Wednesday, December 27, 2017

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, Na2content is controlled at lower level as Na2causes 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 1100C) : 0.30 % max.
α Al2O3 : 90% min.

Typical physical properties of Refractory grade alumina:-
Appearance: White crystalline,
Melting point: 2040oC,
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 1st 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 1400oC 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: 250oC.

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



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