CA1091896A - Production of red phosphorus - Google Patents

Abstract

PRODUCTION OF RED PHOSPHORUS

ABSTRACT OF THE DISCLOSURE
Red phosphorus, usually in amorphous form, is obtained by heating white phosphorus at a temperature of between 250° and 590°C while the phosphorus is subjected to a pressure greater than the vapor pressure of white phosphorus at the heating temperature. In this way, formation of a phosphorus vapor phase during the conversion is avoided.
This may be achieved by feeding liquid white phosphorus into an extrusion head heated to the reaction temperature while sub-jecting the white phosphorus to a pressure in the extrusion head, the red phosphorus being removed into an inert atoms-phere. Alternatively, molten white phosphorus in droplet form may be pumped into an inert immiscible heat exchange fluid at the reaction temperature and particulate red phosphorus separated from the fluid.

Description

~V91896 This invention relates to the production of red phosphorus.

Red phosphorus in mainly amorphous form is pre-pared in a batch procedure by heating white phosphorus in a substantially inert atmosphere above the temperature at which white phosphorus converts to the red form at a practical rate, typically to about 250C. This procedure has many drawbacks, typically of an environmental nature arising from incomplete conversion of white phosphorus to red phosphorus. Thus, toxic phosphorus fumes may be -~ emitted on opening the reactor, removal of the residual white phosphorus, typically by addition of soda ash followed by leaching produces a phosphate solution requiring disposal, and the disintegration of the red phosphorus product for removal from the reactor by chipping leads to dust forr~'cion and hazards to operators.
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In accordance with the present invention, white ,........................... .
;. phosphorus is heated under pressure to form red phosphorus, usually in amorphousform~under conditions such that only a very small phosphorus vapor phase or no vapor phase at all is formed. In this way, the present invention eliminates or at least minimizes the prior art problems and further enables the production of red phosphorus from white phos-.
phorus to be achieved in a continuous manner.

~ The white phosphorus generally is heated, by ; externally applied heat and/or by heat of reaction, in the range of about 250 to about 590C in the process of the invention, preferably in range of about 450 to about 550C,

- 2 -1(~9~896 while applying a pressure greater than the vapo~ pressure of white phosphorus at the reaction temperature. Typical of such vapor pressures are 180 psi at 450 and 465 psi at 550C. The pressure may be applied to the white phosphorus in a variety of ways.
In one embodiment of the invention, a continuous operation may be provided and the pressure may be applied to the white phosphorus by pumping liquid white phosphorus into an extrusion head maintained at the desired reaction temperature and from which the red phosphorus is extruded, generally into an inert atmosphere.
The white phosphorus feed may be admixed with re-cycled red phosphorus to absorb some of the lieat of con-version generated in the reaction.

Any suitable mechanical disintegration technique may be employed on the product, ater suitable cooling of the product, if desired, to provide the red phosphorus in particulate form.
The minimum residence time of the phosphorus in the extrusion head in this embodiment is determined by the kinetics of the conversion reaction at the particular reac-tion temperature. For example, 99.9% conversion to red phosphorus is achieved in 0.1 minutes at 500C.
In another embodiment of the invention, the white phosphorus is pumped in particulate form illtO a pressurized inert immiscible heat exchange fluid which applies the required pressure to the phosphorus during the conversion reaction. Following completion of the conversion to red phosphorus, the particulate red phosphorus is separated from the fluid.

; 30 One example of a suitable heat exchange fluid for use in this embodiment of the invention is a bath of molten lead. Due to the exothermic nature of the con-1()91896 version reaction, after an initial heating of the bath, cooling only m~y be necessary to maintain the desired reaction temperature.
Lead has a sufficiently low melting point, is inert to phosphorus and has an adequate heat conductivity ~hat it serves as an admirable heat exchange fluid.

The temperatures used in the present invention are conducive to reaction of phosphorus with water and hence, it may be necessary to dehydrate the white phos-phorus prior to pumping into the extrusion head. When such dehydration is desired, it may, for example~ be achieved by heating the white phosphorus to a temperature slightly above 100C with an inert gas sweeping the surface.

The invention i~ described further by way of illus-tration with referenc~ to the accompanying drawings, in which:
Figure 1 is a schematic representation of one embodiment of the invention;
Figure 2 is a graphical representation of the extent of conversion of white phosphorus to red ; ~ phosphorus against time at various temperatures; and Figure 3 is a graphical representation of the vapor pressure of phosphorus at various temperatures.

