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Wednesday, April 13, 2022

Bessemerisation : Purification of iron from cast Iron or pig iron :

The iron obtained from a blast furnace is a brittle material called cast iron or pig iron. It contains about 4% elemental C and smaller amounts of other impurities such as elemental Si, P, S, and Mn that are formed from their compounds in the reducing atmosphere of the
The most important of several methods for purifying the iron and converting it to steel is the basic oxygen process or oxidative refining. 
Molten iron from blast furnace is exposed to a jet of pure O2 gas for about 20 minutes in a furnace that is lined with basic oxide such as CaO. The impurities in the iron are oxidized and the acidic oxides that form react with basic CaO to yield a molten slag that can be poured off.
For example Phosphorous,  is oxidized to P4O10, which then reacts with CaO to give molten Ca3(PO4)2.

P4(l) + 5 O2(g)--> P4O10(l)

6 CaO (g) + P4O10(l) -->2 Ca3

Basic oxide acidic oxide slag
Mn also passes into the slag because its oxide is basic and reacts with SiO2 yielding molten manganese silicate.
This process produces steel that contains about 1% carbon but only very small amount of P and S. Usually the composition of liquid steel is monitored by chemical analysis and the amount of oxygen and impure iron used are adjusted to achieve the desired concentration of carbon and other impurities

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