1280
INDUSTRIAL AND ENGINEERING CHE~lIITlRY
increased from 20" to 70" C., the coefficient of friction remained constant. At 70" C. the frictional coefficient began to rise rapidly.
PLASTICITY OF GREASES Although the lubricating greases are usually regarded as plastics, the relationships between unit stress intensity and rate of flow of a grease are not exactly those specified by the laws of ideal plastic flow. There is no very definite yield point or minimum stress intensity below which no deformation or flow of the grease occurs, and the mobility or rate of change of deformation with change in stress intensity is not constant but increases with the rate of shear ( I ) . As the stress on a grease is gradually increased from zero, the rate of shear increases continuously (at first slowly and then more rapidly) until finally the ratio of the rate of deformation to the stress intensity becomes approximately constant. The plasticities of several greases were measured, using the plastometer described by Rhodes and Wells ( 2 ) . The efflux tube of this plastometer was 5.77 cm. long and had a radius of 0.1928 cm. The pressure required to force the grease through this tube was provided by applying nitrogen, under pressure, to the surface of the charge in the plastometer. The instrument was so constructed that the grease within the supply chamber could be stirred if desired. Plasticity measurements mere made on three samples containing 25 per cent of sodium oleate and, respectively, no glycerol, an amount of glycerol equivalent to the soap present, and five
Vol. 23, Yo. 11
times this amount of glycerol. The stirring mechanism was detached, so that the data indicate the characteristics of the material in the unmorked condition. The results are shown in Figure 5 . At 0" C. the presence of the normal amount of giycerol stiffens the mass; with large concentrations of glycerol the grease becomes even thinner than when none is present. At 25.8" C. somewhat similar effects are observed but the differences between the samples are smaller. At 60.5" C. the sample containing no glycerol is much more fluid than either of the others. With increasing concentration of glycerol there is a decrease in the change in consistency with temperature. Determinations were also made (at 25.8" C.) in which the grease was stirred. The agitator in the plastometer was rotated a t 50 revolutions per minute. The results are shown in Figure 6. The sample containing no glycerol broke down quickly and ran through the efflux tube under its own hydrostatic head. The grease containing the normal amount of glycerol showed much less tendency to disintegrate; the one containing five times the normal amount showed practically no change in consistency. The glycerol serves to minimize the change in consistency of the grease with working.
LITERATURE CITED (1) Aneson, ISD. E ~ CHEV., G 24, 71 (1932) (2) Rhodes and Wells, Ibzd., 21, 1273 (1929). (3) Wilson and Barnard, Ihid., 14, 683 (1922) RECEIVED M a y 15, 1933
Reaction between Urea and Gypsum COLINW. WHITTAKER, FRARK 0. LUNDSTROM, AND STERLIXG B. HENDRICKS Bureau of Chemistry and Soils, Washington, D. C.
M
Crea reacts with gypsum i n the presence of moisture to set free the water of hydration the
opportunity for reaction between the urea and various s u b s t a n c e s present, e v e n in gypsum and form the complex CaSO4.4CO(NHd2. the absence of a visible liquid The Physical, optical, and chemical properties Of phase, This paper presents the this compound are described. It has beenfound results of a s t u d y of the reto be less hygroscopic than urea and therefore a c t i o n between urea and one would not in itself impair the mechanical conof the c o m p o n e n t s of superp h o s p h a t e , calcium s u l f a t e phosphate the with the bedifion of a fertilizer mixture in which if was dihydrate or gypsum, and the comes intimately mixed formed. If the reaction Were to 90 to completion properties of the c o m p o u n d other solids present. A kn0w.lin a n acerage commercial fertilizer