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Submerged arc furnaces for phosphorus production are a specialized subtype of electric arc furnaces used for the production of phosphorus and other products.Submerged arc furnaces are mainly used to produce ferroalloys.The term submerged means that the electrodes of the furnace are buried deep in the charge.The reduction reaction takes place near the electrode tip to facilitate the process in the furnace
Furnace construction and individual components
The outer shell or shell of a submerged arc furnace is made of steel.The lower part is lined with strongly calcined hard carbon blocks, and the upper part is lined with refractory bricks.The furnace bottom and lower part are water-cooled.Three electrodes are placed at the corners of an equilateral triangle with rounded corners.These furnaces can be equipped with prebaked electrodes or Söderberg electrodes.The Söderberg electrode is heated by electric current and furnace heat and then baked into a solid in the area of the contact clip.When the electrode is fed down to compensate for its consumption in the furnace (a few centimeters per hour), the electrode must become solid over its entire cross-section (inside the charge).If the electrodes are not fully baked, there is a risk of breakage, especially with long electrodes.Söderberg electrodes contain a higher proportion of organic impurities than prebaked electrodes.Thus, furnaces with Söderberg electrodes produce yellow phosphorus and prebaked electrodes produce white phosphorus.
By automatically raising and lowering the electrodes, the electrode current remains fairly constant during operation.As the current increases, the electrodes rise, increasing the resistance between the electrodes and the furnace floor.So reducing the current has the opposite effect while keeping the voltage constant.Some furnaces are also controlled by constant voltage or power.From one submerged arc furnace, a variety of ferroalloys can be produced.The choice of transformer voltage is based on future shifts in ferroalloy production.
Gravity transports the apatite ore, carbon (coke) and silica (gravel) to the submerged arc furnace through a feed chute located at the furnace roof.This ensures a constant packed bed volume.The gaseous products, a mixture of carbon monoxide and phosphorus, leave the furnace through two symmetrically placed outlets located above the ferrophosphorus tapping hole in the furnace roof.Ferrophosphorus is excreted, usually once a day.However, the slag is continuously tapped through two alternating water-cooled tap holes located 400 mm above the furnace floor.The feed material forms the main resistance and flow resistance of the melting furnace circuit.As the feed materials descend into the hot zone in the furnace, they begin to soften and melt, significantly reducing electrical resistance.Thus providing a conductive path between the electrodes where Joule heat is released to achieve the high temperature and energy levels required to achieve a fundamentally endothermic reaction.
The energy in an industrial phosphorus furnace is distributed between the heating and melting of the material (≈ 40%) and the chemical reaction (≈ 45%).Cooling losses (cooling water), electrical losses (Joule heating) and radiative heat losses account for the remainder (≈ 15%).
Maintenance and security
The lifespan of a phosphorus furnace is linked to the durability of its carbon lining.However, the new state of the liner does not require the furnace to be shut down and completely emptied to monitor the state of the liner. Monitoring is accomplished by incorporating radioactive sources at points where erosion is known to occur.In addition, special thermocouples are inserted at different depths of the carbon bricks to continuously measure the wall temperature.Wall temperature readings can also be used as a maintenance tool, alerting the system to any anomalies.Monitoring the position of the wear line is critical to avoid molten iron or slag breaking through the lining and causing injury to operators, furnaces and nearby equipment.In addition, the gaseous products leaving the furnace consist mainly of phosphorus tetrahedra (P4) and carbon monoxide.It still needs to be cleaned of any dust and is usually sent to an electrostatic gas cleaning system.