Oxidation of Phosphorus
bv Steam J
Investigation of the Gas-Phase Oxidation in the Presence and Absence of Phosphate Rock
STEPHEN BRUNAUER J. F. SHULTZ
AND
Bureau of Plant Industry, U. S. Department of Agriculture, Washington, D. C.
Experimental Procedure The apparatus used is shown in Figure 1; it was a modification of the one developed by Emmett and Shultz (3) for the study of the oxidation of phosphorus by carbon dioxide. It FOR OXIDATION O F PHOWHORUS BY S m . w FIQURE 1. APPARATUS consisted of four Darts: the source of ohosphorus, the source of steam, the reLCtion vessel, and the analytical apparatus. Baker's yellow phos horus, U. S. P., was used. In the early 3 1925 Liljenroth (6) patented a process for the oxidation runs it was purified by %stillation in a stream of nitrogen at about of phosphorus by steam a t temperatures of 1000" C. and 140' C. and 100 mm. pressure. In the later runs undistilled higher, based upon the reaction: phosphorus gave results that agreed with those of the earlier ones within the experimental error. The phosphorus was thermoPc lOHzO = PdOio 10Hs (1) stated in oil bath H at the temperatures that gave the required vapor pressures. The tem erature of the oil bath was kept conThis reaction seemed very promising since i t was believed stant within k0.2" C. dtrogen was used as a carrier gas for the phosphorus vapor. Linde tank nitrogen was deoxidized over that the oxidation product of phosphorus could be used in the a copper catalyst heated to 400' C., dried with phosphorus pentproduction of phosphate fertilizers, while the valuable byoxide, and passed through a flowmeter and then over the molten product hydrogen could be utilized for the synthesis of amphosphorus. In the early runs complete saturation was not obmonia or in other hydrogenation reactions. In 1929 Britzke tained; in later runs container I was used in which nitrogen passed over a larger surface of molten phosphorus and thus comand Pestoff (1) performed some experiments on the above plete saturation was obtained. The nitrogen saturated with reaction in the gas phase and found that the oxidation prodphosphorus was then led t o the top of the electric furnace; this uct was not merely phosphorus pentoxide but a mixture of portion of the glass tubing was heat-lagged to prevent condensathe pentoxide and some lower oxides, and that the hydrogen tion. was contaminated with phosphine. Since the lower oxides The source of the steam was a steam boiler, a modification of the one used by Kuentzel (6). It consisted of two concentric of phosphorus are harmful to plants, and phosphine is a poitubes, J and K , so arranged that the steam coming from bulb L son for the catalysts used in the synthesis of ammonia, the completely filled K and thus surrounded J . Water was boiled in industrial feasibility of the Liljenroth process depends upon L by means of a Bunsen burner; reflux condenser M prevented the successful removal of these two undesirable products. the loss of water from L. The steam in K kept the water in J a t the boiling point, so that when current was passed through coil Various investigators have attempted t o accomplish this by 0 the energy was used entirely for the vaporization of the water the use of low temperatures and catalysts (1, 7), or by low in J. Ground-glass stopper P was used in filling J with water temperatures and high pressures ( 4 ) . The present investigaand also as the exit for the electrical lead wires. The bulbs and capillaries, S, were used to equalize the ressure between the inner tion was undertaken partly to obtain further quantitative inand outer tubes, J and K . The steam Prom J passed out through formation on the oxidation of phosphorus by steam in the tube T and was led to the top of the electric furnace. The glass gas phase and partly t o try out a different method for the tubing from the boiler to the electric furnace was heat-lagged to elimination of the lower oxide of phosphorus and phosphine. prevent condensation of the steam. The two lines, the nitrogenThis method consisted in carrying out the reaction in the phosphorus and the steam line, united at G just before entering the porcelain reaction vessel. presence of phosphate rock. Although the experiments had The reaction vessel consisted of an outer porcelain tube, A , of to be discontinued on account of various circumstances and I-inch (2.5-cm.) internal diameter, and an inner porcelain tube the present work therefore cannot be considered complete, B of 6/s-inch (1.6-om.) internal diameter. In the experiments the authors believe that some results of sufficient interest were with phosphate rock the char e was supported in B. The connections between porcelain a n f Pyrex glass were made by ground obtained to warrant publication. 