High-resolution dielectric characterization of minerals: A step towards understanding the basic interactions between microwaves and rocks

Abstract

Microwave energy was demonstrated to be potentially beneficial for reducing the cost of several steps of the mining process. Significant literature was developed about this topic but few studies are focused on understanding the interaction between microwaves and minerals at a fundamental level in order to elucidate the underlying physical processes that control the observed phenomena. This is ascribed to the complexity of such phenomena, related to chemical and physical transformations, where electrical, thermal and mechanical forces play concurrent roles. In this work a new characterization method for the dielectric properties of mineral samples at microwave frequencies is presented. The method is based upon the scanning microwave microscopy technique that enables measurement of the dielectric constant, loss factor and conductivity with extremely high spatial resolution and accuracy. As opposed to conventional dielectric techniques, the scanning microwave microscope can then access and measure the dielectric properties of micrometric-sized mineral inclusions within a complex structure of natural rock. In this work two micrometric hematite inclusions were characterized at a microwave frequency of 3 GHz. Scanning electron microscopy/energydispersive x-ray spectroscopy and confocal micro-Raman spectroscopy were used to determine the structural details and chemical and elemental composition of mineral sample on similar scale.

Statistical Description of Inhomogeneous Samples by Scanning Microwave Microscopy

“…These features are still too small to be efficiently tested by a conventional miniaturized dielectric probe [6]. A similar situation is found in other applications, ranging from the high-T. Monti power microwave processing of materials [7], where the dielectric features of micrometric inclusions in the treated materials strongly influence the treatment efficiency itself [8], to the diagnosis of 3D-printed circuit elements [9], especially when assembled in a complex board. Engineering applications like these, usually involve natural or man-made multiphase materials at micrometric scale with uncontrolled roughness and partially known composition.…”

Traceable measurement and imaging of the complex permittivity of a multiphase mineral specimen at micron scales using a microwave microscope

“…NSMMs can be used to study multiphase materials, such as composite and functional materials [3][4], biological materials [5] and minerals [6]. In this paper, this is demonstrated by measurements on particulate rock set in an epoxy and polished.…”

“…Permittivity and loss tangent of a specimen placed at the probe tip are determined from measured shifts in resonant frequency and Q-factor. An alternative type of microwave microscope based on an AFM cantilever has recently been applied to measurements on haematite [6].…”

The experimental study of microwave heating on the microstructure of oil shale samples

“…23 Many experimental studies have used nitrogen adsorption-desorption isotherms, 24 mercury intrusion porosimetry (MIP), 25 and scanning electron microscope (SEM) 26 to evaluate the pore structure of oil shale after conventional heating. 28 It is also recognized that microwave heating has the ability to generate new fractures in rock and increase the apertures of existing fractures in coals, [29][30][31] which differs from conventional heating. Wang et al 27 calculated the specific surface area and specific pore volume of oil shale particles by the low-temperature adsorption of nitrogen, but this method did not involve the direct observation of individual pores or address the connectivity of the pore space.…”

“…Moreover, the porous characteristics of oil shale after heating have a great influence on both the emission of pollutants and their recycling selection. 28 It is also recognized that microwave heating has the ability to generate new fractures in rock and increase the apertures of existing fractures in coals, [29][30][31] which differs from conventional heating.…”