APARATOS SANITARIOS, MESONES, DIVISIONES Y GRIFERÍA

The reaction equilibria in the liquid steel-slag systems have been extensively studied, both experi-mentally and theoretically by applying the principles of thermodynamics and physical chemistry.

In a recent reassessment of the available experimental data on steel-slag reactions¹⁰³, it became evi-dent that the equilibrium constants of slag-metal reactions vary with the slag composition in dif-ferent ways, depending on the type of reaction. For some reactions the slag basicity is the key parameter to be considered; for another reaction the key parameter could be the mass concentra-tion of either the acidic or basic oxide components of the slag.

2.7.1.1 Oxidation of Iron

In steelmaking slags, the total number of g-mols of oxides per 100 g of slag is within the range 1.65 ± 0.05. Therefore, the analysis of the slag-metal equilibrium data, in terms of the activity and mol fraction of iron oxide, can be transposed to a simple relation between the mass ratio [ppm O]/(%FeO) and the sum of the acidic oxides %SiO₂ + 0.84 × %P₂O₅ as depicted in Fig.

2.100(a). The experimental data used in the diagram are those cited in Ref. 103. There is of course a corollary relation between the ratio [ppm O]/(%FeO) and the slag basicity as shown in Fig.

2.100(b).

%SiO2 + 0.84 × %P2O5

0 10 20 30

80

40

0

1600°C

xx x x

x

x x

x x

x xx x x x x

(a)

100

80

60

40

20

0 (b)

0 1 2 3 4 5 6

B 1500°C

1600°C 1700°C

x

x

[ppm O] (%FeO)

[ppm O] (%FeO)

Fig. 2.100 Equilibrium ratio [ppm O]/(%FeO) related to (a) SiO₂and P₂O₅contents and (b) slag basicity; experimental data are those cited in Ref. 103.

2.7.1.2 Oxidation of Manganese

For the FeO and MnO exchange reaction involving the oxidation of manganese in steel, formulated below,

(FeO) + [Mn] = (MnO) + [Fe] (2.7.1)

the equilibrium relation may be described in terms of the mass concentrations of oxides

(2.7.2) where the equilibrium relation K′_FeMn

depends on temperature and slag composition.

The values of K′_FeMn derived from the equilibrium constant for reaction 2.7.1, given in Ref. 27 and the activ-ity coefficient ratios γ_FeO/γ_MnOin Fig.

2.67, are plotted in Fig. 2.101 against the slag basicity. In BOF, OBM(Q-BOP) and EAF steelmaking, the slag basicities are usually in the range 2.5 to 4.0 and the melt temperature in the vessel at the time of furnace tap-ping in most practices is between 1590 and 1630°C for which the equi-librium K′_FeMnis 1.9 ± 0.3. The plant analytical data for tap samples give K′_FeMnvalues that are scattered about the indicated slag-metal equilibrium values.

Morales and Fruehan¹⁶¹have recently determined experimentally the equi-librium constant K′_FeMn for reaction 2.7.1 using MgO-saturated calcium silicate melts. Their values of K′_FeMn are plotted in Fig. 2.102 together with some data from the studies of Chip-man et al.¹⁶² and Suito et al.¹⁶³. The broken line curve is reproduced from Fig. 2.101. Resolution of observed differences in the values of K′_FeMn awaits future studies.

2.7.1.3 Oxidation of Carbon

With respect to the slag-metal reaction, the equilibrium relation for carbon oxidation would be

(FeO) + [C] = CO + [Fe] (2.7.3)

Fig. 2.101 Equilibrium relation in equation 2.7.2 related to slag basicity. From Ref. 103.

For 1600°C, γ_FeO= 1.3 at slag basicity of B = 3.2 and p_CO= 1.5 atm (average CO pressure in the vessel), we obtain the following equilibrium relation between the carbon content of steel and the iron oxide content of slag.

2.7.1.4 Oxidation of Chromium

There are two valencies of chromium (Cr²⁺ and Cr³⁺) dissolved in the slag. The ratio Cr²⁺/Cr³⁺

increases with an increasing temperature, decreasing oxygen potential and decreasing slag basic-ity. Under steelmaking conditions, i.e. in the basic slags and at high oxygen potentials, the triva-lent chromium predominates in the slag. The equilibrium distribution of chromium between slag and metal for basic steelmaking slags, determined by various investigators, is shown in Fig. 2.103;

slope of the line represents an average of these data.

(2.7.7)

2.7.1.5 Oxidation of Phosphorus

It was in the late 1960s that the correct formulation of the phosphorus reaction was at last realized, thus [P] + ⁵/²[O] + ³/²(O^2–) = (PO₄^3–) (2.7.8)

%

%Cr . . %

Cr FeO

( )

[ ]

⁼

⁽

^{0 3 0 1±}

⁾

^×

^{( )}

(2.7.6) K_FC= 108.8

a

_FeO= N_FeO ≈ FeO FeO

×

( )

^{= ×}

( )

1 3 1 3

72 1 65 0 011

. .

. % . %

log K'Mn-Fe

0 1 2 3 4 5

Temperature: 1600°C Morales and Fruelhan ¹⁶¹ Chipman et al¹⁶²

Suito et al¹⁶³ From Fig. 2.101 2.0

1.5

1.0

0.5

0.0

(%CaO + %MgO) %SiO2

Basicity, B =

Fig. 2.102 Experimental values of K′_FeMn measured recently by Morales and Fruehan¹⁶¹.

At low concentrations of [P] and [O], as in most of the experimental melts, their activity coeffi-cients are close to unity, therefore mass concentrations can be used in formulating the equilibrium relation K_POfor the above reaction.

(2.7.9) The equilibrium relation K_PO, known as the phosphate capacity of the slag, depends on tempera-ture and slag composition.

From a reassessment of all the available experimental data, discussed in detail in Ref. 103, it was con-cluded that CaO and MgO components of the slag, had the strongest effect on the phosphate capac-ity of the slag. Over a wide range of slag composition and for temperatures of 1550 to 1700°C, the steel-slag equilibrium with respect to the phosphorus reaction may be represented by the equation

(2.7.10) where BO = %CaO + 0.3 (%MgO).

2.7.1.6 Reduction of Sulfur

The sulfur transfer from metal to slag is a reduction process as represented by this equation

[S] + (O^2–) = (S^2–) + [O] (2.7.11)

for which the state of slag-metal equilibrium is represented by

(2.7.12) Κ_SO S

S O

=

( ) [ ] [ ]

^{%% %}

log ,

. .

ΚPO

T BO

= 21 740− + × 9 87 0 071 Κ_PO P

P O

=

( ) [ ][ ]

^{%% % −}

52

FeO (wt %)

0 4 8 12 16 20 24 28

(%Cr) [%Cr]

10

8

6

4

2

Fig. 2.103 Variation of chromium distribution ratio with the iron oxide content of slag, in the (∆) open hearth¹⁶⁴ and (o) electric arc furnace¹⁶⁵at tap is compared with the results of laboratory experiments¹⁶⁶(•).

As is seen from the plots in Fig. 2.104, the sulfide capacities of slags, K_SO, measured in three inde-pendent studies are in general accord. The effect of temperature on K_SOis masked by the scatter in the data. The concentration of acidic oxides, e.g. %SiO₂+ 0.84 ×%P₂O₅, rather than the slag basic-ity seems to be better representation of the dependence of K_SOon the slag composition.

In document UNIVERSIDAD DE ANTIOQUIA ESPECIFICACIONES TÉCNICAS DE CONSTRUCCIÓN (página 108-113)