## 4 Results and Discussion
### Dissolution and Diffusion Processes of Bloedite  
Changes in the saturation degree profiles of bloedite are shown in Figure 5, and changes in the solute saturation degree at different water depths are shown in Figure 6. The diffusion capacities of Na\({}_{2}\)SO\({}_{4}\) and MgO\({}_{4}\) in water are very close, and the local saturation degree of the two is related to the composition of the original ore. The specific manifestation is that the concentration ratio of the two solutes in water is roughly equal to the molar ratio of Na\({}_{2}\)SO\({}_{4}\) and MgO\({}_{4}\) in the original ore. However, there is still a slight difference between the saturation degree or concentration profiles of the two, and this is manifested in the diffusion rate of Na\({}^{+}\) being slightly faster than that of Mg\({}^{2+}\). It is reasonable to expect that when the time for dissolution and diffusion is sufficient, the difference between the concentrations of Na\({}_{2}\)SO\({}_{4}\) and MgO\({}_{4}\) above the water layer will be significant enough for this approach to be used for engineering separation.  
Figure 4: Local saturation degrees of solute at different times and locations (in the dissolution and diffusion processes of single electrolyte in water): (**a**) MgO\({}_{4}\); (**b**) Na\({}_{2}\)SO\({}_{4}\).  
Figure 5: Local saturation degrees of solutes at different diffusion times in the dissolution and diffusion process of bloedite.  
The general rule is that during short-term dissolution, the concentration of different solutes in the bottom water will be slightly different; if the dissolution time is extended indefinitely, the solute concentration of the upper brine will have a greater difference. This phenomenon is generally similar to that in an interactive system [9], but they are still slightly different. The main difference is that in the interactive system, especially in the \"1-1 + 2-2\" type of mixed-salt systems (such as the mixed salt of \"NaCl + MgSO\({}_{4}\)\"), the difference in the mass transfer rates of the two salt solutes is more obvious. This makes it possible for the Mg\({}^{2+}\) concentration to be close to zero somewhere above the solid-liquid interface; however, the Na\({}^{+}\) concentration is high, and thus, the two salts can be separated after a certain diffusion time when the resulting solution is pumped out from a certain water depth. However, when the mixed salt of Na\({}_{2}\)SO\({}_{4}\) and MgSO\({}_{4}\) was used, both of the salts are the \"2-2\" type, and their mass transfer rates are not much different. Therefore, it is not possible to achieve separation unless the dissolution and diffusion time is as long as about one year. Figure 7 shows the quantitative relationship between the average concentration of resulting solution and different immersion depths. When the water immersion depth increases, the total amount of dissolved solute increases because of the increase in water volume; however, the concentration of the resulting solution decreases. Considering factors such as the total amount of dissolution and solute concentration, it is recommended that an immersion depth of 20-40 mm is the optimal value.  
Figure 6: Local concentrations of solute at different times and locations in the dissolution and diffusion processes of bloodite; the experimental data after 2000 h are the test results in the salt pond of Yuncheng Salt Lake, China.  
Figure 7: Saturation degrees and dissolved solute amounts of resulting solution at different times and immersion depths (in the dissolution and diffusion processes of bloodite).