Crystallization of Spherical Common Salt in the Submillimeter Size

Oct 26, 2010 - Indrajit Mukhopadhyay,* Vadake. P. Mohandas, Girish R. Desale, A. Chaudhary, and. Pushpito K. Ghosh*. Discipline of Salt and Marine ...
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Ind. Eng. Chem. Res. 2010, 49, 12197–12203

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Crystallization of Spherical Common Salt in the Submillimeter Size Range without Habit Modifier Indrajit Mukhopadhyay,* Vadake. P. Mohandas, Girish R. Desale, A. Chaudhary, and Pushpito K. Ghosh* Discipline of Salt and Marine Chemicals, Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research, G. B. Marg, BhaVnagar 364 002, Gujarat, India

Nearly spherical morphology of solution-grown NaCl particles in the size range of 300-1000 µm was achieved at 55-60 °C employing a suitable crystallizer equipped with a butterfly wing-shaped impeller operated at 250 rpm. Morphology control was equally effective with synthetic and natural brines and required no habit modifier. Sieved spherical salt of 350-500 µm size exhibited superior flow (ca. 20% greater mass flow rate through a funnel; angle of repose ∼16°)sas compared to commercial vacuum evaporated free flow cubic salt of comparable dimensionsupon treatment with potassium ferrocyanide anticaking additive. The superior flow characteristic was retained even after 3 months of storage. Scanning electron microscopic studies revealed that the round polycrystalline particles were derived from the stacking of minute NaCl cubes and the average size of the spheres was amenable to reduction through use of ethanol as antisolvent. The process was successfully scaled up to 10 kg level. Introduction The shape and size of NaCl crystals can have a profound bearing on basic properties such as their free flow nature and dissolution characteristics. Consequently, crystallization of NaCl from brine has been a subject of fundamental importance for decades. Thermodynamically, crystals in a cubic system like NaCl crystallize in cubic form, on the basis of the chemical bonding theory of single-crystal growth.1-3 It has been shown through a number of studies that the growth of perfect cubes is difficult, especially in the presence of other constituents such as those present in natural brines.4 Perfect cubic crystals have, however, been obtained by the addition of a small quantity of soluble lead salts.5 The cubic morphology has certain disadvantages though. Compared to spherical morphology, the flow characteristics are inferior and the crystals are more prone to caking as a result of large contact area. Consequently, in parallel with the quest for perfect cubic crystals, there has been a constant search for modification of the morphology of common salt.6-13 Fundamentally, the cubic shape of common salt occurs because of slow growth of the {100} crystallographic faces.14-17 A number of methods are known for changing the morphology of crystals grown from solution.18-20 Use of impurities is one of the frequently adopted methods.21,22 Many kinds of impurities have been used for the purpose.6-8,23-32 It has been proposed in the literature that adsorption of impurities onto crystal faces changes the relative surface free energies and may block sites essential to the incorporation of new solute on to the crystal lattice. These factors in turn affect the growth kinetics and, consequently, modify the habit of the crystalline phase.14 It has been seen that the requirement of inorganic ionic impurities are at the parts per million (ppm) level, while organic impurities are generally required in larger amounts, which is a negative factor.7 Cubic system single crystals often crystallize in a general habit with the competing growth rate of {100} and {111} faces.1 * To whom correspondence should be addressed. Tel.: (+91) 278 2567039. Fax: (+91) 278 2567562. E-mail: [email protected] (I. Mukhopadhyay) [email protected] (P. K. Ghosh).

Lian et al. have demonstrated nicely how the morphology of NaCl can be changed by varying the relative growth rate of {100} and {111} faces.33 NaCl crystals of dodecahedron or octahedron shape can be obtained by retarding the relative growth rate of the {110} or {111} crystallographic faces, respectively. The electrostatically polar {111} surface of NaCl, where the bulk structure consists of alternating sheets of anions and cations along the 〈111〉 direction, has been computed to be unstable theoretically.34 Although there are a number of reports on stabilization of the {111} faces of NaCl in the solid-vacuum system,35 it is only recently that Radenovic et al. have investigated the occurrence of NaCl {111} faces during growth from aqueous solution.23 As part of our efforts on morphology control,32 it has been shown recently that the shape of NaCl can be changed to rhombic dodecahedron in a practical manner.28,29 However, the problem of large contact area between crystals was only partially mitigated. Moreover, the crystals that were obtained were large. Spherical morphology of salt would exhibit the lowest contact area between crystals; such a salt is therefore expected to exhibit the best flow properties while, at the same time, mitigating the problem of caking. Spherical salt is occasionally encountered at the edges of the crystallizer during solar salt production. However, the particle size is too large, several millimeters for any practical application. Davey et al. have published the optical micrograph of granular NaCl having ca. 1 mm size.36 In another approach cubic salt crystals having dimension of 500-800 µm were taken together with a binding agent in a centrifuge and fine particles of salt were sprayed, resulting in spherical salt, albeit of fairly large size.37 Nearly spherical NaCl and KCl in the size range of 10-100 µm have been prepared by bringing a suspension of salt in contact with the flame of a burner at 1000-1300 °C.38 The high temperature led to partial melting of the salt particle followed by cooling to yield glassy spherical particles. Moschini and Poggi have reported a method of producing spherical salt in the size range of 100-300 µm through atomization of a supersaturated solution of salt.39 Additives like saccharides at 5% (w/v) concentration have been used to produce spherical morphology in evaporative crystal-

