Simultaneous Crystallization of NaCl and KCl from Aqueous Solution

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Simultaneous Crystallization of NaCl and KCl from Aqueous Solution: Elementary Phenomena and Product Characterization Frederico M. Penha,† Gustavo P. Zago,† Yuri N. Nariyoshi,‡ André Bernardo,§ and Marcelo M. Seckler*,† †

Polytechnic School, Department of Chemical Engineering, University of São Paulo, Av. Prof. Luciano Gualberto, trav.3, n.380, 05508-010, São Paulo, São Paulo, Brazil ‡ Department of Chemical Engineering, Federal University of Espírito Santo, Rodovia BR 101 Norte, Km 60, 29932-900, São Mateus, Espirito Santo, Brazil § Department of Chemical Engineering, Federal University of São Carlos, Rod. Washington Luiz, km 235 - SP 310, s/n, 13565-905, São Carlos, São Paulo, Brazil ABSTRACT: The main elementary processes were identified during simultaneous crystallization of NaCl and KCl from aqueous solutions by the batchwise evaporative method using NaCl seeds. In the early stages of the batch operation, when only NaCl crystallizes, the main elementary processes are molecular crystal growth, agglomeration, and secondary nucleation. When the eutonic condition is exceeded, a KCl primary nucleation event takes place in solution. Subsequently, part of the resulting KCl particles agglomerate with the NaCl particles. Besides, epitaxial growth of KCl upon NaCl crystals takes place. Consequently, the product is comprised of mixed composition polycrystalline particles and single crystals of each salt. The elementary phenomena just described were sensitive to crystallization conditions such as evaporation rate, seed size, and seed content, suggesting the possibility of controlling the morphological features of the particulate product as well as the chemical composition of its size fractions. Such knowledge is important in the context of wastewater treatment, where the particulate product should be suitable either for disposal as a residue or for further processing in easily separable size fractions for exploitation of valuable components.

1. INTRODUCTION In order to reduce the water consumption and the discharge of liquid effluents into the environment, the processing industry will have to develop ever better solutions for water reuse in the coming years. The increase of legal instruments for restricting and charging the use of water resources will make water reuse more economically attractive,1 further driving efforts for its development. In addition, some components in those effluents can also be considered as raw material resources, opening opportunities to develop methods for their recovery along with the water.2,3 Treating industrial saline effluents is important because they are produced in large amounts by several industrial segments. They can be treated by several methods, but most techniques do not allow full water recovery, yielding brines that are harmful for aquatic organisms. In addition, heavy metals, agrochemicals, and other pollutants can have their solubility and other properties changed when mixed with salts, increasing associated environmental problems.4,5 Therefore, interest in zero liquid discharge (ZLD) policies has grown, in which the saline effluent is separated into reuse water and one or more solid streams. To this end, some kind of crystallization process has to be applied.2,6 An economic option is the simultaneous crystallization of all nonvolatile solutes in a single evaporative © XXXX American Chemical Society

crystallization operation to yield a solid residue. Indeed, in recent years such technological solution for ZLD has become commercially available. A potential alternative would be to conduct the crystallization in such a way that valuable components would crystallize as individual particles, so that they might be recovered downstream of the crystallizer in pure form by a physical separation operation exploring size and/or density difference with respect to the other solids. Irrespective of the intended destination for the solids, i.e., either as a residue or as a particulate suitable for downstream recovery of valuable compounds, ZLD crystallizers should be able to cope with simultaneous crystallization. Crystallization from multicomponent systems has been addressed in previous studies since it represents a potential source of impurities incorporation in crystalline products.7 However, the literature about simultaneous crystallization is either scarce or situated in nonchemical engineering contexts. Therefore, current design of ZLD crystallizers is largely empirical. The particulate products of such crystallizers may be comprised of polycrystalline particles of mixed components Received: November 17, 2017 Revised: February 13, 2018 Published: February 14, 2018 A

