developments in electric arc furnace

1. Made by:-
Sanjit Dhar(17120065)
Sarandeep Singh
Khurana (17120066)

2. An electric arc furnace used for steelmaking consists of a refractory-lined
vessel, usually water-cooled in larger sizes, covered with a retractable roof,
and through which one or more graphite electrodes enter the furnace. The
furnace is primarily split into three sections:-
 the shell, which consists of the sidewalls and lower steel "bowl";
 the hearth, which consists of the refractory that lines the lower bowl;
 the roof, which may be refractory-lined or water-cooled, and can be shaped
as a section of a sphere, or as a frustum (conical section). The roof also
supports the refractory delta in its centre, through which one or
more graphite electrodes enter.
The hearth may be hemispherical in shape, or in an eccentric bottom tapping
furnace , the hearth has the shape of a halved egg. In modern melt shops,
the furnace is often raised off the ground floor, so that ladles and slag pots
can easily be maneuvered under either end of the furnace. Separate from
the furnace structure is the electrode support and electrical system, and the
tilting platform on which the furnace rests. Two configurations are possible:
the electrode supports and the roof tilt with the furnace, or are fixed to the
raised platform.

3. A typical alternating current furnace is powered by a three-phase electrical
supply and therefore has three electrodes. Electrodes are round in section,
and typically in segments with threaded couplings, so that as the electrodes
wear, new segments can be added. The arc forms between the charged
material and the electrode, the charge is heated both by current passing
through the charge and by the radiant energy evolved by the arc. The
electric arc temperature reaches around 3000 °C (5000 °F), thus causing the
lower sections of the electrodes to glow incandescently when in
operation. The electrodes are automatically raised and lowered by a
positioning system, which may use either electric winch hoists or hydraulic
cylinders. The regulating system maintains approximately constant current
and power input during the melting of the charge, even though scrap may
move under the electrodes as it melts. The mast arms holding the electrodes
can either carry heavy busbars (which may be hollow water-
cooled copper pipes carrying current to the electrode clamps) or be "hot
arms", where the whole arm carries the current, increasing efficiency. Hot
arms can be added from copper-clad steel or aluminium . Large water-
cooled cables connect the bus tubes or arms with the transformer located
adjacent to the furnace. The transformer is installed in a vault and is water-
cooled.

4. • Use of 100 percent steel
scrap
Conservation
Of resources
• Vary the production rate
• Started and stopped rapidly
Flexibility in
steel
production
• Much feasible if electricity is
available at cheaper rates
Requirement
of electricity

5. Scrap metal is delivered to a scrap bay , located next to the melt shop ( consists of two
main grades) ; shred of light gauge steels and , heavy melt of large beams and slabs ,
along with some direct reduced iron or pig iron for chemical balance . The scrap is
loaded in large buckets in layers , with clamshell doors for the base i.e. heavy melt is
sandwiched between lighter shred . They are pre-heated from the furnace off-gases to
recover the plant efficiency . A lot of potential energy is released by the tonnes of
falling metal ; any liquid metal in the furnace is often displaced upwards and outwards
by the solid scrap , and grease and dust on the scrap is ignited if the furnace is hot ,
resulting in a fireball erupting . In some twin-shell furnaces , the scrap is charged into
the second shell while the first is being melted down , and pre-heated with off-gases
from the active shell . After the roof is swung back , oxygen is blown into the scrap ,
combusting or cutting the steel , and extra chemical heat is provided by the wall
mounted oxygen-fuel burners . Supersonic nozzles enable oxygen jets to penetrate
foaming slag and reach the liquid slag . An important part of steel-making is the
formation of slag , which acts as a thermal blanket and reduce erosion of refractory
lining . Later in the heat , coke or coal is injected , which reacts with FeO to form Fe
and CO to form a foamy slag , allowing better thermal efficiency , better arc stability
and electrical efficiency . The voltage is at a lower level in the initial stages of arcing ,
but reaches to higher levels on reaching to the layer of heavy melt . Electrodes are
raised slightly then to increase the length of the arc and increasing power to the melt .

6. 1. The furnace refractories are often made with calcium carbonates
which are extremely susceptible to hydration from water , so any
suspected leaks from water-cooled components is treated seriously ,
beyond the immediate concerns of potential steam explosions .
2. Because of the very dynamic quality of the arc furnace load , power
systems may require technical measures to maintain the quality of
power to other customers , flicker and harmonic distortion are
common side-effects of arc furnace operation on a power system .
3. Enclosures to reduce high sound levels ;
4. Dust collector for furnace off-gases ;
5. Maintenance of conductive furnace hearth is a bottleneck in
extended operation of a DC arc furnace .
6. The size of DC arc furnaces is limited by the current carrying
capability of the available electrodes , and maximum allowable
voltage .

