INDUSTRIAL AND ENGINEERING CHEMISTRY
973
proximate variation in concentration of alkali is also shown in Figure 2. However, when much gentler agitation, thicker liquid films (0.019 cm.), and slower absorption rates are encountered, as in the experiments of Hitchcock (4), the products (bicarbonate ion and carbonate ion) must attain correspondingly higher concentrations near the interface to cause them to diffuse through the liquid film into the main body of the solution. As the concentrations of the products rise, the concentration of the alkali from which they are produced decreases near the interface, to the point where the assumption that i t is constant throughout the liquid film is no longer justified. The rates of absorption of carbon dioxide obtained by Hitchcock ( 4 ) and by Davis and Crandall (2) depend rather on the rate of diffusion of alkali into the liquid film, and hence on the first power of the alkali concentration, as has already been stated by others (IO).
Nomenclature concentration of COZ, moles per cc. or per liter, at a point 5 cm. from interface concentration of COn at liquid-gas interface total differential
VOL. 32, NO. 7
partial differential increment D = diffusion constant, sq. cm. per second k = reaction rate constant In = Napierian logarithm OH-) = concentration of alkali, moles per liter ( t = time, seconds x = distance into liquid from gas-liquid interface, cm. b = A =
Literature Cited (1) Arnold, J. H., J . Am. Chem. Soc., 52, 3937 (1930). (2) Davis, H. S , and Crandall, G. S., I b i d . , 52. 3757 (1930). (3) Faurholt, C. J. chim. phys., 21, 400 (1924). (4) Hitchcock, L E., IND. ENQ.CEIEM., 26, 1158 (1934); 29, 301
(1937).
(5) International Critical Tables, Vol. 111, p. 260, New York, McGraw-Hill Book Co., 1928. (6) Ledig, P. G.,and Weaver, E. R.. J . Am. Chem. SOC.,46, 050 (1924); Ledig, IND.ENQ.CKEM.,16, 1231 (1924). (7) Lewis and Randall, “Thermodynamics”, p. 576, New York, McGraw-Hill Book Co.. 1923. (8) Saal, R.N.J., Rec. k a v . chim., 47, 264 (1928). (9) Sherwood, T. K., “Absorption and Extraction”, pp. 22-4, New York. McGraw-Hill Book Co., 1937. (10)Ibid., pp. 210-12; Hatta, S.,Tech. Rept. TBhoku Imp. Unic., 8 , 1 (1928-29). (11) Welge, H J., unpublished thesis, Mass. Inst. Tech., 1933.
Problems in Lime Burning A NEW X-RAY APPROACH W. F. BRADLEY
G. L. CLARK
Illinois State Geological Survey, Urbana, 111.
University of Illinois, Urbana, Ill.
V. J. AZBE, GED as is the process of manufacturing limes by the thermal decomposition of limestonesand dolomites, it is
A
generally agreed that there are many unsolved problems, both chemical and physical, relating especially to the accurate prediction of practical behavior of products of the kiln, such as limes or finishing hydrates. It has long been known that some limestones will make a good finishing hydrate whereas other limestones with almost the same chemical composition will not; or again two limestones of different chemical composition will often give equally good hydrates. Many attempts have been made to find physical explanations, and the x-ray diffraction method was applied a dozen years ago in one of the early applications to industrial problems. Even earlier, crystal structure analyses had been made for the principal solid materials involved in the topochemical reactions of lime burning: calcium carbonate (calcite and aragonite), dolomite (calcium magnesium carbonate), calcium and magnesium oxides, and the corresponding hydroxides. Table I lists the best data on these crystal structures. The burning of limestone and dolomite may be represented diagrammatically in Figure 1 in terms of the atomic structure of the cleavage plane. The lesser volume associated with a given group of C a + +ions is thus illustrated, as is the environment provided, on the one hand, by the coplanar CO,-groups, and in the other by 0-- ions. I n 1927 Farnsworth (3) made the first attempt t o explain differences in plasticities of limes by means of x-ray analyses. Chemically pure calcium carbonate and calcium hydroxide
St. Louis, Mo.
were dissociated to controlled degrees and hydrated, the plasticities being correlated with the dissociation conditions. This present study is being directed a t commercially prepared limes and hydrates with a view toward evaluating the applicability of x-ray diffraction methods to the current problems of industry. Such commercial materials came to hand as powders or polycrystalline aggregates, and are best
TABLE I. CRYSTALSTRUCTURES Structure
Space Group
Rhomb Angle, OL
D& = R ~ C101°55’*
Calcite, CaCOa
Rhombohedral
Magnesite, hlgc03
Rhombohedral
D5d = R3;
Dolomite, C a l k (cod2 Aragonite. CaCOa
Rhombohedral
Ci, =
R3
Orthorhombio
Vk6
Pcmn
...