10.1021/ie1016317  2010 American Chemical Society Published on Web 10/26/2010

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lization.40 Spherical sodium chloride of extremely small dimension (6-50 nm) has also been prepared for environmental studies.41 Hasegawa and Masaoka have discussed the effect of mother liquor composition and crystal growth rate on the quality of crystals. The authors have further indicated a correlation between crystal size and crystal shape under agitated conditions, wherein salt crystals having size less than 300 µm are cubic, those having size more than 500 µm are spherical while in the range of 300-500 µm “condensation and wear” is observed.42,43 It appeared to us from the survey of the literature that there is no practical method for the preparation of spherical salt in the size range of 200-500 µm which is important for many practical applications. In particular, such spherical salt can be a substitute for the currently available vacuum-evaporated salts which come in similar size range but are of cubic shape. We report here the simple preparation of nearly spherical NaCl particles from a variety of brines.44 Fifty percent of the total yield was in the size range of 300-500 µm. The process, which requires no additive and instead relies on controlled agitation, was readily scalable to 10 kg scale. The superior flow of the prepared salt was demonstrated. Further, the average particle size could be reduced through use of ethanol as an antisolvent. Experimental Section Crystallization Technique. Laboratory Preparation of Spherical Common Salt. Synthetic brine of 1.208 g · cm-3 density was prepared from AR-grade NaCl. Three hundred milliliters of the filtered brine was taken in a 1 L glass beaker. For experiments with sea and subsoil brines, 300 mL of clear brine of 25° Beaume (F ) 1.208 g · cm-3) was taken instead. NaCl crystals were obtained by evaporation of the brine at 55 ( 5 °C and relative humidity of 50 ( 10% under continuous stirring at 250 rpm with a specially designed impeller (Figure 1b). The brine was evaporated to a final volume of ca. 50 mL and the product crystals were collected by centrifuging the slurry followed by oven drying at 35 ( 10 °C. Laboratory Preparation of Cubic Common Salt. When the above salt crystallization experiments were repeated using instead a Teflon-coated magnetic stirrer operating at low rpm, the salts obtained were cubic in shape. Bench-Scale Preparation of Spherical Common Salt. Crystallization was carried out at 58 ( 2 °C in a custom-made cylindrical SS 316 crystallizer of 100 L capacity. Seventy liters of synthetic or natural brine (having a density of 1.208 g · cm-3) was fed to the crystallizer. During crystallization continuous stirring at 210 rpm was undertaken using a mechanical impeller of similar type to the one used at laboratory scale. When the volume of brine in the crystallizer reduced to 20 L, the slurry containing common salt crystals was transferred to a continuous centrifuge. A part of the centrifuged salt was surface-treated by immersing in saturated brine containing 5 ppm (w/v) potassium ferrocyanide for 1 h and centrifuged once again. The wet product was either sun-dried over a day or dried in an oven at 38 °C for 4 h (R.H. 60%). This manner of drying helped avoid lump formation. A part of the salt was sieved successively using 30, 35, and 45 mesh size sieves to obtain spherical salt crystals having different size distributions. Experimental Procedures. The structural characterizations of the salt samples were performed by optical microscopy, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR) techniques. The free flow ability was measured in two different ways: In the first approach the angle of repose, which is defined

