It is used in electric arc furnaces for the production of steel transforming solid metallic raw material into a liquid state (DRI, HBI and scrap), through the electric arcs generated by graphite electrodes while making contact with the metallic charge.
To increase the thermal efficiency of the furnace during the process, the foam slag technique is used. This is formed on the molten metal and rises covering the arc of the electrodes ensuring that the electrical energy is transferred directly to the molten bath to take advantage of the heat generated, allowing the arc to operate at high voltages without increasing the thermal load by radiation on the refractory of the furnace. This achieves increases in thermal efficiency of up to 90%. Submerging the arc also helps prevent nitrogen from being exposed to the arc where it can dissociate and enter the steel.
The formation of the foamy slag can be divided into two stages:
- Initially the oxygen is blown into the molten metal. This reacts with the existing carbon in the bath forming bubbles of CO. Oxygen also reacts with Fe present in the molten bath:
- 2C + O2 = 2CO
- 2Fe + O2 = 2FeO
- As the iron is oxidized, carbon is injected into the slag through cooled supersonic lances promoting the reduction of iron oxide. This stage generates CO and causes the iron to return to the molten bath:
- C + FeO = CO + Fe
The injected coal must have a high percentage of fixed carbon (> 70%) and an adequate particle size (1/8″ to 250 microns) that allows it to dissolve in the slag without being prematurely consumed and travel through the injection system without complications. In addition to having low abrasive power to avoid damage to injection machinery.