Electric Arc Furnace

Overview

An electric arc furnace, or EAF, melts steel scrap with electricity. Three large graphite electrodes are lowered through the roof until arcs jump between their tips and the scrap, and those arcs, drawing tens of thousands of amps, generate enough heat to melt a furnace full of cold scrap in under an hour. The EAF is the heart of the scrap-based steelmaking route: it turns recycled steel back into liquid metal without the blast furnace and coke ovens that the ore-based route needs. The Furnace Shell holds the charge, the Electrode System strikes the arcs, the Power Supply feeds them, the Injection System system adds chemical energy, the Tilting Platform platform pours the heat, and the Water Cooling System and Fume Extraction systems keep the equipment and the air manageable, all under the Control System.

Shell and hearth

The Furnace Shell is built in two parts. The Lower Shell holds the Hearth Refractory, a magnesia lining that contacts the molten bath and must survive the heat and chemical attack of the slag. The upper shell is lined not with brick but with Water-Cooled Panel water-cooled pipe panels, because nothing refractory survives direct arc radiation for long; the panels build a protective slag skull on their cold faces. Steel is poured through the Eccentric Bottom Taphole, an off-centre taphole whose Taphole Slide Gate gate opens to pour clean steel and closes to hold a heel of liquid metal for the next heat, sealed between heats by Fill Sand. A Slag Door at the bath line lets operators rake slag and insert lances.

Electrodes and arcs

The Electrode System carries three Graphite Electrode columns, each gripped by a Electrode Clamp on a current-carrying Electrode Arm that rides a Lifting Mast. As the electrodes burn down they are extended by screwing on new sections with threaded Electrode Nipple joints. The electrodes pass through the Water-Cooled Roof, which lifts and swings aside on its Roof Lift and Swing so an overhead crane can drop a bucket of scrap into the open shell. Once charged, the roof closes, the electrodes lower, and the arcs bore down into the scrap. A Regulator Cylinder on each electrode adjusts its height many times a second to hold the arc as the scrap collapses and melts.

Power supply

The current an EAF needs is enormous, so its Power Supply is a small power plant in itself. The Furnace Transformer steps grid voltage down through its HV Winding and LV Winding windings to deliver tens of thousands of amps, with an on-load Tap Changer selecting arc voltage as the heat progresses. A Series Reactor steadies the violently fluctuating arc and limits short-circuit current when an electrode touches scrap. Current reaches the swinging arms through water-cooled Bus Tube tubes and Flexible Cable cables, and a Furnace Breaker switches and protects the circuit. The windings sit in oil in the Oil Tank, cooled by the Transformer Cooler.

Chemical energy and tapping

Electricity alone is not the cheapest way to finish a heat, so the Injection System system adds chemical energy. Oxygen Lance lances blow oxygen into the bath to cut scrap and burn off carbon, releasing heat in the process; Wall Burner oxy-fuel burners melt the cold scrap in the corners the arcs reach last; and Carbon Injector injectors blow carbon powder to foam the slag. Foamy slag matters because it buries the arc, so its heat goes into the steel instead of radiating against the cooled panels, which both saves energy and protects the furnace. The Gas Skid meters all these flows. When the heat is ready, the Tilting Platform platform rocks the furnace on its Tilt Cradle using Tilt Cylinder cylinders, tipping it forward to pour through the bottom taphole into a waiting ladle and back to rake slag through the door.

Cooling, fume, and control

Arc heat would destroy the furnace without the Water Cooling System system, where Cooling Pump pumps drive water through the panels and roof, a Heat Exchanger isolates the clean loop, and a Cooling Tower rejects the absorbed heat. Melting scrap throws off a huge volume of fume, captured by the Fume Extraction system: gas is drawn directly from the roof through the fourth-hole elbow and a Cooling Duct that burns off carbon monoxide, a Canopy Hood catches what escapes, and the Baghouse with its thousands of Filter Bag sleeves cleans the gas before the Induced Draft Fan vents it. The heat is run from the Control System station, where a Furnace PLC sequences charging, melting, and tapping, the Electrode Regulator holds arc current from Current Transformer feedback, and the Power Controller runs a voltage and current profile tuned for each stage. Operators work from the Operator HMI. Because the EAF route recycles scrap with far less energy and emission than ore-based steelmaking, it now produces a large and growing share of the world's steel, and its liquid output feeds directly into a ladle refining furnace for final adjustment before casting.