Carbon Reaction During Refining :

Carbon forms the single largest impurities in pig iron to be eliminated during refining. The reaction takes place as the gas metal interface and eliminates the necessity of nucleation of gas bubble.

●  The decarburization rate (dC/ dt) is controlled by the rate of Diffusion of either carbon or oxygen to the gas metal interface. In the basic oxygen furnace processes the formation of slag-gas- metal emulation help in providing fresh gas metal interface and there by decarburization rates are obtained.

●  The carbon reaction in open hearth process is very slow because of no gaseous refining medium used. That way to understand this differences between the processes we know about physics of formation of carbon monoxide bubble

● It is known that the pressure inside a bubble of radius 'r' in molten metal of surface tension ∆ is given
Where "Po" is the static pressure due to the heads of atmosphere, slag and metal compressing the bubble.
"2∆/r" is the pressure term due to surface tension opposing the growth of bubbles.

●  The possibilities of homogeneous nucleation of carbon monoxide bubble in the melt during refining should be entirely ruled out. The effect of crevice size on the uncleation and growth of CO bubble at the metal refractory interface.

● Active Sites :  The bubble attains hemispherical shape before the partial pressure of CO attains it's equilibrium value and hence the bubble are formed and being mechanically unstable are separated.
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●  Inactive Sites :  The CO partial pressure inside the bubble reaches it's equilibrium value before the hemispherical shape is attained and hence no separation is possible.

Mechanism of Oxygen Transport And Kinetics

●  The rate of reaction is affected by diffusion of carbon to the interface. The stirring of the bath caused in this process which help to hasten the carbon transport and then decarburization rate up to 10% per hr.

●  In the furnace oxygen form atmosphere has to diffuse accross the slag and the metal layer, to reach the proper metal interface.

●  The physical solution of oxygen in the slag is negligible and it dissolved in slag in ionic form. The Mechanism of oxygen transport from gas phase is :
At the gas slag interface oxygen dissolved as

and the iron in the slag gets oxidised as
2(Fe+)  =  2(Fe3+) + 2e-
So the overall reaction is
2(Fe2+) + 1/2(O2)  =  2(Fe3+) + (O2-)

●  Due to thermal Diffusion these migrate from gas slag to slag metal interface and a reverse reaction as 2(Fe3+) + (O2-) = 2(Fe2+) + [O] takes place.

●  The ferrous ions again migrate to the gas slag interface and complete the cycle which repeats itself. The oxygen dissolved in metal diffuses to pore metal interface and combined with carbon.

●  The process of Diffusion of oxygen can be hastened by eliminating the diffusion path accross the slag layer by adding lumps of ferric oxide(Fe2O3) which being havier than slag and lighter then metal, settle at the slag metal interface. And make

(Fe2O3) = 2FeO + [O]
which is an endothermic reaction. It also known as oreing of the slag. If oreing is carried out it's maximum extend the rate can increase up to 0.6%C/hr. The limit is due to the inadequate heat transfer across the slag layer in overall reaction.

●  2/3 (Fe2O3) + 2[C] = 4/3 [Fe] + 2{CO}
    ∆H = +65 kcal
is endothermic in nature. If low pressure oxygen is sprayed on the slag surface, the above endothermic reaction is replaced by

   [C] + [O] = {CO}             ∆H = -89 kcal

with a net heat gain and hence dC/dt up to 3% C/ hr have been obtained.

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