GB1560686A - Manufacture of phosphorus - Google Patents

Description

(54) MANUFACTURE OF PHOSPHORUS (71) We, ALBRIGHT & WILSON LI MITED, a British Company of P.O. Box 3, Oldbury, Warley, West Midlands, England, do hereby declare the invention, for which we pray that a Patent may be granted to us and performed to be particularly described in and by the following statement: The present invention releates to the manufacture of phosphorus from phosphatic ores.

Although a number of methods have been proposed from time to time for the production of phosphorus for the last fiftv years commercial phosphorus manufacture has been carried out almost exclusively by reduction of phosphate rock between carbon electrodes. Mixtures of calcium phosphate rock, anthracite and silica have been charged to a furnace and heated between carbon electrodes. The anthracite reduces the phosphate and phosphorous vapour is evolved from the furnace, while the silica forms a slag with the calcium, which may be separately tapped from the furnace.

The conventional furnace has always been associated with a number of problems. For example, the electrodes require seals to prevent escape of phosphorus vapour from the furnace or leakage of air into the furnace, localised regions of very high temperature in the neighbourhood of the electrodes have caused undesirable side reactions including the formation of carbides, phosphides and silicon and, in the case of burdened furnaces, phosphorus vapour flowing rapidly through the channels which form round the electrodes entrains fine solids from the burden to form phosphorus mud in the condenser. In recent years, the increasing size of phosporus furnaces has given rise to further serious problems.For example, the electrodes require expensive control equipment to regulate their vertical movements. while lateral movement frequently causes electrode breakages which may necessitate shutting down the furnace for the difficult operation of removing the broken electrode and fitting a new one.

These difficulties have appeared to set a limit to the maximum size of a commercialy viable phosphorus furnace, yet no acceptable alternative to the conventional furnace has hitherto been suggested.

We have now devised a novel method and apparatus for winning phosphorus from phosphate rock by which all the foregoing problems may be substantially avoided.

Our invention provides a method of winning phosphorus from a phosphatic ore which comprises bringing a mixture of the phosphatic ore, an elementary carboncontaining reducing agent and a siliceous flux into thermal contact with a pool of a molten iron-phosphorus composition at a temperature sufficient to effect reduction of the phosphatic ore by the reducing agent to form phosphorus vapour together with a siliceous slag, the temperature of the pool being maintained either by electromagnetic induction or by passing an electric current through the iron-phosphorus composition.

The phosphatic ore is preferably calcium phosphate rock such as is commonly used in the manufacture of phosporus or an aluminous phosphate e.g. Senegal aluminous phosphate. It may be precalcined to remove organic material. The reducing agent is typically anthracite or any similar material comprising elementary carbon. Generally the mixture may be substantially the same as those normally used in electric reduction furnaces for producing phosphorus. Preferably the feed is finely ground and may be pelletised. The feed components are preferably mixed prior to pelletisation, e.g. prior to grinding.

The iron phosphorus composition is preferably ferro-phosphorus, which is a mixture comprising iron phosphides which is customarily formed as a by-product of phosphor us manufacture. typically ferrophosphorus comprises iron together with from 20 to 25% by wt. of phosphorus and about 1 to 5% silicon and carbon and minor amounts of metallic impurities based on the weight of the ferrophosphorus.

The iron phosphorus composition may alternatively comprise an alloy of iron or steel with ferrophosphorus or phosphorus.

Preferably the phosphorus content of the composition is greater than 3% by weight, most preferably at least 6% e.g. 12 to 30% based on the weight of the composition.

The most preferred range is 15 to 25% by weight of phosphorus. Similar alloys formed by adding phosphorus to iron may also be used. The invention is hereinafter described with reference to the use of ferrophosphorus although it will be appreciated that the alternative iron-phosphorus compositions described above may also be employed.

An important feature of the invention is that energy is supplied to the furnace via the ferrophosphorus pool instead of through electrodes in the reaction mixture. One method of heating the pool is to pass an electric current through the ferrophosphorus, for example between electrodes in the sides of the vessel located at the level of the ferrophosphorus pool. Preferably such electrodes are in the form of graphite inserts in the sides of the vessel near the base. A disadvantage of this system is that the vessel, or at least that part of it which is in contact with the ferrophsophorus, must then be constructed, at least in part, of an electrically non-conductive refractory.

