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I.

INTRODUCTION

THE investigations of the thermodynamic properties of slag systems in the last several years have concentrated on the slag's ability to hold elements, such as sulfur, tl':l phosphorus, t31 and nitrogen. E41The solubilities of these species in slags depend on several variables, of which basicity is the most important. One possibility for a quantitative measure of the basicity is the measurement of the carbonate capacity of different slag systems proposed by Wagner. Ls] Many steelmakers are using CaO-CaF2-based fluxes to obtain low concentrations of the impurities mentioned previously. The aim of the present work was to determine the effect of Li20 on the carbonate capacity of CaOCaF:-A1203 melts over a wide range of compositions and over the temperature interval 1250 ~ to 1350 ~ II.

THERMODYNAMIC CONSIDERATIONS

The dissolution of carbon dioxide in the slag can be represented as follows: CO2 (gas) + 0 2 - = CO 2- (slag) for which the expi'essed as

equilibrium

constant

K1

[1] can

be

Kl = a c o l - / ( a o 2 "Pco2) = Ycot "Xcot-/(Yo2 "Xo2- 9Pco2)

[2]

When we define carbonate capacity as C c = Xco~- /Pco~

the content of C c is expressed as C c = ( K l " ao~ /3'cot-)

SIMEON SIMEONOV, formerly Visiting Research Fellow, Institute of Industrial Science, University of Tokyo, is Lecturer, Sofia Technological University, Sofia, Bulgaria. KOJI FUKITA, formerly with the Institute of Industrial Science, University of Tokyo, is with Phillips Japan Ltd., Tokyo, Japan. MASAFUMI MAEDA, Associate Professor of Metal Source Technology, is with the Institute of Industrial Science, the University of Tokyo, Minato, Tokyo, Japan, 106. Manuscript submitted July 3, 1991. METALLURGICAL TRANSACTIONS B

where PM = XM = aM= 7M = Cc =

partial pressure of M; mole fraction of M; activity of M; activity coefficient of M; and molar basis carbonate capacity.

Based on Eq. [2], the carbonate capacity in terms of weight percent, (Cc), can he defined as Cb = (wt pct CO2)/Pco2

[3] Comparing the sulfide or other capacities with the carbonate capacity, as in Wagner's article, tS1 we may expect higher sulfide capacity for a slag of higher carbonate capacity. III.

EXPERIMENTAL PROCEDURE

The experiments were carried out by using a thermogravimetric technique. The details of the experimental apparatus have been published elsewhere. ~6J The solubility of carbon dioxide was measured by equilibrating the CaO-CaF2-A1203-Li20 melts with a CO2-Ar gas mixture. The experimental fluxes were prepared from reagentgrade CaCO3, CaF2, A1203, and LiaCO3 which were dried at 130 ~ in a vacuum drying oven. After drying, the CaCO3, CaF2, and A1203 were carefully weighed and thoroughly mixed to obtain the required composition. The resultant mixture was transferred to a carbon crucible and melted. The fused flux was crushed to a powder and subsequently heated in a platinum crucible at 1000 ~ for 1 hour to remove all carbonaceous contaminants. Approximately 7 g of this base flux was mixed with Li2CO3 to add the appropriate content of Li20 and melted