ILVDUSTRlAL AND EAVGISEERIiVG CHEMISTRY
1126
1-01, 21, so.11
FIGURE 5
0
10.
20'
3s
.w' co' rmw r n s t n m ~ cLND
40'
DISTANCE
w
90'
iud
since ignition loss persists in appreciable amount to within 5 feet of the kiln front, whereas the lime free to recombine becomes very low several feet farther back. The effects of a change in firing conditions are illustrated by Figure 5. Results obtained in the preheating zone are little different from those in Figure 4. Subsequent calcination is slower and recombination of the lime is not completed, producing an unsatisfactory clinker. Under these conditions the recombination of the lime appears t o be the slow process, lagging behind the calcination of the calcium carbonate. Since rotary kilns vary in design or dimensions, and operating conditions may differ, the results given above serve only to illustrate the use of the method. The curves given may only be considered representative of the specific con-
ditions prevailing. The method would seem, however, to be capable of yielding interesting and valuable results regarding the influence of the various factors involved in kiln operation. Acknowledgment
John Treanor, president of the Riverside Cement Company, and Earle JIacDonald, general superintendent, have given their hearty support and cofiperation in this investigation. Literature Cited (1) Lerch and Bogue, IND. ENG CHEX.,18, 739 (1928). (2) Steinour and Woods, Rock Pvoducl?, 33, 74 (1929)
Reduction of Tricalcium Phosphate b y Carbon Effect of Silica and Alumina on the Reaction1 K. D. Jacob, D. S. Reynolds, and W. L. Hill FERTILIZER AND FIXSD NITROGEN IKVESTIGATIONS, B ~ R E AOF U CHEMISTRY A N D SOILS, WASHINGTOY, D. C.
I
T HAS been known for many years that the rate of re- calcium phosphate and coke with silica and alumina. Ross, duction of tricalcium phosphate by carbon is increased Mehring, arid Jones (27) conducted a rather extensive series of by the presence of silica. This discovery is usually attrib- experiments on mixtures of phosphate rock and carbon with uted to Wohler (37) in 1829, but the credit actually belong. silica and with potash silicates, under closely controlled to Berthier ( I ) , who published a short paper on the subject in conditions. While the last mentioned investigations brought 1826. The process was first applied to the conimercial out some interesting facts regarding the reduction of phoso rock, conclusions as to the individual effect of silica on manufacture of phosphorus in 1890 by Readman ( H ) ,~ ~ hphate used an electric furnace. the reaction cannot be drawn from the results because of the During the past fifteen years considerable work has heen presence of appreciable quantities of alumina and other imdone on the preparation of phosphoric acid by smelting purities in the materials used. A study of the factors affecting the reduction of pure trimixtures of phosphate rock, silica, and carbon in a reducing atmosphere, oxidizing the evolved phosphorus to phosphorus calcium phosphate by pure carbon has been reported in a pentoxide, and finally recovering the latter as orthophos- recent paper by Jacob and Reynolds (I 1 ) . The present inphoric acid by absorption in water, or by means of the electro- vestigation was undertaken to determine the effect of silica and alumina on the reaction. static precipitator. The literature (5,26,3Zt o 36) contains the results of a nuinMaterials ber of semi-commercial investigations on different phases of the process, but these have been directed principally towards the TRICAI,CIUU PHOSPHATE AXD CAnBox-These materials solution of various mechanical difficulties, and the chemical were the same as were used in a previous investigation (11). side of thc process seems t o have received comparatively The variety of carbon qpecified as carbon flour? was used in all little attention in the way of thorough laboratory investi- the experiments. gations. Waggaman, Easterwood, and Turley (34) report the SILICA-A commercial grade of quartz flour was treated on results of a few iaboratory experiments on mixtures of tri2 This material was obtained from the National Carbon Co , Cleveland, 1
Received May 7, 1929.
