Ma Wei, Guo Liyan, Liu Xuehu, Yu Jie
(Dalian Univ. of Tech., Dalian 116024)

Received March14, 2000; Supported by NSFC (29777011)

1. INTRODUCTION
Application of the magnetic field on preventing and removing scales in circulating water has developed for 40 years and it has attracted much attention for its good effect ^[1-4]. In recent years, the magnetic field was increasing applied in industry to prevent and remove scale of calcium carbonate. Ronald Gehr has studied the SO₄^2- contained solution under the magnetic field of 4.75T and the results showed that the magnetic field benefit forming of gypsum so that scales can be prevented and removed easily^[2]. H.E.Lunbager.Madsen ^[5] considered that the magnetic field only had effects on the carbonate and phosphate with diamagnetic metal ions during his research on the crystal of some inorganic salts. Also good results have been obtained in treating scales of sugar medium with electromagnetic field. Several hypotheses about the mechanism have been raised, such as changes of the water structure or the magnetic particles in the solution. But an integrated theoretical system of the magnetic field effects hasn’t formed yet till now.
In recent years, the rich nutrition phenomenon of the water system has become more and more serious, which do the phosphate detergents, phosphate water treatment agent and the synthetic fertilizer cause. The composition of the remaining agent would cause the second pollution when phosphate-dispersing agent is used to solve the problem ^[6,7]. So researchers have shown great interest in the physical methods applying magnet, light, microwave and so on to treat the polluted water ^[6-8]. We just studied the problem of phosphate scales through the Nd-Fe-B magnetic field and tried to make a further explanation of the mechanism of magnetic field effects according to a series of experiments.

2. EXPERIMENTAL
2.1 Materials
A.R grade C_aCl₂, (NH₄)₃PO₄ and KH₂PO₄ were used in the experiment.
2.2 Main Instruments
The main instruments we used were the magnetizing equipment, spectrometer model 721, X-ray diffraction model X/D_max-III A (made in Japan), scanning electron microscope model S-510 (made in Japan), microscope model L2000, conductometer model DDS-11A and the surface stagnation equipment.
2.3 Methods
When the solution of C_aCl₂, or MgCl₂ (NH₄)₃PO₄ and KH₂PO₄ is passed through the magnetic field of certain intensity (0.4T) with the speed of 2.0mL/s, the changes of the conductivity and the surface tension against the solution without magnetic field are determined.
When the equally mixed solution of magnetically treated C_aCl₂ (0.5mol/L) or MgCl₂ and (NH₄)₃PO₄ (0.1mol/L) is passsed the magnetic field of 0.4T, the crystal growth is observed under microscope. The content of phosphorus was analyzed and the results were compared with that of without magnetic field after 48 hours.
Five groups of data were needed to take the mean value and the shapes and structures of the crystals were tested through XRD and SEM after filtration.

3. RESULTS
3.1 The crystallization process and the structure of the phosphate
A large quantity of tiny crystals were separated after the magnetic treated solutions are mixed up. The crystals were uniform and an obvious layer of crystal water was around each of them (just like the ice). As time passing, the particles mostly remained small but a few had gathered to form groups. When we observed the same process of the solution without magnetic field, we found the crystallization speed slower and the particles larger. The large particles were black under the microscope. The results of the further test by SEM showed that the size of the particles with magnetic field treated was smaller (showed in Fig1.).

      a (×6000)                                                                     b (×3000)
Fig.1 SEM photographs of calcium phosphate (a-with magnetic field, b-without magnetic field)

The structures of the crystals were analyzed by XRD. The results (in Fig.2) indicate that the existence of Ca₈H₂(PO₄)₆^.5H₂O ( OCP) except Ca₅(OH)(PO₄)₃ (HAP) and Ca₃(PO₄)₂ (TCP) for the magnetic treated particles, but OCP does not exist in the particles without magnetic treatment. This indicates that the magnetic effects are favourable to the formation of OCP.

We have also studied the sedimentation process of magnesium phosphate by the same method. The result of SEM was showed in Fig.3, which is similar to Fig.2.

3.2 Effects of magnetic field on the physical and chemical parameters of the solution    
The magnetic effects on the physical and chemical parameters were studied by passing the solution through the magnetic field and the results showed some properties of the solution changed after magnetic treatment. The changes of surface tension and conductivity are showed in tab1.