containing formed in that reaction. edge of the reactions that may Urea and superphosphate the increase in free The u r e a a n d calcium s d occur b e t w e e n u r e a and the fate dihydrate used were C. P. various constituents of supermoisture would be only 0.2 to 0.4 per cent. p h o s p h a t e is fundamental to grade. lt7hen heated to conitant weight a t 350" to 400" C., a proper understanding of this problem. The use of urea in place of other forms of nitrogen the calcium sulfate lost weight equivalent to a water content in mixed fertilizers raises new problems among which is the of 20.8 per cent. The theoretical water of crystallization is effect of urea on the mechanical condition or drillability of the 20.93 per cent. Saturated salt solutioiis in desiccators furnished the various relative humidities used in the hygromixture containing it. Urea is known to form complexes with many salts, especially scopicity work and in determining the loss of moisture from with those having water of crystallization, although the fact urea-gypsum mixtures. The desiccators were kept in a that a given salt has no hydrates does not preclude the for- constant-temperature room a t 30" C. mation of a urea complex with that salt. A few typical REACTION BETWEE?; UREA.LVD C a L c I u I l r SULFATE IX THE examples of urea complexes with inorganic salts are as follows PRESENCE OF A LIQUID PHASE (8): Ca(N03)2,4CO(r\"2)2, KH4C1CO(NH2)2, CaI2.6COEqual amounts of calcium sulfate dihydrate and urea were (MI&,C a I ~ C 0 ( N H 2 ) , . 2 H ~HgCl2.CO(NH2)2, O, AgKO&O( S H & The spraying of superphosphate with a solution of added to an aqueous solution of the latter saturated a t 30" urea in ammonia (6) deposits the urea in a finely divided form C., and the whole was maintained a t that temperature in a on the granules of the superphosphate, thus providing thermostat with occasional shaking. Microscopic examina-
ODERN processes of
ureamanufactureprod u c e a s o l u t i o n of urea in a m m o n i a that can be economically used to ammoniate s u p e r p h o s p h a t e . The ammonia reacts with the acid c o n s t i t u e n t s of the s u p e r -
Kovember, 1933
ISDUSTRIAL AND ENGINEERING CHEMISTRY
tion of the solid phase after about 2 days showed only calcium sulfate dihydrate crystals although the original solution had been saturated v-ith urea. Additional amounts of urea were added periodically and it soon became apparent that crystals of a third substance were being formed at the expense of the dihydrate. The urea additions were continued until gypsum crystals could no longer be found in the solid phase; a final careful examination was then made with the petrographic microscope, both of crystals immersed in their own mother liquor and of crystals freed of mother liquor. Inspection of hundreds of crystals failed to reveal a single crystal of either urea or calcium sulfate dihydrate. The absence of urea was indicated by failure to obtain uniaxial int'erference figures. All crystals showed a relatively high birefringence whereas that of gypsum cryst'als is low. Moreover, both urea and gypsum crystals are characteristic in appearance. The crystals were sucked dry on a vacuum filter, washed several times with alcohol, and dried by repeated evacuation. OF CRYSTALS. The composition of the COJIPOSITIOI~ crystals was determined by a n indirect method ( 3 ) , by macroanalysis of a representative sample, and by niicroanalysis of selected crystals.
1281
sulfate) determined microchemically by the methods of Pregl (7) with some of the modifications suggested by Weygand (9). Determinations were also made on a sample of small (about 60-mesh) but otherwise unselected crystals. The results are summarized in Table 111. The small crystals run high in ash (calcium sulfate), as would be expected if etching by alcohol had occurred, whereas the large crystals with bright faces give close to theoretical results. TABLE111. MICROCHEMICAL ANALYSESO F URE.4-CALCIUM SULFATE COMPLEX C
Hz
N?
%
%
%
.40H
%
LARGE CRYSTALS N I T H VERY BRIGHT F I C E S
...