Ohio
November, 1929
I.VDUSTRIAL .4SD ENGINEERING CHEMISTRY
1127
the steam bath with 1 : 1 hydrochloric acid and aqua regia, the quantity of phosphorus volatilized was very small, the respectively, and finally boiled with 70 per cent sulfuric residues were ignited at 800" C. to remove the excess carboii acid. The washed and dried product contained 0.1 per cent prior to treatment with aqua regia, since Jacob and Reyof material non-volatile with hydrofluoric acid. For use nolds (12) have shown that when the residues were treated in the experiments it was passed through a 200-iriesli sieve directly with aqua regia 0.5 to 1.0 per cent of the total phosand ignited at 800-900" C. A few experiments were also run phorus was adsorbed from solution by the finely divided with a high-grade commercial silica gel which was ground to carbon. The following results are, in most cases, each the average pass a 200-mesh sieve and ignited at 800-900" C. The product contained 0.2 per cent of material non-volatile with froin at least two experiments in which the total phosphorus hydrofluoric acid. T7nless otherwise stated, quartz flour was volatilized did not vary more than 2.5 per cent. used in all the esperiments. Effect of Size of Charge ALUVIS~-+i material of c. P. grade wab passed through a 200-mesh iieve antl ignited at 900-1000" C., the product, The effect of the size of the charge on the percentage of which contained 98.33 per cent A1203,being stored in a desic- phosphorus volatilized in 1 hour at 1200" C. was determined on three mixtures containcator until u w l ing the weight ratios CadM I S E R I LA L U U I A L V I'H 0 S P H .I T E "--?\I i I1 e 1' a 1 (PO1)z:C:SiOz = 1:1:0.5833, Silica has a marked effect in increasing the rate of corresponding to a 1:l mol aluminum phosphate from reduction of tricalcium phosphate by carbon. In its ratio of silica to lime. The the Connetahle Iqlands. off presence volatilization of phosphorus begins at 1050" C., boats in which the mixtures the coait of French Guiana, and under favorable conditions the reaction is more were hkated were all of the was ground t o pa-s a 200than 90 per cent complete in 1 hour at 1250" C., or in same dimensions, but owing mesh sic\-e and ignited at 10 minutes at 1350' C. Alumina also accelerates reto their semi-circular cross 600-650" c'. The materialduction of tricalcium phosphate, but its effect is not section the maximum verlost 21.85 pel rent of comso pronounced as that of silica. With a constant tical thickness of the mixbined water on ignition antl initial ratio of tricalcium phosphate to carbon, the tures was not directly prothe final product contained quantity of phosphorus volatilized under given condip o r t i o n a l to their weight. A1203 36.00, Fen03 14.63, tions increases with increase in the silica and alumina The percentages of phosPzOs 45.36 nntl Pi02 2.34 content of the charge. Silica gel and quartz are equally phorus volatilized were 61.6, per cent. effective in accelerating the reaction. Reduction of 57.6, and 45.8 from IO-, 20-, tricalcium phosphate, in the presence of silica, follows Apparatus and Experiand 30-gram mixtures, remental Method the course of a monomolecular reaction. spectively, showing a deAluminum phosphate is rapidly reduced by carbon crease of 16 per cent when The apparatu- a n d exat 1100" C. and addition of silica to the charge has the size of the charge wai perimental method used in very little, if any, effect in promoting the reaction. increased from 10 grams to this inve+tigation have been 30 grams, while in previouq described in detail in previexperiments (11) with mix14. 1 1 ) . The ous *Daners _ temperature of the graphite boat in which the mixtures tures of tricalcium phosphate and carbon alone, at 1250" C., w r e heated did not exceed 950" C. during the initial the total quantity of phosphorus volatilized decreased about period of heating the furnace, thus avoidiiig the possibility of 30 per cent with the same difference in size of charge. premature ~olatilizationof phosphorus. Temperatures were Further evidence on the effect of the thickness of the charge measured on the front and back of the boat, and the average on the rompleteness of the reaction mas obtained in an exof these, which differed by 10" to 25" C. depending on t'he periment in which the percentages of the total phosphoruh final temperature desired, was taken as the temperature of the volatilized from different levels of the same charge mere mixture. Although the temperatures of the mixtures may determined. The mixture was composed of 40 grams of have been somewhat, lower than those actually measured on tricalcium phosphate, carbon, and silica in the proportionthe exterior of the boat, owing to absorption of heat in the used in the foregoing experiments, and was heated for 1 hour reaction, it is quite likely that, these differences xere well at 1250" C. The procedure used in obtaining samples of the within the range of accuracy of bhe general experimental residue at different levels was the same as that described in :L procedure. Esperirncntal evidence on this point will be previous paper (11). obtained in the near future. Excessive temperature differThe results in Table I shorn that the greatest volatilization ences due to conduction of heat into the charge lvere avoided of phosphorus occurred in the top and bottom layers of the by using thin layers of mixtures. Unless othern-ise stated, mixture and the smallest in the center layers. The differences, 10-gram niixtures Tvere used in all the experiments. however, were not so great as those obtained in previous exThe percentage of phosphorus lost by 1-olatiliza t'ion wa,s periments (11) with mixtures of tricalcium phoqphate and carusually calculated from the total phosphorus content of the bon alone. residue as compared witmli that of the original mixture. lT711en- Table I-Volatilization of Phosphorus a t Different Depths of Mixture of Tricalcium Phosphate, Silica, a n d Carbon ever it was evident that t'liere had been any appreciable mechanical loss of the charge prior to weighing the residue, the DEPTHAT WHICH RESIDUE PHOSPHORUS WAS SAMPLED VOLATILIZED percent'age of phosphorus volatilized was determined from the change in t'he Pa05-Ca0 ratio of the residue as compared wit'h Cm. Per cent the ratio for the original mixture. The residues from the 0 to 0 . 5 79.8 0 . 5 to 1.0 62.5 experiments with mineral alumiriuin phosphate were brought L o t o 1.5 55.4 1 . 5 to 2 . 0 55.9 into solution by fusion with sodium carbonate, but this treat2 . 0 to 2 . 5 64.6 ment was not necessary with residues from experiment's 2 . 5 to 3 . 0 85.7 with pure materials, the phosphate being entirely soluble in As pointed out in a previous paper (11), the decrease in the aqua regia. In the experiments at. low temperatures where percentage of total phosphorus volatilized with increase in the 3 This material was kindly furnished by the Warner Chemical Co., Carteret, ?;. J. thickness of the charge, in the case of mixtures of tricalciuin
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1128
phosphate and carbon alone, was due a t least partly t o the fact that temperatures in the interior of the thicker charges were lower than were actually measured on the exterior of the boat. It was not determined whether the more uniform results obtained in the presence of silica were due to an increase in the rate of heat transfer into the interior of the charge or to some chemical effect of the silica in speeding up the reaction. Effect of Silica and Alumina on Reduction of Tricalcium Phosphate
Berthier's discovery that reduction of tricalcium phosphate is increased by the presence of silica has been utilized for the past forty years in the commercial manufacture of phosphorus and for the past ten years in furnace processes for the production of phosphoric acid. With the object of utilizing the slag produced in these processes, a number of patents have recently been issued which propose the manufacture of high-alumina cements as by-products ( 6 , I S ) . Some of these patents ( I d , I 7 , S I ) propose to substitute alumina, in the form of bauxite or other suitable material, for all or a part of the silica customarily added to the furnace charge, claiming that alumina has the same effect as silica in accelerating reduction of the phosphate. I n order to determine whether this is actually the case, a series of experiments was carried out comparing the effect of alumina and silica on the reduction of tricalcium phosphate a t various temperatures. The results in Table I1 show that in mixtures containing the same molecular ratios of reacting constituents alumina has a marked effect in increasing the rate of reduction of tricalcium phosphate by carbon while the effect of silica is still more pronounced. Table 11-Eff ec t of Silica a n d Alumina on Reduction of Tricalcium Phosphate
I
0
c.