as the angle between the base and slope of a heap of salt formed upon dropping of a certain amount of salt from a given elevation, was measured.45-47 100 g of salt crystals of 350-500 µm size range was dropped from a glass funnel with 7 mm tube diameter (i.d.). The tip of the tube was placed 10 cm from the base. The angle of repose of the salt heaps was measured for six consecutive measurements and the average computed. Another measurement was the time taken for all the salt to fall. Results and Discussion Figure 2a-c shows the optical microscopic images of laboratory-prepared NaCl particles obtained by evaporation of different brines (synthetic, sea, and subsoil) at elevated temperature (55 °C), employing an ordinary Teflon-coated magnetic stirrer operating at low rpm (ca. 50 rpm). The brines were evaporated to ca. 20% of initial volume. It can be seen that the salt crystals were cubic for all the three brine samples investigated. The experiment with synthetic brine was repeated with different types of overhead stirrers. The pitch blade-type impeller was employed at different rpm (50-250) and gave cubic NaCl crystals in all the experiments [Figure 1a(i)]. An impeller with flat propeller-type blade was studied next and at 250 rpm it led to partial shaping of the corners and edges of the cubic crystals [Figure 1a(ii)]. When a butterfly wing-shaped blade was employed [Figure 1a(iii)] at 250 rpm under continuous stirring (the direction of rotation of the impeller was such that it led to a downward thrust), the morphology of the particles was transformed to nearly spherical shape (Figure 2d). The clearance between the impeller blade and the side wall and between the impeller tip and the crystallizer bottom were observed to be important. Figure 1b shows the schematic drawing of the experimental setup employed for laboratory preparation of spherical salt along with relevant details. However, even with the above setup when the stirring rate was reduced to 200 rpm or less, only corner-shaped cubic morphology was obtained (Figure S1), pointing to the importance of the impeller speed besides crystallizer geometry. Under the optimum conditions of crystallization, the size distribution of the spherical crystals was in the range of 300-1000 µm. As can be seen from Figure 2e,f, this unit and impeller design gave spherical NaCl with sea and subsoil brines also. The spherical morphology of salt was realized even when evaporation of brine was carried out under ambient conditions employing the special stirring action. However, the time required was 3-4 times longer and the average particle size increased to >1 mm. The effect of “rounding” is presumed to be entirely physical in origin. During the crystallization of salt from brine in the laboratory as described above, the brine initially became increasingly cloudy as the evaporation proceeded but at a certain stage it became relatively more translucent with concomitant formation of NaCl crystals, which remained suspended by the impeller action. These initial crystals were examined by SEM, and as can be seen from Figure 3a, the crystals were found to be basically cubic in shape, albeit with shaping of the corners and edges. The stacking of the “shaped” crystals into a partially formed polycrystalline sphere is evident from the structure indicated with an arrow. Figure 3b shows the SEM of the sieved fraction of the salt having 350-500 µm size range, comprising over 50% of the total yield (to obtain undamaged sieved salt, it is essential to minimize lump formation of the crystals at the time of drying of the centrifuged wet mass). Fusion of partially formed spheres in some cases results in the formation of ellipsoidal/capsule-type morphology. The magnified image of a single granule of spherical salt in Figure 3c shows details of

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Figure 1. (a) Schematic drawings of (a) different types of impeller blades studied in the present work [(i) pitch blade, (ii) flat propeller blade, and (iii) butterfly wing-shaped blade] and (b) details of laboratory-scale experimental setup and dimensions of the butterfly wing-shaped blade identified for spherical salt preparation. The length scale is in mm and the angular scale in degrees.

curvature. At still higher magnification, the surface was found to comprise a self-assembly of tiny cubic crystals of 3-5 µm size; i.e., the spherical particles were polycrystalline in nature. Not surprisingly, the powder XRD pattern of the spherical salt was nearly the same as that of cubic salt as shown in Figure 4. Scale-up of the process was attempted next in a 100 L crystallizer. Besides optimization of the vessel geometry, impeller design, and tip speed, the initial density of brine and evaporating temperature were also optimized to yield the product shown in Figure 5. The bench-scale product was superior in terms of surface smoothness and overall particle shape. As shown in previous work,48,49 ethanol can be used as an antisolvent in solution system; we thereafter attempted further size reduction of spherical salt crystals through instantaneous rise of the number density of nuclei. For this purpose, ethanol was used as antisolvent. Addition of the antisolvent at saturation point in an aqueous medium raises the supersaturation level

suddenly forcing instantaneous nucleation with enhanced number density. This in turn lowers the average size.14 It can be seen from Figure 6 that the largest spherical salt particles were no more than 300 µm in size in our scale-up trials.50 However, there were accompanying cubic crystals of relatively small size. The crude salt could be readily sieved to obtain fully spherical salt with an average size range of ca. 250 µm. What causes the polycrystalline NaCl agglomerates to transform into spherical shape? The process of crystallization is, first and foremost, dependent on ion-solvent interactions, on the one hand, and the tendency toward desolvation of the solvated ions with evaporation.51 However, such molecular level interactions are unlikely to be affected in any significant manner by the crystallizer geometry and impeller action reported herein that lead to the transition from cubic to spherical morphology. As mentioned above, parameters such as (1) type and shape of impeller, (2) vessel bottom to impeller distance, (3) ratio of

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Figure 2. Optical microscopic images of cubic salt crystals obtained through crystallization of (a) synthetic, (b) sea, and (c) subsoil brines in an open beaker under gentle magnetic stirring at 55 °C. When the experiments were repeated using a special impeller operating at 250 rpmswhile maintaining all other conditions identicalsspherical salt particles shown in (d), (e), and (f), respectively, were obtained.