828
I
+
+
INDUSTRIAL A
June, 1941
EMISTRY
joints. The analytical apparatus consisted of two U-tubes and a samplin bulb. The first U-tube, C, served for the collection of the oxifes of phosphorus and the excess water. The second U-tube, D, immersed in liquid air, collected the phosphine as well as some other gases that developed in the experiments with phosphate rock. The gas mixture leaving D consisted of nitrogen and hydrogen only; bulb E was used t o take samples of this mixture for analysis. ~
~~
~~
When phosphorus is oxidized by steam at 1000-llOOo C., the products of the reaction are not merely phosphorus pentoxide and hydrogen, but also phosphorus tetroxide and phosphine. Since compounds of the lower oxides of phosphorus are harmful to plants, the tetroxide must be eliminated if the oxidized phosphorus is to be used in the manufacture of fertilizers. If the hydrogen is to be used for the synthesis of ammonia, the phosphine must be removed because it poisons the catalyst. In the presence of phosphate rock both of these undesirable products are eliminated; the tetroxide is oxidized to pentoxide, which in turn reacts with the rock to give calcium metaphosphate, and the hydrogen becomes free of phosphine. The oxides of phosphorus were analyzed for total and for trivalent phosphorus in the manner described by Emmett and Shultz (9). The condensates in D were analyzed for phosphine by combustion in a Bone-Wheeler gas analysis apparatus; when phosphate rock was present, the total amount of the alkalisoluble gases (hydrogen fluoride, hydrogen sulfide, carbon dioxide, etc.) was also determined. The hydrogen-nitrogen mixtures in E were analyzed for hydrogen in the Bone-Wheeler apparatus. Usually two phosphine analyses and four t o six hydrogen analyses were performed in the course of a run.
Reaction of Steam and Phosphorus in the Absence of Phosphate Rock The results on the oxidation of phosphorus by steam in the
829
oxides of phosphorus (columns 9 and 10) were calculated from the known amounts of hydrogen and phosphine formed and from the ratio of pentavalent to tetravalent phosphorus determined by chemical analysis (column 12). They were calculated as Pa08 and P4O1a,but if the oxides formed were actually PZO4and P~OS, then the values should be multiplied by 2. The volume of the entering phosphorus was calculated from the amounts of phosphorus in the oxides and phosphine. Checking this value against that calculated from the known vapor pressure of phosphorus and the rate of flow of nitrogen showed that complete saturation was obtained in all runs except the earliest ones. Several conclusions can be drawn from Table I. The amount of phosphine formed at 1000° and l l O O o C. is considerable; for each volume of phosphine produced, 3.5 to 4 volumes of hydrogen are lost'. This means a loss of 3 to 3.5 per cent hydrogen with the highest steam-phosphorus ratio of 56, and 4.5 to 5 per cent loss with a ratio of 24. With a ratio of 10.4 some unconverted phosphorus remains in the exit gases, and the loss of hydrogen due t o phosphine formation rises to 8.5-10 per cent. These results are in qualitative agreement with those of Britske and Pestoff (1). From their Table IX we can calculate that the loss of hydrogen due t o phosphine formation was 1.5-17.5 per cent at 1200" C., depending on the ratios of steam to phosphorus. From the present work in conjunction with the results of Britzke and Pestoff two qualitative conclusions can be drawn: (a) The amount of phosphine decreases as the ratio of