I UNIT OPERATIONS REVIEW
CHEMICAL ENGINEERING R E V I E W S
I I
II Crystallization I
T H E subject of crystallization continues to receive increased attention from man)-
investigators. Consequenrl).. the literature is becoming more voluminous and increasingly difficult to summarize thc more pertinent references ivithout verbosity. This makes division into sections also rather arbitrary. The nonstead\-state problem in nucleation has been treated by a method similar to one used in the analogous problem in heat conduction. \fan) investigations have been conducted on the nucleation of ammonium iodide. silver azide. prop>-I pentacontanoate and on the rat? of nucleation in various other supersaturated compounds to dererinine conditions for nucleation. Crystal growth has also received much attention in the literature. Investigations ivere conducted 011 the growth of cadmium iodide and longlifetime germanium crystals. Studirs concerned with the crystaliizarion of silver chloride, fatty acids. ethylene glycol polyterephthalate. urea adducts. and butadiene styrene have been reported. The heats of crystallization of magnesium and cobalt compounds \\'ere determined and reported. The kinetics of cr!-stal gro\\.th has been investigated under such studies as the rates of crystallization of silver chloride, the rate of growth of salol crystals in the region of the vitreous state. and the rate of crystal growth of ammonium chloride. Other subjects considered are the linear crystallization velocity of sodium acetate in supersaturated solutions. the viscosity and linear velocity of crJ-stallization of resorcinol in aqueous and nonaqueous solutions at various temperatures, and the mechanism of precipitation of magncsium oxalate from supersaturated solutions. Growth velocity of crystallization of single faces of hexamethylenetetramine \vas measured. The theor)- of crystal growth is considered as a molecular phenomenon. Theories relating the growth rate to crystallization temperature \cere experimentally verified. An exponential relationship between the crystal groivth rate and supercooling was found. A theoretical discussion of various crystallization theories is included; one of which deals with the decomposition theory of saturated solutions. Empirical equations for the crystallization fields of mirabilite, thenardite, sodium chloride, and epsomite were developed and reported. In 470
another rheor)-. a therinodynaniic rxplanation is given for the critical temprrature a t \vhich epitaxy appears betlveen t ~ v o crystalline phases. The diffusion problems associated with crytal growth from dilute solutions were treated mathematically. hlathematical equaLions \vere derived and devcloped for thr crystallization surface in ternary systems, the rate of nucleation in supercooled solutions, and for tlie tendency of a supersaturated solution t o crystallize spontaneously. The possibility of increasing the true crystal density by inthc nuinher of vacant sites in tal 1:irrice !\-as considered theoretical!!. and confirmed in the carbides of tantalum and niobium. Current cr)-stallization theories, as applied to sugar boiling and crystallizing? were summarized. The theory of crystal nuclcation from vapor, liquid. and solid s>.stems\vas discussed. Certain variables affect crystallization in different \va>.s. The crystal habits of ethylenediamine tartrate !\-ere found to vary \\-ith the concentration of boric acid. Investigations \\.ere conducted on the influence of foreign ions on the groLvth rate of silver chloride froin aqueous solutions and on the nucleation process in superheated liquids. The modifyin? po\ver of dyes on the cr!-stal habits of inorganic salts and the relationship bctwcen rhese dyes and crystal structure have k e n reported. The presence of dyes !vas said to affect both the nucleation rate and
INDUSTRIAL AND ENGINEERING CHEMISTRY
