Solubilities of carbon dioxide in sodium silicate melts
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I.
INTRODUCTION
SODIUM oxide based fluxes have been particularly interesting to metallurgists 1-5 because of their high basicity and consequently strong capability for absorbing sulfur and phosphorus from molten iron and steel alloys. During the past ten years, several fluxes have been investigated 67,8 for the simultaneous removal of sulfur and phosphorus from hot metal. Under these circumstances, the dephosphorization reaction must be performed at a relatively low oxygen potential in order to ensure that the desulfurization reaction is not adversely affected. This consideration is also important in order to minimize decarburization reactions. Thus to accomplish these conflicting reactions, a higher basicity slag is required for these particular treatments than would be used in the case of conventional BOF operations where a lime/silica ratio of about three is common. Sodium carbonate and lime-spar-calcium chloride fluxes are the two most widely used reagents. Although it would appear that refractory erosion may be a major concern, plant data indicate that sodium carbonate based systems give better results than those obtained with lime-spar-chloride fluxes. While such treatments are becoming common practice, there is as yet no good indicator of slag basicity by means of which the relative performance of the different reagents can be compared. The major objective in the present study is to measure the solubility of carbon dioxide in molten sodium silicate slags and thus determine carbonate capacity values which might in turn serve as appropriate indicators of slag basicity. Various researchers such as Tomlinson, 9 Pearce, ~~ and Wagner 11 have suggested that the solubility of carbon dioxide in molten slag can be a good indicator of the basicity. The dissolution process can be represented in the following manner: C02~g ) -~- (O2-)slag = (CO~-)slag
For which the equilibrium constant, K - Yco~- " Xco~Pcoz " ao>Thus K 9 ao 2- _ Xco]- _ Cco]"gCO]-
PCO2
The carbonate capacity ~ as defined above is similar in concept to phosphate" and sulfide 12 capacities, There are very few equilibrium studies which have been conducted to determine the dissolution of carbon dioxide in molten sodium silicates. Pearce ~~has reviewed the available literature and measured the solubility of carbon dioxide in sodium silicate melts, within the acidic range, at temperatures between 900 ~ and 1300 ~ After equilibration in platinum crucibles for 72 hours, the slag melt was quenched and subsequently analyzed to determine the CO2 content using the vacuum fusion technique. In the present study, attention has been directed toward more basic melts within the sodium silicate system. Two different methods have been employed. In one set of experiments the weight change caused by either desorption or absorption of CO2 is measured after the slag melts are quenched. In the second set of experiments, the CO: content of the quenched slags is determined by chemical analysis. As discussed in more detail later, these two different measurements provide infor
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