Systematic Approach for Design of Slurry Mixing Agitators

         Hi Friends,

In various technical forums, process experts as well as equipment manufacturers have opined that the design of agitators for mixing process slurry handled in Chemical industries is complicated and tricky issue. In this paper, we would like to discuss the subject in very simplified manner with systematic approach starting with brief description of involved terminology, associated design parameters and methodology with sample motor rating calculations for the slurry mixing agitator.

Basic Formula Drive Rating Calculations for Agitator:

Power Required for Agitator Drive Motor = N_p X ρ x N³ x D⁵ watt for 1 Impeller.

Where Np = Power number (Dimensionless number dependent on type of impeller),

ρ = Density of Fluid in kg/m3,

N = Revolution per second and

D = Diameter of Impeller in m.

Indicative Power Numbers for Various Types of Impellers:

S.No.	Type of Agitator Impeller	Power Number (Np)
1.	Four bladed 45 degree Pitch Turbine	2.1
2.	Rushtom Turbine	5.5
3.	Propeller Type Impeller	0.70
4.	Hydro-Foil Type Impeller	0.30 to 0.51

Sample Calculations to Arrive at Drive Motor Rating for Agitator: 

Simplified sample calculations have been presented to arrive at the drive motor rating for the agitator of a Process tank having around 3000 m³ gross capacity with key input figures as well as realistic assumptions-

Fluid height in the tank, H = 16 m and Diameter of tank, D_t = 14 m

Slurry volume in tank   = π *D_t²*H/4 =  π * (14)²*16 /4 = 2463 m³   

Solid consistency in Slurry = 50 % (w/w),   Density of slurry, ρ = 1602 kg/m³,

Viscosity of slurry, μ = 550 cp

Agitator Impeller Diameter, D_i= 33 % of tank diameter = 14 * 33% m = 4.62 m

Tip speed of Impeller = π *D_i *N = 3.1416 * 4.62 *20 = 290 m/minute,      

 Drive motor RPM = 1500 rpm, Gear Box Reduction Ratio = 75

∴ Agitator RPM = Drive Motor RPM/Gear Box Reduction Ratio = 1500/75 = 20 rpm,

Thus revolution per second of impeller, N = 20/60 = 0.333 Revolution per second.

Flow Number N_q   = 0.56 and Power Number,     N_p  = 0.51 (assumed figures for Impeller)

∴ Pumping Capacity  =  N_q * N * D_i³  m³/ second

= 0.56 * 0.333 * (4.62)³ = 18.41 m³/sec.

Area of Tank               = π  * D_t²   = π  *(14)² / 4 = 153.94 m²

Bulk fluid Velocity      = pumping capacity/area of tank

= 1104.44 / 153.94= 7.18 m/min. = 23.55 ft./min.

Degree of Agitation   = bulk fluid velocity / 6     

(For 6 ft/min., degree of agitation =1 and Degree of agitation varies from 0 to 10)

= 23.55 / 6 = 3.93 ~ 4

Annular Area = π  * (D_t²- D_i² ) /4  

Where D_t = Diameter of tank      and        D_i = Diameter of impeller in meters.

= 3.14 * (14² – 4.62²) / 4 = 137.18 m²

Rising velocity of particles = pumping capacity / annular area

= 1104.44 / 137.18 = 8.051 m/min.  = 0.1342 m/sec. 

Tank Turnover rate   = Pumping capacity / tank capacity

= 1104.44 / 2463 = 0.45 times / min.

Power Number N_p    = 0.51

Shaft Power,      P      = N_p* ρ *(D_i)⁵ * N³ /1000 kW

Where N_p = Power number of impeller (Dimensionless number),

D_i = Diameter of impeller in meters, Shaft RPS,             N = revolutions per second,

∴ Shaft Power, P   = 0.51 * 1602 * (4.402)⁵ * (0.333)³ /1000 kW = 49.86 kW

Taking Gear Box Efficiency   = 80% and Drive Motor Efficiency = 95%,

Design margin           = 1.15

∴ Drive Motor Rating = 1.15 * 49.86/(0.80 * 0.95) =73.95 kW.

Thus the drive motor of about 75 kW shall be adequate for successful operation of agitator of 3000 m³ Process tank containing fluid of considered solid content. 

Conclusions:

The developed methodology clearly reveals that motor rating calculations for any slurry mixing agitator can be carried out easily by simply replacing the associated input process conditions, operating parameters, dimensions of tanks / vessel and appropriate power number for impeller in above simplified derivation. 

Trust, you will like the methodology adopted in this technical paper. We would like to have your comments.

In case, your company needs assistance of our team in resolving any technological, process, engineering or operational problems of your plant; we may be contacted. We are operating out of Gurugram, India. You may like to have a look at our website.

Regards.

