Refining
Refining operations in the electric
arc furnace have traditionally involved the removal of phosphorus, sulfur,
aluminum, silicon, manganese and carbon from the steel. In recent times,
dissolved gases, especially hydrogen and nitrogen, been recognized as a
concern. Traditionally, refining operations were carried out following meltdown
i.e. once a flat bath was achieved. These refining reactions are all dependent
on the availability of oxygen. Oxygen was lanced at the end of meltdown to
lower the bath carbon content to the desired level for tapping. Most of the
compounds which are to be removed during refining have a higher affinity for
oxygen that the carbon. Thus the oxygen will preferentially react with these
elements to form oxides which float out of the steel and into the slag.
In modern EAF operations, especially
those operating with a "hot heel" of molten steel and slag retained
from the prior heat, oxygen may be blown into the bath throughout most of the
heat. As a result, some of the melting and refining operations occur
simultaneously.
Phosphorus and sulfur occur normally
in the furnace charge in higher concentrations than are generally permitted in
steel and must be removed. Unfortunately the conditions favorable for removing
phosphorus are the opposite of those promoting the removal of sulfur. Therefore
once these materials are pushed into the slag phase they may revert back into
the steel. Phosphorus retention in the slag is a function of the bath
temperature, the slag basicity and FeO levels in the slag. At higher
temperature or low FeO levels, the phosphorus will revert from the slag back
into the bath. Phosphorus removal is usually carried out as early as possible
in the heat. Hot heel practice is very beneficial for phosphorus removal
because oxygen can be lanced into the bath while its temperature is quite low.
Early in the heat the slag will contain high FeO levels carried over from the
previous heat thus aiding in phosphorus removal. High slag basicity (i.e. high
lime content) is also beneficial for phosphorus removal but care must be taken
not to saturate the slag with lime. This will lead to an increase in slag
viscosity, which will make the slag less effective. Sometimes fluorspar is
added to help fluidize the slag. Stirring the bath with inert gas is also
beneficial because it renews the slag/metal interface thus improving the
reaction kinetics.
In general, if low phosphorus levels
are a requirement for a particular steel grade, the scrap is selected to give a
low level at melt-in. The partition of phosphorus in the slag to phosphorus in
the bath ranges from 5 to 15. Usually the phosphorus is reduced by 20 to 50 %
in the EAF.
Sulfur is removed mainly as a
sulfide dissolved in the slag. The sulfur partition between the slag and metal
is dependent on slag chemistry and is favored at low steel oxidation levels.
Removal of sulfur in the EAF is difficult especially given modern practices
where the oxidation level of the bath is quite high. Generally the partition
ratio is between 3 and 5 for EAF operations. Most operations find it more
effective to carry out desulfurization during the reducing phase of
steelmaking. This means that desulfurization is performed during tapping (where
a calcium aluminate slag is built) and during ladle furnace operations. For
reducing conditions where the bath has a much lower oxygen activity,
distribution ratios for sulfur of between 20 and 100 can be achieved.
Control of
the metallic constituents in the bath is important as it determines the
properties of the final product. Usually, the melter will aim at lower levels
in the bath than are specified for the final product. Oxygen reacts with
aluminum, silicon and manganese to form metallic oxides, which are slag
components. These metallics tend to react with oxygen before the carbon. They
will also react with FeO resulting in a recovery of iron units to the bath. For
example:
Mn
+ FeO = MnO + Fe
Manganese will typically be lowered
to about 0.06 % in the bath.
The reaction of carbon with oxygen
in the bath to produce CO is important as it supplies a less expensive form of
energy to the bath, and performs several important refining reactions. In
modern EAF operations, the combination of oxygen with carbon can supply between
30 and 40 % of the net heat input to the furnace. Evolution of carbon monoxide
is very important for slag foaming. Coupled with a basic slag, CO bubbles are
tapped in the slag causing it to "foam" and helping to bury the arc.
This gives greatly improved thermal efficiency and allows the furnace to
operate at high arc voltages even after a flat bath has been achieved. Burying
the arc also helps to prevent nitrogen from being exposed to the arc where it
can dissociate and enter into the steel.
If the CO is evolved within the
steel bath, it helps to strip nitrogen and hydrogen from the steel. Nitrogen
levels in steel as low as 50 ppm can be achieved in the furnace prior to tap.
Bottom tapping is beneficial for maintaining low nitrogen levels because
tapping is fast and a tight tap stream is maintained. A high oxygen potential
in the steel is beneficial for low nitrogen levels and the heat should be
tapped open as opposed to blocking the heat.
At 1600 C, the maximum solubility of
nitrogen in pure iron is 450 ppm. Typically, the nitrogen levels in the steel
following tapping are 80 - 100 ppm.
Decarburization is also beneficial
for the removal of hydrogen. It has been demonstarted that decarburizing at a
rate of 1 % per hour can lower hydrogen levels in the steel from 8 ppm down to
2 ppm in 10 minutes.
At the end of
refining, a bath temperature measurement and a bath sample are taken. If the
temperature is too low, power may be applied to the bath. This is not a big
concern in modern meltshops where temperature adjustment is carried out in the
ladle furnace.
