H. M. SCHOEN
J . VAN DEN BOGAERDE
CRYSTALLIZATION -EMPHASIS
IS O N BASICS
Fundamental aspects oj’ crystallizer design, operation, and scale-up are making great advances.
Considerable attention
is directed to efect o f trace impurities in growing crystals
he dominant factor dictating the a crystallizer system is the nature of the material to be crystallized. Since each chemical combination is unique, the crystallizer system must be designed to take into account these unique properties. Differences among the many possible chemical combinations are such that a specific design may be adapted to a wide range of chemicals. There are, however, a number of units designed to do a specific piece of work, limiting their usefulness, but adding to the understanding of the fundamentals of crystallization. I t would be impossible in a limited space to cover all the aspects of crystallization theory, design, and application in the literature in the past year. Any discussion of past effort or current or future trends must be limited to only the most significant items. Certainly the volume of literature relating to crystallization has grown -in many cases far outstripping other fields. If a particular area could be said to have received more emphasis than others, it must be in the welcome addition of items dealing with the more fundamental aspects of crystallizer design, operation, and scale-up.
Tactual design of
Supporting weight can be found in the substantial number of papers discussing the effect of impurities and additives on crystal growth. Several of these papers consider the mechanism of incorporating impurities in a growing crystal. The distribution of trace impurities in growing crystals also received considerable attention.
Equipment
A number of pieces of equipment were introduced which seem to have the potential for other uses, though designed for specific crystallization applications. Certainly the salient features of some are basis for optimism for wider and more varied uses. Growth of single crystals from a
The scraped surface exchanger has been afifilied to separation by crystallization of the isomers of xvlene, dichlorobenzene, and hexachlorobentene VOL. 5 4
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Need Y2 to 44 Microns? Sturtevant Micronizers* Make 325 Mesh Obsolete
One Operation Reduces, Classifies Sturtevant Micronizers grind and classify in one operation in a single chamber-provide fines in range from 95 to 44 microns to meet today's increased product fineness needs. Can handle heat-sensitive materials. Production Model (15 in. chamber) N o Attritional H e a t Particles in high speed rotation, propelled by compressed air entering shallow chamber at angles to periphery, grind each other by violent impact. Design Uves instant accessibility, easy cleaning. N o moving parts. Classifying i s Simultaneous Centrifugal force keeps oversize material in grinding zone, cyclone action in central section of chamber classifies and collects fines for bagging. Rate of feed and pressure control particle size. Eight M o d e l s A v a i l a b l e Grinding chambers range from 2 in. diameter laboratory size ( f / . to 1 lb. per hr. capacity) to large 36 in. diameter production size (500 to 4000 lbs. per hr. capacity). For full description, request Bulletin No. 091.
Engineered for Special Needs A 30 in. Sturtevant Micronizer is reducing titanium dioxide to under 1 micron at feed rate of 2250 Ibs. per hr. For another firm, a 24 in. model grinds 50% DDT to 3.5 average microns at a solid feed rate of 1200-1400 lbs. per hr. A pharmaceutical house uses an 8 in. model to produce procaine-penicillin fines in the 5 t o 20 micron range. Iron oxide pigment is being reduced by a 30 in. Micronizer to 2 to 3 average microns. Sturtevant will help you plan a FluidJet system for your ultra-fine grinding and classifying requirements. Write today.
