PRODUCTION OF RED PHOSPHORUS

Abstract: Red amorphous phosphorus is formed by thermal conversion of yellow phosphorus by a process wherein thermal conversion is effected while simultaneously the heat of conversion is permitted to effect evaporation of yellow phosphorus. The evaporated yellow phosphorus is conde­ nsed, collected and used in a subsequent conversion batch. The red amorphous phosphorus is obtained substantially free from residual unconverted yellow phosphorus and in a physical form which allows ready further processing to particulate form.

Description: The present invention relates to the production of red amorphous phosphorus by thermal conversion of yellow phosphorus. The production of red amorphous phosphorus by the thermal conversion of yellow phosphorus is well known. In one commercial operation, such conversion is achieved by heating a batch of yellow phosphorus in an enclosed container until significant thermal conversion commences at about 270° to 280°C, thermal conversion is allowed to occur while cooling air or the like is used to remove the heat of conversion until the exotherm subsides, the container is further heated to a temperature of about 370° to 380°C to achieve further conversion of the yellow phosphorus to red phosphorus, and the container then is cooled to room temperature, opened and the red phosphorus recovered. The latter operation is a time consuming one, typically involving about 100 hours for each batch, and does not result in complete conversion of yellow phosphorus to red phosphorus, so that precautions must be taken to avoid release of unconverted yellow phosphorus to the atmosphere and provision for disposal of unconverted yellow phosphorus must be provided. In addition, the red phosphorus is obtained in the form of a hard cake-like compact mass which requires considerable mechanical action to remove the same from the container and to reduce it to particulate form. Efforts have been made to improve on this prior art procedure, in particular to improve the efficiency of conversion of yellow phosphorus to red phosphorus and to provide the process as a continuous one, but to date none of these improvements have reached commercial operation. Representative patents covering such processes are U.S. Patents Nos. 4,152,403 and 4,188,367. In US Patent 2476335 is described a process for separating yellow phosphorus from red phosphorus in which a mixture of red phosphorus in yellow phosphorus is heated while being passed up a heated conveyor to evaporate yellow phosphorus, with crushing of the red phosphorus in the conveyor to give a ground red phosphorus powder. A disadvantage of this process is this crushing operation. Applicants have discovered a process making a soft crumbly red phosphorus. In accordance with the present invention, there is provided a process of forming red amorphous phosphorus by thermal conversion of yellow phosphorus which is a radical departure from existing technology and efforts to date to improve thereon. As mentioned above, the process of conversion of yellow phosphorus to red phosphorus is highly exothermic. In this invention, the heat so generated is permitted to effect volatilization of yellow phosphorus, so that the heat of conversion is dissipated by the volatilization. The volatilized yellow phosphorus is condensed outside the conversion vessel and the condensed yellow phosphorus can be used to form part of the next or subsequent batch which is converted to red phosphorus. Thus from one aspect this invention provides a process for the production of red amorphous phosphorus by the thermal conversion of yellow phosphorus to red phosphorus characterised in that yellow phosphorus is permitted to evaporate from a charge of liquid yellow phosphorus at its boiling point in a reaction vessel and form yellow phosphorus vapour which is evolved from the reaction vessel and is collected in a condenser and simultaneously with the evaporation conversion to red phosphorus takes place in the reaction vessel, with the simultaneous evaporation and conversion occurring subsequently in the reaction vessel until substantial completion of the thermal conversion to red phosphorus. In the present invention, therefore, in complete contrast to prior art procedures, no attempt is made to confine the yellow phosphorus to prevent volatilization but rather this latter is permitted to occur. The volatilization of yellow phosphorus serves to remove heat of conversion, thereby eliminating the necessity for the application of external cooling. At atmospheric pressure, the rate of conversion of yellow phosphorus to red amorphous phosphorus becomes significant at about 260°C with the generation of approximately 86 kcal per kg of yellow phosphorus. Accumulation of the heat increases the mass temperature until the boiling point of yellow phosphorus is reached at 280°C. The temperature stabilizes at the boiling point and vaporization of yellow phosphorus occurs, requiring 96 kcal per kg of yellow phosphorus evaporated. In the present invention, therefore, two processes occur simultaneously, namely thermal conversion of yellow phosphorus to red phosphorus and volatilization of yellow phosphorus which removes the heat of conversion. The volatilized yellow phosphorus can readily be collected by condensation and recycled, so that there is no overall loss of yellow phosphorus. Although a substantial proportion of the initial yellow phosphorus is volatilized, this