DOI: 10.1021/acs.cgd.7b01603 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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with unexpected particle sizes, shapes, and compositions.8 The resulting particulate solid is a slurry that is potentially difficult to filter and to dewater. Besides, the possibility of producing particulate solids that are easily separable from each other downstream the crystallizer is not known. As a model system, the simultaneous crystallization of KCl and NaCl from aqueous solution is considered next. Sodium is very common in several effluents, such as those from the food industry,9 tanneries,10 the textile industry,11 and oil refineries.12 Potassium chloride can also be found in several effluents but in lower amounts, compared to sodium chloride. Both compounds find commercial applications if suitably purified. Matsuoka et al.8 have observed the spontaneous nucleation of KCl on the surface of a dissolving NaCl crystal, even in solutions unsaturated with respect to both salts, due to high concentrations in the mother-liquor surrounding the NaCl crystal in dissolution. According to Takiyama et al.,13 the crystal lattice parameters of NaCl and KCl crystals are substantially different, so that a combination of the structure between the two crystalline phases is hardly expected. Accordingly, they have reported the existence of a continuous interfacial region between the two crystals, which exhibits physical properties (mechanical strength and reactivity with humidity from the air) that differ from those of the pure materials. Epitaxial growth can often be induced by heterogeneous nucleation.14 In literature, epitaxial growthand the regularity of the orientations of the crystalsare mainly reported from a crystallographic point of view. However, correlations with growth conditions or process mechanisms are poorly known. Thus, the exact nature of epitaxy remains unclear. At the same time, it is well-known that the joint growth of pairs of epitaxial crystals can result in both oriented and nonoriented excrescences, and the level of mutual orientation between crystals is influenced by temperature, system supersaturation, and imperfections on the substrate faces.14 In addition, the influence of crystallographic factors is not decisive, since the proximity of structural parameters, e.g., crystalline lattice constants, to several pairs of alkali metal and halogen substrates may increase or decrease the degree of epitaxy, depending on the binding strength between the precipitate and the substrate.15 In studies with the NaCl−KCl−H2O system in crystallization by cooling, with seeds of both salts and in eutonic conditions, Glikin14 has observed that the surface texture of the crystals of KCl has its roughness highlighted with increases in the subcooling, whereas the surface texture of NaCl crystals is smoothed. He has also reported epitaxial growth of both KCl on NaCl and of NaCl on KCl. Besides, a growing crystal of one compound can adhere to an existing crystal of another compound with its edges and vertices. Excrescence growths of this compound are generally irregularly distributed at the periphery of the face of these crystals. This distribution, however, becomes more regular, with higher rates of cooling. Morphological peculiarities in the KCl and NaCl particles, such as nucleation and subsequent growth of crystals of different composition directly on the seed surface, indicate a limitation due to diffusion in the growth process.14 This contribution addresses the knowledge about the interactions between two compounds in simultaneous crystallization. Particular attention is paid to the possibility of the development of isolated particles of each compound as this feature is desirable for their potential downstream separation as valuable products. The simultaneous crystallization of KCl and NaCl was selected as a model system. The crystallization

parameters investigated were the evaporation rate, seed content, and particle size in experiments seeded with NaCl. This information is useful to support the development of reliable design criteria for ZLD crystallizers.

2. EXPERIMENTAL SECTION 2.1. Materials. Saline solutions were prepared with analytical grade reagents (NaCl and KCl) and distilled water. Each assay was performed with a new solution. Produced particles were washed with ethanol (Purity >99.5%). 2.2. Experimental Setup. The crystallizer was a vertical cylindrical jacketed glass vessel, with 0.5 L volumetric capacity, provided with baffles and mechanical stirring (IKA - RW20 Digital). Solvent evaporation was imposed by circulation of a heating fluid on the crystallizer jacket. The boiling temperature was monitored by a thermocouple (K-type) in direct contact with the crystallizing solution/suspension. The heating fluid temperature was controlled with a thermostatic bath (Lauda, Eco RE 620). The crystallizer was connected with an external condenser and the latter to a vessel that collected the evaporated solvent during crystallization. The vessel was placed on a semianalytical scale for determination of the evaporation rate. 2.3. Phase Diagram and Operation Route. The phase diagram for the ternary system KCl−NaCl−H2O was derived from the Pitzer’s thermodynamic model as implemented in the free software PHREEQC (PHREEQC for Windows version 2.18.00). The phase diagram for 108 °C−boiling temperature of the mother-liquoris shown in Figure 1. The dashed and full lines represent the solubility of

Figure 1. Solubility curves at 108 °C. The initial (○) and eutonic (△) operation conditions proposed for the simultaneous crystallization process correspond to concentration of KCl and NaCl of 0.338 and 0.279 kg/kg H2O at the start, and of 0.444 and 0.263 kg/kg H2O at the eutonic condition. NaCl and KCl, respectively. Systems whose thermodynamic states lie below these lines are (clear) solutions. If the NaCl concentration is sufficiently high but the KCl concentration is low, i.e., for states above the dashed line and on the left of the full line, solid NaCl coexists with the solution. Similarly, solid KCl in solution occurs for high KCl and low NaCl concentrations, i.e., for concentrations on the right of the full line and below the dashed line. Finally, solid NaCl, solid KCl, and solution coexist for sufficiently high NaCl and KCl concentrations, i.e., above both lines. In the two-phase regions, the liquid phase composition is given by the solubility lines of either NaCl or KCl. In the three phases region, the solution composition is invariant and corresponds to the so-called eutonic composition. Batchwise evaporative crystallization experiments were designed considering the phase diagram of Figure 1. In the beginning of each batch a solution was saturated in NaCl and highly undersaturated in KCl (start; Figure 1) at 108 °C. This strategy allowed the initial crystallization of NaCl alone. As water was removed, the solution state changed along the NaCl solubility (dashed line) with increasing KCl B



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concentration and decreasing NaCl content. After 18% of the water was removed, the solution reached the eutonic condition (eutonic point; Figure 1). Thereafter, evaporation proceeded until 50% of the water was removed. In this period, NaCl and KCl crystallized simultaneously, but the solution composition did not change. Actual solution concentrations during crystallization differed from the equilibrium conditions mentioned because of supersaturation with respect to each salt, but the differences may be neglected for this analysis, since the metastable limits for these salts are small. Experimental conditions are shown in Table 1. Two seed sizes, two seed contents, and two evaporation rates were investigated.

sieving using Tyler series sieves in a magnetic vibrating platform for 50 min. Mass based population densities were determined. Coefficients of variation (CV) were calculated with eq 1, where L16, L50, and L84 are, respectively, the cumulative characteristic sizes of 16, 50, and 84% of the sample in mass.