12.8 12.9
4.2
12.7 12.9
4.1 4.1
29.6 30.0
36.2 36.3
SMALL CRYSTALS
28.8 28.9
37.2 37.3
CALCULATED
12.75
4.28
29.78
36.18
Although the above results appear to establish the composition conclusively, further checks were obtained from the crystal structure data and from the molecular volumes and The mother liquor in equilibrium with the solid at 30" C. was sampled with a pipet, the contents of vihich were weighed in a refractions. The unit of structure is a parallelopipedon, the closed dish and made up to volume. The wet solid was sampled edges and angles of Ivhich are knonm from a n x-ray study of after decantation of the mother liquor, dissolved in cold dilute this compound ( 5 ) . From these values and the density and hydrochloric acid, and made up to volume. from the fact that the unit of structure contains four molecules Nitrogen was determined by the Gunning method ( 2 ) and the calcium was weighed as calcium sulfate. An aliquot was evapo- of CaS044CO(SH2)*,the molecular weight is calculated to rated to dryness in a platinum dish, ignited gently to drive off be 388 compared with the theoretical of 376. The agreement urea, evaporated again with a little dilute sulfuric acid, and ig- is within the accuracy of the x-ray measurements. nited just to dull redness after driving off sulfur trioxide. In the The molecular volume calculated from the density is 209 case of the samples of mother liquor it was necessary to remove the urea by extraction with alcohol previous to ignition in order whereas that obtained by adding four times the molecular to avoid spattering. After the alcohol extraction the calcium volume of urea to that of anhydrite is 226. The molecular sulfate was brought into solution with hydrochloric acid, evnpo- refraction, taken as the square root of the mean of the squares rated, and treated further as above. All determinations were of the molecular refractions calculated by the Lorenzmade in duplicate. The results are summarized in Table I. Lorenz equation for each refractive index1 is 69.0 while that obtained by adding those for urea and anhydrite calculated OF MOTHERLIQUOR ASI) WET SOLID TABLE I. COMPOSITION in the same manner is 70.5. The agreement in both cases is MOTHERL I Q K O R ~ WET SOLID within that usually obtained on compounds of this type.' Sample .I B -4 B 26 6 26.2 1.2 1.2 CaSOi, % 54.7 60.9 60.9 54.8 Urea, % (calcd. from Si) a Density of mother liquor at 30' C. = 1.157 grams per 1 ~ (from . weight of pipetted samples).
These results, plotted on a triangular diagram, give points through which two lines may be drawn, one intersecting the base line at 63.3 per cent urea, the other at 63.6 per cent. The theoretical urea content of CaSO4.4CO(NHz)z is 63.83 per cent. The dry salt described above was analyzed for nitrogen and calcium by the methods already described and, in addition, for urea by the urease method (4) and for calcium by precipitation as the oxalate and weighing as calcium oxide. The results are summarized in Table 11. TABLE11. AKALYSESO F UREA-CALCIUM SULFATE COMPLEX METHOD
Cas04
%
UREA %
Gunning .. 61.2 Urease 61.5 Calcium sulfate detd. as Cas04 38: 4a .. 38.6 Calcium sulfate detd. as CaO Required for CaS04,4CO(NHz)z 36.18 63:S2 a Determined on a different batch of crystals which were prepared in the same manner.
These results show the absence of water of crystallization but do not agree as closely as might be desired with the composition CaS04,4CO(NH2)2.Examination of the crystals showed many dull faces indicative of etching, and subsequent work indicates t h a t the urea is slowly removed by alcohol. Large crystals with bright faces were therefore carefully selected, and carbon, nitrogen, hydrogen, and ash (calcium
REACTIOX BETWEEN UREAAKD GYPSUM IN LIQUID PHASE
THE
ABSEKCEOF A
The fact that urea combines with gypsum in the presence of a liquid phase does not necessarily mean that the two solids would react when dry, or nearly so, as in an ordinary mixed fertilizer, I n order to clarify this point, 100-mesh urea was mixed with 100-mesh calcium sulfate dihydrate in the ratio of four moles to one and kept at 30" C. for 7 weeks a t various relative humidities. These samples were weighed out separately and mixed carefully with a spatula. The change in weight and condition of the samples were noted weekly. The results are summarized in Table IV. If the urea replaces the water of crystallization of the gypsum to form CaS04.4CO(NHz)2,samples a t lorn relative humidities should lose weight. On the other hand, if no reaction takes place, none of these samples should lose weight other than that caused by loss of free or adsorbed moisture originally present. As will be shown, both CaS04,4CO(NH2)zand urea tend to dissolve when kept above certain relative humidities. I t is evident, then, that a mixture of gypsum and urea mould tend to take up water and go into solution above a certain relative humidity even if no water of crystallization was replaced by the urea. I n Table I V the weight lost each week, expressed as per cent of the total water of crystallization present, is shown for each relative humidity used, It is recognized that part 1 Values for urea and anhydrite were taken from International Critical Tables.
