Preparation of high purity phosphorus

High purity phosphorus and phosphorus compounds are prepared by first reacting H.sub.3 PO.sub.4 with a lead compound such as PbO to form Pb.sub.3 (PO.sub.4).sub.2. The Pb.sub.3 (PO.sub.4).sub.2 is reduced with H.sub.2 at a temperature sufficient to form gaseous phosphorus which can be recovered as a high purity phosphorus product. Phosphorus compounds can be easily prepared by reacting the phosphorus product with gaseous reactants. For example, the phosphorus product is reacted with gaseous Cl.sub.2 to form PCl.sub.5. PCl.sub.5 is reduced to PCl.sub.3 by contacting it in the gaseous phase with solid elemental phosphorus. POCl.sub.3 can be prepared by contacting PCl.sub.5 in the gaseous phase with solid P.sub.2 O.sub.5. The general process is particularly suitable for the preparation of radiophosphorus compounds.

High purity phosphorus is conventionally prepared by heating phosphate rock 
in the presence of sand and coke in an electric furnace at 
2300.degree.-2700.degree. C. See, for example, Mellor's Modern Inorganic 
Chemistry, G. D. Parks, ed., John Wiley and Sons, Inc., New York (1961) pp 
811-813. The high temperatures make the electric furnace process very 
expensive. Crude phosphoric acid, such as wet process phosphoric acid, is 
relatively inexpensive. Wet process phosphoric acid is prepared by 
reacting phosphate rock with sulfuric acid to produce phosphoric acid and 
a precipitate of calcium sulfate. An inexpensive low temperature process 
for producing elemental phosphorus from phosphoric acid has long been 
needed. 
Phosphorus compounds such as PCl.sub.3, PCl.sub.5, POCl.sub.3, etc. are 
useful in the chemical industry for a variety of applications. For 
example, PCl.sub.3 and PCl.sub.5 are routinely used to transform hydroxyl 
groups of organic compounds into chlorides. High purity phosphorus 
compounds are useful in the manufacture of pharmaceuticals, particularly 
radioactively labeled compounds useful in tracer studies and potentially 
useful for detection and treatment of cancerous tissue. Incorporation of 
radioisotopes of phosphorus (.sup.33 P and .sup.32 P) into 
radiopharmaceuticals which concentrate in malignant neoplasms offers 
potential therapeutic advantage because the beta radiation (particularly 
the short range beta of .sup.33 P) should provide maximum irradiation of 
the target cells with minimal damage to contiguous healthy tissue. 
Processes for the manufacture of radioactive chemicals must be capable of 
handling small amounts (millimoles) of materials--since very high specific 
activity is required and should be high in yield since expensive 
radioisotope preparations are used. In addition, such processes must 
require a minimum of manual manipulation since they are ordinarily carried 
out in shielded environments, glove boxes, etc. High temperatures, above 
about 1000.degree. C., are also to be avoided since they complicate remote 
handling, etc. 
Several methods for preparing radiolabeled phosphorus compounds, such as 
.sup.32 PCl.sub.3, .sup.32 PCl.sub.5 and .sup.32 POCl.sub.3 have been 
described in the prior art. The reduction of phosphoric acid or alkaline 
earth phosphates to liberate elemental phosphorus has been carried out 
using carbon at high temperatures as the reductant followed by 
chlorination to produce PCl.sub.3, PCl.sub.5 and POCl.sub.3. See J. A. 
Kafalas, J. Am. Chem. Soc., 79:4260 (1957); J. R. Bocquet, Ann. Soc. Roy. 
Sci. Med. Nat. Brussels, 9:161 (1956); and R. A. Oosterbaan, et al, J. Am. 
Chem. Soc., 78:5641 (1956). Another method involved the direct production 
of phosphorus by carbon reduction of phosphates and simultaneous 
chlorination, see B. Witten et al, J. Am. Chem. Soc., 70:399C (1948). The 
chief difficulties with these prior art processes for preparing 
radiophosphorus compounds were the requirements for high temperatures, 
long reaction times, difficult controls and the direct handling of 
radioactive materials required. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a process for preparing high 
purity phosphorus and phosphorus compounds. 
It is a further object to provide a process which requires minimum mixing 
and manual manipulation. 
It is a further object to provide a low temperature process for preparing 
high purity elemental phosphorus from impure phosphoric acid, such as wet 
process phosphoric acid. 
It is a further object to provide a process which is capable of producing 
high purity phosphorus and phosphorus compounds in very high yields, above 
75%. 
