Effect of a Phosphonate Antiscalant during Eutectic Freeze Crystallization of a Sodium Sulfate Aqueous Stream

Reverse osmosis (RO) plants are widely used for recovering potable water from industrial wastewater streams. The RO retentate stream is highly concentrated in salts, thereby limiting further treatment using RO due to increased scaling potential of the salts. Currently, the brine streams are disposed of in evaporation ponds in which pure salts cannot be recovered. This makes eutectic freeze crystallization (EFC) an attractive separation method for purification of such streams because it can recover both pure water and salt. However, antiscalants are present in RO and other industrial wastewater streams as impurities. These could have an effect on the thermodynamics and crystallization kinetics during EFC. This paper focuses on the effect of a phosphonate-based antiscalant on the crystallization kinetics of both ice and Na2SO4 center dot 10H(2)O in a continuous EFC process under subeutectic conditions. It was observed that the salt nucleation rate remained almost constant, while a general decrease in the ice nucleation rate occurred with an increase in antiscalant concentration. A general increase in the growth rates of both ice and salt was observed. A threshold antiscalant concentration in the range 350-500 mg/L was observed, beyond which the effect was reversed. The morphology of salt crystals from both the control and antiscalant-dosed solutions was monoclinic and prismatic. The morphology of ice could not be detected due to agglomeration. Product washing was very effective in removing impurities, suggesting that adsorption occurred by physisorption.