Variation features of unfrozen water content of coal under low temperature freezing in uncovering coal in cross-cut

In order to discover the unfrozen water content variation of the coal from the low temperature frozen cross-cut passed through seam, a low-field nuclear magnetic resonance technology was applied to the test on three type saturated coal samples of the lignite, bituminous coal and anthracite coal. Meanwhile,in combination with the T2 spectrogram, from a microcosmic view, the paper had an analysis on the unfrozen water distribution and variation in coal pores in the freezing-thawing process. The experimental result showed that in the freezing-thawing process of the three different type coal samples, with the temperature varied, the unfrozen water would appear similar characteristics. In the freezing process, the process could be divided into three stages and they were a fluctuating stage, steeply reduced stage and stable stage. In the thawing process, the process could be divided into two stages and they were a stable increased stage and steep increased stage with an obvious lagging phenomenon. During the freezing period, the water in the big pores would firstly have a phase transition and then be in ice. Late the water in the medium pores, micro pores and small pores would finally be reduced obviously. In the thawing period, the ices in the micro and small pores would firstly be thaw. Within the same freezing temperature zone, the unfrozen water contents in the bituminous coal and anthracite coal would reduce less than the lignite.