It is a further object to provide a process which is readily adaptable to 
the production of radioactively labeled phosphorus and phosphorus 
compounds. 
These and other objects are achieved according to this invention in a 
method for preparing phosphorus or phosphorus compounds comprising the 
steps of reacting H.sub.3 PO.sub.4 with a lead compound to provide a 
Pb.sub.3 (PO.sub.4).sub.2 intermediate and reacting said Pb.sub.3 
(PO.sub.4).sub.2 intermediate with H.sub.2 at a temperature sufficient to 
cause the formation of gaseous phosphorus. The phosphoric acid starting 
material can be pure, radioactively labeled or impure such as wet process 
phosphoric acid. The gaseous phosphorus product can be condensed and 
recovered as high purity solid phosphorus. Alternatively, the phosphorus 
product can be reacted with suitable gaseous reagents to form phosphorus 
salts or other compounds. For example, the evolved phosphorus can be 
reacted with Cl.sub.2 to form PCl.sub.5 or PCl.sub.3. The PCl.sub.5 
product can be converted to PCl.sub.3 by contacting with excess elemental 
phosphorus or it can be reacted with P.sub.2 O.sub.5 to form POCl.sub.3. 
DETAILED DESCRIPTION 
In the first step of this process, H.sub.3 PO.sub.4 is reacted with a lead 
compound such as PbO to form Pb.sub.3 (PO.sub.4).sub.2. The acid 
concentration is not critical to the reaction, with 0.1 M to 100% acid 
suitable for this step. Other lead compounds such as Pb(NO.sub.3).sub.2 
and Pb(CH.sub.3 COO).sub.2 which react with H.sub.3 PO.sub.4 to form 
Pb.sub.3 (PO.sub.4).sub.2 may be used, however, these salts, upon reaction 
with H.sub.3 PO.sub.4, tend to form oxy compounds which form as films or 
indeterminate crystals, and do not react efficiently with gaseous 
reductants. The reaction of H.sub.3 PO.sub.4 with PbO produces a 
crystalline precipitate which is easily handled and which has a high 
surface area for reaction with H.sub.2. The use of PbO in the process 
results in phosphorus or phosphorus compound yields of greater than 90% 
while Pb(NO.sub.3).sub.2 and Pb(CH.sub.3 COO).sub.2 would result in yields 
less than 50%, normally about 25%. Accordingly, the process will be herein 
illustrated using PbO as an initial reactant with the understanding that 
any compound which forms Pb.sub.3 (PO.sub.4).sub.2 upon reaction with 
H.sub.3 PO.sub.4 will be operable in the process with the likelihood of 
lower yields. Of course, phosphoric acid precursors such as P.sub.2 
O.sub.5 and H.sub.2 O may also be reacted with the lead compound to form 
the Pb.sub.3 (PO.sub.4).sub.2 reaction product. 
The Pb.sub.3 (PO.sub.4).sub.2 is reduced to elemental P and Pb with H.sub.2 
at about 600.degree. to 800.degree. C. We have found that the hydrogen 
reduction of Pb.sub.3 (PO.sub.4).sub.2 either proceeds at a lower 
temperature or produces a higher yield than hydrogen reduction of the 
phosphates of Ca, Mg, Al, La, Zn and Bi. The reduction of Pb.sub.3 
(PO.sub.4).sub.2 with CH.sub.4 produced only one-half the yield of H.sub.2 
reduction. According to the process of this invention, gaseous H.sub.2 may 
be passed over a refractory boat such as quartz which contains the 
Pb.sub.3 (PO.sub.4).sub.2 product. Water formed in the reaction passes out 
as vapor in the H.sub.2 stream. If this reaction is carried out in a tube 
furnace or other continuous flow reaction vessel, the phosphorus reaction 
product will collect as a solid in the cool part of the tube or in a 
condenser and Pb metal will remain in the boat. High purity phosphorus 
compounds are easily prepared by contacting the elemental phosphorus 
reaction product with gaseous reactants such as Cl.sub.2, Br.sub.2, etc. 