molten mass with a linear isotherm is accomplished by controlled cooling of a crucible containing the seed crystal ( 4 4 ) . Crystallization is induced by vertical motion of the cooled, thermally shielded cylinder across the gradually heated walls of the crucible. An induction-heated pressure vessel is used to grow ferrite Sufficient oxygen crystals (5A). pressure minimizes decomposition. This furnace is capable of operation at 1600" C. and 75 atm. pressure. A U. S. patent (2,983,589) describes a continuous device for the separation of hydrocarbon mixtures ( 7 A ) . Formation of a crystal bed in the purification zone is obviated by passing the crystals through one end of a chamber whose other end is heated. A filter is so placed as to prevent the crystals from entering the channel through which the melt is withdrawn. A Russian patent (131,336) describes a columnar crystallizer in which alternately inclined plates cause the crystals to flow in a zigzag path countercurrent to the ascending mother liquor (72A). I n a specially designed evaporating vessel 100 kg. per hour of Glauber's salt was produced from a solution containing 28y0 sodium sulfate and lOy0 sulfuric acid ( 3 A ) . A conical bottom design keeps the fine crystals within the unit until they have attained sufficient size to be withdrawn as product. Lower pressure within a vacuum crystallizer can be obtained by locating the condenser within the crystallizer body, above the boiling-liquid surface ( 6 A ) . Crystallization of glucose is accomplished in a screw crystallizer ( 9 A ) . Large crystals of ammonium sulfate are obtained by controlling circulation and agitation of the mother liquors (70A). Conical saturators with variable-speed mixing are used
Can Test or Contract Micronizing Help You? Test micronizing of your own material, or production micronizing on contract basis, are part of Sturtevant service. See for yourself the improvement ultra-fine grinding can contribute to your product. Write for full details.
STURTEVANT MILL CO., 1 0 5 Clayton St.. Boston, Mass.
m R E O I S T E R E D T R A D E M A R K O F S T U R T E V A N T MILL 00.
Herbert M . Schoen is Manager of the Resources Technology Diu., Radiation Applications, Inc. He has authored I B E C ' s annual review on Crystallization f o r the past nine years. Jack Van den Bogaerde is a Chemical Engineer at American Cyanamid's Stamford Research Laboratories. AUTHORS.
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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
in this design. A continuous unit for the concentration of solutions by selective freezing out of the solvent was patented ( 7 4 4 ) . I n this unit the solution flows spirally in a path perpendicular to the axis of the stirrer. Solvent crystals are discharged at the top where the fresh solution is introduced. The effect of stirring intensity, cooling rate, initial concentration, and presence of NaCl was investigated in a threestage continuous vacuum pilot unit producing ammonium chloride (75A). Three recent publications discuss a continuous sectional crystallizer with a cooling coil and a feed and overflow tube in each section (764)' an apparatus for growing cr)-stals from melts (73A), and the operating conditions for heaters in zone-recrystallization furnaces (84). Other equipment includes a lowcost Bridgman-type single-crystal device ( 7 7A), an apparatus for growth from solution ( Z A ) , and one for carrying out crystallization in an inert atmosphere ( 7 A ) . I n d u s t r i a l Practice
A most noteworthy paper was published in which the various factors important to crystallizer design are discussed ( 7 B ) . A means of predicting the operating variables is described. To carry out the prediction, growth rate data are used in the following form:
where
dr -
dt
r
= linear growth rate
equivalent radius of the crystal a = specific growth rate constant s = supersaturation b, n = constants =
Examples are given for various modes of crystallizer operation such as classified suspension, mixed suspension of classified product, and mixed suspension of an unclassified product. A draft tube and baffle-type crystallizer is described in a recent paper by Caldwell ( 3 B ) . Maintenance of
i
