Ultrasound has been shown to promote nucleation of crystals and produce narrow size distribution products without seeding. Crystallisation is initiated in a controlled and reproducible way that provides a well-defined starting point [1-5].  For these reasons, ultrasound has been beneficial in the pharmaceutical industry to enhance crystallisation of organic molecules [2] as well as in the mineral industry to crystallise alumina from Bayer’s liquor [6].  Despite the large body of literature on sonocrystallisation, the exact mechanism behind its effects is still a subject of debate.  Although there are various theories that suggest cavitation bubbles are responsible for sonocrystallisation, most studies use power ultrasonic horns that generate both intense shear and cavitation. The intense shear created can have a significant effect on crystallization, especially in antisolvent crystallization processes, making it difficult to see the role that cavitation bubbles play.  The effect of cavitation bubbles without the intense shear can be examined using high frequency ultrasound, which generates stable cavitation bubbles without the intense shear seen with ultrasonic horns.

This study examined the effect of (i) mixing in the absence of ultrasound, (ii) sonication by an ultrasonic horn and (iii) high frequency ultrasound on the antisolvent crystallization of sodium chloride in ethanol.  The degree of mixing was quantified by calculating the segregation index using a parallel competing reaction involving the reaction between iodide-iodate coupled with a neutralization reaction. The sodium chloride crystals were characterized for surface morphology using scanning electron microscope, size distribution using Malvern Mastersizer and crystal structure using X-ray diffraction.

The results show both mixing and ultrasound irradiation decreased the size of NaCl crystals. However, the crystals produced under mixing were irregular in geometry, whereas, crystals obtained under ultrasound sonication were more cubic and symmetric in geometry (Figure 1).  Shown in Table 1 are the segregation index and the volume weighted mean diameter of the NaCl crystals.  Increasing mixing decreased the segregaton index, which is as expected. The decrease in the NaCl crystal size with increasing mixing is due to the increase in the homogeneous nucleation and rate of nucleation.  The results for a 20 kHz horn agree with what has been reported, which is a reduction in crystal sizes with segregation index. However, the use of 647 kHz ultrasound from a plate transducer provides a high segregation index but produces sodium chloride crystals of similar size distribution as the ultrasonic horn.  At high frequency ultrasound, both large degas bubbles and cavitation bubbles may be responsible for the effect as gas bubble surfaces can act as nucleation sites for heterogeneous nucleation to occur.  The advantage of high frequency ultrasound is that it allows a larger volume of liquid to be sonicated compared to ultrasonic horns where the active region is localized near the tip, making scaling up inefficient and difficult. Further, the reduction in shear is likely to lead to increased energy efficiency.

Table 1: Segregation index and volume weighted mean diameter of particles obtained at various mixing and sonication conditions.

References

[1] L.H. Thompson, L.K. Doraiswamy, The rate enhancing effect of ultasound by inducing supersaturation in a solid-liquid system, Chem. Eng. Sci., 55 (2000) 3085-3090.

[2] G. Ruecroft, D. Hipkiss, T. Ly, N. Maxted, P.W. Cains, Sonocrystallization: The use of ultrasound for improved industrial crystallization, Organic Process Research & Development, 9 (2005) 923-932.

[3] A. Abbas, M. Srour, P. Tang, H. Chiou, H.K. Chan, J.A. Romagnoli, Sonocrystallisation of sodium chloride particles for inhalation, Chem. Eng. Sci., 62 (2007) 2445-2453.

[4] R.K. Bund, A.B. Pandit, Sonocrystallization: Effect on lactose recovery and crystal habit, Ultrason. Sonochem., 14 (2007) 143-152.

[5] M. Ashokkumar, R. Bhaskaracharya, S. Kentish, J. Lee, M. Palmer, B. Zisu, The ultrasonic processing of dairy products - An overview, Dairy Sci. Technol., 90 (2010) 147-168.

[6] G. Ruecroft, D. Hipkiss, M. Fennell, Improving the bayer process by power ultrasound induced crystallization (sonocrystallization) of key impurities, in: H. Kvande (Ed.) 134th The Minerals, Metals & Materials Society Annual Meeting, TMS, San Francisco, California, 2005, pp. 163-166.

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