KINETICS OF STEEL REFINING (PRIMARY)


The partitioning of solutes elements during refining of molten pig Iron take place at the slag-metal and gas-metal interfaces. The structure of the interface is broadly described as

Interface = ( bulk phase no.1 / stagnant boundary layer of phase 1 ) ÷ ( stagnant boundary layer of phase 2 / bulk phase no.2 )

The Mechanism of reaction are heterogeneous system which is generally described in the form of following three steps.

1. Diffusion of reactants accross the stagnant boundary layer to the interface.

2. Chemical reaction at the interface.
3. Diffusion of product accross the corresponding stagnant boundary layer into the bulk phase.

●The overall rate of the reaction is controlled by the slowest steps out of the above three stpes, i.e either by the Maas transport rates across the stagnant boundary layer or by the chemical reaction at the interface.

● In some case the individual atoms participate in the overall reaction must acquire enough energy to overcome the energy barrier before the reaction can occur. This priproc is called as activated process and their rates are related to the temperature of the reaction by

Arrhenius relationship rate = A. exp( -Q/RT )

Where A and Q do not vary sensible with temp.
             R is the gas content.

A is the no. Of particles correctly oriented for reaction and also activation energy to overcome the barrier resisting the reaction.

● Arrhenius relationship can be experimentally established in the form of useful plots of log rate vs 1/T. These are linear relationship with slope equal to Q/R and from which the activation energy Q of the process can be calculated.

● In general the chemical reaction at the interface at steel making temperature is bound to be very fast and the refining reactions are usually controlled by one of the two mass transport processes. The rate of mass transport accross a stagnant boundary layer is

J = Di . {( Cr - Ci )/ x} . A  g per second.

Di = is the Diffusion coefficient
Cr & Ci = are the concentration in bulk and at the interface respectively.
x = is the thickness of stagnant boundary layer.
A = is the interface area.

● The illustrates the interface where the refining reactions predominantly take place in commercial processes

1. Slag metal interface controlled : Open heart and electric without oxygen lancing, kaldo etc.

2. Gas metal interface controlled :  Acid Bessemer.

3. Mixed controlled : Open hearth and electric with oxygen lancing, Thomas, LD, OLP, OBM etc.


Reactions at Slag Metal Interface
:

●  Except the oxidation of carbon the rest of the refining reactions take place at the slag metal interface. Generally these reaction are found to be under Diffusion control and more so.

●  Accross the stagnant boundary of slag rather then the metal phase. Vigorously stirring of the slag metal system is known to accelerate these reaction.

●  In practice Vigorously stirring is caused in Bessemer process by bottom blowing, in open hearth and electric furnace by carbon boil, in KALDO by rotation of the vessel and in LD, rotor process by the super sonic oxygen jet.

●  Stirring reduces the thickness of the stagnant boundary and increase the interfacial area. In addition of every attempt is made to keep the slag as fluid as possible by either raising the temperature of the furnace or by adding suitable fluxes to hasten mass transport in the slag phase.

●  Kinetically the removal of P, S, Si and Mn from iron by slag, therefore, doesn't pose any serious problem of being slow in practice provided the thermodynamic and physical requirement of slag are met.

●  The observation is amply substantiated in practice by the fact of that P, S and Mn are very prone to reversion to metal phase if slag composition change in unfavourable direction or a high temperature even for a short while.

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