ROBERT C. REID GREGORY D. BOTSARIS GEOFFREY MARGOLIS DONALD J. KIRWAN
Papers dealing with crystallization processes are reviewed
T h i s , i s the, second part of the Crystallization Review for 1970, which is in reality a continuation of last year's review. This covers material from the Spring of' 1969 to the Spring of 1970. Part I on "Transport Phenomena of Nucleation and Crystal Growth" appeared in the Xovember issue of I B E C , page 52. Crystallization Processes Involving Suspensions of Crystals-Theory and Experimental Studies
During the past year, there have been relatively febv studies concerned \Titi> t h e overall theory of suspension-type crystallization processes. IIowever, Bccker and Larson (,?Hj have discussed the important but often ncglected effects of mixing and process geometries on crystallizer pcrformaiice, and further applications of population balancc mechanics to crystallizcr aiialysis have bceii presented both by Larson and Randolph (IOHj and Stone aiid Iiaiidolph ( 2 7 H ) . Studies on nucleation occurring in crystal suspcnsions. nevertheless, continue t o be rcported 011: and these h a v e hcen concerned Ivith homogeneous 13H j? heterogeneous i~ i.ciN;: and secondary nucleation ( 1 7 H , 2 0 H ) . T h e iatter studies, however, while confirming vvell-knowm effects (such as higher agitation rates result in higher nucleation rates), still tend to be somc5vhat empirical. ?'he paper by Niclsen (1311')docs howcvcr present valuable data aiid discussion on homogeiicous nuclcatioii and gro\\ th of crystals a t high supersaturations. O n the othcr h'ind, a fair number of experimental studies have hccn presented on cry-stallization processes invoil-ing crystal suspensions. Most of these studies have bcen performed i n batch cryst;rllizers and h a \ e been concerned mainly with the cffects of temperature and supersaturation on gro\vth rates, although the interesting work of hfullin and Gaska ( 7 2 N ) o n the growth, dissolution. and micleation of potassium sulfate \vas performed in a fluidized bed, which pcrmitted carcful control of the intensi\-c Lwiables. Typic,al systems studied batch\vise havc been barium ii i tr a r e ( h" ) , ani I nonium tit any I s u I fa t r i!if$ ) , d ic alc ium phosphate dihydraw ( / / H ) ; strontium sulfatc : - / H ) ,and lead sulfate (2.3H). Finally, a n increased interest has been shown both on the effect of surfacc-active agents as a means of modifying crystal habit and growth rate (5H, O", 75f17)~aiid also o n the variables affccring the kinetics of sodium aluminate crystallizarion (7Z-I: ZHj. Coprecipitalion and Purificotion in Crystallization Proeesses from Solution
T h e continuous coiumii crystalliza~ionprocess as i i technique for purification has b e e n the subjret of a iiumber of papers. Both Bolsaitis (2J)and Lafay ( 4 J ) have presented mathematical analyses of rhe colurnii opcration together kvith experimental data on t h e purification of p-xyicne from a xylene mixturc feed, while Devyatykh (3.7) has iiidicared how a column crystallizer may be used for the separation of salt mixtures. 148
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Coprecipitation studies have included Lvork on thc sodium chloride-sil\rer chloride-water system (61)and on mctal diincthylglyoximates generated from biacctyl monoxime or biacetyl and hydroxylamine ( S J j .
Development and Studies on Industrial Crystallizer Practice
As expected, development and studies on industrial crystallizer practice stili remain of interest to many parties, a n d this is reflected iii the number of articles and patc-nts thar have recently been published and granted. T h c articlcs, how< have either becn of a more general type and concerned bvith crystallizer operarion and design, or have described specific commercial crystallization processes. l'he former group includes several useful papers such as the one by Nyvlt (CiK) that dcscribes mathematically the operation of both batch and continuous crystallizer,. Similarly, in this group, the paper by M a t z (7K) is concerned with the sreps involvcd in the planning of a11 industrial crystallization operation, while the theory and applications of vacuum crystallization has been considered by L'lcssing
(8R). On the other hand, studies concerned \vitii specific compounds iiiciude papers on the thcory and practice of c,itric acid ( 7 K ) , urea (.3Kj: ammonium sulfate / O K ) , and sodium hydroxidc ( 7 1K) crystallization. Finally, many of the crystallization patents granted during 1969-70, Together with a brief description of their mdjor claims, are listcd i n Table I-K. .\s can be seen from the comments in the table, in many cases the patriit claims teiid to dcwribe rclatixly minor changes. $'
production of Single Crystuls from the Melt
T h e needs of the electronics industry continue to stimulate cxtensive research in melt growth techniques iilcludin# the Czochralski, Bridgman-Stockbarger, Verncuil, flux, and traveling zone methods. Here we discuss only those papers whose primary purpose is a process improvement or an explanation of a phenomenon occurring, rather than the application of the process to a particular material. .\ paper by Chinmulgund ( 7 6 L ) reviewcd the methods of growth for the production of single crystals of magnetic materials and concluded that the flux method is presently the best method for growing high quality crystals. .\nother paper ( R L ) reviev\cd growth Techniques for silicon crystals. Of iuterest even to the laym a n was the recent announcement by thc General Electric Company of their ability to grow gem quality diamonds from small seed crystals. T h e high-temperature, high-pressure process was described in a patent ( 7 7 L ) . T h e Czochralski technique has maintained its popularity as a useful and versatile crystal gro\vth technique and, in addition, has
EDWARD G. DENK GUN S. ERSAN JEFFERSON TESTER FRANKLlN WONG
Part II. Crystallization Processes
