Black Out

Large amounts of mixed plastics arise during recycling of household refuse, electronics, automobiles, etc. High-grade black plastics in particular cause problems in subsequent sorting aimed at obtaining clean, reusable and pure fractions of single types of plastic. Black plastics are not detectable with conventional separation processes, and the purity of the separated products therefore is low.
 

Why Black Persists
Black is a trendy, modern color and therefore is used in the plastic surfaces of many pieces of technical equipment, such as telephones and laptops. Furthermore, many plastic parts are made only in black for cost reasons. The “colored surface” is then produced through painting or metallization. Automotive bumpers, for example, are produced exclusively in black or dark grey.

Color hardly plays an aesthetic role, however, with items such as photocopiers, computers or functional automotive parts. Manufacturing black plastic components for these devices makes production and storage easier; but, it also makes recycling more difficult. For example, in recycling of electrical and electronic equipment, large amounts of high grade engineering plastics, such as ABS (acrylonitrile butadiene styrene), PS (polystyrene), etc., arise, of which approximately 50 to 70 percent are black. The share of black plastics in automobiles also is very high. Using the process detailed here, black plastic can be sorted into a pure single material.
 

Beyond Near Infrared
Different processes are commonly used to sort colored plastics. Mixtures of plastics from post-consumer sources (such as post-consumer packaging) are separated into single fractions in sorting plants with the aid of fully automatic sorting equipment. The camera systems used here incorporate near infrared technology (NIR), in which the near-infrared spectrum of light reflected by the plastics is evaluated. Each type of plastic has its own characteristic spectrum that serves to identify the material. The problem with this method, however, is that as black plastics do not reflect the radiation, no spectrum can be recognized, and they cannot be sorted. Work is currently underway on new optical sorting processes to eliminate these drawbacks.

In addition to the problems with NIR sorting methods, the plastics available for sorting are mainly in small pieces because of the preceding mechanical treatment, having to some extent a particle size of less than 2 inches. On top of this, there is severe surface contamination caused by painting as well as by adhered organic or inorganic soil.

As a consequence, there is presently no practical optical sorting equipment for plastic identification available in the market. A number of sorting systems used inline are often required to obtain clean fractions from a mixed plastics material stream.
 

Employing Elecrostatic Separation
Black plastics can be separated easily with electrostatic equipment, which works irrespective of the color of the material being sorted. Electrostatic separators operate on the premise that mixed plastics take on different electrical charges, positive or negative, depending on type. With a mixture of PS and ABS, for example, the PS takes on a negative charge, and the ABS takes on a positive charge. Charged differently in this way, the particles are subsequently separated by an electrode of approximately 35,000 volts. Positively charged ABS is attracted to the negative electrode and negatively charged PS is attracted to the positive electrode.

Purity of individual plastic fractions can be optimized by using a separation sheet. Depending on the composition of the plastics, either a single or two-stage separation process can be useful.

Separators working on the electrostatic principle are used with great success in recycling of window profile off-cuts (separation of rubber from PVC, or polyvinyl chloride), beverage bottles (separation of PVC from PET, or polyethylene terephthalate), electronic scrap and in many other applications.

Typical throughput rates amount to approximately 750 to 1,200 kilograms per hour, or 165 to 2,640 pounds per hour. Higher throughput rates can be easily realized by combining a number of single pieces of equipment operating in parallel.
 

Pretreatment Techniques
As electrostatic separators can only separate positively or negatively charged plastic particles from each other, a mixture consisting of two plastics is ideal. Such mixtures are obtained, for example, by a separation process installed before electrostatic separation, such as separation by density. To sort by density, two separation fluids are usually used, which also clean the surface of heavily soiled plastics.

But, as only dry plastics can be electrostatically charged, a drying stage has to be incorporated. Mechanical shredding in cutting mills (granulators) or similar equipment is often adequate to achieve separation.
 

Practical Examples
The following examples discuss different applications using electrostatic separation with black plastic mixtures.

Separation of mixed plastics from electronic scrap. Typical electronic scrap (excluding televisions and monitors) consists of a predominant share of metals. Mixed plastics comprise approximately 15 percent.