Figure 3. SEM images obtained during the salt crystallization process of Figure 1e above: (a) crystals collected from the bottom of the beaker at an intermediate time, revealing corner shaping of cubic crystals and their aggregation; (b) 350-500 µm spherical salt particles obtained by sieving of the product of Figure 1e; (c) magnified image of a single spherical salt particle showing details of curvature; and (d) still higher magnification revealing self-assembly of tiny cubic crystals leading to the formation of spherical morphology.

impeller diameter to vessel diameter, (4) direction of rotation, (5) speed of impeller, and (6) rate of evaporation of the saturated brine together play a crucial role in the process of obtaining rounded NaCl particles of desired size. Secondary effects such

as interparticle distance, size of particle, primary and secondary flow generated, and so forth are believed to be the dominant parameters in the shaping process. The rate of evaporation controls the primary nucleation and growth processes, which

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Figure 4. Powder XRD patterns of (a) cubic salt and (b) spherical salt generated from synthetic brine.

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Figure 6. Optical microscopic image of salt particles obtained upon addition of ethanol into saturated synthetic brine, otherwise following the methodology adopted in Figure 1d.

Figure 7. Optical microscopic images of a lump of caked up untreated spherical salt stored in a bottle for 3 weeks.

Figure 5. Optical microscopic image of spherical salt obtained at bench scale (14 kg per batch) from saturated sea brine at 60 °C and under continuous stirring at 200-210 rpm in a crystallizer equipped with specially designed impeller.

in turn influence the size distribution of the particle. The evaporation temperature selected in the present study leads to the desired size range. With the increase of evaporation of the brine, the number and average mass of the crystals will increase, leading to a decrease in the interparticle distance.52 Figure 6 suggests that spherical shape is promoted beyond a critical dimension of the salt particles. Thus, the mass of the particles and the resultant momentum may help in the smoothening of surfaces as a result of three body abrasion.52 Abrasion against the wall and impeller may also be contributing factors. Moreover, the increase in the size of the particles may raise the resistance to flow in the brine and eddies formed near the particles would inhibit the formation of crystals with sharp edges. This would no doubt be influenced by the viscosity of the solvent also. Also due to rotation of the impeller, primary and secondary flow patterns are generated in the slurry.53 Hence, besides possessing a linear velocity the particles exhibit selfrotary action as well, i.e., rotation about their own axes, which

would also favor roundness. It is hypothesized that the differences in the primary and secondary flow characteristics induced by different impellerssand at different speeds of operationshave a profound bearing on the process of rounding of the salt crystals. Dissolution and association of salt crystals may facilitate the process of shaping under the above conditions. We next studied the free flow nature of sieved spherical salt of 350-500 µm size. The freshly dried salt exhibited an angle of repose of 19° and the time of fall through the funnel (vide supra) was 5.8 s. Unfortunately, the salt tended to cake up over a period of 3 weeks of storage in a bottle and the free flow was retarded. A lump of such salt was imaged under undisturbed condition and, as can be seen from the optical micrograph of Figure 7, there was clear evidence of interparticle bridge formation. When freshly prepared spherical salt was surfacetreated by immersing it for 1 h in a 5 ppm (w/v) solution of K4Fe(CN)6 habit modifier in saturated brine,12,54 and thereafter centrifuged, dried, and sieved, the salt was seen to retain its spherical morphology (Figure S2). Furthermore, it exhibited an angle of repose of only 16° (this value is close to the value of 15° measured for silica spheres45 (Figure S3) and the time of flow was 4.9 s. The measurements were repeated after 3 weeks and the flow remained satisfactory with little evidence of interparticle bridge formation (Figure 8a). Indeed, even after 3 months of storage, the values of angle of repose and time of fall remained virtually unchanged. Figure 8b shows the optical

