Beyond Density

You can be certain that if the issues of electronic scrap and electronics recycling are being covered by the broader consumer media, such as National Geographic or Time magazine, they are hot and current topics. The same can be said about industry-specific journals, which are featuring more articles on electronics recycling. The reason is simple: Electronic scrap is being collected in larger and larger volumes every month. Electronics are the fastest growing segment of the recycling stream. Several factors are responsible:

Without a doubt, electronic scrap can be difficult to process and to sort, and its complex nature presents many challenges to recyclers.

This article focuses on cutting-edge sorting equipment that is available for and in use at processing facilities. It does not cover the more traditional technologies, such as electro-static separators, fluidized bed, sink-float, hydro-cyclone or any other "density" separators.

From a technical standpoint, the biggest challenge for electronics recyclers is the heterogeneous nature of the incoming material as it arrives at a facility from a variety of sources. Furthermore, the shredding process generates a diverse mix that consists of dozens of different materials that are sometimes interconnected over a wide particle size distribution.

Additionally, processors have to constantly adapt to new and ever-changing legislation while trying to increase the recovery or yield of valuable materials.

Last but not least, processors also have to ensure they can address their customers’ need for quality and quantity of materials, especially for domestic durable plastics markets that are still emerging.

From our experience during the last two years, the following materials are typically targeted for recovery at electronics recycling and sorting facilities today:

While ferrous metals make up about 50 percent of the electronic scrap stream by weight on average, plastics comprise the largest amount by volume. The targeted recovered plastics commodities are generally desired to be free of flame-retardant (FR) materials. However, while technologies are available today to sort FR-containing plastics from non-FR plastics (discussed further below), the high cost of such machinery has thus far prevented it from being used on a larger scale commercially. What this means, in other words, is that plastic scrap from electronics has mostly been exported to Asian countries, with China being the main customer outlet.

A variety of mechanical metal sorting technologies are used in electronics recycling facilities, but magnets and eddy current separators (ECS) have been known to be the most commercially successful. For some dedicated cable/copper sorting applications, air tables, de-stoners and even water tables have been used successfully as well.

However, conventional metal sorting technologies face several challenges in electronic scrap applications. For instance, ECS units sort PCBs with nonferrous metals to a very high degree in light of their copper content. While the smelters don’t mind accepting the PCBs in the aluminum/nonferrous mix, e-scrap recyclers can increase their revenues by separating the PCBs from the nonferrous metals, achieve a selling price that is three to four times higher than when the PCBs are mixed with the nonferrous stream.

Even at small percentages, recovering PCBs from the other nonferrous metals provides a fast payback. This is where optical sorting equipment has been unmatched so far.

In light of the wide size and shape variations of shredded electronics, ECS units have a tendency to cross-contaminate one of the two streams (splitter setting). Additionally, ECS units cannot induce many significant eddy currents into oddly shaped and crumpled particles, thus lowering the recovery of nonferrous metals from the shredded electronic scrap stream. Finally, ECS units cannot separate stainless steel.

One of the more popular technologies available today from several different vendors is induction-based metals sorting equipment.

Where ECS units induce eddy currents into metal particles and then use the subsequently generated magnetic repulsion to physically separate these particles from the other materials, induction-based metal sorters use the eddy current effect merely to sense the presence of metals. Compressed air jets (or, in some instances, mechanical paddles) perform the actual act of separating these metals from the rest of the material stream. This allows induction sorters to detect and sort stainless steel and to do a better job with oddly shaped and distorted particles.

Mechanically, induction sorting machines function quite simply. The shredded incoming material is evenly distributed across the machine’s width (a vibratory feeder is normally used). Depending on the specific application, the material is then transferred onto a wide high-speed conveyor or steep slide. In a slide arrangement, the detection coils are embedded at the very end of the slide, and precisely targeted air jets eject the metals perpendicular to their natural trajectory as the materials come off the slide.

