Thermochemistry of calcium oxide and calcium hydroxide in fluoride slags
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INTRODUCTION
T H E key role of fluoride slags in controlling hydrogen in electroslag remelting (ESR) is very well recognized, tl-4~ All ESR slags are highly basic in nature and can contain a certain amount of moisture. Also, the water vapor present in the gaseous phase above the liquid slag reacts with the basic slags and transfers hydrogen in the form of hydroxyl ion. {HEO} + (02-) ~ 2(OH-)
[11
The slag, in turn, transfers hydrogen to the refined liquid metal pool until the equilibrium ratio of (H)/[H] is reached, tl,2] The reaction at the slag/metal interface can be expressed as 2(OH-) = (02-) + (O) + 2(H)
[2]
The direct sampling of liquid metal poses practical problems, tS~ The most feasible alternative for hydrogen analysis is to sample the liquid slag and analyze it for hydrogen or water. With the subsequent knowledge of the hydrogen distribution coefficient between the slag and metal, it would be possible to estimate the hydrogen content in the metal without incurring the hazards of direct sampling. For a better understanding of hydrogen behavior during electroslag remelting, a thermodynamic investigation of Reaction [1] is necessary. There are three important parameters in that reaction: the partial pressure of moisture in the gas phase, the oxide ion activity in the slag phase, and the hydroxyl ion activity in the slag phase. It is obvious that the reduction of PnEo can result in a lower hydroxyl ion solubility in the slag at any given oxide ion activity. This can be achieved in two ways: by either operating the process under vacuum or using a controlled atmosphere on top of the slag phase. The first S. CHATTOPADHYAY, formerly Graduate Student, University of British Columbia, is Research Officer with National Metallurgical Laboratory, Jamshedpur, India. A. MITCHELL, NiDI/NSERC Research Professor, is with the Department of Metallurgical Engineering, University of British Columbia, Vancouver, BC V6T 1W5, Canada. Manuscript submitted December 20, 1989. METALLURGICALTRANSACTIONS B
option is impractical because of the rapid vaporization of CaF2. The other alternative requires an elaborate and expensive arrangement for supplying a dry atmosphere and a continuous atmosphere monitoring technique to check its efficacy. The hydroxyl ion capacity of the slag is directly related to the oxide activity in the slag. The previous studies t6-1~ carried out on the oxide activity reveal many discrepancies. These arose because of different experimental techniques and also because several workers assumed that the final slag composition was the same as the composition of the initial slag, without allowance for volatilization reactions. It is thus desirable to obtain more reliable estimates of the oxide activity in highly basic ESR slags, as it relates to reactions involving H20 and CO2. Unlike nonfluoride slags, the direct weight gain method to determine the oxide activity poses a problem because of the hydrolysis reaction: CaF2 + HEO = CaO + 2HF
[3]
and in the silicate or aluminate slags, volatilization: 2CaF2 + SiO2 =
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