niic1i.i gro~vili10 p ~ ~ t ~ c ~ psizr. ~ i l i l r'l'lic. rare of cooliny ailected t h y i i n i p y r a i i i r r of crystal forni,rtion I'rom a s a t u r a t ~ ~ l soliirion and ii rrport o f i t 5 rl1'cc.t o n cr!.stal size ha? heen publishrti. .\ I[I\\, frcciumcy \-ihratioii \viis rc.1)1it~1cdto acccleratc crystallization in s o l u t i o n . Studies \vt:re madr OII thr cflict 01 u l t r a sound on [lie cr!.stallimtior1 w t c ' 01 solutions. allo)-s. and s o i n ( ' cirqanic ~ 1 1 1 ) stances. 'The preparation 01' siiiqli, CI variuus types has I ~ c r n h v SL inan>-1)aIjcrs and p d t C I 1 t S . '1'1i-o ]);t[('tlIs describe the processes foi, qo\\in? c r i l o t ~ r d rutile boules. Procrsses fur prc1)ariny large single crystals of etlivlcr~crliarni~~~~ and a titaniuni oxide arc' clescribctl i n several Japanrsc: patents. .\Ieth[ic preparing a cadmium svlenidc cr single zinc and tin cr!-stnls. and a motlocrystalline silica gel have brrn d~~sct~itic~tl in detail. Llethods for improving rhi: crystallization prorcs~csin industry 11;ivc hcrn drscribrd in the literature in I.)o1h 1 1 a t c . 1 1 1 ~ and articles. In an anal>.sis of cr!stalsize control in vacuum c.r!.stalliL:c%rc, ~ h key r to eff'cctive control pi.ocedu~~.s \vas the segregation limr: oi'nuclri in rhr iincsremoval system. The background. ires sent status. c i r d principles of zone mclrinq have bc.c.n discussed rogether \\-iIli tlie inathciiiatic;11 equations for this principle. Extractive crystallization and the principles o f tile unit operation of exvactivc crystalliza-
JOSEPH A. PALERMO of the research and development staff of Colgate-Palmolive Co. works primarily on spray drying of detergents. His Ph.D. dissertation ( 1 952) from Syracuse University, was carried out on crystallization. Palermo is a member of AIChE, ACS, Phi Lambda Upsilon, and Sigma Xi.
C. S. GROVE, JR., received his Ph.D. from the University of Minnesota in 1942. He is professor of chemical engineering and director of engineering research at Syracuse. HERBERT MARTIN SCHOEN received his Ph.D. in 1957 from Syracuse University. He is currently on leave of absence, studying crystallization processes on a Fulbright Fellowship in Germany and other countries of Western Europe.
tion have been discussed. The separation of a- and p-dichlorobenzenes by the above method and the purification of organic substances by the same method has been described. The fundamentals and outstanding applications of temperature zone refining have been outlined. A study was made on the effect of zonerefining variables on the segregation of impurities in indium antimonide. The development of a new method of suspension of the molten zone and the application of the method are detailed. A number of patents and articles describe apparatus and equipment used in crystallization processes, both for specific and general applications. An automated Verneuil furnace to permit growth of crystal rods was developed. Types of apparatus for crystallizing and separating slightly soluble substances and for extractive recrystallization have been disclosed in patents. A patent was obtained for a modified Ridgway tube furnace suitable for the production of discrete crystals. The design of a crystallizer for use in the fine chemicals industry is explained. An apparatus for manufacturing single crystal oxides for use in jewels has been described.
Nucleation and Growth Collins (34 has treated the nonsteady state problem in nucleation by an approximate method, the form of which parallels the analogous problem in heat conduction. The results are especially of interest in nucleation of viscous solutions. Newkirk and Turnbull (73A) have studied nucleation of ammonium iodide from aqueous solutions. They have determined the critical supersaturation ratio required for nucleation and have related it to the interfacial energy between the crystals and the solution. Hirano ( 7 A ) studied the rate of nucleation in supersaturated solution and described some trials of the theoretical estimation of nucleation rate in electrolyte solutions. Sawkill ( 7 4 4 has investigated the nucleation of silver azide by electron microscopy and diffraction. The use of supersaturated silver iodide in triethylene glycol for identifying silver iodide nuclei and compounds of similar structures from the atmosphere was studied by Montmory (77A). Anderson ( 7 4 has reported on an electron microscope study of the nucleation and growth of propyl pentacontanoate. The growth of cadmium iodide crystals from an aqueous solution saturated at 85' was investigated by Newkirk (724). Davies and Jones ( 4 4 studied the crystallization of silver chloride from aqueous solution and observed that this crystallization was delayed when surfacecontaminated seed crystals were added to supersaturated solutions. Flaschen (5.4) investigated the aqueous synthesis of barium titanate.