Many such materials which are commercially available are liable to be attacked by the slag layer which overlies the ferrophosphorus. This problem may preferably be alleviated by cooling the sides of the furnace, or at least such electrically non-conductive parts of it as come in contact with the slag e.g. by means of a circulated coolant such as water.

It is preferred, however, to heat the pool inductively. In order, effectively, to heat the pool of iron phosphorus composition it is possible to surround the reaction vessel with an induction coil, but this has disadvantages in that the reactor may screen the pool from the induction coil unless the reaction vessel is constructed of the electrically non conducting refractory materials whose disadvantages are discussed above. Moreover the size of coil required would be inconvenient in the case of a large scale furnace.

We therefore prefer to employ an indirect forced circulation system of heating whereby part of the molten iron phosphorus composition is continuously withdrawn from the reactor, passed through an alternating electro-magnetic field (e.g. through or around a coil) to raise its temperature, and returned to the pool in the reactor. This may conveniently be achieved by providing one or more loops, depending on the furnace capacity, e.g. by means of a channel furnace, which may be provided with at least one, preferably electrically non-conducting refractory tube surrounded by an induction coil, or constituting the secondary winding of a transformer. One end of the tube may conveniently be connected with an outlet from near the base of the furnace and the other end is connected with an inlet (preferably a jet inlet) to the furnace.Circulation of the molten ferrophosphorus may be forced by the electromagnetic field.

The temperature of the pool is preferably maintained between 1300"C and 1700"C e.g.

1500"C. An important advantage of the invention is that the pool provides a heat sink which ensures more even heating than can be obtained with electrodes and so substantially reduces the occurence of hot spots and undesirable side reactions. The absence of electrodes makes the furnace easier to enclose, so minimising loss of vapour or leakage of air. The reaction proceeds with or without a burdened feed.

There is no problem of electrode breakage, so removing an important constraint on the practicable size of the reactor.

The reactor is preferably lined with carbon, e.g. graphite, to avoid corrosion. The feed may be continuously charged to the reactor, phosphorus vapour being continuously withdrawn through a vapour outlet to be separately condensed as elemental phosphorus. Alternatively, air may be present or may be introduced into the vapour and the product recovered at least partially as phosphorus pentoxide or as thermal phosphoric acid formed by condensing the phosphorus pentoxide vapour with water.

Slag may be continuously or intermittently tapped from the furnace and excess ferrophosphorus may be separately recovered.

Preferably means are provided for draining the furnace prior to shut down for maintenance. Auxiliary heating means may optionally be supplied for starting up the furnace, or the furnace may be started by charging it with molten ferrophosphorus from an external source. As an alternative to the foregoing continuous operation the furnace may be operated batchwise.

The frequency of the alternating electric current will be determined mainly by the cross-sectional area of the iron/phosphorus composition to be heated within the field.

Generally it is possible to use frequencies from 30 to 1800 c/s, or even higher, but the inductive field at higher frequencies penetrates less far through the electrically conductive medium. Thus very high frequencies cause the heat to be generated entlrely on the outside of the iron/phosphorus composi tion, or on the outside of the vessel, where the heated part of the reactor is made of (or lined with) an electrically conductive material such as graphite. Although it is possible to heat the composition in this way, it is more efficient to use lower frequencies which penetrate further into the composition so that heat is generated throughout the volume of composition in the heated part of the furnace. Thus it is possible to use ordinary mains frequency current.Preferably the current has a frequency of from 50 to 500 cs e.g. 50 cs. or 110 cs depending on local mains frequency.

The power of the current will depend on the rate at which heat must be supplied to the furnace to maintain its temperature. In a typical commercial operation the power would be from 500 KW to 100 KW e.g.

2MW to 70MW.

Apparatus for performing the reaction will be described with reference to the drawing which is a sectional elevation of a channel furnace.