is not disadvantageous. The physical form of the red amorphous which results in this invention is considerably different from that produced in the prior art. As noted above, in the cited prior art procedure, the red phosphorus is formed as a hard compact mass which requires considerable physical effort to remove the same from the container and reduce it to particulate form. The red phosphorus product of this invention, in contrast, has considerable porosity and is soft. The product is readily removed from the reaction vessel and requires very little effort to provide the same in useful particulate form. In a preferred embodiment of the invention, the overall rate of production of read amorphous phosphorus is improved, as compared with the cited prior art process, despite the necessity in the present invention to recover and recycle a considerable proportion of the initial charge of yellow phosphorus. In this preferred embodiment, the charge of yellow phosphorus is provided in a vessel which is introduced into a partially preheated reaction zone. The contents of the vessel then are rapidly heated to a temperature at which the rate of conversion become significant by increasing the temperature of the reaction vessel. The evaporated yellow phosphorus is removed from the reaction vessel and condensed while conversion to red phosphorus occurs in the reaction vessel. Once the conversion reaction is complete, the reaction vessel may be removed from the reaction zone without the necessity of awaiting complete cooling of the reaction vessel. The vessel containing converted red phosphorus is cooled outside the reaction zone while a further batch of yellow phosphorus can be introduced to the reaction zone for conversion. An inert gas, such as, nitrogen, usually is passed into the vessel to flush yellow phosphorus vapour therefrom and to provide an inert atmosphere during cooling. This semi-continuous type mode of operation considerably decreases the overall processing time necessary to produce red phosphorus. In this preferred procedure, the vessel housing the yellow phosphorus may take the form of an openable closed vessel having a vent for yellow phosphorus vapour which communicates with a vapour outlet of the reaction zone, which in turn communicates with a yellow phosphorus condenser. Rapid heating of the vessel within the reaction zone may be effected using any convenient heat source, such as, molten lead. In a preferred embodiment a mass of yellow phosphorus covered with water is confined in an enclosed reaction vessel, the enclosed reaction vessel is introduced into a reaction zone in heat conducting relationship with a body of molten lead having an elevated temperature, a vapor flow path is established from the reaction vessel to the exterior of the reaction zone, the reaction vessel is heated by the molten lead as the lead cools from the elevated temperature to a set point of about 290°C to effect firstly evaporation of the water from the enclosed reaction vessel and subsequently simultaneous thermal conversion of yellow phosphorus to red amorphorus and evaporation of yellow phosphorus at the boiling point thereof until the thermal conversion substantially ceases to occur, the yellow phosphorus vapour is conveyed through the vapour flow path and is condensed, the lead and the reaction vessel are heated to a temperature above the atmospheric pressure boiling point of yellow phosphorus, for a time sufficient to evaporate substantially all residual unconverted yellow phosphorus, the reaction vessel is removed from the reaction zone, and the red amorphous phosphorus is recovered from the reaction vessel. The invention is described further, by way of illustration, with reference to the accompanying drawings, in which: Figure 1 is a schematic elevational view of one embodiment of apparatus for carrying out the process of the invention; and Figure 2 is a close-up of a portion of the apparatus of Figure 1. Referring to the drawings, a yellow-to-red phosphorus conversion reactor 10 is provided. This reactor 10 is a modified form of the piece of equipment disclosed in U.S. Patent No. 4,081,333 which is used for the distillation of small quantities of phosphorus from phosphorus mud, a by-product of the conventional thermal conversion process, at high temperatures of typically from about 400° to about 600°C. The conversion reactor 10 has a vapor outlet 12 which communicates with a condenser 14 for condensation of evaporated yellow phosphorus. The reactor 10 has a lid 16 with a depending peripheral flange 18 which is received in a channel 20 formed at the top of the vessel walls 22 to effect sealed closure of the reactor 10. Electrical heating elements 24 are embedded in the base, walls and lid of the reactor 10 to effect heating of the reactor and its contents. The reactor 10 contains a supply of molten lead 26 to act as a heat transfer agent. The use of molten lead in this illustrated embodiment of the invention is but one example of a wide variety of heating sources which may be used to effect thermal conversion of yellow phosphorus. Other heat transfer media which are liquid at the desired temperatures may be used, or, alternatively, an electrically-heated place may be used. When used to effect thermal conversion of yellow to red phosphorus a portable skip 28, housing the yellow phosphorus, is positioned in the reactor 10 suspended from the lid 16 and in contact with the molten lead 26. The skip 28 