Tovbin (76A),in studying the stabilities of supersaturated solutions of barium iodate and lead iodide, determined the maximum relative supersaturation values that can be attained before spontaneous crystallization occurs. The effect of additives were also studied. Jamieson and Frost (9.4) determined the heats of crystallization of vacuum-dehydrated magnesium sulfate heptahydrate and cobaltous chloride hexahydrate. Becker (2A) studied the growth of long-lifetime germanium crystals. Gerasimenko and Golovin (6A) applied the elastic-quartz filament method to measure crystal growth in sucrose solutions within an accuracy of 10-5 grams. Wood (77.4) has reported on crystallization in butadiene-styrene copolymers. von Sydow (75.4)crystallized fatty acids with 12 to 18, 20, 22, 24, and 26 carbon atoms from pentane, ethyl ether, acetone, ethyl acetate, benzene, carbon disulfide, carbon tetrachloride, and chloroform. The solid phases obtained were investigated from the polymorphic point of view by an x-ray powder method. Hirano (8A) determined the time required for the nucleus formation of urea adducts of paraffins under the microscope in the presence of such activators as methyl, ethyl, propyl, and butyl alcohols, and water. Legrand and Coste ( 7 0 4 studied the crystallization of ethylene glycol polyterephthalate from thin films at temperatures between 80' and 260' and by rapid quenching to 0'. They also made x-ray diffraction observations at room temperature.
Kinetics of Growth The rates of crystallization and solution of silver chloride a t 15, 25, and 35' were determined by Davies and Nancollas (3B). Moriyama and Yawataya (8B) in studying the rate of crystal growth of ammonium chloride found that the presence of sodium bicarbonate or ammonia remarkably retarded the crystal growth of ammonium chloride, whereas sodium chloride had very little effect. Dietz (4B) studied the linear crystallization velocity of sodium acetate in supersaturated solutions. Chatterji and Mathur (7B) have determined the viscosity and linear velocity of crystallization of resorcinol in aqueous and nonaqueous solutions a t various temperatures. The values obtained are explained in the light of Frenkel's views on the velocity of crystallization in unicomponent systems. Honigmann and Heyer (5B)measured the growth velocity of crystallization of singles faces of hexamethylenetetramine crystals from the vapor phases. Malkin (7B)investigated the rate of growth of salol crystals in the region of the vitreous state. Peisach and Brescia (9B) studied the mechanism of precipitation of magnesium oxalate from supersaturated solutions.
The magnesium and oxalate form undissociated molecules in a rapid process, followed by a rapid formation of a critical nucleus which then associates with a third molecule by a slow process. The rate controlling step was determined as a function of .time. The formation of the nuclei was found to be third order. Komarova and Figurovskii (6B) investigated the kinetics of crystallization of aluminum-potassium sulfate, potassium chloride, potassium iodide, potassium sulfate, ammonium chloride, sodium carbonate decahydrate, lead nitrate, sodium thiosulfate, and copper sulfate a t various initial degrees of supersaturation from 0.003 to 2.36. Davies and Jones (2B) studied the kinetics of growth of silver chloride seed crystals from aqueous solutions. Schlipf (70B) studied the processes in crystal growth and discussed the kinetics of layer growth, the growth mechanism of dendrites, and the calculation of the diffusion field of a cubic crystal growing from a solution.
Theory Kossel (9C) has considered crystal growth as a molecular phenomenon and has described several growth processes. The primary process consists of the sedimentation of new particles on the crystal. During the growth of silver wires by electrolysis, electric currents showed evidence,of a stepwise growth process. Neuhaus (72C, 73C) has presented a theoretical discussion which includes crystallization in melts and solutions, molecular concepts of growth in solution, complexity and agitation, additives and multiple systems, probability of nucleation and size of growth vessel, and nature of the metastable state. Salli (75C) has reported the decomposition theory of saturated solutions. He made a kinetic study of the growth and the form of a crystal of the new phase which separates from a supersaturated solution. A formula for the growth rate of the center of the new phase was obtained by Salli who also studied the role of surface tension of the liquid a t the crystal solution boundary in the different stages of growth. Falsiev (5C) developed empirical equations for the crystallization fields of mirabilite, thenardite, sodium chloride, and epsomite. He discussed the derivation and use of Pozner equations. Blisnakov ( I C ) has given a thermodynamic explanation for the critical temperature a t which epitaxy appears between two crystalline phases. Danilov and Malkin (2C) have experimentally verified the theories of Stranski and Kaishev (78C) on crystal growth by measuring the linear growth of the crystal b o h d a r y for phenyl salicylate in the region of slight supercooling. They found a n exponential relationship between the growth rate and supercooling,
VOL. 49, NO. 3, PART I1
MARCH 1957
471
The key to effective size control in vacuum crystallizers is segregation time of nuclei in fines-removal system.