7. Electric arc furnaces are also used for the production of ferro-alloys
and non-ferro alloys . Continuous type EAFs generally uses Soderberg
electrodes , to prevent interruptions due to electrode changes .
COOLING METHODS :-
Smaller arc furnaces may be adequately cooled by circulation of air over structural
elements of the shell and roof, but larger installations require intensive forced cooling to
maintain the structure within safe operating limits. The furnace shell and roof may be
cooled either by water circulated through pipes which form a panel, or by water sprayed
on the panel elements. Tubular panels may be replaced when they become cracked or
reach their thermal stress life cycle. Spray cooling is the most economical and is the
highest efficiency cooling method. A spray cooling piece of equipment can be relined
almost endlessly; equipment that lasts 20 years is the norm. However while a tubular
leak is immediately noticed in an operating furnace due to the pressure loss alarms on
the panels, at this time there exists no immediate way of detecting a very small volume
spray cooling leak. These typically hide behind slag coverage and can hydrate the
refractory in the hearth leading to a break out of molten metal or in the worst case a
steam explosion.

8. 1.In the design of the furnace
• Ex. Ultra-high power transformer ,
advanced electrode holder
2.In the furnace operation
• Ex. Foamy-slag practice , Iron-
carbide as charge

9. The latest development in transformer design is the three-legged
transformer which permits adjustment of the voltage of each
phase independently . This provides the ability to control hot-
spots in the furnace while maintaining maximum power input .
An additional item associated with the UHP transformers is
static VAR (volt-ampere reactive) generates to obtain flicker
control .
This is supported by the water-cooled electrodes . This
technology consists of employing water-cooled electrode
sections between the furnace electrode holder and the
conventional graphite electrode which ultimately reduces
graphite losses caused by oxidation at high temperatures

10. An important development in the furnace shell design is the
application of water cooling to those portions of the upper
side wall lining and roof which suffer maximum wear rate
. This avoids mid-campaign patching . The largest areas
of cooling are the “hot spots” where side-wall is closest to
the electrodes and which are subjected to intense
thermal radiation , arc flame and iron-oxide particle which
tend to penetrate and flux the refractory lining material .
Also heat recovery can also be obtained from the cooling
water . In DC electrical system , electrode surface wear
is greatly reduced and arc is more stable as compared to
AC electric system .

12. One great development is the split shell design in which the shell
structure is constructed in two separable sections – lower section
containing the hearth and the upper section containing the balance
of the shell side wall . With this design the upper section with used
side wall lining can be replaced with a new lined upper section ,
thus reducing furnace downtime for a sidewall replacement . This
also allows improved working condition for the actual relining work
. The split shell feature is used mostly on larger size continuously
operated furnace .
Another innovation in this field is the tapered shell , in which the
diameter of hearth and lower side sections of the shell are of larger
diameter than the top opening . The advantages attributed to this
shell design are : a. longer side wall lining life due to the larger
shell diameter at the hot spot areas (as refractory erosion being
inversely proportional to square of the distance); b. improved
thermal efficiency because of reduced heat losses and increased
reverberatory heating of the charge ; c. greater shell volume to
accommodate larger charges.

14. An oxy-fuel burner uses natural gas or oil , together with pure oxygen ,
to produce an extremely high-flame temperature. They are used for
heating , melting unmelted scrap between the electrodes and the
refining . The burners are located in the sidewalls of the furnace
between the electrodes . They may help to increase the rate of scrap
melting in cold spots and thereby make scrap melting more uniform .
A deep foamy slag formed early in the melting operation may envelope
the electrode tips and submerge the arc . With this practice , it is
possible to operate with higher power factor and increased electrical
efficiency . It is also possible to operate at higher power inputs for a
longer part of the melting cycle , thus increasing productivity .
Damage to the furnace resulting from arc flame is reduced because
the arc is buried . Thermal efficiency is improved because the slag
provides an improved medium for energy transfer .

16. For scrap pre-heating , hot off-gases from the arc furnace are
passed through the scrap in the charging bucket and a portion
of heat energy in the off-gases is recovered . The scrap may
also be pre-heated in the charging bucket by burning gaseous
fuel . The benefits of scrap pre-heating are reduction in power
and electrode consumption , and improvement in productivity .
Scrap is heated in a oscillating conveyor in a pre-heating
section by the hot gases from EAF. Carbon monoxide in the
exit gas is also burned in the pre-heater to extract it’s calorific
value . The pre-heated scrap is continuously fed into the liquid
heel of metal in the EAF , where the scrap melts by immersion
. Energy savings of 80-120 kwh/ton of liquid steel produced
has been reported .

17. Another alternative charge material for EAF is iron carbide
which contains more than 6% carbon in the form of Fe3C
. It is introduced into the molten steel by injecting it below
the slag layer pneumatically using a lance . Because of
its high melting point(1873 degree Celsius) , iron carbide
does not melt directly . Instead it dissolves in molten
steel bath . When the carbon introduced into the bath
through iron carbide addition is burnt with oxygen , large
amount of heat is released . This saves equivalent
amount of electric power . In addition , foamy slag
operation becomes easier , submerging the arc and
improving thermal efficiency . It also contributes to
increase in the metallic yield .