FURNACE OPERATIONS
The electric arc furnace
operates as a batch melting process producing batches of molten steel known
"heats". The electric arc furnace operating cycle is called the
tap-to-tap cycle and is made up of the following operations:
Modern operations aim for
a tap-to-tap time of less than 60 minutes. Some twin shell furnace operations
are achieving tap-to-tap times of 35 to 40 minutes.
Furnace Charging
The first step in the
production of any heat is to select the grade of steel to be made. Usually a
schedule is developed prior to each production shift. Thus the melter will know
in advance the schedule for his shift. The scrap yard operator will prepare
buckets of scrap according to the needs of the melter. Preparation of the
charge bucket is an important operation, not only to ensure proper melt-in
chemistry but also to ensure good melting conditions. The scrap must be layered
in the bucket according to size and density to promote the rapid formation of a
liquid pool of steel in the hearth while providing protection for the sidewalls
and roof from electric arc radiation. Other considerations include minimization
of scrap cave-ins which can break electrodes and ensuring that large heavy
pieces of scrap do not lie directly in front of burner ports which would result
in blow-back of the flame onto the water cooled panels. The charge can include
lime and carbon or these can be injected into the furnace during the heat. Many
operations add some lime and carbon in the scrap bucket and supplement this
with injection.
The
first step in any tap-to-tap cycle is "charging" into the scrap. The
roof and electrodes are raised and are swung to the side of the furnace to
allow the scrap charging crane to move a full bucket of scrap into place over
the furnace. The bucket bottom is usually a clam shell design - i.e. the bucket
opens up by retracting two segments on the bottom of the bucket. The scrap
falls into the furnace and the scrap crane removes the scrap bucket. The roof
and electrodes swing back into place over the furnace. The roof is lowered and
then the electrodes are lowered to strike an arc on the scrap. This commences
the melting portion of the cycle. The number of charge buckets of scrap
required to produce a heat of steel is dependent primarily on the volume of the
furnace and the scrap density. Most modern furnaces are designed to operate
with a minimum of back-charges. This is advantageous because charging is a
dead-time where the furnace does not have power on and therefore is not
melting. Minimizing these dead-times helps to maximize the productivity of the
furnace. In addition, energy is lost every time the furnace roof is opened.
This can amount to 10 - 20 kWh/ton for each occurrence. Most operations aim for
2 to 3 buckets of scrap per heat and will attempt to blend their scrap to meet
this requirement. Some operations achieve a single bucket charge. Continuous
charging operations such as CONSTEEL and the Fuchs Shaft Furnace eliminate the
charging cycle.
Melting
The melting period is the
heart of EAF operations. The EAF has evolved into a highly efficient melting
apparatus and modern designs are focused on maximizing the melting capacity of
the EAF. Melting is accomplished by supplying energy to the furnace interior.
This energy can be electrical or chemical. Electrical energy is supplied via
the graphite electrodes and is usually the largest contributor in melting
operations. Initially, an intermediate voltage tap is selected until the
electrodes bore into the scrap. Usually, light scrap is placed on top of the
charge to accelerate bore-in. Approximately 15 % of the scrap is melted during
the initial bore-in period. After a few minutes, the electrodes will have
penetrated the scrap sufficiently so that a long arc (high voltage) tap can be
used without fear of radiation damage to the roof. The long arc maximizes the
transfer of power to the scrap and a liquid pool of metal will form in the
furnace hearth At the start of melting the arc is erratic and unstable. Wide
swings in current are observed accompanied by rapid movement of the electrodes.
As the furnace atmosphere heats up the arc stabilizes and once the molten pool
is formed, the arc becomes quite stable and the average power input increases.
Chemical energy is be
supplied via several sources including oxy-fuel burners and oxygen lances.
Oxy-fuel burners burn natural gas using oxygen or a blend of oxygen and air.
Heat is transferred to the scrap by flame radiation and convection by the hot
products of combustion. Heat is transferred within the scrap by conduction.
Large pieces of scrap take longer to melt into the bath than smaller pieces. In
some operations, oxygen is injected via a consumable pipe lance to
"cut" the scrap. The oxygen reacts with the hot scrap and burns iron
to produce intense heat for cutting the scrap. Once a molten pool of steel is
generated in the furnace, oxygen can be lanced directly into the bath. This
oxygen will react with several components in the bath including, aluminum,
silicon, manganese, phosphorus, carbon and iron. All of these reactions are
exothermic (i.e. they generate heat) and supply additional energy to aid in the
melting of the scrap. The metallic oxides that are formed will end up in the
slag. The reaction of oxygen with carbon in the bath produces carbon monoxide,
which either burns in the furnace if there is sufficient oxygen, and/or is
exhausted through the direct evacuation system where it is burned and conveyed
to the pollution control system. Auxiliary fuel operations are discussed in
more detail in the section on EAF operations.
Once enough scrap has
been melted to accommodate the second charge, the charging process is repeated.
Once the final scrap charge is melted, the furnace sidewalls are exposed to
intense radiation from the arc. As a result, the voltage must be reduced.
Alternatively, creation of a foamy slag will allow the arc to be buried and
will protect the furnace shell. In addition, a greater amount of energy will be
retained in the slag and is transferred to the bath resulting in greater energy
efficiency.
Once the final scrap
charge is fully melted, flat bath conditions are reached. At this point, a bath
temperature and sample will be taken. The analysis of the bath chemistry will
allow the melter to determine the amount of oxygen to be blown during refining.
At this point, the melter can also start to arrange for the bulk tap alloy
additions to be made. These quantities are finalized after the refining period.
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