The basic oxygen furnace (BOF),
whose profile is shown in the figure, is a tiltable vessel lined
with refractories such as magnesia carbon brick. Auxiliary equipment
includes a chute for scrap charging, hoppers for alloys and fluxes,
a lance for injecting pure oxygen gas, a sublance for measuring
the temperature and carbon concentration of the molten steel,
lifting devices for the lance and sublance, equipment for tilting
the vessel, and equipment for recovering and cleaning the exhaust
gas. The BOF capacity is expressed as the weight of crude steel
that can be decarburized per heat. Most BOFs in Japan have a
capacity of 150-300 tons.
The main function of the BOF is to decarburize the hot metal
using pure oxygen gas. In the top-blown BOF, pure oxygen is injected
as a high-velocity jet against the surface of the hot metal,
allowing penetration of the impinging jet to some depth into
the metal bath. Under these conditions, the oxygen reacts directly
with carbon in the hot metal to produce carbon monoxide. The
pure oxygen top-blown BOF can decarburize 200 tons of hot metal
from 4.3% C to 0.04% C in about 20 minutes. As a result of this
high productivity, the BOF replaced the open hearth furnace,
which was a much slower process.
The injected pure oxygen gas first oxidizes silicon and then
carbon in hot metal. When the carbon concentration of the hot
metal is decreased to about 1%, the oxidation of iron begins
in parallel with that of carbon. The oxidation of iron becomes
marked at carbon concentrations of less than 0.1%, decreasing
both the oxygen efficiency for decarburization and the decarburization
rate, while increasing iron loss into the slag. The problem with
the top-blown BOF is thus the oxidation of iron when a low carbon
concentration is reached, and the resulting decrease in the decarburization
rate. When the iron oxide content of the slag increases excessively,
it can react too quickly with carbon in the molten steel and
cause sudden gas evolution, forming a mix of slag and molten
steel that sometimes erupts from the vessel in a phenomenon called
"slopping" or "spitting". The use of an oxygen
lance with multiple holes at the tip has proven very effective
in delocalizing the oxygen supply and increasing the decarburization
rate while restraining excessive oxidation of the molten steel
and preventing slopping and spitting. However, the effectiveness
of this lance was still inadequate, and the bottom-blown oxygen
process was developed, in which pure oxygen gas is injected into
the molten steel from the bottom of the BOF. The bottom blowing
enhances the stirring of the hot metal and thereby shortens the
average mixing time in the molten steel bath, and promotes transport
of solute carbon in the bath, preventing the over-oxidation of
slag, which is the cause of slopping and spitting. Consequently,
the bottom blowing enhances decarburization efficiency, especially
at low carbon concentrations. The bottom-blowing is performed
with bottom tuyeres of concentric double-wall pipe. The inner
pipe is used to blow pure oxygen gas along with pulverized limestone
as a slag-forming agent, while propane gas is blown through the
outer pipe as a coolant to prevent tuyere burn back, since propane
undergoes an endothermic reaction during decomposition, which
results in cooling and reduced burning of the tuyeres. These
improvements have made the production of low-carbon steels much
easier.
The top-and-bottom blown BOF, which combines the advantages of
both types of BOF, has recently become prominent in oxygen steelmaking.
The combined blowing BOFs mostly use bottom-blown inert gases
in place of oxygen gas for stirring. Various methods of bottom-blowing
have been adopted. As one example, a ceramic plug with embedded
multiple small pipes or multiple slits is used in the bottom
tuyeres. Irrespective of the type of the BOF, the exhaust gas,
which is high in CO content, is either combusted in the throat
of the BOF and passes through a waste-heat boiler installed in
the upper part of the throat to recover the sensible heat and
the heat of combustion, or is recovered uncombusted through exhaust-gas
recovery equipment and stored in a gas tank for later use as
fuel. |
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