Minutes
1
PHOSPHORUS VOLATILIZED
Per cent
I
Per cent
1
Per cent
Vol. 21, No. 11
to react with tricalcium phosphate a t about 1150' C. to form a calcium silicophosphate, having the empirical formula Car (P04)2.3Si02. The results given in Table I11 do not indicate whether the tricalcium phosphate and silica first reacted t o form a calcium silicophosphate which was later reduced by carbon, or the tricalcium phosphate was first reduced by carbon to form phosphorus and free lime, the latter then reacting with silica t o form a calcium silicate. The possibility of the formation of a calcium aluminophosphate in the same manner has not been investigated, but the presence of free lime in the residues indicates that the reaction was a t least partly between free lime and alumina. Table 111-Free c O ~ P O S I T I O N OF
Lime i n Residues
TIME
MIXTURE
Grams CaO 2.SOa Si02 3 02 Mol ratio Si02 : CaO = 1 : 1
I
0
c.
Minures
PHOSPHORUS FREE vOLATILCaO IN IZED RESIDUE
Per cent
1000 1200
120 30
....
Per cent 31.0 1.5
Caa(P01)z Carbon Si02 Mol ratio
3.871 3.871 2.258 SiOn : CaO = 1 : 1
1200
60
61.6
0.0
Cas(P01)z Carbon Si02 Mol ratio
4.186 4.186 1.627 SiOz: CaO = 2 : 3
1200
60
54.8
0.0
Caa(P0a)z Carbon Si02 Mol ratio
4.557 4.557 0.885 Si03: CaO = 1 : 3
1200
60
45.1
0.0
Cas(P0a)n 3.346 Carbon 3.346 A1203 3.307 Mol ratio A1203 : CaO = 1 : 1
1200
60
31.7
0.1
Caa(P0a)z Carbon Ah03 Mol ratio
3.857 3.857 2.286 &Os : CaO = 3 : 5
1200
60
30.8
0.4
Cas(P0n)z Carbon 81~0s Mol ratio
4.275 4.275 1.450 AhOa: CaO = 1 : 3
1200
60
25.9
0.6
a
Mixture originally contained 48.8 per cent free CaO.
Table IV-Effect of Variation i n Silica-Lime Ratio COXPOSITION OF MIXTURE
Several of the residues obtained in various experiments during the course of this investigation were ignited a t 900" C. t o burn off excess carbon and were analyzed for free calcium oxide by the ammonium acetate method of Lerch and Bogue (16). Comparat,ive experiments were also made with lime and silica mixtures. The results (Table 111) show that silica reacts with lime at temperatures as low as 1000" C. and a t 1200" C. the reaction is quite rapid. Residues from mixtures of tricalcium phosphate, silica, and carbon, heated for 1 hour a t 1200" C., contained no free lime, while those in which the silica was replaced by alumina contained only a small percentage, which decreased as the quantity of alumina in the mixture was increased. The residue from the experiment with lime and silica a t 1000" C. was caked but could be easily broken up with the fingers, while that from the mixture heated a t 1200" C. was badly caked and sintered but had not fused to a liquid melt. The residues from the mixtures containing silica, carbon, and phosphate were only slightly caked, while those containing alumina were practically in the same condition as the original mixtures. Microscopically, the residues, after removal of the carbon, appeared to be in practically the same condition as the original mixtures: According t o h'ielsen (20) silica begins
Caa(P0n)t
Carbon
Silica
Grams 5,000 4.557 4.364 4.186 3.871 3.600
Grams 5.000 4.557 4.364 4.186 3.871 3.600
Grams
I I
PHOSPHORUS VOLATILIZED
Si02 :CaO
60 Minutes 30 Minutes a t 1200° C. a t 1300' C.
MoLRAT1o
0.0 0.886
1.272 1.627 2.258 2.800
0:1 1:3 1:2 2:3 1:l 4:3
Per cent 19.1 45.1 50.9 64.8 61.6 65.8
Per cent 58.1 76.7 90.5 96.2 97.7
..
