Rapid Dissolution of Quicklime into Molten Slag by Internally Formed Gas
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NTRODUCTION
CAO-BASED slag is used in a steelmaking process for removing impurities, such as phosphorus and sulfur, from steel. The slag is produced by dissolving various solid oxides such as lime and dolomite. The dissolution rates of these oxides in a molten slag play important roles in increasing the reaction rate between steel and slag, decreasing flux and energy consumptions, decreasing slag emission, etc. In particular, the rate of lime dissolution into slag affects the rate for removing phosphorus from hot metal.[1–9] A part of the lime added as flux into slag, however, still exists after the refining, because the dissolution rate of lime into steelmaking slag is limited. Traditionally, the dissolution rate of solid oxide has been measured by a rotating cylinder method.[10–15] In this method, a sintered oxide rod is immersed into molten slag and rotated in the slag, and the decrease in the diameter of sintered rod with time is measured. For example, Matsushima et al. have measured the dissolution rate of sintered lime in a slag.[10] They found that the dissolution rate increased with an increase in the rotation rate and temperature. If the dissolution of lime is controlled by the mass transport of CaO in a liquid phase saturated by C2S, the dissolution rate can be calculated by the following equation:
NOBUHIRO MARUOKA, Assistant Professor, and HIROSHI NOGAMI, Professor, are with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai City 980-8577, Japan. Contact e-mail: maruoka@ tagen.tohoku.ac.jp Manuscript submitted October 29, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
dr dt
kCaO ¼ 100q
lime kCaO qs
ffi 100q
lime
ðpctCaOÞi qi ðpctCaOÞs qs
DðpctCaOÞ
;
½1
where kCaO is the mass transfer coefficient of CaO, (pctCaO) is the content of CaO, and subscripts i, s, and Lime denote the interface, slag, and lime, respectively. D(pctCaO) is the driving force of diffusion assumed to be the difference between the content of CaO in bulk slag and solubility of CaO in the slag. This equation has been often used for steel refining simulators based on a coupled reaction model with lime addition developed by many researchers.[5,6,9, 16–22] These experiments, however, were carried out using dense sintered oxides, whereas industrial fluxes and refractories were porous and nonuniform in structure. As a consequence, only 1 mm in diameter of sintered CaO rod was dissolved for 20 minutes in their experiment, though 20 to 30 mm in diameter of CaO lump is used in actual operation, and it mostly dissolved during refining period. Therefore, the dissolution rate of lime into molten slag might be underestimated if Eq. [1] is employed. Another technique was developed by Deng et al. measured the dissolution rate of cubic lime into slag under forced convection.[23] They also pointed out that an interfacial layer including the C2S layer formed around the lime prevents the lime dissolution, and they concluded that the main dissolution mechanism was detachments of C2S
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