CRYSTALLIZATION C. S. GROVE, JR. SYRACUSE UNIVERSITY, SYRACUSE, N. Y.
HERBERT M. SCHOEN
,
UNITED STATES RUBBER I N T E R N A T I O N A L CORP., B A Y A M O N , PUERTO RlCO
JOSEPH A. PALERMO COLGATE-PALMOLIVE-PEET CO., JERSEY, CITY N. J.
Increased interest in nucleation and the theory of crystallization was evidenced during the year. Work on oriented overgrowth was presented in a few papers. Attention to ~ r o w t h of large single crystals has been shown with emphasis on quartz, germanium, and crystalline ceramics. Fractional crystallization equipment for improved separations of many comDoundr has been described in numerous publications and patents.
mer-Becker theory, and the recent observations on variations due to size of sample on the evaluation of the activation energy for nucleation. They also described the susceptibility . of sugar sirups to prompt nucleation by sonic irradiation a t high intensities and the improvement of the size distribution of the final granulated product by this means. A number of substances, which are ordinarily difficult to crystallize, have been nucleated in this manner. Telkes (68) reviewed the field of heterogeneous nucleation in supersaturated inorganic solutions. It was pointed out that nucleation may occur when the crystallographic data of the nucleation catalyst and the salt to be crystallized agree within 15%. Thie rule, first applied by Hume-Rothery (69)to solid solution formation in metallic alloys, has been applied to the nucleation of sodium sulfate decahydrate from saturated solutions. This can be accomplished by using a small amount of sodium tetraborate decahydrate as the nucleation catalyst. Mel'nik (49) showed epperimentally that green sirup with a purity over 80 will crystallize in 7 to 8 hours upon cooling to 20" and will yield about 36 to 40% of crystals and a mother liquor of about 77 purity. Okawa (62) studied the crystallization and melting of linear high polymers as rate processes. He calculated the rate of crystallization as measured by the rate of volume change as a function of the crystallization temperature. Honigman ( 2 6 ) studied the modification of habit of sodium chloride crystals in saturated solutions under the inEuence of temperature oscillations of 0.1", 2 O , and 5". Honigman and Stranski ( 2 7 ) studied the habit change in hexamethylenetetramine crystals a t constant temperature and also under the influence of temperature fluctuations. Danilov and Ovsienko (If) made an experimental study of the crystallization of hydroquinone on single crystals of calcite.
C
RYSTALLIZATION was emphasized by a "first" chemical engineering symposium (9) on nucleation phenomena held a t the eighteenth annual meeting of the ACS Division of Industrial and Engineering Chemistry in December 1951 a t Northwestern University, Evanston, 111. This symposium represents the first general coverage of the theory and application of nucleation phenomena ever given in this country. Broad review papers bringing together developments from widely different fields of interest demonstrated the similarity of nucleation phenomena whether in gases, liquids, or solids. Reports of new theory and stimulating research results are given. A large number of articles, which are not covered in this review, have been published on crystal constants and habits of certain crystalline compounds, on grain structure of metals, and on nucleation and crystal growth from solid solutions. This review briefly summarizes the more significant advances in the field of crystallization as a unit operation.