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Figure 8. Optical microscopic image of (a) potassium ferrocyaninde-treated free-flowing spherical salt and (b) commercial free-flowing cubic salt of similar size subjected to flow measurements for which the data are provided in Table 1. Table 1. Comparison of Free Flow Nature of Cubic and Spherical NaCl Particles of Comparable Size sample

angle of repose (deg)

time of free fall (s)

angle of repose after 3 months of storage (deg)

time of free fall after 3 months of storage (s)

spherical NaCl without surface treatment spherical NaCl with ferrocyanide treatment commercial cubic NaCl with free flow additives

19.5 16 22

5.7 4.9 6.1

21 16 22

6.8 4.9 6.2

micrograph of a commercial vacuum-evaporated edible salt containing free flow additives. As can be seen from the comparison of (a) and (b) in Figure 8, both salts were of similar sizes but the commercial salt crystals were cubic. The higher values of the angle of repose (22°) (Figure S3) and the time of fall (6.1 s) (Table 1) of the cubic common salt vis-a`-vis the spherical counterpart confirm the superior flow characteristics of the latter. Conclusions We have presented a simple technique for the crystallization of nearly spherical polycrystalline NaCl particles of submillimeter size range relevant to many applications. No habit modifiers were found necessary and the process relied instead on selection of appropriate crystallizer geometry, impeller design, stirring speed, and evaporation temperature, leading to desired particle size as also conducive flow patterns and interparticle interactions promoting the “rounding” process. The process was found to be robust in as much as both synthetic and natural brines yielded similar results. It was also amenable to scale-up from 100 g to 10 kg level. Application of ethanol as an antisolvent in the process was also demonstrated as a means of lowering the size distribution of the spherical salt particles. Free-flowing spherical salt of 350-500 µm diameter was markedly superior to commercial free-flowing vacuumevaporated cubic common salt in terms of (1) the greater mass flow rate through a funnel and (2) the low angle of repose approaching the limiting value reported for solid granular spheres. Besides its utility in conventional edible and industrial applications, the spherical common salt may be of interest as water-soluble filler material. Future studies will address in greater detail the mechanistic aspects of the process and feasibility of further scale-up. Acknowledgment The authors thank Mr. C. K. Chandrakanth and Dr. P. Paul for providing the SEM data, Mr. M. R. Gandhi and Mr. J. R.

Chunawala for assisting with the angle of repose measurements, Dr. A. Kumar and Dr. B. Ganguly for helpful discussions on ion-solvent and ion-ion interactions, Mr. D. Bhattacharjya and Mr. D. J. Patel for experimental assistance, Mr. P. R. Jadav for assistance in the scale-up experiment, and Professor J. B. Joshi for his keen interest in the work. We are also grateful to the reviewers for several helpful suggestions. The authors also thank M/s Allergan Limited, USA, for providing the impetus for scaleup and for evaluating the product. CSIR, New Delhi, and the Research Council are acknowledged for supporting the research as part of an in-house laboratory project. Supporting Information Available: Optical microscopic images of the salt particles obtained at low impeller speed; SEM image of spherical salt particles after ferrocyanide treatment; pictures of the salt heaps formed during free flow measurements. This information is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Zhao, X.; Bao, Z.; Sun, C.; Xue, D. Polymorphology formation of Cu2O: A microscopic understanding of single crystal growth from both thermodynamic and kinetic models. J. Cryst. Growth 2009, 311, 711. (2) Zhao, X.; Sun, C.; Si, Y.; Liu, M.; Xue, D. Crystallization behaviors of ferroelectric and piezoelectric materials. Mod. Phys. Lett. B 2009, 23, 3809. (3) Xu, D.; Xue, D. Chemical bond simulation of KADP single-crystal growth. J. Cryst. Growth 2008, 310, 1385. (4) Van Hook, A. Crystallization - Theory and Practice; ACS Monograph; Rheinhold Publishing Corp.: New York, 1961. (5) Wu, J.; Xue, D. Morphology-Tuned Growth of ZnO Microstructures. Mater. Res. Bull. 2010, 45, 300. (6) Buckley, H. E. Crystal Growth; John Wiley: New York, 1951. (7) de Rome de l’ Isle, J. B. L. Crystallographie; L’Impremerie de Monsieur: Paris, 1783; 379. (8) Xue, D.; Yan, X.; Lei, D. Production of specific Mg(OH)2 granules by modifying crystallization conditions. Powder Technol. 2009, 191, 98. (9) Julg, A.; Deprick, B. J. The influence of glycine, according to pH, upon the growth habit of NaCl and KI. Cryst. Growth 1983, 62, 587. (10) Jacquemain, D.; Wolf, S. G.; Leveiller, F.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. Dynamics of two-dimensional

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ReceiVed for reView July 31, 2010 ReVised manuscript receiVed September 20, 2010 Accepted September 29, 2010 IE1016317