and that the experimental curves coincided with the theoretical curves. Krishtal (70C) experimentally verified existing theories of crystal growth which related growth rate to crystallization temperature. This was in agreement withthe equations given byVolmer (ZOC). Seeger (76C) discussed the diffusion problems associated with the growth of crystals from dilute solutions and mathematically treated an idealized situation for 2- and 3-dimensional crystal growth with plane faces. Elbaum (-IC) has studied crystal growth as related to lattice imperfections. Smirnova (77C) has considered theoretically the possibility of the existence of previously undescribed cases where the true crystal density is larger when the number of vacant sites in the lattice is greater. Confirmation of this rheory \vas found in the carbides of tantalum and niobium. Grigor'ev (8C)has developed mathematical equations for the crystallization surface in ternary systems in which mutual displacement occurs. Rastogi (74C) has derived equations for the rate of nucleation in supercooled solutions. Van Hook (79C) summarized the current crystallization theories as applied to sugar boiling and crystallizing and also discussed their practical implications. Dunning (3C) has discussed the theory of crystal nucleation from vapor, liquid: and solid systems. Frornrey (6C) reviewed the lattice recrystallization Lvith diagrams that demonstrate the therniodynamic and kinetic relations. Gopal (7C) has developed a mathematical expression for the tendency of a supersaturated solution to crystallize spontaneously. Milone (77C) has reported on the relationship between surface tension and crystal habit.
Variables Affecting Crystallization Booth (30) investigated the effect of boric acid on the growth of eth>-lenediamine tartrate crystals. The crystal habits vary with the concentration of boric acid and with the crystallization rate. They decrease in growth rate in comparison to the pure solution and increase in the supersaturation range in which spontaneous crystallization does not occur. Davies and Jones ( 4 0 ) investigated the influence of foreign ions on the rates of growth of silver chloride from aqueous solutions including KOBz, sodium dodecyl sulfate, potassium eosin, potassium naphthalene-2-sulfonate, and (n-CI6H33)Me3NN03. Bertanza (70) studied the effects of ions on the nucleation process in superheated liquids and
472
discussed the modification of the behavior predicted by the usual theories. IVhetstone ( I d D ) , in reporting the crystal habit modification of inorganic salts with dyes: found that the modifying polver of 24 dyes depended on both their anionic and cationic polar substituent groups and the nature of their substitution. He (7.50)also discussed the relationship between crystal structure and habit modifying dyes. Gupta and Buckley (50) experimentally exanlined the change in supersolubility of inorganic salts--e.g., potassium chlorate. potassium dichromate, and potassium chloride-by adding minute amounts of organic dyes. They ( 6 0 ) also studied solutions of ammonium chloratr, potassium sulfate, and potassium-alum. The)- stated that both nucleation rate and nuclei growth to perceptible size are affected by the presrnce of dyes. Hirano ( 7 0 ) determined the reinperature of crystal formation from a saturated solution of potassium chloride (40") and of copper sulfate ( 3 0 " ) as functions of thc rate of cooling which ranged from 0.01 to 0.3' per second. Xlatusevich ( 7 7 0 ) has reported the effect of the rate of cooling on the size of the resultant crystals. Bono ( 2 0 ) has studied the parameters governing the rate of linear crystallization as applied ~~
~
