Use of X-ray fluorescence analysis in modern materials recycling (Part 2)

Valuable resources and energy can be saved by recycling materials, especially metals. Many components and assemblies, such as the bodies of modern vehicles, contain a variety of different types and grades of material. If pure recycling is not possible due to a lack of analysis options, expensive and high-quality alloying elements are often lost in the general scrap. In this article, the physical measuring principle is first discussed and the functionality of XRF is explained using a mobile analyzer. Using concrete examples from stainless steel recycling, the possible applications and also the application limits are also shown. Finally, other methods that can partially compensate for these limitations are briefly discussed.

3 Stainless steel recycling application example

The use of mobile XRF will be demonstrated using the recycling of stainless steels as an example. Precise quantification is generally not required here. Figure 3 shows a handheld device in use at a recycling company.

According to DIN EN 10020, stainless steel is a designation for alloyed or unalloyed steels whose sulphur and phosphorus content must not exceed 0.035% by weight [4]. The term "stainless steel" is a collective term for non-rusting corrosion-resistant steels. They contain at least 10.5% chromium (Cr) and have significantly improved corrosion resistance compared to unalloyed steels [6].

One specific example of the use of stainless steels is cutlery. Two parts from different manufacturers were tested as examples. One was the product of a traditional Swabian company and the other was imported goods from the lowest price segment. Table 1 shows the composition of the relevant elements.

As can be clearly seen, the steels differ significantly in terms of their alloying elements. The 1.4301 (X5CrNi18-10), which is known as a typical stainless, austenitic steel and was also to be expected due to the 18/10 cutlery embossing, has a Ni content of just over 8.5% by weight. Although this value is still within the permissible range of 8.0-10.5% by weight according to DIN EN 10088-3, it also shows how steel manufacturers are deliberately pushing expensive alloying elements to the lower end of the scale.

On the other hand, the material proposal with the designation 1.2080 (X210Cr12) for the inexpensive cutlery part is surprising. This steel with a relatively high carbon content is normally used for highly stressed cutting and punching tools. A comparative measurement with a spark spectroscope (optical emission spectrometry - OES), which is more accurate with regard to the measurability of lighter elements and whose functionality will be briefly discussed in the following section, provides a similar result in terms of composition, but suggests 1.4000 (X6Cr13) as the material. This ferritic stainless steel with its resistance in a moderately aggressive environment appears much more plausible in this context.

4 Conclusion and outlook

This article has described the mode of action and specific application examples of mobile X-ray fluorescence analysis. Both the versatility of this method and its limitations were demonstrated. In one case, the relatively low Mn and Si content in combination with the inability to analyze carbon (due to the low atomic number 6), which can be determined with OES in contrast to XRF, obviously led to an incorrect material suggestion. This is also the reason why spark spectrometry predominates in steel metallurgy for precise analysis. The mode of operation is similar to that of XRF. In simple terms, atoms are stimulated to emit light rays by applying energy. The light spectra are evaluated and, if the wavelengths of the light particles (photons) are known, the respective type of atom can be determined. As the amount of light is approximately proportional to the content of the corresponding element, the content of an element in the test specimen can be determined from the radiation intensity. Spark spectrometry has its origins in spark testing.

As the atoms are excited by an electric arc, the sample has a focal spot after the test, which generally has little effect on the technical properties of the component, but can have a negative visual impact. Due to the high temperature of the arc, the test area must be shielded with shielding gas. In addition, only conductive matrix materials can be tested and more complex surface preparation is required.

This simple example has shown that the XRF method can be used to determine very quickly how a material is composed, whether the composition has changed across batches and to what extent this modification is still within the permissible limits. In many cases, these statements are completely sufficient in practice. This is the reason for the frequency of this test method mentioned at the beginning, particularly in quality assurance and recycling.

Finally, a new mobile measuring method as an alternative to XRF for better determination of lighter elements should be mentioned. This is laser-induced breakdown spectroscopy (LIBS), which currently still has some disadvantages in terms of calibration and measurement accuracy. Users in these fields are recommended to keep an eye on the development of this method over the next few years.

This publication was created as a conference contribution as part of the "Pforzheimer Werkstofftag 2018" event

Literature

[1] Bauch, J.; Rosenkranz, R.: Physical Materials Diagnostics, Springer Vieweg Verlag, 2017
[2] Haschke, M.; Flock, J.: X-ray fluorescence analysis in laboratory practice, Wiley-VCH Verlag, 2017
[3] https://www.analyticon.eu/de/rfa.html
[4] Schlegel, J.: Kleine Stahlkunde - Insights into the world of stainless steels, Rommert Verlag, 2015
[5] Thermo Scientific Niton XL3t GOLDD Alloy Analyzers - Elemental Limits of Detection in Titanium/Iron/Copper-based Alloys, product data sheet, 2008
[6] https://www.edelstahl-rostfrei.de/downloads/iser/MB_821.pdf