Innovative vacuum distillation technology for preparing refined Al from Al–Mg alloy

The saturated vapor pressures of various metallic substances within Al-Mg alloy were theoretically calculated to assess the feasibility of Mg separation. The relationship between the saturated vapor pressure (P*, Pa) and temperature (T, K) for a specific metal can be expressed by Eq. (3):

where A, B, C, and D are the evaporation constants for each metal28. The results of the investigation into the relationship between metal pressure and temperature (Fig. 10) demonstrate that the pressures of Mg are significantly higher than those of Al within the temperature range of 973 K to 1373 K. At a distillation temperature of 1373 K, the saturated vapor pressure of Al is recorded at a mere 2.36 × 10− 4 Pa, in contrast to 6.97 × 104 Pa observed for Mg. Consequently, Mg is preferentially evaporated and separated from Al-Mg alloy at relatively low temperatures. In contrast, Al, characterized by its low saturation vapor pressure, is more challenging to evaporate and tends to remain in the residues. The removal of Mg and the enrichment of Al from Al-Mg alloy can be effectively accomplished at elevated temperatures. Furthermore, the judicious application of VD process can significantly enhance the purification of Al from Al-Mg alloy.

Maximum volatilization rate dynamics

A comprehensive investigation was conducted to examine the dynamics of VD process as it pertains to Al-Mg alloy. The maximum volatilization rates of Al and Mg were assessed at various distillation temperatures utilizing the Langmuir Eq. 28. This equation serves to characterize the maximum rate at which a particular element can evaporate:

where ν is the maximum volatilization rate (g/(cm2∙min)); α is the accommodation coefficient, which is generally considered to be close to 1; P* is the saturated vapor pressure (Pa); M is the molar molecular weight (g/mol); and T is the melt temperature (K). The maximum volatilization rates of Al and Mg at different temperatures are depicted in Fig. 11.

Figure 11 illustrates the maximum volatilization rates of Al and Mg across a range of temperatures. As the distillation temperature increases, the maximum volatilization rate for each metal also rises correspondingly. The ranking of these rates at a given temperature reflects the order observed in saturated vapor pressures. Within the distillation temperature range of 973 K to 1373 K, the disparity in peak volatilization rates among the substances is significant. Notably, the maximum volatilization rate of Mg surpasses that of Al by an five orders of magnitude. At a distillation temperature of 973 K, the maximum volatilization rate for Al is measured at only 1.811 × 10–11 g/(cm2∙min), which is substantially lower than the rate of 2.264 g/(cm2∙min) for Mg. This considerable difference indicates that the initial phase of VD predominantly features the volatilization of Mg, while Al’s volatilization occurs at a slower rate. As the distillation temperature continues to rise, the evaporation of Mg intensifies, concurrently leading to an increase in the evaporation loss of Al. Therefore, it is essential to determine an appropriate duration for the distillation process based on empirical research. This strategy ensures the efficient recovery of Al, reduces energy consumption, and minimizes the processing time required for Al purification.

Fig. 10

Relationship between saturated vapor pressure and temperature for Al-Mg alloy.

Fig. 11

Relationship between maximum volatilization rate and temperature for Al-Mg alloy.

Prediction of activity coefficients

To evaluate the separation efficiency of VD process at a specific temperature, it is necessary to understand the activity coefficients of the constituents within the liquid alloy28,29, as represented in a VLE diagram. This study employed the most widely recognized molecular interaction volume model (MIVM)30,31, non-random two-liquid (NRTL) model32,33, and Wilson Eqs. 34, 35 to calculate the activity coefficients of Al and Mg components. To assess the reliability of the model calculations, the research introduced average standard error (S\(_{i}^{*}\)) and average relative error (Si) between the calculated and experimental values36. Table 1 outlines a comparison of the computed activity coefficients from each model against the experimental data. The results indicate that Al-Mg melt behaves as a non-ideal solution, exhibiting a negative deviation, which suggests the presence of reciprocal suppression between Al and Mg atoms during VD process. In Table 1, NRTL model exhibited the least fitting deviations when comparing the observed and calculated activity coefficient values for Al-Mg alloy. This result implies that NRTL model is more accurate and stable than Wilson and MIVM models, which are the other two local composition models considered. This phase of the investigation provides further insights into the thermodynamic properties of Al-Mg melt. Therefore, NRTL model was selected to predict the activity coefficients of Al-Mg alloy within the temperature range of 973 K to 1373 K for distillation purposes. The calculated activity coefficient values for Al-Mg alloy components, as derived from NRTL model, are detailed in Table 2.

VLE diagram of Al-Mg alloy

VLE diagrams serve as intuitive and effective instruments for comparing saturation vapor pressure and VD capabilities. These diagrams are utilized to assess the efficacy of a process in separating impurity elements and purifying metals37-39. By applying the selected NRTL model to calculate activity coefficients, along with Eqs. (5) and (6), one can obtain VLE data for Al-Mg alloy as follows:

where P*Al and P*Mg are the saturated vapor pressures of Al and Mg related to temperature, respectively; γAl and γMg are the activity coefficients of Al and Mg, respectively, which both had been calculated (Table 2); ωAl,l and ωMg,l are the mass fractions of Al and Fe in the Al-Fe binary alloy system, respectively. Based on Eqs. (5) and (6), the mass fraction of Mg and Al in the vapor phase (ωMg,g and ωAl,g) can be calculated quantitatively (Fig. 12).

Fig. 12

VLE diagram of Al-Mg binary alloy.

In Fig. 12, even a minimal concentration of Mg in the liquid phase tends to evaporate almost entirely into the gas phase. For instance, at a distillation temperature of 1373 K, a liquid phase containing 9.09% Mg results in a vapor phase concentration of 99.9999987% Mg. The concentration of Mg in the gas phase ωMg,g decreases with increasing temperature, indicating that a greater amount of Al evaporates into the vapor phase at a constant ωAl,l. Additionally, as the concentration of Al in ωAl,l increases at a constant temperature, ωMg,g gradually decreases. The precise prediction of VLE diagram demonstrates that VD can effectively separate Mg from Al-Mg alloys, thereby yielding high-purity Al.