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
1282
of the water liberated by the reaction was retained in the sample by either adsorption or solution, especially a t the higher relative humidities, so that the loss in weight is not a true measure of the extent of the reaction except possibly a t the lower humidities; nevertheless, several facts are apparent from the tabulated data.
Vol. 25, No. 11
liquid phase accompanied by large gains in weight. This material is therefore less hygroscopic than urea itself which tends to become wet a t relative humidities above 72.5 per cent (1).
The foregoing experiments do not prove conclusively that urea reacts with gypsum in an actual fertilizer mixture, but it is not improbable that such reaction does occur. I n ordiTABLEIv. WATER LOSTFROM URE.4-CALCIUM SULFATE DIRY- nary practice 30 to 50 pounds of urea are added per ton of DRATE MIXTURES AT VARIOUSRELATIVE HUMIDITIES superphosphate. This urea, if it all reacted as described, RELATIVE Loss O F WATERAFTER (WEEHE): would liberate 5 to 7 pounds of water and would increase HUMIDITY 1 2 3 4 5 6 7 Total the free moisture by only 0.2 to 0.4 per cent. Where urea is % % % % % % % % % 0 0.39 2.00 0.61-0.22a-0.16 -0.06 2 .-56 added in an ammonia-urea solution, this could be readily 59.4 0.22 -0.11 3.05 5.10 6.54 9 . 2 1 l i : f 6 36.77 compensated for, if desired, by using a slightly more concen0.61 1.77 2.11 3.55 4.66 7.43 9 . 7 6 29.89 61.9 64.5 4.22 16.81 43.04 13.48 5.49 3.27 2.33 88.64 trated solution. Since CaSO4,4CO(NH& is less hygroscopic 67.5 11.04 42.10 27.34 5 . 9 9 2.33 1.28 1 . 1 1 91.19 70.1 0.17 1.16 2 . 8 3 10.37 3 9 . 5 0 14.03 4 . 8 8 72.94 than urea itself, it is likely that the net result would be an . .. 72.4 -8.32 21.52 9 . 2 6 52.36 -4.93 11.54 14.03 improvement in the condition of the mixture. 7 5 . 2 -32.00 16.58 20.07 3 . 8 3 1.39 6 . 9 3 -2.93 . . , 7 7 . 2 -59.34 - 6 . 7 1 - 8 . 3 7 - 7 . 2 1 -7.65 .94 ... 7 9 . 2 -22.99 -166.5-159.5 . . . ... 18.14 ... - 2... ... PHYSICAL AND CHEMICAL PROPERTIES OF CaS01.4CO(hTH2)2 Minus signs indicate gains in weight. I
The sample stored a t zero relative humidity (over PzOj) came to constant weight in 6 weeks, having experienced a total loss in weight corresponding to 2.56 per cent of the total water of crystallization present or 0.23 per cent of the total sample. A sample of the calcium sulfate dihydrate used lost 1.1 per cent on being brought to apparent constant weight over P z O ~and , a sample of the urea used lost 0.004 per cent. The difference is perhaps due to moisture taken up while mixing the sample. I n any event it is evident that very little water of crystallization was lost when this mixture was stored over P205. The initial rates of loss of the samples a t 59.4 and 61.9 per cent humidity Tvere low but increased steadily until the end of the seventh week, when the experiment was stopped. The rates of loss of the next three samples rose rapidly to a maximum, then fell off, The samples a t 72.4, 75.2, and 77.2 gave erratic results. Evidently this is the region where the water tends to stay in the material, and the extent of the reaftion a t these humidities cannot be judged by the change in weight. This is further evidenced by the condition of the samples: those a t 70.1 per cent relative humidity and below were slightly caked but dry in appearance; that a t 72.4 was lumpy; and those a t 75.2 and 77.2 were definitely sticky. The samples a t 79.2 and several above that humidity (not tabulated) developed liquid phases and large gains in weight. The highest total loss occurred a t 67.5 relative humidity. These