In the conveyor arrangement, the detection coils are positioned as close as possible to the head pulley directly under the conveyor belt. The metals are detected as they travel across the coils at a speed of 500 to 700 feet per minute and are ejected by air jets after they leave the belt.

Today’s state-of-the-art induction technologies, such as the MSS MetalSort unit, are able to distinguish between ferrous, nonferrous and stainless steel. This enables an operator to sort out specific types of metals, if desired. Induction separation systems can do a better job compared to an ECS when it comes to very thin (wires) and odd shaped particles that are generated by the shredder.

In some installations the MetalSort unit has even replaced an ECS. In many instances, where an ECS is already installed in a facility, an induction sorting system can be an economically feasible addition to the system, as it is capable of recovering any residual metals that are still left in the "non-metal" fraction after the material has passed through the ECS.

Other new technologies, such as magnetic induction tomography, are starting to become available as well, but have not been proven enough technically and economically for e-scrap applications.

Optical sensing and sorting technologies have been used in plastics recycling facilities for more than 20 years. However, only in the past two to three years have optical sorters been introduced to e-scrap applications. Systems like the MSS e-Sort have several key advantages over other, more traditional technologies, such as:

Mechanically, optical sorting machines can be set up the same as induction-based sorters, even in a slide or conveyor arrangement. For e-scrap applications, the conveyor system is definitely the arrangement of choice. One of the additional options with optical sorters is that two arrays of air ejectors can be used, allowing systems like the e-Sort to generate three output products from one input stream in a single pass.

Sorting by color is probably one of the most basic tasks an automated optical sorting unit can perform. For electronic scrap, the separation of PCBs, which are mostly green in color with other specific colors such as blue, red and beige in the mix as well, and of copper "wire nests" will be the main focus. Another application, if the input material is reasonably clean, is the separation of metals by color, for example red metals, such as copper and/or brass, can be separated from gray metals, such as aluminum or zinc.

Optical sorting by type of plastic resin can be done using near-infrared (NIR) sensor technology. In the infrared light spectrum each polymer resin and/or blend shows a very specific "signature" that can be detected by the opto-electronic sensor computer.

The separation of ABS, HIPS, PC/ABS or a combination of any or all of the above is frequently used for the electronics stream. Some organic flame-retardant-containing plastics can be separated by NIR separators as well if the amount of FR in the plastic is high enough.

The optical sorting system’s software can be pre-programmed with different "recipes" that enable the user to tailor the sort setup to the different materials processed (e.g. computer monitor housing vs. cell phones, etc.). The MSS e-Sort actually combines color, NIR and metal sorting capabilities, so all of the above described color, resin and metal type optical sorting applications can be carried out in a facility by one sorting sensor module.

Optical sorting modules based on X-ray transmission, such as the MSS X-Sort, can be used to separate brominated-flame-retardant plastics from non-brominated-flame-retardant plastics very effectively. As with the color and NIR sorting module, the X-ray sorters for this application are employed in a conveyor arrangement with special shielding against the X-rays.

A second application for X-ray transmission technology is the separation of leaded CRT (cathode ray tube) glass from non-leaded glass. In this particular case, a slide arrangement is normally used to process the broken glass cullet.

Last, but not least, X-ray transmission can be used to separate metals of different atomic density (e.g., zinc vs. aluminum) if the materials are of reasonably equal size and shape.

One of the up-and-coming sorting technologies for electronic scrap is X-ray fluorescence.

Today, this technique is most commonly used in handheld "single-shot" metal analyzers. The first machines using a "multi-channel" approach to automatically sort (and not only identify) metals of different composition are now coming into operation. However, so far only larger particle sizes can be sorted at relatively low throughputs.

The processing and sorting of obsolete electronics has by far not yet reached its maximum potential. Many areas and applications are subject to intense research and development by a number of companies in search of the optimum electronic scrap processing system. Some of the areas that will be of interest by equipment manufacturers and recyclers in the upcoming months and years include:

Increasing interest in the area of electronics recycling should ensure continued research and development in the area of sorting and separation equipment.