Temperature-dependence of propagation velocity of ultrasonic wave in coals

To study the characteristics of the change of rheological properties of coals on heating, the propagation velocity of an ultrasonic wave of 100 KHz was determined for 27 coals of different rank (C, 71.2–94.5%) in the temperature range 77–523 K. It was difficult to establish a clear relationship between absolute values of the velocity and rank of coals, but the temperature-dependence of the velocity appears to be a function of the carbon content of the coals, similar to the well-known correlation between hardness and rank of coals. The change of propagation velocity in 17 coals was determined continuously while the coals were heated from room temperature up to 673 K at 0.5 Kmin⁻¹. At temperatures below 523 K the velocity decreased almost linearly with increasing temperature; however it was observed that the velocity suddenly decreased at a certain temperature above 523 K, which was specific for each individual coal. This specific temperature was found for both caking coals and non-caking coals and was significantly lower than the initial softening temperature detectable by the conventional plastometer or dilatometer.