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Operation electric arc furnace
eafh.com
Operation
Scrap metal is delivered to a scrap bay, located next to the melt shop. Scrap
generally comes in two main grades: shred (scrap light enough to have been
passed through a shredder) and heavy melt (large slabs and beams), along with
some direct reduced iron (DRI) or pig iron for chemical balance.
The scrap is loaded into large buckets, with 'clamshell' doors for a base. Care
is taken to layer the scrap in the bucket to ensure good furnace operation;
heavy melt is placed on top of a light layer of protective shred, on top of
which is placed more shred. These layers should be present in the furnace after
charging. After loading, the bucket may pass to a scrap pre-heater, which uses
hot furnace off-gases to heat the scrap and recover energy to increase plant
overall efficiency.
The scrap bucket is then taken to the melt shop, the roof is swung off the
furnace, and the furnace is charged with scrap from the bucket. Charging is one
of the more dangerous operations for the EAF operators. There is a lot of energy
generated by multiple tonnes of falling metal; any liquid metal in the furnace
is often displaced upwards and outwards by the solid scrap, and the grease and
dust that coats the scrap is ignited if the furnace is hot, resulting in a
fireball erupting out of the top of the furnace and the slag door. 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-gas from the active shell. Other
operations have trialled pre-heating scrap on a conveyor belt (leading to
continuous furnace charging) or charging the scrap from a shaft set above the
furnace, with off-gases directed through the shaft. Yet other furnaces can be
charged with hot (molten) metal from other operations.
After charging, the roof is swung back over the furnace and meltdown commences.
The electrodes are lowered onto the scrap, an arc is struck and the electrodes
are then set to bore into the layer of shred at the top of the furnace. Lower
voltages are selected for this first part of the operation to protect the roof
and walls from excessive heat and damage from the arcs. Once the electrodes have
reached the heavy melt at the base of the furnace and the arcs are shielded by
the scrap, the voltage can be increased and the electrodes raised slightly,
lengthening the arcs and increasing power to the melt. This enables a molten
pool to form more rapidly, reducing tap-to-tap times. In more modern furnaces,
oxygen is also lanced into the scrap, combusting or cutting the steel and
burning out carbon, and sometimes chemical heat is provided by wall-mounted
oxy-fuel burners. Both processes accelerate scrap meltdown.
An important part of steelmaking is the formation of slag, which floats on the
surface of the molten steel. Slag usually consists of metal oxides, and acts as
a destination for oxidised impurities, as a thermal blanket (stopping excessive
heat loss) and helping to reduce erosion of the refractory lining. For a furnace
with basic refractories, which includes most carbon steel-producing furnaces,
the usual slag formers are calcium oxide (CaO, in the form of burnt lime) and
magnesium oxide (MgO, in the form of dolomite and magnesite). These slag formers
are either charged with the scrap, or blown into the furnace during meltdown.
Later in the heat, carbon (in the form of coke) is lanced into this slag layer,
partially combusting to form carbon monoxide gas, which then causes the slag to
foam, allowing greater thermal efficiency, and better arc stability and
electrical efficiency. The slag blanket also covers the arcs, prevents damage to
the furnace roof and sidewalls from radiant heat.
Once flat bath conditions are reached, i.e. the scrap has been completely melted
down, often another bucket of scrap is charged into the furnace and melted down.
After the second charge is completely melted, refining operations take place to
check and correct the steel chemistry and superheat the melt above its freezing
temperature in preparation for tapping. More slag formers are introduced and
more oxygen is lanced into the bath, burning out impurities such as silicon,
sulphur, phosphorus, aluminium, manganese and calcium and removing their oxides
to the slag. Metals that have a poorer affinity for oxygen than iron, such as
nickel and copper, cannot be removed through oxidation and must be controlled
through scrap chemistry alone, such as introducing the direct reduced iron and
pig iron mentioned earlier. A foaming slag is maintained throughout, and often
overflows the furnace to pour out of the slag door into the slag pit.
Temperature sampling and chemical sampling (in the form of a 'chill' - a small,
solidified sample of the steel) take place via automatic lances.
Once the temperature and chemistry are correct, the steel is tapped out into a
preheated ladle through tilting the furnace. Here, some alloy additions are
introduced into the metal stream. Often, a few tonnes of liquid steel and slag
is left in the furnace in order to form a 'hot heel', which helps preheat the
next charge of scrap and accelerate its meltdown. At the same time, the furnace
turnaround is started: the slag door is cleaned of solidified slag, repairs may
take place, and electrodes are inspected for damage or lengthened through the
addition of new segments; the taphole is filled with sand at the completion of
tapping. For a 90-tonne, medium-power furnace, the whole process will usually
take about 60-70 minutes from the tapping of one heat to the tapping of the next
(the tap-to-tap time).
Advantages of electric arc furnace for steelmaking
The precise control of chemistry and temperature encouraged use of electric arc
furnaces during World War II for production of steel for shell casings. Today
steelmaking arc furnaces produce many grades of steel, from concrete reinforcing
bars and common merchant-quality standard channels, bars, and flats to special
bar quality grades used for the automotive and oil industry.
A typical steelmaking arc furnace is the source of steel for a mini-mill, which
may make bars or strip product. The steelmaking arc furnace is generally charged
with scrap steel, though if hot metal from a blast furnace or direct-reduced
iron is available economically, these can also be used for steelmaking.
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