Effect of magnetic water treatment on homogeneous and heterogeneous precipitation of calcium carbonate

Effect of magnetic water treatment on homogeneous and heterogeneous precipitation of calcium carbonate

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In this paper are reported experimental results on the effect of a magnetic field on the precipitation process of calcium carbonate scale from a hard water. Carbonically pure water was circulated at a constant flow rate in a magnetic field. After this treatment, calcium carbonate precipitation was induced by degassing dissolved carbonic gas. The nucleation time was identified from the variations of the pH and the Ca2+ concentration. The ratio between homogeneous and heterogeneous nucleation was determined from the measurement of the mass of precipitated calcium carbonate. 

It is shown that the magnetic treatment increases the total amount of precipitate. This effect depends on the solution pH, the flow rate and the duration of the treatment. In addition, the magnetic treatment modifies the ratio between homogeneous/heterogeneous nucleation. Homogeneous nucleation is promoted by an increasing the pH of water, the flow rate as well as the residence time. The magnetic treatment enhances these effects with a maximum for a 15 min treatment time. It is shown that the presence of calcium carbonate colloid particles is not necessary. It is advanced that the main magnetic effects concern the associations of ionic species which are present in the solution and which are involved in the nucleation process of calcium carbonate precipitation 

The antiscale magnetic treatment (AMT) of hard waters is currently used to prevent the build up of deposits on the walls of industrial systems, in particular in heat exchangers, as well in domestic equipments (Proceedings of the International meeting of Antiscale magnetic treatment, 1996; MAG 3, 1999). This kind of physical treatment presents the great advantage to avoid the use of chemicals such as strong acids or polyphosphates which are expensive and can be harmful for human life or deleterious for the environment. AMT has been employed for more than a half century. The first commercial
 device was patented in Belgium in 1945 (Vemeiren, 1958). Powerful electromagnets were used in hot water systems since the 1960s in the Soviet Union (Grutsch, 1977). The application of AMT was reported in the United States since 1975 (Grutsch, 1977; Grutsch and McClintock, 1984). In many cases, the field is delivered by permanent magnets in various geometrical configurations. Several devices are based on AC or pulsed fields. (Oshitani et al., 1999). According to the review paper of Baker and Judd (1996), in spite of this long experience, the efficiency of this treatment is still a con- troversial question and it is not possible to get a clear explanation of the phenomenon.

A great number of experimental researches were carried out on the modification of the CaCO3 precipitation process by AMT. In most cases, a supersaturated solution was prepared by the ‘‘double decomposition method’’, by mixing two equimolar solutions of CaCl2 and Na2CO3, respectively, but this method presents the drawback to introduce systematically an excess of foreign Na2+ and Cl— ions which increase the conductivity. Various electrochemical and non-electrochemical techniques were used to estimate the scaling potentiality of water (Hui and Le´dion, 2002). In the electrochemical methods, the strong increase of the interfacial pH on the cathode plate due to the reduction of dissolved oxygen induced the heterogeneous nucleation of calcium carbonate. The scaling rate can be evaluated from the decrease of the electrochemical current intensity  (Le´dion  et  al.,  1985),  by  measuring  the  deposited mass thanks to a quartz crystal microbalance (Gabrielli et al., 1996b), or even by performing electrochemical impedance analysis (Gabrielli et al., 1996a; Deslouis et al., 1997). Among the non-electrochemical methods, some of them consist in forcing the calcium carbonate precipitation by increasing the pH, some other are based on the heating or the evaporation of the solution (Euvrard et al., 1997).
According to the literature, the efficiency of the magnetic treatment depends on numerous parameters. For example, Chibowski et al. (2003) or Barrett and Parsons (1998) have observed that a magnetic treatment applied during on a hard water decreased the quantity of scale deposited on the wall. The principle of the phenomenon is still not well understood and various contradictory hypotheses were proposed. It was often  attributed  to  the  Lorentz  forces  ~F ¼ q • ~v ~ ~B  exerted
 
In a previous work (Gabrielli et al., 2001), the scaling power of a magnetically treated water was evaluated by mean of an electrochemical method. The crystal growth rate was directly measured with an electrochemical  quartz crystal microbalance. It was shown that the field strength, the composition and the flow velocity of the water have significant effects on the scaling rate. The nature of the material of the tubing, in which the MF was applied, appeared also to have an influence on AMT. It was deduced that electrokinetic phenomena occurring in the vicinity of the walls of the tubing could be also implied (Gabrielli et al., 2001). However, it could be objected that the electrochemical precipitation test induces exclusively an heterogeneous nuclea- tion by a strong interfacial pH shift, this could be in contra- diction with what happens in usual conditions where the homogeneous and heterogeneous precipitation are mainly due to the escape of carbonic gas from the water, under the effect either of a change of pressure or an increase of the temperature. For this reason, the experimental works which are presented in this paper were performed with the same magnetic treatment system which proved its efficiency, but the scaling potentiality of the treated water was evaluated by inducing the calcium carbonate precipitation by a method based on the extraction of dissolved carbonic gas. From the measurement of the variations of the solution pH, the Ca2+ concentration, and the mass of the homogeneous precipitate, it was possible  to evaluate the effect of the magnetic treatment on the nucleation time and on both the homogeneous and heterogeneous precipitation rates. The morphology of the two kinds of precipitate was examined by SEM. Their crystal structure was identified by X-ray diffraction.

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