freeze crystalisation
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Prentec Freeze CrystalisationTRANSCRIPT
7/17/2019 Freeze Crystalisation
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Prentec (Pty) Ltd
Brine Treatment by Freeze Crystalisation
The mining industry in South Africa has made great strides in handling water
affected by mining activities through treatment by high recovery Reverse
Osmosis. Although
these
systems
achieve
world
leading
water
recovery
as
high
as
99.5%, there still remains a small stream of brine which must be dealt with. This
is mostly disposed of in ponds at enormous cost.
The industry, through its research organization Coaltech, invited technology
companies to propose technical solutions to reducing the volume of brine.
In response to this Prentec is commercializing a Freeze crystallization process
which is capable of recovering water from these very concentrated brines. Key to
the process is an efficient refrigeration system.
The energy cost of freezing water with a refrigeration coefficient of performance
(COP) of
3 is
at
best
30kWh/ton.
The
situation
is
far
worse
with
brines
freezing
at
‐5 to ‐20C. This type of performance would make the technology very energy
intensive and viable in only a small number of cases.
To counter these problems Prentec have designed a large scale (1.8t/h) freeze
desalination plant with a dual refrigeration circuit. In this approach part of the
heat is rejected into the ice melt. The compressor in this circuit hence has a very
low condensing temperature and pressure. This dramatically reduces power
consumption. A refrigeration COP’s of up to 10 is achieved, reducing power costs
to between 12 and 20 kWh/t, depending on the brine freezing point.
A large
scale
demonstration
of
this
design
has
been
commissioned
at
Optimum colliery and the plant has been started up. This design is
based on R236fa which is a low pressure refrigerant. The compressor
is a two stage Roots type blower which is able to operate at the very
low (Vacuum) condensing pressures required. The refrigeration
circuits operate correctly and have achieved design levels of
refrigeration for short periods. Problems have been encountered
with the removal of ice from the crystallizer. This is being rectified
and it is expected that the unit would then be able operate
sustainably at or near its design capacity.
Once commercialized
there
are
many
many
applications
in
the
Mine
Water recovery industry. Other crystalisation applications
traditionally dealt with by evaporative means could also open up to
this technology.
The developers of the technology are:
Adrian Viljoen – Prentec,
Kathy Karakatsanis – Prentec,
Martin Pryor – Prentec,
Vivian Moller – Prentec,
Hugh Grantham – Grantham Air.