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Sponge Iron - Manufacturing Process |
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The reduction of Iron Ore can be achieved by using either carbon bearing material, such as non-coking coal or a suitable reducing gas in the form of reformed natural gas. The processes employing coal are known as solid-reluctant of coal-based processes while those employing reducing gases are known as gas-based processes. Sponge Iron produced by gas-based process is normally hot briquette and hence it is also known as Briquetted Iron (HBI).
In the decade 1990-2000 the total sponge iron production was distributed as follows:
| Gas-Based |
: |
4 Million Tonnes |
| Coal-Based |
: |
3 Million Tonnes |
Coal-Based Process
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This process utilizes non-coking coal as reducing agent along with lumpy rich grade iron ore. |
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The reduction is carried out in an inclined horizontal rotary kiln, which rotates at a predetermined speed. A temperature profile ranging from 800-1050 degree centigrade is maintained along the length of the kiln at different zones and as the material flows down due to gravity the ore is reduced. |
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The hot reduced sponge iron along with semi-burnt coal, discharged from kiln is cooled in water-cooled cylindrical rotary cooler to a temperature of 100–200 degree centigrade. |
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The discharge from cooler consisting of sponge iron, char other contaminations are passed on through magnetic separators so that sponge iron can be separated from other impurities. |
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Later the sponge iron is screened into two size fractions i.e. –3 mm & +3 mm. +3 mm fraction directly goes for usage, -3 mm fraction can be either used directly where ever it is possible or is to be briquetted by using molasses and hydrated lime as binders. |
The basic reactions in this process is as follows:
Raw Material Requirements
As in case of any process, here also quality of raw materials plays a vital role in obtaining the rated capacity and product quality. The iron ore shall have resistance for physical & thermal decrepitation and good reducibility and the coal should be of reactive nature along with the characteristics of high fusion temperature and less amount of ash.
» Chemical and Physical Composition of Iron Ore
| Constituent |
% |
| Chemical Composition |
|
| Total iron |
65 - 67 % |
| Sio2 + al2 o3 |
2 - 3 % |
| Cao + mgo |
0.5 - 1 % |
| Sulphur |
0.02 % max. |
| Phosphorous |
0.04 % max. |
| Physical Composition |
|
| Size |
5 - 20 mm |
| Shatter Index |
+ 95 % |
| Tumbler Index |
+ 88 % |
| Abrasion Index |
+ 5 % max |
| Reducibility Index |
+ 94 % |
| Thermal Degradation Index |
5 % |
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» Typical Chemical Analysis of Coal
| Constituent |
% |
| Moisture |
8 % max |
| Total Iron |
27 - 30 % |
| Ash |
23 -25 % |
| Fixed Carbon |
+ 40 % |
| Size |
0 - 20 % |
| Ash Softening Point |
1250 deg cent |
| Calorific Value |
5200 k . Cals / mole |
| Reactivity |
2.2 co / gm . Deg c |
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» Typical Chemical Analysis
| Cao |
40 - 45 % |
| L.O.I |
40 - 45 % |
| Insolubles |
5 - 7 % |
| Size |
0.5 - 4 mm |
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» Typical Analysis of Sponge Iron Produced in Coal Based Process
| Total Iron |
91 - 93 % |
| Metallic Iron |
80 - 84 % |
| Metallisation |
90 - 92 % |
| Gangue |
3 - 4 % |
| Carbon |
0.25 % |
| Sulphur |
0.02 - 0.03 % |
| Phosphorous |
0.035 - 0.05 % |
|
Gas-Based Process
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This process utilizes natural gas the reducing agent. |
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Natural gas is reformed to enrich with H2 and Co mixture and this enriched and reformed gas mixture is preheated in gas-based process. |
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A vertical retort is used for the reduction of iron ore as against a rotary kiln in coal based sponge iron process. |
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When the gases are traveling upwards the charge moves downward by gravity. |
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Gas-based sponge iron is not subjected for magnetic separation, as no contamination with non–magnetic is possible either it can be cooled indirectly or briquetted in hot condition to get hot briquetted iron (HBI). |
The basic reactions are as follows:
There are only two well established processes
1. Midrex
2. Hyl
In gas-based process more preferable feed is pellets but lump ore is also used for with proper size distribution to have required permeability.
