XRF Offers New Metal Sorting Possibilities

The demand of the industry for mixed metal materials is rising, and so does number and amount of metal compounds and alloys. Several sorting methods and systems are on the market, but some of them are already „old standard“, says Engineer Martin Weiss. In the following article he describes the benefits of XRF sorting for metal recycling. 

The global demand for ferrous and nonferrous metals is increasing from year to year. The modern industrial production in all areas of daily life devours an enormous quantity of aluminium and nonferrous metals such as copper, brass, nickel, stainless steels, tin, zinc, lead, etc. The Institute of Scrap Recycling Industries Inc. (ISRI) in the United States defined various terms for mixed metals such as Zorba, Zebra, Zeppelin, Zurik, etc.

The current used sorting method for sorting of Zorba, Zurik or metals of similar composition are sink-float separation processes or sensor-based sorting systems based upon camera technology. The proportion of manual sorting is also still considerable. Manual sorting results in very high purities, the process however is very cost-intensive in industrial countries and should therefore only serve as final check and quality control. Furthermore, not all metals can be sorted manually as the visual difference is simply not existent. Grey metals can only be sorted to a certain extent and e.g. stainless steel cannot be separated visually at all. The sink-float process is used in many treatment plants to separate materials of different density. This process requires huge quantities of water and very expensive additives such as ferro-silicon. The separation efficiency is also very limited, and thus only aluminium can be separated from heavy metals and other impurities of low density (e.g. plastics, wood, etc.).

Sorting by color camera: limited efficiency

The mix of heavy metals must be treated in another way or even has to be sorted manually. Most heavy metals are exported to Asia for further manual sorting, which increases the dependence on these countries, particularly China, and should therefore only be considered an emergency solution. The aluminium gained cannot be further separated either. The use of sensor-based sorting systems represents an economic and reasonable alternative to manual or density sorting. Sorting by color using a color camera is one possibility of the sensor-based sorting technology.

However, the separation efficiency is quite limited, therefore only fractions which are clearly distinguishable by color such as copper and brass can be separated. All grey heavy metals such as zinc, lead, nickel, etc. and stainless steels are not sortable. The sorting by color camera is further very inefficient in case of surface contamination and color impurities. Sorting tests and experience with different input material have shown that only part of the copper and brass fraction is also optically recognizable. Only about one-third of the whole copper and brass fraction corresponds to the color definitions of these two nonferrous metals. The remaining quantity that is two-thirds of the total fraction remains unidentified due to surface contaminations and therefore unsorted within the heavy metal fraction.

rw2XRT technology: alternative to sink-float separation

The XRT technology (X-ray Transmission) is also limited in its sorting efficiency and can therefore only be regarded as an alternative to the sink-float separation process. The transmission technology “shines through” the material and is based upon the detection of density differences (an X-ray image is a very good comparison). Dense material (bones) weakens the X-rays much more than less dense material (tissue). The X-ray transmission is a kind of dense sorting such as the sink-float process.

Another possibility of the sensor-based sorting technology is the sorting with XRF (X-ray fluorescence). A few years ago Redwave developed a sensor-based sorting system based upon X-ray fluorescence in cooperation with Olympus. Olympus is the global market leader of portable XRF-systems for rapid on-site measurements and has extensive experience in the field of X-ray fluorescence. The XRF sorting system of Redwave was initially used in the field of glass sorting, more precisely for the separation of heat-resistant and leaded glass from the waste glass cullet. Soon it became apparent that the fields of application go far beyond the glass sector. The use of this innovative technology together with the development of a new machine design set new standards in the metal sorting, in particular the sorting of nonferrous metals. Using this technology it is possible to separate also material which, up to now, was considered as not sortable.

