Graphite electrodes are used almost exclusively within the electric arc furnace of a steel mill. So, to understand a graphite electrode, first you must understand what an electric arc furnace is.
OK. What is an Electric Arc Furnace?
An electric arc furnace is essentially a big recycling machine. Simply put, it's a large pot into which scrap steel, like old cars, bicycles, and refrigerators, are dumped. The furnace operator loads (called charging the furnace) carefully screened and selected scrap steel into this big pot. Then, the operator swings the furnace lid back over the pot and lowers it into place.
The electrodes are part of the furnace roof structure. Electrodes can be small, 75 millimeters (3 inches) in diameter, or quite large, up to 750 millimeters (30 inches) in diameter and as much as 2800 millimeters long. The largest weigh more than two metric tons (4,400 lbs.). The manufacturer of the furnace determines the size the electrode should be.
The electrodes are assembled into columns, usually 3 to a column. Graphite connecting pins-tapered and threaded on each end screw two electrodes together. Some furnace operators assemble the columns on the shop floor and lift them into place whole. Others add electrodes one at a time to the top of the column while it is still on the furnace. Each electrode column has a large steel arm which moves it up and down.
Furnaces using 3-phase AC (alternating current) electricity have 3 electrode columns; DC (direct current) furnaces only need one column of electrodes, but they are generally larger in diameter.
Once the furnace lid is in place, the electrodes are lowered until the tip of the electrode column almost touches the top of the scrap steel. Huge amounts of electricity--large furnaces use enough electricity for a town of 40,000 people--flow through large water-cooled cables into the electrodes. At the bottom of the electrode column, the electricity jumps (or arcs) from the electrode tip to the nearest piece of scrap steel. The intense heat of this electric arc melts the scrap steel, hence the name of the electric arc furnace.
The tip of the electrode will reach 3,000 degrees Celsius or 5,000 degrees Fahrenheit, half the temperature of the surface of the sun. Electrodes are made of graphite because only graphite can withstand this incredible temperature.
Eventually all the steel is melted. The furnace operator turns off the power and raises the electrode columns. The furnace is then tipped on its side to pour the molten steel into giant buckets called ladles. The ladles quickly carry the molten steel to the steel mill's caster which is the next step in making new and useful products from recycled scrap steel.
To answer the question, then, what is a graphite electrode? A graphite electrode is a device to conduct electricity down into an electric arc furnace which generates sufficient heat to melt scrap steel.
For more information, be sure to look through our new Graphite Electrodes Brochure.