steam to phosphorus increases; (b) the quantity of phosphine produced appears to be nearly independent of the temperature between 1000° and 1200" C. The product of the oxidation of phosphorus is a mixture of the pentoxide and the tetroxide. Table I (column 12) shows that at 1000" and 1100" C. about one tenth to one third of the oxidation product is the tetroxide, depending on the ratio of steam to phosphorus and the time of contact. Britzke and Pestoff (1) found that at 1200" C. as much as one half to two thirds of the produdt was the tetroxide under their experimental conditions. I n addition to introducing an undesirable product, the formation of the tetroxide represents 1 One volume of PHs oorresponds to 0.25 volume of PI; on oxidation to h O a , 1 gram atom of phosphorus would give 2 volumes of hydrogen, on oxi2.5 volumes. In the reaction of P4 and Ha, 1.5 volumes of dation to P~OIO. hydrogen are used per volume of PHs. Thus the total loss is 3.5 to 4 volumea of hydrogen per volume of phosphine produced
gas ohase are summarized in
h b i e I. The flow of phosphorus (column 5) was kept approximately the same in all runs except 33 and 39. The ratio of steam to phosphorus was varied by changing the flow of steam. The amounts of hydrogen and phosphine produced in the reaction (columns 8 and 11) represent the average values of all analyses performed during each run. The quantity of the exit water (column 7) was calculated by subtracting from the amount of entering water the amount converted into hydrogen and phosphine; it indudes the water of hydration of the oxides of phosphorus if they are hydrated in the reaction. The volumes of the
TABLEI. OXIDATION OF PHOSPHORUS BY STEAM R~~ No, 37 38 36
Enterin Gases, T ~ ~ ~Cc.. at, 8. P./Min. O C. Ha0 Na P4 1000 100.0 25.0 1.81 1000 99.8 25.0 1.89 1100 99.8 25.0 1.78
3.
55 53 56
83.2 82.2 83.2
Exit Gases CO. at S. T. P./(Min. H~ P4Os ~~o~~ 16.6 0.56 1.23 0.10 17.4 0.49 1.37 0.11 16.3 0.35 1.38 0.22
23 P4
nzo
Ratio P4od P,O. 2.2 2.8 3.9
22 23 28 29 320 34"
1000 1000 1100 1100 1100 1100
71.0 71.0 70.8 70.8 70.8 70.8
25.0 25.0 25.0 25.0 25.0 25.0
1.64 1.84 1.70 1.72 1.67 1.69
43 39 42 41 42 42
55.7 53.8 54.9 54.6 55.2 54.9
15.0 16.9 15.6 16.0 15.3 15.6
0.28 0.35 0.30 0.27 0.28 0.28
1.31 1.44 1.35 1.41 1.34 1.36
0.20 0.20 0.19 0.15 0.19 0.19
4.7 4.1 4.5 5.3 4.8 4.8
33 39
1100 1100
35.4 35.4
25.0 25.0
0.83 0.86
43 41
27.8 27.3
7.4 7.9
0.14 0.12
0.65 0.71
0.15 0.10
4.5 5.7
17 19 13 16
1000 1000 1100 1100
39.6 39.5 39.5 39.5
24.9 25.0 25.0 25.0
1.66 1.66 1.72 1.68
24 24 23 24
23.8 23.7 23.3 23.6
15.6 15.6 15.8 15.5
0.18 0.17 0.18 0.18
1.44 1.45 1.47 1.44
0.15 0.16 0.27 0.24
8.1 8.4 8.0 8.0
1000 17.4 25.0 1.68 5.0 0.13 18b 10.4 12.0 1.14 0.29 9.1 14b 1100 25.0 1.68 10.4 2.7 17.5 14 2 0.15 1.36 0.38 9.0 15b 1100 3.7 25.0 1.68 10.4 13.2 0.12 17.5 1.28 0.37 10.3 In these t w o runs the phosphine in the exit gases was not determined experimentally. The values in column 11 were obtained b taking the average of runs 22, 23, 28, and 29. b In these runs &ere was unconverted phosphorus in the exit gases. The quantity of unconverted phosphorus WES not determined analytically. The rate of flow of the entering P4 (column 5) was assumed to he equal to the average of runs 13, 16, 17,and 19.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
a great loss in hydrogen, since for every volume of phosphorus oxidized t o the pentoxide, 10 volumes of hydrogen are obtained, whereas oxidation to the tetroxide gives only 8 volumes. If the ratio of steam to phosphorus is too low, some unconverted phosphorus remains in the exit gases. At 1000" and 1100" C. with a ratio of 10.4 to 1 (slightly above the stoichiometric ratio), about 5 to 20 per cent of the phosphorus passes through the furnace unreacted under the conditions of the present experiments (runs 14, 15, and 18, Table I). With a ratio of 24 to 1, all of the phosphorus reacts. No ratios were tried between 10.4 and 24. Britzke and Pestoff found that a t 1200" C. under their experimental conditions some phosphorus remained unconverted even with a ratio of 23.5 to 1. As one would expect, when the exit gases contain unreacted