Abstract

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 Na₂SO₄·10H₂O
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.

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