## 4 Results and Discussion
### Phase Diagram Analysis and Process Optimization Strategy  
As seen in the phase diagram shown in Figure 10, the dissolution of soluble salts containing Na\({}_{2}\)SO\({}_{4}\) and MgSO\({}_{4}\) basically follows the rule that the solute composition in the liquid phase is consistent with the solid ore composition. The slow and incomplete dissolution of Na\({}_{2}\)SO\({}_{4}\) and MgSO\({}_{4}\) from bloedite causes the increase in the solute concentration at different heights and different times to gradually slow down; thus, the concentration of the resulting solution will slowly stagnate in a certain area of Figure 10. This means that the resulting solution does not reach the traditional saturation line in the phase diagram. This is consistent with the actual situation on a project site.  
Figure 8: Calculated local mass transfer coefficient profiles at different times in the dissolution and diffusion processes of bloedite.  
For example, in engineering practices, the dissolution mining time of 2160 h is roughly equivalent to an extensive mining time in autumn and winter; that is, flood irrigation begins in autumn, brine collection begins in spring, and freezing time in winter is excluded if there is no freezing Glauber's salt. In this case, when the depth of the dissolving water was 100 mm, the concentration reached half of the saturation values of solutes. Except for the influence of seasonal temperature changes, this phenomenon in practice is consistent with the predicted values shown in Figure 10.  
Figures 6 and 10 provide references for actual solution mining. If the water source is sufficient, the dissolution can be stopped at 720 h and new fresh water can be introduced to continue solution mining, because the increase in concentration of resulting solution begins to slow down after 720 h of dissolution. However, the disadvantage of this solution extraction method is that the subsequent evaporation and crystallization processes consume more water. Figure 11 shows the amount of water that must be evaporated when the resulting solution is concentrated to a saturated state and begins to precipitate salt product under the conditions of different dissolution times and water depths. The results shown in Figure 11 are related to the technological economy of the dissolution and salt  
Figure 10: The composition of the resulting solution in the phase diagram in the dissolution and diffusion processes of bloedite.  
Figure 9: Comparison of mass transfer coefficients between mixed-electrolyte (Na\({}_{2}\)SO\({}_{4}\)-MgSO\({}_{4}\)-H\({}_{2}\)O system from the bloedite dissolution) and single-electrolyte systems.  
production processes; it is expected that the amount of evaporated water decreases so that the enthalpy consumption of the production process is also reduced. Figure 11a shows that as the depth of dissolving water increases, the amount of water that must be evaporated in the subsequent evaporation process also increases. As seen in Figure 11b, when the optimized brine depth is 40 mm (Figure 7), the required amount of evaporated water decreases roughly linearly with respect to time within the first 720 h; when the dissolution time is more than 720 h, the amount of evaporated water generally tends to be stable with respect to time, and this indicates that it is meaningless to extend the dissolution in terms of process optimization. When the dissolution time is long, such as from autumn to the following spring, it is better for the dissolution time to be controlled to be about 2160 h, because the amount of evaporated water is not further reduced even if the dissolution time is further extended. The information provided in Figure 11 can be used as a quantitative reference for industrial dissolution mining processes.