phosphorus, the phosphine content is also high. The percentage of phosphine, calculated on the basis of the sum of hydrogen and phosphine, rises to 2.5 per cent a t 1000" and 1100" C.; Britzke and Pestoff found as much as 4 to 5 per cent a t 1200" C. It is important t o inquire to what extent the results reported in Table I are influenced by reaction between the porcelain tube and the phosphorus pentoxide, or possibly both oxides. I n the present experiments tubes A and B (Figure 1) were made of unglazed porcelain which already in the course of the first run had become coated with a white glaze. If the porcelain tubes react preferentially with the higher oxide, the ratios of pentoxide to tetroxide given in Table I (column 12) should show a drift from run t o run, the earlier runs giving lower ratios than the later ones. As a matter of fact no such drift was found. Run 28, for example, was performed with new unglazed porcelain inner and outer tubes. I n the course of seven successive runs no drift was observed; the ratio of pentoxide to tetroxide in runs 28, 29, 32, 33, and 34 was 4.5, 5.3,4.8,4.5, and 4.8, respectively. Since the quantity of oxide picked up by the porcelain is probably a n appreciable part of the total, the explanation of the lack of drift may possibly be that the rate of reaction of the two oxides with porcelain is approximately equal.
Effect of Steam-Phosphorus Ratio on Phosphorus Oxidation The most unexpected result obtained in the present experiments was the increase in the ratios of pentoxide to tetroxide with decreasing ratios of steam to phosphorus shown in Table I. The average values of these ratios are 3, 4.8, and 8.1 for steam-phosphorus ratios of about 55, 42, and 24, respectively. With a steam-phosphorus ratio of 10.4 there is unconverted phosphorus in the exit gases, yet the pentoxidetetroxide ratio rises still higher to about 9.5. The results were the same a t 1000" and 1100" C., as Table I (columns 2 and 12) shows. The temperature coefficient of the reaction will be discussed later. Britzke and Pestoff obtained similar results a t 1200" C. I n the last column of their Table IX they reported that with steam-phosphorus ratios of 68, 38, 23.5, and 18.7, the ratio of pentavalent to trivalent2 phosphorus was 2.0, 2.3, 3.7, and 3.2, respectively. (With the last two ratios there was unconverted phosphorus in the exit gases.) However, they apparently overlooked this surprising result and did not discuss its significance in their paper. The decrease in the pentoxide-tetroxide ratios with increasing ratios of steam to phosphorus is real and cannot be In chemical analysis one always obtains the amount of trivalent and pentavalent phosphorus since the tetravalent oxide dissolvea in water t o give equimolar quantities of the trivalent and pentavalent acids. However, the stable oxides at 1000" C. and higher are not the trioxide and pentoxide, b u t the tetroxide and pentoxide. f
Vol. 33, No. 6
attributed t o reaction between the porcelain tubes and phosphorus pentoxide. If it be assumed that all of the phosphorus lost by reaction with the porcelain and otherwise had been in the form of the pentoxide, the ratios of pentoxide t o tetroxide given in Table I would thereby be increased from 3 t o 4.5, from 4.8 t o 6.1, and from 8.1 to 10.0 for steam-phosphorus ratios of 55, 42, and 24, respectively. The true ratios are undoubtedly between these limits and, it is believed, close to the values given in Table I. It may be safely assumed that the phosphorus pentoxide is produced in two successive reactions: Pa
+ 8H20
pzo4
+ SH,
(2)
+ HzO +PzO6 + H P
(3)
2P20,
~h
