Definition and Basic Construction
A graphite electrode is a cylindrical body of manufactured carbon with a threaded socket at each end, used to conduct current into an electric arc furnace. Nominal diameters run from roughly 75 mm for small units up to 750 mm for the largest ultra-high-power installations, and a single section is normally 1,800, 2,100 or 2,400 mm long. The body is produced from a filler of needle coke or regular coke and a binder of coal tar pitch, then formed, baked, graphitised and machined. Each electrode is supplied with a matching nipple that screws into the sockets and joins one section to the next.
How the Electrode Is Made
The filler is crushed, milled, screened and mixed with pitch, then formed either by extrusion or by pressing. The green body is baked at about 800 to 1,000 degrees Celsius so that the binder carbonises and binds the coke particles into a solid mass. Selected grades are impregnated with pitch and re-baked to raise density before the final heat treatment. The baked body then passes through a graphitisation furnace at 2,500 to 3,000 degrees Celsius, where the carbon lattice becomes ordered graphite. Finally the body is machined to its dimension series and the sockets are cut with a tapered thread.
Grades and Dimension Series
Electrodes are supplied as regular power, high power and ultra-high power grades, normally abbreviated RP, HP and UHP. The grades differ mainly in the current density they can carry and in the raw materials used: regular grades accept lower current densities, while ultra-high-power grades are made from needle coke with additives that lower the coefficient of thermal expansion and the electrical resistivity. Grade designations such as UHP400, UHP500 and UHP600 state the nominal diameter in millimetres and imply a current capacity band.
How the Column Works
Three electrodes are suspended over the furnace hearth in a triangle. Each is held by a clamp on an electrode column that also carries current from the transformer through water-cooled busbars. The tip is lowered until the arc strikes between the graphite and the scrap charge. As the tip is consumed the column is slipped down, and when a section becomes too short a new section is screwed onto the top. The assembly is therefore a conductor that is continuously renewed while the furnace stays in operation.
Role in Steelmaking and Other Furnaces
Electric arc furnaces consume the largest share of graphite electrodes worldwide, because the arc is the only practical way to deliver several megawatts of power directly into a scrap charge. The same product is used in ladle furnaces for secondary refining, in submerged arc furnaces producing ferroalloys, silicon metal, calcium carbide and yellow phosphorus, and in some direct current arc installations. In every case the electrode is selected according to the current, the diameter required and the cleanliness demanded by the final product.
Frequently Asked Questions
Q: What is a graphite electrode in simple terms?
It is a cylindrical, consumable carbon rod that carries electric current into an arc furnace and forms the arc that melts scrap steel or other charge materials.
Q: What raw materials are used to make one?
A filler of needle coke or regular petroleum coke and a coal tar pitch binder. The mixture is formed, baked, graphitised at 2,500 to 3,000 degrees Celsius and then machined.
Q: What do the RP, HP and UHP grades mean?
They stand for regular power, high power and ultra-high power, and they describe the current density the electrode can carry. UHP grades use needle coke and carry the highest current density.
Q: What diameter and length are available?
Nominal diameters typically span 75 mm to 750 mm, and standard section lengths are 1,800, 2,100 and 2,400 mm, with longer sections available for very large furnaces.
Q: How are electrode sections joined?
A tapered-thread graphite nipple screws into the socket at each end of the section. The nipples are made from denser graphite because they carry the full current and load at the joint.
Q: Where are graphite electrodes used?
Mainly in electric arc furnace steelmaking, but also in ladle refining, ferroalloy and silicon submerged arc furnaces, calcium carbide and yellow phosphorus furnaces and some direct current arc units.