How to Sort Aluminum Scrap Metal with Magnetic Fields

Aluminum is one of the most widely used metals in the world, thanks to its light weight, strength, and versatility. It is also one of the most recyclable materials, as it can be melted and reused multiple times without losing its quality. Recycling aluminum has many benefits for the environment and the economy, such as saving energy, reducing greenhouse gas emissions, and creating jobs.

However, not all aluminum is the same. There are two main types of aluminum: wrought and cast. Wrought aluminum is made by rolling, extruding, or forging the metal into various shapes and forms, such as sheets, rods, tubes, and wires. Cast aluminum is made by pouring molten metal into molds, such as engine blocks, wheels, and cookware. Wrought aluminum has higher conductivity, ductility, and corrosion resistance than cast aluminum, while cast aluminum has lower cost, higher strength, and better wear resistance than wrought aluminum.

These differences make it important to separate the two types of aluminum before recycling, as mixing them can degrade the quality and performance of the final product. For example, if cast aluminum is mixed with wrought aluminum, the impurities present in the cast aluminum can cause cracks and defects in the wrought aluminum, making it unsuitable for applications that require high strength and durability, such as in the automotive industry.

How to Separate Wrought and Cast Aluminum

There are several methods available for separating wrought and cast aluminum, but they have some limitations and drawbacks. Some of the common methods are:

• Manual sorting: This method relies on human workers to visually inspect and sort the aluminum pieces by hand. This method is slow, labor-intensive, and prone to errors, as it depends on the skill and experience of the workers. It is also not feasible for small and irregular pieces of aluminum that are hard to identify and handle.
• Laser-induced breakdown spectroscopy (LIBS): This method uses a laser beam to vaporize a small portion of the metal surface and analyze the resulting plasma emission spectrum. This method can identify the chemical composition and alloy type of the metal, and thus distinguish wrought and cast aluminum. However, this method is expensive, complex, and sensitive to surface contamination, such as dirt, paint, or oxidation, that can affect the accuracy of the measurement.
• X-ray fluorescence (XRF): This method uses an X-ray source to excite the atoms in the metal and measure the characteristic X-ray emission spectrum. This method can also identify the chemical composition and alloy type of the metal, and thus separate wrought and cast aluminum. However, this method is also costly, complicated, and susceptible to interference from other elements or materials present in the metal, such as iron, copper, or plastic.

A New Method: Magnetic Induction Spectroscopy (MIS)

A new and promising method for sorting wrought and cast aluminum is magnetic induction spectroscopy (MIS). This method uses an oscillating magnetic field to induce eddy currents in the metal, and measures the resulting secondary magnetic field. The secondary magnetic field depends on the conductivity and permeability of the metal, which are different for wrought and cast aluminum. Therefore, by analyzing the frequency response of the secondary magnetic field, wrought and cast aluminum can be distinguished.

MIS has several advantages over the existing methods, such as:

• Simplicity: MIS only requires a coil to generate and detect the magnetic fields, and a signal generator and analyzer to control and process the signals. It does not need any expensive or complex equipment, such as lasers, X-rays, or spectrometers.
• Speed: MIS can measure the secondary magnetic field in milliseconds, and thus sort the aluminum pieces in real time. It can also handle small and irregular pieces of aluminum that are difficult to sort by other methods.
• Robustness: MIS is not affected by surface contamination, such as dirt, paint, or oxidation, that can alter the chemical or physical properties of the metal. It only depends on the intrinsic conductivity and permeability of the metal, which are stable and consistent.

How MIS Works

The basic principle of MIS is illustrated in Figure 1. A coil is placed near a metal piece, and a sinusoidal current is applied to the coil, creating an oscillating primary magnetic field. The primary magnetic field induces eddy currents in the metal piece, which in turn generate a secondary magnetic field that opposes the primary magnetic field. The coil detects the total magnetic field, which is the sum of the primary and secondary magnetic fields. The amplitude and phase of the total magnetic field depend on the frequency of the current, and the conductivity and permeability of the metal piece.

The frequency response of the total magnetic field can be represented by two components: the real component and the imaginary component. The real component is in phase with the primary magnetic field, and the imaginary component is 90 degrees out of phase with the primary magnetic field. The real component reflects the resistive loss of the metal due to the eddy currents, and the imaginary component reflects the reactive energy stored in the metal due to the magnetic induction. The real and imaginary components can be plotted as a function of frequency, forming a curve called the MIS spectrum.

The MIS spectrum can be used to classify wrought and cast aluminum, as they have different conductivity and permeability values, and thus different MIS spectra. Figure 2 shows the MIS spectra of wrought and cast aluminum spheres, measured by a laboratory system. It can be seen that the wrought aluminum has higher real and imaginary components than the cast aluminum, especially at low frequencies, indicating higher conductivity and permeability. Therefore, by comparing the MIS spectra of the metal pieces with the reference spectra of wrought and cast aluminum, they can be sorted accordingly.

How MIS Works in Practice

To test the feasibility and performance of MIS for sorting wrought and cast aluminum in practice, an industrial system was developed and implemented, combining the MIS sensors with a commercial-scale separator system. The system consists of a conveyor belt that transports the aluminum pieces at a speed of 2 m/s, a detection system that measures the MIS spectra of the aluminum pieces, and an ejection system that uses air jets to divert the aluminum pieces into different bins, based on the classification results.

The system was tested with various types and sizes of aluminum pieces, such as cans, foils, wires, tubes, and engine parts. The system was able to sort the aluminum pieces with high accuracy, achieving 89.66% recovery and 94.96% purity for wrought aluminum, and 95.83% recovery and 93.75% purity for cast aluminum. The system also demonstrated high speed and robustness, handling up to 3000 pieces per hour, and being insensitive to surface contamination.

Conclusion

MIS is a new and promising method for sorting wrought and cast aluminum, which are two main types of aluminum that need to be separated before recycling. MIS uses an oscillating magnetic field to induce eddy currents in the metal, and measures the resulting secondary magnetic field. The secondary magnetic field depends on the conductivity and permeability of the metal, which are different for wrought and cast aluminum. Therefore, by analyzing the frequency response of the secondary magnetic field, wrought and cast aluminum can be distinguished.

MIS has several advantages over the existing methods, such as simplicity, speed, and robustness. It only requires a coil, a signal generator and analyzer, and a separator system. It can measure the secondary magnetic field in milliseconds, and sort the aluminum pieces in real time. It is not affected by surface contamination, such as dirt, paint, or oxidation, that can alter the chemical or physical properties of the metal.

MIS has been tested and implemented in an industrial system, combining the MIS sensors with a commercial-scale separator system. The system has shown high accuracy, recovery, and purity for sorting wrought and cast aluminum, as well as high speed and robustness, handling up to 3000 pieces per hour.

MIS is a novel and effective solution for sorting aluminum scrap metal, which can improve the quality and efficiency of aluminum recycling, and contribute to a circular economy and a sustainable future.

Here is a table that summarizes the main differences between wrought and cast aluminum, as well as the methods for sorting them:

ICAS stands for International Annealed Copper Standard, which is a measure of conductivity relative to pure copper.