NUCLEATION AND GROWTH Xiggli (0'0) has reviewed the growth of crystals. Bamforth ( 3 )has reported on recent developments in crystallization; forty references are given. Fullman and Wood (16) have prepared a
color motion picture which shows the stepwise manner in which the surface moves as cadmium iodide crystals grow from sohtion. Preckshot and Brown (67)made a study to determine the effect of crystallographically similar, insoluble, ionic crystals used for nucleating quiescent supersaturated solutions of potassium chloride. The results were referred to the spontaneous nucleation of the Rame solutions which had been saturated at 49.453'. A conductometric instrument was devised to measure the time required for the formation of nuclei on these foreign crystals and in the hulk of the solution; temperature readings were used to determine the saturation concentrations. I t was found that nucleation depends on the insoluble foreign crystal present and that the character of the foreign crystals influences the amount of subcooling required for nucleation. The lattice constants of the foreign crystals used were within 10% of that of potassium chloride and no correlation appeared possible on this basis. Activation energies, surface energies, and side lengths of nuclei were determined for potassium chloride nuclei in the temperature range 32' to 42' C. Van Hook and Frulla (73) outlined established facts and procedures of sugar boiling, indicating the significance of nucleation and crystal growth in this use. They reviewed previous work suggesting the applicability of the essential features of the Vol-
THEORY Pound (65) has reviewed the kinetics of transformation of supersaturated vapor to liquid for which the nucleation rate is represented as a function of the supersaturation ratio of supercooled liquid to crystals. They give a detailed account of the results observed by Pound and LaMer (66) who studied the kinetics of crystalline-nucleus formation in supercooled liquid tin. LaMer (55)has given an excellent theoretical review of nucleation in phase transitions. He emphasizes the fundamental importance of nucleation processes in the preparation of all types of colloidal dispersions by condensation methods. For example, in meteorology, fog formation and artificial rain formation are accomplished by seeding with appropriate nuclei; in metallurgy, the initiation and production of new phases which profoundly affect the physical properties; and in chemical engineering, supercooling, superheating, overcompression, and production of super-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
saturation and precipitation by chemical means. He points out the practical importance of controlled relief of supersaturation and continued maintenance of the unstable phase. The conceptual and phenomenological aspects are reviewed in light of contributions of J. W. Gibbs, Ostwald, Farkas, Frenkel, Volmer, Becker, and Doering. More recent contributions of LaMer and Pound, dealing with the preparation of monodisperse colloids, nucleation rate in polycomponent systems, and further examples of homogeneous and heterogeneous nucleation in gaseous and liquid systems are reviewed. Turnbull and Vonnegut ( 7 1 ) have presented an interesting theoretical review of nucleation catalysis. A nucleation catalysis crystallographic theory predicts that the order of potency of various catalysts should be identical with the reciprocal of the disregistry (1/6) between the catalyst and the forming crystal on low index planes of similar atomic arrangement; that a t relatively small values of 6 nuclei will form coherently with the catalystLe., with a strain e = 6-and the free energy of transition, corresponding to a perceptible rate, will be proportional to a2; and that for S very large 6 > > e the interface between nucleus and catalyst will consist of small regions of good fit separated by a gridwork of dislocations. The interface energy will be proporE. Evidence tional to the dislocation density, therefore to 6 indicates that ice nuclei will form coherently on silver iodide surfaces (6 = 0.0145). It is indicated by experience that in general nuclei form coherently with catalysts only for 670.005 to 0.015. Turnbull (70) has investigated the kinetics of solidification of supercooled liquid mercury droplets and has developed a theory of catalysis (69)by surface patches. By hypothesizing a plausible distribution of units for crystal nucleation with respect to size, and assuming that the size of the units may be of the order of the critical size for growth of a nucleus into a supercooled liquid, the multiplicity in crystal nucleation frequency sometimes observed for the isothermal solidification of small droplets is accounted for with the use of no more than two fundamental frequencies. This theory explains satisfactorily the atherrpal nucleation of crystals for the solidification of small mercury droplets that have “HgX” patches on their surface. Mukherjee (47) has discussed the effect of particle size on the controlled crystallization process. Hartman and Perdok (88) have qualitatively developed a theory of crystal habit based on the continuity of strong chemical bonds in the crystal structure. This theory states that the highest growth velocity of a crystal is in the direction of a periodic bond chain vector. Results of the application of the theory to orthorhombic sulfur are compared with results given by the Donnay-Harker theory. Lonsdale (38)has reviewed the motion of atoms and molecules in crystals with special consideration of thermal vibration, the anistropy of thermal vibration and their influence on the x-ray diagram. A discussion of the possibilities of studying the motions of the structural units of crystals with the aid of x-ray and electron diffraction is included. Ramachandran (68) discussed the theory of optical activity of crystals and applied it to the calculation of the optical rotary power of sodium chlorate and sodium bromate. The values are in reasonable agreement with the experiment. Hosemann and Bagchi (88) have shown that the concept of a distortedlattice, L‘Ideal-paracrystal’’and its diffraction theory leads to a generalization of other well-known interference theories. This in particular, under certain special conditions, degenerates to the theory of crystals by von Laue and Bragg, to that of liquids by Debye and Menke, and Zernike and Prins, and to that of amorphous matter by Guinier, Warren, and Hosemann.
