Electric Arc Furnaces
This furnace can be described as a furnace heating charged materials by the way of an electric arc. The first electric furnaces came into being in 1907, at the hands of Paul Heroult of French origin. The commercial part of these furnaces was established in the USA.
|Electric Arc Furnaces
These furnaces exist in all the sizes-right, from the smallest one having a capacity of around 1 ton to the largest one having a capacity of 400 tons(approx.)
The electric arc furnace can have temperatures risen up to 1800 Celsius.
An electric arc furnace used for steelmaking consists of a refractory-lined vessel, usually water-cooled in larger sizes, covered with a retractable roof, and through which one or more graphite electrodes enter the furnace. The furnace is primarily split into three sections:
The shell, which consists of the sidewalls and lower steel ‘bowl’;
The hearth, which consists of the refractory that lines the lower bowl;
The roof, which may be refractory-lined or water-cooled, and can be shaped as a section of a sphere. The roof also supports the refractory delta in its center, through which one or more graphite electrodes enter.
HEAT ENERGY SOURCE
Furnace can be described as a furnace heating charged materials by the way of an electric arc.
|Heating energy source
A typical alternating current furnace has three electrodes.
They may be lined with acid or basic refractories.
Electrodes are round in section, and typically in segments with threaded couplings, so that as the electrodes wear, new segments can be added.
The arc forms between the charged material and the electrode, the charge is heated both by current passing through the charge & by the radiant energy evolved by the arc.
Electric arc furnaces may be categorized as direct arc or indirect arc. Both types of units are suited for the melting of high melting point alloys such as steels.
The electrodes of direct arc furnace are automatically raised and lowered by a positioning system.
The regulating system maintains approximately constant current and power input during the melting of the charge.
The furnace is built on a tilting platform so that the liquid steel can be poured into another vessel for transport. The operation of tilting the furnace to pour molten steel is called “tapping”.
Often modern furnaces have an eccentric bottom tap-hole (EBT) to reduce inclusion of nitrogen and slag in the liquid steel. It is filled with refractory sand, such as olivine, when it is closed off.
Electric arc steelmaking is only economical where there is plentiful electricity.
1. Scrap metal is delivered to a scrap bay, located next to the melt shop.
(Scrap generally comes in two main grades: shred (whitegoods, cars and other objects made of similar light-gauge steel) and heavy melt (large slabs and beams), along with some direct reduced iron (DRI) or pig iron for chemical balance.)
2. The scrap is loaded into large buckets called baskets, with ‘clamshell’ doors for a base.
(Care is taken to layer the scrap in the basket to ensure good furnace operation; heavy melt is placed on top of a light layer of protective shred, on top of which is placed more shred. These layers should be present in the furnace after charging)
3. The scrap basket is then taken to the melt shop, the roof is swung off the furnace, and the furnace is charged with scrap from the basket.
(Charging is one of the more dangerous operations for the EAF operators. There is a lot of energy generated by multiple tonnes of falling metal; any liquid metal in the furnace is often displaced upwards and outwards by the solid scrap, and the grease and dust on the scrap is ignited if the furnace is hot, resulting in a fireball erupting)
4. After charging, the roof is swung back over the furnace and meltdown commences.
(The electrodes are lowered onto the scrap, an arc is struck and the electrodes are then set to bore into the layer of shred at the top of the furnace. Lower voltages are selected for this first part of the operation to protect the roof and walls from excessive heat and damage from the arcs. Once the electrodes have reached the heavy melt at the base of the furnace and the arcs are shielded by the scrap, the voltage can be increased and the electrodes raised slightly, lengthening the arcs and increasing power to the melt. This enables a molten pool to form more rapidly, reducing tap-to-tap times. Oxygen is also supersonically blown into the scrap, combusting or cutting the steel)
An important part of steelmaking is the formation of slag, which floats on the surface of the molten steel. Slag usually consists of metal oxides, and acts as a destination for oxidized impurities, as a thermal blanket (stopping excessive heat loss) and helping to reduce erosion of the refractory lining. The slag blanket also covers the arcs, preventing damage to the furnace roof and sidewalls from radiant heat.
5. Once flat bath conditions are reached, i.e. the scrap has been completely melted down, another bucket of scrap can be charged into the furnace and melted down.
(After the second charge is completely melted, refining operations take place to check and correct the steel chemistry and superheat the melt above its freezing temperature in preparation for tapping).
6. More slag formers are introduced and more oxygen is blown into the bath, burning out impurities such as silicon, sulfur, phosphorus, aluminium, manganese and calcium and removing their oxides to the slag.
(Removal of carbon takes place after these elements have burnt out first, as they have a greater affinity for oxygen. Metals that have a poorer affinity for oxygen than iron, such as nickel and copper, cannot be removed through oxidation and must be controlled through scrap chemistry alone, such as introducing the direct reduced iron and pig iron mentioned earlier)
7. Once the temperature and chemistry are correct, the steel is tapped out into a preheated ladle through tilting the furnace.
8. During and after tapping, the furnace is ‘turned around’.
(the slag door is cleaned of solidified slag, repairs may take place, and electrodes are inspected for damage or lengthened through the addition of new segments; the taphole is filled with sand at the completion of tapping).
The use of EAFs allows steel to be made from a 100% scrap metal feedstock, commonly known as ‘cold ferrous feed’ to emphasise the fact that for an EAF, scrap is a regulated feed material.
The primary benefit of this is the large reduction in specific energy (energy per unit weight) required to produce the steel.
Another benefit is flexibility: while blast furnaces cannot vary their production by much and are never stopped, EAFs can be rapidly started and stopped, allowing the steel mill to vary production according to demand.
Although steelmaking arc furnaces generally use scraps steel as their primary feedstock, if hot metal from a blast furnace or direct-reduced iron is available economically, these can also be used as furnace feed.