Aluminium scrapUpdated

Laser sorting of aluminium scrap

Laser induced breakdown spectroscopy (LIBS) is an atomic emission spectroscopy procedure that can give rapid chemical analysis of a variety of materials, including all metals. The emergence of more compact, including hand-held, systems makes it possible to use laser spectroscopy directly in technological processes.

Details about sorting and processing of aluminum scrap:

Handbook of aluminium recycling /Ch. Schmitz – 2014

Aluminium recycling /M. Schlesinger – 2017 

Chemical analysis of aluminum scrap

The uncertainty of the chemical composition of the materials that make up aluminium scrap limits the use of recycled aluminium for alloys that have fairly strict and precise restrictions on the chemical composition, such as impurities such as:

  • iron,
  • copper or zinc.

There is the lack of effective methods for sorting aluminium scrap, firstly, by type of metal, and, secondly, by aluminium alloy. That is why only a limited amount of recycled aluminium can be used in the production of aluminum products. Almost all recycled aluminium is used to produce foundry aluminium alloys and aluminium castings. This is because most cast aluminum alloys are very tolerant of high levels of impurities such as iron. Moreover, permanent high-pressure casting requires a higher iron content to ensure that the molten aluminium does not “stick” to the steel molds.

However, to expand the possibilities of using recycled aluminium in foundries, as well as other branches of the aluminium industry, more thorough and efficient classification and sorting of aluminium scrap streams is necessary (Figure 1). This promises great benefits for manufacturers of aluminium products, both from an economic point of view and from the point of view of reducing the negative burden on the environment.

Fifure 1 – Scrap sorting for separating aluminium scrap by type (cast or wrought)
and wrought-alloy family. (From Rosenfeld, A et al) [1]

Laser emission spectroscopy method

The main advantage of laser spectroscopy is that it is capable of doing rapid and remote chemical analysis of a material without sample preparation.

The laser emission spectroscopy method is based on focusing short laser pulses that cause the formation of plasma from a small amount of evaporated material (Figure 2).

Figure 2 – A schematic shows the components of a typical LIBS setup [2]

The very high temperature inside the plasma causes the evaporated material to decompose into excited atoms and ions. As the plasma cools, it emits characteristic atomic emission lines, which can be easily detected by a spectrograph. This method enables rapid and sensitive chemical analysis of virtually any type of substance – solid, liquid and gaseous.

The detection threshold for various chemical elements is usually a few parts per million for heavy metal elements. For light elements the sensitivity of this method is higher than for heavy ones.

Sample preparation is usually not required, so this method can be considered non-destructive in many cases since very little material is removed from the surface. In an application such as sorting aluminium scrap, this feature has no significance. Another advantage of the laser microscopy method is its ability to work through contaminants on the surface of the material, which is very important for sorting aluminium scrap.

Laser analyzers for aluminium scrap

Compactness and the ability to work “in the field” make laser spectroscopy very convenient for sorting aluminium scrap.

Most aluminum scrap on the secondary aluminium market is supplied after processing the scrap in so-called “shredders” – huge shredding machines in which cars and other industrial units and household products are broken into small pieces, no more than 100 mm long, and not always aluminium.

Indeed, this crushed material is very heterogeneous and contains not only pieces of various aluminium alloys, but also various other metals, as well as plastics and other organic materials. Currently, such aluminium scrap is sorted almost entirely by hand, mainly in China and other developing countries, or by very crude and not entirely reliable scrap aluminium sorting methods.

Industrial sorting of aluminum scrap

Laser sorting systems for aluminium scrap have been successfully tested in the laboratory and are already quite widely used in industrial settings. They demonstrate the ability to quickly “recognize” different metals and even different aluminium alloys, which gives them advantages for use in sorting aluminium scrap. An example of a pilot-scale laser spectroscopic sorting system for aluminium scrap is shown in Figure 3.

lazer-sortirovka-alyuminievogo-loma

Figure 3 – A prototype of LIBS system for automatic scrap metal sorting is field-tested [2]

Moreover, laboratory and industrial tests of devices for laser sorting of aluminium scrap demonstrate the ability of this method to work with contaminated aluminium scrap directly in the field conditions of aluminium scrap storage sites. Laser spectroscopy has proven its ability to clearly separate even very contaminated metal components, including those from various aluminium alloys, by chemical composition.

It can be seen that laser emission spectroscopy technology has great potential for use in instruments for industrial monitoring of chemical analysis in the sorting of aluminium scrap. More accurate and full sorting of aluminium scrap will increase the efficiency of its processing and expand the possibilities of using recycled aluminium in various industries.

Sources:

  1. Aluminium recycling /M. Schlesinger – 2017 
  2. www.laserfocusworld.com