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ADDITIVES AFFECTING CRYSTALLIZATION Relatively few new data were published on additives affecting crystallization. Seifert (68) has reviewed modification of the
Vol. 46, No. 1
habit of growing crystals by impurities considered as an adsorption problem and has described the effect of glycine on sodium chloride crystals. He postulates that glycine is adsorbed on edges of 100 faces of sodium chloride in the form of a chain of hydrogen-bonded molecules such as occur in the glycine crystal. Lewin and Vance (37) studied the influence of hydrogen ion concentration, solubility, rate of precipitation, and relative supersaturation on the crystal growth of strontium sulfate. A new technique for batch production of strontium sulfate crystals is described,
ORIENTED OVERGROWTH Oriented overgrowth of crystals of organic compounds as a problem of the chemistry of molecular compounds has been reviewed by Willems (80). Dash (19) has described the growth of single crystals of barium oxide by vapor phase deposition on magnesium oxide crystals at high temperature. Neuhaus (49), in a study of the oriented crystal precipitation of potassium bromide, potassium chloride, sodium bromide, and sodium chloride on orthoclase by sublimation a t high vacuum :ind at temperatures of 340’ to 350”, observed several different oriented overgrowths. Holzman and Moore (85) made an electron diffraction study of the reorientation of certain alkali halides deposited on mica and on mica surfaces covered by organic films. Schulz (61) has described the oriented overgrowths of alkali halides on calcite. Monier (44) has studied the orientation of dimethylglyoxime on fresh cleavage faces of potassium chloride, sodium chloride, calcium fluoride, sodium nitrate, mica, and calcite. He (46, 46) has also studied the orientation of p-aminophenol on crystals of sodium chloride, potassium chloride, sodium nitrate, calcite, barite, and celestite and of o-aminophenol, p-aminophenol, and dimethylglyoxime on blende. Verma ( 7 4 ) has reported new observations of crystal overgrowth on silicon carbide.
CRYSTALLIZATION EQUILIBRIA Wendrow and Kobe (78) investigated the system sodium oxidephosphorus pentoxide-water. The data include the binary systems: trisodium phosphate-water, disodium phosphate-water, and monosodium phosphate-water. Carlson, Chaconsa, and Wells (6) have made a study of the system barium oxide-aluminum oxide-water a t 30’. Lamberger and Paris (34) have reported on the system calcium nitrate-ammonium nitrate-water. Windmaisser and Stock1 (81) have investigated the crvstallization of salts of the type sodium sulfate hydrate Na4 (S05.HzO), H 2 0 and sodium selenate hydrate Na4(SeO~.HzO),H20from aqueous solutions of sodium sulfate (Na2SOd) or sodium selenate (NazSeOd), and sodium hydroxide (NaOH). Hund, Wagner, and Peetz (SO)studied the preparation of mixed crystals over the complete composition range in the system cerium oxide-uranium oxide.
