DC ferrochrome smelting : the arcing zone and its influence on energy transport and exergy dissipation

Abstract

Phenomena between the electrode tip, the arc attachment zone (AAZ), and the bath below the AAZ—taken together as arc attachment volume (AAV)—have a significant impact on the performance and efficiency of DC arc furnaces. This paper investigates the phenomena in the AAV in detail. Phenomena are analyzed with reference to industrial scale furnace viz. a Kazakh DC ferrochrome furnace. The impact of the various reactions in the AAV under different conditions both from an energy and exergy flow perspective is analyzed. The temperature, carbon in feed (thus the carbon to feed ratio called RC), and slag from the slag bath are investigated as the significant variables affecting phenomena in the AAV. Results show that alloy can become the reductant to fume specifically Mg(g), SiO(g), Cr(g) and even Fe(g) from slag containing MgO SiO2, FeOx and Cr2O3. The implications and resulting mass and energy transfer by these fumes can play a significant role in understanding open-bath ferrochrome smelting better as well as the energy balance of the arc and its impact on overall furnace efficiency. The irreversibility of the fuming reactions and reoxidation of metal vapor has a significant negative impact on furnace operation if the temperature in the AAV is not managed well due to poor operation regarding charging of feed. This implies not only that the average slag and metal temperature are of importance, but especially the actual AAV temperature is crucial. To fully understand the effect of chemical potential of the gases coming from the AAV, an exergy analysis uniquely showed that the fumes can be essential to reduce energy consumption if managed well, especially if the chemical potential energy can be harnessed in other zones within the furnace. This investigation is also relevant to other open-bath processes with fumable oxides, for example in smelting of ferronickel, titania slag, direct reduced iron, or even iron ore. Understanding the reaction mechanisms in the AAV in detail will be significant to push efficiencies of these applications to their thermodynamic, kinetic, and technological limits to ensure that their green metallurgical impact is fully realized.