CV = 100 ×

seed size (μm) seed content (wt %)a evaporation rate (min−1)b 295 196 295 196

5/25 5/25 5/25 5/25

a wt %: kgseed·kginitial_solution−1·100. min−1.

b

0.005 0.005 0.003 0.003

min−1: gevaporated·ginitial_solution−1·

2.4. Seeds. NaCl seeds were obtained by sieving chemically pure salt in the size ranges of interest with Tyler series sieves in a magnetic vibrating platform during 1 h. In order to reconstruct damaged crystal edges and faces, as well as to dissolve fragments (fines) adhered to crystal surfaces, seeds were “healed”. To this end, seeds were added to a solution saturated in NaCl at 104 °C and highly undersaturated in KCl. The seeds suspension was stirred for 1 h at this temperature. After healing, the temperature was raised to the boiling point of 108 °C. Here, a slight dissolution of the seeds removed any remaining fines adhered to the crystal surfaces. Seeds dissolution kept the solution saturated. 2.5. Evaporative Crystallization. After the seeds healing procedure, with the system at the boiling point (∼108 °C). Removal of water from solution created the supersaturation necessary for crystallization. The released vapor was condensed and collected at constant time intervals (5 min) to monitor the evaporation rate and the evaporation extent (proportion or evaporated water with respect to the initial water content, in %). The end of the batch was set for an evaporation extent of 50%. Samples of the crystallizer suspension were taken during the crystallization process by suction in a glass tube at the predetermined evaporation extent values shown in Table 2. The suspension samples were immediately filtered in vacuum, washed with ethanol and dried at 50 °C for 8 h.

a

1

2a

3

4

5

6

evaporation extent (%)

9

18

27

36

45

50

nCl− = mNaCl MNaCl + mKCl MKCl

(2)

msample = mNaCl + mKCl

(3)

where M represents the molar mass and m the mass of each salt. As the left-hand sides of eqs 2 and 3 are known, these equations yield the masses of NaCl and KCl in the sample. The NaCl proportion in each sample x (g NaCl·g sample −1·100) is then calculated according to eq 4: mNaCl x= 100 mNaCl + mKCl (4) This titration method was statistically validated for the whole range of salt compositions of interest, using gravimetrically prepared synthetic mixtures of these salts. Statistical analysis−ANOVA−was performed to each range of proportions, using statistical software Minitab 17. Standard deviations were around 5 wt % or less. In order to quantify the variation of solids composition with particle size, a segregation index was defined according to eq 5, where xi is the NaCl proportion in each size range, xaverage is the average proportion of NaCl in the whole product and P is the mass based population density in kg·kg−1·μm−1.

Table 2. Crystal samples Withdrawn According to Evaporation Extent, Defined As the Proportion of Evaporated Water with Respect to Initial Water Content sample

(1)

Dominant sizes were considered as the weighted arithmetic mean size between the two highest populational densities in the particle size distribution. 2.8. Focused Beam Reflectance Measurement (FBRM). Chord length distributions were experimentally determined with an FBRM instrument. The measurements are based on the detection of reflected laser light upon hitting particles in suspension within the crystallizer. The number of particles hit per second separated by chord-length ranges was determined. Selected experiments were repeated with in situ monitoring with Lasentec FBRM (Mettler Toledo). Measurements were taken every three seconds. 2.9. Particles Chemical Composition. Argentometry was applied to determine the mass proportion of NaCl in dry crystallization products in each size range. Titrations were conducted using Mohr’s Method, as described in detail elsewhere.16 Briefly, a known mass of solid (msample) composed of NaCl and KCl is dissolved in water and titrated in triplicate with a solution of silver nitrate (AgNO3). Titration yields the number of chloride ions in solution (nCl−) from the spent volume of a known AgNO3 solution. The number of chloride ions in solution is the sum of sodium and potassium ions (eq 2). Similarly, the sample mass is the sum of its components NaCl and KCl (eq 3).

Table 1. Experimental Conditions E1/E2 E3/E4 E5/E6 E7/E8

(L84 − L16) 2L50

Sample near the eutonic point.

