Conventional process of steel making.
Their are three basic type of conventional process:-
A) Basic Bessemer/ Thomas process
In this process molten pig iron is held in a vessel with perforated bottom called a converter. Cold air or oxygen enriched blast is forced through the metal from below as shown in figure .refining is complete in about 15-20 minutes and taking into account the time required for charging, tapping, repairs, etc. A tap-to-tap time of about 30-35 minutes are required. This is a autogeneous process, no external heat is needed. The exothermic chemical reaction during refining provide the necessary heat in order to maintain the metal well above its melting point, in spite of the fact that, cold air is blown through.
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Basic Bessemer process figure |
B) Open Hearth process
The furnace in this process is fairly shallow basic lined vessel as shown in figure and hence the name. It is heated by either liquid and or gaseous fuels using the heat regeneration principle so as to attain steel making temperature of about 16-1700 degree C. In this modern practice the charge is a mixture of scrap and molten pig iron, the scrap is initially heated to near its softening point and molten pig iron from the blast furnace is poured into it. The charge may contain iron ore and limestone. The early practice of using iron ore for refining is now of only historical significance. Now refining is universally carried out by blowing oxygen gas through lances. This make the process during refining a autogenous one although heating is necessary for heating the scrap initially. The basic slag is prepared by adding lime and iron ore. The atmosphere in the furnace is therefore always oxidising. A tap-to-tap time of about 6-10 hours is usually required although some furnaces have achieved this as low as even 4 hours.
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Open Hearth Furnace Figure |
C) Electric Arc process
Generally a three electrode arc furnace as shown in figure is used the steelmaking temperature is maintained by an electric are stuck between the electrodes and the metallic charge. This process is very similar to open hearth in charging and refining and several hours are needed for one heat. This is the only process wherein either oxidising or reducing conditions can be maintained at will during refining since the furnace does not possess its own ambient oxidising atmosphere. The high cost of electrical energy in general makes this process costly. It is well suited for making alloy steels. It usually runs on cold charges and the furnace is of small capacity. The morden electric are furnaces are however of much bigger sizes and some of them also take hot metal as part of the charge.
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3 Electrode Electric Arc Furnace Figure |
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