is an enclosed container and has a vapor off-take pipe 30 communicating with the vapor outlet 12. The skip 28 may also be provided with a central opening 32 for the passage of inert gas to the skip 28 to remove phosphorus vapor from the skip upon completion of the reaction. In operation of the illustrated apparatus, the skip 28 is filled with yellow phosphorus, usually under a water blanket, and closed. The lead mass 26 is heated to a temperature above its melting point and to a temperature such that the sensible heat of the molten lead is sufficient to effect the desired heating of the skip 28, typically to about 450°C, and the skip 28 is suspended from the lid 16 and inserted into the reactor 10 in contact with the bath of molten lead with the vapor off-take pipe 30 in communication with the vapor outlet 12. As energy is absorbed by the skip 28 and its contents, the lead cools and its temperature stabilizes at the set point of 290°C. When the contents of the skip 28 heat up to 100°C, evaporation of the water occurs, forming an inert atmosphere within the skip 28 and also the reactor 10. Once all the water has been evaporated, the yellow phosphorus is heated by the molten lead to the boiling point of yellow phosphorus at 280°C. Simultaneous conversion of yellow phosphorus to red phosphorus and evaporation of yellow phosphorus occur at this temperature. The evaporated yellow phosphorus is conducted by off-take pipe 30 to the vapor outlet 12 and thence to the condenser 14 for condensation therein. The use of the off-take pipe 30 minimizes phosphorus deposits on the inner walls of the reactor. The distillation of yellow phosphorus is prominent during the early stages of the reaction and the reaction rate may be measured as a function of the quantity of yellow phosphorus recovered in the condenser 14. Monitoring of the reaction temperatures, quality of sampled vapors and the rate of recovery of yellow phosphorus in the condenser 14 may be used to determine the termination of the conversion reaction. The lead bath 26 then is heated to a temperature above the atmospheric pressure boiling point of yellow phosphorus and preferably to about 320° to 330°C, to effect volatilization of residual quantities of yellow phosphorus. The reactor 10 is opened and the skip 28 is lifted from the reactor. A purge of nitrogen or other inert gas is inserted through the cover of the skip 28 through opening 32 to drive out the last traces of phosphorus vapour and to ensure that an inert atmosphere is maintained during cooling. The skip 28 is detached from the lid 16 and allowed to cool. Once the skip 28 is detached from the lid 16, a further skip containing yellow phosphorus may be attached thereto and inserted into the reactor 10, after the lead has been reheated to its initial temperature. Conversion of a further batch of yellow phosphorus to red phosphorus, therefore, can be effected while the first batch is still cooling, thereby considerably decreasing the overall processing time. In a typical operation, the 280°C temperature is reached in about 7 hours, thermal conversion and yellow phosphorus evaporation occurs for about 7 hours at 280°C, heating to 320°C takes about 6 hours and subsequent heating at about 320° to 330°C is effected for about 5 hours, providing a total processing time of 25 hours compared with about 100 hours for the typical commercial operation discussed above. The condensed yellow phosphorus collected in the condenser 14 may be reutilized in a subsequent batch conversion. When the skip 28 has cooled sufficiently, water is normally added and the cover is removed for access to the red amorphous phosphorus. The red amorphous phosphorus is obtained as a soft crumbly, mass which requires little further processing to provide the desired particulate form. It is somewhat surprising that the red amorphous phosphorus is obtained in such a useful form directly from the conversion process. The invention is illustrated further by the following Example: Example Thermal conversion of yellow phosphorus to red amorphous phosphorus (RAP) was carried out using the apparatus of Figure 1. A 3.5 meter diameter and 90 cm deep skip was charged with 4250 kg of yellow phosphorus and covered with 70 kg of water. The lead bath in the reactor was heated to 450°C and the skip was lowered into the reactor into contact with the molten lead bath. Throughout the experiment, a purge of nitrogen of 215 litres/min was maintained. Temperature sensors were applied to the skip and located in the molten lead and the temperatures monitored. In addition, the quality of the off-gas stream and the quantity of condensate collected were monitored during the process. The temperature in the evaporation of the water occurred. Thereafter with the lead bath temperature having dropped to 290°C, the temperature of the phosphorus rose to 280°C at which temperature the bulk of the conversion of yellow phosphorus to red phosphorus and evaporation of yellow phosphorus occurred. Upon completion of the conversion, the lead bath temperature was raised to 330°C to effect further distillation of unconverted yellow phosphorus. The skip thereafter was removed from the reactor and air and water cooled to ambient temperature. The temperature/time profile is reproduced in the following Table I: Temperature (°C) Time (hrs) Notes Skip Lead 20 450 0 100 400 0.5 Evaporation of water commenced 100 350 2 110 350 3.5 Evaporation of water ceased 280 290 7 RAP conversion commenced 280 290 13 RAP conversion ceased 330 330 19 Residual P₄ distillation commenced 330 330 24 Residual P₄ distillation ceased 20 - 30 The red amorphous product obtained was a reddish colour, porous and easily fractured and occupied a depth of 19 cm in the skip, indicating a bulk density of 1.014 g/cc. 1550 kg of red amorphous phosphorus was obtained, corresponding to 36.5 wt% of the initial charge. 