The reaction of solid phosphorus with Cl.sub.2 proceeds rapidly at ambient 
temperatures. If PCl.sub.3 is a desired reaction product, it is preferred 
to react the elemental phosphorus with excess chlorine to provide 
PCl.sub.5, which can then be reduced by reaction with elemental phosphorus 
in the absence of chlorine to provide PCl.sub.3. The reduction of 
PCl.sub.5 to PCl.sub.3 is easily accomplished by heating PCl.sub.5 to 
cause it to sublime into a carrier gas (Ar, N.sub.2, He, etc.) and passing 
the vapor at ambient temperature through a reductor containing red 
elemental phosphorus. An efficient method of carrying out this reaction is 
to pass PCl.sub.5 vapor through a reactor containing inert aggregate 
material such as silica or borosilicate glass beads which have been coated 
with high purity red phosphorus. The reduction product PCl.sub.3 will 
remain in the vapor phase and can then be recovered in a condenser at 
-77.degree. C. One can also produce PCl.sub.3 by contacting PCl.sub.5 with 
P.sub. 2 O.sub.5. In this embodiment, PCl.sub.5 is deposited from the 
vapor phase onto a layer of P.sub.2 O.sub.5 solid, heated to 
.about.100.degree. C., whereupon POCl.sub.3 is vaporized into a carrier 
gas and condensed at 0.degree. C. 
According to this invention, elemental phosphorus can be produced from wet 
process phosphoric acid in a semicontinuous process. The wet process acid 
is reacted with PbO in a continuous reactor, preferably under ultrasonic 
agitation. The Pb.sub.3 (PO.sub.4).sub.2 product can be recovered via 
centrifugation and the unreacted H.sub.3 PO.sub.4 returned to the reactor. 
The separated Pb.sub.3 (PO.sub.4).sub.2 cake can be passed through a 
continuous dryer such as a kiln and then ground to a smaller particle 
size, if desired. The Pb.sub.3 (PO.sub.4).sub.2 product can then be passed 
through a continuous hydrogen reduction furnace where it is contacted with 
H.sub.2 and an inert carrier gas at temperatures up to about 800.degree. 
C. During the H.sub.2 reduction water is evolved. It may be desirable in 
some cases to regulate the reaction to allow all the water to be removed 
prior to phosphorus evolution, e.g., to prevent premature cooling of the 
product. This can be achieved by regulating the heating rate of the 
furnace or utilizing a multitemperature zoned furnace. The reaction 
product gas, which contains elemental phosphorus carrier gas, unreacted 
H.sub.2, and in some cases H.sub.2 O, can be passed through a water trap 
where it is cooled to just above the melting point of elemental white 
phosphorus (44.1.degree. C.). The liquid phosphorus can be tapped and cast 
into sticks. Molten lead is recovered from the furnace and cast. The slag 
is removed and the lead can be reoxidized to provide PbO which is returned 
to the reactor. 
In many situations, it may be desirable to remove certain impurities from 
the wet process acid prior to treatment according to the present process. 
Sulfate and many cations can be removed by conventional cation and anion 
exchange processes. Uranium values can be recovered by any of several 
prior art techniques such as by the method described in commonly assigned 
U.S. Pat. No. 3,835,214.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The process of this invention will be described in detail in reference to 
the preparation .sup.33 P-labeled PCl.sub.3 and it will be apparent to 
those skilled in the art that minor modifications can be made to this 
process to provide for recovery of PCl.sub.5, POCl.sub.3 or elemental 
phosphorus. Of course, purification steps can be eliminated in the 
preparation of non-radioactive materials, where process economics are not 
so dependent upon yield. 
While .sup.32 P has been successfully used in radiopharmaceuticals, .sup.33 
P which has a much lower beta energy and a longer half-life would be 
desirable in many applications. .sup.33 P is produced by an (n, .alpha.) 
reaction with about 60% isotopicly enriched .sup.36 Cl and is recovered 
carrier free (without nonradioactive phosphorus) as the orthophosphate. 
Both .sup.32 P and .sup.33 P in the form of phosphoric acid are available 
from the Isotope Sales Department of the Oak Ridge National Laboratory, 
Oak Ridge, Tenn. 37830. .sup.33 P is normally supplied as a hydrochloric 
acid solution containing phosphate ions. Because of its very high specific 
activity a curie of .sup.33 P has negligible mass and therefore must be 
used with carrier phosphorus or phosphorus compounds to facilitate 
handling during chemical processing. This is readily accomplished merely 
by mixing a known amount of labeled H.sub.3 PO.sub.4 with nonradioactive 
H.sub.3 PO.sub.4. Accordingly, any process for preparing .sup.32 P or 
.sup.33 P compounds should have the capability of using H.sub.3 PO.sub.4 
as a starting material. 
