps.

These are a few major conclusions / observations. Hope, you have also similar opinions. In case, you have some additional comments / remarks, please feel free to place your comments for further improvements in publication of technical posts in future.

Best regards.

Rajendra Kunwar

Key Member of Alumina Technology Team

www.ceti.co.in