Ore analysis in mining involves the identification of elements and minerals present in rocks. This process serves a key purpose: identifying sites worthy of further investigation for exploitation and determining the commercial feasibility of potential mining sites.1
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Many mining sites are located in remote areas that are hard to access. The need for heavy machinery also results in significant costs, even for prospecting a site.2
Analytical methods for the qualitative analysis of the types of elements present in a rock and the quantitative analysis of their concentrations pose a particular challenge for the mining sector. The analysis needs to be conducted on-site, preferably in real-time, without extensive sample preparation.
However, rocks are typically complex mixtures of chemical species, with the materials of interest often buried deeper within the rock, and the elements of interest may only be present in trace amounts.
One popular approach for ore analysis in mining is the use of X-Ray fluorescence (XRF). In XRF, the sample of interest is irradiated with high-energy electromagnetic radiation, and the emitted photons are detected.
Using high-energy X-Rays causes the ejection or promotion of core electrons, rendering the element energetically unstable. During core-hole relaxation, there is a certain probability of photon emission.
XRF is invaluable for mining as it enables element-specific spectral signatures to be obtained. The tightly bound core electrons that interact with X-Ray radiation have binding energies that are generally unique to the element being irradiated.
The energy of the emitted fluorescence can be used to determine both the nature of the elements present and details about the local chemical environment.3 With the use of suitable calibrations, XRF can also quantity the amounts of certain elements present.
An XRF measurement requires an X-Ray light source, a detector, and typically, focusing and steering optics. One of the most popular X-Ray light sources, known for its high photon dose and excellent energy tuneability, is the synchrotron light source.3
The large photon dose makes it possible to detect even trace levels of elements, and the ability to change the incident photon energy means that different elements can be measured. In non-resonant XRF, the incident photon energy must exceed the binding energy of the core electrons of the element; otherwise, they will not be ionized.
While synchrotron sources are ideal for detailed, offline investigations in many mining applications, portable devices are preferable. There are numerous commercially available handheld and benchtop instruments designed for field use, capable of capturing reasonable quality XRF spectra of many elements in approximately 1 minute.4
These sources typically incorporate X-Ray tubes that use a cathode filament to generate electrons, which are then accelerated into a target to produce X-Ray radiation.
X-Ray cathode or tube sources emit at fixed energy determined by the element they are made of and cannot generate the same number of photons as synchrotron sources.
However, they have been shown to provide reasonable accuracy when determining the concentrations of elements such as arsenic, copper, iron, cadmium, and silicon, though species like magnesium and aluminum can pose more challenges.4
The diversity of the sample matrix types (that is, the various rock environments), adds further challenge. Matrix-specific calibrations can improve accuracy, though caution is necessary regarding the potential interference effects between the matrix and sample.4
For elements such as gold, which are typically found at very low concentrations, handheld devices may not always be suitable due to accuracy issues.5 Elements found deeper in soils and rocks can also prove problematic or may require crushing and sample preparation before analysis.
Research and Company Developments
Although there are challenges associated with using handheld measurements for certain elements, portable devices are already being utilized for XRF imaging studies of geological features and element profiling.6
Like many other areas of spectroscopy and image analysis, researchers are actively working to develop more automated image analysis and data extract approaches to improve efficiency.
While XRF is a quantitative technique, the challenge of quantitative measurements lies in relating the peak area to the amount of a certain element present. In theory, such a relationship should be linear, but matrix effects and overlapping features can contribute to peak broadening, which is why matrix-specific calibrations may be necessary.
Research groups are actively incorporating machine learning to improve the accuracy of spectral interpretation. This approach is particularly crucial for combining automated analysis with unmanned aerial vehicles, facilitating entirely independent remote exploratory operations.6
The brightness of the light source determines the sensitivity of an XRF measurement, the detector's collection efficiency, and the detector's sensitivity. The appeal of cost-effective exploratory operations for the mining industry is appealing, which has spurred quick development in this area.7
Future developments are likely to focus on improved sensitivity detectors or automated sample preparation, reducing the necessity for direct contact with the mineral of interest.7
Read More: The Role of Spectroscopy in Modern Ore Analysis
References and Further Reading
- Jung, D., Choi, Y. (2021). Systematic Review of Machine Learning Applications in Mining: Exploration, Exploitation, and Reclamation. Minerals. doi.org/10.3390/min11020148.
- Humphreys, D. (2001). Sustainable Development: Can the Mining Industry Afford It? Resources Policy. doi.org/10.1016/S0301-4207(01)00003-4.
- von der Heyden, BP. (2020). Shedding Light on Ore Deposits: A Review of Synchrotron X-Ray Radiation Use in Ore Geology Research. Ore Geology Reviews. doi.org/10.1016/j.oregeorev.2020.103328.
- Hall, GEM., Bonham-Carter, GF., Buchar, A. (2014). Evaluation of Portable X-Ray Fluorescence (Pxrf) in Exploration and Mining: Phase 1, Control Reference Materials. Geochemistry: Exploration, Environment, Analysis. doi.org/10.1144/geochem2013-241.
- Laperche, V., Lemière, B. (2020). Possible Pitfalls in the Analysis of Minerals and Loose Materials by Portable XRF, and How to Overcome Them. Minerals. doi.org/10.3390/min11010033.
- Rahman, A., Timms, G., Shahriar, MS., Sennersten, C., Davie, A., Lindley, CA., Hellicar, AD., Smith, G., Biggins, D., Coombe, M. (2016). Association Between Imaging and XRF Sensing: A Machine Learning Approach to Discover Mineralogy in Abandoned Mine Voids. IEEE Sensors Journal. doi.org/10.1109/JSEN.2016.2546241.
- Lemière, B., Uvarova, YA. (2020). New Developments in Field-Portable Geochemical Techniques and On-Site Technologies and Their Place in Mineral Exploration. Geochemistry: Exploration, Environment, Analysis. doi.org/10.1144/geochem2019-044.