SI =

Samples 1 and 2 provided information about the period when the only salt crystallizing was NaCl. The system reached the eutonic point shortly after Sample 2; thus KCl was near its saturation point at this moment. The other samples (3−6) were withdrawn after the eutonic point; i.e., the system was saturated with respect to NaCl and KCl. 2.6. Crystal Characterization. Samples of dried crystals were characterized by optical microscopy (Olympus BX60F-3) and by scanning electron microscopy (SEM - JEOL JSM-7401F). SEM was coupled with energy-dispersive X-ray spectroscopy (EDS) for investigation of the composition of polycrystals. 2.7. Particle Size Distribution (PSD). Samples of dried crystals formed near the eutonic point and at the end of operation (final product) were evaluated regarding particle size distribution through

⎡ (xi − xaverage) ⎤ ∑⎢ x × Pi ⎥ ⎣ average ⎦ ∑ Pi

(5)

3. RESULTS AND DISCUSSION 3.1. Supersaturation. Elementary processes related to crystallization were conveniently described considering the supersaturation, since this variable is the driving force for all such processes. The supersaturation is defined as the difference between the solute chemical potential in solution and in the solid phase. Unfortunately, experimental determination of the supersaturation for moderately soluble salts such as NaCl and KCl is difficult because the supersaturation and equilibrium states are very close to each other. Qualitative assessment of C



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Figure 2. Particle size distribution for seeds, samples near the eutonic condition (18% water removal − R18) and final product (50% water removal − R50) for the experiments performed with 5 and 25 wt % content of seeds sized 295 (A−B, respectively) and 196 μm (C−D, respectively), evaporation rate of 0.005 min−1.

supersaturation is nevertheless possible, based on the following reasoning. Since supersaturation is generated by evaporation of the solvent, the supersaturation increases with the evaporation rate. Similarly, supersaturation is reduced by crystal growth which is favored in the presence of a large seeds content or a small seeds size. Using this reasoning it is possible to qualitatively compare the supersaturation in the various experimental conditions covered in this research. The supersaturation is considered to be high in an experiment with a high evaporation rate, in comparison with an experiment with a low evaporation rate. Similarly, a higher supersaturation is associated with a small amount of crystal-seeds, in comparison with a large amount. Finally, large particles (which exhibit a low specific surface area) yield higher supersaturation in comparison with small particles. 3.2. Particle Size Distribution (PSD) and Optical Microscopy. Figure 2 and Figure 4 show the PSDs of the seeds, of the samples near the eutonic point (R18) and of the final products (R50) as well as the CVs for each. Tables 3 and 4 display the corresponding dominant sizes and CVs of the PSD. Experiments with a relatively high evaporation rate of 0.005 min−1 are considered first (Figure 2 and Table 3). Seeds PSD are unimodal and present a narrow size dispersion, as desired. In the initial stage of crystallization, before the eutonic condition is achieved (R18), only NaCl crystallizes. The changes observed with time on the PSDs for 25 wt % seeds indicate that molecular crystal growth occurs, as the dominant size shifts to higher characteristic lengths, and the width of the PSD does not change much. For 5 wt % seeds, the shift in dominant size is too large to be attributed to crystal growth alone, and instead it may be explained by agglomeration, as the attachment of two or more particles often leads to high apparent particle growth rates. Besides, agglomeration is favored at high supersatura-

Table 3. Coefficient of Variation (CV) and Dominant Sizes (LD) near the Eutonic Condition (18% Water Removal − R18) and Final Product (50% Water Removal − R50) for the Experiments Performed with the Indicated Seeds Contents and Sizes, Evaporation Rate of 0.005 min−1 CV (%)

seeds E1/E2 - 295 μm seeds E3/E4 - 196 μm seeds

E1/E2 - 295 μm seeds E3/E4 - 196 μm seeds

12 18

5 wt %

25 wt %

5 wt %

25 wt %

R18

R18

R50

R50

14 22

38 29

21 25 25 wt %

16 17 LD (μm) 5 wt %

25 wt %

5 wt %

seeds

R18

R18

R50

R50

295 196

353 250

315 233

298 280

299 215

tion17,18 such as found in these 5 wt % experiments in comparison with 25 wt % seeds. Indeed, optical microscopy images for particles just before the eutonic is reached confirm extensive agglomeration of the NaCl particles grown at 5 wt % seeds and little agglomeration for 25 wt % seeds (not shown). An increase in the population density of particles smaller than the dominant sizes can be seen in the experiments with 295 μm seeds. This is probably related to the occurrence of secondary nucleation of NaCl due to the high supersaturation, in comparison to experiments with 196 μm seeds. Besides, large particles are known to be more prone to the formation of secondary nuclei than small ones.15 As evaporation proceeds, NaCl crystals grow and the KCl concentration in the mother-liquor increases. After the eutonic condition, both salts are supersaturated so simultaneous D



