Pretreatment of hot metal
desiliconisation, desulphurisation and dephosphorisation
Published on: Mar 4, 2016
Transcripts - Pretreatment of hot metal
PRE-TREATMENT OF HOT METAL
WHAT IS PRE-TREATMENT ?
• Pre-treatment of hot metal is the adjustment of the
composition and temperature of blast furnace
produced hot metal for optimal operation of the
oxygen converter process.
• It is an extra step between the blast furnace and the
• In most of the steelmaking practices hot metal is pre-
treated to remove Si, P and S from hot metal to the
extent it is possible.
• To produce good quality steel, it is necessary to
lower the levels of sulphur and phosphorus to less
than 0.01% .
• Hence, hot metal containing low levels of sulphur
and phosphorus must be charged into the BOF.
• It is not economical to produce these S and P levels
in blast furnace.
• Hence, pre-treatment of hot metal to remove excess
sulphur and phosphorus prior to BOF steelmaking
has gained worldwide acceptance as an intermediate
• It can be carried out either at blast furnace runner, or
in transfer ladle or in torpedo car.
• If silicon is on the higher side (1%) in the basic pig
iron, the lime consumption in a basic process will
increase in order to maintain the required basicity.
• Hence, excess silicon must be removed.
• Removal of excess silicon before the metal enters the
steelmaking furnace is called external
• Silicon in hot metal can be removed by injecting an
oxidising agent like mill scale.
• It is accompanied by lime to help produce a neutral
• Soda ash can also be used to remove silicon:
• When soda ash is added, dense fumes are generated,
which can cause environmental problems.
• Hence, it is not preferred in recent times.
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• Desiliconisation is not normally practised because of
problems in handling the highly siliceous slag .
• Such slags have high temperature and are extremely
fluid, causing aggressive attack on most refractories.
• Instead, efforts have been made to produce low
silicon(0.3-0.6 %) hot metal directly in blast furnace
to the maximum possible extent.
• It can only be done under oxidising conditions and in
the presence of a highly basic slag.
• It is possible only after the total removal of silicon.
• If the basicity falls, phosphorus may soon revert back
to the metal phase.
• The activity of must be decreased using a
strong and excess external basic flux like lime.
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Sulphur is an unwanted element in most of the
applications of steel due to the following reasons:
• Sulphur affects both internal and surface quality of steel
• Sulphur contributes to the steel brittleness.
• It forms undesirable sulphides which promotes granular
weakness and cracks in steel during solidification.
• It lowers the melting point and intergranular strength.
• Dip lance process is the most economical, effective
and reliable method of desulphurisation of hot
• It consists of pneumatic injection of fine grained
desulphurisation reagent into the hot metal.
• Nitrogen gas is normally used as a carrier gas for the
• The injection of desulphurisation reagents through
deeply submerged lance causes an intimate mixing of
the desulphurisation reagent with the hot metal .
• The process allows the use of several
desulphurisation reagents, such as lime, calcium
carbide and magnesium, which remove the sulphur
in the hot metal by chemical reaction and convert it
to the slag.
• Sulphur rich slag generated during the process is
removed immediately after completion of the
• The most common method is to tilt the ladle and
rake the slag off with the help of a slag raking
• Dip lance method can reliably reduce the sulphur
content of hot metal to figures as low as 0.001 %.
The most commonly used desulphurising reagents
are lime (CaO), calcium carbide and
• Lime – Its low cost and easy availability make it an
attractive reagent. The grain size of the lime is to be
restricted to 45 micrometer maximum.
• Magnesium – Due to low boiling point (1090 deg C),
magnesium vapourizes as it enters the hot metal.
• Once in ladle, the magnesium vapour forms bubbles
which rise through the hot metal, dissolve and react
with sulphur in solution, forming magnesium
• This magnesium sulphide then floats on the top of
the ladle and settles in the slag layer, which is
• Calcium carbide - It was once most used
desulphurizing reagent but now it is less prevalent.
• Complicated material handling procedures as well as
stringent environment requirements associated with
the disposal of slag have negatively influenced its
• Sodium carbonate can also be used to remove
sulphur. But, it is not preferred nowadays because of
generation of harmful fumes.
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TORPEDO CAR HOT METAL
• In the recent years, injection metallurgy has been
developed to such an extent that it is now widely
used to pre-treat hot metal.
• Hence, steelmaking is restricted to decarburisation
• Efficient removal of the slag after pre-treatment as
well as the development of suitable refractories for
the ladle are areas in which work is still continuing.
• Nonetheless, it can be concluded that with ever-
increasing demand for lower levels of impurities in
steel, pre-treatment of hot metal will be adopted
even more widely in the years to come.
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