Sorting regardless of color and surface contamination

An X-ray tube emits X-rays (so-called primary X-ray fluorescence radiation) exciting thereby the metal piece. Depending on the composition of the metal piece, this excitation leads to the emission of a characteristic radiation (secondary X-ray fluorescence radiation). This radiation emitted from the metal piece is representative for the composition of the metal. Each element of the periodic table has a unique and distinctive energy and thus can be detected specifically. If, for example, a pure copper piece is excited, only the radiation with the typical energy of copper is emitted. On the other hand, when exciting the element brass the typical radiation of copper and zinc is perceived. This radiation is captured by special sensors and then evaluated. The technology makes an element-specific sorting of mixed metal waste possible. Besides this element-specific detection, the most important benefit of the XRF technology is the metal sorting regardless of color and surface contamination. Compared to the camera technology, also dirty and not characteristic color pieces of copper and brass, for example, can be accurately detected and sorted. Likewise, it is of course possible to detect metals of the same or similar color separately and to sort them. This unique detection mechanism enables it to separate also grey heavy metals into single elements. All elements after vanadium in the periodic table can be exactly identified and sorted. There are also no limits set to the sorting logic. One element, several elements or the combination of two or more elements can be used as sorting criterion. The threshold values and sensitivities of each single element are variably adjustable.

Redwave XRF

Redwave XRF

Zorba XRF sorting: Gaining all components

The application possibilities of the XRF technology within metal sorting are particularly versatile. For example, nickel-based stainless steels can be separated from nickel-free stainless steels (the same applies to molybdenum-based stainless steels), gold, silver, platinum and other precious metals can be sorted out of a mixture of scrap metals, and aluminium can be also separated from heavy metals as well as single heavy metals into pure fractions. The sorting of Zorba shall be mentioned here in more detail: Zorba is usually composed >70 per cent aluminium, brass, zinc, copper, iron, stainless steels in different percentages as well as accompanying metals such as lead, silver alloys, etc. Using an XRF sorting machine it is possible to gain all components of Zorba according to type and to sort aluminium on the basis of the alloying elements copper, zinc and iron. In the first sorting step all metals, besides aluminium, are positively separated and thereby two fractions are generated, a heavy metal fraction and an aluminium fraction. Then all heavy metals are successively recovered in an off-line set up operation and the high-grade and very precious metals such as copper, brass, zinc, stainless steel, etc. are directly fed to the recycling process as secondary raw materials.

Further separation of aluminium

It is also possible to further separate the aluminium fraction from the first sorting step. Based on the detection of the alloying components copper, zinc and iron in the aluminium alloys, it is possible to separate aluminium alloys which are rich in iron, copper and zinc. The generated pure aluminium fraction corresponds to the primary aluminium quality 6061 and can be also directly recycled. Through this sorting process precious heavy metals must neither be sorted manually, nor exported to low-wage countries and a high-purity aluminium fraction can be produced as well.

Impure Copper and Pure Copper

Impure Copper and Pure Copper

Payback reachable in less than a year

The sorting of Zurik is similar to the sorting of Zorba. First of all, the Zurik fraction is separated from metals which are harmful or undesirable for further processing of stainless steel. Copper, brass, tin, zinc, etc. are separated in that way, which results in the generation of a stainless steel and a mixed metal fraction. The mixed metal fraction can be further separated in an off-line set up operation (as described above) to obtain precious fractions such as copper and brass. The stainless steel fraction can be further sorted on the basis of different alloy components (e.g. positive separation of nickel-based or molybdenum-based stainless steels). Besides the high throughput and purity, the flexible and versatile application possibility of the XRF sorting technology is a great advantage compared to sorting techniques such as camera or X-ray transmission. The applicability of the X-ray fluorescence technology is very multifaceted and not limited to one material class or application. Compared to other technologies, moisture, coloring and surface contamination have no significant negative impact on the detection. High-purity metal fractions are produced due to the sorting processes, which can be sold directly and profitably. Assuming the current revenues for the recovered metals for the sorting of Zorba, the payback of the machine including labor and operating costs shall be reached in less than a year. The flexibility of the XRF technique and the sophisticated sorting logic make it possible to respond to changes of the sorting processes as quick as possible. Furthermore, a great variety of sorting steps can be carried out with the same machine and different preset sorting programs.

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Source/Photos: BT-Wolfgang Binder GmbH