stimulated the application of fundamental principles to the under.. standing and control of important physical phcnomena occurring during crystal pulling. Interest remains very high in the general problem of impurity banding in a pulled crystal, which is generally assumed to I x due to thermal fluctuations resulting from natural convection in the melt. Belouet ( 7 L )described a method of overcoining these problems iir thc high-teinpcrature pulling of Nd-doped YA1 garnet single crystals. Zupp, Niclsen, and Vittorio (58L)deinonstrated that a low-thermal gradient would minimize convection during growth of BalNaNbaO:. Another paper ( 1 9 L ) discussed the effect of rotation reversal on tlie impurity distribution in the solid. Thc solid-liquid interface shape during crystal pulling was the subject of two papers (3L, 49L). Brice (7ZL)theoretically analyzed facet formation during crystal pulling and concluded that the most practical method of reducing the size of a facet may be to decrease the growth rate. Liquid encapsulation to prevent vaporization w a s employed to grow GeTe, SnTe, and PbTe by the Czochralski technique (61,). T h e general problem of the effect of volatile impurities during melt grow-th was treated in three papers ( 3 i L , 43L, 54L). I n contrast to the Czochralski method, thc use of the flux technique, although becoming very useful for the production of high quality single crystals, is still for the most part more art than science. Berkcs and IVhite ( 9 L ) cxamiiicd the structural characteristics of alkali borate flux liquids and related them to the influence of the flux on crystal nucleation and growth. DiBenedetto and Mlavsky (271,)dcseribcd a flux method for growing homogeneous solid solutions. T h e effect of flux on the habits of Y3:lljOi~ and Y a G a j O l ~was observed by Chase and Osmer (77L). Other papers described an oscillating tempcrature technique ( E L ) , oxygen partial pressure control of the flux growth of intermediate oxides of titanium (5L), and a furnace for flux growth under pressure (471,). Traveling zone methods arc capable of producing good quality crystals when care is taken in the sclcction of zone sizc and speed. Improved floating zone techniques using a CO2 laser for heating the molten zone (231,) and employing a hollow cathode dc discharge for high-melting materials ( 5 3 L ) were described. Ciszek (2OL) analyzed the solid- liquid interface morphology of floatzoned silicon crystals and concluded that the interface is composed of both rough and singular portions. T h e use of a horizontal traveling zone method for the production of mercury selenide was discussed in a paper by IVhitsett and Nelson (57L). The older Bridgnian and Verneuil methods seem to be declining in popularity, probably because of the difficulty in controlling the growth conditions with these methods. T h e Bridgnan technique is oftcn employed for growth in a controlled atmosphere (28L, 30L, 351,)or in the early prcparations of a new material (44L, 50L). ,I.he Verrieuil technique is still uscd for the production of very high melting point matcrials (18L, 52L),but is rapidly being supplanted by the flux method.
Puriflcotion by Melt Crystallization
Zone refining continues to be widely used for purification of organic, inorganic, metallic, and semiconducting materials on the laboratory scale, but only has applications for large-scale purification of costly semiconducting materials. Column crystallization as a largc-scale method for purification of high volume, low cost materials is developirig a t a steady rate and holds much promise for the future. No attempt will be made here to list the materials most recently purified by zone refining, but rather we will discuss improvements in technique that have been published in the past year. Gravatt and Gross ( 9 M ) described a laboratory apparatus for the zone refining of organic crystals in the temperature range of 25°-300"C. ,\ heater-immersed zone refiner was patented (,?M). IVilcox ( 2 Z M ) described a number of macroscopic phenomena affecting purification that were observed during zone melting under a microscope. Mixing of the molten zone by volatile components present during zone melting was discussed in a Russian paper (77M). ,I number of articles reported purification or separation of organic materials ( 7 A f , 3 M ,8114, 19iM),seawater ( 7 9 M ) , and sulfur ( 7 M )by column crystallizcrs using a rotating spiral to move crystals countercurrent to the melt. "Jbertins and Powers ( 7 M ) concluded that axial diffusion was the primary factor limiting purification in their experiments with benzene, while diffusion in the solid w a s found to be the rate limiting step in the purification of sulfur ( 7 M ) . Bolsaitis (4M) analyzed the operation of a continuous column crystallizer for the separation ofp-xylene from a xylene mixture, while two Frrnch papers f 7 2 M , 21'14) described a countercurrent process employing direct thermal contact with a n immiscible refrigerant. Other papers described a new apparatus for seawater desalination f 14M) and analyzed the ice making operation in a desalination process (6,Vf) and in a sucrose concentration process ( 2 3 M ) . The employment of continuous crystallization from the melt as a large-scale industrial purification process is still in its infancy because of the difficult problems involving heat and mass transfer, solids handling, and separation of crystals from the mother liquor that must be overcome; but its potential usefulness as a n ecoriomic separation technique remains. AUTHORS Robert C. Reid is Professor of Chemical Engineering at 114.Z.T., C a m b r i d p , 'Mass. Gregory D.Botsarzs is Associate Professor of Chemical Engineering at T u f t s University, Medford, M a s s . Geoffrey Margolis is Assistant Professor of Chemical Engineering at M.I.T., and Donald J . Kirwan is Assistant Professor of Chemical Engineering at the University of Virginia, Charlottesville, V a . Other coauthors are Edward G . Denk and Gun S. Ersan, Ph.D. Candzdates in Chemical Engineerzng, T u f t s ; and Jefferson Tester arid Franklin W o n g , P h . D . Candidates i n Chemical Engineering, M . I . T . W i t h the exception of D r . Kirwan, all the authors are members of the M.I. T . - T u f t s C?ystalliration Study Group.
