Numerical simulation on migration effects of heavy metal elements in coal gangue backfilling body caused by the lithology of coal seam floor

In recent years,coal gangue backfill mining technology has been widely applied as one of the green mining technologies. The coal gangue filling in goaf has been in the acidic or alkaline mine water environment for a long time, its heavy metal elements in goaf have the risk of migrating to the floor and polluting groundwater. In this paper,through the static immersion test of coal gangue,the concentration of element in soaking solution was determined by ICP. The main heavy metals components in coal gangue were Be and Mn. A numerical simulation model for the migration of heavy metals in coal gangue under different floor lithology ( mudstone,granite,sand-stone and limestone) was estab- lished by using COMSOL and Multiphysics. Taking Mn as an example,the diffusion distance and concentration distri- bution of heavy metal element in different rock floors were analyzed,and the relation between the diffusion distance of heavy metal element and the permeability coefficient of floor rock was obtained. The results show that the diffusion dis- tance of Mn element in the bottom plate is 5. 2 m,12. 5 m,16 m and 19 m,respectively,when the clay cushion is not laid on the mudstone,granite,sandstone and limestone. After laying clay cushion,the diffusion distance of Mn element is 0. 9 m,2. 8 m,5. 2 m and 9. 1 m,and decreases by 82. 69% ,77. 60% ,67. 50% ,52. 10% respectively.