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line particles. For 5 wt % seeds the particles are formed by a larger number of crystals (primary particles) in comparison with 25 wt %, possibly due to agglomeration, as a low seed content is associated with a low crystal surface area, which keeps the supersaturation relatively high. The polycrystals may also have been formed by epitaxial growth. According to Glikin,14 for eutonic ternary solutions of NaCl and KCl crystallizing by cooling, the topography of both crystals may vary with time. Simultaneously, cubic growing crystals can adhere to a substrate, forming “vertical” textures (perpendicular to the substrate face) which characterize epitaxial growth. According to Croker and Hodnett,20 the existence of another chloride crystal surfacein this case, NaClin the crystallization of KCl reduces the energy necessary for the nucleation and eases the formation of KCl crystals upon NaCl surfaces. Because epitaxial growth starts with heterogeneous nucleation of the new phase upon the parent phase, it is also directly related to supersaturation. Since both epitaxial growth and agglomeration lead to polycrystals and increase with supersaturation, it is sometimes difficult to distinguish these two elementary phenomena. Experiments under a lower evaporation rate of 0.003 min−1 are considered next (Figure 4 and Table 4). In the early stages of the process, when only NaCl crystallizes (R18), experiments under low supersaturation (196 μm) evidence molecular growth, as the dominant size and the CV of the PSD slightly increases (Table 4). However, for high supersaturation (295 μm seeds) particles much larger than the seed size are formed (Table 4), pointing to agglomeration. Furthermore, the proportion of particles with sizes smaller than the dominant size increases, suggesting secondary nucleation of NaCl. All these features of crystallization had also been found for the higher evaporation rate (Figure 2). It is noteworthy, however, that under low evaporation rate much more small crystals survive. Evidence of nucleation may appear contradictory, as the lower evaporation rate promotes lower supersaturation in this set of experiments. However, the crystallization time is longer, which increases the number of collisions undergone by the particles, suggesting that secondary nucleation is the dominant nucleation mechanism. Besides, agglomeration is less important under lower supersaturation. Therefore, it is likely

Table 4. Coefficient of Variation (CV) and Dominant Sizes (LD) near the Eutonic Condition (18% Water Removal − R18) and Final Product (50% Water Removal − R50) for the Experiments Performed with the Indicated Seeds Contents and Sizes, Evaporation Rate of 0.003 min−1 CV (%)

seeds E5/E6 - 295 μm seeds E7/E8 - 196 μm seeds

E5/E6 - 295 μm seeds E7/E8 - 196 μm seeds

12 18

5 wt %

25 wt %

5 wt %

25 wt %

R18

R18

R50

R50

28 23

29 31

19 20

24 22 LD (μm) 5 wt %

25 wt %

5 wt %

25 wt %

seeds

R18

R18

R50

R50

295 196

370 236

355 209

362 283

330 227

crystallization starts. Figure 2 shows that, in the time interval between the eutonic and the end of the batch, the dominant crystal size in general decreases and the width of the distribution increases. Since no seeds of KCl were added, these changes in PSD are probably related to KCl nucleation. The new KCl crystals tend to grow faster than the NaCl crystals,19 their size approaching those of the added NaCl seeds. In this way, the observed increase in the proportion of crystals smaller than the added seeds, but close enough to their size, can be explained. Besides, crystals much larger than the dominant size appear in most experiments, suggesting that agglomeration was important, the only exception being for 25 wt % and 196 μm seeds, when the supersaturation was the smallest. It is remarkable that the NaCl crystals smaller than the dominant size observed on the PSD at the eutonic condition disappear at the end of the experiments. Besides, KCl crystals formed by nucleation after the eutonic do not appear in the small size fractions either. An unlikely high molecular growth rate would be required to provide the observed effect. A better explanation is that these small NaCl and KCl crystals agglomerate with the larger particles of the PSD. Optical microscopy images of the mixed crystals (R50) presented in Figure 3 show a major occurrence of polycrystal-

Figure 3. Optical microscopy images for seeds (i and iv) and final product (50% water removal − R50) for the experiments performed with 5 and 25% content of seeds sized 295 μm (ii−iii, respectively) and 196 μm (v−vi, respectively), evaporation rate of 0.005 min−1. E



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Figure 4. Particle size distribution for seeds, samples near the eutonic condition (18% water removal − R18) and final product (50% water removal − R50) for the assays performed with 5 and 25% content of seeds sized 295 (A−B, respectively) and 196 μm (C−D, respectively), evaporation rate of 0.003 min−1.

