Electric Arc Furnace

Electric Arc Furnace

Introduction

  • EAF is direct arc furnace.
  • Steelmaking temp is maintained by an electric arc struck between the graphite electrodes &the metallic charge.
  • The arc temperature is above 4000°C and is used to heat the bath by radiative heat transfer.
  • Only process wherein either oxidizing or reducing condition can be maintained as desired.
  • Different slag types can be carried out.
  • High cost of electrical energy makes this process costly.

Parts Of EAF

  • Furnace body consisting of the shell,
  • the hearth, the walls, the spout, doors, etc.
  • Gears for furnace body movement
  • Arrangements for moving the roof
  • Electrodes with their holders and supports
  • Electrode lifting and lowering mechanism
  • Electrical equipment

Charging Materials

  • Steel scrap is the principle raw material. It may constitute 60 to 80% of the charge.
  • In some practices sponge iron and or pig iron is also used for chemical balance.
  • In basic furnaces slag formers like limestone, fluorspar, sand, and quartzite are used to form a slag to refine the metal.
  • Oxygen lancing is used for decarburization
  • For the purpose of deoxidation Ferro-manganese, ferrosilicon or aluminum is used.
  • Alloying elements are added for the production of alloy steels.

EAF Operations

Basically EAF operations consists of three steps

  1. Charging
  2. Melt down period
  3. Refining

1. Charging

  • Heavy and light scrap are preheated through the exit gas. Burnt lime and spar are added to help early slag formation.
  • Iron ore may also be added if refining is required during melt- down period.
  • The furnace is charged from the top (Hot metal is also charged as per the requirement).
  • Some furnaces are equipped with continuous charging.

2. MELT DOWN PERIOD

  • Electrodes are lowered and bored into the scrap.
  • To protect the roof and walls from excessive heat and damage from the arcs, Lower voltages are selected.
  • When the arc is shielded by scrap, voltage is increased to form molten metal pool to reduce the meltdown period.
  • During meltdown period, silicon, manganese and carbon oxidizes.
  • Oxidizing and limy slag is produces which promotes dephosphorization as well.

Melt down period depends on Arc conditions and Deep/shallow bath.

  • Larger arc requires lower current and lower heat losses.
  • Deep bath shortens the meltdown period.

3. Refining

  • Refining continues even during melting.
  • Removal of phosphorus must be complete before the rise in temperature and carbon boil.
  • When removal of sulphur is not required single oxidizing slag practice is used.
  • When both Phosphorous and Sulphur is not required double slag practice is used.
  • In double slag practice, oxidizing slag is removed and reducing slag is formed after deoxidation.
  • Reducing slag helps to avoid loss of alloying elements.

Once the bath chemistry and its temperature are attained, heat is deoxidized and finished for tapping.

Advantages of EAF Steelmaking

  • The use of EAFs allows steel to be made from a 100% scrap metal feedstock.
  • The large reduction in specific energy (energy per unit weight) required to produce the steel.
  • EAFs can be rapidly started and stopped, allowing the steel mill to vary production according to demand.
  • Hot metal from Blast furnace can be directly used.
  • Flexibility to produce a variety of grades.

Limitations of EAF Steelmaking

  • Running cost is high because of electrical energy consumptions and other attached equipments.
  • When EAF repairs are necessary they are often quite expensive.
  • Scrap availability as Scrap is principle charge material in EAF.

Modern Developments in EAF Steelmaking

  • Ultra high power supply (UHP)
  • Oxygen lancing
  • Foamy slag practice
  • Use of DRI/Hot briquetted iron
  • Pre-heating of scrap and DRI
  • Bath stirring by argon
  • Eccentric bottom tapping
  • Emission and noise control
  • Process automation and control

———On demand topic by Rupesh Yadav———-

Credit- Rohit Jha

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