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

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