CRYSTAL GROWTH & DESIGN
A Practical Approach To Produce Near-Spherical Common Salt Crystals with Better Flow Characteristics Ballabh,†
Trivedi,†
Amar Darshak R. Parthasarathi Amitava Pramanik,‡ and V. G. Kumar‡
Dastidar,*,†
Pushpito K.
2006 VOL. 6, NO. 7 1591-1594
Ghosh,*,†
Analytical Science Discipline, Central Salt & Marine Chemicals Research Institute, G. B. Marg, BhaVnagar 364 002, Gujarat, India, and Hindustan LeVer Research Centre & UnileVer Research India, 64 Main Road, Whitefield, Bangalore 560 066, India ReceiVed NoVember 30, 2005; ReVised Manuscript ReceiVed April 13, 2006
ABSTRACT: A practical approach to recycle glycine as habit modifier to produce rhombic dodecahedron shaped NaCl crystals from saturated NaCl solution is reported. It is also shown that the method is applicable to solar salt production from natural brines. The modified salt crystals have better flow characteristics than those of their cubic counterparts. Introduction Interest in crystallization and in various ways of altering the shapes and structures of crystals has a long history because an extraordinary range of physical and chemical properties of crystalline solid-state materials is dictated by their crystal form and size.1 Efforts to modify crystallization processes so as to generate new crystalline forms of substances continue to be of considerable importance for various reasons, for example, improvement of mass-handling characteristics of particulate materials, production of materials that are stronger or more durable than existing materials, and improvement in the physical characteristics such as optical clarity, superior flow, and enhanced shelf life. Conventional ways of altering the shape (i.e., the “habit” or the “morphology”) of a crystalline material include (1) using additives,2 (2) changing the crystallization solvent, (3) inducing nonequilibrium behavior such as supersaturation of the crystallizing solution, and (4) altering the rate of evaporation. Common salt, apart from being an essential dietary component, is a basic raw material for the manufacture of a wide variety of industrial chemicals, including bulk chemicals such as sodium carbonate (soda ash), sodium hydroxide (caustic soda), and chlorine. Besides, salt is used in textile, dairy, dyeing, food, fertilizer, paper, and pharmaceutical industries. Caking of water-soluble inorganic salt such as common salt is a common storage problem. Caking is believed to occur because of the formation of solid intercrystalline bridges that cement crystals together. Evaporation of minute amounts of water on the surface of the crystals causes the formation of intercrystalline bridges and consequently caking over the period of storage time. Understandably, caking reduces free-flow properties of common salt and increases storage problems. Besides salt bridge formation, the shape of the crystalline particles has a significant role to play in the free-flow properties of the substance. Larger intercrystalline surface area of contacts, as it is in the native cubic form of NaCl, has a negative influence on the free-flow properties. Obviously, the intercrystalline surface area of contact is greatly reduced in the case of spherical or near-spherical crystallites thereby increasing their free-flow characteristics. Habit modification of NaCl is well-known, and the first discovery of making octahedral crystals of NaCl when grown * To whom correspondence should be addressed. E-mail addresses:
[email protected] (P.D.);
[email protected] (P.K.G.). † Central Salt & Marine Chemicals Research Institute. ‡ Hindustan Lever Research Centre & Unilever Research India.
