In situ ultrasonic spectroscopy study of the nucleation and growth of copper sulfate pentahydrate batch crystallized from supersaturated aqueous solutions

In situ ultrasonic attenuation spectroscopy is applied to the challenging case of monitoring the nucleation and growth of copper sulfate pentahydrate crystallized from supersaturated aqueous solutions, a system not readily amenable to analysis via optical methods due to the intense blue color of the saturated crystallizing solution. In experiments in the 2.8-L rectilinear reactor of a spectrometer, crystallization and dissolution points are reliably detected from the measured attenuation spectra. There are minor differences between data taken at 10 and 50 MHz, notably the lower frequency data appearing to be more sensitive to the particle formation/dissolution process. The nucleation data reveal that the material crystallizes fairly easily as characterized by a metastable zone width of ca. 3-4 °C and a significantly cooling-rate-dependent nucleation order of reaction of ca. 1.7 reflecting the fact that for high cooling rates the nucleation rate is less than that associated with supersaturation generation. The evolving crystal size distribution following nucleation, calculated from ultrasonic attenuation spectroscopy measurements, reveals well-defined oscillations in the observed crystal sizes consistent with the break-up of crystals larger than ca. 250 µm in this crystallizer. From dynamic measurements of changing particle size and concentration during the crystallization process, apparent crystal mass growth rates are calculated to be between 2.3 × 10^-3 kg/m ²·s at the maximum cooling rate of 0.55 °C/min and 2.02 × 10^-4 kg/m²·s at the minimum cooling rate of 0.2 °C/min. On the basis of these data and assuming a spherical particle model, the linear crystal growth rates are estimated to be between 2.0 × 10^-6 m/s at the maximum cooling rate of 0.55 °C/min and 1.8 × 10^-7 m/s at the minimum cooling rate of 0.2 °C/min.