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Pipe in need of descaling

Descaling

Descaling is the removal of limescale for industrial and domestic water users in hard water areas.

 

link Desalination

V Hard water
V Water softening
V Magnetic descaling

Hard water

Hard water is water that has high calcium and/or magnesium ions content (>~1.2 mM ) acquired during its passage through calcium and/or magnesium-containing rock such as limestone or chalk. The amount of dissolved calcium and magnesium in water determines its "hardness", expressed as the equivalent amount of calcium carbonate in parts per million (mg ˣ L-1). Classification into 'hard' or 'soft" water is not internationally standardized; water having < 60 mg ˣ L-1 calcium carbonate equivalent is generally regarded as 'soft whereas water having > 120 mg ˣ L-1 is generally regarded as 'hard'. Rainwater is extremely soft (~ 1 mg ˣ L-1) and seawater is extremely hard (> 6000 mg ˣ L-1). In domestic settings, hard water is often indicated by a lack of suds formation when soap is agitated in water, as the minerals precipitate the soap as scum. Hard drinking water is generally not harmful to health and can usefully serve as a dietary supplement for calcium and magnesium. It can, however, cause the formation of limescale in both domestic and industrial boilers and water treatment.

 

Limescale (consisting of mainly calcium carbonate, b 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 'hard' 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 calcite crystals, which may form directly or subsequent to metastable hexagonal and fibrous vaterite crystal formation. Hard water may be treated and limescale can be removed by water softening or the process of descaling.

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Water softening

Water softening is the process of removing the Ca2+ and Mg2+ ions. The usual method for this is to use an ion exchange resin (for example, sulfonated polystyrene) to replace them by Na+ ions.

Na+··· Resin··· Na+ + Ca2+(aq) = Resin:::Ca2+ + 2 Na+(aq)

 

The resin is normally made of small beads. After it is full of Ca2+ and Mg2+ ions it can be regenerated by washing with an excess of NaCl salt.

Resin:::Ca2+ + 2 Na+(aq)) (excess) = Na+··· Resin··· Na+ + Ca2+(aq)

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Magnetic descaling

It is widely reported that magnetic fields may halt or reverse scale build-up [2213, 2676].. The literature is somewhat confused with some reporting that orthorhombic aragonite crystals have a higher density but, although intrinsically harder, are less prone to form hard scale on surfaces [104], whereas other papers report aragonite is more troublesome forming the harder scale on surfaces [2183]. There seems to be little experimental literature to decide one way or another and in any case it is the relative (kinetic) ability of the precipitating particles to stick to surfaces rather than themselves, under the prevailing physical (e.g. electrical and magnetic fields) and chemical (e.g. Mg2+ and Fe3+ content) conditions, that is of overriding importance.

 

Properties of crystalline polymorphs of CaCO3
CaCO3 crystal
Density, g cm-3
Calcite Trigonal (R3c)
2.701
Aragonite Orthorhombic (Pmcn)
2.904
Vaterite Hexagonal (P63/mmc)
2.560

 

The mechanism by which the magnetic field produces its effect seems down to the presence of disordered hydrated CaCO3 aggregates [1954], a forming liquid emulsions which may be affected by the magnetic field and so convert into different prenucleation clusters and hence different structures on crystallization [1955].

 

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 , gradientB~10 T ˣ m-1, it has been shown that the initial amount of aragonite formed is significantly increased over calcite in samples with and without the presence of dissolved iron [107], although this aragonite eventually changes to calcite [555]. A separate experiment has shown that drawing a pure solution of calcium carbonate and bicarbonate through static magnets (0.16 T) for 5-30 min increases the precipitate formed on degassing the excess dissolved carbon dioxide [1043]. Magnetic treatment has been shown to affect the crystal surfaces of both aragonite and calcite [1689] and increases the rate of aragonite crystallization over that of calcite [2184]. .

 

The direction and variation of the magnetic field has also been shown to be important [555], with crystal size decreasing with increasing magnetic field [623]. A different group has showed agreement in a recent study where under similar conditions (B = 0.5 T, flow rate = 0.1 m ˣ s-1) the magnetic field produces mainly a mixture of aragonite (44%) and vaterite (42%) whereas without it well-crystallized calcite (34%) is formed with little aragonite (14%) [252]. It has been proposed that the smaller water cluster size, being more reactive, hydrates the calcium and carbonate (particularly; see a recent supporting study [793]) ions more effectively and so encourages aragonite nucleation [110]. Alternatively, the magnetic field may cause a surface and/or orientation effects on the growing crystals [980]. 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 [353]. The pipe material and its surface were also found to be important [1586].

 

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: [104]

  1. Reduction in the amount of limescale formed.
  2. Production of a less tenacious limescale due to a change in the crystal morphology.
  3. Removal of existing scale (3 - 6 months).
  4. Retention of anti-scaling properties for hours following treatment.