» Iron Ore Chemical Analysis
| Chemical Property |
% |
| Total Iron |
65 - 67 % |
| Sio2 + al2 o3 |
2 - 3 % |
| Cao + mgo |
1 - 2 % |
| Sulphur |
0.025 % |
| Phosphorous |
0.045 % |
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» Iron Ore Physical Analysis
| Physical Property |
% |
| Size |
6 - 40 mm |
| Moisture |
3 % max |
| Slatter Index |
+ 90 % |
| Reducibility Index |
+ 92 % |
| Bulk Density |
2.7 t / m3 |
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» Chemical Analysis of Iron Ore Pellets
| Total Iron |
64 - 65 % |
| Si O2 + Al2O2 |
4 - 6 % |
| CaO + MgO |
2 - 3 % |
| Sulphur |
0.028 % |
| Phosphorous |
0.045 % |
| Size |
12 - 16 mm |
| Avg. Cold cr. Strength |
200 |
| Bulk Density |
2.2 t / m3 |
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» Typical Chemical Analysis of the Natural Gas
| Constituent |
% After reformation |
| Nitrogen |
0.3 - 1.0 % |
Co + h2 - 90 %And above |
| Carbon dioxide |
0.1 - 5 % |
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| Methane |
85 - 93 % |
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| Ethane |
3 - 8 % |
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| Propane |
1 - 4 % |
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» Typical Analysis of Sponge Iron of Gaseous Process
| Total Iron |
92 % |
| Metallic Iron |
81 - 85 % |
| Metallization |
85 - 93 % |
| Gangue |
6 - 8 % |
| Carbon |
1.2 - 2.5 % |
| Sulphur |
0.003 % max |
| Phosphorous |
0.06 % max |
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Coal-Based Process v/s Gas-Based Process
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The present output of sponge iron in India is about 7 MT/year. Out of which 50 % output is based on non–coking coal. |
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Out of the 73 DRI units in India, 70 units are coal-based and the rest 3 are gas-based. Further the capital cost for setting up of coal-based units is cheaper compared to Gas-based units. Gas deposits are not fully explored and more over allocation of this industry is yet to be demarcated by the government. |
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DRI, nowadays used along with the scrap as a dilutants of tramp elements and also in some cases as complete feedstock instead of scrap. |
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It is cheaper, clean and rosily available raw material for making clean steel. |
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Therefore DRI of coal-based process is of much relevance to steel manufacturing. |
» Comparison between Coal and Gas Based Product
| No. |
Parameters |
Gas Based |
Coal Based |
| 1. |
Metallization |
85 - 93 % |
88 - 92 % |
| 2. |
Carbon |
1.2 - 2.5 % |
0.2 - 0.25 % |
| 3. |
Size of the product |
Mostly uniform as pellets are commonly used |
Wide variation as lump ore is used (3-20) mm |
| 4. |
Recitation |
Prone for re-oxidation unless briquetted |
Relatively more stable |
| 5. |
Melting in EAF |
Higher carbon content is advantageous for reducing residual iron oxide in S.I |
Low carbon content. Extra carbon required for reducing residual iron oxide. |
| 6. |
L.D Converter |
Carbon content will be high. |
Carbon in the liquid metal is reduced because of FeO content in S.I. |
| 7. |
Bulk density MT/ m3 |
1.5-1.9 |
1.6-2.0 |
| 8. |
Non - magnetic (Char) |
Nil |
0.3 to 0.5 % |
| 9. |
% of usage of change mix in EAF |
Max 30%. |
Up to 80 % in continuous charging |
| 10. |
Yield |
1-2 % will be more when compare with coal based sponge iron. |
1-2 % will be less when compared with gas based sponge iron. |
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