Effect of antiscalants during eutectic freeze crystallization of reverse osmosis retentate

摘要

Eutectic Freeze Crystallization (EFC) is a technique that has great potential for purification of industrial waste water and brine streams so as to recover pure water and pure salt(s). This technique makes use of the simultaneous crystallization of both ice and salt from a solution at the eutectic temperature of that particular salt-water system. Gravitational separation of ice and salt crystals can be achieved inside the crystallizer due to the differences in density between the ice, salt and mother liquor. The resulting ice and salt slurries are filtered to separate mother liquor from crystals. The ice crystals are then washed with pre-cooled deionised water so as to remove mother liquor adhering onto crystal surfaces as well as that entrapped within the interstitial spaces of aggregates. This ensures a highly pure ice product. Likewise, salt crystals are washed with a saturated salt solution to remove surface impurities and those entrapped in the interstitial spaces within the salt crystal bed. EFC can be used for further purification of the brine produced by reverse osmosis (RO). However, antiscalants are dosed in RO feed streams to prevent scaling of salts on membrane surfaces which would otherwise reduce operational efficiency and membrane life [1]. Consequently, the antiscalants become concentrated in the retentate brine stream and are likely to have an effect on water freezing point and salt solubility as well as nucleation and growth kinetics of both ice and salt crystallization in an EFC process. This research aims to investigate the effect of antiscalants on thermodynamics of both ice and salt in EFC. Understanding the effect of additives or impurities is paramount to the design and development of crystallization processes [2].