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1970
149
TABLE I-K.
Title
CRYSTALLIZER PATENTS
Country, .Vo., and Date
Assignee &? Company
Cornrnents
Crystallizer
Brit. 1,173,082 3 Dec. 1969
(.%r Liquide, Soc. :Inon. pour 1’Etude et 1’Exploitation des Procedes Georges Claude)
A suspension-type crystdlli7cr is described for the precipitation of N a perborate from S d metaborate and H202 in a liquor
Continuous crystallization apparatus for even grains
U. S. 3,459,509 5 ’ h g . 1969
.1 suspension-type crystallizer with cur\ ed walls for better classification
Crystal size control during continuous crystallization
Ger. Offen. 1,903,447 28 Aug. 1969
Aoyama, Y . (Daido Namari Kakoki Co., Ltd.) Aovama. Y . (Daido Namari Kakoki Co., Ltd.)
Crystallization from solutions by passing them through cooled tubes
Ger. 1,296,125 29 AIay 1969
Bartsch, A. (Nord Deutsche d2ffinerie)
Crystallization process a n d apparatus
F r . 1,539,106 1 3 Sept. 1968
Dambrine, F. (Society Fives LilIe-Cail~
Apparatus for obtaining different grades of course crystals from salt solutions Double salt of sodium sulfatesodium carbonate Crystallization procedure
Ger. 1,298,969 10 July 1969
Domninq, H. (Wintershall A.-G., Celle)
U. S. 3,493,326 3 Feb. 1970
Indicates method for producirig a novrl double salt i N a , S O & - i X a , C O , ~ ) , Process for recovering crystallization substances A continuous scqregating salting out crystallization system is utilized for the high yield production of crystals
A method to produce coarse crystals of uniform size by preventing the spontaneous formation of nuclei, is described Patent indicates that scale can bc prevented from forming in coolinq coils by pumping the fluids through the tubes in highly turbulent flow Describes a process for improving multiple crystallization by the use of a n evaporator t o produce seeds for feeding to thc crystallizer l i u l t i p l r effect suspension crystallizcrs with interstage cooling
Crystallization of salts from solutions by displacement with organic solvents
Fr. 1,562,459 4 Apr. 1969
Steward, D . .l. ( P P G Ind. I n c . ) Furukawd, T.. et al. (Nippon Shiryo Kogyo C o . ‘) Hoppe, H., et a l . iKaliForschungsinstitut )
Sodium chloride crystals
Fr. 1,538,676 6 Sept. 1968
Iiiiperial Chemical Industries 1,td.
Method is described for preparing iarqc YaCl crystals by evaporation of a brinc containing polyvinyl alcohol
Crystal preparation of a slightly soluble inorganic salt
Gen. Offen. 1,804,289 14 .lug. 1969
Irie, K . , rt al. (Fuki Photo Film Co., L t d . )
Uniform crystals arc grown by addition of 2 or inore aqueous solutions to a dispersiny media at controlled rate
Continuous crystallization
S. Afr. 68 02,746 4 Nov. 1968
Javet, ,I.,et al. (2teliers Relges Reunis S. .I.)