Figure 5. Optical microscopy images for seeds (i and iv) and final product (50% water removal − R50) for the experiments performed with 5 and 25% content of seeds sized 295 (ii−iii, respectively) and 196 μm (v−vi, respectively), evaporation rate of 0.003 min−1.

epitaxial growth are not important. Optical microscopy images (Figure 5) confirms the proposed role of agglomeration, as the products from experiments with 5 wt % seeds are mainly formed by polycrystals, whereas those from 25 wt % seeds are mainly single crystals along with few fragments and some polycrystals. The lower occurrence of agglomeration may be explained by the low supersaturation expected under the high solids contents and low evaporation rate. In the same way, as high supersaturation is known to favor agglomeration, it is possible that the few polycrystalline particles observed under

that less new crystals disappear by incorporation in agglomerates, leading to more small crystals in the PSD. After the eutonic condition is reached, for 5 wt % seeds, the PSD changes are similar to the high evaporation rate experiments: the width of the PSD’s increases, particles much larger than the dominant size appear, and particles much smaller than the dominant size disappear. As before, KCl primary nucleation and agglomeration are proposed as important phenomena in simultaneous crystallization. For 25 wt % seeds, the width of the product size distribution remains the same or decreases, indicating that agglomeration and F



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Figure 6. Micrographs of samples taken during batch crystallization for the assay with 5 wt % seed (295 μm) and 0.005 min−1 evaporation rate (E1), identified by EDS, where the red circles represent the crystals of KCl and the yellow circles represent NaCl crystals. Samples 1−6 were taken at increasing evaporation extents as given in Table 2. Note that the magnification used for Samples 1 and 2 is higher than for the rest of the samples.

Figure 7. Micrographs of samples taken during batch crystallization for the assay with 25 wt % seed (196 μm) and 0.003 min−1 evaporation rate (E8), identified by EDS, where the red circles represent the crystals of KCl and the yellow circles represent NaCl crystals. Samples 1−6 were taken at increasing evaporation extents as given in Table 2. Note that the magnification used for Sample 2 is higher than for the rest of the samples.

were respectively taken at 9% of the solvent removed, just before the eutonic point (18 % of the solvent removed) and at successively higher evaporation extents until the final value of 50%, as shown in Table 2. EDS performed in specific points on single crystal surfaces revealed that they were either pure NaCl or pure KCl, as expected. Extensive isomorphic substitution of Na+ ions into the KCl crystal lattice and of K+ ions into the NaCl only occurs for temperatures much higher than those used in this work, at which these materials form solid solutions.21,22 Rough-surfaced crystals were identified as KCl and smoothsurfaced ones as NaCl. These same differences in particles textures were reported by Glikin14 in eutonic systems

this lower supersaturation are mainly formed by epitaxial growth. 3.3. Scanning Electron Microscopy Coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDS). Although optical microscopy images reveal particle shape, no information is derived regarding composition because both salts develop as cubic crystals. This gap may be covered by SEMEDS. Two experiments were selected for detailed examination, one with a high supersaturation (5 wt % seed content of 295 μm seeds, 0.005 min−1 evaporation rate, E1) and another one with comparatively low supersaturation (25 wt % seed content of 196 μm seeds, 0.003 min−1 evaporation rate, E8), shown respectively in Figures 6 and 7. Samples 1−6 are shown, which G



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Figure 8. Chord counts in time for each chord size range for the assay with 5% seed content of 295 μm seeds and evaporation rate of 0.005 min−1 (E1), where the dotted line represents the evaporation (%) along time.

Figure 9. Chord counts in time for each chord size range for the assay with 5 wt % seed content of 196 μm seeds and evaporation rate of 0.003 min−1 (E7), where the dotted line represents the evaporation (%) along time.

KCl crystals are also observed which are unlikely to be originated from this sampling effect. KCl particles that appear just after the eutonic are mainly adhered to surfaces of NaCl parent crystals and occasionally develop as single crystals (Sample 2). These adhered KCl crystals are often crystallographically aligned to the parent NaCl crystals, suggesting that at least some of the polycrystals are formed by epitaxial growth instead of agglomeration. For longer crystallization times, increases can be observed in size and

comprised by NaCl−KCl−H2O. The formation of rough faces is also related to high supersaturations and high growth rates. For high supersaturation, NaCl particles formed before the eutonic are single crystals with cubic habit (Figure 6, Sample 1). Tiny KCl crystals are observed on the NaCl surfaces. These crystals arise due to sampling and can also be observed at later stages of crystallization. As the solubility of KCl is strongly influenced by temperature, during the filtration, the blunt cooling induces its nucleation. After the eutonic, much larger H



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Figure 10. NaCl proportion, particle size distribution and average NaCl proportion of the final samples for the experiments performed with evaporation rate of 0.005 min−1 and (A) 5% of 295 μm seeds; (B) 25% of 295 μm seeds; (C) 5% of 196 μm seeds; and (D) 25% of 196 μm seeds.