INDUSTRIAL PRACTICE The growth of single crystals has received considerable attention in the literature during the past year. Hale (80) has d e scribed the growing techniques of synthetic quartz crystals at elevated temperature and pressure. A specially designed twochamber rocking steel autoclave is used in which chunks of quartz are placed in the dissolving chamber; an array of seed plates is supported in the other chamber; the remainder of the space is filled with aqueous sodium carbonate. This equipment is operated at pressures above 3000 pounds per square inch and a t 350’. The crystals produced are large (about 100 grams), are free from both optical and electrical twinning, and can be produced right or left handed. Brown et al. ( 4 ) have studied the growth and properties of large crystals of synthetic quartz and have found that their method (6) of growing quartz crystals depends on the greater solubility
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January 1954
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
of silica glass compared with quartz under hydrothermal conditions; hence in an autoclave containing both, the solution is saturated with respect to the vitreous phase and supersaturated with respect to the quartz crystalline seed on which growth occurs. The quality of the crystals is dependent on the rate of growth; below a critical value, crystals are free of flaws and twinning. Lawson (56) described the production of oxygen-free single crystals of lead telluride, selenide, and sulfide. The method developed eliminates oxygen from the specimens by melting the elements in a crucible held in a hydrogen atmosphere. Yamaguchi (8.2)outlined the gxperimental procedure and described the furnace used for the preparation of single sodium nitrate crystals of definite orientation exceeding 1 kg. in weight. Czyzak, McCain, and Reynolds (IO) prepared single synthetic cadmium sulfide crystals by a method in which a quartz tube containing pure powdered cadmium sulfide was inserted into a combustion furnace, evacuated, filled with hydrogen sulfide a t 6 pounds per square inch, heated a t 1000" C. for 48 to 72 hours, and then cooled at a rate of 10" C. per hour. Kanai (52)prepared single silicon crystals by completely melting the silicon, rapidly decreasing the input power of the furnace, and withdrawing the single crystal from the melt as the crystal began to grow along the surface and before solidification had prcgreased deeply into the interior of the melt. Ordway (53)studied the techniques for growing and mounting small crystals of refractory compounds. Noda (51) reviewed the preparation of lithium fluoride, calcium fluoride, quartz, rutile, mica, and asbestos crystals. Teal, Sparks, and Buehler ( 6 7 ) describe a process for making large single germanium crystals and discuss the use of single germanium crystals in the development of new types of transistors, photocells, and rectifiers and in the improvement of the reproducibility and reliability of the point-contact transistors. Joyner (51) has reported a fundamental ceramic investigation which reveals that it is possible to regulate the electrical properties of glass by growing various types of crystals in the glass. Schablik (60) has studied crystal formations in stoneware glazes under the phase contrast microscope. Kudo (35') describes the production of sodium chloride and potassium bromide large single crystals by a floating method in which the growing crystal is floated on the surface of the melt. Crystals of the order of 25 em. in diameter and 12 em. in height have been grown in this manner. Watanabe (76) describes, in a Japanese patent, a continuous process for producing large crystals of cryolite. Dauncey (IS),in an illustrated review, has described the large .ecale production of ethylene diamine tartrate crystals for piezoelectric applications. The conditions of crystallization are given. A continuous process is described in a patent by Carney (7) for the separation of organic compounds having a wide range of melting temperatures. The process involves the passing of the mixture to a series of perforated trays that move through the apparatus. The high melting component solidifies on the upper tray that contains solids of lower high melting component content than the feed. Molten material runs down to the next tray repeating the melting-freezing process. The molten low melting fraction is removed continuously from the bottom while the solid high melting component is removed continuously from the top of the apparatus. The system may be employed for the separation of palmitic acid from palm oil; the recovery and purification of quinaldine; and the separation and purification of petroleum hydrocarbons. Hcrrmann (SS) describes a process for the continuous production of ammonium chloride crystals of uniform size containing 0.2% lead chloride for use as a galvanizing flux. Hirohashi (@), in a Japanese patent, describes a process for the crystallization of ammonium chloride. A process for producing almost colorless potassium chlorate crystals by cooling the solution in the pres-
77
ence of sulfonated hydrocarbon compounds is described in a patent by Frejacques ( 1 7 ) . Adamski and Frank1 (1) have described a crystallization process for obtaining sodium hydroxide, containing only small amounts of sodium chloride, from crude technical sodium hydroxide made by diaphragm electrolysis. A process for the purification of alkali metal azides by dissolving and recrystallizing in liquid ammonia is described in a patent by Wehrle, Niehouse, and Burtle ( 7 7 ) .
