A new diagnosis

There are no limits set to the recycling of metals because they can be remelted as scrap infinitely. Of course, metals have to be separated from nonmetallic impurities, and different metals have to be sorted into pure fractions to be used effectively as secondary raw materials.

When scrap is used in place of primary raw materials, not only are resources preserved but energy is saved in most cases as well. Furthermore, production costs can be reduced considerably.

Additionally, the resources of the planet are exhaustible, making it more important to use secondary raw materials. By using scrap metal in the production of aluminium, for example, the energy-intensive electrolysis process is eliminated, thereby reducing the total energy costs to approximately 10% of primary production’s energy price.

It also should be noted that the mining of many ores for primary production occurs in politically unstable regions, and sometimes under unsafe working conditions.

On the environmental front, ore mining and the associated processes produce tailings and even “toxic red mud.”
 

Away from the “old” standard

The global demand for ferrous and nonferrous metals is increasing each year. Industrial production in all areas of daily life devours an enormous quantity of aluminium and other nonferrous metals, such as copper, brass, nickel, tin, zinc and lead.

The Institute of Scrap Recycling Industries Inc. (ISRI) in the United States has created terms and specifications for mixed shredded scrap metals, including zorba, zebra, zeppelin, zurik and others.

The current method used for sorting zorba, zurik and metals of similar composition includes sink-float separation processes and sensor-based sorting systems that use camera technology.

The practice of manual sorting is still considerably widespread. Manual sorting results in high purity levels. The process, however, is cost-intensive in industrial countries and should, therefore, only serve as a final check for quality control purposes. Furthermore, not all metals can be sorted manually because discernible differences are not visible. Gray metals can be sorted to a certain extent, and it can be difficult for stainless steel to be separated visually at all.

The sink-float process is used in many treatment plants to separate materials of different density. This process can require large volumes of water and expensive additives, such as ferro-silicon. The separation efficiency also is limited, but aluminium can be separated from heavy metals and low-density nonmetallic impurities (e.g., plastics, wood, etc.).

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. The aluminium recovered cannot always be easily further separated.

The use of sensor-based sorting systems represents an economic and sometimes reasonable alternative to manual or density sorting.

Sorting by color using a color-detecting camera is one possibility offered by sensor-based sorting technology. However, this form of separation also faces limitations. Only fractions that are clearly distinguishable by color, such as copper and brass, can be separated. All gray, heavy metals, such as zinc, lead, nickel and stainless steels, may not be sortable from each other.

Sorting by color camera also can be negatively affected by surface contamination and color impurities. Sorting tests and experience with different input material also have shown that only part of the copper and brass fraction is optically recognizable.

Only as little as one-third of the whole copper and brass fraction corresponds to the color definitions of these two nonferrous metals. The remaining quantity—as much as two-thirds of the total fraction—remains unidentified because of surface contamination, and therefore is unsorted within the heavy-metal fraction.

XRT (X-ray transmission) technology also faces limitations in its sorting efficiency 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, similar to X-ray imaging. Dense material (bones) weakens the X-rays much more than less dense material (tissue). The X-ray transmission thus differentiates material in a similar way as the sink-float process.

Another possibility of sensor-based sorting technology uses XRF (X-ray fluorescence). A few years ago Austria-based Redwave developed a sensor-based sorting system based upon X-ray fluorescence in cooperation with Japan-based Olympus Corp.

Redwave offers optical sorting machines in the environmental and mineral industries, while Olympus is a global market provider of portable XRF systems for rapid on-site measurements and has experience in the field of X-ray fluorescence.

The Redwave XRF sorting system was initially used in the field of glass sorting, more precisely for the separation of heat-resistant and leaded glass from recyclable glass cullet. Soon it became apparent that the fields of application go far beyond the glass sector.
 

The story on XRF sorting

In an XRF sorting unit, an X-ray tube emits X-rays (so-called primary X-ray fluorescence radiation), thereby “exciting” 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 represents the composition of the metal.

Each element of the periodic table has a unique and distinctive energy and thus can be detected. 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 possible an element-specific sorting of mixed metal scrap.

In addition to this element-specific detection, the most important benefit of XRF technology is metal sorting regardless of color and surface contamination. Compared to camera technology, dirty and/or discolored pieces of copper and brass, for example, can be detected accurately and sorted. Likewise, it is possible to detect metals of the same or similar color separately and to sort them.

This unique detection mechanism enables it also to separate gray heavy metals into single elements. All elements after vanadium (V, element number 23) in the periodic table can be exactly identified and sorted. There are also few limits to the sorting logic. One element, several elements or the combination of two or more elements can be used as sorting criteria. The threshold values and sensitivities of each single element are variably adjustable.
 

Zorba and Zurik

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, and 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 also can be separated from heavy metals and single heavy metals can be sorted into pure fractions.

The sorting of zorba is of particularly keen interest to scrap recyclers. Zorba is usually composed of greater than 70% aluminium, brass, zinc, copper, iron and stainless steels in different percentages, as well as accompanying metals, such as lead and silver alloys.

Using one or more XRF sorting machines, it is possible to recover all components of zorba according to type and to sort aluminium on the basis of its alloying elements, including copper, zinc and iron. In the first sorting step, all metals (other than aluminium) are positively separated and, thereby, two fractions are generated: a heavy metal fraction and an aluminium fraction.

Next, all heavy metals are recovered in an offline setup operation and the high-grade and other nonferrous metals, such as copper, brass, zinc and stainless steel, are fed directly to the recycling process as secondary raw materials. The aluminium fraction from the first sorting step also can be separated further.

Based on the detection of alloying components such as copper, zinc and iron in the aluminium alloys, it is possible to separate aluminium alloys that are rich in these metals. The generated pure aluminium fraction corresponds to primary aluminium alloy 6061 and can be remarketed for recycling.

Through this sorting process, nonferrous metals need not be sorted manually or exported, and a high-purity aluminium fraction also can be produced.

The sorting of zurik with this method is similar to the sorting of zorba. First, the zurik fraction is separated from metals that are harmful or undesirable for the further processing of stainless steel. Copper, brass, tin, zinc and other metals are separated in this stage, which results in the generation of a stainless steel and a mixed metal fraction.

The mixed metal fraction can be further separated in an offline setup (as described above) to create red metals fractions such as copper and brass. The stainless steel fraction can be further sorted on the basis of different alloy components (such as the positive separation of nickel-based or molybdenum-based stainless steels).
 

Taking on all challenges

In addition to the high throughput and purity, the flexible and versatile application possibility of the XRF sorting technology can offer advantages compared with many alternative techniques.

The applicability of the X-ray fluorescence technology is multifaceted. Compared with other technologies, moisture, coloring and surface contamination have no significant negative impact on the detection. High-purity metal fractions that can be sold directly and profitably are produced by the sorting processes. Assuming the current revenue for the recovered metals for the sorting of zorba, the payback of the machine—including labour and operating costs—can be reached in less than one year.

The flexibility and sophistication of the XRF technique make it possible to respond to changes in sorting processes as quickly as possible. Furthermore, a great variety of sorting steps can be carried out with the same machine.

The author is a sales manager with the Redwave division of Austria-based BT-Wolfgang Binder GmbH and can be contacted via www.redwave.at.