By Buhle Xakalashe
Genesis of Electric arc furnaces
Furnaces are arguably the foundation of our entire civilization and are a basic building block for the industrial society. While furnaces have existed for centuries, the genesis of electric arc furnaces (EAFs) is a recent one. The first of any practical importance was constructed by Sir William Siemens in 1878, since that time the development has been rapid. The beginning of the electric arc furnace (EAF) may be traced farther back. In 1800 only a few months after Volta’s discovery of the electric battery Sir Humphry Davy, experimenting with the new battery, produced the first arc light between carbon points, and as the electric arc is the primary source of heat in EAFs its discovery was the first step in their evolution . Today, EAFs occupy a favourable position in the furnace food chain. How did this come about over such a relatively short period of time?
One of the early research undertakings on EAFs was in the 1890s by the Nobel Prize winning scientist, Henri Moissan. He developed a laboratory scale EAF that could attain temperatures of up to 3500oC and used it in high temperature applications, such as to volatilize substances which had been regarded as infusible, the production of diamonds as well as to prepare many compounds including calcium carbide . Up to this day industrial calcium carbide is produced in a similar manner . EAFs are the mainstay to many high temperature industrial processes, as fuel based furnaces cannot reach as high temperatures. Laboratory scale EAFs continue to play a vital role in research and development undertakings, and have also found commercial applications.
Application and Taxonomy
Since inception, EAFs remain one of metallurgy’s most amazingly diversified and flexible processing units which have found a wide range of applications in industry, examples include areas such as ferroalloys, chemical industry, lead, zinc, copper, refractory, Platinum Group Metals (PGMs), slag cleaning, waste recycling, steel production and specialised applications .
The various types of EAF features offer flexibility to process design and has facilitated evolution in EAFs. The taxonomy of EAFs is summarised above. The combination of such basic features allows the design of tailor made furnaces to suit the specific process requirements .
The taxonomy of EAFs is directly related to their mechanism in electrical heating, and is demonstrated in the figure below. On the one extreme of operation, open-arc, a large fraction of energy is dissipated in the arc; with some energy generation through resistive (joule) heating of the slag. This design can accommodate fine feed and is useful for highly conductive slags. The molten bath radiates significant energy to the roof and side-walls. Applications of this design include ilmenite smelting.
On the other extreme of operation, immersed-electrode, the electrode tips are dipped in the molten slag. This requires an electrically resistive slag to generate sufficient energy by resistive heating. This design is applied to PGMs smelting.
Submerged-arc furnaces are also common. These require a lumpy feed to allow gas permeability and coke for structural strength. This is useful when volatile components to be recovered to the metal or slag phase are present. Ferromanganese smelting is a good example of the application of this type of furnace .
Outlook for Electric arc furnaces
Increased productivity in EAFs, which has been the main activity over the last century, is considered as the most important direction in EAF development, especially in the steel industry [6, 7]. A push towards larger furnaces is a lingering consideration for EAFs, mainly for benefits associated with economies of scale .
EAFs are a major consumer of energy; consequently energy efficiency and energy recovery in EAFs are receiving major focus . Health, safety and the environment have also seen increased focus in the EAF development over the last few decades. Previously, operators controlled EAFs by visual inspections on open furnaces and by listening to the sound of the furnace; today most furnaces are closed, few or no operators are present around the furnaces, and control of the furnace is by measuring vital signs of the furnace . Current developments in electric arc furnaces are aligned towards sustainability [11, 12].
The role of electric arc furnaces in iron and steel industry
One industry which has benefited enormously from EAF technology is the iron and steel industry, in its entirety. Ferroalloys used in both the steel and stainless steel industry are mainly produced in EAFs. Furthermore, the steel making process as well as recycling within this industry utilises EAFs.
Titaniferous magnetite ore and ilmenite, a minor iron ore, are mainly composed of oxides of iron and titanium, as well as aluminium and silicon. Smelting of these ores employs EAFs mainly because of high temperature requirements where iron is recovered to the metal and the slag is enriched with titanium oxides, as well as the other gangue components [14, 15].
Bauxite residue or red mud, a waste by-product generated by the extraction of alumina from bauxite ore via the Bayer process, contains significant amounts of iron, aluminium, silicon and titanium as well as smaller concentrations of critical and/or industrially important elements such as Rare Earth Elements, vanadium, chromium and others . An EAF is a feasible smelting unit for red mud to recover iron to the metal phase and to concentrate the gangue components to the slag for downstream processing. My project focuses on process development to achieve this goal efficiently and sustainably.
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