Crystallization processes using reverse osmosis

Abstract

Supersaturation is one of the main factors controlling the growth of crystals in solution. Narrow crystal size distribution is desirable but often is difficult to achieve. This is mainly because of difficulties which occur in the generation and maintenance of the required levels of supersaturation. In reverse osmosis processes, water is removed selectively from salt solution. While the water recovery proceeds, the residual solution can become supersaturated with respect to dissolved salts. In this study reverse osmosis hollow-fibre membranes have been employed as a means of generating supersaturation in a plug flow system and precipitating calcium oxalate. Significant levels of supersaturation were generated in the reject flow, the level of supersaturation being a function of the residence time of the solution within the hollow fibres. Crystallization and precipitation processes subsequently occurred. The crystals which had been formed in the concentrated flow were characterized by employing size distribution measurement, X-ray diffraction and scanning electron microscopy (SEM). The narrow size distribution and the similarity observed in the SEM micrographs (taken during different experiments) appear to indicate reproducible and constant conditions prevailing in the system; there was a constant generation and maintenance of the supersaturation driving force necessary for salt precipitation.