Submitted to: Institute of Electrical and Electronics Engineers
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/8/1998
Publication Date: N/A
Citation: Interpretive Summary: In coal mines, combustible mixtures of coal dust and air constitute explosive hazards. For this reason, the Code of Federal Regulations requires that the incombustible content of combined coal dust, rock dust, and other dust in coal mines shall not be less than 65 percent. Thus, there is need for rapid test methods to determine the rock dust component. Electrical properties, called dielectric properties, of coal and rock dust may possibly provide a means for these measurements. Because of our unique laboratory equipment and experience, we were asked by the U. S. Bureau of Mines (now NIOSH) for assistance in determining the microwave properties of coal and limestone samples. Measurements of the dielectric properties of pulverized Pittsburgh coal and limestone samples were made over the frequency range from 200 MHZ to 20 GHz. Special techniques we had developed earlier were used to determine the bulk densities at which measurements were made, and a proven dielectric mixture equation was used to provide reliable estimates of the dielectric properties of solid coal and limestone. Significant differences were found in the properties of coal and limestone. Further studies are needed to evaluate the feasibility of such techniques for rapidly measuring rock dust content in mixtures of coal and rock dust, but the data are being made available for the benefit of scientists working with new techniques involving coal and minerals.
Technical Abstract: The open-ended coaxial-line probe technique can be used to obtain estimates of the permittivities of some solid dielectrics over broad ranges of frequency by measurements on powdered or pulverized samples, but certain limitations must be recognized. Such a probe was used with a network analyzer to estimate the permittivities of coal and limestone from reflection coefficients measured on pulverized samples. The bulk density of the pulverized samples for the coaxial probe measurements was determined from auxiliary single-frequency permittivity measurements on the samples at known bulk densities, and the permittivities of the solid materials over the frequency range from 0.2 to 20 GHz were then estimated by computations based on the Landau & Lifshitz, Looyenga dielectric mixture equation and solid material densities.