Analysis of Heavy Mineral Sands Using SEM and Raman Spectroscopy

Figure 1 displays the Renishaw structural and chemical analyser (SCA). This enables mineralogists and geologists to benefit from the analytical and imaging capabilities of scanning electron microscopes (SEMs) whilst also combining the structural and chemical characterisation provided by Raman spectroscopy.

The SEM-Raman system enables elemental analysis and rapid observation at both micro and macro scales.

A crucial aspect of commercial minerals’ extraction is process control monitoring. Significant financial implications can result from a failure to recognise deterioration in process efficiency. On the other hand, rewards can be gained in a short period of time from any improvements to the process.

Figure 1. The SEM-SCA at Bologna University - Italy

Efficiency in extraction on an industrial scale has generally been measured via a comparison of the final yield with the amount of starting material that is consumed.

This somewhat unsatisfactory method is increasingly being replaced by in-stream monitoring, however. This allows for the regular monitoring of efficiency which enables process deterioration to be noticed early, and consequently for measures to be implemented which improve the process.

SEM-Raman Investigation of Heavy Mineral Sands

A vital source of iron, zirconium, and titanium is found on the east coast of South Africa, in the form of heavy mineral concentrates from placer sand deposits. The sands are concentrated into Fe-rich, Zr-rich, and Ti-rich streams through electrical separation, magnetic, and flotation methods.

X-ray and SEM analysis as well as Raman microscopy have previously been used in order to investigate the efficiency of this process. Although these approaches were useful, it was not practical to examine the same region of interest using different instruments.

The vibrational frequencies of molecular bonds in an analysed material are measured using Raman spectroscopy. The Raman spectrum which results is not just unique for any given compound, but it is also sensitive to the local environment. This means that structural data can also be revealed by the spectra.

A laser spot is used as the excitation source during Raman spectroscopy. This analysis is non-destructive, and is not affected by the vacuum condition. A spatial resolution which is comparable to that of EDS analysis is guaranteed by the fact that the laser spot’s size is in the order of one micrometre.

Previously conventional Raman microscopy was used to identify the composition of grains. As the grains appear similar when they are optically examined, this has proven to be time-consuming since a high number need to be analysed before composition statistics are able to be considered reliable (for example, see Figures 2 and 3).

The majority of the difficulties which are related to the sample-size can be overcome by stage mapping together with autofocus. However, careful set-up is required for these methods, as well as extended acquisition times in order to yield data which is representative.

Figure 2. Optical microscope image collected from a typical area using 5x objective

Figure 3. Low magnification LV-SEM image of prepared sample

Raman spectroscopy, EDS, and SEM were carried out in situ during this investigation with the use of SEM-SCA. This enabled both chemical identification and classification to be carried out more quickly.

The sample was sourced from a zirconium-rich stream. It was prepared simply by sprinkling sand grains onto an adhesive pad before shaking off any non-adhering particles.

In order to avoid charging, the SEM was used in low-vacuum mode as the sand grains are non-conductive. In this mode denser particles appear brighter as the image contrast is derived mainly from difference in mean atomic number.

Figure 4 is an image from the centre of Figure 3 which is more magnified. It was also the area from which the x-ray maps were gathered.

Figure 4. LV-SEM image from the centre of Figure 3 – this was the area from which x-ray maps were collected

The elements which are present can be identified within just a few minutes through the x-ray mapping of the sample in the SEM. This also allows for the classification of the sample, by identifying the fraction of impurities (here represented by titanium grains) within the Zirconium-rich stream. The precise chemical composition of the grains is often unable to be unambiguously identified by x-ray analysis alone.

Within the region displayed in Figure 4, nearly a dozen elements were detected. However, just the most abundant elements are considered in this study.

X-ray maps for zirconium and titanium are displayed in Figures 5 and 6. Most of the particles analysed contain zirconium, with some containing titanium, and a small minority of others containing neither of the two. This latter type will be discussed later.

Figure 5. Zr map from the region shown in Figure 3

Figure 6. Ti map from the region shown in Figure 3

With the Renishaw SCA, Raman spectroscopy can be carried out inside the SEM. SEM imaging and x-ray analysis locate the grains of interest, and they can be characterised chemically using Raman spectroscopy.

