I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1448 (16)
Meneies, -4.W. C., and Humphrey, E. C., OTig. Corn. 8th Intwn.
CongT. A p p l . Chenz., 2, 175 (1912). (17) Rfuromteev, B. A . . and Nanarova, 1,. A., Bull. m a d . sci. Li.R.S.S, Classe sci. math. nut., Sh.chim., 1938, No. 1, 177-84. (18) Obukhov, A . P., and RIikhailova, M. N,,J . A p p l i e d Chem. (U.S.S.R.), 8, 1149-57 (1935). (19) Parker, E. G., J . P h ~ 8Chem., . 18, G53 (1914). (20)
Purdon, F. F., and Slater, Y. S.,"LYqueous Solution and the Phase Diagram," p . 6 5 , London, Edward Arnold and Co., 1946.
(21) (22) (23)
Quimby, 0. T., Chem. Reus.. 40, 160 (1947). Ravich, M. I., Kalil (U.S.S.R.), 1936, KO. 10, 33-7. Richards, T. W., and Churchill. J. E., Z. p h u s i k . Chem.. 28, 314 (1899).
(24) (25)
Vol. 44, No. 6
Russell, C. H., U. S. Patent 2,436,670 (Feb. 24, 1948). Schroeder, W. C., Berk, A. A , , and Gabriel, A., J. Ant. Chem. SOC.,59, 1783-90 (19373.
Seidell, A., "Solubilities of Inorganic and Organic Compounds," p. 310, New York, D. Van Nostrand Co., 1907. (27) Ibid., 2nd ed., Tal. 2. pp. 1287, 1289, 1290 (1928). (28) Shiomi, T., Mem. C'oZZ. Sci. and Dng., ICuoto I m p . Univ., 1, 406 (26)
(1908).
Smith, J. H., J . SOC.Chem. Ind. (London),36, 420 (1917). (30) Teeple, J. E., "Industrial Development of Searles Lake Brines." p. 164, S e w York, Chemical Catalog Co., 1929. (31) U'addell, M. D., U. S. Patent 2,375,054 (May 1, 1945). (32) T47estbrook,L. R., Ibid., 1,711,707 (May 7, 1929). (29)
RECXIVED for review M a y 29, 1931.
Equilibrium in the
ACCEPTED
January 25, 1952.
Silicon
Tetrafluori F. A. LENFESTY, THAD D. FARR, AND J. C. BROSHEER Tennessee Valley Authority, Wilson Dam, Ala.
ASES containing water vapoi, fluorine compounds, and siliceous dust are evolved in the manufacture of fertilizers from rock phosphate. The fluorine compounds can be a nuisance, and their recovery in conimercially useful forms is desirable. Knowledge of t h e equilibrium SiF, (9)
+ 2Hz0 (g) = Si02 (s) + 4 H F ( g )
(1)
is useful in the development of methods for recovery of the fluorine compounds. Baur ( 1 ) reported experimental data on the equilibrium at 104 O and 270" C. I n t h e present work, equilibrium constants for the reaction shown in Equation 1 were determined experimentally a t several temperatures in the range from 200 ' t o 800 O C. A carrier gas containing hydrogen fluoride and water vapor was passed through an equilibration chamber t h a t was filled with silica. The hydrogen fluoride and silicon tetrafluoride in the effluent were absorbed in sodium fluoride a t 105' C., the water vapor was absorbed in anhydrous calcium sulfate (Diierite), and the volume of carrier gas was measured with a wet-test meter. The hydrogen fluoride and silicon tetrafluoride were separated by heating the absorption tube to 350" C. t o drive off the hydrogen fluoride. The reactions utilized in the determination of hydrogen fluoride and silicon tetrafluoride weie for absorption at 105 ' C.
+ HF = NaHFg 2NaF + SiF, = NazSiFG NaF
(2) (3)
and for desorption a t 350" C. NaHFz = K a F
+ HF
(4)
Wartenberg and Bosse ('7) found t h a t sodium bifluoride decomposes rapidly at 270' C., and Hantke ( 8 )reported that the decomposition pressure of silicon tetrafluoride over sodium fluosilicate at 300" t o 450" C. IS less than 1 mni. of mercury.
TABLEI.