For high supersaturation (Figure 8) evaporation starts (0− 200 s) with a peak in particle counting, probably due to the arising of the first vapor bubbles. During the evaporation period that precedes the eutonic point (200−2500 s), the changes in the counts of chord lengths are small, so it is difficult to relate them to molecular crystal growth or agglomeration. However, when the eutonic point is reached around 15% evaporation (this value differs from the calculated eutonic point of 18% because the vapor mass is determined with a delay between its actual formation and its collection and measurement after it leaves the condenser), an abrupt increase in the number of particles is observed, indicating primary nucleation of KCl. This primary nucleation event explains the presence of monocrystalline particles of KCl in the product. After the nucleation event ceases, the total number of particles decreases. This observation can be attributed to agglomeration and also confirms what is observed in the microscopy images. For low supersaturation (Figure 9) a similar behavior is found, only the variation in the number of counts was lower (compare with Figure 8), as expected. It is noteworthy that, after the nucleation event, not only small particles increased in number, but also larger ones, albeit with a slight delay and lower intensity. Other authors observed similar trends. Mitchell and co-workers23 showed results after nucleation from paracetamol−ethanol solutions for all chord counts lower than 1000 μm. Barthe and co-workers24 observed for paracetamol that the chord counts from 1 up to 251 μm showed a sharp increase, in lower intensities for the higher chord lengths. Barrett and Glennon,25 in their work to characterize the metastable zone and solubility of potash alum in water, also reported a nucleation event using FBRM. They report increases in chord counts for fines (0−20 μm) and coarse particles (50−250 μm). In most cases, the increase in coarse chords counts was associated either to rapid growth or

number of KCl particles bonded to the NaCl surfaces. Small KCl crystals are also present on the surface of NaCl even at late stages of the batch, suggesting the KCl epitaxial growth occurs throughout the batch. Single NaCl crystals can be seen in all samples, as well as single KCl crystals. Notable differences can be seen between high and low supersaturation (Figures 6 and 7, respectively). For high supersaturation, agglomerates constituted by many crystals of various sizes and compositions are formed early in the batch (Sample 3), whereas for low supersaturation agglomerates are mainly formed by few crystals with sizes comparable with the added seeds and more frequently seen at a later crystallization stage (Samples 5 and 6). This observation is consistent with previous findings from PSD determinations of more extensive agglomeration at higher supersaturation. For low supersaturation, crystals at the end of the batch (Sample 6) had rounded edges and corners, whereas for high supersaturation they were sharp. These features are probably due to the increased mechanical collisions associated with the higher seed content and longer residence time. It shows that secondary nucleation of NaCl occurred even in this lower supersaturation and confirms the previous findings from PSD determinations. 3.4. Focused Beam Reflectance Measurements (FBRM). Primary nucleation was investigated by monitoring crystallization with a FBRM-Lasentec probe for experiments with 5 wt % seeds. Results for an experiment under relatively high supersaturation (295 μm seeds and 0.005 min−1, E1), as well as for another one under low supersaturation (196 μm seeds and 0.003 min−1, E7) are shown in Figures 8 and 9, respectively. Data are expressed in chord counts (which are roughly proportional to the number of particles) versus time. The evaporation extent (in g of removed water per g of initial water × 100) is plotted on the secondary axis. The crystallizer temperature and the jacket temperature are also given. I



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Figure 11. NaCl proportion, particle size distribution and average NaCl proportion of the final samples for the experiments performed with evaporation rate of 0.003 min−1 and (A) 5% of 295 μm seeds; (B) 25% of 295 μm seeds; (C) 5% of 196 μm seeds; and (D) 25% of 196 μm seeds.

compositions of the size ranges deviate considerably from the average composition, we refer to the particulate product as segregated. In general, low supersaturations yielded more segregation of NaCl and KCl throughout the size ranges than high supersaturations. This trend can be seen in Figures 10 and 11 and in Table 5, by noting that segregation is, in general, more important for the evaporation rate of 0.003 min−1 than for 0.005 min−1 and more important for 196 μm seeds than for 295 μm seeds. An exception is the lower segregation for 25 wt % compared with 5 wt % of seeds at a lower evaporation rate, probably due to enhanced secondary nucleation of NaCl. The intermediate size range contains a higher NaCl proportion than average for all experiments, as expected, since this is also the size range of the NaCl seeds. In the large size range, the NaCl proportion is generally lower than average. Particles in this size range are predominantly polycrystalline, so they might contain KCl primary particles originated both from agglomeration with single crystals and from epitaxial growth upon NaCl particles. Besides, because KCl grows faster than NaCl, large single crystals of KCl also contribute to the low NaCl proportion measured. Segregation in this size range is particularly evident in experiments carried out with smaller seeds in the high evaporation rate (Figure 10C,D), when agglomeration is less prominent. Therefore, KCl epitaxial growth and KCl single crystal growth are likely to induce segregation in these low supersaturations. For small seeds and low evaporation rate, segregation in the large size range is less evident, probably because enhanced KCl secondary nucleation and little agglomeration reduces the KCl content in the large fractions. In the small size range, the NaCl proportion is variable, as a number of simultaneous phenomena contribute to the

