In response to the basic policy of green and low-carbon circular development to solve resource, environmental and ecological problems, gypsum is considered to be a filling material for mine backfilling. To explore the potential risks of gypsum to the groundwater environment due to the backfilling of abandoned mines, a sequential batch leaching experiment was carried out in this paper, which used three types of industrial waste gypsum, namely, phosphorus gypsum (PG), titanium gypsum (TG) and flue gas desulfurization gypsum (FGDG). COMSOL Multiphysics 5.4 software was used to simulate and solve the migration process of the leached metal elements in the mine floor when these three gypsum types were used as filling materials to observe the concentration distributions and diffusion distances of the metal elements from these three gypsum types in the mine floor. The results show that (1) during repeated contact of the three types of industrial waste gypsum with the leaching medium, the pH levels changed, and the changes in pH affected the leaching patterns for the heavy metal elements in the gypsum. (2) Based on the concentrations of the metal elements that were leached from the three types of gypsum, it can be determined that these three types of gypsum are not classified as hazardous solid wastes, but they cannot be ruled out with regard to their risk to the groundwater environment when they are used as mine filling materials. (3) When the three types of gypsum are used as filling materials, the concentration distributions of the metal elements and their migration distances all exhibit significant changes over time. The concentration distributions, diffusion rates and migration distances of the metal elements from the different gypsum types are affected by their initial concentrations in the leachate. The maximum migration distances of Zn in the floor from the PG, FGDG and TG are 8.2, 8.1 and 7.5 m, respectively.

This paper conducts sequential batch extraction experiments on phosphorus gypsum, titanium gypsum and desulfurized gypsum to explore their leaching characteristics and evaluate the hazards of the three types of gypsum. In addition, COMSOL Multiphysics is used to numerically solve the filling models of the three gypsum materials and the patterns of migration of metal elements in gypsum are observed. The result show that the leaching concentration of metal elements of the three gypsums is much lower than the leaching toxicity identification standard, so none of the three have the characteristics of leaching toxicity, and they are not classified as hazardous solid wastes; phosphorus gypsum and FGD gypsum are easier to release metal elements under low pH conditions, while the release of metal elements in titanium gypsum is not obvious under acidic conditions; the simulation results show that the diffusion concentration of metal elements increases with the passage of time, and its migration ability decreases with the increase of depth. The three gypsums can be considered as filling material for abandoned mines. During the filling process, the diffusion rate, diffusion distance and final concentration of metallic elements in gypsum are all affected by the initial concentration.