Mixed plastics from electronics consist of many different types of plastics, which arise in different amounts. If the “undesired plastics” (e.g. flame retardant plastics, PVC) are taken out of the equation, a good product share of approximately 55 percent can be obtained. Practice has shown that approximately 70 percent of this “good fraction” consists of black plastics.

A combination of dry and wet separation processes is used for processing mixed plastics from electronic scrap. Undesired foils, fibers, dust, etc., are first separated using sieving and air classification. A PS/ABS concentrate and a polyolefin fraction are then recovered by swim/sink separation in fluids of different density.

The mixture of PS and ABS is “selectively dried” in a subsequent shredding process that takes particles of 1.2 inches to 1.6 inches down to less than 0.4 inches. This means that the large amount of wood present in electronics remains moist, while the plastic on the surface is already dry. It is thereby possible to separate the wood fraction and other conductive products with the “corona roll separator” style of electrostatic separator.

Highly concentrated PS and ABS fractions with purity greater than 98.5 percent are produced in the subsequent electrostatic plastic-from-plastic separation process.An additional effect of this separation stage is that filled polypropylene can be separated.

Recycling of dashboards. Up to 20 percent of scrap arises from the production of cockpits and instrument panels for automobiles. This material is primarily highly rigid, solid construction grades, such as glass-fiber-reinforced SMA (styrene maleic anhydrid) clad with PU (polyurethane) foam and an “outer skin” of PVC or PU, for example. However, in most cases, all three plastics—SMA, PU foam and outer skin—are black.

The production scrap is shredded and ground in a cutting mill (granulator). The PU is almost entirely separated in the mill and can be subsequently completely separated by air classification.

The mixture of black SMA with black PVC is then separated. The achievable purity of the SMA amounts to more than 99.5 percent. This purity is so high that this product can be processed directly into new dashboards. Also, the flexible PVC can be easily processed into new products, on account of its high purity.

Accidentally mixed plastics. Lack of attention can lead to different types of plastics of the same color being mixed together. The mixed plastics are charged in an electrostatic separator and then separated into pure single products. The materials recovered from such faulty lots can be so clean after separation that they can be easily processed as virgin materials.

Functional automotive parts. Ventilation and air conditioning parts in cars that are not in view of passengers are produced almost exclusively from black plastics. Composite materials also are often used, such as in a two-component ventilation flap in glass-fiber-reinforced PP, onto which an elastic sealing strip in TPU (thermoplastic polyurethane) or EPDM (ethylene propylene diene monomer) has been injection molded. When recycling production scrap, contamination with rubber gives rise to problems and prevents reuse of the recyclate.

When the materials are shredded in a granulator, the rubber strip is almost entirely released through friction within the mill. Dust is then removed from the ground material and it is subsequently separated electrostatically. A highly concentrated PP fraction is obtained.

PVC window profiles. Plastic window profiles consist of rigid PVC with rubber or flexible PVC seals. As the window profile is often also in dark PVC, clean separation of the undesired rubber seals is problematic. Rigid PVC from window profiles is an ideal thermoplastic material that can be reworked into new profiles as long as the material is free of rubber.

Window profile off-cuts are ground and dust is removed from the regrind. The rubber and flexible PVC seals are then separated via electrostatic separation. The purity of the PVC fraction enables it to be processed into the inner core layer of new multilayer window profiles.

Recycling of toner cartridges. Empty toner cartridges from photocopiers or fax equipment are taken back by some producers and recycled. They consist of different black plastics with or without flame retardants, sealing foam, silicone rubber strips and more. As the valuable plastics should be used again in production of new cassettes, separation of “foreign plastics” and seal materials is vital.

After various initial processes, a black regrind material is obtained that has a high degree of contamination by foreign plastics and rubber. The mainly black rubber fraction can be separated with the aid of the corona-roll type conductive/nonconductive separators. Other foreign plastics are separated in a further tribo-electric separation stage.

These examples show it is possible to separate black plastics into pure, single materials using electrostatic separation. Electrostatic separators can offer high throughput rates, low separation costs and purity of finished products.

The author is managing partner of Hamos GmbH, Penzberg, Germany and owner of Wersag GmbH & Co KG in Siebenlehn, Germany. He acquired Hamos in 1987, and the first KWS electrostatic metal separator was sold by the company in 1992. Hamos presented the first electrostatic separator for black plastics in 2001.