Heat-transfer and pressure-drop considerations in the design of Sirosmelt lances
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I.
INTRODUCTION
TIlE Sirosmelt top submerged combustion smelting process was developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) throughout the 1970s and has been successfully applied to a wide range of processes, such as slag treatment, oxide smelting, and sulfide smelting, fLj The process utilizes a steel annular lance that enters the smelting vessel from above to convey combustion air and fuel into the melt (Figure l). Helical vanes impart a swirling motion to the air, thus increasing the convective heat-transfer coefficient between the lance wall and the process air and cooling the lance wall so that slag can freeze on the outside. This solid slag coating protects the lance from attack by the harsh liquid slag. The two critical requirements of the Sirosmelt lance system are, first, that a solid layer of slag is maintained on the outer surface of the lance to protect it from attack by the melt, and second, that the lance wall is sufficiently cool so as to not fail mechanically. These two aspects of the system are intimately linked, since it is the relatively cold face of the lance wall that allows the solid slag layer to form. However, there is only a very limited amount of control the designer has over the heat-transfer mechanisms occurring and, hence, the slag layer thickness and wall temperature. Heat is transferred from the vessel surrounds to the solid slag layer by combined convection and radiation. The rate of convection is related to the intensity of the stirring and quantity of gases passing into the vessel, while the radiation is a function of both the optical properties of the slag and
C.B. SOLNORDAL, Research Scientist/Engineer, The G K . Williams Cooperative Research Centre for Extractive Metallttrgy. is with the CSIRO Division of Minerals. Clayton, Victoria 3169, Australia. N.B. GRAY, Associate Professor, l'hc G.K. Williams Cooperative Research Centre for Extractive Metallurgy, is with the Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria 3052, Australia. Manuscript submitted March 15. Iq95. MF.TAI,LURGICAL AND MATERIAI,S TRANSACTIONS B
the temperature of the vessel. Maximum gas/liquid contacting is required to ensure efficient rates of reaction, little control is available over the optical properties of the slag, and the vessel itself must be at a temperature to ensure adequate mobility of the melt. Therefore, little can be done to control heat transfer on the vessel side of the slag layer. Similarly, heat transfer through the slag layer could only be controlled by modifying the thermal conductivity of the slag. Therefore, the main avenue for control of the heat transfer is through design of the lance itself. The choice of steel as the lance material is governed by its cost, availability, and robustness. A material that could withstand higher temperatures, such as a refractory, would be far more expensive and also would most likely be too brittle for use in such a harsh environment. Other methods of cooling the lance include
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