Bull. Soc. Sea Water Sci., Jpn., 73, 81 － 84（2019） Bulletin of the Society of Sea Water Science, Japan

Special Issue : “World Symposium Report” （Report） Laboratory Scale Study for the Control Crystallization of Pyramid Crystals（ fleur de sel）, Prepared by Solar Salt

1＊ 1 1 1 Dr. Farhan ULLAH KHAN , Dr. Majid MUMTAZ , Dr. THESEEN and Dr. Atiya HASSAN

In this study, an accelerated method to crystallize well-formed pyramidal crystals of salt is achieved. The square pyramidal crystals of salt, Fleur de Sel salt are known for their unique crystal structure and used in specialty gourmet . The natural crystallization condi- tions include hot summer days on the surface of ponds containing concentrated sea water. The current study has established a con- trolled production of pyramidal crystals as an alternate to natural crystallization which is only possible in summer weathers. A saturated solution of salt（ brine） is prepared by dissolving in water. This brine is evaporated in an open crystallizer by gen- tle evaporation with heating from bottom of the crystallizer. Temperature plays an important role in formation of pyramidal crystals. It was noted that a meta-stable condition in brine for the nucleation and crystal growth is achieved between 55 to 65 ℃ temperature range. Brine purification was also applied for removal of sulphate and subsequent crystallization of uninterrupted pyramidal shaped crystals. The study, when applied, not only yields the well defined shape of pyramidal crystals but also produces a high purity form of salt crys- tals in comparison with naturally crystallized Fleur de Sel.

Keywords : Crystal Structure, Pyramidal Crystal, Stirring, Temperature, Sodium Chloride, Sulphate removal.

The brine was prepared in a small tank at room tempera- Introduction ture and four hours contact time was given with vigorous Fleur de Sel（ Flower of Salt） is the common name of nat- stirring to increase the concentration. Brine treat- urally crystallized sea salt. The pyramidal structure of Fleur ment was performed to eliminate sulphate as barium sul- de Sel is different and unique from others salt. The natural phate 7）. Aluminum sulphate was used as a coagulant agent crystallization of Fleur de Sel is reported on the coast of （pH 4） and all the insoluble impurities were precipitated in Brittany（ ） and in the region of 1）. the next two hours. Qualitative tests were performed to Fleur de Sel is crystallized in ponds containing concentrated check the presence of barium and sulphate ions in the brine seawater. The hollow pyramidal crystals float and grow on solution. 8） The settled brine solution was further filtered the surface of ponds in hot summer days 2）. through vacuum filtration with 20 µm paper to ensure the Fleur de Sel has a unique crystal structure, lower bulk removal of suspended solids. density, large surface area, improved taste and rapid desola- A 10 X 10 inches stainless steel 316 L open square crystal- tion as compared to common cubic salt. Many others salt lizer was heated from the bottom with a hot plate for crystal- are also available in market which has low bulk density es- lization of sodium chloride. Five littrs of prepared brine were pecially in Japan and known as flaky salt 3）. allowed to evaporate at different temperatures from 50 to 90 The micro-meter size pyramids growth has been ob- ℃ without stirring. Observations were taken for the crystal- served by many scientists in a variety of experiments 4, 5）. A lization of square pyramidal salt. Crystallized pyramids were comparison of pyramidal crystallized on earth with collected twice and analyzed for their chemical composition. NaCl crystals grown by the evaporation of an aqueous salt The brine solution was also analyzed at the time of super-sat- solution in microgravity has also been reported 1）. uration and at the end of experiment.

Experimental Procedure Results and Discussion

For the brine preparation salt was selected that had high- The volume of brine at the starting point was 4.91 L in the er levels of calcium and lower levels of magnesium content, crystallizer and temperature was maintained at 64 ℃. No as it has been observed that the high concentration of mag- crystals were observed initially for three hours and as the nesium chloride decreases the solubility of sodium chloride initial supersaturation was obtained few small nuclei were in brine 6）. observed. As a result of gentle evaporation of water and with