Effect of Variation in Silica-Lime Ratio Using Constant Caa(PO&-Carbon Ratio
I n the commercial manufacture of phosphorus and phosphoric acid the composition of the charge is regulated so that the mol ratio of silica to lime is about 1:l. I n order to determine the effectproduced by variations in the silica content, of the charge, a series of experiments was carried out with 10-gram mixtures containing a constant initial Ca3(P04)2carbon ratjo. The results are given in Table IV. As the silica content of the charge was increased there was a progressive increase in the percentage of total phosphorus volatilized. This was particularly noticeable in the experiments a t 1200' C., the percentage of phosphorus volatilized increasing from 45.1 with a mol ratio of Si0z:CaO = 1:3 to 65.8 with a ratio of 4 3 .
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1929
Effect of Variation in Ca3(P0&Carbon Ratio Using Constant Lime-Silica Ratio A series of experiments was carried out to determine the effect produced by varying the Ca3(PO&carbon ratio while maintaining a constant ratio of silica to lime. Ten-gram charges were used and a 1:l mol ratio of silica t o lime was maintained. The results (Table V) show that as the carbon content of the charge was increased there was a progressive increase in the percentage of total phosphorus volatilized. The theoretical weight ratio of Ca3(PO& to carbon for the reaction Cas(PO& 5C 3Si02 = 3CaSiOa 5CO $- PZ is 1 : 0.1933. There was evidence of incipient fusion in the residues from experiments a t 1400' C., this being more pronounced in the mixture containing the smallest quantity of carbon, while the residue from the corresponding experiment a t 1200" C. was only slightly caked. Ross, Mehring, and Jones (27) state that practically no loss of phosphorus occurs when mixtures of tricalcium phosphate and silica alone are ignited in a neutral or oxidizing atmosphere a t 1300" C. for 1 hour.
+ +
Table V-Effect
Ca%(POd? Carbon
Grams 5.607 5.042 4.286 3.871
Grams 1.122 2.017 3.214 3.871
be true when a silicate is used because of the introduction of additional basic elements. Table VII-Effect of Temperature on Volatilization of Phosphorus from Mixtures of Tricalcium Phosphate, Silica, and Carbon (Mixtures heated for 1 hour)
TEMPERATURE
Silica
Grams 3.271 2.941 2.500
I1
c.
1
1
I Per cent 1:0.2:0.5833 1:0.4:0.5833 1:0.75:0.5833 1:1:0.5833
2.258
31.5 46.4
a
Table VIII-Effect
TEMP.
O C. 1200
1250
1300
c.
1350
In these experiments the weight ratio of tricalcium phosphate to carbon was held a t 1 : 1, while the mol ratio of alumina t o lime mas varied from 1 : 3 to 5 : 3. The results given in Table VI show that, in the temperature range of 1200" to 1500" C., increasing the proportion of alumina to lime in the charge results in an increase in the percentage of total phosphorus volatilized. Comparison of the figures given in Tables IV and VI shows that in general the increase in volatilization of phosphorus with increasing aluminz content of the charge is less than that obtained with increasing silica content. Table VI-Effect
of Variation i n Alumina-Lime Ratio
COMPOSITION OF hfIXTURE
I
Car(P0a)z Carbon
Alumina
1
Grams
Grams
I
Grams
MOL Cao
PHOSPHORUS VOLATILIZED 1 60 Min30 Min- 5 Min- 5 Min-
utes a t utes a t utes a t utes a t 11200° C. 1300' C. 1400' C. 1500' C.
I P e r cent Per cent 30.8 31.8
74.6 87.0
Per cent Per cent
..
6::1
70.2
TIME
Minutes 20 40 60 80 100
.
Effect of Variation in Alumina-Lime Ratio Using Constant Ca3(PO&-Carbon Ratio
89:7 96.4
..
Effect of Silica from Different Sources The effect of silica in the forms of silica gel and quartz was compared in experiments with 10-gram charges containing 1 : 1 weight ratios of tricalcium phosphate to carbon and 1 : 1 mol ratios of lime t o silica. The mixtures were heated for 1 hour a t 1200" C. The percentages of total phosphorus volatilized from the mixtures containing silica gel and quartz were 59.3 and 61.6, respectively. These results duplicate each other practically within the limits of experimental error and indicate that a t 1200" C. the form of silica used has no significant effect on the reaction. Undoubtedly this will not
of Time of Heating a t Various Temperatures PHOSPHORUS VOLATILIZED
94.8 I
-
Mixture heated for 2 hours.
15
30 45
60
Per cent 84.1
Per cent 0.5 1.0 2.6 11.7 28.4 61.6 95.3
1050 10505 1100 1150 1200 1250
+
PHOSPHORUS VOLATILIZED WEIGHTRATIO Ca3(P04)z:C:Si03 60 XIinutes 10 Minutes / a t 1200' C. a t 1400" C.
PHOSPHORUS VOLATILIZED
1oooa
of Variation i n C a a ( P 0 a ) ~ C a r b o nRatio
C O M P O S I T I O N OF M I X T U R E
1129
1500
10 20 30 40
Per cent 31.4 47.5 61.6 74.3 82.6
49.7
z::
95.3
..
