Electric arc steel-making furnaces produce carbon and alloy steels from scrap metal along with variable quantities of direct reduced iron (DRI), hot briquetted iron, and cold pig iron. Hot metal may also be added if available. The charge is melted in cylindrical, refractory-lined electric arc furnaces (EAFs) equipped with carbon electrodes (one for DC furnaces, three for AC furnaces). During charging, the roof is removed to place scrap metal and other iron-bearing materials into the furnace. Alloying agents and fluxes are added through doors on the side of the furnace. The electrodes are lowered into the furnace to about an inch above the metal and current is applied, generating heat to melt the scrap. Modern electric arc furnaces use an increasing amount of chemical energy to supplement the melting process. The chemical energy contribution is derived by burning elements or compounds in an exothermic manner. Sources that provide chemical energy include3:
1. Oxy-fuel burners and oxygen lancing
2. Charge carbon
3. Foaming carbon
4. Exothermic constituents in scrap
5. Exothermic constituents in alternate iron sources
Oxy-fuel burners are used to introduce combinations of natural gas, oil, or even coal into the furnace to displace electricity use. The reaction of carbon with oxygen within the bath to produce CO results in a significant energy input into the process. The injection of a carbon source also promotes the formation of a foamy slag, which retains energy that is transferred to the bath. The generation of the CO within the bath is necessary to flush out dissolved gases (nitrogen and hydrogen) in the steel, as well as flush oxide inclusions from the steel into the slag.1 Some EAFs use ferromanganese as a catalyst in the melt to add energy and help stabilize the melt. The efficiency of manganese combustion can be between 90 and 100%.3 Residence time in the furnace for a 100% scrap charge ranges from about 45 min to several hours.1 When the charge is fully molten it is refined to remove unwanted materials (e.g., phosphorus, sulfur, aluminum, silicon, manganese, and carbon), tapped from the tilted furnace, and sent for secondary treatment prior to casting. Because scrap metal rather than molten iron is the primary material charged, EAF steel producers avoid the coke-making and iron-making process steps. Figure 2.5 illustrates electric arc furnace steel making and the Table 2.9 gives its major inputs and outputs. An overview of key environmental and energy facts of electric arc furnace process of steel making is presented in Table 2.10.
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