1 Department of Chemistry University of Karachi, Pakistan ＊ Corresponding author E-mail：[email protected] Tel：+923458208355 81 82 日本海水学会誌 第 73 巻 第 2 号（2019） the reduction of volume more nuclei starts to grow and were nuclei of large numbers were observed to sink down and a observed to have a 3 mm side length of the base. A few crys- smaller number of nuclei appeared and started to grow. At tals started to sink to the bottom of the crystallizer. When the volume of 4.13 L, small nuclei started to sink and over a the volume of crystallizer has been reduced to 0.53 L during period of 20 minutes approximately 80 ％ of the nuclei set- 3 hours evaporation, a sample of small volume 9 mL was col- tled in the bottom and the proper crystallization of pyrami- lected at the same time and analyzed for calcium, magne- dal crystals began. The results of pyramid salt are presented sium, potassium, bromine, sulphate and sodium chloride in Table 2 as result S1. content and result are presented in Table 1, B2. The newly appeared nuclei grew into perfect square base At the time of regular pyramids crystallization the volume pyramid crystals. The majority of crystals were of a size up of brine had been reduced to 4.38 L. Initially a few needle to 10 mm side length of the base. A comparatively better like crystals were also observed, with the small crystal of metastable zone was observed with the temperature at 60 Fleur de Sel（Fig A） 4）. ℃, where larger crystals were grown and lesser numbers of At the volume level of 4.26 L the temperature was de- nuclei formed. A very stable crystals formation and size gain creased slightly and maintained at 60 ℃. At the same time was observed for next 3 hours, and the picture shows the vigorous stirring was applied and all the crystals that float- crystallization of pyramids at 60 ℃ on the surface of brine ing on the surface were collected for analysis. Large num- solution. Collected pyramids were very fragile in nature, bers of small nuclei were observed as the result of agitation. （Fig C）. Once an unstable zone had been generated, the system lost Before increasing the temperature up to 70 ℃, the crys- its metastable region.（Fig B） tallization was again monitored at 60 ℃ and the same phe- As metastable region started to reestablish these small nomena, sinking of small nuclei and crystallization of pyra-

Table 1

Chemical Sodium Sulphate［ a］ Calcium Magnesium Potassium Bromide Elements Chloride （mg/L＝ppm） Result B1 305000 ＊ND 1795 447 290 67 Result B2 315000 ＊ND 2054 546 478 68 Result B3 307000 ＊ND 3632 684 592 62

Figure A Needle like crystal with pyramid. Fig B Small nuclei formation before starting the crust

Table 2 Chemical Sodium Sulphate［a］ Calcium Magnesium Potassium Bromide Elements（％） Chloride Result S1 99.07 ＊ND 0.105 0.032 0.0158 0.0080 Result S2 99.1 ＊ND 0.144 0.0525 0.0094 0.0038 Result S3 98.65 ND 0.264 0.156 0.0104 0.0064 ［a］Not detected in 200 ppm qualitative test 83 mid crystals were observed. All the crystallized pyramids was noted as being very good for proper pyramid crystals were collected before crystallization at 70 ℃ was started. formation and this pH was maintained in the brine as a func- The volume of brine in the crystallizer was noted as 2.9 L tion of aluminum sulphate that were used as coagulant for and the temperature was adjusted to maintain it at 70 ℃. settling of insoluble impurities. Fig E shows the well formed The brine took 20 minutes to reach the temperature of 70 ℃. single crystal floating on the surface of the brine. Initially small nucleation was observed which settled down The pyramid crystals grew in different sizes, and Fig F after a few minutes and small pyramid crystals started to shows a few big size pyramidal crystals up to 24 mm in side grow on the surface. The size of the side length of base for length of the base. these pyramid measure up to 4 mm but most of them were The effect of temperature（ 50 ℃） shows that slow growth smaller pyramids with 1 to 2 mm side length base. of pyramidal crystals provides the best crystallization ob- During the experiment when the temperature of brine was served at 60 ℃. At comparatively high temperatures above monitored at 80 ℃, the crystals started to join together and 70 to 90 ℃, the formation of a crust and of a large number individual pyramid crystals were rarely observed,（ Fig D）. The crust of irregular pyramids joined with each other were observed to rapidly sink into the bottom of crystallizer. This observation was taken when the temperature of brine reached 90 ℃. The brine solution was agitated and the crust settled down, demonstrating a rapid formation of small nu- clei observed which joined together within minutes. At the same moment with this high temperature region a few crys- tals of pyramids were also observed but with the passage of time they jointed with each other and formed a crust. The same brine was allowed to cool and the observation of crystallization was again made between the temperatures of 55 and 65 ℃. The crystallization of regular pyramid crys- Fig C Metastable state of crystallization tals of salt was observed again. During the experiment, the formation of stable pyramidal crystals was observed in the temperature region 55 to 65 ℃ without agitation. The con- centration of brine at the end of experiment is presented in Table 1, B3 and the result of harvested pyramids salt is pre- sented in Table 2, S2. It was observed that other types of crystal like flakes, hopper cube were also crystallized with the pyramid crys- tals with suphate contained brine.1, 2） The calcium sulphate precipitation was noted as one of the cause for improper pyr- amid crystallization. The sulphate free crystallization ensure the compliance of salt with codex standard of grade salt Fig D Crust Formation at high temperature region above 80 ℃. but natural pyramid（ Fleur de Sel） occasionally meet this criteria. The analysis result of natural pyramid is mentioned in Table-2, B3. The concentration of different parameters in brine for crystallization may vary but Calcium rich brine was preferred for good crystallization as the presence of cal- cium sulphate in brine increases the solubility of Sodium Chloride 10）. The presence of Sulphate also causes the pre- cipitation of calcium sulphate, which is the function of tem- perature. 7） The presence of Magnesium Chloride decrease the solubility of Sodium Chloride in brine and thus plays an important role of this impurities’ observed reduction of growth rate of crystal. 6, 11） A range of pH between 4 and 5 Fig E The beautiful single crystal floating and growing 84 日本海水学会誌 第 73 巻 第 2 号（2019）