Cubic Cubic Hexagonal
0,” = Fm3m O E = Fm3m Cjm
... ...
Hexagonal
Csm
...
*
-
(46’7‘) 103O20’* (48’10’) 102’50’* (47’30’)
...
Unit Cell Constant, a 6.4125*
(6.361) 5.84*
(5.61) 6.18* (6 .OO) a = 4.94 b = 7.94 c = 5.72
4.80 4.20 a = 3.58 c = 5.03 a = 3.12 c = 4.73
Based on the conventional rhomb. analogous to th,e unit cell of NaCI. Actually the unit cell contains 2 instead of 4 molecules with a and a shown in brackets.
JULY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
Because of the fact that so many empirical methods are used today in the old process of lime burning, an attempt is made by the x-ray diffraction method to answer some practical problems of accurate prediction of the behavior of products of the kiln, such as lime or finishing hydrates. A clear structural difference is established in soft- and hard-burned high-calcium limes. The following general effects are considered: high magnesia content, crust formation, the composition and mechanism of formation of cores in dolomitic limes, the structure and properties of hydraulic limes as a function of silicate content, the dependence of the properties of plastic hydrates upon particle shape deduced from x-ray patterns, and the effect of rate of hydration upon development of heat and crystal grain growth and inhibiting effects of magnesium oxide. In all cases the results are based upon an examination of a wide variety of actual commercial specimens from many parts of the United States and Canada.
examined by the powder diffraction method. A limitation is immediately imposed that impurities can be detected only when they are crystalline, and when they constitute several per cent of the specimen. The information which is available, however, on the predominant crystalline phases, with regard to the sizes and habits of ultimate particles, constitutes a valuable insight into the known physical behaviors. Rather long x-ray wave lengths are desirable for easy visual inspection for the above purposes, and in this study F e K a radiation was utilized.
Soft- and Hard-Burned High-Calcium Limes Ten samples from widely different sources and covering a wide range in hardness were examined. Limestone begins to dissociate against the normal carbon dioxide partial pressure in air at about 550" C. and is completed readily against lime kiln atmospheres with high carbon dioxide partial pressure in the neighborhood of 950-1000" C. Practical commercial burning is commonly carried to temperatures around 14001500" C. Thus the majority of the commercial products are somewhat overburned. Overburning of limes is indicated b y a shrinkage of the original bulk of the lime accompanied by growth of the individual calcium oxide crystals. The rate of such growth increases with increasing temperature. Thus excessive temperatures overburn a lime rapidly, whereas an experimental batch soaked 12 hours a t 1000" C. exhibited no shrinkage.
913
This crystal growth, which actually is overburning, can be followed by x-ray diffraction methods. Figure 2, a and b, gives patterns from a soft- and a hard-burned lime each from high-quality limestone. I n a, the diffraction lines are smooth and continuous, indicating an immense number of very small individual crystals; in b the lines are composites of spots large enough to be counted, resulting from a smaller number of much larger individuals. Samples intermediate between these two exhibit fewer or more individual spots superposed on a smooth line background; thus the limes can be arranged in order of hardness by inspection. I n general, the limes derived from high-quality high-calcium limestone yielded diffraction lines only for calcium oxide of which just six fall in the range reproduced here. Occasional additional weak lines are due t o the impracticability of entirely excluding some wave lengths other than FeKn from the radiation employed. I n a t least one case, however, a n apparently pure lime was contaminated with detectable amounts of anhydrite, but examination showed that this contamination had been localized in a crust on the lump lime (Figure 2d) and was probably a result of reaction with sulfurcontaining coal gases. A lime derived from a dolomite consisted, as would be expected, of calcium oxide and magnesia. As in high-calcium limes, the calcium oxide is subject to overburning, but no commensurate grain growth is apparent for the more refractory magnesia' (Figure 2 e ) . Such magnesia is indeed "deadburned" and does not commonly rehydrate appreciably on slaking in commercial hydrators over periods as short as onehalf hour. This inactivity in so much smaller size range is presumably a consequence of the much lower solubility of magnesium hydroxide. The curious situation is presented,
T
480 A. I
CAO
-r 4.20 A . I
T
I
CAO
FIGURE 1. LIMEBURNINQ OF CALCITE AND DOLOMITE IN TERMS OF CRYSTAL PLANES
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INDUSTRIAL AND ENGINEERING CHEMISTRY
VOI,. 32, NO,7
D
b
FIGURE2.