Since with steam-phosphorus ratios of 24 and higher no determinable amounts of phosphorus remain in the exit gases, this indicates that the first reaction is probably a t equilibrium or close to it, and that the equilibrium is completely over to the right side of Equation 2. On the other hand, the second reaction cannot be a t equilibrium since in that case increasing partial pressures of water should shift the equilibrium toward greater pentoxide-tetroxide ratios, whereas actually just the opposite happens. If we combine the equilibrium constant obtained by Emmett and Shultz ( 3 ) for the oxidation of phosphorus by carbon dioxide with the equilibrium constant of the water gas reaction, we can calculate the equilibrium constant of reaction 3. The experimental data of Table I show no agreement with the calculated value of the constant. This is a further evidence that reaction 3 is not at equilibrium. The authors wish to suggest two possible mechanisms for the explanation of the role of water. One involves the assumption that reaction 3 is a wall reaction, and that water is so strongly adsorbed on the porcelain glazed over by P205 that it displaces P204from the surface. This leads to the mechanism,
and thus water retards the formation of P205. The second explanation does not require a catalytic mechanism. It is assumed that the phosphorus tetroxide undergoes a hydration reaction besides the oxidation reaction ( 3 ) . At first it will be assumed that the products of the hydration are the pyro acids, Pzo4
+ 2H20 +$H4Pz06 + ~ H ~ P P O T
(4)
If reactions 3 and 4 are far from equilibrium so that the reverse reactions can be neglected, their rates will be: Rate of oxidation
=
ko(P204)(HzO)
(3BB
Rate of hydration
=
kh(P~O~)(H20)~
(44
Thus the two reactions compete for P204. On raising the partial pressure of water, the rate of the oxidation reaction will increase more slowly than the rate of the hydration reaction, since the former depends on the first power, the latter on the second power of the partial pressure of water. Since the hydration reaction produces both trivalent and pentavalent phosphorus while the oxidation reaction only pentavalent phosphorus, this means that increasing the partial pressure of water will decrease the ratio of pentavalent to trivalent (or tetravalent) phosphorus. I n the above discussed case the ratio should vary roughly inversely as the first power of the water concentration. Actually it varies inversely as a power somewhat greater than unity. This sort of variation
lune, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
is due to the fact that the rates of the back reactions of Equations 3 and 4 are not negligible. Indeed the ratio of pentavalent to tetravalent phosphorus may vary from a n inverse second-power relation with respect to water to a direct relation as the first power of water, depending on how near reactions 3 and 4 are to equilibrium. If reaction 3 is near equilibrium and reaction 4 is far from it, the ratio of pentoxide to tetroxide may vary inversely as the second power of the water concentration; if reaction 4 is near equilibrium and reaction 3 far from it, the ratio may vary directly as the first power of water. The observed variation may fall anywhere within these limits. If the tetroxide were hydrated to the acids, we should expect that the pentoxide also would be hydrated. However, this reaction would have no influence on the pentoxide-tetroxide ratio under the assumed conditions. Also we could have used as examples the oxides P408 and P4Ol0 instead of P204and Pz06 in both the catalytic and the gas-phase mechanisms. Similarly we could have postulated the hydration of the oxides to the meta instead of the pyro acids. Using, for example, P408and P4O10 as the oxides, and assuming hydration to the meta acids, we can show that the ratio of pentavalent to tetravalent phosphorus may vary from an inverse secondpower to a direct second-power relation with respect to water, depending on how near the oxidation and hydration reactions are to equilibrium. The present experiments cannot decide which oxides and which acids are produced in the reaction. Tilden and Barnett (8) showed that metaphosphoric acid is quite stable at bright red heat. Although there seems to be no evidence in the literature that would disprove the existence of the pyro acids a t the high temperatures of these experiments, nevertheless the formation of the meta acids is probably more likely. We wish to emphasize that we regard both the catalytic and the gas-phase mechanisms here suggested to explain the behavior of water as tentative, and subject to verification or disproof by further experiments.