HYDROGEN FLUORIDE OVER SODIUX BIFLUORIDE
PARTrAL PRESSURE O F PHF,
I01 126 150 a
200 From (8)
Mm. Hg 0.41 2.8
7.5
87c
Temp.,
c.
205 2.50 275
PHF,
Mm. H g 129 422a 706a
Froning and coworkers ( 2 ) measured the vapor pressure of hydrogen fluoride over sodium bifluoride a t 200", 250 ', and 275 C. Their results and the results of similar measurements made by TVA in the range from 100" to 205" C. are shown in Table I. The two set's of data are represented by the equation log P H F (mm.) = 9.97 - 3830/T (5) The partial pressure of hydrogen fluoride over sodium bifluoride was found t o be less than 1 mm. of mercury a t 100" C. Water vapor had no appreciable effect on the vapor preasure of hydrogen fluoride over sodium bifluoride. Hydrogen fluoride and silicon tetrafluoride thus can be absorbed quantitatively in sodium fluoride a t 105" C. and so freed from water vapor. The hydrogen fluoride can be expelled subsequently a t 350" C. without loss of silicon tetrafluoride. APPARATUS 4 N D MATERIALS
The measuring apparatus is shown in Figure 1. All parts that were exposed t o hydrogen fluoride were of h e silver. The equilibration chamber was sealed with a ring gasket of 1.6-mm. silver wire. When the apparatus was heated, the gasket froze to the silver surfaces. T o open the reaction tube, the gasket was cut out with a hack saw. The surfaces were redressed for subsequent closure. The connection between the equilibration tube and the hydrogen fluoride-silicon tetrafluoride absorption tube was subjected t o temperatures no higher than 350" C., and a Teflon gasket between the flanges, which were held together by a spring clamp, formed a gas-tight seal. The equilibration t'ube was heated electrically in a vertical tube furnace that had two concentrically mound resistance element?. The current applied t o the outer element, which extended the full length of t h e furnace, was controlled manually a t a temperatuxc 20' t o 25' C. below t h e experimental temperature. The power input t o the other element, which was centered on the equilibration tube, was controlled thermostatically a t the experimental temperature. The maximum variation of temperature within the equilibration tube during a run was f 3 " C. The tube connecting the equilibration tube t o the absorption tube was maintained below the equilibration temperature but above 100" C. The absorption tube was heated a t about 105' C. with a small tube furnace. Temperatures in the equilibration tube, hydrogen fluoride generator, and the furnaces were measured with thermocouples. The pressure on the system was measured with a manometer con-
June 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
nected t o the top of the equilibration tube by a capillary. The back pressure in t h e system generally was less than 10 mm. of mercury except in t h e experiments at 800" C. where it was about 30 mm. of mercury.
1449
Hydrogen fluoride was passed through the tube at t h e experimental temperature until t h e silica gel was saturated with fluorine compounds. The adsorption of fluorine compounds was more pronounced at the lower temperatures than at the higher temperatures. From 7 t o 12 liters of carrier gas were passed through t h e system in each experiment. Tests were made at each temperature with hydrogen fluoride alone and with both hydrogen fluoride and water vapor. At the end of an experiment, the sodium fluoride absorption tube and the Drierite tower were weighed. The sodium fluoride
+2
*-
200oc.
0 I I MM.I.D., 1MM.WALL
-2 BALITE p.
Y
w-4 0 A
V I T R E O U S SILICA
-6 ABSORPTION TUBE
-8
FLANGE TO MATCH' ASSORPTION TUBE
I
0
A
THIS WORK BAUR (1)
1
d
EOUlLlBRATlON TUBE
Figure 1.
