- Manufacture of White Phosphorus
White Phosphorus is industrially by far the most important form of elemental Phosphorus. It is produced by the electrothermal reduction of apatite with coke in thepresence of gravel (silica). The main reactions are represented by the following equations:
Carbon monoxide is regenerated from carbon dioxide and coke. The silica acts as a flux, converting the calcium oxide formed into a low melting slag as follows:
This slag also incorporates most of the impurities brought into the process with the ingredients (with the exception of the iron) and part of the tluoride. The iron is converted into Ferrophosphorus with the approximate composition Fe2P.
To ensure an adequate permeability to gases in the phosphorus reduction furnace, the raw materials must have a minimum size of 1 to 2 cm. Gravel and coke (smelting coke with preferably <10 % ash) can be obtained in this size and the fluoroapatite used is preferably finer and should be agglomerated. This can be ccomplished by, for example, pelletization with a pan granulator using the dust from electrostatic filters (see below) as a binder. These pellets then have to be dried and sintered, for which the Carbon monoxide-containing the gases can be used as an energy source.
Modern reduction plants consist of three units:
(1)an electrothermal reactor
(2)a gas purifier
(3)a phosphorus condenser
Figure 1.5-3 shows a process flow sheet for the electrothermal manufacture of phosphorus. The electrothermal reactor, which is round or triangular with rounded corners, is lined with carbon blocks at the bottom and otherwise with firebricks. The symmetrically positioned carbon electrodes (Soederberg self-baking electrodes or, particularly in the USA, preformed carbon electrodes) have a diameter of 1.3 to 1.5 m in a modern reactor. The electricity consumption is 50 to 70 MW.
The feed shutes for the raw materials are positioned symmetrically round the electrodes. The tap-hole for the ferrophosphorus is positioned right at the bottom of the reactor, with the tap-hole for the less dense calcium silicate slag just above it. Carbon monoxide and phosphorus vapor leave by the gas outlet at the top of the reactor.
Gas purification (with hot electrostatic precipitators) has to be carried out at temperatures above 280°C to prevent phosphorus condensation. About 0.04 kg of dust is separated per m3 of gas.
In modern plants the condensation of phosphorus is carried out in two stages. In a first condensation tower water at 50 to 60°C is sprayed from the top and meets the phosphorus vapor being transported countercurrently from below, whereupon the phosphorus condenses as a liquid.
The second condensation tower uses water at 10 to 25°C and solid phosphorus is formed. This is periodically melted and as with the first condensation tower run off into a holding tank. The liquid phosphorus is stored under water, drawn off and transported.
The byproducts can, in part, be further utilized: carbon monoxide is a valuable energy source and can, for example, be used in the sintering of the apatite pellets. The calcium silicate slag has, however, only limited marketability e.g. in road building. The ferrophosphorus contains 15 to 28% phosphorus and is, when the silicon content is below 3%, usable in metallurgy. Ferrophosphorus from plants in the Western USA contains 7 to 14% of vanadium (as V2O5). This ferrophosphorus is processed to vanadium(Ⅴ) oxide or vanadium chloride and represents the second most important source of vanadium in the USA. The phosphorus contained in the ferrophosphorus is simultaneously converted into non-utilizable byproducts.
Phosphorus-containing sludge and process water are also produced, which are difficult to dispose of.
Production of one ton of phosphorus entails the consumption of:
(1)8.0 t Florida phosphate (3 1 % P2O5)
(2)2.8 t of gravel (97% silicon dioxide)
(3)1.25 t of coke (90% carbon)
(4)0.05 t Soederberg electrodes
(5)ca. 13 . lo3 kWh of electricity
and the production of the following quantities of byproducts:
(1)7.7 t silicate slag (90% calcium silicate)
(2)0. 15 t of ferrophosphorus
(3)0.1 t filter dust (20% P2O5)
(4)2500 m3 flue gas (85% carbon monoxide)
Reactors with an effective power of 70MW can therefore produce a maximum of 5.4 t of phosphorus per hour.
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