OPEN HEARTH FURNACE

OPEN HEARTH FURNACE

In
open hearth furnace, pig iron, steel scrap etc. are melted to obtain steel.
This furnace is widely used in
American foundries for steel production. The hearth is surrounded by roof and walls of refractory bricks as shown in
Fig. The charge is fed through a charging door and is heated to 1650ยฐC mainly
by radiation of heat from the burning
of gaseous fuels above it. This
heat is obtained by the burning of sufficiently pre-heated air and gas. Such pre-heated air of gas is obtained
by passing them though arc shaped hot regenerators at a lower
level. This contains fire bricks which are arranged
to extract heat from exhaust gases. In the furnace air and fuel are passed
through a honeycomb of hot firebrick, called checkers. It preheats the air and
fuel so that they are ready for combustion when they enter the hearth.
The products of combustion
at the same time pass through the checkers at the other end of the furnace.
The hot gases
heat the checkers. The process then reverses itself, and the newly
heated checkers now are used to heat the air and the fuel. It is said as a regenerative process.
The products of combustion after giving up their heat to the
checkers pass up through the stack. On firing of coke, the charge is
heated. Part of the heat necessary, results from radiation from the
low hot roof of the chamber. The furnace is raised bricked in with the charging platform, at the rear, also raised so that the charge may be put into the furnace. The melt is tapped off the front into large ladles.ย 




Open hearth furnace
Open hearth furnace




The chemical composition of the end product
depends upon the lining, the charge, and
the
control impurities added
during the melt after the melt has been tapped
off into the ladle. The lining
plays a major roll in the control of impurities. For magnesite lined furnace,
the
charge consists of pig iron,
limestone, and scrap iron. The limestone forms a slag. This slag and
the oxygen
in the air combine to remove impurities. The slag reacts
with the sulfur and theย phosphorus in the metal, while the bubbling
air causes oxidation of the carbon and silicon. If too
much carbon is present in the melt, iron ore is added.
The oxygen from the iron oxide burns out the excess carbon. If the
carbon content is too
low, pig iron
is added. This replenishes
the carbon. Other alloying elements like Cr, Ni. Co,
W, Mo, V etc. are added
as needed. Ferromanganese may be added to the crucible after tapping. For acid lining furnace, the charge should be scrap iron and low-phosphorus pig iron. Limestone is required to keep the slag fluid. As
described above, the basic lining burns phosphorus, silicon, and carbon. The
slag is
tapped off by the molten
metals being allowed to overflow the sides of the crucible into a slag pot.
Oxygen
is one of the most important
elements used in the reduction
of the molten metal. Rust, scale, slag, and limestone are some
of the sources of oxygen. Oxygen is introduced into
the furnace with oxygen lances through
the roof of the furnace. Twice the oxygen input will double the
carbon reduction. This increases the steel production
of the furnace.


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