Submerged-arc furnace for phosphorus production
Encyclopedia
Submerged-arc furnace for phosphorus production is a specific type of electric arc furnace
Electric arc furnace
An electric arc furnace is a furnace that heats charged material by means of an electric arc.Arc furnaces range in size from small units of approximately one ton capacity up to about 400 ton units used for secondary steelmaking...

 used to produce, amongst other products, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

. The term submerged refers to the electrodes that are usually buried deep in the furnace burden where the reduction reactions take place near the tip of the electrodes.

Main reaction

Large-scale production of phosphorus uses the Wöhler process , in which apatites (nearly always fluorapatite) are reduced in the presence of carbon (coke) and silica (gravel) in a submerged-arc furnaces at temperature of between 1150 and 1400C:
Ca10(PO4)6F2 + 15C + 9SiO2 → 3P2(g) + 9[(CaO•SiO2)] + CaF2 + 15CO(g)


This produces a liquid calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

 silicates slag
Slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...

, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 gas and the desired product, phosphorus gas. The process has intermediate reactions and in practice the phosphate rock will always have some impurities, with one of the most important being iron oxide, which will be reduced and form iron phosphides, resulting in a second liquid product namely ferrophosphorus. Iron is undesirable because it requires additional carbon and power for reduction and it locks up a certain percentage of phosphorus. Other impurities include alumina which increases the slag bulk but reduces the melting point somewhat.

Material flow

Gravity delivers the apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...

 ore, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 (coke) and silica (gravel) to the submerged-arc furnace through feed chutes situated in the rood of the furnace ensuring constant packed bed volume. The gaseous product, consisting mostly out of a mixture of carbon monoxide and phosphorus gas, leaves the furnace through two symmetrically spaced 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
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

. A conductive path is thus provided between the electrodes where the Joule heating
Joule heating
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...

 is released to attain the high temperatures and energy levels necessary to effect the endothermic reactions needed to produce the phosphorus gas.

Furnace structure and individual components

The furnace shell or casing is fabricated from steel. The lower part 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. The electrodes are placed in an equilateral triangle with rounded corners. The three electrodes are situated at the corners of a regular triangle. These furnaces can be equipped with either pre-baked electrodes or Söderberg electrodes. The material of the Söderberg electrodes is heated by the electrical current and the furnace heat and is 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 (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 which is the reason why the Söderberg furnace produces yellow phosphorus and the pre-baked electrodes white phosphorus.

Furnace control

The electrode current is kept fairly constant during operation by automatically raising and lowering the electrodes. This means that if the current increases, the electrodes are raised, increasing the electrical resistance between the electrodes and the furnace floor and hence reducing the current, and vice versa (combined with keeping voltage 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.

Energy consumption

The energy in an industrial phosphorus furnace is distributed between heating up and melting of the material (≈ 40%) and chemical reactions (≈ 45%), while cooling losses (cooling water), electrical losses (Joule heating) and radiative heat losses account for the rest (≈ 15%).

Maintenance and safety

The lifetime of a phosphorus furnace is mainly determined by the durability of the carbon lining. Detailed monitoring of the state of the lining does not necessitate shutting down the furnace and completely emptying it. It is achieved by incorporating radioactive sources at points where erosion is known to occur. Special thermocouples are inserted at various depths in the carbon bricks to continuously measure the wall temperature. These temperatures 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. This type of industrial monitoring is not only important for maintaining high standards of safety but also serves an economic purpose. Although the gaseous product leaving a furnace mostly comprises out of P4 and carbon monoxide it still needs to be purified from any dust and is sent to an electrostatic gas purification system.
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