FIG. 1 is a diagram of laboratory scale apparatus suitable for carrying out 
the process of this invention for preparing PCl.sub.3. Reduction tube 1 
contains quartz boat 2 containing Pb.sub.3 (PO.sub.4).sub.2 product from 
the reaction of H.sub.3 PO.sub.4 with suitable lead compound. The 
reduction tube is contained at least partially in a conventional split 
tube furnace 3. In communication with the entrance 4 of the reduction tube 
is a gas manifold 5 to which is supplied Cl.sub.2, H.sub.2 and argon. 
H.sub.2 and Ar are metered through flow meters 6 and 7. Cl.sub.2 need not 
be metered since the progress of the chlorination reaction can be observed 
visually. At the exit of the reduction tube is product collection region 8 
which communicates to U-tube 9 containing aggregate material 10 coated 
with red phosphorus. The exit of the U-tube communicates with product 
collection tubes 11 having condensing regions 12, contained in Dewar 
bottles 13 and 14. Valve 15 permits the recovery of PCl.sub.5 which can 
bypass the U-tube reactor if desired and be recovered as a product. Sweep 
gases and unreacted H.sub.2 escape through exit 16. 
A. Preparation of Pb.sub.3 (PO.sub.4).sub.2 
Carrier-free .sup.33 P obtained in the form of orthophosphoric acid and 1 N 
in HCl is subjected to prepurification by cation exchange to assure that 
no cations other than hydrogen are present. This prepurification step is 
suggested since other cations such as Ca or Mg may form phosphates which 
will not reduce at the low temperature of the process, thus lowering the 
yield, though not necessarily the purity of the product. The carrier 
solution of H.sub.3 PO.sub.4 is made up with a known amount of phosphorus. 
Carrier solution equivalent to 20 mg phosphorus is added to a 50 ml 
polytetrafluoroethylene coated beaker containing the desired amount of 
carrier .sup.33 P-H.sub.3 PO.sub.4 solution. The beaker contents are 
evaporated down to incipient dryness several times with a small volume of 
concentrated nitric acid in order to remove all chlorides. Lead chloride 
is quite insoluble and coprecipitates with Pb.sub.3 (PO.sub.4).sub.2, thus 
lowering the yield. The residual material is taken up in a minimal amount 
of water for transfer to a 76.times.16.times.10 mm silica boat containing 
238 mg PbO (1.06 mmoles) spread in a thin layer on the bottom. The boat is 
partially immersed in an ultrasonic water bath, Cole-Parmer Model No. 
8.845-30, Cole-Parmer Instrument Co., 7425 North Oak Park Ave., Chicago, 
Ill. 60648. After approximately one hour under ultrasonic agitation at 
room temperature the metathesis has proceeded essentially to completion. 
The boat is then placed under a heat lamp, the water is evaporated off, 
and the Pb.sub.3 (PO.sub.4).sub.2 is dried for one hour. The reaction is 
represented by: 
EQU 3PbO+2H.sub.3 PO.sub.4 .fwdarw.Pb.sub.3 (PO.sub.4).sub.2 +3H.sub.2 O (1) 
B. Reduction of Pb.sub.3 (PO.sub.4).sub.2 with Hydrogen 
The boat is transferred to a quartz or Vycor reduction tube and placed in 
the center of the furnace. Argon or other inert gas is passed through the 
entire reaction train at 50 cm.sup.3 /min for 30 minutes to flush air out 
of the system. The inert gas flow is stopped and hydrogen is introduced at 
25 cm.sup.3 /min. Valve 15 is adjusted to bypass the U-tube reactor. The 
electric furnace is turned on. It is preferred that the furnace be 
controlled initially to heat up at a gradual rate until a temperature of 
at least 500.degree. C. is reached, so that all water will be removed from 
the system prior to the evolution of elemental phosphorus. The reaction 
proceeds in stages. First the casual water is removed up to about 
300.degree. C. and then as the hydrogen reduction begins the Pb.sub.3 
(PO.sub.4).sub.2 is thought to be transformed to unstable Pb.sub.3 P.sub.2 
with the release of additional water. The water evolution can be observed 
visually and the temperature must be maintained below about 500.degree. C. 
until water evolution ceases. The furnace temperature can then rise to 
about 650.degree.-800.degree. C. whereupon the Pb.sub.3 P.sub.2 decomposes 
and elemental phosphorus vapor is transported to the cool end of the 
reduction tube where it is deposited as white, yellow, orange and red 
phosphorus. As the temperature rises, the phosphorus is liberated more 
readily. At about 600.degree. C. small amounts of PH.sub.3 may appear. 