a high magma density, removal of excess fines, and controlled circulation of crystals to the supersaturation zone are required to grow crystals up to about 1 6 mesh. Scale-up and performance relations for continuous crystallizers are treated by Saeman (6B). The effect of solution velocity upon crust formation in crystallizers was reported ( 4 B ) . Crust formation was found to be a direct function of the temperature gradient at the wall and an inverse function of the solution velocity. For a number of common salts, the values of velocity and temperature gradient at which crust formation did not occur were determined. A rotating-disk contactor is used for preparing pure potassium nitrate crystals (8B). I n the contactor, the mother liquor, containing potassium chloride, and hydrochloric and nitric acids are displaced by a potassium hydroxide solution. Precipitation rate, yield, and particle size of alumina hydrate are increased by heating the seed slurry prior to its addition to the solution (5B). A plant for salt manufacture from sea water at New South Wales is described (2B). For a two-step process, manufacturing costs range from $11 to $17 per ton of salt. Commercial production of sodium sulfate from Sambhar Lake bitterns is discussed ( 7 B ) . Pilot plant studies have confirmed the feasibility of improving the sodium sulfate recovery by increasing the sulfate content of the bittern, the bitter mother liquor that remains in saltworks after the primary salt has been crystallized. Nucleation
Crystallization is a two-step process involving first nucleation and then the growth of the nucleus to a macro size. In practice, both steps are taking place a t the same time within the crystallizer. Although much has been written on the subject of nucleation, it is difficult to make use of these findings in an industrial crystallizer. I t is important, however, to design a crystallizer so that supersaturation levels for nucleation are such that unwanted nuclei do not form. Particular attention
must often be given to temperature and supersaturation levels in recirculating lines. Cold spots can lead to the formation of nuclei and/ or clogging of lines. Analysis of the nucleating effect of fine particles is complicated, not only by variation in sizes but also by aggregation (3C). The general effect of aggregation is to decrease the total number of particles and also to reduce required supercooling. The effect of agitation upon nucleation was investigated on a laboratory and plant scale (5C). For ammonium dihydrogen phosphate a region was found where an increase in the agitation reduced the tendency to nucleate. All previous investigators have found that increased agitation causes increased tendency to nucleate, or in some cases, no effect was observed. A theoretical analysis of nucleation and growth of crystals during adsorption of foreign substances on their surfaces was carried out (4C). Using a simple cubic lattice as a model, the effect of adsorption on the equilibrium form of the crystals as well as the kinetics of the formation of crystal nuclei was determined. Experiments on the nucleation threshold of alkali halides (2C) indicated that the nucleation threshold temperature T and the true freezing point T , can be related by the expression 9 = T/T,, where 9 0.82. Three hundred ions were estimated to be present in the threshold nucleus. Another study of the nucleation of alkali halides was conducted (8C). Nucleation and growth of mercury crystals at low supersaturation were determined (6C). Nucleation took place at much lower supersaturations than those previously reported. The rate of nucleation at high pressures was investigated ( I C ) . I t is calculated that the rate of nucleation of diamond crystals in graphite is practically zero even at 75,000 atm. and 1000° K., unless foreign inclusions are present. I n a recent study of the kinetics of crystal nucleation in a number of normal alkane liquids, nucleation frequencies were determined quantitatively. The frequencies were found to be propor-
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tional to droplet volume and increased by a factor of 5000 to 8000 per 1 C. temperature decrease ( 7 C ) . Crystal Growth
A possible mechanism for crystal growth from the melt is given ( 4 0 ) . Stable facets are assumed to grow by nucleation a t the point of supercooling, followed by rapid sheet growth across the facet. The theory of crystal growth and interface motion in crystalline materials is discussed in detail by Cahn ( 7 0 ) . I t was found that if a critical driving force is exceeded, the surface is able to advance normal to itself without steps. If the driving force is below the critical value, lateral step motion is necessary. I n another paper (ZD), the rate of spiral growth from solutions and melts is examined. A study of “self-healing’’ kinetics of intentionally damaged crystals was presented (30). Three mechanisms of self-healing are discussed : volume diffusion, surface diffusion, and transfer of matter through gaseous phase or recondensation. Zone Melting
