3. Oxygen enrichment in the blast furnace
Oxygen enrichment by itself has been considered of marginal advantage in the blast furnace except in the production of ferro-alloys. This is because a greatly increased tuyere cone temperature upsets the thermal balance and leads to some difficulty in avoiding the production of high silicon iron. It has been found, however, that oxygen-enriched blast combined with steam injection serves to drive the furnace faster and generate a gas of greater reducing power with no significant change in hearth zone temperature.
Some plants currently practice routine oxygen enrichment of blast furnace air for the production of steelmaking iron. Approximately 65 tons of oxygen per day per furnace are required for each 1 per cent of enrichment. With an available oxygen supply of about 400 tons per day, it has been, found advantageous to spread this amount over four operating furnaces rather than to concentrate it on one furnace.
The economic attractiveness of oxygen enrichment in the blast furnace is greatest under conditions of capacity operation when there is a strong demand for, the additional iron which can be produced. The most promising use of blast furnace oxygen enrichment lies in ferro-alloy smelting. The high hearth temperature produced with oxygen favours the reduction of non-ferrous metal oxides and formation of their high melting point slag.
4. Oxygen for direct reduction of iron ore
Basically the industry is still dependent on the units of production that characterized it in the early years of this century by-product coke ovens, blast furnaces, open hearths, blooming and secondary mills. Direct ore reduction processes may change this since they provide a potential replacement for the coke oven-blast furnace complex. The H-iron process is a typical example. This process consists in bringing into intimate contact, at about 900°F and 400 pounds per square inch, finely divided iron ore and preheated hydrogen gas.
Pressurized hydrogen enters a large cylindrical reducing vessel at the bottom and in passing upward keeps the fine ore particles in a state of turbulent motion. In the reducer hydrogen reacts with the preheated iron oxide to form reduced iron and water:
Fe2O3+3H2==2Fe+3H2O.
The gas leaving the top of the reactor consists of unreacted hydrogen and water vapour. |
Regenerated hydrogen, as well as fresh hydrogen to take the place of the hydrogen converted to water, is recycled to the reducer. The reduced iron product is briquetted, thermally passivated, and can then be charged directly to open hearth or electric furnaces to replace the scrap. The production of hydrogen for such a process may be accomplished b exothermic partial oxidation of natural gas of fuel oil.
5. Crucible furnace
The
crucible furnace is one
of
the oldest furnaces
for melting
non-ferrous metals, and
it
is being used
a great deal
at
the
present time. High carbon
steel
is made
by melting scrap
steel in a
crucible
too. The
construction
of
a crucible furnace
consists of a
shell
which is made pf a steel
plate about
in. thick.It
rests
on a plate made of cast iron.
The
legs support the furnace high enough from
the floor to allow
for draft and ashes.
The grate
works
on hinges so that it can
be
raised
and dropped. The
grate is
turned
by the handle. The cover
holds down
the
flame.
The furnace should be connected
by the flue with a chimney. The
melting capacities
of the crucible in one heat are from 20
to
300
lbs
of brass or
from 10
to
100
lbs
of aluminium. The
linings
are
made of brick
and are
applied
about the same as in
the cupola furnace.
Sometimes the furnaces are set on the floor, but, in most cases, they are set in a pit because they are more easily operated in such a setting. Shavings and kindling are placed on the grate with about 50 lbs of coke on the kindling. After the coke bed is well lighted, one should build up the coke bed so that the top of the crucible, when put in, will be even with the top of the flue. The bottom of the furnace and grates should be kept free from ashes and cinders, or the draft will not get through, causing much trouble in getting the metal enough to run the castings.
The crucible way is used for making high carbon steel and special steels. The percentage of carbon the scrap steel contains must be known.