## 2 Materials and Methods  
The physical model of this work (Figure 1b) is based at the mining site of Yuncheng Salt Lake (Figure 1a). As a sodium sulfate subtype salt lake, there are bloedite deposits in the Yuncheng mining area. It is assumed that the bloedite dissolves and forms a concentration gradient during static water dissolution, which can be modelled in the laboratory (Figure 1c). The dissolution and diffusion experiments of single-component salt and bloedite were carried out in an air-conditioned room at a constant temperature of 20 \({}^{\circ}\)C. To avoid the occurrence of convective mass transfer and the interference of fluid flow in the dissolution and diffusion processes, it is necessary to ensure that the dissolution and diffusion processes are as slow as possible in a static environment. Therefore, either Na\({}_{2}\)SO\({}_{4}\)\(\cdot\)10H\({}_{2}\)O or MgSO\({}_{4}\)\(\cdot\)7H\({}_{2}\)O (analytically pure, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), or bloedite (taken from the Yuncheng salt lake, Shanxi Province, China) were placed in a 1-L graduated cylinder. The amount of salt placed in the cylinder was 300 g. A 60-80-mm high cylindrical piece of foamed plastic was placed above the salt layer, which was about 75-90 mm in height in the graduated cylinder, and then ice cubes were placed on top of that (Figure 1d). After the ice melted, the water flowed down slowly from the gap between the foamed plastic and the inner wall of the graduated cylinder and soaked the salt layer below (Figure 1e). Once the water and salt came into contact, the timer started and the diffusion process of the solute in the liquid phase was timed. At the same time, the foamed plastic floated up (Figure 1f), and finally, the salt layer was fully immersed in water (Figure 1g). The initial distance from the water surface to the salt-water interface was about 225-240 mm.  
When the salt layer is soaked, the processes of dissolution and diffusion are very slow. Therefore, it can be assumed that the saturated solubility of the salt is maintained at the solid-liquid interface; thus, the influence of the particle size of the salt on the dissolution and diffusion processes is neglected.  
Figure 1: The experimental setup of the diffusion test: (**a**) project site of dissolution mining of bloedite; (**b**) schematic diagram of dissolution mining on-site; (**c**) schematic diagram of dissolution in the laboratory; (**d**) experimental step one—placed ice and foamed plastic on the salt; (**e**) experimental step two—the melted ice water soaked the salt; (**f**) experimental step three—the ice melted and the plastic floated to the top of the water; (**g**) experimental step four—removed the plastic and started sampling.  
After a certain period of time of dissolution, sampling by using a long-needle syringe and analysis were carried out at a specific height above the salt-water interface to determine the solute concentration.  
In order to accurately test, the bloedite (Na\({}_{2}\)SO\({}_{4}\)-MgSO\({}_{4}\)-4H\({}_{2}\)O) specially used in this experiment was 95.74% in purity, a grade which was higher than that of bloedite in common salt lake tailings. The impurities in the bloedite sample were mainly insolubles. The resulting solution was used to define the aqueous solution that resulted from the dissolution of bloedite. The concentrations of Na\({}^{+}\), Mg\({}^{2+}\), and SO\({}_{4}\)\({}^{2-}\) (S) were analyzed using inductively coupled plasma emission spectroscopy (ICP-OES, Thermo ICAP 6000, Waltham, MA, USA), and the concentration of Cl\({}^{-}\) was determined via chemical titration using the argentometry method. The saturation degree was used to characterize the amount of salt dissolution, which is expressed as the ratio of the concentration of dissolved electrolyte to its theoretical saturation value.