Fig F Few big size pyramidal crystals Figure G SEM view of Pyramidal crystals of NaCl of nuclei were observed. It was also noted that at high tem- peratures regions due to a boiling effect and a fast change in the concentration of different layers of brine resulted in the loss of the metastable condition for pyramid crystallization. Figures G and H shows the scanning electron micro- scope（ SEM） view of a single pyramidal crystal and the for- mation of building blocks of pyramidal crystals.

Conclusion

The method developed through this study, ensure accel- erated and stable crystallization of regular pyramidal crys- Close building block SEM view of Pyramidal crystals tals of salt with proper control. Figure H of NaCl The temperature range between 55 to 65 ℃ was found to be suitable for crystallization of regular pyramidal crystals. Elimination of sulphate ions from the brine decrease the and slip line of sodium chloride whisker, Tohoku Univ., Sen- crystallization of other types of salt like flakes, hopper cube dai, Sci. Repts. Research Insts., Tohoku Univ., Ser. A 12 （1960）, 258-270. and other irregular crystals. A better crystallization was ob- 5） D. Aquilano, L. Pastero, m. Bruno, M. Rubbo, {100} and {111} served between pH 4 and 5. The method also improved the form of the NaCl crystals coexisting in growth form pure purity of Sodium Chloride pyramid crystals, in compliance aqueous solution, J. Crystal Growth 311（ 2009）, 399-403. with codex standard of food grade salt 9）. By this method the 6） S. B. Zhang, J. J. Yuan, H. A. Mohameed and J. Ulrich, , The Effect of Different Inorganic Salts on the Growth Rate of first ever sulphate free pyramidal crystals of salt were pro- NaCl Crystallized from Sea Water. Crystal Research and duced. Technology, 31（1996） 19-25. 7） M. Harshad, O. Patel, Factor for optimum brine treatment REFERENCES process design. Sixth International Symposium on Salt, vol# 1） C. Donadio, A. Bialecki, A. Valla, L. Dufosse, Carotenoid-de- 2（1983）, 515-533. rived aroma compound detected and identified in brines and 8） British Pharmacopoeia（ International Edition）, HMSO Lon- speciality sea salts（ fleru de sel） produced insolar saterns don, vol. 1（2005）, pp-604-605, Method 7647-14-5. from saint-Armel（ France） Journal of Food Composition and 9） Codex Standard for Food Grade Salt, CX STAN 150-1985, Analysis. 24（ 2011） 801–810. Rev. 1-1997, Amend. 1-1999, Amend. 2-2001, Amend. 3-2006. 2） P.Fontana, J. Schefer, D. Pettit, Characterization of sodium 10） P. M .Synowiec and B. Bunikowska, Application of crystalli- chloride crystals grown in microgravity. J.Cryst. Growth 324 zation with chemical reaction in the process of waste brine （2011） 207-211. purifying in evaporative Sodium Chloride Ind.Eng Chem Res, 3） S. Inoue, N, Sugimoto and M. Yoshida, Akoh Kaisui Corpora- 44（, 2005）, 2273-228. tion 974-Katoh, Kariya, Akoh City, hyogo Prefecture, Japan, 11） Farhan Ullah Khan＊, Majid Mumtaz and Tehseen Ahmed Manufacturing of flaky salt. 8th world salt symposium Vol #1, “Pilot Plant Study to Utilize Waste Brine Generated by Salt （2000） 439-444. Industries“ ,Journal of Basic & Applied Sciences, 2012, 8, 38 4） K. Kan-Ichi, Y. Tsutomu, Growth shape, plastic deformation,