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Advanced sorting of aluminium alloys thanks to XRF-BS

Italian corporation SGM Magnetics, a leader in X-ray sorting technologies- for metal recyclers including x-ray transmission (XRT) and x-ray fluorescence (XRF), is launching its latest proprietary technology called XRF-BS.

The new technology combines XRF with x-ray back scattering and is presented as an alternative to laser induced breakdown spectroscopy (LIBS) for the sorting of aluminium alloys. X-ray back scattering is an underlying phenomenon of fluorescence and is usually considered as a rumour in the XRF spectrographic sorting analysis.

Instead of being represented by a few high intensity peaks of specific energies, it is characterised by a continuous distribution of low intensity photons, the distribution profiles of which are characteristic of the presence of the various metals.

XRF-BS can identify metal concentrations down to 0.2% which makes it a performing solution for sorting the different aluminium alloys and a competitive alternative to LIBS.

What are the main benefits of XRF-BS versus LIBS?

Unlike XRT which performs a readthrough analysis, both XRF-BS and LIBS perform a ‘surface’ analysis. This means that the accuracy of both the LIBS and XRF BS readings is impacted by dirt, painting or oxidation on the surface of the material to e analysed. Accuracy depends on the thickness of the contamination and the penetration of the reading.

XRF-BS penetration is typically around 100-200 microns while LIBS penetration is usually around 10 to 20 microns depending on the level of energy used for the laser source and the type of laser lens. The typical thickness of painting on aluminium profiles is over 40 microns which is less than XRF-BS penetration and greater than LIBS’.

Continuous vs discontinuous

XRF-BS performs a continuous reading while a LIBS reading is discontinuous, making it effectively a sampling analysis. LIBS is certainly good enough on the tailings of aluminium sheets at stamping plants. In this case, the material surface is always clean and every piece is homogenous in composition meaning that a shallow surface sampling analysis provides an accurate analysis of the whole material.

Last but not least in such an application, you know in advance the limited different types of aluminium being processed which helps the sorting. In the case of aluminium scrap, the situation is different meaning that deeper and continuous analysis from the XRF-BS is a plus.

Reading distance

Reading distance is a sensitive element for both XRF-BS and LIBS but is easier to control in the case of XRF-BS. Readings must take place within a certain distance of the surface but this is no problem as the reading is continuous and material is analysed from underneath while in free fall.

With LIBS there is the additional constraint of staying within the focal length of the laser, typically around 20mm, which is not a lot considering the length of the piece and probable misshaped aluminium profiles. Focal length can be increased but that reduces the energy and penetration depth.

Read this story in the latest issue of Recycling International >>

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