The appearance of the sample to the light microscope is depicted in Figure 2. With a comparison to a similar magnification SEM image (such as the one shown in Figure 4), it is possible to see how hard it can be to relocate grains of interest which have been identified in the SEM using an optical microscope-based spectrometer.

Spectral matching against Renishaw’s minerals library followed from in-situ Raman spectra (Figure 8) recorder from typical zirconium and titanium containing grains (see the green and red particles in Figure 7 respectively). This demonstrated that the particles comprise of rutile (red spectrum – a polymorph of titanium dioxide), and zircon (ZrSiO₄  - the blue spectrum)

Figure 7. Figure shows areas where Raman data were collected

Figure 8. Raman analyses of the areas shown in Figure 7

Combining these results with the x-ray mapping data displayed in Figures 5 and 6, provides a measure of the purity of the zirconium-rich stream.

Some grains which are also of interest are those which are neither pure rutile nor zircon.

In Figure 9 shown below (and also in Figures 4 and 5), the bright red coloured particles contain significantly less zirconium than the zircon particles surrounding them. Despite this, its brightness suggests that it is more dense than the majority of them.

Figure 9. Particle containing REEs

Figure 10. X-ray spectrum from particle indicated in Figure 9

Through x-ray analysis, the presence of rare-earth elements (REEs) such as lanthanum and cerium were revealed (see Figure 10). As a result of a strong fluorescence emission, it was impossible to collect Raman spectra with the use of 514 nm excitation. However, the spectrum shown in Figure 11 was given using 785 nm excitation.

Figure 11. Raman spectrum (in red) collected from the particle shown in Figure 9, zircon reference spectrum in blue

The zircon matrix’s peaks (shown in blue is a zircon reference spectrum) are still visible, however photoluminescence peaks from the REEs are superimposed on top of them.

Another grain (coloured red) is displayed in Figure 12. EDS analysis shows that this contains neither zirconium nor titanium, but oxygen, silicon, magnesium, and calcium.

Displayed in Figure 13, Raman analysis of this particle provided a strong match for diopside (Ca, Md)SI₂O₆, one of the pyroxene group of minerals. This result is consistent with x-ray data.

Figure 12. Particle containing Ca, Mg, Si, and O (no Ti or Zr)

Figure 13. Raman analysis (in red) of particle shown in Figure 12, diopside reference in blue

In this study, the combination of EDS and SEM with Raman spectroscopy has facilitated the characterisation of the sand to be carried out more efficiently as compared to using the techniques in isolation.

The elemental composition of dozens of grains can be identified simultaneously and rapidly (just a few minutes). Subsequently, key elements can be used to classify the grains (as shown in Figures 5 and 6). Raman spectroscopy can subsequently be used to analyse typical grains, in order to determine their structural and chemical composition.

As it is just a trivial matter to undertake image analysis of the x-ray maps, it is possible to determine a ratio for the volume or number of titanium-containing grains in the zirconium stream. This ratio can be used as a measure of the efficiency of the separation process.

Grains which differ from the modal types can be detected by the SEM/EDS technique, as it is sufficiently sensitive. These can then be individually analysed with the use of Raman spectroscopy, in order to characterise their chemical structure.

Some grains, such as the rare earth metals, could be important commercially, so it is vital that they are not overlooked. Spectral bands which are otherwise confusing in the Raman spectrum can be explained by the elemental composition provided by EDS.

Advantages of SEM-SCA in Geology and Mineralogy

• Minimal preparation is required in order to view samples which enables them to be analysed using other methods
• Fast and simple characterisation of mineral mixtures can be carried out
• Whilst EDS solely identifies the elements present, Raman determines the chemical composition
• Whilst EDS cannot distinguish between polymorphs such as crystobalite and rutile, Raman can
• The spatial context can be maintained as Raman analysis can be carried out in situ
• Raman spectroscopy and EDS analysis complement each other as they reveal subtle variations in the composition as well as the presence of trace elements

This information has been sourced, reviewed and adapted from materials provided by Renishaw plc.

For more information on this source, please visit Renishaw plc.