Temperature coefficient of phosphorus Oxidation
831
of these quantities. The fact that neither changes appreciably indicates that the explanation cannot be merely that both the oxidation and hydration reactions are accelerated equally by the 100" rise in temperature, but that probably both reactions individually have very low temperature coefficients. I n reaction 3 we would expect a high temperature coefficient, since bimolecular collision reactions that proceed a t measurable rates only around 800" C. must necessarily have high energies of activation. If, however, we deal with P408 molecules instead of P 2 0 4 , the oxidation reaction is: Pa08
+ 2Hz0 +P,Oio + 2Hz
(5)
If this is a trimolecular reaction, we should expect a low-temperature coefficient, since trimolecular reactions have slight (sometimes even negative) temperature coefficients. Summarizing briefly, both the decrease in the pentoxidetetroxide ratios with increasing steam-phosphorus ratios and the negligibly low temperature coefficient of the reaction can be explained on the basis of either the catalytic or the gasphase mechanism. The data do not enable us to state definitely which oxides and which acids are present; evidence seems to favor slightly the formation of Pa08 and P 4 0 1 0 rather than Pz04and Pz06,and the meta acids rather than the pyro acids.
Reaction of Steam and Phosphorus in Presence of Phosphate Rock The reaction between phosphorus pentoxide and phosphate rock was investigated by Curtis, Copson, and Abrams (2) who found that i t takes place readily above 800" C. Emmett and Shultz (3) found that the presence of phosphate rock disturbs the equilibrium between phosphorus tetroxide and pentoxide in the oxidation of phosphorus by carbon dioxide in such manner as to remove the tetroxide from the system. The rock reacts preferentially with the pentoxide to form a product that is essentially calcium metaphosphate; as soon as the pentoxide is removed, more of the tetroxide reacts with the carbon dioxide until finally all of the tetroxide reacts to form pentoxide and all of the pentoxide goes into the phosphate rock. I n the present investigations an attempt was made to eliminate the,undesirable products of the Liljenroth process (phosphorus tetroxide and phosphine) by the presence of phosphate rock.