The subject of zone melting is treated in considerable detail in a recent thesis (5E) titled “Fractional Crystallization from Melts.” Economics for industrial-scale zone refining are developed. Experimental studies were conducted with a zone melter and a cooled drum apparatus. I t is felt that the cooled-drum apparatus has commercial possibilities for cheap separation of bulk materials. A British patent (840,144) was granted for the continuous zone melting of metals and salts (3E). The impure material is fed at the center of a horizontal device. Purer material overflows at the end where the hot zones start with less pure material at the other end. High purification of organic niaterials was obtained in a modified apparatus
(4-9. A patent was granted to Pfann for a cross-flow zone refiner with recycle (2E). T h e process operates in a manner analogous to a distillation column. An analog computer Circle NO. 21 mi Readers’ Service Card
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
was used to simulate the zone melting of small-diameter tungsten rods (723). Predicted time-temperature curves are presented for a rod, 0.4 cm. in diameter, with optimum power input. I t was also shown that unsatisfactory operation would be obtained for rods, 1 cm. in diameter. Impurities and Additives
A U. S.patent (2,954,282) relates the effects of surfactants upon the growth of sodium sesquicarbonate (ZF). I t was found that anionic surfactants such as dodecylbenzenesulfonate, polypropylene, benzenesulfonates (ClO-Cl8, alkyl groups), and taurates improve the crystallization of inorganic salts. In general, it was found that the yield could be increased and that the dewatered crystals contained less water than was obtained in the absence of surfactant additives. I n a plant producing 210 tons per day of sodium sesquicarbonate crystals containing 10 to 15yowater, the addition of 20 p.p.m. of dodecylbenzene sodium sulfonate increased the production to 850 tons per day containing 67, water. A recent British patent ( I F ) and paper ( 5 F ) are concerned with the effect of additives on sodium chloride. In the former a large number of organic and inorganic additives were found to produce crystals resistant to abrasion. I n the latter, acetic acid and alanine had little or no effect on growth, while glycine led to development of dodecahedral faces and formamide to development of octahedral faces. Addition of trace quantities of indium chloride to sodium chloride crystals growing in a slightly saturated solution led to verification of the equation ( 3 F ) : x/y = A Co/(l
+ BC0)112
where x and y are amounts of impurity and macrocomponent in the solid phase, respectively; A = 0.01, B = 400, and Co is the initial concentration of the impurity in the solution. In the crystallization of strontium sulfate brought about by mixing strontium chloride and potassium sulfate solutions, the presence of a third component had a large effect habit ( 4 F ) . Sodium citrate gave
:lliptical disks while ethylenediiminetetraacetic acid gave twolimensional spherulites. leviews and Books
WHAT REALLY MAKES AN
IONKCHANGER “A UT 0 MATC I ”
I n 1943 we used small index valves o n a central panel t o operate the valves of a 600 gpm de-ionizer (a job that is still in operation). This was called “automatic” control at the time, but today it would be correctly termed “remote manual.” Since then, we have built many different types of “automated” deionizers, and hence feel we are capable of defining what you should expect i n “automatic” ionxchange equipment. A fully-antomatic de-ionizer will include bulk storage of regenerants with n o handling of regenerant chemicals; automatic initiation of regeneration by a conductivity meter; automatic service rinse after shutdown, to restore desired quality; and of course, complete automatic control of all regeneration steps, including dilution of regenerants. Since multiple units are commonly used to provide continuous flow, a freshly regenerated unit will usually go o n stand-by, to be put into service automatically when another requires regeneration. Interlocks prevent simultaneous regeneration of two or more units. Such a n installation usually includes monitors which will sound a n alarm to indicate malfunction. T h e control circuits are flexible enough to permit simple adjustments of the regeneration cycle, and the control panel includes visual indicators to inform the operator of the status of all units. While the foregoing indicates the scope of fuZZ “automation,” it is obvious that many modifications are possible, particularly with small units, whereby certain functions are initiated or governed manually rather than automatically. T h e extent of our experience is such that we can provide reliable “automation” equipment of any degree or detail that a particular job calls for. See your I W T representative for details.