. i n apparatus for growing uniform crystals by passin? the crystals uia a helical screw conveyor through a bath of constant supersaturation
Crystallizer
Brit. 1,172,939 3 Dec. 1969
Lafay, R. (Institute Francais d u Petrole, des Carburants et Lubrifiants)
h process for selectively crystallizinq one of
Crystallizer
Fr. 1,556,957 14 Feb. 1969
Lafay, R. (Institute Francais du Petrole)
Indicates a means for separating crystals from a solution
Continuous crystallization
U. S. 3,497,552 24 Feb. 1970
Olsen. G. P. [(Standard Oil Co. (Indiana)]
A process for crystallizing a n organic solutc from a solution without evaporation or otherwise removing solvent and without shock cooling of the solution. Examplc quoted-terephthalic acid, HzO, $-toluic acid mixture
Crystallization of carbonateyielding minerals
S. Afr. 6,902,382 21 O c t . 1969
Port, E. B., e t al. (.\Hied Chemical Corp.)
.i process concerned with the recovery of
Equipment for production of crystalline ammonium sulfate from saturated solution
Czech. 129,967 15 N o v . 1968
Pour, A T .
Produces (NH.$)2S04from N H 3 and H2SO< in a batch process agitated by a pneumatic stirrer that does not crush the crystals
Apparatus for the crystallization of inorganic salts
Ger. (East) 65,554 20 Feb. 1969
Scherzberg, H., et al.
Apparatus for the precipitation of inorganic salts by the addition of a chemical neutral to the solution
Crystallizers
Brit. 1,134,463 27 Nov. 1968
Shionogi and Co. Ltd.
.\pparatus for continuous crystallization of a substance dissolved in a solvent which can be decomposed by a 2nd solvent
Crystallization of phosphates
F r . 1,529,379 14 June 1968
Troost, S. (Unilever
Describes ways of controlling phosphate crystal form by the use of ionic surfaceactive agents
Ger. 1,301,299 21 Auq. 1969
Waldleben, W. (Inst. fuer Leichtbau und Oekonomische Verwendung von Werkstoffen)
Continuous crystallizer with novel way of controlling recirculation and, hence, supersaturation
Continuous vacuum crystallization
150
Fr. 1,533,138 2 .\us.1968
N. v.i
INDUSTRIAL A N D ENGINEERING CHEMISTRY
the constituents of liquid mixture of at least two constituents by direct heat exchange with a n immiscible liquid
crystals of N a H C 0 3 N a sesquicarbonate anhyd. hTa,COa and NagC03 .I320 from trona and other NaaC03-yielding inincrals is described
Miscellaneous Crystallization Syslems, Processes, and Techniques
Vapor-Liquid-Solid growth. Crystals grown by the vaporliquid-solid ( V L S ) technique are much superior in quality than those grown by more conventional means ( i . e . , vapor-solid growth). Descriptions of this process can be found in a number of recent patents by Wagner and associates ( ~ N R31\' , a, 4 N a ) . M a y and Shah ( Z X a ) havc discussed the VLS mechanism o n sapphire whiskers. Gel growth. I n his recent book, "Crystal Growth in Gels," Henisch ( 6 N b ) gives a summary of the mechanisms that describe the nucleation a n d growth of crystals in gels. Also included are a n historical survey of gel growth and a discussion of the solved and unsolved problems connected with crystallization in gels. Armington and O'Connor ( 7 N b ) discussed the conditions employed for the gel growth of crystals by the complex dilution method. Hanoka (5") described the gel cusps that form around the liquid surrounding the crystals in a gel, and showed how the cusps act as a membrane through which solute passes by osmosis to feed the growing crystal. Kurz (7A'b) reported the growth of potassium acid tartrate crystals by a chemical reaction in acidic gels. Gels made with tartaric acid u p to 6 N gave imperfect crystals, whereas gcls with higher normalities yielded clear, perfect crystals. Kurz also reported (8iVb) the growth of veiled cupric furoate crystals by a chemical reaction in silica gcls. T h e growth of filamentary crystals in gels has been reportcd by Boulin and Ellis (2117b), who grew siiver acetate crystals u p to 5 cm in length. Olsen ( / & l i b ) grew .11Cl3 crystals in gels and proposed a n explanation for the dendritic growth he observed. Ranadive et al. ( 7 7A:b) grew a-Pb(Ns)s single crystals 1-2 m m in length by the controlled diffusion of a NaN3 solution into a silica gcl containing Pb(NO3)2. TIN3 crystals were also prepared by the same method. Glocker and Soest ( 4 N h ) reported that single NI-14HzP04crystals u p to several centimeters long could be grown in gels by reducing the solubility of NI-I4H?POa in the gel with NH4Cl. Roy ( 7 2 S b ) presentcd a summary of the gel growth of apatite crystals, while Nick1 (9iVb) described procedures for the growth of calcitc crystals in gels. Fahrig et ~ l ( .3 N b ) reported some preliminary experiments on the growth of CdS and ZnS in gels. These papers represent progress in the field of gel growth, b u t much rcmains to be done