EXPERIMENTAL
Fine Silver Apparatus
-IC C
I
1.0
I
I.5 IOOO/T
I
2.0
2.5
The equilibration tube was charged with minus 10- plus 20mesh silica gel. Fused quartz of the same particle size was Figure 2. Equilibrium in the System Silicon unsatisfactory, because equilibrium was not attained a t practiTetrafluoride-Water cable rates of flow of the reactant gases. The carrier gas was nitrogen. Hydrogen fluoride was introduced by passing the nitrogen through TABLE 11. EXPERIMENTAL DATAAND EQUILIBRIUM CONSTANTS heated sodium bifluoride in a copper tube. When Wt., a t Equilibrium Carrier Val. of Gas, Pressurea, Total water vapor was added, the nitrogen was bubbled Temp., through water at 52" C. upstream from the hyO c. HF SiFa HzO Liters, S.T.P. Mm. Hg Log K p (atm.) drogen fluoride generator. 200 0.0073 0.8551 0.2366 9.68 749.0 9.438 0.0818 1.4819 1.0416 9.96 749.8 g. 046 The sodium fluoride absorbent was prepared 0.0218 0.7328 0.5817 754.5 7.00 7.732 from a paste of reagent-grade sodium fluoride and 0.0073 0.8936 0.7136 744.8 9.99 -5.416 water. The paste was dried a t 110" C., and the 300 0.0437 1.2745 0.9243 9.88 756.7 6.140 resultant cake was broken into lumps and sieved 0.0408 1.2023 0.4398 9,99 757.2 c.713 t o minus 8 plus 14 mesh. 0.0348 1.1622 0.4141 9.91 758.3 5.510 ( 3 . 9
OPERATING PROCEDURE
T o charge the equilibration chamber, two layers of platinum gauze, separated by a 2-cm. layer of single-turn silver helices, were placed in t h e bottom. The tube was filled with silica gel in 2-cm. layers t h a t were separated by silver washers. Each washer had a segment removed from one edge for passage of gas, and these openings were staggered t o prevent channeling of the gas.
0.0544 1.5591 1.2077 0.0441 0.9836 0.8272 0.0362 1.1791 0.4345 400 0.0826 1.2745 0.3102 0.0715 1.1691 0.3464 0.1499 1.5434 1.0280 600 0.3894 1.5049 0.5615 0.2883 1.0610 0.3897 0.3933 0.9453 0.7774 800 0.4066 0.5734 0.2477 0.6204 0.3732 0.6226 Atmospheric pressure plus back pressure.
Q
12.23 8.98 9.82 9.96 7.35 12.37 12.40 9.85 9.78 9.87 9.81
752.9 755.7 750.7 749.8 742.2 750.0 751.3 747.5 744.6 787.1 782.9
6.105 6.410 6.479 4.217 s.024 4.008 2.212 z.262 2.230 1.548 1.641
-
1450
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 6
Baur ( 1 ) for 104" C. and 2iO" C. are given in Figure 2 ; the value for 270" C . is in good agreement (SiF4 ( g ) + 2Hz0 (9) = 4 H F (9) + Si02 (8)) wit,h t,he results of the present work, but t,he value Constants in Thermodynamic Functions for 104" C. is incorrect. Temp. Range, F o r m of Si02 (s) K. a b c x 103 d x 10-6 i Ryss (6) calculated equilibrium constant,sfor the Vitreous 298-2000 26,220 -6.470 0.41 -1.08 -1.x1 reaction from thermodynamic data. His calculaa-Quartz 298-848 -1.497 tions mere based upon data for amorphous silica 24,165 -8.842 @Quartz 848-2000 a-Cristobalite 298-523 26,810 14.483 -8.28 -2.80 -68.20 and 6-quartz at several temperatures between 25" 24,320 -8.819 1.23 -2.80 4.91 6-Cristobalite 623-2000 a-Tridymite 298-390 27,150 16.809 -10.1,j -2.80 -64.02 and 330" C., a range in which P-quartz is not a 6-Tridymite 390-2000 24,830 -7.069 0.93 -2,80 -0.52 stablephase. Theconstantsdeterminedin thepresa The functions ent experiments fall between those calculated by A H o (calcd.) = a - 0.4343 bT - cT? f 2 dT-1 Ryss for amorphous silica and quartz. AFO (calcd.) = a + bT log T + c T 2 + d T - 1 4- iT 106 K (atm.) = -0.2186 (aT-1 + b log 2' + c T + d T - ? + i) The results of calculations based upon more rewere derivezfrorn entropies and heats of formation given, by the National Bureau of Standwith the ards (6) and heats of transition and heat capacity equat:ons given by.Kelley (41, cent and more complete thermodynamic data (,$, 5 ) assumptions that Si02 has unit activity, the total equilibrium pressure IS 1 atmosphere, and are shown in Table 111. The logarithms of the the gaseous components behave ideally. equilibrium constants for reaction 1 with vitreous