• PDF / 812,067 Bytes
• 7 Pages / 576 x 792 pts Page_size
• 43 Downloads / 210 Views

DOWNLOAD

REPORT

---

(J/moI)

02 (g) = Ba3(PO4)2 (s)

AG~ = -2,523,000 + 580.0T (--- 16,600)

(J/mol)

The stability and the thermodynamic behavior of barium compounds as reaction products of dephosphorization of steel were discussed in terms of the oxygen partial pressure and the activity coefficient of BaLsP in molten Ba saturated with CaO.

I.

INTRODUCTION

DEPHOSPHORIZATION of steel has been traditionally carried out under oxidizing conditions using basic fluxes. However, the conventional technique is not applicable to alloys which contain a large amount of less noble elements than iron, such as chromium or manganese. Recently, a new technique for this purpose has been developed in which another reaction mechanism holds. It is featured by reducing phosphorus in metals by CaCaF2 or CaC2-CaF2 fluxes; many favorable results have been reported. [L2,3] It is expected that dephosphorization of steel, either by oxidation or by reduction, is performed more effectively by using barium-based fluxes because BaO is more basic than CaO. [41 The dephosphorization product of steel by using bariumbased fluxes is Ba3P2 or Ba3(PO4)2. In order to characterize the efficiency of dephosphorization by barium-based fluxes, thermodynamic properties of those reaction products must be well known. However, extensive discussions on the reaction mechanism have not been undertaken because there are few available data, particularly of the standard Gibbs energies of formation of barium compounds. Accordingly, the present work has been carried out in order to determine the standard Gibbs energies of formation of Ba3P2 and Ba3(PO4)2 by using a chemical equilibration technique.

D.J. MIN, formerly Graduate Student, Department of Metallurgy, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. N. SANO, Professor, is with the Departmentof Metallurgy, The Universityof Tokyo, Bunkyo-ku, Tokyo 113, Japan. Manuscript submitted December 16, 1988. METALLURGICALTRANSACTIONSB

II. THE PRINCIPLE OF DETERMINATION OF STANDARD GIBBS ENERGIES OF FORMATION A. Ba3P2

The reaction of formation of Ba3P2 is expressed by Eq. [11: 3Ba (1) + P2 (g) = Ba3P2 (s)

[11

The values of AG~ for Eq. [1 ] can be derived by measuring AG~ for Eq. [21 by equilibrating molten silver with Ba3P2 and by combining it with Eq. [41: 3Ba (in liquid silver) + 2P (in liquid silver) = BaaP2 (s)

[21 AG~ = - R T In K2 = RT(3 In T~3a+ 3 In XBa + 2 In Xe Ba + 3eB~XB~ + 2e~Xp + 2eapaXB~+ 3e~aXp)

[31 1 P2 (g) = P (X, in liquid silver) 2 AG ~ = - 2 0 , 5 0 0 + 28.5T

(J/mol) r

[4]

where yo and Xi are the activity coefficient at infinite dilution and the mole fraction of species, i, in silver, respectively, and e~ is the interaction parameter between i and j in silver. The reference states of barium and phosphorus are liquid barium and infinite dilute solution of phosphorus in silver, respectively. When Ba3P2 is solid at experimental temperatures (i.e., aBa~P2 = 1), the Gibbs energy of Eq. [