COMMERCIAL LIMES
0.
Soft-burned cium lime
high-oal-
6 . Hard-burned high-oatO i U m lime c.
Soft-burned lime me-
pazed in laboratory d . Cmstfromhsrd-burned high-eaioium lime e. Medium-burned doiomite (lines additignsi to thme of ~slmum oxide due to magnesia) I. Under;burnedoorcfrom dolomite Linea marked C CaCO,: s = cnso.
ion
j.+
++++
7!
iovlvlvl
-
v d
c
I
FIQURE4. FINISKING HYDRATES Poor D l v t i o hydrate from high-eelcium lime b. Goodplaatio hydrate froin high-calcium lime c. Aiidsked high-enieium (1.
lime
d. Good plastic hydrate from
dolomite
Lines marked M m MgO: indices refer to Ce(OH,.
+
z
+
E
4 E
JULY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
then, that underburned dolomite can consist, as in Figure 2f, of dead-burned magnesia and recarbonated calcium carbonate, neither of which is subject to any appreciable degree of hydration over short periods of time under ordinary conditions. This does not mean that magnesia cannot be hydrated under other conditions. Wells and Taylor (4)found in a study of six commercial hydrates that 70 to 90 per cent of the magnesia was still unhydrated after 1 day of soaking; and that 2 to 4 months were required to hydrate 95 per cent of the magnesia on soaking a t room temperature, the rate increasing rapidly a t higher temperatures.
975
!
0 -
iE a
Hydraulic Limes
tion lines from spacings perpendicular to the layers, as contrasted with lines from spacings parallel to or inclined to the layers. In Figure 4 the indices 100, 110, and 200 relate to translations within a layer and are correlated with cross sections of particles, whereas 001 repre-
Three commercial hydraulic limes (Figure 3) were examined whose compositions varied such that, for the first, too little lime was present for complete reaction with the silica and alumina; for the second, a moderate excess was present; and for the third, several fold excess ensured a substantial balance of free calcium oxide. Analyses of the three samples furnished by courtesy of the Riverton Stone and Lime Company are as follows: Sample Silica and insoluble Iron oxide Alumina Calcium oxide Magnesia
so3
Loss on ignition
LowCalcium 27.42Y0 1.88 8.78 53 06 5.56 1.16 0.85
99.21
IntermediateCalcium 19.34% 2.60 7.52 62.16 4.56 1.85 0.85
98.88
HighCalcium 6.84Y0 0.90 2.38 84.44 3.02 0.29 0.66
98.53
In each case weak magnesia diffraction lines appeared, consistent with the analytically determined values above. The low-calcium specimen was inhomogeneous, consisting of nodules of P-Ca2SiO4with some free lime excess embedded in a yellow matrix of lower calcium content. Diffraction patterns of the matrix alone indicated a mixture of P-Ca2Si01 with considerable amounts of a second unidentified phase, presumably a lower calcium silicate. The two higher calcium specimens were homogeneous, the percentages of free calcium oxide and @-Ca2Si0b being essentially supplementary as judged from the relative intensities of their patterns. S o evidence of Ca3SiO5was observed although temperatures of burning were near the lower limit of stability for this compound. However, Brownmiller and Bogue (1) reported that a t least S per cent is necessary for its identification in diffraction patterns, so that lesser amounts might be present. I n the two higher calcium specimens a few weak lines were observed attributable to 3Ca0.Al2O3. The lower calcium specimen probably contained pentacalcium aluminate (12Ca0.7&03). The hydraulic properties of these limes are obviously attributable to the hydrolytic reactivities exhibited by these same compounds in portland cement ( 2 ) .
Plastic Hydrates Four high-calcium and three magnesian commercial finishing hydrates were examined. Minor amounts of calcium carbonate were encountered in some samples, possibly precipitated by dissolved carbon dioxide in the slaking water, but no tendency toward influence on the comniercial grade was noticed. Two factors which are known to influence the plastic properties of materials are size distribution and particle shape. Since calcium hydroxide has a layer lattice structure, the latter factor can be expected to exercise the predominating influence. Also, for layer lattice structures, the shapes of individual particles, even of submicroscopic size, can be in-
(u
somewhat broadened and
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Whether magnesia merely reduces heating as an inert diluent or actually inhibits the grain growth of the hydrate particles can hardly be inferred from examination of the hydrates.