results indicate that the reaction between solid urea and gypsum tends t o occur a t ordinary temperatures if a small amount of moisture is present and that the rate of reaction increases with increasing moisture content up to a certain point. It is evident also that the reaction goes nearly to completion in a fairly short time. At a relative humidity of 67.5 per cent it was 80 per cent complete in 3 weeks as judged by the loss of weight. HYGROSCOPICITY OF CaSO4,4CO(NHz)z One-gram samples of CaS04.4CO(NH& were placed in thin layers in small weighing dishes and stored a t 30” C. a t relative humidities ranging from 59.4 to 87.8 per cent in steps of about 2.5 per cent. Each sample was weighed weekly and its condition noted. Those samples a t 59.4 and 61.9 relative humidity experienced no change in weight, and, although those between 61.9 and 79.2 showed small initial gains, all samples a t humidities below 79.2 reached constant weight in 3 weeks. All these samples remained free-flowing, except those a t 75.2 and 77.2 per cent relative humidity which were slightly caked. Those a t 79.2 per cent relative humidity and above became wet and developed a definite
The urea in the CaS04,4CO(NH2)2molecule is dissolved readily by water but only slowly by 95 per cent alcohol; hence the crystals may be freed of mother liquor by rapid washing with alcohol. On gentle heating over a free flame, urea is volatilized from the compound without fusion. The complex is nonhygroscopic a t ordinary humidities and can be dried a t 100” C. without perceptible loss of urea if the heating is not too prolonged. The density determined by t h e centrifugal suspension method ( 5 ) is 1.8006. Well-formed prismatic crystals are obtained when prepared as described here. The refractive indices, determined by the immersion method a t 25” C. for the sodium D line are cy = 1.523, p = 1.583, and y = 1.615. The crystals are triclinic and pinacoidal, and are optically negative with 2V, calculated, 70” C. The acute bisectrix is approximately parallel to the axis of the prism zone. ACKXOWLE DGNEXT The writers wish to acknowledge their indebtedness to Mildred S. Sherman who made the microchemical determinations. LITERATURE CITED (1) Adams, J. R., and hlerz, A. R., IND. ENQ.C H E x f . , 21, 305 (1929). (2) Assoc. Official Agr. Chem., Methods, 3rd ed., p. 20 (1930).
(3) Findlay, A., “The Phase Rule and Its Applications,” 6th ed.. p. 264, Longmans, 1927. (4) Fox, E. J., and Geldard, W., IND. ENG.CHEM.,15, 743-45 (1923). (5) Hendricks, S. B., J. Optical SOC.Am., 23, 299 (1933). (6) Parker, F. W., and Keenan, F. G . ,Am. Fertilizer, 77, 11 (1932). ( 7 ) Pregl, Fritz, “Quantitative Organic Microanalysis,” 2nd English ed., tr. from 3rd rev. German ed. by Ernst Fyleman, Blakiston, 1930.
(8) Thorpe, T. E., ”Dictionary of Applied Chemistry,” Vol. VII, p 275, Longmans, 1927. (9) Weygand, G . , “Quantitative analytische Mikromethoden der
organischen Chemie in vergleichender Darstellung,” Akademische Verlagsgesellschaft, Leipzig, 1931. RECEIVED June 7 , 1933. Presented before the Division of Phyeical and Inorganic Chemistry at the 85th Meeting of the American Chemical Society. Washington, D. C , March 26 t o 31, 1933.
Correction Through an error in typing, the units of columns 4, 5, and 6 of Table I, and columns 2 and 3 of Table IV, of the article b y Fancher and Lewis, “Flow of Simple Fluids through Porous Materials” [IND.ENG.CHEM.,25, 1139-1147 (1933)], should be Ff. X I O 4 , Ft. X 1G6, Ff. x 105, and Av. DIAMETER ( X IO3), respectively, instead of Ft. x 10-4, Ff. X 10-6, Ff. X 10-5, and Av. DIAMETER ( x 10-3).
GEORGEH. FANCHER
