Iron occurrence in coal measures and its release mechanism during water-rock interactions: Taking a mine in the eastern grassland as an example

The mine water with iron out of limit corrodes the water distribution pipeline and the industrial equipment, thereby damaging the liver of human body, and further threatening the human health. The enrichment or dilution of iron in mine water is determined by the iron content and its occurrence in coal measures, as well as its release mechanism during water-coal and water-rock interactions. Herein, 18 groups of coal and rock samples were collected from a mine in the eastern grassland, and had the iron content, occurrence and carrier in the coal measure analyzed with an inductively coupled plasma mass spectrometry (ICP-MS), an X-ray diffractometer, a scanning electron microscope with energy spectrum (SEM-EDS) and a sequential chemical extraction experiment (SCEE). Meanwhile, laboratory water-coal and water-rock simulation tests were carried out to study the release mechanism of iron from coal and rock. The results show that: (1) The iron content is high in coal and roof sandstone, and the enrichment factors of iron in the 18 coal and rock samples are all greater than 1. (2) The iron in coal and sandstone is mainly presented in the residue, followed by the organics, carbonate and the exchangeable form in descending order. (3) The occurrence carriers of iron in residual, carbonate and exchangeable form in coal are kaolinite, siderite and illite respectively, while those in sandstone are chlorite, siderite and illite respectively. The maximum release amount of iron from coal and sandstone during the water-coal and water-rock interactions is 1.26 mg/L and 2.75 mg/L, respectively. Thus, it is indicated that the release amount of iron from sandstone is significantly greater than that from coal. As shown by the comparison of changes in the occurrence of iron in coal and rocks before and after the water-coal and water-rock interactions, the iron content in the residue, organics and iron-manganese oxide is basically unchanged, while the iron content in the exchangeable form and carbonate is reduced. This is because the iron is prone to being released in water due to the poor stability and strong mobility of iron in these two forms, which is also the main reason for iron enrichment in mine water. The results will provide a basis for the treatment and utilization of high iron mine water.