10
2.5
Ca3(POd)z 3.346 g. Carbon 3.346 g. A1203 3.307 g. Mol ratio: C a 0 : A l r O ~= 1:l
Ca3(POd)z 3.871 g. Carbon 3.871 g. Si02 2.258 g. Mol ratio: CaO:SiOz= 1:l
"'O
I
P e r cent
1 II
39.7 68.9 87.0 95.8
.. 83.8
Effect of Temperature and Time of Heating The percentages of total phosphorus volatilized in 1 hour when 10-gram charges containing 1 : 1 weight ratios of tricalcium phosphate t o carbon and 1 : 1 mol ratios of lime t o silica were heated a t various temperatures were determined, with the results given in Table VII. The figures show that, under the conditions of the experiments, volatilization of phosphorus certainly begins a t a temperature as low as 1050" C. and may begin a t 1000" C. Although every precaution was observed in carrying out the experiments a t the lower temperatures, very small errors would easily account for the result obtained a t 1000" C. The reaction is comparatively slow a t temperatures below 1200' C. but is 95 per cent complete in 1 hour a t 1250" C. Hempel (9) states that, qualitatively, volatilization of phosphorus from mixtures containing 5 parts bone ash, 1.5 parts wood charcoal, and 3 parts sand begins a t 1150" C. Blome (2) noted that evolution of phosphorus began a t 1300" C. when mixtures of tricalcium phosphate and silica were heated in carbon crucibles. Ross, Mehring, and Jones (27)found that slight volatilization of phosphorus occurred at 1050" C. when mixtures of phosphate rock and coke with sand or natural potash silicates were heated for 1 hour. Jacob and Reynolds (11)have shown that in the absence of silica reduction of pure tricalcium phosphate by pure carbon begins a t about 1150" C. Table VI11 shows that with mixtures containing equimolecular proportions of lime and silica more than 90 per cent of the phosphorus is volatilized in 45 minutes a t 1250' C., in 30 minutes a t 1300' C., or in 10 minutes a t 1350" C. When the silica is replaced by an equimolecular quantity of alumina, the reaction is 95 per cent complete in 40 minutes a t 1300" C. With mixtures of tricalcium phosphate and carbon alone (11) 87 per cent of the phosphorus is volatilized in 1 hour a t 1300" C.
IJVDUSTRIAL A,VD EYGIXEERIXG CHEMISTRY
1130
I t has been shown (11) that reduction of pure tricalcium phosphate by pure carbon follows the course of a monomolecular reaction in the temperature range 1250" to 1400" C. Using the results given in Table VIII, the velocity coefficient for the reaction between tricalcium phosphate, carbon, and silica a t 1200" C. was calculated from the equation for a niouomolecular reaction. The uniformity of the results (Table 1X) shows that the reduction takes place in such a n a y that its velocity corresponds t o that for a moiiomolecular reaction. The same explanation applies in this case as in the case of tricalcium phosphate and carbon alone (11)namely, that the velocity of the composite reaction by which elemental phosphorus is produced is determined by the velocity of the dissociation of tricalcium phosphate into lime and
I
0
025
050
Nof Raho
.?to,
/OO 125 : CaO and A l z g : CaO
075
I50
3 .;/
phosphorus pentoxide, in the presence of silica and carbon. and not by the velocity of the rapid reaction between carbon and phosphorus pentoxide vapor. The velocity of the reaction may be affected to some extent by the rate of diffusion of phosphorus from the mixtures, but the experimental results do not give any definite indications that this is the case. Table IX-Velocity Coefficient for Reaction between Tricalcium Phosphate, Carbon, a n d Silica a t 1200° C.
TIME Minules 0
20 40 60 80
100
PZOS .I
-
x
l / t loge
Grams 1 751 (.1) 1 201 0 920 0 674 0 450 0 304
A A - x
....
Av
0.0188 0,0161 0,0159 0.0170 0.0175 0 0170
In each of the experiments the mixture was heated t o the desired temperature in 3 minutes under uniformly regulated conditions, and the time of heating a t the particular temperature was then measured from the end of this initial heating period. A number of experiments showed that with a final temperature of 1200" C. a negligible quantity of phosphorus was volatilized during the %minute initial period of heating, and therefore no appreciable error mas introduced into the calculations by measuring the time of reaction from the end of this initial heating period. Such a procedure was not justified, however, in the experiments a t higher temperatures, because of the rapidity of the reaction and the comparatively large volatilization of phosphorus during the initial period of heating. Consequently, it was not possible to calculate the velocity coefficients for the reaction a t temperatures above 1200" C. Reduction of Mineral Aluminum Phosphate
Mineral aluminum phosphate in the form of wavelli te mas used for a short time about twenty years ago in the manufacture of phosphorus a t Yorkhayen, Pa. (30). Aluminum phosphate was also formerly used to a certain extent in the
1-01, 21, K O .
