Water vapor solubility in ladle-refining slags
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THE presence of a few parts per million of hydrogen in molten steel can have a serious effect upon the properties of the final products. A recent study has shown that a hydrogen concentration even as low as 1 to 2 ppm would have a detrimental impact on the mechanical properties of tool steels.[1] The primary source of hydrogen pickup in steelmaking is the moisture in the furnace atmosphere and the raw materials charged into the ladle furnace. Although most hydrogen is removed as vapor, low pressure is required to bring down the hydrogen concentrations below 1 ppm. In most industrial practices, dehydrogenation is performed by vacuum degassing treatment. While both the level of vacuum and the argon flow rate are of great importance in determining the rate of hydrogen removal, the partition of hydrogen between slag and molten steel plays another crucial role. It is common knowledge that steel produced in humid weather usually contains a higher hydrogen concentration. Water vapor dissolves in liquid slag in the form of hydroxyl ion or hydroxyl radical. The presence of hydroxyl ions in the ladle slag will result in hydrogen transfer from the slag back into the steel bath.[2] Dor et al. have reported that the water content in the slag after vacuum treatment could be 200 times higher than the value predicted from the metalgas-slag equilibrium.[3] The reason for this is that the dehydrogenation kinetics in the steel is much faster than it is in the slag.[4] Hence, the initial concentration of hydroxyl ions in ladle slag would seriously affect the kinetics of the dehydrogenation process and therefore the hydrogen content in the final product. To optimize the dehydrogenation process, a good knowledge of the hydrogen solubility of the ladle slag is essential. Solubilities of gases in slags have been the subject of many researchers. Elemental hydrogen is soluble only in ferrous and manganese slags. In acidic slags, water vapor reacts with a covalently bonded oxygen by silicons, thereby breaking a silicate network structure forming a hydroxyl radical. In basic slags, water vapor reacts with a free oxygen ion and exists as a hydroxyl ion.[2] The solubility of water has been found to be proportional to the square root of the water vapor pressure.[2,4–7] This JENNY BRANDBERG, Graduate Student, and DU SICHEN, Professor, are with the Department of Material Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. Contact e-mail: [email protected] and [email protected] Manuscript submitted September 6, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
finding supports that the water vapor is present as free hydroxyl ions in basic slags.[2] By increasing the basicity, the solubility of water is increased.[4,5,8,9] The temperature, on the other hand, has been found to have a negligible effect on the solubility of water vapor in the slags.[5,6,10] This can be attributed to low heat of solution of water vapor in metallurgical slags.[6,11] Usually the major oxide components of a ladle slag are Al2O3, CaO, MgO, a
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