A volume on crystallization covers :rystallography, phase equilibria, md industrial crystallization (2G). rhree papers appeared as published orms of an ACS symposium on :rystallization equipment. These 3apers treated theory (4G), design :3G), and applications ( 7G). A recent volume, “Crystallization : rheory and Practice,” should prove :o be an excellent reference source :5G). In addition to covering kheories of growth and nucleation, laboratory and industrial practice is discussed. The practice of crystallization is treated by the various industries employing the operation, including sugar, fertilizer chemicals, glass, ceramics, and others. LITERATURE CITED
Apparatus and Equipment (IA) Arens, J. L., Ratje, J. D., U. S. Patent 2,983,589 (May 9, 1961). (2A) Belyaev, L. M., Karpenko, A. G., Vitovskii, B. V., Dobrzhanskii, G. F., Russ. Pat. 132,615 (Oct. 20, 1960). 3A) Ebner, K., Brit. Pat. 835,659 (May 25, 1960). (4A) Eckstein, J., Wachtl. Z., Czech. Pat. 94,996 (April 15, 1960). (5A) Ferretti. A., Wickham, D. G., Wold, A., Rev. Sci. Instr. 32, 566 (1961). (6A)- Garbato, C., Ital. Pat.-605,152 (May 17, 1960). (7A). Hauptman, Z . , Chem. lzsty 55, 70 (1961). (8A) Krapukhin, V. V., Vigdorovich, V. N., Imest. Vysshikh Ucheb. Zauedenii, Ysvetnaya Met., 3, No. 4, 122 (1960). (9A) Lebedev, N. V., Lyubetskaya, M. N., Bannikova, A. A., Russ. Pat. 135,836 (Feb. 15, 1961). (10A) Lebedeva, G. N., Gol’dvasser, G. L., Koks i Khim., No. 10, 42 (1960). ( l l A ) Olson, E. H., U. S. Atomic Energy Comm., IS-178,1960. (12A) Rodionov, V. A., Russ. Pat. 131,336 (Sept. 10, 1960). (13A) Sheftal, N., Stepanov, I. V., Vasil’eva, M. A., Khaimov-Mal’kov, V. Ya., Russ. Pat. 132,613-14 (Oct. 20, 1960). (14A) Steinbacher, F. Schmerbeck, C. S., U. S. Pat. 2,956,414 (Oct. 18, 1960). (1 5A) Synowiec, J., Pabis-Machej, J.: Przemysl Chem. 39, No. 3, 161 (1960). (16A) Vol’fson, B. N., Russ. Pat. 131,335 (Sept. 10, 1960). Industrial Practice (1B) Bransom, S. H., Brit. Chem. Eng. 5 838 (1960). (2B) Buchanan, R. H., Chem. Processin& (Sydney) 13, No. 4,39 (1960).
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THERMODYNAMICS OF SOLIDS By R. A. SWALIN, Univ. of Minnesota. Interweaves atomistic and thermodynamic interpretations to show h o w thermodynamic measurements can be used to derive information on the mechanisms involved. 1962. 343 pages. $12.50 POLYMER PROCESSsNG
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From INTERSCIENCE RADIATION CHEMISTRY OF POLYMERIC SYSTEMS By A. CHAPIRO. Stresses radiation-initiated polymerization, radiation effects in solid polymers, radiation effects in polymer solution, and radiation-initiated graftcopolymerization. (High Polymer Series: TTolume lj.) 1962. Approx. 718 pages. Prob. $20.00 POLYETHEWS In three parts. Edited by N. G .
GAYLORD, Gaylord Associates, Inc. Chemistry, technology, and applications. (High Polymer Series: Volume 13.) Parts I and I1 cover polyalkylene oxides and epoxy resins. Part I11 (to be published first) presents polyethers containing sulphur. Part 111: 1962. Approx. 320 pages. Prob. $11.00. Parts I and 11in Press AUTOXIDATION AND ANTIOXIDANTS Two Vols. Edited by W . 0 . LUNDBERG,Univ. of Minnesota. Surveys theoretical and practical aspects. Vol. I deals primarily with theory of the fuiidamental chemistry; Vol. I1 applies this material in great detail to preservation of many organic products. 