Geology Reference
In-Depth Information
The product obtained from the blast furnace is called pig iron, with a carbon
content of around 4%. This must be lowered to less than 0.01-0.4% to produce
steel. Therefore the liquid hot metal is discharged on a railed torpedo ladle that
directly carries it to a basic oxygen furnace (BOF). The objective of the BOF is to
oxidise the carbon excess as well as removing other impurities including sulphur,
magnesium, phosphorus and manganese. Other metals are added in the case of
alloy production. The steps are as follows: first, the hot metal is desulphurised and
de-slagged with caustic soda amongst others. Subsequently, the hot metal enters the
BOF, a furnace operating in a semi-continuous process that involves the charging
of molten pig iron and scrap; oxygen blowing, sampling, temperature recording and
tapping. The energy required is supplied by the exothermic oxidation reactions and
the heat balance is controlled through additions of scrap or ore. Subsequently, the
hot metal receives a secondary metallurgical refining treatment and the process ends
with either continuous casting or the production of ingots. The main gas exiting
the BOF is carbon monoxide, which after being cleaned is mixed with blast furnace
gas and burnt in a cogeneration plant (Stubbles, 2011).
The electric arc furnace (EAF) plays an increasingly important role in integrated
steelworks as recycling becomes more commonplace. It is used to convert scrap
iron into steel. An EAF heats the charged scrap and some ferroalloys to adjust
the desired composition of the finished steel. It essentially consists of a refractory-
lined hemispherical vessel with typically three graphite electrodes. The furnace is
built on a tilting platform to discharge the liquid steel with voltage set to increase
as melting progresses. Oxygen is then blown in order to decarbonise the melt
and remove impurities. Therefore the furnace is also charged with slag formers
like limestone and dolomite, as fluxes, at temperatures around 1600
o
C. At these
temperatures the fluxes decarbonise and melt the iron oxide and other undesired
oxides such as those of phosphorus and silicon, producing a slag which floats on
the surface of the molten steel and maintains its temperature. As iron oxide forms
in the oxygen blown, granular carbon is added as a reducing agent. This reaction
produces carbon monoxide where bubbles convert the slag to foam which serves to
protect the furnace walls and improve the energy transfer from the arc to the bath.
The refining operations for adjusting steel composition take place once the scrap
has been completely melted down. This melting process takes around 37 minutes
(IPPC, 2001; Kawatra, 2011)
2
. A general scheme of stainless steel production is
shown in Fig. 8.2.
8.2.2 Energy and environmental issues
Even if iron and steel production requires a very intensive consumption of raw
materials and energy, its long establishment as an industrial process means that it
is also a highly optimised and integrated one. Yet despite this, almost half of the
2
See also: http : ==en:wikipedia:org=wiki=Electric_arc_furnace. Accessed May 2013.
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