from urine was reported as early as 1783.3 Since then many efforts have been made to modify the crystal habit of NaCl using various additives.4 For examples, a list of organic and inorganic additives tested on NaCl crystal growth have been reported by L. Phoenix as early as 1966;5 effect of polysaccharides on NaCl crystallization has been reported by Birchall and Davey.6 Three different crystal faces, namely, [100], [111], and [110], are known to be important in NaCl crystallization. While the [100] face is found in its native cubic form, [111] and [110] faces are observed in its octahedron and rhombic dodecahedron forms. Since alternating cationic and anionic sheets are stacked along the [111] direction, the [111] surfaces must have a very high divergent electrostatic energy that makes them theoretically unstable,7 and many interesting properties8 can be realized if a well-ordered [111] face can be grown. Therefore, efforts have been made to grow octahedral NaCl crystals using various additives.9 Although the rhombic dodecahedron form of NaCl obtained from aqueous solution containing glycine as an additive has been reported10 as early as 1949 and theoretical considerations suggested the plausible mechanism of the habit transformation from [100] to [110],11 no reports pertaining to a practical method for the production of the glycine-modified rhombic dodecahedron form of NaCl have been found. In this paper, we report a practical method of recycling glycine to produce near-spherical rhombic dodecahedron shaped NaCl crystals, which indeed show better flow characteristics compared to their native cubic counterpart crystallized under identical conditions. We also show that the method can be extended to natural brine systems.12 Results and Discussion In the presence of glycine, the [100] faces of the cubic form of NaCl have been reported to grow faster than the [110] faces, which eventually leads to the formation of a rhombic dodecahedron habit consisting of only [110] faces via an intermediate habit of octadecahedron in which both [100] and [110] faces are present.11 It is clear that the intercrystal contact area in the cubic form is greatly reduced in the regular rhombic dodecahedron form, which resembles a sphere (Scheme 1). The main problem of producing the [110] form of NaCl using glycine as habit modifier is the requirement of a high concentration of the additive to effect the habit modification.10 Since glycine is highly soluble in water and hardly soluble in organic solvents, it is not possible to use any solvent extraction method
10.1021/cg050633v CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
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Scheme 1. Transformation of NaCl Crystals from Cube to Rhombic Dodecahedron via Octadecahedron in the Presence of Glycine as Crystal Habit Modifier
to effect its removal from the crystallization mixture for reuse. Therefore, it was necessary to invent a method that would allow us to recycle glycine so that a practical method of producing the [110] form of NaCl can be realized. Before making such attempts, we sought to address the following questions: (1) What is the minimum initial concentration of glycine required in saturated brine to effect the modification to rhombic dodecahedron habit to the maximum extent? (2) What is the effect of temperature on the process? (This information is necessary since crystallization temperatures are very different for solar salt production and forced evaporative crystallization.) Since the reported experiments on glycine as additive had dealt with crystallization on a microscopic slide,10 it was important to investigate crystallization effects in bulk systems, such as in a beaker, to simulate practical salt production. In our initial work, we undertook studies under ambient conditions to ascertain the required glycine concentration for habit modification. To a solution containing a given amount of glycine, an excess amount of NaCl was added, and the solution was stirred at room temperature for about an hour to obtain saturated brine. The filtrate was then kept for crystallization under ambient condition. In about 2 weeks time, crystals were grown. It was observed that when the starting concentration of glycine in the saturated NaCl was in the range of 15-19% (w/v), mainly octadecahedron habit with prominent [110] faces, along with some cubes, are obtained. Figure 1 depicts an isolated octadecahedron NaCl crystal displaying well developed [100] and [110] face, grown in the presence of 17% initial concentration of glycine in the saturated brine. When the initial glycine concentration was raised to 20-23% (w/v) in the saturated brine, mainly rhombic dodecahedron crystals were obtained, along with some octadecahedron crystals. Only when the initial glycine concentration was raised to 24-25% did all the salt crystals appear to be of rhombic dodecahedron shape (Figure 2a). The scanning electron micrograph (Figure 2b) of one such isolated [110] crystal of NaCl reveals more clearly the presence of regular rhombic [110] faces.
Figure 1. Eighteen-faced octadecahedron crystal of NaCl crystal viewed from the top. The crystal was grown from a saturated NaCl solution containing 17% (w/v) glycine as starting concentration.
Figure 2. (a) Rhombic dodecahedron crystals of NaCl grown using glycine (25% w/v), (b) SEM of an isolated rhombic dodecahedron NaCl crystal depicting regular rhombic [110] faces (six faces can be seen from this view), and (c) rhombic dodecahedron crystals of NaCl grown from subsoil brine using glycine (25%).
