Surface World April 2019 Surface World April 2019 | Page 46
PUMPS & FILTRATION
Pure water for a perfect powder coat
Powder coating is
going green. In some
cases this is the result
of legislation which
requires existing
process cleaning
solutions to be
replaced with more
environmentally
friendly ones.
Increasingly, however,
it’s coating quality
that is the driver.
Some coating
specifi cations now
demand salt spray
testing for more
than 1000 hours,
and that means that
surface preparation
is absolutely critical.
One of the proven ways
of achieving high quality
surface preparation is the
use of pure water. Mains
water contains a wide range
of dissolved salts. These are
harmless in drinking water
but can promote corrosion
and poor adhesion of
powders to metal surfaces.
Using pure water increases the life
expectancies of cleaning tanks and sprays
and is a proven way of making both the
cleaning and protective barrier chemical
more effective. This means longer periods
between tank drain downs and cleans,
extending process operating time and less
maintenance. It also means less wastewater
generation and lower operating costs.
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APRIL 2019
This generates a waste stream
with volume equivalent to about
5 - 15% of the plant throughput
depending on the raw water
quality. RO is a continuous process
that uses no chemicals but depends
on a high operating pressure
generated by an electrically driven
pump. This makes it appear more
environmentally friendly than ion
exchange. The ions removed from
the raw water are concentrated
into a waste stream that is typically
25% of the throughput suggesting
that it is less sustainable. If the
raw water is hard, RO can be
fouled by scaling and this is
normally prevented by softening
upstream of the membranes,
but this requires salt and produces
more wastewater.
A simple two-stage ion exchange
demineralisation system can
produce purified water of >1µS/
cm conductivity which is equivalent
to about 0.5mg/l total dissolved
solids (TDS) whilst RO permeate
is typically about 5µS/cm (about
2mg/l TDS).
A more environmentally and sustainable
process. All this and improved product
quality too.
There are two main processes to produce
pure water: ion exchange demineralisation
and reverse osmosis (RO). Ion exchange uses
a chemically active synthetic resin to remove
dissolved ions from water, in exchange for
hydrogen in the case of cations and, hydroxyl
in the case of anions. The resulting hydrogen
and hydroxyl ions form water molecules.
On the other hand, RO passes the water
through a semi-permeable membrane
which effectively filters out the dissolved ions.
Selecting the best process depends on the
quality of the mains water supply and
the water purity required.
Ion exchange demineralisation is a batch
process. The ions removed from solution
accumulate on the resins and then have
to be removed by regeneration. This uses
hydrochloric acid for the cation resin and
caustic soda for the anion. The regeneration
process isn’t 100% efficient, so some of the
chemical – typically about 20% - is wasted,
and the regenerated resins also have to
be rinsed prior to returning to service.
RO has a higher capital cost than ion
exchange but operating costs for both
processes depend on the system flow rate
and the TDS of the raw water. The chemical
consumption in ion exchange is directly
proportional to the raw water TDS whilst the
power consumption of RO remains fairly
constant, so RO becomes more economic
for higher TDS raw waters. The actual
cut-off point will depend on the raw water
composition, the costs of chemicals and
the anticipated lifetime of the plant.
Using pure water in all stages of surface
preparation will not only improve the
adhesion of the coating but will also
extend chemical life and reduce waste.
Not only sustainable but also cost effective.
Visit www.adeptpwt.co.uk for more
information on water purifi cation
for surface fi nishing.
Telephone: 01933 677181
Email: [email protected]
read online: www.surfaceworld.com