The Submerged-arc furnace for phosphorus production is a particular sub-type of electric arc furnace used to produce phosphorus and other products. Submerged arc furnaces are mainly used for the production of ferroalloys. The nomenclature submerged means that the furnace's electrodes are buried deep in the furnace burden. A reduction reaction takes place near the tip of the electrodes to facilitate the furnace's process.
Large-scale production of phosphorus uses the Wöhler process. [1] In this process, apatites (nearly always fluorapatite) are reduced in the presence of carbon (coke) and silica (gravel). This is performed in a submerged-arc furnace at temperatures of between 1150 and 1400C. The main internal reaction is described below:
This main reaction produces a liquid calcium silicates slag, carbon monoxide gas and the desired product, phosphorus gas.
This process also has intermediate reactions, and as such, the phosphate rock created has impurities. One such impurity—and the most important one—is iron oxide. Iron oxide impurities are reduced and form iron phosphides. The resulting second liquid product is called ferrophosphorus. Iron is considered an impurity and undesirable because it requires additional carbon and power for reduction. Thus, it locks up a certain percentage of the phosphorus.
Another impurity is alumina - this increases the slag bulk but reduces the melting point.
A submerge-arc furnace's shell or casing is fabricated from steel. The lower part is lined with hard blocks of strongly calcined carbon, and the upper part with firebrick. The floor and lower section of the furnace are water-cooled. Three electrodes are placed at the angles of an equilateral triangle with rounded corners. These furnaces can be equipped with either pre-baked electrodes or Söderberg electrodes. The Söderberg electrodes are heated by the electric current and the furnace heat, then baked solid in the region of the contact clamps. The electrode must become baked solid over its entire cross section (inside the charge) when it is fed downwards to compensate for its consumption in the furnace (which is a few centimeters per hour).
If the electrodes are not completely baked, there is a risk of breakage, especially with long electrodes. Söderberg electrodes have a higher proportion of organic impurities when compared to the pre-baked electrodes. Thus, Söderberg electrode-equipped furnaces produce yellow phosphorus and pre-baked electrodes produce white phosphorus.
The electrode current is kept fairly constant during operation by automatically raising and lowering the electrodes. When current increases, the electrodes are raised, increasing the electrical resistance between the electrodes and the furnace floor. Hence reducing the current, produces the opposite effect when the voltage is kept constant.
Some furnaces are also controlled through either constant voltage or power. From one submerged-arc furnace, it is possible to produce several ferro alloys. Transformer voltage selection is based on future changeover of ferro alloys production.
Gravity delivers the apatite ore, carbon (coke) and silica (gravel) to the submerged-arc furnace through feed chutes situated in the roof of the furnace. This ensures a constant packed bed volume. The gaseous product, a mixture of carbon monoxide and phosphorus gas, leaves the furnace through two symmetrically placed outlet vents situated above the ferrophosphorus tap hole in the roof of the furnace. The ferrophosphorus is tapped off, usually once per day. Slag, however, is continuously tapped through two alternating, water-cooled tap holes located 400 mm above the furnace floor. The feed material forms the major electrical and flow resistance of the smelting furnace circuit. As the feed materials descend towards the hot zone in the furnace, they start to soften and melt, significantly lowering the electrical resistance. A conductive path is thus provided between the electrodes where the Joule heating is released to attain the high temperatures and energy levels needed to effect the essential endothermic reactions. [2]
The energy in an industrial phosphorus furnace is distributed between heating up and melting of the material (≈ 40%) and chemical reactions (≈ 45%). Cooling losses (cooling water), electrical losses (Joule heating) and radiative heat losses account for the rest (≈ 15%). [3] [4]
The lifetime of a phosphorus furnace correlates with the durability of its carbon lining. However, new state of the lining does not require shutting down the furnace and completely emptying it in order to monitor the status of the lining. Monitoring is instead achieved by incorporating radioactive sources at points where erosion is known to occur.
Also, special thermocouples are inserted at various depths in the carbon bricks to continuously measure wall temperature. Wall temperature readings also serve as a maintenance tool, alerting the system to any irregularities. It is crucial to monitor the position of the wear line in order to avoid hot metal or slag from breaking through the lining and cause damage to the operators, the furnace and nearby equipment.
In addition, the gaseous product leaving a furnace mostly comprises out of phosphorus tetrahedron (P4) and carbon monoxide. It still needs to be purified of any dust and is typically sent to an electrostatic gas purification system.
Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as brazing and soldering, which do not melt the base metal.
The Hall–Héroult process is the major industrial process for smelting aluminium. It involves dissolving aluminium oxide (alumina) in molten cryolite, and electrolyzing the molten salt bath, typically in a purpose-built cell. The Hall–Héroult process applied at industrial scale happens at 940–980 °C and produces 99.5–99.8% pure aluminium. Recycled aluminum requires no electrolysis, thus it does not end up in this process.