COURTESY STRUTHERS WELLS CORP.. WARREN. PA,
Vacuum Crystallizer Unit Producing Potassium Chloride at Duval Sulphur & Potash Co.0 N e w Plant, Carlsbad, N. M.
Shafor and Catterson (65) describe the crystallization of glutamic acid derived from hydrolyzates; Massey ( 4 2 ) describes the crystallization of fumarase. Fetterly (14) describes in a British patent a process for separating, by extractive crystallization with urea, organic compounds containing six or more carbon atoms. Ketones, esters, amines, amides, sulfides, disulfides, mercaptans, acids, halogen compounds, ethers, or nitro compounds, which form crystal complexes with urea under the working conditions of the process, have been separated. A multistage crystallization process for the separation of mixtures of compounds is described in a patent by Hachmuth (19). Robinson (69)has described an evaporative crystallization process and apparatus for manufacture of crystalline materials utilizing a cone-bottomed crystallizer for maximum growth of crystals; the crystallization of ammonium sulfate is also described. A process was patented by Gray (18) for manufacturing crystals of inorganic and organic compounds by recycling a portion of the magma to the barometric leg connecting a vacuum evaporator with a crystallizer; the production of ammonium sulfate is described. Werkespoor ( 7 9 ) patented a process for obtaining crystals having another habit which are larger than ordinary crystals of sodium carbonate and which cannot be mistaken for sodium chloride. This was accomplished by the addition of one or more salts of alginic acid to the saturated solution of sodium carbonate. Carreras (8) patented a process for the purification of crystals in which small amounts of solvent descend continuously by gravity through a layer of crystals to be purified forming three principal
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INDUSTRIAL AND ENGINEERING CHEMISTRY
zones; a top layer of pure crystals; a middle layer of crystals and impurities; and a lower layer of crystals, solvent, and most of the impurity.
EQUIPMENT FOR CRYSTALLIZATION A number of papers have been published and several patents have been issued describing equipment and apparatus used in crystallization processes. A number of patents have been awarded for fractional crystallization equipment and apparatus. Findlay (16) describes a continuous fractional crystallizer adapted to separate binary mixtures that form solid solutions on freezing. Shelby ( 6 4 ) describes a fractional crjstalliaer, designed for obtaining high purity benzene from a benzene-heptane feed, which features replaceable rings used to scrape solid material from the inner wall of the crystallizer. Wecdman (78) describes a multistage fractional crystallizer designed to obtain high purity benzene from a benzene-heptane feed and operated with a porous piston conveyor which does not become bridged over with formed crystals. Pankrate, Dutcher, and Alleman (54)describe another type of continuous fractional crystallizer which provides increased efficiency, improved separation of mixtures, recovery of high purity compounds and improved fractionation. McKay (40) describes a multistage fractional crystallizer and Vance (12) describes a fractionation crpstalli~erseparator adapted to separate eutectic solid mixtures such as cis- and trans 2-butene. Still another type of continuous fractional crystallizer described by IvIaclrlin (41) consists of a column surrounded by coolant, containing glass wool crystallization beds separated by spacers and a series of heaters that are activated in series. Other patents describe various types of cr~~stallization equipment. These include a crystallization vessel (W), a roll for the crystallization of liquids (.GS), an apparatus for purifying crystals (667,and a room-temperature-control device especially for use in crystallizer baths (39). In another type of crystallizer described by Simms (65), the crystallizer bottom is equipped with several outlets and an internal baffle device for more efficient removal of crystals suspended in the mother liquor. Harms (bl) describes a crystallizer with control of crystal size primarily adapted to produce crystal,line aluminum oxide (Aid&) hydrate from sodium aluminate (Na2A1104).