to xylenols. Nasledov ( 7 2 0 ) studicd thc process of change of selenium from amorphous and liquid phases into tlic: crystalline phase under 4000-atm. pressuse. Sanchez (730) acceleratrd cr tion in solution by means of a lo\^ Crrqucncy vibration transmitted b!- a inenibrane or piezoelectric disk siibrnerqcd in the fluid and connected to an rirctromagnetic vibrator. Kapustin (SDj studied the effect of ultrasound in the 720 kilocycle to 6-mc. rang' on thc crystallization rate of solutions and alloys. The rate increase is due to forination o f large numbrrs of crystallization centers in the solution exposed to thr ulvasound. He ( S D ) has also reported the rffcct of ultrasound on the crystallization o f some organic substances. Le\vin ( 7 0 0 ) has correlated t l i c sizc,. habit, and transparTncy of lead chloridc crystals obtained from aqueous solutions of sodium chloride, hydrochloric acid, and perchloric acid bvith lead chloride solubility in these media. IVolff ( / f i D ) has reported uii cr)-stal habit of silic~inarid germanium. Single Crystals The growth of colored rutilt: h u l v s has been described in t\\-o patrnts. hlerker ( 7 0 F ) has described conditions fur groivth of rutile boules using titanium
~
Systems
Crystallization Equilibria Equi1itJria studied
Sucrose-fructose-potassium chloridewater Sucrose-glucose-potassium chloridewater Sucrose-fructose-glucose-water Glucose-fructose-potassium chloridewater Sucrose-glucose-fructose-potassium chloride-water Na?SOI-Li?SOa-HrO CaCOa-MgCOa-H>O Na. S-Nai C 02-Hr 0 KsCOj-K2SOa-HzO KHC O I - K ~ S04-H: 0 NanS04-Na?C0,-H?0 KzSOI-K~COI-HZO NanNOa-NaC1-HpO NaN02-KNOZ-H:O LiF-HF-HZO H~BO;-K?SOI-HZO Sodium chlorite-sodium chlorate-water
Phase equilibria
Isotherm at 0 ' C. Solubility systems at various temperatures and CO? pressures At 30' C." At 50' C. Solubility isotherms At 150' C. Polytherms Equilibrium isotherm at 70' C. Solubility isotherm at 2 5 O C. Solubility polytherms at 25' to 1000 c. Determinations at 15, 2 5 , 35, and 450
NalSOa-NaOH-HnO
Referciire (.jE-Rl? I
c.
Solubility isotherms at 70" and 2000 C.h Phase equilibrium at 200' to 600' C. and 10 to 250 kg./sq. cm. Solubility isotherm at 25' C.
Ammonium molybdate-ammonium sulfate-water Solubility of Nad203 in saturated aqueous Na2S decreases with increazing tcmpeiatuie. * Also established the limits of crystallization of Na:SOa.
INDUSTRIAL AND ENGINEERING CHEMISTRY
CRYSTALLIZATION dioxide with 0.005 to 0.1% aluminum oxide in one patent. Moore (74F) patented a process for producing colored rutile boules by adding cobalt oxide in amounts of 0.005 to 0.13% or nickel oxide in amounts of 0.005 to 1.0% to titanium dioxide. Kunitomi (7F)has described a Japanese patented process for making a titanium oxide which is suitable as a raw material for synthetic jewels. Heinz and Banks ( 6 F ) prepared a cadmium selenide crystal weighing 17 grams by slowly cooling the melt in a graphite crucible at 250-pounds per square inch pressure, and determined the resistivity-temperature curves and other physical properties. Brown and others (4F) have investigated the influence of impurities on the growth of quartz crystals from flint and quartzite. Tilenschi (76F) has described the method of preparing a monocrystalline silica gel and has investigated its physical properties. Merker (SF,7 7F) described and patented a process for making monocrystalline strontium titanate, suitable for manufacturing gems, lenses, and prisms, and has studied its physical properties. Japanese patents obtained by Misumi, Ishikawa, and Tanaka (72F, 73F) describe processes for making large single crystals of ammonium chloride. Booth ( 3 F ) studied the growth rates of single crystals of ethylenediamine tartrate. Maslennikov and Likhtman (8F) have reported a new method of growing metallic monocrystals from a melt. A device is presented (with drawings and photographs) for the preparation of filiform monocrystals of tin, lead, zinc, bismuth, and aluminum. Chigvinadze and Chkhaidze ( 5 F ) have described the apparatus and conditions of growth in detail for single zinc crystals of fixed orientation and length. Takaki and others (75F) have made experimental studies of conditions required for the preparation of single tin crystals. Billig ( 2 F ) has investigated the growth ofmonocrystab of germanium from an undercooled melt. Alexopoulos ( 7 F ) has described a crucible in which a metal can be cooled from the melt while it is subjected to a strong temperature gradient.
