Low Phosphorus Partition Ratio Models

A real steelmaking process involves many complex physical and chemical coupled phenomena such as oxidation, decarburization, dephosphorization, and slag formation.

Phosphorus is an element of representative impurities that should be kept as low as possible in conventional steel grades. Low phosphorus content steels are essential for steel applications where high ductility is required, such as thin sheets, deep drawn structures, pipelines, and automobile exteriors. Phosphorus’ ability to strengthen and embrittle ferrite imposes restrictions on the maximum phosphorus content for the aforementioned applications. The main source of phosphorus in steel is from the raw materials; thus, raw material selection is a way to maintain the phosphorus content within the desirable range.

In (BOF) oxygen steelmaking it is difficult to predict the distribution of phosphorus between slag and metal by direct application of models (listed in Table 1), even though a pseudo-steady state is usually attained between slag and metal towards the end of the blow. This is because under practical (shop-floor) conditions, besides the slag composition and temperature, the phosphorus distribution is found to depend also upon other parameters, such as slag mass, turndown carbon, initial phosphorus content of metal, intensity of bottom stirring, lance height, oxygen flow rate, addition scheme and timing of addition of fluxes and iron ore.

Therefore, each plant usually develops its own blowing practice and, on the basis of analysis of plant data, adapts a suitable model for control and prediction of phosphorus at tap. The selection of a control model is not a simple task. It is to be noted that the models listed in Table I do not explicitly take into account the effect of precipitation and/or dissolution of C₂S on phosphorus distribution.

Table 1: Phosphorus Distribution log Lp Equations

The efficiency of dephosphorization process may be given by the distribution coefficient for phosphorus between slag and the metal phase L_P = (%P₂O₅)/[%P], and dephosphorization degree, which is equal to the ratio of the phosphorus concentration removed from the metal to the initial phosphorus concentration ηP = [%∆P]/[%P_ini].

On the other hand, it is well known that the phosphorous distribution between slag and metal (L_P) is generally lower (10 to 50) for EAF steelmaking than for oxygen steelmaking (50 to 200).

The phosphorus [P] removal in BOS steelmaking is generally described by the ionic reaction shown in equation (1). From (1) it can be seen that a high [P] activity, high oxygen potential [O] and high basicity O^2- all promote phosphorus removal. Also, [P] removal may also be represented by the simple molecular reaction, given in equation (2), allowing a Gibbs free energy to be calculated using equation (3).

The equilibrium constant of this reaction is given by equation (4)

It is generally agreed that L_P increases with slag basicity and oxygen activity and decreases with increasing temperature.

References

1. Hassan A.I, Kotelnikov G. I, Semin A. E, Megahed G. M: Phosphorous behavior in Electric Arc Furnace steelmaking with the melting of high phosphorous content direct reduced iron, METAL 2015, Jun 3rd - 5th 2015, Brno, Czech Republic, EU;

2. D. Y. Shin, C. H. Wee, M. S. Kim, B. D. You, J. W. Han, S. O. Choi, D. J. Yun: Distribution Behavior of Vanadium and Phosphorus between Slag and Molten Steel, Metals and Materials International, Vol. 13, No. 2 (2007), p. 171-176;

3. A. N. Assis, M. A. Tayeb, S. Sridhar, R. J. Fruehan: Phosphorus Equilibrium Between Liquid Iron and CaO-SiO2-MgO-Al2O3-FeO-P2O5 Slag Part 1: Literature Review, Methodology, and BOF Slags, Metallurgical and Materials Transactions B, 2015, DOI: 10.1007/s11663-015-0408-9;

4. Deo. B, Halder. J, Snoijer. B, Overbosch. A, Boom. R.: Effect of MgO and Al2O3 variations in oxygen steelmaking (BOF) slag on slag morphology and phosphorus distribution, VII International Conference on Molten Slags Fluxes and Salts, The South African Institute of Mining and Metallurgy, 2004;

5. C.P.Manning, R.J.Fruehan: The rate of the phosphorus reaction between liquid iron and slag, Metallurgical and Materials Transactions B, Vol.44B, February 2013, p.37-44;

6. K. Kunisada, H. Iwai: Effects of CaO, MnO, and Al2O3, on Phosphorus Distribution between liquid iron and Na2O-MgO-FeO—SiO2 slags, Transactions ISIJ, Vol. 27, 1987, p.332-339;

7. P.B. Drain, B. J. Monaghan, G. Zhang, R. J. Longbottom, I. Murgas, M.W. Chapman: Development of a New Phosphorus Partition Relation for Australian Steelmakers, Paper no. 3397176, Chemeca 2016, 25 – 28 September 2016, Adelaide, Australia;