The apparatus comprises a furnace (1) provided with a graphite lining (2) and a refractory lagging (3). The furnace (1) has an inlet (4) for solid feed at or near the top, a vapour outlet (5) in the upper part, a slag tap (6) in the lower part, a ferrophosphorus tap (7) below the level of the slag tap (6), and a heating channel (8) communicating at each end with the furnace (1) and disposed coaxially around an electromagnet (9).

In use, the lower part of the furnace (1) is filled with molten ferrophosphorus up to the level of the ferrophosphorus tap. The ferrophosphorus is maintained at a temperature of, for example, 1600"C by passing an alternating electric current through the coil of the electromagnet (9) which produces a forced circulation of molten ferrophosphorus within the furnace (1). A mixture, preferably pelletised, of calcium phosphate rock, anthractie and silica is fed continuously through the inlet (4). Phosphorus vapour is evolved and is withdrawn through the vapour outlet (5) and slag and excess ferrophosphorus are intermittently or continuously tapped through the taps (6) and (7) respectively.

The invention will be illustrated by the following example.

Example I 320 g of pellets containing phosphate rock (28.5% P205), silica and anthracite in the proportions 250:58.2:33.8 respectively. by weight, were placed on the surface of a pool of ferrophosphorus maintained at 16000C with induction heating, by passing a current of 3,000 cs and 25MW helically around the pool. Phosphorus vapour was evolved and air was introduced into this to give P205 as the final product. After 15 minutes no further fume of P20 was noticeable. A representative sample of the 270 g of slag formed was analysed and shown to contain 1.6% of phosphorus. Thus 85% of the total input of phosphorus had been evolved.

WHAT WE CLAIM IS: 1. A method for winning phosphorus from a phosphatic ore which comprises bringing a mixture of the phosphatic ore, an elementary carbon-containing reducing agent and a siliceous flux into thermal contact with a pool of a molten ironphosphorus composition at a temperature sufficient to effect reduction of the phosphatic ore by the reducing agent to form phosphorus vapour together with a siliceous slag, the temperature of the pool being maintained either by electromagnetic induction or by passing an electric current through the iron-phosphorus composition.

2. A process according to Claim 1 wherein the iron phosphorus composition comprises ferrophosphorus formed as a by-product of phosphorus manufacture.

3. A process according to Claim 2 wherein the ferrophosphorus comprises iron together with from 20 to 25% by weight of phosphorus based on the weight of the ferrophosporus.

4. A process according to Claim 1 wherein the iron-phosphorus composition is an alloy of iron or steel with phosphorus or ferrophosphorus which alloy comprises at least 3% by weight of phosphorus based on the weight of the composition.

5. A process according to Claim 4 wherein the alloy comprises from 12 to 30% by weight of phosphorus.

6. A process according to any of the preceding claims wherein the ironphosphorus pool is heated by the passage of an electric current between two electrodes positioned to extend into the pool.

7. A process according to Claim 6 wherein the electrodes take the form of graphite inserts in the side of the reaction vessel at the level of the ferrophosphorus pool.

8. A process according to any of Claims 1 - 5 wherein a part of the molten iron phosphorus composition is continuously withdrawn from the reactor, passed through an alternating electromagnetic field thereby raising its temperature and returned to the reactor.

9. A process according to any of Claims 1 - 5 and 8 wherein the electromagnetic field is generated by passage of an alternating electric current through a coil at a frequency of from 30 to 1,800 cycles per second.

10. A process according to any of Claims 1 - 5 and 8 and 9 wherein the power of the electric current passed through the coil is from 500KW to 100MW.

11. A process according to Claim 10 wherein the power of the electric current is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   tion, or on the outside of the vessel, where the heated part of the reactor is made of (or lined with) an electrically conductive material such as graphite. Although it is possible to heat the composition in this way, it is more efficient to use lower frequencies which penetrate further into the composition so that heat is generated throughout the volume of composition in the heated part of the furnace. Thus it is possible to use ordinary mains frequency current. Preferably the current has a frequency of from 50 to 500 cs e.g. 50 cs. or 110 cs depending on local mains frequency.