Industrial Practice Saeman (20G) has analyzed crystalsize control in vacuum crystallizers. The key to effective size-control procedures is the segregation time of nuclei in the fines-removal system. Speyerer (25G) has reported on crystallization in the sugar industry in a comprehensive review which describes crystallization methods and inherent problems and difficulties. Buch (4G) has presented a survey of the Turkish sugar industry and has described the new German sugar factories a t Konya and Amasya.
Carter (6G)has described in detail the factory setup and processing steps of domestic white sugar manufacture a t Entumeni mill. Bah and Su (7G),reporting on the recovery of sugar from cane molasses, described an improvement of the Olivarius process. In two Japanese patents, Suzuki (27G, 28G) has described sugar crystallization processes in which the treatment of 60% aqueous solutions of cane sugar with 0.001% cobalt as cobalt sulfate, 0.0001% cobalt-manganese mixture (equal weight) or 0.00001yo cobalt-manganese mixture results in an increased crystallization yield. Elias (9G) has described a process for invert sugar in a Spanish patent. Hrubisek' (74G) has discussed process conditions in sugar-beet refining for greater recovery of sugar. Zhvirblyanskif, Volobueva, and Abragam (342)have made experimental investigations which showed the possibility of crystallizing sugar without boiling the sirup. Yarmolinskii (3%") studied the rate of crystal formation of sugar from its aqueous solutions and the effect of supersaturation on crystallization. Zelazny and Nitschke (33G) investigated the crystallizability of supersaturated sugar sirups after adding powdered sugar and found that the added powder induced nucleation in the sirup and that its extent depends on the coefficient of supersaturation. Munekata (76G) has surveyed the saltmanufacturing industry in Japan. Hester and Diamond (73G)have described in detail the vacuum-pan and grainer processes of sodium chloride manufacture. Valyashko (30G) investigated the crystallization of sylvite (potassium chloride) upon evaporation of sea water and (29G) discussed the process conditions for the production of potassium chloride by freezing the concentrated brines of Inder Lake. Peisach (7%) has studied the system sodium chloride-odium sulfate-water for application to the recovery of sodium chloride and sodium sulfate from South African natural brines. A three-stage process is required:
1. Concentration of brine by evaporation 2. Refrigeration yield pure sodium sulfate, and 3. Solar evaporation followed by recycling of solution to yield pure sodium chloride. Three commercial processes (7G) for the production of ammonium nitrate are compared with the aid of flowsheets:
1. Stengel, which features casting of a thin film of ammonium nitrate on water-cooled stainless steel belts 2. The prilling process, in which a concentrated aqueous solution of ammonium nitrate is sprayed downward in a prilling tower against a countercurrent conditioned air stream, and
3. The crystallization process in which ammonium nitrate is formed in a vacuum crystallizer. Saeman (27G) has described a patented process for crystallization of ammonium nitrate. Dorsey (8G) has described, in a patent, an improvement in the Stengel process for making ammonium nitrate. Simms (23G) has described a patented process for the concentration of ammonium nitrate solutions. Whetstone and Bell (37G) have obtained an ammonium nitrate process patent. Otto (78G)has described the manufacture of ammonium sulfate a t the BerlinMariendorfgas coke-oven plant. Stamicarbon (26G) has described a process for coarse crystals of ammonium sulfate. Fukumoto and others ( 7 7G, 72G) have described two processes for producing ammonium sulfate and sodium bicarbonate from sodium sulfate. France (70G) has described a process for the recovery of anhydrous sodium sulfate from the viscose process by fractional crystallization of the concentrated mother liquors, A process for the simultaneous production of 'sodium sulfate and basic magnesium carbonate from natural waters of central Spain has been described (22G). Bundy and others (5G) have developed a process for making diamonds. Hyodo, Maeda, and Uemura (75G) have discussed the operating conditions for the cyclic process for manufacturing sodium bicarbonate and ammonium chloride in the ammonia-soda plant. Ball and others (ZG, 3G) have described processes for crystallization of sodium borate by heating sodium borate mixtures in an autoclave a t 150' C. to equilibrium. Oliver (77G) described a patented process for the production of anhydrous sodium cyanide. A French patent ( 2 6 ' ) has described a process for crystalline sodium metasilicate which is useful in 'manufacture of detergents.