Steelmaking is the process of producing steel from iron ore and/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon are removed from the sourced iron, and alloying elements such as manganese, nickel, chromium, carbon and vanadium are added to produce different grades of steel.
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. Blast refers to the combustion air being supplied above atmospheric pressure.
Arc welding is a welding process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals, when cool, result in a binding of the metals. It is a type of welding that uses a welding power supply to create an electric arc between a metal stick ("electrode") and the base material to melt the metals at the point of contact. Arc welding power supplies can deliver either direct (DC) or alternating (AC) current to the work, while consumable or non-consumable electrodes are used.
Basic oxygen steelmaking, also known as Linz-Donawitz steelmaking or the oxygen converter process, is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing oxygen through molten pig iron lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as basic because fluxes of burnt lime or dolomite, which are chemical bases, are added to promote the removal of impurities and protect the lining of the converter.
An open-hearth furnace or open hearth furnace is any of several kinds of industrial furnace in which excess carbon and other impurities are burnt out of pig iron to produce steel. Because steel is difficult to manufacture owing to its high melting point, normal fuels and furnaces were insufficient for mass production of steel, and the open-hearth type of furnace was one of several technologies developed in the nineteenth century to overcome this difficulty. Compared with the Bessemer process, which it displaced, its main advantages were that it did not expose the steel to excessive nitrogen, was easier to control, and permitted the melting and refining of large amounts of scrap iron and steel.
An electric arc is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An arc discharge is initiated either by thermionic emission or by field emission. After initiation, the arc relies on thermionic emission of electrons from the electrodes supporting the arc. An arc discharge is characterized by a lower voltage than a glow discharge. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".
An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc.
Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable metals. Pyrometallurgical treatment may produce products able to be sold such as pure metals, or intermediate compounds or alloys, suitable as feed for further processing. Examples of elements extracted by pyrometallurgical processes include the oxides of less reactive elements like iron, copper, zinc, chromium, tin, and manganese.
Puddling is the process of converting pig iron to bar (wrought) iron in a coal fired reverberatory furnace. It was developed in England during the 1780s. The molten pig iron was stirred in a reverberatory furnace, in an oxidizing environment to burn the carbon, resulting in wrought iron. It was one of the most important processes for making the first appreciable volumes of valuable and useful bar iron without the use of charcoal. Eventually, the furnace would be used to make small quantities of specialty steels.
Direct reduced iron (DRI), also called sponge iron, is produced from the direct reduction of iron ore into iron by a reducing gas or elemental carbon produced from natural gas or coal. Many ores are suitable for direct reduction.
A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminum and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.
Deoxidization is a method used in metallurgy to remove the oxygen content during steel manufacturing. In contrast, antioxidants are used for stabilization, such as in the storage of food. Deoxidation is important in the steelmaking process as oxygen is often detrimental to the quality of steel produced. Deoxidization is mainly achieved by adding a separate chemical species to neutralize the effects of oxygen or by directly removing the oxygen.
Aluminium smelting is the process of extracting aluminium from its oxide, alumina, generally by the Hall-Héroult process. Alumina is extracted from the ore bauxite by means of the Bayer process at an alumina refinery.
A cupola or cupola furnace is a melting device used in foundries that can be used to melt cast iron, Ni-resist iron and some bronzes. The cupola can be made almost any practical size. The size of a cupola is expressed in diameters and can range from 1.5 to 13 feet. The overall shape is cylindrical and the equipment is arranged vertically, usually supported by four legs. The overall look is similar to a large smokestack.
Zinc smelting is the process of converting zinc concentrates into pure zinc. Zinc smelting has historically been more difficult than the smelting of other metals, e.g. iron, because in contrast, zinc has a low boiling point. At temperatures typically used for smelting metals, zinc is a gas that will escape from a furnace with the flue gas and be lost, unless specific measures are taken to prevent it.
In metalworking, a welding defect is any flaw that compromises the usefulness of a weldment. There are many different types of welding defects, which are classified according to ISO 6520, while acceptable limits for welds are specified in ISO 5817 and ISO 10042.
Archaeometallurgical slag is slag discovered and studied in the context of archaeology. Slag, the byproduct of iron-working processes such as smelting or smithing, is left at the iron-working site rather than being moved away with the product. As it weathers well, it is readily available for study. The size, shape, chemical composition and microstructure of slag are determined by features of the iron-working processes used at the time of its formation.
Ferrophosphorus is a ferroalloy, an alloy of iron and phosphorus. It contains high proportion of iron phosphides, Fe2P and Fe3P. Its CAS number is 8049-19-2. The usual grades contain either 18 or 25% of phosphorus. It is a gray solid material with melting point between 1050-1100 °C. It may liberate phosphine in contact with water. Very fine powder can be combustible.