Electric arc furnace

An electric arc furnace, often called an EAF, is a powerful industrial tool used to melt materials.

It works by using an electric arc to generate intense heat. This process allows manufacturers to turn solid scrap metal into liquid steel. These machines are vital for modern steelmaking and come in many different sizes. Small units used by dentists or in research labs might only hold a few dozen grams.

Industrial furnaces are much larger. Some hold about one tonne for making cast iron. The largest units can hold up to 400 tonnes for secondary steelmaking.

The furnace is built with several specific parts. It has a shell, which includes the sidewalls and a lower steel bowl. The bottom of this bowl is called the hearth. The hearth is lined with refractory brick to protect the shell from extreme heat.

Above the hearth is a retractable roof. This roof supports the electrodes, which are long rods made of graphite. In many large furnaces, the shell is water-cooled to manage the heat. The electrodes are often moved by a positioning system using electric winches or hydraulic cylinders.

To melt the metal, the furnace uses a process involving an electric arc. The arc forms between the graphite electrodes and the material inside, known as the charge.

As electricity passes through the charge, it creates immense heat. The electric arc itself can reach temperatures of approximately 3,500 degrees Celsius. This heat comes from both the electric current and the radiant energy of the arc. The electrodes often glow white-hot during this stage. To keep the temperature even, modern plants may use oxygen-fuel burners in the sidewalls. They might also inject oxygen and carbon into the furnace to add chemical energy.

History shows that many scientists worked to perfect this technology. In 1803, Vasily Vladimirovich Petrov demonstrated that an electric arc could melt metal. Sir Humphry Davy conducted an experimental demonstration in 1810. Later, Sir William Siemens took out patents for arc-type furnaces between 1878 and 1879.

The first successful, operational furnace was invented by James Burgess Readman in Edinburgh, Scotland, in 1888. His machine was used to create phosphorus. Paul Héroult from France later developed more advanced furnaces. In the United States, the Sanderson brothers installed the first electric arc furnace in Syracuse, New York.

Efficiency and scale are very important in modern steelmaking. A mid-sized modern furnace might produce 80 tonnes of liquid steel in about 50 minutes.

This includes the time from when cold scrap is added to when the steel is poured. For comparison, a massive DC furnace in Japan can have a tap weight of 420 tonnes. These large machines use many transformers to reach huge power levels. Melting one tonne of scrap steel requires about 440 kilowatt-hours of energy. This is much more efficient than some other global steel production methods.

There are different ways to manage the liquid steel once it is melted. One method is called tapping, which is when the furnace tilts to pour the metal out.

Some furnaces use an eccentric bottom tap-hole, or EBT, to help with this. This taphole is located in the narrow "nose" of the egg-shaped hearth. It is filled with refractory sand, such as olivine, when the furnace is closed. This method helps reduce the amount of nitrogen and slag that enters the liquid steel. Other designs, like the Stassano furnace, might rotate to mix the molten bath.

Electric arc furnaces have changed how the world produces metal. In the past, "electric steel" was a specialty product for things like spring steel. During World War II, these furnaces were used widely to produce alloy steels. In 1969, a company called Nucor used a mini-mill with an EAF to enter the steel market. Mini-mills are much cheaper to build than integrated steel mills. It costs about US$140–200 per ton of capacity to build a mini-mill. In contrast, an integrated mill costs about US$1,000 per ton. This allowed smaller companies to compete with giant steelmakers. Today, EAFs are used globally to produce many types of steel products.