As seen in Figure 1, white phosphorus having a temperature of above about 100C is situated in a con-tainer 10 through which an inert gas is passed through inlet 12. A submerged pump 14 pumps the white phosphorus into and through an extrusion head 16, having a heater jacket 18.

109~89~i The extrusion head 16 preferably has a tempera-ture of about 450 to 550C and extrudes the product red phosphorus into a cooling chamber 20, into which an inert gas is passed through inlet 22.
The pumping pressure and residence time in the extrusion head 16 are controlled first to minimize phosphorus vapor volume at the prevailing temperature,as determined from Figure 3, and second to allow substantially complete conversion to red phosphorus as determined from Figure 2.
A cutting head 24, or other mechanical disin-tegration means, is located in the cooling chamber 20 to break up or comminute the red phosphorus as it is extruded to provide an accumulation 26 of particulate product in the cooling chamber 20 for removal from time to time.
The gaseous materials from the container 10 and the cooling chamber 20 pass by pipes 28 and 30 into a water seal 32 which prevents the escape from the system of any phosphorus fumes which may be present in the gas streams.
; 20 The gases are cooled by a condensor 34 prior to entry into the water seal 32.
It will be seen that this procedure is non-polluting since substantially complete conversion of white to red phosphorus is achieved and the escape of phosphorus fumes is avoided.
Further, continuous production of red phosphorus may be carried out with a minimum of labor requirements and in simple manner.
Additionally, by using filtered white phosphorus and stainless steel extrusion equipment, it is possible to prepare a red phosphorus product which is low in iron and copper, which traditionally have caused instability and firing of red phosphorus.

The present invention, therefore, provides a simple and convenient non-polluting manner of forming red phosphorus by exothermic conversion of white phosphorus.
Modifications are possible within the scope of the invention.

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the production of red phosphorus which comprises heating white phosphorus at a temperature above the white to red phosphorus conversion temperature while subjecting said white phosphorus to a pressure above the vapor pressure of white phosphorus at the heating tem-perature, and confining said white phosphorus during said heating step such that no phosphorus vapor phase is formed.

2. The process of claim 1 wherein said heating temperature is about 250° to about 590°C.

3. The process of claim 2 wherein said heating temperature is about 450° to about 550°C.

4. The process of claim 1 carried out continuously by feeding liquid white phosphorus into an extrusion head heated to the reaction temperature, subjecting the liquid white phosphorus in the extrusion head to a pressure exceeding the vapor pressure of white phosphorus at said reaction temperature, maintaining said white phosphorus in said extrusion head for a period of time exceeding that required for substantially complete conversion of white phosphorus to red phosphorus at said reaction temperature, and removing red phosphorus from said extrusion head.

5. The process of claim 4, including subdividing said red phosphorus as it is extruded from the extrusion head into an inert atmosphere.

6. The process of claim 4, including drying said white phosphorus before feeding the same to said extrusion head.

7. The process of claim 6 wherein said drying is carried out by heating the white phosphorus to a temperature above 100°C and sweeping an inert gas over the surface of the heated white phosphorus.

8. The process of claim 7, including extruding said red phosphorus into a cooled vessel through which an inert gas flows, removing inert gas from said cooled vessel, combining said removed inert gas with inert gas from said sweeping and forwarding said combined stream through a water seal to scrub phosphorus therefrom.

9. The process of claim 4 including filtering said white phosphorus prior to feeding the same into said extrusion head and using stainless steel as the material of construction of said extrusion head.

10. The process of claim 4 including mixing said liquid white phosphorus with recycled red phosphorus prior to feed to said extrusion head.

11. The process of claim 1 carried out by feeding liquid white phosphorus in droplet form into an inert immiscible heat exchange medium heated to the reaction temperature, subjecting said droplets of white phosphorus to a pressure exceeding the vapor pressure of white phosphorus at said reaction temperature in said heat exchange medium, maintaining said droplets in said heat exchange medium at said reaction temperature for a period of time exceeding that required for substantially complete conversion of white phosphorus to red phosphorus, and separating the resulting particulate red phosphorus from said heat exchange medium.

12. The process of claim 11 wherein said heat exchange medium is a molten bath of lead.