Unfrozen Water Content and Ice–Water Thawing Mechanism in Cryogenic Frozen Coal

The content and distribution of unfrozen water in coal affect directly its pore structure and macroscopic mechanical properties. It is key to break through the theory of cryogenic mining technology to study the ice–water phase transition process of frozen coal. Taking bituminous coal in Yuan Zhuang, China, as the research object, this paper studied pore thawing characteristics of coal frozen by liquid nitrogen through nuclear magnetic resonance technology. The experimental results demonstrate that the unfrozen water content was exponentially related to temperatures. In the early thawing stage of the coal sample, the thawing speed of micropore was the fastest. The cumulative porosity has a linear relationship with the cumulative pore-throat distribution, and an exponential relationship with the unfrozen water content. According to the analysis of thermodynamics, pore water with high pressure and small aperture has low freezing point, causing the micropore structure was first generated with thawing. In general, during the thawing process of bituminous coal frozen by liquid nitrogen, there are more micropores and strong connectivity, which contain less ice and water. There are less mesopores and macropores, while the connectivity in the early thawing stage is poor, but the content of ice and water in that is large.