11
manufacture of sodium phosphate by heating the miueral with sodium hydroxide. Because of its limited use, no extensive search has been made for deposits of aluminum phosphate in this country. but the mineral is known to occur in commercial quantity in the vicinity of RIount Holly Springs, Pa. (.?O), and deposits of unknown extent are said to exist near Coal City, St. Clair County, Ala. (23). It should be possible to use aluminum phosphate in the manufacture of phosphoric acid by furnace processes, piovided it is no more difficult to reduce by carbon than is tricalcium phosphate. In addition to the production of phosphoric acid, the residue would contain a high percentage of alumina, which could probably be used for the manufacture of metallic aluminum. The literature contains no definite information on tlie reduction of aluminum phosphate, although Schloesing (28) states that the "earthy metal" phosphates, presumably includiug aluminum phosphate, are not reducible by carbon alone. In the present investigation several experiments were made with 10-gram mixtures of equal weights of mineral aluminum phosphate and carbon. The figure.; in Table X shoiv the very interesting fact that only 5.4 per cent of the total phosphorus was volatilized in 1 hour at 1050" C. while 71.9 per cent was volatilized in 1 hour a t 1100" C., as compared a-ith 11.7 per cent from mixtures of tricalciurn phosphate, carbon, and silica a t the same temperature. With mixtures of tricalcium phosphate and carbon alone (11) only 2.3 per cent of the total phosphorus was volatilized in 1 hour at 1150" C. The percentage of phosphorus volatilized from aluminum phosphate increased gradually from 83.2 a t 1150" C. to 90.2 a t 1400" C., the incompleteness of the reaction iindoubtedly being due to the formation of iron phosphide, since the aluminiun phosphate originally contained 14.63 per cent of Fe20a. Table X-Reduction of A l u m i n u m P h o s p h a t e by Carbon (Mixtures composed of 5 grams mineral aluminum phosphate and 5 Prams carbon. heated for 1 hour) TEMPERATURE
c. 1050 1100 1150 1200 1300 1400
PHOSPHORUS V O L A T I L I Z E D
Per cent 5 4 il 9 83 2 83 9 88 6 90 2
To determine whether addition of silica t o the charge increases the rate of reduction of aluminum phosphate, 10-gram mixtures containiiig equal weights of aluminum phosphate and carbon and sufficient silica to give a 1: 1 mol ratio of alumina to silica were heated for 1 hour a t 1050" and a t 1100" C. The percentages of phosphorus volatilized were 5.3 and 73.7, respectively, showing that silica has very little, if any, effect in accelerating the reduction of aluminum phosphate a t these temperatures. All the residues from experiments with aluminum phosphate were obtained i n the form of uncaked powders which showed 110 evidence of having been fused or sintered a t any time. The rapidity with which aluminum phosphate is reduced by carbon at comparatively low temperatures and the non-fusion of both the aluminum phosphate and the residual aluminum oxide indicate that it should be admirably adapted to the manufacture of phosphoric acid by the use of rotary kilns or similar types of furnaces in which solid materials can be treated continuously. Mechanism of Reactions
The reaction between tricalcium phosphate and carbon, producing free phosphorus, in the absence of silica is usually represented by the equation Cas(POa)z
+ 5C = 3Ca0 -t 5CO + P?
(1)
I S D U S T R I A L A\?D ESGISEERILY'G CHEMISTRY
Sovenibcr, 1929
while in the pregence of silica the reaction may be represented RS follo\Ts: Ca3(P04)?
+ 3Si01 + 5C
=
YCaSiOs
+ 5'20 + PJ
(2)
I3y varying the quantitieq of silica in the niisturr I 3 we may forinnlate a seriey of equation. differing from Ekpation 2
I /
/!
I
I
(1n1yin the molecular ratios of liine to silica. By substitutillg alumina for silica a siinilar series of equations may be formulated, a possible reaction being represented by the equation
+
Ca3(P04)~ 3A1203
+ 5C = 3Ca(A10J2 + 5CO -t
PS ( 3 )
Honever, these equations give no indication a? to the actual mechanism of the reaction. As a possible explanation for the completentieh of the reduction of solid tricalciuin phosphate by solid carbon. .Jacob and Reynolds (11) have assumed that tricalcium phosphate has certain definite dissociation pressures a t high temperatures. Reactions 1, 2. and 3 may then he ri3preiented A- occurring in tn-o stages, ai: Ca3(P0& = 3Ca0 5c = 5 c o
P206
+
++ P205 PB
1131
The rapid reduction of aluminum pliosphate at 1100" C , as compared with that of tricalcium phosphate under similar conditions, may be explained by the assumption that aluminum phosphate dissociate\ more rapidly and completely at this temperature than does tricalciuin phosphate, which would lie expected in view of the less basic character of aluminum oxide. The fact that addition of silica does not appreciably accelerate reduction of aluminum phosphate a t 1050-1100" C. indicates that compounds of alumina and silica are not formed to any extent at these temperatures and consequently the dissociation of aluminum phosphate is not aflected. According to Rankin and Wright (.24), only one compound. A%1203.sio2, melting a t 1800" C., appears in the binary system alumina-silica. The lowest melting eutectic in this system has the weight percentage composition, &03 13, SiOs S5 and melts at 1610' C., which is about 200" C. higher than the melting points of the lowest melting eutectics in the binary systems lime-silica and lime-alumina. Xielsen (20) attributes the action of silica in promoting rcduction of calcium phosphate to the forination of a calcium silicophosphate, which is more easily reducible by carbon than is the tricalcium phosphate itself. He ascribes to thir compound the formula Ca3(P0J2.3SiO?, and concludes from a study of the cooling curves of mixtures of tricalcium phoiphate and silica that its formation begins a t 1150" C. whilc its melting point is 1630" C. Even if such a compound k formed, it is still necessary to provide an explanation for the fact that it is reducible by carbon at temperatures so far below its fusion point. This can be done on the assumption that the compound dissociates into phosphorus pentoxide and calcium silicate.
