ELECTRIC ARC FURNACE PROCESS


Introduction :

Electricity is an alternative source of liquid and gaseous fuel to generate heat. In steel making electricity is used to supply necessary head for refining process of steel. Generally there are four types of electric furnace to perform metal process.
Electric arc furnace operation
Electric arc furnace

1. Electric resistance furnace
2. Indirect Arc furnace
3. Direct Arc furnace
4. High frequency Induction furnace
Last two type 3 and 4 furnaces are widely used in steel making industry.

ELECTRIC ARC FURNACE


Furnace Construction :

● Direct arc electric furnace is made up of a heavy steel shell lined inside with acid or basic refractory bricks as required by the refining process perform. For acidic process acidic lining and basic lining for basic process is required. Most of the furnace are lined with basic magnesite bricks, because this lining permits variety of charge materials for the steel making.
Electric furnace details and lining
Electric arc furnace lining and details

● The cover or the spherical roof is lined with silica bricks and through roof. The carbon electrodes are inserted in to furnace, it can be lower or raised up during the process. The spout is lined with the fire clay bricks. The whole furnace rest on the bearing and turnion arrangements and tilting purpose due the charging, tapping and splashing the slag.

Working Principle :

The metallic charge in the hearth act as cathode and the carbon electrodes are made anode. When current is allowed to pass through the carbon electrode, a high temperature electric arc is produced between the charge and the carbon electrode which melts the material and refining processed.

Furnace Operation :

1. Charging :

Prior to charging, furnace hearth is pre-heated at first by broken carbon electrode pieces are kept in the hearth and arcing is done. Due to burning of carbon pieces, the furnace interior is heated. After completion or pre-heating, the burn piece of carbon product are removed out of the hearth. At first steel scrap charged to the hearth followed by the pig iron, iron ore and flux (limestone and dolomite)

Melting :

After charging the carbon electrodes are lowered in to suitable position and arcing is carried out. The arc produces, generally enormous heat which is transmitted to charge by radition then charge starts melting.

During arcing the position of the electrode is frequently changed to have greater affect of heating and melting. When the charge are completely melted, electrode are kept at a constant position till the refining.

Refining :

Actually refining starts with beginning of melting. Refining in electric arc furnace is carried out by producing two or more type of slag. Generally oxidizing slag is prepared by adding mill scale (Fe3O4) and lime, most of the Si, Mn, and P are removed during oxidizing slag formation. The oxidizing slag is then removed completely from the melt by tilting the furnace. The oxidizing refinination process via following chemical reaction as :
Illustration of arc furnace
Illustration of arc furnace

2Si + 2FeO = SiO2 + 2Fe
Mn + FeO = MnO + Fe
SiO2 + MnO = (MnO.SiO2)
[FeS] + MnO + C = MnS + Fe + CO
FeO + SiO2 = FeO.SiO2
2P + 5FeO = P2O5 + 5Fe
P2O5 + 3FeO = FeO3.P2O5
FeO3.P2O5 + CaO = CaO.P2O5 + 3FeO

● Oxidising slag or black slag does not have sulphur in any from hence it remain in the refine from steel melt. Next of refining in electric arc process is carried out by producing reducing slag. Lime powder, coke and Flurspar are added to the melt to from new slag or white slag. The chief purpose of coke is to react with the lime to form calcium chloride (CaCl2)

● The reducing slag has strongly deoxidized the bath and removes sulphur as CaS. From the arcing CaC2 form from lime and coke as
CaO + 3C = CaC2 + C
Which have high temperature and thus CaC2 affectively removed sulphur as CaS. This stage of refining gives a very high degree of desulphurisation and very good deoxidation.

● This reducing atmosphere provides alloying addition for alloy steel making or the time of tapping. The reducing reaction involved in this stage of refining, some residual MN is left in the slag are :

Mn + FeO = MnO + Fe
MnO + C = Mn + CO
CaO + 3C = CaC2 + CO
FeS + CaO +C = Fe + CO + CaS
3FeO + CaC2 = 3Fe + CaO + 2CO
FeS + 2CaO + CaC2 = 3Fe + 2CO + 3CaS

Samples of this second slag is taken and tested. After sometime it is lighter and liberates acetylene gas who dipped in water. It is an indication that the bath is deoxidising and desulphurising.

Tapping :

After reducing, refining and slag removal the metal bath is deoxidized with FeSi and Al. Alloying addition is done and sample check for specification. The furnace is then tilted forward and molten metal is tapped in the ladle. But in modern practices total alloying addition is to be done in steel ladle.

Advantage :

1. Temperature during the process is controlled effectively.
2. Thermal efficiency is higher in direct arc furnace.
3. Refining sequence can be followed at well.
4. The melt is refined to a proper specification and can be heated to solid pouring temperature.
5. Furnace atmosphere is well control.
6. Alloy recovery is higher then ld process.
7. Wide variety of charge can be melted and refined.
8. Quicker readiness of the furnace.
9. Longer hearth life and easily repairable.
10. Sponge iron can also be refined to steel.

Disadvantage :

1. Higher heating cost due to high consumption of electricity.
2. High erosion of carbon electrodes during arcing.
3. Electrical saftey is required.
4. Higher noise and emissions during the process.






References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty

4. Sr. Professors from IIT's & BPUT. (INDIA)

Author :
Subir Kumar Sahu
Metallurgist.

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