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Scrap melting in a continuous process rotary melting furnace Zhang, Yanjun

Abstract

Based on the preliminary modeling study, an improved heat-transfer model has been developed in this study to further examine the viability of the oxy-fuel-fired continuous process rotary melting furnace (CPRMF) as a replacement of the electric arc furnace (EAF) in minimill steelmaking. The model treats the furnace as three domains: the freeboard space, the liquid metal bath and slag, and the refractory structure. Based on certain physical correct assumptions for the gas flow and combustion patterns, radiative exchange within the freeboard is solved by the zone method in combination with a clear-plus-3-gray emissivity/absorptivity model for the gas phase thus the model allows axial temperature variations in the gas phase and the refractory hot-face. Assuming an isothermal metal bath condition, heat transfer to the exposed bath is simplified by a specified temperature difference between the slag/freeboard and slag/metal interfaces, while regenerative heat transfer to the covered bath is calculated using the local refractory temperature and the local heat-transfer coefficients. The refractory structure is solved by 1-D transient conduction in the radial direction. The three domains are linked by shared boundary conditions and the requirement that the furnace itself operates at steady-state. The model was partially validated using experimental results from copper melting trials on a bench-scale CPRMF, which was designed and constructed as a part of work in this study. The trials explored two operating variables, i.e., oxygen and slag. Both experimental and model results indicate an increase in furnace thermal efficiency with increasing oxygen enrichment in the combustion air and a decrease in the efficiency with increasing slag thickness. The partially validated model was then employed to evaluate the commercial viability of the CPRMF. According to the model predictions, a melting rate in the order of 100 ton h⁻¹ can be achieved by a 4 m ID x 16 m furnace with a natural gas firing rate of 6000 Nm³ h⁻¹. Under the baseline conditions, the furnace thermal efficiency is 66%. Without scrap preheating, this configuration consumes less direct energy at 619 kWh t⁻¹ than the typical EAF (662 kWh t⁻¹) and can save at-source energy by about 45%.

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