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Dr. Dmitri Kopeliovich
Electric Arc Furnace (EAF) is a steel making furnace, in which steel scrap is heated and melted by heat of electric arcs striking between the furnace electrodes and the metal bath.
Two kinds of electric current may be used in Electric Arc Furnaces: direct (DC) and alternating (AC).
Three-phase AC Electric Arc Furnaces with graphite electrodes are commonly used in steel making.
The main advantage of the Electric Arc Furnaces over the Basic Oxygen Furnaces (BOF) is their capability to treat charges containing up to 100% of scrap. About 33% of the crude steel in the world is made in the Electric Arc Furnaces (EAF).
Capacity of Electric Arc Furnace may reach 400 t.
The scheme of a Electric Arc Furnace (EAF) is presented in the picture.
The furnace consists of a spherical hearth (bottom), cylindrical shell and a swinging water-cooled dome-shaped roof.
The roof has three holes for consumable graphite electrodes held by a clamping mechanism. The mechanism provides independent lifting and lowering of each electrode.
The water-cooled electrode holders serve also as contacts for transmitting electric current supplied by water-cooled cables (tubes). The electrode and the scrap form the star connection of three-phase current, in which the scrap is common junction.
The furnace is mounted on a tilting mechanism for tapping the molten steel through a tap hole with a pour spout located on the back side of the shell.
The charge door, through which the slag components and alloying additives are charged, is located on the front side of the furnace shell. The charge door is also used for removing the slag (de-slagging).
The scrap is charged commonly from the furnace top. The roof with the electrodes is swung aside before the scrap charging.
The scrap arranged in the charge basket is transferred to the furnace by a crane and then dropped into the shell.
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Refractory linings of Electric Arc Furnaces are made generally of resin-bonded magnesia-carbon bricks. Fused magnesite grains and flake graphite are used as raw materials.
When the bricks are heated the bonding material is coked and turns into a carbon network binding the refractory grains, preventing wetting by the slag and protecting the lining the from erosion and chemical attack of the molten metal and slag.
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Melting
Melting process starts at low voltage (short arc) between the electrodes and the scrap. The arc during this period is unstable. In order to improve the arc stability small pieces of the scrap are placed in the upper layer of the charge. The electrodes descend melting the charge and penetrating into the scrap forming bores. The molten metal flows down to the furnace bottom.
When the electrodes reach the liquid bath the arc becomes stable and the voltage may be increased (long arc). The electrodes are lifting together with the melt level. Most of scrap (85%) melt during this period.
Temperature of the arc reaches 6300ºF (3500ºC).
Oxidizing stage
At this stage excessive carbon, phosphorous, silicon and manganese oxidize.
The process is similar to that in Basic Oxygen Furnace.
Basic oxidizing slag composed of lime (CaO) and ion ore (FeO) is used during the oxidizing period.
Gaseous oxygen may be blown into the melt for additional oxidizing.
Iron oxide causes increase of Oxygen content in the molten steel according to the reaction:
(square brackets [ ] - signify solution in steel, round brackets ( ) - in slag, curly brackets {} - in gas)
(FeO) = [Fe] + [O]
Oxygen dissolved in the melt oxidizes carbon, phosphorous, silicon and manganese:
[C] + [O] = {CO}
[Si] + {O2} = (SiO2)
[Mn] + 1/2{O2} = (MnO)
2[P] + 5/2{O2} = (P2O5)
Carbon monoxide partially burns in the atmosphere:
{CO} + {O2} = {CO2}
The formed oxides are absorbed by the slag. CO bubbles floating up through the melt result in refining of the steel from non-metallic inclusions and hydrogen removal.
Gaseous products CO and CO2 are removed by the exhausting system. Oxidizing potential of the atmosphere is characterized by the post-combustion ratio: {CO2}/({CO2}+{CO}).
The oxidizing slag enriched with phosphorous and other oxides formed during this period is removed from the furnace to a slag pot (de-slagging).
Reducing stage
New slag composed mainly of lime (CaO), CaF2 (as slag fluidizer) is added at this stage for formation of basic reducing conditions.
The function of this slag is refining of the steel from sulfur and absorption of oxides, formed as a result of deoxidation (”killing”).
The excessive oxygen dissolved in the melt during oxidizing period is removed by metallic deoxidizers Mn, Si, Al:
[Mn] + [O] = (MnO)
[Si] + 2[O] = (SiO2)
2[Al] + 3[O] = (Al2O3)
Basic reducing slag is favorable for desulfurization in accordance to the reaction:
[S] + (CaO) = (CaS) + [O]
Oxide and sulfide non-metallic inclusions are absorbed by the slag.
Alloying elements (Cr, Ni, Mo, V, etc.) are added after deoxidation.
In many cases the processes of “killing” (deoxidation), desulfurization, alloying and final heating are performed outside of the furnace - Ladle refining.
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