Conclusions Precise quantitative control or evaluation of lime or finishing hydrate properties could entail some refinements in the examination of the diffraction patterns and a wide practical experience in the desirability of various characteristics in the products. The indications described in this paper, however, are promising. The structural properties desired in a burned lime undoubtedly depend upon the use for which the lime is intended. A hard-burned lime indicated by its pattern as possessing large grains, possibly cemented by glassy impuri-
VOL. 32, NO. 7
ties melted a t high temperatures, is evidently undesirable. A highly active soft-burned high-calcium lime has the disadvantage of deterioration on storage. Lime to be sold and used as quicklime undoubtedly requires different burning conditions and structure than that which is to be hydrated. Further studies correlated with the practical experience of lime manufacturers and users will clarify these uncertainties.
Literature Cited (1) Brownmiller, L. T., and Bogue, R. H., Am. J . Sci., 20, 241 (1930). (2) Bussem, W., Symposium on Chemistry of Cements, Stockholm. 1938. (3) Farnsworth, Marie, IND. ENG.CHEX.,19, 583 (1927). (4) Wells, L. S., and Taylor, K., J. Research S a t l . Bur. Standards, 19, 215 (1937).
Thermodynamic Properties of Fluorochloromethanes and -Ethanes Heat Capacity of the Liquid and Vapor of Three Fluorochloromethanes and TrifluorotrichloroetharPe' cooled receivers, K and K ' , over A. F. BENNING, R. C. MCHARNESS, HE heat capacities of the Of time. liquid and vapor are W. H. RIARKWOOD, JR,, AND W. J, SMITH2 Heat losses in the caloquantities which are esKinetic Chemicals, Inc.. Wilmington, Del. rimeter were minimized as folsential to the calculation and lows: Losses due to gaseous checking of accurate tables of conduction and convection were eliminated by maintaining a thermodynamic properties of a compound. This paper covers pressure of less than 0.001 mm. in the jacket. Radiation in detail the work done on these properties as previously losses were minimized by surrounding the heated portion of outlined in the first paper of this series (1). the calorimeter with two concentric silver shields. The outer Heat Capacity of Vapor at Constant Pressure shield was electrically heated and maintained a t the same temperature as the inner shield. Differential thermocouples The heat capacity of the vapor a t a constant pressure of attached to these shields were used to indicate temperature 1 atmosphere was measured in a constant-flow type of calodifferences and the degree of equilibrium established throughrimeter. A diagram of the calorimeter and the necessary auxili* out the calorimeter. ary equipment is shown in Figure 1: The quantities necessary for the calculation of the heat The compound under investigation was placed in the 4-liter capacity of the vapor are the flow rate, the heat input t o the Dewar flask, A , and vaporized at a constant rate by introducing vapor, the temperature rise, and the average temperature of a constant amount of electrical energy. Slight fluctuations in the vapor. The procedure followed in making these measurethe vapor flow rate were smoothed out with the aid of equalizing ments and the precautions taken are discussed below. flask C . The vapor then passed through copper coil F and quartz calorimeter G, both of which were immersed ifj-a bath maintained The flow rate of the vapor was determined from the a t a temperature constant t o within *0.01 C. As the vapor weights of material collected in chromium-plated steel conentered the calorimeter, its temperature was measured by means tainers over measured intervals of time. The flow of vapor of a platinum resistance thermometer, T I , projected into the gas was obtained by the introduction of heat through the coil in stream. The vapor then passed through a baffle and over a heating coil, H, where its tem erature was raised approximately Dewar flask A. This was kept constant by maintaining a 10" C. by the introduction o f a constant measured amount of constant potential drop across the coil. Slight variations in electrical energy. After passage through a second baffle the flow due t o superheating or splashing on the walls of the temperature of the vapor was measured with a second resistance flask were smoothed out with the aid of equalizing flask C in thermometer, Tz. The flow rate of the vapor was determined from the weights of material collected in the solid-carbon-dioxidewhich the vapor was maintained under a constant pressure of about 10 mm. of mercury. 1 The first four papers in this series appeared in July, 1939, and April, May, The heat input t o the vapor was determined b y measuring and June, 1940, respectively. the current flowing through the calorimeter heater coil, H , 2 Present addreas, 823 West Church Street, Elmira, N. Y.
T
