Selective salt crystallization from a seeded ternary eutectic system in Eutectic Freeze crystallization

Abstract

As salt and ice are crystallized from multi-component saline solutions, the concentrations of the non-crystallizing salt species increase until the system is saturated with more than two species. This study showed that, in such a situation, seeding can be employed to control the crystallization kinetics of the salts. The aim of this study was to determine the effect of seeding on the yield and purity of the salt product in a eutectic MgSO4-Na2SO4-H2O system. A continuous 2L jacketed, scraped and stirred glass crystallizer was seeded simultaneously with Na2SO4·10H2O and ice, and MgSO4·11H2O and ice, respectively. The initial salt seed mass was varied. A suspension residence time of 30 min and operating temperature of approximately −5°C were used. It was found that MgSO4·11H2O crystallized out at fractions higher than the eutectic thermodynamic ratio, indicating a higher selectivity towards this salt. An increase in seeding mass was found to increase the total salt yield until it reached a plateau of up to 9.5% of the thermodynamic maximum. Furthermore, the proportion of the seeded salt in the underflow slurry also increased. The introduction of a large mass of MgSO4·11H2O seeds (30 g) produced a pure salt product (above 99.4 wt% purity) in both the underflow and overflow. This was due to the higher selectivity of the system towards MgSO4·11H2O salt, combined with a large seed surface area, which increased the growth rate of MgSO4·11H2O. This led to an increased average particle size, and suppressed growth of the unseeded salt. Therefore, this study showed that selective recovery of one salt is possible in a multi-supersaturated system through seed engineering.