Source and release law of fluorine in water-rock system of coal mine goaf in contiguous area of Shaanxi and Inner Mongolia

The coal mine goaf is the key area for the formation of high-fluoride mine water, and the source and release of fluorine in goaf water-rock system determines the enrichment or depletion of fluorine in mine water. Aiming at the unclear source and release rule of fluorine in the mine water of 3-1 coal goaf in the contiguous area of Shaaxi and Inner Mongolia, hydrochemical test, mineral composition analysis, mineral scanning electron microscopy and energy spectrum detection, as well as laboratory water-rock simulation experiment, were conducted to determine the source and occurrence carrier of fluorine in coal mine water, and to reveal the release rule of fluorine in the rock. The results show that the main hydrochemical types of mine water in 3-1 coal goaf of the study area are HCO₃-Na and SO₄-Na. The concentration of fluorine in mine water of 3-1 coal goaf is higher than that in the newly born mine water of 3-1 working face. Besides, the pH and Na⁺/Ca²⁺ ion ratio of mine water in the coal goaf are higher than that in the working face. Therefore, the fluoride in mine water in 3-1 coal goaf mainly comes from mudstone and siltstone, with the average fluorine content of 741 mg/kg and 610 mg/kg, respectively. The main carriers of fluorine in solid phase are smudge, kaolinite, illite and chlorite. Above all, the release of fluorine in the water-rock system of the goaf is controlled by lithology, particle size, pH, temperature and Na⁺/Ca²⁺ concentration. Specifically, fluorine in mudstone and siltstone is more easily released into water than that in the fine sandstone. Regardless of lithology, the concentration of fluorine ions released increases with the decrease of rock particle size. Moreover, the hydrochemical environment characterized by alkaline, high temperature, high sodium and low calcium can promote the dissolution and release of fluorine from rocks to water.