agglomeration. In the present work, besides these phenomena, epitaxial growth could also be associated with this sharp increase in the number of larger particles. 3.5. Particles Chemical Composition for Each Size Range. Products of crystallization formed under 0.005 and 0.003 min−1 evaporation rates were classified by sieving. The mass proportion of NaCl for the retained fraction in each sieve is presented in Figures 10 and 11 (solid lines) along with the particle size distribution (dashed lines). The figures also indicate the average compositions for the products before sieving (dotted lines), as calculated from the mass of each salt produced by the evaporation of 50% of the water increased by the mass of NaCl seeds added. The resulting values are a NaCl proportion of 0.59 for 5 wt % seed content and 0.74, for 25 wt % seed content. Therefore, the figures also allow the assessment of the composition in each size fraction in comparison with the average value. Furthermore, Table 5 shows the segregation indexes, indicating the overall segregation trend for each experimental condition. For simplicity of analysis, it is convenient to classify the PSD in three size ranges, small, intermediate and large, the intermediate size range corresponding to the dominant size and the other ones to its neighborhood. When the Table 5. Segregation Indexes for All Evaporation Rates, Seed Sizes, and Seed Contents segregation index E1/E2 E3/E4 E5/E6 E7/E8

- 295 μm seeds - 196 μm seeds - 295 μm seeds - 196 μm seeds

evaporation rate (min−1)

5 wt %

25 wt %

0.005 0.005 0.003 0.003

0.19 0.21 0.23 0.29

0.08 0.22 0.20 0.22 J


Crystal Growth & Design composition. Primary nucleation of KCl takes place shortly after the eutonic is reached irrespective of the crystallization conditions. For high supersaturation, these freshly formed particles leave the small size range due to agglomeration, contributing to a high NaCl proportion such as found for high evaporation rate and low solids content (Figure 10A,C). Enhanced secondary nucleation of NaCl crystals populate the small size range, as observed for low evaporation rate and small particles (Figure 11C,D). From the above, segregation depends on the dominant processes. Since agglomeration and secondary nucleation are controlled by different parameters (secondary nucleation depends on power input, solids content; agglomeration depends on supersaturation and particle size and number), it should be possible to find optimum conditions to improve segregation.

ACKNOWLEDGMENTS

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REFERENCES

Authors are grateful to Brazil’s National Council of Scientific and Technologic Development − CNPq and to Petrobras for the financial support.

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4. CONCLUSIONS Simultaneous crystallization of NaCl and KCl was studied in the NaCl−KCl−H2O system by the batchwise evaporative method using NaCl seeds. The main elementary phenomena involved in simultaneous crystallization were identified. NaCl crystallization up to the eutonic point happens mainly by molecular crystal growth and agglomeration, the latter being enhanced at high supersaturation. Secondary nucleation also plays a role for high seed contents and long batch times, in spite of the corresponding low supersaturation. When the eutonic condition is exceeded, a KCl primary nucleation event takes place in solution and on the surface of NaCl crystals. The latter induces epitaxial growth of KCl upon NaCl crystals. Both epitaxial growth and further agglomeration occur, leading to the formation of particles of mixed chemical composition. Thus, the product is comprised by mixed composition polycrystals and single crystals of each salt. In general, it was observed that NaCl−KCl segregation was favored under low supersaturation conditions, i.e., low evaporation rates, small seed size, because single crystals of NaCl and KCl predominate. Under high supersaturations, segregation is limited as most particles are polycrystalline of mixed composition. This highlights a possibility of inducing the preferential formation of single crystals of each component in the solution by choosing the appropriate process conditions. Ideal parameters to achieve this goal should prevent epitaxial growth and provide better control of agglomeration and secondary nucleation, which can be identified as the main hindrances to obtaining size separable fractions rich in each salt. The knowledge developed in this research suggests that simultaneous crystallization may be controlled to yield particulate products with tunable morphological properties, which may be useful to develop particulates suitable either to disposal as a residue or to downstream separation for recovery of components of economic value.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Frederico M. Penha: 0000-0001-7614-8448 Marcelo M. Seckler: 0000-0001-9842-4896 Notes

The authors declare no competing financial interest. K



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(21) Bakker, R. J. Package FLUIDS. Part 4: thermodynamic modelling and purely empirical equations for H2O-NaCl-KCl solutions. Mineral. Petrol. 2012, 105 (1), 1−29. (22) Priya, M.; Mahadevan, C. K. Studies on multiphased mixed crystals of NaCl, KCl and KI. Cryst. Res. Technol. 2009, 44 (1), 92− 102. (23) Mitchell, N. A.; Frawley, P. J.; Ó ’Ciardhá, C. T. Nucleation kinetics of paracetamol−ethanol solutions from induction time experiments using Lasentec FBRM®. J. Cryst. Growth 2011, 321 (1), 91−99. (24) Barthe, S. C.; Grover, M. A.; Rousseau, R. W. Observation of Polymorphic Change through Analysis of FBRM Data: Transformation of Paracetamol from Form II to Form I. Cryst. Growth Des. 2008, 8 (9), 3316−3322. (25) Barrett, P.; Glennon, B. Characterizing the Metastable Zone Width and Solubility Curve Using Lasentec FBRM and PVM. Chem. Eng. Res. Des. 2002, 80 (7), 799−805.

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Simultaneous Crystallization of NaCl and KCl from Aqueous Solution

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