2550 kg of yellow phosphorus were collected in the condenser, corresponding to 60 wt% of the initial charge. Total power consumption during the run was 5500 MJ, or approximately 3500 MJ/tonne of red amorphous phosphorus, which is considerably less than a typical power consumption of 10,000 MJ/tonne of red amorphous phosphorus in the case of the cited prior art procedure. Samples of the red amorphous phosphorus from different parts of the skip were tested for residual yellow phosphorus content. The results are reproduced in the following Table II: No. Sample Location P₄ (ppm) 1 Core at top 258 2 Core at bottom 230 3 at surface on opposite side from take-off port 229 4 at surface below take-off line 195 5 at surface at edge 206 The results reproduced in Table II indicate a relatively low residual yellow phosphorus content in all samples. In summary of this disclosure, the present invention provides a novel method of forming red amorphous phosphorus which does not suffer from the drawbacks of the cited prior art procedure and permits formation of the product in a readily-processable form in a much shorter reaction cycle. Modifications are possible within the scope of this invention.

Claims: A process for the production of red amorphous phosphorus by the thermal conversion of yellow phosphorus to red phosphorus characterised in that yellow phosphorus is permitted to evaporate from a charge of liquid yellow phosphorus at its boiling point in a reaction vessel and form yellow phosphorus vapour which is evolved from the reaction vessel and is collected in a condenser and simultaneously with the evaporation conversion to red phosphorus takes place in the reaction vessel, with the simultaneous evaporation and conversion occurring subsequently in the reaction vessel until substantial completion of the thermal conversion to red phosphorus. A process according to claim 1 characterised in that the thermal conversion is initiated by heating the yellow phosphorus in an enclosed reaction vessel by means of an external heating source to the boiling point of the yellow phosphorus and the yellow phosphorus vapour is evolved from the reaction zone and condensed. A process according to claim 1 or 2 characterized in that a mass of yellow phosphorus covered with water is confined in an enclosed reaction vessel, the enclosed reaction vessel is introduced into a reaction zone in heat conducting relationship with a body of molten lead having an elevated temperature, a vapor flow path is established from the reaction vessel to the exterior of the reaction zone, the reaction vessel is heated by the molten lead as the lead cools from the elevated temperature to a set point of about 290°C to effect firstly evaporation of the water from the enclosed reaction vessel and subsequently simultaneous thermal conversion of yellow phosphorus to red amorphorus and evaporation of yellow phosphorus at the boiling point thereof until the thermal conversion substantially ceases to occur, the yellow phosphorus vapour is conveyed through the vapour flow path and is condensed, the lead and the reaction vessel are heated to a temperature above the atmospheric pressure boiling point of yellow phosphorus, for a time sufficient to evaporate substantially all residual unconverted yellow phosphorus, the reaction vessel is removed from the reaction zone, and the red amorphous phosphorus is recovered from the reaction vessel. A process according to claim 3 characterised in that yellow phosphorus is evaporated at its atmospheric pressure boiling point of above about 280°C and the temperature above the atmospheric pressure boiling point to which the reaction vessel is subsequently heated is about 320°C to about 330°C. A process according to any of the preceding claims characterised in that the external heating source comprises a heat transfer medium having a temperature sufficient to permit the yellow phosphorus to be heated to the boiling temperature, said heat transfer medium being liquid at the boiling temperature. A process according to claim 5 characterised in that the heat transfer medium is molten lead and the molten lead cools from an initial temperature towards its set point as the yellow phosphorus is heated to the boiling temperature. A process according to any of claims 3 - 6 characterised in that an inert gas stream flows into the reaction vessel after the evaporation of the residual yellow phosphorus to remove residual traces of yellow phosphorus vapour and to provide an inert atmosphere in the reaction vessel. A process according to any of claims 3 to 7 characterised in that the reaction vessel is removed from the reaction zone, cooled and opened to permit the recovery of red amorphous phosphorus. A process according to claim 8 characterised in that after the removal of the reaction vessel from the reaction zone a second reaction vessel is introduced into the reaction zone.

Inventors: RYAN, KEVIN PATRICK, YANK, CARL DENIS
Applicants: TENNECO CANADA INC.
Application year: 1984
Main classification: C01B25
IPC codes: C01B25