(4)
(5)
Although the dissociation pressures of tricalciuin phosphate in the temperature range 1050" to 1200" C., for instance, are undoubtedly quite small, qualitative experiments indicate that at these temperatures free phosphorus pentoxide is reduced very rapidly by carbon which would correspondingly increase the rate of dissociation of tricalciuin phosphate. According to Guernsey, Yee, Richardson, and Brahain (8) 110 loss of phosphorus occurs when tricalcium phosphate alone is heated for 30 minutes at the temperature of the oxyhydrogen flame (above 1800" C.) in a lion-reducing atmoqphere. Silica and alumina are both acidic oxides, the latter being less acid than the former, and their presence would undoubtedly tend to accelerate the dissociation of tricalcium phosphate through the formation of calcium silicates and aluminates, consequently accelerating the composite reactions as represented by Equations 2 and 3. The results given in Table I11 shorn that silica and alumina react quite readily with lime a t temperatures as low as 1200" C. Even in the presence of silica and alumina, however, the dissociation pressures of tricalcium phosphate are probably small below 1500' C. Guernsey, Yee, Richardson, and Braham ( 8 )did not obtain decisive evidence of loss of phosphorus when mixtures of tricalcium phosphate and silica, or alumina, mere heated at 1500" C., but with mixtures containing silica the loss was quite rapid a t the temperature of the oxyhydrogen flame. The fact that the rate of reduction at low temperatures is dependent on the quantity of silica or alumina present (Tables IV and TI)would seem to offer further evidence that these oxides increase the dissociation of tricalcium phosphate into lime and phosphorus pentoxide.
It is possible that the carbon monoxide formed in reaction1. 2, 3, and 5 may itself act as a reducing agent toward-
tricalciuin phosphate according to the equations Ca3(PO& Caz(P04)2
+ 5CO
=
3Ca0
+ 3SiOL + 5CO
+ 3CaSi03 ~ C O J , +Pz + 5c02 + PL
=
((2 (7)
Sielsen (20) did not obtain any reduction when pure tricalciuin phosphate alone was heated a t 1250" C. in a stream of carbon monoxide, and Lassieur (15) reports no reaction a t 1300" C. Segative results were obtained by Meyer (18) in attempts to reduce Thomas slag by carbon monoxide at temperatures up to 1200" C., but according to Xielsen (20) the phosphorou. content of a mixture of tricalcium phosphate and silica decreased 2 per cent on heating for 30 minutes a t 1175" C. in a nickel boat in a stream of carbon monoxide, and SchloeFing (28) states that calcium and aluminum phosphates when mixed with silica are reduced by carbon monoxide a t EL white heat. The above statements furnish some evidence that in the presence of silica carbon monoxide does act as a reducing agent towards tricalcium phosphate. The fact that the
1132
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
presence of silica appears to be necessary in order to obtain any reduction, a t least at relatively low temperatures, would seem to indicate that the reaction is between carbon monoxide and phosphorus pentoxide vapor rather than between carbon monoxide and undissociated tricalcium phosphate. A thorough investigation of the dissociation pressures of tricalcium phosphate and of the action of carbon monoxide on phosphorus pentoxide vapor and on solid tricalcium phosphate is necessary before any positive statements can be made as to the exact mechanism of the reduction of tricalcium phosphate by carbon. In addition to these explanations of the mechanism of the process, we have the possible reaction between carbon and the phosphate in the solid state, which may occur to a certain extent. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11)
Berthier, Ann. chim. phys., 121, 83, 178 (1826). Blome, Metallurgie, 7, 659, 698 (1910); Stahl Eisen, SO, 2161 (1910). Britzke and Dunaev, J. Chem. Ind. (Moscow), 6, 161 (1928). Bryan, Mehring, and Ross, IND. END.CHEJI.,16, 821 (1924). Carothers, Ibid., 10, 35, 239 (1918). Dyes, Chem.-Zlg., 61, 973, 994 (1927). Grandeau, “Trait6 d’analyse des matieres agricoles,” p. 70, Paris, 1877. Guernsey, Yee, Richardson, and Braham, U. S. Dept. Agr., Fixed Nitrogen Research Lab., Mimeographed Report 65, and Appendix (1921). Hernpel, Z . angew. Chem., 18, 132, 401 (1905). Hoeflake and Schaeffer, Rec. h v . chim., 45, 191 (1926). Jacob and Reynolds, INP. ENG.CHEM.,20, 1204 (1928).