Whereas all previous studies of glycine-induced NaCl habit modification have focused on pure NaCl solution, natural brine systems contain other ions such as Mg2+, Ca2+, K+, SO42-, and Br- to different extents. Experiments were therefore performed with natural subsoil brine in the presence of 25% glycine to study the effect of such impurity ions. Remarkably, no appreciable effect was seen, and the crystals of NaCl obtained were found to be rhombic dodecahedron, that is, these impurity ions do not interfere with the habit modification process (Figure 2c). Although, the crystallization method to produce habitmodified NaCl crystals as described above is highly appropriate for solar salt production, which is commonly practiced, it was considered worthwhile to carry out the crystallization at higher temperature relevant to salt production under forced evaporation conditions. However, similar crystallization experiments with initial glycine concentration of 25% in saturated brine show that the additive has no effect when evaporative crystallization is carried out at 50 °C. Mainly cubes with small amounts of edge-affected NaCl (octadecahedron) crystals were noticed in such experiments. At the still higher temperature of 80 °C, only cubic crystals of NaCl could be obtained. Therefore, glycine may not be a useful additive for preparation of habit-modified NaCl crystals through forced evaporation at elevated temperature. The altered morphology in the presence of glycine is on
Approach To Produce Near-Spherical Common Salt Crystals
account of selective adsorption on the (110) plane of NaCl.11 However, the adsorption is almost certainly very weak since high concentrations of glycine are required. As a result, the adsorption process can be adversely affected by disturbances such as convection. We believe this is the reason elevated temperatures do not yield encouraging results. Since the initial concentration of glycine required to achieve the desired habit modification is high, a sizable amount of the glycine crystallizes out along with the salt during the crystallization process. Such glycine crystals could be seen under the microscope. It occurred to us that if the crude habit-modified salt is washed with a fresh lot of saturated brine, the solid glycine would dissolve in the brine leaving behind NaCl that is comparatively purer. Moreover, the fresh saturated brine would also yield habit-modified salt because it contains the glycine. It remained to be established, however, whether the original habit-modified salt retains its habit after washing or whether crystallized salt dissolves and recrystallizes in a dynamic equilibrium process. We observed, as expected, that most of the crystallized glycine could indeed be dissolved away in the fresh lot of saturated brine without salting out NaCl from the solution. To our delight, we also found that this process did not affect the crystallized salt, which retained its rhombic dodecahedron shape. The steps of this process are described in detail below for pure NaCl solution and also subsoil brine.12 Crystallization from NaCl Solution. Step 1. An excess amount of commercially available NaCl was added to 150 mL of distilled water, and the mixture was stirred at room temperature for 0.5 h. The solid/liquid mixture was then filtered, and 25 g of glycine was added into 100 mL of such filtered saturated brine. The solution was then kept for crystallization in an open beaker under ambient conditions in the laboratory. After 90% evaporation, the resulting crystals were harvested by filtration, dried in a fluidized bed-type drier and subjected to microscopic observation. The crystals of NaCl thus obtained were mainly of rhombic dodecahedron shape. The additive glycine was also crystallized along with NaCl as examined by an optical microscope equipped with a cross-polarizer; NaCl, being a cubic crystal system, does not show any extinction effect, whereas glycine crystals can easily be identified due to their extinction effect. Step 2. The crystals obtained in step 1 were washed with 90 mL of saturated brine prepared as in step 1. After washing, the crystals were isolated and dried as described in step 1. Observation of such crystals revealed that the crystals of salt retained the rhombic dodecahedron shape, whereas most of the glycine crystals had disappeared (microscopic observation). Step 3. The mother liquor obtained in step 1 was combined with the washings of step 2 and left for evaporation under ambient conditions. Crystals were then harvested and dried. The salt crystals were once again found to be rhombic dodecahedron in shape and contained significant quantities of glycine crystals in a manner identical to that of the salt described in step 1. This process of recycling the mother liquor and washings was repeated several times, and each time, the salt crystals were found to be rhombic dodecahedron in shape containing ∼0.83% glycine as determined from an IR calibration plot (see Supporting Information). Approximate yield of modified NaCl in each cycle was 24.0 g. This amounts to about 0.19 g of glycine that adhered to the modified NaCl crystals. In other words, the wash liquor after step 1 contains 24.8 g of glycine. Thus, loss of glycine in each cycle is negligible, and therefore, at least seven cycles can be performed to obtain rhombic dodecahedron NaCl crystals without topping up with additional glycine.
