## 1 Introduction  
Water solution mining activity in salt lake mining areas is an important factor that affects water quality and mineral quality. Low-grade bloedite (mNa\({}_{2}\)SO\({}_{4}\):nMgSO\({}_{4}\):xH\({}_{2}\)O) is a sediment [1] of sodium-magnesium sulfate in salt lake areas [2; 3]. It is also a low-grade soluble evaporite mineral that remains after sun salting, and its use is necessary for balancing the exploitation of salt lake minerals with sustainable development of the mining area. Bloedite has a microstructure that can undergo substitution reactions [4] and it can be used to assist in the preparation of electrode materials [5] and whiskers [6]. In addition to Na\({}_{2}\)SO\({}_{4}\) and MgSO\({}_{4}\), bloedite in tailings also contains insoluble substances and a small amount of NaCl. Thus, it needs to be purified, and its purification generally uses water-soluble mining methods that are commonly used in salt mines [7], such as open-pit leaching, in-situ leaching, and underground tunnel mining [8]. However, the chemical components and concentration of resulting brine (the aqueous solution of bloedite) are usually variable, and a mechanism to explain the variation in the dissolution process is lacking. This is one of the problems that inhibits the recycling and use of low-grade bloedite. One of the most common topics is the dissociation mechanism at the solid-liquid interface in the study of salt dissolution. There is generally no real-time change in the concentration at the solid-liquid interface. In this case, it can be presumed that the instability of the concentration of the leached solution results from the difference in solute diffusion and mass transfer in the bulk liquid phase. This phenomenon is more obvious in the immersion-type dissolution process in static water [9], resembling element leaching in a flooding irrigation [10]. This phenomenon is specifically manifested in the changes in chemical composition or concentration of the resulting solution at different times, even when the original ore remains unchanged. This leads to uncertainties in the later purification, crystallization, and separation processes. In-situ water dissolution of bloedite is a static exploitation method because stirring does not occur, and this is also what enables diffusion to affect concentration changes in the aqueous solution. The problem of how the dissolution process changes over time has attracted more attention in the research of evaporite dissolution. The dissolution process of salt is not a steady-state process [11]; the dissolution rate constant changes over time [12], and the ratio of solutes in the dissolution liquid phase is also affected by operating conditions [13]. For example, in static water dissolution of mixed evaporites, such as halite (NaCl)-epsomite (MgSO\({}_{4}\)\(\cdot\)7H\({}_{2}\)O) or Glauber's salt (Na\({}_{2}\)SO\({}_{4}\)\(\cdot\)10H\({}_{2}\)O)-bischofite (MgCl\({}_{2}\)\(\cdot\)6H\({}_{2}\)O) [9], the amount of dissolved electrolytes continuously changes over time. Because the diffusion rate of Na\({}^{+}\) is higher than that of other ions, the composition of the resulting water solution is not stable. Similarly, in the process of dissolving solids, such as polyphosphate [14], the bulk water phase also experiences real-time changes in solute concentration. Most dissolution research has focused on the solid-liquid equilibrium [15] or phase diagram [16; 17] or thermochemical characteristics of the phase transition [18]. The experimental methods used to study phase diagrams are all based on equilibrium systems [19], but nonequilibrium data are usually required in the actual dissolution and crystallization processes [20]. Changes in the solute concentration of the resulting solution in the dissolution process are different from those in the evaporation process. Furthermore, the predictability of these changes is poor because the water solution is not saturated at this point, and calculation methods, such as the Pitzer model [21], are inconvenient to apply.