## 1 Introduction  
The problems on production safety and equipment maintenance caused by calcium carbonate scale have becoming increasingly serious. Calcium carbonate is an inversely soluble salt [1], that is, its solubility decreases with increasing temperature. Calcium and carbonate ions in the circulating cooling water combine to form calcium carbonate scale and are deposited on the surface of heat exchangers. Calcium carbonate has a thermal conductivity of 2.9 W/mK, which is less than 1% of that for metal copper (401 W/mK) [2]. Therefore, the calcium carbonate deposited on the heat exchanger can remarkably reduce heat exchange efficiency, likewise reducing the operating efficiency of the equipment and increasing the operating cost [3; 4; 5; 6; 7]. The average annual cost of cleaning and maintaining industrial production equipment due to scale is billions of dollars [2; 4]. Adding a 1 mm-thick limestone deposit can double the power consumption of a power plant [2]. Therefore, inhibiting the formation and deposition of calcium carbonate scale is highly valuable. Scale inhibition methods are mainly classified into chemical and physical techniques. The former generally has disadvantages, such as high cost and secondary pollution to the environment [2; 6], whereas the latter is economic and environmentally friendly [1; 2; 6; 8; 9; 10; 11]. Hence, physical water treatments may replace chemical methods. The main physical scale inhibition techniques include the use of magnetic fields, electric fields, ultrasonic waves, and RF(Radio Frequency) electric fields [2; 4; 6; 7; 12; 13]. Amongthem, the electromagnetic field method is particularly interesting. Calcite scaling was easily produced, and the fouling resistance decreased remarkably after high-frequency electric field treatment [2]. [PERSON] et al. [14] studied the pulsed electromagnetic field water treatment device and found its effect on the crystal morphology of calcium carbonate. [PERSON] et al. [6] reported that the grain size and the proportions of aragonite and calcite change under the action of high-voltage electric field and variable frequency magnetic field. Moreover, [PERSON] et al. [15] stated that the proportions of calcite and aragonite change after magnetic field treatment. After electromagnetic field treatment, calcium carbonate is found in bulk water rather than being attached to the wall of the heat exchanger [3; 8; 11; 16; 17; 18]. Magnesium ions affect the crystallization and growth of calcium carbonate [19; 20; 21]. Low concentrations of magnesium ions are easily incorporated into calcite, but high concentrations have an inhibitory effect on the formation of calcite [20]. [PERSON] et al. [19] proposed that the combination of magnesium ions and calcium carbonate crystal lattice changes the equilibrium thermodynamic properties of the new growth surface and inhibits the growth of calcite. [PERSON] and [PERSON] [21] suggested that the inhibitory effect of magnesium ion on the calcite is due to the uneven distribution of magnesium ions on the surface of the calcite crystal, leading to the formation of a new crystal surface and the reduced new surface growth rate. However, studies on the effects of magnesium ions on calcium carbonate mostly focused on the formation of natural minerals. The mechanism of magnesium ions on the scale formation of calcium carbonate in heat exchangers is unclear. Although many studies confirmed the scale inhibition effect of electromagnetic fields, some showed the effect of magnesium ions on the growth of calcium carbonate. Whether magnesium ions and electromagnetic fields have a synergistic relationship with scale inhibition remains unclear. In the present experiment, we added AR-grade(Analytical Reagent grade) magnesium chloride to distilled water to prepare circulating cooling water. The following three different circulating cooling waters were prepared: without magnesium chloride, with 4 mmol/L magnesium chloride, and with 6 mmol/L magnesium chloride.