Study on association solubilization and inhibition of scale in recirculating cooling water system under S-HGMF

The present study conducted an investigation on the effect of a superconducting high gradient magnetic field (S-HGMF) on the association solubilization of recirculating cooling water and the crystal form change of scale. The effects of magnetic flux density, flow rate and cycle-index on the solubility of scale-forming ions were investigated, and the effects of viscosity and surface tension on the molecular internal energy and order degree of the circulating water were analyzed. The scale was ground and mixed with water and placed in an S-HGMF system to study the effect of S-HGMF on the crystal form change of CaCO3. The experimental results showed us that S-HGMF could increase the solubility of scale-forming ions. It could enhance the interaction between water molecules by increasing viscosity and reducing surface tension, so as to improve the stability of water quality, reduce ion precipitation, and achieve the effect of scale inhibition. At the same time, it could also change the crystal structure of CaCO3, promote the transformation of calcite to aragonite, and realize the purpose of scale inhibition. In a word, S-HGMF treatment can effectively solve the scaling problem of a recirculating cooling water system, which provides a reference for scale inhibition of recirculating cooling water.


INTRODUCTION
Recirculating cooling water contains a lot of bicarbonate, carbonate and chloride compounds, and also contains a lot of scale-forming ions, such as Ca 2þ , Mg 2þ , etc. (Huang & Lu ). Since the HCO 3 À in the water are not stable, efficiency, but also increases pipe resistance and maintenance costs. It will also affect the cooling effect and cause under-scale corrosion of the system. How to achieve scale inhibition in a recirculating cooling system is an urgent problem to be solved (Alimi et al. ).
At present, the commonly used scale inhibition methods in industry include chemical treatment methods and physical treatment methods (Zhang et al. 

The recirculating cooling water and scale analysis
The recirculating cooling water was obtained from a steel company located in Hebei, China. The characteristics of the water are listed in Table 1. It can be seen that the hardness and alkalinity of the recirculating cooling water were high, and it was easy to scale.
Taking the scale for analysis, it can be seen that the scale contains a certain amount of Ca, Mg, Si, and other elements.
This told us that it was a composite scale and the main element was Ca, as shown in Table 2.

Characterization
The viscosity and surface tension changes of the recirculating cooling water were measured by viscometer and surface tension meter. X-ray fluorescence (XRF) and X-ray   According to Eyring theory, the viscosity of liquid water η (mPa·s) satisfies the following equation (Eyring ): where: h is Planck's constant, J·s; N A is Avogadro's constant, 1/mol; V m is the molar volume of the liquid, mL/mol; E 0 is the molar activation energy, J/mol; R is the gas constant, J/(mol·K); and T is the absolute temperature, K.
Regarding the activation energy E 0 as the energy required for the space that can accommodate a water molecule in the recirculating cooling water, the change in the molar energy of recirculating cooling water ΔE and the change in the molar activation energy ΔE 0 satisfy the equation (Tabor ): Through Equations (1) and (2), we can get: where: η 0 and η are the viscosity of the water before and after treatment by S-HGMF, mPa·s; and E 0 and E 0 0 are the molar activation energy of liquid water when the viscosity is η and η 0 , J/mol. It can be known from Equation (3)

Effect of S-HGMF on the order degree of water molecules
The surface tension of a liquid is related to its microscopic molecular structure (Amiri & Dadkhah ; Lee et al.

).
When the surface tension changes, the microscopic structure of the molecules in the liquid also changes, which reflects the change in the order degree of molecules inside the liquid. The effect of S-HGMF on surface tension was analyzed, and the order degree of recirculating cooling water was explored in this study.
Under certain temperature and pressure conditions, surface tension σ (mN/m) is equal to the partial differential of Gibbs free energy G (J) to surface area A (m 2 ) (Tabor ): According to the thermodynamic relationship between Gibbs free energy, enthalpy value H (J), and entropy value S (J/K), at a certain pressure P (Pa) and temperature T, the surface entropy S A (J/(K·m 2 )) satisfies the equation: In addition, the surface tension coefficient and temperature satisfy the following equation: where: σ a is a constant related to the liquid; n is the experimental factor; and T c is the dissolution temperature.
Combining Equations (4) and (5), the relationship between surface entropy S A and surface tension can be obtained (Tabor ): Let the relative change of surface entropy be ΔS It can be concluded: S A 0 and σ 0 are the surface entropy (J/(K·m 2 )) and surface tension coefficient of the untreated water, respectively. It can be seen from Equation (8) that by measuring the surface tension coefficient, the relative change in the surface entropy (ΔS A /S A 0 ) of the recirculating cooling water can be obtained.
The flow rate was an important factor affecting the S-HGMF treatment. With the flow rate increased, the surface tension was also increased. It can be seen from Equation (8) that ΔS A /S A 0 increased and the order degree of the water decreased, as shown in Figure 4  In order to analyze the morphological changes of the CaCO 3 , SEM was used to observe the morphology of the scale, as shown in Figure 6. It can be seen that CaCO 3 was mainly composed of calcite, as shown in Figure 6(a).
Under the action of S-HGMF, calcite transformed into aragonite with low crystal hardness and loose structure, as shown in Figure 6 According to the analysis of Figure 3, S-HGMS could reduce the internal energy and resulted in lower molecular energy than that of calcite. Aragonite has a metastable structure, and its molecular energy is lower than calcite. Therefore, when the internal energy was small, it could promote the transformation of calcite to aragonite. In other words, the lower the molecular energy, the easier it was to produce aragonite. Under the action of S-HGMF, the scale was transformed from calcite to aragonite.

CONCLUSION
The present study indicates that S-HGMF technology is a good method for recirculating cooling water treatment, and has a good effect in terms of scale inhibition. After S-HGMF treatment, the internal energy of the recirculating cooling water is reduced, the order degree of the water molecules is increased, the water quality is stabil-