Rajendra Kunwar

E.Mail: rajendra@ceti.co.in

Our Website: www.ceti.co.in

System Design for Setting up Refractory Grade Alumina Production Unit

Hi Friends,

In earlier technical papers, we have already covered the operation, technological options, broad design basis, process engineering and process calculations for metallurgical grade Alumina refineries. Now, we would like to present the glimpse of specifications and production processes including plant design as well as engineering aspects for non-metallurgical grades of 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 purer grades of 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.

Previously, we have already published a technical paper on "Market potential of Refractory grade alumina" which was well appreciated by the readers.

In present paper, we would like to discuss about the product specifications, production process route and system design calculations for major equipment of a typical RGA plant of  1 tph 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 minimize 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 Calculations for 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. We will welcome your comments and remarks.

Kunwar Rajendra

Principal Consultant - Engineering

Consultancy, Engineering and Training Institute (CETI), India.

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

Design Calculations for Chemical Cleaning Heat Exchangers for Alumina Refinery

Hi Friends,

Hard scale formation in Red and White areas of Alumina Refinery causes interruptions in plant operations badly affecting the production, productivity and efficiency of the plant. Manual cleaning of these scale is time taking which causes huge loss time for the plant. Adoption of chemical cleaning system attributes in faster cleaning process by dissolving hydrates in hot caustic soda forming Sodium aluminate.

In present post, we will discuss the detailed sample calculations for design of heat exchangers used in white area of Alumina refinery for cleaning of vessels, equipment and process lines. In some Alumina refineries, these heat exchangers are also known as Caustic cleaning heat exchangers. Here, we have taken certain assumed input figures required for a typical Alumina refinery for explanation purposes. Please follow these engineering calculations for the plant of any capacity of your choice. You will find these calculations unique blended with simplified methodology for carrying out equipment design calculations accurately. Step by step derivations and used formula have also been presented here as guideline and reference for easy understanding-

 

You will appreciate the efforts put by our team in developing such complicated design calculations in simplified manner. You will find it as an asset in your knowledge bank as such unique derivations are not available in any published literature. In case, you find similar derivations any where please let me know for my reference.

Please put your views / suggestions / remarks / comments, if any, for further improvements in methodology and method of presentation as well for easy understanding by our young generation. In case of any doubt, please feel free to contact us at www.bauxite2aluminium.blogspot.com.
Regards.

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

Minimum Feasible Capacity of Specialty Grades of Alumina Refinery

Hi Friends,

In earlier post, we have already discussed about minimum feasible capacity of smelter grade Alumina refinery which is techno-economically viable to set up in any part of the globe. In present post, we will briefly discuss about minimum feasible capacity of specialty grades of  Alumina Plant.

It is well known fact that specialty grades of hydrate and alumina are marketed at premium price ranging from three times to five times of those of chemical grade hydrate and standard calcined alumina where as the production of specialty grades of hydrate and special alumina are approximately 15% to 20% higher than those of chemical grade hydrate and smelter grade alumina respectively. Because of increasing demand of specialty grades of hydrate and calcined alumina for various non-metallurgical applications all over the World has tremendous market potential with reasonably high profit margin. Thus, it gives us opportunity to think about the minimum feasible capacity of specialty grade alumina plant which can give high returns resulting in low pack back period. 

The preliminary study reveals that specialty grade alumina plant with 25,000 tonnes per annum production capacity is the minimum feasible capacity considering the availability of basic technological equipment for continuous operation on commercial scale. The estimated capital investment for such plant will be around US$ 25 million excluding the investment towards land and infrastructure facilities. The average manufacture cost of specialty grade alumina is estimated at around US$ 310 per tonne inclusive of all financial charges where as minimum  selling price will be ranging from US$ 600 to US$ 1000 per tonne of specialty grade alumina. Taking average level of sales realization at around US$ 800 per tonne of specialty grade alumina, the pay back period for total capital investment works out to less than 3 years. Specialty grades of hydrate have more profit margin compared to specialty grades of calcined alumina.

The preliminary financial indices prompt us to go ahead for execution immediately. However, It is always advisable to check the following basic information before taking final decision for execution-

1. Availability of about 3.0 million tonnes of chemical grade bauxite,
2. Required utilities and services,
3. About 20 hectares of land for plant,
4. About 50 cubic meters per hour of industrial grade water,
5. Other basic input raw materials and skilled workforce.

In addition to all stated above requirements, the detailed feasibility study is a must to confirm the market scenario, fixing of product-mix, finalizing the required process technology and re-look at outcome of final financial parameters. Execution of such plants definitely require technical competence in process expert as a process technology supplier for the project, otherwise execution with superficial knowledge about the process may be detrimental for the stake holders in the project - this is just a word of caution from our expert team.

We feel that you will like the article on "Marketing potential of Special Hydrates and Aluminas".

These are just the basis to initiate working on this lucrative project. Please put your views / suggestions / remarks / comments, if any.

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

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