Escaping phosphorus species can be absorbed in a water scrubber and 
activated carbn trap located downstream of exit 16. The heating rate 
depicted in FIG. 2 has been found to be suitable for this reaction, 
permitting complete removal of water prior to evolution of gaseous 
phosphorus. The phosphorus product changes form as indicated under the 
influence of time, temperature, and light exposure. 
About 1 to 11/2 hrs. at 650.degree.-800.degree. C. is usually sufficient 
for the reduction of all Pb.sub.3 (PO.sub.4).sub.2. The gas flow is 
reduced 10-20 cm.sup.3 /min during the collection of elemental phosphorus 
in order to minimize losses by evaporation and entrainment. At the end of 
the reduction the heat is turned off and the furnace is removed from 
around the reduction tube. The hydrogen flow is stopped and 10-20 ml/min 
inert gas is turned on to flush hydrogen out of the tube. At this point 
the process is most vulnerable to oxygen contamination and the pressure of 
the gas in the tube should not fall below atmospheric pressure. The 
reaction for this step is represented by: 
EQU Pb.sub.3 (PO.sub.4).sub.2 +8H.sub.2 .fwdarw.3Pb+2P+8H.sub.2 O (2) 
Reaction 2 is more fully described in J. C. Hutter, Ann. Dev. Chim., 12 
Serie, 1.8 (1953), a treatise on hydrogen reduction of phosphates. The 
elemental phosphorus product can be directly recovered by passing the 
vapor directly into cold water causing white phosphorus to condense under 
water, i.e., eliminating U-tube 9. 
C. Chlorination of Elemental Phosphorus to PCl.sub.5 
An excess of oxygen free chlorine gas is introduced through the gas 
manifold 5 into the inert gas filled tubes at a very low flow rate, 
virtually zero. The reaction proceeds at room temperature and its progress 
is easily observable by the formation of yellowish-white PCl.sub.5 rapidly 
on the walls of the reduction tube. One-half hour is allowed to ensure 
that the reaction goes to completion. Valve 15 should be closed during the 
chlorination period in order to minimize PCl.sub.5 losses. Excess chlorine 
is then flushed out by passing 1-20 cm.sup.3 /min of inert gas through the 
tube. The reaction is represented by: 
EQU 2P+5Cl.sub.2 .fwdarw.2PCl.sub.5 (3) 
D. Reduction of PCl.sub.5 
Valve 15 is readjusted to allow carrier gas to flow through the reductor. 
The Dewar bottles 13, 14 can be maintained at -77.degree. C. by filling 
them with a solid CO.sub.2 -trichloroethylene slush so that the collection 
tubes are immersed in the slush. The PCl.sub.5 deposit on the reduction 
tube can be heated with a hot air gun and caused to sublime. A 10-20 
cm.sup.3 /min stream of inert gas is passed through the U-tube reductor 
containing phosphorus deposited on borosilicate glass beads. The reductor 
is also heated with the hot air gun to at least about 
70.degree.-80.degree. C. PCl.sub.3 is formed in the gas phase within the 
U-tube reductor and collects in the bottom of the condenser within the 
Dewar bottles. The reaction is represented by: 
EQU 3PCl.sub.5 +2P.fwdarw.5PCl.sub.3 (4) 
The radiophosphorus specific activity of the PCl.sub.5 is diluted 3:5 by 
this reaction. The .sup.33 PCl.sub.3, approximately 0.090 ml, which is 
collected in Dewar bottle 13 may be redistilled (heating with warm water) 
to the second trap 14, if desired. This redistillation step should be 
carried out gradually to prevent decomposition of PCl.sub.3, i.e., 
gradually enough to prevent the formation of phosphorus. Inert gas flow 
should be continued throughout the system to transport the reaction 
products. This redistillation slightly decreases the POCl.sub.3 content, 
however, any elemental phosphorus carried over from the U-tube reductor is 
left behind in condenser 13. The source of POCl.sub.3 contamination is 
normally O.sub.2 and H.sub.2 O present in the gases and the phosphorus 
employed in the U-tube reductor. A convenient way of recovering the 
.sup.33 PCl.sub.3 product is to seal off the base of the condenser tube 
using a small gas oxygen torch, forming an ampule. Chemical yields for the 
preparation of .sup.33 PCl.sub.3 have averaged better than about 90%. 
It will be apparent that several of the features of this process for 
preparing high purity phosphorus or phosphorus compounds can be modified 
without departing from the concept of this invention, and such 
modifications are contemplated as equivalents of the specific applications 
described herein and covered by the appended claims.