Fast precise moisture measurement in mineral process plants, has long been a difficult problem. In this master thesis a foundation for a new way of measurement using transmission line theory, is laid out. Macroscopic electro-magnetic...
moreFast precise moisture measurement in mineral process plants, has long been a difficult problem. In this master thesis a foundation for a new way of measurement using transmission line theory, is laid out. Macroscopic electro-magnetic properties of the measured sample can be linked to those of the individual constituents of the sample. Thus one is believed to be able to do a precise estimate of the amount of moisture in a sample. In our case the samples are moist magnetite concentrate.
The characterization of sample macroscopic electro-magnetic properties is done by using a coaxial cell, performing frequency spectra measurement. In this master thesis, the electrical behavior that links the measured data to the macroscopic electro-magnetic properties is examined. It exists two setups, one inductive and one capacitive.
The DC resistivity of the moist magnetite concentrate is determined and some interesting phenomena, suggested to be self potential were discovered. This phenomenon is in general known from geochemistry of mineral rich clays. This made the measurements of resistivity tricky to perform, some measurements were achieved and a new relation between moisture of the moist magnetite concentrate and resistivity is presented.
Studies have been performed on the history of attempts of solving this problem, as well as current relevant research. Reference samples have been prepared in a geotechnical laboratory. Impedance frequency behavior of the measurement equipment is measured and presented, with different curves for different moisture content. From these measurements it is obvious that the moisture affects the impedance of the measurement equipment and thus it is possible to use this method to measure moisture content, if one can determine the electrical model which describes the measurement system.
A few equivalent circuits of transmission lines have been discussed, and one was chosen to represent the purely coaxial part of the coaxial cell. Other parts have been modeled with other elements of AC theory.
A new electrical model for the coaxial probe is proposed, based on LC resonances. Some Comsol FEM simulations have been performed on the coaxial cell, as well as on electric property sensors. Ideas to designs of electrical sensors are discussed. It is mainly the geometry which has been investigated.
First it was proposed that a phenomenon of resonance occurs at the interface of the bottom and coaxial part of the cell, LC resonance. The resonance effect has later been investigated and identified as quarter wavelength resonance, due to the length of the measurement cell and change of wave velocity due to wave propagation in MUT.
Simulations of proposed circuit equivalents in Orcad with PSpice have given some correlation with measured data for dry and non-conducting samples.
There is still work to be done, to complete an accurate mathematical model of the system.