   The power of the current will depend on the rate at which heat must be supplied to the furnace to maintain its temperature. In a typical commercial operation the power would be from 500 KW to 100 KW e.g.

   2MW to 70MW.

   Apparatus for performing the reaction will be described with reference to the drawing which is a sectional elevation of a channel furnace.

   The apparatus comprises a furnace (1) provided with a graphite lining (2) and a refractory lagging (3). The furnace (1) has an inlet (4) for solid feed at or near the top, a vapour outlet (5) in the upper part, a slag tap (6) in the lower part, a ferrophosphorus tap (7) below the level of the slag tap (6), and a heating channel (8) communicating at each end with the furnace (1) and disposed coaxially around an electromagnet (9).

   In use, the lower part of the furnace (1) is filled with molten ferrophosphorus up to the level of the ferrophosphorus tap. The ferrophosphorus is maintained at a temperature of, for example, 1600"C by passing an alternating electric current through the coil of the electromagnet (9) which produces a forced circulation of molten ferrophosphorus within the furnace (1). A mixture, preferably pelletised, of calcium phosphate rock, anthractie and silica is fed continuously through the inlet (4). Phosphorus vapour is evolved and is withdrawn through the vapour outlet (5) and slag and excess ferrophosphorus are intermittently or continuously tapped through the taps (6) and (7) respectively.

   The invention will be illustrated by the following example.

   Example I

   320 g of pellets containing phosphate rock (28.5% P205), silica and anthracite in the proportions 250:58.2:33.8 respectively. by weight, were placed on the surface of a pool of ferrophosphorus maintained at 16000C with induction heating, by passing a current of 3,000 cs and 25MW helically around the pool. Phosphorus vapour was evolved and air was introduced into this to give P205 as the final product. After 15 minutes no further fume of P20 was noticeable. A representative sample of the 270 g of slag formed was analysed and shown to contain 1.6% of phosphorus. Thus 85% of the total input of phosphorus had been evolved.

   WHAT WE CLAIM IS: 1. A method for winning phosphorus from a phosphatic ore which comprises bringing a mixture of the phosphatic ore, an elementary carbon-containing reducing agent and a siliceous flux into thermal contact with a pool of a molten ironphosphorus composition at a temperature sufficient to effect reduction of the phosphatic ore by the reducing agent to form phosphorus vapour together with a siliceous slag, the temperature of the pool being maintained either by electromagnetic induction or by passing an electric current through the iron-phosphorus composition.
2. 2. A process according to Claim 1 wherein the iron phosphorus composition comprises ferrophosphorus formed as a by-product of phosphorus manufacture.
3. 3. A process according to Claim 2 wherein the ferrophosphorus comprises iron together with from 20 to 25% by weight of phosphorus based on the weight of the ferrophosporus.
4. 4. A process according to Claim 1 wherein the iron-phosphorus composition is an alloy of iron or steel with phosphorus or ferrophosphorus which alloy comprises at least 3% by weight of phosphorus based on the weight of the composition.
5. 5. A process according to Claim 4 wherein the alloy comprises from 12 to 30% by weight of phosphorus.
6. 6. A process according to any of the preceding claims wherein the ironphosphorus pool is heated by the passage of an electric current between two electrodes positioned to extend into the pool.
7. 7. A process according to Claim 6 wherein the electrodes take the form of graphite inserts in the side of the reaction vessel at the level of the ferrophosphorus pool.
8. 8. A process according to any of Claims 1 - 5 wherein a part of the molten iron phosphorus composition is continuously withdrawn from the reactor, passed through an alternating electromagnetic field thereby raising its temperature and returned to the reactor.
9. 9. A process according to any of Claims 1 - 5 and 8 wherein the electromagnetic field is generated by passage of an alternating electric current through a coil at a frequency of from 30 to 1,800 cycles per second.
10. 10. A process according to any of Claims 1 - 5 and 8 and 9 wherein the power of the electric current passed through the coil is from 500KW to 100MW.
11. 11. A process according to Claim 10 wherein the power of the electric current is

    from 2MW to 70MW.
12. 12. a process according to Claim 1 substantially as hereinbefore described in example 1.