**3. Results**

At the first stage of experimental studies, DTA of the reduction process for neodymium from a technological salt mixture with a magnesium–zinc alloy was carried out. Figure 1 shows the thermograms obtained in the first heating cycle (green line of the curve) and the second heating cycle (purple line of the curve) of the Mg–Zn–KCl–NaCl–CaCl2–NdF3 sample up to 800 ◦C.

**Figure 1.** Thermograms of the first (green) and second (purple) heating cycle of the sample Mg–Zn–KCl–NaCl–CaCl2–NdF3 up to 800 ◦C.

During the first cycle of heating the charge, the beginning of zinc melting was recorded at a temperature of 413 ◦C. It was accompanied by an endothermic effect with a maximum at 431 ◦C. The interaction with magnesium begins after the zinc melts, which is confirmed by a strong exothermic effect with a minimum at 476 ◦C. It is typical for the process of formation of intermetallic compounds from pure elements. The exothermic peak with a minimum at 486 ◦C cannot be interpreted since it is associated with the uneven melting of the charge and the peculiarity of filling the melting pot. The onset of the reduction of neodymium by magnesium–zinc melt from fluoride in the chloride melt was recorded at a temperature of 522 ◦C after the end of the interaction of magnesium and zinc. This is confirmed by an extended exothermic effect with a minimum at 566 ◦C. The endothermic effect with a maximum at 703 ◦C corresponds to the melting of the components of the technological salt mixture, and at the temperature of 731 ◦C all interactions in the system under study cease. During the second heating cycle, the thermogram recorded endothermic effects of melting of the magnesium-zinc alloy with a maximum at 347 ◦C and a ternary master alloy with a maximum at 532 ◦C It was found that this effect corresponds to the melting of a ternary compound MgxNdyZnz [7].

Figure 2 shows the thermograms obtained in the first cooling cycle (blue line) and the second cooling cycle (brown line) of the Mg–Zn–KCl–NaCl–CaCl2–NdF3 sample up to 200 ◦C.

**Figure 2.** Thermograms of the first (blue) and second (brown) cooling cycle of the sample Mg–Zn–KCl–NaCl–CaCl2–NdF3 up to 200 ◦C.

During the first and second cooling cycles, two thermal effects were revealed on the thermograms. The first with the minimum at 326 ◦C, corresponding to the crystallization of the double magnesium–zinc eutectic. The second at 511–512 ◦C, corresponding to the crystallization of the MgxNdyZnz ternary compound. In addition, thermal effects were recorded at the temperature of 494 ◦C, corresponding to the crystallization of the reacted technological salt mixture. Elemental analysis of the obtained sample for the magnesium–zinc–neodymium master alloy showed the presence of 18.11 wt.% neodymium, which proves the fundamental possibility of obtaining the master alloy at temperatures up to 700 ◦C and indicates the process of the reduction of neodymium from the fluoride–chloride melt with the magnesium–zinc alloy (Table 1).


**Table 1.** Chemical composition of Mg–Zn–Nd master alloy.

In the first series of experiments, the temperature effect on the degree of neodymium extraction was studied during experimental research on the neodymium reduction from a fluoride–chloride melt in a shaft electric furnace. In this case, in order to reduce the temperature for the synthesis of the ternary master alloy was taken the constant value of the Mg:Zn ratio equal to 1:2 [31]. As a result of processing the obtained data, the dependences of the degree of extraction of neodymium on the residence time at temperatures of 550, 600, 650, 700 ◦C were assessed (Figure 3).

**Figure 3.** Dependence of the degree of extraction of neodymium and the residence time at temperatures of 550–700 ◦C.

During the experiments, it was confirmed that the addition of zinc to the charge helps to reduce the temperature of the neodymium reduction process from the molten salt, in contrast to the temperature ranges for obtaining the double magnesium–neodymium master alloy [32]. It was found that the degree of extraction of neodymium of up to 60% is achieved at a synthesis temperature of 550 ◦C. Moreover, the degree of extraction of neodymium increases to 92.2–93.2% with an increase in temperature to 650 ◦C

In the second series of experiments, studies were carried out in order to identify the most optimal technological parameters for conducting melts. In this case, a high degree of neodymium extraction into the maser alloy is achieved. The initial data and the results of experiments on obtaining the master alloy are shown in Table 2.


**Table 2.** Results of synthesis of Mg–Zn–Nd master alloy.

It was determined that the neodymium fluoride reduction is accompanied by the formation of the homogeneous magnesium–zinc–neodymium master alloy. According to experimental data, it has been proved that the neodymium yield increases to 99.6% with an increase in the ratio of chlorides to NdF3

up to 6:1 in the technological salt mixture and with continuous stirring of the melt. With an increase in temperature up to 700 ◦C, the neodymium yield does not change significantly. As a result of the performed melts, master alloys with a neodymium content of 10 to 25 wt.% were obtained. Analysis of the quality of the resulting master alloy showed that its macrostructure is characterized by the absence of gas pores and non-metallic inclusions (Figure 4).

**Figure 4.** Macrostructure of master alloy 25Mg–50Zn–25Nd.

Since there are no regulatory requirements for the ternary master alloy of the studied composition, the comparison was made with the requirements for the content of impurities for the magnesium–neodymium alloy; this one is also used for the production of special-purpose magnesium alloys. The Mg–Zn–Nd master alloys meets the requirements for Mg–Nd magnesium master alloys according to specification TU 48-4-271-91 (Table 3).


**Table 3.** Chemical composition of Mg–Zn–Nd master alloy.

The microstructure of all tested master alloys in the cast state consisted of a magnesium–zinc matrix, and well-distinguishable individual intermetallic compounds, mainly polygon-shaped. Regular shapes and some coagulation of the edges can be detected, which ultimately leads to the absence of local defects in the microstructure in the matrix. A significant part of the thin section for the obtained magnesium–zinc–neodymium master alloy with a content of 24.8 wt.% neodymium (Figure 5a) is occupied by eutectic colonies (dark areas). It is located along the boundaries of the individual intermetallic compounds (light areas). In some cases, areas of accretion of intermetallic compounds are observed. The average grain diameter is 35 μm. Zones of eutectics are revealed at ×500 magnification (Figure 5b). They are located within the individual intermetallic compounds.

**Figure 5.** Microstructure of master alloy 25Mg–50Zn–25Nd. Zoom (**a**) ×100, and (**b**) ×500.

Micro X-ray spectral analysis of the structure sections shows (Figure 6a) that individual intermetallic compounds contain about 21 wt.% neodymium, which corresponds to the Mg3,4NdZn7 phase. Double eutectic (Figure 6b), alloyed with neodymium, contains 32.41 wt.% magnesium, 60.72 wt.% zinc and 6.88 wt.% neodymium (Table 4).

**Figure 6.** Electronic images of the microstructure of the 25Mg–50Zn–25Nd master alloy (**a**) and (**b**)—×2000.


**Table 4.** Values of spectra of Mg–Zn–Nd master alloy.