Table I shows that the rate of the over-all reaction, as judged by the amounts of hydrogen formed and the pentoxidetetroxide ratios, was the same a t 1000" as a t 1100" C. within the experimental error. The negligibly low temperature 00efficient can be explained on the basis of either of the two mechanisms suggested. If the rate-determining step is catalytic, the apparent energy TABLE 11. OXIDATIONOF PHOSPHORUS BY STEAM IN PRESENCE OF PHOSPHATE ROCK of activation differs from the Alkalitrue energy of activation by Ratio, PHJ Oxides of ~401o/~a0s c c . a t S. T. P . / M i n . GS,iks, a n amount that depends on P4 U,nreacted In In In In Cc. a t the fractions of the surface Run Temp., Time Ratio with Rock, presence absence presence absence S. T P./ No. C. Min.' % of rock of rocka of rock of rookQ HsO/$a Min. covered by the reactants and 9a 1100 113 67 .. 9.9 2 0.07 .. reaction products and on the 9b 1100 129 67 16 1.3 2 010 0.07 .. heats of adsorption of reacQc 1100 461 67 23 1.3 2 0.03 0.07 .. tants and products. It is not loa 1100 120 45 15 14.6 4.6 0.0 0.12 .. 10b 1100 134 45 8 3.5 4.6 0.0 0.12 .. uncommon in catalytic reac1oc 1100 45 1.6 4.6 .. 0.12 .. tions to find that these energy lla* 1100 120 as 10 13.1 5.5 0.0 0.15 .. quantities balance each other 39 8 llb 1100 120 2.5 5.5 0.0 0.15 llc 1100 335 39 27 1.3 5.5 0.12 0.15 0102 so as to make the apparent energy of activation approxi12a* 1100 123 40 14 4.7 5.3 0.0 0.14 0.12 12b 1100 90 40 11 2.3 5.3 0.0 0.14 0.04 mately zero. 12c 1100 120 40 28 1.3 5.3 0.02 0.14 0.01 12d 1100 381 40 .. 1.2 5.3 0.10 0.14 0.01 If the reaction proceeds by the gas-phase mechanism, 2Oa 1000 127 25 16 1.7 8.0 0.12 0.21 20b 1000 120 25 30 2.9 8.0 0.17 0.21 0:02 one would expect a shift with .. 3.2 8.0 .. 0.21 .. 2oc 1000 67 25 temperature in the amount 21a* 1000 132 25 14 14 8 0 012 021 0 11 of hydrogen formed and in a Estimated by interpolation and extrapolation. the ratio of pentoxide to * Phosphate rock was air-treated prior t o run. tetroxide, or a t least in one
...
..
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
The results of the experiments with phosphate rock are shown in Table 11. I n all of the runs approximately 6-gram samples of Florida phosphate rock were packed into the inner porcelain tube B (Figure 1); the apparent volume of the charge was about 5.5 CC. The time of contact of the gases with the charge was from 0.5 t o 1 second. Each run was divided into three or four parts; in each part analysis of the oxides and several analyses of the hydrogen and phosphine were obtained. As the calcium metaphosphate that forms in the reaction is a liquid a t 1000° C., it runs down the porcelain tube and solidifies on a lower and cooler part of the wall. In the course of time this results in the blocking of the flow of gases by the solid calcium metaphosphate; a t that point the run was considered completed. Under the present experimental conditions a portion of the oxides of phosphorus always passed the phosphate rock without reaction. Table I1 (column 5) gives the volume percentages of the oxides that remain in the exit gases. These percentages increase as the reaction proceeds; for example, in run 20 in the first two hours 16 per cent, in the next two liours 30 per cent of the oxides passed the rock without reaction. However, the fact should be noted that in runs 10, 11, and 12 more of the oxides passed without reaction in the first two hours than in the next two hours. Perhaps this was due to the fact that in the first period the phosphorus pentoxide had to diffuse to the phosphate rock against the gases coming out of the rock. I n runs 11, 12, and 21 the phosphate rock was air-treated prior to the run. This consisted in passing air over the charge for one hour a t 800" to 1100" C. and for one hour at 1100O C. This air treatment, however, seemed to have very slight effect either on the amounts of oxides passing by unreacted or on the amounts of hydrogen and phosphine in the exit gases. Needless to say, we can eliminate the oxides from the exit gases by sufficiently slowing down the rate of flow of the gases over the rock. The present experimental setup was unsuitable to produce longer times of contact than one second. I n actual industrial operation, however, the process can be so arranged that the reacting gases pass upward through a long bed of phosphate rock that is continuously replaced by feeding in new charge at the top while the molten