Treatment of Pig Iron

The removal of sulfur in the blast furnace from the molten iron into the slag is a partition process. As purer supplies of iron oxide have been obtained, there has been a progressive reduction in the quantity of slag produced in the blast furnace from 700 to 250kg/t of pig iron (see section 11.1.3). This has reduced the amount of sulfur removed from the iron by the slag, and levels of 0.05 % S are typical [27.5]. 

This effect, coupled with the increasing need to produce low sulfur steels (less than 0.001 %) has necessitated the introduction of a desulfurisation stage between the blast furnace and the BOS process. In Germany, for example, some 80 % of pig iron is desulfurised [27.5]. 

Several elements are capable of removing sulfur from molten iron. In order of decreasing effectiveness, they are Ce, Ca, Sr, Ba, Mg, Na [27.4]. They may be used directly as metals, as oxides or carbonates, or indirectly via a synthetic slag. 

Calcium carbide was initially used as the desulfurising agent. It was added, in a granular form, to the torpedo ladle (used to transfer the molten iron to the basic oxygen steel making process). It is now generally injected as a powdered mixture with lime (typically 60 to 80 % of calcium carbide) into the molten pig-iron in the transfer ladle. At an addition level of 7 kg/t of pig iron, such mixtures can reduce the level of sulfur to 20 % of the initial value. 

Other commonly used desulfurising agents include powdered mixtures of magnesium or aluminium with lime or calcium carbide, and sodium carbonate [27.6]. The choice of reagent depends on economic factors and on the required sulfur level in the treated iron — typically in the range of 0.02 to 0.005 % [27.6]. 

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The use of lime products in the treatment of pig iron and its conversion into steel are described in chapter 27. 

Particularly as a result of (b), the roles of the primary steelmaking processes have changed, with the BOF being used mainly for the removal of carbon from pig iron, and the EAF being used mainly to melt steel scrap. This places increased emphasis on refining the hot metal either before the primary steelmaking operation (i.e., treatment of pig iron — see section 27.3), or after it (i.e., secondary steelmaking). 

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