bcfore all the potentials and drawbacks of this crystal growth technique are fully understood. Whisker growth. Recent advances in industry have created a demand for materials with different properties than are available now. This is especially true for aerospace enginecring applications where high strength a n d light weight are a must. This demand has led to the use of composite materials such as reinforced polymers a n d metals. Because of their high tensile strength, whiskers have often been used as the filamentary component of the composite. A drawback of whiskers is that their dimensions make them hard to handle. If the mechanism of whisker growth could be established, it might be possible to grow materials having the desirable properties of whiskers but having dimensions that are easier t o work with. A comprehensive review of thc growth and properties of whiskers was given by Yamamoto et d.( Z I N C ) . Kotler and Tarshis (72Nc) gave a n analysis of dendritic growth in pure systems, while Berg and McDonnell ( ? N e )explained a special form of whisker growth. A Dutch patent (75iVc) described a method for producing constant diameter whiskers by controlling the growth rate. Diamond whiskers were grown by Deryagin et al. (3Arc, dive), and Stolin (20Nc) reported the growth of diamond whiskers from the vapor phase on a diamond substrate. T h e growth rate Stolin observed was faster than those reported in the literature. Metals have been grown as whiskers by reduction of their halides. For example, Grange and Jourdan ( 7 N c ) prepared N i whiskers by the reduction of NiI2. A helicoidal whisker mechanism was proposed to describe the growth process. Kittaka and Kaneko ( 7 0 N c ) reported the growth of Fe whiskers on a large scale by the reduction of Fe halides. They also p u t forward a theory attempting to explain the growth mechanism. Morita ( 7 4 N c ) prepared Co whiskers by reducing CoClz. Shimazu et al. (79Nc) grew C u whiskers by the reduction of cuprous chloride and studied the effect of temperature and pressure on growth. Budnikov and Sandulov (2Nc) obtained M g O whiskers u p to 3 c m in length by
chemical transport, and Sharma (781Vc) grew Z n O whiskers from the vapor phase u p to 100 fi in diameter. Regis and Calviac ( 7 7 N c ) measured the growth rate of Cu whiskers both from the tip and the base of the whisker. Kahlweit (8Nc, SNc) determined the growth velocity of NH4Br and NHlCl whiskers as a function of temperature and supersaturation. A mechanism describing the growth of tin whiskers from AI-Sn alloys was presented by Furuta and H a m a m u r a ( 5 i V ~ ) . A patent ( 6 N c ) told how a-alumina whiskers could be grown by passing a mixture of hydrocarbon and water over molten A1 a t high temperatures. Another patent ( 7 7 h ' c ) reported the growth of S i c whiskers in a special crucible of Sic containing some CeOz. Levi and Gavanovich ( 7 3 i V ~ were ) able to grow ice whiskers a t - 1 5 O C in a solution of Formvar and ethylene dichloride. Pate1 and Deshapande ( 1 6 X ' c ) reported the growth of disubstituted ureas on mica and glass substrates. T h c mechanism of .\g whisker growth in a n unsteady electrolytic system has been studied by Zubov et al. ( 2 Z N c ) . As of now, whiskers offer great promise for industry, but more quantitative d a t a o n their growth is needed, and the mechanisms by which they grow must be establishcd before this potential can be realized. Hydrothermal growth. Hydrothermal growth is the synthesis of crystals in aqueous solvents under high temperatures and pressures. At ,these elevated conditions, materials that are normally insoluble dissolve and can subsequently be recrystallized. A numbcr of foreign reviews havc come out. Ikornikova ( S S d ) has rcviewcd the structure and physiochemical properties of some common solvents, while Kabeiiau and R a u (27,Vd) have summarized the current use of alkaline solvents. T h e latter authors also discuss the new application of hydrohalic acids as a solvent for materials that can rarely be grown from alkaline solutions. Examples of alkaline solvents can be found in a number of sources-( IiVd, ZiYd, 6,Vd, 7,l'd, :JAVd, I 7!Vd--141Vd, 76.Yd-1BA\'d, numbcr of strictly aqueous solvents have 22 X d - 2 4 N d , 2 S N d ) . also been reported (3A 5,ITd, lO)\-d, 1.9.Vd, 20.)-d). T h e equipment for hydrothermal growth should satisfy certain requirements. I t must be constructed to withstand high temperatures and pressures, be resistant to alkaline and acidic solvents, and b e designed in such a way that thcrc is rapid transfer of solute from the saturates to the growing crystal. I-Iclpful information about the design of such equipment can be found in a n article by Litvin and Tules ( 7 5 N d ) and a patent of Smid et R Z . (25,Vd). Several interesting studics on thc kinetics of hydrothermal growth have been published. Chernov et 01. (2,Vd) ha\'e measured the crystallographic growth rates of quartz in alkaline solvents in which the growth was not mass transfer limited. They hypothesized the existence of a n adsorbed solvent film o n the growing face of the crystal to explain their observations. For the first time, Voitsekhovskii et al. ( 2 7 N d ) have established the nature of thc microblock structure of corundum crystals (i,e., A \ 1 2 0 a ) . They attributed the microblock production to definite physiochemical