LITERATURE CITED Adamski, Tadeusy, and Frankl, Zycmunt, Prace Gldwnego Inst. Chem. Przemyst., No. 2, 1-4 (1951). Airaku, Hideo, and Kaoru, Ono, Japan. Patent 419 (Feb. 13, 1952). Bamforth, A. W.,M f g . Chemist,23, 323-5, 329 (1952). Brown, C. J., Kell, R. C., Thomas, L. A,, Wooster, Nora, and Wooster, W. A,,Mineralog. Mag., 29, 858-74 (1952). Brown, C. J., Kell, R. C., Thomas, I.,. A,, Wooster, Nola, and Wooster, W. A., Nature, 167, 940 (1951). Carlson, E. T., Chaconas, T. J., and Wells, L. S., J . Research Natl. Bur. Standards, 45, 381-98 (1950). Carney, C., U. S. Patent 2,622,114 (Dec. 16, 1952). Carreras, J. M., Span. Patent 194,537 (Sept. 11, 1950). ENG.CHEY., 44, 1269 (1952). Cooper, C. M., IND. (10) Cayaak, S. J., McCain, C. E., and Reynolds, D. C., J . d p p l . Phys., 23, 932-3 (1953). (11) Danilov, V. I., and Ovsienko, D . E., Dopouidi Akud. Nauk Ukr. R. S.E., 1950, 205-8. (12) Dash, W. C., Phys. Rez., 82, 314 (1951). (13) Dauncey, L. A,, “The Times,” Rev. ofInd., 6,24-5,27 (1952). (14) Fetterly, L. C., Brit. Patent 671,456 (May 7, 1952). (15) Findlay, Robert A., U. S. Patent 2,617,273 (Nov. 11, 1952). (16) Fullman. R. L., and Wood, D. L., “Research in Progress,” J. Metals, 4, 103944 (1952). (17) Frejacques, J. L. M ~U.,S. Patent 2,595,238 (May 6, 195q) (18) Gray, Worth, Ibzd., 2,623,814 (Dec. 30, 1952).
Vol. 46, No. 1
(19) Haohmuth, K. H., Ibid., 2,593,300 (April 15, 1952). (20) Hale, D. R., Brush Strokes (Brush Development Co.), 1-8 (December 1952). (21) Harms, V., U. S. Patent 2,606,820 (Aug. 12, 1952). (22) Hartman, P., and Perdok, W. G., Proc. Kkoninki, Ned. Akad. Wetenschap., 55B, 134-9 (1962). (23) Herrmann, C. V., U. S.Patent 2,591,067 (April 1, 1952). (24) Hirohashi, Soriaki, Japan. Patent 3482 (Oct. 30, 1950). (25) Holzman, G. R., and Moore, K. H , J . Colloid Sei., 7, 396-406 (1952). (26) Honigman, B., 2. Elektrochem , 56, 342-5 (1952). (27) Honigman, B., and Stranski, I. N., Ibid., 56, 338-42 (1952). (28) Hosemann, R., and Bagchi, R., Acta Cryst., 5 , 612-14 (1952). (29) Hume-Rothery, W., “.4tomic Theory,” London, Institute of Metals, 1946. (30) Hund, F., Wagner, R., and Peeta, U.,2. Elehtrochem., 56, 61-5 (1952). 131) Joyner, B. L., Engineering Reseorch News (North Carolina State College), 3, No. 2, 1 (January 1953). (32) Kanai, Jasuo, J . Phys. SOC.,7, 534-6 (1952). (33) Kudo, Keiei, Science of Light (Japan),1, No.2, 75-79 (1951). (34) Lamberger, Josette, and Paris, R. A., BUZZ. soc. chim. France, 1951,984-7. (35) LaMer, V. K., IND.ENG.CHEM.,44, 1270-77 (1952). (36) Lawson, W. D., J . A w l . Phys., 23, 495-6 (1952). (37) Lewin, S. Z.. and Vance, J. E., J. Am. Chem. Soc.. 74. 