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Limescale (consisting of mainly calcium carbonate, plus calcium
sulfate, barium sulfate, calcium phosphate, magnesium hydroxide,
zinc phosphate, iron hydroxides and silica, dependent on the geographical
area) is a problem in heated water systems wherever water is obtained
from limestone or chalk countryside. It is formed primarily because
the solubility of calcium carbonate decreases with increasing temperature.
Limescale is only a problem if calcium carbonate deposits rhombohedral
calcite crystals, which may form directly or subsequent to metastable
hexagonal and fibrous vaterite crystal formation. Orthorhombic aragonite
crystals have a higher density and, although intrinsically harder,
are less prone to form hard scale, but are only about one kJ mol-1
less stable and favored at higher temperatures.
Once formed, crystals are kinetically (if not thermodynamically)
stable for hundreds of hours. By drawing water through a static
magnetic field (B ~0.1 T , ÑB~10 T m-1, it has been shown
that the amount of aragonite is significantly increased over calcite
in samples with and without the presence of dissolved iron . Agreement
has been shown in a recent study by a different group where under
similar conditions (B = 0.5 T, flow rate = 0.1 m s-1) the magnetic
field produces mainly a mixture of aragonite (44%) and vaterite
(42%) whereas without it well crystallised calcite (34%) is formed
with little aragonite (14%). It has been proposed that the smaller
water cluster size, being more reactive, hydrates the calcium and
carbonate (particularly) ions more effectively and so encourages
aragonite nucleation . It is possible, however, that the major effect
is the magnetically-induced competitive formation of hydrated silica
in suitable solutions that then absorbs calcium ions .
There are many devices on the market for the magnetic treatment
of water for the removal of such limescale. The sales success of
these devices would seem to indicate that some work as promoted,
at least under some circumstances. Magnetic treatment of water is
claimed to cause four effects:
Reduction in the amount of limescale formed.
Production of a less tenacious limescale due to a change in the
crystal morphology.
Removal of existing scale (3 - 6 months).
Retention of anti-scaling properties for hours following treatment.
Many tests mainly utilizing single pass systems, however, have
proved negative . Recirculatory systems, with prolonged magnetic
exposure, give more supportive results. Rapid movement (1200 rpm)
in a strong magnetic field (4.75 T) had a significant effect compared
with the movement or field alone . A smaller number of larger crystals
causes the effect as nucleation is suppressed and crystal growth
is enhanced. It is possible that the effect is due to magnetically
enhanced corrosion promoting the release of Fe2+ that, even at ppm
reduces calcite but has no effect on aragonite production; inhibiting
the thermodynamic transformation of aragonite into calcite. (Fe2+
may be used as a threshold inhibitor by industry). However the amount
of dissolution, required by this theory, has not been found.
Magnetic treatment devices that create additional turbulence may
enhance anti-scaling effect [109], perhaps by encouraging precipitation
in the bulk rather than by deposition. A potential (= v.B.L where
L is the distance between detecting electrodes) is generated when
a dilute electrolyte flows (v) through a transverse magnetic field
(B, greatest when the flow is orthogonal to the magnetic field).
This may increase colloidal coagulation.
A recent well-controlled study has shown that scaling can be reduced
by a few percent by even one pass though a simple magnetic device
but that it is difficult to increase this effect to more than about
20% even with extensive recirculation . This study also showed an
optimum in the flow rate as at too high a flow rate the magnetic
field was encountered only briefly.
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