TABLE I-Nd. H Y D R O T H E R M A L S Y N T H E S I S OF VARIOUS M A T E R I A L S Ailaterial Ref. bluterial Ref. A l O ( 0 H ) (boehm- (7Nd) MgO (28'Yd) (NaiilSiO.i)o. (7,Vd) ite) A1203 (corundum) ( 2 7 : V d ) 2 NaCl BeAl204 (22Nd) N a germanates ( 7 2 N d ) C a C 0 3 (arago(IgNd) Na,Sc,Si,O, ( /XLVd) nite) Na,Y,,Si,O, (771Vd) C a C 0 3 (calcite) (g2Vd, 7 9 S d ) N a zeolite (74.Vd) CdMoOc (4 N d ) Nd(0H)zCI i51Vd) CdW04 (3h'd) SbzS3 (20A'd) HgS (cinnabar) (241Vd) S i o s (quartz) ( 2 N d , 6')Vd) K germanates (72Nd) Y3Alj012 (26iVd) K,Sc,Si,O, ( 78h'd) Zn gernianates ( 7 2 h ' d ) K,Y,Si,O, ( I 71\id ) Zn h l o 0 4 i4Nd) KZnFl 73"Vd 1 ZnO LinFe(WO4)z (70Nd j Li2W04 ( I0A-d) Li,Y,Si,O, ( 7 711:d) ~~
VOL. 6 2
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conditions of growth, and noted the importance of the microblock structure in determining the growth directions. Experiments have been done to determine the influence of impurities on the growth rares of different crystallographic faces (17-Y~':16iYd, 23.Yd, 26.Yd). Timofceva et a / . ( X A Y d )in\,estigatcd the distribution of iYd'j* axid Cr3+in yttrium garnets (Y.\G; and found that the sections of the crystal with maximum chromium concentration had a miiiimunl neodymium concentration (i,e., the maximum of one impurity corresponded to the iniiiinium for the other impurity). 1,ukina and Chernyaex (76-Vdj studied the uptake of iron o n zincite crystals ( Z n O ) and discovered that iron was adsorbed to different degrees on the crystallographic Paces of zincite. They also iioted that the slowest growiiig face '0001) captured iron the most. Processes related to crystallization: scaling. Scale is the term commonly applied to unwanted hard crystalline deposits which adhere to heat transfer surfaces. T h e presence of scale significantly increases the resistance to heat transfer and lowers the thermal efficiency of a process, somctiiiies to such a n extent that the scale must be removed. T h e rcnioval of scale results in unnecessary interruptions in a process, which makes it less efficient. Scaliiig is a problem common t o many industries and is of particular importance i n the evaporative processes for the desalination (if seaMiater with \vhich this rci-icw is concerned. T h e scope of this review covers thc papers o n scaliiig that have been published from January 1969 t o July 1970. For scaling to occur. the solution must be supcrsaturdted. T h u s , only those compounds which have invcrred solubilities (i,t., their solubilities decrease with the increase in temperature) will form scale. T h e principal components in scale arc caicium sulfate, aiid the alkaline scales-calcium carbonate and magnesium hydroxide. I n seawater, thc alkaline scales form from thc reaction of the bicarbonate ions.
2 HCOB- = COa2- + C O , f HyO
+ COa'-
=
COY
t
+ H?
+ 2 01-1-
il i (21
Calcium carbonate can exist in two crystallinc f o r m s : a r a o i i i t c and calcite, while calcium sulfate can exist i n three forins, di11.0). and aiihydrate ( C a S 0 4 2 I-IzO), hemihydrate (CaSO: hydrite (CaSO;). Because of the iniportancc of supersatiiratioii i n s c a l i n g . the solubility diagrams for the above compou1ids ha\ c brcn clctcrmined in varying scaivatcr concentrations. \\ark is still contiiiuiiig and, recently, the solubilities of calcium sulf,ite anhydrite in Ilos~vcll ~ v a t c r (72.Yej (i.e,, brackish water"', and sornc silicatc minerals in scawater ( I e ) have been iiicasuiccl. hkCIIANIshI. Scaling is the result of nucication a n d crystal growth. Experimental studies ( ~ . V P >17.\ e, 20.\.