1433-6 (1952). (38) Lonsdale, K., Advancement of Sei., 6, 112-14 (1939). (39) Maasa, Heinrich, German Patent 813,846 (Jan. 7, 1952). (40) McKay, D. L., U. S. Patent 2,613,136 (Oct. 7, 1952). (41) Macklin, R. L., Ibid., 2,620,263 (Dee. 2, 1952). (42) Massey, V., Biochem. J . (London),51, 490-4 (1952). (43) Mel’nik, P. A., Sakharnaya Prom., 25, No. 10, 12-13 (1951). (44) Monier, J. C., Compt. Rend., 234, 1185-6 (1952). (45) Ibid., pp. 2375-7. (46) Ibid., PP. 2456-8. Mukherjee, N. R.. Trend Eng. Uniu. Wash., 4, No. 4, 8-10 (1952). Neuerburg-Obsterei K.-G. (hdolf Zumann, Inventor), German Patent 817,137 (Oct. 15, 1951). Neuhaus, h., Fortschr. Mineral., 29-30, 18-23 (1951). Niggli, P., V%erteljahrsschr.naturforsch. Oes. Zllrich, 97. No. 3, 5-35 (1952). Noda, Takichi, Proc. Phys. SOC.Japan, 8 , 37-49 (1953). Okawa, A,, J . Phys. SOC.Japan, 6, 473-8 (1951). Ordway, Fxed, J . Research Null. Bur. Standards, 48, 152-8 (1952). Pankrata, H. J., Dutoher, H. R., and Alleman, C. E., U. S. Patent 2,603,667 (July 15, 1953). Pound, G . AI., IND. ENG.CHEM.,44, 1278-83 (1952). Pound, G. hl.. and LaMer, 1‘.K.. J . Am. Chem. SOC.,74, 232332 (1952). Preckshot, G. W., and Brown, G. G., IND.ENG.CHEM.,44, 1314-21 (1952). Ramachandran, G. N., PTOC. Indian Acad. Sci , 33A, 309-15 (1951). Robinson, S. P., U. S. Patent 2,614,035 (Oct. 14, 1952). Schablik, Alphons, Sprechsaal, 85,427-32 (1952). Schulz, L. G., Phys. Rev., 82, 314 (1951). Seifert, H., 2.Elektrochem., 56, 331-8 (1952). Shafor, R. W., and Catterson, F. H., Brit. Patent 670,003 (April 9, 1952). Shelby, A. O., U. S. Patent 2,615,794 (Oct. 28,1962). Simms, R. K., Ibid,, 2,602,023 (July 1, 1952). Soler, Careras, J. M., Spanish Patent 194,538 (Sept. 11, 1950). Teal, G. K., Sparks, M., and Buehler, E., Proc. I . R. E., 40, 906-9 (1952). (68) Telkes, Maria, IND. ENG.CHEM.,44, 1308-10 (1952). (69) Turnbull, David, Acta Metallurgica, 1, 8-14 (January 1953). (70) Turnbull, David, J . Chem. Phys., 20, S o . 3, 411-24 (1952). ENC.CHEM.,44, (71) Turnbull, David, and Vonnegut, Bernard, IND. 1292-7 (1952). (72) Vance, F. P., Jr., U. S. Patent 2,632,314 (Xarch 24, 1953). (73) Van Hook, Andrew, and Frulla, Flora, IND.ENG.CREM., 44, 1305-8 (19521. (74) Verma, A. R., Proc. Phya. S O ~(London), . B65, 525-8 (1952). (75) Watanabe, Tsutomu, Japan. Patent 1662 (June 7, 1950). (76) Weedman, J. A., U. S. Patent 2,615,793 (Oct. 28, 1952). (77) Wehrle, J. J., Niehouse, 0. L., and Burtle, J. G., Ib;d., 2,591,664 (April 8, 1952). (78) Wendrow, Bernard, and Kobe, K. A,, IND.ENG. CHEM., 44, 1439-47 (1952). (79) Werkespoor, N. V., Dutch Patent 70,417 (July 15, 1952). (80) Willems, J., 2. Elektrochem., 56, 345-50 (1952). (81) Windmaisser, F., and Stockl, F., Monatsh, 83, 151-3 (1952). (82) Yamaguehi, Tasaburo, J . Phys. SOC.Japan. 7, 113 (1952).
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