Here, electric arc is produced between the electrodes.
This electric
arc is used for melting the metal. The arc furnaces are used to
produce mini steel structural bars and steel rods. The electric
furnace is in form of a vertical vessel of fire brick. There are mainly
two types of electric furnaces. They are alternating current (AC)
and direct current (DC) operated electric furnaces.
DC Electric Arc Furnace
DC Arc Furnace is recent and advanced furnace compared to AC
Arc Furnace. In DC Arc Furnace, current flows from cathode to
anode. This furnace has only a single graphite electrode and the
other electrode is embedded at the bottom of the furnace. There
are different methods for fixing anode at the bottom of the DC
furnace.
The first arrangement has a single metal anode at the bottom,
cooled by water due to fast heating. In the next method, the anode
is a conducting hearth lined with C-MgO, with current supplied to a
Cu plate at the bottom and air cooling the anode. The third setup
uses metal rods as the anode, embedded in MgO mass. The fourth
uses thin sheets as the anode, also embedded in MgO mass.
Advantages of DC Electric Arc Furnace
Decrease in electrode consumption by 50%.
Melting is almost uniform.
Decrease in power consumption (5 to 10%).
Decrease in flicker by 50%.
Decrease in refractory consumption
Hearth life can be extended.
AC electric Arc Furnace
In an AC electric furnace, current flows between the electrodes
through the metal charges. This furnace uses three graphite
electrodes as cathodes, with the scrap acting as the anode.
Compared to a DC arc furnace, it is cost-effective and commonly
used in small-scale operations.
Construction of Electric Arc Furnace
As mentioned above, the electric furnace is a large firebrick lined
erect vessel. It is demonstrated in figure 2.
The main parts of an electric furnace are the roof, hearth (where
molten metal is collected), electrodes, and side walls. The roof has
three holes for electrode insertion and is made of alumina and
magnesite-chromite bricks. The hearth contains metal and slag.
The furnace uses a tilting mechanism to pour molten metal. A roof
retraction mechanism allows for electrode removal and furnace
charging. Fume extraction is provided for operator health. In AC
furnaces, there are three round graphite electrodes due to their
high electrical conductivity. Carbon electrodes are also used, with
an automatic system to raise and lower them. Electrodes oxidize
quickly at high current density.
Transformer: –
The transformer provides the electrical supply to the electrodes. It
is located near to the furnace. It is well protected. The rating of
large electric arc furnace may be up to 60MVA.
Working Principle of Electrical Arc Furnace
The operation of an electric furnace involves charging the
electrode, melting the metal, and refining. Heavy and light scrap in
a large basket is preheated using exhaust gas. Burnt lime and spar
are added to speed up slag formation. The furnace is charged by
swinging its roof, and hot metal can also be added as needed.
Next is the meltdown period. The electrodes are moved down onto
the scrap in this period. Then the arc is produced between the
electrode and metal. By considering the protection aspect, low
voltage is selected for this. After the arc is shielded by electrodes,
the voltage is increased for speeding up the melting process. In this
process, carbon, silicon, and manganese get oxidized. The lower
current is required for large arc production. The heat loss is also
less in this. Melting down process can be fastening by deep bathing
of electrodes.
Refining process starts during melting. The removal of sulfur is not
essential for single oxidizing slag practice. Only phosphorous
removal is required in this. But in double slag practice, both (S and
P) are to be removed. After the deoxidizing; in double slag practice,
the removal of oxidizing slag is performed. Next, with the help of
aluminum or ferromanganese or ferrosilicon, it gets deoxidized.
When the bathing chemistry and required temperature is reached,
the heat will get deoxidized. Then, the molten metal is ready for
tapping.
For the cooling of the furnace, tubular pressure panels or hollow
annulus spraying can be used.