(-) with calcium sulfate iiidicatc that the phenomenon is very c o m p l c x . Iiiitinlly, minutc crystals or clusters of crystals are deposited on t h c srirfxe, gradually developing into a n iiitcrlockiiig mesh, I Iassoii cl ai. (SA\e)conducted calcium sulfare scaliiig cxpcrinicnts i n a tubular heat exchanger aiid noticed that the scale initially formed i i i the exit region of the exchanger propagated against the direction of the fluid flow, and stopped before reaching the ciitraiicc. T h e y \+-ere able to correlate thcir data by assuming that nuclcation occurred in the thcrma! boundary iayer and that these nuclei wcrc instantaneously deposited on thr surface whcrc they grew into crysrals. Separate studies 011 nucleation and crystal growth have resulted in a better understanding of the factors that affect scaling. XUCLIXYIOX.Besides the influence of a hiSher supcrsaturation, scalc forms more readily on surfaces that contain impcrfections (z.e., pit marks, grain boundaries, etc.). These observations have been interpretcd using the Volmer equation originally developed for homogeneous iiucleation and modified for the hetcrogciieous nucleation cases like scaliiig. This equation cssentially says that the iiucleation rate is related to the chaiigc in free energy associated with rhe formation of a "critical niicleui" and that, i n scaling, thcre exist iiucieation sites on the surface that low-er this frcc-energy chaiige aiid increase the nucleation rate. These nucleation sites catalyze the formation of scale, thus explaining why surfacrs that are' imperfect and, hence, contain many nucleation sites, teiid to scale more easily than smoother surfaces. Usiiig radioactive methods, Marcus ut n i . (13,Ye) have measured rhe number density (sites;cm') of these sites for calcium 152
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
sulfate nucleating o n a platiiiiiin surface. They also found that the number density could be altercd if the surface \vcrc electrically charged. Kecht < 79.Yt) did similar experiments o n a copprr surface and noted that the degree of siirfacc oxidation affected the uptake of scale. CRVSI A L GROIVITI.I n the evaporalive process for the desaiination of scatcater: the optimal operating tcniperariirc rnngc is 12O0-15O0C. I n this temperature range, necdlc-like crystals of the metastable hemihydrate prcferenrially precipitate on the surface r a t h e r than the imore stabir xihydritc. Coriiidcring the prevalence of the hemihydrace in scaliiis, it is siirprisii the crystal growth kinetics of this substance ha\.? iiot t-~eeii iiie Some \\.ark, ho\vevcr, ha5 been done oii the crystal grobvth of dihydrate and anhydritc. 1,iu and Nancol1;rs ( I L Y P ) have stridied the crystallization of calcium sulfate dihydrate o n rhc addition of seed crystals to stable supersaturated solutions. l'he i?roi+tti \vas found to be surface controlled bvith dn acti\farioii enerqy of 1.5 kcal g niole. Similar measarenie1lts on the gro\rth of calciiiiii sulfdtc anhydritc by I,u et d.(72.Ve) indicatctl that i t w a s surface coiitrollcd but \vith a n actimtion energy 3 times that of the dihydrate. CURKEN r ME1HOLE O P Sc.4r.r C o ~ ~ r n c i i .Lllintt . Jlasoil (7.\.e 1 prrscnt exccllcnL rc\icws o n thc present control in sca\\.atcr cvaporalors. I n contrast, hlcCutchan ~t i d . (75.j.e) @\,e n d(,tailed siurim,iry of the .atin?, condirions currentl), being used iii a iiiiiii1xr of sea r c ~ , . q o r a t o r sin the est in ecology, a parCJnitcd States. Because of rhc public ticularly interesting rr\.icw is thar by O h e c h t ( /(?:Ye ) > who looks a t t h e c u r r e n t mcthods of scale control from a11 e n \ iroiinieiital cngiiicer's viewpoint. Oiic of the niorc popular methods of controlling scale is with thc usc of additives (GAY?, 78.\~e, 2.1.3-e). .I fcw parts per millioii of additive can effectively illhibit the precipitati~ii~ of scalc, and brcausc such low roiiceiitratioiis a r c used this 1echniquc is callcd a threshold treatment. 1 low thcsc a d d i t i l e xvork has n o t bcrii resolved b u t it can bc said thar the ddditivc does iiot form d comp!cx with t h e scale because the concentrations used arc much smaller than the chemical equivalents of scale in s e a ~ i i t e r . Ilccently, though, there is evidence (,FAVe)that there inighr be sonic reaction between the additivc aiid the scale. Instead of keepiiiq the scale in solution as in the thrcshold treatment, methods have been devclopecl in which the scalr is precipitated from the scawatcr with chemicals before t h c scawater is fcd to thc evaporator. I)i Luzio e l d.(2.Ye) ha\.c found that by reacting phosphoric acid with ammonia iii scawarcr, csscntially all the magncsium anti up to 995; of the calcium cotild be removed. Sicder (22.3-r) has patented a process in which the fccdLvatcr is first heated t o the point of incipient sca!iiil: before a flocculating agent is added to precipitate the scalc. 1\11 interesting application of the principle of hc:tc:rogelicous nuc1e;ition is t o offer a n alternative surface for thc scalc to deposit on. This surface must contain inally more nucleation sites than the heatin? surface for it to coliipetc favorably for thc scale. Sieder (27,Ye), in his patent, describes t h r usc of a fluidized bed of granular solids to remove rhc scale from seawater. .