Technique for Measuring the Dielectric Properties of Minerals as a Function of Temperature and Density at Microwave Heat
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TECHNIQUE FOR MEASURING THE DIELECTRIC PROPERTIES OF MINERALS AS A FUNCTION OF TEMPERATURE AND DENSITY AT MICROWAVE HEATING FREQUENCIES Johanna B. Salsman* *U. S. Bureau of Mines, Tuscaloosa Research Center, P. 0. Box L, Tuscaloosa, AL 35486 ABSTRACT As part of the research effort on investigating the effects of microwave energy absorption on the chemical and physical properties of minerals and ores, the Bureau of Mines, Tuscaloosa Research Center has developed a technique of measuring the dielectric constant and loss tangent of minerals at the common microwave heating frequencies. The objective was to establish a reliable data base to aid in predicting the effects of microwave heating on minerals. In this phase of microwave research, the Bureau measured the dielectric properties of powdered minerals with medium to high electrical
conductivities (a > 0.02 Mho/m) in the frequency range of 300 MHz to 3 GHz using an open-ended coaxial line probe connected to an HP 8753A network analyzer. Since the minerals were prepared as powders, techniques were used to relate the measured dielectric properties of the powdered minerals to the dielectric properties of the mineral at Its theoretical or natural density. Also, these measurements were performed as a function of temperature, from 250 to 3250 C. The measured values of the dielectric constants and loss tangents using this method were accurate within +5 percent. This report describes the method of measurement and discusses the results of the Bureau's investigations into dielectric properties of minerals. INTRODUCTION Major research efforts are underway worldwide on the use of microwave energy In various processing techniques. Deterrents to these research efforts include the lack of sufficient data on the electrical properties of materials (including minerals) and the lack of personnel trained In technology relating to microwave processing of materials. It is very likely that microwave energy may offer many advantages to minerals beneficiation and processing techniques; however, it is difficult to exploit these advantages without an understanding of the electrical properties of minerals. The electrical properties of interest in this study are those properties that characterize how a mineral is affected by an applied electric field. These properties generally change with frequency, and are also temperature dependent. At the microwave heating frequencies, 300 MHz to 3 GHz, these properties (namely, the dielectric constant, _', and the r loss tangent, tan6), are often referred to as the dielectric properties of minerals. The dielectric constant determines how much electric energy can be stored in a material while the loss tangent is proportional to the amount of electric energy that is lost as heat. The product of these two terms, the dielectric loss factor, determines how well a material will absorb microwave energy. Since a hiatus exists in the literature on these properties at the higher microwave heating frequencies [1), It i1 imperative to develop techniques to determine t