Tab.1 Changes of surface tension and conductivity with magnetic field

* m. --abbreviation of magnetic field

4. DISCUSSIONS
Surface tension is an important physical property of solution. In terms of thermodynamics, the surface contraction of solution Gibbon's energy is a spontaneous process of the decrease in the system. So, a certain quantity of energy was needed to form the new surface. The solution with magnetic field separated out a large quantity of tiny crystals in a short time just because of the decrease of surface tension, which is showed in table1. The conductivity can reflect the concentration and state of electronic particles in the solution. The magnetic field had no obvious effect on the conductivity of CaCl₂ while it increased the conductivity of phosphate according to table1. The effects of magnetic field on the ionization of weak electrolyte were clearly proved.
    Phosphoric acid is polyprotic undergoes stepwise dissociation as given by the following equilibrium:

H₃PO₄=H₂PO₄^-+H⁺ [H⁺][H₂PO₄^-]=K₁[H₃PO₄] ----(1)

H₂PO₄^-=HPO₄^2-+H⁺ [H⁺][HPO₄^2-]=K₂ [H₂PO₄^-] ----(2)

HPO₄^2-=PO₄^3-+H⁺ [H⁺][PO₄^3-]=K₃[HPO₄^2-] ----(3)

    The values of ionization constants are reported as follows: K₁=7.5×10^-3, K₂=6.2×10^-8, K₃=2×10^-13. Thus depending on the [H⁺], different species like H₃PO₄, H₂PO₄^-, HPO₄^2- and PO₄^3- may exist in the solution. The mass balance on the phosphate as follows:

C_T = [H₃PO₄]+[H₂PO₄^-]+[HPO₄^2-]+[PO₄^3-] ----(4)

    Where C_T is the total concentration of the phosphate. According to our experiments, C_T of the mixed solution was 80mg/L with magnetic treatment of 48hrs, which was almost 2.2 times higher than that without magnetic field. The latter was 35mg/L when other conditions ware identical.
It can be seen from equation (1)-(3) that major proton transfer from weak acid to water. M.L.Mikhdson and Z.Kolloidn^[7] published that the Lorentz force on charged particles should have considered. which is given by the vector product:

F=qv×B ----(5)

    Where F is the Lorentz force (N); q the charge of particles(C); v the velocity (m/s) and B the magnetic induction (T). M.L.Mikhelson thought that the Lorentz force caused by magnetic field on moving ions in solution is far too weak to influence the crystallization process by Simple calculation ^[4]. But H.E.Lunbager.Madsen discovered that the magnetic field really had some effects on the diamagnetic particles in spite of no obvious effects on the paramagnetic particles through his research on the inorganic salt crystallization process ^[5]. The results of our experiments showed that magnetic field effects on the weak electrolyte were more evident than that on the strong electrolyte. So, the magnetic effects are determined not only by the magnetic properties but also by the state of the particles in the solution. The effects on the diamagnetic positive ion and the negative ion of weak acid and weak base were more evident. Just as the effects on the phosphate, the magnetic field promoted the solubility because the phosphoric acid was weak acid and undergoes stepwise dissociation. The polyphone resulted in the gathering of tiny particles against the shaping of large regular crystals. (Showed in Fig.1 and Fig.2). Kelvin effect improved the deliquescent performance of the particles.
    In addition, the Lorentz force probably effected the surface electric charge distribution of the crystal cores (electric particles) when they passed through the magnetic field. So there were some changes on the crystal surface and the diffuse layer between the crystal and the solution, which would result in the changes of the shape and structure. The changes were beneficial to removing and preventing of the scales.

5. CONCLUSIONS
The effect of magnetic field on the crystallization process prompt the separation of tiny particles and Kelvin effect increases the solubility of the phosphate.
    The effects of magnetic field have close relations to the properties of both the magnetic field and the particles in the solution. The effects on the diamagnetic positive ion and the negative ion of weak acid and weak base are more evident and result in the difference of the products.
    The difference of the water ring around the crystals show the change of the electric charge distribution on the crystal surface. Mechanism studies and experimental further verification remained to be carried on.

REFERENCES
[1] Donaldson J, Grimes S. New Scientist, 1988, 18 (4): 43.
[2] Ronald G et al. Wat. Res., 1995, 3 (29): 933.
[3] Ahamad H M, Dixit S G. Wat. Res., 1992, 6 (66): 845.
[4] Mikhelson M L, Kolloidn Z. Chem. Absstr., 1977, 87: 577.
[5] Lundager H E, Madsen. Journal of Growth, 1995, 152: 94.
[6] Safarik I. Wat. Res., 1995, 29 (1): 101.
[7] Yasozo Y S, Takashi F. Wat. Res., 1994, 28 (5): 1175.
[8] Broomberg. J. Magneic and Electrical Separation, 1999, 3 (9): 169.

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