\ coinmon method of control alkaline scaliiig is called In this proress, carbon carbon dioxidc injection ( I O l - e , 7 dioxidc gas is injected under pressurc: into seavxter to reverse reaction (7.3-e). .\ ilew technique which shows promise for controlling scale is to cover the heat transfer surface with a polymer coating. I t has been shown that a polyfluorocarbon coating (9iVe, 2.7.Ve) will siynificantly reduce the formation of scale, perhaps by covering the nucleation sites. This technique will undoubtedly become increasingly more important as inore effective ways of bonding the coating to the surface and bktter heat resistant polymer coatings a r e found. Ice. Research into all aspects of ice crystallization rcmainr as usual at a high level, although three major areas appear to havc been favored in papers published diiring the preceding year. These areas cover the growth of ice from solution, the growth and forination of ice in the atmosphere, anti the inorphological stability of ice crystals. Higashi et n l . (5A\y)used a n X-ray diffraction topographical method to examine dislocation structures in large single crystals of ice grown from water by a modified Czochralski method, and from their observations were able t o infer rhat a two-diineiisiorial
nuclei mechanism was the predominant growth mode in the case of c-axis parallel growth, while spiral growth around screw dislocations was the predominant growth mechanism in the case of growth perpendicular to the c-axis. Ryan (77,Vj) o n the other hand measured the growth of ice parallel to the basal plane in water arid metal fluorides at supercoolings ranging from 0.1’ to 2.5”C. H e observcd that in pure water the radius of curvature of the tip decreased with increasing supercooling, and suggested further that the enhancement of the growth rate of ice dendrites in supercooled aqueous metnl fluoride solutions was due to preferential orientation of water molecules by the interface freezing potential. I t has becn pointed out, howcver, by Miksch (i’2:Vf) that the morphology and growth rate of ice grown in supercooled water w a s markcdly affected by conLection currents in the water, and this well-known obsrrvation should further emphasize the experimental hardships that must be overcome t o obtain good kinetic growth data. T h e growth of “atmosphcric” ice, however, has been studicd by Fukuta ( 2 h 7 ) , and Isono PI al. (7:’~’’) has indicated that ice mm Hg grown on solid particles in air at pressure of IO+ to have a quasi-spherical shape with specular facets of high indices which differ from the shapc of ice grown at higher pressures. Ice crystal agglomeration has bccn the subject of a report by Smith-Johannsen (2 1.17) who obser\red numerous agglomerates somewhat like the letter T among replicas of ice crystals collected from laboratory ice clouds. IIe proposed that these T agglomerates were formed by a hydrodynamic process. Finally, a comprehensive theoretical and experimental study on ice morphological stability has becn completed by Sekerka et al. (79,i:f), while a n experimental study on the stability of cylindrical ice crystals by Hardy and Coriell (4.VJ) has confirmed that the growth rate of perturbations in the circular shape of the cylinder are, in fact, in agrremcnt with the predictions of a Mullins-Sekcrka stability analysis.
(22H) Sutherland, D . N., Crystalliz,ition-1)issolutioti Cycling of Sucrose Cryst.ils, Chem. J h g . Sci., 24, 192 ( 1 9 4 9 ) . (23H) Takiyama, K., Ynnxid.i, F., I‘urukaw.i, F , , Formition of I.e,id Sulfate P r e cipitntes, Aunseii Kagaku, 18, 7 2 8 (1969). ( 2 4 I I ) Tyutyunniko\.a, T. V., I i ) b k i n , Y . IC., Soloin;ikh,i, Y.A . Crvstallizaiion of Lithium Fluoride from Aqueous Solutions, V k r . Khim. Zli,, 35, S k 4 (i 969).
Coprecipitation a n d Purificafion in Crystallization Processes From Solution (1 J ) Abashidzc, F.. I , Shpunt, S. Y.:hfctastable l’hdsc in thc CaHPO4 f CnSOi H s 0 svstem a t 80’ (;>pplicnhle to thr production of CaHPOi), Zit. P n k l , Khtm., 42,’732 (1969). (2J) Bolsaitis, P., Continuous Column C ~ y a t ~ i l l m ~ t i oCn , (1969). (3J) Dcvyatykh G. G.. Umilin, V. A , >B o y x k i n , A \’., Separation of Salt Sllatilres b y Counte-r Chrrcnt Cryst.illiz:ition f r o m Solution, Y e o r . O T Khzm. ~ 7Pkhnol., 3,225 (1969). (45) I.af.iy, R . , Usc of Back-Flow i n Ilcicluping (’ryst,illir;ition Procrss .In
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Developmenl and Studies on Industrial Crystallizer Practice Agrcb., I.. M., l.yiikinx, T. V , Novotcl’no\,.i, N. \‘A, t l oi., ‘Ihroi y . i n c l l ’ r c i c t i ~ c (lFf)Citric Acid Crysl;illiz.ition, l i / i l t b o p c i . Kondiler. l ’ i ~ i i i . , 13, 26 ( I 9 6 9 ) . ( 2 K ) Barnforth, A W.. H o b , !o Sprcily Crystallizer, Chetn Proceir.. 16, 8(1970) f3K) Bcnnctt. K . C., V,in liurcn. h i . , Comiricrci.il IJrc
