X-ray Fluorescence Analysis of Complex Materials

Abstract

Portable, in-situ and on-stream X-ray fluorescence (XRF) is widely used in minerals industry applications to analyse materials with little or no sample preparation. These XRF techniques have been found particularly useful for measuring the concentrations of precious and base metal such as gold, platinum, nickel and copper in mineral slurries. The consequence of measuring mineral samples in slurry form is that physical sample effects can limit the accuracy of XRF analysis. This thesis describes two different approaches for improving the XRF analysis of in-situ slurries: thorough characterisation of heavy element L-shell spectra for improved spectral fitting, and the development of a particle size effect correction technique.

L-shell X-ray spectra have been measured for 8 elements with atomic numbers between 68 and 79, and the measured line intensity ratios and total subshell intensity ratios are compared to existing theoretical and experimental values. The spectra were carefully fitted to determine line energies and intensities, accounting for Lorentzian line broadening, Compton scattering, incomplete charge collection and the silicon escape effect. A Monte Carlo approach was used to calculate geometry, attenuation and detector efficiency corrections. Up to 15 line intensity ratios and total L1/L3 and L2/L3 subshell intensity ratios are reported for each element. Substantial disagreement is found in both magnitude and trend with atomic number when compared to theory. The measured results are used to predict the errors introduced during elemental composition determination using theoretical basis-function fitting when measured XRF spectra are analysed with incorrect theoretical X-ray emission intensities.

The intensity of characteristic fluorescent radiation from mineral phases in particulate materials such as slurries decreases as the particle size of the ore being measured increases. The particle size effect can lead to significant analysis errors, but is usually ignored in on-stream applications where there is limited control over the particle size. This thesis describes measurements of the particle size effect for copper and iron powders in a weakly absorbing matrix. The measured results are compared to a theoretical model and Monte Carlo simulations. A preliminary correction method involving measurements using dual exciting radiation energies is discussed and evaluated using measured and simulated data.