What is Slag ?

The molten oxide by product of smelting and refining process is known as a slag. The oxide melt containing at least two oxides of opposite chemical nature should be called a slag. By convection the oxides like SiO2, P2O5, B2O etc are called acid oxides and CaO, MgO, FeO, MnO, N2O, K2O etc are called basic oxides. The opposite site chemical nature result in the formation of complex silicate, phosphate etc.

2RO + SiO2 = 2RO . SiO2
4R2O + P2O5 = 4R2O . P2O5

(Where R represent monovalent or divalent cation) in a manner analogous to the aqueous acid base neutralization.

The basic oxides are large ionic solids where in the ionic characters predominant over the covalent character of the bond.

In an orthosilicatemelt the dissociation appears to be as

Which indicate as if there is no activity of oxygen ions in the melt. This composition can convenience by referred to as neutral silicate melt.
As the basic oxide content increase the free oxygen ions paper in the melt which is known as basic melt. On the other side of the neutral composition since the structure is deficient in oxygen ions which is termed as acid melt.

Slag Properties :

The main slag properties are it's ability to retain oxide product (also gangue) of reaction and it's oxidizing/ reducing power I.e. viscosity, thermal conductivity, surface tension etc. of a slag. Which are also important from the point of view of rate of slag-metal, slag-gas and slag-refractory reaction.

Thus the ability of slag to retain oxide is generally expressed as the ratio of basic to acid oxides.

V ratio =  %CaO / %SiO2

Basicity =   % all basic oxides / % all acid oxides

So common basicity is =
%[CaO + MgO] / %[SiO2 + Al2O3 + P2O5]

Theory of Slags :

Besides the structure, the liquidious temperature and the thermodynamic properties of a slag, attention has also been given to understand the factors that affect the reaction of a slag with other phase. The theories of slags are nothing but a set of principle to evaluate as far as possible the true equilibrium constant of a reaction between the slag and any other phase like metal, refractory wall of a container, furnace gases etc.

● Molecular Theory :

It assume the existence of neutral molecules such as various silicate, phosphate, sulphides, oxides etc., in the molten slag. Schenck assumed the complex molecules to be dissociated to a definite external in to correspondence simpler molecules e.g. as

2(FeO.SiO2) = 2(FeO) + (SiO2)

From which the concentration of the free oxide could be calculated.

1. The knowledge of the free oxide concentration in a slag is a useful parameters in the calculation of equilibrium constant of a reaction of slag with any other phase. The free oxide concentration can be calculated from the dissociation constant of such reaction :

D = (%FeO) . (%FeO) . (%SiO2) / (%2FeO.SiO2)

2. A ssrise of temperature dependent value of dissociation constant were computed by schenck and co-workers from laboratory and works data. They also derived equilibrium constant for partitioning reaction between slag and metal. A value of zero for the dissociation constant of calcium phosphate is high stability and it explain the phosphorus distribution.

3. Since activity values of oxides in complete silicate slags deviate much less form ideality of  the free oxide concentration.

4. Molecular theory was quite simple in the early days since a limited number of complex compound in slag were known. This type of such compounds have found out in slag and dissociation constant for each one of them needs to calculate the free oxide concentration.

● Ionic Theory :

The molecular constitution of molten slags cannot explain the phenomenon observed of slag e.g. electrical conductivity, electrolysis, electrocapillarity etc. Galvanic elements is formed if a metal bath is in contact with molten slag. It means that the reaction between slag and any other phase is an electrochemical process and can be represented in the form of a partial electrochemical reaction such as

[S] + 2e = (S2-)
[Fe] =  (Fe2+) + 2e
[O] + 2e = (O2-)
The overall reaction can be represented as

[S] + [Fe] = (S2-) + (Fe2+)
So that the electrical netrality of the phase is maintained. The equilibrium constant if the above reaction.

K =  a. (Fe2+) . a.(S2-) / a[Fe] . a[S]

The ionic theories attempt to compute the activity of ions to determine tgetequilibrium constant.

[S] + (O2-) = (S2-) + [O]
[S] + (CaO) = (CaS) + [O]
(Fe2+) + [Mn] = (Mn2+) + [Fe]
(2Fe . SiO4) + [Mn] = (2Mn . Sio4) + [Fe]

Since the precise nature of the anions are known only in basic slags the cationic equilibria can be analysis for these slags with tolerable accuracy. The major steel making reaction are between basic slag and metal. It can be analysed with tolerable accuracy.

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