silica and with a- and P-cristobalite are plotted in Figure 2. The equilibrium constants for a-quartz, a-cristobalite, and a-tridyniite are practically the same, as are tube then was heated a t 350' C. and flushed Tvith dry nitrogen t o those for P-quartz, 8-eristohalite, and 8-tridymite. The fact constant weight. From the weights of the absorption tubes the t h a t t,he experimental line in Figure 2 crosses both the vitreous amounts of hydrogen fluoride, silicon tetrafluoride, and water in silica line and the 6-cristobalite line indicates that equilibrium the effluent gas n-ere obtained. in the system silicon tetrafluoride-water is influenced by the The measured volume of carrier gas was corrected to standard form of the solid silica. The agreement of the experimental conditions, and partial pressures of the reactants were calculated. data with the vitreous silica line a t low temperatures and with The equilibrium constant, K , was expressed as the P-cristobalite line a t high temperatures indicates that Equation 7 is of practical utility over the temperature range from 100" t o 1 i O O " C. When the apparatus was opened a t the conclusion of the study, where the p terms are partial pressures in atmospheres. a small amount of finely divided silica was found among the silver helices in the bottom of the equilibration tube. This silica EVALUATION O F RESULTS n-as identified as cristobalite by microscopic and x-ray examinaThe experimental data and the logarithms of the equilibrium tion. The silica in the main portion of the equilibration chamber constants calculated therefrom are shown in Table 11. The equiwas largely cristobalite with some unchanged silica gel. librium constants are plotted in Figure 2. The results obtained LITERATURE CITED at 200' C. are erratic, but those for higher temperatures are reasonably consistent. The absence of a systematic variation in (1) Baur, E., 2.physik. Chem., 48,483-503 (1904). K p with variation in pHPoindicates that equilibrium was attained (2) Froning, J. F., Richards, M. X., Stricklin, T. W., a n d Turnbull, S.G., IND.EKG.CHEM.,39, 275-8 (1947). and that the variations in K , are due to inherent analytical errors. (3) H a n t k e , G., 2. angew. Chem., 39, 1065-71 (1926). The equation (4) Kelley, K. K., U. 8.Bur. Mines, Bull. 476 (1949).
TABLE 111. THERMODYNAMIC FUXCTIONS~ 7
7
-;,;;
243010
log K p ( a t m . ) = 5.547 - 6383/T
-'!:!:
(7)
represents the experimental data for 300 O , 400 ', 600 ', and 800' C. with a maximum deviation of 4.6%. The average experimental values for the four temperatures and the values calculated from the equation agree, however, within 0.6%. The average values of the equilibrium constants determined by
( 5 ) Kational Bureau of Standards, "Selected Values of Chemical Thermodynamic Properties," Sei-. I, 1947-48. (6) Ryss, I. G., J . P h y s . Chenz. (U.S.S.R.), 14, 571-81 (1940). (7) Wartenberg, H. v., a n d Bosse, O., 2. Elektrochenz., 28, 384-7 (1922 j
.
RECEIVED for review December 3, 1961. ACCEPTED January 28, 1962. Presented before the Southmide Chemical Conference, Wilson D a m , .41a., October 18-20, 1951.
CORRESPONDENCE Algebraic Representation of Vapor-Liquid Equilibria SIR: I n connection Kith the paper entitled "An Algebraic Representation of Vapor-Liquid Equilibria" by Prahl @),it should be mentioned t h a t although the author of t h e article presents an excellent correlation for vapor-liquid equilibrium data, he has neglected t o point out its important thermod3mamic significance and the limitations of the method which are brought out thereby. Starting out with Equation 3 of the Prahl article and applying the method outlined in another paper ( I ) , this thermodynamic significance may be shown as follow^:
From Equation 3 of Prahl ( 2 ) a21
=
- 22 c BA+ 22
(1)
Taking logarithms and differentiating d 1%
0121
[-
-1 1 = 2.303 __ A -
~2
1 + By dx2 ] - 2
From Equation 9 of Gilmont et nl. ( 1 )
(2)