 

Many tests mainly utilizing single pass systems, however, have proved negative [212]. 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 [105]. 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 (due to a combination of magnetically modifies hydration and the shifting of charged particles in the magnetic field [645]) rather than by deposition. Another contributing factor may be the lowering of the surface tension, multiple passes producing increased lowering up to about 8% [735]. A potential (= v ˣ B ˣ 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 [259]. This study also showed an optimum in the flow rate as at too high a flow rate the magnetic field was encountered only briefly, an effect recently confirmed [555]. Recently, the presence of dissolved oxygen has been shown important for the production of the magnetic effect for forming aragonite rather than calcite [970], and for initiating scaling [1046]. It may be assumed that many of the studies described on this page did not control for oxygen content, so their effects may have been moderated by the varying dissolved oxygen contents. c

 

There are 'electronic' devices, related to the above purely magnetic devices, that use weak electromagnetic signals utilizing a coil wound around a pipe. A square-wave pattern is often used as it effectively contains many frequencies from a few Hz to 100 kHz [657]. This changes the magnetic field in a manner similar to a number of rapid passes past a very weak static magnet. However the changing electric field will also contribute to the effect, as shown using a pulsed electric field [799]. Recently,13.56 MHz at 2 V has been found to work well [1626].

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Footnotes

a These sloppy objects are known as DOLLOPs (dynamically-ordered liquid-like oxyanion polymers) [1954]. [Back]

 

b On heating a solution containing dissolved bicarbonate, the following reaction occurs irreversibly due to the loss of gaseous CO2.

Ca(HCO3)2 (aqueous) = CaCO3 (solid)↓ + H2O + CO2 (gas)

 

The thermodynamic formation/dissolution of solid CaCO3 from/in aqueous solution involves the four CO2 equilibria given elsewhere plus water dissociation plus the four further relationships given below (data at 25 °C) [2182].

CaCO3 (solid)= Ca2+(aq) + CO32-(aq)

KS is the calcium ion concentration times the carbonate concentration over the concentration of solid calcium carbonate

KS = 3.35 nM (calcite)

KS = 4.49 nM (aragonite)

CaCO3 (aq, ion-pair) = Ca2+(aq) + CO32-(aq)

KD is the calcium ion concentration times the carbonate concentration over the concentration of ion-paired calcium carbonate

KD = 0.703 mM
CaHCO3+(aq, ion-pair)= Ca2+(aq) + HCO3-(aq)

KD2 is the calcium ion concentration times the bicarbonate concentration over the concentration of ion-paired calcium bicarbonate

KD2 = 96.6 mM
CaOH+(aq, ion-pair)= Ca2+(aq) + OH-(aq)

KD3 = 71.0 mM

 

There is a total of nine equilibria in eleven unknowns (CaCO3 (aq), H3O+, Ca2+(aq), CaHCO3+ (aq), CaOH+ (aq), OH-, HCO3-, CO32-, CO2 (g), CO2 (aq), H2CO3). A further equation is the charge balance (positive charges = negative charges)

 

H3O+ + 2 Ca2+(aq) + CaHCO3+ (aq) + CaOH+ (aq) = OH- + HCO3- +2 CO32-

 

so allowing all of the equilibrium concentrations to be calculated, given one of them. However, it is not straightforward as there are three different KS values dependent on the crystal form, the equilibrium constants are disputed/inaccurate and the concentration-dependent hydration equilibria involving bound water are not included.

 

Solubility of CaCO3 with temperature [2178]

[CaCO3](S) is given the value unity in the equation. Note that Ca(HCO3)2 is not thought to exist as solid or by ion association and evaporating stoichiometric solutions gives CaCO3 and CO2 rather than a solid Ca(HCO3)2. Slightly different values for the solubility product (KS) are given in the literature. The crystal form of the CaCO3 also has an effect with aragonite being about 16% more soluble (KS 34% greater) and vaterite being twice as soluble (KS four times greater) [2178]. All these solubilities reduce with increasing temperature, which is why dissolved CaCO3 forms scale in heated pipes. Also these solubilities increase with increasing dissolved CO2 and more acidic pH. KS is an equilibrium value and does not relate to the kinetics of any dissolution/ precipitation. As calcite is the least soluble, at equilibrium at ambient temperature where aragonite is about one kJ ˣ mol-1 less stable [107], aragonite and vaterite in water convert to calcite. Aragonite is, however, kinetically if not thermodynamically, favored to precipitate at higher temperatures.

 

Water saturated with CaCO3 in equilibrium with pCO2 = 0.0003 atm forms about 10 µM CO2 (aq) solution of pH 8.3, containing about 0.5 mM Ca2+, 5 µM CaCO3 (aq), 5 µM CaHCO3+(aq), 20 nM CaOH+(aq), 10 µM carbonic acid (H2CO3), 1.0 mM HCO3-, 10 µM CO32-. This is close to the reaction (CaCO3 + O2 + H2O --> Ca2+ + 2 HCO3-) of 0.5 mM CaCO3 dissolved to give 0.5 mM Ca2+ and 1 mM HCO3-. [Back]

 

c Oxygen can be removed using sodium sulfite plus cobalt chloride (catalyst). [Back]

 

 

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This page was established in 2002 and last updated by Martin Chaplin on 6 October, 2016


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