1701. I : 1961. 464pages. $15.50. Vol. 11: 1962. Approx. 490 pages. Prob. $16.00 EXPERIMENTAL THERMOCHEMISTRY, Vol. II Edited by H . A. SKINNER, Diziv. of Manchester. Foreword by F. D. ROSSINI, Carnegie Inst. of Technology. Articles by leading world authorities, developing further the topics in Vol. I and giving detailed descriptions of new techniques. 1962. Approx. 488 pages. Prob. $15.50 THE PYRIMIDINES AND THEIR DERIVATIVES Vol. 16 in Heterocyclic Compounds Series. By D. J. BROWN, Australian Nat’l University. A critical review of pyrimidine chemistry with emphasis on practical rather than theoretical -aspects. 1962. Approx. 800pages. Prob. $32.00 INTRODUCTION TO THERMODYNAMICS OF IRREVERSIBLE PROCESSES By I. PRIGOGINE, Univ. of Brussels. 1962. Approx. 132 pages. Prob. $4.00
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53, 115 (1961\. (4B).Karetnikov, Y u . P., Tarasova, V. N., Z h r . Priklad. Khim. 34, 282 (1961). (5B) Roberts, R . V., Jr., U. S. Pat. 2,935,376 (May 3. 1960). (6B) Saeman, W. C., “Separation Procrsses in Practice,” collected papers, p. 84, Rheinhold, New York, 1961. (7B) Seshadri, K., Vyas, R . P.: RrsParch and Ind. (India) 5 , 282 (1960). (8B) Vermande, C . , Dutch Pat. 94,714 ( J u l y 15, 1960). I
,
Nucleation (1C) Bradley, It. S., J . Colloid Sci. 15, 525 (1960). (2C) Buckle, 1:. R . , .Vature 186, 875 (1960). (3C) Head, R. B., Sutherland, K . L., Au.rtralian ,J. Phys. 13, 584 (1960). (4C) Kaishev, K., Mutafchiev, B., Zzoest. Khim. Z m f . Rzilgar. Akad. iVauk 7, 145 (1960). (5C) MLllin. ,J. I\’., R vrn, K. D., .Vu&u,e 190. 251 11961) --,. (6C) Sears: G . lV., J. Chnn. Ph.ys. 33, 563 (1960). (7C) Turnbull, D.: Cormia, R . L., J . C‘hhrm. Phys. 34. 820 (1961\. \ , (8‘2) It‘ilkrns, JV., 4th Znlern. Kongr. Elrctrorirnmikrorko~ie, Berlin, 1958; Verhandl. 1 , p. 506 (1960). \
Crystal Growth (1D) Cahn, J. W., Acta M e & . 8, 554 (1960). (2D) Chernov, A. A,, Doklady Akad. llieuk U.S.S.R. 133, No. 6, 1323 (1960). (3D) Geguzin, Ya. E., Kulik, I. O., Fiz. Metal. i Meta!Zorrd.> Akad. Nauk S.S.S.R. 9, 379 (1960). (4D) Trainor, A., Bartlett, B. E., SolidStat? EIPrtl-onics 2, Nos. 2/3, 106 (1961). Zone Melting (1E) Mason, H. L., ,J. Research Nat. Bur. Standards 65C, h’o. 2, 97 (1961). (‘E) Pfann, I\;. G.? U. S. Pat. 2,949,348 ( A 4 u 16. ~ , 1960). (35) \Vestern Electric Co., Brit. Pat. 840,144 (July 6: 1960). :4E) ITynne, E. A,, Microchem. J . V, No. 2, 175 (1961). :5E) S2’ilcox, FV. R., “Fractional Crystallization from Melts,” Ph.D. thcsis, Univ. of California? June 1960. mpurities and Additives ‘1F) Allday, C.: Booth, C. L., Brit. Pat. 848,328 (Sept, 14, 1960). 2F) Bauer. W. C., McCue, A. P., Rule, K. C . , U. S. Pat. 2,954,282 (Sept. 27, 1960). 3F) Melikhov, I. V., Ch’iu, H., Merkulova: M. S., Doklady Akad. ‘%-aukS.S.S.R. 133, 401 (1960). 4F) Otani, S., Bull. Chem. Soc. Ja$an 33, 1543 (1960). 5F) Speidel, R., Neues Jahrb. Mineral.! Monotsh. 1961, p. 81. .
I
keviews and Books 1G) Garrett, D. E., IND. ENC. C H m i . 53, 623 (1961). 2G) Mullin, J. FV., “Crystallization,” Butterworth, Washington, D. C., 1961. 3G) Saeman, IV. C., IND. ENG. CHmi. 53, 612 (1961). IG) Schoen, H. M., Zbid., 607. 5G) Van Hook, A., “Crystallization: Theory and Practice,” ACS Monograph 152, Rheinhold, New York, 1961.
