Freezing is a crystallization
process that can be used to purify water. When salts or other constituents
are dissolved in water, the freezing point of the solution is lowered
below 32F, the freezing point of pure water. Partial freezing occurs
when the solution is cooled to below 32F, but not below the freezing
point of thesolution.
Relatively pure ice crystals form, and an unfrozen solution, or brine,
containing elevated concentrations of the chemical constituents is also
Because of the presence of these chemical constituents in the brine, it
has a higher density than that of the purified ice and, therefore, readily
flows from the ice. Thus, the purified ice and the brine are naturally
separated. The advantages of natural freezing for water purification are
that the required refrigeration is provided at no cost and the ice pack
is repeatedly subjected to freeze-thaw (IT) cycling. This repeated FI
cycling promotes the formation of large ice crystals, which, in turn,
increase the permeability of the ice pack. This increased permeability
allows the brine to flow more readily through the purified ice pack.
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