calcium metaphosphate is tapped off a t the bottom of the furnace. If there is always some unreacted phosphate rock a t the top, the flow can be so adjusted that only a negligible fraction of the oxides passes by. I n the presence of phosphate rock the amount of hydrogen increased in the expected manner, although this is not shown in Table 11. At the same time it would be expected that the ratio of pentavalent to tetravalent phosphorus in the exit gases should decrease, since the rock reacts preferentially with the pentoxide. Table I1 (columns 6 and 7) shows that this is actually the case, except that in runs 9, 10, and 11 the ratio increased in the first two hours instead of decreasing. After the first two hours the ratios show the expected decrease in every run. The explanation of the initial increase is by no means clear; perhaps the tetroxide is oxidized by some constituent of the gas mixture coming from the rock (possibly carbon dioxide). Table I1 (columns 8 and 9) shows that the phosphine is efficiently removed when phosphate rock is present. The results of runs 9, 10, 11, and 12 indicate that in the first four hours of the run no phosphine appeared in the exit gases. Later small quantities of phosphine appeared, which increased as the rock was gradually consumed in the reaction. In runs 20 and 21 the phosphine was not completely removed even in the first two hours; in these runs the temperature and the steam-phosphorus ratio were both lower. On the basis of the data of Table I we believe that the appearance of phosphine was due not so much to the lower temperature but
Vol. 33, No. 6
rather t o the low steam t o phosphorus ratio. The mechanism of the removal of phosphine is uncertain; it may just be catalytic cracking of the gas, or i t may be that the rock shifts the equilibrium by a more complete removal of the phosphorus from the system. Table I1 (column 10) shows that, although the phosphate rock removes the phosphine, the issuing hydrogen is not pure but contains other gaseous impurities coming from the rock. No attempt was made to obtain a complete analysis of these impurities; only the amount of phosphine and the amount of alkali-soluble gases (hydrogen fluoride, hydrogen sulfide, carbon dioxide, etc.) were determined in the issuing hydrogen-nitrogen mixture. As expected, the amount of the alkali-soluble gases decreases as the phosphate rock is consumed in the reaction. It is believed that since phosphine is removed in the reaction, the purification of hydrogen from the other gases for the synthesis of ammonia would be a relatively simple process. However, only further experimentation can establish this definitely. The calcium metaphosphate obtained in the reaction is a clear, transparent, glasslike substance. The P205 content of the Florida phosphate rock used was 31.1 per cent, the P2OScontent of the metaphosphate obtained was 67.4 per cent. This percentage is higher than would be obtained by converting all of the calcium in the rock to calcium metaphosphate, in agreement with the results of Curtis, Copson, and Abrams (2). The practical advantages of carrying out the oxidation in the presence of phosphate rock may be briefly summarized as (a) complete oxidation of phosphorus to the pentavalent stage, thereby removing the harmful lower oxides and obtaining more hydrogen; ( b ) the removal of the harmful phosphine and a t the same time obtaining in its place more phosphorus pentoxide and hydrogen.
Acknowledgment The authors wish to express their thanks to Ellen K. E s t for analyzing all the oxide samples for total and trivalent phosphorus, and to T. H . Tremearne for analysis of the calcium metaphosphate samples.
Literature Cited Britzke and Pestoff, Trans.Sci. Inst. Fertilizers (U. S. S. R.), No. 59 (1929). Curtis, Copson, and Abrams, Cham. & Met. Eng., 44, 140-2 (1937). Emmett and Shults, IXD.ENQ.CHEM.,31, 108-11 (1939). Ipatieff, V. N., U. S. Patent 1,848,295 (1932); Brit. Patent 308,684 (1930). Kuentael, J. A m . Chem. SOC.,52, 437-44 (1930). Lilienroth, F. G . , Canadian Patent 247,164 (1925); U. S. Patent 1,594,372 (1926). Liljenroth, F. G . , U. S. Patent 1,673,691 (1928). Tilden and Barnett, J. Chem. SOC.,69, 154-60 (1896).
PBBSBNTBD before the Division of Fertilizer Chemistry at the 100th Meeting of the American Chemical Society, Detroit, Mich. A patent has been a p plied for on the process of oxidizing phosphorus by steam in the presence of phosphate rock.