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Crystallization from Subsoil Brine. The experiments described above were repeated with 500 mL of subsoil brine instead of pure brine. The density of the brine was 1.208 kg/L. The brine was evaporated up to a density of 1.239 kg/L. The resulting crystals of salt were of rhombic dodecahedron shape with significant quantities of glycine crystals. The salt was washed with 500 mL of fresh 1.208 kg/L subsoil brine, and the crystal morphology was found to be retained, while the glycine crystals were found to have largely disappeared. When compared with the native cubic form crystallized under the same conditions, rhombic dodecahedron NaCl as obtained above displays better flow characteristics. We have performed an experiment to assess semiquantitatively the relative free-flow characteristics of modified and cubic NaCl crystals. For this purpose, we have followed a technique used in industry known as “angle of repose” (http://www.answers.com/topic/angle-ofrepose). The smaller the value of angle of repose, the higher is the free-flow characteristic. While rhombic dodecahedral NaCl yields an angle of repose of 22.6°, the corresponding value for the native cubic form is 28.8° indicating superior flow characteristics of the glycine-modified NaCl crystals (Supporting Information). The free-flowing nature remained unaffected when stored in a plastic container for over a year. The cubic shaped salt crystals obtained in control experiments showed some degree of caking under identical conditions. Presumably, apart from habit modification, glycine also imparts some degree of hydrophobicity to the salt crystals. The small quantities of residual glycine in habit-modified salt would presumably have no deleterious effect in dietary applications. Glycine is known to have a refreshing, sweetish flavor and occurs abundantly in mussels and prawns. It is considered to be an important flavor component of these products. When used as an additive for vinegar, pickles, and mayonnaise, it attenuates the sour taste and lends a note of sweetness to their aroma. Glycine is reported to be used in masking the aftertaste of the sweetener saccharin.13 Glycine is also reported to exhibit a special preservative effect.14 The above information suggests that glycine is not harmful and may, in fact, impart a beneficial effect to certain foods. Conclusions We have shown that glycine is an effective habit modifier of common salt when crystallization of such salt is carried out under ambient conditions such as undertaken during solar salt production. Moreover, the modification is equally effective with pure NaCl solution and natural brines such as subsoil and sea brines. A process has been devised to wash away glycine that crystallizes along with salt while enabling the salt itself to retain its rhombic dodecahedron habit. Such washing is accomplished with a fresh lot of saturated brine, which then can be made to yield a fresh crop of modified salt crystals. In this manner, the glycine can be recycled. The salt crystals were confirmed to have superior free-flow characteristics compared to the normal cubic salt crystals. Our studies as described above, and particularly the applicability to natural brines, indicate that the process of habit modification as developed may be useful in production of free-flowing NaCl crystals. Experimental Procedures All the chemicals were commercially available (Aldrich). Details of crystallization experiments are given case by case in the Results and Discussion section. Grown crystals were visually observed and photographed under an optical microscope (Leica, WILD M3Z). Scanning electron microscopy (Leo) was performed using paper-tissue-
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dried crystals selected under an optical microscope and subsequently mounted on the SEM sample holder.
Acknowledgment. A.B. and D.R.T. thank Hindustan Lever Research Centre for financial support under the collaborative project. Continuing support of CSIR for research on salt is also gratefully acknowledged. Supporting Information Available: IR calibration plot for determining glycine concentration in the modified NaCl crystals and experimental methods and data for flow characteristics measurements. This material is available free of charge via the Internet at http:// pubs.acs.org.
(5) (6) (7) (8) (9) (10)
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