Vol. 21, No. 11
(12) Jacob and Reynolds, J. Assocn. Official Agr. Chem., 11, 128 (1928). (13) Jacob and Reynolds, A m . Fertilizer, 70, No. 6, 19 (1929). (14) Kyber. British Patent 256,622 (1927) ; Canadian Patent 274,862 (1927). (15) Lassieur, Ovig. Communications, 8th Intern. Cong. APPlied Chem., 2, 171 (1912). (16) Lerch and Bogue, IND.ENG.CHBM.,18, 739 (1926). (17) Mehner, British Patent 235,924 (1925) ; Canadian Patent 262,339 (1926). (18) Meyer, Mill. Kaiser-bilheZmInsl. Eisenforsch. Dusseldorf, 9, 273 (1927). (19) Neumann, Z . angeiu. Chem., 18, 289, 735 (1905). (20) Nielsen, Ferrum, 10, 97 (1912); A m . Fertilizer, S9, No. 6 , 63 (1913). (21) Orlandi, Notiz. chim. ind., 3, 343 (1928). (22) Partington, “Textbook of Inorganic Chemistry for University Students,” p. 610, London, 1926. (23) Phalen, U. S . Geol. Survey, Mineral Resources of U,S.,1911, Pt. I, 937 (1912). (24) Rankin and Wright, Am. J . Sci., [41, 39, 1 (1915). (25) Readman, J. Soc. Chem. Ind., 9, 163, 473 (1890) Ibid., 10, 445 (1891). (26) Ross, Carothers, and Merz, J. IND.ENG.CHEM.,9, 26 (1917). (27) Ross, Mehring, and Jones, Ibid., 16, 563 (1924). (28) Schloesing, Compt. uend., 69, 384 (1864). (29) Smits and Rutgers, J. Chem. Soc., 125, 2673 (1924). (30) Stose, U. S. Geol. Survey, Bull. 315, 474 (1907); Mineral Resources of U.S., 1906, 1084 (1907). (31) Urbain, British Patent 280,763 (1927). (32) Waggaman, IND.END.CHEM.,16, 176 (1924). (33) Waggaman, Easterwood, and Turley, Chem. Met. Eng., 25, 517 (1921). (34) Waggaman, Easterwood, and Turley, U. S. Dept. Agr., Bull. 1179 (1923). (35) Waggaman and Turley, J. I N D . END.CHRM.,12, 646 (1920); Chcm. Met. Eng., 23, 1067 (1920). (36) Waggaman and Wagner, J. IND.E s o . CHEM.,10, 353 (1918). (37) Wohler, Ann. Phys. Chem. Poggendorff, 17, 178 (1829).
New Solvents for the Removal of Arsenical Spray Residue’ Y
R. H. Robinson INSECTICIDEDIVISION, BUREAU OF CHEMISTRY AND SOILS,WASHINGTON, D. c.
A
are stronger it is difficult t o CHEMICAL washing The action of aqueous solutions of different classes of rinse the fruit sufficiently in process f o r the recompounds and combinations of compounds as solthe water wash to remove all moval of a r s e n i c a l vents for lead arsenate spray residue has been studied. spray residue from fruit has the solvent from the calyx No one compound was found superior to hydrochloric end. This may subsequently been used commercially duracid for removal of the spray residue. result in calyx injury or dising the past two years. Two Combinations of certain sulfates or chlorides with coloration. solvents have been employed hydrochloric acid dissolve far larger amounts of lead The importance of finding -(1) a dilute solution of hyarsenate than hydrochloric acid alone, when the acid a new solvent or combination d r o c h l o r i c acid, and (2) a was used in equivalent concentrations. Furthermore, of s o l v e n t s that would be s o l u t i o n of a mixture of the solvent action rapidly reaches its maximum since more effective and yet not s o d i u m carbonate, sodium little more, if any, of the lead arsenate dissolves in 30 cause injury to fruit so easily hydroxide, and borax. While minutes than in 5 minutes. The combination of is o b v i o u s . A study was remarkable success has atsodium sulfate and hydrochloric acid appears to be the t h e r e f o r e undertaken with tended the use of these solmost practical of those studied. the hope of finding such a solvents, sperial conditions have On account of the few commercial washing tests comvent. made it difficult, and in some pleted thus far, general recommendations cannot be cases impossible, to remove made at this time. The results obtained, however, Solubility of Lead Arsenate the residue below the tolerwarrant further commercial tests to learn whether the in Different Solvents ance of 0.00143 gram arsenicombinations may be advisable. I n a search for a more effecous oxide (AszOJ per kilogram tivesolvent for arsenical mrav of fruit (0.01 grain per pound) (3). Whenever the f&t packer or orchardist experienced dif- residues, the method of estimating its effectiveness is of iara“ficulty in effectively removing the residue, the strength of the mount importance. With some solvents the reaction between solvent was increased, and frequently concentrations that the lead arsenate and solvent may be very rapid or even inwould cause injury to the fruit under unfavorable conditions stantaneous, while with others it may take several hours or were used. As much as 2.5 per cent hydrochloric acid has days to dissolve the maximum amount of the arsenical and been used, while injury may occur if 1.0 per cent is used on for the reacting materials to come to equilibrium. I n the susceptible varieties of apples washed before enough wax has different commercial fruit-washing machines the fruit is exformed to protect the fruit. Furthermore, when the solutions posed to the action of the solvent for periods of time varying between 30 seconds and 2 minutes. Solvents, therefore, that 1 Received June 13, 1929.