Zone Melting Pfann (7OH) has discussed the principles of zone melting and derived mathematical equations for this principle. Pfann ( 8 H ) also discussed the origin and present state of zone melting, zone leveling, and zone remelting and listed applications to systems other than those involving semiconductors. Kobe (5H, 6 H ) has discussed extractive crystallization and has reviewed the thermodynamics involved in the adduct formation of urea and thiourea. Chivate and Shaf ( 7 H ) have discussed the principles of the unit operation of extractive crystallization and its application in separating rn-cresol andp-cresol from their mixtures. Nichols ( 7 H ) studied the separation of 0- and p-dichlorobenzenes by continuous fractional crystallization.
VOL. 49, NO. l,PART II
MARCH 1957
473
UNIT OPERATIONS REVIEW Pfann ( Q H ) has outlined the fundamentals and exemplary applications of temperature gradient zone refining. This phenomenon can be utilized in fabricating semiconductive devices, grolving single crystals: joining. boring fine holes in solids. measuring diffusivities in liquids, small-scale alloying. and purification. Pfann and Hazelbargrr (72H) have developed a new method of suspension of the molten zone. the so-called method of magnetic suspension \\-it11details of the application. -4 continuous zone-refining process h'is been patented by Pfann ( 1 7 H ) . This process utilizes the variations in salute solubility in adjacent liquid and solid phases to segregate solutes in fusible systems-e.g., metals and alloys. salts. and salt solutions. The purification of organic substances by fractional crystallization has been described by Kienitz ( i H ) . Christian (I")has described the application of zone refining to organic materials. Harman ( 3 H ) made a study of the effect of zone-refining variables on the segregation of impurities in indium antimonide.
Equipment for Crystallization Verma, Sirkar. and C:hatteijcr ( 7 /.J) have developed an automated T'erneuii furnace to permit growth of cr>-stalrods such as calciuni and cadmium tungsrates. 2. to 4 mm. in diameter and 5 10 10 cin. long. Pluim. Sipkes, and van Sweden (77J) describe a patented apparatus for crystallizing and separating slightly soluble substances from liquids. Herington, Handle?. and Cook (1J. 6 J ) havr developed an apparatus for the purification of organic compounds by zone melting. Continuous sugar crystallizers havr been patented by Hastings (5J) and Kopke ( 9 J ) . Dobrowsky (X) has described. in an Austrian patent, a laboratory apparatus for extractive recrystallization. X parent has been obtained by \'an der Pyl ( 7 3 4 for a modified Ridg\\.ay tube furnace which is suitable for the continuous production of silicon carbide from carbon andsilicon dioxide (silica) as discrete crystals. Bamforth ( 7 J ) has explained the design of a crystallizer for use in the fine chemicals industrv. Various types of crystallization equipment are described in other patents. These include an apparatus for continuous crystallization ( 8 J ) .a revolvingtype crystallizer ( 7 5 4 , a multistage crystallizer ( 3 4 ,and a mixer-crystallizer (705). Saiki and Funaki ( 7 2 4 describe a n apparatus for manufacture of single-crystal oxides for use in j e n d s . Hoekstra, Ke, and Trivich ( 7 4 describe a simple lathe attachment for the machining of srnall single-crystal spheres.
474
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(2B) Davies. C:. \ V , , .Jones, ;I, I,.. T i a n s . b-aS;lliidaJSOC. 51, 812-1' (1955). ( 3 H ) Davies, C. IY., Nancollas. C;. H., Ibid.. 51. 818--23 1 1 9 5 3 14B I Dietz. 'P.L.. Jr.. Cniv. Microfilms (.Ann .Irbor. M c h ) Publ. 15086, 141 pp. [nirrrrintlori Ibstr. 16, 31 2 I~. 1 9 -5 -h ,) ,l . Honi5mann. B.. Hcyer. H . , %. I
